Characteristic Molecular Structures of Electronically Excited Species Studied by Ultrafast Time-Resolved Infrared Spectroscopy in the Fingerprint Region.

在本研究中，我们试图提高我们的仪器在皮秒红外光谱指纹区域的灵敏度，并测量了几种光化学上重要的电子激发物质，以获得它们的结构信息。在灵敏度提高方面，除了对仪器进行了许多微小的改动外，我们还成功地提出了一种基于光学外差检测吸收各向异性的新方法。通过这种方法，吸收信号的幅度增加了近（或有时超过）一个数量级。然后，我们对几个光化学上重要的激发态进行了皮秒红外测量。在乙腈和正庚烷溶液中测量了反式二苯乙烯S态和S^1态的瞬态红外光谱，并与同种类的拉曼光谱进行了比较，讨论了其激发态的分子结构。在正庚烷中，S^1保持着中心对称结构，而在乙腈中，S^1的结构与中心对称结构有轻微的扭曲。对这一结果的一种可能解释是，在乙腈溶液中，激发态在烯烃C=C键周围呈弱极化。作为一个相关的化合物，我们也成功地获得了激发的二苯乙炔的红外光谱。本文研究了4-（二甲氨基）苯腈在乙腈溶液中S^1态的瞬态红外光谱，该光谱与分子内电荷转移（TICT）结构有关。在单键伸展波数区发现了一个强带，这可能是苯环C原子与二甲氨基N原子之间的键的C-N伸展。这一结果表明，碳氮键在S^1态下具有单键特征。

In this research we have tried to improve sensitivity of our apparatus for picosecond infrared spectroscopy in the fingerprint region and to measure several photochemically important electronically excited species to obtain their structural information. As for the sensitivity improvement, we have succeeded in proposing a novel method based on optically heterodyned detection of absorption anisotropy, in addition to many minor changes of apparatus. By this method amplitude of the absorption signal increases in nearly (or sometimes more than) one order of magnitude. We have then performed picosecond infrared measurements in the fingerprint region for several photochemically improtant excited species. Transient infrared spectra of S state of S^1 state of trans-stilbene are measured in acetonitrile and n-heptane solutions, and from the comparison between them and the Raman spectra of the same species, the excited molecular structures are discussed. It has been suggested that the S^1 species retains the center of symmetry in n-heptane, while the S^1 state in acetonitrile is slightly distorted from the centrosymmeric structure. A possible interpretation for this result is that the excited species is weakly polarized around the olefinic C=C bond in acetonitrle solution. As a related compound, we have also succeeded in obtaining an infrared spectrum of the excited diphenylacetylene. We have investigated transient infrared spectrum of S^1 state of 4-(dimethylamino)benzonitrile in an acetonitrile solution, which attracts much attention in relation to the TICT (twisted intramolccular charge transfer) structure. A strong band has been found in the single-bond stretching wavenumber region, which may be assigned to the C-N stretch of the bond between the C atom of the benzene ring and the N atom of the dimethylamino group. This result suggests that the C-N bond has a single-bond character in the S^1 state.