Vibronic structure of the circular dichroism and absorption spectra of bianthryls

It will be shown that factors of virtually no influence on the vibronic structure of the absorption spectra in biaryls may profoundly change the correspoding vibronic structure in the CD spectra. This is illustrated by the theoretical analysis of the experimental vibronic absorption and CD spectra of some bianthryl derivatives.     

Theoretical studies on the electronic structures and absorption spectra of substituted benzenes with one- and two-dimensional charge transfer character

The electronic structures and absorption spectra of one- and two-dimensional charge transfer (CT) molecules based on para-nitroaniline (pNA) and 1,3-diamino-4,6-dinitro- benzene (DADB) have been studied theoretically via semi-empirical and ab initio methods. It is found that the behaviors of optical absorption are strongly influenced by the dimension of CT. Different from the well-known one-dimensional CT molecule of pNA, which shows one intense absorption related to the π → π^* CT transition, two-dimensional CT molecule of DADB exhibits more absorption peaks associated with various low-lying CT transitions in near ultraviolet range. In addition, the relative orientations of transition dipole moment and ground state dipole moment in one- and two-dimensional charge transfer molecules were also discussed.     

Femtosecond broadband time-resolved fluorescence and transient absorption study of the intramolecular charge transfer state of methyl 4-dimethylaminobenzoate

A combined application of femtosecond broadband time-resolved fluorescence (fs-TRF), fluorescence anisotropy (fs-TRFA) and fs to microsecond (μs) transient absorption (TA) have been used to probe directly the dynamics, nature, formation and decay paths of the singlet intramolecular charge transfer ((1)ICT) state of methyl 4-dimethylaminobenzoate (1a) in acetonitrile. The result reveals explicit evidence for a common electronic origin (the L(a) nature) of the (1)ICT state and its precursor the locally excited ((1)LE) state to account jointly for the dual florescence known to this system. It also shows that the ICT reaction from the (1)LE to (1)ICT state occurs with time constant of ~0.8 ps and the (1)ICT state formed decays with a ~1.9 ns time constant leading mainly to a ππ* natured triplet state ((3)T(1)). The (3)T(1) then relaxes with a ~4 μs lifetime under deoxygenated condition resulting in full recovery of the ground state (S(0)). As a case study, this work contributes novel experimental data for improved understanding of the mechanism of ICT reaction; it also reveals a distinct deactivation pattern for this prototype para-amino substituted aromatic carbonyl compound in acetonitrile.     

Assignment of charge transfer absorption band in optical absorption spectra of the adsorbate-silver colloid system

In this paper we reported the UV-visible-NIR optical absorption properties of silver colloid, employed as a high efficient substrate in surface-enhanced Raman spectra (SERS), under various conditions. Experimental results revealed that the new absorption band, usually appearing in the longer wavelength region due to the addition of molecules, was related to the direct adsorption of molecules on colloidal silver surface. When the adsorption occurred, this new band would appear. Once the molecules were desorbed from silver surface, the new band could not be observed. Some evidences inferred that the new absorption band was associated with the effect of charge-transfer transition between adsorbates and colloidal silver particles, while not with the effect of the surface plasma resonance due to the silver particles aggregation which was usually attributed to in previous research work.     

Relating absorption, emission, and resonance Raman spectra with electron transfer rates in photoinduced charge transfer systems: promises and pitfalls

In molecules or complexes in which a photoinduced electronic transition corresponds to a formal electron transfer process, there exists a close relationship between the theoretical expressions for the absorption, fluorescence, and resonance Raman spectra and the nonphotochemical return electron transfer rate in the coupling limit. In this paper, the theoretical expressions cast in both the sum-over-vibrational-eigenstates picture and the equivalent time-domain picture are reviewed. Several recent efforts by ourselves and other groups to make use of such relationships in the analysis of experimental spectroscopic and electron transfer rate data are then described and critiqued. Finally, prospects for the further development of this approach are discussed, as are some of its limitations and potential pitfalls.     

Intramolecular charge transfer and electronic absorption and luminescence spectra of fluoroquinolinones

TD-DFT study on the effect of donor and acceptor substituents on molecular orbital localization and charge distribution in fluoroquinolinone molecules showed that their photoexcitation is accompanied by electron density redistribution over particular fragments. Depending on the protolytic form, frontier molecular orbitals are localized on different fragments, whereas variation of substituents weakly affects localization of these orbitals.     

