Theoretical reconsideration of the ESPT process of the pheomelanin building block in methanol

Abstract

Pheomelanin is responsible for enhancing UV sensitivity and its proton transfer (PT) process is crucial to reveal the complex mechanisms of pheomelanin phototoxicity. Nahhas et al. studied the PT process of the pheomelanin building block 6-(2-amino-2-carboxyethyl)-4-hydroxy-1,3-benzothiazole (BT) in methanol. Based on the small Stokes shift corresponding to the absorption and fluorescence peaks, it is deemed that BT cannot perform PT process in methanol [J. Phys. Chem. Lett. 5 (2014) 2094–2100]. However, protic methanol solvent has both oxygen atom that attract proton and hydrogen atom that supply proton. It is worth reconsidering about the intermolecular proton transfer (inter-PT) process of BT with the aid of methanol. In our work, the inter-PT process of BT and methanol molecules are deciphered by density functional theory and time-dependent density functional theory. The structures, non-covalent interactions and infrared spectra testify that the double hydrogen bonds are reinforced under photoexcitation. The potential energy curve of S1 state without barrier attests that the inter-PT process can occur in the BT molecule where methanol acts as the proton transporter. In addition, the fluorescence of BT-methanol complex is emitted by enol* form. In the light of the frontier molecular orbitals, the non-fluorescence of keto* form is explained by the reduced charge coupling compared to the enol* form and the competition of the twisted intramolecular charge transfer state. Our results provide an insight for understanding the photochemical process of the pheomelanin in protic systems.