Quantum chemical calculations of the electronic states and fluorescence properties of carbazolyl- and carbazolymethylene-substituted diacetylenes

The carbazolyldiacetylenes show very different fluorescence behaviors depen ding on how the carbazolyl (Cz) moiety is connected to the diacetylenic bac kbone. In fact, when Cz is attached to the backbone through one or more CH2 spacer(s) fluorescence properties are observed which are closely related t o those of the fluorescent Cz group. On the contrary, almost no fluorescenc e is observed when Cz is directly attached to the backbone. In this paper, these phenomena are studied quantum chemically and an explanation for the o bserved facts is provided. The relevant excited states are studied in detai l, showing that in the lowest singlet excited state a considerable intramol ecular charge transfer occurs from Cz to the backbone when they are directl y bonded.