Excited-State Mixed-Valence Distortions in a Diisopropyl Diphenyl Hydrazine Cation (original) (raw)
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Journal of Physical Chemistry A, 2005
A quantitative model of mixed-valence excited-state spectroscopy is developed and applied to 2,3-diphenyl-2,3-diazabicyclo[2.2.2]octane. The lowest-energy excited state of this molecule arises from a transition from the ground state, where the charge is located on the hydrazine bridge, to an excited state where the charge is associated with one phenyl group or the other. Coupling splits the absorption band into two components with the lower-energy component being the most intense. The sign of the coupling, derived by using a neighboring orbital model, is positive. The transition dipole moments consist of parallel and antiparallel vector components, and selection rules for each are derived. Bandwidths are caused by progressions in totally symmetric modes determined from resonance Raman spectroscopic analysis. The absorption, emission, and Raman spectra are fit simultaneously with one parameter set. *
Journal of The American Chemical Society, 2003
A model for the quantitative treatment of molecular systems possessing mixed valence excited states is introduced and used to explain observed spectroscopic consequences. The specific example studied in this paper is 1,4-bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)-2,3,5,6-tetramethylbenzene-1,4diyl dication. The lowest energy excited state of this molecule arises from a transition from the ground state where one positive charge is associated with each of the hydrazine units, to an excited state where both charges are associated with one of the hydrazine units, that is, a Hy-to-Hy charge transfer. The resulting excited state is a Class II mixed valence molecule. The electronic emission and absorption spectra, and resonance Raman spectra, of this molecule are reported. The lowest energy absorption band is asymmetric with a weak low-energy shoulder and an intense higher energy peak. Emission is observed at low temperature. The details of the absorption and emission spectra are calculated for the coupled surfaces by using the time-dependent theory of spectroscopy. The calculations are carried out in the diabatic basis, but the nuclear kinetic energy is explicitly included and the calculations are exact quantum calculations of the model Hamiltonian. Because the transition involves the transfer of an electron from the hydrazine on one side of the molecule to the hydrazine on the other side and vice versa, the two transitions are antiparallel and the transition dipole moments have opposite signs. Upon transformation to the adiabatic basis, the dipole moment for the transition to the highest energy adiabatic surface is nonzero, but that for the transition to the lowest surface changes sign at the origin. The energy separation between the two components of the absorption spectrum is twice the coupling between the diabatic basis states. The bandwidths of the electronic spectra are caused by progressions in totally symmetric modes as well as progressions in the modes along the coupled coordinate. The totally symmetric modes are modeled as displaced harmonic oscillators; the frequencies and displacements are determined from resonance Raman spectra. The absorption, emission, and Raman spectra are fit simultaneously with one parameter set. The coupling in the excited electronic state Hab ex is 2000 cm -1 . Excited-state mixed valence is expected to be an important contributor to the electronic spectra of many organic and inorganic compounds. The energy separations and relative intensities enable the excited-state properties to be calculated as shown in this paper, and the spectra provide new information for probing and understanding coupling in mixed valence systems.
Journal of Physical Chemistry A, 2005
Photolysis into the longest wavelength absorption band of 2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl hydrazine (Hy) substituted naphthalenes causes aryl group reduction electron transfer to give + Hy-Ar -. Electrooptical absorption measurements characterize the charge separation properties from these bands. Emission studies demonstrate that the separation between absorption and emission maxima for symmetrically disubstituted compounds is smaller than that for monosubstituted compounds, which is attributed to excited-state intervalence. The excited-state diabatic surfaces may be described as a Hy + -NA --Hy 0 , Hy 0 -NA --Hy + pair, for which electronic interaction produces a double minimum that qualitatively resembles that in the ground state of the disubstituted intervalence radical cations.
Theoretical Studies of Structure, Spectroscopy, and Properties of a New Hydrazine Derivative
Journal of Chemistry, 2013
We will report a combined experimental and theoretical study on molecular structure, vibrational spectra, and energies of (E)-1-(2,4-dinitrophenyl)-2-[(4-methylphenyl)methylidene]hydrazine (1). The molecular geometry and vibrational frequencies and energies in the ground state are calculated by using HF and DFT levels of theory with 6-311G basis sets. The calculated HOMO and LUMO energies also confirm that charge transfer occurs within the molecule. The harmonic vibrational frequencies were calculated, and the scaled values have been compared with experimental FTIR and FT-Raman spectra. The observed and the calculated frequencies are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically constructed bar-type spectrograms.
Inorganic Chemistry, 1989
The factors that govern the resonance Raman intensities of fundamentals, overtones, and combination bands are quantitatively evaluated. The calculations and interpretation are based on the time-dependent theory of Lee, Tannor, and Heller. From the time-dependent point of view, the Raman intensities are governed by the overlap of the time-dependent wave packet with the final Raman wave function of interest as a function of time. The most important factors are the magnitude of the overlap and the time development of the overlap. The magnitudes of the overlaps for overtones of a given mode are smaller than that for its fundamental, and the magnitude for a combination band is smaller than those of the fundamentals of the modes comprising the combination band. Thus, the overtone and combination bands are weaker than fundamentals. The time development of the overlap depends on both the frequency of the vibration and the displacement of the excited potential surface relative to the ground potential surface along the normal coordinate. The damping of the overlap determines whether short-time or long-time processes dominate the intensities. For large molecules where short-time processes dominate, the larger the initial overlap and the faster the overlap increases with time, the higher the Raman intensity. The intensities of fundamentals, overtone bands, and combination bands will be discussed in terms of the overlap. Qualitative rules for interpreting excited-state molecular properties from the Raman intensities are developed. The spectra of W(CO)5(pyridine) and Rh2(02CCH3)4L2, where L = PPh3 or AsPh3, are analyzed.
