Direct Observation of Rapid Triplet Harvesting by Radical Emitters

Spin doublet radical organic semiconductors can show near unity luminescence yield from their lowest energy excited state and are attractive as the emissive component in organic light-emitting diodes (OLEDs). <br/> <br/>Here we report our recent measurements of direct, rapid, spin-allowed energy transfer from triplet excitons generated within a closed-shell organic host to a doublet chromophore. We use a carbene-metal-amide (CMA-CF₃) as a model host, since following photoexcitation it undergoes extremely fast intersystem crossing to set up a population of triplet excitons within a few picoseconds. We track the subsequent energy transfer to the TTM-3PCz radical using transient absorption and temperature-dependent transient photoluminescence spectroscopies. These show that direct triplet-to-doublet energy transfer is the dominant channel that accounts for over 90% of all radical emission. OLEDs based on the CMA-CF₃:TTM-3PCz blend show improved device characteristics compared to radical OLEDs without triplet-enhanced energy transfer.<br/> <br/>Our design overcomes triplet-imposed performance limits for optoelectronics by activating spin-allowed triplet-doublet transfer on picosecond–nanosecond timescales, with light emission obtained orders of magnitude faster than derived from conventional triplet(-singlet) management technologies.<br/> <br/>This method allows photophysical studies to reflect the working mechanisms in OLEDs under operating conditions by mimicking spin statistics under electrical charge injection, which may be a powerful tool for the wider organics community.