Vibronic Absorption Spectra Research Articles

Numerical Tests of a Fixed Vibrational Basis/Gaussian Bath Theory for Small Molecule Dynamics in Low-Temperature Media

A recently framed quantum/semiclassical treatment for the internal nuclear dynamics of a small molecule and the induced small-amplitude coherent motion of a low-temperature host medium (Chapman, C. T.; Cina, J. A. J. Chem. Phys.2007,127, 114502) is further analyzed and subjected to initial tests of its numerical implementation. In the illustrative context of a 1D system interacting with a 1D medium, we rederive the fixed vibrational basis/gaussian bath (FVB/GB) equations of motion for the parameters defining the gaussian bath wave packet accompanying each of the energy eigenkets of the quantum mechanical system. The conditions of validity for the gaussian-bath approximation are shown to coincide with those supporting approximate population conservation. We perform initial numerical tests of the FVB/GB scheme and illustrate the semiclassical description it provides of coherent motion in the medium by comparing its predictions with the exact results for a high-frequency system harmonic oscillator bilinearly coupled to a lower-frequency bath oscillator. Linear vibronic absorption spectra or, equivalently, ultrafast wave packet interferometry signals are shown to be readily and accurately calculable within the FVB/GB framework.

(TD-)DFT Calculation of Vibrational and Vibronic Spectra of Riboflavin in Solution

The photophysics and photochemistry of flavin molecules are of great interest due to their role for the biological function of flavoproteins. An important analysis tool toward the understanding of the initial photoexcitation step of flavins is electronic and vibrational spectroscopy, both in frequency and time domains. Here we present quantum chemical [(time-dependent) density functional theory ((TD-)DFT)] calculations for vibrational spectra of riboflavin, the parent molecule of biological blue-light receptor chromophores, in its electronic ground (S(0)) and lowest singlet excited states (S(1)). Further, vibronic absorption spectra for the S(0) --> S(1) transition and vibronic emission spectra for the reverse process are calculated, both including mode mixing. Solvent effects are partially accounted for by using a polarizable continuum model (PCM) or a conductor-like screening model (COSMO). Calculated vibrational and electronic spectra are in good agreement with measured ones and help to assign the experimental signals arising from photoexcitation of flavins. In particular, upon photoexcitation a loss of double bond character in the polar region of the ring system is observed which leads to vibronic fine structure in the electronic spectra. Besides vibronic effects, solvent effects are important for understanding the photophysics of flavins in solution quantitatively.

Application of quantum mechanical method to analysis of the intensity distribution in spectra of resonant hyper-Raman scattering of low-symmetry molecules

The quantum-mechanical method of calculating the relative intensities of lines in the spectra of resonant hyper-Raman scattering of polyatomic molecules in the Herzberg-Teller approximation is tested with respect to low-symmetry molecules. The method makes it possible to describe resonant Raman and hyper-Raman scattering spectra, as well as vibronic absorption spectra, from the same viewpoint based on a common set of parameters. The particular features of the implementation of the method are discussed based on the calculation of the spectra of resonant hyper-Raman scattering of chlorobenzene and adenine, and the expedience of the application of the method is illustrated. Satisfactory agreement is obtained between calculation results and available experimental data.

Vibronic Transitions and Quantum Dynamics in Molecular Oligomers: A Theoretical Analysis with an Application to Aggregates of Perylene Bisimides

Vibronic absorption spectra of molecular aggregates consisting of up to N = 9 monomer units are calculated employing methods of time-dependent quantum mechanics. Taking one vibrational degree of freedom for each monomer into account and treating one-exciton excited electronic states leads to a problem with N vibrations and N electronically coupled states. The demanding quantum propagation is carried out within the multiconfiguration time-depended Hartree method (MCTDH). Spectral features of and population transfer in the aggregates are analyzed as a function of the aggregate size and the strength of the electronic coupling. With a model for oligomers of perylene bisimides, it is shown how measured temperature-dependent absorption spectra correlate with the aggregate size. Furthermore, the exciton localization and dynamics in these aggregates are investigated.

