Phototriplet mechanism

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. 

Further revealing information is obtained from individual monitoring of the polarization evolution profiles of the separate components of the CIDEP spectrum. These are given in Figure 4b. The results clearly indicate that the "new" radical is the primary intermediate, formed before the phenacyl radical. Furthermore, the relative relaxation rate of the spin polarization of the phenacyl radical is faster than that of the "new" radical. A final critical experiment is that when very dry acetonitrile was used as solvent, no CIDEP observations could be obtained for any radicals. From all these observations we are now ready to formulate the primary phototriplet reactions mechanism to account for all these facts. 

With the establishment of the primary phototriplet reduction mechanism we now turn to the explanation for the effect of flow rates and the formation of polarized phenoxy radicals. Since reaction [2] is a relatively fast secondary process it is readily understood that the observation of the primary ketyl radicals will be dependent upon flow rate. The triplet polarization (E) of the secondary phenacyl radical should not have been affected but the increased contribution of the E/A Radical-Pair polarization altered the overall appearance of the polarization pattern. The diffusion model of the Radical-Pair theory relates the E/A polarization magnitude to the radical concentration within the reaction zone. Since the phenacyl radical is considered to be very chemically reactive, and the product phenol "accumulated" within the reaction zone is also a much better hydrogen donor, the following reactions will proceed within the reaction zone ... 

In photolysis of ketones CIDNP studies have confirmed that the Norrish type I split occurs predominantly from a phototriplet state (32,38,118), although some of the reactions with aliphatic ketones exhibit polarization involving both the excited singlet and the triplet (47,118) states as well as the postulated exci-plex intermediates (71). An exciplex mechanism has also been postulated in the CIDNP observation of the photolysis of tri-fluoroacetophenone with dimethoxybenzene in acid solutions (117). 

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