Excited-state Geometry Changes Research Articles

Dual fluorescence of (E)-N-(4-(dimethylamino)benzylidene)-2H-1,2,4-triazol-3-amine (DMABA-Amtr): A ground state perspective

We present the absorption and fluorescence spectra of (E)-N-(4-(dimethylamino)benzylidene)-2H-1,2,4-triazol-3-amine (DMABA-Amtr), an electron donor-π-acceptor system. The molecule shows a single fluorescence emission band in non-polar solvents while dual emissions were observed in polar aprotic solvents. Although several researchers over the years provide different explanations for the mechanism of the phenomena, based on solvent assisted excited state geometry changes of such systems, it is still a matter of controversy since such systems are unique as they contradict Kasha's rule. The emission spectrum of the molecule shows strong dependence on solvent polarity and excitation wavelength. This observation together with a single iso-emissive point found in the area normalize emission spectra indicates the presence of two ground state equilibrium structures of the compound which are both fluorescent. Density functional theory (DFT) and time-dependent (TD-DFT) calculations also support the experimental findings.

The photodissociation of N,N-dimethylnitrosamine at 355 nm: The effect of excited-state conformational changes on product vector correlations

In a photodissociation experiment, the dynamics associated with creating reaction products with specific energies can be understood by a study of the product vector correlations. Upon excitation to the S1 state, N,N-dimethylnitrosamine (DMN) undergoes an excited-state geometry change from planar to pyramidal around the central N. The significant geometry change affects the vector correlations in the photoproducts. Using polarized lasers for 355 nm photodissociation of DMN and for NO photoproduct excitation in a velocity-mapped ion imaging apparatus reveals new vector correlation details among the parent transition dipole (μ), photofragment velocity (v), and photofragment angular momentum (j). The dissociation of DMN displays some μ-v correlation [β02(20)=-0.2], little μ-j correlation [β02(02)∼0], and, surprisingly, a v-j [β00(22)] correlation that depends on the NO lambda doublet probed. The results point to the importance of the initial excited-state conformational change and uncover the presence of two photolysis channels.

Resonance Raman overtones reveal vibrational displacements and dynamics of crystalline and amorphous poly(3-hexylthiophene) chains in fullerene blends

Resonance Raman spectra of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester blend thin films display progressions of overtone and combination bands (up to two harmonics) involving the dominant symmetric C=C backbone stretching mode of P3HT that encode excited state vibrational displacements and dynamics information. Contributions from both crystalline (aggregated) and amorphous (unaggregated) P3HT domains are resolved and intensities are analyzed using the time-dependent theory of spectroscopy. Raman spectra, excitation profiles, and absorption spectra are simulated with the same parameters using a single electronic state description for each P3HT form. Time-dependent wavepacket overlaps expose vibrational coherence on sub-100 fs timescales, which is usually difficult to extract from conventional ultrafast pump-probe spectra and transients of polymer∕fullerene blends. The results demonstrate the potential of simpler CW resonance Raman approaches to uncover excited state geometry changes and early vibrational dynamics from distinct morphological forms in polymer∕fullerene blends.

Theoretical Study of the Relationships between Excited State Geometry Changes and Emission Energies of Oxyluciferin

In order to find a relationship between firefly luciferases structure and bioluminescence spectra, we focus on excited substrate geometries which may be affected by rigid luciferases. Density functional theory (DFT) and time dependent DFT (TDDFT) were employed. Changes in only six bond lengths of the excited substrate are important in determining the emission spectra. Analysis of these bonds suggests the mechanism whereby luciferases restrict more or less the excit-ed substrate geometries and to produce multicolor bioluminescence.

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

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.

