Photochromic Mechanism of a Bridged Diarylethene: Combined

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Photochromic Mechanism of a Bridged Diarylethene: Combined Electronic Structure Calculations and Nonadiabatic Dynamics Simulations Yating Wang, Yuan-Jun Gao, Qian Wang, and Ganglong Cui J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b11682 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 5, 2017

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The Journal of Physical Chemistry

Photochromic Mechanism of a Bridged Diarylethene: Combined Electronic Structure Calculations and Nonadiabatic Dynamics Simulations Ya-Ting Wang, Yuan-Jun Gao, Qian Wang, and Ganglong Cui∗ Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China E-mail: [email protected]

Abstract Intramolecularly bridged diarylethenes exhibit improved photocyclization quantum yields because the anti-syn isomerization that originally suppresses photocyclization in classical diarylethenes is blocked. Experimentally, three possible channels have been proposed to interpret experimental observation, but many details of photochromic mechanism remain ambiguous. In this work we have employed a series of electronic structure methods (OM2/MRCI, DFT, TDDFT, RI-CC2, DFT/MRCI, and CASPT2) to comprehensively study excited state properties, photocyclization, and photoreversion dynamics of 1,2-dicyano[2,2]metacyclophan-1-ene. On the basis of optimized stationary points and minimum-energy conical intersections, we have refined experimentally proposed photochromic mechanism. Only an S1 /S0 minimum∗ To

whom correspondence should be addressed

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energy conical intersection is located; thus, we can exclude the third channel experimentally proposed. In addition, we find that both photocyclization and photoreversion processes use the same S1 /S0 conical intersection to decay the S1 system to the S0 state, so we can unify the remaining two channels into one. These new insights are verified by our OM2/MRCI nonadiabatic dynamics simulations. The S1 excited-state lifetimes of photocyclization and photoreversion are estimated to be 349 and 453 fs, respectively, which are close to experimentally measured values: 240±60 fs and 250 fs in acetonitrile solution. The present study not only interprets experimental observations and refines previously proposed mechanism, but also provides new physical insights that are valuable for future experiments.

Introduction Photochromism is a light-induced reversible chemical transformation between two stable forms of a compound with different absorption wavelengths. 1 This kind of reactions are of great significance due to their actual implications for the development of photomemories, photoswitches, and data storage materials. 2,3 Among available photochromic families, diarylethenes are a class of the most efficient photoswitch compounds because of their high thermodynamic stability, high fatigue resistance, and good photoconversion yield. 4,5 For this reason numerous advances on diarylethenes in various environments have been reported, including chemical synthesis of various derivatives, multi-photon absorption, luminescence, Förster resonance energy transfer, magnetic properties, electronic transport properties, laser flash photolysis, photocyclization dynamics, etc. 6–41 However, photocyclization yields of traditional diarylethenes are not satisfactory because of the co-existence of open-form anti and syn configurations (only the latter is active for photocyclization, see Eq. 1). In addition, both isomerization and cyclization reactions can take place from the syn configuration; thus, photocyclization is to certain extent suppressed by the competing photoisomerization. 1,4 2 ACS Paragon Plus Environment

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Figure 1: Photochemical reactions in classical diarylethenes: isomerization between synand anti-open configurations, and photocyclization and photoreversion (ring-opening) between syn-open and syn-closed configurations.

Figure 2: Bridged photochromic diarylethene (meta-cyclophan-1-ene) synthesized by Takeshita et al. 42–44 In this series of compounds, two reacting aryl groups are maintained in an antiparallel conformation by the presence of an additional alkyl bridge of variable length n.



