Photoprotection Mechanism of p-Methoxy Methylcinnamate: A

Article pubs.acs.org/JPCA

Photoprotection Mechanism of p‑Methoxy Methylcinnamate: A CASPT2 Study Xue-Ping Chang, Chun-Xiang Li, Bin-Bin Xie, and Ganglong Cui* Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China S Supporting Information *

ABSTRACT: p-Methoxy methylcinnamate (p-MMC) shares the same molecular skeleton with octyl methoxycinnamate sunscreen. It is recently found that adding one water to pMMC can significantly enhance the photoprotection efficiency. However, the physical origin is elusive. Herein we have employed multireference complete active space self-consistent field (CASSCF) and multistate complete active-space secondorder perturbation (MS-CASPT2) methods to scrutinize the photophysical and photochemical mechanism of p-MMC and its one-water complex p-MMC−W. Specifically, we optimize the stationary-point structures on the 1ππ*, 1nπ*, and S0 potential energy surfaces to locate the 1 ππ*/S 0 and 1 ππ*/1nπ* conical intersections and to map 1ππ* and 1nπ* excited-state relaxation paths. On the basis of the results, we find that, for the trans p-MMC, the major 1ππ* deactivation path is decaying to the dark 1nπ* state via the in-plane 1ππ*/1nπ* crossing point, which only need overcome a small barrier of 2.5 kcal/ mol; the minor one is decaying to the S0 state via the 1ππ*/S0 conical intersection induced by out-of-plane photoisomerization. For the cis p-MMC, these two decay paths are comparable 1ππ* deactivation paths: one is decaying to the dark 1nπ* state via the 1 ππ*/1nπ* crossing point, and the second is decaying to the ground state via the 1ππ*/S0 conical intersection. One-water hydration stabilizes the 1ππ* state and meanwhile destabilizes the 1nπ* state. As a consequence, the 1ππ* deactivation path to the dark 1nπ* state is heavily inhibited. The related barriers are increased to 5.8 and 3.3 kcal/mol for the trans and cis p-MMC−W, respectively. In comparison, the barriers associated with the photoisomerization-induced 1ππ* decay paths are reduced to 2.5 and 1.3 kcal/mol for the trans and cis p-MMC−W. Therefore, the 1ππ* decay paths to the S0 state are dominant relaxation channels when adding one water molecule. Finally, the present work contributes a lot of knowledge to understanding the photoprotection mechanism of methylcinnamate derivatives.

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INTRODUCTION Sunscreens are believed to be a valuable tool in providing photoprotection against the detrimental effects of UV radiation.1,2 Both acute and chronic UV exposure can lead to sunburn, photocarcinogensis, and photoaging. These days, there are several photoprotection methods, e.g., sun avoidance, seeking shade, use of protective clothing, and application of sunscreens. Among them, using sunscreen remains the most prevalent protection strategy. However, a number of controversies have developed regarding their safety and efficacy. A growing body of studies that demonstrate the beneficial effects of vitamin D on health outcomes have called into question the potential for vitamin D deficiency with sunscreen use. The possibility of adverse biological effects from various ingredients in sunscreens has also been developed recently.3−9 Therefore, in order to enhance the safety and efficacy of sunscreens in a bottom-up means, understanding their microscopic photoprotection mechanism is prerequisite.7−9 One kind of widely used sunscreen is octyl methoxycinnamate. Its main molecular skeleton is p-methoxy methylcinna© XXXX American Chemical Society

mate (p-MMC), which is structurally similar to p-coumaric acid (p-CA), 4-hydroxycinnamate (MeOpCA), and sinapoyl ester derivatives.10−28 Recently, the excited-state dynamics of pMMC attracts a lot of experimental attention. Tan et al. employed high-resolution spectroscopic techniques to study the excited-state dynamics of p-MMC.29 They found that excitation to the initial “bright” ππ* state does not directly lead to repopulation of the electronic ground state. Instead, internal conversion to another electronically excited state that is identified as the “dark” nπ* state is a major decay pathway and thus impedes the fast energy dissipation. However, adding a single water changes the corresponding excited-state properties. The one-water complex of p-MMC (p-MMC−W) undergoes a rapid internal conversion from the initial “bright” ππ* state to the electronic ground state that is mediated by the double-bond photoisomerization. Miyazaki et al. studied the S1 Received: August 30, 2015 Revised: October 28, 2015