Using Internal Coordinates to Describe Photoinduced Geometry Changes in MLCT Excited States

A resonance Raman intensity analysis of the metal-to-ligand charge-transfer (MLCT) transition for the rhenium compound Re(2-(2'-pyridyl)quinoxaline)(CO)(3)Cl (RePQX) is presented. Photoinduced geometry changes are calculated, and the results are presented using the vibrational normal modes and the redundant internal coordinates. A density functional theory calculation is used to determine the ground-state nonresonant Raman spectrum and a transformation matrix that transforms the redundant internal coordinates into the normal modes. The normal modes nu(37) (rhenium coordination sphere distortion) and nu(75) (ligand skeletal stretch) show the largest photoinduced geometry change (Delta = 1.0 and 0.7, respectively). A single carbonyl mode is enhanced in the resonance Raman spectra. Time-dependent density functional theory is used to calculate excited-state geometry changes, which are subsequently used to determine the signs of the photoinduced normal mode displacements. Transforming to internal coordinates reveals that all the CO bond lengths are displaced in the excited state. The Re-C and C-C ligand bond lengths are also displaced in the excited state. The results are discussed in terms of a simple one-electron picture for the electronic transition. Many bond angles and torsional coordinates are also displaced by the metal-to-ligand charge transfer, and most of these are associated with the rhenium coordination sphere. It is demonstrated that using internal coordinates presents a clear picture of the geometry changes associated with photoinduced electron transfer in metal polypyridyl systems.

Resonance Raman spectroscopic study of fused multiporphyrin linear arrays

For prospective applications as molecular electric wires, triply linked fused porphyrin arrays have been prepared. As expected from their completely flat molecular structures, π-electron delocalization can be extended to the whole array manifested by a continuous redshift of the HOMO-LUMO transition band to infrared region up to a few μm as the number of porphyrin units in the array increases. To gain an insight into the relationship between the molecular structures and electronic properties, we have investigated resonance Raman spectra of fused porphyrin arrays depending on the number of porphyrin pigments in the array. We have carried out the normal mode analysis of fused porphyrin dimer based on the experimental results including Raman frequency shifts of two types of C13-isotope substituted dimers, Raman enhancement pattern by changing excitation wavelength, and depolarization ratio measurements as well as normal-mode calculations at the B3LYP/6-31G level. In order to find the origins for the resonance Raman mode enhancement mechanism, we have predicted both the excited state geometry changes (A-term) and the vibronic coupling efficiencies (B-term) for the relevant electronic transitions based on the INDO/S-SCI method. A detailed normal mode analysis of the fused dimer allows us to extend successfully our exploration to longer fused porphyrin arrays. Overall, our investigations have provided a firm basis in understanding the molecular vibrations of fused porphyrin arrays in relation to their unique flat molecular structures and rich electronic transitions.

Resonance Raman Intensity Analysis of Cresyl Violet Bound to SiO2Colloidal Nanoparticles

Resonance Raman spectra and absolute cross sections have been measured for cresyl violet in aqueous solution and bound to the surface of SiO 2 colloidal nanoparticles. The absorption spectra of cresyl violet on SiO 2 havepreviously been interpreted to show formation of H-type dimers on the surface. The spectra of the monomers in solution are simulated to obtain the excited-state geometry change along each normal mode and the electronic spectral broadening parameters. The spectra of the dimers on SiO 2 are then simulated with a model that assumes transition dipole coupling between the vibronic transitions on the two monomers. The simulations require increases in both the electronic homogeneous line width and the inhomogeneous line width upon binding to the colloid. The general features of the resonance Raman spectra are reproduced fairly well, but significant differences in the relative intensities of certain Raman lines upon binding to the surface suggest specific vibrational or vibronic effects not considered in this simple model.

Aromatic versus polyenic linkers in push–pull chromophores: electron–phonon coupling effects

The linear absorption spectra and absolute resonance Raman excitation profiles of [(2 E,4 E,6 E,8 E)-9-[4-(dibutylamino)phenyl]-2,4,6,8-nonatetraenylidene]propanedinitrile, an electron donor–acceptor push–pull chromophore with a polyenic inker group, have been measured in cyclohexane and acetonitrile solution. Numerical simulation of the spectra yields solvent reorganization energies and the excited-state geometry changes along the strongly Raman-active vibrations. The ground-state vibrational frequencies and excited-state reorganization energies are considered within the context of two-state valence bond models. Comparison of the results for this chromophore with p-nitroaniline, which has a purely aromatic linker group, suggests that the two-state model is more applicable to molecules with polyenic rather than aromatic linkers.

