Light-induced hydrogen abstraction from isobutanol by thienyl phenyl, dithienyl, and thienyl pyridyl ketones

In this paper the bi-molecular interactions of some heteroaromatic (thienyl phenyl, dithienyl, and thienyl pyridyl) ketones with isobutanol were studi ed by using nanosecond laser flash photolysis in trifluoroethanol at room t emperature. The absorption spectra of the triplets and ketyl radicals and t heir decay dynamics were investigated. Depending on the thienyl ring positi on (2 or 3), two types of photobehavior were found: (1) the 2-thienyl keton es, which have lowest triplets of pi,pi* character, abstract hydrogen at a slow rate (k(H) < 2 x 10(4) dm(3) mol(-1) s(-1)) and (2) the 3-thienyl keto nes, which have lowest triplets of n,pi* character, abstract hydrogen at a faster rate (k(H) similar to 3 X 10(5) dm(3) mol(-1) s(-1)). For most of th e latter compounds, both n,pi* and pi,pi* triplets are spectrally and kinet ically observed. While in 2-thienyl ketones the upper reactive n,pi* triple t is thermally populated by the lowest pi,pi* one, in 3-thienyl ketones the two states are independent populated from the singlet state and vibronic m ixing between them is not efficient.