Temperature effects on dual emission from ion pairs produced by excited-state proton transfer in the 1-naphthol–triethylamine system

Abstract

1-Naphthol (ROH)–triethylamine(NEt₃) hydrogen-bonded systems in non-polar rigid matrices at 77 K show dual fluorescence ascribable to the contact ion pair (CIP), with an in-plane orientation between excited naphtholate and alkylammonium ions, and the separated ion pair (SIP) with an out-of-plane orientation between them. However, at room temperature it was found that there was remarkable quenching of the excited singlet state of the ROH–NEt₃ hydrogen-bonded system. Using nanosecond time-resolved emission spectroscopy, temperature effects on the fluorescence-decay kinetics of the ROH–NEt₃ system has been studied in polyethylene film and methylcyclohexane–isopentane (MP, 3:1 vol.: vol.) to obtain the excited state behaviour of CIP and SIP. The fluorescence intensities and peaks did not change in the temperature range 77–100 K. Above 100 K, a gradual red shift of the total fluorescence peak with loss of intensity was observed, the effect being more pronounced in MP, especially after the glass-softening temperature (ca. 115 K). Two different quenching processes were found to be operative. Rapid quenching competing with excited-state proton transfer in both hydrogen-bonded systems occurred at high temperatures (> 100 K). This resulted in a considerable decrease of the total fluorescence quantum yield (ϕ^Σ) with no significant change in fluorescence lifetimes in the temperature range 100–130 K. The quenching mechanism might be a fast internal conversion due to the out-of-plane bending motion of O–H ⋯ N bond. Above 130 K, a new quenching process resulting in the decrease of fluorescence lifetimes of both SIP and CIP also appeared. This dynamic quenching of excited ion pairs is probably caused by a charge–transfer interaction. In addition, from the appearance of neutral ROH phosphorescence, enhanced intersystem crossing and subsequent reverse proton transfer in the triplet state might be taking place.