Excited-state intramolecular proton transfer in photochromic jet-cooled N-salicylideneaniline

The laser induced fluorescence (LIF) and dispersed fluorescence spectra of jet-cooled N-salicylideneaniline have been observed in a supersonic free jet for the first time. The spectrum shows a very broad feature with no vibronic structure in the 24,800–28,800 cm−1 region except for a broad peak at 28,200 cm−1. The broad feature of the spectrum has been ascribed to the homogeneous broadening of vibronic bands due to very fast internal conversion from the 1(ππ∗) to 1(nπ∗) state which is located below the 1(ππ∗) state as well as to the excited-state intramolecular proton transfer (ESIPT) reaction. No resonance emission has been detected following the excitation of the 1(ππ∗) state of the enol form, while anomaly Stokes-shifted fluorescence was observed at 15,000–20,500 cm−1. The dispersed fluorescence spectra show two broad maxima separated by 700–1000 cm−1, suggesting that two keto tautomers are produced via the ESIPT reaction. The existence of the corresponding two cis-keto tautomers with similar energies has been suggested by density functional theory (DFT) calculations at the B3LYP/6-31G∗∗ level and ab initio calculations at the HF/6-31G∗∗ level.