Photochemical dynamics of indolylmaleimide derivatives

On-the-fly nonadiabatic ab initio molecular dynamics simulations have been carried out for three anionic species of indolylmaleimides (3-(1H-3-indolyl)-2,5-dihydro-1H-2,5-pyrroledione, IM) to clarify the mechanisms of photochemical reactions. The results are obtained for (i) a monovalent anion with a deprotonated indole NH group (IM-'), (ii) a monovalent anion with a deprotonated maleimide NH group (IM-'') and (iii) a divalent anion with doubly deprotonated indole and the maleimide NH groups (IM2-). Quantum chemical calculations are treated at the three state averaged complete-active space self-consistent field level for 6 electrons in 5 orbitals with the cc-pVDZ basis set (CAS (6, 5) SCF/cc-pVDZ). Molecular dynamics simulations are performed with electronically nonadiabatic transitions included using the Zhu-Nakamura version of the trajectory surface hopping (ZN-TSH) method. It is found that the nonadiabatic transitions occur accompanied by the stretching and shrinking motions of the N(7)-C(8) bond in the case of IM-' and the C(11)-N(12) bond in IM2- rather than the twisting motion of the dihedral angle. We also found that the ultrafast S-2 -> S-1 nonadiabatic transitions occur through the conical intersection (CoIn) right after photoexcitation to S-2 in IM-' and IM2-. Furthermore, the S-1 -> S-0 nonadiabatic transitions are found to take place in IM-'. It is concluded that IM2- would mainly contribute to the photoemission, because the S-1 <- S-0 and S-2 <- S-0 transitions of IM-'' are dipole-forbidden transitions and, moreover, IM2- is found to be the only species to stay in the S-1 state without non-radiative decay.