Effects of Structural Changes in the Amino group on the Relaxation Processes of Elecronically Excited Aromatic Amines

用时间分辨荧光、瞬态吸收和时间分辨光热测量研究了N，N-二甲基-1-氨基萘及其苯胺衍生物的氨基结构变化对弛豫过程的影响。对于N，N-二甲基-1-氨基萘，由于激发态结构由扭曲转变为准平面，导致了紧密排列的S_1和S_2态之间的电子振动耦合，导致了在非极性溶剂中的快速内转换。这种快速的非辐射过程也被发现苯胺衍生物，其中有一个扭曲的氨基在基态。结果表明，溶剂对芳香胺弛豫过程的影响是由于氨基在激发态的结构变化引起的，氨基在电子激发下从金字塔结构变为准平面结构，导致分子内电荷从氮原子转移到环上.结果发现苯胺衍生物在266或308 nm光激发下的光电离阈值降低，单光子电离成为可能。

Effects of structural changes in the amino group on the relaxation processes of N,N-dimethyl-1- aminonaphthalenes and aniline derivatives were investigated by time-resolved fluorescence, transient absorption and time-resolved photothermal measurements. For N,N-dimethyl-1-aminonaphthalenes it was revealed that structural changes from trwisted to quasi-planar from in the excited state induce vibronic coupling between closely-lying S_1 and S_2 states, resulting in very fast internal conversion in nonpolar solvents. Such a fast nonradiative process was also found for aniline derivatives which have a twisted amino group in the ground state. This work showed that characteristic solvent effects on the relaxation processes of excited aromatic amines could be attributed to structural changes of the amino group in the excited state.The structural change of the amino group from a pyramidal to quasi-planar upon electronic excitation results in intramolecular charge transfer from the nitrogen atom into the ring. As a result photoionization threshold is lowered and one-photon ionization upon 266 or 308nm photon excitation was found to become possible for aniline derivatives in water.