Unusual Alternating Crystallization-Induced Emission Enhancement Behavior in Nonconjugated ω-Phenylalkyl Tropylium Salts

Abstract

The alternating physical properties, especially melting points, of α,ω-disubstituted n-alkanes and their parent n-alkanes had been known since Baeyer’s report in 1877. There is, however, no general and comprehensive explanation for such a phenomenon. Herein, we report the synthesis and examination of a series of novel ω-phenyl n-alkyl tropylium tetrafluoroborates, which also display alternation in their physicochemical characters. Despite being organic salts, the compounds with odd numbers of carbons in the alkyl bridge exist as room temperature ionic liquids. In stark contrast to this, the analogues with even numbers of carbons in the linker are crystalline solids. These solid nonconjugated molecules exhibit curious photoluminescent properties, which can be attributed to their ability to form through-space charge-transfer complexes to cause crystallization-induced emission enhancement. Most notably, the compound with the highest photoluminescent quantum yield in this series showed an unusual arrangement of carbocationic dimer in the solid state. A combination of XRD analysis and ab initio calculations revealed interesting insights into these systems.

The locations of triphenylamine and tetraphenylethene on a cyclohexyl ring define a luminogen as an AIEgen or a DSEgen

“…It is postulated that the aggregate emission could stem from the clusteringresulted spatial proximity of the electron-rich oxygen and nitrogen atoms in the polymer molecule. [34][35][36] Given that TPEcTPA and TPAcTPE have large TPE and TPA moieties that may contribute to spatial overlap and also have abundant oxygen and nitrogen atoms, the underlying reason for the long-band absorption and the long, intense PL of TPAcTPE might be associated with the extended conjugation and/or the through-space internal charge transfer (ICT) 37 within the molecule resulting from the effective through-space interactions.…”

Highly Efficient Sky‐Blue π‐Stacked Thermally Activated Delayed Fluorescence Emitter with Multi‐Stimulus Response Properties

“…[38] Moreover, intra/ intermolecular interactions (such as CÀ H•••CÀ H, CÀ H•••π, and π•••π interaction) are observed in a unit cell (d 1 ~d5 ) and packing structures (d 15 ), which can stabilize crystal packing and limit the intramolecular motions that give rise to AIEE behavior. [16,32,[38][39][40][41] Details of the photophysical properties will be described later.…”

Alkyl-Engineered Dual-State Luminogens with Pronounced Odd–Even Effects: Quantum Yields with up to 48% Difference and Crystallochromy with up to 22 nm Difference

“…However, alkyl chains are found to be able to tune the solid-state fluorescence (FL) by influencing the molecular packing modes . Particularly, the odd–even alternation of the FL quantum yield (FLQY) caused by alkyl chains is also observed in OLMs. − However, such an odd–even effect of FLQY is much less reported as compared to that of melting point. In addition, the odd–even effect in solid-state FLQY is very small in the reported literature, with only a single-digit difference in most cases.…”