Synthesis and emissive properties of a series of tetrahydro (imidazo[1,5-a]pyrid-3-yl)phenols: A new class of large stokes shift organic dyes

A series of 2-(5,6,7,8-tetrahydroimidazo[1,5-a]pyridin-3-yl)phenols with different substituents (R) in the para position with respect to the hydroxyl group, was synthesized. The photophysical properties of these species have been investigated in dichloromethane solution: they displayed intense fluorescence when excited with UV light (310–340 nm), with λmax of emission varying in the range 430–530 nm and lifetime decay of few nanoseconds. A very large Stokes shift (up to 189 nm) and high absolute quantum yields (up to 0.72) were recorded for most of the compounds. Moreover, good tuning of the emission is observed according to the electronic features of the substituent R, and a linear correlation was found between the emission maxima of the compounds and the Hammett σp constants of R. Two of these (tetrahydroimidazo[1,5-a]pyridin-3-yl)phenols were then chosen for preliminary measurements of their photophysical properties when dispersed in a host polymeric matrix (PMMA).