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DOI: 10.1021/acs.jpca.5b08434 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Table 1. Computed Vertical Excitation Energies (Unit: eV) to the First Two Excited Singlet States (i.e., 1ππ* and 1nπ*) at the Franck−Condon Points of p-MMC and p-MMC−W in Trans and Cis Conformers TD-CAM-B3LYP state

p-MMC

MS-CASPT2 (IPEA0)

p-MMC−W

ππ* nπ*

4.52 4.96

4.41 5.19

ππ* nπ*

4.41 4.84

4.32 4.93

1 1

1 1

p-MMC Trans 4.12 5.23 Cis 4.13 5.13

MS-CASPT2 (IPEA25)

p-MMC−W

p-MMC

p-MMC−W

4.19 5.49

4.72 5.50

4.67 5.75

4.12 5.28

4.74 5.41

4.73 5.57

field (CASSCF) method, which has been shown to give a balanced description for both excited and ground electronic states in our previous applications.37−42 In the CASSCF calculations, equal state weights are used; the active space of 10 electrons in 8 molecular orbitals is employed, which is referred to as CASSCF(10,8) hereinafter. Specifically, for the 1 ππ* state, five occupied π molecular orbitals and three unoccupied π* molecular orbitals are used, whereas for the 1 nπ* state, one occupied π molecular orbital is replaced by the n orbital of the O atom of the carbonyl group (see Supporting Information). Since the CASSCF approach cannot provide sufficient correlation energy, the multistate complete active-space second-order perturbation (MS-CASPT2) approach43,44 is exploited to re-evaluate the energies of all optimized structures, minimum-energy potential energy profiles, and linearly interpolated internal coordinate paths. In the MS-CASPT2 computations, a larger active space of 12 electrons in 9 molecular orbitals is used, i.e., five occupied π molecular orbitals, one occupied n orbital of the O atom of the carbonyl group, and three unoccupied π* molecular orbitals. In addition, the Cholesky decomposition technique with unbiased auxiliary basis sets is used for accurate two-electron integral approximations.45 The imaginary shift technique (0.2 au) is employed to avoid intruder-state issues.46 The ionization potential− electron affinity shift of 0.25 (IPEA25) and 0.0 (IPEA0)47 are used in the computations of vertical excitation energies, whereas in the single-point energy refinements, the default IPEA25 is adopted unless otherwise stated. Vertical excitation energies at the Franck−Condon points are calculated using MS-CASPT2 and TD-CAM-B3LYP methods. 48,49 The 6-31G* basis set is employed for all computations.50,51 All DFT and TD-DFT calculations, and SA-CASSCF calculations for conical intersections are carried out using GAUSSIAN09;32,48,52 all MS-CASPT2 calculations and SA-CASSCF calculations for minima and minimum-energy reaction paths are performed using MOLCAS8.0.53,54

state dynamics of p-MMC, o-MMC, and m-MMC under supersonic jet-cooled conditions using laser-induced fluorescence and mass-resolved resonant two-photon ionization spectroscopy.30 They found that the S1 dynamics of p-MMC is different from those of o-MMC and m-MMC. The S1 decay of p-MMC is very faster and is excess energy-dependent, whereas the S1 decay of o-MMC and m-MMC is on the nanosecond time scale and exhibits little tendency of excess energy dependence. Furthermore, they found that the hydration significantly accelerates the S1 excited-state decay. Very recently, Miyazaki et al. again investigated the photoisomerization of p-MMC using low-temperature matrixisolation Fourier transform infrared spectroscopy.31 It was found that the trans−cis and cis−trans photoisomerizations happen in the S1 state upon irradiation with the wavelength of light longer than 300 and 275 nm, respectively. Moreover, they proposed that the photoisomerization occurs in the S1 potential surface and not after taking the internal conversion to the dark nπ* state. On the computational side, Miyazaki et al.30 have employed the single-reference MP2, CIS(D), PBE0, and TD-PBE0 methods to optimize ground-state conformers of p-MMC and p-MMC−W and to scan the initial ππ* isomerization paths and the ππ* decay paths to the dark nπ* state. However, only initial trans−cis photoisomerization paths (from 180° to 140°) are calculated due to the convergence issue of single-reference electronic structure methods in the vicinity of intersection region of potential energy surfaces. To our best knowledge, there is no systematic multireference high-level electronic structure calculations on the photophysics and photochemistry of p-MMC and p-MMC−W until now. In this work, we have employed high-level electronic structure methods (DFT, CASSCF, and MS-CASPT2) to optimize the trans and cis S0 and S1 minima and the S1/S0 conical intersections, to map the S1 and S0 potential energy profiles relevant to the whole S1 photoisomerization paths and the ππ* decay paths to the dark nπ* state. On the basis of the results of our electronic structure calculations, we explain very well the excited-state properties of p-MMC and p-MMC−W observed in experiments and proposed the corresponding photophysical and photochemical mechanism of p-MMC and p-MMC−W.

