Triplet spin-lattice

CIDEP Initial Polarization. The establishment and the development of the photoexcited triplet mechanism in CIDEP of transient radicals in solution had been rather controversial, if not as turbulent and exciting as the photoexcitation process itself. The early objections centered around two very important questions. The first one concerns the uncertainty of whether the spin polarization in the molecular frame can be effectively transferred to the laboratory frame for triplet systems in liquid solution. The second related question involves the fact that the polarized triplet molecules are rotating rapidly with respect to the laboratory axes and the triplet spin lattice relaxation time T x (normally between 10 and lO-- - s) would be too short for the polarization to be retained in the radical pair. The earlier photoexcited triplet mechanism developed by Wong et al. (136,137) is based on a "static model" with the excited triplet molecules being randomly oriented. Such a static model cannot deal satisfac-... 

Since the amount of polarization that can be retained in the radical pair after the triplet reaction will depend upon both the chemical reaction rate and the intrinsic triplet spin lattice relaxation rate, the spin polarization of the radical pair will be given by

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A technique for measuring triplet spin-lattice relaxation times in fluid solution, based on the observation of chemically induced electron spin polarization, in the presence of a triplet quencher, has been applied to duroquinone with triethylamine as the triplet quencher.237 E.s.r. studies of the triplet states of 9-aza-bicyclo-... 

K. W. Rousslang and A. L. Kwiram, Triplet state decay and spin-lattice relaxation rate constants in tyrosinate and tryptophan, Chem. Phys. Lett. 39, 226-230 (1976). 

In the experiment done under conditions of inefficient spin-lattice relaxation the triplet spin states originally produced are retained during internal conversion and vibrational relaxation. Important conclusions about the routes of intersystem crossing can thus be obtained from a study of the population of the triplet sublevels, the so-called "spin alignment . 

This change in the triplet decay rate constant was verified by ESE measurements where spin lattice relaxation effects can be minimized from the decay rate constant measurements (14). Observed decay rate constants are given in Table I for the y triplet level... 

In this mechanistic scheme, the CIDNP intensities of reactant and product are determined by the competition of key steps at each stage of the reaction. For the system discussed here, the qualitative features of the observed polarization suggest that nuclear spin lattice relaxation during the lifetime of the olefin triplet state is negligible, that singlet and triplet pairs recombine with similar efficiencies, and that the triplet state decays to each of the isomers with equal efficiency. 

Second, it is conceivable that amines quench triplet ketones before spin-lattice relaxation takes place within the three triplet sublevels 159>, which are populated unevenly 160>. In that event, radicals can be produced with their electron spins polarized. The CIDEP phenomenon 161>, whereby EPR emission is observed upon irradiation of ketones and very reactive substrates, may involve this mechanism. In fact, certain CIDNP observations may depend on rapid quenching of spin-polarized triplets 162>. 

Based upon the current theories of CIDEP and CIDNP, we propose that in many photochemical systems the primary photochemical reaction of the excited triplet state contributes to magnetic polarization via the triplet mechanism. The secondary reaction of the polarized primary radicals may transfer their initial polarization to the "secondary radicals" provided that the radical reactions can compete with the radical spin-lattice relaxation process (59,97). On the other hand, secondary reactions of the primary radical pair or the uncorrelated F pair contribute to polarization by the radical-pair mechanism. A general scheme showing the possible and simultaneous operations of both the... 

The introduction of the photochemically excited triplet mechanism leading to CIDEP of the resulting radicals has added a new dimension to the potentials of the CIDEP techniques in photochemistry. In liquid photochemical systems, very little is known experimentally about the exact nature of the intersystem crossing process, but the rate or efficiency of such ISC process can sometimes be estimated by chemical (86) and optical methods (51,105). The treatment of the phototriplet mechanism in CIDEP of radicals in liquid solution is consistent with the following conclusions (1) ISC occurs mainly by the spin-orbit coupling mechanism in carbonyl compounds, (2) spin polarization of the triplet sub-levels is obtained via the selective ISC processes, and (3) the chemical reaction rate of the triplet is at least comparable to its depolarization rate via spin-lattice relaxation. 

Satisfaction of the third condition above depends on the rates of the spin-lattice relaxation processes between the spin sublevels. These rates are highly temperature dependent and depend also on the environment within which the molecular system is placed. In order to maintain a steady-state triplet sublevel population imbalance, the rates of spin-lattice relaxation must be slower than the rates at which the sublevels decay to the ground state. To reduce the spin-lattice relaxation rates to this level requires temperatures of the order of 4.2°K or lower, in the proper solvent. Whether or not spin-lattice processes can be "frozen" at liquid helium temperatures may even depend on the solid phase of the... 

L. Hall, Ph.D. dissertation, "Spin-Lattice Relaxation and Zeeman Effects in the Triplet State of Pyrazine d4," UCLA (1971)... 

Keywords Triplets in organometallic compounds. Chemical tunability. Spin-lattice relaxation. Excited state binding properties, Vibronic coupling. Spatial extensions of electronic states, ODMR results... 

These latter discussions in 1. and 2. show that effects of spin-lattice relaxation can in principle be studied by an investigation of the temperature dependence of decay properties, even when the spectral resolution is one order of magnitude smaller than the zero-field spHtting of the triplet. (Compare also Sect. 3.1.) However, since more adequate methods, such as the techniques of microwave double resonance are available, one should use these latter methods with preference. (See the reviews [32,90,130]). But when the optical resolution is sufficient due to a larger zero-field spHtting, as is found for Pt(2-thpy)2, optical investigations of effects of spin-lattice relaxation become highly successful [24,65] and thus will represent the methods of preference as will be shown in detail in Sects. 4.2.7 to 4.2.9. 

With temperature increase from T = 1.2 K to T > 5 K, the decay behavior changes drastically. At T = 5 K, the decay is already monoexponential with a decay time of r(5 K) = (230 10) ps (Plot (b) of Fig. 6). Within limits of experimental error this value is constant at least up to T = 40 K [57]. Obviously, temperature increase induces an efficient spin-lattice relaxation between the three triplet substates. This leads to a fast thermalization. The observed monoexponential decay demonstrates that the sir is much faster than the shortest emission decay component. 

The results discussed above have shown that time-resolved emission spectroscopy can provide detailed insight into vibronic deactivation paths of triplet substates, even when the zero-field splitting is one order of magnitude smaller than the obtainable spectral resolution (= 2 cm ). This is possible at low temperature (1.3 K), because the triplet sublevels emit independently. They are not in a thermal equilibrium due to the very small rates of spin-lattice relaxation between these substates. In the next section, we return to this interesting property by applying the complementary methods of ODMR and PMDR spectroscopy to the same set of triplet substates. 

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