Modeling the Modulation of Emission Behavior in E ... - ACS Publications

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

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Modeling the Modulation of Emission Behavior in E/Z Isomers of Dipyrrolyldiphenylethene: From Molecules to Nanoaggregates Laura Le Bras,† Carlo Adamo,† and Aurélie Perrier*,†,‡ †

Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris (IRCP), F-75005 Paris, France Université Paris Diderot, Sorbonne Paris Cité, 5 rue Thomas Mann, F-75205 Paris Cedex 13, France

‡

S Supporting Information *

ABSTRACT: The dipyrrolyldiphenylethene (DPYDPE) molecule shows a switching between aggregation-inducedemission (AIE) and crystallization-induced emission (CIE) upon modification of the stereochemistry of the molecule. Herein, we propose a theoretical study based on molecular dynamics, time-dependent (TD-) density functional theory (DFT), and QM/QM′ calculations to investigate the structural and optical properties of the E and Z isomers in three different phases (solution, crystal, and aggregate). By computing the Huang−Rhys factors and the reorganization energies, we demonstrate that the fluorescence quenching observed in solution for both isomers is due to a nonradiative decay process involving low-frequency vibrational modes assigned to scissoring motions. In the crystal and in the aggregates, the effects of steric hindrance strongly modify the topology of the potential energy surface of the first excited state, and this results in a restriction of the vibrational modes involved in the energy dissipation. The modulation of the emission behavior (CIE/AIE) as a function of the stereochemistry can be rationalized by analysis of the inter- and intramolecular interactions.

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films. AIE and CIE have been widely studied over the past few years given the promising number of practical applications in highly efficient optoelectronic devices, fluorescent sensors, cell imaging, and so on.8 AIE and CIE luminogens are usually classes of conjugated molecules with propeller-shaped structures, and among all of the molecules that exhibit AIE and/or CIE, tetraphenylethene (TPE) has been particularly studied.11−13 This molecule consists of a central double bond surrounded by four phenyl rings. In dilute solutions, TPE molecules are weakly emissive, and this behavior is due to the almost free rotation of the phenyl moieties around the central olefinic group, resulting in a radiationless relaxation after excitation. Upon aggregation, the restriction of intramolecular rotation (RIR) along with huge steric constraints can be observed, and the emission phenomenon is consequently enhanced. In this study, we are interested in a recently synthesized TPE-like molecule, namely, dipyrrolyldiphenylethene (DPYDPE).14 Like TPE, this system is a propeller-shaped molecule, but with two phenyl rings replaced by pyrrole moieties. Two different isomers, E-DPYDPE and Z-DPYDPE (Figure 1), were synthesized, separated, and characterized. Importantly, no E/Z isomerization (EZI) process occurs in this

INTRODUCTION Over the past few years, a significant effort has been made to understand and optimize the luminescent properties of molecules and materials for use in innovative high-tech devices such as organic light-emitting diodes (OLEDs).1−3 To incorporate these luminophores into such optoelectronic devices, luminescent molecules should be assembled in thin films or aggregates.4 An exception is found in the area of biomedicine, where luminescent materials are diluted in a physiologic liquid. In this case, because of the nature of luminescent materials, which are mainly composed of conjugated units, molecules tend to aggregate, and solubility problems have to be circumvented. The necessity of using luminescent materials in the solid phase and the aggregation of luminescent materials in inappropriate solvents have highlighted the phenomenon of aggregation-caused quenching (ACQ).5 Indeed, for aromatic systems, intermolecular interactions arising from π−π stacking induce a nonradiative relaxation and thus a quenching of the emission.5−7 This phenomenon is commonly observed for conventional fluorophores, thus restricting their use in practical applications. Fortunately, in the past decade, opposing phenomena called aggregation-induced emission (AIE) and crystallization-induced emission (CIE) were observed in a series of nonemissive dyes.8−10 These molecules are weakly luminescent or nonluminescent in dilute solutions but emit efficiently when aggregated in concentrated solutions or assembled into solid © XXXX American Chemical Society

Received: September 19, 2017 Revised: October 23, 2017 Published: October 24, 2017 A

DOI: 10.1021/acs.jpcc.7b09310 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Structures and bond numbering for E- and Z-DPYDPE. Angles (A) and dihedral angles (D) are also defined. Hydrogen, carbon, and nitrogen atoms are represented in white, gray, and blue, respectively.

(DFT) calculations combined with the QM/QM′ approach to model the structural and optical properties of the molecules in the different environments. We also use molecular dynamics (MD) to simulate the aggregate structures. First, we investigate and compare the absorption and emission spectra of the EDPYDPE and Z-DPYDPE isomers in solution with an implicit solvent model. In a second step, these results are used as a reference to rationalize the modulation of the emission in the crystalline and aggregate environments for both isomers.

molecule under continuous irradiation with UV light. It has been shown that E-DPYDPE and Z-DPYDPE exhibit different emission behaviors depending on the phase in which they are found (solution, aggregate, and crystal). Although they are both weakly emissive in dilute tetrahydrofuran, E-DPYDPE is an AIE luminogen, whereas Z-DPYDPE exhibits CIE. The fluorescence quantum yields measured in the different environments are provided in Table 1. Interestingly, DPYDPE shows a switching

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Table 1. Experimental Emission Quantum Yields Φfluo (%) for E- and Z-DPYDPE in Solution,a Crystal, and Aggregate14 solution crystal aggregate a

