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Sep 22, 2016 - ... pathways through the T2 state in the time scale of 100 ps: (a) stepwise ISC followed after the internal conversion (IC) from S1 to ...
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Multistep Intersystem Crossing Pathways in Cinnamate-Based UV‑B Sunscreens Kaoru Yamazaki,† Yasunori Miyazaki,‡ Yu Harabuchi,† Tetsuya Taketsugu,† Satoshi Maeda,*,† Yoshiya Inokuchi,‡ Shin-nosuke Kinoshita,‡ Masataka Sumida,‡ Yuuki Onitsuka,‡ Hiroshi Kohguchi,‡ Masahiro Ehara,*,§,∥ and Takayuki Ebata*,‡ †

Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan § Institute for Molecular Science and Research Center for Computational Science, 38 Myodaiji, Okazaki 444-8585, Japan ∥ Elements Strategy for Catalysts and Batteries, Kyoto University Katsura, Kyoto 615-8520, Japan ‡

S Supporting Information *

ABSTRACT: The nonradiative decay pathways of jet-cooled para-methoxy methylcinnamate (p-MMC) and para-methoxy ethylcinnamate (p-MEC) have been investigated by picosecond pump−probe and nanosecond UV-Deep UV pump−probe spectroscopy. The possible relaxation pathways were calculated by the (time-dependent) density functional theory. We found that p-MMC and p-MEC at low excess energy undergo multistep intersystem crossing (ISC) from the bright S1 (1ππ*) state to the lowest triplet T1 (3ππ*) state via two competing pathways through the T2 state in the time scale of 100 ps: (a) stepwise ISC followed after the internal conversion (IC) from S1 to the dark 1nπ* state; (b) direct ISC from the S1 to T2 states. These picosecond multistep ISCs result in the torsion of CC double bond by ∼95° in the T1 state, whose measured adiabatic energy and lifetime are 16577 cm−1 and ∼20 ns, respectively, for p-MMC. These results suggest that the ISC processes play an indispensable role in the photoprotecting sunscreens in natural plants.

P

ara-methoxy methylcinnamate (p-MMC) and para-methoxy ethylcinnamate (p-MEC) shown in Scheme 1 are

not only to reveal their role in nature, but also to develop new UV absorbing compounds for sun screening cosmetics and medicines. Thus, the mechanism of NRD and photoisomerization of cinnamate derivatives has been the subject of many experimentalists and theoreticians.1−8,11−22 Three possible pathways have been proposed for NRD of the S1 state: the trans → cis isomerization via S1 (1ππ*)/S0 minimum energy conical intersection (MECI),1,16 internal conversion (IC) to the 1nπ* state,13−19 and intersystem crossing (ISC) to the lowest triplet excited (T1) state20 via minimum energy seam of crossings (MESXs). Several experimental groups have recently been investigating the NRD mechanisms of p-MMC and its derivatives: A matrixisolation infrared spectroscopic study at 6 K reported by Miyazaki et al. suggested that trans → cis isomerization via S1 (1ππ*)/S0 MECI takes place only when the wavelength of the UV light for the 1ππ* excitation is shorter than 300 nm.16 By using picosecond UV−UV pump−probe spectroscopy under the jet-cooled condition, they also reported that the S1 state of the trans-p-MMC decays with the lifetime of 80−280 ps and exhibits strong dependencies on the conformation and excess energy.13 Tan et al. found some dark state for trans-p-MMC14

Scheme 1. Schematic Molecular Structure of the trans-pMMC and trans-p-MEC

known to exhibit an efficient nonradiative decay (NRD) involving trans → cis isomerization.1 Such photoinduced trans → cis isomerization of cinnamate derivatives is widely used in nature. For example, photoactive yellow protein (PYP) existing in Halorhodospira halophila has coumaric acid as a chromophore. By absorbing blue right, coumaric acid isomerizes from trans to cis, which is an initial step of the negative phototaxis.2−7 Lignin in the plant skin is synthesized by the polymerization of cinnamate derivatives, such as p-coumaryl, coniferyl, and sinapyl alcohol. Lignin works as a UV filter for protecting photodegradation of DNA. 8−11 In our life, cinnamate derivatives are good candidates of ultraviolet (UV) screening to reduce the skin damage from sunlight.11,12 Understanding the NRD mechanism of the cinnamate derivatives is important © 2016 American Chemical Society

Received: July 25, 2016 Accepted: September 22, 2016 Published: September 22, 2016 4001

