A Series of 2,5-Bis(5-aryl-2-thienyl)pyrazines with a Linear-Shaped (D-π)2-A System: Synthesis and Study of the Optical Properties Including Fluorosolvatochromism and Proton-Base-Sensing

A series of 2,5-bis(5-aryl-2-thienyl)pyrazines with distinct optical properties tuned by the intramolecular charge transfer (ICT) degree based on the donor strength of the aryl groups have been synthesized. It was found that 2,5-bis[5-(4-N,N-dibutylaminophenyl)-2-thienyl]pyrazine exhibits a fluorosolvatochromism that originates from its ICT excited state and proton sensibility with the result that the two-step protonation of the amino nitrogen atoms and pyrazinyl nitrogen atom leads to stepwise solution and emission color changes. Additionally, it should be noted that 2,5-bis[5-(4-hydroxyphenyl)-2-thienyl]pyrazine exhibits a base sensitivity through the obvious emission color change based on the combined process consisting of the deprotonation process of the phenolic hydrogen atoms and the subsequent ICT process. Consequently, by incorporating the N,N-dibutylaminophenyl group (acts as a strong electron donor and proton acceptor) or the phenolic group (acts as a proton donor) into the pyrazine (acts as an electron acceptor and proton acceptor)-cored (D-π)2-A structure, we have constructed an environmentally responsive optical probe.