Sulfone-Functionalized Chichibabin’s Hydrocarbons: Stable Diradicaloids with Symmetry Breaking Charge Transfer Contributing to NIR Emission beyond 900 nm

Abstract

While monoradical emitters have emerged as a new route toward efficient organic light-emitting diodes, the luminescence property of organic diradicaloids is still scarcely explored. Herein, by devising a novel radical−radical coupling-based synthetic approach, we report a new class of sulfone-functionalized Chichibabin's hydrocarbon derivatives, SD-1−3, featuring varied substituent patterns and moderate to high diradical characters of 0.44−0.70, as highly stable diradicaloids with rarely seen NIR emission beyond 900 nm. Via comprehensive experimental and theoretical investigations, we reveal that the optoelectronic and magnetic properties of these materials are significantly tuned by the variations of substitutions (H/CF 3 /OMe) on the molecular skeletons. More importantly, quantum chemical computations indicate that the embedding of sulfone groups has contributed to a breaking of their quasi-C 2 symmetry of these diradicaloid molecules and results in an excited-state charge transfer character. Therefore, a remarkably deep NIR emissive wavelength of up to 998 nm, together with a large Stokes shift (∼386 nm), is achieved for the CF 3 -based SD-2 molecule in tetrahydrofuran. To the best of our knowledge, such a luminescent wavelength of SD-2 has represented the longest wavelengths among the currently reported organic fluorescent radicals. Overall, our work not only establishes a new synthetic approach toward stable Chichibabin's hydrocarbons but also paves the way for designing NIR emissive open-shell materials with both fundamental understanding and feasible control of their luminescent properties.

Spin-Correlated Luminescence of a Carbazole-Containing Diradical Emitter: Single-Molecule Magnetoluminescence and Thermally Activated Emission

“…The highest Φ values for 1 and 2 were 0.28 in CH 2 Cl 2 and 0.53 in iodobenzene (PhI), respectively. The Φ value of 0.28 is among the highest for diradical emitters, ,,− whereas that of 0.53 is comparable with those of carbazole-containing luminescent monoradicals such as TTM-3PCz 9 and TTM-3NCz 9 (see also a review for individual Φ values of other luminescent radicals 4 ). We note that no obvious emission wavelength dependence of the emission lifetimes was detected for 1 and 2 in all of the solvents used in the photophysical measurements.…”

“…The development of simple luminescent polyradicals − is a promising approach to study spin-correlated luminescence phenomena. In the last five years, stable luminescent diradicals have been gradually developed and their luminescence properties have been unveiled. ,,− Figure summarizes emissive open-shell molecular systems with two radical units showing MagLum. ,, A prominent example is the diradical emitter, THDBA-PyBTM′ 2 (THDBA = 5,6,8,9-tetrahydro-7-phenyldibenz­[ c , h ]­acridine, PyBTM′ = (2-chloro-3-pyridyl)­bis­(2,4,6-trichlorophenyl)-methyl radical), which has an intramolecular exchange interaction of 2 J / k B = 8.3 ± 11.3 K . The diradical dispersed in poly­(methyl methacrylate) (PMMA) exhibited MagLum (i.e., single-molecule MagLum), indicating that the minimum number of radicals required for MagLum is two.…”

“…For diradicals with strong exchange interactions (i.e., |2 J / k B | > ∼10 K), more quantitative information on their MagLum behavior may be provided by luminescence measurements under stronger magnetic fields of energy comparable with the | J | values (>∼10 T). Investigation of ISC has rarely been considered in polyradical emitters ,,− and would help to elucidate spin-correlated luminescence properties, including excited-state dynamics. Spin degrees of freedom in both ground and excited states are unique features of organic radical emitters, and detailed analysis of their luminescence properties is critical for a basic understanding of polyradical emitters and the development of novel photofunctions unique to open-shell electronic structures.…”

Variable Peripheral Ligand Donation Tunes Electronic Structure and NIR II Emission in Tetrathiafulvalene Tetrathiolate Diradicaloids

“…Leading NIR II-emitting lumiphores rely on complex scaffolds with large, conjugated motifs to shift absorption and emission energies into this low-energy region; ,, however, these complex scaffolds prevent facile synthetic tuning of photophysical properties. Scaffolds with tunable NIR absorption/emission would be valuable as they would also allow for targeted and multicolor imaging. , Furthermore, multicolor imaging can additionally be enhanced and leveraged with dyes that are responsive to various environments; this approach can give additional environmental information as a result of emission or absorption shifts. − In addition to NIR II bioimaging, complexes that emit in the low attenuation telecom bands (∼1260–1550 nm) are of special interest for communication-related applications. , In particular, open-shell complexes that emit in this region would be ideal candidates for sensing and transmitting quantum information.…”

Spin-State Manipulation in a Luminescent Diradical Polymer

“…Organic diradicals offer the relative independence of the two unpaired electrons within one molecule. This characteristic allows for the control of electron spin–spin interaction through external stimuli such as temperature, pressure, and magnetic fields, rendering diradicals an ideal material system for spin-optical modulation (Figure c). − Nevertheless, a challenge persists as many diradicals are nonluminescent or exhibit poor luminescence efficiency. ,− Additionally, they tend to form dimers or aggregates in the condensed state, posing difficulties in achieving spin-optical modulation.…”