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Twist of C=C Bond Plays a Crucial Role for Quenching of AIE-Active Tetraphenylethene Derivatives in Solution Kenta Kokado, Takashi Machida, Takeshi Iwasa, Tetsuya Taketsugu, and Kazuki Sada J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11248 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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The Journal of Physical Chemistry

Twist of C=C Bond Plays a Crucial Role for Quenching of AIEActive Tetraphenylethene Derivatives in Solution Kenta Kokado,†,‡,* Takashi Machida,† Takeshi Iwasa,†,‡ Tetsuya Taketsugu, †,‡ and Kazuki Sada†,‡,* †

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita10 Nishi8, Kita-ku, Sapporo, 060-0810, Japan. ‡ Department of Chemistry, Faculty of Science, Hokkaido University

ABSTRACT: Aggregation-induced emission (AIE) has emerged as a new class of attractive photoluminescence behavior. Understanding the precise mechanism of AIE phenomenon will lead to rational molecular design of novel molecules with AIE property (AIEgens). In this Article, we selected disubstituted tetraphenylethene (TPE) derivatives, a well-known archetypal AIEgen, as the model compounds to elucidate the AIE mechanism. As the result of photochemical experiments and quantum chemical computations, π bond twist (π twist), including (E)-(Z) isomerization (EZI), was found to be the major factor for quenching the photoexcited state of TPE derivatives in solution state, differently from the well-accepted propeller-like rotation of the side phenyl groups. in precedent researches. In the photochemical experiments, the prepared TPE derivatives exhibited EZI in the solution state upon photoirradiation, and a negative correlation was observed between this isomerization and the AIE phenomenon. The theoretical computations verified the crucial role of π twist triggered by photoirradiation in the solution state, rather than intramolecular rotation. In the crystal state, π twist was efficiently suppressed by the surrounding molecules. Our results will realize novel smart AIEgens that can respond to various external stimuli.

Introduction Aggregation-induced emission (AIE) is a photoluminescence phenomenon in which a so-called AIE luminogen (AIEgen) exhibits intense emission in the aggregated or solid state but only weak or no emission in the solution state.1-5 Since the definition of AIE by Tang et al. in 2001,6 AIEgens have become a new class of optical materials because of this attractive switching property. Many studies on molecules exhibiting AIE properties have been reported, leading to practical applications in optoelectronics,7-9 fluorescent probes,10-12 and biosensors.13-15 Understanding the underlying mechanism of AIE phenomenon in an explicit fashion is obviously significant to attain rational molecular design of AIEgens, which will provide novel AIEgens without depending on trial-and-error approach or accidental discovery. Tetraphenylethene (TPE) is an archetypal AIEgen and also a source of various AIEgens.1,2 The restriction of intramolecular rotation (RIR) of the phenyl rings, thus the restriction of thermal perturbation, is generally accepted as the main cause for AIE phenomenon of TPE derivatives in AIE community.16 The suggested RIR process can occur not only in aggregated or solid states but also in solution or dispersed states. Several studies have thus focused on the occurrence of emissive phenomenon of TPE derivatives through supramolecular interaction.17-20 For example, Shinkai and coworkers reported “cyclization-induced emission” of a TPE derivative with zinc dipicolylamine groups interacting with dicarboxylic acids in homogeneous buffer solution.18 Very recently, Hahn and coworkers reported the emission of an N-heterocyclic carbene (NHC)-tethering TPE derivative through metal complexation of the NHC moiety.20 In addition, we have previously demon-

strated emission control of TPE derivatives through a supramolecular interaction via incorporation in network polymers or liquefaction.21-26 These findings verify the presumption that aggregate formation is not a requirement for AIE in TPE derivatives but only a sufficient condition for it. Note that, even after these supramolecular association events, the rotation of phenyl rings can still occur, because most of reported molecular systems have TPE derivatives with substitution at the paraposition. TPE is a symmetrical α,α’-diphenyl stilbene; thus, it is reasonable to consider that TPE undergoes (E)-(Z) isomerization (EZI) via π bond twist (π twist) after photo-excitation in a similar manner as stilbene and its derivatives.27-38 Indeed, in the earliest studies, Barbara et al. and Greene independently reported π twist of TPE in the excited state by using a picosecond spectroscopy as well as temperature and viscosity dependent fluorescence in solution,39,40 followed by femtosecond spectroscopy by Wiersma et al.41,42 The study on quenching mechanism of TPE derivatives in solution will provide a deep insight of the AIE mechanism, since the quenching can be regarded as the complementary event of emission from the aggregation. Nonetheless, studies that consider π twist of TPE derivatives, including EZI, in the excited state as referred above remain very limited in the field of AIE. 43-46 In earlier reports, Tang and coworkers synthesized pure (E) and (Z) isomers of TPE derivatives, and they denied the involvement of EZI under conventional photoluminescence measurement conditions.43,44 On the other hand, in a recent report, they recognized the minor involvement of EZI in quenching the excited state of TPE derivatives,45 leading to a confusion in understanding the AIE mechanism of TPE derivatives. Recent computational studies have contributed to the clarification of the