DOI: 10.1021/acs.jpclett.6b01643 J. Phys. Chem. Lett. 2016, 7, 4001−4007

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The Journal of Physical Chemistry Letters

Figure 1. (a) (upper) S1−S0 R2PI spectrum of p-MEC cooled in a supersonic beam. (lower) UV−UV hole-burning spectra by fixing the probe laser at bands a−d. The stick bars indicate the vertical transition energies of the four conformers, which are obtained at the TD- PBE0/cc-PVZ level of theory. The frequencies are scaled by a factor of 0.9448 to fit the position of the sTC(g) conformer. The labelings sTC(g) and sTC(p) mean syn/ trans/s-cis(gauche) and syn/trans/s-cis(planar), respectively. (b) Decay rate constants of p-MEC (square) and p-MMC (circle) vs UV excitation energy.

and trans-p-hydroxy methylcinnamte15 by using a nanosecond pump−probe scheme with 193 nm probe laser, and proposed the dark state would be the 1nπ* state.14,15 However, the energy level as well as character of the dark state has not been revealed yet up to now. What is the nature of the dark state? In this paper, we focus on the assignment of the dark state observed by Tan et al., and the mechanism of the whole NRD pathways for p-MMC and p-MEC. We first discuss what kind of factor controls the NRD rate of the S1 state by comparing the S1 lifetimes of p-MMC and p-MEC. We next proceed to the assignment of the dark state: We first determine the energy level of the dark state via the ionization threshold of the dark state and the ionization potential (IP0) measured by nanosecond UV- tunable deep-UV (DUV) pump−probe spectroscopy and picosecond pump−probe scheme via the S1 state, respectively. Nanosecond UV- tunable DUV pump−probe spectroscopy is newly developed in the present study to determine the energy level of the dark state. The observation of the dark state of gas phase molecules has been carried out by the measurement of photoelectrons combined with ultrafast UV-vacuum UV (VUV) pump−probe study by Suzuki and coworkers.23−25 However, the molecules investigated so far are limited to small molecules. In the present study, instead of observing the photoelectrons with wavelength-fixed ionization laser light, we observe the threshold of the ionization from the dark state by using a tunable DUV ionization laser light. This method provides us with the information on the dark state similar to that obtained by photoelectron spectroscopy. We then discuss the character of the dark state by considering the possible ionization scheme from the dark state with the help of the adiabatic energy level calculations of the D0 and D1 states at the symmetry-adapted cluster-configuration interaction (SACCI) level of theory. We finally propose the whole NRD pathways based on the results of extensive exploration of the MECIs and MESXs at the (TD-)B3LYP/6-311G(d) level of time-dependent density functional theory (TDDFT).

First, the spectral feature of the S1 excited state of p-MEC is described. The upper panel of Figure 1a shows the resonance enhanced two-photon ionization (R2PI) spectrum of the S1−S0 electronic transition of p-MEC in a supersonic beam. The UV− UV hole-burning (HB) spectra at the lower panel indicate that four different species, corresponding to bands a−d, contribute to the electronic spectrum. We assigned them to (a) sTC(g), (b) sTC(p), (c) aTC(g), and (d) aTC(p) by the comparison of the vertical transition energies at the TD-PBE0/cc-pVDZ level of theory26,27 shown as a stick diagram at the lower panel of Figure 1(a). Here, the labeling of the conformers such as sTC(g) and sTC(p) means syn/trans/s-cis (gauche) and syn/ trans/s-cis(planar), respectively. Syn and anti represent that the −OCH3 group is oriented either in the same side or opposite side of the substituent at para-position, respectively. T in the middle indicates the trans form along the CC double bond. S-cis and s-trans discriminate the relative orientation of the C O group and CC double bond along the C−C single bond. Finally, planar and gauche indicate that C2H5 is located in either the molecular plane or out-of-plane (gauche) position. The optimized structures, S0 energies, and S1−S0 vertical transition energies of the all conformers considered are shown in Figure S3 in Supporting Information (SI). The assigned four conformers are lying within the energy of 130 cm−1 in the S0 state. The calculated vertical transition energies (with frequencies scaled by a factor of 0.9448) show good agreement with the positions of the observed band origins as shown in Figure 1a. The ethyl-ester group is very flexible so that the ethyl group is tilted almost 90 deg out-of-plane in the aTC (g) and sTC(g) conformers (not Cs point group). Figure 1b shows the decay rate constants (inverse of lifetime) of the vibronic bands of p-MEC versus the excitation energy measured by picosecond pump−probe spectroscopy. Decay profiles of the vibronic bands are shown in Figure S4 in the SI, which were fitted by the convolution assuming single exponential decay function with laser pulse width (12 ps). 4002

