Luminescence studies of proton transfer in the excited electronic states of hydrogen bonded quinoline and isoquinoline

Abstract

In the absence of a hydrogen bonder, quinoline only éxhibits phosphorescence in a 3-methypentane glass at 77 K. Under these conditions isoquinoline both fluoresces and phosphoresces weakly. The addition of a weak hydrogen bonder such as ethanol induces structured ultraviolet fluorescence originating from the lowest excited singlet state. More acidic alcohols such as trifluoroethanol and hexafluoro-2-propanol induce both the structured ultraviolet fluorescence, and a now broad, violet fluorescence. This new fluorescence resembles, but differs from, that of the corresponding protonated cation. An analysis of the excitation spectrum, which differs from the excitation spectrum of the structured fluorescence, leads to the conclusion that the proton has been displaced toward the nitrogen heterocycle in the excited state.

Steady State and Time-Resolved Fluorescence Study of Isoquinoline: Reinvestigation of Excited State Proton Transfer

“…At this stage, we can anticipate that in aqueous medium in the ground state, IQ may exist in two formsneutrals and cations and, of course, as is evident from the absorption spectrum in water, the relative number of neutrals to the cations should be large enough. Our observation of cation in the ground state is in contrast to the suggestion of Moomaw and Anton. , …”

“…In water, IQ exhibits an absorption band with structural vibronic progressions with a maximum at ∼320 nm. These results are similar to that observed by Huber et al − in ethanol and methyl pentane, respectively. However, in basic medium, a slight but marked difference in the spectral region of 325–350 nm is observed.…”

“…The electronic states and luminescence properties of isoquinoline (IQ) and its derivatives have been the subject of considerable interest and study. − Being isoelectronic with naphthalene, these molecules provide useful comparisons for checking the electronic and vibrational state assignments, ionization potentials, and other properties of the parent hydrocarbon.…”

“…Moomaw and Anton , reported the effect of hydrogen bonding solvent on the spectral properties of IQ and proposed the mechanism of proton transfer reaction in the excited state. They observed broad emission, which was explained in terms of cation-like fluorescence and a double well potential for proton tunnelling was suggested.…”

Synthesis and Fluorescent Properties of Novel Isoquinoline Derivatives

“…An inspection of the literature data indicates that isoquinoline-3-amine derivatives exhibit fluorescent properties and may constitute potential fluorophores [13,14,15]. Moreover, for many years the electronic states and optical properties of isoquinoline and its derivatives have been an object of particular interest and study [16,17,18,19,20,21,22,23,24,25]. Recently, notable progress has been made in the synthesis and development of novel organic fluorophores with diverse applications and uses in in vivo imaging.…”