E-DPYDPE

Z-DPYDPE

0.2 10 69

6 89 37

METHODOLOGY Solvated Molecules. Our calculations were performed using both the Gaussian 0926 and Gaussian 1627 packages. We employed the density functional theory (DFT) and timedependent DFT (TD-DFT) methods to model the properties of the ground state (GS, state S0) and excited state (ES, Sn with n = 1, 2, 3), respectively. In the course of both the geometry optimizations and the calculations of the optical properties, we used three different exchange-correlation (XC) functionals, the global hybrid functional PBE028 and the two range-separated hybrid (RSH) functionals CAM-B3LYP29 and ωB97X-D.30 The last of these functionals includes empirical dispersion corrections. We used these three functionals in combination with the extended 6-311+G(d,p) basis set. For all of the optimized structures, vibrational frequencies were computed to ensure that the geometries correspond to true minima of the potential energy surfaces. The localizations of the transition states (TSs) were achieved with the synchronous transit-guided quasi-Newton (STQN) method and, more specifically, with the QST331 approach. All of the geometry optimizations were carried out in tetrahydrofuran (THF), using the integral equation formalism polarizable continuum model (IEF-PCM) to quantify the impact of the environment.32 The GS and ES geometry optimizations were performed in the equilibrium limit using the linear-response (LR) PCM scheme. To predict the absorption and emission properties, we carried out singlepoint calculations for the optimized GS and ES structures and tested three different approaches to account for the solvent: the LR model; the corrected linear-response model (cLR, also

Tetrahydrofuran, THF.

between AIE and CIE behaviors upon modification of the stereochemistry of the molecule, and there is significant interest in understanding the outstanding properties of this luminogen. Within this framework, theoretical studies constitute a valuable tool for understanding the behavior and guiding the design of novel AIE/CIE organic functional materials. Computational (photo)chemistry is now capable of accurately representing large systems,15−21 and recent works have investigated the AIE effect with the help of quantum mechanics/molecular mechanics (QM/MM) calculations. It was shown that aggregation effects can hinder the nonradiative decay channel by blocking low-frequency movements.22−24 Other works, based on excited-state dynamics combined with QM calculations, attributed the nonradiative decay of isolated molecules to the existence of a conical intersection between the ground state and the first excited state.25 The present work aims at understanding (i) the modulation of the emission properties in the three different environments (solution, aggregate, and crystal) and (ii) the impact of the stereochemistry on the emission properties. Toward this end, we employ time-dependent (TD-) density functional theory B


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Figure 2. Representation of the QM/QM′ model based on the stacking structures of (a) E-DPYDPE and (b) Z-DPYDPE with the central molecule defined as the high-level region.

Figure 3. QM/QM′ model for DPYDPE aggregates sampled with MD simulations. (a) Conformation for E-DPYDPE aggregates extracted from MD simulations with a representation of the water box. (b) QM/QM′ model for each aggregate: The embedded molecule is defined as the QM region (in red), and the other molecules are defined as the QM′ region.

known as the PisaLR model);33 and the approach due to Improta, Barone, Scalmani, and Frisch (IBSF).34 cLR and IBSF are both state-specific (SS) models, but they differ in terms of their perturbative (PisaLR) and self-consistent (IBSF) natures.35 The results of a benchmark study of the geometries and absorption properties of both isomers are provided in the Supporting Information (SI). Based on these results, we selected the TD-cLR-ωB97X-D//ωB97X-D computational scheme to investigate the structural and optical properties. Indeed, with this computational strategy, for the maximum absorption wavelength λmax, the absolute deviations between the calculated excitation energies and the experimental data, ΔEexp−calc, are −0.21 and 0.02 eV for E- and Z-DPYDPE, respectively. The Huang−Rhys (HR) factors (i.e., dimensionless electron−vibration coupling constants) and the reorganization energies were calculated with Gaussian 16.27 For an emission process, the factor HRj represents the variation of the jth vibrational mode in the course of the Sn (n = 1, 2) → S0 deexcitation

HR j =

ωjDj 2 2ℏ

(1)

where ωj is the vibrational frequency and Dj2 is the displacement along normal mode j between the equilibrium positions of the two electronic states of interest.24,36 For each mode j, the reorganization energy, Ereorg,j, is defined as the product of the HR factor and the corresponding vibrational energy Ereorg, j = HR j ωj

(2)

Crystalline Systems. To determine the excited-state properties of crystalline systems, the QM/QM′ ONIOM method was used.37 In this approach, a sufficiently large cluster (27 molecules) is extracted from the crystal supercell14 and treated at the QM/QM′ level (see Figure 2). The central DPYDPE molecule constitutes the high-level (QM) region, whereas the low-level (QM′) region corresponds to the surroundings (26 other molecules). In the course of the geometry optimizations of the ground and excited states, the structure of the central molecule was C


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Figure 4. Molecular orbitals of E-DPYDPE (top) and Z-DPYDPE (bottom) calculated at the PCM(THF)-ωB97X-D level (isodensity = 0.025 au).

