Dual Excitation and Dual Emission in a 1,4-Diazepine Bearing an Extended π-System

Abstract

We present the synthesis of a benzo[b]benzo[11,12]tetracene[5,6-ef][1,4]diazepine (BBTDZ) and a benzo[g]phenanthro[9,10,1-cde]indazolium (BPIZ), two novel N-heterocyclic nanographenes. Both molecules were independently isolated from 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-mediated cyclodehydrogenation reaction of the previously reported benzo[f,g]tetracene β-diketiminate (BT-BDI). Our mechanistic investigation shows the instrumental role of Sc(OTf)3 in allowing for C─N bond formation over N─N bond formation. Unexpectedly, the BBTDZ compound shows unique dual emission and dual excitation (DE/DE) properties. A combination of spectroscopic and computational studies provides a rationalization of these features. Charge transfer followed by geometric relaxation were found to cause the dual, strongly red-shifted emission. The understanding of both the reaction mechanism and mechanism of DE/DE can contribute to the rational design of materials with unique photophysical properties.