Molecules present a versatile platform for quantum information science1,2 and are candidates for sensing and computation applications3,4. Robust spin-optical interfaces are key to harnessing the quantum resources of materials5. To date, carbon-based candidates have been non-luminescent6,7, which prevents optical readout via emission. Here we report organic molecules showing both efficient luminescence and near-unity generation yield of excited states with spin multiplicity S > 1. This was achieved by designing an energy resonance between emissive doublet and triplet levels, here on covalently coupled tris(2,4,6-trichlorophenyl) methyl-carbazole radicals and anthracene. We observed that the doublet photoexcitation delocalized onto the linked acene within a few picoseconds and subsequently evolved to a pure high-spin state (quartet for monoradical, quintet for biradical) of mixed radical-triplet character near 1.8 eV. These high-spin states are coherently addressable with microwaves even at 295 K, with optical readout enabled by reverse intersystem crossing to emissive states. Furthermore, for the biradical, on return to the ground state the previously uncorrelated radical spins either side of the anthracene shows strong spin correlation. Our approach simultaneously supports a high efficiency of initialization, spin manipulations and light-based readout at room temperature. The integration of luminescence and high-spin states creates an organic materials platform for emerging quantum technologies.
Triplet excited states in organic semiconductor materials and devices are notoriously difficult to detect and study with established spectroscopic methods. Yet, they are a crucial intermediate step in next-generation organic light emitting diodes (OLED) that employ thermally activated delayed fluorescence (TADF) to upconvert non-emissive triplets to emissive singlet states. In organic photovoltaic (OPV) devices, however, triplets are an efficiency-limiting exciton loss channel and are also involved in device degradation. Here, we introduce an innovative spin-sensitive method to study triplet states in both, optically excited organic semiconductor films, as well as in electrically driven devices. The method of transient optically detected magnetic resonance (trODMR) can be applied to all light-emitting materials whose luminescence depends on paramagnetic spin states. It is thus an ideal spectroscopic tool to distinguish different states involved and determine their corresponding time scales. We unravel the role of intermediate excited spin states in opto-electronic and photovoltaic materials and devices and reveal fundamental differences in electrically and optically induced triplet states.
Carbon chains with an odd number of C atoms are reactive intermediates with a high biradical character. Here we report a joint experimental and computational investigation of the dynamics of diphenylpropynylidene, C6H5-C3-C6H5, in dichloromethane and ethanol. The biradical is generated by ultraviolet light from 1,3-diphenyldiazopropyne. Electron paramagnetic resonance spectra are recorded to elucidate the spin multiplicity and geometry of the biradical. In both solvents a triplet ground state at 4 K is verified. Transient absorption spectra provide insight into the fate of the biradical. A study in deaerated dichloromethane permits us to follow the photophysics of diphenylpropynylidene and to extract time constants for its vibrational as well as electronic relaxation. In the presence of oxygen, a more complex photochemistry is observed that permits us to derive a model for the reaction of the biradical with O2. In ethanol, the spectra recorded in the presence and absence of O2 are very similar, which can be explained by the similarity of the chromophores of the reaction products.
