Design and Synthesis of Cofacially-Arrayed Polyfluorene Wires for Electron and Energy Transfer Studies

A study of cofacially arrayed π-systems is of particular importance for the design of functional materials for efficient long-range intra-chain charge transfer through the bulk semiconducting materials in the layers of photovoltaic devices. The effect of π-stacking between a pair of aromatic rings has been mainly studied in the form of cyclophanes, where aromatic rings are forced into a sandwich-like geometry, which extensively deforms the aromatic rings from planarity. The synthetic difficulties associated with the preparation of cyclophane-like structures has prevented the synthesis of many examples of their multi-layered analogues. Moreover, the few available multi-layered cyclophanes are not readily amenable to the structural modification required for the construction of D–spacer–A triads needed to explore mechanisms of electron and energy transfer. In this review, we recount how a detailed experimental and computational analysis of 1,3-diarylalkanes led to the design of a new class of cofacially arrayed polyfluorenes that retain their π-stacked structure. Thus, efficient synthetic strategies have been established for the ready preparation of monodisperse polyfluorenes with up to six π-stacked fluorenes, which afford ready access to D–spacer–A triads by linking donor and acceptor groups to the polyfluorene spacers via single methylenes. Detailed 1H NMR spectroscopy, X-ray crystallography, electrochemistry, and He(I) photoelectron spectroscopy of F2–F6 have confirmed the rigid cofacial stacking of multiple fluorenes in F2–F6, despite the presence of rotatable C–C bonds. These polyfluorenes (F2–F6) form stable cation radicals in which a single hole is delocalized amongst the stacked fluorenes, as judged by the presence of intense charge-resonance transition in their optical spectra. Interestingly, these studies also discern that delocalization of a single cationic charge could occur over multiple fluorene rings in F2–F6, while the exciton is likely localized only onto two fluorenes in F2–F6. Facile synthesis of the D–spacer–A triads allowed us to demonstrate that efficient triplet energy transfer can occur through π-stacked polyfluorenes; the mechanism of energy transfer crosses over from tunneling to hopping with increasing number of fluorenes in the polyfluorene spacer. We suggest that the development of rigidly held π-stacked polyfluorenes, described herein, with well-defined redox and optoelectronic properties provides an ideal scaffold for the study of electron and energy transfer in D-spacer-A triads, where the Fn spacers serve as models for cofacially stacked π-systems.


Introduction
Through-space interactions between cofacially stacked aromatic rings are at the origin of many phenomena of photovoltaics and biological chemistry. For example, the role of cofacially stacked π-systems has recently played out prominently in studies of long-range charge transfer in DNA through stacked aromatic bases, as well as in the exploration of DNA's use in futuristic molecular electronic devices ( Figure 1) [1]. After much controversy exploration of DNA's use in futuristic molecular electronic devices ( Figure 1) [1]. After much controversy surrounding the efficiency of long-range charge transfer through stacked bases in DNA, the current consensus, reached largely through the work of researchers at Northwestern University [2], is that DNA can transport charge via a hopping mechanism through π-stacked bases [3][4][5][6][7]. While the suggested application of DNA in modern photovoltaic devices is yet to be realized, usage of cofacially stacked organic π-systems could instead be potentially important in photovoltaic devices, i.e., organic solar cells and light emitting diodes [1]. For example, since the discovery of conducting polyacetylenes [8], a variety of π-conjugated organic polymers, e.g., poly-p-phenylenes, polythiophenes, polyphenylenevinylenes, and polyfluorenes [9][10][11][12][13][14], have been employed in charge-transport layers of the photovoltaic devices. It is expected that the efficiency of charge transport in the conducting layers of these devices is governed by not only the intra-chain charge transport through the backbone of π-conjugated polymers but also via inter-chain charge transport through the bulk material, where the assembly of π-conjugated molecules is expected to be randomized ( Figure 2) [15][16][17][18][19]. Therefore, development of the next generation of conductive organic materials for photovoltaic applications requires careful study of the model compounds to develop a fundamental understanding of structure-function relationships for the interchain charge transport through cofacially stacked aromatic π-systems.  While the suggested application of DNA in modern photovoltaic devices is yet to be realized, usage of cofacially stacked organic π-systems could instead be potentially important in photovoltaic devices, i.e., organic solar cells and light emitting diodes [1]. For example, since the discovery of conducting polyacetylenes [8], a variety of π-conjugated organic polymers, e.g., poly-p-phenylenes, polythiophenes, polyphenylenevinylenes, and polyfluorenes [9][10][11][12][13][14], have been employed in charge-transport layers of the photovoltaic devices. It is expected that the efficiency of charge transport in the conducting layers of these devices is governed by not only the intra-chain charge transport through the backbone of π-conjugated polymers but also via inter-chain charge transport through the bulk material, where the assembly of π-conjugated molecules is expected to be randomized ( Figure 2) [15][16][17][18][19]. Therefore, development of the next generation of conductive organic materials for photovoltaic applications requires careful study of the model compounds to develop a fundamental understanding of structure-function relationships for the interchain charge transport through cofacially stacked aromatic π-systems.
Molecules 2023, 28, x FOR PEER REVIEW 2 of 29 exploration of DNA's use in futuristic molecular electronic devices ( Figure 1) [1]. After much controversy surrounding the efficiency of long-range charge transfer through stacked bases in DNA, the current consensus, reached largely through the work of researchers at Northwestern University [2], is that DNA can transport charge via a hopping mechanism through π-stacked bases [3][4][5][6][7]. While the suggested application of DNA in modern photovoltaic devices is yet to be realized, usage of cofacially stacked organic π-systems could instead be potentially important in photovoltaic devices, i.e., organic solar cells and light emitting diodes [1]. For example, since the discovery of conducting polyacetylenes [8], a variety of π-conjugated organic polymers, e.g., poly-p-phenylenes, polythiophenes, polyphenylenevinylenes, and polyfluorenes [9][10][11][12][13][14], have been employed in charge-transport layers of the photovoltaic devices. It is expected that the efficiency of charge transport in the conducting layers of these devices is governed by not only the intra-chain charge transport through the backbone of π-conjugated polymers but also via inter-chain charge transport through the bulk material, where the assembly of π-conjugated molecules is expected to be randomized ( Figure 2) [15][16][17][18][19]. Therefore, development of the next generation of conductive organic materials for photovoltaic applications requires careful study of the model compounds to develop a fundamental understanding of structure-function relationships for the interchain charge transport through cofacially stacked aromatic π-systems.

Cyclophanes as Models for the Study of π-Stacking
The π-stacking or through-space interactions between a pair of cofacially stacked aromatic rings has been studied primarily in the cyclophanes, molecules where two π-systems are forced into a sandwich-like geometry that generally imparts extensive deformation of the cofacial aromatic moieties (Figure 3) [20]. The chemistry of cyclophanes was first initiated by Cram [21], who showed that the two benzene rings in paracyclophane 1 lie at a distance shorter than the van der Waals contact. Expectedly, such a close cofacial approach of a pair of benzene rings in 1 alters its electronic and photophysical properties owing to the efficient electronic coupling [21].

