Triphenylamine–boron complexes: molecular thermometry and alkyl chain controlled molecular fluorescent liquids

Abstract

Molecular structural tailoring is one of the versatile approaches for developing functional organic fluorescent molecules with desired properties for practical applications. In this manuscript, we report the synthesis of triphenylamine based boron complexes (TPA-BF₂) with unusual thermofluorochromism and alkyl chain length controlled molecular fluorescent liquids/solids. TPA-BF₂ complexes with ethyl, propyl, butyl and pentyl groups produced fluorescent solids whereas with hexyl, heptyl and octyl groups yielded fluorescent liquids. The longer-alkyl chain compounds significantly improved the fluorescence efficiency (quantum yield (Φ_f = 12.6%)) by inhibiting close arrangement of fluorophores. TPA-BF₂-2 and TPA-BF₂-3 showed very weak fluorescence (Φ_f = 1.9%). On the other hand, all complexes exhibited strong solvent polarity dependent tunable fluorescence in the solution state. Importantly, the ability to form multiple excited states (locally excited (LE), charge transfer (CT) and twisted intramolecular charge transfer (TICT) states) resulted in unusual fluorescence tuning and reversible thermofluorochromism. The conversion of CT emission at RT to LE state emission at 77 K (under liquid nitrogen) of TPA-BF₂ complexes in CHCl₃/toluene resulted in strong blue shifting, while warming towards RT, the TPA-BF₂ CHCl₃ solution showed unusual fluorescence tuning. The fluorescence λ_max of TPA-BF₂-2 was red shifted from 476 to 600 nm and then blue shifted to RT fluorescence (550 nm). Interestingly, TPA-BF₂ complexes in solution and in the PMMA matrix showed clear phosphorescence at 77 K. The unusual fluorescence tuning while warming from 77 K is attributed to the phosphorescence emission coupled with fluorescence at room temperature. Notably, the fluorescence intensity at RT was relatively stronger compared to red shifted fluorescence intensity while warming. TPA-BF₂ complexes in toluene showed strong linear enhancement of fluorescence intensity (1.9 to 2.5 times) with a blue shifted peak position upon increasing the temperature from 0 to 100 °C. The alkyl chain length exhibited a strong influence on the rate and extent of fluorescence tuning with temperature. Thus, alkyl group variations in simple Schiff base–boron complexes produced unusual thermofluorochromism and molecular fluorescent liquids.