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RESULTS AND DISCUSSION Local Spectroscopic Properties. Table 1 compiles TDCAM-B3LYP and MS-CASPT2 (IPEA0 and IPEA25) computed vertical excitation energies to the lowest excited 1 ππ* and 1nπ* states at the Franck−Condon points of the trans and cis p-MMC and p-MMC−W (Figures 1 and 2). For the 1 ππ* state of p-MMC [p-MMC−W] in the trans and cis conformers, TD-CAM-B3LYP results are 4.52 [4.41] eV and 4.41 [4.32] eV; MS-CASPT2 computations with default IPEA shift of 0.25 give 4.72 [4.67] and 4.74 [4.73] eV. The prediction of these two approaches is a little higher than those computed at the TD-PBE0/cc-pVDZ level (4.29 eV for the trans p-

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SIMULATION DETAILS Ground-state conformers of p-MMC and p-MMC−W are first optimized using the DFT method32 with the B3LYP exchangecorrelation functional.33−36 Minima, conical intersections, minimum-energy potential energy profiles, and linearly interpolated internal coordinate paths are computed using the two-root state-averaged complete active space self-consistent B


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The vertical excitation energy to the 1nπ* state is complicated. For the trans p-MMC, it is predicted to be 4.96 eV at the TD-CAM-B3LYP level, 5.23 eV at the MS-CASPT2 (IPEA0) level, and 5.50 eV at the MS-CASPT2 (IPEA25) level, which are increased to 5.19, 5.49, and 5.75 eV when adding one water to p-MMC. In comparison, the 1nπ* energy shift of the cis conformer due to the one-water hydration is a little smaller compared with those of the trans conformer (see Table 1). Figures 3 and 4 illustrate molecular orbitals involved in the vertical excitation transitions of the trans and cis p-MMC and pMMC−W to the 1ππ* and 1nπ* states. The 1π → π* electronic transitions of either trans or cis p-MMC conformers, the bonding π and antibonding π* molecular orbitals are all delocalized within the whole molecular space; however, for the 1 n → π* electronic transition, the n lone-pair electron mainly stems from the O atom of the carbonyl oxygen. Trans and Cis 1ππ* and 1nπ* Minima. Starting from the trans S0 minima of p-MMC and p-MMC−W, we have optimized the corresponding trans 1ππ* and 1nπ* minima, which are shown in Figure 1. As discussed above, in Figure 3, the 1nπ* state is caused by the n → π* electronic transition in which the lone-pair electron is from the O6 atom of the C5 O6 group (some from the pz orbital of H2O for p-MMC−W) and the π* molecular orbital has a node between the C3 and C4 atoms. Thus, in comparison with the trans S0 minima, the C5O6 and C3−C4 bond lengths of the trans 1nπ* minima are elongated very much, specifically, 1.352 [1.445] Å in 1nπ* versus 1.191 [1.345] Å in S0 for p-MMC and 1.357 [1.446] Å in 1 nπ* versus 1.199 [1.329] Å in S0 for p-MMC−W. Conversely, the C2−C3 and C4−C5 bond lengths are decreased (see Figure 1). Furthermore, it can be found that the water molecule is bound to p-MMC much weaker in the 1nπ* state than in the S0 state. The corresponding hydrogen-bond lengths are increased from 2.025 [2.411] Å in S0 to 2.461 [2.658] Å in 1 nπ*. Different from the n → π* electronic transition, the π → π* excitation primarily involves the phenyl group and the central double bond. Hence, the C5O6 bond length does not change from the S0 minima to the 1ππ* minima (see Figure 1). In this process, the most remarkable structural change is the central C2−C3 and C3−C4 bond lengths. For p-MMC [pMMC−W], the former is shortened significantly from 1.471 [1.471] to 1.397 [1.441] Å, while the latter is elongated from 1.345 [1.329] to 1.394 [1.338] Å. In addition, we have found that the water molecule is bound to p-MMC in the 1ππ* state as strong as in the S0 state, but both are much stronger than in the 1nπ* state. Adding one water stabilizes the 1ππ* state and destabilizes the 1nπ* state of the trans conformer. At the MS-CASPT2 level, the energies of the 1nπ* and 1ππ* minima relative to the S0 minima are computed to be 100.8 and 105.2 kcal/mol for the trans p-MMC conformers and 106.1 and 103.1 kcal/mol for p-MMC−W, respectively. Specifically, the hydration only stabilizes the 1ππ* state by 2.1 kcal/mol and destabilizes the 1 nπ* state by 5.3 kcal/mol. In addition, it should be noted that the 1ππ* state is lower than the 1nπ* state at the Franck− Condon points (either p-MMC or p-MMC−W). However, for adiabatic excitation energies, the order of the 1nπ* and 1ππ* states is variational. For p-MMC, the 1nπ* adiabatic excitation energy is the lowest, but it is reversed when adding one water molecule. This is consistent with experimental viewpoint.29,55

