【作者】 伍贤方；

【导师】 郑旭明；

【作者基本信息】 浙江理工大学 ， 应用化学， 2009， 硕士

【摘要】 噻吩、呋喃等五元芳香杂环化合物的激发态动力学,受到了国内外科学研究小组的广泛关注。随着光电材料的研究发展,因为聚噻吩通过π共轭体系能很好地完成电子和能量的转移,所以大量用于发光二极管、激光器和场效应晶体管等方面。从此,如何利用外部刺激有选择性控制噻吩聚合物的电子和能量转移成为当前最重要的主题之一。同时噻吩低聚物S₁态的寿命也随着聚合长度的增加而变长,而电子势能面的锥型交叉在超快的内转换、无辐射衰变等方面扮演着重要的角色,因此,研究激发态势能面的锥型交叉可以揭示分子的激发态动力学。本文采用共振拉曼光谱技术,结合量子化学计算方法,研究了在环己烷和甲醇溶液中的噻吩,硒吩,呋喃,噻唑等化合物在Frank-Condon区域的结构动力学,并结合密度泛函理论和完全活化空间的自洽场方法（CASSCF）洞察了噻吩,硒吩,呋喃和噻唑的光解离途径,取得了一些有意义的研究成果。（1）获得了噻吩在环己烷溶剂中的电子吸收光谱,其最大吸收波长在240nm左右。获得了噻吩在环己烷溶剂中的239.5 nm和266.0 nm共振拉曼光谱。共振拉曼光谱显示,噻吩环的对称伸缩振动（ν₃,1394cm₁、ν₄,1344cm₁）,C-H面内的摇摆振动（ν₅,1074cm₁）,C₃-C₄伸缩振动+C=C-C_面内弯曲振动（ν₆,1028cm₁）,C-S对称伸缩振动+C=C-C面内弯曲振动（ν₇,835cm₁）,C-S不对称伸缩振动+C=C-C面内弯曲振动（ν₂1,754cm₁）这6个拉曼带的泛频和组合频占据了共振拉曼光谱强度的主要部分。ν₄,ν₂1振动模及其组合频占据共振拉曼光谱强度的绝大多数,表明噻吩激发态结构动力学主要沿着C-S不对称伸缩振动和噻吩环的对称伸缩振动这2个活性模展开,而9个活性振动模的同时存在表明其在Frank-Condon区域的光解离短时动力学具有多维性。不对称性模ν₂1的共振拉曼强度要比对称性模ν₇的强,预示着S₁态噻吩分子的C-S键键长要发生变化,其中一个增长,一个缩短,这暗示噻吩在S₁态的Frank-Condon区域有可能与某个高激发态Sn发生锥型交叉,并导致开环反应。因此,采用CASSCF方法获得了噻吩S₁、S₂、S₃电子激发态和势能面交叉点CI（S₃/S₁）、CI（S₂/S₁）的电子跃迁能和几何结构,这些结果与噻吩S1态在Frank-C_ondon区域的光解离动力学密切相关。开环光解离反应通道是通过锥型交叉点CI（S₃/S₁）导致C-S键发生断裂被证明在S₁态噻吩超快光解离的反应途径中占据主要地位;通过锥型交叉点CI（S₂/S₁）的内转换反应通道也是噻吩S₁激发态衰变（光反应的）一种途径。（2）测得硒吩在环己烷溶剂中的最大吸收带在250nm左右,采用239.5nm和252.7nm的激发光获得其共振拉曼光谱。研究结果显示,硒吩环的对称伸缩振动（ν₃,1416cm₁、ν₄,1345cm₁）,C-H面内的摇摆振动（ν₅,1073cm₁）,C₃-C₄伸缩振动+C=C-C面内弯曲振动（ν₆,1012cm₁）,C-Se对称伸缩振动+C=C-C面内弯曲振动（ν₇,755cm₁）,C-Se不对称伸缩振动+C=C-C面内弯曲振动（ν₂1,624cm₁）这6个拉曼带的泛频和组合频占据了共振拉曼光谱强度的主要部分。而7个活性振动模的同时存在表明其在Frank-Condon区域的光解离短时动力学具有多维性。通过比较硒吩与噻吩的FT-Raman光谱、共振拉曼光谱和光谱指认我们断定,硒吩在Frank-Condon区域的光解离短时动力学与噻吩非常相似。（3）测得呋喃在环己烷溶剂中的最大吸收带在209nm左右,采用217.8nm和208.8nm的激发光获得其共振拉曼光谱,并进行了强度分析。研究结果显示,呋喃在Frank-Condon区域的光解离短时动力学具有多维性。主要沿着C₃-C₄的伸缩振动+C=C-C面内弯曲振动ν₄ （|Δ|=1.01,λ=702 cm₁）和C₂=C₃/C₄=C-5的伸缩振动+C-O-C的面内弯曲振动ν₃ （|Δ|=0.98,λ=712 cm₁）这2个振动模展开。呋喃的共振拉曼光谱没有不对称性模的出现,光谱指认也和噻吩相差很大,这表明呋喃在Frank-Condon区域的光解离短时动力学与噻吩不一样。（4）测得噻唑在环己烷和甲醇溶剂中的最大吸收带在230nm左右,采用239.5nm和228.7nm的激发光获得其共振拉曼光谱,并进行了强度分析。研究结果显示,噻唑在Frank-Condon区域的光解离短时动力学具有多维性。主要沿着CH面外摇摆振动ν₁5 （|Δ|=1.20,λ=572 cm₁）,噻唑环对称伸缩振动ν5 （|Δ|=1.12,λ=864 cm₁）和C-S不对称伸缩振动ν₁2 （|Δ|=0.95,λ=341 cm₁）这3个振动模展开。由于氮原子的取代,使得C-S不对称伸缩振动模ν₁2的强度增强;在噻唑中,基频ν₁2比基频C-S对称伸缩振动模ν₁3要强很多,从振动重组能上也能体现出来。这表明C被N取代后除了一些振动模式产生了影响之外,噻唑在Frank-Condon区域的光解离短时动力学特征和噻吩也非常相似。更多还原

【Abstract】 There has been interest for many years in excited state dynamics of thiophene, furan and other pentaatomic heterocycles because of their role in the pharmaceutical synthesis, resins and organic synthesis, etc. With the photoelectric materials research and development, thiophene oligomers have used extensively in light-emitting diodes, lasers and field-effect transistors because of their excellent electron and energy transfer properties through theπ-conjugation. From then on, how to optically control such molecular wires of thiophene oligomers by external stimuli becomes an important theme. As is now generally recognized, the S₁ state lifetimes of thiophene oligomers increases with the aggregate length. conical intersection of electronic potential energy surfaces（PES） play an central role in ultrafast