The Journal of Physical Chemistry A
Resonance Raman scattering studies in the extended near-infrared region show that six modes are coupled to the intramolecular charge-transfer transition in the mixed-valence radical cation diazatetracyclodiene. Spectral analysis based on time-dependent scattering theories shows that all six modes make substantial contributions to the vibrational reorganization energy. Measured Raman scattering cross sections were found to be considerably less than would be predicted by the conventional time-dependent scattering theories utilized here, evidently owing to complications arising from avoided surface crossings and anharmonic wave packet propagation. An examination of the classical and nonclassical kinetic effects of the coupled modes indicated that a considerable influence on the rate of intramolecular electron transfer is exerted by the modes collectively.
Charge-Localized Naphthalene-Bridged Bis-hydrazine Radical Cations
Journal of The American Chemical Society, 2006
Electron transfer (ET) in four symmetrically substituted naphthalene-bridged bis-hydrazine radical cations (1,4; 1,5; 2,6; and 2,7) is compared within the Marcus-Hush framework. The ET rate constants (kET) for three of the compounds were measured by ESR; the 2,7-substituted compound has an intramolecular ET that is too slow to measure by this method. The kET values are significantly dependent upon the substitution pattern of the hydrazine units on the naphthalene bridge but do not correlate with the distance between them. This is contrary to an assumption that is frequently made about intervalence compounds that the bridge serves only as a spacer that fixes the distance between the charge-bearing units. The internal vibrational and solvent portions (λ v and λs) of the total reorganization energy (λ) have been separated using solvent effects on the intervalence band maximum, resulting in a λv that is the same, 9900 cm -1 , for the differently substituted naphthalenes. This is in accord with the general assumption that λv is primarily dependent upon the charge bearing unit and not the bridge. However, the trends in λs cannot be explained by dielectric continuum theory.
The Journal of Physical Chemistry A, 2004
Characteristic patterns of infrared bands in the ν(CO) region have been observed in the time-resolved infrared (TRIR) spectra of fac-rhenium tricarbonyl complexes that allow for identification of transient states that result following laser flash excitation. These patterns can be interpreted by combining experimental TRIR data with density functional theory (DFT) calculations. The DFT calculations are particularly valuable as they provide vibrational energy shifts between the ground and excited states and an analysis of the electronic interactions in terms of the orbitals involved in the excitation. TRIR and DFT results for four different transient excited states, intraligand π f π*, metal-to-ligand charge transfer (MLCT), intramolecular (dππ) f π* excited states, and a redox-separated (RS state), are presented here. A unique example of competing excited states studied by TRIR is also presented. The complexes studied include fac-[Re I (CO) 3 (Me 2 dppz)(4-Etpy)] + , fac-[Re I (CO) 3 (bpy)(4-Etpy)] + , fac-[Re I (CO) 3 (4,4′-(CH 3) 2 bpy)(OQD)] , fac-[Re I (CO) 3 (Me 2 dppz)(py-PTZ)] + , and fac-[Re I (CO) 3 (dppz)(py-PTZ)] + (Me 2 dppz is dimethyl dipyrido[3,2-a:2′,3′-c]phenazine; dppz is dipyrido-[3,2-a:2′,3′-c]phenazine; 4Etpy is 4-ethylpyridine; bpy is 2,2′-bipyridine; 4,4′-(CH 3) 2 bpy is 4,4′-(CH 3)-2,2′bipyridine; OQD is 1-methyl-6-oxyquinone; py-PTZ is 10-(4-picolyl)phenothiazine). In addition to the DFT studies on the lowest triplet states probed by TRIR spectroscopy, time-dependent DFT (TD-DFT) calculations were also performed to analyze several of the lowest singlet and triplet excited states for each of the complexes.
Resonance Raman De-Enhancement Caused by Excited State Mixed Valence
The Journal of Physical Chemistry A, 2007
Resonance Raman and absorption spectra of 9,10-bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)-anthracene (2) are measured and analyzed. The contribution of the individual vibrational normal modes to the reorganization energy is investigated. Excited-state mixed valence in this system is analyzed using density functional theory electronic structure calculations. The resonance Raman excitation profiles exhibit a resonance de-enhancement effect around 20 725 cm -1 , but a corresponding feature is not observed in the absorption spectrum. This unusual observation is attributed to the presence of a dipole-forbidden, vibronically allowed component of the split mixed valence excited state. The de-enhancement dip is calculated quantitatively and explained in terms of the real and imaginary components of the polarizabilities of the two overlapping excited states.