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Review

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

by
Rajendra Rathore
1,† and
Sameh H. Abdelwahed
2,*
1
Department of Chemistry, Marquette University, Milwaukee, WI 53233, USA
2
Department of Chemistry, Prairie View A&M University, Prairie View, TX 77446, USA
*
Author to whom correspondence should be addressed.
†
Deceased author.
Molecules 2023, 28(9), 3717; https://doi.org/10.3390/molecules28093717
Submission received: 16 March 2023 / Revised: 17 April 2023 / Accepted: 18 April 2023 / Published: 25 April 2023
(This article belongs to the Special Issue Molecular Wires and Organic Materials for Energy Storage)
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Abstract

:
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.

1. 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 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 inter-chain charge transport through cofacially stacked aromatic π-systems.

2. 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].
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.

3. 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 (11–13) 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 the redox potential 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°).
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+•, TΔSo = ~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 cation radical (i.e., ~250 mV) compared with the corresponding model electron donor 27 (Table 1) [28].

4. 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).
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).
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].
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).

5. 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).
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).
Unfortunately, the two conceived Friedel-Crafts precursors to F3, i.e., 44 and 48, from easily synthesized 43 or 47 (Scheme 4), could not be prepared due to the failure of the reactions of either 43 or 47 with 2-biphenylmagnesium bromide or 2-biphenyllithium, most likely due to steric hindrance.
The failed synthesis of bis-alcohol 44—a precursor to the dicationic intermediate 49—led us to explore if it can be accessed by protonation of a corresponding bis-alkene derivative 50 (Scheme 5).
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 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 tBuOK (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).
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.
Surprisingly, a reaction of equimolar (commercially available) fluorenomethanol 60 and fluorene in the presence of tBuOK 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 CH2Cl2/C2H5OH, CH2Cl2/C2H5OH 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 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 versatility of the synthetic strategies for the preparation of F1–F6 (Scheme 6, Scheme 7, Scheme 8, Scheme 9 and Scheme 10) was further delineated by preparation of the selectively deuterated polyfluorenes (e.g., Scheme 11) [39] needed for EPR spectroscopic studies as well as 1H NMR spectral assignments (vide infra).

6. Demonstration of the Cofaciality of the Fluorene Rings in F1–F6 by 1H NMR Spectroscopy

The 1H 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 1H 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].

7. 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.

8. 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].
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).

9. Electrochemistry/Photoelectron Spectroscopy of Polyfluorenes

Besides F1, polyfluorenes F2–F6 undergo reversible electrochemical oxidation under ambient conditions (Figure 15a) [35,40]. The first oxidation potentials (Eox1) of Fn decrease in order from F1 to F6 (i.e., ~1.27, 1.01 0.90, 0.84, 0.81, and 0.79 V vs. Fc/Fc+, respectively), and they show that a single cationic charge is stabilized by ~0.5 V by delocalization over six cofacially stacked fluorenes in F6 compared with model F1.
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 Eox1 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 1H 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.

10. Generation and Spectroscopy of Polyfluorene Cation Radicals

The cofacially stacked F2–F6 form stable cation radicals when exposed to oxidants such as NO+SbCl6− [46,47], Et3O+ SbCl6− [48], and SbCl5 [49] in CH2Cl2. 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 CH2Cl2 (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 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.

11. 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 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].

12. 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 Scheme 6, Scheme 7, Scheme 8, Scheme 9 and Scheme 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 single-step 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.

13. Materials and Methods

13.1. 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 (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 2.83 (s, 6H), 4.25 (m, 4H), 4.42 (m, 4H), 7.28–7.68 (m, 8H); GC-MS: m/z = 274 (M+) calcd for C21H22.

13.2. Preparation of 1-[9-(2-Oxo-propyl)-9H-fluoren-9-yl]propan-2-one (43)

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); 1H NMR (CDCl3) δ 1.94 (s, 6H), 3.25 (m, 4H), 7.25–7.75 (m, 8H); 13C NMR (CDCl3) δ 31.98, 50.87, 120.06, 124.54, 127.6, 127.76, 139.44, 149.61, 206.78; GC-MS: m/z = 278 (M+) calcd for C19H18O2.

13.3. Preparation of 2-Biphenyl-2-yl-1-[9-(2-biphenyl-2-yl-2-hydroxy-propyl)-9H-fluoren-9-yl]propan-2-ol (44)

A solution of 2-biphenylmagnesium bromide was prepared from 2-bromobiphenyl (0.92 g, 3.96 mmol) and excess magnesium turnings (0.23 g, 8 mmol) in anhydrous tetrahydrofuran (25 mL) under an argon atmosphere by refluxing for 4 h. The Grignard solution thus obtained was transferred after cooling to room temperature to another Schlenk flask containing 43 (0.5 g, 1.79 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 × 15 mL). The combined organic extracts were dried over anhydrous magnesium sulfate and filtered. After evaporation of the solvent under vacuum, the resulting residue was purified by chromatography on silica gel using a 97:3 mixture of hexanes/ethyl acetate as eluent to recover biphenyl, the starting material.

