Effect of bulkiness on the triplet state of carbazole-benzophenone-based dyad systems

• A series of carbazole-benzophenone dyad systems was prepared to vary a bridging unit. • The different photophysical properties depending on the bulkiness of the dyad system were studied using transient absorption spectroscopy and DFT calculations. • The bulkiness in the dyad system can significantly affect the lowest triplet energy state (T 1 ). In this work, a series of carbazole–benzophenone-based dyad system, (4'-(9 H -carbazol-9-yl)-[1,1'-biphenyl]-4-yl)(phenyl)methanone ( PCBP ) and (4'-(9 H -carbazol-9-yl)-2,3,5,6-tetramethyl-[1,1'-biphenyl]-4-yl)(phenyl)methanone ( PCTBP ), was designed and prepared to study the structure–property relationship of the bulkiness with the benzophenone unit. For comparison, (4'-(9 H -carbazol-9-yl)-2',5'-dimethyl-[1,1'-biphenyl]-4-yl)(phenyl)methanone ( XCBP ), which has a xylyl bridging unit between the carbazole and benzophenone units, was also prepared. The photophysical and electrochemical properties of PCBP and PCTBP differ significantly depending on the bulkiness of the benzophenone unit. These results demonstrate that bulkiness in the carbazole–benzophenone-based dyad system can significantly affect the photophysical properties because of the different degrees of twisting between the two units. In particular, the lowest triplet energy state (T 1 ) of the dyads was affected by the bulkiness of the benzophenone unit; the T 1 state for PCBP and XCBP was confined to the benzophenone unit, whereas the T 1 state for PCTBP was confined to the carbazole unit. The results obtained in this study will provide useful information in related fields owing to the importance of the excited T 1 state in various optoelectronic applications.