Origin of Red-Shifted Phosphorescence from Triphenylamines: Triplet Excimer or Impurity?

Triphenylamine (TPA) was reported to exhibit temperature-dependent dual phosphorescence, where the red-shifted band was assigned as the excimeric phosphorescence with an energy shift of >3000 cm^-1 (J. Phys. Chem. 1991, 95, 7189). Here we show that purified TPA (purity: >99.97 %) shows a single phosphorescence band with a small energy shift of <200 cm^-1 under the same experimental conditions. The new experimental results, along with theoretical calculations, suggest that the previously reported triplet excimer of TPA is probably not valid and is most likely due to an unidentified impurity. As-received TPA samples, however, do exhibit temperature-dependent dual phosphorescence bands, and the wavelength, relative intensity, and temperature dependence of the lower-energy phosphorescence band vary significantly depending on dopant structures. It was found that dopant phosphorescence could still become dominant even in dilute third-party solutions of the host at low temperature.