Charge Resonance Band Research Articles

Observation of intramolecular dimer radical anion of 1,1-diarylmethanols bearing electron withdrawing groups at room temperature

Transient absorption measurement of radical anions of 1,1-diarylmethanols bearing electron-withdrawing groups in dimethyl-formamide at room temperature has revealed that symmetric compounds form intramolecular dimer radical anions as an intermediate of their reductive C–O bond dissociation giving diarylmethyl radicals. The dimer radical anions showed an absorption band in the near infrared region (NIR) (1000–1600 nm) characteristic to their charge resonance (CR) properties. On the contrary to 1,1-diarylmethanols, corresponding radical anion of 1,3-diarylpropan-1-ol showed no CR band. It is suggested that formation of dimer radical anions is energetically favorable for diaryl compounds with one-atom spacer but unfavorable for them with three-atom spacer being different from the n=3 rule for the favorable formation of intramolecular excimers.

Spectroscopic studies on molecular interactions of photogenerated tricyanovinylstyrylpyridinyl radicals in acetonitrile at room temperature

Photoinduced electron transfer reactions of newly synthesized 1-hexadecyl-4-(4-tricyanovinylstyryl)pyridinium tetraphenylborate salts in acetonitrile were studied at room temperature by steady-state photolysis and ESR spectroscopy. Upon irradiation, stable tricyanovinylstyrylpyridinyl radicals were formed by interionic electron transfer accompanied with color change from pale yellow to blue. The photogenerated radicals showed a strong tendency to interact with each other or tricyanovinylstyrylpyridinium cations. A fairly strong charge resonance (CR) band was observed in the near infrared region with a peak at 924nm, which was attributed to the electronic interaction between a tricyanovinylstyrylpyridinyl radical and a tricyanovinylstyrylpyridinium cation. Observed and simulated ESR spectra clearly confirmed the formation of both radical dimers and dimer radical cations in solution. An additional electron-withdrawing cyano group in tricyanovinylstyrylpyridinium tetraphenylborate salts greatly stabilized its dimer radical cation compared with that of dicyanovinylstyrylpyridinium tetraphenylborate salts.

Photoinduced electron transfer between C 60 and carbazole dimer compounds in a polar solvent

Photoinduced electron transfer processes between carbazole (Cz) dimer compounds and C 60 in benzonitrile have been investigated by observing the transient absorption spectra in the visible and near-IR regions. By excitation of C 60 with 532 nm-laser light, electron transfer occurred via the triplet excited state of C 60 ( T C 60 ∗ ) from the Cz-dimer compounds, which was confirmed by observing characteristic absorption bands due to C 60 − and the Cz-dimer radical cation. The Cz-dimer compounds, forming sandwich-like conformation of radical cation, showed the charge resonance (CR) bands in the near-IR region longer than 1200 nm. Rate constants for electron transfer seem to depend on the conformation of the Cz-dimer radical cation. The Cz-dimer compounds taking sandwich conformation donate the electron easily to T C 60 ∗ , because of the stabilization energies of the sandwich dimer radical cations of the Cz-dimer compounds. By excitation of the Cz-dimer compounds with 355 nm-laser light, the absorption bands due to the Cz-dimer radical cations appeared immediately after the laser irradiation and the absorption of C 60 − is formed at diffusion controlled rate. This observation suggests that direct photoejection from the Cz-dimer compounds occurs in benzonitrile, yielding the Cz radical cations and solvated electron, in which the latter was trapped with C 60.

Spectroscopic Studies of Radicals Formed upon Photoexcitation of Bis(nitrostyrylpyridinium)alkane Ditetraphenylborate Salts.

The charge resonance (CR) interactions of a series of bis(4-(4-nitrostyryl)pyridinium)alkane derivatives upon steady photoexcitation through a band pass filter (405nm) were studied by absorption spectroscopy at room temperature. The CR bands due to the intermolecular or intramolecular dimer radical cations were observed with peaks extending from 950nm to 1740nm. The CR band of intermolecular dimer radical caton was common to all these compounds. The relative constant of dimer radical cations depended on the alkyl chain length and the irradiation time. The CR band of intramolecular dimer radical cation was sensitive to their geometrical structure and showed the formation of different dimer radical cations.

Photodissociation spectroscopy of (benzene–toluene)+. Charge delocalization in the hetero-dimer ion

The electronic spectrum of the benzene–toluene hetero-dimer ion is measured in the 380–1400 nm region. The spectrum shows intense bands around 1175 and 670 nm and a weaker band around 920 nm, which correspond to charge resonance (CR) bands of homo-dimer ions. The observation indicates that the positive charge stays on the benzene part in some probability, although the ionization potential of benzene is 0.4162 eV higher than that of toluene. A local excitation (LE) band is observed around 420 nm, where a π←π transition is locally excited in the charged benzene or toluene molecule. On the basis of the positions of the CR-like bands, as well as the intensity of the LE band relative to that of homo-dimer ions, the probability of finding the charge on the benzene molecule is analyzed to be approximately 36%.

