Study Of The Excited State Dynamics In Resonance Raman Spectrum Of Aminouracil And Its Substructure

Photochemical reaction dynamics of nucleic acid base which is an importantcomponent of nucleotide plays a significant role in the UV photodamage of nucleicacids. The electrons in base transit from ground state to excited state byphotochemical reaction, while excited electronic state sometimes is apt to decay intoharmful photochemical products, which can cause mutations and interfere with thenormal cellular processing of DNA.In recent decades, efforts have been made in the study of the relaxation dynamicsof excited-state nucleic acid bases using fluorescence relaxation techniques. Thelifetimes of the first electric dipole-allowed state (S1) have been observed to bearound1ps, and fast decay to the ground state via internal conversion (IC) has alsobeen proposed. Based on this transformation of the complexity of the process in thedecay, in recent years the decay many different mechanisms have been put forward,and the major breakthrough has been obtained. Fluorescence relaxation techniquescan not detect to excited state structure dynamics in the Franck-Condon (F-C) region(in50fs), because of their low resolution (100fs). So that the lifetime in the processof electronic excited and the position of potential surfaces intersection are not clear.Resonance Raman spectral technique has a unique advantage on the research ofthe conical intersection and vibronic coupling in the F-C region, on other aspects, itprovides irreplaceable contributions for electronic excited states decay mechanism.In this paper, the excited state structural dynamics of5-aminouracil (5Au),6-aminouracil (6Au),3-amino-2-cyclohexen-1-one (ACyO) and relatedanthraquinones (PBQ/BQ/HQ) in different solvents systems have been studied by theResonance Raman spectra in combination with DFT calculation and discussed theinfluential factors to tune the photochemical reactions. Main conclusions of thepresent work are summarized as follows,(1) The FT-IR and FT-Raman spectra of5-aminouracil and6-aminouracil in solid state. Density functional theory calcualtions were done to help elucidate thevibrational band assignments. Resonance Raman spectra were respectively obtainedfor5-aminouracil and6-aminouracil in water solution with252.4,228.4,273.9, and266nm excitation wavelength in resonance with the CT-band absorption spectrum toexamine the excited state structural dynamics and the state-mixing or curve-crossingtuned by substituent groups. There are many of the same vibration modes such asHN9H scissor, N3H/C5N/N1H bend asysm NCN stretch in Resonance Raman spectra.While there are also some differences in ring deformation. It suggusts that the excitedstate structural dynamics and reaction coordination of the ring are inflution indifference of substituent positions, and then lead to different life of the electronicexcited states decay.(2) To find out the substructure which is the factor to tune the photochemicalreactions of aminouracil ring, the study of3-amino-2-cyclohexen-1-one that is modelcompound of aminouraci has been completed. The FT-IR and FT-Raman spectra of3-amino-2-cyclohexen-1-one in solid state and/or in solvents of water and acetonitrilehave been researched. Density functional theory calcualtions were done to helpelucidate the vibrational band assignments. Resonance Raman spectra were obtainedfor ACyO in different solutions with299.1,282.4,273.9, and266nm excitationwavelength in resonance with the CT-band absorption spectrum to examine theexcited state structural dynamics and the state-mixing or curve-crossing tuned bysolvents. A preliminary resonance Raman intensity analysis using the time-dependentwave-packet theory and simple model was done for ACyO in acetonitrile solvent.Resonance Raman spectroscopic probing of the excited state curve-crossing orstate-mixing was proposed. The largest changes in the displacements take place withthe anti-symmetric C1C2=C3stretch+C1=O7stretch11(1588cm-1,=0.92, λ=672cm-1). The vibrational reorganizational energy of this mode accounts for56.6%of thetotal vibrational reorganizational energy. Moderate changes in the displacements takeplace for H11C4H10/H14C6H15scissor15, the ring breath34+37, and the H16N8H17rock25vibrational degree of freedom. In acetonitrile solvent the Franck-Condonregion structural dynamics of ACyO have multidimensional character. Compared with 6Au, it is clear that O=C-C=C-plays a significant role in excited state structuraldynamics of uracil ring. This discovery provides a new perspective to the study ofexcited states relaxation dynamic mechanism.(3) As the compound which also contains O=C-C=C-, anthraquinones have beenstudied. The FT-IR and FT-Raman spectra of related anthraquinones which are alsothe model compounds of uracil in solid state. Density functional theory calcualtionswere done to help elucidate the vibrational band assignments. Resonance Ramanspectra were respectively obtained for related anthraquinones with the CT-bandabsorption spectrum to examine the excited state structural dynamics and thestate-mixing or curve-crossing, and subsequently offer some useful help to the studyof aminouracil by the comparison between parent compound and6-aminouracil that isthe lord compounds research system.