Solution Photochemistry Research Articles

Investigating the phase-dependent photochemical reaction dynamics of chlorine dioxide using resonance Raman spectroscopy

Recent progress in understanding the phase-dependent reactivity demonstrated by halooxides is outlined. Specifically, resonance Raman intensity analysis (RRIA) and time-resolved resonance Raman (TRRR) studies of chlorine dioxide (OClO) photochemistry in solution are presented. Using RRIA, it has been determined that the excited-state structural evolution that occurs along the asymmetric-stretch coordinate in the gas phase is restricted in solution. The absence of evolution along this coordinate results in the preservation of groundstate symmetry in the excited state. The role of symmetry in defining the reaction coordinate and the solvent-solute interactions responsible for modification of the excited-state potential energy surface are discussed. TRRR studies are presented which demonstrate that geminate recombination of the primary photoproducts resulting in the reformation of ground-state OClO is a central feature of OClO photochemistry in solution. These studies also demonstrate that a fraction of photoexcited OClO undergoes photoisomerization to form ClOO, with the ground-state thermal decomposition of this species resulting in Cl production on the subnanosecond timescale. Finally, time-resolved anti-Stokes experiments are presented which demonstrate that the OClO vibrational-relaxation dynamics are solvent dependent. The current picture of OClO photochemistry derived from these studies is discussed, and future directions for study are outlined.

Photochemistry of Anils in NaY Zeolite

A series of anils (1−6) have been incorporated in NaY zeolites and the resulting samples characterised spectroscopically. Raman spectral analysis showed that anils exist in the zeolite microcavities predominantly in a zwitterionic form. In agreement, diffuse reflectance UV/Vis spectra showed the presence of a significant concentration of the zwitterionic form, characterised by an absorption band around 400 nm. A sharp contrast is drawn between their photochemistry in solution or in the crystalline form with that in NaY zeolites. Thus, steady state irradiation at 400 nm led to persistent changes in the diffuse reflectance UV/Vis spectra with decrease of the reflectance of the band at 400 nm and the appearance of a new absorption band at longer wavelengths. No photochromism is observed and this is attributed to the isomerization of the zwitterionic form to other stereoisomers, possibly by rotation about C−C or C−N bonds with partial double bond character. This photochemical isomerization has not been reported so far. Even though the hydrogen bonding ability and polarity of NaY zeolite has been reported to be similar to that of polyfluorinated alcohols, based on the enolisation equilibrium of anils, the photochemical behaviour of these compounds incorporated within NaY is different to that reported in polyfluorinated alcohols. Specifically, no excited-state proton transfer or proton catalysed re-enolization of the zwitterionic form is observed with anils in NaY. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Investigating the Phase-Dependent Reactivity of Chlorine Dioxide Using Resonance Raman Spectroscopy

Recent progress in understanding the phase-dependent reactivity of chlorine dioxide (OClO) is outlined. Specifically, resonance Raman intensity analysis (RRIA) and time-resolved resonance Raman (TRRR) studies of OClO photochemistry in solution are presented. The RRIA studies demonstrate that the optically prepared excited-state undergoes significant modification in solution relative to the gas phase. Specifically, the substantial evolution that occurs along the asymmetric-stretch coordinate in the gas phase is restricted in solution. The absence of substantial evolution along the asymmetric-stretch coordinate results in the preservation of ground-state symmetry in the excited state. The role of symmetry in defining the reaction coordinate and the solvent−solute interactions responsible for modification of the excited-state potential energy surface are discussed. TRRR studies performed with pump and probe wavelengths at 390 nm are presented which demonstrate that geminate recombination of the primary photo...

Synthesis and Photochemistry of Tertiary Amine Photobase Generators

A family of tertiary amine photobase generators have been prepared and studied. The compounds investigated were quaternary ammonium salts of benzhydrylamine (aminodiphenylmethane) and 9-aminofluorene. The compounds were prepared by the following methods: methylation of the benzhydryl or fluorenylamines, reaction of a tertiary amine with 9-bromofluorene, and reaction of a primary or secondary amine with 9-bromofluorene followed by exhaustive methylation. Alkylation was limited to methylation in the benzhydryl system, as larger alkyl groups would not react. This appears to be a result of steric hindrance. The fluorenyl system allowed for a wider variation in the synthesis of tertiary amine photobase generators. Examination of the solution photochemistry by NMR spectroscopy supported a heterolytic mechanism for photodecomposition.