DOI: 10.1021/acs.jpclett.6b01643 J. Phys. Chem. Lett. 2016, 7, 4001−4007

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Figure 2. (a) Ionization efficiency curve by nanosecond UV−tunable DUV spectroscopy with ν1 fixed to S1(0,0) at 32667 cm−1. (b) Picosecond pump−probe ionization curve of p-MMC from S1(0,0) of the s-cis/anti conformer at 32667 cm−1. The ionization threshold (IP0) is obtained to be 63427 cm−1. (c) Case 1: The energy levels diagram of p-MMC by assuming the DUV ionization from the pure 1nπ* state to the D1 state of ion. Case 2: The energy levels diagram by assuming the dark state has the ππ* character and the state is ionized to the D0 state.

observed decay rate constants of the 0,0 bands between p-MEC (sTC(g)) and p-MMC (sTC) is 4, which qualitatively agrees with that predicted by the density of states. In any case, both pictures can interpret the faster IC decay of p-MEC. IC therefore is the main NRD process governing the decay of S1. We then investigate the dark state of p-MMC and p-MEC generated after the excitation to the bright S1 state. As reported by Tan et al.,15 the UV energy to ionize the S1 state is not high enough to ionize the dark state, and we need DUV light. Figure 2a shows the ionization efficiency curves of p-MMC obtained by nanosecond UV−tunable DUV pump−probe spectroscopy. Here, ν1 is fixed to the (0,0) band of the aTC conformer of pMMC at 32667 cm−1, and tunable DUV light (ν2), introduced at the delay time of 10 ns after ν1, is scanned with an interval of 250 cm−1 (1.0 nm). Because the lifetime of the S1 state is shorter than 280 ps, ν2 is not able to ionize molecules in the S1 state under this condition. Thus, the ion signal is attributed to the ionization from the dark state, and the observed state is essentially the same as the one reported by Tan et al. by using 193 nm for ionization laser, although they did not determine the ionization threshold. The enhancement of the ion signal starts at ν2 = 46850 cm−1, and gradually increases with the energy. We determine the energy 46850 cm−1 as the ionization threshold from the dark state. Figure 2b shows the picosecond pump−probe ionization signal from the S1 zero-point level of aTC conformer at 32667 cm−1. The threshold appears at 30760 cm−1 giving the ionization potential IP0 = 63427 cm−1. We determine the energy of the dark state from the obtained IP0 and the ionization threshold from the dark state. To assign the dark state and to evaluate its energy level, we must discuss the character of the dark state. If we assume the dark state has a pure 1nπ* character, the ionic state should be the D1 state due to dipole transition. Since the D1 energy is not experimentally determined, we estimated the adiabatic D1 energy to be 14550 cm−1 by SAC-CI28/cc-pVDZ calculation in Level Three accuracy. The SAC-CI calculation is well established for the

Also shown in Figure 1b are the plots of the decay rate constants of p-MMC for comparison.5 As seen in the figure, the S1 state of p-MEC decays much faster than p-MMC, and its excess energy dependence is more prominent. The relative energies of the electronic states are expected to be similar between p-MMC and p-MEC. In fact, the potential energy curves along the CC double bond in the S1 state at the CIS(D)/aug-cc-pVDZ level of theory, and the S1/1nπ* potential curve crossing by linear interpolation internal coordinate (LIIC) plot at the TD-PBE0/cc-pVDZ level of theory (Figure S5 in the SI) exhibit almost the same energy barriers between p-MMC and p-MEC. This means that the difference of the decay constants between them must be attributed to the difference in the magnitude of matrix element of the NRD process and/or the density of states of the dark state. In the former case, the ethyl-ester group in p-MEC is flexible so that it is easily directed out of plane, but the methylester group in p-MMC stays in plane. Such situation stimulates p-MEC to undergo faster S1 (1ππ*) → 1nπ* internal conversion (IC). For planar p-MMC, it is essential to promote vibrations of the out-of-plane vibrational mode(s) to go through conical intersection (CI) since the symmetry of S1 is A′ and that of 1 nπ* is A″ in the Cs point group. On the other hand, S1 and 1 nπ* belong to the same representation at C1 point group, so that the S1 → 1nπ* IC will be more efficient than p-MMC. In the latter case, p-MEC has lower frequency vibrations than pMMC, so that the density of states ρ of the 1nπ* of p-MEC at the crossing point will be higher than p-MMC. Since the decay rate constant is the product of the coupling matrix element and ρ, the IC decay rate constant of p-MEC will be larger than pMMC assuming equal coupling matrix element. We calculated the density of states of the 1nπ* state for p-MEC and p-MMC at the S1/1nπ* crossing point, which is located at ∼2000 cm−1 from the lowest energy level of 1nπ*. The calculated results show that the density of states of p-MEC is about 8 times larger than that of p-MMC at the crossing point. The ratio of the 4003

