Photoinduced bond dissociation of 4-methylcoumarin derivatives in solution studied by laser flash photolysis and DFT calculations.

Photochemical properties of 4-coumarinylmethyl derivatives (CM-X, X = Br, Cl, OH, OPh, SH, and SPh) in solution were studied by laser flash photolysis and DFT calculations. It was found that CM-Br and CM-SPh undergo photoinduced bond dissociation in the lowest excited singlet state, yielding the 4-coumarinylmethyl radical (CMR) and the corresponding radical with quantum yields of approximately 0.25. Laser flash photolysis of CM-Cl, OH, OPh, and SH provided no or very little transient absorption of the corresponding triplet state. Upon triplet sensitization of CM-X using benzophenone (BP) as a triplet sensitizer, efficient formation of triplet CM-OH and -OPh was seen, whereas CM-SH and -SPh underwent the C-S bond cleavage in the lowest triplet (T(1)) state, resulting in production of CMR and the corresponding radicals with efficiencies (alpha(rad)) of >or=0.66. CM-Br and -Cl efficiently quenched triplet BP without formation of appreciable intermediates. On the basis of the results of laser flash photolysis and DFT calculations, photochemical features of triplet CM-X were discussed in detail.

The facile generation of a tetramethyleneethane type radical cation and biradical utilizing a 3,4-di(alpha-styryl)furan and a photoinduced ET and back ET sequence.

Product analyses and nanosecond time-resolved spectroscopy on laser flash photolysis were studied for the photoinduced electron-transfer reaction of 3,4-di(alpha-styryl)furan (6a). A combination of these results, kinetic, density functional theoretical (DFT), and time-dependent DFT analyses enabled assignment of the absorption to the tetramethyleneethane (TME)-type radical cation (7a*+, lambda(max) = 392 nm) and the corresponding singlet biradical ((1)7a**, lambda(max) = 661 nm). These two intermediates were mechanistically linked to each other with a facile back electron-transfer reaction. The present studies provide a new method for the generation of aryl-substituted TME-type intermediates.

Spin dynamic study on the electric field dependence of carrier generation.

Using an acceptor-doped poly(N-vinylcarbazole) film, the magnetic field effect (MFE) on the generation efficiency of photoinduced charge was measured under various electric fields in order to clarify how the applied electric field affects the elementary processes in the photocarrier generation in photoconductive polymeric molecular solids. The external magnetic field influenced the electron spin dynamics among the geminate electron-hole pairs within a scale of a few nanometers and decreased the photocarrier generation efficiency. The observed MFE due to a hyperfine mechanism was almost independent of the electric field. By employing the stochastic Liouville equations based on a one-dimensional lattice model, we performed some model calculations for the dissociation, hopping, and recombination rate dependence of MFE on the generation efficiency. From a comparison between the observed and calculated MFE, it was concluded that the electric field affects the dissociation more than the hopping and the recombination. This coincides with the concepts in the Onsager model that is used to analyze the electric field dependence of carrier generation efficiency so far. The one-dimensional lattice model is a proper model for the carrier generation in polymeric molecular solids, which is qualitatively consistent with the Onsager model except for the long-range hole jump.

Carrier generation process on photoconductive polymer films as studied by magnetic field effects on the charge-transfer fluorescence and photocurrent.

We have studied the magnetic field effects (MFEs) on the charge-transfer fluorescence and transient photocurrent of a 1,2,4,5-tetracyanobenzene-doped poly(N-vinylcarbazole) film, which reflect the recombination and escape yields of the carriers, respectively. The recombination yield dependence of the external magnetic field (B) clearly shows two types of the MFEs, growth with increasing B due to the hyperfine mechanism (HFM) and a negative dip due to the level-crossing mechanism (LCM). On the other hand, the escape yield indicates complementary MFEs with a sharp decrease in yield with increasing B and then a positive dip. Simultaneous observation of the HFM- and LCM-MFEs proves the stepwise hole-hopping mechanism rather the long-range hole-jumping one. The quantitative analysis of the recombination and escape MFEs is performed using the stochastic Liouville equations (SLE) for a one-dimensional lattice model in which the stepwise hole hops take place between the nearest neighbor carbazole units with spin conservation. The SLE analysis provides the recombination and hole transfer rate constants of 7.0 x 10(7) and 4.5 x 10(8) s(-1), respectively. The boundary site number for the ion pairs in the one-dimensional model is estimated by the best fit to the experimental results. The interionic distance of the boundary ion pair in the one-dimensional model including eight sites agrees with the thermalization distance in the Onsager model. Hence, it is concluded that the elementary processes in the Onsager model applied to molecular amorphous solids are the stepwise hole hops rather than a long-range hole jump.

