Xue-Ping Chang

Photoprotection Mechanism of p-Methoxy Methylcinnamate: A CASPT2 Study

p-Methoxy methylcinnamate (p-MMC) shares the same molecular skeleton with octyl methoxycinnamate sunscreen. It is recently found that adding one water to p-MMC 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 second-order 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 (1)ππ*, (1)nπ*, and S0 potential energy surfaces to locate the (1)ππ*/S0 and (1)ππ*/(1)nπ* conical intersections and to map (1)ππ* and (1)nπ* excited-state relaxation paths. On the basis of the results, we find that, for the trans p-MMC, the major (1)ππ* deactivation path is decaying to the dark (1)nπ* state via the in-plane (1)ππ*/(1)nπ* 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 (1)ππ*/S0 conical intersection induced by out-of-plane photoisomerization. For the cis p-MMC, these two decay paths are comparable (1)ππ* deactivation paths: one is decaying to the dark (1)nπ* state via the (1)ππ*/(1)nπ* crossing point, and the second is decaying to the ground state via the (1)ππ*/S0 conical intersection. One-water hydration stabilizes the (1)ππ* state and meanwhile destabilizes the (1)nπ* state. As a consequence, the (1)ππ* deactivation path to the dark (1)nπ* 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 (1)ππ* decay paths are reduced to 2.5 and 1.3 kcal/mol for the trans and cis p-MMC-W. Therefore, the (1)ππ* 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.

Excited-State Proton-Transfer-Induced Trapping Enhances the Fluorescence Emission of a Locked GFP Chromophore

The chemical locking of the central single bond in core chromophores of green fluorescent proteins (GFPs) influences their excited-state behavior in a distinct manner. Experimentally, it significantly enhances the fluorescence quantum yield of GFP chromophores with an ortho-hydroxyl group, while it has almost no effect on the photophysics of GFP chromophores with a para-hydroxyl group. To unravel the underlying physical reasons for this different behavior, we report static electronic structure calculations and nonadiabatic dynamics simulations on excited-state intramolecular proton transfer, cis–trans isomerization, and excited-state deactivation in a locked ortho-substituted GFP model chromophore (o-LHBI). On the basis of our previous and present results, we find that the S1 keto species is responsible for the fluorescence emission of the unlocked o-HBI and the locked o-LHBI species. Chemical locking does not change the parts of the S1 and S0 potential energy surfaces relevant to enol–keto tautomerization; hence, in both chromophores, there is an ultrafast excited-state intramolecular proton transfer that takes only 35 fs on average. However, the locking effectively hinders the S1 keto species from approaching the keto S1/S0 conical intersections so that most of trajectories are trapped in the S1 keto region for the entire 2 ps simulation time. Therefore, the fluorescence quantum yield of o-LHBI is enhanced compared with that of unlocked o-HBI, in which the S1 excited-state decay is efficient and ultrafast. In the case of the para-substituted GFP model chromophores p-HBI and p-LHBI, chemical locking hardly affects their efficient excited-state deactivation via cis–trans isomerization; thus, the fluorescence quantum yields in these chromophores remain very low. The insights gained from the present work may help to guide the design of new GFP chromophores with improved fluorescence emission and brightness.

Mechanistic photodecarboxylation of pyruvic acid: excited-state proton transfer and three-state intersection.

Photodissociation dynamics of pyruvic acid experimentally differs from that of commonly known ketones. We have employed the complete active space self-consistent field and its multi-state second-order perturbation methods to study its photodissociation mechanism in the S0, T1, and S1 states. We have uncovered four nonadiabatic photodecarboxylation paths. (i) The S1 system relaxes via an excited-state intramolecular proton transfer (ESIPT) to a hydrogen-transferred tautomer, near which an S1/S0 conical intersection funnels the S1 to S0 state. Then, some trajectories continue completing the decarboxylation reaction in the S0 state; the remaining trajectories via a reverse hydrogen transfer return to the S0 minimum, from which a thermal decarboxylation reaction occurs. (ii) Due to a small S1 -T1 energy gap and a large S1/T1 spin-orbit coupling, an efficient S1 → T1 intersystem crossing process happens again near this S1/S0 conical intersection. When decaying to T1 state, a direct photodecarboxylation proceeds. (iii) Prior to ESIPT, the S1 system first decays to the T1 state via an S1 → T1 intersystem crossing; then, the T1 system evolves to a hydrogen-transferred tautomer. Therefrom, an adiabatic T1 decarboxylation takes place due to a small barrier of 7.7 kcal/mol. (iv) Besides the aforementioned T1 ESIPT process, there also exists a comparable Norrish type I reaction in the T1 state, which forms the ground-state products of CH3CO and COOH. Finally, we have found that ESIPT plays an important role. It closes the S1-T1 and S1-S0 energy gaps, effecting an S1/T1/S0 three-state intersection region, and mediating nonadiabatic photodecarboxylation reactions of pyruvic acid.

