Steady State and Transient Phosphorescence Microwave Double Resonance (PMDR) Studies of Aromatic Carbonyl Molecules; Studies of Experimental Conditions and Properties of π-π* Triplet States

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Abstract
Steady state and transient PMDR studies of several π‐π* type aromatic carbonyl molecules were made. The conditions influencing the steady state PMDR signals were investigated and ways of enhancing the steady state signal intensities were discussed using 4‐chlorobenzaldehyde (CBA) in 1,4‐dibrombenzene host as an example. We studied the effects of microwave sweep width, sweep rates and power levels on the PMDR signals. The experimental conditions to optimize the steady state signals were discussed. The zero field splittings (ZFS) of four π‐π* type aromatic carbonyl molecules were determined and compared. It was observed that ZFS are quite different amongst the individual systems. Possible explanations for these large differences are given. The transient PMDR studies were made on CBA in DBB and trimethylbenzaldehyde (TMBA) in durene. The populating and decay rates of CBA in DBB and TMBA in durene were compared. Various cases of nearly complete and partial inversion, partial and near saturation of spin sub‐levels were reported. Methods of determining the degree of inversion in different cases were discussed. The roles of local electronic symmetry, triplet state energy transfer and external heavy atom effects in determining these rates are discussed briefly.