The development of aryl-substituted 2-phenylimidazo[1,2-a]pyridines (PIP) with various colors of excited-state intramolecular proton transfer (ESIPT) luminescence in the solid state

Abstract

A series of solid-state luminescent dyes based on 2-phenylimidazo[1,2-a]pyridine (PIP) displaying a wide range of emitting colors from blue to red have been developed. Whereas 2′-methoxy PIP (2′MeOPIP, 10) shows blue luminescence, 2′-hydroxy PIP (HPIP, 1) exhibits emission with large Stokes shift at around 500 nm that is known as the excited-state intramolecular proton transfer (ESIPT) luminescence, which can be tuned from blue-green to red by simply introducing aryl group(s) into HPIP through Pd-catalyzed cross coupling reactions. It is shown that the energy of ESIPT luminescence decreases as the electron-withdrawing nature of the para-substituent on the aryl group increases. Varying the substitution position is also an effective tuning method, because the ESIPT luminescence wavelength is in the order 6-aryl < 8-aryl < 6,8-diaryl. Although the quantum yields of these compounds are quite low in organic solutions (Φ ∼ 0.01), they generally display a much stronger ESIPT luminescence in the solid state. For all compounds except for 9 having long C₆-alkyl chains, the similar emission properties in the dilute frozen matrix and the solid state indicated that ESIPT emission in the solid state is from the monomeric species, even though π-stacked motifs of the HPIP cores and the aryl groups introduced are confirmed by crystallographic analysis. Time-dependent DFT calculations reasonably explained the effect of substitution on ESIPT luminescence in the solid state. The results show that aryl-substitution is a convenient approach to tuning the radiation energy of the ESIPT luminescence of HPIPs without suffering the quenching effect due to intermolecular interactions, and thus a series of PIP compounds that exhibit a wide range of luminescence colors can be realized.