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Mechanisms of the Aggregation-Induced Emission and Photo/Thermal E/Z Isomerization of a Cyanostilbene Derivative: Theoretical Insights Norifumi Yamamoto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02147 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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The Journal of Physical Chemistry

Mechanisms of the Aggregation-Induced Emission and Photo/Thermal E/Z Isomerization of a Cyanostilbene Derivative: Theoretical Insights Norifumi Yamamoto* Department of Applied Chemistry, Faculty of Engineering, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan

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ABSTRACT

In this study, the aggregation-induced emission (AIE) of a cyanostilbene derivative, called CN-MBE, was investigated using molecular simulations. The E-form of CN-MBE has been known to exhibit AIE, which is non-emissive in dilute solutions but becomes highly emissive in aggregated states, while its Zform is non-emissive even in its crystalline form. In addition, upon UV light irradiation, the Z-form exhibits a Z-to-E isomerization, while the E-form keeps its conformation at room temperature and undergoes a nonradiative E-to-Z isomerization only at a high temperature. The results from the electronic structure calculations employed in this work showed that the potential energies of CN-MBE for the electronic ground (S0) and first excited (S1) states were degenerate at a twisted conformation around the ethylenic C=C π-bond, which led to the fluorescence quenching of the molecule. Molecular dynamics simulations and free-energy analyses revealed that the E-form molecules assembled closely, with the C=C bond rotation markedly restricted. This, in turn, prevented the fluorescence quenching via the S0/S1 conical intersection. In contrast, the Z-form molecules aggregated relatively sparsely, allowing for the nonradiative Z-to-E isomerization to proceed. All in all, the theoretical insights presented herein give a clear picture on the AIE and photo/thermal isomerization mechanisms of CNMBE.


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INTRODUCTION Fluorescent molecules with aggregation-induced emission (AIE), which are non-emissive in dilute solutions but become highly emissive in solid or aggregate phases, have come to attract a lot of attention as novel luminogenic materials for various optical devices,1,2 as well as chemical and biological sensing.3,4 The cyanostilbene derivative 2,3-bis(4′-methylbiphenyl-4-ly)acrylonitrile (CNMBE) is a prototypical AIE luminogen (AIE-gen),5,6 which has a small fluorescence quantum yield (Φ < 0.01) in dilute solutions that increases remarkably (Φ = 0.69) after nanoparticle formation.5 CN-MBE has two different geometric isomers called E- and Z-forms (Figure 1). While (E)-CN-MBE has been known to exhibit AIE, (Z)-CN-MBE is non-emissive even in crystalline phase.7 In addition, the Z-form is nonfluorescent but exhibits a Z-to-E isomerization upon UV light irradiation, while the E-form keeps its conformation upon UV light illumination at room temperature and undergoes a nonradiative E-to-Z isomerization only after UV light irradiation at a high temperature of about 500 K.7 Therefore, CNMBE has bright prospects as a novel functional material that exhibits AIE and photo/thermal E/Z isomerization beyond conventional molecular crystals and/or soft matters. The AIE of CN-MBE has been attributed to the combined effects of aggregation-induced planarization and J-aggregate formation in the solid phase.5,6,8,9 However, no definite view on the nature of AIE in CN-MBE has been given hitherto.


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Figure 1. Chemical structures of (E)- and (Z)-CN-MBE.

Apart from CN-MBE, tetraphenylethylene (TPE) is another archetypal AIE-gen.10 Because of its simple structure and easy synthesis, the AIE mechanism of TPE has been more widely investigated, both experimentally and theoretically, than that of CN-MBE.11-16 The main cause of AIE in TPE and its derivatives has been attributed to the restriction of intramolecular rotation (RIR) of their phenyl rings that are connected to the central ethylenic moiety via C−C σ-bonds.11,12 The RIR hypothesis suggests that in dilute solutions TPE undergoes fluorescence quenching through rotations of the phenyl rings by friction with solvent molecules. Upon aggregate formation, these rotations become restricted from the surrounding molecules, and a strong light emission is induced. Recently, however, it has been recognized that the ethylenic C=C π-bond rotation in TPE plays a crucial role in fluorescence quenching.14-16 Several computational studies based on electronic structure calculations revealed that the ground- and excited-states of TPE have the same energy with a twisted geometry about the ethylenic C=C bond site, and intersecting each other to form conical intersections (CIs).14,15 Even though this demonstrates that the fluorescence quenching in TPE can be caused by a rapid internal


