Simulated T ← S spectrum of 2,4,5-trimethylbenzaldehyde in durene

The T_1,2 ← S₀ phosphorescence excitation spectrum of 2,4,5-trimethylbenzaldehyde in durene has been simulated using forty-five zero-order Born-Oppenheimer product states of which thirty-two belong to T₁ (ππ*), the others to T₂ (nπ*). The spectrum is very complicated in the region 400–600 cm^?1 above the T₁ (ππ*) ←3 S₀ origin band at 24150 cm^?1. In this tangled region conventional vibrational analysis is not useful. Several comments on the physical properties of the excited triplet states of 2,4,5-trimethylbenzaldehyde are given.     

Vibrational Spectrum of o‐Dimethoxybenzene in the S1 and D0 States

The optimized molecular geometries of o‐dimethoxybenzene (ODMB) in the S₀ state were predicted by ab initio and density functional theory calculations. Its vibrational spectra in the S₁ and D₀ states were studied by one color resonant two photon ionization (1C‐R2PI) and mass analyzed threshold ionization (MATI) experiments. The results indicated that trans rotamer was most stable. Only one rotamer of ODMB was detected by the 1C‐R2PI spectra, and its band origin was (35750±2) cm^?1, its ionization energy was (61617±5) cm^?1. Most of the observed vibrations in the D₀ state resulted from the in‐plane ring and substituent sensitive modes.     

A novel spectroscopy using ultrafast two-pulse excitation of large polyatomic molecules

A first infrared pulse at frequency ν₁ interacts with vibrational states in S₀ and a second visible pulse at ν₂ promotes the excited molecules to the S₁ state from where they fluoresce. Tuning the frequency ν₂ over 600 cm^?1 allows the observation of a detailed spectrum which gives information on vibrational states in S₀ and on vibronic states in S₁ together with corresponding Franck—Condon factors. The spectra differ drastically from the common broad and featureless absorption and fluorescence bands.     

Molecular Structure, Vibrational Spectrum and Ionization Energy of m-Dimethoxybenzene

The optimized molecular geometries of the three rotamers of m-dimethoxybenzene in the ground So and electronically excited Sl states were predicted by ab initio and density functional theory （DFF） calculations. Their vibrational spectra in the St state were studied by one color resonant two photon ionization （1C-R2PI） method, and their ionization energies were measured by two color resonant two photon ionization （2C-R2PI） experiment. The optimized molecular geometries showed that the total energy of conformer a was the lowest in the So state. Most of the active vibrations assigned from the 1C-R2PI spectrum were found to be of the in-plane ring modes. The ionization energies （IE） of conformers a, b and c were determined to be 63521, 64487 and 63755 cm^-1, respectively.     

Moderately high resolution fluorecence spectrum of the S1 → S0 transition of azulene

We report an Ar/Kr ion laser induced spectrally resolved S₁ → S₀ emission from an azulene in a host naphthalene crystal observed at the helium λ-point and at 77 K. Less well resolved S₁ → S₀ emission from crystalline azulene dispersed in a KBr pellet at 300 K is also reported. For the 6471 Å excitation the emission from the azulene in naphthalene system is analyzed in terms of three components: a resonance enhanced Raman emission originating from a nonstationary laser photon energy state 800 cm^?1 above the S₁ origin, a partially relaxed fluorecence originating from the 665 cm^?1 vibrational level of S₁ and a totally relaxed fluorecence from the S₁ origin (14651 cm^?1). The interpretation of the spectral lines is based on totally symetric vibrational modes (406, 679, 825,902, 1203, 1269, 1401, and 1586 cm^?1) the most prominent of which is the progression forming 825 cm^?1 mode. On the basis of both energies and intensities, correlations are made between ground and excited state vibrations and are compared with earlier results. Based on our results, a discussion is given on a plausible relaxation scheme for our system including the influence of Franck—Condon factors on the observability of unrelaxed emission.     

