Moorthy Jarugu Narasimha, Koner Apurba L, Samanta Subhas, Roy Ankur, Nau Werner M
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India.
Chemistry. 2009;15(17):4289-300. doi: 10.1002/chem.200801925.
A ready synthetic accessibility of a series of 6- and 7-arylchromenes via Pd(0)-catalyzed Suzuki coupling protocol has permitted a comprehensive investigation of the thermal decay behavior of a broad set of photogenerated o-quinonoid reactive intermediates. It is shown that substantial mesomeric effect between the benzopyran nucleus and the aryl ring at C6 or C7 position of the former renders significant absorption beyond 350 nm such that they are readily photoactivated to yield colored o-quinonoid intermediates. The absorption spectra of the latter are found to be strongly influenced by the substituents on C2-, C6- and C7-aryl rings; indeed the colored absorptions can be conveniently tuned by appropriate choice of substituents. The thermal decay (bleaching phenomenon), which is important from the point of view of their application in ophthalmic lenses, was investigated in each case by micros-ms as well as real-time absorption spectroscopy. By careful experimentation, we have extracted the decay rate constants for Z,E and E,E o-quinonoid isomers of all 6- and 7-arylchromenes in an attempt to establish a correlation between the electronic attributes with their thermokinetic behavior. From a combined analysis of micros-ms (laser flash) and real-time kinetic data, it is shown that the colored o-quinonoid intermediates decay faster when the C2-aryl and C6-/C7-aryl rings contain electron-donating and electron-accepting groups, respectively. In the same vein, the decay was found to occur slowly for the reversed scenario, while intermediate decay rates are observed when both substituents are electron-donating. Thus, any substituent on the C2-aryl ring that contributes mesomerically to the development of charge on the quinonoid oxygen, and any substituent on the C6-/C7-aryl ring that exerts -I effect appear to expedite thermal decay. Furthermore, evidence is obtained for the first time from mus time-resolved laser-flash spectroscopy for the formation and characterization of the trans,cis (E,Z) o-quinonoid isomer, which has heretofore eluded spectral characterization in the photochromic phenomena of pyrans.
通过钯（0）催化的铃木偶联反应，一系列6-和7-芳基苯并二氢吡喃能够方便地合成，这使得对大量光生邻醌型反应中间体的热衰变行为进行全面研究成为可能。结果表明，苯并吡喃核与前者C6或C7位的芳基环之间存在显著的中介效应，使得在350nm以上有明显吸收，从而它们很容易被光激活，生成有色的邻醌型中间体。发现后者的吸收光谱受C2-、C6-和C7-芳基环上取代基的强烈影响；实际上，通过适当选择取代基，可以方便地调节有色吸收。从它们在眼科镜片中的应用角度来看很重要的热衰变（漂白现象），在每种情况下都通过微秒级质谱以及实时吸收光谱进行了研究。通过仔细的实验，我们提取了所有6-和7-芳基苯并二氢吡喃的Z,E和E,E邻醌型异构体的衰变速率常数，试图建立电子属性与其热动力学行为之间的相关性。通过对微秒级质谱（激光闪光）和实时动力学数据的综合分析表明，当C2-芳基和C6-/C7-芳基环分别含有供电子基团和吸电子基团时，有色邻醌型中间体衰变更快。同样地，对于相反的情况，发现衰变发生得很慢，而当两个取代基都是供电子基团时，观察到中间的衰变速率。因此，C2-芳基环上任何对醌型氧上电荷发展有中介贡献的取代基，以及C6-/C7-芳基环上任何产生-I效应的取代基，似乎都能加速热衰变。此外，首次从微秒时间分辨激光闪光光谱中获得了反式,顺式（E,Z）邻醌型异构体形成和表征的证据，该异构体在吡喃的光致变色现象中迄今尚未得到光谱表征。
