Alkyl chain regulation: distinctive odd–even effects of mechano-luminescence and room-temperature phosphorescence in alkyl substituted carbazole amide derivatives

Abstract

A series of carbazole amide derivatives were synthesized by adjusting the n-alkyl chain with different lengths from methyl to octyl (CAC-N, N = 1–8). Their photophysical behaviors were highly dependent on the parity of their alkyl chain. That is, compounds with even-numbered alkyl chains exhibited efficient mechanoluminescence (ML) and strong room temperature phosphorescence (RTP), however, no ML and inferior RTP were observed in the odd-numbered counterparts. To make the results more clear, the important starting material, carbazole, was synthesized in the lab, from which, all the above eight carbazole amide derivatives were re-synthesized for comparison. Although RTP properties were disturbed by impurities in commercial carbazole, the ML property remained unchanged. Single crystal analyses revealed that the crosslink network and isolated dimers were formed in even- and odd-numbered CAC crystals respectively through hydrogen-bond interactions, which should account for the odd–even alternating ML phenomena. Additionally, multicolor ML from blue to red was realized by doping various organic dyes in the ML host matrix of CAC-8. To the best of our knowledge, this work is the first observation of parity effects in the ML property.