With their unusual electronic structures, organic radical molecules display luminescence properties potentially relevant to lighting applications; yet, their luminescence quantum yield and stability lag behind those of other organic emitters. Here, we designed donor–acceptor neutral radicals based on an electron-poor perchlorotriphenylmethyl or tris(2,4,6-trichlorophenyl)methyl radical moiety combined with different electron-rich groups. Experimental and quantum-chemical studies demonstrate that the molecules do not follow the Aufbau principle: the singly occupied molecular orbital is found to lie below the highest (doubly) occupied molecular orbital. These donor–acceptor radicals have a strong emission yield (up to 54%) and high photostability, with estimated half-lives reaching up to several months under pulsed ultraviolet laser irradiation. Organic light-emitting diodes based on such a radical emitter show deep-red/near-infrared emission with a maximal external quantum efficiency of 5.3%. Our results provide a simple molecular-design strategy for stable, highly luminescent radicals with non-Aufbau electronic structures.
Subscribe to Journal
Get full journal access for 1 year
only $17.42 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
The data that support the results of this study are available at https://doi.org/10.17863/CAM.41550.
Simao, C. et al. Molecular platform for non-volatile memory devices with optical and magnetic responses. Nat. Chem. 3, 359–364 (2011).
Morita, Y. et al. Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers. Nat. Mater. 7, 48–51 (2008).
Dhimitruka, I., Bobko, A. A., Eubank, T. D., Komarov, D. A. & Khramtsov, V. V. Phosphonated trityl probes for concurrent in vivo tissue oxygen and pH monitoring using electron paramagnetic resonance-based techniques. J. Am. Chem. Soc. 135, 5904–5910 (2013).
Peng, Q., Obolda, A., Zhang, M. & Li, F. Organic light-emitting diodes using a neutral π radical as emitter: the emission from a doublet. Angew. Chem. Int. Ed. 54, 7091–7095 (2015).
Obolda, A., Ai, X., Zhang, M. & Li, F. Up to 100% formation ratio of doublet exciton in deep-red organic light-emitting diodes based on neutral π-radical. ACS Appl. Mater. Interfaces 8, 35472–35478 (2016).
Ai, X. et al. Efficient radical-based light-emitting diodes with doublet emission. Nature 563, 536–540 (2018).
Hattori, Y., Kusamoto, T. & Nishihara, H. Enhanced luminescent properties of an open-shell (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical by coordination to gold. Angew. Chem. Int. Ed. 54, 3731–3734 (2015).
Hattori, Y., Kusamoto, T. & Nishihara, H. Luminescence, stability, and proton response of an open-shell (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical. Angew. Chem. Int. Ed. 53, 11845–11848 (2014).
Heckmann, A. et al. Highly fluorescent open-shell NIR dyes: the time-dependence of back electron transfer in triarylamine-perchlorotriphenylmethyl radicals. J. Phys. Chem. C 113, 20958–20966 (2009).
Velasco, D. et al. Red organic light-emitting radical adducts of carbazole and tris(2,4,6-trichlorotriphenyl)methyl radical that exhibit high thermal stability and electrochemical amphotericity. J. Org. Chem. 72, 7523–7532 (2007).
Fox, M. A., Gaillard, E. & Chen, C. C. Photochemistry of stable free radicals: the photolysis of perchlorotriphenylmethyl radicals. J. Am. Chem. Soc. 109, 7088–7094 (1987).
Armet, O. et al. Inert carbon free radicals. 8. Polychlorotriphenylmethyl radicals: synthesis, structure, and spin-density distribution. J. Phys. Chem. 91, 5608–5616 (1987).
Ai, X., Chen, Y., Feng, Y. & Li, F. A. Stable room-temperature luminescent biphenylmethyl radical. Angew. Chem. Int. Ed. 57, 2869–2873 (2018).
Hattori, Y., Kusamoto, T. & Nishihara, H. Highly photostable luminescent open-shell (3,5-dihalo-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radicals: significant effects of halogen atoms on their photophysical and photochemical properties. RSC Adv. 5, 64802–64805 (2015).
Kimura, S. et al. Organic radical with two pyridyl groups: high photostability and dual stimuli-responsive properties, with theoretical analyses of photophysical processes. Chem. Sci. 9, 1996–2007 (2018).
Hicks, R. G. What’s new in stable radical chemistry?. Org. Biomol. Chem. 5, 1321–1338 (2007).
Ballester, M., Riera-Figueras, J., Castaner, J., Badfa, C. & Monso, J. M. Inert carbon free radicals. I. Perchlorodiphenylmethyl and perchlorotriphenylmethyl radical series. J. Am. Chem. Soc. 93, 2215–2225 (1971).
Schweyen, P., Brandhorst, K., Wicht, R., Wolfram, B. & Broring, M. The corrole radical. Angew. Chem. Int. Ed. 54, 8213–8216 (2015).
Sun, Z., Ye, Q., Chi, C. & Wu, J. Low band gap polycyclic hydrocarbons: from closed-shell near infrared dyes and semiconductors to open-shell radicals. Chem. Soc. Rev. 41, 7857–7889 (2012).
Bohr, N. Über die Anwendung der Quantentheorie auf den Atombau. I. Die Grundpostulate der Quantentheorie. Z. Phys. 13, 117–165 (1923).
Mallion, R. B. & Rouvray, D. H. Molecular topology and the Aufbau principle. Mol. Phys. 36, 125–128 (1978).
Wang, Y. et al. Radical cation and neutral radical of aza-thiahelicene with SOMO−HOMO energy level inversion. J. Am. Chem. Soc. 138, 7298–7304 (2016).
