Evaluation of single-photon-counting measurements of excited-state lifetimes.

An extremely short instrumental response function for a single-photon-counting system has been obtained by using a low-jitter photomultiplier tube, fast amplification of the single photoelectron pulse from this photomultiplier, a constant fraction discriminator with a wide bandwidth input, and a stable reference timing signal. This synchronously mode-locked laser system has been shown to be capable of measuring the fluorescence lifetimes of compounds from 66 psec to 4.5 nsec. The estimated error in these determinations is 6% or 10 psec, whichever is greater. This apparatus has proved itself to be superior to the previously used nitrogen flashlamp apparatus for the determination of subnanosecond lifetimes. The validity of using deconvolution to determine lifetimes shorter than the instrumental response function has been verified.

New reactions and theory in organic photochemistry: The 1,3-vinyl migration and its relevance to exchange integral control.

The photochemistry of 1,1,4-triphenyl-1,4-pentadiene was investigated. Sensitized irradiation of this compound labeled at carbon 3 with (2)H led to a degenerate rearrangement wherein the diphenylvinyl moiety migrated 1,3, thus affording diene with (2)H substitution at carbon 5. In contrast, direct irradiation of the triphenyl pentadiene led to 1,3,3-triphenylbicyclo[2.1.0]pentane by a [2 + 2] cycloaddition. This multiplicity dependence of the photochemistry is understood on the basis of triplet reactants preferring a linearly conjugated diradical mechanism with a large exchange integral (K) and singlet reactants preferring an electrocyclic mechanism with a small exchange integral.