Cyclophanes as Models for the Study of π-Stacking
The π-stacking or through-space interactions between a pair of cofacially stacked omatic rings has been studied primarily in the cyclophanes, molecules where two π-s tems are forced into a sandwich-like geometry that generally imparts extensive de mation of the cofacial aromatic moieties (Figure 3) [20]. The chemistry of cyclophanes first initiated by Cram [21], who showed that the two benzene rings in paracyclophan lie at a distance shorter than the van der Waals contact. Expectedly, such a close cofa approach of a pair of benzene rings in 1 alters its electronic and photophysical proper owing to the efficient electronic coupling [21]. Since this initial discovery, a variety of cyclophanes containing polynuclear arom hydrocarbons (2)(3)(4)(5)(6) with varying bridge sizes have been synthesized (Figure 3) [20 recent successful multi-step synthesis of a six-bridge "superphane" 9 ( Figure 3) provi testament to the continually evolving power of modern synthetic methodologies; h ever, electronic coupling between aromatic rings in 9 is dramatically modulated via sigma framework [22], rendering it a non-ideal model for the evaluation of π-stacking

Multi-Decker Cyclophanes
Due the synthetic difficulties, examples of cyclophanes with multiple layers scarce (e.g., Figure 3b) [23]. Unfortunately, the few reported systems (11)(12)(13) exist a mixture of stereoisomers, and the benzene rings are significantly distorted from plana [23]. Examples of multi-layered cyclophanes in which the benzene rings are largely pla have also been prepared via multi-step syntheses ( Figure 4); however, these studies h been generally limited to cyclic voltammetry and donor-acceptor complexation with racyanoethylene as an electron acceptor [24][25][26].  Since this initial discovery, a variety of cyclophanes containing polynuclear aromatic hydrocarbons (2-6) with varying bridge sizes have been synthesized (Figure 3) [20]. A recent successful multi-step synthesis of a six-bridge "superphane" 9 ( Figure 3) provides testament to the continually evolving power of modern synthetic methodologies; however, electronic coupling between aromatic rings in 9 is dramatically modulated via the sigma framework [22], rendering it a non-ideal model for the evaluation of π-stacking.

Multi-Decker Cyclophanes
Due the synthetic difficulties, examples of cyclophanes with multiple layers are scarce (e.g., Figure 3b) [23]. Unfortunately, the few reported systems (11)(12)(13) exist as a mixture of stereoisomers, and the benzene rings are significantly distorted from planarity [23]. Examples of multi-layered cyclophanes in which the benzene rings are largely planar have also been prepared via multi-step syntheses ( Figure 4); however, these studies have been generally limited to cyclic voltammetry and donor-acceptor complexation with tetracyanoethylene as an electron acceptor [24][25][26].  A hallmark of electronic coupling in these systems is the lowering of th tential by efficient stabilization of the cationic charge. While the aromatic ring clophane-like molecules 14-22 in Figure 4 do not suffer distortions from pla interplanar angles are far from the sandwich-like geometries found in cyclop ( Figure 3). It is therefore of interest that the reversible oxidation potentials o cyclophane-like bichromophoric molecules 28-30 (Table 1) all show a similar namic stabilization of the cationic charge by ~350 ± 30 mV, despite varied inte gles (0-30°). Table 1. The Eox1 of cyclophane-like bichromophoric molecules with varying interplana tained from the X-ray structures) and their corresponding model compounds [12].

Monomer
Eox (  A hallmark of electronic coupling in these systems is the lowering of the redox potential by efficient stabilization of the cationic charge. While the aromatic rings in the cyclophanelike molecules 14-22 in Figure 4 do not suffer distortions from planarity, their interplanar angles are far from the sandwich-like geometries found in cyclophanes 1-13 ( Figure 3). It is therefore of interest that the reversible oxidation potentials of a series of cyclophane-like bichromophoric molecules 28-30 (Table 1) all show a similar thermodynamic stabilization of the cationic charge by~350 ± 30 mV, despite varied interplanar angles (0-30 • ).
In comparison, an intermolecularly formed dimer cation radical of octamethylbiphenylene (32), displaying a perfect sandwich-like geometry and free of distortions of aromatic rings, also showed an enthalpic stabilization of a single cationic charge by~350 mV ( Figure 5) [16]. It is noted that the dimerization constants of formation of intermolecular (32) 2 +• (K~300 M −1 or ∆Go~−150 mV) as well as other dimeric cation radicals [16,27] are relatively low because of the significant entropic penalty for the formation of the intermolecular dimer (i.e., for (32) 2 +• , T∆S o =~200 mV at 22 • C) [16], a penalty that is absent in the rigid cyclophane-like donors 28-31.
Electrochemical evaluation of various cofacially arrayed bichromophoric systems (Table 1) against the intermolecularly formed dimer cation radical of 32 ( Figure 5) clearly suggests that a close but not complete sandwich-like geometry between interacting πsystems is necessary for effective electronic coupling. Consistent with this expectation, the drastically reduced overlap between the aryl moieties in ethanoanthracene 31, due to the large interplanar angle of 120 • , leads to reduced stabilization of the cationic charge in its cation radical (i.e.,~250 mV) compared with the corresponding model electron donor 27 (Table 1) [28]. Table 1. The E ox1 of cyclophane-like bichromophoric molecules with varying interplanar angles (obtained from the X-ray structures) and their corresponding model compounds [12].

