David Creed

THE PHOTOPHYSICS AND PHOTOCHEMISTRY OF THE NEAR‐UV ABSORBING AMINO ACIDS–II. TYROSINE AND ITS SIMPLE DERIVATIVES

Abstract— The photophysics and photochemistry of tyrosine and some of its simple analogues and derivatives are comprehensively reviewed. Topics discussed include emission spectroscopy, formation of photoproducts in direct and sensitized reactions, photooxidations, and mechanistics studies using steady state kinetic methods, flash photolysis, and electron spin resonance. Energy and electron transfer processes involving tyrosine and other molecules of biological interest are briefly summarized.

Effect of Molecular Structure on Mesomorphism. 18.1 Twin Dimers Having Methylene, Ethylene Oxide and Siloxane Spacers

Abstract Six Twin dimeric compounds having two rigid cores connected by a central spacer group are described. Two types of rigid units, 4-pentyloxyphenyl cinnamates and 4-cyano-4-oxybiphenyl, were employed. The spacer groups were pentamethylene (PM), diethylene oxide (EO) and disiloxane (DS). For each rigid core the nematic-isotropic temperature followed the order PM > EO > DS. Investigation of the ΔH and ΔS parameters for the nematic-isotropic (clearing) transition revealed that for these compounds the clearing transition was driven by ΔS. The EO mesogens have larger ΔS values than for the PM mesogens, which result in lowered clearing temperatures due to relatively constant ΔH values. The conformational preference for gauche isomers in the spacer of the EO compounds is seen as conducive to proper orientational alignment of the rigid cores in the nematic phase, better so than for the PM and DS spacers.

Photochemical Crosslinking of Novel Polycinnamate Main Chain Mesogens

Abstract A prototype cinnamate based main chain liquid crystalline polymer was synthesized and characterized. Its photochemistry was studied in the form of a thin film. The polymer undergoes photochemical crosslinking. Major photoreactions are postulated as 2+2 cycloaddition and photo-Fries rearrangement.

The mechanism of photooxidation of the menaquinones

Abstract Irradiation of menaquinone-1 under nitrogen gives rise to metastable isomeric o-quinone methides, one of which reacts with oxygen in the dark to form the same hydroperoxide obtained previously from direct or dye sensitized photolysis of the quinone in the presence of oxygen.

PHOTOCHEMICAL ELECTRON TRANSFER REACTIONS and EXCIPLEXES

This review covers the literature on photochemical electron transfer (p.e.t.) reactions from January 1983 to the end of June 1984. Because of space considerations, certain topics have been excluded: reactions of excited semiconductors and at other solid surfaces, electron transfer initiated photooxidations unless the work provided insights into p.e.t. per se, and in vivo processes including studies of photosynthetic reaction centers. Of the many published papers, several are especially noteworthy. Pulse radiolysis has been used to obtain the first evidence for the Marcus inverted region (Miller et al., 1984) in electron transfer processes and for long range intramolecular electron transfer across rigid saturated hydrocarbon spacers (Calcaterra etal., 1983). In contrast, Indelli et al. (1984) were unable to observe the Marcus inverted region for outer sphere electron transfer reactions exergonic by up to 58 kcal/mol. Strauch et al. (1983) have demonstrated exponential dependence of p.e.t. rates on distance leading to the observation (Guarr et al., 1983) of collisionless p.e.t. Other noteworthy reports are those of Moore et al. (1984) on photoinduced charge separation in a model for photosynthetic reaction centers, a series of studies by Hub et d. (1983, 1984a,b,c) of ion pairs and exciplexes in the stilbene-amine system, and direct observation by picosecond flash photolysis of dissociation of geminate ion pairs and multiple exciplexes and ion pairs (Mataga, 1983; Hirata et al., 1983a). Several reviews of general interest have appeared. Most noteworthy are those of Juillard and Chanon (1983,1985) in which the connections between p.e.t. and thermal electron and proton transfer are imaginatively explored with reference to a wide range of organic and inorganic examples, and that of Davidson (1983) on reactions of excited state complexes. Meyer (1983a,b) and Endicott et al. (1983) have also reviewed p.e.t. processes with emphasis on theory and examples mainly from inorganic and organometallic chemistry. Finally, Balzani et al. (1983) and Sutin and Creutz (1983) have written useful introductory level reviews. Less general reviews are mentioned in the appropriate sections below.