Vibronic spectra, ab initio calculations, and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states. Part I: Reanalysis of the 3680Å and 4100Å absorption systems of oxalyl chloride

The vapor-phase absorption spectrum of oxalyl chloride in the 3000–4180 Å region has been re-examined at high resolution. Singlet–singlet A ˜ 1 A u ← X ˜ 1 A g and singlet–triplet a ˜ 3 A u ← X ˜ 1 A g electronic transitions of the trans- conformer found in the spectrum are in agreement with earlier works [W.J. Balfour, G.W. King, J. Mol. Spectrosc. 26 (1968) 384–397; ibid. 27 (1968) 432–442]. Torsion levels of trans -oxalyl chloride in the ground X ˜ 1 A g and excited A ˜ 1 A u and a ˜ 3 A u states were found for the first time. Ab initio calculations of structures for conformers of oxalyl chloride in the ground and lowest excited electronic states explain the absence of second conformer transitions in the vibronic absorption spectrum.

Semiclassical on-the-fly computation of the S→S1 absorption spectrum of formaldehyde

The anharmonic S(0)-->S(1) vibronic absorption spectrum of the formaldehyde molecule is computed on the fly using semiclassical dynamics. This first example of an on-the-fly semiclassical computation of a vibronic spectrum was achieved using a unit prefactor modified frozen Gaussian semiclassical propagator for the excited state. A sample of 6000 trajectories sufficed for obtaining a converged spectrum, which is in reasonable agreement with experiment. Similar agreement is not obtained when using a harmonic approximation for the spectrum, demonstrating the need for a full anharmonic computation. This first example provides a resolution of approximately 100 cm(-1). Potential ways of improving the methodology and obtaining higher resolution and accuracy are discussed.

Microsolvation Effects on the Optical Properties of Crystal Violet

We present a joint experimental and theoretical study of the photoabsorption and photodissociation behavior of crystal violet, that is, the tris[p-(dimethylamino)phenyl]methyl cation. The photodissociation spectra of isolated and microsolvated crystal violet have been measured. A single band is observed for the bare cation. This is in good agreement with the calculated vibronic absorption spectrum based on time-dependent density functional theory calculations. The interaction of crystal violet with a single water molecule shifts and broadens the photodissociation spectrum, so that it approaches the spectrum obtained in solution. Theoretical calculations of the structure of the complex suggest that the shift in the absorption spectrum originates from a water molecule bonding with the central carbon atom of crystal violet.

Correlations of instantaneous transition energy and intensity of absorption peaks during molecular vibration: toward potential hyper-surface

Time-resolved spectrum after ultrashort pulse excitation revealed fine structure of instantaneous vibronic absorption spectra in a thiophene derivative. The probe photon energy-dependent amplitudes of molecular vibration coupled to the induced absorption were composed of several peaks. An absorbance-change peak-tracking method revealed four vibronic transitions buried in the time-integrated spectra over several vibrational periods of typical molecular vibration. Four vibronic transitions located at 2.024, 1.921, 1.818 and 1.731 eV were found to be correlated among themselves with respect to the photon energies and intensities of the peaks in the difference absorbance change spectra. From the size and sign of the correlation strengths the mechanism of the vibronic coupling was related to non-Condon mechanism and Herzberg–Teller vibronic coupling.

Vibronic spectra of charge-transfer excitons in donor–acceptor molecular crystals

The linear absorption spectra of a molecular stack of alternatingly and regularly arranged donor and acceptor (D, A) molecules have been studied theoretically for the vibronic spectral regions. The vibronic excitations consist of charge-transfer excitons (CTEs) plus one or two quanta of the intramolecular vibrational mode of the D (A) molecule. The model of CTEs on a DA molecular stack considered by Haarer, Philpott and Morawitz is extended by including the linear and quadratic coupling between CTEs and intramolecular vibrations and represents the basis for the theoretical calculations of the linear optical susceptibility and of the absorption coefficient using the methods of Green’s functions and of canonical transformations. The parameters of the CTEs and of the vibrations obtained in the experimental studies of the absorption, reflection and electroreflectance spectra of the anthracene-PMDA crystal have been introduced in the numerical simulations of the absorption coefficients in pure excitonic, in one- and in two-phonon vibronic spectra. In the case of a relatively weak transfer of the components of the CTEs (of electrons between acceptors and holes between donors) the vibronic spectra will manifest the bound (one-particle) exciton–phonon states only. In the case of stronger transfer, the vibronic absorption spectra will be modified by the impact of many-particle (unbound) exciton–phonon states and the absorption curve will be asymmetrical and broadened (non-Lorentzian). The calculated derivative of the absorption coefficient gives information on vibronic states in both cases of weak and strong transfer.

