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Combining the Advantages of Alkene and Azo E-Z Photoisomerizations? Mechanistic Insights into Ketoimine Photo Switches Qingqing Su, Yuanying Li, Bin Wang, Minjuan Liu, Hongjuan Wang, Wenliang Wang, and Fengyi Liu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b01674 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 20, 2017

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The Journal of Physical Chemistry

Combining the Advantages of Alkene and Azo E-Z Photoisomerizations?

Mechanistic

Insights

into

Ketoimine Photo Switches Qingqing Su, Yuanying Li, Bin Wang, Minjuan Liu, Hongjuan Wang, Wenliang Wang and Fengyi Liu* Key Laboratory for Macromolecular Science of Shaanxi Province, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi 710062, P. R. China. AUTHOR INFORMATION Corresponding Author * Corresponding Author: [email protected] (Fengyi Liu)

ACS Paragon Plus Environment

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ABSTRACT

We carried out CASPT2//(TD)DFT and CASPT2//CASSCF studies on the working mechanism of imine switches, including a camphorquinone-derived ketoimine (shortened as kImine) switch designed by Lehn as well as a model camphorquinone alkene-imine (a-Imine) proposed in this study. Under the experimental condition (light irradiation with 455 and 365 nm for E and Z, respectively), k-Imine is excited to the S1:(nN,π*) state and then decays towards a perpendicular intermediate following the C=N bond rotation coordinate. During the bond rotation, a mild energy barrier caused by the strong interaction of S1:(nN,π*) and S2:(nO,π*) states will more or less slow down the rotation speed of k-Imine. The large difference in irradiation light wavelength supports k-Imine as a two-way photo switch. The photoisomerization of aImine obeys a similar but fully barrierless pattern, while requires higher excitation energy to reach the (nN,π*) state. The good directionality of thermal isomerization towards E(a-Imine), plus the barrierless photoisomerization, allow for the design of a thermal and photo-operated switch. For both imines, a MECI located at the perpendicular region, with low relative energy and close neighborhood to the rotary path, insures the directionality of C=N bond rotation, and confirms imines as optimal candidates for photo switches and motors.


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1. Introduction Light-driven molecular switches and motors have attracted extensive attention in recent years due to their great successes gained in a wide range of application. Besides the bio-related or bio-mimicking photoactive systems (such as retinal protonated Schiff bases,1,2 fluorescent protein chromophore,3,4 fulgides,5 etc), artificially designed light-driven molecular switches and motors6-12 have been successfully synthesized. Among them, the overcrowded alkenes (e.g., stilbene)13-19 and azo compounds (e.g., azobenzene)

20-27

possessing a central C=C and N=N

bond, respectively, are two shiniest pieces of the jigsaw. Owing to the joint efforts of both experimentalists and theoreticians, deep knowledge about the Z/E photoisomerization mechanism of stilbene28-35 and azobenzene36-45 (as well as related systems) have been acquired, which provides a solid base and opens new opportunities for developing alkene and azo-based molecular switches and motors, as well as fine tuning their performance. For either alkene or azo systems that have been applied as photo switches/motors, their advantages are always accompanied by some inherited drawbacks. For alkene systems, the bonding capability of ethylenic (C=C) carbon allows for two single bonds in addition to the C=C double bond, therefore constructing of relatively complex structure is possible. So far, they have been utilized as not only bistable switches but also rotary motors with multistable states.7 In the meantime, the topology of the excited- and ground-state potential energy surfaces (PESs) of Z-E isomerization of alkene systems around C=C bond, e.g., the excited and ground-state surface crossing points (conical intersection, CI)46,47 are found to be away from the rotary path and high in energy,33,35 will more or less hurt the unidirectionality of rotation. In addition, the overcrowded structures used in the currently designed alkene switches and motors make the