Dependence of mechanochemically induced decomposition of mono-chlorobiphenyl on the occurrence of radicals.

Mechanochemically induced dechlorination of mono-chlorobiphenyl (BP-Cl) on the surfaces of metal oxides was compared with that on metal hydroxides, using the three metals of Mg, Al, and La as examples. The metal oxides, such as gamma-alumina (gamma-Al2O3), magnesium oxide (MgO) and lanthanum oxide (La2O3) showed an efficient ability to dechlorinate the BP-Cl; however, BP-Cl remained in the ground samples when the hydroxides were used. From the product analysis using gas chromatography/mass spectrometry (GC/MS) and electron spin resonance (ESR) measurements, it was confirmed that the charge transfer from the O2- site on the surface of the oxide additives due to the intense grinding has plays a significant role in the decomposition of the chlorinated compound. Based on the observed dependence of the dechlorination on the radical occurrence, some practical methods were proposed to improve the destruction efficiency of the chlorinated organic compounds.

Formation of superoxide anion during ferrous ion-induced decomposition of linoleic acid hydroperoxide under aerobic conditions.

We studied the mechanism of formation of oxygen radicals during ferrous ion-induced decomposition of linoleic acid hydroperoxide using the spin trapping and chemiluminescence methods. The formation of the superoxide anion (O2*-) was verified in the present study. The hydroxyl radical is also generated through Fenton type decomposition of hydrogen peroxide produced on disproportionation of O2*-. A carbon-centered radical was detected using 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO) as a spin trap. Alkoxyl radical formation is essential for the conversion of linoleic acid hydroperoxide into the peroxyl radical by ferrous ion. It is likely that the alkoxyl radical [R1CH(O*)R2] is converted into the hydroxylcarbon radical [R1C*(OH)R2] in water, and that this carbon radical reacts with oxygen to give the alpha-hydroxyperoxyl radical [R1R2C(OH)OO*], which decomposes into the carbocation [R1C+(OH)R2] and O2*-.

Ferromagnetic Interactions between Imino Nitroxides through Diamagnetic Metal Ions: Crystal Structures, Magnetism, and Electronic Properties of [M(I)(imino nitroxide)(2)](PF(6)) (M = Cu(I) and Ag(I)).

An orthogonal arrangement of imino nitroxide ligands in Cu(I) and Ag(I) complexes results in propagation of an intramolecular ferromagnetic interaction between the coordinated imino nitroxides through these diamagnetic ions. Reaction of [Cu(I)(CH(3)CN)(4)](PF(6)) and Ag(I)(PF(6)) with imino nitroxides in ethanol solution gave [Cu(I)(immepy)(2)](PF(6)) (1) and [Ag(I)(impy)(2)](PF(6)) (2), respectively (immepy = 2-(2-(6-methylpyridyl))-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy and impy = 2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy). Complex 1 crystallizes in the orthorhombic space group Pba2, which has a unit cell with a = 11.251(2) Å, b = 14.745(3) Å, c = 9.747(2) Å, V = 1617.0(5) Å(3), and Z = 2. Refinement with 991 reflections observed [|I(o)| > 3sigma(I(o))] gave R = 0.044 (R(w) = 0.053). In 1, imino nitroxides coordinate to the Cu(I) ions in a tetrahedral fashion, the dihedral angle between coordinating imino nitroxides being 88.7 degrees. Magnetic susceptibility measurement for 1 shows typical triplet-singlet behavior with strong ferromagnetic interactions (J = 55.1(6) cm(-)(1) with g = 2.0; H = -JS(1)().S(2)()). The UV-visible absorption spectrum of 1 shows strong bands at 464 and 766 nm assignable to charge-transfer bands from the Cu ion to the ligand NLUMO and SOMO, respectively. The pi-back-donation to the SOMO induces the spin on the Cu ion and results in the strong ferromagnetic interaction. Complex 2 crystallizes in the orthorhombic space group P2(1)cn which has a unit cell with a = 10.535(6) Å, b = 31.295(3) Å, c = 8.921(4) Å, V = 2941(1) Å(3), and Z = 4. Refinement with 1882 reflections [|I(o)| > 3sigma(I(o))] gave R = 0.058 (R(w) = 0.074). In 2, the four coordination sites of the Ag(I) ion were completed with two imino nitroxides and the dihedral angle between the imino nitroxides is 79.2 degrees. The EPR spectrum of a frozen ethanol solution of 2 shows a typical triplet signal with zero field splitting (|D| = 0.043 cm(-)(1) and |E| = 0.014 cm(-)(1)). Curie plots for the Deltam = 2 signal below 80 K gave a positive Weiss constant (theta = 4 K), suggesting an intramolecular ferromagnetic interaction in 2.