13.4. 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: 4.9 g (90%); mp 61–63 °C (CH2Cl2/MeOH) (mp 62–64 °C); 1H NMR (CDCl3) δ 1.52 (d, 3H), 3.97 (q, 1H), 7.12–7.80 (m, 8H); 13C NMR (CDCl3) δ 18.98, 42.91, 119.63, 123.76, 126.62, 140.10, 148.47; GC-MS: m/z = 180 (M+) calcd for C14H12.

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 (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 1.28 (s, 6H), 2.11 (s, 4H), 6.28 (m, 1H), 3.75 (s, 2H), 7.16–7.63 (m, 16H); 13C NMR (CDCl3) δ 27.99, 46.92, 51.68, 116.81, 119.94, 123.63, 126.79, 126.93, 140.03, 141.09, 151.29; GC-MS: m/z = 412 (M+) calcd for C32H28.

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). The combined organic extracts were washed with water, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure. Yield: 4.1g (77.8%); mp 96–98 °C (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 1.31 (s, 6H), 2.53 (s, 4H), 7.23–7.68 (m, 16H); 13C NMR (CDCl3) δ 26.16, 49.22, 52.92, 119.77, 123.19, 126.98, 127.04, 139.13, 150.35, 204.61.

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.
(B) 2-Biphenyl lithium was prepared according to a published procedure [39]. A quantity of tert-butyllithium (3.13 mL, 4.84 mmol, 1.7 M in pentane) was added to a solution of 2-iodobiphenyl (0.677 g, 2.42 mmol) in ether (10 mL) at −78 °C over 30 min. The resulting slurry was stirred at −78 °C for 1 h, then 47 (1.0 g, 2.42 mmol) dissolved in ether (10 mL) was added dropwise over 10 min. The solution was warmed to room temperature for over 30 min and poured into water. The aqueous layer was extracted with ether (3 × 15 mL), and the combined organic layers were washed with brine and dried over magnesium sulfate. 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.

13.8. Preparation of 9-[(Fluoren-9-yl)-methyl]flourene (55)

Fluorene (1.66 g, 10 mmol) was placed in a 100 mL Schlenk flask under an argon atmosphere. After addition of DMF (12.5 mL) and potassium tert-butoxide (1.12 g, 10 mmol), the mixture turned red. Then, 9-Methanol-fluorene (1.96 g, 10 mmol) was added to the reaction mixture and stirred for 15 min at room temperature. The mixture was poured onto water (20 mL) and then filtered to provide 55. Yield: 2.9 g (86%); mp 210–212 °C (CH2Cl2/MeOH) (mp 212–213 °C); 1H NMR (CDCl3) δ 2.23 (t, J = 7.6 Hz, 2H), 4.45 (t, J = 7.6 Hz, 2H), 7.25 (t, J = 7.4 Hz, 4H), 7.39 (t, J = 7.4 Hz, 4H), 7.54 (d, J = 7.5 Hz, 4H), 7.81 (d, J = 7.5 Hz, 4H); 13C NMR (CDCl3) δ 39.37, 46.31, 119.87, 124.75, 126.65, 126.98, 140.53, 146.95; GC-MS: m/z = 344 (M+) calcd for C27H20.

13.9. Preparation of 9,9-Bis((9H-fluoren-9-yl)methyl)-9H-fluorene (63)

A sample of 55 (1.73 g, 5 mmol) was placed in a 100 mL Schlenk flask under an argon atmosphere. After addition of DMF (12.5 mL) and potassium tert-butoxide (0.56 g, 5 mmol) the mixture was stirred for 5 min at room temperature, then 9-methanol-flourene (0.98 g, 5 mmol) was added to the reaction mixture and it was stirred for another 1h. The mixture was worked up by quenching it with dilute hydrochloric acid (5%, 30 mL) and then filtered. The crude product was purified by column chromatography on silica gel using a 90:10 mixture of hexanes/ethyl acetate as eluent to produce 66. Yield: 2.5 g (72%); mp 274–276 °C; 1H NMR (CDCl3) δ 2.80 (d, J = 4.5 Hz, 4H), 3.25 (t, J = 4.7 Hz, 2H), 6.62 (d, J = 7.1 Hz, 4H), 7.04 (t, J = 4.7 Hz, 4H), 7.22 (t, J = 4.7 Hz, 4H), 7.48–7.57 (m, 8 H), 7.72–7.5 (m, 2H), 7.86–7.88 (m, 2H); 13C NMR (CDCl3) δ: 44.74, 46.02, 55.70, 119.37, 120.93, 124.99, 125.50, 126.70, 126.76, 127.58, 128.26, 140.53, 141.95, 148.58, 148.90.