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excited-state decay process of TPE in the solution state, and the results indicated the presence of conical intersection (CI) on the ultrafast photophysical quenching process of TPE including cyclization and/or π twist, rather than the propellerlike rotation of the side phenyl groups.47-49 Motivated by these various reports, in this work, we carried out photochemical experiment of disubstituted TPE derivatives, as well as theoretical computations to elucidate the origin of the efficient quenching mechanism of TPE derivatives in the solution state. This work will be beneficial in an important and clear-cut design guide for finding novel AIEgens.

terns differed. Differential scanning calorimetry (DSC) measurements also revealed the dissimilar behavior of isomers of TPE-2OMe upon heating; therefore, the crystallizing and melting peaks were observed only for (E)-rich TPE-2OMe (Figure S4). In contrast, both isomers of TPE-2F exhibited similar melting points.

Results and Discussion

Figure 2. UV-Vis and photoluminescence (PL) spectra of (a) (E)TPE-2OMe and (b) (Z)-TPE-2OMeUV-Vis spectra were measured in THF (10 µM), and PL spectra were in THF/H2O mixed solvent (10 µM).

Figure 1. (a) Synthesis of (E) and (Z) isomers of TPE-2OMe, TPE-2Me, and TPE-2F. (b) Molecular structures of TPE-4OMe, TPE-2O, and TPE-4oM.

The synthesis of disubstituted TPE derivatives, TPE-2OMe, TPE-2Me, and TPE-2F, was performed using standard McMurry coupling with titanium(IV) chloride and zinc powder as the reagents, followed by purification via silica gel column chromatography (Figure 1). The substituent of the paraposition mainly affected the reaction yield rather than the (E)(Z) ratio of the product, unlike the case of bulky substituent at the ortho-position.50 In our system, the (E)-(Z) ratio of the products was consistently close to 50/50. The ease of purification using silica chromatography was mainly dependent on the substituents at the para-position. The large dipole moment derived from heteroatomic substituents enabled purification (TPE-2OMe and TPE-2F), whereas hydrocarbon-substituted TPE (TPE-2Me) showed no difference in retention time, consistent with previously reported results.43-46 The (E) and (Z) isomers were distinguishable in 1H NMR spectra; for example, the protons on the methoxy group of TPE-2OMe resulted in a 0.02 ppm chemical shift difference between the (E) and (Z) isomers (Figure S1). The isomers could be identified by nuclear Overhauser effect (NOE) spectra between protons at the ortho-position of different and vicinal phenyl groups (Figure S2). These isomers exhibited different crystallinity, as revealed by X-ray diffraction (XRD) measurements (Figure S3). In particular, (Z)-rich TPE-2OMe was observed to be amorphous, whereas (E)-rich TPE-2OMe was crystalline. Both isomers of TPE-2F were also crystalline; however, their pat-

Figure 3. Photoisomerization of TPE-2OMe and TPE-2F in chloroform solution under (a) deep-UV lamp irradiation, (b) ambient light irradiation, and (c) in the dark. (d) Photoisomerization of TPE-2OMe and TPE-2F in the solid state under deep-UV lamp irradiation.

The enriched isomers were then subjected to photophysical experiments. Both (E)- and (Z)-rich TPE-2OMe and TPE-2F exhibited typical AIE properties; therefore, they were not emissive in the solution state but highly emissive in the aggregated tate. These isomers exhibited similar absorption and emission spectra. TPE-2F showed hypsochromically shifted absorption and emission maxima compared with TPE-2OMe (Figure 2 and Figure S5). These similar absorbing features provided similar photoreaction efficiencies under the same reaction condition. After exposing the (E)- or (Z)-rich isomers to deep ultraviolet (UV) lamp irradiation (6.2 mW/cm2), they exhibited isomerizing behavior approaching photostationary state, as revealed by 1H NMR measurements. In addition, photostationary state was achieved after 1–4 h (Figure 3a, see also Figure S6). Even under ambient light conditions (0.32


