Triphenothiazinyl triazacoronenes: donor–acceptor molecular graphene exhibiting multiple fluorescence and electrogenerated chemiluminescence emissions

Abstract

We synthesized and studied a new class of A–(π-D)₃ type donor–acceptor molecular graphene, triphenothiazinyl triazacoronenes, [2,3,6,7,10,11-hexamethoxy-4,8,12-tri-(10-alkyl-phenothiazine)-1,5,9-triazacoronene] (TPTZ-TAC derivatives). These molecules have been synthesized by employing three electron-rich triphenothiazine (PTZ) groups as electron donors, which were linked to an electron acceptor of an electron-deficient triazacoronene core (2,3,6,7,10,11-hexamethoxy-1,5,9-triazacoronene, TAC). These donor–acceptor molecular graphenes exhibited unique multiple fluorescence and electro-generated chemiluminescence (ECL) emissions that are dependent on the concentration of these molecules, attributed to strong π-stacking interactions. The electrochemical behaviour showed two closely spaced consecutive reversible one-electron oxidations occurring on the PTZ groups and a reversible one-electron reduction localizing on the TAC core. The absorption and fluorescence emission spectra reveal that the electronic properties are affected by the intramolecular charge transfer (ICT) interaction from the PTZ donors to TAC acceptors in the excited state. The effect of π-stacking interaction was noticed for the excimer emission at the lower energy region. The ICT properties of the TPTZ-TACs have been analyzed by concentration-dependence and solvatochromism of fluorescence spectral studies. Remarkably, multiple ECL emissions were produced from the TPTZ-TAC derivatives via a radical ion annihilation and coreactant process through the formation of a charge-transfer excimer state. This work demonstrates that the attachment of electron-rich PTZ groups as electron donors to an electron-deficient TAC core as an electron acceptor, is a promising route to improve the optoelectronic properties of the molecular graphene TAC core. The D–A molecule excimers will represent a new approach to red luminescence and a means to enhance the fluorescence efficiency.