13.10. Preparation of Bis(9-((9H-fluoren-9-yl)methyl)-9H-fluoren-9-yl)methane (64)

A sample of 55 (1.73 g, 5 mmol) was placed in a 100 mL Schlenk flask under an argon atmosphere. After addition of DMF (12.5 mL) and potassium tert-butoxide (0.56 g, 5 mmol), the mixture was stirred for 5 min at room temperature. Then, 9-methanol-flourene (1.96 g, 10 mmol) was added to the reaction mixture and it was stirred for another 1h. The mixture was worked up by quenching it with dilute hydrochloric acid (5%, 50mL), and then filtered. The crude product was purified by column chromatography on silica gel using a 90:10 mixture of hexanes/ethyl acetate as eluent to produce 64. Yield: 2.5 g (72%); mp 253–255 °C (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 2.59 (d, J = 4.6 Hz, 4H), 2.80 (t, J = 4.4 Hz, 2H), 3.30 (s, 2H), 6.39 (d, J = 7.6 Hz, 4H), 6.93–7.22 (m, 32 H), 7.49 (d, J = 7.6 Hz, 4H); 13C NMR (CDCl3) δ 43.77, 47.76, 49.78, 54.03, 61.41, 119.178, 119.66, 124.95, 124.99, 126.16, 126.52, 126.64, 126.71, 140.36. 141.37, 147.65, 148.88.

13.11. Preparation of 9-Methyl-9H-fluorene-d9 (70)

Anhydrous THF (20 mL) was added to a Schlenk flask containing fluorene-d8 (0.49 g, 2.78 mmol) under an argon atmosphere and cooled to −30 °C. A quantity of n-Butyllithium (1.12 mL, 3.1 mmol) was added dropwise, with the color changing to orange during the addition. The reaction was stirred for 10 min and then iodomethane (0.5 mL, 8 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 dilute hydrochloric acid (5%, 20 mL) followed by extraction with dichloromethane (3 × 20 mL). The combined organic extracts were dried over anhydrous magnesium sulfate and evaporated under reduced pressure. Yield: 0.41g (78%); mp 61–63 °C (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 1.58 (s, 3H); 13C NMR (CDCl3) δ 18.43, 42.86, 140.10, 148.47; GC-MS: m/z = 189 (M+) calcd for C14H3D9.

13.12. Preparation of 9-(2-Bromoallyl)-9-methyl-9H-fluorene-d8 (71)

To a suspension solution of 70 (0.41 g, 2.17 mmol) in toluene (15 mL) under an argon atmosphere at room temperature, a sodium hydroxide 50% solution (15 mL) was added, then 2,3-dibromopropene (0.4 mL, 4 mmol) was added to the reaction mixture. Tetra-n-butylammonium bromide (80 mg) was added as a phase-transfer catalyst. The reaction mixture was stirred overnight at room temperature. The work-up was performed by adding ethyl acetate (10 mL) and separating the organic layer. The aqueous layer was washed with ethyl acetate (2 × 10 mL). The combined organic extracts were washed with dilute hydrochloric acid (5%, 15 mL) and water, dried over magnesium sulfate, and evaporated under reduced pressure. The oily crude product was purified by column chromatography on silica gel using a 98:2 mixture of hexanes/ethyl acetate as eluent to yield 71. Yield: 0.16 g (30%); (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 1.58 (s, 3H), 3.18 (s, 2H), 4.94 (d, 1H), 5.17 (d, 1H,); 13C NMR (CDCl3) δ 27.27, 50.68, 51.01, 120.39, 128.63, 140.21, 148.42; GC-MS: m/z = 306 (M+) calcd for C17H7D8Br.

13.13. Preparation of 9-(2-Biphenyl-2-yl-allyl)-9-methyl-9H-fluorene-d8 (72)

A solution of 2-biphenylmagnesium bromide was prepared from 2-bromobiphenyl (0.25 g, 10 mmol) and excess magnesium turnings (0.30 g, 12.5 mmol) in anhydrous THF (20 mL), and 5 drops of dibromoethane were added to activate the reaction under an argon atmosphere and refluxing for 4 h. The Grignard solution thus obtained was transferred at room temperature to another Schlenk flask containing 42 (0.16 g, 0.50 mmol), and a catalytic amount of bis(triphenylphosphine)palladium dichloride (60 mg) was added. The resulting yellow 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 × 15 mL). The combined organic extracts were dried over magnesium sulfate and filtered. Evaporation of the solvent under reduced pressure yielded a brown residue that was filtered through a short pad of silica gel with hexanes as eluent, after which the resulting product was further purified by column chromatography on silica gel using a 95:5 mixture of hexanes/ethyl acetate as eluent. Yield: 0.15 g (76%); mp 127–129 °C (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 1.07 (s, 3H), 2.40 (2H, s), 4.42 (d, 1H), 4.66 (d, 1H), 6.01 (d, 1H) 6.64–7.7.34 (m, 8H); 13C NMR (CDCl3) δ 27.52, 45.59, 51.33, 119.25, 126.76, 126.82, 126.90, 128.27, 129.13, 129.15, 129.29, 130.01, 138.05, 140.15, 142.22, 147.54, 148.51.

