Stable π-Extended Thio[7]helicene-Based Diradical with Predominant Through-Space Spin-Spin Coupling.

In this work, a novel π-extended thio[7]helicene scaffold was synthesized, where the α-position of the thiophene unit could be functionalized with bulky phenoxy radicals after considerable synthetic attempts. This open-shell helical diradical, ET7H-R, possesses high stability in the air, nontrivial π conjugation, persistent chirality, and a high diradical character (y0 of 0.998). The key feature is a predominant through-space spin-spin coupling (TSC) between two radicals at the helical terminals. Variable-temperature continuous-wave electron spin resonance (cw-ESR) and superconducting quantum interference device (SQUID) magnetometry in the solid state reveal a singlet ground state with a nearly degenerate triplet state of ET7H-R. These results highlight the significance of a stable helical diradicaloid as a promising platform for investigating intramolecular TSCs.

Peri-Alkylated Terrylenes and Ternaphthalenes Building-Blocks Towards Multi-Edge Nanographenes.

Since its first synthesis by Clar in 1948, terrylene - a fully connected ternaphthalene oligomer via naphthalene's peri-positions - has gained special focus within the rylene family, drawing interest for its unique chemical, structural, optoelectronic and single photon emission properties. In this study, we introduce a novel synthetic pathway that enhances the solubility of terrylene derivatives through complete peri-alkylation, while also facilitating extensions at the bay-positions. This approach not only broadens the scope of terrylene's chemical versatility but also opens new avenues for developing solution processable novel multi-edge nanographenes and tailoring electronic energy levels through topological edge structures. Our findings include a comprehensive structural and spectroscopic characterization along with transient absorption spectroscopy and photophysics of both the synthesized peri-alkylated terrylene and its phenylene-fused derivative.

Nonalternating π-System Mediated Spin Coupling in Azulene Nitronyl Nitroxide Diradicals.

To explore the distinctions in spin coupling between the molecular bridges of alternating and nonalternating π-systems, we synthesized a pair of isoelectronic compounds, namely, 2,6-Na-NN and 2,6-Az-NN, by utilizing naphthalene and azulene (naphthalene = Na and azulene = Az) as the bridges, respectively. Moreover, we conducted assessments to predict the coupling paths for nonalternating azulene. Variable-temperature EPR (VT-EPR) and SQUID results consistently reveal that both 2,6-Na-NN and 2,6-Az-NN exhibit antiferromagnetic coupling interactions, with coupling constants of J(2,6-Na-NN) = -22.3 cm-1 and J(2,6-Az-NN) = -30.1 cm-1, respectively. Density functional theory computations support these discoveries by revealing negative coupling constants (J < 0) and the spin densities population of the diradicals are observed to delocalize into the molecular bridges. This work suggests the most suitable coupling path for 2,6-Az-NN. In addition, we have investigated the potential spatial resistance of the diradicals in conjunction with single-crystal data. Theoretical calculations underestimating the torsion angle of the diradicals and overestimating the value of the magnetic coupling provide an explanation for this phenomenon. The final experimental results and theoretical calculations show that the 2,6-Az-NN coupling path prefers short paths.

TADF Blue Emitters with Balanced π-Conjugation─Design, Synthesis, Spectral Characterization, and a Model OLED with 8-(5-(tert-Butyl)-1,3,4-oxadiazol-2-yl)-N,N-bis(4-(tert-butyl)phenyl)dibenzo[b,d]furan-2-amine.

Blue organic light-emitting diodes (OLED) suffer from relatively short lifetimes and a comparatively low lighting efficiency. One of the approaches to improving their characteristics is the development of luminophores with the potential for thermally activated delayed fluorescence (TADF). Herein, a set of donor-spacer-acceptor compounds with potential for TADF are designed, synthesized, and computationally and spectroscopically characterized. The excited state dynamics of the most prospective dye is monitored by time-resolved fluorescence and transient absorption spectroscopy. The experimental data are obtained and processed by a newly developed method and supplemented by quantum chemical calculations. The comprehensive approach allowed rationalization of the complex cascade-type photophysical behavior. The most promising emitter is included in an OLED displaying a blue color with a maximum EQE of 4.9% and negligible efficiency roll-off at higher luminance.

Synthesis and photoinduced behavior of DPP-anchored nitronyl nitroxides: a multifaceted approach.

Understanding and controlling spin dynamics in organic dyes is of significant scientific and technological interest. The investigation of 2,5-dihydropyrrolo[4,3-c]pyrrolo-1,4-dione derivatives (DPPs), one of the most widely used dyes in many fields, has so far been limited to closed-shell molecules. We present a comprehensive joint experimental and computational study of DPP derivatives covalently linked to two nitronyl nitroxide radicals (DPPTh-NN2). Synthesis, single crystal X-ray diffraction study, photophysical properties, magnetic properties established using steady-state and pulse EPR, fast spin dynamics, and computational modelling using density functional theory and ab initio methods of electronic structure and spectroscopic properties of DPPTh-NN2 are presented. The single-crystal X-ray diffraction analysis of DPPTh-NN2 and computational modeling of its electronic structure suggest that effective conjugation along the backbone leads to noticeable spin-polarization transfer. Calculations using ab initio methods predict a weak exchange interaction of radical centers through a singlet ground state of DPPTh with a small singlet-triplet splitting (ΔEST) of about 25 cm-1 (∼0.07 kcal mol-1). In turn, a strong ferromagnetic exchange interaction between the triplet state of DPPTh chromophore and nitronyl nitroxides (with J ∼ 250 cm-1) was predicted.

Inverted device architecture for high efficiency single-layer organic light-emitting diodes with imbalanced charge transport.

Many wide-gap organic semiconductors exhibit imbalanced electron and hole transport, therefore efficient organic light-emitting diodes require a multilayer architecture of electron- and hole-transport materials to confine charge recombination to the emissive layer. Here, we show that even for emitters with imbalanced charge transport, it is possible to obtain highly efficient single-layer organic light emitting diodes (OLEDs), without the need for additional charge-transport and blocking layers. For hole-dominated emitters, an inverted single-layer device architecture with ohmic bottom-electron and top-hole contacts moves the emission zone away from the metal top electrode, thereby more than doubling the optical outcoupling efficiency. Finally, a blue-emitting inverted single-layer OLED based on thermally activated delayed fluorescence is achieved, exhibiting a high external quantum efficiency of 19% with little roll-off at high brightness, demonstrating that balanced charge transport is not a prerequisite for highly efficient single-layer OLEDs.