The effect of molecular configuration and donor on the photophysical properties of 2-phenylfuro[2,3-b]quinoxaline based emitters

The Y-type compound 67dTPA-FQ combining 2-phenylfuro[2,3-b]quinoxaline (FQ) with triphenylamine (TPA) demonstrated to be a green fluorescence emitter (519 nm) with a Delta ES1T1 of 0.40 eV. Herein, TPA units in 67dTPA-FQ were replaced with 10-phenyl-10H-phenoxazine (Ph-PXZ) units to shift the S1 energy to a lower direction to reduce Delta ES1T1 and realize thermally-activated delayed fluorescence (TADF). Thus, Y-shaped (67dPXZ-FQ) and T-shaped (58dPXZ-FQ) compounds were synthesized. As the donor changed from TPA to PXZ, the emission bands of the Y- and T-shaped compounds in toluene shifted to 585 nm and 631 nm, respectively. As we expected, Delta ES1T1 for Y- and T-shaped compounds decreased to 0.12 eV and 0.15 eV, respectively, well lower than 0.3 eV. Unfortunately, the TADF feature was not observed. Theoretical calculations revealed that the HOMO and LUMO orbitals of both compounds were completely located at one of the PXZ fragment and 2-phenylfuro[2,3-b]quinoxaline moiety, respectively, yielding a complete electron separation by phenylene bridges between the acceptor FQ and PXZ. As a result, the oscillator strength (f) of the S1 -> S0 transition was only 0.0001-0.0002, leading to weak radiative transition capability of the S1 state. As a result, a low fluorescent yield of 25-36% was obtained. Moreover, the calculated spin-orbital coupling value between the S1 and T1 was only 0.13 cm^-1 for both 67dPXZ-FQ and 58dPXZ-FQ, suggesting weak driving capability for the reverse intersystem crossing process. As such, the TADF features of the two compounds were not observed.