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

Abstract

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. • A series of novel 1,5-benzodiazepin-2-ones have been designed and synthesized. • Some compounds have shown fluorescence response due to ESIPT process as long as their hydroxyl group remains free. • O-acetylation prevents emission by blocking proton transfer. • It is hypothesized that distant OH/C O proton transfer can occur leading to unexpected ESIPT mechanism in aqueous mixtures. • Computational studies provide a rationalization of proton transfer.