Anti-cooperative Self-Assembly with Maintained Emission Regulated by Conformational and Steric Effects

Abstract

Herein, we present a strategy to enable a maintained emissive behavior in the self-assembled state by enforcing an anti-cooperative self-assembly involving weak intermolecular dye interactions. To achieve this goal, we designed a conformationally flexible monomer unit 1 with a central 1,3-substituted (diphenyl)urea hydrogen bonding synthon that is tethered to two BODIPY dyes featuring sterically bulky trialkoxybenzene substituents at the meso-position. The competition between attractive forces (H-bonding and aromatic interactions) and destabilizing effects (steric and competing conformational effects) limits the assembly, halting the supramolecular growth at the stage of small oligomers. Given the presence of weak dye-dye interactions, the emission properties of molecularly dissolved 1 are negligibly affected upon aggregation. Our findings contribute to broadening the scope of emissive supramolecular assemblies and controlled supramolecular polymerization.

Keywords: Anti-Cooperativity; BODIPY Dyes; Conformational Changes; Hydrogen Bonding; Self-Assembly.

Scheme 1

Top: Summary of different types of transfer of emissive properties in supramolecular self‐assembly. Bottom: Molecular structure of urea‐based BODIPY 1 and cartoon representation of its anti‐cooperative self‐assembly into discrete species with maintained emission.

Figure 1

Absorption (a) and emission spectra (b, λ _exc=500 nm) of compound 1 in a molecularly dissolved (CHCl₃, inset: right solution) and aggregated state (MCH, inset: left solution) (c=10 mM, T=298 K). VT‐Absorption (c) and VT‐emission spectra (d, λ _exc=500 nm) in MCH (c=20 μM, 10 K min⁻¹, 363 K–268 K). e) Concentration‐dependent absorption studies in MCH at 333 K. f) Goldstein–Stryer fit with variable σ at 333 K.

Figure 2

a) Molecular structure of 1. b), c) 2D ROESY NMR experiments of 1 in CDCl₃ (c=10 mM, 333 K). d) Optimized geometry and relative stability of BODIPY conformations using ωΒ97Χ‐D/6‐31G* method in the gas phase. The dodecyloxy chains have been reduced to methoxy to decrease computational costs.

Figure 3

VT‐¹H NMR (a) and absorption spectra (c) of 1 in MCH‐d₁₄/CDCl₃ (1/1 (v/v), c=10 mM cooling (5 K min⁻¹) from 328 K to 298 K. b) Optimized geometry of 1 anti–anti without geometric constraints using ωΒ97Χ‐D/6‐31G* method in the gas phase. d) Optimized structure using ωΒ97Χ‐D/6‐31G* method in the gas phase by freezing the linker urea and the attached benzene with variable intramolecular hydrogen bond length between CH(H_k)‐CO(O).

Figure 4

VT‐¹H NMR (a), FT‐IR (b, d) and absorption spectra (c, e) of 1 in MCH‐d₁₄/CDCl₃ (1 : 1; v/v, c=1 mM, a) and in MCH (c=1 mM, b–e) cooling from 363 K to 283 K, using a cooling rate of 5 K min⁻¹ (a, c and e). f) Optimized monomer, dimer and tetramer structures of 1 highlighting the different steps in the self‐assembly process (calculations were perfomed using the ωΒ97Χ‐D/6‐31G* method (monomer) or the semi‐empirical PM6 method (dimer and tetramer) respectively).

Figure 5

a) Cartoon representation of the molecular orientation of 1 on HOPG. b) AFM studies of 1 with corresponding height‐profile histogram (inset). Solutions used for these studies: MCH at 20 μM and 273 K. c) Experimental SAXS profiles (circles) for Agg1 in MCH at ≈2 mM (blue) and ≈4 mM (red) and the corresponding fittings (solid lines) to the customized model spheres. d) Correlation function of the DLS studies at 100 μM, 298 K in MCH.