High resolution electronic spectroscopy of 1-aminonaphthalene: S0 and S1 geometries and S1←S0 transition moment orientations

Fluorescence excitation spectroscopy at both vibrational and rotational resolution has been used to probe the changes in energy, electronic distribution, and geometry that occur when 1-aminonaphthalene (1AN) absorbs light at ∼332 nm. The 000 band of the S1←S0 transition of 1AN is red shifted by nearly 2000 cm−1 with respect to the corresponding band of naphthalene. Additionally, it is mainly b-axis polarized, unlike the corresponding bands of naphthalene and other 1-substituted naphthalenes. Thus, 1La/1Lb state reversal occurs on 1-substitution of naphthalene with an NH2 group. The S0 state of 1AN is pyramidally distorted at the nitrogen atom. Additionally, the NH2 group is rotated by ∼20° about the C–NH2 bond. Excitation of 1AN to the zero-point vibrational level of its S1 state reduces the C–NH2 bond length by ∼0.2 Å and flattens the NH2 group along both out-of-plane coordinates. Other vibronic bands in the S1←S0 transition exhibit significantly different rotational constants, inertial defects, and transition moment orientations. An explanation for these findings is given that is based on the well-known conjugative properties of the NH2 group in chemically related systems.