Figure 1 (a) Chemical structures and (b) optimized geometries (top: top view; bottom: side view) of the two NFREAs. (c) Single crystal structures of the two model compounds TzDT-Br and TzClDT-Br. (d) GIWAXS patterns of neat Tz-H and Tz-Cl films and (e) the corresponding scattering profiles in the out-of-plane (solid line) and in-plane (dashed line) directions.
The optimized molecular geometries of Tz-H and Tz-Clwith simplified side-chains were investigated with DFT calculations at the B3LYP/6-31G(d,p) level. As shown in Figure 1b, both Tz-Hand Tz-Cl possessed near-coplanar π-conjugated backbones with small dihedral angles (θ 1 andθ 2). Especially, Tz-Cl exhibited bothθ 1 and θ 2 of near 0°, which is favorable for the intermolecular π–π stacking and charge transport. The intramolecular distance between the sulfur and nitrogen atoms (d intra, S···N) were calculated to be ~3.04 and ~3.05 Å for Tz-H andTz-Cl , respectively, and the intramolecular distance between the sulfur and chlorine atoms (d intra, S···Cl) was calculated to be ~3.21 Å for Tz-Cl , much shorter than the sum of the van der Waals radius (r w, S···N = 3.35 Å and r w, S···Cl = 3.65 Å), indicating the existence of S···N and S···Cl NoCLs. Thus, Tz-Cl possessed a more planar conformation mainly due to the incorporation of multiple NoCLs.
To gain more structural information on the thiazole- and 4-chlorothiazole-based NFREAs, the single crystals of two model compounds TzDT-Br and TzClDT-Br were grown by solvent vapor diffusion using chloroform/methanol (Table S1). As shown in Figure 1c, the single crystal structure of TzClDT-Br shared a more co-planar conformation with dihedral angles θ 1 of 1.8° andθ 2 of 3.8°, much smaller than those of 2.1° and 5.5° in TzDT-Br. In addition, the short d intra, S···N and d intra, S···Cl again indicated that the planar conformations could be enforced by S···N and S···Cl NoCLs. Such a planar conjugated backbone of TzClDT-Br is expected to favor the stacking of Tz-Cl .
Table 1 Basic properties of Tz-H and Tz-Cl