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The Journal of Physical Chemistry mW/cm2), isomerization was observed, and the samples reached photostationary state after 48 h for TPE-2OMe (Figure 3b). The differences observed for the deep-UV and ambient light irradiation originated from not only the irradiation power but also the wavelength range of the light source. Thus, the observed slow isomerization of TPE-2F under ambient light can be attributed to the difference in absorbing wavelength. We would like to emphasize that the (E) and (Z) isomers exhibited almost identical UV-vis absorption behavior, as observed in Figure S6; therefore, both isomers should be equally excited. Under dark condition, the isomers did not exhibit the isomerization behavior even after a long time (Figure 3c). However, in the solid state, TPE-2OMe and TPE-2F did not exhibit any isomerization behavior even under deepUV lamp irradiation as well as in solution under dark condition (Figure 3d). Note that no evidence of photocyclization was observed in our experiment setup, as revealed by 1H NMR measurements. These results indicate that the isomerization of TPE-2OMe and TPE-2F can occur under typical fluorescent measurement conditions (0.64 mW/cm2) and that the isomerization is suppressed in the solid state even under UV irradiation, analogous to the solution state under dark condition. In other words, a negative correlation between the isomerization and occurrence of AIE was implied. EZI could be spectroscopically observed in disubstituted TPE derivatives, but of course EZI is not possible in tetrasubstituted TPE or bare TPE. Moreover, although EZI in disubstituted TPE derivatives should be caused by π twist in the photo-excited state, the transition from (E) to (E) or (Z) to (Z) isomers cannot be detected and thus ignored, despite through π twist. To obtain deeper insight into the AIE mechanism, theoretical computations were conducted for TPE-2OMe and TPE-2F in the ground state (S0) and singlet excited state (S1) in solution using the polarizable continuum model (PCM)51 by density functional theory (DFT) and time-dependent DFT (TDDFT) at the B3LYP52,53/6-31+G(d) level using Gaussian 16.54 S0min and S1min represent the most stable optimized structures in the S0 and S1 states, respectively. In chloroform solution, the calculated energies of the electron transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) (3.38 eV, f = 0.4757 for TPE-2OMe and 3.58 eV, f = 0.4456 for TPE-2F) correspond well to the observed peak in the ultraviolet-visible (UV-vis) absorption spectra. The dihedral angle about the C=C bond θ(C2C3C4C5) (see also Figure 1a) in S0min was calculated to be 167° for TPE-2OMe and 168° for TPE-2F. After excitation to S1 state, the dihedral angle about the C=C

bond in S1min significantly differed from that of S0min, i.e., θ(C2C3C4C5) was calculated to be 111° for TPE-2OMe and 105° for TPE-2F (Table 1 and Figure 4). Additionally, the central C=C bond considerably elongated from 1.368 Å (S0min) to 1.490 Å (S1min) for TPE-2OMe and from 1.366 Å (S0min) to 1.490 Å (S1min) for TPE-2F, enabling a large structural variation around the central C=C bond. However, the dihedral angle related to the central C=C bond and phenyl rings was calculated to be 49° in S0min and 20° in S1min for TPE-2OMe and 51° (S0min) and 20° (S1min) for TPE-2F. These results indicate that both dihedral angles, θ(C2C3C4C5) and θ(C1C2C3C4), changed in S1min compared with those in S0min; however, the variation was larger in θ(C2C3C4C5), implying the predominant contribution of π twist to the AIE mechanism.

Figure 4. S0min and S1min of TPE-2OMe, TPE-2F, TPE-2O, and TPE-4oM in solution (CHCl3) using the polarizable continuum model (PCM) by density functional theory (DFT) and timedependent DFT (TDDFT) at the B3LYP/6-31+G(d) level using Gaussian 16.



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Table 1. The calculated geometrical parameters for TPE compounds at S0min and S1min at the B3LYP/6-31+G(d) level. θ(C2C3C4C5) (º) S0min TPE-2OMe TPE-2F TPE-4OMe in crystal

a

a

θ(C1C2C3C4) (º)

S1min

S0min

S1min

C2=C3 (Å) S0min

S1min

C1=C2 (Å) S0min

S1min

167

111

49

20

1.368

1.490

1.493

1.443

168

105

51

20

1.366

1.490

1.496

1.447

175

167

63

45

1.366

1.440

1.496

1.445

TPE-2O

130

106

9

-1

1.415

1.486

1.470

1.435

TPE-4oM

167

136

59

43

1.370

1.468

1.511

1.467

Calculated by the ONIOM method where the high layer at the B3LYP/6-31+G(d) level while the low layer at the UFF level.

