Studying internal conversion in molecules using branch-point instanton theory

Internal conversion constitutes an important non-radiative relaxation pathway, and therefore plays a significant role in determining a molecule’s quantum efficiency. This is important for the development of organic electronic devices, which motivates the development of accurate methods for computing internal conversion rate constants. Many studies on this are based on overlaps of vibrational wavefunctions, and in order to calculate these for a molecule, they need to rely on global harmonic approximations to the potential energy surfaces. We avoid this approximation by applying (branch-point) instanton theory [1,2], which is a semiclassical path-integral method that naturally takes into account the anharmonicity along the optimal tunneling path. We demonstrate the potential of instanton theory in the study of nonradiative relaxation processes by using it to calculate the rate of internal conversion in napthalene.

[1] Ansari, I. M., Heller, E. R., Trenins, G., & Richardson, J. O. (2024). Nat. Commun. 15(1), 4335.

[2] Richardson, J. O. (2024). J. Phys. Chem. Lett., 15(29), 7387-7397.