Solvent effect on electronic absorption, fluorescence, and phosphorescence of acetone in water: revisited by quantum mechanics/molecular mechanics (QM/MM) simulations.

The accurate simulation of fluorescence and phosphorescence spectra in solution remains a huge challenge due to the difficulty of simulating excited state dynamics in condensed phase. In this work we revisit the solvent effect on the electronic absorption, fluorescence, and phosphorescence of acetone by virtue of quantum mechanics/molecular mechanics (QM/MM) equilibrium state dynamics simulations for both the ground state (S0) and the lowest excited singlet (S1) and triplet (T1) states of aqueous acetone, which use periodic boundary conditions and hundreds of explicit solvent molecules and are free of empirical electrostatic fittings for excited states. Our calculated solvent effects on acetone's n → π* (S0 → S1) absorption (0.25-0.31 eV) and n ← π* (S1 → S0) emission (0.03-0.04 eV) as well as the Stokes shift (0.22-0.27 eV) are in good accordance with the experimental results (0.19 to 0.31, -0.02 to 0.05, and 0.14 to 0.33 eV, respectively). We also predict small water effects (-0.05 to 0.03 eV) for S1 → T1 and T1 → S0 phosphorescence emissions of acetone, which have no experimental data to date. For the recent dispute about the magnitude of the solvent effect for acetone's S1 → S0 fluorescence, we confirm that such effect is very small, agreeing well with the experimental determinations and most recent theoretical calculations. The large solvent effect for electronic absorption and small or negligible one for fluorescence and phosphorescence are shown to be related with much reduced dipole moments of acetone and accordingly much less hydrogen bonds for aqueous acetone in the electronic excited states S1 and T1 comparing to the ground state S0. We also disclose that solvent polarization effects are relatively small for all the electronic transitions of aqueous acetone involved in this work through the investigation of the QM region size effect on QM/MM results.