Knowledge Bank

Two simultaneous biphotonic chemistries in a rigid organic solution are treated quantitatively. The fractional fluorescence loss” of the parent solute molecule is monitored as the key experimental parameter for quantifying the photochemistry. The fluorescence loss results both from the depletion of the parent molecule and from competitive light absorption by the photoproducts. Furthermore the photoproducts are not homogeneously distributed in the sample because of its high viscosity. A general formalism is developed for handling this general problem with the aim of obtaining quantum yields or cross-sections for the several elementary steps. Durene as the solute in 3-methylpentane at $77^{\circ} K$ is the example studied. Here the two photochemistries are $\beta$-bond scission to give the duryl radical, and a one-electron ionization to give the durene cation. Both photochemistries proceed via a one-photon excitation from lowest durene triplet state. The two simultaneous photochemical channels are differentiated by reversing the ionization step through infrared induced charge recombination and measuring a fractional fluorescence recovery” of durene. Perdeuterated durene is studied as well. The $\beta$-bond cleavage shows a strong isotope effect. The quantitative results, though crude, yield the following results: The quantum yields for the second photon step at 275 nm out of the triplet state are $\sim$ 0.4 ($\beta$-bond scission) and $\sim$ 0.6 (ionization). Thus every excitation appears to lead to photochemistry. Molar decadic extinction coefficients at 270 nm in nonpolar solution at $77\circ K$ are found to be $1.3\times 104$ for the triplet state, $2.5\times 103$ for the duryl radical, and $\sim 0.5\times 103$ for the durene cation. The efficiency for the infrared induced charge recombination to generate durene via its excited singlet and triplet states is found to be of the order of unity. Finally, a previously discovered one-photon channel for the $\beta$-bond cleavage via upper vibrational levels of the first excited singlet state is seen to proceed with a quantum yield of $\sim$ $10-4$.