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In order to improve their photocyclization yields, Takeshita et al. successfully synthesized a series of meta-cyclophan-1-ene photochromic compounds (Fig. 2) in which the two reacting aryl groups are maintained in an antiparallel conformation by an additional alkyl bridge. 42–44 In their early work, only a marginal improvement of photocyclization yield was reported. 45 To understand the reason for this moderate improvement and to figure out parameters controlling photoswitch efficiency, Aloïse et al. re-investigated photochemical reactivities of some bridged diarylethenes in pure acetonitrile and mixed hexane-ethanol (12:1) solutions using stationary and femtosecond transient absorption spectroscopy. 27 Many spectroscopic properties were obtained, as well as photochromic quantum yields at different wavelengths and solutions. Interestingly, the photocyclization quantum yield of a bridged diarylethene is close to unity when the wavelength is longer than 400 nm, but decreases to 0.69 at 313 nm in acetonitrile solution. Furthermore, in terms of time-resolved absorption spectra, time-dependent density functional theory (TD-DFT), and semiempirical configuration interaction with single excitation (AM1/CIS) calculations, they proposed three possible photocyclization pathways to interpret their experimental observation. In the first channel, Eq. 1, the S1 state is directly populated in the Franck-Condon region, followed by an ultrafast relaxation (less than 100 fs) to the open-form S1 minimum [S∗1 (OF)]; then the system decays to the S0 state via a conical intersection [CI1 (S1 /S0 )] within less than 120 fs, eventually arriving at the closed-form [S0 (CF)]. In the second channel, Eq. 2, the higher excited state [SFC n (OF)] is first populated and then this state quickly decays to the open-form hot S1 minimum [S∗1 (OF)]. After overcoming a transition state [S1 (TS)], the system arrives at a closed-form S1 minimum [S1 (CF)], followed by internal conversion to the S0 state [S0 (CF)]. Finally, they proposed the third channel (Eq. 3) to be as an alternative to the second channel because there is no clear isosbestic point involved in the closed-form visible band observed in their transient spectra. In such channel, another S1 /S0 conical intersection [CI2 (S1 /S0 )] directly funnels the open-form hot S1 minimum [S∗1 (OF)] to the closed-form S0 minimum [S0 (CF)]. Nevertheless, one can easily find that


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many mechanistic details at the atomistic level are elusive in these three pathways, for example, what do these two different S1 /S0 conical intersections look like? etc.

∗ SFC 1 (OF) → S1 (OF) → CI1 → S0 (CF) → S0 (CF)

(1)

∗ SFC n (OF) → S1 (OF) → S1 (TS) → S1 (CF) → S0 (CF)

(2)

∗ SFC n (OF) → S1 (OF) → CI2 → S0 (CF)

(3)

Computer simulations are complementary to experiments. They can provide invaluable insights and give more mechanistic details that cannot be gained by experiments alone. Theoretical computations on photocyclization and photoreversion reactions of bridged diarylethenes are rarely reported. To our best knowledge, there are merely a few AM1/CIS and TD-DFT electronic structure calculations by Aloïse et al., 27 who aimed at interpreting experimentally observed time-resolved transient absorption spectra. Even though the AM1/CIS computations already gave insights into the photochromic mechanism of bridged diarylethenes, many atomistic details remain ambiguous. Hence further electronic structure calculations and nonadiabatic dynamics simulations are very meaningful. In this work, we employed a series of electronic structure methods i.e. TDDFT, RI-CC2, DFT/MRCI, and CASPT2 to re-investigate the S1 photocyclization and photoreversion processes of one bridged diarylethene studied experimentally. 27 Subsequently, we carried out trajectory-based nonadiabatic surface-hopping dynamics simulations to verify our proposed mechanism.



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Figure 3: Photocyclization and photoreversion (ring-opening) reactions of the bridged diarylethene proposed by Aloïse et al. 27 The corresponding π orbitals are also shown.