Table 2. Calculated Absorption Wavelengths (λ, nm), Oscillator Strengths ( f), and Electronic Excitation Assignmentsa for EDPYDPE and Z-DPYDPE in Different Environments E-DPYDPE λ

a

f

S1 S2 S3 expt14

337 264 263 357

0.569 0.001 0.371

S1 S2 S3

304 275 263

0.762 0.053 0.000

S1 S2 S3

354 272 264

0.363 0.031 0.003

Z-DPYDPE assignment

Solution HOMO → LUMO HOMO → LUMO + 1 HOMO → LUMO + 2 Crystal HOMO → LUMO HOMO → LUMO + 1 HOMO → LUMO + 4 Aggregate HOMO → LUMO HOMO − 1 → LUMO HOMO → LUMO + 1

λ

f

assignment

341 258 255 340

0.467 0.003 0.142

HOMO → LUMO HOMO → LUMO + 1 HOMO → LUMO + 2

333 257 252

0.430 0.003 0.110

HOMO → LUMO HOMO → LUMO + 1 HOMO → LUMO + 2

305 270 252

0.764 0.013 0.010

HOMO → LUMO HOMO − 1 → LUMO HOMO → LUMO + 1

For the assignments, only the major contributions are given (strongly dominant electronic excitations).

ca. 1.0 mol %. The DPYDPE molecules were more concentrated at the center of the simulation box to avoid a time-consuming aggregation process starting from a uniform distribution.42 The general Amber force field (GAFF)43 was employed to describe the atom types and interaction parameters of DPYDPE, whereas the TIP3P model44 was used for the water molecules. To check the validity of the selected force field for DPYDPE molecules, for each isomer, we compared the structure of the molecule optimized in a vacuum with GAFF with that obtained by DFT at the ωB97X-D/6-311+G(d,p) level. This comparison is provided in the Supporting Information. The particle mesh Ewald (PME) method45,46 was used to model the Coulomb interaction effects in the whole system. After an energy minimization using the steepest descent algorithm, a MD simulation was first performed in the NPT ensemble for 1 ns with a 1-fs time step. The reference pressure (1 bar) and temperature (300 K) were incorporated by the Berendsen method.47 At the end of the NPT simulation, we checked that the system was at equilibrium and found a small

relaxed, whereas the positions of the molecules in the low-level region were kept frozen. For both geometry optimizations and calculations of absorption/emission properties, the ωB97X-D/ 6-311+G(d,p) level was considered for the high-level QM region. Following previous investigations, the low-level region was treated at the Hartree−Fock (HF) level coupled with 3-21G basis set within the charge-embedding framework.38,39 For the optimized GS and ES structures, analytical frequencies were calculated with the Gaussian 16 code.27 One should note that it was not possible to calculate the HR factors within the ONIOM framework. Aggregated Phase. To model the aggregated phase, molecular dynamics (MD) simulations were performed with the AMBER16 package.40 The initial configurations were generated with the PACKMOL41 package: For this purpose, 21 DPYDPE (E or Z) molecules were placed in a cubic simulation box of 3.0 nm, as displayed in Figure 3. An additional layer of pure water with a width of 0.5 nm was added to avoid interference with the boundaries, leading to a final simulation box with a 4.0-nm edge length. The number of water molecules was 2119, corresponding to a molar concentration of D


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Figure 5. Structure superposition of optimized ground and excited states of E-DPYDPE (left) and Z-DPYDPE (right) in solution. S0, S1,opt, S2,opt, and S3,opt are represented in black, red, dark blue, and light blue, respectively.

with these virtual orbitals localized on the phenyl rings. The charge-transfer characters of these two excited states are thus more pronounced. We investigated the impact of the rotation of one pyrrole group (variation of the angle χ1 in Figure 1) on the energetics and on the absorption properties (see Figure S4). The GS rotation barriers are equal to 4.3 and 4.8 kcal mol−1 (ΔG = 3.4 and 3.8 kcal mol−1) for E-DPYDPE and Z-DPYDPE, respectively, and we can thus conclude that both systems are flexible. For Z-DPYDPE, the intramolecular bond between the nitrogen and hydrogen atoms induces a small rigidification of the structure compared to that of its E counterpart, as the value of the rotation barrier is increased by +0.5 kcal mol−1. This weak interaction is revealed through the analysis of the bond distances and angles collected in Table S4: The N···H bond lengths (2.891 and 2.862 Å for N2···H1 and N1···H2, respectively) are in the range of weak or moderate hydrogen bonds, and the values of the NH−N angles are close to 90°. Upon rotation of one pyrrole moiety against the ethylene stator, there are two stable conformers for both E-DPYDPE and Z-DPYDPE, as the relative Boltzmann populations of these two conformers are 72:28 for E-DPYDPE and 63:37 for ZDPYDPE. There is no impact of the pyrrole rotation on the value of λmax, with both stable conformers presenting the same absorption properties. For instance, for Z-DPYDPE, the λmax value is 340 nm for the conformer depicted in Figure 1 and 337 nm for the conformer obtained after the rotation of one pyrrole moiety (see Figure S4). Along the rotation path, there is a decrease of the λmax value due to the loss of conjugation. As an illustration, at the TS, the λmax values are 268 and 277 nm for EDPYDPE and Z-DPYDPE, respectively. Similarly, we investigated the rotation of one phenyl moiety against the vinyl stator (Figure S5). In that case, for E-DPYDPE and ZDPYDPE, the values of the rotation barriers were found to be 10.8 and 5.4 kcal mol−1, respectively (ΔG = 11.1 and 6.9 kcal mol−1, respectively). Because of steric hindrance, the rotation of one phenyl ring thus requires more energy than the rotation of one pyrrole ring.