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cross-plane thermal transformations relatively difficult and thus also slow down the speed of rotation.48-51 For azo compounds, the existence of lone-pair (LP) electrons at the sp2 nitrogen atom causes the covalent modification at N=N site less amendable with respect to their C=C counterparts, which has limited their applications to bistable molecular switches20-27 (rather than multistable motors). Also, the lone-pair electrons introduce a dark (nN,π*) excited state in addition to the bright (π,π*) state, as well as an in-plane inversion type of Z/E transformation in addition to N=N bond rotation, thus may increase the complexity of reaction for both the experimental controlling of intramolecular motions and the spectroscopic/theoretical characterizations of the photochemistry and photophysics behind them.40-42 Besides continuous efforts to overcome the imperfectness in alkene and azo-based systems, searching for new systems that may adopt the advantages from both sides seems to be an alternative solution. Actually, in addition to the above-mentioned homonuclear C=C and N=N double bonds, the photoisomerization around heteronuclear C=N bond, which can be regarded as the composite of C=C and N=N halves, has been known for several decades.52-56 Considering the remarkable switching performance in the N-containing biological chromophores (e.g., retinal Schiff base, though which isomerizes around the C=C bond rather than C=N bond), imines are expected to be an optimal candidate for molecular photo switches. Meanwhile, imines have been extensively used as key reactants or intermediates in organic chemistry, the convenience in synthesis will be another advantage for imine as molecular switches and motors.57,58 The ideas of light-driven imine switches and motors have been originated from Lehn.59 In his recent works, several imine compounds have been synthesized; they serve as either photo


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switches, or molecular rotary motors via the-state-of-the-art chemical modifications.60,61 Lehn’s works demonstrated that imines can act as a new type of molecular switches and motors that are distinct from the known alkene and azo-based systems, and the photo switching of one of his chiral imines (See Chart 1) has been further validaded by other authors with a matrix-isolation vibrational circular dichroism (MI-VCD) spectroscopy;62 however, at the mechanistic side, knowledge on the mechanism of imine photoisomerization is far from profound, with respect to that for alkene and azo systems. The few documented theoretical works focus either on the thermal isomerization of imine,63-66 or on the excited-state properties at narrow region (e.g., at Franck-Condon, FC) or with rather approximative approaches. The ground- and excited-state PESs along the extensive isomerization paths, as well as the detailed information about nonadiabatic transition, are urgently required in order to understand the working mechanism of imine switches as well as to improve their performance.

Chart 1. The Z/E isomerization of a camphorquinone imine photo switch (ref 61).


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Chart 2. Definitions of geometry parameters for the camphorquinone imine switch (k-Imine) (in ref 61) and a model camphene imine (alkene-imine) switch (a-Imine) purposed in this work.

In this work, we carried out density functional theory (DFT) and multireference CASSCF/CASPT2 studies on one of the simplest imine photo switch, that is, a camphorquinone imine switch (a ketoimine derivative, hereinafter named as k-Imine, see Chart 1) designed by Lehn et al.61 In addition to the extensive experimental measurements on the thermodynamics, kinetics and photochemical properties of the Z/E isomerization reaction, Lehn and co-workers have carried out quantum chemistry calculations on the ground-state Z/E configurations of kImine and thermal transformations between them. Still, critical information about the excited states is to be revealed. In order to clarify the role of the carbonyl substituent (and the LP electrons introduced by the carbonyl oxygen atom) in the photoisomerization process, a model camphene imine (an alkene imine derivative, namely, a-Imine in Chart 2) in which the carbonyl oxygen is substituted by a methylene (=CH2) group were also investigated for comparison. Thus the purposes of the current study are to reveal the working mechanism of the k-Imine switch, as well to verify if the a-Imine can serve as a prototype photo switch. We hope the findings of this work will shed light on mechanistic understandings of the imine-based switches and inspire new experimental developments and applications in this field.


