Examination of Photoluminescence in Rhodamine 6G Using Excited State Dynamics

Rhodamine 6G is a highly versatile dye that has been observed to enhance Raman scattering, exhibit two-photon absorption and fluorescence, and can be directly purchased. Further, it has been observed that rhodamine 6G exhibits a photoluminescence quantum yield (PLQY) of close to 100%, but the reason for this large quantum yield has been left largely unexplored. Here we use density functional theory and Redfield based excited state dynamics to explore the reasons for the quantum yield. Explicit solvation in methanol is included to account for solvent effects during the calculations. To account for perturbations of the lowest excitation oscillator strength along the molecular dynamics trajectory, we implement the molecular dynamics photoluminescence method,¹ based on Kasha’s rule, to obtain line widths of the photoluminescence. Nonradiative relaxation rates are computed in terms of Redfield theory by propagating the excited-state reduced density matrix for electronic degrees of freedom weakly coupled to a heat bath.² Nonadiabatic couplings between electronic and nuclear degrees of freedom, computed ‘on-the-fly’, are used to parametrize the rates of population transfer.³ Einstein coefficients for spontaneous emission is used to compute the radiative relaxation rates of charge carriers. The quantum yield is then determined from the nonradiative and radiative relaxation rates.⁴ The poor overlap of vibrational states with the transition energy results in larger nonradiative lifetimes resulting in the high PLQY.

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