expansion of the cubic box to edge lengths of 4.10 and 4.12 nm for the E and Z isomers, respectively (+2.7% and +2.9%, respectively). For each system, the subsequent MD simulations were carried out in the NVT ensemble for 10 ns. The results of the MD simulations were then analyzed with CPPTRAJ software48 within the AMBER16 package. Ten snapshots were randomly collected for each system for the analysis of the aggregate geometries and spectral properties. The number of snapshots was limited by the computationally demanding optimization of the GS and ES geometries. In detail, we followed the strategy of employing the ONIOM model that was already used for the crystalline phase: The GS and ES structures of a selected embedded molecule (Figure 3) were optimized at the ωB97X-D/6-311+G(d,p) level, whereas the positions of the molecules in the low-level region (treated with HF/3-21G) were kept frozen. The absorption and emission properties were then calculated at the same level. We focused our attention on the snapshot with the largest Boltzmann factor and treated the properties calculated for the other snapshots as comparative data.

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RESULTS AND DISCUSSION Properties of the Isolated Isomers in Solution. Electronic Structures and Absorption Properties. For both E-DPYDPE and Z-DPYDPE, the maximum absorption wavelength (λmax) corresponds to a transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). As shown in Figure 4, for both isomers, the HOMO is mainly centered on the pyrrole rings, whereas the LUMO is mainly delocalized on the phenyl rings with a small contribution on the pyrrole rings. The S0 → S1 transition thus corresponds to a charge transfer from the pyrrole rings to the phenyl rings (see also the representation of the density variation in the SI). Two other transitions were calculated at higher energies and present weaker oscillator strengths (Table 2). The nearly degenerate S2 and S3 states arise from HOMO → LUMO + 1 and HOMO → LUMO + 2 electronic excitations, respectively, E


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Figure 6. Calculated HR factors for E-DPYDPE (top) and Z-DPYDPE (bottom) versus the normal-mode wave numbers in the S1 state (left) and S2 or S3 state (right). Representations of the vibration modes yielding the largest HR factors are also shown.

Emission Properties. We optimized the low-lying excited states for both E-DPYDPE and Z-DPYDPE. The corresponding optimized structures are denoted as Sn,opt with n = 1, 2, 3. Note, however, that, for E-DPYDPE, the S3,opt structure could not be found. For E-DPYDPE, the strongest structural modification between the optimized ground state S0 and the first excited states (S1,opt and S2,opt) arises from a torsion of the phenyl groups with respect to the ethylene stator (dihedral angles ξ1 and ξ2 in Figure 1). In fact, the value of ξ1 is −121°, −156°, and −88° for the S0, S1,opt, and S2,opt structures, respectively (see Table S4). Moreover, for the S1 state, this rotation motion induces a concomitant “opening” of the molecular structure while moving from S0 to S1,opt. Indeed, the values of the dihedral angles ϕ and ψ, which account for the torsion against the central double bond and thus the flatness of the structure, are 12° and −170°, respectively, for the ground state and 61° and −121°, respectively, for S1,opt. To illustrate this structural modification, a superposition of the two structures is shown in Figure 5. For S2,opt, except for the torsion of the phenyl groups, there is less structural modification with respect to S0. For Z-DPYDPE, the structural modifications involve the same moieties. For Sn,opt with n = 1, 2, 3, the phenyl rings are still the most impacted groups. The torsion motion of these rings induces an opening of the structure: The values of ϕ and ψ for the optimized ES structures present strong variations from 0° or 180°, which are the values for a planar system. This trend is observed for S1,opt in particular and to a lesser extent for S2,opt and S3,opt. For both isomers, the strong structural modifications between S0 and S1 lead to large Stokes shifts with calculated emission wavelengths of 755 and 779 nm for E- and ZDPYDPE, respectively. There is a large discrepancy between these values and the experimental emission data (461 nm for EDPYDPE, 470 nm for Z-DPYDPE), and we can conclude that S1 is not the emissive state. This conclusion is supported by the calculation of the HR factors. In fact, nonradiative processes are in competition with emission phenomena. The nonradiative

decay rate is dominantly influenced by the internal conversion rate, which depends on the HR factors. As shown in Figure 6, for both isomers, for the S1 state, the HR factors at low frequencies are larger than those at high frequencies. For EDPYDPE, the larger HR factors (HR = 91 and 71) correspond to wavenumbers of 44 and 16 cm−1. (The complete list of HR factors is given in the SI.) These two modes correspond to scissoring motions either in the plane of the vinyl stator with phase opposition (44 cm−1) or out of the plane with all of the rings in phase (16 cm−1). The same conclusion holds for ZDPYDPE, where the largest HR factor (151) corresponds to the in-plane scissoring mode (43 cm−1). As previously shown,24 the large HR factors at low frequencies strongly participate in the nonradiative decay process, and the corresponding vibrational modes increase the energy dissipation. For each vibrational mode j, the reorganization energy Ereorg,j is given in the SI, and the total reorganization energy, Ereorg, is given in Figure 6. Recent works have proposed another interpretation of the origin of the nonemissive behavior of AIE molecules in solution.24,25,49 Indeed, in some cases, the lack of fluorescence in solution is due to the presence of an energetically accessible conical intersection leading to an ultrafast internal conversion from the excited state to the ground state. We thus plotted, along the S1 geometrical relaxation path (Figure 7), the energies of the ground and first excited states versus the rotation angle of the two phenyl rings ξ. We conclude that there is no S1/S0 crossing along this relaxation path, which strengthens our conclusion that the fluorescence quenching in solution is due to the photophysical energy dissipation caused by scissoring motions. The weak emission signal (Φfluo = 0.2% for E-DPYDPE and 6% for Z-DPYDPE) might arise from an emission from higherenergy excited states. Indeed, in Table 2, the comparison of the emission wavelengths calculated for the S 2,opt and S3,opt geometries shows better agreement with experiment, but a hypsochromic shift can still be observed. For instance, for EDPYDPE, the calculated λem value is 358 nm from the S2 state, F


