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

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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.