Thermally activated delayed fluorescence carbazole-triazine dendrimer with bulky substituents

Carbazole-triazine dendrimers with a bulky terminal substituent were synthesized,and the thermally activated delayed fluorescence(TADF)property was investigated.Compared to unsubstituted carbazole dendrimers,dendrimers with bulky terminal substituents showed comparable to better photoluminescence quantum yields(PLQY)in neat films.Phenylfluorene(PF)-substituted dendrimers showed the highest PLQY of 81%,a smallΔEst of 0.06 eV,and the fastest reverse intersystem crossing(RISC)rate of∼1×10^(5 )s^(−1) compared to other dendrimers.Phosphorescence measurements of dendrimers and dendrons(fragments)indicate that the close proximity of the triplet energy of phenylfluorene-substituted carbazole dendrons(^(3)LE)to that of phenylfluorene-substituted dendrimers(^(1)CT,^(3)CT)contributes to RISC promotion and improves TADF efficiency.Terminal modification fine-tunes the energy level and suppresses intermolecular interactions,and this study provides a guideline for designing efficient solution-processable and non-doped TADF materials.

Phenyl-triggered photophysical switching between normal fluorescence and delayed fluorescence in phthalonitrile-based luminophores

Herein,a facile strategy for switching luminescent properties between normal fluorescence and thermally activated delayed fluorescence(TADF)is presented.Two luminophoric molecules,VPN-Ph and VPN-H,combining phthalonitrile as an electron-accepting core and triphenylamines as electron-donating peripheries with and without two phenyl groups,are newly developed.A comparative study on their structural and photophysical properties is conducted.While non-phenyl VPN-H does not exhibit TADF but normal fluorescence,phenyl-introduced VPNPh exhibits TADF with a high photoluminescence quantum yield as a consequence of the phenyl-triggered steric congestion.By virtue of the TADF feature,an organic light-emitting diode(OLED)incorporating VPN-Ph as an emitter achieves a maximum external electroluminescence(EL)quantum efficiency as high as 28.0%,which is five times higher than that of the VPN-H-based OLED.Thus,phenyl-triggered geometric modulation has a drastic impact on the resulting photophysical and EL properties,leading to TADF on/off switching.