The Design Strategy for an Aggregation- and Crystallization-Induced Emission-Active Molecule Based on the Introduction of Skeletal Distortion by Boron Complexation with a Tridentate Ligand

: We describe here a new design strategy for obtaining boron complexes with aggregation-and crystallization-induced emission (AIE and CIE, respectively) properties based on the introduction of skeletal distortion. According to our recent results, despite the fact that an almost planar structure and robust conjugation were obtained, the boron azomethine complex provided a slight emission in solution and an enhanced emission in aggregation and crystal. Quantum calculation results propose that unexpected emission annihilation in solution could be caused through intramolecular bending in the excited state. Herein, to realize this unique molecular motion and obtain AIE and CIE molecules, the phenyl quinoline-based boron complexes BPhQ and BPhQm with distorted and planar structures were designed and synthesized, respectively. BPhQm showed emission in solution and aggregation-caused quenching (ACQ, BPhQm : Φ F,sol. = 0.21, Φ F,agg. = 0.072, Φ F,cryst. = 0.051), while BPhQ exhibited a typical AIE and CIE ( BPhQ : Φ F,sol. = 0.008, Φ F,agg. = 0.014, Φ F,cryst. = 0.017). The optical data suggest that a large degree of molecular motion should occur in BPhQ after photo-excitation because of the intrinsic skeletal distortion. Furthermore, single-crystal X-ray di ﬀ raction data indicate that the distorted π -conjugated system plays a positive role in presenting solid-state emission by inhibiting consecutive π – π interactions. We demonstrate in this paper that the introduction of the distorted structure by boron complexation should be a new strategy for realizing AIE and CIE properties.


Introduction
Organic fluorescent dyes have attracted considerable attention as a platform for constructing fluorescent probes to monitor biological events because of their various advantages, such as high sensitivity, rapid response, and property tenability [1][2][3]. In addition, organic fluorescent dyes can also be utilized for modern electronic devices, such as organic light-emitting diodes [4][5][6] and lighting panels [7]. Therefore, the development of a new type of fluorescent organic dye is a topic of interest in organic chemistry as well as material science. Recently, we proposed the concept of "element-blocks", which are minimum functional units containing heteroatoms, to create functional materials according to preprogrammed designs [8][9][10]. In the series of studies on organic luminescent dyes containing heteroatoms, unique optical properties originating from heteroatoms have been revealed [11]. In particular, boron-containing element-blocks are promising building blocks for obtaining luminescent materials. Boron-fused structures with bidentate π-conjugated ligands can enhance the rigidity and planarity of molecular frameworks because of their strong coordination ability originating from the electron-acceptor character of boron [12][13][14][15][16]. Thus, boron complexation is recognized as an established tactic for constructing highly-emissive fluorescent dyes [17][18][19][20][21][22][23][24]. Indeed, numerous studies have been accomplished on the development of emissive materials based on boron complexes with rigid and planar skeletons [25][26][27][28][29][30]. However, in solid states, planar organic chromophores often show the consecutive π-π stacking and subsequent physical interactions (e.g., energy transfer, inter-or intramolecular charge transfer, and excited states reactions. As a result, emission annihilation, called aggregation-caused quenching (ACQ), inevitably occurs [31,32]. Thus, most organic luminescent materials suffer from ACQ, especially in applications in luminescent devices.
As one of the critical solutions for overcoming ACQ, aggregation-induced emission (AIE) properties [33,34] and crystallization-induced emission (CIE) properties [35][36][37] have attracted attention. The AIE-and CIE-active molecules show enhanced emission by aggregation and crystallization, respectively. In dilute solutions, crucial intramolecular motions such as rotation and vibration occur, and their excited states are readily decayed through non-radiative pathways. These motions would be restricted in solid form, followed by significant luminescence. Furthermore, by modulating the degree of molecular motions in the excited state, environment-responsive behaviors are often observed [38,39]. Therefore, the deep understanding of molecular motions in the excited state and the relationship with these motions and photochemical processes is necessary for designing advanced AIE and CIE-active dyes.
In our previous work, we synthesized a fused azomethine-boron complex with a slightly-distorted π-conjugated system and a protruded substituent on the boron atom [40]. Although these structural features are favorable for obtaining solid-state emission by disturbing nonspecific π-π interactions in the condensed state, the boron azomenthine complex is an AIE and typical CIE-active molecule. From mechanistic studies, it is proposed that intramolecular bending should proceed in the excited state, followed by rapid excitation decay. In other words, the "flexible" molecular skeleton is the origin of the deactivation of excitation energy and subsequent emission annihilation through non-radiative decay processes [38]. From this fact, we presume that intrinsic local distortion in the conjugated ligand, which can be readily generated by boron complexation, could be the driving force for presenting emission annihilation in solution through molecular bending activated in the excited state and the key factor in exhibiting solid-state emission by disturbing intermolecular interactions by skeletal distortion. To evaluate the validity of this speculation, a comparison study with potentially distorted and planar molecules is needed.
Herein, we report opposite changes in emission efficiencies between the solution and solid of the modified azomethine-boron complexes BPhQ and BPhQm with or without skeletal distortion around the boron complexation moiety, respectively. In general, quinoline-based boron complexes have been considered to be a class of highly efficient luminescent dyes in solution owing to their rigid and planar structures [41][42][43][44]. Indeed, BPhQm possesses a planar structure, while BPhQ intrinsically involves distortion, although electronic conjugation can be developed through both molecules. Interestingly, increases and decreases in emission efficiencies by aggregation and crystallization were obtained from BPhQ and BPhQm, meaning that BPhQm shows ACQ in solid, whereas BPhQ is an AIE-and CIE-active molecule. From the series of optical measurements, we propose that skeletal distortion plays a critical role in enhanced emission behaviors in solids as well as emission annihilation in solution. This is the first example, to the best of our knowledge, to offer a new design strategy based on the introduction of distortion with boron complexation for realizing AIE and CIE properties.

