In this work we carefully investigate the relationship between computed data and experimental electronic spectra. To that end, we compare both vertical transition energies, EV, and characteristic frequencies of the spectrum like the maximum, νmax, and the center of gravity, M1, taking advantage of an analytical expression of M1 in terms of the parameters of the initial- and final-state potential energy surfaces. After pointing out that, for an accurate comparison, experimental spectra should be preliminarily mapped from wavelength to frequency domain and transformed to normalized lineshapes, we simulate the absorption and emission spectra of several prototypical chromophores, obtaining lineshapes in very good agreement with experimental data. Our results indicate that the customary comparison of experimental νmax and computational EV, without taking into account vibrational effects, is not an adequate measure of the performance of an electronic method. In fact, it introduces systematic errors that, in the...

Insights for an Accurate Comparison of Computational Data to Experimental Absorption and Emission Spectra: Beyond the Vertical Transition Approximation.

Azobenzene E⇆Z photoisomerization, following excitation to the bright S(ππ*) state, is investigated by means of ab initio CASSCF optimizations and perturbative CASPT2 corrections. Specifically, by elucidating the S(ππ*) deactivation paths, we explain the mechanism responsible for azobenzene photoisomerization, the lower isomerization quantum yields observed for the S(ππ*) excitation than for the S1(nπ*) excitation in the isolated molecule, and the recovery of the Kasha rule observed in sterically hindered azobenzenes. We find that a doubly excited state is a photoreaction intermediate that plays a very important role in the decay of the bright S(ππ*). We show that this doubly excited state, which is immediately populated by molecules excited to S(ππ*), drives the photoisomerization along the torsion path and also induces a fast internal conversion to the S1(nπ*) at a variety of geometries, thus shaping (all the most important features of) the S(ππ*) decay pathway and photoreactivity. We reach this conclus...

The Different Photoisomerization Efficiency of Azobenzene in the Lowest nπ* and ππ* Singlets: The Role of a Phantom State

Resonance Raman intensity analysis was used to investigate the initial excited-state nuclear dynamics of cis- and trans-azobenzene following S1 (npi*) excitation, and fluorescence quantum yield measurements were used to estimate the excited-state lifetimes. trans-Azobenzene exhibits the strongest Raman intensities in its skeletal stretching and bending modes, while torsional motions dominate the nuclear relaxation of cis-azobenzene as indicated by intense Raman lines at 275, 542, 594, and 778 cm(-1). The very weak fluorescence quantum yield for cis-azobenzene is consistent with its approximately 100 fs electronic lifetime while trans-azobenzene, with a fluorescence quantum yield of 1.1 x 10(-5), has an estimated S1 lifetime of approximately 3 ps. The absorption and Raman cross-sections of both isomers were modeled to produce a harmonic displaced excited-state potential energy surface model revealing the initial nuclear motions on the reactive surface, as well as values for the homogeneous and inhomogeneous linewidths. For cis-azobenzene, this modeling predicts slopes on the S1 potential energy surface that when extrapolated to the position of the harmonic minimum give excited-state changes of approximately 6-20 degrees in the CNNC torsion angle and a < or =3 degrees change in the CNN bending angle. The relatively large excited-state displacements along these torsional degrees of freedom provide the driving force for ultrafast isomerization. In contrast, the excited-state geometry changes of trans-azobenzene are primarily focused on the CNN bend and CN and NN stretches. These results support the idea that cis-azobenzene isomerizes rapidly via rotation about the NN bond, while isomerization proceeds via inversion for trans-azobenzene.

