Phosphorescence, monitoring triplet state

Because excited triplet states decay more slowly than excited singlet states, it is much easier to determine the excited triplet-state lifetime 3t than H. Phosphorescence emission from a degassed sample at low temperature (77K) lasts for longer than 1ms and may even be several seconds. The molecules in the sample are irradiated with a short ( 1 ps) flash and the decay of the phosphorescence signal is monitored using an oscilloscope. Any accompanying fluorescence signal will decay too rapidly to be observed. The excited triplet-state lifetime is obtained as the time taken for the emission intensity to fall to 1/e of its initial value. 

Reticulum ATPase [105,106], Owing to the long-lived nature of the triplet state, Eosin derivatives are suitable to study protein dynamics in the microsecond-millisecond range. Rotational correlation times are obtained by monitoring the time-dependent anisotropy of the probe s phosphorescence [107-112] and/or the recovery of the ground state absorption [113— 118] or fluorescence [119-122], The decay of the anisotropy allows determination of the mobility of the protein chain that cover the binding site and the rotational diffusion of the protein, the latter being a function of the size and shape of the protein, the viscosity of the medium, and the temperature. 

The effect on the triplet state kinetics of binding MeHg to tryptophan or benzimidazole has been monitored using o.d.m.r. and polarized phosphorescence... 

Slow-passage ODMR signals frequently are observed by the continuous wave method in which the optical effect is monitored using broadband detection. On the other hand, if the triplet state decay constants are sufficiently large, the microwave power may be amplitude modulated at an audio frequency which results in modulated phosphorescence when the microwave frequency is at resonance. The phosphorescence is then monitored with narrow-band phase-sensitive detection, for a great improvement in the signal/noise ratio. The latter detection method is frequently used to produce a magnetic resonance-induced phosphorescence spectrum by a technique referred to as phosphorescence-microwave double resonance (PMDR). The microwave frequency is fixed at resonance,... 

At the same time collisions induce the thermally equilibrated phosphorescence from the lowest vibronic levels of the T, state, but the overall triplet character of the system (monitored by the T-T absorption or the T-T transfer yield) remains unchanged. The phosphorescence induction may thus be considered as resulting from vibrational relaxation, transferring the molecules from short-lived and weakly fluorescing mixed states to the pure triplet states, with a relatively higher phosphorescence yield (Lahmani et ai, 1974 van der Werf, 1976). We consider, therefore, that the term collision-induced intersystem crossing, often used in this case, is not appropriate. Collisions do not induce, but only sample (by transfer to phosphorescent levels) inherent triplet character of states prepared by optical excitation and unimolecular evolution. 

Figure 7.14 Possible radiationless processes following creation of 6 benzene, one of the vibronic levels which is El-accessible from ground-state benzene (cf. Fig. 7.13). This level may undergo internal conversion (IC) to an isoenergetic, vibration-ally hot So molecule, or it may undergo intersystem crossing (ISC) to an isoenergetic level in triplet state T.,. The T - So phosphorescence transition can be monitored for experimental evidence of ISC. Time-dependent S — Sq fluorescence decay furnishes a probe for depopulation of S., through radiative (fluorescence) and nonradiative (IC, ISC) decay.

Electron transfer kinetics from the triplet excited state of TMPD to PA in polystyrene has been monitored by phosphorescence emission decay in ref. 85. The rate constant has been found to be invariant over the temperature interval 77-143 K. Parameters ae and ve calculated from the phosphorescence decay using eqn. (12) were found to be ae = 3.46 A and vc = 104 s 1. 

---