Vibronic effects on solvent dependent linear and nonlinear optical properties of push-pull chromophores: Julolidinemalononitrile

The linear absorption spectra and absolute resonance Raman excitation profiles of the “push-pull” chromophore julolidinemalononitrile have been measured in cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol solution at excitation wavelengths spanning the strong visible charge-transfer absorption band. Numerical simulation of the spectra using time-dependent wave-packet propagation methods yields the excited-state geometry changes along the ∼15 strongly Raman-active vibrations as well as the solvent reorganization energies. The distribution of the total vibrational reorganization energy among the various normal modes is solvent dependent, indicating solvent polarity effects on the electronic structure. These results are compared with those previously obtained for two other push-pull chromophores, p-nitroaniline and julolidinyl-n-N,N′-diethylthiobarbituric acid. The frequency dispersion of the molecular first hyperpolarizability, β, is also calculated in each solvent using a time-domain form of the standard Oudar–Chemla two-state model modified to incorporate solvent reorganization, inhomogeneous broadening, and the vibronic structure of the charge-transfer state. We show that accurate extrapolation of β measured at frequencies in the near-infrared to zero frequency requires a realistic description of the excited state as the measuring wavelength approaches a two-photon resonance. This is particularly relevant to the high chromophore concentrations needed for device applications, where intermolecular interactions can strongly perturb the electronic transitions.

Solvent Effects on Ground and Excited Electronic State Structures of the Push-Pull Chromophore Julolidinyl-n-N,N‘-diethylthiobarbituric Acid

Resonance Raman spectra and cross sections of a “push−pull” chromophore containing a julolidine donor and a thiobarbituric acid acceptor have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong visible charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the ∼30 Raman-active vibrations as well as the solvent reorganization energies. Several vibrational modes undergo significant (5−15 cm-1) frequency changes as the solvent is varied, signaling solvent polarity effects on the ground-state electronic structure. The excited-state geometry changes are solvent dependent for some vibrational modes but not for others. The total vibrational reorganization energy decreases, and the solvent reorganization ...

Solvent effects on ground and excited electronic state structures of p-nitroaniline

Resonance Raman intensities of p-nitroaniline, a prototypical “push–pull” chromophore with a large first hyperpolarizability (β), have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong near-ultraviolet charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the five or six principal Raman-active vibrations as well as estimates of the solvent reorganization energies. The total vibrational reorganization energy decreases and the solvent reorganization energy increases with increasing solvent polarity in all solvents except methanol, where specific hydrogen-bonding interactions may be important. The dimensionless normal coordinate geometry changes obtained from the resonance Raman analysis are converted to actual bond length and bond angle changes with the aid of normal mode coefficients from a ground-state density functional theory calculation. The geometry changes upon electronic excitation involve predominantly the Cphenyl–Nnitro, N–O, and phenyl C2–C3 bond lengths, with little involvement of the amino group. Nonresonant Raman spectra in 1,4-dioxane, dichloromethane, ethyl acetate, acetone, acetonitrile, and methanol show only a very small solvent dependence of the vibrational frequencies. This suggests that changing the solvent affects the excited state more than the ground state, calling into question two-state models that treat the ground and charge-transfer excited states as linear combinations of neutral and zwitterionic basis states with solvent dependent coefficients.

Symmetry breaking effects in NO3−: Raman spectra of nitrate salts and ab initio resonance Raman spectra of nitrate–water complexes

Ground-state structures and vibrational frequencies are calculated for complexes of the nitrate anion with one and two water molecules at the ab initio Hartree–Fock level with a basis set including diffuse and polarization functions. Two local minimum geometries are found for each complex. Calculations of the electronically excited states at the CIS level are then used to find the forces on each of the atoms upon vertical excitation to the two lowest-lying (near-degenerate) strongly allowed electronic transitions. These forces are converted to gradients of the excited-state potential surfaces along the ground-state normal modes and compared with the parameters obtained previously from empirical simulations of the experimental resonance Raman intensities of NO3− in dilute aqueous solution. The calculations on two-water clusters agree well with the experimental excited-state geometry changes along the totally symmetric N–O stretch. The calculations underestimate the frequency splitting of the antisymmetric stretching vibration (degenerate in the isolated D3h ion) and the resonance Raman intensity in this mode, suggesting that bulk solvent polarization enhances the asymmetry of the local environment for NO3− in water. Comparison of the ground-state vibrational frequency splitting of the antisymmetric stretch with the corresponding values for the nitrate ion in salts having known crystal structures suggests that the rms difference among the three N–O bond lengths for nitrate anion in water probably exceeds 0.01 Å.

