Deciphering intramolecular charge transfer in fluoranthene derivatives

Abstract

An intramolecular charge transfer (ICT) state evolves via excited state structural change and solvent reorganization, where the charge distribution in the excited state is markedly different from that in the ground state. Due to its ubiquitous nature, this intriguing photophysical phenomenon offers promising applications in the realm of optoelectronics. Judicious choice of donor-acceptor-based (D-A) push-pull chromophores is the most fundamental strategy to achieve ICT state formation. In the present work, we introduce a unique class of fluoranthene-based chromophores that do not belong to the conventional D-A design principle, as the fluoranthene core is seldom used as an acceptor. Nevertheless, we observe ICT state formation upon attaching strong donor triphenylamine to the fluoranthene backbone (TPF-2TPA). Theoretical studies demonstrate that the hole and electron densities are localized over the triphenylamine and fluoranthene core, respectively, which induces ICT character in the lowest energy transition of TPF-2TPA. Solvent polarity-dependent steady-state and time-resolved spectroscopic studies confirm the formation of the ICT state. Furthermore, viscosity-dependent study of TPF-2TPA reveals the involvement of a structural relaxation during ICT state formation. The present study sheds light on the rational design of unconventional ICT chromophores based on fluoranthene, thereby widening the applications of fluoranthene-based molecular systems in optoelectronic devices.