13.14. Preparation of Bis(9-methyl-9H-fluoren-9-yl)methane-d8 (F2-d8)

To a cold (0 °C) solution of 72 (75 mg, 0.2 mmol) in dry dichloromethane (10 mL), a few drops (5) of methanesulfonic acid were added, and the reaction mixture was allowed to warm up to room temperature and stirred for 6 h. The work-up was performed by adding saturated solution of sodium bicarbonate (10 mL) to remove the excess acid, followed by extraction with dichloromethane (3 × 15 mL). The combined organic extracts were dried over magnesium sulfate and evaporated under reduced pressure, and the resulting product was further purified by column chromatography on silica gel using a 98:2 mixture of hexanes/ethyl acetate as eluent. Yield: 55 mg (74%); mp 175–177 °C (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 1.23 (s, 3H), 3.07 (s, 2H), 6.83–7.12 (m, 8H); 13C NMR (CDCl3) δ 29.93, 49.35, 49.92, 118.61, 119.14, 123.37, 125.92, 136.10, 150.68.

13.15. Preparation of Di(9H-fluoren-9-yl)methane-d16 (73)

Fluorene-d10 (1.0 g, 5.7 mmol) was placed in a 100 mL Schlenk flask under an argon atmosphere. After addition of DMF (10 mL) and potassium tert-butoxide (0.64 g, 5.7 mmol), the mixture turned red. Paraformaldehyde (85 mg 2.8 mmol) was added to the reaction mixture, and it was stirred for 1 h at room temperature before it was poured into dilute hydrochloric acid (5%, 20 mL) and then filtered. Yield: 1.01 g (98%); mp 210–212 °C (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 2.23 (t, 2H), 4.45 (t, 2H); GC-MS: m/z = 361 (M+) calcd for C27H4D16.

13.16. Preparation of 9,9-Bis-(2-bromo-allyl)-9H-fluorene-d8 (74)

To a suspension solution of fluorene-d8 (0.5 g, 2.85 mmol) in toluene (15 mL) under an argon atmosphere at room temperature, sodium hydroxide 50% solution (15 mL) was added, then 2,3-dibromopropene (1.0 mL, 8.55 mmol) was added to the reaction mixture. Tetra-butyl ammonium bromide (100 mg) was added as a phase-transfer catalyst. The reaction mixture was stirred overnight at room temperature. The work-up was carried out by adding ethyl acetate (15 mL) and separating the organic layer. The aqueous layer was washed with ethyl acetate (2 × 15 mL). The combined organic extracts were washed with dilute hydrochloric acid (5%, 15 mL) and water, dried over magnesium sulfate, and evaporated under reduced pressure. The oily crude product was purified by column chromatography on silica gel using a 98:2 mixture of hexanes/ethyl acetate as eluent to produce 74. Yield: 0.53 g (44%); 1H NMR (CDCl3) δ 3.24 (s, 2H), 4.89 (d, 1H), 5.12 (d, 1H,); 13C NMR (CDCl3) δ 49.95, 54.89, 120.99, 126.88, 141.03, 148.53.

13.17. Preparation of 9,9-Bis(2-biphenyl-2-yl-allyl)-9H-fluorene-d8 (75)

A solution of 2-biphenylmagnesium bromide was prepared from 2-bromobiphenyl (0.75 g, 3.21 mmol) and excess magnesium turnings (0.38 g, 15.6 mmol) in anhydrous THF (25 mL), and 5 drops of dibromoethane were added to activate the reaction under an argon atmosphere and refluxing for 4 h. The Grignard solution thus obtained was transferred at room temperature to another Schlenk flask containing 74 (0.53 g, 1.29 mmol), and a catalytic amount of bis(triphenylphosphine)palladium dichloride (0.1 g) was added. The resulting yellow mixture was refluxed overnight, cooled to room temperature, and quenched with dilute hydrochloric acid (5%, 30 mL). The organic layer was separated and the aqueous phase was extracted with dichloromethane (3 × 50 mL). The combined organic extracts were dried over magnesium sulfate and filtered. Evaporation of the solvent under reduced pressure yielded a brown residue that was filtered through a short pad of silica gel with hexanes as eluent, after which the resulting product was further purified by column chromatography on silica gel using a 95:5 mixture of hexanes/ethyl acetate as eluent. Yield: 0.56 g (79%); mp 151–153 °C (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 2.07 (s, 4H), 4.23 (d, 2H), 4.51 (d, 2H), 5.93 (d, 2H) 6.65–7.47 (m, 16H); 13C NMR (CDCl3) δ 45.51, 55.77, 119.51, 126.57, 126.79, 128.16, 128.99, 129.98, 137.81, 140.12, 141.07, 141.98, 142.14, 146.79, 148.31, 148.63.

