Energy-resolved and thermalized photoisomerization rates of diphenylbutadiene

Recent measurements of energy-resolved photoisomerization rates of jet-cooled diphenylbutadiene molecules are analyzed in terms of RRKM calculations. Perfect agreement over a wide energy range is obtained. A comparison of the thermally averaged rates with liquid-phase data is made.     

Steric effects in hula-twist photoisomerization. 1,4-dimethyl- and 2,3-dimethyl-1,4-diphenylbutadienes

1,4-Dimethyl-1,4-diphenylbutadiene was shown to be able to execute regiospecific HT-1 photoisomerization around a methyl group when irradiated in a low-temperature organic glass, albeit at reduced efficiency. 2,3-Dimethyl-1,4-diphenylbutadiene exhibited a different type of steric effect, causing the E,E-isomer to undergo regiospecific HT-1 photoisomerization.     

Photochemistry of 3-Substituted 1-Iminopyridinium Ylides. Regiospecific versus non-regiospecific photoisomerization patterns [1]

3-Substituted 1-iminopyridinium ylides 1 undergo photo-induced ring enlargement to 1H-1,2-diazepines. With strongly electron-withdrawing substituents the ring expansion process is regiospecific and leads exclusively to 4-substituted 1 H-1, 2-diazepines. Weak electron-donating substituents, like a methyl group and halogen atoms, do not have any directing effect since both 4- and 6-substituted 1 H -1,2-diazepines are obtained. With strong electron-donating substituents no diazepines are formed; instead one observes photo-induced isomerization to the 2-aminopyridine derivatives, the process being non-regiospecific. Regiospecific photo-induced ring expansion processes are explained in terms of a simple HMO model.     

Systematic control of photochemistry: the dynamics of photoisomerization of a model cyanine dye

On-the-fly CASSCF nonadiabatic dynamics have been used to model the trans-cis isomerization of a model cyanine dye. Our results show that the photochemical generation of the trans versus cis product is dynamically controlled by the presence of an extended cis-trans conical intersection seam that persists along all torsional angles. This in turn suggests that the photochemistry could be completely controlled by controlling the distribution of momentum components in a wave packet excited by laser photolysis in a coherent control experiment.     

New liquid crystal polyethers and polyesters based on diphenylbutadiene mesogens

New liquid-crystalline (LC) polymers and small molecules have been synthesized based on the 1,4-diphenylbutadiene mesogen, a chromophore with χ⁽³⁾ nonlinear optical properties. Polyethers were formed by phase-transfer reaction, and polyesters were prepared by Schotten-Bauman polymerization from these 4,4'-bisphenolic mesogenic structures. Liquid crystallinity was examined using differential scanning calorimetry (DSC) and optical microscopy. Most of the model compounds were found to be nematic. All mesomorphic polymers were nematic and displayed LC ranges of up to 50°C as well as a wide diversity of melting points. The melting points varied with the flexible (methylene) spacer length and were strongly dependent upon the presence of substituents in the mesogenic core. Crosslinking and cis-trans isomerization were observed under the influence of heat and ultraviolet radiation. © 1992 John Wiley & Sons, Inc.     

Regiospecific photoisomerization of fluorinated (E,E)-1,4-diphenyl-1,3-butadienes

Photoisomerization of five fluorinated E,E-1-(R-phenyl)-4-phenyl-1,3-butadienes in solution (R = 1: p-monofluoro, 2: m,m′-difluoro, 3: m,m′,p-trifluoro, 4: o,o′,m,m′-tetrafluoro, 5: o,o′,m,m′,p-pentafluoro) was investigated via direct irradiation. Our results indicated that cis-trans photoisomerization of the fluorinated 1,4-diphenyl-1,3-butadienes in the excited singlet state took place exclusively at the CC bonds closer to the fluorine substituents.     

New insight into photochemistry of ferrioxalate

Optical spectroscopy and nanosecond flash photolysis (Nd:YAG laser, 355 nm, pulse duration 5 ns, mean energy 5 mJ/pulse) were used to study the photochemistry of Fe(III)(C2O4)3(3-) complex in aqueous solutions. The main photochemical process was found to be intramolecular electron transfer from the ligand to Fe(III) ion with formation of a primary radical complex [(C2O4)2Fe(II)(C2O4(*))](3-). The yield of radical species (i.e., CO2(*-) and C2O4(*-)) was found to be less than 6% of Fe(III)(C2O4)3(3-) disappeared after flash. [(C2O4)2Fe(II)(C2O4(*))](3-) dissociates reversibly into oxalate ion and a secondary radical complex, [(C2O4)Fe(II)(C2O4(*))](-). The latter reacts with the initial complex and dissociates to Fe(II)(C2O4) and oxalate radical. In this framework, the absorption spectra and rate constants of the reactions of all intermediates were determined.     

