Self-assembly of emissive supramolecular rosettes with increasing complexity using multitopic terpyridine ligands

Abstract

Coordination-driven self-assembly has emerged as a powerful bottom-up approach to construct various supramolecular architectures with increasing complexity and functionality. Tetraphenylethylene (TPE) has been incorporated into metallo-supramolecules to build luminescent materials based on aggregation-induced emission. We herein report three generations of ligands with full conjugation of TPE with 2,2':6',2″-terpyridine (TPY) to construct emissive materials. Due to the bulky size of TPY substituents, the intramolecular rotations of ligands are partially restricted even in dilute solution, thus leading to emission in both solution and aggregation states. Furthermore, TPE-TPY ligands are assembled with Cd(II) to introduce additional restriction of intramolecular rotation and immobilize fluorophores into rosette-like metallo-supramolecules ranging from generation 1-3 (G1-G3). More importantly, the fluorescent behavior of TPE-TPY ligands is preserved in these rosettes, which display tunable emissive properties with respect to different generations, particularly, pure white-light emission for G2.

Fig. 1

Self-assembly of supramolecular rosettes G1−G3. a L1 assembled with Cd²⁺ to form a mixture of trimer, tetramer, pentamer, and hexamer macrocycles (G1); b L2 assembled with Cd²⁺ to form a discrete hexamer (G2); c L3 assembled with Cd²⁺ to form a discrete heptamer (G3)

Fig. 2

¹H-NMR spectra. a L2 and G2; b L3 and G3; c DOSY of G1; d DOSY of G2; e DOSY of G3 (500 MHz, 300 K, CDCl₃ for ligands and CD₃CN for supramolecules)

Fig. 3

ESI/TWIM-MS spectrum. a ESI-MS and b TWIM-MS plots of G2; c ESI-MS and d TWIM-MS plots of G3. The peaks of Fⁿ⁺ denote fragments [G3–L3–nPF₆⁻]ⁿ⁺

Fig. 4

TEM and AFM images of G2 and G3. Representative energy-minimized structure from molecular modeling of a G2 and e G3 (alkyl chains are omitted for clarity); TEM images of single molecular b G2 (scale bar, 30 nm and 10 nm for zoom-in image) and f G3 (scale bar, 100 nm and 30 nm for zoom-in image); proposed stacking structure of c G2 and g G3; TEM images of nanotubes assembled by d G2 (scale bar, 200 nm) and h G3 (scale bar, 200 nm) 0.5 mg mL⁻¹ in acetonitrile solution under isopropyl ether vapor; AFM images of i, j G2 (scale bar, 2 μm and 100 nm, respectively) and l, m G3 (scale bar, 2 μm and 100 nm, respectively); 3D AFM images of k G2 and n G3

Fig. 5

AIE of G2. a Fluorescence spectra (λ_ex = 320 nm, c = 1.0 µM), b CIE 1931 chromaticity diagram, (the crosses signify the luminescent color coordinates), c quantum yields, and d photographs of G2 in CH₃CN/methanol with various methanol fractions; e fluorescence spectra (λ_ex = 320 nm, c = 1.0 µM), f CIE 1931 chromaticity diagram, g quantum yields, and h photographs of G2 in CH₃CN/water with various water fractions. G2 samples were excitation at 365 nm on 298 K (c = 1.0 μM)

Fig. 6

AIE of G3. a Fluorescence spectra (λ_ex = 320 nm, c = 1.0 µM), b quantum yields, and c photographs of G3 in CH₃CN/methanol with various methanol fractions; d fluorescence spectra (λ_ex = 320 nm, c = 1.0 µM), e quantum yields, and f photographs of G3 in CH₃CN/water with various water fractions. G3 samples were excitation at 365 nm on 298 K (c = 1.0 μM)

Fig. 7

DLS data and TEM images of G2 aggregates. Size distribution of G2 in a acetonitrile/methanol, and b acetonitrile/water mixtures by DLS (the percentages in the graphs are the poor solvent contents); TEM images of the aggregates of G2 formed in acetonitrile/methanol mixtures containing c, d 20%, e, f 40%, g, h 60%, and i, j 80% methanol (scale bar 500 nm for the upper images and 100 nm for the lower images, respectively), and aggregates of G2 formed in acetonitrile/water mixtures containing k, l 20% (scale bar, 2 μm and 500 nm, respectively), m, n 40% (scale bar, 2 μm and 500 nm, respectively), o, p 60% (scale bar, 1 μm and 500 nm, respectively), and q, r 80% water (scale bar, 1 μm and 500 nm, respectively)