First generation of polyradicals was synthesized to compare the performance of radicals in the polymer main chain versus radical as the pendant group. From these two systems, main chain polyradicals performed better, but the external quantum efficiencies of light-emitting diodes were low <1% for emission peaking beyond 700 nm in the near-infrared spectral region. The low performance originated from low solid-state photoluminescence yields of these polymers as well as solution processing of the devices, which commonly delivers lower efficiencies as compared to vacuum-deposited devices. Importantly, however, these experiments were the first demonstration of light-emitting diodes fabricated from conjugated polyradicals using solution-based processing. Second generation of polyradicals focused on tuning the emission wavelength. Depending on the extent of conjugation, polyradicals show either monomer-like emission below 700 nm or highly redshifted emission peaking beyond 800 nm. The latter originates from highly conjugated polyradicals, demonstrating that conjugation can be used as an effective tool to tune the wavelength of doublet emission.
After demonstrating the feasibility of polyradicals for use in solution-processed devices, the research extended to small molecular radicals and their device applications. Spectroscopic characterization aided the materials design towards mesityl-substituted TTM radicals (called MxTTM series). Mesitylation made symmetric radicals highly emissive, which contrasts with the previous understanding that symmetric radicals must be dark. Mesitylation also enhanced the emission of charge-transfer type radicals like TTM-3PCz, and the beneficiary obtained record-performing radical light-emitting diodes with an external quantum efficiency of 28% at 690 nm (internal quantum efficiency reaching 100%). Furthermore, coupling two charge-transfer radicals together boosted the photoluminescence yield to 100%, while the degree of conjugation between the two radicals affected the ground state spin multiplicity of the system and its response to an external magnetic field. Better understanding of conjugation and its relation to emission properties and spin–spin interactions will enable the access to higher spin states in small molecular and polymeric π-radicals.
The above-discussed results have been compiled into one submitted manuscript and two additional manuscripts currently being drafted. The beneficiary has also published a review article of the field in 2022. The beneficiary has presented the project results in two international conferences during 2022, and parts of the project have also been presented in conferences by other members of the host university.