Monomer
Eox tential by efficient stabilization of the cationic charge. While the aromatic rings in the cyclophane-like molecules 14-22 in Figure 4 do not suffer distortions from planarity, their interplanar angles are far from the sandwich-like geometries found in cyclophanes 1-13 ( Figure 3). It is therefore of interest that the reversible oxidation potentials of a series of cyclophane-like bichromophoric molecules 28-30 (Table 1) all show a similar thermodynamic stabilization of the cationic charge by ~350 ± 30 mV, despite varied interplanar angles (0-30°). Table 1. The Eox1 of cyclophane-like bichromophoric molecules with varying interplanar angles (obtained from the X-ray structures) and their corresponding model compounds [12]. tential by efficient stabilization of the cationic charge. While the aromatic rings in the cyclophane-like molecules 14-22 in Figure 4 do not suffer distortions from planarity, their interplanar angles are far from the sandwich-like geometries found in cyclophanes 1-13 ( Figure 3). It is therefore of interest that the reversible oxidation potentials of a series of cyclophane-like bichromophoric molecules 28-30 (Table 1) all show a similar thermodynamic stabilization of the cationic charge by ~350 ± 30 mV, despite varied interplanar angles (0-30°). Table 1. The Eox1 of cyclophane-like bichromophoric molecules with varying interplanar angles (obtained from the X-ray structures) and their corresponding model compounds [12]. tential by efficient stabilization of the cationic charge. While the aromatic rings in the cyclophane-like molecules 14-22 in Figure 4 do not suffer distortions from planarity, their interplanar angles are far from the sandwich-like geometries found in cyclophanes 1-13 ( Figure 3). It is therefore of interest that the reversible oxidation potentials of a series of cyclophane-like bichromophoric molecules 28-30 (Table 1) all show a similar thermodynamic stabilization of the cationic charge by ~350 ± 30 mV, despite varied interplanar angles (0-30°). Table 1. The Eox1 of cyclophane-like bichromophoric molecules with varying interplanar angles (obtained from the X-ray structures) and their corresponding model compounds [12]. tential by efficient stabilization of the cationic charge. While the aromatic rings in the cyclophane-like molecules 14-22 in Figure 4 do not suffer distortions from planarity, their interplanar angles are far from the sandwich-like geometries found in cyclophanes 1-13 ( Figure 3). It is therefore of interest that the reversible oxidation potentials of a series of cyclophane-like bichromophoric molecules 28-30 (Table 1) all show a similar thermodynamic stabilization of the cationic charge by ~350 ± 30 mV, despite varied interplanar angles (0-30°). Table 1. The Eox1 of cyclophane-like bichromophoric molecules with varying interplanar angles (obtained from the X-ray structures) and their corresponding model compounds [12]. tential by efficient stabilization of the cationic charge. While the aromatic rings in the cyclophane-like molecules 14-22 in Figure 4 do not suffer distortions from planarity, their interplanar angles are far from the sandwich-like geometries found in cyclophanes 1-13 ( Figure 3). It is therefore of interest that the reversible oxidation potentials of a series of cyclophane-like bichromophoric molecules 28-30 (Table 1) all show a similar thermodynamic stabilization of the cationic charge by ~350 ± 30 mV, despite varied interplanar angles (0-30°). Table 1. The Eox1 of cyclophane-like bichromophoric molecules with varying interplanar angles (obtained from the X-ray structures) and their corresponding model compounds [12]. tential by efficient stabilization of the cationic charge. While the aromatic rings in the cyclophane-like molecules 14-22 in Figure 4 do not suffer distortions from planarity, their interplanar angles are far from the sandwich-like geometries found in cyclophanes 1-13 ( Figure 3). It is therefore of interest that the reversible oxidation potentials of a series of cyclophane-like bichromophoric molecules 28-30 (Table 1) all show a similar thermodynamic stabilization of the cationic charge by ~350 ± 30 mV, despite varied interplanar angles (0-30°). Table 1. The Eox1 of cyclophane-like bichromophoric molecules with varying interplanar angles (obtained from the X-ray structures) and their corresponding model compounds [12]. In comparison, an intermolecularly formed dimer cation radical of octamethylbiphenylene (32), displaying a perfect sandwich-like geometry and free of distortions of aromatic rings, also showed an enthalpic stabilization of a single cationic charge by ~350 mV ( Figure 5) [16]. It is noted that the dimerization constants of formation of intermolecular (32)2 +• (K~300 M −1 or ΔGo~−150 mV) as well as other dimeric cation radicals [16,27] are relatively low because of the significant entropic penalty for the formation of the intermolecular dimer (i.e., for (32)2 +• , TΔS o = ~200 mV at 22 °C) [16], a penalty that is absent in the rigid cyclophane-like donors 28-31. In comparison, an intermolecularly formed dimer cation radical of octamethylbiphenylene (32), displaying a perfect sandwich-like geometry and free of distortions of aromatic rings, also showed an enthalpic stabilization of a single cationic charge by ~350 mV ( Figure 5) [16]. It is noted that the dimerization constants of formation of intermolecular (32)2 +• (K~300 M −1 or ΔGo~−150 mV) as well as other dimeric cation radicals [16,27] are relatively low because of the significant entropic penalty for the formation of the intermolecular dimer (i.e., for (32)2 +• , TΔS o = ~200 mV at 22 °C) [16], a penalty that is absent in the rigid cyclophane-like donors 28-31. In comparison, an intermolecularly formed dimer cation radical of octamethylbiphenylene (32), displaying a perfect sandwich-like geometry and free of distortions of aromatic rings, also showed an enthalpic stabilization of a single cationic charge by ~350 mV ( Figure 5) [16]. It is noted that the dimerization constants of formation of intermolecular (32)2 +• (K~300 M −1 or ΔGo~−150 mV) as well as other dimeric cation radicals [16,27] are relatively low because of the significant entropic penalty for the formation of the intermolecular dimer (i.e., for (32)2 +• , TΔS o = ~200 mV at 22 °C) [16], a penalty that is absent in the rigid cyclophane-like donors 28-31. In comparison, an intermolecularly formed dimer cation radical of octamethylbiphenylene (32), displaying a perfect sandwich-like geometry and free of distortions of aromatic rings, also showed an enthalpic stabilization of a single cationic charge by ~350 mV ( Figure 5) [16]. It is noted that the dimerization constants of formation of intermolecular (32)2 +• (K~300 M −1 or ΔGo~−150 mV) as well as other dimeric cation radicals [16,27] are relatively low because of the significant entropic penalty for the formation of the intermolecular dimer (i.e., for (32)2 +• , TΔS o = ~200 mV at 22 °C) [16], a penalty that is absent in the rigid cyclophane-like donors 28-31. In comparison, an intermolecularly formed dimer cation radical of octamethylbiphenylene (32), displaying a perfect sandwich-like geometry and free of distortions of aromatic rings, also showed an enthalpic stabilization of a single cationic charge by ~350 mV ( Figure 5) [16]. It is noted that the dimerization constants of formation of intermolecular (32)2 +• (K~300 M −1 or ΔGo~−150 mV) as well as other dimeric cation radicals [16,27] are relatively low because of the significant entropic penalty for the formation of the intermolecular dimer (i.e., for (32)2 +• , TΔS o = ~200 mV at 22 °C) [16], a penalty that is absent in the rigid cyclophane-like donors 28-31. In comparison, an intermolecularly formed dimer cation radical of octamethylbiphenylene (32), displaying a perfect sandwich-like geometry and free of distortions of aromatic rings, also showed an enthalpic stabilization of a single cationic charge by ~350 mV ( Figure 5) [16]. It is noted that the dimerization constants of formation of intermolecular (32)2 +• (K~300 M −1 or ΔGo~−150 mV) as well as other dimeric cation radicals [16,27] are relatively low because of the significant entropic penalty for the formation of the intermolecular dimer (i.e., for (32)2 +• , TΔS o = ~200 mV at 22 °C) [16], a penalty that is absent in the rigid cyclophane-like donors 28-31. In comparison, an intermolecularly formed dimer cation radical of octamethylbiphenylene (32), displaying a perfect sandwich-like geometry and free of distortions of aromatic rings, also showed an enthalpic stabilization of a single cationic charge by ~350 mV ( Figure 5) [16]. It is noted that the dimerization constants of formation of intermolecular (32)2 +• (K~300 M −1 or ΔGo~−150 mV) as well as other dimeric cation radicals [16,27] are relatively low because of the significant entropic penalty for the formation of the intermolecular dimer (i.e., for (32)2 +• , TΔS o = ~200 mV at 22 °C) [16], a penalty that is absent in the rigid cyclophane-like donors 28-31. Electrochemical evaluation of various cofacially arrayed bichromophoric systems (Table 1) against the intermolecularly formed dimer cation radical of 32 ( Figure 5) clearly suggests that a close but not complete sandwich-like geometry between interacting π-systems is necessary for effective electronic coupling. Consistent with this expectation, the drastically reduced overlap between the aryl moieties in ethanoanthracene 31, due to the large interplanar angle of 120°, leads to reduced stabilization of the cationic charge in its

Rationale for the Design of a New Class of Cofacially π-Stacked Polyfluorenes
From these studies, it is clear that the design and synthesis of new π-stacked molecular arrays without the necessity for perfect sandwich-like arrangement are needed for the study of wire-like materials. This insight led us to explore structures that could be accessed via simpler and more versatile synthetic routes. The description below highlights the evolution of design principles involved in the development of cofacially arrayed π-stacked polyfluorenes with the aid of insights gleaned from 1,3-diarylalkane-type structures. It had been speculated [29] that phenyl groups in stereo-regular polystyrenes (33) could acquire a cofacial π-stacked arrangement ( Figure 6a). Unfortunately, experimental/computational studies of various model 1,3-diarylalkanes ( Figure 6b) clearly showed that folding of extended 34e to cyclophane-like 34f is unfavorable [30][31][32]. For example, the (observed) lack of folding of 34e +• into appreciable amounts of 34f +• , together with the similar oxidation potentials of 34 and its model compound 35, suggests that the anticipated enthalpic gain (~350 mV) is mostly negated by the entropic penalty expended in organizing a structure containing four C-C single bond rotors ( Figure 6b). (Table 1) [28].

