Davydov Components Research Articles

Exciton coupling in oligothiophenes: A combined experimental/theoretical study

The influence of exciton coupling on the optical properties of oligothiophenes is studied by means of optical and circular dichroism (CD) spectroscopy. The CD spectrum of a model compound, formed by two terthienyl chromophores arranged in a chiral orientation, displays two bands with opposite signs that can be related to the two Davydov components of the first optical transition. From correlated quantum-chemical calculations, the Davydov splitting is estimated to be on the order of 0.15-0.2 eV.

Polarized fluorescence in α-sexithienyl single crystal at 4.2 K

The polarized fluorescence spectra of the bc plane of α-sexithienyl (αT6) single crystal at T=4.2 K have been measured and analyzed. The spectra show a sharp component superimposed on a broad component. The origin of fluorescence at 18332 cm−1 with a rather narrow linewidth (FWHM=8 cm−1) is completely polarized along b. We assign it to a shallow X-trap 18 cm−1 below the bottom of the 1 1Bu exciton band. The 1 1Bu molecular electronic excited level splits in the crystal in four Davydov components (ag, bg, au, and bu) of which au and bu are dipole allowed. The calculated exciton band structure based on the Ewald sums, assuming the point dipole–dipole approximation, predicts two lowest degenerate Davydov components: an au b polarized component and a forbidden ag component. The fluorescence with the intrinsic sharp vibronic progressions is analyzed in terms of ground state totally symmetric modes which are in excellent agreement with Raman scattering data of the single crystal. Sharp c polarized false origins are identified at 18 167, 18 084, and 18 026 cm−1 and discussed either in terms of X-traps or Herzberg–Teller vibronic coupling with the second higher 2 1Bu molecular level polarized along the in plane short axis. A broad c polarized component is attributed to aggregates.

Theoretical study on the absorption spectra of pseudoisocyanine bromide (PIC-Br) molecular J-aggregates

The linear dichroic absorption spectrum of pseudoisocyanine bromide (PIC-Br) molecular J-aggregates is theoretically analyzed by using the dynamical coherent potential approximation and numerical calculations. The absorption spectra with the electric field polarization parallel and perpendicular to the one-dimensional axis of the oriented J-aggregates are well explained in terms of Davydov components. Above the one-phonon continuum in the dispersion curve of the exciton–phonon system, splitting off of an exciton polaron band is observed. This exciton polaron state is assigned to the largest peak in the case of the latter polarization. The absorption intensity differences between the lowest peak and the 2nd peak observed in two polarization conditions are simply explained by the character of relevant states.

Relaxation dynamics of excitons in J-aggregates revealing a two-component Davydov splitting

The J-aggregates of the thiacarbocyanine dye THIATS revealing a clear-cut two-component Davydov splitting have been investigated by different optical methods. The presence of two molecules per unit cell was proved both by polarised fluorescence and linear dichroism in the absorption of aligned aggregates in a rotating cell. The fluorescence excitation spectrum of the aggregates and their quantum yield (QY) were measured across the entire exciton band. The QY was found to be 0.1 and 0.4 for excitation via the upper and lower Davydov components respectively. The radiative lifetime of emission of the J-band was estimated to be about 15 ns combining fluorescence decay data with the fluorescence quantum yield. The excitation intensity dependence of the fluorescence decays suggests the occurrence of exciton–exciton annihilation (EEA). EEA occurs even at an excitation photon fluxes below 10 10 photons/(pulse cm 2) corresponding to less than one absorbed photon per 10 6 monomer units.

Excitonic and Molecular Properties of the Triplet T 1-State in Diphenylpolyene Single Crystals

Excitonic properties of the excited triplet state in single crystals of diphenylpolyenes all-trans(1, 4-diphenyl)-1, 3-butadiene (DPB) and all-trans(1, 6-diphenyl)-1, 3, 5-hexatriene (DPH) are derived from T 1←S 0 photoexcitation spectra at low temperatures. The 0-0-transition is split into 2 (DPH monoclinic) and 4 (DPB orthorhombic) Davydov components in zero-field. Measurements with an applied external magnetic field made accessible the respective fine-structure components and their assignment to the principal axes of the fine-structure tensor. In the case of DPB orthorhombic the determination of the excitonic D* and E* fine-structure parameters was possible due to the small linewidths occuring in the spectra. The intensity ratios of the Davydov components were evaluated for different polarizations of the excitation light with respect to the crystal axes. The conclusion is drawn that in both DPH and DPB the transition moment is oriented mainly perpendicular to the molecular plane and that the total symmetry of the T 1-state is Au . The vibrational structure in the photoexcitation spectra was assigned by using selectively deuterated crystal samples for comparison. Since the vibrations involving the phenyl endgroup are absent in the spectra of DPH and carry only very weak intensity in the spectra of DPB it is concluded that the T 1-excitation energy is localized mainly on the polyene chain.

