芳香族與生物分子光分解動態學的分子束研究

Molecular Beam Studies on the Photodissociation Dynamics of Aromatics and Biomolecules

曾建銘(Chien-Ming Tseng)

指導教授 ： 李遠哲

共同指導教授 ： 倪其焜(Chi-Kung Ni)

國立臺灣大學/理學院/化學研究所/博士(2007年)

https://doi.org/10.6342/NTU.2007.03087

摘要

利用多重離子影像技術來研究芳香族以及生物分子。對於苯胺(Aniline)、四甲基砒啶(4-methyl-pyridine)以及酚(Phenol)的光致異構與光致分解機構的探究。實驗觀測到許多分解途徑包含有出口能障與無出口能障的基態分解，以及沿著N-H/O-H鍵長的激發態分解。關於”七員環異構化”機構有所討論並且與甲苯做比較。這一系列研究幫助我們了解芳香族分子在照射紫外光後可能發生的反應。芳香族及生物分子在吸收紫外光後躍遷 pp*電子激發態經過位能面上二個不同的圓錐型交叉點(conical intersections)，分子過渡到ps*電子激發態，該相斥的位能面使得分子快速分解並且有極快的碎片動能分佈。此一反應途徑被認為是有O-H或N-H官能基的 (氨基酸，DNA鹼基等)照射紫外光後的新典範。對於phenol與1-napthol 的光分解研究來測試上述反應機構。更進一步我們對於 p-methylphenol, p-ethylphenol, phenylethylamine以及 p-(2-Aminoethyl)phenol研究去了解不同官能基對於發光團的光分解動態學所扮演的角色。包含內轉換(internal conversion)，系統間跨越(intersystem crossing)與上述ps*分解反應途徑的相互競爭。此外，對於以吲哚(indole)為發光團(chromophore)的3-methylindole，tryptoamine與色氨酸(tryptophan)也作了探討。左旋色氨酸(L-tryptophan)的光分解動態學可能與地球上生命的起源的關聯性也一起被討論。N-methyl-pyrrole, anisole與N-methylindole的光分解研究證明了對於非氫化雜環分子其N-CH3/O-CH3 ps*電子激發態的存在。7-azaindole, 2-aminopyridine與8-Hydroxyquinoline的研究則提供了DNA典型分子其激發態光物理化學的新觀念。

關鍵字

分子束 ； 雷射

並列摘要

The photodissociation dynamics of aromatics and biomolecules have been investigated by multimass ion imaging technique. The photoisomerization and photodissociation mechanisms of aniline, 4-methylpyridine and phenol were studied. Various dissociation channels including the ground state dissociation with or without exit barrier and excited state dissociation along with N-H/O-H bond distance were observed. The “seven membered ring isomerization” of these aromatic molecules was discussed and compared with toluene. These series of studies established the basic understanding of aromatic molecules under UV irradiation. The suggestion of the deactivation from the optical bright state (π π* state) to the dark state ( repulsive πσ* state) through conical intersection has been made to explain the ultrafast decay pathway to dissipate the absorbed UV photon energy of aromatic and biomolecules. The results are the H-atom transfer (proton-electron concerted transfer) on the excited state or internal conversion to the ground state. The detail studies of photodissociation dynamics of phenol and 1-napthol was performed to test the idea in these chemical systems. By extending the research to biomolecules, the chromophores of amino acid tyrosine and tryptophan were studied. The p-methylphenol, p-ethylphenol, phenylethylamine and p-(2-aminoethyl)phenol reveal the affects of “floppy” side chain and different functional groups (e.g., amino and hydroxyl groups) on the photodissociation dynamics of these chromophores. Likewise, the studies of indole, 3-methyl-indole, tryptoamine and tryptophan are also the way to go deep into the photochemistry issue of amino acid gradually. The photodissociation dynamics of L-tryptophan in a molecular beam and its relevance to the origin of life on earth will be briefly discussed. The photodissociation of N-methyl-pyrrole, anisole and N-methylindole is the another illustration of the non-hydride heteroaromatic molecules with N-CH3/O-CH3 πσ* state. 7-azaindole, 2-aminopyridine and 8-Hydroxyquinoline were studied to clarify the repulsive N-H/O-H πσ* state which opens up a new aspect to describe the excited-state photophysics of DNA model compounds.

並列關鍵字

photodissociation ； amino acid

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