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Kinetics of electron transfer, following variation of actinic light intensity, for photosynthetic reaction centers (RCs) of purple bacteria (isolated and membrane-bound) were analyzed by measuring absorbance changes in the primary photoelectron donor absorption band at 865 nm. The bleaching of the primary photoelectron donor absorption band in RCs, following a sudden increase of illumination from the dark to an actinic light intensity of I exp, obeys a simple exponential law with the rate constant $$ (\alpha I_{\exp } \; + \;k_{\text{rec}} ) $$ , in which α is a parameter relating the light intensity, measured in mW/cm2, to a corresponding theoretical rate in units of reciprocal seconds, and k rec is the effective rate constant of the charge recombination in the photosynthetic RCs. In this work, a method for determining the α parameter value is developed and experimentally verified for isolated and membrane-bound RCs, allowing for rigorous modeling of RC macromolecule dynamics under varied photoexcitation conditions. Such modeling is necessary for RCs due to alterations of the forward photoexcitation rates and relaxation rates caused by illumination history and intramolecular structural dynamics effects. It is demonstrated that the classical Bouguer–Lambert–Beer formalism can be applied for the samples with relatively low scattering, which is not necessarily the case with strongly scattering media or high light intensity excitation.

Nonequilibrium dynamic effects in membrane-bound reaction centers (RCs) from photosynthetic bacteria are studied. It is shown that the accumulation of slow conformational changes triggered by charge separation events in the RCs control system dynamics.

A study was conducted to analyze the preparation, spectroscopy and photochemistry of s-dipentacene. The symmetric addition product dimer of pertacene (s-dipentacene) was determined upon the irradiation of a solution of pentacene. Spectroscopic measurements were carried out to describe the ultraviolet photochemistry of s-P2. Results indicated that crystals grown from a dichloromethane solution supported a couple of solvent molecules per unit cell.

Spectroscopic studies of the electronic structure of psuedotetrahedral oxovanadium groups dispersed in a silica xerogel matrix reveal the localization of the highest occupied molecular orbital, a nonbonding a2 symmetry orbital, on the basal plane ligands. The first excited state resulting from a one-electron promotion has an E symmetry. This electronic structure, which differs from the previously reported structure, accounts for the catalytic and photocatalytic activities of the oxovanadium center.

The kinetics of photochemical spectral hole burning are modeled considering low temperature photochemical mechanisms requiring both one and two photons. The calculated results are compared with experimental results on photochemical hole burning in the anthracene–tetracene photoadduct (AT) using single color irradiation. Better agreement is obtained for the two photon mechanism while assuming the second photon is absorbed by the lowest triplet state of AT. The effects of irradiation source bandwidth and polarization are included in the analysis. Photochemical and transient saturation effects are evident in the calculated hole shapes. [ABSTRACT FROM AUTHOR]

Two-color, photon-gated photochemical hole burning (PHB) is observed in the absorption spectrum of a photoadduct of anthracene and tetracene (A–T) in a polymer matrix at 1.5 K. A–T undergoes PHB when irradiated with narrow band exciting light into the 0–0 band of the S1 ← S0 absorption. The efficiency of this PHB process is enhanced by simultaneous irradiation near the maximum of the Tn ← T1 absorption of A–T. Holewidths of less than 0.07 cm-1 have been observed for this photochemical cleavage of A–T. PHB was not observed for the related photoadduct dianthracene (A2) under similar irradiation conditions. [ABSTRACT FROM AUTHOR]

Picosecond excited-state singlet–singlet absorption spectra (Sn ← S1) and excited-state decay kinetics are reported for several diazanaphthalenes and a triazanaphthalene in room temperature solution. New Sn (1nπ*) state energies are obtained empirically from these spectra. INDO-PSDCI molecular orbital calculations are used to confirm the Sn ← S1 nature of the excited state transitions observed. Decay rates are calculated from models based on a semiempirical molecular orbital formalism, and these calculated rates are compared with the observed ones. Agreement of the calculated and observed rates are good except in cases of substantial nπ*–ππ* mixing of the actual excited states and in a particular case of high molecular symmetry (1,5-naphthyridine). [ABSTRACT FROM AUTHOR]

Absorption and emission spectra of 4-amino-4’-nitrodiphenyl sulfide in polar and nonpolar solvents were used to characterize and assign the low-energy excited states of the molecule. Fluorescence-excitation anisotropy spectra, fluorescence and phosphorescence lifetimes, and fluorescence quantum yields were also used to characterize the photophysics of these states. The lowest-energy, fluorescent singlet state was determined to be an intramolecular charge transfer (ICT) state involving transfer of a full electron charge from the amino to the nitro group yielding a dipole moment of ∼50 D. A low-energy, intense absorption band is assigned as a transition to a different ICT state involving a partial electron charge transfer from sulfur to the nitro group. [ABSTRACT FROM AUTHOR]

The arc-fault circuit interrupter is beginning to emerge as a new product in residential applications in the U.S. to protect against conditions that may cause fire. This paper provides some of the background regarding its emergence as a product and the function it is intended to provide. The evolving technology may also lend itself to industrial applications, as the potential applications become sufficiently well understood.

Presents information on a study which focused on the emergence of arc-fault circuit interrupter products. Types of arc-faults; Arc characteristics; Contrast with ground-fault circuit interrupter protection.