Figure 1. CASSCF-optimized minimum-energy structures of the trans p-MMC and p-MMC−W in the S0, 1nπ*, and 1ππ* states.

Figure 2. CASSCF-optimized minimum-energy structures of the cis pMMC and p-MMC−W in the S0, 1nπ*, and 1ππ* states.

MMC)30 and at the TD-B3LYP/6-311++G** level (4.17 and 4.03 eV for the trans and cis p-MMC).31 In comparison, MSCASPT2 computations with a zero IPEA shift give much lower values compared with those with the default IPEA value (see above, Table 1). It is as well found that adding one water merely change a little the vertical excitation energy to the 1ππ* state at either cis or trans S0 minima, for example, 4.12 eV of pMMC versus 4.19 eV of p-MMC−W for the trans conformer at the MS-CASPT2 (IPEA0) level, 4.72 eV of p-MMC versus 4.67 eV of p-MMC−W for the trans conformer at the MS-CASPT2 (IPEA25) level. The same feature is also found for the cis conformer (see Table 1). C


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Figure 3. TD-DFT computed molecular orbitals involved in the vertical electronic transitions to the 1ππ* and 1nπ* states of the trans p-MMC and p-MMC−W. See the Supporting Information for molecular orbitals calculated by the CASSCF method.

Figure 4. TD-DFT computed molecular orbitals involved in the vertical electronic transitions to the 1ππ* and 1nπ* states of the cis p-MMC and pMMC−W. See the Supporting Information for molecular orbitals calculated by the CASSCF method.

In addition, we have optimized the corresponding cis 1ππ* and 1nπ* minima starting from the cis S0 minima of p-MMC and p-MMC−W, which are shown in Figure 2. The changes of geometric and electronic structures of these cis minima are similar to those of the trans minima discussed above.

Conical Intersections. At the CASSCF(10,8)/6-31G* level of theory, we have optimized two minimum-energy S1/ S 0 conical intersections for p-MMC and p-MMC−W, respectively, which are referred to as 1ππ*/gs and 1ππ*/gs (W) in Figure 5. The most visible structural feature of these D