Rationale for the Design of a New Class of Cofacially π-Stacked Polyfluorenes
From these studies, it is clear that the design and synthesis of new π-stacked molecular arrays without the necessity for perfect sandwich-like arrangement are needed for the study of wire-like materials. This insight led us to explore structures that could be accessed via simpler and more versatile synthetic routes. The description below highlights the evolution of design principles involved in the development of cofacially arrayed πstacked polyfluorenes with the aid of insights gleaned from 1,3-diarylalkane-type structures. It had been speculated [29] that phenyl groups in stereo-regular polystyrenes (33) could acquire a cofacial π-stacked arrangement ( Figure 6a). Unfortunately, experimental/computational studies of various model 1,3-diarylalkanes ( Figure 6b) clearly showed that folding of extended 34e to cyclophane-like 34f is unfavorable [30][31][32]. For example, the (observed) lack of folding of 34e +• into appreciable amounts of 34f +• , together with the similar oxidation potentials of 34 and its model compound 35, suggests that the anticipated enthalpic gain (~350 mV) is mostly negated by the entropic penalty expended in organizing a structure containing four C-C single bond rotors (Figure 6b).  Table 1.
In order to stabilize a folded cyclophane-like structure, a reduction in the number of C-C bond rotors was required. Thus, we prepared 1,3-diarylalkane 36, which contains only two C-C single bond rotors ( Figure 7a) [33]. Interestingly, the lowering of the first oxidation potential of 36 (~100 mV) compared with that of the model 37 ( Figure 7b) suggests that upon 1-electron oxidation, the extended 36e +• must transform instantaneously into a cyclophane-like 36f +• , presumably due to the reduced entropic penalty (Scheme 1) [33]. A comparison of the energetics of the 36e +• → 36f +• transformation by experiment and DFT calculations ( Figure 8) clearly demonstrate that the entropic penalty for its folding (TΔS° = 230 mV) is significantly reduced due to the reduced number of C-C bond rotors in 36 in comparison with 34 ( Figure 6c). It was assumed that E ox of 34f is the same as cyclophane 28 in Table 1.
In order to stabilize a folded cyclophane-like structure, a reduction in the number of C-C bond rotors was required. Thus, we prepared 1,3-diarylalkane 36, which contains only two C-C single bond rotors ( Figure 7a) [33]. Interestingly, the lowering of the first oxidation potential of 36 (~100 mV) compared with that of the model 37 ( Figure 7b) suggests that upon 1-electron oxidation, the extended 36e +• must transform instantaneously into a cyclophane-like 36f +• , presumably due to the reduced entropic penalty (Scheme 1) [33]. A comparison of the energetics of the 36e +• → 36f +• transformation by experiment and DFT calculations ( Figure 8) clearly demonstrate that the entropic penalty for its folding (T∆S • = 230 mV) is significantly reduced due to the reduced number of C-C bond rotors in 36 in comparison with 34 ( Figure 6c). The absorption spectrum of 36 +• shows a characteristic intervalence (charge-reso nance) transition [27] at λmax = ~1600 nm, while the spectrum of model 37+• lacks any absorption in this region (see Figure 7c), thus confirming the 36e +• → 36f +• transformation in Scheme 1 [33]. The absorption spectrum of 36 +• shows a characteristic intervalence (charge-resonance) transition [27] at λmax = ~1600 nm, while the spectrum of model 37+• lacks any absorption in this region (see Figure 7c), thus confirming the 36e +• → 36f +• transformation in Scheme 1 [33].
We then carefully scrutinized the intermediate (transient) structures formed during the 36f → 36e transformation by DFT calculations. Figure 9a compiles a series of intermediate structures formed by rotation around one of the two equivalent C-C bond rotors, which shows that the rotation around the C-C bond in 36f is not significantly impeded by steric crowding of the aromatic rings. Moreover, the slightly lower energy (~2 kcal mol −1 ) of 36e in comparison with 36f indicates that it will exist largely in an extended conformer in the neutral state [16,34]. We then carefully scrutinized the intermediate (transient) structures formed during the 36f → 36e transformation by DFT calculations. Figure 9a compiles a series of intermediate structures formed by rotation around one of the two equivalent C-C bond rotors, which shows that the rotation around the C-C bond in 36f is not significantly impeded by steric crowding of the aromatic rings. Moreover, the slightly lower energy (~2 kcal mol −1 ) of 36e in comparison with 36f indicates that it will exist largely in an extended conformer in the neutral state [16,34].   We then carefully scrutinized the intermediate (transient) structures formed during the 36f → 36e transformation by DFT calculations. Figure 9a compiles a series of intermediate structures formed by rotation around one of the two equivalent C-C bond rotors, which shows that the rotation around the C-C bond in 36f is not significantly impeded by steric crowding of the aromatic rings. Moreover, the slightly lower energy (~2 kcal mol −1 ) of 36e in comparison with 36f indicates that it will exist largely in an extended conformer in the neutral state [16,34].  This analysis suggested that increasing the size of the aromatic 'plate' in 36 may lead to a 1,3-diarylalkane derivative in which the rotation around the C-C bond will be sterically impeded. Indeed, a conformational analysis of such a derivative, i.e., the difluorene F2 in Figure 9b, showed that the large size of the fluorene rings and their peri-hydrogens in F2 pose severe steric crowding for transformation into an extended conformer. Indeed, DFT calculations predict that the extended conformer of neutral F2 is~10 kcal mol −1 higher in energy than the more stable cofacial conformer. We therefore undertook synthesis of the cofacially arrayed difluorene F2 and its higher homologues (Fn).

Development of Synthetic Strategies for the Preparation of π-Stacked Polyfluorene Oligomers
Fluorene is readily available and contains a planar biphenyl unit which can be easily functionalized at its active methylene carbon 9 by nucleophilic substitution, as well as at the activated aromatic carbons 2 and 7 by either electrophilic aromatic substitution or palladium-catalyzed coupling reactions (i.e., Scheme 2). Figure 9b, showed that the large size of the fluorene rings and their peri-hy in F2 pose severe steric crowding for transformation into an extended conformer. DFT calculations predict that the extended conformer of neutral F2 is ~10 kcal mol − in energy than the more stable cofacial conformer.

F2 in
We therefore undertook synthesis of the cofacially arrayed difluorene F2 higher homologues (Fn).

Development of Synthetic Strategies for the Preparation of π-Stacked Polyflu Oligomers
Fluorene is readily available and contains a planar biphenyl unit which can b functionalized at its active methylene carbon 9 by nucleophilic substitution, as w the activated aromatic carbons 2 and 7 by either electrophilic aromatic substitution ladium-catalyzed coupling reactions (i.e., Scheme 2).

Scheme 2. Different functionalization sites in fluorene.
It was first thought that the synthesis of F2 (or its higher homologue F3) c accomplished by use of the intramolecular Friedel-Crafts reaction of the carbocat termediate 39, generated from an alcohol 38 (Scheme 3).

Scheme 3. Construction of fluorene ring system by intramolecular Friedel-Crafts cyclizatio
Unfortunately, the two conceived Friedel-Crafts precursors to F3, i.e., 44 and easily synthesized 43 or 47 (Scheme 4), could not be prepared due to the failur reactions of either 43 or 47 with 2-biphenylmagnesium bromide or 2-biphenyl most likely due to steric hindrance.
in F2 pose severe steric crowding for transformation into an extended conformer. Indeed, DFT calculations predict that the extended conformer of neutral F2 is ~10 kcal mol −1 higher in energy than the more stable cofacial conformer.
We therefore undertook synthesis of the cofacially arrayed difluorene F2 and its higher homologues (Fn).

Development of Synthetic Strategies for the Preparation of π-Stacked Polyfluorene Oligomers
Fluorene is readily available and contains a planar biphenyl unit which can be easily functionalized at its active methylene carbon 9 by nucleophilic substitution, as well as at the activated aromatic carbons 2 and 7 by either electrophilic aromatic substitution or palladium-catalyzed coupling reactions (i.e., Scheme 2).

