Modelling the Fluorescence Quantum Yields of Aromatic Compounds: Benchmarking the Machinery to Compute Intersystem Crossing Rates

The from-first-principles calculation of fluorescence quantum yields (FQYs) and lifetimes of organic dyes remains very challenging. In this manuscript we extensively test the static machinery to calculate FQYs. Specifically, we perform an extensive analysis on the parameters influencing the intersystem crossing (ISC), internal conversion (IC), and fluorescence rates calculations. The impact of i) the electronic structure (chosen exchange-correlation functional and spin-orbit Hamiltonian), ii) the vibronic parameters (coordinate system, broadening function, and dipole expansion), and iii) the excited-state kinetic models, are systematically assessed for a series of seven rigid aromatic molecules. Our studies provide more insights into the choice of parameters and the expected accuracy for the computational protocols aiming to deliver FQYs values. Some challenges are highlighted, such as, on the one hand, the difficulty to benchmark against the experimental non-radiative rates, for which the separation between the IC and ISC contributions is often not provided in the literature and, on the other hand, the need to go beyond the harmonic approximation for the calculation of the IC rates.