Excited-State Structure and Dynamics of cis-and trans-Azobenzene from Resonance Raman Intensity Analysis

The vibrational structures of trans- (TAB) and cis-azobenzene (CAB) are of interest due to their importance in optoelectronic applications as well as due to the unique isomerization mechanism involving the inversion process (at the nitrogen site). In this paper, we report the equilibrium structures, harmonic frequencies, and mode assignments for TAB and CAB and their isotopic analogues, using restricted Hartree-Fock (RHF), hybrid Hartree-Fock/density functional (HF/DF), and pure density functional theoretical (DFT) methods utilizing the 6-31G* basis set. The results of the optimized molecular structure obtained on the basis of RHF and all the DFT calculations are presented and then critically compared with the experimental electron diffraction results. It is observed that best structural parameters are predicted by the hybrid HF/DF method, viz. B3LYP and B3P86 followed by the pure DFT method BP86. In the case of harmonic vibrational frequencies (unsealed) and the normal modes, it is found that the BP86/6-31G* is the most accurate. The data obtained here has been used to reassign (in contrast to the previously reported MP2 results) some of the vibrational frequencies, particularly, for the main N=N and C-N vibrations of TAB. On the basis of BP86/6-31G* force field, the infrared intensities for both TAB and CAB and their isotopomers have also been calculated. Moreover, the main differences in the vibrational spectra of the two isomers of azobenzene have been discussed from normal mode analysis.

Density functional calculations of structures, vibrational frequencies, and normal modes of trans- and cis-azobenzene

The vibrational structures of 4-nitro-azobenzene (NAB), 4-(dimethylamino)-azobenzene (DAB), and 4-nitro,$4^{\prime}$-(dimethylamino)-azobenzene (NDAB) are of interest due to their importance in optoelectronic applications as well as the unique isomerization mechanism involving the inversion process (at the nitrogen site). In this paper, we present the Fourier transform infrared (FTIR) and Raman spectral studies of NAB, DAB, and NDAB and also report their equilibrium structures, harmonic frequencies, and normal mode assignments, employing the hybrid Hartree-Fock/density functional (HF/DF) method with the 6-31G basis set. The results of the optimized molecular structure obtained on the basis of B3LYP with 6-31G basis set suggest a greater conjugation and \pi-electron delocalization for the substituted azo dyes in comparison to the parent azo molecule, viz. trans-azobenzene (TAB). It is found that the B3LYP/6-31G method is very accurate in predicting harmonic vibrational frequencies and the normal modes for the substituted azo dyes and their isotopic analogues. On the basis of the B3LYP/6-31G force field, the infrared intensities for NAB, DAB, and NDAB, and their isotopomers, are calculated and then compared with those observed experimentally. Finally, the main differences in the vibrational spectra of the substituted azobenzene derivatives are discussed from an analysis of the normal modes.

Structures, Vibrational Frequencies, and Normal Modes of Substituted Azo Dyes: Infrared, Raman, and Density Functional Calculations

The excitation profiles of several Raman bands of trans-azobenzene solution in carbon tetrachloride were measured in resonance with the first allowed electronic transition peaked around 320 nm. The experimental results were analysed by means of a multi-mode Franck–Condon model to obtain some excited state molecular parameters; such parameters were found to reproduce well the red side of the absorption band whereas some inconsistencies between experiment and calculation persist on the UV side. This has been tentatively explained as due to the contribution of a higher energy electronic state whose absorption band is peaked around 287 nm and which may slightly affect the parameters obtained.

Resonance Raman investigation of trans-azobenzene in the lowest energy 1Au excited state

Polarized raman scattering of highly oriented trans polyacetylene

The Raman excitation profile for the 220 cm −1 mode of trans-azobenzene in cyclohexane solution was measured in resonance with the π-π* transition peaked around 320 nm. A strong modulation of the band intensity across the first vibrational feature of the poorly resolved absorption spectrum was detected. The collected data were analysed by means of the time-correlator theory and of a Franck-Condon model to derive some molecular parameters of the excited electron state

Resonant Raman excitation profile of the 220 cm−1 mode in trans-azobenzene

Resonance raman excitation profiles of CrO2−4 from K2CrO4/K2SO4 lyophilised solid mixtures

Direct observation of a 220 cm−1 structure in the lowest π–π* absorption band in the vapour phase of trans-azobenzene

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