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2. Computational Details The geometries of stationary points and reaction paths were explored at both the density functional CAM-B3LYP67 and complete active space self-consistent field (CASSCF)68 levels of theory. In DFT calculations, the ground-state (S0) and excited states were treated by CAMB3LYP and time-dependent (TD)69-71 CAM-B3LYP functional, respectively. Four lowest singlet states were investigated for k-Imine and a-Imine to make sure that all nN→π*, nO→π* and π→π* excitations are included. Correspondingly, a four-root state-averaged (SA4)-CASSCF wave function was used in CASSCF calculations. The cc-pVDZ basis set72 was used in all geometry optimizations. In CASSCF calculations, the active space was constructed with 8 electrons distributed over 6 frontier π orbitals, namely CAS(8e,6o) for k-Imine, including all bonding and antibonding π orbitals and lone-pair orbitals from both nitrogen and oxygen atoms. For a-Imine, a smaller CAS(6e,5o) active space, composed by all π orbitals and the lone-pair orbitals of nitrogen atom, was used. Detailed illustrations of active orbitals are found in Figure S1 of Supporting Information. During the exploration of reaction paths, the minimal-energy paths (MEPs) on the S0 to S1-state PESs were obtained using the MEP-search technique.73 The S1-state MEP-search calculations were started from FC region of S1 state with a step size of 0.5 a.u., and naturally terminate at the perpendicular region where the S1 and S0 states become degenerate in energy. Then, the minimal-energy crossing points (MECIs) between these two states were located at the CASSCF level by using a penalty function method.74 Finally, another set of S0-MEPs were


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carried out, using the S1/S0 MECIs as starting geometries, and lead to either Z or E isomers of the molecule, respectively. In order to consider the solvent effects, the polarizable continuum model (PCM)75-78 using acetonitrile solvent (CH3CN, ε=35.688) was employed. Our preliminary calculations show that while the solvent effect has unneglectable contributions to relative energies, it hardly affect the optimized geometries (as seen in Figure S2 & S3 of Supporting Information). Thus, in this study, the solvent contributions are considered by single-point PCM energy corrections on top of the gas-phase optimized geometries (except for key stationary points where relative energies and/or excitation energies are critical). For both the (TD-)CAM-B3LYP and CASSCF-optimized geometries, energies are refined by the multi-state complete active space second-order perturbation theory (MS-CASPT2) calculations to consider both the multi-reference effect and dynamic electron correlation. The MS-CASPT2 energy corrections were done using the same active space as being employed in CASSCF geometry optimizations but with a larger triple-ζ basis set (cc-pVTZ).72 An imaginary level shift of 0.1 Hartree is used, and in addition, no orbitals are frozen in CASPT2 calculations. Although CASPT2//CASSCF calculations are widely employed in the treatments of photochemical reactions, due to the absence of dynamical correlation, CASSCF-optimized geometries may not be adequate when dynamic correction puts significant influence on geometries, as revealed in some recent studies.79,80 To evaluate the effects of the dynamic correlation on molecular geometry, we carried out MS-CASPT2 optimization on truncated models, in which all methyl groups (-CH3) are replaced with hydrogen (-H) atoms. The relatively good consistencies between the geometries by CASSCF and CASPT2 suggest that dynamic


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correction has some but no vital effect on the current system. Detailed comparisons are found in Figure S4 in Supporting Information. The (TD)DFT calculations were performed using the

Gaussian 09 program,81 and

CASSCF and CASPT2 calculations were done by MOLCAS@UU package.82,83

3. Results and Discussions In the following sections, we first present the computational results on the ground-state Z and E configurations of k-Imine and a-Imine, respectively, as well as their thermal isomerization in subsection 3.1; then, we will show the vertical excitations at both Z and E configurations in section 3.2, as well as the subsequent Z→E and E→Z photoisomerizations of the two molecules in 3.3; next, we will discuss the S1/S0 MECIs that closely relate to the nonadiabatic transition in detail in 3.4; finally, conclusions are drawn based on the all computational results. 3.1 Bistable ground-state Z/E configurations and thermal isomerization Ground-state k-Imine: We first optimized the geometries of S0-state Z and E configurations for k-Imine at the CAM-B3LYP/cc-pVDZ and SA4-CASSCF(8e,6o)/cc-pVDZ levels of theory and summarized the gas-phase results in Figure 1 (upper panel).


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Figure 1. The CAM-B3LYP(gas)/cc-pVDZ and CASSCF(gas)/cc-pVDZ optimized ground-state minima with important geometry parameters, as well as the MS-CASPT2(sol)/cc-pVTZ corrected energies for kImine and a-Imine. For clarity, atoms and bonds less relevant to the isomerization process are illustrated in wireframe; bond lengths are in angstrom, (dihedral) angles are in degree and relative energies are in kcal/mol. Experimental data (Exp.) are obtained from ref. 61.

