Department of Chemistry, Indian Institute of Science Education and Research (IISER)-Pune, 900, NCL Innovation Park, Dr. Homi Bhabha Road, Pune -411008, Maharashtra, India.
J Phys Chem B. 2010 Oct 7;114(39):12508-19. doi: 10.1021/jp105839f.
Here, we report a unique design strategy to trace the role of aromatic π-stacking and van der Waals interactions on the molecular self-organization of π-conjugated building blocks in a single system. A new series of bulky oligophenylenevinylenes (OPVs) bearing a tricyclodecanemethylene (TCD) unit in the aromatic π-core with flexible long methylene chains (n = 0-12 and 16) in the longitudinal position were designed and synthesized. The OPVs were found to be liquid crystalline, and their enthalpies of phase transitions (also entropies) showed odd-even oscillation with respect to the number of carbon atoms in alkyl chains. OPVs with an even number of methylene units in the side chains showed higher enthalpies with respect to their highly packed solid structures compared to odd-numbered ones. Polarized light microscopic analysis confirmed the formation of cholesteric liquid crystalline (LC) phases of fan shaped textures with focal conics in OPVs with 5 ≤ n ≤ 9. OPVs with longer alkyl chains (OPV-10 to OPV-12) produced a birefringence pattern consisting of dark and bright ring-banded suprastructures. The melting temperature followed a sigmoidal trend, indicating the transformation of molecular self-organization in OPVs from solid to ring-banded suprastructures via cholesteric LC intermediates. At longer alkyl chain lengths, the van der Waals interactions among the alkyl chains became predominant and translated the mesogenic effect across the lamellae; as a consequence, the lamellae underwent twisted self-organization along the radial growth direction of the spherulites to produce bright and dark bands. Scanning electron microscope (SEM) analysis of cholesteric LC and ring-banded textures strongly supported the existence of twisted lamellae in the OPVs with ring-banded textures. Variable temperature X-ray diffraction analysis confirmed the reversibility of the molecular self-organization in the solid state and also showed the existence of the higher ordered lamellar structure in ring-banded OPVs. Photophysical characterizations such as excitation, emission, and time resolved fluorescence decay measurements were employed to trace molecular self-organization in their liquid crystalline phases. The emission spectra of the OPV samples showed odd-even oscillation in their emission wavelengths with respect to the length of alkyl chains. Highly packed even-OPVs showed more blue shift compared to that of less crystalline odd-OPVs. Time dependent fluorescence decay of OPVs followed a biexponential fit, and their lifetimes (τ(1) and τ(2) values) revealed that the decay is faster for odd-OPVs compared to even-OPVs. Among all the OPVs, the τ(2) values for OPV-8 and OPV-12 were found to be much higher, indicating their high luminescent characteristics. In a nut shell, bulky liquid crystalline OPV chromophores were cleverly utilized, for the first time, to probe the aromatic π-stacking versus van der Waals interactions on the molecular self-organization of π-conjugated system.
在这里，我们报告了一种独特的设计策略，用于追踪芳香 π-堆积和范德华相互作用在单个系统中对π-共轭构建基元分子自组装的作用。我们设计并合成了一系列新型的含三环癸烷亚甲基（TCD）单元的大体积寡聚苯乙烯基（OPV），其芳香π-核中含有柔性长亚甲基链（n = 0-12 和 16）。发现 OPV 具有液晶性，其相变焓（也为熵）随烷基链中碳原子数呈奇数-偶数振荡。与奇数相比，侧链中具有偶数亚甲基单元的 OPV 具有更高的焓，表明其高度堆积的固体结构。偏光显微镜分析证实，在 OPV-5 到 OPV-9 中形成了扇形纹理的胆甾相液晶（LC）相，具有焦点双折射。OPV-10 到 OPV-12 的熔点呈类正弦趋势，表明 OPV 中从固态到胆甾相 LC 中间相的分子自组装的转变。在更长的烷基链长度下，烷基链之间的范德华相互作用变得占主导地位，并将介晶效应传递到层片中；因此，层片沿球晶的径向生长方向发生扭曲自组织，产生亮带和暗带。胆甾相 LC 和带状纹理的扫描电子显微镜（SEM）分析强烈支持带状纹理 OPV 中扭曲层片的存在。变温 X 射线衍射分析证实了固态中分子自组织的可逆性，并显示了带状 OPV 中更高有序层状结构的存在。采用激发、发射和时间分辨荧光衰减测量等光物理特性来追踪其液晶相中的分子自组织。OPV 样品的发射光谱显示出其发射波长相对于烷基链长度的奇偶振荡。高度堆积的偶数 OPV 与结晶度较低的奇数 OPV 相比，蓝移更大。OPV 的时间依赖性荧光衰减遵循双指数拟合，其寿命（τ(1)和τ(2)值）表明奇数 OPV 的衰减速度快于偶数 OPV。在所有的 OPV 中，OPV-8 和 OPV-12 的τ(2)值都非常高，表明它们具有很高的发光特性。总之，首次巧妙地利用了大体积液晶状 OPV 发色团来探测芳香 π-堆积与范德华相互作用对π-共轭体系分子自组装的影响。
