  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 an important mechanism of such nature. It is shown that the first
derivative nonadiabatic coupling terms serve as major coupling pathways for
nonadiabatic decay processes exhibiting the energy gap law behavior 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.
