The breadth of scientific and technological interests in the general topic of photochemistry is truly enormous and includes, for example, such diverse areas as microelectronics, atmospheric chemistry, organic synthesis, non-conventional photoimaging, photosynthesis, solar energy conversion, polymer technologies, and spectroscopy. This Specialist Periodical Report on Photochemistry aims to provide an annual review of photo-induced processes that have relevance to the above wide-ranging academic and commercial disciplines, and interests in chemistry, physics, biology and technology. In order to provide easy access to this vast and varied literature, each volume of Photochemistry comprises sections concerned with photophysical processes in condensed phases, organic aspects which are sub-divided by chromophore type, polymer photochemistry, and photochemical aspects of solar energy conversion. Volume 34 covers literature published from July 2001 to June 2002. Specialist Periodical Reports provide systematic and detailed review coverage in major areas of chemical research. Compiled by teams of leading authorities in the relevant subject areas, the series creates a unique service for the active research chemist, with regular, in-depth accounts of progress in particular fields of chemistry. Subject coverage within different volumes of a given title is similar and publication is on an annual or biennial basis.
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Photochemistry Volume 17
A Review of the Literature Published Between July 1984 and June 1985
By D. Bryce-Smith
The Royal Society of ChemistryCopyright © 1986 The Royal Society of Chemistry
All rights reserved.
Photophysical Processes in Condensed Phases
BY R. B. CUNDALL
Theoretical aspects of photophysics have not been the subject of a concerted effort although notable contributions have been made. Elliott has produced an authoritative review of the Jahn-Teller effect and three papers deal with the details of ultrafast non-radiative processes in molecules and the time evolution of fluorescence. Fluorescence and absorption by large anharmonic molecules have been analysed by the use of a reduced correlation formulation without having to consider individual eigenstates. An extensive review dealing with the quantum theories of solvent-effects is basic to most experimental studies of environmental influences . Amongst other topics covered are polarization effects in molecular crystals, effects of local heating from radiationless deactivation of electronically excited molecules in rigid media, fluorescence depolarization for a probe in a cylindrical phase, determination of moments of inertia of prolate luminescent molecules, the relaxation time of the orientation polarization and its influence on the luminescence intensity distribution, the effect of quenching upon rotational depolarization of fluorescence, and also a critical contribution to the theoretical analysis of concentration depolarization of fluorescence.
The design of equipment and the processing of data is crucial to development of research in photophysics and its applications. A design of an inexpensive subnanosecond fluorimeter is a noteworthy example. Another paper analyses conventional methods of obtaining fluorescence spectra and shows how sensitivity of detection can be enhanced. The precision of fluorescence measurements has been subject to a fairly detailed experimental examination. The determination of fluorescent decay kinetics still needs improvements of technique and examples are a design for a gating circuit for repetition pulsed light sources and the advantages of a pulsed semiconductor laser as a source for lifetime measurements demonstrated. The latter is smaller and less expensive than conventional lasers as well as being capable of providing picosecond pulses. Amongst papers dealing with fluorescence decay data are an application of the λ invariance test on nonexponential decays, treatment of multiexponential decay data by the method of zero determinants, and the use of Z transforms and method of spike recovery, and a comparison of different fitting techniques. The time resolution of single photon counting of fluorescence from excimer/exciplex systems has been improved by dual analysis using an iterated reconvolution program. A recently published book deals in detail with the whole subject. Advantages of phase-resolved spectroscopy for lifetime measurements have been increasingly recognised especially when lifetime selectivity is combined with wavelength selectivity. Very detailed information about a variable-frequency phase modulation fluorimeter system has been provided by Lakowicz and Maliwal. Phase resolution can enhance signals in both luminescence and Raman spectroscopy and four component determinations have been reported. The so called "colour delay error" can also be overcome and lifetimes with the accuracy of a few picoseconds can be obtained. Amongst papers which deal with improvement of picosecond pulse generation is one dealing with distributed feedback dye lasers and another on pulse on shortening by travelling wave amplified spontaneous emission Dye laser intracavity absorption can also be used as a method for monitoring local environmental change. Improvements in streak cameras also are valuable in the picosecond flash excitation experiments.
