Fluorescence and Rotational Dynamics of a Crystalline Molecular Rotor Featuring an Aggregation-Induced Emission Fluorophore

Abstract

Recent studies have shown that "crystal fluidity" in the form of fast conformational motions is critical for largeamplitude rotational motion in crystals. To explore this concept, we designed a crystalline assembly featuring two diethynylbenzene (DEB) molecular rotators linked to tetraphenylethylene (TPE), a fluorophore known to emit with intensities that depend on the rigidity of the medium. We envisioned that an increase in crystal fluidity as a function of increasing temperature would facilitate rotational motion of the DEB while diminishing the fluorescence intensity of the TPE. The aggregation-induced emission of the TPE moiety was confirmed when its fluorescence intensity increased by the addition of water to a THF solution. While bulk solids showed a relatively strong TPE emission with a lifetime of 4 ± 1 ns, no significant changes were observed between measurements carried out from 77 to 298 K, indicating that the crystal environment has limited motion within the excited-state lifetime. This conclusion was confirmed by the quadrupolar echo 2 H NMR line-shape analysis of a deuterium-labeled sample between 198 and 298 K, which revealed rotational correlation times in the microsecond regime, suggesting that rotational fluidity is 3 orders of magnitude too slow to affect fluorescence emission.

Aggregation‐Induced Emission: New Vistas at the Aggregate Level

“…Garibay et al. designed a series of molecules with dumbbell‐shaped architecture and successfully detected the molecular rotation in crystals . During the study of CIE, Tang et al observed that the CIE molecule BpPDBF exhibited bright emission in the crystal state, while its amorphous powder showed no emission, possibly due to the excited‐state molecular motion (SSMM) in the amorphous powder .…”

The butterfly effect in bisfluorenylidene-based dihydroacenes: aggregation induced emission and spin switching

“…The AIE uorescence quantum yield was found to be up to 180 times higher for bFP in the solid-state than in solution. While AIE typically occurs due to RIR around the connecting bond of the anking phenyl rings and the polyaromatic hydrocarbon core in aggregates or in the solid, 24,32 the uorenoquinoidal molecules shown have no such degrees of freedom. These materials provide blueprints for a new class of luminogens where AIE arises solely from RIV of the strained buttery-like structures.…”

Aggregationsinduzierte Emission: Einblicke auf Aggregatebene

“…Garcia‐Garibay et al. entwickelten eine Serie von hantelförmigen Molekülen und konnten in deren Kristallen molekulare Rotationen nachweisen . Bei Arbeiten zur clusterinduzierten Emission (CIE) beobachteten Tang et al., dass das CIE‐Molekül BpPDBF in kristalliner Form hell emittiert, während für die amorphe Pulverform keine Emission zu beobachten ist.…”