Electronic structure of the lowest excited triplet state of 5,12-naphthacenequinone.

Continuous-wave time-resolved EPR (cw-TREPR) and pulsed electron nuclear double resonance (ENDOR) studies have been carried out to clarify the electronic structure of the lowest excited triplet (Tl) state of 5,12-naphthacenequinone (5,12-NpQ) as well as 1,4-anthraquinone (1,4-AQ) and 6,13-pentacenequinone (6,13-PeQ). The Tl energy level and the D value of the zero-field splitting (ZFS) parameters only slightly decreased with the increasing pi-conjugated system. The Tl states of these linear para-acenequinones were assigned to the pi pi* character. In triplet 5,12-naphthacenequinone, more than 80% of the unpaired electron spins are localized on the naphthalene aromatic sub-system.

Direct Observation of a Distant Ion Pair Generated in a Photoconductive Poly(N-vinylcarbazole) Film with Dopant.

Photoconduction in charge transfer materials apparently occurs through a distant geminate ion pair with an interionic separation of about 8 Å, as demonstrated by time-resolved EPR spectroscopy of a doped poly(N-vinylcarbazole) film.

Synthesis and intramolecular pericyclization of 1-azulenyl thioketones.

Several azulene-substituted thioketones, 1-thiobenzoylazulene (1a) and di(1-azulenyl) thioketone (2a) and their derivatives (1b and 2b-d) with alkyl substituents on each azulene ring, were prepared and their intramolecular pericyclization reaction was examined. The thioketones with a 3-alkyl substituent on each azulene ring exhibited the presumed pericyclization reaction under thermal and acid-catalyzed conditions, although the cases of the 1-azulenyl thioketones without the 3-alkyl substituents afforded a complex mixture under similar conditions. The intramolecular reaction following the intramolecular hydrogen transfer afforded the products 13b, 14b, and 14c. The products 13b and 14b were converted into the corresponding cations 18(+) and 19(+), which have structural similarity with that of the phenalenyl cation. These cations exhibited the expected two-step reduction waves upon CV, although the ESR analysis revealed that the neutral radical state did not have the presumed high stability.

Photoinduced omega-bond dissociation of m-halomethylbenzophenones studied by laser photolysis techniques and DFT calculations. Substituted position effects.

Photochemical profiles of omega-cleavage of carbon-X (X = Br and Cl) bonds in m-bromo- and m-chloromethylbenzophenones (m-BMBP and m-CMBP) were investigated by laser photolysis techniques and DFT calculations. m-BMBP and m-CMBP were found to undergo omega-bond cleavage to yield the m-benzoylbenzyl radical (m-BBR) at 295 K, and the quantum yields were determined. No CIDEP signal was detected upon 308 nm laser photolysis of both the compounds. From these observations, it was inferred that the omega-bond of these m-halomethylbenzophenones (m-HMBP) cleaves in the lowest excited singlet state (S(1)(n,pi(*))) upon direct excitation. Upon triplet sensitization of acetone (Ac), the m-BBR formation was observed in transient absorption for an Ac-m-BMBP system, and an efficiency of the C-Br bond cleavage in the lowest triplet state (T(1)(n,pi(*))) of m-BMBP was determined. In contrast, formation of triplet m-CMBP was seen for an Ac-m-CMBP system. Absence of C-Cl bond cleavage in the triplet state of m-CMBP indicated the reactive state of m-CMBP for omega-cleavage is only the S(1)(n,pi(*)) state. Based on the efficiencies and DFT calculations for excited state energies, photoinduced omega-bond dissociation of m- and p-HMBPs was characterized.

Characterization of the phenoxyl radical in model complexes for the Cu(B) site of cytochrome c oxidase: steady-state and transient absorption measurements, UV resonance raman spectroscopy, EPR spectroscopy, and DFT calculations for M-BIAIP.

