Charge Resonance Band Research Articles

2,5-Dianilinoterephthalate bridged MM quadruply bonded complexes of molybdenum and tungsten

From the reactions between 2,5-dianilinoterephthalic acid and M2(O2CBut)4 in toluene the dicarboxylate bridged complexes [(ButCO2)3M2]2{micro-1,4-(CO2)(2)-2,5-(NHPh)2C6H2}, (M=Mo) and (M=W) have been isolated. The compounds are air sensitive, sparingly soluble in aromatic hydrocarbons but appreciably soluble in tetrahydrofuran. Electronic structure calculations employing density functional theory on the model compounds [(HCO2)3M2]2{micro-1,4-(CO2)(2)-2,5-(NHPh)2C6H2}, indicate that the ground state structure contains a planar bridge and that for molybdenum the HOMO is a bridge based molecular orbital. However, the compounds show reversible oxidation waves (CV and DPV) that for both M=Mo and W are metal based oxidations. Furthermore, the cations + and + are shown to be valence trapped and fully delocalized respectively. The magnitude of the electronic coupling of the two M2 centers, Hab, can be estimated as 383 cm-1 for + and 1500 cm-1 for + based on the corresponding low energy IVCT or charge resonance bands.

Photodissociation of Naphthalene Dimer Radical Cation during the Two-Color Two-Laser Flash Photolysis and Pulse Radiolysis−Laser Flash Photolysis

Photodissociation of naphthalene (Np) dimer radical cation (Np2*+) to give naphthalene radical cation (Np*+) and Np and the subsequent regeneration of Np2*+ by the dimerization of Np*+ and Np were directly observed during the two-color two-laser flash photolysis in solution at room temperature. When Np2*+ was excited at the charge-resonance (CR) band with the 1064-nm laser, the bleaching and recovery of the transient absorption at 570 and 1000 nm, assigned to the local excitation (LE) and CR bands of Np2*+, respectively, were observed together with the growth and decay of the transient absorption at 685 nm, assigned to Np*+. The dissociation of Np2*+ proceeds via a one-photon process within the 5-ns laser flash to give Np*+ and Np in the quantum yield of 3.2 x 10(-3) and in the chemical yield of 100%. The recovery time profiles of Np2*+ at 570 and 1000 nm were equivalent to the decay time profile of Np*+ at 685 nm, suggesting that the dimerization of Np*+ and Np occurs to regenerate Np2*+ in 100% yield. Similar experimental results of the photodissociation and regeneration of Np2*+ were observed during the pulse radiolysis-laser flash photolysis of Np in 1,2-dichloroethane. The photodissociation mechanism can be explained based on the crossing between two potential surfaces of the excited-state Np2*+ and ground-state Np*+.

Formation of Trimer and Dimer Radical Cations of Methyl-Substituted Benzenes in γ-Irradiated Low-Temperature Matrices

Dimer and trimer radical cations of benzene, toluene, and xylenes were produced selectively after gamma-irradiation in low-temperature 2-methylpentane matrices with electron scavengers: oxygen (O(2)) and sec-butyl chloride (sec-BuCl). The charge resonance (CR) band of the trimer radical cation (M(3)(+)) produced via the corresponding dimer radical cation (M(2)(+)) is clearly seen in the solution containing O(2) as the temperature increases over a range from 80 to 90 K. In o-xylene solution, a fairly strong and distinct M(3)(+) CR absorption is observed; this is due to the large M(3)(+)/M(2)(+) relative extinction coefficient. All benzene derivatives show an equilibrium between dimer and trimer radical cations at approximately 90 K; however, the equilibrium constants of toluene and the xylenes are considerably lower than that of benzene. Formation of the trimer radical cation is inhibited in sec-BuCl, which has commonly been used as a low-temperature optical matrix for producing cationic species. An ab initio DFT method is applied to predict the geometry of M(3)(+), giving "slipped sandwich" (for benzene, m-xylene, and p-xylene) and "slipped fan-shaped" (toluene and o-xylene) structures as the most plausible geometries. The experimentally observed spectroscopic parameters reflect well those predicted by TD-DFT calculation based on geometry, suggesting strong dependence of the geometry of M(3)(+) on substitution patterns. This is the first report not only of direct spectroscopic observation of aromatic trimer radical cations in the condensed phase but also on the quantitative analysis of their equilibria.

