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.