DOI: 10.1021/acs.jpclett.6b01643 J. Phys. Chem. Lett. 2016, 7, 4001−4007

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Figure 3. Ionization efficiency curves and time profiles of the dark state of (a) p-MMC and (b) p-MEC monitored at several DUV frequencies. The delay time corresponds to the interval between the UV laser (ν1) and DUV laser (ν2). The red circles are observed pump−probe ion signals and the black solid curves are the deconvoluted decay curves fitted by a single exponential decay with the laser pulse width of 6 ns.

the ionization efficiency curve is due to the large difference of the structure between the dark state and the ionic state. Even if we take into account the hot band transitions of the ionization efficiency curve, it is clear that the energy 16577 cm−1 is too low to assign this dark state to the 1nπ* but could be assigned to the T1 (3ππ*) state. A similar result is obtained for p-MEC, and the energy of the dark state is determined to be 17024 cm−1 (See Figure S6 in the SI). Both p-MMC and p-MEC experiments thus indicate that the dark state observed in the nanosecond time scale is the T1 (3ππ*) state. The energy of T1 (3ππ*) state of these species in the gas phase has not yet been reported, but the obtained values in this work are in relatively good agreement with the reported value for methyl cinnamate (MC) by Herkstroether et al., 54.8 kcal/mol (19200 cm−1), in solution.20 They also measured the T1 lifetime of MC to be 10.3 ns. The time profiles of the dark state of (a) p-MMC and (b) pMEC are shown in Figure 3. Tan et al. reported the lifetime to be 24 ns by using 193 nm (51800 cm−1) for ionization.14 In the present study, we observed time profile at three different DUV frequencies. The decay profiles at different frequencies are very similar and can be fitted with a single exponential decay curves with the lifetime of 20−25 ns for p-MMC and 27 ns for pMEC. The single exponential decay means that this triplet state is populated in very short time scale from the S1 state. All these

energy level calculations of the ionization states. The benchmark calculations with TDDFT for the singlet and triplet transitions have been recently done.29,30 Case 1 of Figure 2c shows the energy level diagram of pMMC if the dark state has a pure 1nπ* character and the ionization occurs to the D1 state. The 1nπ* state is located at 1640 cm−1 below S1. The TD-PBE0 calculation reported by Miyazaki et al.13 also suggest that the difference of the adiabatic energy between the S1 state and the 1nπ* state is 0.2 eV (∼1600 cm−1). However, it is quite possible that the 1nπ* state and the S1(ππ*) state strongly mix with each other. In fact, Zwier and co-workers suggested that the strong S1 (ππ*)/1nπ* mixing causes a band broadening of the S1−S0 absorption spectra of sinapoyl malate derivatives even under supersonically jet-cooled gas phase condition.11 Following this argument, 1nπ* should have a ππ* character, and the ionization occurs to the D0 state. Case 2 of Figure 2c shows the energy level diagram when the observed ionization from the dark state is the D0 state. The energy of the dark state from the S0 state becomes 16577 cm−1. The dark state(s) is thought to have large internal energy after NRD from S1, so that the observed ionization efficiency curve involves many hot bands, resulting in the gradual increase at the threshold as is observed in Figure 2. This will cause uncertainty of the energy level. However, as will be discussed later, we found that the major reason for the shape of 4004

DOI: 10.1021/acs.jpclett.6b01643 J. Phys. Chem. Lett. 2016, 7, 4001−4007

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Figure 4. Potential energy profile of the ISC pathways of p-MMC from the S1 (1ππ*) to T1 state calculated at the (TD-)B3LYP/6-311G(d) level of theory. The characters of the electronic states are written in square parentheses for EQs. The values with zero-point energy corrections are also written in parentheses for EQs and TSs.

