Photochemical dynamics of indolylmaleimide derivatives

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Abstract
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 S2 → S1 nonadiabatic transitions occur through the conical intersection (CoIn) right after photoexcitation to S2 in IM−′ and IM2−. Furthermore, the S1 → S0 nonadiabatic transitions are found to take place in IM−′. It is concluded that IM2− would mainly contribute to the photoemission, because the S1 ← S0 and S2 ← S0 transitions of IM−′′ are dipole-forbidden transitions and, moreover, IM2− is found to be the only species to stay in the S1 state without non-radiative decay.

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