The optimized geometries in gas phase, at either the DFT or CASSCF level, are sufficiently good compared with the available crystal structure of E(k-Imine) shown in the same


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figure.

61

The dihedral angles shown in Figure 1 clearly indicate that the ketoimine moiety

(O=C1-C2=N) and tert-butyl carbon (C3) in both E and Z(k-Imine) are roughly in one plane. Therefore, no distortion of the conjugated region caused by the steric repulsion between the bulky caged substituent (connected to C1 and C2 positions) and the tert-butyl group (attached on N atom) is observed, which is different from those found in overcrowded alkene switches and rotary motors. 48-51 Without external constraints and distortions, the relative stabilities of Z/E configurations are intrinsically governed by the O=C1-C2=N chromophore; and it is no surprise to see that the E and Z(k-Imine) are very similar in energy. In gas phase, Z is more stable than E by about 2.4 kcal/mol at CAM-B3LYP/cc-pVDZ level (and 1.3 kcal/mol at CASSCF level of theory), while in line with the reported gas-phase computational result (0.6 kcal/mol) at TPSSD3/def2-TZVP level, they are in contrast to the experimentally measured thermodynamic favorability (towards E) in CD3CN solution.61 Indeed, the CAM-B3LYP geometry optimizations with polarizable continuum model (PCM) using acetonitrile solvent (CH3CN, ε=35.688) reverse the E/Z thermodynamic balance of k-Imine, in turn, E becomes more stable than Z by about 0.2 and 1.8 kcal/mol, with the ccpVDZ and cc-pVTZ basis sets, respectively. The solvent effect and size of basis sets hardly affect the geometries of k-Imine, while they do put significant influence on the relative Z/E stability (See Figure S2 and Table S1 for the geometry parameters and relative energies computed at various levels of theory; and Figure S5 for the dipole moments of E and Z configurations, in which the larger dipole moment of E suggests the corresponding configuration will be more stabilized in polar solvent than Z). Therefore, during the PES explorations, structures were approximately obtained by gas-phase geometry optimizations with a moderate cc-pVDZ basis set at the (TD)CAM-B3LYP and CASSCF levels, respectively; while energies


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are always refined by MS-CASPT2(sol) approach with an adequate cc-pVTZ basis set. The relative energies of Z(k-Imine) with respect to E, computed at our best level of theory (i.e. MSCASPT2(sol)/cc-pVTZ) on top of the CAM-B3LYP(sol) and CASSCF(gas) optimized geometries, are 2.5 and 2.3 kcal/mol, respectively. The results are in agreement with the experimental value 2.7 kcal/mol,61 and suggest a Z→E trend in thermal isomerization of kImine. As found by Lehn et al, at room temperature, Z(k-Imine) isomerizes to E with a half-life (t1/2) of 10 min and an activation energy of 21.2 kcal/mol.61 The transition state optimized at the CAM-B3LYP level provides a collinear C2-N-C3 structure (See Figure S6), and thus suggests an in-plane N inversion mechanism for k-Imine thermal isomerization. The Z→E isomerization barrier predicted at CAM-B3LYP level with PCM solvent-effect correction is 21.1 kcal/mol; after MS-CASPT2(sol) refinements, the barrier is 23.0 kcal/mol. Both are in agreement with the experimental measurement and the reported B2PLYP(+COSMO) result (21.7 kcal/mol).61 Ground-state a-Imine: The CAM-B3LYP and SA4-CASSCF(8e,6o) optimized S0-state Z and E configurations for a-Imine are also shown in Figure 1(lower panel). Replacing the electron-withdrawing carbonyl O(=C) atom with electron-donating CH2 group, on one hand, slightly alternates the bond lengths of conjugated moiety (i.e., C1-C2 bond is shortened while C2=N is elongated) and introduces larger steric repulsion between the tert-butyl and methene (CH2) group for Z configuration, as indicated by the simultaneous increasing of θ and φ dihedral angles in Figure 1; on the other hand, decreases the dipole moments (See Figure S5) and thus leads to smaller response to solvent polarization. In turn, the E(a-Imine) is more stable than configuration Z by about 5.6~6.5 kcal/mol at the various levels of theory employed (See Table


