Phosphorescence microwave double

It is relatively easy to decide which vibronic bands have a common origin. This is accomplished by observing the phosphorescence intensity change of each band upon microwave saturation at a frequency that corresponds to transitions between rz and tx. This is known as phosphorescence-microwave double resonance (PMDR) spectroscopy. These frequencies for 2,3-dichloroquinoxaline are given in Table 6.3. 

J. Olmsted and M. A. El-Sayed, "Experimental Methods in Phosphorescence-Microwave Double Resonance," to be published in The Creation and Detection of the Excited State, Dekker,... 

However, it is possible, to record emission spectra of individual substates by applying the methods of time-resolved spectroscopy. This has been shown, to our knowledge for the first time for transition metal complexes, by Yersin et al. in Ref. [58]. Having these time-resolved spectra available, it becomes possible for example, to elucidate individual vibronic radiative deactivation paths, as will be shown in this section. Interestingly, results that are deduced from a complementary method, namely from phosphorescence microwave double resonance (PMDR) studies [61], provide a nice agreement with the results deduced from time-resolved investigations. (Compare also Sect. 3.1.5.)... 

In this section, results of three different experimental methods that have been apphed to Pd(2-thpy)2 are reported. Information from optically detected magnetic resonance (ODMR spectroscopy), microwave-recovery measurements, and phosphorescence microwave double resonance (PMDR spectroscopy) is presented. These methods complement each other to a large extent. The discussion presented here can be fimited to the basic impfications of the methods, since a comprehensive review by Max Glasbeek concerning these aspects is found in Volume 213 of this series [90] and a detailed report by Glasbeek, Yersin et al. [61] concerning Pd(2-thpy)2 has only recently been published. 

The method of phosphorescence microwave double resonance (PMDR) spectroscopy is based, like the two other methods discussed above, on c.w. excitation of the Pd(2-thpy)2 compound at low temperature. Additionally, micro-wave irradiation is applied, whereby the frequency is chosen to be in resonance with the energy separation between the two substates I and III of 2886 MHz. With this set-up, one monitors the phosphorescence intensity changes in the course of scanning the emission spectrum. Technically, the phosphorescence spectrum is recorded by keeping the amplitude-modulated microwave frequency at the constant value of 2886 MHz and by detecting the emission spectrum by use of a phase-sensitive lock-in and signal averaging procedure (e.g. see [61, 75,90]). 

The information obtained from the phosphorescence microwave double resonance (PMDR) spectroscopy nicely complements the results deduced from time-resolved emission spectroscopy. (See Sect. 3.1.4 and compare Ref. [58] to [61 ].) Both methods reveal a triplet substate selectivity with respect to the vibrational satellites observed in the emission spectrum. Interestingly, this property of an individual vibronic coupling behavior of the different triplet substates survives, even when the zero-field splitting increases due to a greater spin-orbit coupling by more than a factor of fifty, as found for Pt(2-thpy)2. 

Fig. 10. (a) Time-integrated emission spectrum and (b) PMDR (phosphorescence microwave double resonance) spectrum of Pd(2-thpy)2 at T = 1.4 K dissolved in an n-octane Shpol skii matrix. Concentration = 10 mol/1. Aexc = 330 nm. The PMDR spectrum is induced by a microwave irradiation with a frequency of 2886 MHz, which is in resonance with the energy difference between the triplet substates I and III. An intensity increase (+) signifies vibrational satellites that belong to an emission from the short-lived substate III, while a decrease (-) characterizes satellites of the emission spectrum from the long-lived substate I (Compare Ref. [61])... 

Pd(2-thpy)2 was investigated by applying the methods of time-resolved emission (Fig. 8) and phosphorescence-microwave double resonance (PMDR) spectroscopy (Fig. 10). By both methods, the vibrational satellite structures are resolved and reveal spin selectivity in these satellites. Thus, vibrational satellites can be assigned to the respective triplet substates. The complementary character of these two methods is demonstrated for the first time for transition metal compounds [58,61] (Compare Sects. 3.1.4 and 3.1.5.). 

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. 

Fig. 7. Phosphorescence microwave double resonance (PMDR) spectrum as observed for the 5.0 GHz transition in the excited triplet state of (a) [Rh(phen)3](Cl04)3 and (b) [Rh(phen)2 (bpy)](Gl04)3. Photoexcitation at 320 nm T = 1.4 K...

For each of the two sites, labeled 1 and 2 respectively, two zero-field ODMR transitions could be observed. The resonance frequencies for these transitions are given in Table 9. Conversely, the emission spectrum belonging to each of the ODMR transitions was also measured in a phosphorescence microwave double resonance (PMDR) experiment. The PMDR spectra obtained for the two resonances at 2356 MHz and 2329 MHz, as well as the normal emission spectrum, are presented in Fig. 23. As illustrated in the figure, in PMDR one can separate the emission spectra for sites 1 and 2 in the matrix. Table 9 summarizes the main optical, ODMR, and PMDR results. 

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,... 

If the separation among the sublevels is in the range of microwave frequency, sublevel properties can be obtained by observing the effect of microwave resonance on the emission from this state. The zero-field splitting is of the order of microwave frequency for most of rr/r states. Thus, the sublevel properties can be obtained by analyzing the effect of microwave resonance on the phosphorescence intensity. The method is called phosphorescence-microwave double resonance (PMDR) or optical detection of magnetic resonance (ODMR). 

El-Sayed, M.A. and Olmsted, J. 11 (1971) Intersystem crossing relative rates from pulsed-exdtation phosphorescence microwave double resonance. Chem. Phys. Lett., 11, 568. 

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