Simulation Details Ab Initio and Density Functional Methods Stationary points of the S0 state were optimized using the B3LYP method. 46–48 Vertical excitation energies were computed using the TD-DFT method with different exchangecorrelation functionals (B3LYP, 46–48 CAM-B3LYP, 49 ω B97XD, 50 M06-2X 51 ), the coupled cluster theory (RI-CC2) method, 52–54 the complete active space second-order perturbation theory (CASPT2) method, 55 and the DFT-based multireference configuration interaction (DFT/MRCI) method. 56 In the DFT/MRCI calculations, the BHLYP functional 57 was used in order to match with the original parametrization. To achieve more accurate potential energy profiles, the CASPT2 method was employed to refine single-point energies of all optimized structures. In the CASPT2//CASSCF calculations, 10 π electrons were placed into the active space that is comprised of 10 π and π ∗ orbitals; three roots with equal weights were used as well as the cholesky decomposition technique 58 and the imaginary shift technique (0.1 a.u. in our calculations). 59 The following basis sets and packages were used: cc-pVDZ for CASPT2//CASSCF (MOLCAS7.6), 60,61 SVP for RI-CC2 (TURBOMOLE6.3), 62,63 TZVP for DFT and TDDFT (GAUSSIAN09), 64,65 and TZVP for DFT/MRCI (TURBOMOLE5.7.1). 56,64,66


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Semiempirical Methods All semiempirical calculations (geometric optimizations and nonadiabatic dynamics) were performed using the OM2/MRCI method implemented in the MNDO99 program. 67–72 The restricted open-shell Hartree-Fock formalism was applied in the self-consistent field (SCF) treatment (i.e., the orbitals were optimized for the leading electronic configuration of the S1 state with two singly occupied π orbitals). The active space in the MRCI calculations included six electrons in six orbitals: in terms of the SCF configuration, it comprised the two highest doubly occupied π orbitals, the two singly occupied π orbitals, and the two lowest unoccupied π ∗ orbitals. For the MRCI treatment, three configuration state functions were chosen as references, namely the SCF configuration and the other two closed-shell configurations derived therefrom (i.e., all singlet configurations that can be generated from HOMO and LUMO of the closed-shell ground state). The MRCI wavefunction was built by allowing all single and double excitations from these three references. During OM2/MRCI geometry optimizations and dynamics simulations, all required gradients and nonadiabatic coupling elements were computed analytically. 67–69

Nonadiabatic Dynamics Simulations Nonadiabatic dynamics simulations with Tully’s fewest-switches surface hopping (FSSH) scheme 73,74 were carried out at the semiempirical OM2/MRCI level. 67–69,75,76 The initial atomic coordinates and velocities were randomly selected from 5 ps trajectories of ground-state Born-Oppenheimer molecular dynamics. The number of excited-state dynamics runs for each chosen snapshot was chosen according to the computed S0 → S1 transition probabilities. A total of 272 (142 for photocyclization and 130 for photoreversion) surface-hopping trajectories were successfully finished and thus evaluated in the present nonadiabatic OM2/MRCI simulations, with all relevant energies, gradients, and nonadiabatic coupling elements computed “on the fly” when needed. The fewest-switches



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criterion of Tully et al. was applied to decide when to hop and how to adjust the related velocities. 73,74 Time step was chosen to be 0.2 fs for nuclear motion and 0.002 fs for electronic propagation. The unitary propagator evaluated at middle-point was used to propagate electronic motion. 75,76 Translation and rotation motions were removed in each step. The empirical decoherence correction (0.1 in our simulations) proposed by Granucci et al. was employed. 77 Further technical details were given in previous publications. 78–92

Results and Discussion Open- and Closed-Form S0 Minima The bridged diarylethene has two stable conformers i.e. open and closed forms as shown in Fig. 3. Its two reacting phenyl groups are linked by the −C2 H4 − group. This chemical modification removes the anti configuration that originally exists in nonbridged diarylethenes (e.g. stilbene in Fig. 1). As a result, It blocks the anti-syn isomerization, in such way enhancing the quantum yield of photocyclization. Fig. 4 shows two stable conformers optimized by the OM2/MRCI method, which are referred to as S0-OF and S0-CF. In S0-OF, the C1 and C2 atoms are not bonded (2.457 Å); whereas, this bond is formed in S0-CF (1.530 Å). At the CASPT2/cc-pVDZ and OM2/MRCI levels, the potential energy of S0-CF is predicted to be 8.0 and 14.5 kcal/mol higher than that of S0-OF, respectively.

