Optical Pumping of the Lowest Triplet State and Multiple Resonance Optical Techniques in Zero Field

The principles involved in the optical detection of zero-field transitions, electron spin–electron spin double resonance (EEDOR) transitions, and electron spin–nuclear spin double resonance (ENDOR) transitions in zero-field of the lowest triplet state are briefly summarized. Equations are then derived which relate the changes in the phosphorescence intensity originating from any of the zero-field levels to the radiative and nonradiative intramolecular rate constants of processes involving the triplet state upon: (a) saturation of any of the two zero-field transitions involving the emitting level with microwave radiation; (b) simultaneous saturation of two zero-field transitions, only one of which involves the emitting level optical detection of electron–electron double resonance (EEDOR); (c) simultaneous saturation of a zero-field transition involving the emitting level and a hyperfine transition involving either of the two electron spin levels being saturated with microwave (optical detection of electron–nuclear double resonance, ENDOR). These equations are derived for three different systems, in all of which the spin-lattice relaxation processes are assumed negligible (i.e., systems at very low temperatures). In System I, the triplet state is pumped by singlet–singlet absorption followed by the intersystem-crossing processes in the molecule studied (guest). In System II, pumping of the lowest triplet state is carried out by singlet→triplet absorption of the guest. In System III, pumping of the lowest triplet state of the guest is carried out by exciting either the singlet or the triplet states of the host, which is followed by triplet–triplet energy transfer. In System I, the changes in intensity are related to the rate constants of the intersystem-crossing processes to and from the individual zf levels of the molecule being studied. In System II, the radiative rate constants of the optical transition used for pumping, as well as the decay constants of the zf levels of the lowest triplet state of the guest molecule, determine the changes in the phosphorescence intensity upon microwave or radiowave saturation. In System III, in addition to the decay constants of the emitting triplet state, the intersystem-crossing (S1 ↝ T1) rate constant or the singlet→triplet absorption cross section of the host molecule determines the predicted changes in the phosphorescence intensity in the multiple resonance experiment. The relative orientation of the magnetic principal axes of the host to those of the guest is also important in System III. The conservation of spin direction in triplet–triplet energy transfer is briefly discussed. From the equations derived and the observed intensity changes upon microwave saturation, the ratios of important unimolecular rate constants for radiative and nonradiative singlet–triplet processes involving the individual zero-field levels can be determined. Comparison between theoretical predictions and observed effects is made whenever results are available.