Photo-induced Charge Transfer Dynamics Study On 2,2'-bithiophene And Its Carbonyl Derivatives

Oligo-thiophene and its carbonyl derivatives have been widely studied. Scientists in domestic and abroad have focused on 2,2'-bithiophene , the simplest oligo-thiophene, as the model molecule of Photo-induced charge-transfer study. Polythiophene have the excellent ability to accomplish charge-transfer and energy-transfer by there ownÏ€-conjugation system. oligo-thiophene materials are not only used as nonlinear and linear optical materials,but also be used as semiconductors. From then on, how to optically control such charge-transfer and energy-transfer processes of thiophene oligomers by external conditions have become an important theme.A lot of advanced techniques have been used to study the photo-induced charge-transfer dynamics. In this paper, by the aid of DFT, we have studied the photo-induced charge-transfer dynamics of 2,2'-bithiophene, Bis(2-thienyl)ketone and 2,2'-bithiophene-5-carboxaldehyde. In those studies, Resonance Raman spectra is the most important method which can probe photo-induced decay in Franck-Conden region happened in tens to dozens of femtoseconds. Main contributions of the work are summarized as follws.(1) We acquired absorption spectra of 2,2'-bithiophene in acetonitrile solution, there are two charge-transfer bands, A-band near 302.6nm while B-band band near 247nm. And we also acquired Resonance Raman spectra of 2,2'-bithiophene in acetonitrile solution,all of the excited wavelength are localized in A-band and B-band. The Raman spectra shows there are many apparent differences in A-band and B-band.Ï…₅ (1554cm^-1,Ring breathing(â… â…¡) +β_CH +ν_C(5)C(9)) is the predominant vibrational modes in A-band,while in B-bandÏ…₄ (1441cm^-1,β_CH +ν_C(1)C(5)+ν_C(2)C(3)+ν_C(7)C(8)+ν_C(5)C(9)) is the predominant vibrational modes.The phenomenon shows 2,2'-bithiophene in A-band and B-band have very different dynamics in Franck-condon region. Research results also support the inference, A-band transition is mainly caused byπ→π* transition, while n→π* B-band transition is attributeis to n→π* transition.(2)We also acquired absorption spectra of Bis(2-thienyl)ketone in cyclohexane solution. It also has two charge-transfer bands, A-band near 301nm and B-band band near 267nm. Then we acquired resonance Raman spectra of Bis(2-thienyl)ketone in cyclohexane solution. The Raman spectra shows there are differences in A-band and B-band. In A-bandÏ…₁₀ (1414 cm^-1,Ring breathing+ν_{SynC(1)C(11) C(9)} stretch) is the predominant vibrational modes,while in B-band isÏ…₇ (1616 cm^-1,ν_CO+ν_C(2)C(3)+ν_C(8)C(9) stretch) .We make a distinction of photo-induced charge-transfer dynamics in different excited wavelength by comparing the the Raman Excitation Profiles (REPs). The result gives two information: A-band photochemical reaction is mostly along the Ring breathing+ν_{SynC(1)C(11) C(9)} stretch reaction coordinate, B-band photochemical reaction is mostly along theν_CO+ν_C(2)C(3)+ν_C(8)C(9) stretch. The information is also consistent with the transition modes of A-band and B-band.Futhermore, we acquired the Raman spectra of Bis(2-thienyl)ketone in acetonitrile and methanol. Supporting by the experiments, although the solvents with different polarity and permittivity, the reaction's dynamics have nearly no distinctions. But for some vibrational modes, the excited state's lifetime have changed in different solvents.(3) We obtained the absorption spectrum of 2,2'-bithiophene-5-carboxaldehyde in cyclohexane, methanol and acetonitrile, all of the maximum absorption is near 350nm. Then we obtained the Resonance Raman spectra under the excited wavelength of 309.1nm,319.9nm and 341.5nm. The study presents the fruit that solvents with different polarity and permittivity nearly have no inffluence on the dynamics properties in A-band. The photochemical reaction is mainly along theÏ…₁₀ (1445 cm^-1,Ring breathing(â… ,â… )+β_C(11)H);Ï…₈(1545cm^-1,β_CH(â… ) +ν_C(1)C(2)+ν_C(3)C(4)+ν_C(6)C(7)+ν_C(8)C(9));Ï…₇ (1650 cm^-1,ν_CO);Ï…₉ (1508 cm^-1,β_CH+ν_C(1)C(2)+ν_C(3)C(4)+ν_C(6)C(7)+ν_C(8)C(9)). Therefore, the photo-induced charge-transfer dynamics of 2,2'-bithiophene-5-carbonyl in different solvents have multidimension in Franck-Condon region.(4) We have compared the dynamics in A-band of 2,2'-bithiophene,bis(2-thienyl) ketone and 2,2'-bithiophene-5-carboxaldehyde in acetonitrile solution.The results indicate that when 2,2'-bithiophene combined with carbonyl, not only the dynamics mechanism in Franck-condon region changed, but also the photochemical reaction coordinates changed in a certain extent.