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conversion via the CIs in a similar manner as in stilbene,17 the AIE mechanism is still a controversy.12,13 Considering that CN-MBE is also a substituted stilbene like TPE, the key to understanding the AIE mechanism of CN-MBE would be to examine whether and how the fluorescence quenching occurs, and if it involves an ethylenic C=C bond rotation. However, no study has elucidated this problem so far. Moreover, even though the steric hindrance from the surrounding molecules is essential to the AIE process, computational studies on the AIE-gens have been limited to single-point potential energy (PE) calculations of the molecules that ignore the thermodynamic influence from the environment. In this study, the mechanisms of AIE and photo/thermal E/Z isomerization of CN-MBE were investigated based on the free-energy (FE) profiles of the molecule in condensed phases as functions of the ethylenic C=C bond rotation.

COMPUTATIONAL DETAILS Electronic Structure Calculations. In order to elucidate the characteristics of the PE profiles for the photochemical processes of CN-MBE, the minimum energy paths (MEPs) for the E/Z isomerization reaction of CN-MBE in an isolated phase were investigated using electronic structure calculations. Constrained geometry optimizations were performed by fixing the torsional angle of the ethylenic C=C bond in CN-MBE, φ, (Figure 1) over a range of 0° to 180° for the electronic ground (S0) and first excited (S1) states. The spin-flip approach18 within the time-dependent density functional theory (SFTDDFT) method19 was used to compute the PEs and analytical gradients for the S0 and S1 states of the molecule. Since the SF-TDDFT method is known to suffer from spin-contamination, the lowest two electronic states with the values of 〈S S 〉 smaller than 0.5 were classified as the S0 and S1 states. The BHHLYP hybrid functional (50% Hartree-Fock plus 50% Becke exchange,20 the Lee-Yang-Parr correlation21) and 6-31G(d) basis set were employed in the SF-TDDFT calculations. In previous studies, the SF-TDDFT method was utilized to examine the photochemical reactions of ethylene22 and ACS Paragon Plus Environment

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stilbene.23 The results from these investigations were comparable to those obtained using the multiconfigurational ab initio molecular orbital methods, e.g., at the complete active space selfconsistent field (CASSCF)24 and second-order multireference perturbation theory (CASPT2)25 levels. Therefore, taking into account that CN-MBE is considered to be a substituted stilbene, the SF-TDDFT method would be a promising approach in this study. The ground- and excited-state PE surfaces of ethylene and stilbene are known to be nearly degenerate with twisted geometries about the central C=C bond site, and intersecting with each other to form CIs.17 Therefore, in this work, we investigated the minimum energy point of the conical intersection (MECI) between the S0 and S1 states of CN-MBE at the SF-TDDFT level using the penalty-constrained optimization method.26,27 All electronic structure calculations of CN-MBE presented herein were performed using the GAMESS program.28 Molecular Dynamics Simulations. The FE profile of the E/Z isomerization of CN-MBE in condensed phase was computed using molecular dynamics (MD) simulations based on an empirical force field representation. All MD simulations of CN-MBE presented herein were carried out using the GROMACS 2016.3 program with a GPU acceleration.29 Following the energy minimization and equilibration processes, production runs were performed under the isothermal-isobaric (NpT) ensemble. The temperature was maintained either at room temperature (300 K) or higher (500 K) using a velocityrescaling thermostat.30 The pressure was kept constant at 1 bar using a Parrinello−Rahman barostat.31 The time step used in all MD simulations was 2 fs and the list of nonbonded interaction pairs was updated every 10 steps with a cutoff radius of 1 nm. The electrostatic interactions were computed using the particle mesh Ewald algorithm32 with a cutoff radius of 1 nm, in which the reciprocal part was performed by fast Fourier transformations. All bond lengths were constrained using the linear constraint solver (LINCS) algorithm.33 Fitting Force Field Parameters. As demonstrated by previous studies on TPE-based AIE-gens,14-16 the rotation around the ethylenic C=C bond can play an important role in the photochemical processes of CN-MBE. Therefore, the empirical force fields used in the MD simulations were specifically ACS Paragon Plus Environment