The S 1 ← S 0 vibronic spectra and structure of the (CH3)2 CHCHO and (CH3)2 CHCDO 2-methylpropanal molecules

A multipass cell with an optical path up to 120 m long was used to measure the vibronic absorption spectra of 2-methylpropanal-h1 (MPA-h1, (CH3)2CHCHO)) and 2-methylpropanal-d1 (MPA-d1, (CH3)2CHCDO)) over the frequency range 28200–31600 cm−1. The most intense spectral lines were assigned to transitions from vibrational levels of the cis and gauche MPA-h1 and MPA-d1 conformers in the ground electronic state (S 0) to vibrational levels of conformers 1 and 3 in the lowest singlet excited electronic state (S 1). According to our estimates, the origins (0 0 0 ) of the 1 S 1) ← cis(S 0) and 3(S 1) ← cis(S 0) and also 1(S 1) ← gauche(S 0) and 3(S 1) ← gauche(S 0) electronic transitions were situated at 29147 and 29177, 29391 and 29417 cm−1, respectively, for MPA-h1 and at 29226 and 29240, 29480 and 29500 cm−1 for MPA-d1. The structure of conformers 1 and 3 in the S 1 state was shown to differ from the structure of the cis and gauche conformers in the S 0 state by the angle of rotation of the (CH3)2CH-isopropyl top and “pyramidal distortion” of the CCHO/CCDO carbonyl fragment. A series of fundamental frequencies of MPA conformers in different electronic states were found. The potential functions of inversion were determined for the conformer 1-conformer 3 pairs of MPA-h1 and MPA-d1 from the experimental energy levels of inversion vibrations. The potential barriers to inversion and equilibrium displacements of the CH/CD bond out of the CCO plane were found to be 735/675 cm−1 and ±34°/±32° for MPA-h1 and MPA-d1, respectively.

Electronic spectra of cationic forms of meso-tetrapropylporphin in a nanoporous silicate gel matrix

We have studied the fluorescence and fluorescence excitation spectra at 300 K, 77 K, and 4.2 K for silicate gel matrices colored with meso-tetrapropylporphin by impregnation of the matrix with a solution of the pigment. Comparison of the data obtained with the absorption spectra in acidified solutions and analysis of the low-temperature fine-structure vibronic spectra, and also taking into account data obtained earlier for octaethylporphin in a xerogel showed formation of two cationic forms of meso-tetrapropylporphin in the gel matrix: the short-wavelength form has a dicationic structure, while the long-wavelength form has a monocationic structure. We have traced out the correlations of the vibrational structure in the spectra of the dicationic form with data for the porphin dication, and we have drawn a number of conclusions concerning the normal vibrational modes that are active in the vibronic fluorescence and absorption spectra of the studied cationic forms. Using the AM1 semiempirical quantum chemical method, we optimized the geometry of the mesotetrapropylporphin dication: the most stable of the possible conformers is the dication structure with saddleshaped macrocycle nonplanarity.

Effect of a solid substrate on orientational ordering and the process of associate formation in thin interlayers of solutions of some organic compounds

We have studied the vibronic absorption spectra of thin submicron interlayers of anisole and nitrobenzene solutions as a function of the concentration and thickness of the interlayer. We have observed isosbestic points in a series of spectra obtained for different concentrations and thicknesses. This in turn is due to the existence in the system of two types of absorbing centers, which we connect with monomers and molecular associates (dimers). We discuss the relative contribution of different types of interactions leading to dimer formation.

