Computational analysis of photoisomerization of unsubstituted spirooxazine by TD-DFT: solvent effect and functional choice

The effects of hybrid functionals and solvents (nonpolar, polar aprotic, and polar protic) on the results of calculating the photochemical transformations of unsubstituted spirooxazine (TMINSO) in solution were analyzed. A preliminary selection of functionals showed that the predominant S₀→S₁ transition observed in the experiment for the closed form gives BMK, CAM-B3LYP, LC-ωHPBE, M052X, M062X, M08HX, M11, MN15, SOGGA11X, ωB97, ωB97X, and ωB97XD functionals in each of the three solvents considered (cyclohexane, acetonitrile, methanol). The functionals that did not break the C_spiro-O bond upon TMINSO excitation (LC-ωHPBE, M11, and SOGGA11X in all solvents considered, and ωB97 in the methanol) nonetheless weakened it. Most of the functionals that provided the photoinduced breaking of the C_spiro-O bond in all three solvents under consideration gave excited intermediate X* forms in which the indoline and naphthoxazine units are approximately perpendicular to each other. The exceptions were M052X and ωB97XD in both polar solvents. The outcome of X*→X relaxation is determined by the X* conformation, which, in turn, is the interplay of the used functional and the solvent. For intense photobleaching in a nonpolar solvent, the excitation of the planar merocyanine form must be accompanied by its twisting into the X* form, causing the C_spiro and O atoms to approach each other, which, in turn, makes it possible to recover the C_spiro-O bond. In polar solvents, TMINSO photobleaching, on the contrary, is weak, which corresponds to the preservation of the planar excited open structure. Therefore, functionals giving both of these effects (M052X and ωB97XD) are recommended for modeling the phototransformations of spirooxazine in polar and nonpolar solvents. The possibility of recovering the C_spiro-O bond does not depend directly on the distance between these atoms but on the electron densities on them and the conformation of the linker connecting the aromatic systems in the X form. Three solvating methanol molecules for most of the used functionals stabilize the closed form of TMINSO. Preservation of the planar MC structure during its excitation in the composition of the solvated complex gives only the M052X functional, so it is recommended for modeling the excitation of spirooxazine in methanol. The effect of excitation on the H-bonds of spirooxazine with methanol in the solvated complex was also analyzed.