Review papers deal with picosecond laser photolysis studies of charge-transfer phenomena and Raman processes. Femtosecond optical techniques allow ultrafast photochemical reactions to be followed and the methodology exemplified by photoionization in micellar solutions.
Multidimensional luminescence measurement enormously increases the information which can be obtained and used to advantage in chemical analysis. Computing methods have also been used to develop a quantum counter comparator and calibration of quantum counters. The usefulness of room-temperature phosphorimetry is reviewed in the book by Vo-Dinh. An interesting example of the use of this technique is the design of a passive monitor for the presence of polynuclear hydrocarbons. Investigations directed towards improvement of the technique continue. Micellar enhanced analytical fluorimetry is another useful method in which the consequences of interfacial effects and molecular organization are exploited. Other reviews of specific applications of fluorescence spectroscopy include biological systems, determinations in clinical and agricultural samples, and fluoro-immunoassay
A very important application of fluorescence in biology is flow cytometry, a subject reviewed by Steinkamp. Since all aspects of fluorescence phenomena can be exploited much development is likely in this subject in the near future.
The study of transient absorption in optically dense materials by diffuse reflectance laser flash photolysis has been surveyed and the theory of absorption presented.
A specific example of the use of luminescence lifetime measurements to determine formation constants has been given for the terbium acetate complex.
A review of the now widely used technique of two-photon spectroscopy applied to aromatic molecules draws attention to the selection rules (parity, symmetry, and pairing) and likely future developments.
2 Excited Singlet-state Processes
The 1.27 µm luminescence from O2 (1Δg) allows direct measurements of relaxation of singlet oxygen to be made. Liquid mixtures are different from pure liquids in their quenching ability and this may provide a method for studying liquids and liquid mixtures. Reversible exciplex formation between O2 (1Δg) vitamin E is deduced from the experimental activation enthalpy and a model of the quenching process suggested.
The fluorescence lifetimes of 33 alkanes have been measured at 25°C by pulse radiolysis and the relationship between molecular structures, fluorescence lifetime, quantum yield, and radiative rate constant discussed. The lifetime of the fluorescent excited state in solid, liquid, and vapour phase cyclohexane has been measured using VW synchrotron radiation.
The formation time of the solvated electron in water and alcohols has been measured by picosecond 266 nm multiphoton excitation. Similar studies have also been made with liquid CC14 and related alkyl chlorides. The participation of excimer formation in the photolysis of 2-pentanone in iso-octane has been deduced from the concentration dependence of product and triplet state quantum yields and from the fluorescence decay kinetics.
Benzene and its simple derivatives continue to attract interest. The Rydberg spectrum of benzene has been explored by linear and circular polarized UV two-photon absorption of the jet-cooled molecule61 and in a related paper symmetry assignments of six gerade Rydberg states made by four-photon spectroscopy Excitation of benzene, pyridine, and hexafluorobenzene with the fourth harmonic of a passively mode-locked Nd3+ YAG laser causes a red fluorescence emitted from an electronically excited state of a biradicaloid intermediate which may be related to the decay channel III of benzene. Picosecond time-resolved absorption studies of several neat liquid benzenes have been made by multiphoton laser photolysis. Excimer formation proceeds by ionization and recombination processes and a lifetime 10-20 ps determined for the ionized (ion pair) state in neat benzene. The photophysics of chlorobenzene in cyclohexane have been examined in some detail. The triplet which can be studied by laser flash photolysis has a lifetime of 1.6 [+ or -] 0.1 µs and rate constants for the various excited processes assigned. Intramolecular 1(2pπ [right arrow] 3dπ charge transfer occurs very rapidly (< 10 ps) in the excited state of phenyldisilane. At room temperature a long lived 425 nm transient appears with a rise time of 30 ps from the 1(2pπ, 3dπ) CT state. The photophysics and photochemistry of lepidopterene and its dimethyl derivatives (tetracyclic benzenoid hydrocarbons) provide an example of systems with coupled chromophores. A considerable insight into the behaviour of this system has been obtained. The photophysical behaviour of exciplexes of 1,4-dicyanonaphthalene with methyl- and methoxy-substituted benzenes shows that the radiative rate constant for emission from the exciplex decreases as the amount of charge-transfer character increases whilst both intersystem crossing and internal conversion increase. The results are explained by the effect of locally excited configurations and the width of the energy gap between states involved in the radiationlesss transitions.