Vertical Excitation Energies The computed S0 → S1 vertical excitation energies of the open- and closed-form bridged diarylethene in vacuo are compiled in Table 1. At S0-OF, TD-B3LYP (2.96 eV), DFT/MRCI (3.17 eV), and AM1/CIS (3.20 eV) to different extent underestimate the S0 → S1 vertical excitation energy compared with CASPT2 (3.59 eV), CC2 (3.49 eV), TD-M062X (3.48 eV), TD-CAM-B3LYP (3.43 eV), and OM2/MRCI (3.87 eV). This implies that the S1 state 8 ACS Paragon Plus Environment

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4

5

3

6

2 457

3 1.5

. 12

S0-OF

1.66

0

S0-CF

28

7

1.7

S1-CF

S1S0-CF

Figure 4: S0 and S1 minima and S1 /S0 conical intersection optimized at the OM2/MRCI level (OF: open form; CF: closed form). Also shown are the C1-C2 bond lengths. See supporting information for Cartesian coordinates.



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has partial charge-transfer character because the conventional and non-range-separated functionals fail to give good prediction for charge-transfer states. At S0-CF, TD-B3LYP (1.97 eV) and DFT/MRCI (2.17 eV) still underestimate in comparison with CASPT2 (2.51 eV); but, AM1/CIS (2.33 eV) is close to OM2/MRCI (2.28 eV), TD-CAM-B3LYP (2.26 eV) and CC2 (2.28 eV). Table 1: Vertical S0 → S1 Excitation Energies (eV) of the Open- (first row) and ClosedForm (second row) Bridged Diarylethene in Vacuo Calculated by Various Electronic Structure Methods† OM2/MRCI CAM-B3LYP M062X CC2a DFT/MRCIa AM1/CISb 3.87 3.43 3.48 3.49 3.17 3.20 2.28 2.26 2.24 2.28 2.17 2.33 † TDDFT computations use the B3LYP/TZVP optimized structures; a TZVP basis set with CC2/SVP optimized structures; b from the work of Aloïse et al. (AM1-CIS//AM1); c CASPT2(10,10)/cc-pVDZ with the OM2/MRCI optimized structures.

CASPT2c 3.59 2.51

Table 2: Single-Point Energies Calculated by OM2/MRCI (first row) and SA3CASPT2(10,10)/cc-pVDZ//OM2/MRCI (second row) Methods S0-OF S1-OF-FC 0.0 89.2 0.0 82.8

S0-CF 14.5 8.0

S1-CF-FC 64.7 65.0

S1-CF 49.3 53.3

S1S0-CF(S0/S1) 59.0/59.0 55.2/61.1

Table 3: CASPT2/cc-pVDZ Calculated Dipole Moments (Debye) at the Franck-Condon Points of Open- (column 2-3) and Closed-Form (column 4-5) (i.e. S0-OF and S0-CF) structure S0-OF S0-CF electronic state S0 S1 S0 S1 in vacuoa 8.1 13.1 9.5 8.7 a OM2/MRCI optimized structure; To understand electronic structures of the S1 state at the open- and closed-form Franck-Condon points i.e. S0-OF and S0-CF, we analyzed their gas-phase S1 electronic wavefunctions at the CASPT2 level, which can be expressed:

ψS1 (OF) = −0.903719ΦHOMO→LUMO + 0.134895ΦHOMO→LUMO+2 + ... 10 ACS Paragon Plus Environment

(4)

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ψS1 (CF) = −0.887181ΦHOMO→LUMO + 0.073979ΦHOMO→LUMO+1 + ...