13.18. Preparation of 9,9-Bis((9-methyl-9H-fluoren-9-yl)methyl)-9H-fluorene-d8 (F3-d8)

To a cold (0 °C) solution of 75 (0.56 g, 1 mmol) in dry dichloromethane (20 mL), a few drops (10) of methanesulfonic acid were added, and the reaction mixture was warmed up to room temperature and stirred for 4 h. The work-up was carried out by adding a saturated solution of sodium bicarbonate (30 mL) to remove the excess acid followed by extraction with dichloromethane (2 × 20 mL). The combined organic extracts were washed with water, dried over magnesium sulfate, and evaporated under reduced pressure. The resulting product was further purified by column chromatography on silica gel using a 95:5 mixture of hexanes/ethyl acetate as eluent. Yield: 0.22 g (40%); mp 227–229 °C (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 1.10 (s, 6H), 2.90 (s, 4H), 6.62–7.02 (m, 16H); 13C NMR (CDCl3) δ 30.23, 49.46, 50.88, 52.96, 118.93, 123.19, 125.67, 125.75, 139.48, 150.25.

13.19. Preparation of 9-(2-Bromo-allyl)-9-[(9-(2-bromo-allyl)-fluoren-9-yl)methyl]fluorene-d16 (76)

To a suspension solution of 73 (1.01 g, 2.9 mmol) in toluene (15 mL) under an argon atmosphere at room temperature, sodium hydroxide 50% solution (15 mL) was added, and then 2,3-dibromopropene (1.0 mL, 8.55 mmol) was added to the reaction mixture. Tetra-butyl ammonium bromide (100 mg) was added as a phase-transfer catalyst. The reaction mixture was stirred overnight at room temperature. The work-up was performed by adding ethyl acetate (15 mL) and separating the organic layer. The aqueous layer was washed with ethyl acetate (2 × 15 mL). The combined organic extracts were washed with dilute hydrochloric acid (5%, 15 mL) and water, dried over magnesium sulfate, and evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel using a 95:5 mixture of hexanes/ethyl acetate as eluent. Yield: 0.51 g (35%); mp 172–175 °C (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 3.01 (s, 2H), 3.26 (s, 2H), 4.59 (d, 2H), 4.96 (d, 2H).

13.20. Preparation of 9-(2-Biphenyl-2-yl-allyl)-9-[(9-(2-biphenyl-2-yl-allyl)fluoren-9-yl)methyl]fluorene-d16 (77)

A solution of 2-biphenylmagnesium bromide was prepared from 2-bromobiphenyl (0.4 g, 1.7 mmol) and excess magnesium turnings (0.2 g, 8.5 mmol) in anhydrous THF (20 mL), and 5 drops of dibromoethane were added to activate the reaction under an argon atmosphere and refluxing for 4 h. The Grignard solution thus obtained was transferred at room temperature to another Schlenk flask containing 76 (0.51 g, 0.85 mmol), and a catalytic amount of bis(triphenylphosphine) palladium dichloride (0.1 g) was added. The resulting yellow 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 × 15 mL). The combined organic extracts were dried over magnesium sulfate and evaporated under reduced pressure to produce a brown residue that was filtered through a short pad of silica gel with hexanes as eluent. The resulting product was further purified by column chromatography on silica gel using a 95:5 mixture of hexanes/ethyl acetate as eluent. Yield: 0.48 g (76%), mp 146–148 °C (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 2.13 (s, 4H), 2.33 (s, 2H), 4.12 (d, 2H), 4.51 (d, 2H), 5.76 (2H, d), 6.98–7.43 (m, 16H); 13C NMR (CDCl3) δ 47.29, 48.18, 53.94, 119.75, 126.43, 126.73, 126.82, 128.25, 128.98, 129.14, 129.90, 137.69, 140.56, 142.95, 142.45, 146.25, 147.16.

