Dual-State Fluorescence in Some Salicylaldehyde-triphenylene Discotic Liquid Crystal Derivatives Induced by Excited State Intramolecular Proton Transfer

Abstract

A new family of excited state intramolecular proton transfer (ESIPT) molecules based on the enol/ketone tautomerism is reported. The molecules were designed to display columnar mesophases and consist of the archetypical 3,6,7,10,11-penta(alkoxy)triphenylene discotic core functionalized in the vacant "2-position" by a salicylaldehyde group (TPn-SA). For comparison and confirmation that the ESIPT effect is solely due to the salicylaldehyde group, structurally related triphenylene derivatives bearing benzaldehyde (TPn-S) or aldehyde (TPS6) side groups were also synthesized. These molecules were synthesized either by Suzuki−Miyaura cross-coupling (TPn-SA, TPn-S) or by the Bouveault aldehyde synthesis (Ar−Li and DMF, TPS6) in overall good yields. All triphenylene compounds show an enantiotropic columnar hexagonal mesophase over large temperature range. Furthermore, TPn-SA compounds undergo a mesophase to crystalline phase transformation upon UV light irradiation at 365 nm, caused by ESIPT-induced enol− ketone tautomerism. TP12-SA also self-assembles into room temperature gels in toluene and ethyl acetate at rather low critical gel concentration. TPn-SA and TPn-S present strong fluorescence in both solution and solid state, with fluorescence quantum yields for the TPn-SA compounds around ∼20%; TPS6, in contrast, does not show fluorescence in solid state because of aggregation-caused quenching. The dual emission observed for TPn-SA was supported by DFT calculation. The ketone emission is dominant in nonpolar solvents and aggregated states, while the enol emission is dominant in polar solvents or in dilute solutions. Finally, the performances of TP6-SA and TP6-S in solid-state lighting LED devices have been investigated.