Scheme 2. Different functionalization sites in fluorene.
It was first thought that the synthesis of F2 (or its higher homologue F3) could be accomplished by use of the intramolecular Friedel-Crafts reaction of the carbocationic intermediate 39, generated from an alcohol 38 (Scheme 3).  The desired bis-alkene 50 was easily synthesized in two steps (Scheme 6); indeed, it underwent a facile acid-catalyzed intramolecular Friedel-Crafts cyclization to produce F3 in quantitative yield (Scheme 6) [35]. This three-step strategy for the preparation of F3 from parent fluorene was also applicable to synthesis of F2 from 9-methylfluorene in~85% overall yield (Scheme 6).  The desired bis-alkene 50 was easily synthesized in two steps (Scheme 6); indeed, it underwent a facile acid-catalyzed intramolecular Friedel-Crafts cyclization to produce F3 in quantitative yield (Scheme 6) [35]. This three-step strategy for the preparation of F3 from parent fluorene was also applicable to synthesis of F2 from 9-methylfluorene in ~85% overall yield (Scheme 6).  The desired bis-alkene 50 was easily synthesized in two steps (Scheme 6); indeed, it underwent a facile acid-catalyzed intramolecular Friedel-Crafts cyclization to produce F3 in quantitative yield (Scheme 6) [35]. This three-step strategy for the preparation of F3 from parent fluorene was also applicable to synthesis of F2 from 9-methylfluorene in ~85% overall yield (Scheme 6).  The desired bis-alkene 50 was easily synthesized in two steps (Scheme 6); in underwent a facile acid-catalyzed intramolecular Friedel-Crafts cyclization to pro in quantitative yield (Scheme 6) [35]. This three-step strategy for the preparatio from parent fluorene was also applicable to synthesis of F2 from 9-methylfluorene overall yield (Scheme 6). Scheme 6. Successful syntheses of F2 and F3 via a reliable three-step strategy. Scheme 6. Successful syntheses of F2 and F3 via a reliable three-step strategy.
The di-fluorene F2 can also be prepared by methylation of the difluorenemethane 55 (Scheme 7). Unfortunately, 55 was obtained in low yield (>20%) after tedious purification of a complex mixture from a reaction of fluorene with paraformaldehyde in the presence of t BuOK (potassium tert-butoxide) in DMF (Scheme 7) [36]. Notwithstanding the poor yield of 55, its availability permitted the preparation of the tetra-fluorene F4 with~70% yield from 55 using the three-step strategy outlined in Scheme 6, i.e., alkylation, Kumada coupling, and Friedel-Crafts cyclization (Scheme 7).
Molecules 2023, 28, x FOR PEER REVIEW 11 The di-fluorene F2 can also be prepared by methylation of the difluorenemethan (Scheme 7). Unfortunately, 55 was obtained in low yield (>20%) after tedious purifica of a complex mixture from a reaction of fluorene with paraformaldehyde in the pres of t BuOK (potassium tert-butoxide) in DMF (Scheme 7) [36]. Notwithstanding the p yield of 55, its availability permitted the preparation of the tetra-fluorene F4 with ~ yield from 55 using the three-step strategy outlined in Scheme 6, i.e., alkylation, Kum coupling, and Friedel-Crafts cyclization (Scheme 7). Scheme 7. Preparation of F4 from 55 using a three-step strategy.
A simple mechanistic analysis of the reported preparation of 55 [36] suggested an initial aldol-type condensation between a fluoranyl anion (58) and formaldeh should produce 9-fluorenomethanol 60 as the initial product followed by formal elim tion of water to produce 9-methylenefluorene 61. A subsequent Michael-type additio 58 to 61 can produce 55 after protonation, i.e., Scheme 8. Unfortunately, all the react depicted in Scheme 8 are expected to be reversible, and therefore, an optimization of reaction conditions may allow access to 55 and its higher homologues with good yiel Surprisingly, a reaction of equimolar (commercially available) fluorenomethano and fluorene in the presence of t BuOK as a base in DMF at 22 °C for ~2 min yielded p 55 after a simple pouring of the reaction mixture into water, filtration, and recrystal tion from CH2Cl2/C2H5OH, CH2Cl2/C2H5OH with >86% yield (Scheme 9) [37,38]. Mo ver, the higher homologues 63 and 64 can be similarly prepared from a reaction of 55 w one and two equivalents of 60, respectively (Scheme 9) [37,38]. A simple mechanistic analysis of the reported preparation of 55 [36] suggested that an initial aldol-type condensation between a fluoranyl anion (58) and formaldehyde should produce 9-fluorenomethanol 60 as the initial product followed by formal elimination of water to produce 9-methylenefluorene 61. A subsequent Michael-type addition of 58 to 61 can produce 55 after protonation, i.e., Scheme 8. Unfortunately, all the reactions depicted in Scheme 8 are expected to be reversible, and therefore, an optimization of the reaction conditions may allow access to 55 and its higher homologues with good yields.
(Scheme 7). Unfortunately, 55 was obtained in low yield (>20%) after ted of a complex mixture from a reaction of fluorene with paraformaldehyd of t BuOK (potassium tert-butoxide) in DMF (Scheme 7) [36]. Notwithst yield of 55, its availability permitted the preparation of the tetra-fluore yield from 55 using the three-step strategy outlined in Scheme 6, i.e., alk coupling, and Friedel-Crafts cyclization (Scheme 7). Scheme 7. Preparation of F4 from 55 using a three-step strategy.
A simple mechanistic analysis of the reported preparation of 55 [36 an initial aldol-type condensation between a fluoranyl anion (58) an should produce 9-fluorenomethanol 60 as the initial product followed by tion of water to produce 9-methylenefluorene 61. A subsequent Michael 58 to 61 can produce 55 after protonation, i.e., Scheme 8. Unfortunately, depicted in Scheme 8 are expected to be reversible, and therefore, an op reaction conditions may allow access to 55 and its higher homologues w Surprisingly, a reaction of equimolar (commercially available) fluo and fluorene in the presence of t BuOK as a base in DMF at 22 °C for ~2 m 55 after a simple pouring of the reaction mixture into water, filtration, a tion from CH2Cl2/C2H5OH, CH2Cl2/C2H5OH with >86% yield (Scheme 9 ver, the higher homologues 63 and 64 can be similarly prepared from a re one and two equivalents of 60, respectively (Scheme 9) [37,38]. Surprisingly, a reaction of equimolar (commercially available) fluorenomethanol 60 and fluorene in the presence of t BuOK as a base in DMF at 22 • C for~2 min yielded pure 55 after a simple pouring of the reaction mixture into water, filtration, and recrystallization from CH 2 Cl 2 /C 2 H 5 OH, CH 2 Cl 2 /C 2 H 5 OH with >86% yield (Scheme 9) [37,38]. Moreover, the higher homologues 63 and 64 can be similarly prepared from a reaction of 55 with one and two equivalents of 60, respectively (Scheme 9) [37,38]. The availability of 63 and 64 then allowed easy access to F5 and F6 using th three-step strategy outlined above (Scheme 10). It is noted that somewhat low yields of F5 and F6 in Scheme 10 arise largely due to partial de-polymerization o 64 during alkylation. Fortunately, a combination of column chromatography and lizations easily produce pure 65 and 67 for further high-yielding transformatio and F6 (Scheme 10) [39,40]. The versatility of the synthetic strategies for the preparation of F1-F6 (Schem was further delineated by preparation of the selectively deuterated polyfluoren Scheme 11) [39] needed for EPR spectroscopic studies as well as 1 H NMR spectra ments (vide infra). The availability of 63 and 64 then allowed easy access to F5 and F6 using the robust three-step strategy outlined above (Scheme 10). It is noted that somewhat low overall yields of F5 and F6 in Scheme 10 arise largely due to partial de-polymerization of 63 and 64 during alkylation. Fortunately, a combination of column chromatography and crystallizations easily produce pure 65 and 67 for further high-yielding transformations to F5 and F6 (Scheme 10) [39,40]. The availability of 63 and 64 then allowed easy access to F5 and F6 using the robus three-step strategy outlined above (Scheme 10). It is noted that somewhat low overal yields of F5 and F6 in Scheme 10 arise largely due to partial de-polymerization of 63 and 64 during alkylation. Fortunately, a combination of column chromatography and crystal lizations easily produce pure 65 and 67 for further high-yielding transformations to F5 and F6 (Scheme 10) [39,40]. The versatility of the synthetic strategies for the preparation of F1-F6 (Schemes 6-10 was further delineated by preparation of the selectively deuterated polyfluorenes (e.g. Scheme 11) [39] needed for EPR spectroscopic studies as well as 1 H NMR spectral assign ments (vide infra). The versatility of the synthetic strategies for the preparation of F1-F6 (Schemes 6-10) was further delineated by preparation of the selectively deuterated polyfluorenes (e.g., Scheme 11) [39] needed for EPR spectroscopic studies as well as 1 H NMR spectral assignments (vide infra). ecules 2023, 28, x FOR PEER REVIEW Scheme 11. Preparation of selectively deuterated polyfluorenes.

