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

Abstract

The molecular packing architecture parameter θT (the angle of the projected hopping path of dimers relative to the reference axis) and the ratio R (the ratio between the transfer integrals...

Comprehensive Study on the Mobility Anisotropy of Benzothieno[3,2-b][1]benzothiophenes: Toward Isotropic Charge-Transport Properties

“…Previously, anisotropic charge-transport characteristics of OCSs have been classified into seven levels, including (a) low anisotropy (μ max /μ min < 1.5), (b) mid anisotropy (μ max /μ min = 1.5−2.0), (c) high anisotropy (μ max /μ min = 2.0−5.0), (d) higher anisotropy (μ max /μ min = 5.0− 10.0), (e) huge anisotropy (μ max /μ min = 10.0−50.0), (f) huger anisotropy (μ max /μ min = 50.0−100.0), and (g) complete anisotropy (μ max /μ min > 100.0). 23 To easily distinguish the mobility orientation of the studied BTBT molecules, we refer to the above seven levels for classifications. The unsubstituted BTBT shows an LHB packing structure (θ T = 53.5°) with face-to-edge interactions, as shown in Figure 1a.…”

“…Long alkyl chains and bulky groups can not only effectively stabilize the LHB packing but also enhance the intermolecular hole-transport properties at transverse contacts, which can promote low anisotropy in mobility. 23 Short alkyl chains are not recommended in the rational design of isotropic BTBTs as they may impede the LHB packing (e.g., mono-C 2 -BTBT). In short, we suggest that it is difficult to realize low anisotropy in hole mobility with monosubstituted BTBTs.…”

“…It is worth mentioning that the phenyl group has a significant effect on the reduction of mobility anisotropy in comparison with the results of mono-C n -BTBTs (Figure 2h,i). The introduction of phenyl groups has been found to enhance intermolecular C•••H interactions at the transverse contact, 18,22,23 which not only promotes the molecular alignment within the respective layers to achieve an LHB packing motif but also improves the charge-transport properties at the transverse contact to fulfill the key requirement of V P ≈ V T in the development of low anisotropy OSCs. Meanwhile, PE-BTBT-C n (n = 4, 8, 10, and 12) (Figure 3b) were studied to determine the effect of the phenylethynyl (PE) group on mobility anisotropy.…”

“…Recently, the control of the molecular packing architecture parameter θ T and ratio R (R = V P / V T ) was studied by a structure−property analysis of alkyl and bulky end-capped BTBTs based on the θ T − R model. 23 In this work, most of the symmetrically substituted BTBTs have been studied, and it is summarized that symmetric substitutions of alkylated-phenyl groups may be beneficial for realizing low anisotropy in BTBTs. To clarify this hypothesis, a novel symmetrically substituted BTBT, namely, DBEP-BTBT, was synthesized and is shown in Figure 5a.…”

“…Up to now, only a few symmetrically substituted BTBTs have been found to satisfy the LHB-IS requirement. 23 Thereby, we plotted a contour map (R: range 0−4; and θ T : range 30−80°) for systemically analyzing the relationship between the mobility anisotropy of BTBT derivatives (which satisfied the LHB-IS requirement) and the key parameters of R and θ T values, as shown in Figure 6. Meanwhile, a contour map with a wide R range (0−11) was also provided in ESI (Figure S6).…”