Modulation of Spectrokinetic Properties of o‐Quinonoid Reactive Intermediates by Electronic Factors: Time‐Resolved Laser Flash and Steady‐State Photolysis Investigations of Photochromic 6‐ and 7‐Arylchromenes

A variety of differently substituted 6‐ and 7‐arylchromenes such as that depicted undergo photoinduced CO bond cleavage to yield colored o‐quinonoid intermediates. A combined analysis of μs–ms (laser flash) and real‐time kinetic data show that the o‐quinonoid intermediates decay faster when the C2‐aryl and C6‐/C7‐aryl rings contain electron‐donating and electron‐accepting groups, respectively. Similarly, the decay occurs slowly for the reversed scenario, while intermediate decay rates are observed when both substituents are electron donating. A ready synthetic accessibility of a series of 6‐ and 7‐arylchromenes via Pd0‐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 μs–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 μs–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 μs time‐resolved laser‐flash spectroscopy fo