The photophysical behaviour of biphenyl and its derivatives is complicated by the influence of molecular conformation on nature of the excited state. The effects of temperature and change of solvent have been examined in some detail by two groups.
The photophysics and photochemistry of naphthalene in alcohols and alkanes have been studied at pressures up to 6 GPa at room temperature. Photoreduction, photoaddition, and subsequent polymerization reactions indicate that the primary step is a two-photon process, possibly involving a triplet pathway. The laser-excited Shpol'skii spectroscopy has been studied in some detail for methylnahthalene derivatives in low-temperature durene crystals. Unstable excimer formation has been studied in cis-1,2-di(2-naphthyl) cyclobutene. The instability of the excimer is due to the inability of the naphthalene rings to form the stable parallel sandwich conformation at a distance of 0.3-0.4 nm. The conformation of S0 and S1 states of diphenylnaphthalenes have also been analysed. The effects of inter-chromophore interaction on the spectroscopy and photochemistry of anthracene and naphthalene have been examined in a phane compound. The equilibrium geometries of 9-phenylanthracene in S0, S1, and T1 states and electronic transition energies and oscillator strengths have been calculated theoretically and used to interpret the behaviour in absorption and fluorescence. The S0 and S1 torsional potentials of 9-phenylanthracene have also been measured by laser-induced fluorescence in a pulsed supersonic expansion. Gustav has also carried out similar calculations with 9,10-diphenylanthracene.
In recent years reports of oscillatory behaviour during fluorescence analysis have been attributed to a convective unstability. A recent paper reports a correlation between hydro-dynamic and fluorescence oscillations for 9,10-dimethylanthracene in chloroform. There has been much debate and confusion over such systems.
Solute-solute interactions occur in centres responsible for the Shpol'skii effect in solid solutions of phenanthrene and dibenzofuran in n-alkanes, and a model for the luminescent centres has been proposed. The dynamics of relaxation from upper levels of the S1* state and upper vibronic states S3* and S4* of perylene dissolved in organic solvents have been investigated. Slow decay times have been measured: these are about 30 ps for vibrational relaxation of S1* and 50-60 ps for internal conversion from states S3* and S4*. Significant solvent effects have not been observed.
An exciplex with an emission around 480 nm is found in the quenching of perylene fluorescence by Ag+ ions. With several aromatic hydrocarbons and derivatives quenching by closed-shell heavy-metal ions Zn2+, Ag+, Cd2+, In3+, Sn2+, Cs+, Hg2+, Tl+, and Pb2+ indicates that fluorescence quenching involves nonfluorescent complex formation followed by rapid intersystem crossing. Photo-ionization of perylene in polar solvents shows both electron and hole injection are involved in electrophilic (acetonitrile) and nucleophilic (dimethylformamide) solvents. Under supersonic jet conditions perylene in the presence of alkane vapour forms van der Waals clusters of varying sizes. This type of experiment gives precise control of conditions for studying molecular relaxation.
The fluorescence lifetime of pyrene in hexane has been measured as a function of concentration of ethyl iodide and CBr4 and influence of hydrostatic pressure up to 343 MPa. The equilibrium constant for exciplex formation increases with increasing pressure. Intramolecular electron transfer in 9,9'-bianthryl changes mechanism as a result of pressure change. Fluorescence studies on solvent-involved intramolecular charge separation in symmetric biaryls show a high dipole moment is developed in a twisted charge-transfer state.
Details of the electronic spectra and structure of triphenylene, terphenyls, p-quaterphenyls, o-durylene and pentaphene have also been published. The resonance Raman spectrum of all-trans diphenylbutadiene (DPB) in its lowest S1 state provides some support of an assignment of 1Ag symmetry. The efficiency of 1O2 generation by oxygen quenching of polyene singlets and triplets has been measured. 1O2 is formed from both the singlet and triplet excited state of 1,6-diphenylhexatriene (DPH). The fluorescence behaviour of 1,1-diphenylethylene has also been analysed . Fluorescence spectra and quantum yields of a series of cyclophanes with unsaturated bridges show shifts which are related to the planarity of the excited state. Laser flash photolysis studies have also been made on aromatic carbenes. The photo-physics and photochemistry of arylmethyl radicals, produced by pulse radiolysis, were excited by 347 nm laser pulses and their fluorescence properties and reactions examined. The effects of solvent and heavy-atom perturbation show intersystem crossing from the triplet to singlet state prior to chemical reaction for the radical 1,1-biacenaphthene-2,2'-diyl.