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in a small value of the fluorescence quantum yield. One should also expect a wavelength effect: The fluorescence quenching phenomenon might be modulated using different irradiation wavelengths. One can draw exactly the same conclusion for ZDPYDPE. We include in Figure 6 the HR factors for the S3 state because the oscillator strength f computed for this state is much larger than that calculated for the S2 state (see Table 3). Therefore, S3 is potentially the emissive state, which is supported by the fact that the values of the HR factors are much smaller than the ones calculated for the S1 state. Table 3. Calculated Emission Wavelengths (λem in nm) for E-DPYDPE and Z-DPYDPE in Different Environments (Solution, Crystalline Phase, and Aggregate), along with Experimental Emission Wavelengths (λem,exp, nma) and Fluorescence Quantum Yields (Φfluo)14 E-DPYDPE state

λem

f

λem,exp

S1 S2 S3

755 358

0.240 0.041

461

S1

396

0.488

483

S1

504

0.313

475

a

Figure 7. Relative energies (eV) along the S1 geometrical relaxation path of the two states S0 and S1 for (a) E- and (b) Z-DPYDPE. The potential energy surfaces are plotted versus the rotation angle of one phenyl ring, ξ1. For both isomers, the reference energy corresponds to the energy of the optimized GS structure.

Z-DPYDPE Φfluo (%)

λem

Solution 0.2 779 303 311 Crystal 10 427 Aggregate 69 409

f

λem,exp

Φfluo (%)

0.224 0.012 0.250

470

6

0.375

489

89

0.525

450

37

λexcitation = 350 nm.

Why is the value of the fluorescence quantum yield Φfluo larger for Z-DPYDPE than for E-DPYDPE in solution? Indeed, Figure 6 shows that the reorganization energy for E-DPYDPE is larger than that for Z-DPYDPE. Hence, the photophysical energy dissipation caused by vibrational motions might induce a stronger fluorescence quenching for E-DPYDPE. We want to stress that we do not compare the same states for the two isomers because it was not possible to characterize the S3,opt structure for E-DPYDPE. A modification of the electronic structure for the E isomer might lead to a dissociative S3 potential energy surface (with no minimum) and thus impede the radiative relaxation process from this more emissive state. (The comparison of the oscillator strength values in Table 3 also shows that S3 is more emissive than S2.) An investigation of the topology of the excited-state potential energy surfaces with multiconfigurational approaches, which goes beyond the scope of the present study, would be necessary to clarify this point. Solid-State Systems. Structural and Absorption Properties. Key structural parameters for E-DPYDPE and Z-DPYDPE in the crystalline phase are collected in Tables S9 and S10, respectively. First, a comparison with X-ray crystallography data14 is provided in Figure S6 to ensure that the cluster that we used (Figure 2) was appropriate for mimicking the crystalline structure. This comparison shows that, for a ZDPYDPE monomer in the crystal, there are no major differences between the X-ray structure and our calculated structure. For E-DPYDPE, after QM/QM′ optimization, there is a small deviation in the positions of the pyrrole rings. Indeed, the angles α1 and α2 move from 119° for the X-ray structure to 127° for the optimized structure, leading to a slight opening of the structure. However, these structural modifications remain minor, and this comparison validates our model.

whereas the experimental value reaches 461 nm. This shift is partially related to the single-excitation nature of TD-DFT. To test this hypothesis with an affordable computational strategy, we performed configuration interaction singles (CIS) and configuration interaction singles and doubles CIS(D) calculations for the emission energy of each isomer in a vacuum.50 We found that the difference in calculated emission energies, ΔECIS − ΔECIS(D), is 0.48 eV for E-DPYDPE (S2 state). For ZDPYDPE, this difference is about 0.51 eV for the S2 state and 0.45 eV for S3. Therefore, CIS overestimates the emission energies compared to CIS(D), and we thus conclude that the neglect of double excitations in CIS yields an overestimated value of the emission energy. Of course, this result, based on a comparison of the CIS and CIS(D) methods, provides an indication of the role of double excitations but does not allow for a definitive determination of this effect. Coupled-cluster with single and double excitations (CCSD) calculations, which could not be performed on our system, would provide a definitive conclusion. We next investigated possible photophysical energy dissipation from these higher-energy excited states. For EDPYDPE, as shown in Figure 6, the HR factors calculated for the S2 state are much smaller than those calculated for S1. This finding supports the conclusion that S2 is more emissive than S1. However, experimentally, the irradiation wavelength was set to 350 nm, and this excitation was not sufficiently energetic to populate S2 (λ = 263 nm in Table 2). After irradiation, a complex relaxation pathway is thus necessary to induce an emission phenomenon from the S2 state, and this should result G


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Figure 8. Molecular packings of E-DPYDPE (left) and Z-DPYDPE (right). Weak interactions (CH−π and NH−π) and the corresponding distances (in angstroms) are given.

