Molecular Photophysics under Shock Compression: Ab Initio Nonadiabatic Molecular Dynamics of Rhodamine Dye

Photoluminescent chromophores can be used as probes for shock-wave propagation, provided that luminescence wavelength and/or lifetime depend on the increased pressure and temperature generated by the propagating shock-wave. Recent experiments have observed such effects with organic dyes and inorganic semiconductor quantum dots. We employ ab initio density functional theory and nonadiabatic molecular dynamics to study the effects of pressure and temperature on fluorescence properties of rhodamine-6G encapsulated in silica. Increase of pressure alone already decreases the luminescence wavelength and lifetime. The combined effect of enhanced pressure and temperature, representing shock conditions, is even stronger with temperature growth having a particularly strong influence on the nonradiative lifetime. The excitation wavelength is red-shifted due to changes in the highest occupied molecular orbital and lowest unoccupied molecular orbital energies caused by mechanical distortions of the π-electron plane of the organic chromophore. The nonradiative relaxation accelerates due to enhancement in the nonadiabatic electron-vibrational coupling, that is particularly sensitive to temperature. The difference in the sensitivity of luminescence wavelength and lifetime to pressure and temperature suggests that it may be possible to determine the thermodynamic properties separately on the basis of the two measured quantities. A broad spectrum of vibrational modes couples to the electronic transition with lower frequency modes exhibiting stronger coupling. Higher-frequency modes become more important under the shock conditions. The simulations show excellent agreement with experiment, characterize how temperature and pressure influence electron-hole recombination in rhodamine-6G encapsulated in a silica matrix, and provide important details on the mechanism of nonequilibrium dynamics in the fluorescent chromophore.