The role of X–H bonds (X = C, N and O) in internal conversion processes: dibenzoterrylene as an example

Abstract

We have developed a theoretical framework for calculating rate constants of internal conversion (k_IC) in the Franck–Condon (FC) and Herzberg–Teller (HT) approximations. The method accounts for anharmonic vibrational contributions and the Duschinsky effect. Our approach employs recursive dynamic programming to sum over multiple vibrational quantum number combinations and uses a Lagrange-multiplier technique with dispersion broadening to improve the accuracy of the calculated rate constants. We validate the methods by performing calculations on dibenzoterrylene (DBT), which is a molecule emitting in the near-infrared spectral range. The calculations confirm that anharmonic vibrational effects are the main contribution to k_IC, while the Duschinsky effect is significant only for molecules whose lowest excitation energy exceeds 22 000 cm⁻¹. The contributions of the individual X–H bonds are quantified by using the X–H mode approximation (k_IC-XH) and the XH bond approximation (k_IC-proton). The calculations show that the CH bonds of the tetracene moiety of DBT have the largest contribution to k_IC. Deuteration of these bonds leads to a significant decrease in k_IC with complete deuteration resulting in the largest overall effect. The calculated rate constants highlight the important role of the X–H bonds as acceptors of electronic excitation energy, offering strategies for modulating the k_IC through selective substitution of the hydrogen atoms with heavier atoms such as D, F or Cl.