Concentration-dependent luminescence of ionic liquids containing trialkyl(pentafluorocyclotriphosphazenyl)ammonium moieties

Room-temperature ionic liquid compounds (CpzNR3+X−) consisting of trialkyl(pentafluorocyclotriphosphazenyl)ammonium (CpzNR3+) and anions (X−) such as chloride and bis(trifluoromethylsulfonyl)imide (TFSI−) emitted blue luminescence under an excitation of 360 nm. The luminescent quantum yields (Φem) of CpzNR3+X− were determined to be 0.012–0.044 in methanol solution at 360 nm excitation. The excitation spectra and luminescent lifetime (τ) measurements indicated the existence of two luminescent species. The appearance of luminescence from the pentafluorocyclotriphosphazenyl (Cpz) chromophore at longer wavelengths and the dependence of the luminescent intensity upon the concentration of CpzNR3+X− revealed that the observed luminescence was attributed to the J-aggregates of the Cpz chromophore. The formation of these aggregates was also evidenced by the concentration-dependent 1H-NMR chemical shift. The J-aggregates involved both luminescent aggregates of smaller sizes with shorter τ (1.0 ns) and those of larger sizes with longer τ (5.0 ns). The Φem of the aggregates with larger sizes were enhanced with increasing CpzNR3+X− concentration. Thus, the luminescence stemming from the luminescent aggregates can be reasonably explained by the “aggregation-induced enhanced emission” (AIEE) mechanism.