Fluorescence behavior of 2,6,10-trisubstituted 4,8,12-triazatriangulene cations in solution and in the solid state

Abstract

Triazatriangulenium (TATA⁺) is a cationic aromatic dye with C₃ symmetry, which shows an isolated absorption and emission band at the visible region (λ_max,FL = 558 nm, fluorescence quantum yield Φ_f = 0.25). A number of TATA derivatives have been reported until now; however, substitution at the 2-, 6-, and 10-positions has not been adequately investigated yet. In this work, we synthesized TATA derivatives having various substituents at the 2-, 6-, and 10-positions and investigated their photophysical properties in solution and in the solid state. The emission wavelength was red-shifted by ca. 25 nm in the solution phase when a 4-nitrophenyl group was directly introduced into the TATA framework (λ_max,FL = 585 nm, Φ_f = 0.17). The emission band was further red-shifted by ca. 140 nm in the solid state and a near-infrared (NIR) emission was observed (λ_max,FL = 728 nm, Φ_f = 0.008). Analysis based on the radiative (k_f) and non-radiative (k_nr) rate constants suggests that the substituent effect on Φ_f in the solution phase is mainly attributed to the change in the k_nr value. On the other hand, the strong fluorescence quenching in the solid state is attributed to the decrease of the k_f value, suggesting that intermolecular interactions in the solid state become prominent when the π-conjugation system of the TATA framework is extended by direct functionalization.