Physicochemical properties of the covalently cross-linked tyrosine-histidine-Cu(B) (Tyr-His-Cu(B)) unit, which is a minimal model complex [M(II)-BIAIPBr]Br (M = Cu(II), Zn(II)) for the Cu(B) site of cytochrome c oxidase, were investigated with steady-state and transient absorption measurements, UV resonance Raman (UVRR) spectroscopy, X-band continuous-wave electron paramagnetic resonance (EPR) spectroscopy, and DFT calculations. The pH dependency of the absorption spectra reveals that the pK(a) of the phenolic hydroxyl is ca. 10 for the Cu(II) model complex (Cu(II)-BIAIP) in the ground state, which is similar to that of p-cresol (tyrosine), contrary to expectations. The bond between Cu(II) and nitrogen of cross-linked imidazole cleaves at pH 4.9. We have successfully obtained UVRR spectra of the phenoxyl radical form of BIAIPs and have assigned bands based on the previously reported isotope shifts of Im-Ph (2-(1-imidazoyl)-4-methylphenol) (Aki, M.; Ogura, T.; Naruta, Y.; Le, T. H.; Sato, T.; Kitagawa, T. J. Phys. Chem. A 2002, 106, 3436-3444) in combination with DFT calculations. The upshifts of the phenoxyl vibrational frequencies for 8a (C-C stretching), 7a' (C-O stretching), and 19a, and the Raman-intensity enhancements of 19b, 8b, and 14 modes indicate that UVRR spectra are highly sensitive to imidazole-phenol covalent linkage. Both transient absorption measurements and EPR spectra suggest that the Tyr-His-Cu(B) unit has only a minor effect on the electronic structure of the phenoxyl radical form, although our experimental results appear to indicate that the cross-linked Tyr radical exhibits no EPR. The role of the Tyr-His-Cu(B) unit in the enzyme is discussed in terms of the obtained spectroscopic parameters of the model complex.

Effects of localized triplet exciton on reactivity of photoinduced omega-bond dissociation in naphthyl phenyl ketones having pi,pi* lowest triplet (T1) states studied by laser flash photolysis.

Photochemical properties of photoinduced omega-bond dissociation in naphthyl phenyl ketones having a phenylthiyl moiety as a leaving group, p-(alpha-naphthoyl)benzyl phenyl sulfide (NBPS) and 4-benzoyl-1-naphthylmethyl phenyl sulfide (BNMPS), in solution were investigated by laser flash photolysis techniques. Both ketones were shown to undergo photoinduced omega-bond cleavage of the C-S bond to release the phenyl thiyl radical (PTR) at room temperature. Irrespective of excitation wavelengths of NBPS, a quantum yield (Phi(rad)) of the PTR formation was obtained to be 0.1, whereas that for BNMPS was found to depend on the excitation wavelength, i.e., absorption bands from the ground state (S0) to the excited singlet states, S3, S2, and S1 of BNMPS; Phi(rad)(S3) = 0.77 and Phi(rad)(S2) = Phi(rad)(S1) = 1.0. By using triplet sensitization of p-phenylbenzophenone (PBP), efficiencies (alpha(rad)) of the radical formation in the lowest triplet state (T1(pi,pi*)) of NBPS and BNMPS were determined to be 0 and 1.0, respectively. The agreement between Phi(rad)(S1) and alpha(rad) values for BNMPS indicates that the C-S bond dissociation occurs in the T1 state via the S1 state via a fast intersystem crossing from the S1 to the T1 state. The wavelength dependence of the radical yields upon direct excitation of BNMPS was interpreted in terms of the C-S bond cleavage in the S3 state competing with internal conversion from the S3 to the S2 state. The smaller value of Phi(rad)(S3) than those of Phi(rad)(S1) and Phi(rad)(S2) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S3 state. Photoinduced omega-cleavage of NBPS was concluded to take place only in the S1(n,pi*) state. Difference in reactivity of omega-cleavage between the triplet states of NBPS and BNMPS was interpreted in terms of localized triplet exciton in the naphthoyl moieties.

Azulene-substituted aromatic amines. synthesis and amphoteric redox behavior of N,N-Di(6-azulenyl)-p-toluidine and N,N,N',N'-tetra(6-azulenyl)-p-phenylenediamine and their derivatives.

