Radiochromism of spiropyran via the radical ions studied by pulsed electron radiolysis and DFT calculation

Spiropyrans and spirooxazines (SP) are known to undergo photochromism to form merocyanines (MC). In this study, upon pulsed electron radiolysis of SP in organic solvents, we observed the transient absorption spectra of SP radical ions in the visible wavelength region. Based on first-order kinetics in the microsecond time domain, the absorption spectra changed to different ones, which were plausibly due to the radical ions of the corresponding MC. The reaction rates from SP radical ions to MC ones were determined. Using heats of formation for the radical ions of SP and MC evaluated with DFT calculations, the differences in heats of formation between the radical ions of SP and MC were determined to be exothermic. The reaction rate correlated with the difference in heat of formation, which depended on the parity of the radical ions. Time-dependent DFT calculations provided the SOMO surfaces of the radical ions of SP and MC. Comparing the SOMO with the LUMO, HOMO, and HOMO-1 surfaces of the neutral SP, we found that the attached electron locates on the LUMO upon one-electron reduction in SP, whereas the HOMO-1 of SP is involved in one-electron oxidation of SP rather than the HOMO.