Ralph S. Becker

Erratum: Comprehensive Investigation of the Electronic Spectroscopy and Theoretical Treatments of Ferrocene and Nickelocene

New data concerning the electronic absorption and emission spectra of ferrocene and the absorption spectrum of nickelocene are presented. Assignment and discussion of the transitions are made based on the experimental results. No ferrocene emission is observed upon excitation into the lowest energy absorption band. An emission occurs upon excitation into the next higher energy band. These results are interpreted in terms of crossing of the first excited state and the ground state. The luminescence is long lived and is interpreted as phosphorescence paralleling an assigned singlet‐triplet absorption at 5280 A. No nickelocene emission was noted. Assignments of the absorption bands of ferrocene and nickelocene are proposed within the framework of each of the major theoretical approaches.

Extension of Electron Affinities and Ionization Potentials of Aromatic Hydrocarbons

The absolute molecular electronegativities for 14 aromatic hydrocarbons are reported. For the hydrocarbons considered, a linear relationship exists between the electron affinity and the energy of the 1La transition. The molecular electronegativity for some eight aromatic hydrocarbons is approximately constant. On this basis, it is possible to predict some ionization potentials which are in excellent agreement with recent experimental values. It does not appear reasonable to relate the intercept resulting from extrapolation of the aforementioned plot to hν=0 to the work function of graphite. Based on the measured electron affinities, it is possible to deduce a relationship with methyl affinities and calculate ΔEsol between neutral hydrocarbons and their mononegative ions. Certain theoretical approaches can be successfully employed to estimate electron affinities of aromatic hydrocarbons.The absolute molecular electronegativities for 14 aromatic hydrocarbons are reported. For the hydrocarbons considered, a linear relationship exists between the electron affinity and the energy of the 1La transition. The molecular electronegativity for some eight aromatic hydrocarbons is approximately constant. On this basis, it is possible to predict some ionization potentials which are in excellent agreement with recent experimental values. It does not appear reasonable to relate the intercept resulting from extrapolation of the aforementioned plot to hν=0 to the work function of graphite. Based on the measured electron affinities, it is possible to deduce a relationship with methyl affinities and calculate ΔEsol between neutral hydrocarbons and their mononegative ions. Certain theoretical approaches can be successfully employed to estimate electron affinities of aromatic hydrocarbons.

The visual process: photophysics and photoisomerization of model visual pigments and the primary reaction

Abstract— A systematic comprehensive consideration of the emission spectroscopy, emission lifetimes, transient spectroscopy, as well as quantum yields of fluorescence, triplet occupation and photoisomeriz‐ation is given for a wide variety of polyene derivatives including retinyl and longer, as well as shorter chainlength homologs. Absorption spectral properties and the results and significance of theoretical calculations are also included. Chainlength, solvent and temperature effects on state order and photophysical as well as photoisomerization properties are evaluated. The mechanism for the primary light step in vision is considered in light of photophysical and photoisomerization data on model visual pigments and rhodopsin.

Comprehensive Spectroscopic Investigation of Polynuclear Aromatic Hydrocarbons. I. Absorption Spectra and State Assignments for the Tetracyclic Hydrocarbons and their Alkyl‐Substituted Derivatives

The absorption spectra have been investigated and state assignments made for benz (a) anthracene, benzo (c) phenanthrene, chrysene, triphenylene, naphthacene, and pyrene. In addition, these results are compared with existing theoretical and experimental studies. The identification of electronic transitions have been made for all monomethyl drivatives of the above hydrocarbons except for naphthacene. The study extends over the spectral region 16 500–52 500 cm—1. In addition, vibrational analyses are proposed for all cases considered. Certain dimethyl‐, alkyl‐ and fluorosubstituted derivatives have been studied. General spectral characteristics are included.

Effect of Metal Atom Perturbations on the Luminescent Spectra of Porphyrins

The low‐temperature emission spectra of the dimethyl ester of mesoporphyrin IX and its divalent derivatives of cobalt, nickel, copper, zinc, palladium, cadmium, and barium have been obtained. Magnesium etioporphyrin(II) and zinc phthalocyanine were also investigated. The divalent silver mesoporphyrin was also studied, but no luminescence was obtained for this compound. Considerations of the relative lifetimes of fluorescence and phosphorescence of the various derivatives permit qualitative deductions to be made regarding magnetic susceptibility. Certain qualitative information on metal‐porphyrin bonding is presented based on the comparative intensities of fluorescence and phosphorescence. Anomolous emission characteristics of several of the metallo‐porphyrins are tentatively interpreted in terms of crossing of a first excited state and the ground state.