The Dewar Benzene Radical Cation and Its Ring-Opening Reaction

The radical cation of Dewar benzene, 1*+, has been generated and observed by optical spectroscopy in cryogenic matrices. 1*+ distinguishes itself by a charge resonance band at 600 nm, very similar in shape and position to that observed for the related radical cation of norbornadiene. This coincidence indicates that in ground-state 1*+ the odd electron is also located in a pi-MO. The energy of the charge resonance transition, which is very sensitive to the dihedral angle between the four-membered rings in 1*+, is predicted consistently too low by TD-DFT and CASPT2. Probably this angle is too large in the B3LYP and CASSCF geometries. As 1*+ can be observed at 77 K, it must be separated by a barrier of at least 7-8 kcal/mol from its very exothermic decay to the radical cation of benzene, 2*+. An analysis shows that the ring-opening of 1*+ is a multistep process involving two avoided crossings between potential surfaces of different symmetry and electronic nature. Owing to the orbital symmetry-forbidden nature of the process, the energy of 1*+ starts by increasing steeply on stretching the central C-C bond, but then the system undergoes a crossing to a 2A1 surface which leads adiabatically to an excited state of 2*+. Therefore, another avoided crossing must be transited before the molecule can decay on the ground-state surface of 2*+. The rearrangement of 1*+ to 2*+ is an example of a "pseudodiabatic" thermal reaction that transits between potential surfaces representing very different electronic structures.

Transannular Distance Dependence of Stabilization Energy of the Intramolecular Dimer Radical Cation of Cyclophanes

The intramolecular dimer radical cation and charge-transfer complex of various cyclophanes were investigated by using pulse radiolysis measurements. The charge resonance band due to the dimer radical cation of cyclophanes appeared in the near-IR region, which showed a blue-shift as the distance between the two benzene rings of cyclophane decreased. The stabilization energy of the dimer radical cation, which was estimated from the peak position of the charge resonance band, was explained by the exchange interaction, while the substituent effect was small. The absorption peak of the charge-transfer complex with chlorine atom also showed the shift in accordance with the oxidation potential of cyclophanes.

Formation of Highly Stabilized Intramolecular Dimer Radical Cation and π-Complex of [3n]Cyclophanes (n = 3, 5, 6) during Pulse Radiolysis

Formation of radical cation and charge-transfer complex of [3n]cyclophanes (n = 3, 5, 6) was investigated by transient absorption spectroscopy during pulse radiolysis. Radical cations of [3n]cyclophanes showed the charge resonance band around 700 nm which exhibited a blue-shift as the number of trimethylene bridges increased, indicating formation of highly stabilized intramolecular dimer radical cation of [3n]cyclophanes. The absorption peak of the charge-transfer complex with chlorine atom also showed the shift in accord with the oxidation potential of [3n]cyclophanes.

Formation of benzene trimer radical cation in γ-irradiated low temperature 2-methylpentane matrices

Benzenes in γ-irradiated low temperature 2-methylpentane matrices with O 2 show clear optical absorption attributed to charge resonance bands (CR band) of benzene dimer radical cations ( Bz 2 + ) . New absorption bands attributed trimer radical cations ( Bz 3 + ) are also observed with an isobestic point by an increase in the temperature from 82 to 85 K. The derived transition energy of the CR bands by DFT calculation in both Bz 2 + and Bz 3 + (1.43 and 0.89 eV) is in agreement with the values obtained experimentally (1.41 and 0.90 eV), suggesting that delocalized charge distribution in Bz 3 + observed in the present study.

