Triplet energy acceptors

However, cfc-stilbene does not behave as a classical triplet energy acceptor inasmuch as the rate of energy transfer is much larger than would be predicted from Eq. (9.7) for donors with triplet energies less than 57 kcal/mole. This is then additional information consistent with Eq. (9.5). 

One might use spectral recording in experiments in which one adds a triplet energy acceptor to the reacting system. Results from a study of the fluoran-thene(-)/10-methylphenothiazine(+) reaction are shown in Figure 28.15 [86,99]. This process is an energetically deficient system that is thought to yield its blue fluoranthene-like emission via redox-produced fluoranthene (FA) triplets ... 

Sometimes the fluorescence and phosphorescence spectra of a compound in solution overlap. They may be separated as follows. If a suitable triplet energy acceptor is added, this will quench the phosphorescence, leaving the fluorescence unaffected, while a suitable triplet donor will sensitize the phosphorescence in the absence of any fluorescence. Back-strom and Sandros have analyzed the total luminescence spectra of biacetyl, benzil, and anisil in this way, using pyrene as the triplet acceptor and benzophenone as triplet donor.3... 

DeGraff and Calvert (66) have carried out careful steady-state photolyses of H2CO and D2CO with added pressures of propylene, neopentane, and biacetyl at 334.0, 313.0, 265.4, and 253.7 nm. Propylene was used as an olefinic radical scavenger, neopentane as an "inert" gas for vibrational deactivation, and biacetyl as a possible triplet energy acceptor. 

It is well established that the triplet ENT occurs at the diffusion-controlled rate (7tdin) when ET of the triplet energy donor is 13 kJ mol-1 higher than that of the triplet energy acceptor [41, 52]. In the present case, the ET values of DCB and DCNB in the Ti state (335 and 305 kJ mol-1, respectively) are much higher... 

Other examples of NVET include the triplet energy acceptors 1,3,5,7-cyclooctate-traene,116 which has a puckered structure in the ground state but relaxes to a planar structure in the triplet state, and a cyclobutadiene derivative,117 which changes from a rectangular to a square structure. 

Stilbene, for which several of the above criteria have been applied, has already been considered. Examination of some of the other cases is instructive. We start with a comparison of the behavior of 1-phenylcyclohexene (PC) with that of l-(l-naphthyl)cyclohexene (1-NC). PC triplets are short-lived (= 63 ns), are not quenched by low-triplet-energy acceptors, decay to give cis and trans isomers (71), and are quenched very efficiently by O2 (I = 9 x s ). Taken... 

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