Photodegradation of Polyimides .4. Mechanism for the Photooxidation Process Based on a Model-Compound

Abstract Based on analysis of the photochemistry of a model compound, N -phenylphthalimide, the photolysis of a polyimide derived from 4,4′-oxydianiline (ODA) and a diarylanhydride with a hexafluorinated isopropylidine bridging group (6F) is proposed to proceed via formation of a phthalic anhydride type photoproduct. The quantum yield for product formation of N -phenylphthalimide is quite low ( −3 ) indicating a rather inefficient first step in the overall oxidative degradation process. However, based on the efficiency of the photo-oxidation of the primary photoproducts, a process leading to the ultimate failure and complete ablation of the 6F-ODA films in air upon exposure to the unfiltered output of a mercury lamp is postulated.

Photophysics and photochemistry of side-chain substituted liquid crystalline polyaryl cinnamates

The synthesis and photochemistry of comb-like thermotropic liquid-crystalline (L.C.) polymers containing UV-sensitive cinnamate ester chromophores incorporated as part of the pendant groups is described. The methacrylate monomer was polymerized using AIBN as a free radical initiator. The acrylate monomer was resistant to polymerization under the same reaction conditions. The principle photochemical reactions on photolysis (313 nm) of solutions of the polymer as well as freshly cast films are photocycloaddition and photo-Fries rearrangement of the aryl cinnamate chromophore. Aggregation of chromophores was studied in films as a function of phase type at different temperatures. These results are compared with those obtained from main chain L.C. polyaryl cinnamates.

The mechanism of photo-Fries fragmentation of aryl cinnamates in polymer films and in solution

Abstract The photo-Fries rearrangement has been observed for the aryl cinnamate chromophore in both polyaryl cinnamates and small-molecule models. Laser flash photolysis of solutions of aryl cinnamates and the corresponding polymers leads to phenoxy and cinnamoyl radicals from the cleavage of the CO ester bond. The formation of these radicals has been verified by their independent generation by flash photolysis of phenols (e.g. p-pentyloxyphenol) and aldehydes (e.g. p-methoxycinnamaldehyde). The effects of triplet sensitizers and quenchers on the formation of these transient intermediates and on the formation of the photo-Fries rearrangement products are reported. It is concluded that these products arise from either the aryl cinnamate singlet state or a short-lived upper triplet state.

Triplet-Sensitized Irradiation of a Main-Chain Liquid Crystalline Poly(aryl Cinnamate) in Three Different Phases

Visible light irradiation of thin films of a main-chain liquid crystalline poly(aryl cinnamate) using ketocoumarins as triplet sensitizers leads to photochemical crosslinking and UV-vis and FTIR spectroscopic changes associated with saturation of the cinnamate double bond, most likely by 2 + 2 photocycloaddition. The triplet sensitizers are themselves photolabile and are lost by photochemical reactions during the sensitization process. A new ketocoumarin sensitizer with decyloxy substituents and a reduced tendency to phase separate from the polymer is reported. A simple calculation of the sensitization stoichiometry shows that a single molecule of this ketocoumarin sensitizes the destruction of approximately 90 cinnamate chromophores in the “as cast” films below Tg and about 300 chromophores in the more-ordered glassy nematic films and in “as cast” films of poly(vinyl cinnamate). Triplet sensitization of fluid nematic films leads, upon initial irradiation, to UV-vis hyperchromism that is attributed to disruption of chromophore aggregation and, possibly, to disruption of the nematic mesophase as photoproducts begin to form.

Photoreduction of 3,7-diaminophenoxazinylium chloride (‘oxonine’) by iron(II). A model system for singlet-state driven photogalvanic cells

Fluorescence of the title dye is quenched by iron(II) in water with concomitant photoreduction via a mechanism which does not involve significant induced intersystem crossing by the quencher.