internal conversion, radiationless decay etc. The information about conical intersection gained from examination of the excited-state PES helps to investigation of the excited-state dynamics.In this paper, the short-time photodissociation dynamics of thiophene, furan, selenophene and thiazole in cyclohexane and methanol solution have been investigated by the Resonance Raman spectra in combination with DFT and CASSCF calculation. We also discussed the photodissociation process in the photochemical reactions. Main contributions of the present work are summarized as follows,（1） We obtained the absorption spectrum of thiophene in cyclohexane solution which has a charge-transfer（CT） band near 240nm. Resonance Raman spectrums were obtained for thiophene in cyclohexane solution with 239.5nm and 266nm excitation wavelength in resonance with the CT-band absorption spectrum. The results indicate that the Franck-Condon region short-time photodissociation dynamics of thiophene have multidimensional characters with motion predominantly along the nominal C₂=C₃-C₄=C₅ in-plane symmetric stretch modesν₃ （A₁） at 1394 cm₁ andν₄ （A₁） at 1344 cm₁, the nominal C-H in-plane wag modeν₅ （A₁） at 1074 cm₁, the nominal C₃-C₄ stretch +C=C-C bend modeν₆ （A₁） at 1028 cm₁, the nominal C-S in-plane symmetric stretch+C-C=C bend modeν-7 （A₁） at 835 cm₁, the nominal C-S anti-symmetry stretch+C-C=C bend modeν₂1 （B₂） at 754 cm₁. The nominal C-S anti-symmetric stretch + C-C=C bend modeν₂1 （B₂） at 754 cm₁ possesses much more Raman intensity than the nominal C-S in-plane symmetric stretch + C-C=C bend modeν₇ （A₁） at 835 cm₁.This indicates that thiophene molecule in the S₁ state undergoes large bond length changes along the two C-S bond lengths with one C-S bond becoming much longer while the other becomes somewhat shorter. This kind of short-time dynamics suggests that a ring opening reaction may occur in or nearby the Franck-Condon region due to a curve crossing between S₁ and Sn state.The electronic transition energies, the excited state structures and the conical intersection points CI（S-3/S₁） and CI（S₂/S₁） between the S₁ and the S₂ or S₃ potential energy surfaces of thiophene were determined by using complete active space self-consistent field theory computations. These results were correlated to the Franck–Condon region photodissociation dynamics of thiophene in its S₁ state. The ring opening photodissociation reaction pathway through cleavage of one of the C-S bonds and via a conical intersection point CI（S₃/S₁） was revealed to be the predominant ultrafast reaction channel for thiophene in the S₁ state. The internal conversion pathway via a conical intersection point CI（S₂/S-1） was also found to be another reaction channel for thiophene.