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We have analyzed the S1 electronic structures at the 1ππ*/gs and 1ππ*/gs (W) conical intersections, as shown in Figure 5. There the S1 state is of a typical diradical character. Its two singly occupied electrons are separately located on the C3 and C4 atoms. However, due to the conjugation interaction of the pz atomic orbitals of the C3 atom and the phenyl group, the one singly occupied electron will delocalize within the whole phenyl moiety including the C3 atom. In comparison, the other singly occupied electron is much more localized, only around the C4 atom. Isomerization Paths along the C3−C4 Bond Rotation. We have computed the S1 minimum-energy isomerization paths of p-MMC and p-MMC−W along the C2−C3−C4−C5 dihedral angles. As shown in the left panel of Figure 6, there is a small barrier of 4.9 kcal/mol at 140° for the C3−C4 bond rotation of p-MMC from the trans S1 conformer; in contrast, from the cis S1 conformer, the barrier is decreased to 1.8 kcal/ mol at 30°. After overcoming these two barriers, the S1 energy immediately drops approaching the S0 energy, but both states do not intersect (they are parallel to each other). Similar to our multireference MS-CASPT2 calculations, the CIS(D) approach in the work of Miyazaki et al. also gives a small but much lower barrier, ca. 0.5 kcal/mol at 150° from the trans S1 minimum. It is noteworthy that they did not compute the whole isomerization path due to the convergence issue of the single-reference method (TD-DFT therein), only from 180° to 140° (i.e., the initial stage of the trans−cis isomerization).30 The shapes of the S1 and S0 potential energy surfaces of pMMC−W along the C2−C3−C4−C5 dihedral angle are similar to those of p-MMC except that the S1 barriers are reduced to 2.5 kcal/mol at 150° from the trans S1 conformer and to 1.3 kcal/mol at 30° from the cis S1 conformer, respectively. The former is close to but a little higher than 0.1 kcal/mol at the CIS(D) level.30 In addition, the S1 and S0 states are more close to each other in p-MMC−W than in p-MMC. Therefore, it is clear that the one-water hydration decreases the S1 barriers and narrows the energy gap of the S1 and S0 states. This topological feature agrees with experimental observation that the hydration significantly accelerates the S1 nonradiative decay. Furthermore, we do not find real conical intersections at about 100.0° along the S1 minimum-energy isomerization paths in both p-MMC and p-MMC−W, which implies that the 1ππ*/ gs and 1ππ*/gs (W) conical intersections are not located in the

Figure 5. Minimum-energy 1ππ*/gs conical intersections of p-MMC and its one-water complex p-MMC−W computed by the CASSCF method.

two conical intersections is the almost perpendicular orientation of the phenyl and methyl ester moieties, as characterized by the C2−C3−C4−C5 dihedral angle of 107.2° for p-MMC and 100.2° for p-MMC−W. Furthermore, the C3−C4 bond lengths of the 1ππ*/gs and 1ππ*/gs (W) conical intersections are greatly elongated compared with those of the 1ππ* minima. At the conical intersections, it is 1.451 [1.451] Å, which is longer than 1.394 [1.338] Å at the 1ππ* and 1 ππ* (W) minima. Moreover, the C5O6 bond lengths of the 1 ππ*/gs and 1ππ*/gs (W) conical intersections are also increased to 1.243 and 1.248 Å, respectively. The changes of the other geometric parameters can be found in Figures 1 and 5. Finally, it can be found that the hydrogen bonding interaction at the 1ππ*/gs (W) conical intersection is enhanced because the O···H hydrogen bond length is shortened to 1.834 Å from 2.024 Å of the 1ππ* (W) minimum. Energetically, these two conical intersections are about 28 and 34 kcal/mol lower than their corresponding 1ππ* minima, respectively; thus, they are approachable in view of energy. Mechanistically, the 1ππ*/ gs and 1ππ*/gs (W) conical intersections play important roles for the 1ππ* decay to S0 (see below).

Figure 6. MS-CASPT2//CASSCF computed S1 minimum-energy isomerization paths of p-MMC (left) and p-MMC−W (right) along the C2−C3− C4−C5 dihedral angle. E