Semiclassical simulations of azomethane photochemistry in the gas phase and in solution.

We present semiclassical simulations of the dynamical events which follow the n --> pi excitation of an azomethane molecule: decay to the ground state, isomerization, and dissociation. Ab initio potential energy surfaces and couplings, slightly modified according to available experimental data, are employed in conjunction with a trajectory-surface-hopping method. For an isolated molecule, we show that dissociation takes place in the ground state, because the radiationless decay is very fast. The dissociation is mainly sequential: the methyldiazenyl radical first produced fragmentates very rapidly to N(2) + CH(3)*. The results of previous experiments by Lee's and Zewail's groups, from which partially conflicting conclusions had been drawn, are reproduced and interpreted. Molecular dynamics, coupled to the surface-hopping algorithm, allows us to simulate the photochemistry in acqueous solution. Here the fragmentation is suppressed because of vibrational energy loss to the solvent: only the isomerization takes place.

Solution and Nujol Matrix Photochemistry of (η5-C5H5)2Os2(CO)4 and Nujol Matrix Photochemistry of (η5-C5H4CH3)2Ru2(CO)4 and (η5-C5H5)M(CO)2H, Where M = Ru and Os

Lowering the temperature of a Nujol solution of (η5-C5H4CH3)2Ru2(CO)4 containing bands of the trans- and cis-bridging and trans-nonbridging isomers from room temperature to ca. 90 K results in complete disappearance of the bands of the trans-terminal isomer. The photochemistry of (η5-C5H4CH3)2Ru2(CO)4 in frozen Nujol at 90 K differs subtly from that of (η5-C5H5)2Ru2(CO)4 in inert gas matrixes at 12 K in that the trans-bridging isomer is found to undergo photolysis at high energy (λirr > 250 nm) to yield both the triply bridged carbonyl-loss species, (η5-C5H4CH3)2Ru2(μ-CO)3, and the cis-bridged species isomer. There is no evidence for formation of the trans-nonbridged isomer upon photolysis as observed for (η5-C5H5)2Ru2(CO)4. The solution photochemistry of (η5-C5H5)2Os2(CO)4 is shown to closely parallel that of (η5-C5H5)2Ru2(CO)4. Photolysis of (η5-C5H5)2Os2(CO)4 in CHCl3 yields (η5-C5H5)Os(CO)2Cl, while photolysis in benzene yields hydride products believed to arise from metal-to-ring coupling of radicals. Photolysis of (η5-C5H5)2Os2(CO)4, which exists only in a nonbridged form, in frozen Nujol (90 K) results in CO loss and formation of (η5-C5H5)2Os2(μ-CO)3 as the only photoproduct. Photolysis of (η5-C5H5)M(CO)2H, where M = Ru or Os, in frozen Nujol yields (η5-C5H5)M(CO)H and a second product that appears to be either (η5-C5H6)M(CO)2 or the (η5-C5H5)M(CO)2 radical.

Excited State Properties of Rh2(O2CCH3)4: Solution Photochemistry and Photoinitiated DNA Cleavage

The photophysical properties of Rh2(O2CCH3)4(L)2 (L = CH3OH, THF = tetra-hydrofuran, PPh3 = triphenylphosphine, py = pyridine) were explored upon excitation with visible light. All the complexes exhibit a long-lived transient absorption signal (τ = 3.5–5.0 μs) assigned as an electronic excited state of each molecule. An optical transition at −760 nm is observed in the spectra of the transients, whose position is relatively independent of axial ligand. No emission from the Rh2(O2CCH3)4(L)2 (L = CH3OH, THF, PPh3, py) systems was observed at room temperature or at 77 K, but energy transfer from excited Rh2(O2CCH3)4(PPh3)2 to tetracene and perylene takes place to form the 3ππ* excited state of each acceptor. Electron transfer from *Rh2(O2CCH3)4(PPh3)2 to dimethyl viologen (MV2+) and chloro-p-benzoquinone (Cl-BQ) takes place with quenching rate constants (kq) of 8.0 × 106 M−1s−1 and 1.2 × 106 M−1s−1 in methanol, respectively. A kq value of 2 × 108 M−1s−1 was measured for the quenching of the excited state of Rh2(O2CCH3)4(PPh3)2 by O2 in methanol. The results of the energy and electron transfer experiments are consistent with the production of an excited state of Rh2(O2CCH3)4(PPh3)2 with energy, E00, between 1.34 eV and 1.77 eV. The excited state of Rh2(O2CCH3)4 is not able to undergo hydrogen abstraction chemistry. However, the photoproduced one-electron oxidized complex, Rh2(O2CCH3)4 +, is able to convert isopropanol to acetone and to efficiently cleave DNA with λirr≥ 610 nm.

