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 (IM^2-). 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 IM^2- rather than the twisting motion of the dihedral angle. We also found that the ultrafast S ₂ → S₁ nonadiabatic transitions occur through the conical intersection (CoIn) right after photoexcitation to S₂ in IM^-′ and IM^2-. Furthermore, the S₁ → S₀ nonadiabatic transitions are found to take place in IM ^-′. It is concluded that IM^2- would mainly contribute to the photoemission, because the S₁ ← S₀ and S₂ ← S₀ transitions of IM ^-′′ are dipole-forbidden transitions and, moreover, IM^2- is found to be the only species to stay in the S₁ state without non-radiative decay.