Vibronic coupling with low-frequency vibrations in quasi-forbidden Frenkel exciton states

A simple dimer model is used to demonstrate that in weakly allowed Frenkel exciton states, where the factor-group splitting is largely due to charge transfer interactions, the exciton-phonon coupling constant may be considerably different for the two Davydov components. The model is applied to the S 1 transition of the naphthalene crystal and suggests a smaller Davydov splitting than normally assumed (60 cm −1 rather than 150 cm −1).

Vibron dynamics in naphthalene crystal

The polarized infrared spectrum of naphthalene single crystal has been studied between 800 and 1600 cm−1. The dependence on temperature of the linewidth of eight Davydov components of internal modes has been studied in the 10–240 K temperature range. The deconvolution of the experimental line shapes allows us to extract a constant inhomogeneous contribution to the line broadening, due probably to crystal defects, over all the temperature range. The bandwidth data are interpreted on the basis of elementary relaxation mechanisms. In particular, at low temperature, the vibrational decay is dominated by three-phonon depopulation processes. Above 80 K, higher order dephasing mechanisms generally contribute to the vibron dynamics. The calculated decay compares well with experimental results. The infrared results are compared with the relaxation data presented for the Raman modes in previous papers.

Magnetic Davydov splitting in 2D AFM (CH2)2(NH3)2MnCl4 (abstract)

Recently it was shown that the light absorption spectrum of (CH2)2(NH3)2MnCl4 (EDAMnCl4) differs strongly from those of other compounds of this family (CH2)n(NH3)2MnCl4. Additional peculiarities in the spectral distributions of two-magnon Raman scattering and the coefficient of exciton-magnon absorption were found. It was also determined that the pure exciton band in the absorption spectrum is of the electric dipole type of polarization. All these peculiarities are out of the framework of the usual tetragonal two-sublattice antiferromagnet model and indicate the inverse center loss and magnetic cell doubling. Results of studying the absorption spectrum of EDAMnCl4 presented in the report show that not less than three Davydov components of the exciton band σA1g(σS)→4Eg(4G) optical transition may be detected with confidence in magnetic fields oriented along the c axis. It confirms the four-sublattice magnetic structure of EDAMnCl4. The presence of exciton line splitting at H=0 and its nonlinear character at H∥α (that is practically parallel to l) allows us to make a conclusion about the resonance interaction between ions with antiparallel spins and, probably, about the initial noncollinearity of magnetic structure. Parameters characterizing the magnetic Davydov splitting are determined using comparison of the experimental data and theoretical analysis.

Optical properties of a streptocyanine dye single crystal with tubelike aggregation: bis(dimethylamino)-heptamethinium tetrafluoroborate BDH+BF−4

Normal incidence reflectance spectra from different faces of an optically biaxial dye single crystal, bis(dimethylamino)-heptamethinium tetrafluoroborate, have been measured for two orthogonal polarizations of light. The spectra are very anisotropic with a wide band of quasi-metallic reflectance for one polarization of light. Strong coupling of light to electronic states leads to splitting of transverse and longitudinal excitons as large as 1.5 eV, resulting in strong directional dispersion of the spectra. The data were analyzed by Kramers-Kronig transformation of spectra from appropriate faces and checked by comparison of solutions of the wave equation and the experimental spectra. Electronic transition energies of two Davydov components were found at 2.07 and 2.18 eV shifted by 0.32 and 0.24 eV with respect to absorption spectra of molecules in solution. The results are compared with those obtained from the chloride of the same dye molecule.

