Polymorphic Phase-Dependent Optical and Electrical Properties of a Diketopyrrolopyrrole-Based Small Molecule

Four different polymorphic conformations
of diethyl 5,5′-[5,5′-[2,5-bis­(2-ethylhexyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo­[3,4-c]­pyrrole-1,4-diyl]­bis­(thiophene-5,2-diyl)]­difuran-2-carboxylate
(DPP-(CF)2), namely, DPP-(CF)2-α, DPP-(CF)2-β, DPP-(CF)2-γ, and DPP-(CF)2-ω, were identified from X-ray diffraction analysis conducted
on their thin films and single crystals. Highly crystalline and well-textured
thin films of these four polymorphs were successfully prepared via
postgrowth solvent vapor and thermal annealing treatments to investigate
the polymorphic phase-dependent optical and electrical properties
of DPP-(CF)2. Interestingly, during the phase transition
from DPP-(CF)2-α to DPP-(CF)2-ω,
the optical band gap decreases from 1.75 to 1.5 eV because of the
enhanced π–π interaction between the neighboring
molecules. Except for DPP-(CF)2-γ, the other three
phases show ambipolar charge transport. Although DPP-(CF)2-β and DPP-(CF)2-γ exhibit a similar way of
packing, a small increment in the π–π-stacking
distance (0.006 Å) and twist conformation of the grafted electron-donating
moieties of DPP-(CF)2-γ are found to reduce its hole
mobility.

通过对薄膜和单晶的X射线衍射分析，成功鉴定了四种二乙基5,5′-[5,5′-[2,5-双(2-乙基己基)-3,6-二氧-2,3,5,6-四氢吡咯[3,4-c]吡咯-1,4-二基]双(噻吩-5,2-二基)]二呋喃-2-羧酸酯（DPP-(CF)2）的多晶形态，即DPP-(CF)2-α、DPP-(CF)2-β、DPP-(CF)2-γ和DPP-(CF)2-ω。通过后生长溶剂蒸气和热退火处理，成功制备了这四种多晶形态的高度结晶和良好纹理的薄膜，以研究DPP-(CF)2的多晶相依赖的光学和电学性质。有趣的是，在DPP-(CF)2-α向DPP-(CF)2-ω的相转变过程中，由于邻近分子间π-π相互作用的增强，光学带隙从1.75 eV降至1.5 eV。除了DPP-(CF)2-γ之外，其他三个相表现出双极性电荷传输。尽管DPP-(CF)2-β和DPP-(CF)2-γ的堆积方式相似，但DPP-(CF)2-γ的π-π堆积距离（0.006 Å）的微小增加及其嫁接电子供体基团的扭曲构象被发现降低了其空穴迁移率。