Materials and Methods
According to Scheme 1, the quinoline-based boron complex BPhQ was synthesized from the tridentate quinoline ligand 3 (Figures S1-S6 and Schemes S1-S3, See Supplementary Materials). The skeletal distortion was introduced into the quinoline scaffold by the formation of a five-membered ring with a four-coordinating boron atom. We also synthesized the methylene-inserted quinoline boron complex BPhQm as the planar molecule ( Figures S7-S9 and Scheme S4, See Supplementary Materials). In the case of BPhQm, the double six-membered ring systems were formed by a boron complexation reaction for releasing distortion. To improve the solubility and inhibit undesired aggregation in the solution, BPhQ and BPhQm were modified with tert-butyl groups. Indeed, the synthesized complexes have good solubility in common organic solvents such as chloroform, THF, and dichloromethane. The structures of BPhQ and BPhQm were confirmed by 1 H, 11 B, 13 C NMR, ionization mass measurements and elemental analyses. Fortunately, we obtained single crystals of the complexes suitable for X-ray analyses.

Single Crystal X-ray Diffraction Analysis
From the comparison with the single-crystal X-ray diffraction (SCXRD) data ( Figure 1 and Tables S1-S2, See Supplementary Materials), it was shown that BPhQ had a relatively distorted conformation according to the dihedral angle between the quinoline and the adjacent phenyl ring, which was 154.7°. The skeletal distorted conformation should originate from the tightly fused structure involving five-and six-membered rings. In contrast, it was observed that BPhQm has a relatively planar structure (the dihedral angle = 168.8°), owing to the loosely fused six-membered ring of the methylene group. From these data, it is evident that both complexes have expected structural features.