13.21. Preparation of Bis(9-((9-methyl-9H-fluoren-9-yl)methyl)-9H-fluoren-9-yl)methane-d16 (F4-d16)

To a cold (0 °C) solution of 77 (0.48 g, 0.67 mmol) in dry dichloromethane (25 mL), a few drops (5) of methanesulfonic acid were added, and the reaction mixture was allowed to warm up to room temperature and stirred for 4 h. The work-up was carried out by adding a saturated solution of sodium bicarbonate (30 mL) to remove the excess acid followed by extraction with dichloromethane (3 × 15 mL). The combined organic extracts were dried over magnesium sulfate and evaporated under reduced pressure. Yield: 0.15 g (36%); mp 215–217 °C (CH2Cl2/MeOH); 1H NMR (CDCl3) δ 1.04 (s, 6H), 2.71 (s, 6H), 6.09–6.99 (m, 16H); 13C NMR (CDCl3) δ 30.28, 49.35, 50.87, 52.90, 118.88, 123.09, 124.21, 124.26, 125.58, 125.69, 139.38, 140.38, 146.40, 150.21.

14. 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 1H 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.

Author Contributions

Conceptualization, R.R. and S.H.A.; data curation, R.R. and S.H.A.; methodology, R.R. and S.H.A.; supervision, R.R. and S.H.A.; writing—original draft, S.H.A.; writing—review and editing, R.R. and S.H.A. All authors have read and agreed to the published version of the manuscript.

Funding

We thank the NSF and NIH for their financial support. Also, we thank the Robert A. Welch Foundation grant number L-0002, and Prairie View University Faculty grant number 552509-00018.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article references.

Conflicts of Interest

The authors declare no conflict of interest.

