William G. Herkstroeter

The fate of oxygen in the quenching of excited singlet states

Abstract Measurement of the efficiency of rubrene-sensitizied photooxidation of 1,3-diphenylisobenzofuran imply that direct formation of singlet oxygen via oxygen quenching of excited rubrene singlets is inefficient. This contrasts with recent publications based upon studies of self-sensitized rubrene photooxidation. The observed inefficiency can be rationalized in terms of the spin-allowed decay of an initially formed 3(T1 + 1Δ) complex state to a lower energy 3(T1 + 3Σ) state prior to complex dissociation.

Inclusion complexes of pyrenylbutyrate with γ-cyclodextrin

Mixing 4-pyren-1-ylbutyrate ion with γ-cyclodextrin in aqueous solution leads to inclusion complexes that become evident from changed absorption spectra as well as protection of the included pyrene moieties from fluorescence quenchers. The inclusion proceeds in stages, yielding first a 1:1 complex, which then dimerizes to form a 2:2 complex. At 25 °C the respective equilibrium constants of complex formation are 1.3 × 103 and 5.2 × 104 l mol–1. The dimerization equilibrium has an unusual temperature dependence, with ΔH≠–16.7 kcal mol–1 and ΔS≠–35 cal K–1 mol–1.

Photodimerization — relevant triplet state parameters of methyl cinnamate, diethyl 1,4-phenylenediacrylate and methyl 1-naphthylacrylate

Abstract A number of photo-cross-linkable polymers are based on triplet state dimerizations of aryl acrylate esters, such as cinnamate (C), phenylene-1,4-diacrylate (PDA) and 1-naphthylacrylate (NA). A detailed study of the triplet state properties and photodimerization kinetics of these chromophores was conducted. The energy transfer method was applied to determine the triplet energies and the extent of configurational changes between the ground and the triplet states of these compounds. The so-determined 0-0 triplet energies of C, PDA and NA are 54.8 kcal mol −1 , 49.3 kcal mol −1 and 48.5 kcal mol −1 respectively. The triplet state configurational change is largest for C (1.6 kcal mol −1 ), decreases with extended conjugation over two ethylenic bonds in PDA (0.9 kcal mol −1 ) and is immeasurably small for NA, where the conjugation involves a larger aryl group. Triplet lifetimes (10.3 ns, 6.9 μs and 10 μs for C, DPA and NA respectively), dimerization rate constants and dimerization efficiencies were determined from steady state kinetics of the photodimerization.

The triplet state of methyl 1,2-diphenylcyclopropene-3-carboxylate

Abstract Methyl 1,2-diphenylcyclopropene-3-carboxylate (CP), which is of special interest because of its role as a monomer in photo-cross-linkable polymers, was selected as a representative example of a substituted cyclopropene for investigation of its triplet state. Flash photolysis was used to view the CP triplet directly and to measure the rate constants for energy transfer to CP from a series of triplet sensitizers. Because the intersystem crossing quantum yield of CP is only 5 × 10−3, the triplet state can be populated efficiently only by energy transfer from an appropriate sensitizer. In benzene solution the triplet lifetime of CP is 360 μs, and the rate constant for reaction with ground state CP to form dimers is 3.8 × 107 M−1 s−1. The 0−0 energy of the triplet state is 51.0 kcal mol−1 (17.8 kilokaysers), determined from a plot of the energy transfer data according to the Balzani equation. The profile of this plot shows that the ground and triplet state geometries of CP are different, probably because of partial relief in the triplet state of the considerable ground state strain energy.

A study of isomeric retinal triplets by low temperature spectroscopic and kinetic flash photolysis

Abstract Spectroscopic and kinetic data of triplets of all-trans retinal and several of the cis isomers obtained with frozen samples at liquid nitrogen temperature are reported. They are believed due to unisomerized triplets of the corresponding ground state isomers. Interestingly, decay of transients from 7-cis-retinal was found to be wavelength dependent.

Viscocity-dependent phosphorescence spectra of 1-benzoyl-8-benzylnaphthalene and 1-benzoylnaphthalene

Abstract Two distinct phosphorescence spectra were observed in 1-benzoyl-8-benzylnaphthalene in rigid glass solvents at low temperature that depend upon the viscocity of the host medium. These spectra differ from each other in their profiles, the positions of their band maxima, and their radiative lifetimes. 1-benzoylnaphthalene, but not 1-benzylnaphthalene, also displays a similar change in its phosphorescence spectrum as a function of solvent viscocity. These results are interpreted in terms of triplet-state configurational changes in low-viscocity solvents that involve the carbonyl group; this freedom of molecular reorientation would be forbidden at higher solvent viscosities.

Deuterium effects on the phosphorescence spectrum and low-temperature photochemical reactivity of 1-benzoyl-8-benzylnaphthalene

Abstract Because of interest in the intramolecular hydrogen abstraction that 1-benzoyl-8-naphthalene undergoes from a triplet having the π-π * configuration, we also prepared modifications of this molecule where one and two deuterium atoms have replaced the hydrogen atoms of the photolabile methylene group. As measured in glass-forming, hydrocarbon solvents at liquid-nitrogen and liquid-argon temperatures, the deuterium atoms in these benzoylbenzylnaphthalenes make themselves noticeable by shifting the phosphorescence spectrum hypsochromically, lengthening the phosphorescence lifetime, and reducing the photochemical reactivity.