Single Crystal X-ray Diffraction Analysis
From the comparison with the single-crystal X-ray diffraction (SCXRD) data ( Figure 1 and Tables S1-S2, See Supplementary Materials), it was shown that BPhQ had a relatively distorted conformation according to the dihedral angle between the quinoline and the adjacent phenyl ring, which was 154.7 • . The skeletal distorted conformation should originate from the tightly fused structure involving five-and six-membered rings. In contrast, it was observed that BPhQm has a relatively planar structure (the dihedral angle = 168.8 • ), owing to the loosely fused six-membered ring of the methylene group. From these data, it is evident that both complexes have expected structural features.

Optical Properties
To evaluate the electronic structures of the complexes in the ground state, the UV-vis absorption spectra were measured with the dilute chloroform solutions (Table 1 and Figure 2a). Similar spectral shapes and molar extinction coefficients were obtained from the absorption bands around 380 nm, indicating that both complexes should have similar electronic structures. It should be noted that the skeletal distortion in BPhQ slightly affects the electronic conjugation in the ground state.
In contrast to the absorption properties, the emission behaviors, representing electronic structures in the excited state, were largely different between both complexes ( Figure 2b). We evaluated the photoluminescence (PL) properties of BPhQ and BPhQm with the same chloroform solutions. The emission band of BPhQ was detected in the remarkable red-shifted wavelength region compared to that of BPhQm (BPhQ: λem = 550 nm; BPhQm: λem = 493 nm). In particular, a wider Stokes shift was observed from BPhQ than from BPhQm, meaning that a larger degree of structural relaxation should proceed in BPhQ after photo-excitation.