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Molecules 28 03717 g001 550
Figure 1. Donor–DNA–acceptor assemblies for electron-transfer studies through DNA wires.
Figure 1. Donor–DNA–acceptor assemblies for electron-transfer studies through DNA wires.
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Figure 2. A cartoon diagram showing the intra-chain versus inter-chain charge transfer in bulk semiconductors.
Figure 2. A cartoon diagram showing the intra-chain versus inter-chain charge transfer in bulk semiconductors.
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Molecules 28 03717 g003 550
Figure 3. (a) Structures of representative cyclophanes derived from the benzenoid and polynuclear hydrocarbons, and (b) examples of multi-decker cyclophanes.
Figure 3. (a) Structures of representative cyclophanes derived from the benzenoid and polynuclear hydrocarbons, and (b) examples of multi-decker cyclophanes.
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Molecules 28 03717 g004 550
Figure 4. Structures of representative multi-decker cyclophanes.
Figure 4. Structures of representative multi-decker cyclophanes.
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Molecules 28 03717 g005 550
Figure 5. Formation of dimeric (32)2+• and its crystallographic packing diagram [16].
Figure 5. Formation of dimeric (32)2+• and its crystallographic packing diagram [16].
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Molecules 28 03717 g006 550
Figure 6. (a) Structure of stereo-regular polystyrene. (b) The calculated [M06/6-31G* + PCM(CH2Cl2)] structures of conformers 34e and 34f. (c) Comparison of electrochemical potentials of 34 and model 35. It was assumed that Eox of 34f is the same as cyclophane 28 in Table 1.
Figure 6. (a) Structure of stereo-regular polystyrene. (b) The calculated [M06/6-31G* + PCM(CH2Cl2)] structures of conformers 34e and 34f. (c) Comparison of electrochemical potentials of 34 and model 35. It was assumed that Eox of 34f is the same as cyclophane 28 in Table 1.
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Molecules 28 03717 g007 550
Figure 7. Synthesis (a), CVs of 36 (Eox1 = 0.68 V, Eox2 = 1.05 V) and 37 (Eox1 = 0.78 V) in CH2Cl2 at v = 200 mV s−1 at 22 °C (b), and transient absorption spectra of 36+• (blue) and 37+• (red) generated by laser flash photolysis using triplet chloranil as an oxidant (c). Inset: The absorption spectrum of chloranil–• generated using DABCO as the electron donor whose cation radical absorbs at <350 nm.
Figure 7. Synthesis (a), CVs of 36 (Eox1 = 0.68 V, Eox2 = 1.05 V) and 37 (Eox1 = 0.78 V) in CH2Cl2 at v = 200 mV s−1 at 22 °C (b), and transient absorption spectra of 36+• (blue) and 37+• (red) generated by laser flash photolysis using triplet chloranil as an oxidant (c). Inset: The absorption spectrum of chloranil–• generated using DABCO as the electron donor whose cation radical absorbs at <350 nm.
Molecules 28 03717 g007
Molecules 28 03717 sch001 550
Scheme 1. Electron transfe-induced folding of 36e into 36f+•.
Scheme 1. Electron transfe-induced folding of 36e into 36f+•.
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Molecules 28 03717 g008 550
Figure 8. Comparison of the experimental (a) and calculated [M06/6-31G* + PCM(CH2Cl2)] (b) oxidation potentials of 36 and model 37. The oxidation potential of 36e was assumed to be same as for model 37.
Figure 8. Comparison of the experimental (a) and calculated [M06/6-31G* + PCM(CH2Cl2)] (b) oxidation potentials of 36 and model 37. The oxidation potential of 36e was assumed to be same as for model 37.
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Molecules 28 03717 g009 550
Figure 9. Conformational analysis of (a) 36 and (b) F2, [M06/6-31G* + PCM(CH2Cl2)].
Figure 9. Conformational analysis of (a) 36 and (b) F2, [M06/6-31G* + PCM(CH2Cl2)].
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Molecules 28 03717 sch002 550
Scheme 2. Different functionalization sites in fluorene.
Scheme 2. Different functionalization sites in fluorene.
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Molecules 28 03717 sch003 550
Scheme 3. Construction of fluorene ring system by intramolecular Friedel-Crafts cyclization.
Scheme 3. Construction of fluorene ring system by intramolecular Friedel-Crafts cyclization.
Molecules 28 03717 sch003
Molecules 28 03717 sch004 550
Scheme 4. Attempted syntheses of F3 by Friedel–Crafts cyclization.
Scheme 4. Attempted syntheses of F3 by Friedel–Crafts cyclization.
Molecules 28 03717 sch004
Molecules 28 03717 sch005 550
Scheme 5. Alternative Friedel–Crafts precursor for F3.
Scheme 5. Alternative Friedel–Crafts precursor for F3.
Molecules 28 03717 sch005
Molecules 28 03717 sch006 550
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.
Molecules 28 03717 sch006
Molecules 28 03717 sch007 550
Scheme 7. Preparation of F4 from 55 using a three-step strategy.
Scheme 7. Preparation of F4 from 55 using a three-step strategy.
Molecules 28 03717 sch007
Molecules 28 03717 sch008 550
Scheme 8. Proposed mechanism for the formation of 55 from 60 and 58.
Scheme 8. Proposed mechanism for the formation of 55 from 60 and 58.
Molecules 28 03717 sch008
Molecules 28 03717 sch009 550
Scheme 9. Preparation of 55 and higher homologues.
Scheme 9. Preparation of 55 and higher homologues.
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Molecules 28 03717 sch010 550
Scheme 10. Preparation of F5 and F6.
Scheme 10. Preparation of F5 and F6.
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Molecules 28 03717 sch011 550
Scheme 11. Preparation of selectively deuterated polyfluorenes.
Scheme 11. Preparation of selectively deuterated polyfluorenes.
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Molecules 28 03717 g010 550
Figure 10. (a) Partial 1H NMR spectra of F1–F6 in CDCl3 at 22 °C, the cofacial stacked fluorene units were colored based on the anisotropic deshielding (b) showing 1/n dependence of the chemical shift of H4 in F2–F6 and (c) the dynamic displacement of the central fluorene in F3 from ideal sandwich-like geometry.