ChemInform Abstract: Is the Translational Diffusion of Organic Radicals Different from That of Closed-Shell Molecules?

The translational diffusion of photochemically created intermediate radicals is measured by the transient grating technique. The diffusional behavior of these intermediates is different from that of stable molecules, which have already been investigated extensively. The investigation of the diffusion of these species will provide an opportunity to reveal the unique intermolecular interaction between the intermediates and matrix. This information will be valuable for understanding photochemistry in solutions.

Is the translational diffusion of organic radicals different from that of closed-shell molecules?

The translational diffusion of photochemically created intermediate radicals is measured by the transient grating technique. The diffusional behavior of these intermediates is different from that of stable molecules, which have already been investigated extensively. The investigation of the diffusion of these species will provide an opportunity to reveal the unique intermolecular interaction between the intermediates and matrix. This information will be valuable for understanding photochemistry in solutions.

Oxidation of Aryl- and Diarylcyclopropanes in a Pentasil Zeolite: Ring Opening with Deprotonation or Net Hydrogen Migration

Incorporation of trans-1,2-diphenylcyclopropane (1) and its 3,3-D2-isotopomer into the channels of a redox-active pentasil zeolite (Na-ZSM-5) resulted in the formation of exo,exo-1,3-diphenylallyl radical (2•) and its 2-D1 isotopomer, respectively. This conversion requires oxidation, ring opening, and deprotonation, in an unspecified sequence. The allyl radical 2• is also formed upon incorporation of trans-1,3-diphenylpropene (3). A comparison with the solution photochemistry, in the presence or absence of added base, shows the conversion of 1 into 2• to be a zeolite-specific reaction. Incorporation of arylcyclopropanes 9 (R = H, OCH3) into ZSM-5 generated trans-propenylbenzene radical cations 10•+ (R = H, OCH3); the 2,2-D2 isotopomer of 9 (R = OCH3) gave rise to three different isotopomers of 10•+ (R = OCH3).

Latent chemical behavior revealed in the crystalline state: novel photochemistry of a cis-9-decalyl aryl ketone

The case of a ketone whose photochemistry in solution is markedly different from that observed in the crystalline state is reported.

A resonance Raman study of Cl 2O photochemistry in solution: evidence for ClClO formation

The first resonance Raman spectra of Cl 2O dissolved in CCl 4 are reported. The dependence of the resonance Raman spectrum on incident energy is investigated and found to be consistent with the photochemical production of both ClO and ClClO. Both of these photoproducts appear linearly with incident energy at low power, but the scattering intensity of ClO is found to increase quadratically at higher energies. This behavior is proposed to be consistent with photodissociation of Cl 2O into ClO and Cl followed by partial geminate recombination of these fragments, resulting in ClClO formation. At higher excitation energies, photodissociation of ClClO is proposed to occur resulting in ClO production. Finally, the photochemical behavior of Cl 2O is compared to that of OClO, where geminate recombination does not result in formation of the analogous ClOO isomer. This comparison suggests that although geminate recombination is a characteristic feature of halo-oxide photochemistry in condensed environments, the products formed by recombination are species dependent.

Photophysical Features of the M(2)(CO)(10), M = Mn and Re, Solution Photochemistry.

In the UV photolysis of M2(CO)10, M = Mn or Re, flash photochemical methods were used for the determination of the primary product, i.e., nonacarbonyl (M2(CO)9) and pentacarbonyl (M(CO)5) quantum yields. The quantum yield based action spectra are very metal dependent and indicative of different mechanisms for the Re and Mn photochemistry. In the figure, the action spectra for the Re decacarbonyl photoproducts, φP for Re(CO)5 and φN for Re2(CO)9, are superimposed with the absorption spectrum. The results in the photochemistry of Re2(CO)10 suggest that the upper electronic states, i.e., populated by irradiating below 340 nm, produce some Re(CO)5.

