Somlai Alline P, Cozad Richard A, Page Kirt A, Williams Holly R, Creed David, Hoyle Charles E
Department of Chemistry and Biochemistry, University of Southern Mississippi, Hattiesburg, Mississippi 39406-5043, USA.
Photochem Photobiol Sci. 2008 May;7(5):578-87. doi: 10.1039/b719409a. Epub 2008 Mar 4.
Several aspects of the photochemistry and photophysics of four main chain liquid crystalline polyesters with a rigid trans-stilbene 4,4'-dicarboxylate mesogen as chromophore and flexible spacer groups are reported. The three polymers with the longest 'spacer' groups are liquid crystalline at room temperature, two have smectic phases. Chromophore aggregation has a dramatic effect on the photophysics and photochemistry of these polymers. Each of the polymers in poor solvents or as films has greatly perturbed UV-Vis absorption and fluorescence spectra due to aggregation of the stilbene chromophore. These effects are more pronounced upon annealing above the glass transition temperature, T(g), and in the mesophase. Film fluorescence is excitation wavelength dependent and is suppressed at elevated temperatures. The stilbene 'environment' in both films and solution is clearly heterogeneous and energy transfer processes relatively slow. The dominant photochemical reaction upon direct excitation above 300 nm is 2 + 2 photocycloaddition rendering polymer films insoluble. No significant trans-to-cis photoisomerization can be detected upon initial irradiation of the polymer films. There is evidence for the formation of aldehyde and carboxylate functionality upon irradiation in the presence of air. Loss of the aggregate UV-Vis absorption and fluorescence occurs during irradiation. Difference UV-Vis spectra of irradiated films suggest preferential initial consumption of dimeric aggregates. Loss of stilbene UV-Vis absorption upon irradiation above 300 nm can be partly photoreversed upon subsequent 254 nm irradiation. The rate of stilbene chromophore loss from films increased significantly above Tg and in the smectic phase above room temperature.
报道了四种以刚性反式二苯乙烯4,4'-二羧酸酯介晶作为发色团和柔性间隔基团的主链液晶聚酯的光化学和光物理的几个方面。具有最长“间隔”基团的三种聚合物在室温下是液晶态，其中两种具有近晶相。发色团聚集对这些聚合物的光物理和光化学有显著影响。由于二苯乙烯发色团的聚集，每种聚合物在不良溶剂中或制成薄膜时，其紫外-可见吸收光谱和荧光光谱都受到极大干扰。在高于玻璃化转变温度T(g)以及在中间相进行退火时，这些影响更为明显。薄膜荧光与激发波长有关，并且在升高的温度下受到抑制。薄膜和溶液中二苯乙烯的“环境”明显不均匀，能量转移过程相对较慢。在300nm以上直接激发时，主要的光化学反应是2 + 2光环加成，使聚合物薄膜不溶。在最初照射聚合物薄膜时，未检测到明显的反式到顺式光异构化。有证据表明在有空气存在的情况下照射会形成醛和羧酸盐官能团。在照射过程中，聚集态的紫外-可见吸收和荧光会消失。照射薄膜的差分紫外-可见光谱表明二聚体聚集体优先被初始消耗。在300nm以上照射后二苯乙烯紫外-可见吸收的损失在随后254nm照射时可部分光逆转。高于Tg以及在室温以上的近晶相中，薄膜中二苯乙烯发色团的损失速率显著增加。