Figure 5. The energy variations of the S1 and S0 states along the steepest descent (SD) pathway in the S1 state of TPE-2OMe. The horizontal and vertical axes correspond to the π twist angle θ(C2C3C4C5) and relative energy, respectively. The SD pathways in the S1 state were calculated from the FC structures (= S0min) of the (E) and (Z) isomers.

To verify the relaxation mechanism of TPE derivatives in the excited state, we calculated the steepest descent (SD) pathways in the S1 states for the (E) and (Z) isomers of TPE2OMe, starting from the Franck-Condon (FC) structures, by TDDFT calculations at the B3LYP/def-SV(P) level55 under the resolution-of-identity approximation56 using 57,58 TURBOMOLE. Figure 5 shows energy profiles of the S1 and S0 states along the SD pathways as a function of the dihedral angle starting from the FC structures of the (E) and (Z) isomers until their midpoint. It should be noted that the two geometries obtained from the (E) and (Z) isomers do not coincide with each other at the midpoint (i.e., θ(C2C3C4C5) = 90°). Along the SD pathways, the phenyl rings initially rotate with a decrease of the dihedral angle θ(C1C2C3C4), and then, the molecule shows a rotational motion around the central C=C bond leading to the perpendicular structure with θ(C2C3C4C5) of 90° (see also Figure S7). As the S1 energy gradually decreases, the S0 energy increases accordingly, and the energy difference of S1 and S0 states is finally reduced to 0.5 eV at θ(C2C3C4C5) ~ 90°, suggesting the existence of a CI around there. It should be remarked that in general, TDDFT/DFT is not an appropriate approach to describe the electronic structures around the CI of

S0 and S1 states, since the electronic structure in the S0 states acquires a multi-configurational character leading to difficulty in the SCF convergence of DFT calculations, but anyway the SCF calculations in the present SD-pathways converged and the SD-pathways indicate that the S1-excited TPE-2OMe should return to the ground state via non-radiative way with CI (θ(C2C3C4C5) ~ 90°), followed by the rotational pathway about the central C=C bond. Very recently, there were reported two CIs, i.e., one having the twisted C=C bond and the other with cyclization among two phenyl rings, in theoretical studies for similar systems.48,49 To reach the former CI, there is a small barrier, while the latter CI can be reached without a barrier from the FC structure. It is noted that a simple SD-pathway calculation from the FC structure cannot find the CI structure from the minimum of FC state if a barrier is present, and additional calculations are required. Hence, we show HOMO(π) and LUMO(π*) at θ(C2C3C4C5) = 12º, 93º, and 168º in Figure 6. At the perpendicular structure with θ = 93º (close to the CI), HOMO(π) and LUMO(π*) have almost the identical shape, indicating the near-degeneracy of the S1 and S0 states. The HOMO and LUMO energies are also close to each other near this midpoint region compared with those at S0min. The employment of the more accurate quantum chemical multireference theory can rigorously describe the electronic structures of the geometries around CI.38,59 These results indicate that TPE derivatives show a non-radiative decay through a CI in solution; thus, from a theoretical viewpoint, π twist including EZI should occur, in accordance with our experiment described above.


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The Journal of Physical Chemistry in the S1 state (Figure 4). The dihedral angle θ(C2C3C4C5) is 167° at S0min and 136° at S1min. The bulkiness of the methyl groups effectively suppresses π twist upon photo-excitation, resulting in the smaller change of θ(C2C3C4C5) at between S0min and S1min. ∆θ(C1C2C3C4) in TPE-4oM is observed to be 18°. These results also suggest the importance of π twist in the excited state in solution for AIEgens and related compounds.

Figure 6. HOMO and LUMO of TPE-2OMe at θ(C2C3C4C5) = 12º, 93º, and 168º, located on the steepest descent pathway in the S1 state by using B3LYP/def-SV(P).

