Evidence for ¹L_a, ¹L_b Dual State Emission in 1‐naphthol and 1‐methoxynaphthalene Fluorescence in Liquid Solutions

Abstract

The steady‐state fluorescence spectra of 1‐methoxynaphthalene and 1‐naphthol were measured in pure organic solvents and in binary mixtures of water and several organic solvents. The 1‐isomers exhibited a much larger fluorescence Stokes‐shift than the corresponding 2‐isomers. The emission spectra of 1‐methoxynaphthalene and 1‐naphthol in pure nonpolar organic solvents exhibited roughly the same structured spectral features, while the fluorescence spectra in water and formamide exhibited much broader red‐shifted and less structured spectral features. In accord with previous observations, we attribute these spectral changes to two emitting states, 1Lb and 1La, whose relative fluorescence intensities are sensitive to solvent polarity. Our analysis of the fluorescence spectra of 1‐naphthol and 1‐methoxynaphthalene by Pekarian functions have demonstrated that the 1La state becomes the dominant emitting state in polar solvents. In addition, the 1La state was found to be further stabilized in hydrogen‐bond‐donating solvents. In contrast to previous suggestions, the onset of the excited‐state proton transfer reaction from 1‐naphthol occurred at higher solvent polarities than those required for the establishment of a dominantly 1La emitting state.

Solvent‐Dependent Photoacidity State of Pyranine Monitored by Transient Mid‐Infrared Spectroscopy

“…A similar procedure was previously used to model the 1 L b and 1 L a transitions of 1-and 2-naphthols. [39] Interestingly, in aprotic DMSO (Figure 14 b) the lowenergy Pekarian already resembles more a transition to a 1 L a than to a 1 L b state as judged by the appearance of the vibronic features (spacing of about 1220 cm À1 and width at half maxi- mum of the first vibronic feature of about 700 cm À1 ) in the absorption spectra. [71] The high-energy Pekarian appears featureless and consists of a much larger area than the low-frequency one.…”

“…For 1-naphthol [36,37] the 1 L b ! 1 L a transition is considered to be the ESPT-determining step [9,38,39] In contrast, in a recent combined experimental/theoretical study of ESPT of pyranine [40][41][42] the rate determining step is not the conversion from the optically accessible locally excited (LE) state (or alternatively the 1 L b state), to the second photoacid electronic excited n-p* CT state (or 1 L a state), but the transition of the photoacid CT to photobase CT states.…”

Solvent Assisted Photoacidity

“…12.6. The situation is more complex when two different singlet states are involved in the photon-absorption and photon-emission processes of the photoacid [76,[89][90][91][92][93][94][95][96]. In cases where the vertically accessed excited state level of the photoacid and the relaxed excited state level of the photoacid are both the relatively nonpolar 1 L b state the difference between the calculated value and the thermodynamic value of pK * a is expected to be small.…”

Intermolecular Excited‐State Proton Transfer in Clusters of 1‐Naphthol with Water and with Ammonia

“…In this context, it is very interesting to note that Magnes, Strashnikova, and Pines show evidence for inversion without proton transfer in naphthols. 37 The current description of the naphthol-water cluster Time, fs Fig. 5.…”