Charge transfer induced enhancement of near-IR two-photon absorption of 5,15-bis(azulenylethynyl) zinc(II) porphyrins

Intramolecular charge transfer in 5,15-bis(azulenylethynyl) substituted zinc(ii) porphyrin leads to a significant enhancement of two-photon absorption at near-IR region, which has been investigated by femtosecond Z-scan method.     

Excited state two photon absorption of a charge transfer radical dimer in the near infrared

Nonlinear transmission measurements of a solution of radical dimers of tetramethyl-tetrathiafulvalene, (TMTTF+)2, recorded with 9 ns laser pulses at 1064 nm are reported and interpreted on the basis of a multiphoton absorption process. One finds that the process can be interpreted with a sequence of three photon absorption, the first being a one photon absorption related to the intermolecular charge transfer process characteristic of the dimers and the second a two photon absorption from the excited state created with the first process. A model calculation allows one to obtain the value of the two photon absorption cross section which is found to be several orders of magnitude larger than those usually found for two photon absorbing systems excited from the ground state. These results show the importance of an excited-state population for obtaining large nonlinear optical responses.     

Signature OH absorption spectrum from cluster models of solvation: a solvent-to-solute charge transfer state

Ab initio electronic structure theory calculations on cluster models support the characterization of the signature absorption spectrum of a solvated hydroxyl OH radical as a solvent-to-solute charge transfer state modulated by the hydrogen-bonding environment. Vertical excited states in OH(H2O)n clusters (n = 0-7, 16) calculated at the TDDFT level of theory (with companion calculations at the EOM-CCSD level of theory for n 相似文献   

One-phonon spectra in absorption and fluorescence spectra of molecular impurities: breakdown of mirror symmetry

Conditions for breakdown of the structural and intensity mirror symmetry relationship between the one-phonon spectra in the absorption and fluorescence spectra of molecular impurities are established. A criterion is given for determining when the contribution of quadratic coupling to the zero-phonon bandwidth, for a given linear electron— phonon coupling strength, is expected to be minimal.     

Electrochemistry and photophysics of donor-substituted triarylboranes: symmetry breaking in ground and excited state

We synthesized a series of amino substituted triarylboranes (TABs) 1-3 by copper(I)-catalyzed cross-coupling reactions. The title compounds were investigated by means of cyclic voltammetry (CV) and UV-visible absorption and fluorescence spectroscopy. Electrochemical oxidation of tris(4-carbazolyl-2,6-dimethylphenyl)borane (3) leads to the formation of an electroactive polymer film on the electrode surface. The charge-transfer (CT) absorption band of all three TABs shows a pronounced negative solvatochromism, while the emission is positively solvatochromic. By combining Jortner's theory, AM1 computations, and electrooptical absorption measurements (EOAM), this unexpected behavior was shown to be due to a dipole inversion upon S0-->S1 excitation. Furthermore, polarized steady-state fluorescence spectroscopy and EOAM prove that the ground-state geometry of 3 is of lower symmetry than D3 and that the excitation energy can be transferred from one subchromophore to another within the lifetime of the excited state. Exciton-coupling theory was used to quantitatively analyze this excitation transfer.     

Real-time observation of intramolecular proton transfer in the electronic ground state of chloromalonaldehyde: an ab initio study of time-resolved photoelectron spectra

The authors report on studies of time-resolved photoelectron spectra of intramolecular proton transfer in the ground state of chloromalonaldehyde, employing ab initio photoionization matrix elements and effective potential surfaces of reduced dimensionality, wherein the couplings of proton motion to the other molecular vibrational modes are embedded by averaging over classical trajectories. In the simulations, population is transferred from the vibrational ground state to vibrationally hot wave packets by pumping to an excited electronic state and dumping with a time-delayed pulse. These pump-dump-probe simulations demonstrate that the time-resolved photoelectron spectra track proton transfer in the electronic ground state well and, furthermore, that the geometry dependence of the matrix elements enhances the tracking compared with signals obtained with the Condon approximation. Photoelectron kinetic energy distributions arising from wave packets localized in different basins are also distinguishable and could be understood, as expected, on the basis of the strength of the optical couplings in different regions of the ground state potential surface and the Franck-Condon overlaps of the ground state wave packets with the vibrational eigenstates of the ion potential surface.     