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designed to comply with the SF-TDDFT calculations on the ethylenic C=C bond rotation by varying the parameters of the energy functions. In this study, an analytic linear least-square fitting method34 was employed for the optimization of the force field parameters. These parameters for the rotation around the ethylenic C=C bond of CN-MBE, φ, (Figure 1) were derived separately for the S0 and S1 states. Moreover, since the four C−C σ-bonds that connect the four phenyl rings in CN-MBE are rotatable, a set of corresponding dihedral terms were reparametrized based on the electronic structure calculations for model systems at the MP2/6-31G(d)//HF/6-31G(d) level. Computational details on the optimization of the force field parameters are included in the Supporting Information. In addition, other force field parameters, apart from the aforementioned dihedral terms, i.e., bond, angle, and nonbonding parameters, were determined based on the OPLS-UA force field.35 Free-Energy Calculations. The structural transition between the E- and Z-forms of CN-MBE was examined and the corresponding FE changes were computed with a well-tempered (WT) variant36 of the metadynamics (MetaD) algorithm37,38 by biasing the MD simulations using the PLUMED 2.4.0 plugin.39 While conventional MD simulations tend to do oversampling of the same conformations, MetaD avoids the oversampling problem by constructing a history-dependent bias potential, VG(s, t), as a sum of Gaussians centered along the MD trajectory followed by collective variables (CVs), s, up to time t.37,38 The dihedral angle of the ethylenic C=C bond rotation (φ) was used as a CV in the process. A bias Gaussian with a width of 10° was deposited every 250 timesteps of the simulations and the height of the bias was set to 1 kJ mol−1 with a bias factor of 40. The WT-MetaD simulations were parallelized using the multiple walker method,40 in which four 100-ns simulations, starting from different initial coordinates, were run in parallel. The FE profiles along the CVs, F(s), were obtained from the accumulated bias potential, VG, according to the equation F(s) = −VG(s, t → ∞) + C, where C is an irrelevant additive constant. Statistical errors in the FE profile determination were estimated through a block-averaging analysis.38


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Predicting Aggregated Structures. The aggregated structures of CN-MBE were predicted using the CONFLEX program.41 Although the crystal structures of other cyanostilbene derivatives similar to CNMBE have been determined,42,43 those of the (E)- and (Z)-CN-MBE molecules have not yet been clarified. Therefore, in this study, we explored possible crystal structures of these two molecules by randomly generating trial structures and subsequently optimizing the lattice parameters. In total, 3,000 trial molecular orientations were generated by rotating the rigid molecule in the unit cells. The spacegroup symmetries were constrained in 1 and P21/c during the lattice optimizations because the previously reported crystal structures of CN-MBE-like molecules were contained in one of these space groups.42,43 The crystal energies of all trial crystal structures were estimated based on the Merck Molecular Force Filed 94 (MMFF94)44 and the most stable ones were utilized in the MD simulations.