[reaction: see text] N,N-Di(6-azulenyl)-p-toluidine (1a) and N,N,N',N'-tetra(6-azulenyl)-p-phenylenediamine (2a) and their derivatives with 1,3-bis(ethoxycarbonyl) substituents on each 6-azulenyl group (1b and 2b) were prepared by Pd-catalyzed amine azulenylation and characterized as a study into new aromatic amines for multistage amphoteric redox materials. The redox behavior of each compound was characterized by cyclic voltammetry. These compounds undergo facile reduction to stable anion radicals and dianion diradicals owing to the resonance stabilization between the 6-azulenyl groups and exhibit electrochemical oxidation depending on the amine subunits. The ESR measurement of anion radicals and a dianion diradical generated by the electrochemical reduction of amine 1b and diamine 2b revealed that the unpaired electron of these radicals delocalizes over the entire azulene ring including the central nitrogen atoms. UV-vis spectral analysis of amines 1a,b and diamines 2a,b, taken during the electrochemical reduction, exhibited a gradual decrease of the absorption bands of the neutral species along with an increase of the new absorption maxima at 625, 605, 640, and 610 nm, respectively, with the development of well-defined isosbestic points at 502, 562, 478, and 545 nm, respectively. As indicated by a combined ESR and UV-vis spectral study, the species giving rise to the new absorption maxima are concluded to be the generation of anion radicals and dianion diradicals of aromatic amines and diamines with high thermodynamic stability.

Divergent oxidative rearrangements in solution and in a zeolite: distal vs proximal bond cleavage of methylenecyclopropanes.

Irradiation of 9,10-dicyanoanthracene (DCA) or p-chloranil in the presence of E-1-benzylidene-2-phenylcyclopropane (E-5) in CH(2)Cl(2) causes E-5 to undergo methylenecyclopropane rearrangement. An adduct, Z-7, between DCA and 5 firmly supports the involvement of a bifunctional trimethylenemethane radical cation. In contrast, incorporation of E-5 into HZSM-5 produces trans,trans-1,4-diphenyl-1,3-butadiene radical cation sequestered in the HZSM-5 interior, tt-8(.+)@HZSM-5, identified by ESR and diffuse reflectance spectroscopy. In addition, low yields of tt-8, its cis,trans-isomer (ct-8), and 1-phenyl-1,2-dihydronaphthalene (9) were isolated from the supernatant solution. The sharp contrast between the photoinduced electron-transfer reaction with photosensitizers in solution and the spontaneous reaction with redox-active acidic zeolite offers the prospect of further zeolite-induced regiodivergent reactions in a range of additional substrates.

Primary charge-recombination in an artificial photosynthetic reaction center.

Photoinduced primary charge-separation and charge-recombination are characterized by a combination of time-resolved optical and EPR measurements of a fullerene-porphyrin-linked triad that undergoes fast, stepwise charge-separation processes. The electronic coupling for the energy-wasting charge recombination is evaluated from the singlet-triplet electronic energy gap in the short-lived, primary charge-separated state. The electronic coupling is found to be smaller by approximately 40% than that for the primary charge-separation. This inhibition of the electronic interaction for the charge-recombination to excited triplet state largely results from a symmetry-broken electronic structure modulated by configuration interaction between 3(b1u,b3g) and 3(au, b3g) electronic states of the free-base porphyrin.

Symmetry switching of the fluorescent excited state in alpha,omega-diphenylpolyynes.

By using MO calculations based on DFT, absorption, and fluorescence spectroscopy, we have comprehensively studied the low-lying excited singlet states of alpha,omega-diphenylpolyynes (DPY) having 1-6 triple bonds. The a(g) vibrational modes of the C(triple bond)C stretching and of the phenyl ring motion were observed in the fluorescence spectra of diphenylacetylene and 1,4-diphenylbutadiyne. On the other hand, in the fluorescence spectra of the long DPY with the triple-bond number (N) more than two, the phenyl ring motion with a(g) symmetry disappeared and the b(1g) modes of the phenyl ring twisting (approximately 400 cm(-1)) and of the C-H bending (approximately 900 cm(-1)) were detected. The observed fluorescent states of DPY with N < or = 2 and N > or = 3 are assigned to the 1(1)B(1u) (pi(x)pi(x*)) and 1(1)A(u) (pi(x)pi(y*) and/or pi(y)pi(x*)) states, respectively, based on the vibronic structures, the relatively short lifetimes, and the solvatochromic shifts of the fluorescence spectra. Not only the allowed transition of 1(1)B(1u) <-- S(0) but also the forbidden transition of 1(1)A(u) <-- S(0) was detected in the fluorescence excitation spectra of the long DPY with N > or = 3. The low-lying excited state with A(u) symmetry is characteristic in polyyne, which does not exist in polyene. The oscillator strength (f) of the first absorption band in DPY decreases with an increase in N, which is the opposite behavior of the all-trans-alpha,omega-diphenylpolyenes. The N-dependence of the f value is understood by the configuration interaction between the 1(1)B(1u) and 2(1)B(1u) (pi(y)pi(y*)) states, which is consistent with the reduction of the nonlinear optical response of polyyne.