Study Of The Photoinduced Dynamics Of EDDT And Its Derivatives

Over20years, the π-conjugated sulfur-rich oligomers and polymers such as dithiolene-－tetrathiafulvalenes derivatives have been intensively studied，because they show a variety ofproperties in the preparation of organic optical and electrically conducting materials and have, assuch, been synonymous with the development of molecular organic metals. The introduction ofchalcogen atoms in π-donors is for an increase in dimensionality of the conduction pathway,which was illustrated by the structural characteristics of the bis（ethylenedithio）-tetrathiafulvalenebased superconductors with nearly isotropic conductivity in the donor sheet including theintermolecular S…S network with two-dimensional character. The design of new materials withoptimal performance in oligomers electronics and photonics requires a much better fundamentalunderstanding of the nature of electronic excitations and charging transfer phenomena in orderedπ-conjugated materials. It is useful to point out that the electron-transfer process is driven byelectron-electron and electron-vibration interactions. The transition mechanism involvesvibrational motions driving the reaction coordinates from reactants to products. So it has a greatsignificance to study the typical sulfur-rich compound4,5-Ethylenedithio-1,3-dithiole-2-thione（EDDT） and its derivatives.In this paper, we have researched the short-time photo-induced dynamics of EDDT and itsderivative,1,3-dithiole-2-thione（DTT） using the Resonance Raman spectra in combination withDensity Functional Calculations（DFT）.The comparison of them has also been accomplished. Ingeneral, we could summarize our present work as three follows:（1） Resonance Raman spectra of DTT was obtained with341.5,354.7, and368.9nmexcitation wavelengths. Theoretical and experimental absorption spectra,FT-IR,FT-Raman wereacquired simultaneously by our devices and computation. The results indicated that clearly theA-band absorption of DTT was aroused of π（C=S）�'π^*（C=S）electronic transition,which could beexplained by the frontier orbital theory.The resonance Raman spectra of DTT in cyclohexane,acetonitrile and methanol solvent indicated the short-time photo-induced dynamics in theFranck-Condon region occurs mostly along the nominal C=C stretch υ₂in A₁irreducible representative, the C=S stretch（weaker）+H-C=C-H scissor υ₃, accompanied by C=Sstretch（stronger）+H-C=C-H scissor υ₄, the nominal S-C-S symmetric stretch+H-C=C-H scissorυ₆, and the two times frequcency of non-total symmetry S=C-SS out of plane deformationvibrational mode υ₁1in B₁irreducible representative. This indicates besides the total symmetryvibrational modes υ₂, υ₃, υ₄and υ₆, there is one non-total symmetry vibrational mode2υ₁1whichdoes not present in the foundmental region.We can predict that the existence of theFranck-Condon region of DTT has vibronic coupled between the A₁and B₁states. Theappearance of2υ₁1and4υ₁1（out of plane S=CS2deformation） suggests that geometrydeformation or symmetry breaking occurs in the initial S2excited state which is very similar tothe S3state of ethylene or the S3state of methyl xanthate anion.To our surprise,we have foundFermi resonance phenomemon among the C=S stretch（weaker）+H-C=C-H scissor υ₃, C=Sstretch（stronger）+H-C=C-H scissor υ₄, the two times frequcency of nominal S-C-S symmetricstretch+H-C=C-H scissor υ₆when we get a deeper insight of the Resonance Raman spectra.They have the same vibrational mode （symmetry stretch+scissor） and very closefrequency(υ₃,1074cm^-1,υ₄,1032cm^-1,2υ₆,1022cm^-1),the two conditions of Fermi resonanceoccuring. Theoretical calculations have done by Hongkong University, the results also supportour determination.The further work has done by us to discuss the influencing factors of theFermi resonance here,excitation wavelengths change, variation of solvents and its concentrationhave tried. We acquired ultimately that the influencing factors of the Fermi resonance amongυ₃,υ₄,and,2υ₆were free of （or tiny influenced by）the excitation wavelength and the solventconcentration while that was susceptible to the dielectric constant of the solvent.The distancebetween the new peaks which were produced by the Fermi resonance increased along with theenhancement of the solvent dielectric constant,which was consistent with the datas in literatures.（2） Resonance Raman spectra was acquired for EDDT in chloroform solvent with397.9and416nm excitation wavelengths and the detailed resonance Raman intensity analysis weredone. The results indicate that most of the short time photochemistry dynamics of EDDT in theFranck–Condon region occurs along the C₄=C₅stretch and C₄=C₅twist out of plane modes in the5-menber-heterocycle. We assign the CT-band absorption of EDDT to the π（S-C-S）�'π^*（S-C-S）electronic transition. The calculated and experimental absorption spectra and acomparison between the calculated and the experimental resonance Raman cross sections for 397.9and416nm resonances Raman spectrum in chloroform solvent. The calculated spectra isconsistent with the oscillator strength of the A-band transition and also provide a good fit to theabsolute Raman intensities of the397.9and416nm simultaneously. Examination of the Δ dimensionless parameters and total vibrational reorganizational energy of EDDT could explainthe reduction of the charge density of C₄=C₅double bond in view of the intensity pattern of theC₄=C₅twist out of plane mode overtone progression. The multidimensionality of theFranck-Condon region A-band photodissociation dynamics of EDDT suggest that the time scalefor the wave packet motion on the initial excited state potential energy surface is comparable tothe vibrational motion. This is most likely due to the delocalized π（S-C-S）�'π^*（S-C-S）transition, which is characteristically a very strong excited state electronic coupling. Non-totallyanti-symmetric stretchRing deformation+H-C-H rock25（B） gained the intensity in Franck-Condonregion. This implies that S2excited state undergoes rather large geometry deformation along S-Canti-symmetric stretch in S1-C2-S3, even all the ring.（3） The photochemistry dynamics of EDDT were compared to the results for1,3-Dithiole-2-thione （DTT）, the latter indicates that the Franck-Condon region structuraldynamics of DTT have multidimensional character with motion the nominal C=C stretch υ₂inA₁irreducible representative, the C=S stretch（weaker）+H-C=C-H scissor υ₃, accompanied byC=S stretch（stronger）+H-C=C-H scissor υ₄, the nominal S-C-S symmetric stretch+H-C=C-Hscissor υ₆, and so on, while the Franck-Condon region structural dynamics of EDDT havemultidimensional character with motion predominantly in the C₄=C₅stretch and the C₄=C₅twistout of plane, accompanied by moderate contributions from the C=S stretch, C7-C8stretch, Ringdeformation+H-C-H rock, H-C-H twist,which would be explained from the structural changeand the polarity center migration. It was reflected on the red shift of the electronic absorptionspectra.Properties are determined by structure, in the same way, nature reaction structure.