Upper Excited State Photochemistry: Solution and Gas Phase Photochemistry and Photophysics of 2- and 3-Cyclopropylindene1

The cyclopropylcarbinyl radical rearrangement has been used to probe the photochemistry and photophysics of S2 and S1 in 2-cyclopropylindene (2CPI) and 3-cyclopropylindene (3CPI). Studies in solution and the gas phase are described. Population of S2 with 254 nm light excitation in the gas phase produces the anticipated ring expansion products 2,3,3a,8-tetrahydrocyclopenta[a]indene (1, Φ1 = 0.1) and 1,3,3a,8-tetrahydrocyclopenta[a]indene (2, Φ2 = 0.06) from 2CPI and 3CPI, respectively (Scheme 3). Direct excitation into the S2 state (254 nm) of 2CPI in solution also produces compound 1. The efficiency of the solution phase chemistry is a function of excitation wavelength (Φ1 = 0.022 and 0.006 for 254 and 280 nm, respectively). The solution phase excitation spectrum of 2CPI shows an anomalous dependency on monitoring wavelength which is attributed to a conformational equilibrium. The S1 singlet lifetimes of the cyclopropylindenes are quite short (0.39 and 1.0 ns for 2CPI and 3CPI, respectively) relative to t...

Destruction of the Urea Host Lattice by the Photochemical Fragmentation of Molecular Guests

Photochemistry in confined environments can lead to both different products and different product distributions from those observed in solution photochemistry, leaving open questions concerning the chemical involvement of the host lattice, including whether it maintains its structure. We have found that photolysis of the urea inclusion compound of 5-nonanone shows destruction of the hexagonal urea host lattice and concomitant conversion to the close-packed tetragonal urea structure. In addition, irradiation of urea-d4/5-nonanone gives substantial deuterium incorporation, especially at the α-CH3 of the fragmentation product, 2-hexanone. By studying both the rates of photochemical conversion of 5-nonanone and the loss of the urea host lattice in urea/5-nonanone, we deduce that the structural changes of the urea lattice are associated with fragmentation products that are too small (i.e., too volatile) to support urea inclusion compounds. Our observation of even more degradation of the urea host lattice upon photolysis of urea/2-hexanone, a reaction which leads to a greater proportion of smaller guests than for urea/5-nonanone, supports this conclusion.

Effect of pH on Photopolymerization Reaction of Aniline Derivatives with the Tris(2,2‘-bipyridyl)ruthenium Complex and the Methylviologen System

Photopolymerization of aniline derivatives was revealed to be induced by photoinduced electron transfer (PIET) between the tris(2,2‘-bipyridyl)ruthenium complex [Ru(bpy)32+] and electroactive molecules. Since the initial photopolymerization reaction depended on PIET between Ru(bpy)32+ and N-phenyl-p-phenylenediamine (PPD), the effect of pH of the polymerization solution on the PIET was investigated to improve the efficiency of the photopolymerization reaction. At lower pH, the quenching reaction of [Ru(bpy)32+]* by PPD proceeded with low efficiency due to electrostatic repulsion between molecules concerning the PIET. This was clarified by comparison with the results concerning the photochemistry in a higher pH solution. On the other hand, a lower pH should be required to induce the propagation because a lower pH inhibited the deprotonation of both aniline derivative and growing poly(aniline) chain. From the detailed analyses for both initial and propagation reactions, the increase in ionic strength was revealed to be a solution to improve the photopolymerization efficiency even in low pH.