Demonstration of the Cofaciality of the Fluorene Rings in F1-F troscopy
The 1 H NMR spectroscopy results provide clear evidence of c orenes in F2-F6 in solution at temperatures from +30° C to −70 °C.
For example, a comparison of the 1 H NMR spectra of F1-F6 ( the fluorene protons in F2-F6 are shifted up-field compared with the corresponding protons in F1, which is due to anisotropic deshie of all protons in F1-F6 was confirmed by 2D NMR spectroscopy as of the NMR spectra of selectively deuterated analogues (e.g., Figur Scheme 11. Preparation of selectively deuterated polyfluorenes.

Demonstration of the Cofaciality of the Fluorene Rings in F1-F6 by 1 H NMR Spectroscopy
The 1 H NMR spectroscopy results provide clear evidence of cofacial stacking of fluorenes in F2-F6 in solution at temperatures from +30 • C to −70 • C.
For example, a comparison of the 1 H NMR spectra of F1-F6 ( Figure 10) shows that the fluorene protons in F2-F6 are shifted up-field compared with the chemical shifts of the corresponding protons in F1, which is due to anisotropic deshielding. The assignment of all protons in F1-F6 was confirmed by 2D NMR spectroscopy as well as by comparison of the NMR spectra of selectively deuterated analogues (e.g., Figure 11) [35,39].  This deshielding of the protons in F2-F6 arises not due to the perfect sandwich structure but to slightly displaced sandwiches (Figure 10c) which must rapidly interconvert in solution, as the protons on the two benzenoid rings of all fluorenes in F2-F6 are equivalent on the NMR time scale [35].

X-ray Crystallography of the π-Stacked Polyfluorenes
The molecular structures of F2-F4, as determined by X-ray crystallography, showed in each case a close cofacial juxtaposition of the fluorene moieties, with interplanar angles of 18-24 • and contacts amongst many of the fluorene carbons that were as close as 2.71-3.18 Å (Figure 12a) [35]. Moreover, a typical space-filling representation of F3 (Figure 12b) illustrates the close van der Waals contact amongst the cofacially juxtaposed fluorene rings in polyfluorenes. As expected, based on the NMR studies (Figure 10), the solid-state structures of Fn show substantial displacement of the fluorene moieties from the ideal sandwich-like geometries, and the alternating direction of this displacement produced achiral structures ( Figure 12). Repeated attempts to use chiral additives to crystallize the chiral structures of F3-F6, in which the fluorene moieties are systematically displaced on one side, have thus far been unsuccessful. increases with an increasing number of adjacent fluorenes (Figure 10b). This systematic variation of the chemical shifts due to anisotropic deshielding by adjacent stacked fluorenes also confirms that all the fluorene rings in F3-F6 are fully π-stacked [35].
This deshielding of the protons in F2-F6 arises not due to the perfect sandwich structure but to slightly displaced sandwiches (Figure 10c) which must rapidly interconvert in solution, as the protons on the two benzenoid rings of all fluorenes in F2-F6 are equivalent on the NMR time scale [35].

X-ray Crystallography of the π-Stacked Polyfluorenes
The molecular structures of F2-F4, as determined by X-ray crystallography, showed in each case a close cofacial juxtaposition of the fluorene moieties, with interplanar angles of 18-24° and contacts amongst many of the fluorene carbons that were as close as 2.71-3.18 Å (Figure 12a) [35]. Moreover, a typical space-filling representation of F3 (Figure 12b) illustrates the close van der Waals contact amongst the cofacially juxtaposed fluorene rings in polyfluorenes. As expected, based on the NMR studies (Figure 10), the solid-state structures of Fn show substantial displacement of the fluorene moieties from the ideal sandwich-like geometries, and the alternating direction of this displacement produced achiral structures ( Figure 12). Repeated attempts to use chiral additives to crystallize the chiral structures of F3-F6, in which the fluorene moieties are systematically displaced on one side, have thus far been unsuccessful.

Electronic and Emission Spectroscopy of Polyfluorenes
Despite the close van der Waals contacts amongst the fluorene moieties in F2-F6, their electronic absorption spectra were rather invariant and were similar to that of monomeric F1 (Figure 13a) [28]. However, the fluorescence emission spectra of F2-F6 were strikingly different compared with the spectrum of F1. The emission spectrum of F1 showed a characteristic structured band (307 nm), whereas the remarkably similar emission spectra of F2-F6 showed a Gaussian band at 395 ± 3 nm (Figure 13b) [35,40].

Electronic and Emission Spectroscopy of Polyfluorenes
Despite the close van der Waals contacts amongst the fluorene moieties in F2-F6, their electronic absorption spectra were rather invariant and were similar to that of monomeric F1 (Figure 13a) [28]. However, the fluorescence emission spectra of F2-F6 were strikingly different compared with the spectrum of F1. The emission spectrum of F1 showed a characteristic structured band (307 nm), whereas the remarkably similar emission spectra of F2-F6 showed a Gaussian band at 395 ± 3 nm (Figure 13b) [35,40]. 52,000; and 61,000 M −1 cm −1 ; respectively, and emission spectra (b) in CH2Cl2. Note that the relative emission intensity of F1 was ~10 times higher than for F2-F6.
The striking constancy in the position of the excimer emission band for F2-F6 suggests that the exciton must reside over only two fluorene moieties [41,42]. The energetic penalty for excimer formation, which requires an ideal sandwich-like arrangement of a Figure 13. Normalized absorption spectra of (a) F1-F6 with ε 265 = 17,000; 27,300; 36,000; 45,400; 52,000; and 61,000 M −1 cm −1 ; respectively, and emission spectra (b) in CH 2 Cl 2 . Note that the relative emission intensity of F1 was~10 times higher than for F2-F6.
The striking constancy in the position of the excimer emission band for F2-F6 suggests that the exciton must reside over only two fluorene moieties [41,42]. The energetic penalty for excimer formation, which requires an ideal sandwich-like arrangement of a pair of interacting units [43], must be easily overcome by the energetic gain due to the excimeric stabilization in F2. However, in larger Fn, the delocalization of the exciton beyond two units would require energetically demanding rearrangement of three or more fluorene units in ideal sandwich-like geometries. In F3-F6, one can expect an ensemble of multiple excimeric structures of similar energy where the exciton is localized only onto two fluorene moieties (e.g., Figure 14). emission intensity of F1 was ~10 times higher than for F2-F6.
The striking constancy in the position of the excimer emiss gests that the exciton must reside over only two fluorene moieti penalty for excimer formation, which requires an ideal sandwic pair of interacting units [43], must be easily overcome by the en excimeric stabilization in F2. However, in larger Fn, the delocali yond two units would require energetically demanding rearrang fluorene units in ideal sandwich-like geometries. In F3-F6, one ca multiple excimeric structures of similar energy where the excito two fluorene moieties (e.g., Figure 14).
The He(I) photoelectron spectra of F1-F4 provided vertical ionization potentials, which decrease in order from F1 to F4 (7.85, 7.52, 7.33, and 7.28 eV, respectively), and both E ox1 values and vertical IPs showed a familiar 1/n relationship, with strikingly similar slopes (Figure 15b). Moreover, a Koopmans-like linear relationship (Figure 15c) [44] between thermodynamic oxidation potentials, measured on a submillisecond time scale, and vertical ionization potentials, measured on a subfemtosecond time scale, suggests that stacked Fns do not suffer from any conformational reorganization upon one-electron detachment [35,44,45].
Thus, the linear 1/n dependence of vertical and adiabatic ionization energies (Figure 15b) and anisotropic chemical shifts (in the 1 H NMR spectra, Figure 10b) of F2-F6, together with X-ray crystallography (Figure 12), demonstrate that this new class of polyfluorenes, designed based on the 1,3-diarylalkane framework, retain their cofacial arrangement in the gas phase, solution, and solid state. The He(I) photoelectron spectra of F1-F4 provided v which decrease in order from F1 to F4 (7.85, 7.52, 7.33, and 7.2 Eox1 values and vertical IPs showed a familiar 1/n relation slopes (Figure 15b). Moreover, a Koopmans-like linear relat tween thermodynamic oxidation potentials, measured on a su vertical ionization potentials, measured on a subfemtoseco stacked Fns do not suffer from any conformational reorganiz tachment [35,44,45].
Thus, the linear 1/n dependence of vertical and adiabati