Figure 10. (a) Partial 1H NMR spectra of F1–F6 in CDCl3 at 22 °C, the cofacial stacked fluorene units were colored based on the anisotropic deshielding (b) showing 1/n dependence of the chemical shift of H4 in F2–F6 and (c) the dynamic displacement of the central fluorene in F3 from ideal sandwich-like geometry.
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Molecules 28 03717 g011 550
Figure 11. Comparison of 1H NMR spectra of F3-d8 and F4-d16 with those of F3 and F4. A further analysis showed that the extent of deshielding of the protons on the central fluorene ring/s in F2–F6 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].
Figure 11. Comparison of 1H NMR spectra of F3-d8 and F4-d16 with those of F3 and F4. A further analysis showed that the extent of deshielding of the protons on the central fluorene ring/s in F2–F6 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].
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Figure 12. Comparison of the X-ray structures of F1–F4 (a) and space-filling representation of F3 (b).
Figure 12. Comparison of the X-ray structures of F1–F4 (a) and space-filling representation of F3 (b).
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Molecules 28 03717 g013 550
Figure 13. Normalized absorption spectra of (a) F1–F6 with ε265 = 17,000; 27,300; 36,000; 45,400; 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.
Figure 13. Normalized absorption spectra of (a) F1–F6 with ε265 = 17,000; 27,300; 36,000; 45,400; 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.
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Molecules 28 03717 g014 550
Figure 14. Representative structures showing the localization of the excimeric exciton onto two perfectly sandwiched fluorenes in F2–F4. Note that multiple excimeric structures are possible for F4–F6, as exemplified by F4′ and F4”.
Figure 14. Representative structures showing the localization of the excimeric exciton onto two perfectly sandwiched fluorenes in F2–F4. Note that multiple excimeric structures are possible for F4–F6, as exemplified by F4′ and F4”.
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Molecules 28 03717 g015 550
Figure 15. (a) Cyclic (black) and square-wave (red) voltammograms of 2 mM F1–F6 in CH2Cl2 containing 0.1 M (nBu)4NPF6 at v = 200 mV/s. (b) A plot of the IP (black axis) and Eox1 (blue axis) values versus 1/n. Note that dashed lines include F1 in correlations. (c) A plot of IP versus Eox1 values for F1–F4.
Figure 15. (a) Cyclic (black) and square-wave (red) voltammograms of 2 mM F1–F6 in CH2Cl2 containing 0.1 M (nBu)4NPF6 at v = 200 mV/s. (b) A plot of the IP (black axis) and Eox1 (blue axis) values versus 1/n. Note that dashed lines include F1 in correlations. (c) A plot of IP versus Eox1 values for F1–F4.
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Figure 16. Optical spectra of F1+•-F6+• generated using NO+SbCl6− in CH2Cl2 at 22 °C. Note that F1+• was generated via laser flash photolysis.
Figure 16. Optical spectra of F1+•-F6+• generated using NO+SbCl6− in CH2Cl2 at 22 °C. Note that F1+• was generated via laser flash photolysis.
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Figure 17. (a) The representative EPR spectra of Fn-• in HMPA at 22 °C. (b) The EPR spectrum of F4−•(int) obtained immediately after mixing F4 with potassium (top), the spectrum of the same solution recorded after 1 h (middle), and its simulated spectrum (lower) [39].
Figure 17. (a) The representative EPR spectra of Fn-• in HMPA at 22 °C. (b) The EPR spectrum of F4−•(int) obtained immediately after mixing F4 with potassium (top), the spectrum of the same solution recorded after 1 h (middle), and its simulated spectrum (lower) [39].
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Figure 18. (a) Structures of the Bp–Fn–Nap triads, Bp–Fn and Fn–Nap diads, and representative X-ray structure of the Bp–F3–Nap triad. (b) The 3D transient absorption spectra of Bp–Fn diads (A–C) and Bp–Fn–Nap triads (D–F), where white ovals identify absorption due to the triplet Fn bridge-occupied state. (c) Energy diagram of Bp–Fn–Nap and Bp–Fn with observed triplet-energy transfer time constants (Fn* triplet energy of the Fn).
Figure 18. (a) Structures of the Bp–Fn–Nap triads, Bp–Fn and Fn–Nap diads, and representative X-ray structure of the Bp–F3–Nap triad. (b) The 3D transient absorption spectra of Bp–Fn diads (A–C) and Bp–Fn–Nap triads (D–F), where white ovals identify absorption due to the triplet Fn bridge-occupied state. (c) Energy diagram of Bp–Fn–Nap and Bp–Fn with observed triplet-energy transfer time constants (Fn* triplet energy of the Fn).
Molecules 28 03717 g018
Table
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].
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].
MonomerEox (V) aCyclophaneEox (V) aD (V)
Molecules 28 03717 i001
23		0.66Molecules 28 03717 i002Molecules 28 03717 i0030.330.33
Molecules 28 03717 i004
24		0.73Molecules 28 03717 i005Molecules 28 03717 i0060.350.38
Molecules 28 03717 i007
26		0.77Molecules 28 03717 i008Molecules 28 03717 i0090.420.35
Molecules 28 03717 i010
27		0.82Molecules 28 03717 i011Molecules 28 03717 i0120.570.25
a The Eox vs. Fc/Fc+ were measured in CH2Cl2 containing (n-Bu)4NPF6 at v = 200 mV/sec at 22 °C. (n-Bu)4NPF6 is tetrabutylammonium hexafluorophosphate.
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Rathore, R.; Abdelwahed, S.H. Design and Synthesis of Cofacially-Arrayed Polyfluorene Wires for Electron and Energy Transfer Studies. Molecules 2023, 28, 3717. https://doi.org/10.3390/molecules28093717

AMA Style

Rathore R, Abdelwahed SH. Design and Synthesis of Cofacially-Arrayed Polyfluorene Wires for Electron and Energy Transfer Studies. Molecules. 2023; 28(9):3717. https://doi.org/10.3390/molecules28093717

Chicago/Turabian Style

Rathore, Rajendra, and Sameh H. Abdelwahed. 2023. "Design and Synthesis of Cofacially-Arrayed Polyfluorene Wires for Electron and Energy Transfer Studies" Molecules 28, no. 9: 3717. https://doi.org/10.3390/molecules28093717

APA Style

Rathore, R., & Abdelwahed, S. H. (2023). Design and Synthesis of Cofacially-Arrayed Polyfluorene Wires for Electron and Energy Transfer Studies. Molecules, 28(9), 3717. https://doi.org/10.3390/molecules28093717

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