Evidence of anti-parallel dimer formation of 4-cyano-4′-alkyl biphenyls in isotropic cyclohexane solution
Evidence of anti-parallel dimer formation of 4-cyano-4′-alkyl biphenyls in isotropic cyclohexane solution
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4-氰基-4-烷基联苯在各向同性环己烷溶液中形成反平行二聚体的证据
DOI：
10.1039/c4cp04933k
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发表时间：
2015
期刊：
影响因子：
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通讯作者：
Shunzo Takabatake and Toshiyuki Shikata
中科院分区：
文献类型：
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作者：
Kengo Arai;Misumi Okuzono and Toshiyuki Shikata;大久保 敬;前田大光・甲田直也;狩俣 歩，鈴木修一，小嵜正敏，舘 祥光，岡田惠次;Shunzo Takabatake and Toshiyuki Shikata
在各向同性环己烷溶液（nCB - ch）中证实了两种液晶形成4-氰基-4′-烷基联苯（nCB, n = 5（戊基）和n = 8（辛基））的反平行二聚体的形成。除了50 GHz的高频介质弛豫（DR）测量外，还进行了280 ~ 500 nm波长范围内的荧光发射（FE）实验来研究nCB分子的分子动力学。中、高浓度溶液的DR谱由两种动态模式组成。nCB单体分子的自由旋转具有一个弛豫时间约为90 ps的快速模式。弛豫时间约为400 ps的慢模式归因于反平行二聚体的解离过程（(nCB)2）。测量氰基偶极矩方向相关性的Kirkwood因子（gK）在慢模式下明显小于1，这表明形成了反平行二聚体（nCB）2。各向同性溶液中反平行二聚体形成的平衡常数，即通过DR数据确定的反应2nCB↔（nCB）2的平衡常数随着nCB浓度的增加而增加。在极稀的条件下，在325 nm的FE光谱测量中观察到nCB单体的荧光发射信号。然而，在DR测量中使用的中至高浓度中，可以清楚地观察到慢模式，在390 nm处观察到宽的FE信号，这被分配到准分子发射（包括激发态二聚体的发射）。准分子发射相对于单体发射的相对强度随着浓度的增加而显著增加。此外，准分子形成的平衡常数与反平行二聚体形成的平衡常数基本一致。结果表明，在各向同性溶液中形成的nCB和（nCB）2二聚体的准分子是相同的化学物质。
The formation of anti-parallel dimers of two liquid crystal forming 4-cyano-4′-alkyl biphenyls (nCB, n = 5 (pentyl) and n = 8 (octyl)) was confirmed in an isotropic cyclohexane solution (nCB–cH). In addition to high frequency dielectric relaxation (DR) measurements up to 50 GHz, fluorescence emission (FE) experiments in the wavelength range from 280 to 500 nm were carried out to investigate the molecular dynamics of the nCB molecules. The DR spectra of solutions at intermediate to high concentrations were comprised of two dynamic modes. A fast mode with a relaxation time of ca. 90 ps was assigned to the free rotations of monomeric nCB molecules. The slow mode with a relaxation time of ca. 400 ps was attributed to the dissociation process of the anti-parallel dimers ((nCB)2). The Kirkwood factor (gK), a measure of the orientational correlation between the dipole moments of the cyano groups, was markedly less than unity for the slow mode, which demonstrated the formation of anti-parallel dimers, (nCB)2. The equilibrium constant for anti-parallel dimer formation in an isotropic solution, i.e., for the reaction, 2nCB ↔ (nCB)2, which was determined via the DR data increased with an increase in the concentration of nCB. Under extremely dilute conditions, a sharp fluorescence emission signal attributed to the nCB monomer was observed at 325 nm in FE spectrum measurements. However, at the moderate to high concentrations used in the DR measurements where the slow mode was clearly observed, a broad FE signal at 390 nm was observed, which was assigned to excimer emission (including emission from the excited ground state dimers). The relative intensity of the excimer emission to the monomer emission significantly increased with an increase in the concentration. Moreover, the equilibrium constant for the excimer formation reasonably agreed with that for the anti-parallel dimer formation evaluated by the DR data. Consequently, the results of the FE measurements evidently revealed that excimers of nCB and the (nCB)2 dimers formed in isotropic solution are identical chemical species.