Hence, to discuss an emission from aggregated or crystal TPE derivatives, we performed theoretical calculations for TPE-4OMe in the crystal state (Figure 1b)26 because the single crystal was not obtained for the pure isomer of TPE2OMe. From the experimental crystal structure, 30 molecules were extracted, and the central molecule was treated as the high layer at the B3LYP/6-31+G(d) level, whereas the surrounding 29 molecules were treated as the low layer at the universal force field (UFF) level by the ONIOM method (Figure 7). By geometrical optimization, the dihedral angle θ(C2C3C4C5) at S0min was calculated to be 175°, which is similar to that in solution (167°). In contrast, θ(C2C3C4C5) at S1min was calculated to be 167°; thus, it is almost identical to that at S0min. In addition, the dihedral angle of θ(C1C2C3C4) shows a similar variation (63° at S0min → 45° at S1min) compared with that in solution. Therefore, the access to the CI is strictly inhibited by the other surrounding molecules in the ground state. Indeed, when the calculation was conducted with fixed θ(C2C3C4C5) from 140° to 200°, the total energy abruptly increases with only a 10° or 20° rotation (Figure S8a), indicative of the severely restricted π twist of the central molecule. The intramolecular rotation of phenyl groups in the excited state, directing a planar conformation of the phenyl rings and the central C=C bond, are also reduced to some extent (∆θ(C1C2C3C4) = 29° in solution and 18° in the crystal); however, the restriction of rotation around the central C=C bond mainly governs the occurrence of AIE (∆θ(C2C3C4C5) = 56° in solution, and 8° in the crystal). Moreover, the S0 state in the S1min structure is 2.63 eV lower than the S1 state (Figure S8b), which corresponds well to the observed value (442 nm), validating our calculations. Our hypothesis, in which π twist predominantly affects the efficient quenching of AIEgens in solution, is also proved by the contrapositive results of other AIE-inactive TPE derivatives. For example, oxygen-linked TPE (TPE-2O, Figure 1b) is known to be AIE-inactive, i.e., it is emissive both in solution and in the aggregated states.60 Based on our calculations, θ(C2C3C4C5) of TPE-2O is observed to be far from 180° (planar structure) even at S0min (130°, Figure 4), similar to that at S1min (106°). The phenyl rings are tightly linked in TPE-2O, resulting in a highly twisted structure in the S0 state; therefore, the dye cannot accompany a large structural change upon photo-excitation. Tetra ortho-substituted TPE (TPE-4oM, Figure 1b) has been also reported to be AIE-inactive because of the four bulky methyl groups.61 In our calculation, the optimized molecular structure in the S0 state is largely different from that

Figure 7. (a) S0min and (b) S1min of TPE-4OMe in the crystal state determined by the ONIOM method, with the high layer (ball and stick model) at the B3LYP/6-31+G(d) level and the low layer (stick model) at the UFF level.

Conclusion Based on the present observations and theoretical computations, we conclude that π twist (in other words, twist of C=C bond) relaxes the excited state and crucially unstabilizes the ground state, leading to the non-radiative decay of TPE derivatives in the solution state.62 The rotation of the phenyl ring is also activated by light irradiation to achieve the planar conformation of the phenyl rings and C=C double bond, analogous to the twisted intramolecular charge transfer phenomenon rather than thermal perturbation. In contrast, in the crystal, solid, and aggregated states, the isomerization is severely restricted by the surrounding molecules, thereby resulting in strong emission, while the photo-activated rotation of phenyl rings can take place even in such confined situation. Therefore, π twist in the excited state plays a crucial role in causing the occurrence of the AIE phenomenon, which is totally similar with the quenching and isomerization process of stilbene.6365 In other words, a molecule with a significant structural difference between its ground and excited states will generally exhibit the AIE property, where the large structural change such as π twist would be suppressed in the crystal or solid state, or even in non-aggregated media. This deduction is consistent with the AIE phenomenon triggered by a supramolecular association event in the solution state,17-20 network polymer,21-25 or liquid,26 thus AIE is not ‘induced’ by aggregation, raising a question about the validity of the word “AIE”. The exploration of the AIE mechanism for AIEgens without an isomerizable moiety is currently underway.

ASSOCIATED CONTENT Supporting Information. Experimental section, 1H NMR and NOE spectra, XRD patterns, DSC measurements, UV-vis and photoluminescence spectra, photoisomerization experiment, and details of theoretical computations. This material is available free of charge via the Internet at http://pubs.acs.org. (file type, PDF)



AUTHOR INFORMATION Corresponding Author *[email protected], [email protected]

ACKNOWLEDGMENT The authors acknowledge financial support received from JSPS KAKENHI JP15K17861 and from the Asahi Glass Foundation and the Ogasawara Foundation for the Promotion of Science & Engineering. The authors greatly appreciate the assistance received from Prof. Kato and Prof. Kobayashi for the powder XRD measurements and from Prof. Takeda and Dr. Kageyama for the DSC measurements. The high-resolution mass spectrometry measurements were performed at the OPEN FACILITY, Hokkaido University Sousei Hall. The computations were partly performed at the Research Center for Computational Science, Okazaki, Japan.

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Twist of C=C Bond Plays a Crucial Role for Quenching of AIE-Active Tetraphenylethene Derivatives in Solution Kenta Kokado,* Takashi Machida, Takeshi Iwasa, Tetsuya Taketsugu, Kazuki Sada,*

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