Formation of Pyrene Dimer Radical Cation at the Internal Site of Oligodeoxynucleotides

Doubly internally pyrene (Py) modified oligodeoxynucleotides (ODNs) were synthesized, and the formation rates of the Py dimer radical cation (Py2•+) were measured upon one-electron oxidation during pulse radiolysis. The formation of Py2•+ with an optical absorption at 1550 nm (charge resonance band) was observed in the time range of 1 μs to 1 ms after an electron pulse during the pulse radiolysis of a D2O solution of the doubly internally Py modified ODN in the presence of K2S2O8. The formation rate of Py2•+ in DNA reflected the dynamics of the DNA, which allows the interaction between Py•+ and Py, and was affected by the distance between the two Py's and the local environment of each Py. The trapping of the transiently formed DNA conformation by the attractive charge resonance interaction was demonstrated to be useful to obtain structural and dynamic information of the fluctuating DNA in the time range of 1 μs to 1 ms.

How delocalized is N,N,N',N'-tetraphenylphenylenediamine radical cation? An experimental and theoretical study on the electronic and molecular structure.

The electronic and molecular structure of N,N,N',N'-tetraphenylphenylenediamine radical cation 1(+) is in focus of this study. Resonance Raman experiments showed that at least eight vibrational modes are strongly coupled to the optical charge resonance band which is seen in the NIR. With the help of a DFT-based vibrational analysis, these eight modes were assigned to symmetric vibrations. The contribution of these symmetric modes to the total vibrational reorganization energy is dominant. These findings are in agreement with the conclusions from a simple two-state two-mode Marcus-Hush analysis which yields a tiny electron-transfer barrier. The excellent agreement of the X-ray crystal structure analysis and the DFT computed molecular structure of 1(+) on one hand as well as the solvent and solid-state IR spectra and the DFT-calculated IR active vibrations on the other hand prove 1(+) adopts a symmetrical delocalized Robin-Day class III structure both in the solid state and in solution.

Formation and decay of pyrene radical cation and pyrene dimer radical cation in the absence and presence of cyclodextrins during resonant two-photon ionization of pyrene and sodium 1-pyrene sulfonate

Formation and decay of pyrene radical cation and pyrene dimer radical cation during resonant two-photon ionizations (TPI) of pyrene (Py) and sodium 1-pyrene sulfonate (NaPySA) in the absence and presence of cyclodextrins were studied with visible and near IR transient absorption measurement using 355-nm laser flash photolysis technique. Ionization of Py and 1-pyrene sulfonate anion (PySA−) occurred within the laser flash (5 ns) to give Py radical cation (Py˙+) and PySA− radical cation (Py˙+SA−) having Py˙+ chromophore. Dimerizations of Py˙+ and Py and of Py˙+SA− and PySA− were observed to give the dimer radical cations (Py2˙+ and Py2˙+(SA−)2) based on time-resolved measurements of the charge resonance (CR) band. The TPI of PySA− was also examined in the presence of β- and γ-cyclodextrins (βCD and γCD, respectively). Py2˙+(SA−)2 in the cavity of two γCDs having fully overlapped structure showed the CR band around 1700 nm, while Py2˙+(SA−)2 in the cavity of two βCD having partially and fully overlapped structures indicated the CR bands around 1500 or 1700 nm, respectively. On the basis of formation and decay rates of Py2˙+(SA−)2, bimolecular formation of Py2˙+(SA−)2 and the neutralization are inhibited in the cavities of CDs. Selective formation of the fully overlapped structure of Py2˙+(SA−)2 in the cavity of two γCDs within a laser flash is explained by direct TPI of two PySA−s with the parallel structure of two Py chromophores in two PySA−s in the cavity of two γCDs.

Intervalence (charge-resonance) transitions in organic mixed-valence systems. Through-space versus through-bond electron transfer between bridged aromatic (redox) centers.