Excited state dynamics and spectroscopy of the 1A″ state of NSF vapor — comparisons with SO2

The fluorescence emission spectrum and analysis of NSF vapor is presented. Single vibronic level excitation near the S₁ origin gives rise to a 10 μs radiative decay. The fluorescence lifetime for excitation of levels with ? 4500 cm^?1 excess vibrational energy becomes controlled by a unimolecular radiationless process which is likely photodissociation; the dependence of this radiationless rate on energy and vibrational mode is investigated. The perturbations resulting from coupling of zero-order S₁ states with other vibronic levels which control the excited state dynamics of SO₂ are apparently not operative for NSF. Attempts are made to rationalize the grossly different dynamic behavior of the S₁ levels of these two otherwise very similar systems.     

Excited States of Weak Interacting Complexes of Formaldehyde and Alkali Metal Ions

The electronically excited states of formaldehyde and its complexes with alkali metal ions are investigated with the time-dependent density functional theory (TD DFT) method. Vertical transition energies for several singlet and triplet excited states, adiabatic transition energies for the first singlet and triplet excited states S₁ and T₁, the adiabatic geometries and vibrational frequencies of the ground state S₀ and the first singlet and triplet excited states S₁ and T₁ for formaldehyde and its complexes are calculated. Better agreement with the experiment than that of the CIS method is obtained for CH₂O at the TD DFT level. The nonlinear C=O?M⁺ interaction in the excited states S₁ and T₁ is weaker than the linear interaction in the ground state. In the S₀ and S₁ states, the C=O bond is elongated by cation complexation and its stretching frequency is red-shifted, but in the T₁ state the C=O bond is shortened and its frequency is blue-shifted.     

Investigation of excited‐state properties of fluorene–thiophene oligomers by the SAC‐CI theoretical approach

Excited states of fluorene‐ethylenedioxythiophene (FEDOT) and fluorene‐S,S‐dioxide‐thiophene (FTSO2) monomers and dimers were studied by the symmetry‐adapted cluster (SAC)‐configuration interaction (CI) method. The absorption and emission peaks observed in the experimental spectra were theoretically assigned. The first three excited states of the optimized conformers, and the conformers of several torsional angles, were computed by SAC‐CI/D95(d). Accurate absorption spectra were simulated by taking the thermal average for the conformers of torsional angles from 0° to 90°. The conformers of torsional angles 0°, 15°, and 30° mainly contributed to the absorption spectra. The full width at half‐maximum of the FEDOT absorption band is 0.60 eV (4839 cm^?1), which agrees very well with the experimental value of 0.61 eV (4900 cm^?1). The maximum absorption wavelength is located at 303 nm, which is close to those of the experimental band (327 nm). The calculated absorption spectrum of FTSO2 showed two bands in the range of 225–450 nm. This agrees very well with the available experimental spectrum of a polymer of FTSO2, where two bands are detected. The excited‐state geometries were investigated by CIS/6‐31G(d). These showed a quinoid‐type structure which exhibited a shortening of the inter‐ring distance (0.06 Å for FEDOT and 0.04 Å for FTSO2). The calculated emission energy of FEDOT is 3.43 eV, which agrees very well with the available experimental data (3.46 eV). The fwhm_E is about 0.49 eV (3952 cm^?1), while the experimental fwhm is 0.43 eV (3500 cm^?1). For FTSO2, two bands were also found in the emission spectrum. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Theoretical DFT study of phosphorescence from porphyrins