Fermi resonance states in the natural isotopically mixed CS2 crystal

High-resolution Raman spectroscopy (HRRS) is used to analyze the anharmonic interactions between the modes ν1 and ν2 in the CS2 crystal and its natural isotopic aggregates. Statistics of aggregates (mainly formed with molecules containing 34S and 33S isotopes) are calculated. Using the exciton theory formalism, a unified description of the Fermi resonance going from the molecule to the crystal is provided and used to calculate the various eigenmodes of aggregates. HRRS allows an accurate analysis, in the ν1 frequency domain, of the complex band structure due to the various configurations of isotopic aggregates (monomers, dimers, trimers,...). In the crystal, the weak Fermi resonance interaction (ν1, 2ν2 ) leads to the appearance of bound states ν+ and ν−. HRRS allows one to clearly separate the two Raman active Davydov components (Ag and B3g ) in both the ν− and ν+ bands. Their thermal evolution is also given and compared to other incoherent as well as coherent Raman data.

Vibrational relaxation of Davydov components of the 940 cm −1 mode in KClO 4 crystal

We report on the temperature dependent linewidth of the B 3u component of gn 1 mode in KClO 4 crystal measured by FTIR spectroscopy. These results are compared with previous ones obtained for the A g component. At temperatures higher then 80 K both components have a linear dependence on T showing that three-phonon scattering relaxation processes are driving the decay of this mode. The difference in the residual linewidths at low temperature (20 K) is interpreted in terms of different anharmonic couplings between the two componens and the thermal bath.

Electroabsorption study of vacuum-deposited films of thionaphthenindole

Electric-field modulated absorption spectra in the range 270-380 nm have been studied in polycrystalline layers of thionaphthenindole (TNI) formed by vapour deposition onto room-temperature quartz substrates. The results allowed to refine several weak absorption transitions: (i) in addition to well-resolved vibronic satellites two Davydov components of the first 0-0 transition in the long-wavelength region were distinguished, (ii) the involvement of charge-transfer excitons resulted from electroabsorption behaviour in the short-wavelength region. The revealed nature of the excited states of crystalline TNI confirms previous suggestions that intrinsic photogeneration of charge carriers in the near-electrode layers within S 0→S 1 transition proceeds via dissociation of Frenkel excitons on specific structural defects. The large values of the electroabsorption at the fundamental frequency of an applied field indicate the existence of a high degree of the orientational order in the layers.

High resolution Raman loss spectra of solid α-nitrogen and of matrix-isolated molecules

Stimulated Raman spectroscopy (SRS) has been used for the first time to record spectra of cold molecular crystals of N 2 and of matrix-isolated N 2 and CO at 0.003 cm −1 resolution. For solid α-phase N 2, accurate vibrational frequencies of the two Davydov components favor one of several proposed intermolecular potential models. The linewidth of the 0.74% 15N 14N impurity shows that the spread of site energies in the disordered crystal is less than 0.033 cm −1. Matrix studies of a few percent CO in N 2 and N 2 in argon show broader lines than in α-N 2, due in part to overlapping contributions of dimers and larger clusters in the matrix.

Wavelength‐selective resonance Raman spectroscopy of a finite polyene: Solid‐state all‐trans‐β‐carotene

AbstractTwo samples of solid all‐trans‐β‐carotene which differed in their degrees of crystallinity were studied using visible absorption and resonance Raman spectroscopy.The absorption spectrum of a commercial crystalline β‐carotene sample revealed two bands at 18 450 and 20 040 cm−1 which were not present in the absorption spectrum of a prepared amorphous sample. Instead, the spectrum of the amorphous material showed a band at 19 268 cm−1 which is in a position approximately half way between the 18 450 and 20 040 cm−1 bands. From these observations these two latter bands are assigned to the Davydov components of the crystalline β‐carotene.The resonance Raman spectrum of crystalline all‐trans‐β‐carotene revealed several frequencies for the backbone CC stretching mode, νcc, as the laser excitation wavelength was varied. This frequency was observed to vary between 1516 and 1525 cm−1. The amorphous carotene sample also exhibited a wavelength‐dependent Raman spectrum where νcc ranged from 1516 to 1522 cm−1 initially. On further exposure to laser radiation (30–60 min) at 514.5 nm, the range increased to an upper limit of 1525 cm−1, which was attributed to a photochemical product formed in both the crystalline and amorphous samples. The wavelength‐dependent νcc frequency of these carotene samples is interpreted as being due to resonance‐enhanced Raman bands of β‐carotene molecules with different effective double bond chain lengths originating from separate micro‐domains in the solid samples.

