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

在这项研究中，我们试图提高我们的皮秒红外光谱装置在指纹区的灵敏度，并测量几个具有重要光化学意义的电子激发物种，以获得它们的结构信息。在灵敏度的提高方面，我们成功地提出了一种基于吸收各向异性的光学外差检测的新方法，并对仪器进行了许多微小的改变。通过这种方法，吸收信号的幅度增加了几乎(或有时超过)一个数量级。然后，我们在指纹区对几个重要的光化学激发物种进行了皮秒红外测量。测量了反式二苯乙烯的S^1态在乙腈和正庚烷溶液中的S态的瞬时红外光谱，并与同类分子的拉曼光谱进行了比较，讨论了激发态的分子结构。结果表明，在正庚烷中，S^1物种保持了对称性中心，而在乙腈中，S^1物种的中心对称结构略有扭曲。对这一结果的可能解释是，在乙腈溶液中，激发物种在烯烃C=C键周围被弱极化。作为相关化合物，我们还成功地获得了激发态二苯乙炔的红外光谱。研究了4-二甲氨基苯甲腈的S^1态在乙腈溶液中的瞬时红外光谱，该光谱与分子内扭转电荷转移结构有关。在单键伸缩波数区域发现了一个较强的带，这可能是苯环上的C原子与二甲氨基上的N原子之间的键的C-N伸展。这一结果表明，在S^1状态下，C-N键具有单键性质。

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.