Geometrical structure of free-base porphin (H₂P) and Mg- and Zn-porphyrins together with their vibrational frequencies and vibronic intensities in phosphorescence are investigated by density functions theory (DFT) with the standard B3LYP functional. These molecules have a closed-shell singlet ground state (S₀) and low-lying triplet (T₁) excited states of ππ* type. The S₀–T₁ transition probability and radiative lifetime of phosphorescence (τ_p) of these molecules are calculated by time-dependent DFT utilizing quadratic response functions for account of spin–orbit coupling (SOC) and electric-dipole transition moments including displacements along active vibrational modes. The infrared and Raman spectra in the ground singlet and first excited triplet states are also studied for proper assignment of vibronic patterns. The long radiative lifetime of free-base porphin phosphorescence (τ_p ∼ 360 s at low temperature limit, 4.2 K) gets considerably shorter for the metalloporphyrins. An order of magnitude reduction of τ_p is predicted for Mg-porphyrin but no change of phosphorescence polarization is found. A forty times enhancement of the radiative phosphorescence rate constant is obtained for Zn-porphyrin in comparison with the H₂P molecule which is accompanied by a strong change of polarization and spin-sublevel radiative activity. A strong vibronic activity of free-base porphin phosphorescence is found for the b_2g mode at 430 cm⁻¹, while the 679 and 715 cm⁻¹ vibronic bands of b_3g symmetry are less active. These and other out-of-plane vibrations produce considerable changes in the radiative constants of different spin sublevels of the triplet state; they also promote the S₁ → T₁ intersystem crossing. Among the in-plane vibrations the a_g mode at 1614 cm⁻¹ is found very active; it produces a long progression in the phosphorescence spectrum. The time-dependent DFT calculations explain the effects of the transition metal atom on phosphorescence of porphyrins and reproduce differences in their phosphorescence and EPR spectra.     

The two-photon phosphorescence excitation spectrum of triphenylene

The two-proton excited phosphorescence of triphenylene in PMMA matrix at 77 K was measured using a tunable flashlamp-pumped rhodamine dye laser in the effect spectral region 32000–36000 cm^?1. The band origin of the S₀S₂ transition, which is uncertain in the one-photon absorption spectrum, was observed at 33200 cm^?1. The vibronic features in the one-photon spectrum were reinterpreted.     

Comparative analysis of the vibrational structure of the absorption spectra of acrolein in the excited (<Emphasis Type="Italic">S</Emphasis><Subscript>1</Subscript>) electronic state

The assignments of absorption bands of the vibrational structure of the UV spectrum are compared with the assignments of bands obtained by the CRDS method in a supersonic jet from the time of laser radiation damping for the trans isomer of acrolein in the excited (S ₁) electronic state. The ν_00trans = 25861 cm⁻¹ values and fundamental frequencies, including torsional vibration frequency, obtained by the two methods were found to coincide in the excited electronic state (S ₁) for this isomer. The assignments of several absorption bands of the vibrational structure of the spectrum obtained by the CRDS method were changed. Changes in the assignment of (0-v′) transition bands of the torsional vibration of the trans isomer in the Deslandres table from the ν_00trans trans origin allowed the table to be extended to high quantum numbers v′. The torsional vibration frequencies up to v′ = 5 were found to be close to the frequencies found by analyzing the vibrational structure of the UV spectrum and calculated quantum-mechanically. The coincidence of the barrier to internal rotation (the cis-trans transition) in the one-dimensional model with that calculated quantum-mechanically using the two-dimensional model corresponds to a planar structure of the acrolein molecule in the excited (S ₁) electronic state.     

Theoretical study on geometry and spectroscopic properties of 1,1′-binaphthyl in the electronic ground and first excited singlet states

Equilibrium geometries for the electronic ground and first excited singlet states of 1,1'-binaphthyl have been calculated by minimization of the total energy with respect to all internal coordinates. Using these results, an interpretation of the fluorescence S₁→ S₀ and absorption spectra S_m ← S₀ and S_n ← S₁ in rigid and fluid solutions is given.For the first time the equilibrium geometry of the first excited singlet state of 1, 1′-binaphthyl has been calculated. On excitation to the S₁ state the dihedral angle θ between the two naphthalene moieties is de- creased from 61 ° to 41 °. A detailed survey of CH bond lengths in the S₀ and S₁ states has been given. This result should be of particular importance for the theoretical treatment of radiationless transitions.Using equilibrium geometries for the S₀ and S₁ states a satisfactory interpretation of the S_m ← S₀ and S_n ← S₁ absorption spectra as well as of the fluorescence spectra in fluid and rigid solutions can be given. Concerning the S_n ← S₁ absorption spectrum in fluid solution, the calculations predict a strong absorption (A ← B transition) in the still uninvestigated region of energies lower than 11000 cm^?1.From the results of this paper and of other calculations it can be concluded that the Warshel-Karplus method yields reliable equilibrium geometries for electronic ground and excited states of unsaturated hydrocarbons [22,23].     

