Ethynylene-Bridged Conjugate Carbazole Trimers: Synthesis and their Structural, Photophysical, and Electrochemical Properties

A series of six new ethynylene-bridged conjugate carbazole trimers (1-6) were synthesized through sequential Pd-catalyzed Sonogashira cross-coupling reactions. The structural properties of these trimers were examined by using DFT calculations. Compound 6, in which the two terminal carbazole moieties were substituted at the 1-position, exhibited the longest-wavelength UV/Vis absorption maximum in CH2Cl2, whereas compound 1, in which the two terminal carbazole moieties were substituted at the 3-position, exhibited the shortest-wavelength absorption maximum. These trends were rationalized by using TDDFT calculations. Electrochemical measurements and DFT calculations revealed that trimers 1-6 had smaller HOMO-LUMO gaps compared with that for the corresponding monomer (14), which was caused by a lowering of the LUMO levels, rather than by a raising of the HOMO levels. Trimers 2-6 exhibited excellent emission properties with high fluorescence quantum yields, which were derived from their rigid structures.