First intramolecular charge resonance band observed at room temperature in solution by steady photolysis of 1,3-bis(4-(4-nitrostyryl)pyridinium)propane tetraphenylborate

1,3-Bis(4-nitrostyrylpyridinium)propane tetraphenylborate salt showed a remarkable color change upon steady photoexcitation due to photoinduced electron transfer. It gave two stable charge resonance bands with peaks at 948 and 1742 nm in acetonitrile at room temperature. During storage in the dark, the former decayed monotonically, while the latter gradually changed its shape and showed a peak at 1505 nm after 3 h. The 948 and 1742–1505 nm bands were attributed to sandwich-type and partially overlapped configurations of intramolecular dimer radical cations, respectively.

Photodissociation spectrum of cyanobenzene dimer cation. Absence of intermolecular resonance interaction

Electronic spectra of a homo-molecular dimer cation, (C 6 H 5 CN) 2 + , are measured by photodissociation spectroscopy in the gas phase. Broad features appeared in the 450-650 nm region are characteristic of π 3 → π CN transitions of the C 6 H 5 CN + chromophore. No intense band is observed in the 650-1300 nm region, where other aromatic dimer cations usually show charge resonance bands. Two component molecules of (C 6 H 5 CN) 2 + cannot take a parallel sandwich configuration suitable for the resonance interaction, because of geometrical constraints due to other stronger interactions.

Intracluster Ion−Molecule Reactions of Phenylsilane Dimer Cation following the Charge Resonance Band Excitation

Intracluster Ion−Molecule Reactions of Phenylsilane Dimer Cation following the Charge Resonance Band Excitation

Formation of benzene dimer cations in neat liquid benzene studied by femtosecond transient absorption spectroscopy

Photoionization of neat liquid benzene has been studied by femtosecond pump-probe experiments in the near infrared region. A transient species is formed within 1 ps following two-photon excitation of liquid benzene at 268 nm, which shows a broad absorption band with a peak at 920 nm and extended intensities at wavelengths longer than 1000 nm. This band is attributed to a charge resonance band of a localized benzene dimer cation. The transient species show essentially no evidence of the recombination with electrons in a time shorter than 50 ps.

Electronic Structures and Photoevaporation Dynamics of Benzene Cluster Ions

The electronic spectra of benzene cluster ions, (C6H6)n+ with n = 3−6, are measured through mass-selected photodissociation spectroscopy. The spectra in the 400−1100-nm region show three distinct absorption maxima centered around 430, 590−620, and 950 nm, which are analogous to the spectrum of (C6H6)2+. The 950-nm band is assigned to a charge resonance (CR) band characteristic of the dimer ion, while the other two bands are attributed to local excitation bands. The position of the CR band is found to be almost independent of cluster size. The result suggests that the cluster ions have a charge-localized structure involving a strongly bound dimer ion core. In addition, the number and translational energy of neutral molecules ejected following photoexcitation are measured for (C6H6)n+ with n = 3−8 in the photon energy range of 0.5−3.0 eV. The average number of ejected molecules increases linearly with increasing photon energy, suggesting that the fragmentation proceeds via the sequential ejection of neutral...

Pump-probe photodepletion spectroscopy of (C 6H 6) 2+. Identification of spectrum in the charge resonance band region

Pump-probe photodepletion (hole-burning) experiments are performed on mass-selected cluster ions in the gas phase. An electronic spectrum of (C 6H 6) 2 + in the near-infrared region comprises two charge resonance bands at 920 and 1160 nm. The intensities of these two bands monitored by the probe laser are shown to be reduced by the same amount with the introduction of the pump laser. The behavior proves that the CR bands are due to two distinct transitions of a single isomer. No evidence is found for the existence of other isomers of (C 6H 6) 2 +.

Direct observation of the carbazole hole trap in polymer solid films by the charge-resonance band

The carbazole (Cz) hole trap was directly observed at 77 K in the copolymer of N-vinylcarbazole (VCz) with methyl methacrylate (the mole fraction of VCz < 50%) by the measurement of the charge-resonance band. It is found that the Cz hole trap in solid films is mainly stabilized by charge resonance between two Cz chromophores and that the fraction of more stable Cz hole traps increases with Cz concentration.

Observation of charge-resonance band of naphthalene dimer cation confined in cavities of faujasite zeolites. Near-IR diffuse reflectance laser photolysis study

The naphthalene dimer cation was formed on 266 nm nanosecond laser excitation of naphthalene adsorbed in faujasite zeolites at high loading levels and the charge-resonance band of the dimer cation was measured for the first time by use of diffuse reflectance laser photolysis with a near-infrared detection technique. The dimer cations were found to form exclusively inside the cage networks, not at the external surface of the zeolite and are expected to take a twisted form, conformation in which two naphthalene moieties are partially overlapped, enforced by the cage walls.

