Henry Linschitz

Actinometry in monochromatic flash photolysis: the extinction coefficient of triplet benzophenone and quantum yield of triplet zinc tetraphenyl porphyrin

Abstract— The extinction coefficient εT, of triplet benzophenone in benzene has been directly determined by absolute measurements of absorbed energy and triplet absorbance, ΔD0T, under demonstrably linear conditions where incident excitation energy, E0, and ground state absorbance, A0, are both extrapolated to zero. The result, 7220 ± 320 M‐1 cm‐1 at 530 nm, validates and slightly corrects many measurements relative to benzophenone of triplet extinction coefficients made by the energy transfer technique, and of triplet yields obtained by the comparative method.

Extinction coefficients of C60 triplet and anion radical, and one-electron reduction of the triplet by aromatic donors

Abstract The extinction coefficients of the triplet state of C60 are obtained by energy-transfer techniques in flash photolysis studies. The values permit actinometric measurement of the triplet quantum yield (φR=0.93±0.07) and give rates of T—T annihilation close to diffusion-controlled. Rates and radical quantum yields in reversible one-electron reduction of 3C60 by tri-p-tolylamine, TTA, and hydroquinone are determined as well as extinction coefficients in the visible of the C−60 anion radical. Back electron transfer from C−60 to TTA+ occurs at a distance markedly greater than van der Waals contact.

PHOTOADDITION OF ALCOHOLS TO PURINE

Abstract— A new photoreaction is described, in which alcohols add to purine. The adducts of methanol, ethanol (two isomers), and isopropanol have been isolated and characterized by chemical analysis, and their u.v., i.r., NMR and mass spectra. Mainly on the basis of the NMR data and using deuterated‐purine adducts, it is shown that the addition involves bonding between the α‐carbon of the alcohol and the 6‐carbon of the purine ring. The quantum yield of the reaction, at 254 ran, is about 0·2 in dry, deoxygenated methanol, ethanol or isopropanol. The reaction appears to be rather general, as shown by experiments on pyrimidine and several other simple nitrogen heteroaromatic molecules.

ANION AND SOLVENT EFFECTS ON THE RATE OF REDUCTION OF TRIPLET EXCITED THIAZINE DYES BY FERROUS IONS

Abstract— The lifetimes of the triplet excited states of thionine and methylene blue were measured in aqueous and 50 v/v% aqueous acetonitrile solutions acidified with 0.01 N sulfuric or trifluoromethyl‐sulfonic acid. The rate constants for reaction of the triplet excited dyes with ferrous ions were measured in the same solutions. The triplet lifetimes in the absence of added quenchers were insensitive to a change in acid from trifluoromethylsulfonic to sulfuric or to a change in solvent from water to 50v/v% aqueous acetonitrile (τ for triplet thionine ˜7.5 μs, τ for triplet methylene blue ˜4.5 μs). In contrast, the rate constant for reaction of the triplet dyes with ferrous ions increased by nearly a factor of 10 with a change in acid from trifluoromethylsulfonic to sulfuric. In solutions containing sulfate ions this reaction rate constant increased with increasing sulfate concentration and with a change in solvent from water to 50 v/v% aqueous acetonitrile. The results are discussed in terms of the possibility of association of the positively charged reactive ions with sulfate anions. Quenching of the triplet excited dyes by ferric ions or by ground state dye molecules was shown to be negligible at the concentration used for the ferrous ion quenching study.

Photo‐Ionization and Electron‐Capture Processes in Solution: The Flash Photolysis of Ether Solutions of Potassium Metal and of Naphthacene Dianion

Flash excitation of solutions of potassium in tetrahydrofuran or dimethoxyethane leads to a new substance with a broad infrared absorption. This transforms rapidly to a second intermediate, which finally returns to the original state by a slow first‐order process (k = 12.8 sec—1). These changes are interpreted as photodissociation of doubly trapped electrons (e2) followed by fast recombination to a triplet state of e2 and slower return of the triplet to the original singlet. Flashing solutions of naphthacene dianion yields the monoanion and near‐infrared bands attributable to e1. The recombination process is second‐order (k = 1.5×109 moles—1 sec—1). Much faster electron capture by neutral naphthacene is also observed.

FT-EPR study of triplet state C60. Spin dynamics and electron transfer quenching

Abstract A FT-EPR study was made of paramagnetic species formed by pulsed-laser excitation of C 60 in fluid solution. Earlier findings that photoexcitation of C 60 in fluid solution gives rise to an EPR signal with narrow linewidth were confirmed. The lifetime of the signal corresponds to that of the C 60 triplet as measured by flash photolysis. In the presence of donors, the rate of signal decay is increased and matches the growing in of EPR signals from oxidized donors. The time dependence of the FT-EPR spectra gives values for electron transfer rate constants which agree with those derived from flash photolysis measurements on the same systems. Based on these findings, we assign the narrow line signal to the triplet of C 60 . The time evolution of the FT-EPR signal establishes that the triplets are born with less spin polarization that the thermal equilibrium value. As a result, signal growth is controlled by spin-lattice relaxation.

Charge-transfer interactions of excited molecules with inorganic anions: the role of spin-orbit coupling in controlling net electron transfer

Abstract Radical yields in simple anion-organic triplet charge-transfer interactions are interpreted in terms of spin-orbit coupling in an incipient product radical, as evidenced by increasing yield with increasing [X − ] in cases where X − 2 radicals with zero orbital momentum are formed. Nitrite may also quench by energy transfer.

Multiple decay pathways and electron transfer in excited ion-paired zinc-copper porphyrins: laser photolysis and time-resolved EPR spectroscopy

Abstract Oppositely charged zinc and copper porphyrins form ion-paired dimers which show biphasic decay of flash transients in fluid solution. In a glass at low temperature, two photoexcited components are also seen by time-resolved EPR. On the basis of lifetime, absorption spectrum, oxygen sensitivity and EPR spectrum, the long-lived transient is assigned to the zinc porphyrin triplet. The short-lived species is attributed to an ion radical-pair formed by electron transfer from an excited singlet precursor.

Fluorescence of Photosynthetic Organisms at Room and Liquid Nitrogen Temperatures

Fluorescence spectra of algae and higher plants show two bands, ascribable to monomer and aggregate forms of chlorophyll. At low temperature, the long wavelength emission is greatly enhanced and often appears as a new band. Photosynthetic bacteria, on the other hand, show no new bands at low temperature, within the spectral coverage and sensitivity of these measurements. A green fluorescence is also found in algae, which we attribute to carotenoids.

Photochemistry of Metal Solutions. II. Flash Photolysis of Alkali Metals in Ethylamine

Flash‐photolysis studies have been made on Li, K, Rb, and Cs solutions in ethylamine, over a range of temperature and concentration. Such solutions typically show three bands near 650 (V), 850 (R), and 1300 (IR) mμ. These are assigned, respectively, to a cation‐centered monomer, a dimer, and a solvated electron. In K, Rb, and Cs solutions, flashing at wavelengths shorter than 500 mμ causes initially a bleaching of V and enhancement of IR. This is followed by a growing in of R, but at a rate quite different from that of IR decay. Another precursor X of the R transient is therefore postulated. The R transient finally reverts back to V. The ratio of the yields of the IR and R transients is constant, independent of excitation wavelength, metal concentration and fractional conversion. From this it is inferred that X and IR have a common origin. Direct reversible photolysis of the original R‐band also occurs, leading to an infrared transient which decays back to R in a second‐order process. The rate constants a...