Synthesis, structure and material properties of thiopyranylidene-based asymmetrical squaraines

In this paper, we synthesized a new class of thiopyranylidene-based NIR absorbing asymmetrical squaraines (IPSQ 1–4), and carefully investigated their photophysical, thermal, electrochemical and semiconducting properties. A high hole mobility close to 10−4 cm2/Vs can be realized for IPSQ-3 based organic field effect transistors (OFETs), attributed to the strong intermolecular π-π interactions induced by pyranylidene-type end-groups with large π-system. More interestingly, we find through subtle structural variation, totally different molecular arrangements, herringbone and “brick-wall” -type, can be achieved for IPSQ-1 and -4, respectively, which therefore enables us to study the influence of molecular packing structure on semiconducting properties. For IPSQ-1, the hole mobility was found to be ∼50 times that of IPSQ-4, indicating herringbone-type arrangement is more beneficial to charge carrier transport here. Furthermore, IPSQ-1 also showed a better photovoltaic performance than IPSQ-4. Thus, our findings in this work can help us to understand the implications of tuning structural properties of squaraines, which will be very useful for designing high-performance squaraine-based semiconductors.