Triplet states nurture the long-lived emission at room temperature of a chlorine-containing tetraphenylimidazole derivative when aggregation occurs in water/acetonitrile mixtures.

Abstract

In recent years, there have been numerous reports on the synthesis and applications of 1,2,4,5-tetraphenylimidazole (TEPI) derivatives, particularly due to the photophysical properties of such systems. However, the long-lived emission behavior of TEPIs has not been studied, with research largely limited to attempts at 77 K. In this study, the compound 1-(4-chlorophenyl)-2,4,5-triphenyl-1H-imidazole (TEPI-Cl) was prepared and characterized using experimental techniques (i.e., absorption spectra, steady-state, and long-lived emission), as well as computationally, using a combination of Extended Tight Binding (xTB), Density Functional Theory (DFT), and Time-Dependent (TD-DFT) methods. TEPI-Cl exhibited a low solvatochromic effect, both in absorption and steady-state emission in organic solvents, which is typical of a locally excited transition; this was confirmed by the performed calculations. However, in its aggregated state (observed in water/acetonitrile mixtures with >80% of water, in volume), the compound displayed the emergence of a new band in the absorption spectrum, as well as aggregate-induced enhanced emission in the steady-state emission analysis. The long-lived emission spectrum of TEPI-Cl recorded at room temperature shows two signals (at 380 and 540 nm) and the presence of benzil enables the generation of triplet excited states of the latter, likely through an energy transfer process. The sensitivity of these signals to the presence of oxygen suggested that the related excited states are of a triplet nature; moreover, the calculated electronic transitions for the optimized structures of the T₁ and T₂ states are comparable to the experimentally observed long-lived emission wavelengths. This newly observed behavior of TEPI-Cl comes as a novel photophysical property added to this class of molecules, demonstrating its significant potential for further applications in complex matrices.