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S1), with or without solvent effect, and the Z→E thermal isomerization is thermodynamically more favorable than that of k-Imine. Thermal isomerization of a-Imine takes place in the same Z→E direction and follows the same N inversion mechanism as being observed in k-Imine. The corresponding MSCASPT2(sol)-corrected Z→E activation barriers on top of the CAM-B3LYP and SA4CASSCF(8e,6o) optimized structures, respectively, are 20.9 and 20.8 kcal/mol, slightly lower than that of k-Imine. 3.2 Photoexcitation and absorption maxima of k-Imine and a-Imine The mechanism of photo-initiated reactions depends critically on the initial conditions, that is, which state the molecules can be excited to and how the consequent photophysical and photochemical processes occur, is primarily controlled by the energy/wavelength of light irradiation. Photoexcitation of k-Imine: For k-Imine, our calculations suggest that the S1, S2 and S3 states majorly correspond to nN →π*, nO→π* and π→π* excitations, respectively (See Table S2). The computed S1-state vertical excitation energy of E(k-Imine) is 79.6 (PT2(sol)//TDCAM-B3LYP(sol)/cc-pVTZ) and 78.1 kcal/mol (PT2(sol)//CAS(gas)), respectively. The computed values are very close to the experimental irradiation energy (365 nm in wavelength, corresponding to 78.3 kcal/mol; since no absorption spectra have been reported experimentally);61 while the S2 and S3 states are found to be ~10 and ~80 kcal/mol higher in energy than S1 state, therefore they are energetically inaccessible under the experimental condition (Details of excitation energy calculations at various l of theory are summarized in Table S3). The situations for Z(k-Imine) are similar, except for the lower S1 vertical excitation


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energies as well as the larger S1-S2 gaps (~40 kcal/mol) being overserved. The computed absorption maxima, 65.9 (PT2(sol)//TD-CAM-B3LYP(sol)/cc-pVTZ) and 65.1 kcal/mol (PT2(sol)//CAS(gas)), respectively, are also consistent with experimental irradiation wavelength (455 nm, 62.9 kcal/mol).61 Therefore, to mimic the experimental operating of imine switch, we assigned the initial excitations to S1:(n(N), π*) state and tracked the photoisomerization path along the S1 PES. Photoexcitation of a-Imine: For a-Imine, the absence of carbonyl oxygen eliminates the n(O)→π* excitation, correspondingly, the S1 and S2 majorly possess (nN, π*) and (π, π*) nature, respectively. The decreased charge polarization in a-Imine causes the energy of (nN, π*) state being less stabilized with respect that in k-Imine. The vertical excitation energies at E and Z are 100.8 and 94.6 kcal/mol, respectively, at the PT2(sol)//TD-CAM-B3LYP(sol)/cc-pVTZ level. Thus, it requests shorter UV light at wavelength 284 and 302 nm to excite the molecules to the S1-state FC region of E and Z(a-Imine), respectively. The difference in irradiation wavelength for E and Z configurations, ~20 nm, is not as large as that of k-Imine (~90 nm), therefore, the two-way light switching of a-Imine switch may be less feasible. Considering the good Z→E trend in thermal isomerization, it is still promising to construct an a-Imine switch by combining the reversible photo- and thermal-induced transformation steps together. 3.3 S1-MEPs and photo switching mechanism of k-Imine and a-Imine Figure 2a and c show the CASSCF(gas)-optimized S1-MEP and PT2(sol)-corrected energy profile for k-Imine; and b and d show the CASSCF(gas) optimized S1-MEP and PT2(sol)corrected energy profile for a-Imine, respectively.


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Figure 2. The CASSCF(gas)/cc-pVDZ computed S1-MEP for (a) k-Imine and (b) a-Imine, and the MS-

PT2(sol)/cc-pVTZ//CASSCF(gas)/cc-pVDZ energy profiles for (c) and (d), respectively.