(5)

where Φ represents an electronic configuration, e.g. ΦHOMO→LUMO corresponds to a single-excitation configuration due to an electron excited from HOMO to LUMO. It can be seen that the S1 electronic state is mainly comprised of the HOMO→LUMO electronic configuration at both structures. In acetonitrile solution, the ΦHOMO→LUMO electronic configuration still contributes overwhelmingly to the S1 state. In addition, we have plotted HOMO and LUMO (Fig. 5). At S0-OF, the HOMO→ LUMO electron transition causes partial charge transfer. In HOMO, the center of electronic cloud is located around the C1 and C2 atoms, whereas in LUMO, it moves toward the C4 and C5 atoms. This electron redistribution at S0-OF is also reflected in the CASPT2 computed dipole moments in vacuo/acetonitrile solution: 8.1/10.7 Debye [S0 ] vs. 13.1/16.4 Debye [S1 ]. By contrast, there just exists little charge transfer in the S1 state at S0-CF (see Table 3).

S1 Minimum and S1 /S0 Conical Intersection We tried to optimize the open-form S1 minimum from the open-form S0 minimum at the OM2/MRCI level. However, all attempts converge toward a closed-form minimum with the C1-C2 distance of 1.667 Å. This structure is actually the closed-form S1 minimum, which is referred to as S1-CF in Fig. 4. This implies that the relaxation process from the opento closed-form is essentially barrierless in the S1 state. At the OM2/MRCI level, we have also optimized an S1 /S0 minimum-energy conical intersection, which is referred to as S1S0-CF in Fig. 4. In terms of the C1-C2 reaction coordinate, one can easily find that S1S0-CF (1.728 Å) is structurally inbetween the openform Franck-Condon point (2.457 Å, S0-OF) and the closed-form S1 minimum (1.667 Å, S1-CF). In other words, it is in the middle of the S1 relaxation process. In addition, S1S0-CF is energetically close to S1-CF: 65.0 [64.7] kcal/mol vs. 53.3 [49.3] kcal/mol



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OF

HOMO

LUMO

CF

Figure 5: HOMO and LUMO molecular orbitals (isosurface: 0.02) responsible for the S0 → S1 electronic transition at the open- and closed-form Franck-Condon points, respectively (i.e. S0-OF and S0-CF).

Figure 6: Time-dependent population of the S1 and S0 states for the open-form→ closedform photocyclization and closed-form→ open-form photoreversion processes of the bridged diarylethene. See text for discussion.


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at the CASPT2/cc-pVDZ [OM2/MRCI] level. Therefore, this S1 /S0 conical intersection is accessible both structurally and energetically and should play an important role for the photocyclization and photoreversion of the bridged diarylethene (see below).

Photocyclization and Photoreversion Dynamics To explore the S1 excited-state lifetime of the open-form bridged diarylethene, we have carried out 1 ps OM2/MRCI nonadiabatic dynamics simulations (142 successful runs). In the end of 1 ps simulations, 96.5% trajectories decay to the S0 state; 3.5% ones still survive in the S1 state. In terms of our trajectories, the photocyclization quantum yield is estimated to be 0.47, which is close to the experimentally measured value of 0.49 in methanol upon irradiation of 313 nm. 27 The left panel of Fig. 6 illustrates the time-dependent S1 and S0 populations. Two curves intersect at 242 fs, based on which the S1 lifetime is estimated to be 349 fs (see the fitting model in our previous work 78 ). Our in-vacuo prediction is a little longer than experimentally measured 240±60 fs in acetonitrile solution. 27 This difference could be due to the lacking of solvent effects in our dynamics simulations.

Figure 7: Distribution of the C1-C2 distance at all S1 →S0 hopping points of both (left) photocyclization and (right) photoreversion processes. The distribution of the C1-C2 distance (reaction coordinate of photocyclization) at the 13 ACS Paragon Plus Environment


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S1 → S0 hopping points is shown in the left panel of Fig. 7. Most of the S1 → S0 hoppings take place around the S1 /S0 minimum-energy conical intersection S1S0-CF (1.728 Å), which proves that only a hopping region that centers around S1S0-CF is responsible for the S1 decay of the trajectories starting from the open-form bridged diarylethene. This dynamical feature is well consistent with the topological structure of the S0 and S1 potential energy surfaces (see above).