Figure 9. Structure superposition of the ground and excited states of E-DPYDPE (left) and Z-DPYDPE (right) optimized in solution and in the crystalline environment.

The molecular packings are depicted in Figure 8. For both isomers, the molecules are not tightly packed, and there is no π−π stacking.14 For E-DPYDPE, weak CH−π interactions are observed, with calculated (experimental) distances of 3.051 (3.16) and 3.145 (3.31) Å. In the case of Z-DPYDPE, there are intermolecular NH−π interactions (calc, 3.558 and 3.608 Å; expt, 3.3 Å), and CH−π interactions are also present (calc, 2.729 and 3.162 Å; expt 2.9 and 3.5 Å).

We next systematically compared the structures optimized in the crystalline phase with those obtained in solution. These comparisons are provided in Figure 9. For the ground state, in the case of Z-DPYDPE, the comparison of these two structures shows minimal deformations. This finding can be explained by the presence of the weak intramolecular N−H···N hydrogen bonds that prevent strong structural deformations. In contrast, for E-DPYDPE, there is a structural rearrangement, and the H


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Table 4. Calculated Maximum Absorption Wavelengths (λmax, nm), Oscillator Strengths ( f), and Major Contributions to the Electronic Excitations for E-DPYDPE and Z-DPYDPE in Different Environments E-DPYDPE λmax

f

monomer

337

0.569

monomer, ONIOM monomer, no environment dimer, ONIOM

304 305 305

0.762 0.745 0.679

monomer, ONIOM monomer, no environment dimer, ONIOM

359 355 358

0.356 0.363 0.298

Z-DPYDPE assignment

Solution HOMO → LUMO Crystal HOMO → LUMO HOMO → LUMO HOMO − 1 → LUMO Aggregate HOMO → LUMO HOMO → LUMO HOMO − 1 → LUMO + 1

λmax

f

341

0.467

HOMO → LUMO

333 330 327

0.430 0.420 0.766

HOMO → LUMO HOMO → LUMO HOMO − 1 → LUMO

307 306 302

0.740 0.770 0.887

HOMO → LUMO HOMO → LUMO HOMO − 1 → LUMO

assignment

Figure 10. Low-frequency vibrational modes (0−500 cm−1 region) calculated for the S1 state of E-DPYDPE in solution (top), crystal (middle), and aggregate (bottom). The vibrational modes corresponding to large HR values in solution are given, and the modes presenting similarities in the crystal or in the aggregate are depicted.

phase mainly arises from the structural deformations and that, with this loose packing, the electronic intermolecular couplings are not sufficient to alter the observed photophysical properties. Emission Properties and the CIE Effect. The first excited state (S1,opt) was optimized for both isomers (Tables S9 and S10), and the resulting structures were compared to those obtained in solution. As shown in Figure 9, there is a strong deformation of the S1,opt structure on going from the solution to the crystalline phase. Indeed, this structure is very similar to the S2,opt (and S3,opt for Z-DPYDPE) geometries obtained in solution. This modification of the S1,opt structure is due to the restriction of the rotation and the movement of the different rings in the crystal. At the same time, the electronic structure of S1 in the crystal environment is directly comparable to the solution because this state still arises from a HOMO → LUMO electronic excitations at the S1,opt geometry (The frontier orbitals are given in Figure S9). The calculated emission energies for the S1 state are reported in Table 3 and exhibit a blue shift compared to experiments (deviations of 0.5 and 0.3 eV for E-DPYDPE and Z-DPYDPE, respectively). This could be due to the single-excitation character of TD-DFT calculations. In contrast to the case for

torsion angles around the phenyl rings are the most impacted parameters, with variations of ca. +25°. Calculations of the bond-length-alternation (BLA) parameters (see Tables S8 and S10) show that the structure of EDPYDPE is less conjugated in the crystal phase (BLA value of 0.041) than in solution (0.039). Consequently, as reported in Table 2, the λmax value (304 nm) is blue-shifted with respect to the solution value (337 nm). The same conclusion holds to a smaller extent for Z-DPYDPE, with calculated λmax values of 341 and 333 nm in solution and the crystal phase, respectively. To understand whether this hypsochromic shift arises from an electronic or steric effect, we performed TD-DFT calculations on the crystalline optimized structure without any surroundings. The values obtained for λmax are very close to those obtained with an environment, namely, 305 nm for E-DPYDPE and 330 nm for Z-DPYDPE (Table 4). Finally, we calculated the absorption properties of a central dimer within the ONIOM framework and obtained λmax values of 305 and 327 nm for EDPYDPE and Z-DPYDPE, respectively. Therefore, the effect of excimer formation on the calculated absorption spectrum is negligible. These results show that the modification of the absorption properties in going from solution to the crystal I


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Figure 11. Low-frequency vibrational modes (0−500 cm−1 region) calculated for the S1 state of Z-DPYDPE. See Figure 10 for more details.