S1←S0 Vibronic spectrum and structure of 2,2-difluoroethanal in the S1 state

The vibronic spectrum of the 2,2-difluoroethanal vapor was recorded using a multipass optical cell with an optical length of at least 140 m. The spectrum in the region of 300—364 nm was assigned to the S₁S₀ electronic transition (from the ground S₀ to the first excited singlet S₁ electronic state); the vibrational structure of the spectrum was analyzed. The spectrum bands were assigned to two systems of vibronic transitions, namely, transitions between the levels of the cis-conformer (S₀) and of the S₁ conformers, with the origins (0₀ ⁰ transitions between the zero vibrational levels of conformers) at 29192 and 29087 cm^–1, respectively. Analysis of the spectrum showed that the S₁S₀ electronic excitation of the cis-conformer was followed by rotation of the CHF₂ top and pyramidal distortion of the carbonyl fragment. A number of fundamental frequencies were found for S₁ conformers, in particular, torsion and inversion energy levels. The experimental data are in satisfactory agreement with the results of quantum-chemical calculations for the 2,2-difluoroethanal molecule in the S₀ and S₁ states.     

Electronic absorption spectra of phenol dimer and toluene—cyanobenzene complex as studied by mass-selected MPI—nozzle beam method

S₀–S₀ absorption spectra of phenol dimer and toluene-cyanobenzene complex in a supersonic free jet have been observed by the use of the multiphoton ionization technique. Two low-frequency vibrations at 15 and 112 cm^-1 appear in the spectrum of phenol dimer, which were assigned as the bending and stretching mode of hydrogen bonding, respectively. Toluene—cyanobenzene complex also exhibits a low-frequency vibrational progression which indicates the dissociation of the complex in the S₁ state.     

Laser Spectroscopy and Lifetime Measurements of the S1 State of Tetracyanoquinodimethane (TCNQ) in a Cold Gas-Phase Free-Jet

The S₁ electronic state of 7,7,8,8-Tetracyanoquinodimethane (TCNQ) has been investigated by laser induced fluorescence (LIF), dispersed fluorescence (DF) spectroscopy, and lifetime measurements under jet-cooled conditions in the gas-phase. The LIF spectrum showed a weak origin band at 412.13 nm (24262 cm⁻¹) with prominent progression and combination bands involving vibrations of 327, 1098, and 2430 cm⁻¹. In addition, very strong bands appeared at ∼363.6 nm (3300 cm⁻¹ above the origin). Both the LIF and DF spectra indicate considerable geometric change in the S₁ state. The fluorescence lifetime of S₁ at zero-point level was obtained to be 220 ns. This lifetime is 40 times longer than the radiative lifetime estimated from the S₁−S₀ oscillator strength. Furthermore, the lifetimes of the vibronic bands exhibited drastic energy dependence, indicating a strong mixing with the triplet (T₁) or intramolecular charge-transfer (CT) state. This study is thought to disclose intrinsic nature of TCNQ, which has been well known as a component of organic semiconductors and a versatile p-type dopant.     

