Kumar Bharat, Suresh K A
Raman Research Institute, Sadashivanagar, Bangalore 560 080, India.
Phys Rev E Stat Nonlin Soft Matter Phys. 2009 Aug;80(2 Pt 1):021601. doi: 10.1103/PhysRevE.80.021601. Epub 2009 Aug 3.
We have studied the kinetics of trans to cis isomerization under the illumination of ultraviolet light, in the Langmuir monolayer of mesogenic azobenzene dimer, bis-[5-( 4' -n-dodecyloxy benzoyloxy)-2-( 4''-methylphenylazo)phenyl] adipate, at an air-water interface. We find that the trans to cis isomerization reaction of the molecules in the monolayer shows deviation from the first-order kinetics unlike those reported on Langmuir monolayers of azobenzene molecules. We attribute the deviation from first-order kinetics to the simultaneous photoisomerization of trans isomers to form cis isomers and the reverse thermal isomerization of cis isomers to form trans isomers. Our analysis of the rate of change of mole fraction of trans isomers to form cis isomers indicates a first-order kinetics for trans to cis photoisomerization reaction and a second-order kinetics for cis to trans thermal isomerization reaction. This second-order kinetics mechanism is similar to the Lindemann-Hinshelwood mechanism for the unimolecular reactions at low concentration of reactants. The formation of the activated cis isomer by collisions is a slow process as compared to the decay of the activated cis isomer to trans isomer in the liquid expanded phase. This results in the second-order kinetics for the thermal isomerization of cis isomers.
我们研究了在空气 - 水界面处，介晶偶氮苯二聚体双[5 - (4'-正十二烷氧基苯甲酰氧基)-2-(4''-甲基苯基偶氮)苯基]己二酸酯的Langmuir单分子层在紫外光照射下反式到顺式异构化的动力学。我们发现，与报道的偶氮苯分子的Langmuir单分子层不同，单分子层中分子的反式到顺式异构化反应偏离一级动力学。我们将偏离一级动力学归因于反式异构体同时发生光异构化形成顺式异构体以及顺式异构体发生反向热异构化形成反式异构体。我们对反式异构体形成顺式异构体的摩尔分数变化率的分析表明，反式到顺式光异构化反应为一级动力学，顺式到反式热异构化反应为二级动力学。这种二级动力学机制类似于反应物低浓度下单分子反应的Lindemann - Hinshelwood机制。与液体膨胀相中活化的顺式异构体衰变为反式异构体相比，通过碰撞形成活化的顺式异构体是一个缓慢的过程。这导致了顺式异构体热异构化的二级动力学。
