Synthesis and AIE properties of benzene fused cyclooctetrathiophenes

We have synthesized a series of benzene fused cyclooctetrathiophenes (COThs), including BCOTh, 2BCOTh, 3BCOTh, and 4BCOTh with rotor-free molecular structures, which are similar to the luminogens with the aggregation-induced emission (AIE) property based on the mechanism of restriction of intramolecular vibration (RIV). Investigation on the photophysical properties in THF-H2O mixture solvent showed that 4BCOTh exhibits the aggregation-caused quenching (ACQ) properties, while the others exhibit AIE effect. With increasing the number of the fused benzene rings, the emission behaviors of COThs showed hypsochromic shift in both rigid solution (77 K) and AIE state, which are attributed to the gradually weakened intramolecular through-space conjugation (ITSC). Theoretical calculations and crystal packing analysis indicated that the emission difference in solid between 2BCOTh and 4BCOTh is mainly affected by the factors of molecular conjugation and rigidity. In addition, the model compound COThMe, bearing two methylene groups locking the thiophene rings of COTh, was employed for the experimental investigation of RIV mechanism in the AIE case of COThs.