（2） We obtained the absorption spectrum of selenophene in cyclohexane solution which has a charge-transfer（CT） band near 250nm. Resonance Raman spectrums were also obtained for selenophene in cyclohexane solution with 239.5nm and 252.7nm excitation wavelength in resonance with the CT-band absorption spectrum. The results indicate that the Franck-Condon region short-time photodissociation dynamics of selenophene have multidimensional characters with motion predominantly along the nominal nominal C₂=C₃-C₄=C₅ in-plane symmetric stretch modesν₃ （A₁） at 1416 cm₁ andν₄ （A₁） at 1345 cm₁, the nominal C-H in-plane wag modeν₅ （A₁） at 1073 cm₁, the nominal C₃-C₄ stretch +C=C-C bend modeν₆ （A₁） at 1012 cm₁, the nominal C-Se in-plane symmetric stretch+C-C=C bend modeν₇ （A₁） at 755 cm₁, the nominal C-Se anti-symmetry stretch+C-C=C bend modeν₂1 （B-2） at 624 cm₁. FT-Raman spectroscopy, Resonance Raman spectroscopy and spectral assignments of thiophene is similar to selenophene. so we conclude that the Frank-Condon region short-time photodissociation dynamics of selenophene almost completely seems the thiophene.（3） We obtained the absorption spectrum of furan in cyclohexane solution which has a charge-transfer band near 209nm. Resonance Raman spectrums were obtained for furan in cyclohexane solution with 217.8nm and 208.8nm excitation wavelength in resonance with CT-band absorption spectrum and Resonance Raman analysis was done. The results indicate that the Franck- Condon region short-time photodissociation dynamics of furan have multidim- ensional characters with the reaction coordinates with the nominal C3-C4 stretch+C=C?C bendν₄ （Δ=1.01,λ=702 cm₁）, the nominal C₂=C₃/C₄=C₅ stretch + C-O-C bendν₃ （Δ=0.98,λ=712 cm₁）. The asymmetric mode is not appear on the Resonance Raman spectra of furan and spectral assignments of furan is very different from thiophene, for these reason we conclude that the Frank-Condon region short-time photodissociation dynamics of furan is quite different from thiophene.（4） We obtained the absorption spectra of thiazole in cyclohexane and methanol solution which has a charge-transfer band near 230nm. Resonance Raman spectrums were obtained for furan in cyclohexane solution with 228.7nm and 239.5nm and in methanol solution with 239.5nm excitation wavelength in resonance with CT-band absorption spectrum and Resonance Raman analysis was done. The results indicate that the Franck- Condon region short-time photodissociation dynamics of thiazole have multidimensional characters with the reaction coordinates or displacements occurring with the nominal CH rockν₁5 （Δ=1.20,λ=572 cm₁）, the nominal C₂=N₃-C₄=C₅ in-plane symmetric stretchν₅ （Δ=1.12,λ=864 cm₁）, the nominal C-S anti-symmetry stretch+C-C=C bendν₁2 （Δ=0.95,λ=341 cm₁）. As the replacement of nitrogen atoms, the nominal C-S anti-symmetric stretch possesses much more Raman intensity than the nominal C-S in-plane symmetric stretch, similarly the nominal C-S anti-symmetric stretch of thiazole possesses much more Raman intensity than thiophene. We note that the substitution of the C atom by the N atom fewly modulates the Frank-Condon region photodissociation dynamics. Which indicated thiazole and thiophene have the similar photodissociation process in the Frank-Condon region.更多还原