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

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.