We next focused our attention on the photophysical properties of TPPQ, 4DMAC-TPPQ and 4PXZ-TPPQ doped in poly(methylmethacrylate) (PMMA) and in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and in neat thin films. Initially, we measured the photophysical properties of the three emitters as 10 wt% doped films in PMMA as the polarity of this host closely mimics that of toluene. The absorption bands of all three compound are of similar energy to those in toluene, with the weak CT band observed in the range of 451-650 nm for 4DMAC-TPPQ while it is more red-shifted for 4PXZ-TPPQ (Figure ). Upon photoexcitation at 315 nm, TPPQ shows a structured emission profile at 466 nm (FPL = 4.7%), similar to that measured in toluene, while the CT-based unstructured emission is at 635 nm for 4DMAC-TPPQ (FPL = 14.3%) and at 681 nm for 4PXZ-TPPQ (FPL = 3.0%), emission that is blue-shifted by 15 and 81 nm, respectively, compared to the λPL in toluene (Figure ). The emission of TPPQ decays with biexponential kinetics, with average lifetime, tPL, of 2.1 ns. The time-resolved PL of 4DMAC-TPPQ and 4PXZ-TPPQ shows multiexponential decay kinetics, with average prompt lifetimes, tp, of 25.2 and 15.8 ns, respectively (Figure ), and average delayed lifetimes, td, of 390.9 and 342.5 µs, respectively (Figures S32 and Table ). The contribution from the delayed emission increases with increasing temperature, indicating that its origin is TADF. 2.7 a spin-coated film and annealed at 55 o C for 1 minute; b drop-cast film and annealed at 70 o C for 1 minute; c PL measured under vacuum, λexc = 340 nm for all films except 100 wt% 4PXZ-TPPQ, which has λexc = 465 nm; d photoluminescence quantum yield (FPL) of thin films were determined using an integrating sphere (λexc = 305 nm or 340 nm) under a N2 atmosphere at 298 K. The values in parentheses are in the presence of O2 at 298 K. e ES1 and ET1 were obtained from the onset of the steady-state and phosphorescence spectra (time window: 1 ms -9 ms), respectively, of doped films in CBP at 77 K, λexc = 340 nm, and for 100 wt% 4PXZ-TPPQ, λexc = 465 nm. f ∆EST = ES1 -ET1. g prompt lifetime was measured by TCSPC, λexc = 375 nm, the average lifetime were calculated by using equation 𝜏 012 = Σ𝐴 3 𝜏 3 4 /Σ𝐴 3 𝜏 3 , (where 𝐴 3 is the pre-exponential for lifetime 𝜏 3 ); h delayed emission was measured by MCS, using a microsecond flash lamp as an excitation source, λexc = 340 nm. i ks,nr, ks,r, kRISC and kISC were calculated according to the method given in Refs. To best evaluate whether these compounds could act as promising emitters in red-to-NIR OLEDs, we next turned our attention to assessing the photophysics of these three compounds in an OLED-relevant host, CBP, as this host matrix has a sufficiently high triplet energy (T1 = 2.6 eV) to confine excitons onto the emitters. The lPL of the 10 wt% doped film of TPPQ in CBP is 548 nm and the PL spectrum is much broader (FWHM = 100 nm) and around 80 nm bathochromically shifted compared to that in PMMA (λPL = 466 nm) and in solutions (λPL = 456 nm), Figure . There is only a small red-shift (only 10 nm) of the lPL and moderate decrease in the FPL upon increasing the doping concentration of TPPQ from 5 to 30 wt% in CBP (Figure , Table ). To understand the large red-shit in the emission band of TPPQ in CBP in comparison to both in solution and in PMMA, we checked the PL spectrum of TPPQ in another host, 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi), Figure . The 5 wt% doped films of TPPQ in TBPi emits at λPL of 470 nm and has a FPL of 4.8%, which is very similar to the properties in toluene and in the 10 wt% doped film in PMMA. Notably, the remarkably red-shifted emission of the 10 wt% doped film of TPPQ in CBP is reminiscent of the emission profile after UV exposure in ethereal solvents (vide-supra). This implies that either there is an exciplex between CBP and TPPQ that emits at a similar energy to the TPPQ excimer or that TPPQ preferentially forms excimers in the CBP host. The photophysical properties of the doped films (5-50 wt%) of 4DMAC-TPPQ and 4PXZ-TPPQ in CBP and the neat films are shown in Figures , S34-S39 and Table . Of these films, the highest FPL was found to be 21.0% for 4DMAC-TPPQ and 10.0% for 4PXZ-TPPQ as 10 and 5 wt% doped films in CBP, respectively. The lPL of the 10 wt% doped film of 4DMAC-TPPQ in CBP is 635 nm, which is very similar to that of the 10 wt% doped film in PMMA (Figures and). The emission for this compound does not significantly change across a doping range from 5-50 wt% and even as the neat film (lPL ranging from 627 to 654 nm, Figure and Table ). The λPL of 4PXZ-TPPQ is significantly shifted to the NIR region compared to that of 4DMAC-TPPQ, at 682 nm for the 5 wt% doped film, at 713 nm for the 50 wt% doped film, and 770 nm for the neat film (Figure ). The substantially red-shifted emission as a function of doping concentration of 4PXZ-TPPQ in comparison to 4DMAC-TPPQ may be attributed to the formation of aggregates in the former resulting in strong interactions between adjacent chromophores. The emission profile of the neat films of 4DMAC-TPPQ and 4PXZ-TPPQ match well those in THF:H2O (fw = 80 %), indicating that the species responsible for the emission in each is the excimer.