Acid-base-responsive intense charge-transfer emission in donor-acceptor-conjugated fluorophores

Herein we report on the synthesis and acid-responsive emission properties of donor-acceptor (D-A) molecules that contain a thienothiophene unit. 2-Arylthieno[3,2-b]thiophenes were conjugated with an N-methylbenzimidazole unit to form acid-responsive D-A-type fluorophores. The D-A-conjugated fluorophores showed intense intramolecular charge-transfer (ICT) emission in response to acid. The effect of the substitution on their photophysical properties as well as their solvent-dependence indicated non-twisting ICT emission in protonated D-A molecules. The quinoidal character of 2-arylthienothiophene as a donor part is discussed, as it is assumed that it contributes to suppression of the molecular twisting in the excited state, therefore decreasing the nonradiative rate constant, thereby resulting in the intense ICT emission. Acid-base-sensitive triple-color emission was also achieved by the introduction of a base-responsive phenol group in the donor part.