Figure 8: Time-dependent C1-C2 distance in four typical trajectories starting from the open form. Also shown are the hopping times (numbers, in fs). Four typical trajectories were seen in our dynamics simulations (see Fig. 8). In Trajectory_33, the system quickly relaxes to the S1 minimum (S1-CF) from the open-form Franck-Condon point S0-OF within about 50 fs; then, it oscillates around S1-CF until 146 fs, followed by an S1 → S0 de-excitation; finally, it reversely goes to the open-form S0 minimum (S0-OF). Trajectory_39 is similar to Trajectory_33 except that the system finally goes 14 ACS Paragon Plus Environment

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to the closed-form S0 minimum (S0-CF). However, in Trajectory_16 and Trajectory_173, the system oscillates around S1-CF for a longer time before it decays to the S0 state (762 fs in Trajectory_16 and 628 fs in Trajectory_173). In addition to photocyclization, the photoreversion process from the closed-form bridged diarylethene is also simulated using the OM2/MRCI nonadiabatic dynamics simulations (130 runs with 1 ps time). The S1 excited-state lifetime, based on the time-dependent S1 and S0 populations in the right panel of Fig. 6, is estimated to be around 453 fs, which is a little bigger than the experimentally estimated 250 fs in acetonitrile solution. 27 Different from the photocyclization situation, 80% trajectories decay to the S0 state in the end of 1 ps simulation time. It can be seen that the S1 excited-state lifetime for the photoreversion reaction is a little longer than that for the photocyclization reaction. This difference could be interrelated with the energy of the S1 /S0 minimum-energy conical intersection (S1S0-CF) relative to their respective starting structures in the reaction pathways. According to our computations, S1S0-CF is 21.7 and 3.9 kcal/mol lower than S1-OF-FC and S1-CF-FC (see Table 2). With this in mind, it is rational that accessing to S1S0-CF from S1-OF-FC is faster than that from S1-CF-FC. The distribution of the C1-C2 distance at all S1 → S0 hopping points is shown in the right panel of Fig. 7. Similarly, the S1 → S0 radiationless transition mainly takes place near the same S1 /S0 minimum-energy conical intersection S1S0-CF. There exists as well four typical trajectories for the photoreversion process (see Fig. 9). Trajectory_21 and Trajectory_68 relax quickly to the S1 minimum (S1-CF) within 20 fs. After several periods of oscillation around S1-CF, the system decays to the S0 state (102 fs for Trajectory_21 and 182 fs for Trajectory_68), finally decaying to the open- and closed-form S0 minima S0-OF and S0-CF, respectively. In comparison, Trajectory_8 and Trajectory_58 oscillate around S1-CF much longer before they decay to the S0 state.



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Figure 9: Time-dependent C1-C2 distance in four typical trajectories starting from the closed form. Also shown are the hopping times (numbers, in fs).


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Mechanism In terms of our present static electronic structure calculations and nonadiabatic dynamics simulations, we suggest the following S1 relaxation mechanism for the photocyclization and photoreversion processes of the bridged diarylethene (Fig. 10). It should be noted that both processes adopt a similar photochromic mechanism in the S1 state except starting from different Franck-Condon regions. Specifically, the photocyclization starts from the open-form conformer, while the photoreversion does from the closed-form one. For the photocyclization or photoreversion process, the S1 state is directly populated at S0OF or S0-CF (of course, the S1 state can be indirectly populated via internal conversion from higher excited states; not considered in our work); then, the system quickly relaxes to S1-CF; finally, it decays to the S0 state around S1S0-CF. Once decaying to the S0 state at this S1 /S0 conical intersection, the system bifurcates into either S0-OF or S0-CF.

