| Photo-induced electron transfer, hydrogen transfer and energy transfer reactionsof aromatic carbonyl compounds play an important role in chemical, biologicaland materials science. In this paper, the1ππ*excited state structural dynamics ofN-Methylpyrrole-2-carboxaldehyde(NMPCA),2-Acetyl-1-methylpyrrole(2-Ac-NMP)and3-Acetyl-1-methylpyrrole(3-Ac-NMP) in various kinds of solutions(cyclohexane,acetonitrile and water) had been investigated by the Resonance Raman spectra andDFT calculations. Through the study of the S2(1ππ*)-S1(1nÏ€*) coupling mechanismand solvent effects of heterocyclic aromatic carbonyl compounds, main conclusionswere summarized as follows:(1) We obtained the absorption spectrum of NMPCA in solvents of cyclohexane,acetonitrile and water. In these three solvents, as the polarity of solvent increases,the maximum of absorption wavelength λmaxmoves to the lower wavenumber.Resonance Raman spectra of NMPCA were obtained and seven excitations coveredthe A-, and B-band electronic absorptions. The theory computations showed that theA-, and B-band electronic absorptions were assigned as the Ï€â†'Ï€*transitions. Theresonance Raman spectra suggested the Raman intensity in the fundamentals,overtones and combination bands for about11–13vibrational modes (for A-bandexcitation) or7–11vibrational modes (for B-band excitation). These werepredominately due to the C=O stretch mode ν7, the ring deformation+N1-C6stretch ν17,the ring deformation mode ν21, the C6-N1-C2/C2-C3-C4anti-symmetry stretch modeν14. This indicated that the Franck-Condon region SÏ€structural dynamics of NMPCAmainly occurs along the C=O stretch and ring deformation and N1-C6stretch reactioncoordinates. In a certain solvent and under different excitation wavelengths, therelative intensity of C=O stretch mode ν7versus the C6-N1-C2/C2-C3-C4anti-symmetry stretch mode ν14suggested intense to weak to intense change as theexcitation wavelengths increased. The ν7versus the ν14depends on the solvent, theratio was highest in cyclohexane (nonproton, nonpolar), and middlein acetonitrile (nonproton, polar) and least in water (proton, polar).(2) We obtained the absorption spectrum of2-Ac-NMP in solvents ofcyclohexane, acetonitrile and water. In these three solvents, as the polarity of solvent increases, the maximum of absorption wavelength λmaxmoves to thelower wavenumber. The electronic excitations and the Franck-Condon regionstructural dynamics of2-Ac-NMP were studied by using the resonance Ramanspectroscopy and density functional theory calculations. The A-, and B-band bandabsorptions were assigned as the Ï€â†'Ï€*transitions. The resonance Raman spectrashowed the Raman intensity in the fundamentals, overtones and combination bands ofabout13vibrational modes (for A-band) or8vibrational modes (for B-band),predominately in the C=O stretch mode ν8,C3-C4-C5anti-symm. Stretch+C2-C6stretchν14, ring CH in-plane bend ν18. The relative intensity of the C=O stretch mode ν8versus the C3-C4-C5anti-symm. Stretch+C2-C6stretch ν14depends on solvent andlaser excited wavelength. The intensity variation of the C=O stretch mode ν8directlyreflects the Sn/SÏ€state-mixing or crossing of potential energy surfaces inFranck-Condon region. Solvents can efficiently control the Franck-Condon regionSn/SÏ€state-mixing or crossing processes.(3) We obtained the absorption spectrum of3-Ac-NMP in solvents ofcyclohexane, acetonitrile and water. In these three solvents, as the polarity ofsolvent increases, the maximum of absorption wavelength λmaxmoves to thelower wavenumber. Resonance Raman spectra of3-Ac-NMP were obtained and sevenexcitation wavelengths covered the A-, B-and C-band electronic absorptions. The A-,and B-band electronic absorp tions were assigned as the Ï€â†'Ï€*transitions. Theresonance Raman spectra displayed the Raman intensity in the fundamentals,overtones and combination bands of about9vibrational modes,predominately due tothe C=O stretch mode ν10, the C2=C3/C4=C5/C3=C6stretch ν11and the ringdeformation mode ν19,. This indicated that the Franck-Condon region SÏ€structuraldynamics of3-Ac-NMP mainly occurs along the C=O stretch and ring deformationreaction coordinates. The intensity of A-, and B-band spectra were different,becausethe Ï€â†'Ï€*transitions of A-, and B-band have different contribution. The C-bandtransition was different with A-, and B-band, it contains Ï€â†'Ï€*, nâ†'Ryd and Ï€â†'Rydtransitions. The Raman spetra showed there are many differences in C-band comparedwith A-, and B-band. We also study the intensity change of Resonance Raman spectrain a certain solvent and under different excitation wavelengths and in a certainexcitation wavelengths and under different solvent. The relative intensity of the C=Ostretch mode ν10versus the ring deformation mode ν19dependends on solvent and laser excitation wavelength. |
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