Carbazole-Terpyridine Donor-Acceptor Dyads with Rigid π-Conjugated Bridges

A series of molecules in which 9H-carbazole (electron donor, D) and 2,2 ':6 ',2 ''-terpyridine (electron acceptor, A) are connected through rigid pi-conjugated bridges (D-pi-A systems) have been synthesized and their photophysical properties examined in detail, with the support of DFT calculations. The bridges are made of different sequences of ethynylene, phenylene, and anthracene groups. The synthetic strategies involve condensation of 2-acetylpyridine with the aromatic aldehyde moiety on different functionalized pi-conjugated bridges and couplings with carbazole derivatives. The system incorporating anthracene in the bridge shows the typical absorption and emission fingerprints of this polycyclic hydrocarbon. The other systems have HOMOs and LUMOs centred, respectively, over the carbazole and the bridge and exhibit solvatochromic charge-transfer (CT) luminescence with high photoluminescence yield up to 70 %, except when an ethynylene unit is directly attached to the carbazole ring, due to a trans-bent non-emissive pi-sigma* excited state.