Intervalence absorption bands appearing in the diagnostic near-IR region are consistently observed in the electronic spectra of mixed-valence systems containing a pair of aromatic redox centers (Ar(*)(+)/Ar) that are connected by two basically different types of molecular bridges. The through-space pathway for intramolecular electron transfer is dictated by an o-xylylene bridge in the mixed-valence cation radical 3(*)(+) with Ar = 2,5-dimethoxy-p-tolyl (T), in which conformational mobility allows the proximal syn disposition of planar T(*)(+)/T redox centers. Four independent experimental probes indicate the large through-space electronic interaction between such cofacial Ar(*)(+)/Ar redox centers from the measurements of (a) sizable potential splitting in the cyclic voltammogram, (b) quinonoidal distortion of T(*)(+)/T centers by X-ray crystallography, (c) "doubling" of the ESR hyperfine splittings, and (d) a pronounced intervalence charge-resonance band. The through (br)-bond pathway for intramolecular electron transfer is enforced in the mixed-valence cation radical 2a(*)(+) by the p-phenylene bridge which provides the structurally inflexible and linear connection between Ar(*)(+)/Ar redox centers. The direct comparison of intramolecular rates of electron transfer (k(ET)) between identical T(*)(+)/T centers in 3(*)(+) and 2a(*)(+)( )()indicates that through-space and through-bond mechanisms are equally effective, despite widely different separations between their redox centers. The same picture obtains for 3(*)(+) and 2a(*)(+)( )()from theoretical computations of the first-order rate constants for intramolecular electron transfer from Marcus-Hush theory using the electronic coupling elements evaluated from the diagnostic intervalence (charge-transfer) transitions. Such a strong coherence between theory and experiment also applies to the mixed-valence cation radical 7(*)(+), in which the aromatic redox S center is sterically encumbered by annulation.

Charge delocalization of naphthalene cations formed in triad molecules studied by near-IR transient absorption spectroscopy

Charge delocalization of radical cations formed in naphthalene monomer, dyad, and triad molecules in solutions was studied by near-IR transient absorption spectroscopy. The triad molecules were 1,5-di(2-naphthyl)-3-phenylpentane (NBN), 1,3-di(2-naphthyl)-5-phenylpentane (NNB), and 1,3,5-tri(2-naphthyl)pentane (NNN). Charge resonance bands were observed around 900 nm for NBN, 1250 nm for NNB, and 1300 nm for NNN. The absorption bands for the triad molecules were ascribed to hetero-dimer cation of naphthalene and benzene, dimer radical cation, and trimer radical cation, respectively.

Formation of Pyrene Dimer Radical Cation in DNA Reflecting DNA Dynamics in the Time Range of 1 μs to 1 ms

Doubly pyrene (Py)-conjugated oligodeoxynucleotides (ODNs) were synthesized and used for measurement of the formation rates of Py dimer radical cation (Py(2)(.+)) upon one-electron oxidation during the pulse radiolyses. Formation of Py radical cation (Py(.+)) in the time scale of less than 5 micros was monitored at 470 nm after an electron pulse during pulse radiolysis of D(2)O solution of doubly Py-conjugated ODN in the presence of K(2)S(2)O(8). Concomitant with the decay of Py(.+), formation of Py(2)(.+) with an absorption peak at 1500 nm (charge resonance band) was observed in the time range of approximately 100 micros. The formation rate of Py(2)(.+) in DNA reflected the dynamics of DNA which allows the interaction between Py(.+) and Py, since transiently formed DNA structure is trapped by the attractive charge resonance (CR) interaction to give Py(2)(.+). The formation rate of Py(2)(.+) with a characteristic CR absorption band in the near-infrared (near-IR) region was demonstrated to be useful to obtain the structural and dynamical information of transiently formed DNA in the time range of 1 micros to 1 ms.

