Linked Rotaxane Structure Restricts Local Molecular Motions in Solution to Enhance Fluorescence Properties of Tetraphenylethylene

Abstract

The restriction of local molecular motions is critical for improving the fluorescence quantum yields (FQYs) and the photostability of fluorescent dyes. Herein, we report a supramolecular approach to enhance the performance of fluorescent dyes by incorporating a linked rotaxane structure with permethylated α-cyclodextrins. Tetraphenylethylene (TPE) derivatives generally exhibit low FQYs in solution due to the molecular motions in the excited state. We show that TPE with linked rotaxane structures on two sides displays up to 15-fold higher FQYs. Detailed investigations with variable temperature 1 H NMR, UV-Vis, and photoluminescence spectroscopy revealed that the linked rotaxane structure rigidifies the TPE moiety and thus suppresses the local molecular motions and non-radiative decay. Moreover, the linked rotaxane structure enhances the FQY of the dye in various solvents, including aqueous solutions, and improves the photostability through the inhibition of local molecular motions in the excited TPE.

Mechanically interlocked luminescent molecules

“…Terao et al [44] synthesized a linked rotaxane 4 with permethylated α-cyclodextrins (PM α-CDs), where the macrocycles were linked to a TPE-containing axle via covalent bonds (Figure 1b). A TPE-containing molecule 5 without the linked PM α-CDs was also synthesized, and the optical properties of 4 and 5 were compared.…”

“…Terao et al [44] . synthesized a linked rotaxane 4 with permethylated α‐cyclodextrins (PM α‐CDs), where the macrocycles were linked to a TPE‐containing axle via covalent bonds (Figure 1b).…”

Enhancing Fluorescence in Both Solution and Solid States Induced by Imine Cage Formation

“…[34] Confinement of the TPE unit in a supramolecular structure can result in a significant enhancement in its emission properties due to restrictions in π twist and distortion in the excited state leading to minimum energy conical intersection (MECI). [35][36][37][38][39][40][41] Moreover, these supramolecular structures can be made emissive both in solution and solid phase by tuning their solubility. Notably, Stang et al have widely explored supramolecular coordination complexes (SCCs) that exhibit efficient luminescent properties in both dilute solution and solid states by combining TPE-based tetrapyridyl donors with Pt-metal acceptors.…”

“…AIE‐active moieties such as TPE can be readily integrated into the final supramolecular structure by this approach [34] . Confinement of the TPE unit in a supramolecular structure can result in a significant enhancement in its emission properties due to restrictions in π twist and distortion in the excited state leading to minimum energy conical intersection (MECI) [35–41] . Moreover, these supramolecular structures can be made emissive both in solution and solid phase by tuning their solubility.…”

Deciphering the ultrafast dynamics of a new tetraphenylethylene derivative in solutions: charge separation, phenyl ring rotation and CC bond twisting

“…20,22 These can be achieved by increasing the viscosity of the environment 1,3 or by confining (caging) the molecular system in cavities or channels as those offered by several chemical and biological hosts (micelles, cyclodextrins, proteins, silica-based materials and metal-organic frameworks (MOFs) to name a few). [23][24][25][26] The internal motions of the chemical moieties can also be affected or prohibited at high concentrations of the molecular system of interest, leading to an increase in the emission intensity and observation of an aggregation-induced emission (AIE) phenomenon. 22,[27][28][29][30] Within the myriad of molecular rotors, tetraphenylethylene (TPE) is the archetypal molecule showing an outstanding AIE efficiency.…”