Fluorescence of methyl salicylate has been used as a probe to examine the geometry of complexation in the α, β, and γ-cyclodextrins. Heavy-atom effects on the excited-state properties of 9-acetoxy-10-acetoxyhalogenophenyllanthracene derivatives have been examined and interpreted. The quenching of methoxynaphthalene fluorescence by protons in H2O-CH3CN mixtures and included in β-cyclodextrin has also been studied.
New twisted internal charge-transfer (TICT) molecules have been reported for aniline and N,N-dimethylaniline. An excited charge-transfer state has been reported for 4-(9-anthryl)-N,N-dimethyl-aniline. The concept of excited group moments has been supported by observation of excited-state dipole moments of chloroanilines and chlorophenols determined by solvatochromic shifts in absorption. Absorption and emission spectra of isomeric tolunitriles have also been investigated in detail. Several alkyl nitrnbenzenes have been studied by picosecond transient spectroscopy. Excited-state proton transfer reactions of the excited singlet state of naphthyl/ammonium ion/18-crown-6 complexes in MeOH-H2O mixtures have been studied by single-photon counting fluorimetry. Multi-exponential analysis of the fluorescence decay of 9-cyano-10-t-butylanthracene in PMMA shows a very fast radiationless process from S1 to S0, (> 1010 s-1). Excimer formation in aromatic cholesteric liquid crystals has been related to molecular structure.
The role of proton transfer reactions and proton-induced quenching of aromatic compounds has been thoroughly reviewed by Shizuka. A new method for estimating excited-state pK* from rates of proton transfer has been described. Conductimetric flash photolysis has been used to observe the photoionization and subsequent reactions of β-naphthol. Solvent dependence and kinetics of proton transfer reaction of 9-phenanthrol in the excited singlet state have been reported. Intramolecular excited-state proton transfer in 3-hydroxyflavone has been examined in some detail since it serves as a model for phenomena of importance in thin-layer chromatography. Structural effects on the proton transfer kinetics of the same compound have also been studied. Transient absorption and two-step laser excitation fluorescence studies have been made on proton transfer and relaxation in methanol solutions of 7-hydroxyflavone.
Excerpted from Photochemistry Volume 17 by D. Bryce-Smith. Copyright © 1986 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents
ContentsPART I PHYSICAL ASPECTS OF PHOTOCHEMISTRY,
Chapter 1 Photophysical Processes in Condensed Phases By R.B. Cundall, 3,
Chapter 2 Gas-phase Photoprocesses By J.E. Baggott, 44,
PART II PHOTOCHEMISTRY OF INORGANIC AND ORGANOMETALLIC COMPOUNDS,
Chapter 1 The Photochemistry of Transition-metal Complexes By A. Cox, 111,
Chapter 2 The Photochemistry of Organometallic Complexes By A. Cox, 167,
Chapter 3 Photochemistry of the Main Group Elements By A. Cox, 195,
PART III ORGANIC ASPECTS OF PHOTOCHEMISTRY,
Chapter 1 Photolysis of Carbonyl Compounds By W.M. Horspool, 207,
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By W.M. Horspool, 235,
Chapter 3 Photochemistry of Alkenes, Alkynes, and Related Compounds By W.M. Horspool, 302,
Chapter 4 Photochemistry of Aromatic Compounds By A. Gilbert, 344,
Chapter 5 Photo-reduction and -oxidation By A. Cox, 419,
Chapter 6 Photoreactions of Compounds containing Heteroatoms other than Oxygen By S.T. Reid, 437,
Chapter 7 Photoelimination By S.T. Reid, 482,
PART IV POLYMER PHOTOCHEMISTRY By N.S. Allen, 529,
PART V PHOTOCHEMICAL ASPECTS OF SOLAR ENERGY CONVERSION By A. Harriman, 601,
Author Index, 632,