We next investigated the photophysical properties of DOBDiKTa in doped films in a host of suitably high triplet energy (ET1 = 2.81 eV), 1,3-bis(N-carbazolyl)benzene (mCP) as this was the host used in our prior studies of DiKTa. The emission in mCP doped film red-shifts with increasing doping concentration from 1.5 to 10 wt% (Figure ), reflecting increasing contributions from aggregates to the emission. The FPL likewise drops from 75% to 47% with increasing doping concentration (Figure ). Thus, 1.5 wt% doped film in mCP were used for the solid-state photophysical study as shown in Figure . The steady-state PL shows narrowband blue emission at lPL of 460 nm with FWHM of 37 nm Figure , which is redshift and broader compared to the spectrum in dilute toluene solution (lPL= 449 nm, FWHM= 26 nm). These spectral changes can be attributed to the presence of aggregates as well as hostguest interactions. The emission is also of intermediate wavelength to those of DiKTa (467 nm) and tBuDOBNA (407 nm), Figure . The S1 energy was inferred from the high-energy onset of the steady-state PL at 77 K to be 2.77 eV while the T1 energy was determined from the onset of the time-gated phosphorescence at 77 K to be 2.60 eV; thus, the DEST is 0.17 eV (Figure ). The PL spectrum is somewhat quenched in air, with FPL decreasing from 75% to 67%. The transient PL decay under vacuum shows biexponential kinetics with a tp of 2.6 ns and a td of 43 µs (Figure ). By contrast, DiKTa shows prompt and delayed lifetimes of 4.8 ns and 242 µs, while tBuDOBNA only shows a prompt lifetime of 9.8 ns (Figure ). A rate constant analysis is summarized in Table . The singlet radiative transition rate constant (k S r) of 6.0×10 7 is slower than rate constants of intersystem crossing (kISC= 2.73×10 8 ), which is due to the strong spin orbital coupling (SOC) induced by the carbonyl groups. The rate constant for RISC, kRISC, at 8.8×10 4 is almost 3.5-fold faster than that of DiKTa (kRISC, at 2.75×10 4 ).