Optical Properties
To evaluate the electronic structures of the complexes in the ground state, the UV-vis absorption spectra were measured with the dilute chloroform solutions (Table 1 and Figure 2a). Similar spectral shapes and molar extinction coefficients were obtained from the absorption bands around 380 nm, indicating that both complexes should have similar electronic structures. It should be noted that the skeletal distortion in BPhQ slightly affects the electronic conjugation in the ground state.  To elucidate the difference in emission wavelength between BPhQ and BPhQm, a solvatochromism study was executed (Figures S10-S11, See Supplementary Materials). Accordingly, significant shifts in the absorption bands were hardly observed by altering the solvent polarity, whereas the emission bands were bathochromically shifted from 542 to 567 nm for BPhQ and from 478 to 512 nm for BPhQm as the solvent polarity increased. These results suggest that the emission bands of both boron complexes should be derived from the intramolecular charge transfer (ICT) states. Meanwhile, even in cyclohexane, which is a typical non-polar solvent, there were still pronounced differences between the two emission peak wavelengths (BPhQ: λem = 542 nm; BPhQm: λem = 478 nm). In order to obtain deep insight into the origin of the longer-wavelength emission of In contrast to the absorption properties, the emission behaviors, representing electronic structures in the excited state, were largely different between both complexes (Figure 2b). We evaluated the photoluminescence (PL) properties of BPhQ and BPhQm with the same chloroform solutions. The emission band of BPhQ was detected in the remarkable red-shifted wavelength region compared to that of BPhQm (BPhQ: λ em = 550 nm; BPhQm: λ em = 493 nm). In particular, a wider Stokes shift was observed from BPhQ than from BPhQm, meaning that a larger degree of structural relaxation should proceed in BPhQ after photo-excitation.
To elucidate the difference in emission wavelength between BPhQ and BPhQm, a solvatochromism study was executed ( Figures S10-S11, See Supplementary Materials). Accordingly, significant shifts in the absorption bands were hardly observed by altering the solvent polarity, whereas the emission bands were bathochromically shifted from 542 to 567 nm for BPhQ and from 478 to 512 nm for BPhQm as the solvent polarity increased. These results suggest that the emission bands of both boron complexes should be derived from the intramolecular charge transfer (ICT) states. Meanwhile, even in cyclohexane, which is a typical non-polar solvent, there were still pronounced differences between the two emission peak wavelengths (BPhQ: λ em = 542 nm; BPhQm: λ em = 478 nm). In order to obtain deep insight into the origin of the longer-wavelength emission of BPhQ, the emissive properties were examined with polystyrene films, including 1 wt% of BPhQ or BPhQm, where molecular motion would be suppressed in each isolated molecule (Table 2 and Figure S12, See Supplementary Materials). The PL spectrum of the film of BPhQ showed an emission band with a clear blue shift from that of the solution (λ em,sol. = 550 nm, λ em,film. = 531 nm). In general, the blue-shifted emission in solids is caused by suppressing geometry relaxation in the excited state under a spatially restricted environment compared to the solution [45]. Thus, it is suggested that the longer-wavelength emission of BPhQ than that of BPhQm should originate from the intrinsic larger degree of molecular mobility in the excited state. Correspondingly, in the PL spectra of BPhQm in the polystyrene film, the degree of blue-shift from the solution state was much smaller than that observed in BPhQ. We evaluated the emission efficiencies of the complexes in the solution, aggregation, and crystalline samples (Table 1 and Figures S13-S14, See Supplementary Materials). From a comparison with the emissive properties in the solution and solid form, it was found that BPhQ and BPhQm showed opposite behaviors: BPhQ had AIE and CIE characteristics, while BPhQm showed typical ACQ. We prepared aggregated samples of BPhQ and BPhQm from THF/H 2 O solutions with a water fraction of 99%, and the emission efficiencies were determined as the absolute fluorescence quantum efficiencies (Φ F ). The Φ F values of the aggregation and crystalline sample for BPhQ were significantly higher than those of the solutions (Φ F,sol. = 0.008 → Φ F,agg. = 0.014, Φ F,cry. = 0.017). On the other hand, the Φ F values of the aggregated and crystalline sample for BPhQm were lower than those of the solution state (Φ F,sol. = 0.21 → Φ F,agg. = 0.072, Φ F,cry. = 0.051). The reduction in the Φ F s in the solid of BPhQm is likely to be caused by non-specific intermolecular interaction in the condensed state, followed by ACQ.
To examine in detail the photophysical processes especially in BPhQ, the radiative (k r ) and non-radiative rate constants (k nr )-which represent the sum of the rates in each process with or without emission, respectively-were determined from the lifetime measurements (Table 2 and Figure S15, See Supplementary Materials). Accordingly, it was found that BPhQ had a smaller k r and larger k nr than BPhQm in solution, meaning that both the radiative and non-radiative processes should be quite different between BPhQ and BPhQm. In addition, the k nr s of BPhQ in the aggregated and crystalline samples were lowered from those of the solution state, suggesting that the nonradiative decay channels related to the intramolecular motion of BPhQ in the solution state were efficiently restricted under rigid environments, such as in the aggregation and crystal. A similar enhancement of Φ F was also observed in 1 wt% polystyrene dispersed film (Table 2 and Figure S15, See Supplementary Materials). The Φ F of BPhQ in the polystyrene film was determined as 0.021, which is higher than that of the solution state (Φ F = 0.008), whereas it was found that the k nr in the polystyrene film was one order lower than that in the solution state, representing that the improvement of the emission property in film should be associated with the restriction of molecular motions by polymer chains. Contrary to BPhQ, the Φ F of BPhQm decreased in the polystyrene film compared with that in the solution, and both of the k nr values were in the same range, suggesting that the molecular motions of BPhQm in the excited state hardly play a role in the emission properties of the solution state because of its rigid molecular structure. In summary, from the comparison with both complexes, it was figured out that BPhQ had a larger non-radiative rate constant. Possibly, the distorted structure in BPhQ induced by boron complexation may cause geometrical perturbation in the excited state, and thus the molecular motions promoted non-radiative decay processes.

