Carbazole-based D-π-A molecules: Determining the photophysical properties and comparing ICT effects of π-spacer and acceptor groups

Abstract 4-(9’-Hexylcarbazol-3’-yl)benzaldehyde (Cz-Ph-CHO: 4) and 4-(9’-hexylcarbazol-3’-yl)benzylidenemalononitrile (Cz-Ph-CN: 5) were synthesised with the structure of D-π-A, where carbazole, phenylene and formyl/dicyanovinyl groups act as electron donor (D), pi-spacer (π) and electron acceptor (A) units, respectively. The thermal, electrochemical, optical and intramolecular charge transfer (ICT) properties of compounds 4 and 5 were investigated. Compounds 4 and 5, in particular their ICT behaviour, were also compared with the closely related structure, 2-(9’-hexylcarbazol-3’-yl)-5-pyridinecarbaldehyde (Cz-Py-CHO: 7). For the purpose of tuning chemical structure to obtain targeted properties, electrochemical data and absorption and emission measurements suggest that the dicyanovinyl unit in compound 5 is a better acceptor than formyl in compound 4, and that pyridine in compound 7 is a better π-spacer than benzene in compound 4, in exerting ICT characteristics such as fluorosolvatochromism and Stokes shifts.