Excited-state Structural Dynamics And Decay Mechanism Of Conjugated Molecules Containing Heteroatoms

Due to the dual role of the molecule heteroatoms (O and N) electronegativity and the lone pair of electrons for α,β-conjugated enone,α,β-conjugate acid esters and isothiocyanate, their excited state properties are different from the corresponding conjugated diene and vinyl aromatic groups. In this paper, the excited states and nonadiabatic decay dynamics in different solvents of methyl trans crotonate (MTCA), methyl2,4-pentadienoate (M24PDA), y-crotonolactone, phthalide,5-bromophthalide,5-cyanophthalide and phenyl isothiocyanate (PITC) have been studied by resonance Raman spectra and complete active space self-consistent field (CASSCF) calculation. The role of hetero atoms in excited state charge transfer and nonadiabatic dynamics was studied, the results as follow:(1) The vibrational spectra, electron spectra and resonance Raman spectra of MTCA, M24PDA, y-crotonolactone, phthalide,5-bromophthalide,5-cyanophthalide and PITC were assigned. We obtained the Ultraviolet (UV) absorption spectra, Fourier transform (FT)-Raman, FT-infrared (IR) and resonance Raman spectra of these moleculars. They were also studied by density functional theory(DFT) and CASSCF calculation. The vibrational spectra of ground states, geometric structures and electronic absorption bands characteristic transitions of the excited states and the curve-crossing points were obtained.The resonance Raman spectra of MTCA was assigned as strong peaks v7and v8, weak peaks v10, V14, V15, V16, v17, V21and V33. The resonance Raman spectra of M24PDA was assigned as strong peaks v8, v9and V16, weak peaks V12, V19, v20and V23. The resonance Raman spectra of γ-crotonolactone was assigned as strong peaks V4, V5, v8, v9and v10, weak peak V12. The B band resonance Raman spectra of phthalide was assigned as strong peaks v6, v7, v8, V15, v21and V23, weak peaks v10, V12, V14, v17, v18, V22and V24. The B band resonance Raman spectra of5-bromophthalide was assigned as strong peaks v5, v6, v7, V18and v21, weak peaks v8, V10, V11, V13, V14, V16, V19, V20, V22, V24and V25. The B band resonance Raman spectra of5-cyanophthalide was assigned as strong peaks v6, v7, V15, v18and V22, weak peaks v11, v12, v16, v19, v20, v25and v26.The A band resonance Raman spectra of PITC was assigned as strong peaks v6, v7, v13and v14, weak peaks v9, v18, v19, v20, v22, v28and v32.(2) Some problems in the vibrational spectra assignment were solved. The biggest difference between γ-crotonolactone, phthalide and MTCA, M24PDA, PITC is that the former had only one peak v4(C=O stretch) in the region of1700-1900cm-1in the calculated Raman spectra, but the counterpart had two peaks in the FT-IR and FT-Raman spectra. The dual peaks1738,1770cm-1and1747,1768cm-1were assigned to dimer and monomer of y-crotonolactone and phthalide. The wrong usage of the point group and the wrong assignment of PITC were corrected.(3) The structural effects on electronic absorption bands characteristic transitions were studied. The excited states S2(A’) of MTCA and M24PDA were caused by Ï€Hâ†'Ï€L*electronic transitions. But the same excited state of y-crotonolactone was caused by Ï€H-1â†'Ï€L*electronic transitions. It was because of ring effect, which lead to exchanging relative energy of n and Ï€ orbitals. Three absorption bands in the region of200-400nm of phthalide were assigned as S2(A’), S3(A’) and Se(A’), respectively. And the S3(A’) excited state was caused by B band, the same as S2excited states of MTCA, M24PDA and y-crotonolactone. The S2(A’) excited state was caused by A band Ï€Hâ†'Ï€L*electronic transitions, which delocalized between benzene and carbon of carbonyl. In the structure of phthalide, the unique A band was caused by benzene and y-crotonolactone, which had a significant effect on the excited state decay dynamics and decay mechanism of phthalide.(4) The substituent effects of α,β-conjugated enone in the Franck-Condon region were studied. MTCA and M24PDA had the same S2(A’) excited state characteristic transitions, but their structural dynamics had important differences. The former was dominated by the Cα=Cβ and C=O stretch coordinate, while the latter was mostly along Cα=Cp-C4=C5stretch motion. The variation of the Raman intensity ratios for v7/v8,(v7+v8)/2v8,(v7+2vs)/3v8and (v7+3vs)/4v8in A band resonance Raman spectra of M24PDA and MTCA were investigated. We realized that the structural dynamic of M24PDA was different from the charge transfer mechanism of MTCA and3M3P2O. The conclusion that the substitution of methyl group in the a’-position of α,β-enones by methoxyl group does not substantially affect the short-time structural dynamics, while the substitution of vinyl group in the β-position changes significantly the short-time structural dynamics was proposed.(5) The relation between the intensity mode of resonance Raman spectra and CASSCF calculations were studied systematically. The excited state decay mechanisms of moleculars mentioned above were proposed. The major S2(A’) excited state decay routes of MTCA, M24PDA, y-crotonolactone and PITC were S2â†'S1via conical intersection points CI(S2/S1), then get ground state via conical intersection points CI(S1/S0). The major S3(A’) excited state decay channel of phthalide:S3,FC(ππ*)â†'S3(2)/S2â†'S2(ππ*)â†'S2,min(ππ*)â†'S0(radiative).5-bromophthalide and5-cyanophthalide had the same radiative dacay channel S2,min(ππ*)â†'S0(radiative).