Condon active in the T1−D0 ionization efficiency curve with very weak (0,0) band, and therefore explains the observed gradual increase of the ionization efficiency curve from the ionization threshold (Figures 2 and S6). We further investigated the NRD pathways of p-MMC and p-MEC with low excess energy conditions by the TD-B3LYP/ 6-311G(d) level of theory32,33 utilizing the artificial force induced reaction method.34,35 The NRD pathways were searched with the developer version of the GRRM program35 interfaced with the Gaussian 09 quantum chemistry package.36 It is found that the NRD process mostly undergoes two competing ISC pathways via the MESXs 1 (1nπ*/T2) and 2 (1ππ*/T2) as shown in Figures 4 and S7 (SI) for p-MMC and p-MEC, respectively. We focus on p-MMC since the potential energy profiles of the p-MMC and p-MEC are almost the same (Figures 4 and S7 (SI), and Table S1 (SI)). The major channel is the stepwise ISC pathway through the nonplanar MESX 1 of 1nπ*/T2 (1nπ*/3ππ*) at 30336 cm−1 following after IC from the 1ππ* state to the 1nπ* state. The IC occurs through the nonplanar transition state TS 1 at 32341 cm−1, which corresponds to an avoided crossing between these two states. This IC is a barrier-less process since the zero-point energy corrected value of the potential energy height at the TS 1 is lower than that of EQ 1 (S1 (1ππ*)). We also found a nearly planar MECI 1 (1ππ*/1nπ*) at 32641 cm−1, but the IC via the MECI 1 is energetically less favorable than that via the TS 1 by 300 cm−1. The energy barrier of the TS 1 in p-MMC is higher than that in p-MEC by 19 cm−1. This makes the IC process of p-MMC slower than that in p-MEC. The IC via the TS 1 moves the OMe group out of molecular plane and results in the ISC at the MESX 1 (1nπ*/T2), where φ(OCOC) becomes ±80°. We roughly estimated the lifetime of the ISC, τISC, via the MESX 1 by means of the Fermi’s golden rule37 using the spin−orbit coupling constant (SOC) between the crossing singlet (in this case, the 1nπ* state) and T2 states at

results indicate that p-MMC and p-MEC excited to the S1 (ππ*) state relaxes to the T1 state shorter than 1 ns by IC via the 1nπ* state followed by ISC. The rapid production of T1(3ππ*) state from 1ππ* state is also suggested for thymine and uracil by de Vries and co-workers.31 The observed dark state has a lifetime of 50−70 ns for uracil and 220−300 ns for thymine. They also reported that the NH vibrational frequencies of the IR spectra of this state agree with the calculated one of T1 (3ππ*). To confirm that the final dark state is the T1 state, we searched the stable conformers in the T1 state by the UB3LYP/ 6-311G(d)32,33 method. For p-MMC, we found that the CC twisted conformers EQ 6 and EQ 7 shown in Figure 4 are more stable than planar EQ 5 by ∼500 cm−1. The calculated values of the (0,0) transition energy from the trans conformer in the S0 state is 18034 cm−1 for EQ 6 and 17983 cm−1 for EQ 7, both of which are in agreement with the experimental value of 16577 cm−1. The computed energy difference between EQ 6 and EQ 7 is only 51 cm−1. There is hence an equilibrium between these two conformers, and they coexist with the ratio of EQ 6:EQ 7 ≈ 4:5 assuming the Boltzmann distribution at 600 K, which corresponds to the excess energy in the T1 state. For p-MEC, we obtained the qualitatively the same twisted conformers in the T1 state. The (0,0) transition energies of the conformers are 18009 and 17947 cm−1, both of which are consistent with the experimental value of 17024 cm−1. We also calculated EQs of the D0 state at the UB3LYP/6311G(d) level of theory, and found that EQs 8 and 9 of D0 become planar forms as shown in Figure S8 in SI. Thus, the vertical ionization from the twisted EQs 6 and 7 of T1 induces the structural relaxation to the planar equilibrium structures EQs 8 and 9 in the D0 state, respectively. The energy difference between the vertical position of EQs 6 and 7 and zero-point level of the planar D0 is ∼10 000 cm−1. This large structural difference causes the twisting mode to be highly Franck− 4005