Vibrational analysis and excited-state geometric changes of betaine-30 derived from Raman and infrared spectra combined withab initio calculations

We present a combined experimental and theoretical study of vibrational modes and excited-state geometry changes of betaine-30 (B-30). Infrared and Raman spectra were recorded under electronic off-resonant excitation conditions and also in resonance with the charge transfer transition of B-30. Comparing these spectra with the corresponding vibrational patterns calculated by Hartree–Fock methods we obtained the assignments of the vibrational modes. Excited-state geometry changes were calculated for a smaller model system of B-30 using configuration interaction with single excitations. The calculations predict that the central phenoxide and pyridinium rings move from a twisted conformation in the electronic ground state into a perpendicular position in the first excited state. In addition, the pyridinium ring tilts and the nitrogen atom becomes pyramidalized. In correspondence, we measured two strong low-wavenumber Raman bands with large origin shifts at 291 and 133 cm−1. They were assigned to N-inversion and torsional vibrational modes and are expected to mediate the excited state and back-electron transfer reaction of B-30. Copyright © 2000 John Wiley & Sons, Ltd.

Effects of solvent and cyclodextrin on the photophysical properties of 4-acetylbiphenyl: intramolecular charge transfer associated with hydrogen-bonding effect

The photophysical properties of 4-acetylbiphenyl (ACBP) in various solvents and in aqueous α-cyclodextrin (α-CD) solution have been investigated using steady-state and time-resolved fluorescence spectroscopy. In non-polar solvents, the fluorescence spectrum with an emission maximum at 310 nm exhibits a vibrational structure with a non-mirror image to the absorption spectrum, reflecting the change in excited-state geometry toward the coplanarity of the biphenyl moiety. As the solvent polarity increases, the fluorescence spectrum denotes the structureless normal emission and an additional large Stokes' shift band around 380 nm. This behavior indicates the formation of an intramolecular charge transfer (ICT) state through relaxation from the normal excited state where the excited-state geometry change is hindered. Especially in water, the ICT emission is further red shifted to 400 nm with the normal emission band at 310 nm, and the relative intensities between 310 and 400 nm emission bands are affected by the excitation wavelength. However, this excitation wavelength dependence is not so large in organic polar solvents and aqueous α-CD solutions. The 400 nm emission in water exhibits a single exponential decay with a very short decay time (34 ps) while it shows a bi-exponential or triple-exponential decay in organic solvents and aqueous α-CD solutions. These results suggest that the ICT state in water is stabilized through exciplex formation by the hydrogen-bonding interaction between the acetyl group and water. © 1997 Elsevier Science S.A.

Cyclodextrin effects on excited-state geometry change and intramolecular charge transfer of 4-biphenylcarboxylic acid

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTCyclodextrin effects on excited-state geometry change and intramolecular charge transfer of 4-biphenylcarboxylic acidDae Won Cho, Yong Hee Kim, Seong Gwan Kang, Minjoong Yoon, and Dongho KimCite this: J. Phys. Chem. 1994, 98, 2, 558–562Publication Date (Print):January 1, 1994Publication History Published online1 May 2002Published inissue 1 January 1994https://doi.org/10.1021/j100053a034RIGHTS & PERMISSIONSArticle Views167Altmetric-Citations51LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (583 KB) Get e-Alerts

Excited state geometry changes from preresonance Raman intensities: Isoprene and hexatriene