RESULTS AND DISCUSSION PE Profile of CN-MBE. Figure 2A shows the PE profiles along the S0- and S1-MEPs of CN-MBE in isolated phase. The change in PE of the S0 state along the S0-MEP (denoted as S0//S0-MEP) demonstrated that the isomerization between the E- (φ = 180°) and Z-forms (φ = 0°) of the molecule proceeded very slowly in the S0 state due to the large energy barrier (34 kcal mol−1) at φ = 90°. In contrast, the S1 PE change along the S1-MEP (denoted as S1//S1-MEP) indicated that the torsional motion about the ethylenic C=C bond reduced the energy from the vertically excited Franck-Condon (FC) points of the E- (φ = 180°) or Z-form (φ = 0°) to the minimum energy point (φ = 90°) with no energy barrier. Figure 2B shows a geometry corresponding to the S1-MEP minimum at φ = 90°. As the ethylenic C=C bond rotated along the S1-MEP, the PE gap between the S0 and S1 states decreased, with values within 1.23 eV in the region of φ = 60°−130° and a minimum energy difference of 0.84 eV at

φ = 120°. This result demonstrated that the S1-MEP passed the region where the S0 and S1 PE surfaces approach each other but do not intersect. Moreover, the S0/S1 MECI, in which the PEs of the S0 and S1


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states were almost identical (within 1 kcal mol−1), was discovered. Figure 2C shows a geometry corresponding to the S0/S1 MECI of CN-MBE, which was characterized not only by the twisting (φ = 75°) but also the pyramidalization of the ethylenic C=C bond, thereby indicating that the S0/S1 MECI did not lie on the S1-MEP only with the ethylenic C=C bond rotation. This phenomenon is similar to the one occurring in the ethylene and stilbene molecules.17 All in all, these results indicated that the rotation around the ethylenic C=C bond of CN-MBE was one of the principal coordinates that led to the S0/S1 CIs, where the molecule dissipates the excitation energy nonradiatively via a rapid S0/S1 internal conversion to cause fluorescence quenching.

Figure 2. (A) Potential energy profiles of the S0 and S1 states along the S0- and S1-MEPs with a change in the torsional angle (φ) of the ethylenic C=C bond of CN-MBE in an isolated phase. Geometries corresponding to the S1-MEP minimum at φ = 90° (B) and the S0/S1 MECI (C).

Notably, the 〈S S 〉 values of the lowest two states of geometries on the S0-MEP were almost constant, with average 〈S S 〉 values of 0.17 and 0.38, and were thus successfully classified as the S0 and S1 states, respectively. In contrast, along the S1-MEP, the 〈S S 〉 values for the lowest state, which was supposed to be the S0 state, increased with the twisting about the ethylenic C=C bond. As a result, the value of 〈S S 〉 was 1.35 at the twisted geometry (φ = 90°) and around 0.2 at the E- (φ = 180°) and Z-form (φ = 0°) ACS Paragon Plus Environment

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structures. This indicates that the lowest singlet state was strongly mixed with the triplet state at the twisted geometry of the S1-MEP, which was due to the fact that, at the 90°-twisted geometry, the ethylenic C=C π-bond is broken to produce two degenerate nonbonding p-orbitals containing two electrons, called diradicaloid.45

FE Profile of CN-MBE in THF Solution. Figure 3A shows the FE changes accompanying the rotation around the ethylenic C=C bond of CN-MBE in THF solution, with the overall errors in the FE profiles estimated to be smaller than 1 kJ mol−1. These were obtained via the WT-MetaD simulations of a system in which one CN-MBE molecule was solvated in 434 THF molecules enclosed in a cubic box of 62.8 nm3 under periodic boundary conditions (Figure 3B). According to the obtained results, no energy barrier existed in the FE profile of the S1 state from the vertically excited FC geometries of the E- and Z-forms (φ = 180° and 0°) to the twisted geometry (φ = 90°) (Figure 3A). Importantly, neither the S0/S1 CI nor the MECI of CN-MBE was determined from the FE profiles in THF solution and aggregated state. Instead, the MECI was obtained in an isolated phase, which was characterized by the twisting and pyramidalization of the ethylenic C=C bond. Presumably, the intermolecular interaction with the surrounding solvent or aggregated molecules had an important contribution to the entropy term of free-energy through steric hindrance but did not significantly affect the electronic structures. In other words, both in THF solution and as aggregates, the essential characteristics of the PE surfaces observed in isolated phase might be maintained. Taking this into account, we assumed that the twisting about the ethylenic C=C bond could be one of the principal coordinates that leads to the S0/S1 CIs both in THF solution and aggregated state. Hence, the FE profile shown in Figure 3A indicates that the corresponding structural change, i.e., the torsional motion of the ethylenic C=C bond site, occurred spontaneously even in THF solution, which, in turn, induced the nonradiative de-excitation transition from the S1 to S0 states in the vicinity of the 90°-twisted geometry. Therefore, it was concluded that


