Vibronic effects in the 7000 Å state of azulene

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.