Photoisomerization of k-Imine: Starting from FC point of E(k-Imine), the S1-MEP for E→Z isomerization (following the purple arrow from right to left side) follows a C=N bond torsional mechanism, which is nearly spontaneous except for a mild barrier met at reaction coordinate ~1.6 a.u. (for simplicity of disscussion, the reaction coordinate of Z(k-Imine) is always set to 0). It is caused by the interaction between the S1:(nN,π*) and S2:(nO,π*) states that varies significantly from the planar to perpendicular geometries. The barrier height at the MSCASPT2 level is about 3.0 kcal/mol, which will more or less slow down the C=N rotary speed;


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still, it is conquerable considering the excessive irradiation energy. Once the barrier is surpassed, the molecules can barrierlessly decay to a perpendicular region near 1.0 a.u., where the energies of S1 and S0 states are nearly degenerate in an extended region (due to a technical problem known as “root flipping” caused by the degenerate S1 and S0 states, the perpendicular S1 minimum has been failed to be obtained). Thus, the S1 trajectories originated from E(k-Imine) have a chance to non-adiabatically transit to the ground state to form either Z or E(k-Imine), or in another word, reach a photostationary state (PSS). The photoisomerization of k-Imine for the other direction, that is, the S1:Z→E isomerization, takes place in a similar pattern. Following the cyan arrow from left to right side, the excited Z(k-Imine) molecules conquer a lower barrier (less than 2.0 kcal/mol) to reach the S1-S0 degenerate region, then radiationlessly decay to S0 state and finally reach a PSS state. We suggested that the same nonadiabatic transition mechanism is responsible for both S1:E→Z and S1:Z→E photoisomerizations; the different wavelengths used in excitation of Z(k-Imine) and E(k-Imine) determine the overall direction of photoisomerization. Photoisomerization of a-Imine: Figure 2b and d show the CASSCF(gas) optimized S1MEP and PT2(sol)-corrected energy profile for a-Imine. For both S1:E→Z and S1:Z→E photoisomerizations in a-Imine, the photo-induced C=N rotary paths are straightforward compared with those of k-Imine. Due to the absence of (nO,π*) state, the (nN,π*) decay channels at both the CASSCF and CASPT2 levels, from FC region of either E or Z to perpendicular region, are downhill, thus photoisomerizations take place barrierlessly. The narrow S1-S0 gaps at the perpendicular region, again suggest possible nonadiabatic transition to the ground state. More energy profiles based on (TD)DFT geometries are presented in Figure S7 & S8 in Supporting Information.


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3.4 S1/S0 Conical Intersection and nonadiabatic transition The optimized minimal-energy crossing points (MECIs) between S1 and S0 states are illustrated in Figure 3.

Figure 3. The CASSCF(gas)/cc-pVDZ optimized S1/S0 MECIs with important geometry parameters, as well as the MS-CASPT2(sol)/cc-pVTZ corrected energies for (a) k-Imine and (b) a-Imine. Bond lengths are in angstrom, (dihedral) angles are in degree and relative energies are in kcal/mol.

As seen in Figure 3, both the S1/S0-CI(k-Imine) and S1/S0-CI(a-Imine) feature a nearly perpendicular structure with a C=N bond torsional angle (θ) around 105º, which is close to the terminal structures of S1-MEPs (Figure S9) as well as in good consistence with the 105º geometries obtained by constraint optimizations (Figure S10). In addition, their relative energies perfectly overlap with the CASSCF and MS-CASPT2 energy profiles, as illustrated in Figure 2 (where the MECIs are shown as yellow cones). For instance, the relative energies of the S1 state (the higher root in two energetically-degenerate roots at MECI) at S1/S0-CI(k-Imine) are 55.4 and 59.7 kcal/mol at the CASSCF(gas) and MS-CASPT2(sol) level, respectively, slightly lower