Generation and Spectroscopy of Polyfluorene Cation Radicals
The cofacially stacked F2-F6 form stable cation radicals when exposed to oxidants such as NO + SbCl 6 − [46,47], Et 3 O + SbCl 6 − [48], and SbCl 5 [49] in CH 2 Cl 2 . The F1 cation radical, which is highly unstable, was generated by laser flash photolysis using photoexcited triplet chloranil as the oxidant [33,35]. The absorption spectra of F2 +• -F6 +• in CH 2 Cl 2 (22 • C) showed characteristic intervalence transitions in the NIR region (Figure 16), which is absent in F1 +• . The intervalence transitions intensify and undergo a red shift with an increasing number of fluorene units, thus confirming increased hole delocalization in the π-stacked fluorene array in Fn [50]. This observation of hole delocalization over multiple fluorenes i trasted with excimer formation, which is limited to only two fluorene requirement for an ideal sandwich-like geometry [43]. Significantly, Xhas shown that perfect sandwich-like geometry is not a necessity for the cationic charge in a number of dimeric cation radicals [16]. These fa that the nature of bonding in excimers and cation radicals is different different spin multiplicities, and a careful computational study togeth tallography of Fn +• will be needed to address this question.

Generation of Polyfluorene Anion Radicals and Study of Charge EPR Spectroscopy
To access the anion radicals, we carried out one-electron reductio tassium in HMPA, which was examined using EPR spectroscopy [39 thermodynamically preferred state was formed, where the electron sively on one external fluorene (Figure 17a), as judged by the similarit This observation of hole delocalization over multiple fluorenes in Fn should be contrasted with excimer formation, which is limited to only two fluorene units owing to the requirement for an ideal sandwich-like geometry [43]. Significantly, X-ray crystallography has shown that perfect sandwich-like geometry is not a necessity for the stabilization of the cationic charge in a number of dimeric cation radicals [16]. These facts together suggest that the nature of bonding in excimers and cation radicals is different, possibly due to the different spin multiplicities, and a careful computational study together with X-ray crystallography of Fn +• will be needed to address this question.

Generation of Polyfluorene Anion Radicals and Study of Charge Delocalization by EPR Spectroscopy
To access the anion radicals, we carried out one-electron reduction of F2-F6 with potassium in HMPA, which was examined using EPR spectroscopy [39,51]. In all cases, the thermodynamically preferred state was formed, where the electron is localized exclusively on one external fluorene (Figure 17a), as judged by the similarity of the EPR spectra with that of F1. Interestingly, the initially formed (i.e., kinetically controlled) F4 −• -F6 −• do allow rapid exchange of the electron amongst the internal fluorenes, as discerned by the appearance of a single broad line in their time-dependent EPR spectra (e.g., Figure 17b). These initially formed anion radicals eventually transform into anion radicals where the electron is localized onto a single external fluorene owing to the stringent solvation and ion-pairing requirements for the stabilization of the anionic charge. This observation was further substantiated by an experiment with partially deuter ated F4, where the two internal fluorenes were perdeuterated. Here, the transformation o the kinetic to thermodynamically stabilized anion radical occurred at least an order o magnitude faster than in perprotiated F4 due to the fact that deuterated hydrocarbon stabilize the unpaired electron less effectively [39].

Donor-Spacer-Acceptor Triads with Polyfluorenes as Spacers and Demonstration of the Triplet Energy Transfer
The ready availability of polyfluorenes F2-F6 via versatile synthetic routes, with de fined structures and redox and optoelectronic properties, makes them ideal candidates fo the study of electron and energy transfer using donor-spacer-acceptor triads, where th Fn spacers serve as models for cofacially stacked aromatic π-systems. For example, a se ries of D-spacer-A molecules-where D is benzophenone (Bp, a triplet energy donor), A is naphthalene (Nap, a triplet energy acceptor), and F1-F3 are spacers (Figure 18)-and the model compounds were synthesized by adaptation of the strategies outlined in Schemes 6-10. 27.30 Linking of D and A to Fn via a single methylene bridge retains thei cofacial juxtaposition with the spacer as demonstrated by X-ray crystallography (Figur 18a) [38,45]. This observation was further substantiated by an experiment with partially deuterated F4, where the two internal fluorenes were perdeuterated. Here, the transformation of the kinetic to thermodynamically stabilized anion radical occurred at least an order of magnitude faster than in perprotiated F4 due to the fact that deuterated hydrocarbons stabilize the unpaired electron less effectively [39].

Donor-Spacer-Acceptor Triads with Polyfluorenes as Spacers and Demonstration of the Triplet Energy Transfer
The ready availability of polyfluorenes F2-F6 via versatile synthetic routes, with defined structures and redox and optoelectronic properties, makes them ideal candidates for the study of electron and energy transfer using donor-spacer-acceptor triads, where the Fn spacers serve as models for cofacially stacked aromatic π-systems. For example, a series of D-spacer-A molecules-where D is benzophenone (Bp, a triplet energy donor), A is naphthalene (Nap, a triplet energy acceptor), and F1-F3 are spacers (Figure 18)and the model compounds were synthesized by adaptation of the strategies outlined in Schemes 6-10. 27.30 Linking of D and A to Fn via a single methylene bridge retains their cofacial juxtaposition with the spacer as demonstrated by X-ray crystallography (Figure 18a) [38,45]. Selective photoexcitation of benzophenone moiety at 355 nm in Bp-Fn-Nap molecules was feasible because Fn and Nap do not absorb at wavelengths longer than 300 nm. The initial nanosecond laser spectroscopic analysis showed that photoexcitation of various Bp-Fn-Nap molecules at 355 nm produces identical transients with the characteristic spectral signature of the naphthalene triplet. These transients are formed within the laser pulse (<10 ns) [30].
Detailed probing of these molecules by femtosecond laser spectroscopy provided complete dynamics of the energy transfer events, beginning from the initial excitation of Bp to the trapping of triplet exciton by Nap (Figure 18b,c). An examination of the time constants for energy transfer events in model Bp-Fn diads showed that the exciton is injected into the Fn bridge (τ = 151, 119, and 104 ps for n = 1, 2, and 3, respectively), preceded by the rapid interconversion (5 ps) of singlet Bp to its triplet. In cases of Bp-Fn-Nap triads, the triplet exciton was injected into the spacer with similar time constants followed by its transfer to Nap in cases of F2 and F3 spacers (Figure 18b). Surprisingly, however, in the case of the Bp-F1-Nap triad, the transfer of the triplet exciton from Bp to Nap occurred much faster (62 ps) than the 151 ps observed for the Bp-F1 diad (Figure 18c).
An examination of the complete set of time constants in Figure 18c for various Bp-Fn-Nap triads and Bp-Fn diads demonstrates that a crossover occurs between singlestep tunneling and multi-step hopping of triplet energy transfer as the spacer length increases [38]. Studies are underway to construct similar D-Fn-A triads, where D is an electron donor and A is an electron acceptor, for the study of electron transfer dynamics through cofacially arrayed polyfluorene spacers.

