Computational chemistry analysis of fluorescence properties of terpyridines

Abstract

Organic luminescent materials in solid state have been widely studied from the view point of their abundant applications to opt-electronics. We recently found that 2,2':6',2"-terpyridine (tpy) undergoes fluorescence on/off switching via heat-mode reversible conversion between the two different crystal structure (typ-N and tpy-P). While the switching is derived from the reversible transformation between two chemical structures, its detailed mechanism in molecular level is still unresolved. In this computational study, we calculated the ratio of non-radiative decay rate based on Fermi Golden Rule, where electronic coupling and Franck-Condon (FC) term were independently examined in detail. The slight difference in the molecular structure of tpy-N and tpy-P indicates the differences in the electronic coupling term is too small to play a definitive role in their drastic fluorescent behavior. We thefore focused on the FC-term and found the delicate balance between inner- and outer- reorganization energies and the driving force (the difference of Gibbs free energy between initial- and final states), which decisively affect the non-radiative decay rate.