Table . Extinction coefficients and energetic positions of the photoluminescence obtained from the optical spectroscopy of 4TABN1, 5TABN2 and 6TABN3 in toluene and as 10 wt% DPEPO films. With a view to assessing the potential of these compounds as emitters for OLEDs, we next evaluated their photophysical properties as 10 wt% blend films in DPEPO (bis [2-(diphenylphosphino)phenyl] ether oxide), which has a suitably high triplet energy of around 3.0 eV. The room temperature and 77 K PL spectra of the films are presented in Figure . Similar to the solution-state PL spectra, all compounds show a broad and structureless high energy band, with lPL that are red-shifted by around 0.1 eV relative to their solution-state PL spectra (Table ). The slightly lower energy of the S1 states of the DPEPO films than the corresponding values in solution, obtained using the onset of the prompt fluorescence spectra, could be attributed to the greater polarity of the DPEPO film than toluene. Similar to that observed in solution, the phosphorescence spectra are red-shifted, yielding DEST gaps ranging between 0.17 and 0.21 eV. In addition to the high energy band at around 2.65 eV, 4TABN1 shows a low energy band at 2.12 eV (585 nm), which was not observed in solution. For 5TABN2 and 6TABN3, this low energy band is absent. To understand the origin of the low energy band in 4TABN1, its steady-state emission was measured at a number of different concentrations (Figure ). Figure shows the steady-state PL spectra of 4TABN1, 5TABN2 and 6TABN3 at 1 wt%, 10% and 30 wt% doped DPEPO films, as well as neat films. The observed red-shift of the high energy band (around 2.7 eV)