The primary concern on high power conservation lead to the development of organic electroluminescent (EL) materials from polycyclic aromatic fluorescence (FL), [3] noble-metal-involved phosphorescence (PH) [4] to donor-acceptor (D-A)-featured thermally activated delayed fluorescence (TADF) molecules. [5] Different to 25% electrogenerated excitons, namely singlet excitons, utilized by FL emitters, both PH and TADF materials can harvest 100% excitons in virtue of mutual singlet-triplet conversion through intersystem crossing (ISC) [6] and reverse ISC (RISC). [7] However, FL molecules characteristic of locally excited (LE) states commonly reveal high chromatic purities, whose FWHM values are less than 50 nm. In contrast, charge-transfer (CT) excitedstate components of PH and TADF molecules markedly broaden their emissions by 50-100 nm.Recently, multi-resonance (MR) featured TADF emitters emerge, because of their potential to overcome the challenge in combining high efficiency and emission color purity. [8] This kind of compound has polycyclic aromatic structures with electron-donating and withdrawing atoms, for example, nitrogen and boron, at ortho-positions to form accordant and amplified resonance effects on electron-cloud distribution. [9] In this case, on the one hand, frontier molecular orbitals (FMOs) of MR molecules are separated to facilitate RISC and achieve TADF characteristics; on the other hand, their fused-ring structures limit vibrational relaxation, leading to unique narrowband emissions with FWHM < 30 nm. [10] To balance optoelectrical properties, MR cores were further substituted with functional groups, [11], for example, cyan, [12] fluorine, [13] carbazole, [14] and diphenylamine, [15] which significantly improved carrier injection and transport, resulting in increased luminance and reduced roll-offs. However, to avert spectral broadening, these functional groups were mostly simple, rigid, and finitely extended, which limit the optimal space of MR materials. [16] Nonetheless, it is noticed that most MR diodes should adopt extremely low doping concentrations (commonly <5%) [17] to avoid bimolecular quenching processes, for example, triplet-triplet annihilation (TTA) and triplet-polaron quenching Emerging multi-resonance (MR) thermally activated delayed fluorescence (TADF) emitters can combine 100% exciton harvesting and high color purity for their organic light-emitting diodes (OLED). However, the highly planar configurations of MR molecules lead to intermolecular-interactioninduced quenching. A feasible way is integrating host segments into MR molecules, namely a "self-host" strategy, but without involving additional charge transfer and/or vibrational components to excited states. Herein, an ambipolar self-host featured MR emitter, tCBNDADPO, is demonstrated, whose ambipolar host segment (DADPO) significantly and comprehensively improves the TADF properties, especially greatly accelerated singlet radiative rate constant of 2.11 × 10 8 s −1 and exponentially reduced nonradiative rate constants. C...

Cyclic paraphenylenes, [n]CPPs, and linear paraphenylenes, [n]LPPs, formed by n benzenes, are investigated by Raman spectroscopy for n = 5 to 12 and density functional theory (DFT) for n = 4 to 20. The information on the experimental Raman frequencies and intensities, combined with DFT computations and reported X-ray diffraction structures, provides a consistent interpretation of the Raman spectra and allows establishing relevant structure-property trends. Structural and electronic effects such as benzene ring bending, inter-ring torsions, π-conjugation (aromaticity) and orbital energy gaps as a function of the linear elongation in [n]LPPs versus the macrocyclic curvature in [n]CPPs and of the molecular size (i.e., polymer limit) are systematically analyzed on the basis of the vibrational Raman properties. Changes in the BLA as an indicator of the degree of quinonoid character are analyzed and linked to the Effective Conjugation Coordinate (ECC) model. The BLA patterns involved in twisted and non-twisted conformations and in different species (bipolarons, quinonoid tautomers, and ECC active modes) are compared and their differences are discussed. This paper offers a unified interpretation of structural and electronic aspects in relation to the evolution from linear 1D π-systems to cyclic 2D structures.

Conducting polymers can be synthesized by irreversible diradical monomer polymerization. A reversible version of this reaction consisting of the formation/dissociation of σ-dimers and σ-polymers from a stable quinonoidal diradical precursor is described. The reaction reversibility is made by a quinonoidal molecule which changes its structure to an aromatic species by forming weak and long intermolecular C-C single bonds. The reaction provokes a giant chromic effect of about 2.5 eV. The two opposite but complementary quinonoidal and aromatic tautomers provide the Janus faces of the reactants and products which produces the observed chromic effect. A reaction mechanism is proposed to explain the variety of final products starting with structurally very similar reactants. These reversible reactions, covering an unusual regime of weak covalent supramolecular bonding, yield products which might be envisaged as novel molecular and polymeric soft matter phases.

Raman spectroscopy under high pressures up to 10 GPa and density functional computations up to 30 GPa are combined to obtain insights into the behavior of a prototypical nanohoop conjugated molecule, [6]cycloparaphenylene ([6]CPP). Upon increasing pressure, the nanohoop undergoes deformations, first reversible ovalization and then at even higher pressures aggregates are formed. This irreversible aggregation is caused by the formation of new intermolecular σ-bonds. Frequencies and derivatives of the Raman frequency shifts as a function of pressure are well reproduced by the computations. The frequency behavior is tied to changes in aromatic/quinonoid character of the nanohoop. The modeling at moderate high pressures reveals the deformation of the [6]CPP molecules into oval-like and peanut-like shapes. Surprisingly the pressure derivatives of the observed Raman mode shifts undergo a sudden change around a pressure value that is common to all Raman modes, indicating an underlying geometrical change extended over the whole molecule that is interpreted by the computational modeling. Simulations predict that under even larger deformations caused by higher pressures, oligomerization reactions would be triggered. Our simulations demonstrate that these transformations would occur regardless of the solvent, however pressures at which they happen are influenced by solvent molecules encapsulated in the interior of the [6]CPP.