HYDROGEN-BONDING, PROTONATION AND TWISTING IN THE SINGLET EXCITED-STATE OF SOME 2-(4-AMINOPHENYL)PYRIDO-OXAZOLE, 2-(4-AMINOPHENYL)PYRIDO-THIAZOLE, AND 2-(4-AMINOPHENYL)PYRIDO-IMIDAZOLE

The absorption and fluorescence characteristics of 2-(4-aminophenyl)py rido[3,2-d]oxazole (1), of its thiazole 2 and imidazole 3 analogues, a s well as of the corresponding pyrido[3,4-d]imidazole 4 have been exam ined. S1 is a planar pipi ICT state, similarly to p-electron-withdraw ing substituted anilines. In the protonated form, the chromophore is t he heterocyclic moiety. With compounds 3 and 4 in alcohols, hydrogen b onding depending on proton donating and accepting properties of the me dium determines the fluorescence. In this case, a red-shifted emission attributed to a twisted TICT state is also observed.