Molecularly Smooth Single-Crystalline Films of Thiophene–Phenylene Co-Oligomers Grown at the Gas–Liquid Interface

Single
crystals of thiophene–phenelyne co-oligomers (TPCOs)
have previously shown their potential for organic optoelectronics.
Here we report on solution growth of large-area thin single-crystalline
films of TPCOs at the gas–liquid interface by using solvent–antisolvent
crystallization, isothermal slow solvent evaporation, and isochoric
cooling. The studied co-oligomers contain identical conjugated core
(5,5′-diphyenyl-2,2′-bithiophene) and different terminal
substituents, fluorine, trimethylsilyl, or trifluoromethyl. The fabricated
films are molecularly smooth over areas larger than 10 × 10 μm2, which is of high importance for organic field-effect devices.
The low-defect structure of the TPCO crystals is suggested from the
monoexponential kinetics of the PL decay measured in a wide dynamic
range (up to four decades) and from low crystal mosaicity assessed
by microfocus X-ray diffraction. The TPCO crystal structure is solved
using a combination of X-ray and electron diffraction. The terminal
substituents affect the crystal structure of TPCOs, bringing about
the formation of a noncentrosymmetric crystal lattice with a crystal
symmetry Cc for the bulkiest trimethylsilyl terminal
groups, which is unusual for linear conjugated oligomers. Comparing
the different crystal growth techniques, it is concluded that the
solvent–antisolvent crystallization is the most robust for
fabrication of single-crystalline TPCOs films. The possible nucleation
and crystallization mechanisms operating at the gas–solution
interface are discussed.

单晶硫代苯基-苯基共寡聚物（TPCOs）在有机光电器件领域的潜力已得到先前验证。本研究通过溶剂-抗溶剂结晶、等温慢溶剂蒸发以及等容冷却方法，在气-液界面实现了TPCOs大面积薄膜的溶液生长。所研究的共寡聚物包含相同的共轭核（5,5'-二苯基-2,2'-双硫代苯）和不同的末端取代基，包括氟、三甲基硅基或三氟甲基。所制备的薄膜在超过10×10 μm²的区域内分子平滑，这对于有机场效应器件至关重要。低缺陷结构的TPCOs晶体通过在宽动态范围（高达四个数量级）内测得的单指数衰减动力学以及通过微焦点X射线衍射评估的低晶体 mosaicity 得以证实。TPCOs晶体结构是通过X射线和电子衍射的结合方法解决的。末端取代基影响了TPCOs的晶体结构，导致了非中心对称晶体格的形成，对于最庞大的三甲基硅基末端基团，其晶体对称性为Cc，这在线性共轭寡聚物中颇为罕见。比较不同的晶体生长技术，得出结论：溶剂-抗溶剂结晶对于单晶TPCOs薄膜的制备最为稳固。讨论了在气-液界面处可能发生的成核和结晶机制。