Synthesis and Photophysical and Anion-Sensing Properties of Triarylborane-Substituted Cross-Conjugated and Conjugated Thienothiophenes

Two novel organoboron compounds BBTT1 and BBTT2 possessing conjugated thieno[3,2-b]thiophene and cross-conjugated thieno[2,3-b]thiophene, respectively, as electron donors (D) and bis-triarylboron units as electron acceptors (A) linked through thiophene π spacers have been synthesized and structurally characterized by various spectroscopic methods. The donor capabilities of the thienothiophenes were examined in view of their electrochemical and photophysical properties. Also, their solvatochromic behaviour in different solvents as well as their anion-recognition ability have been thoroughly investigated. Compounds BBTT1 and BBTT2 act as colorimetric and fluorescent chemosensors with high sensitivity and selectivity towards F^–. Furthermore, BBTT1 displays a peculiar redshift in fluorescent titration experiments upon the binding of F^– to the borane moiety, and “turned-on” chemosensor character for the detection of fluoride, as illustrated by the accompanying change in emission colour. On the other hand, the binding of F^– to BBTT2 reveals a “turned-off” sensor property. The electronic and structural characteristics of the two compounds and their fluoride-sensing properties were assessed by means of DFT computations.