New insight to the catechol photochemistry: The role of different monomer and dimer configurations in radiationless decay of S1 electronic excited state

The equilibrium geometries of the ground and first electronic excited states as well as the radiationless deactivation channels of catechol in its monomer and dimer configuration have been investigated using the standard linear-response and the spin-flipped TDDFT methods as well as by the similarity transformed equation-of-motion coupled cluster built with the domain-based local pair natural orbitals (DLPNO-STEOM-CCSD). For the monomer, it was found that there is a new conical intersection geometry that can explain why catechol exhibits different photochemical behavior. This deactivation pathway involves almost simultaneously, an excited state intramolecular proton transfer between the two O atoms and an O–H bond breaks at the proton that is not between the two O atoms. From the energy balance point of view, these geometries are not associated with high potential barriers, so radiationless relaxation can be achieved through these geometries. For cyclohexane solvent, the lowest CI geometry shows a potential barrier with about 4 kcal/mol lower than that found for acetonitrile, making even more easier the relaxation. In the case of catechol dimer structures, it was found several so-called dimer-type CI geometries where both monomers exhibit substantial geometric distortions together with the formation of a weaker C–C bond between the two catechol monomers. These CI geometries are energetically more favorable and, in the case of aggregation processes, more likely to decay the excited states of the catechol through these radiationless deactivation channels.