Packing Structures
To comprehend the solid-state luminescent properties, we compared the molecular packing structures of the complexes determined by the SCXRD data ( Figure 3). Although both complexes belong to the triclinic system (P1 space group), the packing structures were different, probably because of the planarity of BPhQ and BPhQm. In the crystalline packing of BPhQ, each molecule formed π-π intermolecular interactions between dimers, with distance of 3.596 Å. However, in another molecular surface, the distance to the adjacent molecule was 4.148 Å, and thus the π-π intermolecular interactions through three or more molecules were inhibited, owing to the steric hindrance imposed by the distorted π-conjugated system and the protruded fluorine atom. In contrast, the molecules of BPhQm with a planar structure were densely packed into molecular columns that were perpendicular to the quinoline moieties and formed face-to-face π-π interactions between both surfaces, with distances of 3.723 and 3.807 Å. From these structural data, it is proposed that π-π interactions through multiple BPhQ molecules can be disturbed owing to the distorted structure, followed by avoiding ACQ. On the other hand, because of the intrinsic planar structure of BPhQm, a critical ACQ should be induced through tight intermolecular interactions, such as π-π stacking, in the solid state.
Crystals 2020, 10, x FOR PEER REVIEW 7 of 12 another molecular surface, the distance to the adjacent molecule was 4.148 Å, and thus the π-π intermolecular interactions through three or more molecules were inhibited, owing to the steric hindrance imposed by the distorted π-conjugated system and the protruded fluorine atom. In contrast, the molecules of BPhQm with a planar structure were densely packed into molecular columns that were perpendicular to the quinoline moieties and formed face-to-face π-π interactions between both surfaces, with distances of 3.723 and 3.807 Å. From these structural data, it is proposed that π-π interactions through multiple BPhQ molecules can be disturbed owing to the distorted structure, followed by avoiding ACQ. On the other hand, because of the intrinsic planar structure of BPhQm, a critical ACQ should be induced through tight intermolecular interactions, such as π-π stacking, in the solid state.

Theoretical Calculation
In order to support the emissive properties and radiative decay processes of BPhQ and BPhQm, we conducted time-dependent density functional theory (TD-DFT) calculations at the B3LYP/6-311 + G(d,p) level with the optimized structures in the S1 states (Table 3). Figure 4 represents highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) at the S1 state. The LUMOs of both molecules are delocalized on the entire molecules, while the HOMOs are localized on the quinoline moiety and the tert-butyl group-substituted phenyl ring, respectively.

