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Excited‐state kinetics of azoalkanes some further studies with azoethane, azo‐ n ‐propane, and azoisopropane
Author(s) -
Pritchard G. O.,
Marchant P. E.,
Steel C.
Publication year - 1979
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550110903
Subject(s) - chemistry , intersystem crossing , photodissociation , dissociation (chemistry) , isomerization , excited state , photochemistry , singlet state , ground state , triplet state , photoisomerization , singlet fission , kinetics , atomic physics , molecule , catalysis , organic chemistry , physics , quantum mechanics
The photochemistry of azo‐ n ‐propane is investigated at 366 nm up to 1 atm pressure, and over a range of temperature from 50 to 190°C. Some additional experiments with azoethane at room temperature and azoisopropane at 180 and 190°C are also reported. From a consideration of the pressure dependence of the quantum yields for photodissociation a generalized mechanism is proposed which accounts for the known experimental observations in acyclic azoalkane photochemistry. These observations include the extensive photoisomerization data which were previously obtained for azoisopropane. In the mechanistic scheme dissociation at low pressures is believed to occur mainly from S 1 vand T 1 v , the vibrationally excited and randomized first excited singlet and triplet states. At high pressures and low temperatures (≤100°C) the major dissociation channel is probably a nonrandom S 1 state. In direct or singlet sensitized photolysis isomerization occurs predominatly at high pressure and is postulated to occur by internal conversion from S 1 0 , the thermalized singlet, to the ground state. During the process partitioning to the cis or trans isomer is equally probable. In triplet sensitized photolysis isomerization occurs via intersystem crossing from T 1 to the ground state. At elevated temperatures (>150°C) dissociation from S 1 0 , which has a significant activation energy, can compete with return to the ground state.