A method is presented for using a single preresonance Raman spectrum and an absorption spectrum to obtain changes in equilibrium geometry upon electronic excitation. The relative displacements along each of the vibrational normal coordinates are obtained from the Raman intensities, while the overall scaling of the displacements is determined by the absorption band shape. The absorption spectra, as well as Raman excitation profiles, are calculated using either a sum over vibronic states or a formally equivalent time-dependent method [S.-Y. Lee and E. J. Heller, J. Chem. Phys. 71, 4777 (1979)]. The time-dependent method is computationally much faster than the vibronic sum for large multidimensional systems. Our analysis, which assumes isolated molecules and separable, harmonic surfaces, yields a good fit to the vapor phase absorption spectrum of trans-hexatriene with a Lorentzian linewidth of 175 cm−1. However, the diffuse absorption spectrum of isoprene cannot be adequately reproduced using Lorentzian line shapes, even when all 33 normal modes are included. Finite temperature and excited state frequency changes are also found to have little effect on the calculated band shapes. These results suggest that inhomogeneous broadening may be a major factor, but calculations using Gaussian broadening fail to accurately reproduce the experimental spectrum.

Studies on the temperature dependence of methyl anthroate fluorescence

The temperature dependence of the fluorescence quantum hield φ f of methyl-1-, methyl-2- and methyl-9-anthroate was investigated. For the former two esters, the trend of φ f versus temperature is consistent with the mechanism proposed earlier to account for the temperature dependence of φ f for meso-substituted anthracene. In this mechanism, an activated intersystem crossing to a nearby upper triplet is the only significant non-radiative decay mode. The activation energy E A for fluorescence is a measure of the gap between the emitting singlet S 1 and the accepting triplet. Since the former state is more solvent dependent, solvents which lower the S 1 energy increase E A and thereby increase φ f. Thus E A and φ f are inversely related to the S 1 energy and directly related to each other. The situation for methyl-9-anthroate is quite different from that of the other esters. While E A is inversely related to the S 1 energy, φ f is not, and hence there is no correlation between E A and φ f. It appears that the φ f of methyl-9-anthroate is more dependent on the nature of the solvent than on the measured E A in that solvent. The inclusion of a temperature-independent non-radiative decay mode in the analysis does not produce results consistent with the above mechanism. It is suggested that the difference in behavior for these esters may in some way be related to an excited state geometry change which occurs for methyl-9-anthroate but not for the other two compounds.

Spectral Studies on Aromatic Esters of 9-Anthroic Acid

Abstract Substitution of a carboxyl group onto the 9 position of anthracene, as in 9-methyl anthroate (9-COOMe), results in some significant differences in the spectral properties of the ester relative to those of the anthracene parent molecule. For example, the fluorescence quantum yield (⊘f) and fluorescence maximum (υmax) of 9-COOMe become quite solvent dependent due to an excited state geometry change in which the excited singlet takes on significant charge transfer character (1,2).

Vacuum ultraviolet absorption spectra of dimethylsulfide, dimethylselenide, and dimethyltelluride

The vapor phase absorption spectra of dimethylsulfide, dimethylselenide, and dimethyltelluride are reported for the 40 000–80 000 cm−1 vacuum ultraviolet spectral region. Three Rydberg series for each compound were assigned which converge on the ionization potentials of 8.706 ± 0.010, 8.400 ± 0.010, and 7.926 ± 0.010 eV for dimethylsulfide, dimethylselenide, and dimethyltelluride, respectively. These ionization potentials and corresponding Rydberg series members are assigned as originating from the central atom valence p orbital, which is perpendicular to the plane of the molecule. A correlation of the ionization potentials calculated for these molecules to ionization potentials of corresponding rare gases is presented. The term values of Rydberg members of the dimethyl chalcogens are also correlated to the term values of allowed atomic Rydberg series in the rare gases, thus atomic nomenclature is used in the assignment of the molecular Rydberg series. Higher energy absorptions in the spectra of dimethylselenide and dimethyltelluride are assigned as originating from a nonbonding orbital of a1 symmetry localized on the central atom. In the three spectra, vibrational progressions are analyzed with respect to ground state totally symmetric vibrational frequencies. Utilizing Franck-Condon factors and harmonic oscillator potentials, the excited state geometry changes were found to be similar, where the C–X–C (X=S, Se, or Te) bond angles are essentially unchanged and the C–X bond distances are changed approximately 0.040 Å from the ground state bond lengths for all three compounds.