Fluorescence excitation and excited state intramolecular proton transfer of jet-cooled naphthol derivatives: Part 1. 1-hydroxy-2-naphthaldehyde

The S₀ → S₁ fluorescence excitation spectrum of jet-cooled 1H2N with origin at 25484 cm^-1 has been measured. Twelve totally symmetric modes and five non-totally symmetric modes have been assigned in the excitation spectrum. Theoretical calculations at DFT B3LYP/6-31G ** and CIS/6-31G** levels indicate that the 1H2N molecular geometry is more planar in the S₁ state than in the ground state. The geometry of the naphthalene ring changes upon excitation and promotes a number of totally symmetric ring stretching modes, in the excitation spectrum. As a result of the geometry change upon excitation a number of non-totally symmetric modes gain intensity. Based on a rotational envelope fitting procedure the average excited rotational state lifetime was estimated to be between 7 and 16 ps for 0 ≤ E ≤ hc × 800 cm^-1 (E is excess energy above the S₁ origin). The decay rate coefficients, k, of the rotational S₁ states, are not constant over this range of excess energies. By applying a Golden Rule model, it was determined that internal conversion to S₀ is unlikely to be the sole non-radiative process contributing to the decay of the excited states. It was concluded that excited state intramolecular proton transfer (ESIPT) plays a role in the observed behaviour of the rate co-efficient with excess energy. The observation of (i) a sharp increase in rate coefficient, k, above an excess energy of ∼550 cm ^-1, and (ii) a significant number of high intensity fluorescence excitation spectrum features above an excess energy of ∼700 cm^-1, may indicate the presence of an energy barrier of ∼550 cm^-1, between the enol and keto geometries in the S₁ state. This result supports the conclusions of S. De, S. Ash, S. Dalai and A. Misra, J. Mol. Struc. Theochem, 2007, 807, 33-41, who estimated a barrier to ESIPT of ∼750 cm^-1. It was concluded that ESIPT occurs in 1H2N, across an energy barrier with a rate constant, k_pt ≤ 10¹¹ s ^-1. Hence, at low excess energies (≤ 550 cm^-1), the observed emission band originates predominantly from the keto tautomer. Above an excess energy of ∼1600 cm^-1, 1H2N decays predominantly via a non-radiative mechanism.