Theoretical Insights into the Ultrafast Deactivation Mechanism and Photostability of a Natural Sunscreen System: Mycosporine Glycine

Abstract

In this work, different levels of quantum computational models such as MP2, ADC(2), CASSCF/CASPT2, and DFT/TD-DFT have been employed to investigate the photophysics and photostability of a mycosporine system, mycosporine glycine (MyG). First of all, a molecular mechanics approach based on the Monte Carlo conformational search has been employed to investigate the possible geometry structures of MyG. Then, comprehensive studies on the electronic excited states and deactivation mechanism have been conducted on the most stable conformer. The first optically bright electronic transition responsible for the UV absorption of MyG has been assigned as the S₂ (¹ππ*) owing to the large oscillator strength (0.450). The first excited electronic state (S₁) has been assigned as an optically dark (¹nπ*) state. From the nonadiabatic dynamics simulation model, we propose that the initial population in the S₂ (¹ππ*) state transfers to the S₁ state in under 100 fs, through an S₂/S₁ conical intersection (CI). The barrierless S₁ potential energy curves then drive the excited system to the S₁/S₀ CI. This latter CI provides a significant route for ultrafast deactivation of the system to the ground state via internal conversion.

Figure 1

Chemical structure and numbering pattern of MyG applied in this work. Hydrogen atoms over the ring and other C atoms have been omitted for clarity.

Figure 2

Optimized structures of the 14 most stable structures of MyG at the B3LYP/cc-pVDZ level of theory (the energetic values have been corrected by ZPE). The most stable structure (MyG-A) is shown in the center with other structures labeled according to decreasing stability. Their energies (in kJ mol^–1) are relative to MyG-A.

Figure 3

Selected valance MOs of MyG-A, determined at the SCF/cc-pVDZ level of theory, playing a prominent role in the lowest lying electronic transitions.

Figure 4

Optimized geometries of (a) S₀ ground state and (b) S₁ (¹nπ*) state determined, respectively, at the MP2 and ADC(2) theoretical levels.

Figure 5

Optimized geometries of the CIs located for MyG-A: (a) CI₁ (S₂/S₁) and (b) CI₂ (S₁/S₀) determined at the SA-CASSCF(6,6)/cc-pVDZ theoretical level. (c) PE profile of the ground (black) and 2 singlet excited states of MyG-A calculated at the ADC(2)/cc-pVDZ level of theory along the LIIC reaction path.

Figure 6

(a) UV absorption spectrum simulated at the TD-ωB97XD/6-31G* method based on the S₂ ← S₀ electronic transition in the PCM/ethanol implicit solvent with 300 points for MyG-A. (b) Energy profiles of a selected trajectory for MyG-A. The black, red, and blue curves, respectively, indicate the ground (S₀), S₁, and S₂ excited states. The inset in panel a represents the geometry of S₁/S₀ CI (out-of-plane movement of glycine as the prominent alteration).