Vibronic effects in the 7000 Å state of azulene

Abstract

Vibronic perturbations in the 7000 Å S₁(B₁) ← S₀(A₁) absorption system of azulene due to a large number of b₁ vibrations have been identified. The total induced oscillator strength for this transition is 1.2×10⁻³ while the allowed oscillator strength is 7.8×10⁻³. Accurate intensities for the b₁ vibronic origins relative to the allowed origin are reported. Calculations are presented which indicate that the two most probable coupling routes for the vibronic interactions (intensity) involve the second excited ¹A₁ state (S₄) and the ground state, S₀. The latter route exists due to the large change in permanent dipole moment between S₀ and S₁, ∼ 2 D. Assignments for all 12 of the excited state a₁ fundamentals (excluding CH) are given. The differences between the 7000 Å ¹B₁ ← ¹A₁ and 3500 Å ¹A₁ ← ¹A₁ absorption systems are discussed in relation to the anomalous fluorescence behavior of azulene.