The origin of fluorescence from trans-trans diphenylbutadiene

Fluorescence decay time and yield of trans-trans diphenylbutadiene in methylcyclohexane/isohexane and propylene glycol were measured between ?50 and +50°C. The radiative rate parameter is independent of solvent and temperature, with a mean value of 8.2 × 10⁸ s^?1. This is similar to trans-stilbene but unlike longer polyenes, and is consistent with emission from ¹B^*_u rather than the low-lying ¹Ag^*_g state.     

Determination of diphenylbutadiene by liquid chromatography-UV-fluorescence in foodstuffs

Diphenylbutadiene (DPBD) is an optical brightener incorporated into a wide range of polymeric materials. Framed in the FOODMIGROSURE project, it was chosen as a model migrant to study the migration kinetics from polymeric materials in relevant foodstuffs. An analytical method was developed and optimized for the DPBD determination in foods. The sample preparation procedure uses both hexane and acetonitrile as extraction solvents, followed by high-performance liquid chromatography (HPLC) analysis. HPLC analysis was performed using UV detection at a wavelength of 330 nm, and fluorescence detection achieved with excitation and emission wavelengths of 330 and 375 nm, respectively. Good linearity and recovery were achieved. Data are reported.     

The photoisomerization of dihydrocostunolide

Photoisomerization of the germacranolid dihyrocostunolide (2) proceeds with a high degree of regio-and stereoselectivity to give the guaianolide photunolide (16) as the only major product. The conversion of photunolide to two guaianolides of known structure (8 and 9) and synthesis from O-acetylisophotosantonic lactone (11) provide unambiguous proof of both the structure and the stereochemistry of photunolide. The photoisomerization of 2 to 16 appears to involve an intramolecular hydrogen shift.     

Regiospecific metalation of oligobromobenzenes

The metalation of selected oligobromobenzenes with lithium diisopropylamide (LDA) was investigated. 1,3-Dibromo-substituted benzenes were metalated without special precautions since the resultant 2,6-dibromophenyllithium intermediates are relatively stable under reaction conditions: corresponding benzaldehydes were obtained in good or moderate yields after subsequent quench with N,N-dimethylformamide (DMF). Aryllithium compounds derived from 1,4- and 1,2-dibromobenzene are much less stable, but they could be trapped by the in situ use of chlorotrimethylsilane. The one-pot metalation/disilylation of 1,4-dibromo- and 1,2-dibromobenzene afforded 1,4-dibromo-2,5-bis(trimethylsilyl)benzene and 2,3-dibromo-1,4-bis(trimethylsilyl)benzene, respectively.     

Regiospecific Synthesis of Bikaverin

A regiospecific synthesis of bikaverin (1) is described starting from methyl 2-hydroxy-4-methoxy-6-methyl benzoate (2) and 2-chloro-5,7-dimethoxy-1,4-naphoquinone (4).     

Regiospecific photohydroxyalkylation of pentafluoropyridine

Irradiation of 1-hydroxyalkane solutions of pentafluoropyridine in the presence of benzo-phenone resulted in regiospecific substitution at position 4, thus forming 2,3,5,6-tetrafluoro-4-(1-hydroxyalkyl)pyridine.     

The photoisomerization of retinal

Abstract— –Quantum efficiencies have been measured for the photoisomerization of four stereoisomers of retinal (all-trans, 13-cis, 11 cis, and 9-cis) in two solvents at different wavelengths of irradiation and at various temperatures. In heane at 25°C the quantum efficiencies for isomerization at 365 nm are: 9-cis to trans, 0.5; 13-cis to trans, 0.4; 11-cis to trans, 0.2; all-trans to monocis isomers, 0.2-0.06, depending upon assumptions made regarding the stereo-isomeric composition of the product. These values vary somewhat with the wavelength of the irradiating light. The quantum efficiency for the photoisomerization of all-trans retinal in hexane decreases by a factor of 30 when the temperature is lowered from 25° to – 65°C; the activation energy for this photoisomerization is about 5 kcal/mole. The quantum efficiencies for the isomerization of the monocis isomers to all-trans retinal in hexane are virtually independent of temperature. In ethanol the rates of photoisomerization from trans to cis or cis to trans depend only slightly on the temperature between 25° and – 65°C. The photosensitivities of the stereoisomers of retinal are of the same order of magnitude as those of the retinylidene chromophores of rhodopsin (11 -cis), metarhodopsin I (all-trans), and isorhodopsin (9-cis); but it is not yet possible to derive the photochemistry of rhodopsin uniquely and quantitatively from that of retinal.