Design, synthesis and photoluminescent studies of new 1,5-benzodiazepines derivatives: Towards new ESIPT compounds

Abstract A series of novel N1-triazolo-4-(2-hydroxyphenyl)-1,5-benzodiazepin-2-ones 7a-e and N5-triazolo-4-(2-acetoxyphenyl)-1,5-benzodiazepin-2-ones 8a-e were designed and synthesized in good yields via a Cu(I) catalyzed 1,3-dipolar alkyne-azide coupling reaction (CuAAC) between the N1- and N5-propargyl-1,5-benzodiazepines 2 and 5 respectively and various arylazides 6a-e. Photophysical properties were investigated for all the obtained triazolo-benzodiazepine hybrids by mean of absorption and fluorescence spectral techniques. Thus, a fluorescent emission was detected for the derivatives 7a-e in aggregated state. On another hand, the O-acetylated derivatives 8a-e were found to be not emissive. Finally, we have chosen a model reaction to demonstrate that upon deprotecting the -OAc group in the N1, N5-disubstituted benzodiazepine 8d a moderate fluorescence reappeared in the obtained product 9d proving that an ESIPT process can take place as long as the hydroxyl group remains free, allowing the OH/C=O proton transfer to occur. A computational study of compounds 7e and 9d in vacuo provide further details and arguments to rationalize the fluorescence of these compounds in aqueous mixtures.