S1-OF-FC (82.8) S1S0-CF (61.1)

S1-CF-FC (65.0) S1-CF (53.3)

S0-CF(8.0) S0-OF(0.0) Figure 10: Proposed photocyclization and photoreversion mechanism for the bridged diarylethene system. Energies are refined by the SA3-CASPT2(10,10)/cc-pVDZ method. The blue lines represent the S1 and S0 adiabatic potential energy surfaces, while the black one represents the S1 /S0 crossing seam. Our suggested S1 photocyclization and photoreversion mechanism overall matches with experimentally proposed one (Eq. 1). However, we did not find an open-form S1 min17 ACS Paragon Plus Environment


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imum at the OM2/MRCI level and all optimizations starting from the open-form structure converge toward the closed-form S1 minimum (S1-CF). This essentially barrierless S1 relaxation from the open- to closed-form region is also confirmed by our OM2/MRCI-based nonadiabatic dynamic simulations (Fig. 8). In all four typical photocyclization trajectories, the C1-C2 distance rapidly decreases to ca. 1.6 Å from the initial ca. 2.5 Å within 50 fs. Actually, in the work of Aloïse and coworkers, they have merely estimated a negligible barrier of 0.7 kcal/mol at the AM1/CIS level. 27 It is also helpful to note that in many similar 6π electrocyclic systems, this kind of photocyclization reaction is always barrierless. Moreover, Aloïse et al. also proposed another two indirect S1 photocyclization pathways where the S1 state is populated via internal conversion from higher excited states. 27 In the former, Eq. 2, a transition state was suggested. However, this viewpoint is not supported by our present work: in static electronic structure calculations, we do not locate such kind of transition state; in dynamics simulations, such region is also not encountered. The latter, Eq. 3, can also be excluded because we only find a structurally and energetically allowed S1 /S0 conical intersection at the OM2/MRCI level. The most convincing evidence is that in our photocyclization and photoreversion dynamics simulations, only a hopping region that centers around the S1 /S0 conical intersection S1S0-CF is observed (see discussion above and Fig. 7). Taken these reasons, it is clear that these two indirect S1 relaxation channels are in essence the same as the S1 photocyclization and photoreversion mechanism in Fig. 10. In comparison with previous experiments and computations, 27 our current work not only refines previous mechanism but also provides new insights. Due to the impurity of initial samples and the nonnegligible contribution from the open-form bridged diarylethene, it was experimentally difficult to precisely assign the characteristic time for the S1 (CF)→ S0 (CF) internal conversion. 27 Instead, experiments only estimated an upper limit of 250 fs, which is close to our estimated excited-state lifetime of 453 fs. In addition, we have demonstrated that both photocyclization and photoreversion processes share an ener-


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getically easily accessible conical intersection namely S1S0-CF to funnel the S1 excited state. In combination with that the S1 relaxation process from both S0-OF and S0-CF is barrierless, the photocyclization and photoreversion virtually adopt a similar mechanism, just with different initial conditions (see Fig. 10).

Conclusions By using electronic structure calculations and nonadiabatic dynamics simulations we have for the first time explored the S1 photocyclization and photoreversion dynamics of a recently studied bridged diarylethene. 27 In the first phase, we have characterized the S0 and S1 minima and the S1 /S0 minimum-energy conical intersection, and mapped the relevant S1 and S0 potential energy surfaces, based on which the photocyclization and photoreversion mechanism was suggested. In the second phase, we verified our proposed mechanism and predicted the S1 decay times with resort to trajectory-based nonadiabatic dynamics simulations. The estimated excited-state lifetimes are in good agreement with experimentally measured values. The present simulations interpret experimental observations and refine previously proposed pathways. Furthermore, although present electronic structure calculations and dynamics simulations provide new important insights into the mechanistic photochromism of bridged diarylethenes, further experimental and theoretical studies are still needed to understand wavelength dependent photocyclization quantum yields and solvent effects.

Supporting Information Product analysis and Cartesian coordinates of all optimized structures.



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Acknowledgments This work was supported by the National Natural Science Foundation of China (21522302, 21520102005, and 21421003); G.C. is also grateful for financial support from the "Recruitment Program of Global Youth Experts" and "Fundamental Research Funds for Central Universities".

Corresponding Author Phone number: 86-010-58806770. Email: [email protected] (G.C.)

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Photochromic Mechanism of a Bridged Diarylethene: Combined

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