Synthesis and photophysical properties of phenanthroimidazole–triarylborane dyads: intriguing ‘turn-on’ sensing mediated by fluoride anions

Abstract

Phenanthroimidazole-based triarylborane compounds with an N-phenyl (1Ph, 2Ph) or N-biphenyl (1BP, 2BP) bridge were synthesized and characterized. All four compounds exhibit a dual emission pattern in their photoluminescence (PL) spectra, which can be separated into high- (λ_em = ca. 380 nm in THF) and low-energy (λ_em = ca. 480 nm) emissions. While the high-energy emission remains largely unchanged in different organic solvents, the low-energy emission exhibits clear signs of positive solvatochromism. The results of the photophysical analysis and theoretical calculations suggest that the high-energy emission corresponds to a π–π* transition band arising from the phenanthroimidazole, whereas the low-energy emission originates from an intramolecular charge transfer (ICT) transition between phenanthroimidazole and the triarylborane moiety. UV-vis titration experiments examining the association of 1Ph, 2Ph, 1BP, and 2BP with fluoride demonstrate that these compounds associate with a 1 : 1 binding stoichiometry in THF and binding constants (K_a) that are estimated to be around 1.0–3.0 × 10⁴ M⁻¹. These compounds show a ratiometrically increased fluorescence response in PL titration experiments upon binding of fluoride to the borane moiety, thereby giving rise to a ‘turn-on’ chemosensor for detection of fluoride anions. The ‘turn-on’ properties can be judged as a result of the reinforcement of π–π* transition on phenanthroimidazole and the restriction of ICT transition to triarylborane.