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

A series of novel N-1-triazolo-4-(2-hydroxyphenyl)-1,5-benzodiazepin-2-ones 7a-e and N-5-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 N-1- and N-5-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 N-1, N-5-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. (C) 2021 Elsevier Ltd. All rights reserved.