Theoretical Calculation
In order to support the emissive properties and radiative decay processes of BPhQ and BPhQm, we conducted time-dependent density functional theory (TD-DFT) calculations at the B3LYP/6-311 + G(d,p) level with the optimized structures in the S 1 states (Table 3). Figure 4 represents highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) at the S 1 state. The LUMOs of both molecules are delocalized on the entire molecules, while the HOMOs are localized on the quinoline moiety and the tert-butyl group-substituted phenyl ring, respectively. These alterations in the electronic orbital distributions between HOMO and LUMO correspond to the ICT characters of the emission bands of both complexes. The electron-rich oxygen atom might contribute to the localization of HOMO of BPhQ at the quinolone moiety, while the electron donation by the quinoline moiety might be weak in BPhQm, and subsequently HOMO might be able to spread in the tert-butyl group-substituted phenyl ring.  Finally, we examined the electronic transitions with theoretical calculations. The oscillator strength (f) for the S1 to S0 transition, which is associated with kr, was quite small (f = 0.0262) in BPhQ, proposing that the lowest-lying excited state S1 should be a weak emissive state. In contrast, the f value of BPhQm was about six times higher than that of BPhQ (f = 0.1521)-that is, the lowest-lying excited state S1 is a highly emissive state. These theoretical results were in good agreement with the experimental kr data (Tables 2 and 3). Additionally, to reveal the origin of the difference in the oscillator strength for the S1 → S0 transition between BPhQ and BPhQm, the transition densities (ρ) for the corresponding transitions were calculated for a simple and effective representation of the electronic transition ( Figure 5). The magnitude of the f for an electronic transition is proportional to the square of the transition dipole moment (μ). On the other hand, μ can be expressed by the Finally, we examined the electronic transitions with theoretical calculations. The oscillator strength (f ) for the S 1 to S 0 transition, which is associated with k r , was quite small (f = 0.0262) in BPhQ, proposing that the lowest-lying excited state S 1 should be a weak emissive state. In contrast, the f value of BPhQm was about six times higher than that of BPhQ (f = 0.1521)-that is, the lowest-lying excited state S 1 is a highly emissive state. These theoretical results were in good agreement with the experimental k r data (Tables 2 and 3). Additionally, to reveal the origin of the difference in the oscillator strength for the S 1 → S 0 transition between BPhQ and BPhQm, the transition densities (ρ) for the corresponding transitions were calculated for a simple and effective representation of the electronic transition ( Figure 5). The magnitude of the f for an electronic transition is proportional to the square of the transition dipole moment (µ). On the other hand, µ can be expressed by the following equation: where ρ is the transition density for the S 1 state and x denotes a point in three-dimensional space. From the equation, the µ value becomes large when ρ has a value at x at which these elements are large-i.e., when ρ is distributed in the regions that are distant from the coordinate origin [46,47]. Figure 5 shows a more widely distributed ρ in BPhQm, indicating that the overlap between the HOMO and LUMO was larger than that in BPhQ. Owing to this wide ρ distribution spread to the entire molecule, BPhQm has moderate µ of 2.415 (a.u.). In contrast, BPhQ has a small µ of 0.4656 (a.u.), since the ρ distribution range is limited in the small regions around the quinoline moiety. This is because the HOMO of BPhQ is mainly distributed in the quinoline group (Figure 4), and the resulting HOMO-LUMO overlap is thus small. In the presence and absence of the methylene group, the electronic structures are also influenced.
The isosurface value for ρ is 0.0004 atomic units (purple regions: positive; blue regions: negative).

Conclusions
We demonstrate that the introduction of skeletal distortion by boron complexation with the tridentate ligand is a facile strategy for obtaining AIE and CIE-active molecules. From the comparison with phenyl quinoline-based boron complexes BPhQ and BPhQm with or without critical skeletal distortion around the boron complex moiety, it was found that with the molecular motions, represented by the wider Stokes shift, a smaller kr and larger knr are feasible in the excited state, followed by emission annihilation, even though electronic conjugation can be developed through planar molecular structures in the ground state. Further, molecular distortion seems to play a positive role in avoiding ACQ. Consequently, we observed from both complexes contrary changes in emission efficiencies by aggregation, such as AIE and CIE from BPhQ and ACQ from BPhQm. So far, boron complexation has been applied for constructing a robust conjugated system, while we herein present a new manner of boron complexation through the generation of distortion for a potential movable point in the excited state as well as for realizing emission enhancement in the solid state. Thus, it is implied that conjugated element-blocks with distortion might be a promising platform for obtaining advanced stimuli-responsive optical materials.

Conclusions
We demonstrate that the introduction of skeletal distortion by boron complexation with the tridentate ligand is a facile strategy for obtaining AIE and CIE-active molecules. From the comparison with phenyl quinoline-based boron complexes BPhQ and BPhQm with or without critical skeletal distortion around the boron complex moiety, it was found that with the molecular motions, represented by the wider Stokes shift, a smaller k r and larger k nr are feasible in the excited state, followed by emission annihilation, even though electronic conjugation can be developed through planar molecular structures in the ground state. Further, molecular distortion seems to play a positive role in avoiding ACQ. Consequently, we observed from both complexes contrary changes in emission efficiencies by aggregation, such as AIE and CIE from BPhQ and ACQ from BPhQm. So far, boron complexation has been applied for constructing a robust conjugated system, while we herein present a new manner of boron complexation through the generation of distortion for a potential movable point in the excited state as well as for realizing emission enhancement in the solid state. Thus, it is implied that conjugated element-blocks with distortion might be a promising platform for obtaining advanced stimuli-responsive optical materials.