Vibrational spectra of crystalline organic sulphides: dimethyl sulphide and diethyl sulphide

The vibrational spectra of crystalline dimethyl sulphide and diethyl sulphide have been measured at 77 K. The comparison with the liquid-state spectra has provided the correlation between the normal motions of the free molecules and those of the molecules in the Bravais unit.All Davydov components in the solid-state spectra have been assigned with the aid of polarized infrared radiation, and the analysis of the unit-cell symmetry of both molecules has been derived.

Temperature dependence of the Raman spectrum of tetraphosphorus triselenide

The Raman spectrum of P 4Se 3 has been measured as a function of temperature in the range 20–370 K, at ambient pressure. The internal and external first-order Raman-active phonons vary smoothly in wave number over the temperature interval 20–350 K, but the crystalline-plastic phase change at 355 K. results in the loss of the external mode manifold from the 370 K Raman spectrum, leaving only a wing to the Raleigh-scattered component. The temperature coefficients of most external phonons are larger than those of the energetically well-separated internal phonons. The lowest wave number rigid layer phonon displays soft behaviour (d ṽ/d T positive) whereas the remaining members of the external mode manifold have negative temperature coefficients which increase in magnitude approximately as the phonon wave number increases. Some of the internal mode Davydov components show near-zero or positive temperature coefficients, a behaviour similar to that observed for other chalcogenide semiconductors.

Spin-spin and spin-lattice relaxation in the triplet state ofAB-pairs in isotopically mixed crystals of naphthalene

Electron spin echo techniques in combination with laser flash excitation have been used to study the spin relaxation processes in the triplet state of inequivalent naphthalene dimers in zero field down to 0·35 K. The spin-spin relaxation is dominated by a fast spin-lattice relaxation induced by a thermally activated exchange process between the two Davydov components. The technique allows us to obtain selectively S 1 ← S 0 excitation spectra which show that energy transfer is slow compared to the spin relaxation rates.

Triplet States in Isotopically Mixed Naphthalene Crystals: High Resolution Optical Spectroscopy

The triplet 0.0 transitions (T1↔ S0) of guest and host in various isotopically mixed naphthalene crystals, with guest states below and above the host exciton band, are investigated applying both laser excitation spectroscopy and phosphorescence emission spectroscopy. Guest monomer and aggregate spectra are analyzed at helium temperatures (1.6-4.2 K) with regard to line positions, splittings, polarizations and intensities. Shift and broadening of the host Davydov components are studied as a function of the guest concentration (at 1.6 K). In particular, the analysis provides precise values for the individual isotopic shifts U0 and the exciton band structure parameters including the critical isotopic shift. Site splitting is spectrally resolved for several monomers and also for the AB-dimer of 1,4-Nd2 in Nd8

13C isotope shifts of anthracene and naphthalene isomers in triplet state optical spectra

Position-specific 13C isotope shifts in the T 1←S 0 photoexcitation spectra of anthracene and naphthalene crystals containing 13C substituted molecules in natural abundance and of material enriched with various synthesized 13C isomers are compared. From the spectra of matrix isolated A-h10 in A-d10 and N-h8 in N-d8 the individual isotope shifts U 0 of all the 13C monosubstituted isomers are determined (with values U 0 between 0·7 and 3·5 cm-1). An additivity rule is established for the U 0 of the twofold substituted 13C 13C isomers similar to that found for multiple deuteration. Applying these results also the 13C substructure of the excitonic Davydov components in anthracene and naphthalene crystals is understood quantitatively. A correlation between the specific 13C isotope shifts and the triplet state spin density distribution is discussed.

Spectra of direct excitation of phosphorescence in an isotopic mixed naphthalene crystal. High resolution excitation of the two Davydov components of the N- d8 host crystal

Direct excitation spectra of the triplet exciton band of deuterated naphthalene are presented. The high resolution spectra (0.1 cm −1) of the two Davydov components show a substructure that we attribute to the presence in the crystal of natural traps and local strains. A model based on the calculation of the eigenstates of a finite plane of molecules is provided for lineshape simulations. In this work the artificial edge effects introduced by the finite size of the model crystal are eliminated by appropriate boundary conditions. Our method allows us to introduce a random distribution of traps, with variable energies, located inside and outside the exciton band, thus featuring a real crystal. Comparison between calculations and experimental results accounts for the salient effects of the traps on the spectroscopic properties of a mixed crystal.