Nonadiabatic derivative couplings through multiple Franck-Condon modes dictate the energy gap law for near and short-wave infrared dye molecules

TL;DR

This paper reveals that nonadiabatic derivative couplings involving multiple vibrational modes govern the energy gap law in NIR and SWIR dye molecules, explaining their nonradiative decay rates.

Contribution

It demonstrates that nonadiabatic derivative couplings through multiple Franck-Condon modes are key to understanding the energy gap law in NIR/SWIR dyes, a novel mechanistic insight.

Findings

Nonadiabatic derivative couplings are major decay pathways.

Vibrational modes beyond the highest frequency ones significantly influence decay rates.

Theoretical results align with experimental data for deuterated molecules.

Abstract

Near infrared (NIR, 700 - 1,000 nm) and short-wave infrared (SWIR, 1,000 - 2,000 nm) dye molecules exhibit significant nonradiative decay rates from the first singlet excited state to the ground state. While these trends can be empirically explained by a simple energy gap law, detailed mechanisms of the nearly universal behavior have remained unsettled for many cases. Theoretical and experimental results for two representative NIR/SWIR dye molecules reported here clarify the key mechanism for the observed energy gap law behavior. It is shown that the first derivative nonadiabatic coupling terms serve as major coupling pathways for nonadiabatic decay processes from the first excited singlet state to the ground state for these NIR and SWIR dye molecules and that vibrational modes other than the highest frequency ones also make significant contributions to the rate. This assessment is corroborated by further theoretical comparison with possible alternative mechanisms of intersystem crossing to triplet states and also by comparison with experimental data for deuterated molecules.