show some modes presenting some similarities, but the scissoring motion is limited to the phenyl rings. We thus expect that no vibrational mode is involved efficiently in the nonradiative decay process, and this should explain the strong increase of Φfluo in the crystal (89%). Aggregated Systems. In the course of the molecular dynamics simulations, among the 10 randomly selected snapshots, we chose the system with the largest Boltzmann factor for analysis of the calculated absorption and emission wavelengths (Tables 2 and 3), geometries of the GS and ES (Figure 13 and Tables S8 and S9), and low-frequency vibrational modes (Figures 10 and 11). The absorption and emission properties calculated for the other snapshots are given as comparative data in Tables S13 and S14. We focused our attention on one embedded molecule as displayed in Figure 3b. Structural and Absorption Properties. The large aggregate depicted in Figure 3 was found to be stable and not to split into smaller clusters. The radial distribution functions (RDFs) between water molecules and E-DPYDPE and Z-DPYDPE molecules are shown in Figure 12a. The RDFs actually correspond to average values (considering all of the solute molecules within the aggregate) and present a typical evolution behavior.24,50 The analysis of these RDF curves reveals two peaks at about 5 and 7 Å for both isomers. A detailed analysis revealed that the peak at 5 nm corresponds to a hydrogen bond between the water molecules and the nitrogen atoms of the pyrrole groups, whereas the peak at 7 nm corresponds to an interaction between the water molecules and the phenyl groups. The first peak shows a larger intensity for E-DPYDPE than for Z-DPYDPE because the N−H groups of the pyrrole rings of the former are not involved in intramolecular interactions and can more easily form H bonds with water. Figure 12b provides the RDFs as a function of the center-of-mass distance between two E-DPYDPE or Z-DPYDPE molecules. These distributions show large fluctuations, but three peaks can be identifiied in the 0−10-nm range for the two isomers at ca. 5.0, 6.5, and 9.5 Å. The peak intensities are not the same for the two isomers, and for Z-DPYDPE, the shortest-distance interactions (5 nm) are prominent.

the isolated molecule in solution, we believe that the emission band can be attributed to radiative emission from the S1 state because (i) the comparison between experiments and calculations is more favorable than that in solution and (ii) the experimental−calculated deviation is in the line with those of previous TD-DFT calculations carried out on emission phenomena in molecular crystals.38 We also conclude that, for the S1 state, the role played by the molecular environment in the emission process is much more important than that played in the absorption process. Indeed, for absorption (Table 2), in going from the solution to the crystal phase, there is a small difference in the excitation energies and corresponding transition dipoles. In contrast, for emission (Table 3), there is a large blue shift of the emission band corresponding to the radiative relaxation from the S1 state, which is due to a strong structural deformation of the ES minimum. At the same time, the oscillator strength values are multiplied by factors of 2.0 and 1.7 for E-DPYDPE and Z-DPYDPE, respectively (for instance, the value of f for the E isomer is 0.240 in solution and 0.488 in crystal). Therefore, the crystalline environment should enhance the molecular emissive efficiency by increasing the transition dipole moment of the corresponding state. Finally, after calculating the vibrational spectrum of the S1,opt structure, we examined the molecular vibrational modes showing pronounced rotation motions at low frequencies and compared them with those obtained in solution. For EDPYDPE in solution, the mode with the largest HR factor (44 cm−1 mode) is due to a scissoring motion and can be directly compared to the 87 cm−1 value in the crystal (see Figure 10). However, the other mode presenting a large HR factor (16 cm −1 mode) could not be identified in the crystal. Consequently, for E-DPYDPE, the fluorescence quantum yield increases from 0.2% in solution to 10% in the crystal because, of the two scissoring vibrational modes greatly contributing to the nonradiative decay process, one still exists in the crystal and could participate in energy dissipation. For Z-DPYDPE, the comparison with solution reveals that the mode contributing to the energy dissipation in solution (43 cm−1 mode) is no longer present in the crystal. In Figure 11, we J


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there is no variation of the λmax value if the absorption properties are computed for a dimer cluster embedded in the aggregate. Hence, the modification of the absorption spectra is only due to strong structural deformations within the aggregate. Emission Properties and the AIE Effect. The optimized S1,opt structure is compared to those obtained in solution in Figure 13. As for the GS, there is a strong deformation of the S1,opt structure on going from the solution to the aggregate phase. In contrast to the crystalline phase, this structure is not similar to the S2,opt (and S3,opt for Z-DPYDPE) geometries obtained in solution (see Figure S7). If the GS and S1,opt geometries in the aggregate are compared, the structural modifications obtained in the course of the relaxion process imply mainly the torsions of the phenyl rings for E-DPYDPE and the pyrrole groups for Z-DPYDPE. The calculated emission wavelengths, λem, are reported in Table 2 and compare nicely with the experiment results, with deviations from the experimental data of −0.12 and 0.28 eV for E-DPYDPE and Z-DPYDPE, respectively. To understand the AIE effect, we first compared the oscillator strength values of the emission process arising from the S1 state in solution and in the aggregate. This value is multiplied by 1.3 and 2.3 for E-DPYDPE and Z-DPYDPE, respectively ,which shows that the environment should enhance the molecular emissive efficiency. Then, we calculated the vibrational spectrum of S1,opt and examined the molecular vibrational modes that were involved in the nonradiative decay process in solution. In Figures 10 and 11, we show some modes presenting some similarities with the modes presenting large HR factors in solution, and we conclude that these modes do not correspond to scissoring motions. Therefore, for both EDPYDPE and Z-DPYDPE, no vibrational mode potentially contributes to the energy dissipation, and this should explain the strong increase of Φfluo in the aggregates. Why is the value of the fluorescence quantum yield Φfluo larger for E-DPYDPE than Z-DPYDPE in aggregates? This can be explained by the restriction of the motion of the different rings in the aggregates due to three types of interactions: (1) intermolecular interactions with the solvent, (2) intermolecular interactions with other DPYDPE molecules, and (3) intramolecular interactions between the pyrrole rings. For ZDPYDPE, there is a subtle competition between these three effects, and the analysis of MD simulations shows that the interactions of types 2 and 3 are predominant. In contrast, for E-DPYDPE, there is no intramolecular hydrogen bonding, and Figure 12 shows that the intermolecular interactions with the solvent prevail. The large AIE effect for E-DPYDPE might thus arise from stronger interactions with the solvent.