Positive charge distribution in (benzene)1(toleune)2+ and (benzene)2(toluene)1+ studied by photodissociation spectroscopy

The positive charge distribution in benzene–toluene heterotrimer ions is investigated by photodissociation spectroscopy in the near-infrared (6000–14 000 cm−1) and infrared (2800–3150 cm−1) regions. The electronic spectra of (benzene)1(toluene)2+ and (benzene)2(toluene)1+ in the near-infrared region display a strong band at 9430 and 8330 cm−1, respectively. These bands are ascribed to the charge resonance band; the positive charge is not localized on a single molecule. The vibrational spectrum of (benzene)1(toluene-d8)2+ shows three distinct bands at 3054, 3084, and 3108 cm−1; these bands are assigned to the CH stretching vibrations of the benzene moiety. The similarity of the spectral features to those of the neutral benzene monomer suggests that the benzene molecule in the (benzene)1(toluene)2+ ion has a neutral character. The positive charge is localized on the toluene dimer unit with a structure written as (toluene)2+⋯(benzene)1. The vibrational spectrum of (benzene)2(toluene)1+ bears a resemblance to that of (benzene)2+. The vibrational spectrum of (benzene-d6)2(toluene)1+ shows dissimilar features to the spectrum of the neutral toluene monomer, suggesting that a certain amount of the positive charge is carried by the toluene moiety. These results are explained by the charge resonance interaction between (benzene)2 and (toluene)1. A simple perturbation theory is applied for determining the positive charge distribution in (benzene)2(toluene)1+. The probability of finding the charge on the (benzene)2 and (toluene)1 moieties is analyzed to be 58% and 42%, respectively.

First Observation of Monomer and Dimer Radical Cation upon Photoreduction of Cyanopyridinium Derivatives in Solution by Electron Spin Resonance and Absorption Spectroscopy

The monomer radical and the dimer radical cation of cyanopyridinium derivatives have been studied by absorption and electron spin resonance (ESR) spectroscopies at room temperature in acetonitrile upon steady photolysis. The stable cyanopyridinyl radical of 1,3-bis(4-cyanopyridiniumyl)propane (acceptor) formed by one-electron transfer from tetraphenylborate anion (donor) interacted intramolecularly with a parent cation, forming a dimer radical cation. A charge resonance band was observed at 1360 nm and assigned to the formation of intramolecular dimer radical cations. The ESR spectra revealed that one unpaired electron in the intramolecular dimer radical cation was completely shared between two chromophores of photoreduced 1,3-bis(4-cyanopyridiniumyl)propane (1). On the other hand, the intermolecular dimer radical cation of 1-(4-cyanopyrdiniumyl)hexadecane (2) as a monochromophoric compound was not formed because no charge resonance band was observed in the near-infrared region. Clearly resolved ESR spectra of both the dimer radical cation and the monomer radical were observed, for the first time, upon photoreduction of these cyanopyridinium derivatives. The effect of the chromophore size on the charge resonance band in the near-infrared region was also demonstrated by homo- and hetero-type bichromophoric compounds containing 4-cyano- and 4-nitrostyryl-pyridinium.

Ionic Species in Pulse Radiolysis of Supercritical Carbon Dioxide. 2. Ab Initio Studies on the Structure and Optical Properties of (CO2)n+, (CO2)2-, and CO3- Ions

Gas-phase (CO2)+2-4 cations and (CO2)2-, CO3-, CO3-·H2O and CO3-·CO2 anions have been examined using density functional theory calculations. It is shown that the lowest energy (CO2)n+ cations are C2h symmetric “ladder” structures with complete delocalization of charge and spin between the monomer units. These cations absorb in the visible due to a charge resonance band that involves their lowest 2Bu and 2Ag states. The “ladder” structure accounts for several trends observed in the photodestruction spectra of the (CO2)n+ cations. For the (CO2)2+ dimer cation, the energy and the oscillator strength of the charge resonance band compare favorably with these parameters for the solvent radical cation in supercritical (sc) CO2. A close similarity between the VIS spectra of the (N2O)2+ dimer cation (in sc CO2) and the solvent cation suggests that the latter has the (CO2)2+ dimer cation as the chromophore core. It is demonstrated that optical, Raman, and magnetic resonance spectra of the carbonate radical anion, CO3-, can be consistently accounted for by a C2v symmetric structure with the unique O−C−O angle of ≈113° (in water) to ≈100° (in carbonate minerals). The oscillator strength of the A 2A1 ← X 2B2 transition in the visible correlates with this O−C−O angle. In aqueous CO3-, the trigonal distortion is due to hydrogen bonding to a single water molecule. In sc CO2 (and other nonpolar liquids), the trigonal distortion is weak, and CO3- should be a poor light absorber. This may be the reason no dissociative electron capture in sc CO2 has been observed by optical spectroscopy.

