Smitha P, Asha S K
Polymer Research Group, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram 695019, Kerala, India.
J Phys Chem B. 2007 Jun 14;111(23):6364-73. doi: 10.1021/jp071303j. Epub 2007 May 23.
New fluorescent azobenzene dyes and side-chain polymers have been synthesized and characterized and their photophysical properties studied. A series of azobenzene dyes having different fluorophores such as phenol (S1), phenylphenol (S2) and naphthol (S3) incorporated in them were synthesized. S2 had unusually high fluorescence with a quantum yield of phi f = 0.2 recorded in dichloromethane (DCM), whereas S1 and S3 were found to be weakly fluorescent. The azobenzene dyes were converted into methacrylate monomers having short ethyleneoxy spacers and then free radically polymerized. Phenylphenol-based azobenzene polymer (P2) continued to show fluorescence, whereas fluorescence was completely quenched in the case of phenol (P1)- and naphthol (P3)-based polymers. Phenylphenol, though twisted in the ground state is known to have a more planar geometry in the excited state--a factor that enables it to retain its fluorescence behavior even when it is incorporated as part of an azobenzene unit. In contrast, naphthol, which is a better fluorophore compared to phenylphenol, loses much of its emissive behavior upon coupling to the azobenzene unit. The extent of trans to cis photoisomerization in solution was very low (approximately 17%) for P2 after 30 min of continuous irradiation using 365 nm light, in contrast to approximately 40% for P1 under identical conditions. This is attributed to the steric repulsion brought about by the bulky phenylphenol units that restrict rotation. A 2-fold enhancement in fluorescence emission was observed for P2 upon irradiation by UV light at 360 nm, which relaxed to the original intensity in about 7 day's time. The higher emission of the cis azobenzenes is generally attributed to an inhibition of photoinduced electron transfer (PET) mechanism. The emission of P2 showed a concentration dependence which increased initially and then decreased in intensity with the formation of a new red-shifted peak at higher concentration due to aggregation. Irradiation of the fluorescence quenched highly concentrated (1 x 10(-3) M) sample of P2 showed an enhancement in emission from aggregates at 532 nm.
新型荧光偶氮苯染料和侧链聚合物已被合成、表征并研究了它们的光物理性质。合成了一系列含有不同荧光团（如苯酚（S1）、苯基苯酚（S2）和萘酚（S3））的偶氮苯染料。S2在二氯甲烷（DCM）中的量子产率为φf = 0.2，具有异常高的荧光，而S1和S3荧光较弱。偶氮苯染料被转化为具有短乙氧基间隔基的甲基丙烯酸酯单体，然后进行自由基聚合。基于苯基苯酚的偶氮苯聚合物（P2）继续显示荧光，而基于苯酚（P1）和萘酚（P3）的聚合物的荧光则完全猝灭。苯基苯酚虽然在基态是扭曲的，但已知在激发态具有更平面的几何结构——这一因素使其即使作为偶氮苯单元的一部分也能保持其荧光行为。相比之下，萘酚作为比苯基苯酚更好的荧光团，在与偶氮苯单元偶联后失去了大部分发射行为。使用365 nm光连续照射30分钟后，P2在溶液中的反式到顺式光异构化程度非常低（约17%），而在相同条件下P1约为40%。这归因于庞大的苯基苯酚单元带来的空间排斥，限制了旋转。在360 nm紫外光照射下，P2的荧光发射增强了2倍，约7天后恢复到原始强度。顺式偶氮苯的较高发射通常归因于光诱导电子转移（PET）机制的抑制。P2的发射显示出浓度依赖性，强度最初增加，然后随着在较高浓度下由于聚集形成新的红移峰而降低。对荧光猝灭的高浓度（1×10⁻³ M）P2样品进行照射，显示在532 nm处聚集体的发射增强。