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those associated with the C3−C4 bond isomerization; hence, the rotations related to the C2−C3 and C4−C5 bonds become mechanistically unimportant with low excitation energies. Paths to the Dark 1nπ* State. Figure 9 shows MSCASPT2//CASSCF computed linearly interpolated internal coordinate (LIIC) paths connecting the 1ππ* minima and the 1 nπ* minima of the trans p-MMC and p-MMC−W, respectively. For p-MMC, the 1ππ*/1nπ* crossing point is located near point 5, based on which the barrier is estimated to be ca. 2.5 kcal/mol, whereas for p-MMC−W, the crossing sits close to point 6 and the barrier increases to ca. 5.8 kcal/mol. Thus, adding one water near the O atom of the carbonyl group increases the barrier and thus heavily inhibits the population of the dark 1nπ* electronic state from the initial bright 1ππ* state. Our present multireference electronic structure calculations are consistent with recent results of Miyazaki et al.30 Their singlereference TD-PBE0 calculations predict lower barriers for the trans 1ππ*/1nπ* crossing points, 1.1 kcal/mol for p-MMC and 5.0 kcal/mol for p-MMC−W. The excited-state relaxation paths of the cis p-MMC and pMMC−W to the dark 1nπ* state are similar to those of the trans conformers, as shown in Figure 10. The S1 barrier associated with p-MMC−W is higher than that of p-MMC (3.3 vs 1.5 kcal/mol at the MS-CASPT2//CASSCF level). In addition, it is easily found that the barriers of the cis conformers, either p-MMC or p-MMC−W, are relatively lower than the counterparts of the trans ones. Thus, it is expected that, for the cis conformer, it is relatively easier for the 1 ππ* system to populate the dark 1nπ* state. Mechanism. On the basis of the results of our electronic structure calculations, we can summarize the photophysical and photochemical mechanism of the trans and cis p-MMC and pMMC−W in Figure 11. Upon irradiation to the S1 state at the Franck−Condon point of the trans S0 minimum, the system first relaxes quickly to its S1 minimum. Starting from this point, there exists two competitive relaxation pathways. The first one is the out-ofplane trans−cis isomerization pathway around the rotation of the central C2−C3−C4−C5 dihedral angle. When overcoming a small barrier, 4.9 [2.5] kcal/mol for p-MMC [p-MMC−W] at the MS-CASPT2 level, the trans S1 system approaches the S1/ S0 conical intersection. At this funnel, the system hops to the S0 state, further bifurcating into the trans and cis ground-state

minimum-energy S1 isomerization paths (but should be close to these paths). Unlike the S1 photoisomerization, the S0 isomerization reaction has a large barrier. At the B3LYP and CASSCF levels, we have optimized the corresponding transition states of pMMC and p-MMC−W, which are referred to as S0-TS and S0TS (W) in Figures S4 and S5. The associated barriers are predicted to be 57.5 and 57.7 kcal/mol at the MS-CASPT2 level (see Table 2). Table 2. MS-CASPT2 Refined Single-Point Energies (kcal/ mol) of All CASSCF-Optimized Structures in This Work

a

p-MMC

energy

p-MMC−W

energy

S0 (trans) 1 nπ* (trans) 1 ππ* (trans) S0 (cis) 1 nπ* (cis) 1 ππ* (cis) 1 ππ*/gsa S0-TS

0.0 100.8 105.2 5.2 106.1 111.4 77.9/73.0 57.5

S0 (trans) 1 nπ* (trans) 1 ππ* (trans) S0 (cis) 1 nπ* (cis) 1 ππ* (cis) 1 ππ*/gs S0-TS (W)

0.0 106.1 103.1 6.1 110.5 112.2 70.2/66.7 57.7

Two-state single-point energies (S1/S0).

Rotation Paths along the C2−C3 and C4−C5 Bond Rotations. In addition to the C3−C4 bond isomerization, we have computed the S1 minimum-energy reaction paths of pMMC and p-MMC−W with respect to the rotations of the C2−C3 and C4−C5 bonds at the MS-CASPT2//CASSCF level, respectively. When the paths in Figures 7 and 8 are compared, it can be found that the one-water hydration nearly does not alter the barriers of the C2−C3 bond rotation in the S1 state, 13.4 kcal/mol of p-MMC versus 13.7 kcal/mol of pMMC−W. However, the one-water hydration increases a little the S1 barrier of the C4−C5 bond rotation. At the MS-CASPT2 level, it is computed to be 13.2 kcal/mol for p-MMC−W, which is 1.7 kcal/mol higher than that for p-MMC. This differs from the situation of the C3−C4 bond isomerization in which the hydration lowers the S1 barrier and also differs from the situation of the C2−C3 bond rotation where only a negligible change is seen when adding one water. Nevertheless, whether for p-MMC or p-MMC−W, the S1 barriers associated with the C2−C3 and C4−C5 bond rotations are much higher than

Figure 7. MS-CASPT2//CASSCF computed S1 minimum-energy isomerization paths of p-MMC (left) and p-MMC−W (right) along the C1−C2− C3−C4 dihedral angle. F


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Figure 8. MS-CASPT2//CASSCF computed S1 minimum-energy isomerization paths of p-MMC (left) and p-MMC−W (right) along the C3−C4− C5−O7 dihedral angle.

Figure 9. MS-CASPT2//CASSCF computed linearly interpolated internal coordinate (LIIC) paths connecting the 1ππ* minima and the 1nπ* minima of the trans p-MMC (left) and p-MMC−W (right).