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since the S0/S1 CIs could be reached efficiently and facilitate fluorescence quenching after CN-MBE photoexcitation, the molecule would show no emission when dispersed in dilute solutions.

Figure 3. (A) Free-energy profile of the changes in torsional angle (φ) of the ethylenic C=C bond site of CN-MBE in THF solution. The point at which the torsion angle becomes φ = 90° on the S1-FE profile is indicated by a filled circle, while local minima on the S0-FE profile and FC points on the S1FE profile at the E- (φ = 180°) and Z-form (φ = 0°) geometries are indicated by crosses. (B) CN-MBE in THF obtained from MD simulations.

Structures of the CN-MBE Aggregates. Figure 4 shows the crystal structures of the E- and Z-forms of CN-MBE predicted in this study. The most stable structures of the E- and Z-form aggregates were found to be monoclinic systems with a P21/c space group, containing four molecules in the unit cell. Structural details of the CN-MBE aggregates are included in the Supporting Information. MD simulations of the aggregated states of CN-MBE were performed for the systems consisting of 196 Eand 192 Z-form molecules in monoclinic 1×7×7 and 6×2×4 supercells with sizes of 108.3 and 109.1 nm3, respectively, under periodic boundary conditions. Figures 5A and 6A present snapshot pictures of the (E)- and (Z)-CN-MBE aggregates, respectively, which were obtained from these simulations. The fractional free volumes (FFVs)46 were analyzed from the results of the MD simulations performed at ACS Paragon Plus Environment

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300 K. The FFVs of the E- and Z-forms of CN-MBE were found to be 22.1% and 24.2%, respectively, which indicated that the E-forms were more densely packed than the Z-forms in the aggregated phase.

Figure 4. Crystal structures of the E- (A) and Z-forms (B) of CN-MBE predicted in this study.

FE Profile of the (E)-CN-MBE Aggregates at 300 K. Figure 5B shows the FE profile of (E)-CNMBE in its aggregate phase obtained from the WT-metaD simulations performed at 300 K. As can be observed, the resulting FE profile is significantly different from that of CN-MBE in THF (Figure 3A). For the E-form aggregate, the conformational change leading to the twisted geometry (φ = 90°) from the vertically excited FC geometry (φ = 180°) on the S1 energy profile was revealed to be energetically unfavorable and experienced an increase in the FE of 57 kJ mol−1. Therefore, it was demonstrated that when (E)-CN-MBE was photoexcited in the aggregate phase, the rotation around the ethylenic C=C bond required to reach the S0/S1 CIs was energetically demanding because of the steric hindrance from the firmly stacked surrounding molecules. As a result, it was concluded that the sizeable steric hindrance that prevented the nonradiative transition via the S0/S1 CIs at the twisted conformations was responsible for the strong emission of (E)-CN-MBE in the aggregated phase. ACS Paragon Plus Environment

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Figure 5. (A) The (E)-CN-MBE aggregated structure obtained from MD simulations. Free-energy changes accompanying the rotation around the ethylenic C=C bond site of (E)-CN-MBE in the aggregated structure at 300 K (B) and 500 K (C).

