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Considerations on the Molecular Structures and Electronic Properties of 1-Methyl-2 (1H)-pyridinimine in the Ground and Lowest Excited Singlet States

Kozo INUZUKA, Naoya IWASAKI and Akira FUJIMOTO^*

Department of Materials Science and Engineering, Faculty of Engineering, Tokyo Denki University; Kanda, Chiyoda-ku, Tokyo 101-8457 Japan

In order to determine the molecular geometries of 1-methyl-2(1H)-pyridinimine (1MPI : abbrev.) in the ground and lowest excited singlet (S₁ : abbrev.) states, the RHF/6-31+G(d) and RCIS/6-31+G(d) methods were carried out.
And also the IR spectrum was measured to determine the existence of the cis-form of 1MPI with respect to the configuration of the C=NH group in the ground state. From calculated and spectroscopic results the following conclusions were drawn: (1) The ring frame-work of 1MPI in the ground state is planar within the precision of the present calculation. (2) In the S₁ state the ring frame-work is nonplanar. The energy difference between the nonplanar and planar structures is 0.34 kJ mol⁻¹. (3) The S₁ state of 1MPI is of π, π^* character. There are three n, π^* singlet states between the S₁ and second π, π^* singlet states. These n, π^* singlet states are close lying to the S₁ state. (4) Although the S₁ state is of π, π^* character, its state is partially of n, π^* character due to the nonplanarity of the ring frame-work. Therefore, the S₁ state may be mixed easily with the near-lying n, π^* excited states through the vibronic-coupling and spin-orbit coupling. These couplings lead to the nonradiative deactivation of the S₁ state. The reason of low fluorescence quantum yield of 1MPI may be attributed to the nonplanar structure in the S₁ state and existence of the n, π^* singlet states close to the S₁ state. (5) The trans-form with respect to the H atom of C=NH group of 1MPI to the ring N atom is more stable than the cis-form. The calculated potential barrier height for the trans-cis isomerization process suggests that the trans-cis isomerization is more easy in the lowest excited singlet state than in the ground state. The IR spectrum of 1MPI irradiated by the UV light showed the existence of the small amount of cis-form increased with the UV irradiation and with the concentration of 1MPI. (6) The degree of deformation of molecular structure of 1MPI is smaller than those of 2-pyridone and 1-methyl-2-pyridone in the lowest excited single state. The shape of vibrational structure of fluorescence spectrum of 1MPI is different from those of the other ones. This result is related to the difference in the degree of deformation of molecular structure between 1MPI and 2-pyridone and 1-methyl-2-pyridone.

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