Polarity-Dependent Thermochromism and Solvatochromism in Aryl Substituted Quinoxalines

Quinoxaline derivatives with donor-acceptor architectures exhibit interesting photophysics and are suitable for many optical applications. This study investigates the effect of solvent polarity and temperature on the fluorescence characteristics of three prototypical quinoxalines substituted with the aryl moieties, naphthalene, anthracene, or pyrene. Apart from excellent solvatochromism, which is a property common to many donor-acceptor-type fluorophores, these quinoxaline derivatives also show intriguing polarity-dependent thermochromism. It is revealed that this phenomenon arises because of polarity-dependent switching in the nature of the emissive state. In polar solvents, the quinoxalines with strong donor substituents can emit from both a less polar locally excited (LE) state and a highly polar intramolecular charge transfer (ICT) state, whose relative populations can be changed by temperature variation. Furthermore, it is found that variation in solvent polarity can change the nonradiative deactivation pathway of the excited state from a very effective decay due to intramolecular rotation that is coupled with the LE state, to intersystem crossing (ISC), which is accessible from the ICT state. Overall, this study provides an in-depth understanding of the excited-state dynamics of this class of molecules. The possibility of many excited-state processes, whose relative participations can be engineered by chemical substitution, temperature variation, or polarity changes, is expected to be useful for designing quinoxaline derivatives with desired fluorescence and stimuli-responsive properties.