<title>Simulation of photophysical processes of indoles in solution</title>

Abstract

Computational studies of photophysical processes in solution require accurate representations of the potential energy function. The interactions in a polar solute-solvent system are primarily electrostatic in nature, and they are, in molecular simulations, typically mediated by partial atomic charges on the solute and the solvent atoms. We have developed procedures to create partial atomic charges for any electronic state in a solute molecule from a least squares fit to the molecular electrostatic potential. The quantum mechanical electrostatic potential for the electronically excited 1La state of 3-methylindole derived from a semiempirical INDO/S configuration interaction wavefunction is presented. Partial atomic charges for the 1La state are derived from the quantum mechanical potential, and the classical and quantum mechanical electrostatic potentials are compared. Molecular dynamics simulations have been carried out on the ground (So) and two lowest excited singlet states (1Lb, 1La) of 3-methylindole in water using potential derived partial atomic charges. Solvent-induced inversion of the gas phase excited state ordering (1Lb below 1La) is computed for the excited states. A method for introducing polarization by the solvent into the solute electronic wavefunctions, and hence into the solute partial atomic charges, is introduced. A significant increase in solute dipole moment is computed for the 1La state, leading to dramatic increases in solute-solvent interaction energies and additional preferential stabilization of the 1La state over the 1Lb state.