Jancy B, Asha S K
Specialty Polymers, Chemical Sciences & Technology Division, Regional Research Laboratory, CSIR, Thiruvananthapuram 695019, Kerala, India.
J Phys Chem B. 2006 Oct 26;110(42):20937-47. doi: 10.1021/jp063943c.
We report here, for the first time, the role of the molecular design on the liquid crystalline and solid-state photoluminescent properties of soluble and thermally stable liquid crystalline perylenebisimide derivatives. A new series of perylenebisimides were designed and developed for this purpose by adopting the stoichiometry-control approach, and amine-, hydroxyl-, ester-, and amide-functionalized molecules were synthesized. Various types of spacers with different lengths (C(2) to C(12)), types (linear, cyclohexyl, and tricyclodecane), and end-capped by phenyl or tridodecyloxy gallic units were introduced in the perylenebisimide core. The molecules were completely characterized by NMR, FT-IR, SEC, and MALDI-TOF mass techniques. Thermal analysis revealed that the perylenebisimide derivatives were thermotropic liquid crystalline, and threadlike nematic phases were observed under a polarizing light microscope. The spacer length and the rigidity of the spacers play a major role in the liquid crystalline properties of the materials. In phenyl systems, the C(6) chain with ester- and the C(12) chain with amide-end-capped molecules showed a nematic phase, whereas the C(6) chain with an amide end cap and their cyclic and tricyclic counterparts did not show any LC property. The introduction of a tridodecyloxy gallic unit induced the LC property in C(12) and the cyclohexyl system; however, it failed to do so in the tricyclodecane molecule. The absorption properties of the molecules were almost unchanged by the structural variation; however, the emission quantum yield in solution and photoluminescent (PL) intensity in the solid state were significantly different. Though the gallic unit induced liquid crystallinity in the perylenebisimide core, the quantum yield and PL intensity are 4-5 times less compared to those of the simple phenyl-capped liquid crystalline system. Among the various types of spacers, the tricyclodecane induced strong molecular aggregates via pi-stacking, which in turn increased the rigidity of the entire perylenebisimide core, resulting in the absence of liquid crystallinity and low luminescence compared to their linear and cyclohexyl analogues. The molecular aggregates were very stable even at very dilute concentration and also at high temperatures. The aggregates disappeared immediately upon addition of trifluoroacetic acid, thus confirming the strong hydrogen bonding in the aggregated states. In a nutshell, the present report demonstrates the importance of molecular design for introducing liquid crystalline phases in perylenebisimides and also the development of novel highly luminescent n-type pi-conjugated material for application in optoelectronics.
我们首次在此报告分子设计对可溶性和热稳定的液晶苝二酰亚胺衍生物的液晶及固态光致发光特性的作用。为此，采用化学计量控制方法设计并开发了一系列新的苝二酰亚胺，合成了胺基、羟基、酯基和酰胺基功能化的分子。在苝二酰亚胺核心引入了各种不同长度（C(2)至C(12)）、类型（线性、环己基和三环癸烷）且以苯基或三十二烷氧基没食子酸单元封端的间隔基。通过核磁共振、傅里叶变换红外光谱、尺寸排阻色谱和基质辅助激光解吸电离飞行时间质谱技术对这些分子进行了全面表征。热分析表明苝二酰亚胺衍生物是热致液晶，在偏光显微镜下观察到丝状向列相。间隔基的长度和刚性对材料的液晶性能起主要作用。在苯基体系中，带有酯基的C(6)链和带有酰胺封端的C(12)链分子呈现向列相，而带有酰胺封端的C(6)链及其环状和三环对应物未表现出任何液晶性质。引入三十二烷氧基没食子酸单元在C(12)和环己基体系中诱导了液晶性质；然而，在三环癸烷分子中却未能如此。分子的吸收性质几乎不受结构变化的影响；然而，溶液中的发射量子产率和固态下的光致发光（PL）强度却有显著差异。尽管没食子酸单元在苝二酰亚胺核心诱导了液晶性，但与简单苯基封端的液晶体系相比，量子产率和PL强度低4至5倍。在各种类型的间隔基中，三环癸烷通过π-堆积诱导形成强分子聚集体，这反过来增加了整个苝二酰亚胺核心的刚性，导致与它们的线性和环己基类似物相比不存在液晶性且发光较低。即使在非常稀的浓度和高温下，分子聚集体也非常稳定。加入三氟乙酸后聚集体立即消失，从而证实了聚集态中存在强氢键。简而言之，本报告证明了分子设计对于在苝二酰亚胺中引入液晶相的重要性，以及开发用于光电子学的新型高发光n型π共轭材料的重要性。
