The photophysical properties of two bisindolylmaleimide derivatives, 3,4-bis(3-indolyl)-1-H-pyrrole-2,5-dione (arcyriarubin A) and indolo[2,3-a]pyrrolo[3,4-c] carbazole-5,7-(6 H)-dione (arcyriaflavin A), are investigated by using ab initio molecular orbital (MO) and multireference perturbation theory. These compounds are suggested to exist as monovalent anions deprotonated from an indole NH group in aprotic polar solvents. The analysis of MOs shows that the electronic structures of the S1 and S2 states are described by the singleor double-electron excitation between the naturally localized MOs on an indole moiety and on the maleimide part. This indicates that the intramolecular charge transfer (ICT) transfer may occur by photoexcitation. The minimum-energy structure of the arcyriarubin A anion is twisted; the dihedral angles between the indole and maleimide rings are 83.4° and 20.2° for the S1 and S0 states, respectively. The analysis of the minimum energy path along the coordinate of the twist angle is performed to explore the emission process from the Si state. It has been shown that the magnitude of the Stokes shift increases with increasing the twist angle, but the oscillator strength decreases. It has been suggested that the experimentally observed fluorescence arises on the way toward the energy minimum of the S1 state. The Stokes-shifted emission of arcyriaflavin A is contributed by the S1-S0 electronic relaxation after the excitation in the S2 state.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry