Prediction of Fluorescence Quantum Yields using the Extended Thawed Gaussian Approximation

Spontaneous emission and internal conversion rates are calculated withinharmonic approximations and compared to results obtained within thesemi-classical extended thawed Gaussian approximation. This is the firstapplication of the ETGA in the calculation of internal conversion and emissionrates for real molecular systems, namely formaldehyde, fluorobenzene, azuleneand a dicyano-squaraine dye. The viability of the models as black-box tools forprediction of spontaneous emission and internal conversion rates is assessed.All calculations were done using a consistent protocol in order to investigatehow different methods perform without previous experimental knowledge.Contrasting the results with experimental data shows that there are furtherimprovements required before theoretical predictions of emission and internalconversion rates can be used as reliable indicator for the photo-luminescenceproperties of molecules. We find that the extended thawed Gaussianapproximation performs rather similar to the vertical harmonical model.Including anharmonicities in the calculation of internal conversion rates has amoderate effect on the quantitative results in the studied systems. Theelectronic structure calculations were done using the B3LYP, PBE0,$\omega$B97XD and CAM-B3LYP functionals. The choice of the functional does notappear to be a major limiting factor for a black-box approach, when it comes tothe prediction of radiative and nonradiative rates for organic molecules. Theemission rates are fairly stable with respect to computational parameters, butthe internal conversion rate reveals itself to be highly dependent on thechoice of the spectral lineshape function, particularly the width of theLorentzian function, associated with homogeneous broadening.