Frequency shift in the NQR spectra of charge transfer complexes

Zusammenfassung Im Rahmen der Mullikenschen Theorie wird eine Frequenzverschiebung im NQR-Spektrum untersucht. Es wird gezeigt, da\ in der MO-LCAO-NÄherung die Frequenzverschiebung proportional dem Grad des charge transfer ist. Für Pikrylchlorid-Komplexe wird die LinearitÄt dieser AbhÄngigkeit bestÄtigt. Experimentell wurde gezeigt, da\ die Verschiebung stark durch sterische Effekte beeinflu\t wird.

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Frequency shift in the NQR spectra of charge transfer complexes

In the framework of Mulliken's theory a frequency shift in NQR spectra has been considered. It has been shown that in MO LCAO approximation the frequency shift is proportional to the degree of the charge transfer. For the picrylchloride complexes the linearity of this dependence has been confirmed. It has been shown experimentally that the shift is appreciably affected by steric interactions.

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Résumé Dans le cadre de la théorie de Mulliken on a étudié le déplacement de. fréquence dans le spectre NQR. Il a été montré que dans l'approximation MO LCAO le déplacement de fréquence est proportionnel à l'importance du transfert de charge. Pour les complexes du chlorure de picryle cette proportionalité a été confirmée par l'expérience. Le déplacement est affecté par ailleurs par les interactions stériques.

     

Nature of the intense near-IR absorption and unusual broad UV-visible-NIR spectra of azulenocyanines: density functional theory studies

The nature of the broad and intense near-IR (NIR) absorptions of azulenocyanine (H(2)Azc) in the range of 720-1300 nm due to the π→π* transitions is clearly revealed on the basis of density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. The electron moving between peripheral conjugated systems and the eighteen electron-π-conjugated core due to different types of electron transitions has also been clarified. Because of the small gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of all four H(2)Azc isomers, their Q absorption bands in the region of 850-1300 nm are significantly red-shifted relative to its isoelectronic analogue naphthalocyanine H(2)Nc. Observation of the new absorption bands between Q and Soret bands from 720 to 1050 nm for H(2)Azc is considered to be the result of electron transitions from the fused azulene rings to the 18 electron-π-conjugated core. In addition, the introduction of electron-withdrawing or electron-donating substituents onto the periphery of the azulenocyanine ring is revealed to further reduce the LUMO-HOMO gap, leading to a further red-shifted broad and intense NIR absorption band from 900 to 1700 nm for H(2)(F(8)Azc) and H(2)[(NH(2))(8)Azc].     

Skeletal relaxation effect on the charge transfer state formation of 4-dimethylamino,4'-cyanostilbene

Ab initio complete active space self-consistent field (CASSCF) calculations combined with polarized continuum model (PCM) have been performed to examine the charge transfer (CT) state formation of trans-4-dimethylamino,4'-cyanostilbene (DCS) in a solvent. In a polar solvent, the globally stable geometry in S1 takes a twisted conformation where the electron-donating dimethylanilino group is highly twisted against the other part of the electron-withdrawing 4-cyanostyryl group. In addition, skeletal relaxation where the aromatic benzene rings turn to be a nonaromatic quinoid structure is essential to stabilize the CT state. In a nonpolar solvent, the stable geometry in S1 takes a nontwisted conformation, though the skeletal relaxation is also an essential factor. By means of the free energy decomposition analysis, it is found that the stable CT geometry which depends on solvent polarity mainly comes from two factors: the linkage bond between the dimethylanilino and the 4-cyanostyryl group and the electrostatic interaction. In a polar solvent, the linkage bond has a single bond character to slightly prevent the torsional motion. This twist geometrically assists the charge separation so as to reinforce the electrostatic interaction. In consequence, the twisted internal CT (TICT) conformation is stable. In a nonpolar solvent, on the other hand, a nontwisted CT state is stable because the linkage bonds greatly increase a double bond character so as to prevent the torsional motion, while the electrostatic interaction is not so enhanced even by the geometrical twist.