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The Journal of Physical Chemistry Letters the MESX 1. The SOCs were evaluated at the TD-B3LYP/6311G(d) level of quadratic response theory38,39 using the DALTON 2015.1 quantum chemistry package.38 The resultant lifetime is 51 ps based on the norm of the SOC (|SOC|) of 42.7 cm−1, which is consistent with the experimental results that the ISC processes complete within 1 ns. After ISC via the MESX 1, the character of the T2 state switches from 3ππ* to 3nπ* by passing through a 3ππ*/3nπ* avoided crossing on the steepest decent relaxation pathway to the EQ 4 (T2 (3nπ*)) at 27986 cm−1, whose dihedral angle around the ester group φ(OCOC) = ± 76° (Figure 4). The minor channel is the direct ISC pathway from the 1ππ* to T2 state via the planar MESX 2 (1ππ*/T2, 1ππ*/3nπ*) at the potential energy height of 32285 cm−1, which is almost as high as that of EQ 1 (S1 (1ππ*), 32284 cm−1). The lifetime τISC via the MESX 2 is estimated to be 921 ps (|SOC| = 14.2 cm−1), which is longer than the experimental lifetime of the 1ππ* state (80 ps for the corresponding conformer13). p-MMC subsequently relaxes to the EQ 4 with its OMe group moving out of the molecular plane. These stepwise and direct ISC pathways are unified at the EQ 4, and p-MMC further relaxes to the twisted EQs 6 and 7 of the T1 (3ππ*) state via the nonplanar MECI 2 (T2/T1, 3nπ*/ 3 ππ*) MECI (φ(OCOC) = ± 83°) and planar EQ 5 (T1). The trans → cis isomerization via the S1/S0 MECI is not energetically accessible for the present low excess energy condition1,16,18,19 both in p-MMC and p-MEC as discussed in the SI. The major NRD pathway is therefore the stepwise ISC to the dark T1 state through 1ππ* → 1nπ* IC both in p-MMC and p-MEC. In summary, the nonradiative decay pathways of jet-cooled para-methoxy methylcinnamate (p-MMC) and para-methoxy ethylcinnamate (p-MEC) was studied by picosecond pump− probe and nanosecond UV-DUV pump−probe spectroscopy. The possible relaxation pathways were calculated by the (timedependent) density functional theory. We found that p-MMC and p-MEC at low excess energy undergoes multistep intersystem crossing (ISC) from the bright S1 (1ππ*) state to the lowest triplet T1 (3ππ*) state via two competing pathways through the T2 state in the time scale of 100 ps: (a) Stepwise ISC followed by the internal conversion from the S1 state to the dark 1nπ* state; (b) Direct ISC from the S1 state to the T2 state. These picosecond multistep ISCs result in the torsion of CC double bond by ∼95° in the T1 state, whose measured adiabatic energy and lifetime are 16577 cm−1 and ∼20 ns, respectively for p-MMC. These results suggest that the ISC processes play an indispensable role on the photoprotecting sunscreens in natural plants.

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p-MEC at the (TD-)B3LYP/6-311G(d) level of theory (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(S.M.) [email protected]. *(M.E.) [email protected]. *(T.E.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.E., Y.I., M.E., and H.K. acknowledge Grants-in-Aid for Scientific Research (KAKENHI) by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Nos. 25410017, 16H04098, 16H04104, and 15KT0065, respectively. S.M., T.T., K.Y., and Y.H. acknowledge a grant from the Japan Science and Technology Agency with Core Research for Evolutional Science and Technology (CREST) in the Area of “Establishment of Molecular Technology towards the Creation of New Functions” at Hokkaido University. This work is also partially supported by the Joint Studies Program (2014-2016) of IMS. Part of the calculations in this paper was carried out by using the supercomputers at Academic Center for Computing and Media Studies, Kyoto University and Okazaki Research Facilities (Research Center for Computational Science).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01643. Experimental and computational details, table about the relative potential energy of the selected electronic excited states, supplemental figures, potential energy profile, nonradiative decay pathways of p-MEC, additional discussions about structural relaxation triggered by the ionization from the T1 state to D0 state and the trans → cis isomerization via the S1/S0 MECI, and the XYZ coordinates of the optimized structures of p-MMC and 4006

DOI: 10.1021/acs.jpclett.6b01643 J. Phys. Chem. Lett. 2016, 7, 4001−4007

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DOI: 10.1021/acs.jpclett.6b01643 J. Phys. Chem. Lett. 2016, 7, 4001−4007