The ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) spectra of 2,7-tBuCzNB in dilute toluene are shown in Figure . The low-energy absorption band at a peak wavelength (labs) of 465 nm (molar absorptivity, ɛ, of 4.8 × 10 4 M -1 cm -1 ), similar to those of both Cz-SCz (labs of 451 nm) and DtBuCzB (labs of 467 nm) , can be attributed to the SRCT S0-S1 absorption transitions. 2,7-tBuCzNB emits at lPL of 493 nm with a relatively narrow FWHM of 32 nm/0.16 eV. The bathochromic shifting of the absorption and emission spectra compared to Cz-SCz (lPL of 472 nm) indicates that the additional carbazole and secondgeneration carbazole donors affect the electronic properties of the Cz-SCz core. The slightly broader emission compared to that of Cz-SCz (FWHM of 28 nm) can be attributed to larger degree of geometry relaxation because of the additional donors. The FPL value of 2,7-tBuCzNB in degassed toluene is 93%. There is a very small degree of positive solvatochromism, which indicates that the SRCT character of the S1 state is conserved across this family of solvents (Figure ). The steady-state PL and phosphorescence spectra of 2,7-tBuCzNB in 2-methyltetrahydrofuran (2-MeTHF) at 77 K were measured to determine the DEST (Figure ). The singlet and triplet energies, determined from the difference in the energy of the onsets of the steady-state PL and phosphorescence spectra, are 2.59 and 2.43 eV, respectively, resulting in a DEST of 0.16 eV, a value that is similar to the calculated value (DEST of 0.15 eV) and that of Cz-SCz (DEST of 0.18 eV). Time-resolved PL studies in toluene revealed monoexponential decay kinetics, with a fast prompt lifetime (τp) of 6.6 ns (Figure ). No delayed emission was observed in toluene solution, which is due to the magnitude of the DEST and the competing non-radiative decay processes to ISC/RISC cycling. The photophysical properties of thin films were then investigated. 1,3-bis(N-carbazolyl) benzene (mCP) was first used as the host material because of its suitably high triplet energy (ET = 2.9 eV). The absolute FPL values of 2,7-tBuCzNB at different doping concentrations in mCP were measured under a nitrogen atmosphere (Table ). The 1, 5 and 10 wt% doped films of 2,7-tBuCzNB in mCP have similar yet slightly red-shifting PL spectra, peaking at lPL of 496, 500 and 504 nm (FWHM of 41, 44 and 48 nm), while the corresponding FPLs remain high at 80, 81 and 77% (Figure ). Although FPLs are slightly lower than that in toluene, concentration quenching is negligible in this concentration range. When increasing the doping concentrations to 20 and 50 wt%, the PL spectra broaden and further red-shift to lPL of 509 and 516 nm (FWHM of 52 and 65 nm), respectively, as aggregation of the emitters becomes pronounced; the FPL values decrease precipitously to 61 and 36%, respectively (Figure ).