Mechanism for the Nonadiabatic Photooxidation of Benzene to Phenol: Orientation-Dependent Proton-Coupled Electron Transfer

An efficient catalytic one-step conversion of benzene to phenol was achieved recently by selective photooxidation under mild conditions with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as the photocatalyst. Herein, high-level electronic structure calculations in the gas phase and in acetonitrile solution are reported to explore the underlying mechanism. The initially populated (1)ππ* state of DDQ can relax efficiently through a nearby dark (1)nπ* doorway state to the (3)ππ* state of DDQ, which is found to be the precursor state involved in the initial intermolecular electron transfer from benzene to DDQ. The subsequent triplet-state reaction between DDQ radical anions, benzene radical cations, and water is computed to be facile. The formed DDQH and benzene-OH radicals can undergo T1→S0 intersystem crossing and concomitant proton-coupled electron transfer (PCET) to generate the products DDQH2 and phenol. Two of the four considered nonadiabatic pathways involve an orientation-dependent triplet PCET process, followed by intersystem crossing to the ground state (S0). The other two first undergo a nonadiabatic T1→S0 transition to produce a zwitterionic S0 complex, followed by a barrierless proton transfer. The present theoretical study identifies novel types of nonadiabatic PCET processes and provides detailed mechanistic insight into DDQ-catalyzed photooxidation.

Excited-State Decay Paths in Tetraphenylethene Derivatives

The photophysical properties of tetraphenylethene (TPE) compounds may differ widely depending on the substitution pattern, for example, with regard to the fluorescence quantum yield ϕf and the propensity to exhibit aggregation-induced emission (AIE). We report combined electronic structure calculations and nonadiabatic dynamics simulations to study the excited-state decay mechanisms of two TPE derivatives with four methyl substituents, either in the meta position (TPE-4mM, ϕf = 0.1%) or in the ortho position (TPE-4oM, ϕf = 64.3%). In both cases, two excited-state decay pathways may be relevant, namely, photoisomerization around the central ethylenic double bond and photocyclization involving two adjacent phenyl rings. In TPE-4mM, the barrierless S1 cyclization is favored; it is responsible for the ultralow fluorescence quantum yield observed experimentally. In TPE-4oM, both the S1 photocyclization and photoisomerization paths are blocked by non-negligible barriers, and fluorescence is thus feasible. Nonadiabatic dynamics simulations with more than 1000 surface hopping trajectories show ultrafast cyclization upon photoexcitation of TPE-4mM, whereas TPE-4oM remains unreactive during the 1 ps simulations. We discuss the chances for spectroscopic detection of the postulated cyclic photoproduct of TPE-4mM and the relevance of our findings for the AIE process.

QM/MM Study on Mechanistic Photophysics of Alloxazine Chromophore in Aqueous Solution

Compared with isoalloxazine, the core chromophore of biologically important flavins, alloxazine exhibits much lower fluorescence quantum yield and larger intersystem-crossing quantum yield. However, its efficient radiationless relaxation pathways are still elusive. In this work, we have used the QM(MS-CASPT2//CASSCF)/MM method to explore the mechanistic photophysics of alloxazine chromophore in aqueous solution. On the basis of the optimized minima, conical intersections, and crossing points in the lowest (1)ππ*, (1)nπ*, (3)ππ*, and (3)nπ* states, we have proposed three energetically possible nonadiabatic relaxation pathways populating the lowest (3)ππ* triplet state from the initially populated excited (1)ππ* singlet state. The first is the direct (1)ππ*→ (3)ππ* intersystem crossing via the (1)ππ*/(3)ππ* crossing point. The second is an indirect (1)ππ* → (3)ππ* intersystem crossing relayed by the dark (1)nπ* singlet state. In this route, the (1)ππ* system first decays to the (1)nπ* state via the (1)ππ*/(1)nπ* conical intersection, followed by an (1)nπ*→ (3)ππ* intersystem crossing at the (1)nπ*/(3)ππ* crossing point to arrive at the final (3)ππ* state. The third is similar to the second one; but its intersystem crossing is relayed by the (3)nπ* triplet state. The (1)ππ* system first decays to the (3)nπ* state via the (1)ππ*/(3)nπ* crossing point; the generated (3)nπ* state is then de-excited to the (3)ππ* state through the (3)nπ*→ (3)ππ* internal conversion at the (3)nπ*/(3)ππ* conical intersection. According to the classical El-Sayed rule, we suggest the second and third paths play a much more important role than the first one in the formation of the lowest (3)ππ* state.

Quantum Mechanics/Molecular Mechanics Study on the Photoreactions of Dark‐ and Light‐Adapted States of a Blue‐Light YtvA LOV Photoreceptor

The dark- and light-adapted states of YtvA LOV domains exhibit distinct excited-state behavior. We have employed high-level QM(MS-CASPT2)/MM calculations to study the photochemical reactions of the dark- and light-adapted states. The photoreaction from the dark-adapted state starts with an S1 →T1 intersystem crossing followed by a triplet-state hydrogen transfer from the thiol to the flavin moiety that produces a diradical intermediate, and a subsequent internal conversion that triggers a barrierless C-S bond formation in the S0 state. The energy profiles for these transformations are different for the four conformers of the dark-adapted state considered. The photochemistry of the light-adapted state does not involve the triplet state: photoexcitation to the S1 state triggers C-S bond cleavage followed by recombination in the S0 state; both these processes are essentially barrierless and thus ultrafast. The present work offers new mechanistic insights into the photoresponse of flavin-containing blue-light photoreceptors.