A metal free blue emission by the protonated 2,2′:6′,2"-terpyridine hexafluorophosphate

Mono-protonated 2,2':6',2 ''-terpyridine (terpy) compound, 2,2':6',2 ''-terpyridinium hexafluorophosphate {[terpyH]PF6}, was prepared and characterized by electrospray ionization mass spectrometry, UV-Vis and emission spectroscopies, cyclic voltammogram (CV), and X-ray crystallography. The proton is located at the central pyridine nitrogen atom of terpy and was found in electron density difference map, which is completely different from the previously reported mono-protonated species, such as [terpyH]ReO4 and [terpyH]CF3SO3. The H+ in the latter species is located at the terminal pyridine nitrogen. A small difference of proton positions was found to give a drastic difference of crystal patterns. For [terpyH]PF6 in CH3CN, a new pi-pi* band in the UV region and a blue emission (Phi = 0.052 at room temperature) appeared whose maxima were red-shifted relative to those of the corresponding neutral terpy ligand. CV for the proton adduct showed the first reduction wave at around -0.6 V, being more positive than that of the neutral species. Finally, the density-functional theory (DFT) approach was used to investigate isomerization processes of the protonated terpy. Copyright (C) 2008 John Wiley & Sons, Ltd.