Preparation and Acid-Responsive Photophysical Properties of T-Shaped [pi]-Conjugated Molecules Containing a Benzimidazole Junction

T-shaped π-conjugated molecules with an N-methyl-benzimidazole junction have been synthesized and their acid-responsive photophysical properties owing to the change in the π-conjugation system are discussed. T-shaped π-conjugated molecules consist of two orthogonal π-conjugated systems including a phenyl thiophene extended from the 2-position and alkyl phenylenes connected through various π-spacers from the 4,7-positions of the N-methyl-benzimidazole junction. The π-spacers, such as thiophene, ethyne, and ethane, have an effect on the acid response of photophysical properties in terms of changes in conformation, excited-state energy and charge-transfer (CT) characteristics. In particular, the π-conjugated molecule with ethynyl spacers exhibited a marked redshift in the fluorescence spectrum with a large Stokes shift upon the addition of acid, whereas the other molecules showed substantial quenching. The redshift in emission was studied in detail by temperature-dependent fluorescence measurements, which indicated the transition to a CT state over the finite activation energy at the excited state. The change in the frontier molecular orbitals upon acid addition was further discussed by means of DFT calculations. [PUBLICATION ABSTRACT]