Insight into the effect of alkyl chain length and substituent bulkiness on the mobility anisotropy of benzothieno[3,2-b][1]benzothiophenes

The molecular packing architecture parameter theta(T) (angle of the projected hopping path of dimers relative to the reference axis) and ratio R (ratio between the transfer integrals at the parallel and transverse contacts) have been found to be two crucial factors influencing the mobility anisotropy in organic semiconductors (OSCs). Herein, we report insight into the variation rules of theta(T) and R based on the structure and property analysis of alkyl and bulky end-capped benzothieno[3,2-b][1]benzothiophenes (BTBTs). Our investigation on alkyl side-chain substitution shows that a longer alkyl chain is more beneficial for balancing the intermolecular charge-transfer properties than a short chain, which is directly reflected in the decreasing R-value with increasing chain length. Moreover, the angle theta(T) of the bulky end-capped BTBTs (theta(T) = similar to 60 degrees), obviously larger than that of the symmetric alkylated BTBTs (theta(T) = similar to 53 degrees), decreases with increasing chain bulkiness. Furthermore, the bulkiness of the side chains can not only adjust the intermolecular distance between adjacent dimers but also promote the balance of the intermolecular charge-transfer property. After statistical analysis, we reveal that the theta(T) of OSCs generally ranges from 50 degrees to 62 degrees, suggesting that achieving low-anisotropy mobility requires that the bulky end-capped BTBTs satisfy 0.9 < R < 1.2 (theta(T) = 62 degrees) and alkylated BTBTs satisfy R < 0.85 (theta(T) = 50 degrees). Finally, we propose that the tBu groups have the most suitable bulkiness for accomplishing nearly isotropic mobilities of the bulky end-capped BTBTs, while the alkylated-phenyl and -thiophene groups may be beneficial for realizing isotropic charge transport properties of the symmetrical alkylated BTBTs. This work may inspire the design and improvement of isotropic charge transport characteristics for BTBTs and even other OSCs.