Engineering perfluoroarenes for enhanced molecular barrier effect and chirality transfer in solutions

Abstract

Noncovalent forces have a significant impact on photophysical properties, and the flexible employment of weak forces facilitates the design of novel luminescent materials with a variety of applications. The arene-perfluoroarene (AP) force, as one type of π-hole/π interaction, shows unique directionality, involving an electron-deficient π-hole interacting with a π-electron-rich region, facilitating precise orientation and stabilization in supramolecular structures. Here we present an amination engineering protocol to build a perfluoroarene library based on an octafluoronaphthalene skeleton with various steric and electronic properties. In diluted solution-based assemblies, the perfluoroarenes perform as efficient molecular barriers to perylene building units, lighting up the luminescence. Enhanced steric effects, hydrophobicity and appended aromatic pendants are pivotal structural factors to boost the molecular barrier effect. Highly affinitive AP coassemblies transfer chirality from perfluoroarenes to achiral perylene moieties, inducing the appearance of chiral microstructures with tailored circularly polarized luminescence. Application as luminescent ink for enhanced water-resistance in displays and anti-counterfeiting is successfully realized. This work greatly extends the potential of molecular engineering in noncovalently bonded luminescent materials, and clearly reveals structure-property correlations.

Scheme 1. (A) A series of OFN derivatives was developed as guest molecules using a molecular engineering strategy. (B) Two perylene-based building units served as host molecules, enabling the modulation of certain properties through AP interactions. (C) An illustration depicting the chiral effects achieved by adding OFN-based compounds as barrier molecules.

Fig. 1. (A) Bar chart showing the fluorescence enhancement multiples of different systems. (B) Photographs of different system solutions under UV irradiation (0.05 mmol L⁻¹ in MeCN : H₂O = 2 : 8, v/v). (C) Fluorescence emission spectra and UV-vis absorption spectra of 1-EBA (0.05 mmol L⁻¹ in MeCN : H₂O = 2 : 8). (D) Summary table of data for each system (Mol. Vol. = molecule volume/cm³ mol⁻¹, ΔE = energy gap = E_LUMO − E_HOMO/eV mol⁻¹, BE = binding energy/kcal mol⁻¹, ΔG^θ = change in Gibbs free energy/kJ mol⁻¹).

Fig. 2. (A) Plot of the relationship between molecular volume and fluorescence enhancement factor. (B) Plot illustrating the correlation between molecular volume and fluorescence enhancement for Host 1. (C) Plot depicting the correlation between molecular volume and fluorescence enhancement for Host 2. (D) Temperature-dependent fluorescence spectra for 1-OFN (0.05 mmol L⁻¹ in MeCN : H₂O = 2 : 8, and 5 eq. OFN as 1, excitation wavelength 400 nm). (E) Fluorescence decay curve of 1 and 1-OFN (0.05 mmol L⁻¹ in MeCN : H₂O = 2 : 8, and 5 eq. OFN as 1, excitation wavelength 400 nm, and probe wavelength 550 nm). (F) NOESY spectra of DHIA with Host 1 (1.0 mmol L⁻¹1 in DMSO-d₆ : CDCl₃ = 1 : 1, and 15 eq. DHIA as 1).

Fig. 3. MD simulation. (A) System of host 1. (B) System of host 1 and OFN. (C) System of host 1 and ADA. (D) Potential energy of the process. (E) Radial distribution function (RDF) profiles. (F) Solvent accessible area.

Fig. 4. (A) Energy decomposition of the AP interaction between 1 and OFN. (B) Energy decomposition of the AP interaction between 1 and OFN. (C and F) Transient absorption spectrum and kinetic fitting of 1, following photoexcitation with ∼70 fs ultraviolet pulses at 350 nm. (D and G) Transient absorption spectrum and kinetic fitting of 1-OFN, following photoexcitation with ∼70 fs ultraviolet pulses at 350 nm. (E and H) Transient absorption spectrum and kinetic fitting of 1-DHIA.

Fig. 5. (A) Scanning electron microscope (SEM) images of the assemblies of different systems (0.1 mmol L⁻¹ in MeCN : H₂O = 2 : 8, and 5 eq. guest as the host). (B) Small-angle X-ray scattering (SAXS) pattern of 1-OFN. (C) SAXS spectra of 1-DHIA. (D) SAXS spectra of 2-OFN. (E) SAXS spectra of 2-DHIA.

Fig. 6. (A–F) Circular dichroism (C and D) spectra of different systems in an MeCN : H₂O = 2 : 8 (v/v) solvent. (G) Fluorescence of system 1 in thin film. (H) Fluorescence of system 1 in thin film. (I) CPL spectra of 2-DHIA. (J) CD spectra of 2-DHIA in thin film. (K) CPL spectra of 2-DHIA in thin film (all the solutions mentioned above are 0.05 mmol L⁻¹ MeCN : H₂O = 2 : 8 (v/v) solutions, with a concentration of 5 × 10⁻⁷ mol g⁻¹ in the film and the guest molecules present in a 5-fold excess in the assembly systems).

Fig. 7. (A) Images of the impregnated filter paper under 365 nm UV lamp. (B) Images of the PMMA film containing 1 under sunlight and 365 nm UV light. (C) Fluorescent images of the filter paper contaminated with 1, displayed with DHIA and after washing with water. (D) Fluorescent images showing the effects of stamping and washing. (E) Fluorescent images of drawings made on filter paper 1 using DHIA ink.