Tuning the Electronic Properties and Acid-Response Behavior of N-Heteroacene-Based π-Conjugated Liquids by Changing the Number of π-Conjugated Substituents.

We have designed and synthesized two room-temperature-fluorescent π-conjugated liquids based on the N-heteroacene framework (1 and 2). These two π-conjugated liquids, which contained one and two thiophene rings, respectively, exhibited different electronic properties and rheology behaviors. Single-crystal X-ray analysis of dithiophene-appended compound 4 revealed that two thiophene rings hindered the interactions of the imino N atoms with acids through the formation of interactions between the S atoms of the thiophene rings and the imino N atoms of the pyrazine group. On the other hand, monothiophene-appended molecules 1 and 3 each contained an unhindered imino N atom on the opposite site to the thiophene ring. Upon dissolving various acids with different pKa values in compounds 1 and 2, these slight structural differences gave rise to marked differences in their acid-response behaviors, thereby resulting in the emission of variously colored fluorescence in the liquid state. Furthermore, when acids with lower pKa values was dissolved in compounds 1 and 2, phase transition occurred from an isotropic liquid state to a self-organized liquid-crystalline phase.