Vibronic absorption, fluorescence, and phosphorescence spectra of psoralen: a quantum chemical investigation

Excited state potential energy hypersurfaces of 7H-furo[3,2-g][1]benzopyran-7-one (psoralen) have been explored employing (time-dependent) Kohn-Sham density functional theory. At selected points, we have determined electronic excitation energies and electric dipole (transition) moments utilizing a combined density functional/multireference configuration interaction method. Spin-orbit coupling has been taken into account employing an efficient, non-empirical spin-orbit mean-field Hamiltonian. Franck-Condon factors have been computed for vibrational modes with large displacements in the respective Dushinsky transformations. The simulated band spectra closely resemble experimental band shapes and thus validate the theoretically determined nuclear structures at the S(0), S(1), and T(1) minima. In the S(1) (pi(HOMO)-->pi*(LUMO)) state, the lactone bond of the pyrone ring is significantly elongated. From excited vibrational levels of the S(1) state a conical intersection between a (pi-->sigma*) excited state and the electronic ground state may be energetically accessible. Fast non-radiative decay via this relaxation pathway could explain the low fluorescence quantum yield of psoralen. The T(1) (pi(HOMO-1)-->pi*(LUMO)) exhibits a diradicaloid electronic structure with a broken C(5)-C(6) double bond in the pyrone ring. A variational multireference spin-orbit configuration interaction procedure yields a phosphorescence lifetime of 3 s, in excellent agreement with experimental estimates.

Exciton dissociation dynamics in model donor-acceptor polymer heterojunctions. I. Energetics and spectra

In this paper we consider the essential electronic excited states in parallel chains of semiconducting polymers that are currently being explored for photovoltaic and light-emitting diode applications. In particular, we focus upon various type II donor-acceptor heterojunctions and explore the relation between the exciton binding energy to the band offset in determining the device characteristic of a particular type II heterojunction material. As a general rule, when the exciton binding energy is greater than the band offset at the heterojunction, the exciton will remain the lowest-energy excited state and the junction will make an efficient light-emitting diode. On the other hand, if the offset is greater than the exciton binding energy, either the electron or hole can be transferred from one chain to the other. Here we use a two-band exciton to predict the vibronic absorption and emission spectra of model polymer heterojunctions. Our results underscore the role of vibrational relaxation and suggest that intersystem crossings may play some part in the formation of charge-transfer states following photoexcitation in certain cases.

Dye aggregates formed in Langmuir–Blodgett films of amphiphilic merocyanine dyes

Abstract The electronic absorption spectra of pure Langmuir (L) films of 6MeDS prepared upon MgCl2, CaCl2, CdCl2, and CuCl2 aqueous solutions have shown that the Mg2+, Ca2+, and Cd2+ cations promote the J-aggregation of the dye, but the Cu2+ cation inhibits it. Instead, the Cu2+ cation promotes the formation of another type of dye aggregate (D-aggregate), which exhibits a split electronic absorption band. These L films have been transferred onto solid substrates by the Langmuir–Blodgett (LB) technique. The vibronic absorption spectra observed for the obtained LB films have shown the similarity between the metal–cation-containing J-aggregate and the metal-free J-aggregate of the dye, the latter of which was characterized in the previous study [K. Ikegami, J. Chem. Phys. 121 (2004) 2337]. These spectra have also indicated that the bimolecular metal chelation plays an important role in the J-aggregation, like the intermolecular hydrogen bonding in the metal-free case.

Vibronic energies and spectra of molecular dimers

We consider three distinct methods of calculating the vibronic levels and absorption spectra of molecular dimers coupled by dipole-dipole interactions. The first method is direct diagonalization of the vibronic Hamiltonian in a basis of monomer eigenstates. The second method is to use creation and annihilation operators leading in harmonic approximation to the Jaynes-Cummings Hamiltonian. The adiabatic approximation to this problem provides insight into spectral behavior in the weak and strong coupling limits. The third method, which serves as a check on the accuracy of the previous methods, is a numerically exact solution of the time-dependent Schrodinger equation. Using these methods, dimer spectra are calculated for three separate dye molecules and show good agreement with measured spectra.