FE Profile of the (Z)-CN-MBE Aggregates at 300 K. Figure 6B shows the FE profile of the Z-form of CN-MBE in its aggregate phase obtained from the WT-metaD simulations at 300 K. In contrast to the (E)-CN-MBE profile (Figure 5B), the FE profile of the Z-form aggregates resembled that of CNMBE in THF (Figure 3A). Notably, no energy barrier was detected in the FE profile of the S1 state from the vertically excited FC geometry of the Z-form (φ = 0°) to the twisted geometry (φ = 90°). This indicated that after the photoexcitation, (Z)-CN-MBE could relax from the FC point to the S0/S1 CIs on the S1 energy surface in a barrierless manner, which was similar to that of CN-MBE in THF but different from that of the (E)-CN-MBE aggregate. Therefore, it was concluded that the Z-form of CNMBE did not exhibit AIE due to the weakly packed structure that allowed for the rotation around the ethylenic C=C bond to reach the S0/S1 CIs. In this process, the molecules that reached the S0/S1 CIs separated in two paths - one that returned to the Z-form and one that changed to the E-from. Thus, it was evident that the Z-to-E isomerization reaction of CN-MBE occurred readily even at room temperature after photoexcitation.


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Figure 6. (A) The (Z)-CN-MBE aggregated structure obtained from MD simulations. Free-energy changes accompanying the rotation around the ethylenic C=C bond site of (Z)-CN-MBE in the aggregated structure at 300 K (B) and 500 K (C).

FE Profiles of the CN-MBE Aggregates at 500 K. Finally, MD simulations of the (E)- and (Z)-CNMBE aggregates were performed at a higher temperature (500 K). As a natural consequence, the aggregated structures became loose due to the dynamic motion of the molecules induced by the heat. The resulting FFVs at 500 K were determined to be 33.7% and 29.4% for the E- and Z-form aggregates, respectively. As can be observed from Figure 6B and C, the FE profile of the Z-form aggregate obtained from the WT-metaD simulations at 500 K was almost identical to that at 300 K. In contrast, the FE profile of the E-form aggregate at 500 K appeared very different from that at 300 K (Figure 5C and B, respectively). These results indicated that the photoexcited molecule could reach the S0/S1 CIs and cause fluorescence quenching due to the marked reduction in the energy barrier of the ethylenic C=C bond torsion. Therefore, it was concluded that at the higher temperature (500 K) the rotation around the ethylenic C=C bond was allowed even in the E-form aggregate, which led to a nonradiative E-to-Z isomerization.


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CONCLUSIONS In this study, we investigated the mechanisms of AIE and photo/thermal E/Z isomerization of CNMBE using molecular simulations. The PE profiles showed that the S0 and S1 states were degenerate at the twisted conformation around the ethylenic C=C bond, which led to the fluorescence quenching of the molecule via the S0/S1 CIs. The FE profiles showed that the rotation around the ethylenic C=C bond of the (E)-CN-MBE aggregate was markedly restricted at room temperature, thus preventing the quenching through the S0/S1 CIs at its twisted conformation and leading to strong fluorescence. In contrast, the ethylenic C=C bond in the Z-form aggregate could easily rotate to reach the S0/S1 CIs, which led to the barrierless nonradiative transition and subsequent Z-to-E isomerization reaction. However, the rotation of the ethylenic C=C bond in the E-form aggregate was allowed when exposed to high temperatures, thereby creating a nonradiative E-to-Z isomerization. All in all, the method employed in this study combines electronic structure calculations and MD simulations to compute the PE and FE profiles of the photochemical processes in CN-MBE, which can not only be used for analysis of various other AIE-gens but also for rational design of new photochemical materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Computational details about the optimization of the force field parameters, structural details of (E)and (Z)-CN-MBE aggregates (PDF)

AUTHOR INFORMATION Corresponding Author ACS Paragon Plus Environment

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*N.Y.: E-mail: [email protected] ORCID Norifumi Yamamoto: 0000-0001-9900-8288 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grantin-Aid for Scientific Research (C) 17K07315), and the JGC-S Scholarship Foundation. The molecular simulations were carried out on the TSUBAME 3.0 supercomputer in Tokyo Institute of Technology, and the Research Center for Computational Science in the National Institutes of Natural Sciences (NINS).


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