Dual emission from ion pairs produced by excited-state proton transfer in the naphthol–amine system at 77 K. Steric orientation of contact and separated ion pairs
Fluorescence in the 1-naphthol (ROH)–aliphatic amine (NR3) hydrogen-bonded system in non-polar rigid matrices at 77 K is found to be dual in nature. By means of nanosecond time-resolved emission spectroscopy the total fluorescence spectra can be resolved into two component spectra ascribable to contact and separated ion pairs produced by excited-state proton transfer in the hydrogen-bonded complex. The contact ion pair (CIP) is revealed to have a relatively shorter lifetime (5.3 ns) and a shorter-wavelength fluorescence peak (395 nm) compared to those (13.8 ns and 419 nm, respectively) of the separated ion pair (SIP). The fraction of SIP formation (α) is obtained as 0.27 in a PE film and 0.20 in MP glass, showing that CIP formation is greater than that of SIP in the ROH–NEt3 system. A significant increase in the value of α for SIP formation in the MeROH–NEt3 system is observed in comparison with that in the ROH–NEt3 system. By comparing the α values for SIP formation in different naphthol–amine systems it is possible to assign an out-of-plane (but not perpendicular) relative orientation between the excited naphtholate anion RO–* and the ammonium ion H[graphic omitted]R3 to SIP and an in-plane orientation to CIP. Changing the size of the alkyl group R of the amine gives no significant effect on excited-state ion-pair formation.