Inherent singlet lifetime

In Scheme 10 electron transfer to singlet state is portrayed. The plots obtained are also consistent for a process which involves electron transfer to the triplet state. However, using the values of the (slope)/(intercept) for plots of l/4> vs. 1/[A] for 17,18 and 19 in the exciplex dominant region and the values of the triplet lifetimes obtained from phosphorescence measurements (112, 36, and 29 ms), values of kex 10 - 102M 1s 1 are obtained which are too low. Estimating the inherent radiative lifetime from the molar absorptivity, kisc must be 109 - 1010 s 1, since no detectable fluorescence was observed. The slope over the intercept using Eq. 16 now yields values 109 — 1010 M "1 s 1 for kex which is more reasonable [39]. 

The weak acid HNO (pA a = 11.4) has a singlet gronnd state (X A ). The nitroxyl anion NO has a triplet ground state (X ), therefore the dissociation of HNO to NO and H+ is spin forbidden and inherently slow. In physiological systems, HNO is the dominant form, but NO, once formed, has sufficient lifetime to react with other species. HNO is like NO a transient intermediate in physiological systems. 

Compared to type I fluorescent polymers, the conjugated polymer backbone is the active chromophore. The monomer units that make up the polymer might not be inherently fluorescent. The absorption and emission of photons involve electronic transitions between a ground-state singlet (So) and a excited-state singlet (typically Si) as shown in Fig. 1.3. Radiative (kT) and nonradiative (km) transitions result in relaxation to the ground state and the observed kinetic lifetime t of these systems is governed by the relationship... 

The trans stereoselectivity may easily be rationalized as a result of sterics, although direct irradiation of the 1-phenylcyclohexene gives a similar ratio of trans cis stereoselectivity. The singlet reaction presumably does not involve a biradical intermediate of significant lifetime. Thus, the trans selectivity may be enhanced by sterics in the case of the triplet reaction. The preference for trans isomer may be inherent to the approach of the alkenes or, perhaps, it is a function of selective reversibility of biradical intermediates (see Scheme 1). Caldwell has published a formula for predicting the potential for photocycloaddition of alkenes and arenes in the singlet excited state [42], His analysis implicates an exciplex. 

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