Thieno[3, 4‐b]thiadiazole Based Donor‐Acceptor Cyclic Oligomers: Ring Size‐Dependent Photophysical Properties and Efficient Near‐Infrared Electroluminescence Beyond 900 nm

Abstract Organic donor‐acceptor (D‐A) conjugated macrocycles, which are regarded as terminal‐less counterparts of linear π‐systems, have emerged as a promising class material for organic optoelectronics. Nevertheless, circular symmetry generally makes the lowest electronic transition symmetry forbidden, which is a huge obstacle to developing bright near‐infrared macrocyclic dyes. Here, a series of cyclic D‐A oligomers (e.g., cyclic D‐A dimer ( [2]C 16 TT‐TPA ), trimer ( [3]C 16 TT‐TPA ), and tetramer ( [4]C 16 TT‐TPA )) are synthesized in which triphenylamine and 4,6‐bisthienylthieno[3,4‐c]thiadiazole are alternately coupled. The ring size‐dependent photophysical properties of D‐A cyclic oligomers, such as photoluminescence quantum yield and radiative decay rate, are revealed through both experiments and theoretical calculations. Owing to conformational flexibility and dominant twisted conformation, [3]C 16 TT‐TPA shows increasing S 0 →S 1 transition and thereby produces a champion photoluminescence quantum yield of 6.35 % in solution with emission beyond 900 nm, much higher than that of acyclic oligomer (1.56%). Furthermore, the [3]C 16 TT‐TPA ‐based organic light‐emitting diode achieves a relatively high external quantum efficiency of 0.15% with a maximum radiance of 2897 mW sr −1 m −2 . The study not only provides insights into the mechanism of ring size‐dependent photophysical properties but also demonstrates the potential of D–A cyclic oligomers for bright near‐infrared emitters.