Gryn’ova, G., Coote, M. L. & Corminboeuf, C. Theory and practice of uncommon molecular electronic configurations. WIREs Comput. Mol. Sci. 5, 440–459 (2015).
Franchi, P., Mezzina, E. & Lucarini, M. SOMO–HOMO conversion in distonic radical anions: an experimental test in solution by EPR radical equilibration technique. J. Am. Chem. Soc. 136, 1250–1252 (2014).
Kusamoto, T., Kume, S. & Nishihara, H. Cyclization of TEMPO radicals bound to metalladithiolene induced by SOMO–HOMO energy-level conversion. Angew. Chem. Int. Ed. 49, 529–531 (2010).
Gryn’ova, G., Marshall, D. L., Blanksby, S. J. & Coote, M. L. Switching radical stability by pH-induced orbital conversion. Nat. Chem. 5, 474–481 (2013).
Martin, R. L. Natural transition orbitals. J. Chem. Phys. 118, 4775–4777 (2003).
Komamine, S., Fujitsuka, M., Ito, O. & Itaya, A. Photoinduced electron transfer between C60 and carbazole dimer compounds in a polar solvent. J. Photochem. Photobiol. A 135, 111–117 (2000).
Niu, Y., Peng, Q., Deng, C., Gao, X. & Shuai, Z. Theory of excited state decays and optical spectra: application to polyatomic molecules. J. Phys. Chem. A 114, 7817–7831 (2010).
Hayashi, M., Mebel, A. M., Liang, K. K. & Lin, S. H. Ab Initio calculations of radiationless transitions between excited and ground singlet electronic states of ethylene. J. Chem. Phys. 108, 2044–2055 (1998).
Shizu, K. et al. Enhanced electroluminescence from a thermally activated delayed-fluorescence emitter by suppressing nonradiative decay. Phys. Rev. Appl. 3, 014001–014007 (2015).
Shizu, K. et al. Highly efficient blue electroluminescence using delayed-fluorescence emitters with large overlap density between luminescent and ground states. J. Phys. Chem. C 119, 26283–26289 (2015).
Chen, X.-K., Ravva, M. K., Li, H., Ryno, S. M. & Brédas, J.-L. Effect of molecular packing and charge delocalization on the nonradiative recombination of charge-transfer states in organic solar cells. Adv. Energy Mater. 6, 1601325 (2016).
Sugawara, T., Komatsu, H. & Suzuki, K. Interplay between magnetism and conductivity derived from spin-polarized donor radicals. Chem. Soc. Rev. 40, 3105–3118 (2011).
Kim, D.-H. et al. High-efficiency electroluminescence and amplified spontaneous emission from a thermally activated delayed fluorescent near-infrared emitter. Nat. Photon. 12, 98–104 (2018).
Ye, H. et al. Near-infrared electroluminescence and low threshold amplified spontaneous emission above 800 nm from a thermally activated delayed fluorescent emitter. Chem. Mater. 30, 6702–6710 (2018).
Körzdörfer, T. & Brédas, J.-L. Organic electronic materials: recent advances in the DFT description of the ground and excited states using tuned range-separated hybrid functionals. Acc. Chem. Res. 47, 3284–3291 (2014).
Sun, H. et al. Impact of dielectric constant on the singlet–triplet gap in thermally activated delayed fluorescence materials. J. Phys. Chem. Lett. 8, 2393–2398 (2017).
Gaussian 09 rev. D01 (Gaussian, Inc., 2009).
H.G., Q.P., S.D., X.A., M.Z. and F.L. are grateful for financial support from the National Natural Science Foundation of China (grant nos. 91833304, 51673080 and 11804156), the National Key R&D Programme of China (grant no. 2016YFB0401001), the National Key Basic Research and Development Programme of China (973 programme, grant no. 2015CB655003) and the programme ‘JLUSTIRT’ (grant no. 2019TD-33). Q.P. acknowledges support from the Nanjing Tech Start-up Grant (38274017104). X.-K.C., V.C. and J.-L.B. acknowledge support from the Georgia Institute of Technology, Georgia Research Alliance, Vasser-Woolley Foundation and Kyulux. Q.G., D.C., E.W.E., A.J.G. and R.H.F. would like to thank the EPSRC for funding this work (EP/M01083X/1, EP/M005143/1). Q.G. is also grateful for financial support from the China Scholarship Council and Cambridge Trust. D.C. also acknowledges support from the Royal Society (grant no. UF130278). F.L. is an academic visitor at the Cavendish Laboratory, Cambridge, and is supported by the Talents Cultivation Programme (Jilin University, China).
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
Guo, H., Peng, Q., Chen, X. et al. High stability and luminescence efficiency in donor–acceptor neutral radicals not following the Aufbau principle. Nat. Mater. 18, 977–984 (2019). https://doi.org/10.1038/s41563-019-0433-1
Materials Science and Engineering: R: Reports (2020)
Radiative and Nonradiative Recombinations in Organic Radical Emitters: The Effect of Guest–Host Interactions
Advanced Functional Materials (2020)
Quantum Dynamics Simulation of Doublet Excitation and Magnetic Field Effect in Neutral Radical Materials
The Journal of Physical Chemistry Letters (2020)
Exploiting the versatile alkyne-based chemistry for expanding the applications of a stable triphenylmethyl organic radical on surfaces
Chemical Science (2020)
Electroactive Organic Building Blocks for the Chemical Design of Functional Porous Frameworks (MOFs and COFs) in Electronics
Chemistry – A European Journal (2020)