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than the corresponding relative energies of S1 rotary profile at θ = 105º (61.3 and 63.4 kcal/mol, respectively). It thus suggests that the S1 trajectories can easily access the MECIs, requires no additional energy and involvements of other vibrational modes (than C=N torsional mode), in contrast to the MECIs in alkene switches and motors that are both away from rotary path and higher in energy. It is noted that the MS-CASPT2(sol) calculated S1-S0 gap at the S1/S0-CI(k-Imine) is ~7 kcal/mol, while at the S1/S0-CI(a-Imine) increases to ~16 kcal/mol, suggesting the discrepancies of the CASSCF(gas) and CASPT2(sol) methods in optimizing geometries of MECIs. Although the two levels of theory employed in this study have been unable to give a exactly matched geometries at the specific crossing point (or energetics at the same geometries), the CASSCFoptimized S1-MEPs and MS-CASPT2-corrected energy profiles do show consistent trend at the perpendicular region, that is, the S1 and S0 PESs are degenerate in energy in the vicinities of the S1/S0-CI (Figure 2). These sufficiently support that efficient nonadiabatic transition via MECIs can take place in the vicinities of the located critical points. The low-energy and closely neighboring MECIs in k-Imine and a-Imine insure the speed and directionality of bond rotation, thus confirming the imine systems as a good candidate for light-driven molecular switches and rotary motors. Once the nonadiabatic S1→S0 transition occurs, the successive reaction taking place in the S0 state becomes straightforward. As seen in Figure 4, the S0-MEPs starting from either the S1/S0-CI(k-Imine) or S1/S0-CI(a-Imine) goes smoothly downhill towards E or Z isomers, respectively, still majorly following the C=N bond rotation coordinates (the geometry variations along S0-MEP are shown in Figure S11).


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Figure 4. The CASSCF(gas)/cc-pVDZ computed S0-MEP for (a) k-Imine and (b) a-Imine, and the MS-

PT2(sol)/cc-pVTZ//CASSCF(gas)/cc-pVDZ energy profiles for (c) and (d), respectively.

4. Conclusions In summary, the photoisomerization mechanism of imine switches, including a ketoimine (k-Imine) switch designed by Lehn as well as a model alkene-imine (a-Imine) switch proposed in this study, have been studied at the CASPT2//(TD)DFT and CASPT2//CASSCF levels of theory. The photo-induced geometry relaxation and nonadiabatic decay started from the


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S1:(nN,π*) state are considered according to the experimental condition. Some important conclusions are drawn as below: (1) For the ketoimine switch (k-Imine), both E and Z configurations are excited to their lowest (nN,π*) state by light irradiation in different wavelengths, then decay towards a perpendicular intermediate following the C=N bond rotation coordinate; during the relaxation, a mild barrier, ~4.0 kcal/mol and ~2.0 kcal/mol for E→Z and Z→E relaxation, respectively, has to be conquered. A MECI located at the perpendicular region is proposed to be responsible for the nonadiabatic S1→S0 transition. The low energy and closely neighborhood (to the rotary path) of the MECI insure the directionality of bond rotation, thus confirming the imine systems as good candidates for light-driven molecular switches and rotary motors. (2) The photoisomerization of alkene-imine switch (a-Imine) obeys a similar mechanism found in ketoimine switch. The absence of carbonyl group in a-Imine makes the molecule less polar and leads to higher excitation energy to (nN,π*) state, therefore, the light required to excite the molecule is blue-shifted to UV region; in addition, the difference in light wavelengths for E and Z configurations becomes smaller, which makes a-Imine to be operated as a two-way photo switch less feasible. Fortunately, the good directionality in ground-state thermal isomerization, combined with the photoisomerization, suggest alkene-imine may be able to be utilized as a light/thermal bistable switch.

Supporting Information. Detailed results of the CASSCF and (TD)DFT calculations, seen in Table S1-S3, Figure S1-S17; the Cartesian coordinates of optimized stationary points.


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ACKNOWLEDGMENT This work is supported by grants from the NSFC (Grant Nos. 21473107, 21473108), Natural Science Basic Research Plan in Shaanxi Province of China (Grant No. 2015JM2056), Fundamental Research Funds for the Central Universities (Grant No. GK201502002) and Program for Changjiang Scholars and Innovative Research Team in University (IRT_14R33).

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