Photodissociation spectroscopy of phenol dimer ion. Lack of resonance interaction between the aromatic rings

Electronic spectra of a homo-molecular but inequivalent dimer ion, (C 6H 5OH) 2 +, are measured by photodissociation spectroscopy. A broad band appeared in the 340–440 nm region is characteristic of the C ← X transition of a phenol ion chromophore, suggesting that the dimer ion consists of a phenol ion and a neutral phenol. No strong band is observed in the 450–1400 nm region, whereas other aromatic dimer ions usually show charge resonance bands in near-infrared region. Owing to the geometrical constraint of an OH · O hydrogen bond in (C 6H 5OH) 2 +, the two aromatic rings cannot be in a parallel configuration suitable for the resonance interaction.

Specific Interaction of 4-Nitrostyrylpyridinium Ions Upon Photoexcitation in Solutions

Abstract Ion-pair charge-transfer complexes of 4-nitrostyrylpyridinium with tetraphenylborate as its counter ion gave fairly stable styrylpyridinyl radicals in 1,2-dimethoxyethane at room temperature due to photoinduced electron transfer reactions between ion pairs. In addition to the remarkable colour changes due to a characteristic absorption of styrylpyridinyl radicals in the visible region, a charge resonance (CR) band was also observed in the near infrared region. The CR band indicated specific interaction of photogenerated neutral radicals with their parent styrylpyridinium cations. Prolonged irradiation and/or storage in the dark after irradiation caused further colour changes depending on the initial concentrations and irradiation times. These results indicated that nitrostyrylpyridinyl radicals had stronger tendency of association in the dark.

Photoinduced electron transfer and charge resonance band in ion-pair charge-transfer complexes of styrylpyridinium tetraphenylborate

Photoinduced electron transfer reactions were observed at room temperature in organic solvents by steady and flash photolysis of ion-pair charge-transfer complexes between styrylpyridinium derivatives having electron-withdrawing groups at para-position of a styryl group and tetraphenylborates as their counter ions. In addition to the remarkable colour changes due to a characteristic absorption of styrylpyridinyl radicals in the visible region, a charge resonance absorption band (CR band) was observed in the near infrared region depending on the substituent. The latter was due to the electronic interaction of photogenerated neutral radicals with their parent styrylpyridinium cations. The rise of the CR band upon ns laser excitation was as fast as that of the radical itself.

Hole Resonance among More Than Two Carbazole Chromophores in Poly(N-vinylcarbazole)

Hole resonance among more than two chromophores for the radical cationic carbazole (CZ) group of poly(N-vinylcarbazole) (PVCZ) in solution was shown by the radical ion transfer method and charge resonance (CR) band measurements. The sample was PVCZ with its oligomers (degree of polymerization (DP) = 2-100). The hole transfer rate from the radical cationic CZ of the oligomers to a 2,5-dimethoxyaniline, which has a lower ionization potential than the CZ chromophore, decreased with an increase in the oligomer molecular weight due to the hole resonance among neighboring chromophores. Thus, the stabilization energy of N-vinylcarbazole oligomers increases with increase in DP. The peak position of the CR band can be a good measure of the degree of charge resonance. The peak wavelength of the CR band was red-shifted for the larger molecular weight oligomers. This shift also suggested the existence of charge resonance in PVCZ among more than two CZ chromophores.

Photodissociation spectroscopy of (C 6H 6) 3+: Charge localization on a dimer unit of (C 6H 6) 3+

Photodissociation spectra of (C 6H 6 3 + are measured in the 700–1400 nm range. The charge resonance (CR) bands characteristic of (C 6H 6) 2 + are found to be kept almost intact in (C 6H 6) 3 + ; the spectra show the main band at ≈ 960 nm and relatively weak one at ≈ 1220 nm. The photodissociation cross section of (C 6H 6) 3 + at the 960 nm band is estimated to be comparable with that of the corresponding dimer band at 920 nm. These results suggest that the charge is localized on a dimer subunit, which acts as a chromophoric core for the CR transition in (C 6H 6) 3 +.

Photodissociation Spectroscopy of

The photodissociation spectrum of is obtained from the yields of fragment ion as a function of photodissociation wavelength in the 400–1400 nm region. Two bands at 440 and 580 nm are attributed to the C ← X and the B ← X local excitation (LE) bands, respectively. Both the most intense band at 920 nm and relatively weak one at 1160 nm are assigned to charge resonance (CR) bands. The red‐shift of the B ← X band from that of and the cross sections at the CR bands much larger than those at the LE bands are consistent with a sandwich structure for . The appearance of the two CR bands is explained on the basis of displaced sandwich structures for .

First charge resonance band observed by steady photolysis at room temperature in solution

The steady excitation of an ion-pair charge-transfer band of 1-octadecyl-4-(4-nitrostyryl)pyridium tetraphenylborate formed a fairly stable 1-octadecyl-4-(4-nitrostyryl)pyridinyl radical accompanying, for the first time, a charge resonance band owing to its interaction with a parent cation at room temperature in organic solutions.