Figure 10. MS-CASPT2//CASSCF computed linearly interpolated internal coordinate (LIIC) paths connecting the 1ππ* minima and the 1nπ* minima of the cis p-MMC (left) and p-MMC−W (right).

to that starting from the trans conformers except a little different barriers related to the photoisomerization and stateswitch processes. For example, the S1 cis−trans isomerization barriers are computed at the MS-CASPT2 level to be 1.8 and 1.3 kcal/mol for p-MMC and p-MMC−W, respectively, which are lower than those corresponding to the trans−cis photo-

conformers. The second one is the in-plane state switch from the initially populated 1ππ* state to the dark 1nπ* state. The barriers associated with this process are 2.5 and 5.8 kcal/mol for p-MMC and p-MMC−W, respectively. The photophysical and photochemical mechanism starting from the cis conformers of p-MMC and p-MMC−W is similar G


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Figure 11. Photophysical and photochemical mechanism suggested based on the present high-level electronic structure calculations. Also shown are MS-CASPT2//CASSCF computed single-point energies of p-MMC (p-MMC−W) relative to their most stable trans S0 minima.

water hydration significantly accelerates the 1ππ* nonradiative decay via photoisomerization.29

isomerization (see above). Moreover, the state-switching barriers to the dark 1nπ* state in the vicinity of the cis region are decreased, which are predicted to be 1.5 and 3.3 kcal/mol at the MS-CASPT2 level, lower than 2.5 and 5.8 kcal/mol for the trans conformers. When the related barriers of p-MMC are compared with those of p-MMC−W, it is easy to deduce that adding one water molecule facilitates the photoisomerization relaxation channels of either trans or cis conformers and inhibits the relaxation paths to the dark 1nπ* state. This can be understood very well concerning the changes of the S1 potential energy profiles with respect to the isomerization and state-switch processes. As shown in Figure 11, for either trans or cis conformers, the isomerization barriers of p-MMC−W are lower than those of pMMC, e.g., 2.4 kcal/mol lower for the trans S1 conformer and 0.5 kcal/mol lower for the cis one. By contrast, the corresponding state-switch barriers from the 1ππ* state to the dark 1nπ* state are increased from 2.5 [1.5] kcal/mol of pMMC to 5.8 [3.3] kcal/mol of p-MMC−W in the trans and cis regions, respectively. These results are well-consistent with experiments available. Experimentally, it is observed that one-

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CONCLUSIONS We have computationally studied the photophysical and photochemical mechanism of p-MMC and its one-water complex, for the first time at the multireference CASSCF and MS-CASPT2 levels. At the CASSCF level, we have optimized the minima in the 1ππ*, 1nπ*, and S0 states, the 1ππ*/1nπ* and 1 ππ*/S0 conical intersections, and a variety of the 1ππ* excitedstate relaxation paths. There are two competitive relaxation pathways starting from the initially populated 1ππ* state. The first one is decaying to the S0 state via photoisomerization, and the second one involves the state switching to the dark 1nπ* state via the 1ππ*/1nπ* crossing point. The state-switching path is dominant for either trans or cis p-MMC. Nevertheless, adding one water makes the photoisomerization relaxation path favorable. This change is ascribed to the fact that hydration stabilizes the 1ππ* state and destabilizes the 1nπ* state; as a result, it increases and decreases the barriers related to the 1 ππ*/1nπ* state switches and the 1ππ* photoisomerization, respectively. This is the atomistic origin so that adding one H


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water molecule can significantly enhance the photoprotection efficiency of p-MMC. Our proposed 1ππ* decay mechanism is overall similar to that proposed by Miyazaki et al. at a simpler level (TD-PBE0 and CIS).30 However, our current calculations provide more details. In previous work only the initial stage of trans−cis photoisomerization is explored (from 180° to 140°); cis−trans photoisomerization is not studied. In addition, conical intersections responsible for the 1ππ* → 1nπ* internal conversion are not reported. The present work provides important mechanistic implications for understanding the photophysics and photochemistry of UV filters such as methylcinnamate derivatives.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b08434. Active space of CASSCF and CASPT2 computations, transition state of ground-state isomerization, and Cartesian coordinates of all optimized structures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-010-58806770. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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

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REFERENCES

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Photoprotection Mechanism of p-Methoxy Methylcinnamate: A

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