Theoretical study of structure,vibrational and electronic spectra of isomers of methyl-3-methoxy-2-propenoate

A systematic quantum mechanical study of the possible conformations, their relative stabilities, vibrational and electronic spectra and thermodynamic parameters of methyl-3-methoxy-2-propenoate has been reported for the electronic ground (S₀) and first excited (S₁) states using time-dependent and time-independent Density Functional Theory (DFT) and RHF methods in extended basis sets. Detailed studies have been restricted to the E-isomer, which is found to be substantially more stable than the Z-isomer. Four possible conformers c′Cc, c′Tc, t′Cc, t′Tc, of which the first two are most stable, have been identified in the S₀ and S₁ states. Electronic excitation to S₁ state is accompanied with a reversal in the relative stability of the c′Cc and c′Tc conformers and a substantial reduction in the rotational barrier between them, as compared with the S₀ state. Optimized geometries of these conformers in the S₀ and S₁ states are being reported. Based on suitably scaled RHF/6-31G^** and DFT/6-311G^** calculations, assignments have been provided to the fundamental vibrational bands of both these conformers in terms of frequency, form and intensity of vibrations and potential energy distribution across the symmetry coordinates in the S₀ state. A complete interpretation of the electronic spectra of the conformers has been provided.     

On the “anomalous” flourescence of 3,4-benzypyrene in the vapour phase

An analysis of the fluorescence of 3,4-benzpyrene in the vapour phase shows two contributions to the “anomalous” fluorescence: (i) the emission from the second excited state to the ground state and (ii) a vibronically induced S₁ → S₀ emission originating from the +520 cm^?1 vibrational level. A comparison between the intensities of the emissions indicates that in the vapour phase the vibrational redistribution from the 520 cm^? vibrational level of S₁ to modes of lower frequencies is relatively slow.     

The rotational spectrum of the hydrogen-bonded heterodimer H2O…HF in the frequency range 40–80 GHz

The μ_a R-branch rotational spectra of the ground and first excited states of the three lowest vibrational modes of the H₂O…HF heterodimer have been observed in the frequency range 40–80 GHz. Coriolis perturbations between the ground vibrational state (υ = 0) and the first excited state of the out-of-plane bending vibration (υ_β(0) = 1) show that for a given J the K₋₁ = 2 levels of υ_β(0) = 1 lie approximately 3 cm⁻¹ above the K₋₁ = 3 levels of υ = 0. The vibrational separation between these two states is estimated to be 70±3 cm⁻¹. This value is consistent with those determined by other methods and reinforces the conclusion that ν_β(0) is governed by a double-minimum potential energy function with the quantitative form previously published. A perturbation is also observed in the first excited state of the hydrogen-bond stretching vibration υ_σ = 1. This manifests itself as a large, negative centrifugal distortion constant D_JK = −8.5 MHz compared with 2 MHz in the other vibrational states.     

Ir absorption spectrum of CrO2Cl2 molecules for high-lying states of the vibrational quasi-continuum

A new approach to the spectroscopy of highly excited vibrational states of polyatomic molecules has been elaborated. The molecules of CrO₂Cl₂ were prepared in states with a vibrational energy of the ground electronic term A₁ of ≈ 19000 cm⁻¹ by means of internal conversion of electronic energy from the electronic state B₁ excited by laser radiation. The spectroscopy of the vibrationally excited molecules has been carried out in the region of the ν₆ and ν₁ bands with diode and CO₂ lasers. The fwhm of the obtained spectrum was ≈ 15 cm⁻¹. The intermode interaction in CrO₂Cl₂ has been theoretically analyzed, and the calculated spectrum compared with that measured experimentally. The time evolution of the spectrum of vibrationally excited CrO₂Cl₂ molecules has been studied. The average energy transferred per one collision with unexcited CrO₂Cl₂ molecules was equal to 〈δE〉 ≈ 1200 cm⁻¹.     

A density functional study of the electronic spectrum of permanganate

Multiplet splittings for six excited electronic configurations of the permanganate ion, MnO₄⁻, are calculated. Earlier density functional calculations on the same subject are improved upon by the numerical evaluation of some two-electron integrals to resolve certain multideterminantal states. Excellent agreement with the experimental spectrum is obtained, and a reassignment of bands in the 25,000–35,000 cm⁻¹ range is proposed. Fully symmetric (a₁) vibrational frequencies are calculated, and the origin and magnitude of the most significant Jahn-Teller distortions of the excited states are discussed. © 1996 John Wiley & Sons, Inc.