Abstract
Nitrogen heterocycles of azulene have currently been under study to examine the effects of nitrogen substitution in the azulene system. We have determined the dipole moments of three different electronic states of 1,3-diazaazulene: the ground state, the first excited singlet state (1ππ*, B1), and the phosphorescent state (3ππ*). Measurements have been performed on the origin band and several prominent vibronic bands of the 1B1 ← 1A1 transition. The dipole moment of the ground state was determined from dielectric constant measurements, and the change in dipole moments upon excitation to higher electronic states was measured from the pseudo-Stark effect of molecular crystals on diazaazulene in a naphthalene host. The measured values are μ(GS) = 2.98 ± 0.01 D, |Δμ(GS−S1)| = 1.80 ± 0.05 D, and |Δμ(GS—T1)| = 1.10 ± 0.07 D. The measurements enable us to examine the role of nitrogen atoms participating the charge distribution of the σ core and the π-electron cloud in the 1B1 ← 1A1 transition in comparison with the known dipole moment of the corresponding first excited singlet state of azulene. Discussion on the direction of the change in dipole moment and the relative magnitude of the dipole moments of the origin bands of S1 and T1 states as well as the vibronic bands 264, 539, and 575 cm−1 are presented.
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