Microwave, and Related Studies

Huber and Schwoererdescribe e.s.r. experiments which show the existence of quintet states (S = 2) after u.v. irradiation at 4.5 K in perdeuteriated / -toluene-substituted diacetylene. Nine e.s.r. transitions per quintet were detected and were attributed to bicarbenes of short oligomers. E.s.r. has also been applied to the study of photochemical reactions of fluoro-substituted ketones with amines, tetraphenylborates, and organometals.  

Time-resolved e.s.r. is shown to be a useful technique in the direct observation of k- k scattering in a one-dimensional triplet exciton system, 1,2,4,5-tetrachlo-robenzene. This technique appears to be well suited to the determination of time constants involved in the scattering process. 

Two papers have been presented on the photochemistry of 5-methylphena-zinium salts in aqueous solution. Fluorescence, optical flash photolysis, and electron paramagnetic resonance (e.p.r.) techniques have been used to elucidate various aspects of product formation and quantum yield. Two products have been identified, namely the 5-methyl-10-hydrophenazinium cation radical (MPH ) and the pyocyanine (l-hydroxy-5-methyl-phenozinium) cation (PyH ) in a stoicheiometric ratio of 2 1. The quantum yield of formation of (MPH ) was found to be 0.29 0.03 at pH 7.0 and 1.1 0.1 at pH 3.0. The triplet state of MP (Ti) has also been detected by triplet-triplet absorption and is found to have a lifetime of 0.5 ns. Flash photolysis and e.p.r. have also been used to study a geminate triplet radical pair obtained from hydrogen abstraction by excited triplet acetone from propan-2-ol. The authors demonstrate that the geminate pairs contribute most of the polarization in photochemically-induced dynamic electron polarization (CIDEP) as compared with free random-phase pairs. 

The nearest-neighbour excited-state exchange matrix elements in dimers of 1,2,4,5-tetrachlorobenzene (TCB) have been independently obtained using high-resolution phosphorescent - microwave double resonance (p.m.d.r.) and o.d.m.r. techniques. The authors conclude that the matrix element for energy transfer in the triplet state of TCB is not the same for dimer and exciton states. 

The assignment of the orbital symmetry of the phosphorescent triplet state of iV,A, A, jV -tetramethyl-p-phenylenediamine (TMPD) has been studied by microwave-induced delayed phosphorescence (MIDP) and the assignment of B2 for the orbital symmetry established.  

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