Search

The purpose of this work is to suggest a way to utilize image statistics to guide optimization of brightness distributions, towards a goal of complete systematization of image processing to achieve a purely aesthetic objective, with entropy as a response metric. We start with a survey of classic pictorial photographs, proceed to a heuristic theoretical treatment of the brightness distribution function, and follow with several pictorial illustrations of the proposed concept.

Photoexcitation of the title compound (DMBA) in polar solvents leads to sequential formation of at least three excited species as detected by laser transient absorption spectroscopy: the lowest Franck–Condon singlet, a twisted singlet (P-state), and the lowest triplet states. Only the initially formed, localized singlet is emissive under our conditions. Interystem crossing to triplet occurs primarily from the twisted P-state in acetonitrile and in methanol. Photo-CIDNP and quantum yield measurements indicate that electron transfer occurs exclusively from the triplet in acetonitrile or methanol to sensitize decomposition of diphenyliodonium cation and to bleach DMBA. Analysis of the photochemistry is complicated by formation of a charge transfer complex of enhanced photolability. Using tris(trichloromethyl)-sym-triazine as the electron acceptor in hydrocarbon solvent, bleaching of DMBA again occurs, somewhat more efficiently than with the 'onium salt in polar solvent. We infer that under these conditions electron transfer occurs from the P-state. Keywords: photochemistry, fluorescence, electron transfer, TICT, chalcone.

Pulsed picosecond laser photolysis of the diphenyliodonium cation indicates that excitation of the allowed transition is followed by rapid (ps) deactivation to a homolytically dissociative state, presumably T1. Steady-state photolysis yields iodobenzene and biphenyl from the primary radical products. Iodobiphenyl is also observed. We infer in-cage recombination of these radical products, with intervention of a carbocationic intermediate. Laser flash photolysis of diphenyliodonium iodide in its charge transfer band yields an initial transient with a lifetime of ca. 200 ps. A strongly absorbing secondary transient is observed on the nanosecond time scale. Iodobenzene is the exclusive reaction product. We infer that it is not formed directly from the primary photoproduct, but through in-cage recombination of phenyl cation with iodide anion, as in benzenediazonium iodide.

We have studied the anthracene-sensitized photolyses of both diphenyliodonium and triphenylsulphonium salts in solution using both steady-state and laser flash photolysis techniques. Photoproducts, namely, phenylated anthracenes along with iodobenzene or diphenylsulphide, respectively, are obtained from both salts with quantum efficiencies of ca. 0.1 at 375 nm. We infer the intermediacy of diphenyliodo and triphenylsulphur radicals formed by single electron transfer from the singlet-excited anthracene. We have developed a quantitative model of this chemistry, and identify the principal sources of inefficiency as back electron transfer, which occurs at nearly the theoretically limiting rate, intersystem crossing from the initially formed sensitizer–'onium salt encounter complex, and in-cage radical recombination.