Preparation of 9,9-Bis-(2-methyl-allyl)-9H-fluorene (42)
Anhydrous THF (25 mL) was added to a Schlenk flask containing fluorene 41 (2.0 g, 12 mmol) under an argon atmosphere and cooled to −40 • C. Dropwise addition of n-Butyllithium (5 mL, 12.5 mmol) changed the color to orange, and the reaction was stirred for 10 min. Addition of 3-Chloro-2-methylpropene (1.25 mL, 12.5 mmol) to the reaction mixture, which was then stirred for 30 min, whereupon the orange color disappeared. Aliquots of n-Butyllithium (5 mL, 12.5 mmol) and 3-chloro-2-methylpropene (1.25 mL, 12.5 mmol) were added again as described earlier. The work-up was carried out by pouring the mixture into water and extracting the aqueous layer with dichloromethane (2 × 30 mL); the organic extracts were dried over anhydrous magnesium sulfate and evaporated under reduced pressure. The resulting residue was purified by chromatography on silica gel using a 97:3 mixture of hexanes/ethyl acetate as eluent. Yield: 2.60 g (78.8%); mp 60-61 • C (CH 2 Cl 2 /MeOH); 1  Ozone was bubbled into a solution of 42 (2.5 g, 9.12 mmol) in dry dichloromethane (45 mL) at −78 • C; after 35 min, the reaction mixture turned blue, which indicated that the reaction had reached completion. Next, argon gas was bubbled into the solution until the solution became clear and colorless. Zinc (4.0 g) and glacial acetic acid (10.0 mL) were added at −78 • C and the reaction mixture was stirred for 2 h. Then, it was warmed up to room temperature while stirring for another 2 h and then filtered through a short layer of celite. The filtrate was poured into water (30 mL) and extracted with ethyl ether (3 × 40 mL). The combined organic extracts were washed with water, dried over anhydrous magnesium sulfate and evaporated under reduced pressure. The resulting residue was purified by chromatography on silica gel using a 94:6 mixture of hexanes/ethyl acetate as eluent. Yield: 0.9 g (36%); mp 65-67 • C (Ether / MeOH); 1

Preparation of 9-Methyl-9H-fluorene (45)
Anhydrous THF (50 mL) was added to a Schlenk flask containing fluorene (5.0 g, 30.12 mmol) under an argon atmosphere and cooled to −40 • C. Dropwise addition of n-Butyllithium (13.2 mL, 33 mmol) was carried out, during which the color changed to orange. The reaction was stirred for 10 min and then iodomethane (2.5 mL, 40 mmol) was added. The mixture was stirred for 15 min during which the color disappeared. The work-up was carried out by pouring the reaction mixture into water (50 mL) followed by extraction with dichloromethane (3 × 30 mL). The combined organic extracts were dried over anhydrous magnesium sulfate and evaporated under reduced pressure. Yield 13.5. Preparation of 2,3-Bis(9-methylfluorene)methylpropene (46) Anhydrous THF (50 mL) was added to a Schlenk flask containing 45 (5.0 g, 27.8 mmol) under an argon atmosphere and cooled to −30 • C. A quantity of n-Butyllithium (11.5 mL, 28.5 mmol) was added dropwise, during which the color changed to orange. The reaction was stirred for 10 min, and 3-chloro-2-chloromethylpropene (1.7 mL, 20 mmol) was added. The reaction mixture was stirred for 1 h and the color disappeared. The work-up was carried out by pouring the reaction mixture into water followed by extraction with dichloromethane (3 × 20 mL). The combined organic layers were dried over anhydrous magnesium sulfate and evaporated under reduced pressure. Yield: 5.6 g (97%); mp 85-87 • C (CH 2 Cl 2/ MeOH); 1 H NMR (CDCl 3 ) δ 1.28 (s, 6H), 2.11 (s, 4H), 6.28 (m, 1H), 3.75 (s, 2H), 7.16-7.63 (m, 16H); 13  13.6. Preparation of 1,3-Bis(9-methyl-9H-fluoren-9-yl)propan-2-one (47) Ozone was bubbled into a solution of 46 (5.50 g, 13.35 mmol) in dry dichloromethane (55 mL) at −78 • C. After 2 h, the reaction mixture turned blue, which indicated that the reaction had reached completion, and then argon gas was bubbled through it until it became clear and colorless. Zinc powder (5.8 g) and glacial acetic acid (14 mL) were added, and the mixture was warmed up to room temperature and stirred overnight. The reaction mixture was filtered through a short layer of celite. The filtrate was poured into water (50 mL), and the aqueous layer was extracted with ethyl ether (3 × 40 mL 13.7. Preparation of 2-Biphenyl-2-yl-1,3-bis(9-methyl-9-H-fluoren-9-yl)propan-2-ol (48) (A) A solution of 2-biphenyl magnesium bromide was prepared from 2-bromobiphenyl (1.5 g, 6.2 mmol) and excess magnesium turnings (0.45 g, 18.6 mmol) in anhydrous tetrahydrofuran (40 mL) under an argon atmosphere and refluxing for 4 h. The Grignard solution thus obtained was transferred at room temperature to another Schlenk flask containing 47 (2.0 g, 4.83 mmol). The mixture was refluxed overnight, cooled to room temperature, and quenched with dilute hydrochloric acid (5%, 10 mL). The organic layer was separated, and the aqueous phase was extracted with dichloromethane (3 × 20 mL). The combined organic extracts were dried over magnesium sulfate and filtered. After evaporation of the solvent under reduced pressure, the resulting residue was purified by chromatography on silica gel using a 97:3 mixture of hexanes/ethyl acetate as eluent to recover the starting material and biphenyl.

Conclusions
Syntheses of cofacially arrayed multi-layered π-systems based on traditional cyclophanes are scarce and not readily amenable to the structural modifications required for construction of the D-spacer-A triads owing to the synthetic difficulties associated with the preparation of sandwich-like structures. By controlling the number of C-C bond rotors and utilizing a larger aromatic π-system in a 1,3-diarylalkane platform, we developed versatile synthetic strategies for the preparation of monodisperse cofacially stacked polyfluorenes. These synthetic strategies also allow the ready incorporation of polyfluorenes into D-spacer-A triads by linking donor and acceptor groups to the polyfluorene spacers via single methylenes. A combination of 1 H NMR spectroscopy, X-ray crystallography, electrochemistry, and photoelectron spectroscopy have demonstrated the cofacial stacking of multiple fluorenes in F2-F6 and hole and electron delocalization amongst the stacked fluorenes. We also demonstrated, using D-spacer-A triads, that triplet energy transfer through π-stacked polyfluorenes occurs via tunneling and hopping mechanisms depending on the number of fluorenes in the polyfluorene spacer.
We believe that the ready availability of rigidly held π-stacked polyfluorenes, described herein, with well-defined redox and optoelectronic properties provides an ideal scaffold for the study of electron and energy transfer in D-spacer-A triads where the Fn spacers will serve as models for cofacially stacked π-systems.