Dynamical Formation of Intramolecular Carbazole Dimer Cations in Poly(methyl methacrylate) Solids below the Glass Transition Temperature

Structures and dynamical formation of carbazole (Cz) dimer cations in solid states were investigated by absorption spectroscopy of dicarbazolyl compounds doped in PMMA solids. Charge resonance (CR) bands originating from Cz dimer cations were observed even in PMMA solids at room temperature, which is below the glass transition temperature. Peak wavelengths of the CR bands observed were similar to those previously reported for Cz dimer cations formed in N,N-dimethylformamide (DMF) solutions at room temperature. This result shows that Cz dimer cations formed in PMMA solids have structures similar to those formed in DMF solutions. These Cz dimer cations were still observed even at 150 K, although the ratios of dimer cations to monomer cations were lower than those at room temperature. The ratios were kept almost constant below 170 K and steeply increased in the temperature range 200−290 K. This observed temperature corresponds to the transition temperature of the β-relaxation in PMMA solids. We conclude that...

Intramolecular Charge Resonance Interaction in meso−2,4-Bis(4-(4-nitrostyryl)pyridinium)pentane ditetraphenylborates upon Steady Photolysis in Solution

A new ωω'- type bichromophoric model compound, meso-2,4-bis (4-(4-nitrostyryl)pyridinium)pentane (meso-B4SPC52+) ditetraphenylborates, has been prepared. A conformational analysis of meso-B4SPC52+ was performed by using 1H NMR spectroscopy. Its charge resonance bands were also studied by absorption spectroscopy upon steady photolysis at room temperature.

Dynamic infrared band–band spectroscopy of peripheral light-harvesting complexes from R. acidophila

Infrared (IR) kinetic spectroscopy measurements have been employed to investigate and compare the electronic structure of B800 and B850 bacteriochlorophyll (BChl) pigment aggregates within peripheral (LH2) antenna complexes of photosynthetic purple bacteria. Following initial excitation of the B800 chromophores at 800 nm, a strong absorption band centred at ∼1.3 μm was observed in the transient spectrum which is characteristic of monomeric BChls. Evolution of a transient spectrum in the near-IR and IR regions was then tracked while the excitation energy migrated to the B850 chromophores on a ∼0.9 ps time scale. The B850 excited-state IR spectrum was found to differ substantially from that of B800, exhibiting a broader absorption band in the near-IR (centred at 1.1 μm) and new transient absorption features at 2.9 and 3.7 μm which are absent in B800. These results support a localised description of excitations for B800 but indicate the presence of considerably stronger inter-chromophore interactions between the B850 pigments. Differences between the excited-state spectra of B800 and B850 are interpreted as reflecting the formation of delocalized exciton and charge-resonance bands in B850.

Studies of Charge Resonance Bands Formed in .ALPHA.- and .GAMMA.-Styrylpyridinium Tetraphenylborate Derivatives.

The charge resonance (CR) interactions in α-substituted-(4-nitrostyryl)pyridinium derivatives have been investigated. The CR bands of dimer radical cation in α-substituted bichromophoric compounds were observed with a peak around 1010nm upon irradiation at 405nm in acetonitrile (ACN) at room temperature. These bands were weak because of their restricted geometrical conformation as compared with γ-substituted one. The CR band of dimer radical cation in α-substituted monochromophoric compound was observed at 993nm in 1, 2-dimethoxyethane. No CR band for this compound in ACN as a polar solvent strongly suggested that radical is difficult to be generated because of increased distance between the tetraphenylborate anion as a donor to the styrylpyridinium chromophore as an acceptor.