Neutral and Charged Excited States in Polar Organic Films: Origin of Unusual Electroluminescence in Tri-p-tolylamine-Based Hole Conductors

Singlet-Triplet Inversion in Triangular Boron Carbon Nitrides

The discovery of singlet-triplet (ST) inversion in some π-conjugated triangle-shaped boron carbon nitrides is a remarkable breakthrough that defies Hund's first rule. Deeply rooted in strong electron-electron interactions, ST inversion has garnered significant interest due to its potential to revolutionize triplet harvesting in organic LEDs. Using the well-established Pariser-Parr-Pople model for correlated electrons in π-conjugated systems, we employ a combination of CISDT and restricted active space configuration interaction calculations to investigate the photophysics of several triangular boron carbon nitrides. Our findings reveal that ST inversion in these systems is primarily driven by a network of alternating electron-donor and electron-acceptor groups in the molecular rim, rather than by the triangular molecular structure itself.

Shining Light on Inverted Singlet-Triplet Emitters

The inversion of the lowest singlet and triplet excited states, observed in several triangle-shaped organic molecules containing conjugated carbon and nitrogen atoms, is an astonishing result that implies the breakdown of Hund's rule. The phenomenon attracted interest for its potential toward triplet harvesting in organic LEDs. On a more fundamental vein, the singlet-triplet (ST) inversion sheds new light on the role of electron correlations in the excited-state landscape of π-conjugated molecules. Relying on the celebrated Pariser-Parr-Pople model, the simplest model for correlated electrons in π-conjugated systems, we demonstrate that the ST inversion does not require triangle-shaped molecules nor any specific molecular symmetry. Indeed, the ST inversion does not require strictly non-overlapping HOMO and LUMO orbitals but rather a small gap and a small exchange integral between the frontier orbitals.

Alkoxy-Substituted Quadrupolar Fluorescent Dyes

Polar and polarizable π-conjugated organic molecules containing push-pull chromophores have been investigated extensively in the past. Identifying unique backbones and building blocks for fluorescent dyes is a timely exercise. Here, we report the synthesis and characterization of a series of fluorescent dyes containing quadrupolar A-D-A constitutions (where A = acceptor and D = donor), which exhibit fluorescence emission at a variety of different wavelengths. We have investigated the effects of different electron-withdrawing groups, located at both termini of a para-terphenylene backbone, by steady-state UV/vis and fluorescence spectroscopy. Pyridine and substituted pyridinium units are also introduced during the construction of the quadrupolar backbones. Depending on the quadrupolarity, fluorescence emission wavelengths cover from 380 to 557 nm. Time-resolved absorption and emission spectroscopy reveal that the photophysical properties of those quadrupolar dyes result from intramolecular charge transfer. One of the dyes we have investigated is a symmetrical box-like tetracationic cyclophane. Its water-soluble tetrachloride, which is non-cytotoxic to cells up to a loading concentration of 1 μM, has been employed in live-cell imaging. When taken up by cells, the tetrachloride emits a green fluorescence emission without any hint of photobleaching or disruption of normal cell behavior. We envision that our design strategy of modifying molecules through the functionalization of the quadrupolar building blocks as chromophores will lead to future generations of fluorescent dyes in which these A-D-A constitutional fragments are incorporated into more complex molecules and polymers for broader photophysical and biological applications.

Colour-tunable fluorescence of single molecules based on the vibration induced emission of phenazine

Combining the vibration of the phenazine unit (S0) and the intramolecular energy transfer (IET) effect, compound M1–M4 exhibited color-tunable fluorescence even white emission in two different ways.

Charge instability of symmetry broken dipolar states in quadrupolar and octupolar triphenylamine derivatives

The quadrupolar and octupolar cyano triphenylamines shows symmetry broken dipolar charge transfer state, however, the thus formed state is distinctly different from that of its dipolar analogue.

Excimer formation in organic emitter films associated with a molecular orientation promoted by steric hindrance

White emission was obtained by introducing a terphenyl group into a highly twisted core, which promoted a molecular orientation suitable for excimer formation.