Figure 12. (a) Radial distribution functions between water molecules and DPYDPE molecules. (b) Radial distribution functions between two DPYDPE molecules.

We next compared the structures of the optimized GS structures within the aggregate with those obtained in solution. For both isomers, the comparison of these two structures shows strong distortions, with the largest deviations occurring in the dihedral angles χ and ξ that represent the torsions of the pyrrole and phenyl rings, respectively. Moreover, because of the anisotropic constraints imposed by the other DPYDPE molecules and the solvent, the values of the torsion angles χ1 and χ2 for the two pyrrole rings are no longer identical. The same conclusion holds for the ξ1 and ξ2 torsion angles. The evolution of these dihedral angles over the course of the MD simulation (Figures S10 and S11) does not show large fluctuations. For Z-DPYDPE, the asymmetry related to the values of the χ1 and χ2 torsion angles can be explained by the formation of a more specific N···H bonding interaction (see Figure 13) characterized by a bond distance of 2.665 Å (2.862 Å in solution), whereas the two N atoms are more separated (3.306 Å in the aggregate, 2.988 Å in solution). This intramolecular N···H bond interaction is stable over the course of the MD simulations (Figure S10). Because of these large structural modifications compared to the structures in solution, one can observe a variation of the maximum absorption wavelength λmax in Table 2. Indeed, for EDPYDPE, the λmax value is red-shifted (−0.62 eV), whereas for Z-DPYDPE, there is a blue shift (+0.40 eV). One can draw the same conclusions for the λmax values of the 10 different snapshots (Tables S13 and S14). Table 4 also shows that (i) there is no impact of the environment on the λmax value and (ii)

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CONCLUSIONS In conclusion, we have employed MD, QM/QM′, and TDDFT to study the absorption and emission properties of the E and Z isomers of dipyrrolyldiphenylethene in three different phases: solution, crystal, and aggregate. We first carefully investigated the properties of the two isomers in solution, and we showed that the S1 state is not emissive. Indeed, the fluorescence quenching observed in solution for E-DPYDPE and Z-DPYDPE is due to the photophysical energy dissipation caused by low-frequency vibrational modes corresponding to scissoring motions. There is also a large structural reorganization along the relaxation path on the S1 potential energy surface but no S0/S1 potential energy surface crossing. The observed weak emission signal might arise from a radiative relaxation from higher-energy states. K


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Figure 13. Structure superpositions. Top: Ground and excited states of E-DPYDPE (left) and Z-DPYDPE (right) optimized in solution and in the aggregate. Bottom: Ground- and excited-state structures optimized in the aggregate.

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In the crystal and in the aggregates, because of the steric hindrance effect, the emission band is now attributed to a radiative relaxation from the S1 state. The compact environment enhances the molecular emissive efficiency by strongly modifying the topology of the S1 potential energy surface, which results in (i) an increase in the transition dipole moment and (ii) the blocking of the vibrational modes mainly involved in the energy dissipation during the internal conversion process. In the crystalline environment, for E-DPYDPE, a vibrational mode with a large HR factor is still present, and this could explain the smaller increase of the fluorescence quantum yield compared to Z-DPYDPE. In the aggregates, for both isomers, all of the nonradiative decay channels are blocked, so the quantum efficiency rises compared to that of the isolated molecules in solution. There is, however, a difference between the two isomers that is due to a subtle competition among intermolecular interactions with the solvent, intermolecular interactions with other DPYDPE molecules, and intramolecular interactions between the pyrrole rings. This study thus reveals that a careful modeling of the different environmental effects is crucial for understanding and reproducing experimental observations in the context of AIE and CIE. Theoretical works can actively participate in the design of novel AIE/CIE functional materials also presenting modulations of their emission properties depending on the molecular stereochemistry.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09310. Validation of the GAFF parameters. Solvated systems: setup of the computational scheme, density variation, impact of the rotation on the absorption properties, structural parameters for the ground and first excited states, frequencies, and HR factors. Molecules in crystal and in aggregates: structural parameters for the ground and first excited states, molecular orbitals of the dimer, frontier molecular orbitals for the S1,opt structure, frequencies, and HR factors. Analysis of the MD simulations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Aurélie Perrier: 0000-0002-0516-9986 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was granted access to the HPC resources of CINES and IDRIS under allocation 2017-A0010810135 made by GENCI (Grand Equipement National de Calcul Intensif). Dr S. L


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Di Tommaso, K. Chaitou, and R. Berthin are acknowledged for helpful scientific discussions.

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