Theoretical investigation of ground and excited states of the methylene amidogene radical (H(2)CN).

The excited states and the absorption spectrum of the methylene amidogene radical are studied by high-level ab initio calculations. The multireference configuration interaction method was used in combination with different basis sets and basis set extrapolation to compute equilibrium geometries, harmonic frequencies, and excitation energies of the four lowest doublet electronic states of the title species. Potential curves and transition dipole moment functions were determined along the normal mode coordinates of the electronic ground state. These functions were employed to determine vibronic absorption spectra. The intensities of dipole forbidden but vibronically allowed transitions were calculated by explicitly evaluating integrals over the vibrational wave functions and the transition dipole functions of the involved electronic states. By this method the oscillator strengths of the dipole allowed (2)A(1)<--(2)B(2) and the dipole forbidden (2)B(1)<--(2)B(2) bands were computed. It turns out that the dipole forbidden transition is two orders of magnitude weaker than the dipole allowed one. The 0-0 excitation energies are found to be 30 256 cm(-1) for the (2)B(1) state and 34,646 cm(-1) for the (2)A(1) state. From the combined results of the excitation energies and oscillator strengths it is concluded that the experimentally observed peaks must be due to the (2)A(1) state, in contradiction to earlier assignments.

Pulsed discharge nozzle cavity ringdown spectroscopy of cold polycyclic aromatic hydrocarbon ions

The gas-phase electronic absorption spectra of the naphthalene (C10H8+) and acenaphthene (C12H10+) cations have been measured in the visible range in a free jet planar expansion in an attempt to collect data in an astrophysically relevant environment. The direct absorption spectra of two out of four bands measured of the gas-phase cold naphthalene cation along with the gas-phase vibronic absorption spectrum of the cold acenaphthene cation are reported for the first time. Direct absorption spectra of their van der Waals complexes with argon are also reported for the first time. The study has been carried out using the ultrasensitive and versatile technique of cavity ringdown spectroscopy (CRDS) coupled to a pulsed discharge slit nozzle (PDN). The new PDN-CRDS set up is described and its characteristics are evaluated. The direct-absorption spectra of the polycyclic aromatic hydrocarbon (PAH) ions are discussed and compared to the gas-phase and solid-phase data available in the literature. The analysis of the results show that cold, free flying PAH ions are generated in the argon discharge primarily through soft Penning ionization. This enables the intrinsic band profiles to be measured, a key requirement for astrophysical applications.

Infrared vibronic absorption spectrum and spin–orbit calculations of the upper spin–orbit component of the Au3 ground state

Laser-ablated gold co-deposited with excess neon at 3.5 K produces a new sharp 2025.5-cm−1 absorption. Annealing to 8 K increases this absorption ten-fold and produces six weaker associated vibronic bands with 172- and 118-cm−1 intervals. Relativistic configurations (RCI) computations are carried out for several electronic states lying below 4.4 eV. These calculations show that the spin–orbit effect stabilizes the D3h structure and quenches Jahn–Teller distortion. They also predict a 0.2-eV spin–orbit splitting of the Au3 ground state, which is in excellent agreement with the 2025.5-cm−1 neon matrix band origin. We have also confirmed the assignment of the A-X system at 1.8 eV observed by Bishea and Morse. The observed vibronic intervals are in accord with calculated a1′ and e′ stretching fundamentals and they confirm the D3h geometry for Au3. This work reports the first observation of the ground-state spin–orbit splitting of a heavy metal trimer.

Luminescence and absorption spectra of polyatomic molecules subjected to conformational transitions

A general approach and results of specific calculations of conjugated vibronic absorption and fluorescence spectra of “soft” molecules subjected to conformational transitions are presented. The vibronic spectra of these molecules exhibit significant anomalies such as a large energy gap between the fluorescence and absorption spectra, a strong deviation from the mirror symmetry, etc., which cannot be explained within the framework of the theory of vibronic spectra based on single-well adiabatic potentials.