Macrocyclic Amphiphiles. 3. Monolayers of O-Octacarboxymethoxylated Calix[4]resorcinarenes with Azobenzene Residues Exhibiting Efficient Photoisomerizability

O-Octacarboxymethoxylated calix[4]resorcinarenes having four azobenzene residues at the lower rim were prepared to study photoisomerizability in monolayers. UV irradiation of the amphiphilic macrocycles at 365 nm on a water surface resulted in the formation of about 60% of the Z-isomer even in densely packed monolayers while 99% of the Z-isomer was formed in solution photochemistry. Langmuir−Blodgett monolayered films deposited on a quartz substrate displayed about 72% of E-to-Z conversion at a photostationary state. The amphiphilic macrocycles adsorbed firmly on a quartz plate to give relatively densely packed monolayers. About 86% of Z-isomer was formed when these monolayers were exposed to 365 nm light. The efficient E-to-Z photoisomerizability in monolayers is ascribable to large difference in cross-sectional areas between the cylindrical framework of calix[4]resorcinarene (1.7 nm2) and azobenzenes (0.25 × 4 = 1.0 nm2) to ensure a two-dimensional free volume. The E-to-Z photoisomerization in adsorbed monolayers deviated from the first-order plots, indicating that steric constraint exists more or less.

Solution Photoreactivity of Phenanthrenequinone Diimine Complexes of Rhodium and Correlations with DNA Photocleavage and Photooxidation

The transient absorption spectra of Rh(III) complexes containing one or two phi ligands (phi ) 9,10-phenanthrenequinone diimine) and various ancillary ligands were measured at pH 5.0 utilizing visible and UV excitation. The spectra were all consistent with a primarily phi ligand-centered (LC) n﷿* transition. The spectral profile obtained with visible excitation of the complexes is slightly different and significantly weaker at pH 8.0, where now Rh(phi)2(phen) 3+ , Rh(phi)2(bpy) 3+ , and Rh(phen)2(phi) 3+ are deprotonated. Irradiation of these complexes in basic media with 308 nm laser excitation leads to irreversible ligand-loss photochemistry. The LC excited states of Rh(phi)2(phen) 3+ , Rh(phi)2(bpy) 3+ , and Rh(phen)2(phi) 3+ are reductively quenched by the purine DNA bases, A, dA, dAMP, AMP, G, dG, dGMP, and GMP, with rate constants ranging from 1.4 10 9 M -1 s -1 to 4.7 10 9 M -1 s -1 at pH 5.0, but no quenching was observed for dC or dT. Absorbances assigned to the reduced Rh(II) complex and dG ¥ were observed in the transient absorption spectrum. There are some parallels between the observed photochemistry in solution and the DNA photocleavage results, in particular the presence of oxidative damage to the DNA bases.

Photochemical features of intermolecular C–H bond activation

The photophysical and photochemical properties of the C–H activating ( η 5-C 5H 5)Rh(CO) 2, ( η 5-C 5Me 5)Rh(CO) 2, ( η 5-C 5H 5)Ir(CO) 2 and (HBPz 3 *)Rh(CO) 2 (Pz *=3,5-dimethylpyrazolyl) complexes in room-temperature hydrocarbon solutions are compared and contrasted. Recent quantitative measurements of the solution photochemistry and kinetic analyses of the reaction pathways are discussed. The data reveal that the C–H and Si–H activation reactions occur via similar photochemical routes, whereas the ligand photosubstitution reaction proceeds by a different pathway. Several key mechanistic features of the light-induced intermolecular C–H bond activation process are identified.

Quantitative Measurements of CpRh(CO)2 (Cp = η5-C5H5) Photochemistry in Various Hydrocarbon Solutions: Mechanisms for Ligand Photosubstitution and Intermolecular C−H and Si−H Bond Activation Reactions

The quantitative solution photochemistry of CpRh(CO)2 (Cp = η5-C5H5) involving ligand substitution and intermolecular C−H and Si−H bond activation processes has been investigated in several hydrocarbon solvents at room temperature following excitation in the region 313−458 nm. These photoreactions have been monitored by UV−vis and FTIR spectroscopy, and the absolute quantum efficiencies (φcr), determined to be in the 0.0007−0.31 range, are dependent on the entering ligand concentration, excitation wavelength, and solvent. The observed wavelength dependence is consistent with distinct reaction pathways occurring from two rapidly dissociating ligand-field (LF) excited states. Analysis of the quantitative photochemical results has led to a comprehensive mechanistic description for all of the various competing reaction pathways in the photochemistry of CpRh(CO)2. In the absence of an entering ligand, a carbonyl-bridged trans-Cp2Rh2(CO)3 complex is identified as the major photochemical reaction product; this s...