Properties of Triplets

The experiment which unequivocally demonstrated that the metastable phosphorescent state of organic compounds is an excited triplet was performed by Lewis and Calvin in 1945.3 They actually measured the paramagnetic susceptibility of fluorescein dissolved in boric acid crystals under intense illumination. 

In the past few years the ESR technique has been applied to demonstrate the triplet nature of many highly reactive organic biradicals and of various metastable photoexcited states, to estimate the rates of their decay, and to evaluate their electronic distribution. These metastable states are normally produced and observed either frozen in glassy matrices at 77°K or aligned in a host crystal. Spectra have also been obtained of triplet species dissolved in a translucent plastic. 

A fairly direct quantum mechanical treatment results in the following spin Hamiltonian4-5  

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To summarize, the properties of triplet and singlet diradicals are closely related to the effectiveness of through-bond and through-space interactions, which are governed by the orbital phase continuity/discontinuity properties. In the next two sections, we will utilize this simple model to predict the spin preference and intramolecular reactivity for a broad range of diradicals. 

Several examples of the application of energy transfer to determine the properties of triplet states are given below. The reasons why triplet energy transfer can be so successfully applied are ... 

Photophysical Properties of Triplet Emitters Controlled by Metal Participation... 

Knowledge of the magnetic (and optical) properties of triplet states has been greatly enhanced by the development of zero-field (zf) resonance techniques, especially those employing optical detection. In what follows, we review the selection rules which govern the transitions in the zf experiment. We then present recent results from this laboratory on the lowest (nTc ) states of 1-halonaphthalenes and discuss in some detail the analysis of these spectra and their significance with respect to the intramolecular heavy-atom effect on the properties of the parent molecule. Next, we survey some representative results from other laboratories, including zf EPR, ODMR, ENDOR, and ELDOR experiments, and close with a brief description of other zf applications. 

It should be apparent from the above that considerable information about the magnetic properties of triplet states can be obtained from a zf experiment. Table 6 summarizes the spin Hamiltonian parameters measured for 1-halonaphthalenes (IXN X = Cl, Br, I) in zero field additional parameters are known fi-om the high-field experiments (Kothandaraman et al., 1974b, 1975,1979) and are included for discussion purposes. We also list the parameters of the lowest triplet states of the parent molecule (Hutchison and Mangum, 1961) and its 1-fluoro derivative (Mispelter et al., 1971) for comparison. Measurements of many of the relevant optical properties of these species have been reported (Schwoerer and Sixl, 1969 Sixl and Schwoerer, 1970a Kothandaraman et al., 1975, 1979 Saigusa and Azumi, 1978) and some of these are listed in Table 7. 

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