Photochemical dynamics of indolylmaleimide derivatives

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.