Microwave-induced delayed phosphorescence

Schmidt, J., Antheunis, D. A., van der Wtials, J. H. The dynamics of populating and depopulating the phosphorescent triplet state as studied by microwave induced delayed phosphorescence. Mol. Phys. 22. 1 (1971). 

Burland and Schmidt [190] performed microwave induced delayed phosphorescence (MIDP) experiments using a sample preparation with a benzene host crystal for both pyrazine and pyrimidine. With this technique they found that the relative radiative rates closely resembled the relative total rates for the different spin sublevels, indicating a high quantum yield and low non-radiative rate. The spin sub-level rates of pyrazine for Tr and rv were given as 284 and 6.3 ms, respectively, to be compared with the best response theory result of 282 and 12 ms. The z (out-of-plane) spin component is symmetry forbidden, and its lifetime (400 ms) derives from the lattice perturbation. 

Clarke (326) has studied the optical electron spin polarization in triplet anthracene and has observed ESR emission at 1.5°K which was attributed to a non-Boltzman distribution over the triplet spin levels at low temperature. The dynamics of optical spin polarization in triplet naphthalene at 1.6°K was also reported by Sixl and Schwoerer (327a) and van der Waals et al. (327b). have used a general method to study dynamics of populating and depopulating triplet spin levels by microwave-induced delayed phosphorescence. These experiments enable measurements of the lifetimes of each triplet spin state and thus can provide important information about intramolecular decay processes and intermolecular triplet energy transfer. 

Microwave-Induced Delayed Phosphorescence (MIDP) Method... 

Metalloporphyrins Optically detected ESR, 87, 88 microwave induced delayed phosphorescence, 87, 89 Zeeman spectroscopy, 94 MCD, 95 resonance Raman spectra, 97, 100, 101 ESR, 88,96 photolysis 142 ... 

TTie porphyrin ZnP in a crystalline n-octane matrix at 1.2 K was investigated by optically detected magnetic resonance (ODM) and microwave induced delayed phosphorescence (MIDP) . ... 

Table 10-1. ZF splittings and rate constants determined with the technique of microwave induced delayed phosphorescence for individual triplet sub-levels of pyrimidine in benzene at 4.2 K [2].

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. ... 

In the method of microwave-induced delayed phosphorescence (MIDP), the excitation is suddenly extinguished at t = 0. After a time interval, a microwave pulse or rapid passage is applied to induce transitions between T and T. The sudden change in the phosphorescence intensity is described by... 

Fig. 7.26 Microwave-induced delayed phosphorescence. Above two superposed phosphorescence-intensity decays of quinoline in a durene crystal at T = 1.35 K and Bq = 0. The phosphorescing triplet component is Tz). The delayed phosphorescence signals, delayed in the first experiment by ca. 2 s and in the second by ca. 4 s, are produced by resonant 1000.5 MHz pulses, which saturate the zero-field transition Ty Tz at these times after the end of the UV excitation and thus...

A second direct optical-detection method for selective population and depopulation is microwave-induced delayed phosphorescence in zero field (Bq = 0) [25]. Figure 7.26 shows the phosphorescence intensity from quinoline in a durene (tet-ramethyl benzene) host crystal at T= 1.35 K as a function of the time after the end of the UV excitation. The phosphorescing zero-field component here is Tz). Its lifetime is considerably shorter than those of the other two zero-field components, from which furthermore no phosphorescence is emitted. If the zero-field transition... 

Because the relaxation rates of zero-field sub-levels were found to be much reduced by lowering the temperature from 4.2 K, the spin sub-levels in many molecules could be isolated from each other at 1.25 K. During the decay of such an isolated triplet spin system, drastic changes could be induced in the phosphorescence intensity by the sudden irradiation of a microwave transition between a pair of levels. Through such microwave-induced delayed... 

Microwave-induced delayed fluorescence studies have provided information on the triplet states of toluene 230 and p-chloroaniline.231 Magnetic resonance saturated phosphorescence decay measurements on tyrosine,232 and cidnp studies on but-2-enone and pentan-3-one,233 1,4-benzoquinone in propanol,234 9-methylacridine,235 and the dye-sensitized photo-oxidation of phenols230 have been reported. 

Complementary to the method of microwave-recovery, the method of micro-wave-induced delayed phosphorescence (MIDP) is sometimes used for studying population kinetics. MIDP is particularly suited for two-level systems in which only one of the levels is radiative, whereas the other, dark level, is long-living. The microwave recovery is mostly applied when both levels are radiative. In the MIDP experiment the exciting light is chopped (or pulsed). In the dark time, after optical excitation, a resonant microwave pulse is applied at the delay time f(j. At t(j> the population still present in the non-radiative level is in part transferred to the radiative level. Thus a phosphorescence intensity change is induced, the amplitude of which is proportional to the population present in the non-radiative level, at time Measurement of the amplitude decay with gives the decay transient for the non-radiative level. The (fast) decay of the delayed phosphorescence transient at times t>t is typical of the decay of the radiative level. 

Thus from the initial intensity and the leading edge of the microwave-induced phosphorescence signal resulting from the microwave pulse given at a delay time t after cutting off the excitation, the ratio K /K can be determined. 

Ty -o- Tz is rapidly saturated by a short resonant microwave pulse (1000.5 MHz) after the end of the UV excitation, thus when the radiative component Tz) has already mostly decayed, then the radiative component Tz) will again be populated from the non-radiative component Ty), thus inducing renewed phosphorescence. With a still longer delay of the microwave pulse, the increase in the phosphorescence intensity is smaller. From this, the lifetime of Tf) can be derived. With this method, the decay constants of all three zero-field components can be determined. 

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