Spectroscopy and Mechanisms of the Photo‐ and Thermal Reactions of Photochromic Anils

The principal emission from the salicylideneanilines, salicyclidene‐2‐aminoanthracene, and 2‐hydroxynaphthylideneaniline is assigned as a fluorescence originating from a π, π* singlet state of a short‐lived intermediate, cis†. The cis† arises as the first product of the photochemistry and is a species in which proton transfer has occurred but essentially retains the geometry of the enol tautomer. The cis† species, in the salicylideneanilines and salicylidene‐2‐aminoanthracene, converts to the final photocolored product. This final product is a nonequilibrium form of the true cis–keto tautomer. The latter is produced from the photocolored species by a thermal reaction. The cis–keto tautomer cannot be produced by a photoreaction from either the enol or the photocolored species. However, for the 2‐hydroxy‐naphthylideneaniline, the cis–keto tautomer is the final photocolored product from the enol. No photocolored products can be produced from the nitrosalicylideneanilines. Viscosity barriers are present that ...

Photophysics, photochemistry and kinetics of photochromic 2H-pyrans and chromenes

Abstract Nine naturally occurring and synthetic chromenes and 2 H -pyrans (oxo and sulfo) derivatives have been examined using microsecond and nanosecond laser flash techniques. Triplet transients have only been observed for the coumarin-substituted chromenes (xanthyletin and seselin). Photochromism of the coumarin-substituted chromenes arose dominantly via an excited triplet state in about 800 ns. For the other chromenes and 2 H -pyrans, the colored forms arose only from an excited singlet state and subnanosecond rise times were observed. Fluorescence lifetimes have been determined for some chromenes at room temperature as well as 77 K. For xanthyletin, a fluorescence lifetime of less than 30 ps at 295 K in hexane was observed.

TRIPLET EXCITED STATES OF PYRIMIDINE NUCLEOSIDES AND NUCLEOTIDES

Abstract— The triplet absorption spectra, lifetimes, extinction coefficients, eTT, and intersystem crossing quantum yields to the lowest triplet T1, øT1, of thymidine, thymidine monophosphate, uridine and uridine monophosphate, have been determined in various solvents at 300 K.

Vibronic Effects in Photochemistry—Competition between Internal Conversion and Photochemistry

An emission spectroscopic investigation was carried out on molecules capable of undergoing photochemistry as well as on those that cannot. Where photochemistry does occur, the quantum yield of fluorescence (φF) depends upon the particular vibrational level excited within a vibrational manifold of an excited electronic state. That is, φF generally decreases with excitation into successively higher vibrational levels. The mechanism responsible for these results involves competition between internal conversion and photochemistry at each vibrational level within a vibrational manifold. Based on this mechanism, corresponding equations have been developed from which the ratio of the rate constant of internal conversion to that of photochemistry, kIC / kPC, can be evaluated. The value of this ratio is dependent upon the particular vibration involved as well as the particular excited electronic singlet state involved. It appears as if internal conversion may proceed only within a particular vibrational manifold.

Synthesis and photoelectrochemistry of In2S3

Abstract In2S3 was prepared by chemical vapor phase transport and gradient freeze techniques. A variety of differently doped samples were also synthesized. Both n, and n plus p photoresponses were observed depending on the dopant, quantity of excess sulfur, or sulfur atmosphere annealing. Photoetching significantly increased the photocurrent density at short circuit. In a polysulfide couple, the maximum power efficiency obtained with white light was 0.3% and the quantum efficiency of carrier collection at short circuit was 43%. Photocurrent onset occured near - 1.1 V which was shifted significantly positive in nonsulfide-sulfur containing couples. Band gap determinations were made. Stability studies as measured by In3+ in solution indicate a high (to 100%) stabilization which appears to be in conflict with visual observations. The behavior of photocurrent with time varies depending upon whether a negative or positive value of applied potential is involved.