Porphyrin-Based Multicomponent Metallacage: Host-Guest Complexation toward Photooxidation-Triggered Reversible Encapsulation and Release

Abstract

The development of supramolecular hosts with effective host-guest properties is crucial for their applications. Herein, we report the preparation of a porphyrin-based metallacage, which serves as a host for a series of polycyclic aromatic hydrocarbons (PAHs). The association constant between the metallacage and coronene reaches 2.37 × 10⁷ M^-1 in acetonitrile/chloroform (ν/ν = 9/1), which is among the highest values in metallacage-based host-guest complexes. Moreover, the metallacage exhibits good singlet oxygen generation capacity, which can be further used to oxidize encapsulated anthracene derivatives into anthracene endoperoxides, leading to the release of guests. By employing 10-phenyl-9-(2-phenylethynyl)anthracene whose endoperoxide can be converted back by heating as the guest, a reversible controlled release system is constructed. This study not only gives a type of porphyrin-based metallacage that shows desired host-guest interactions with PAHs but also offers a photooxidation-responsive host-guest recognition motif, which will guide future design and applications of metallacages for stimuli-responsive materials.

Figure 1

Different strategies for the self-assembly from tetrapyridyl porphyrin 1, cis-Pt(PEt₃)₂(OTf)₂2, and multicarboxylate ligands 3 or 3′.

Figure 2

(a) Cartoon representations of metallacage 4 by multicomponent self-assembly; partial (b) ³¹P {¹H} and (c) ¹H NMR spectra (243 or 600 MHz, CD₃CN, 295 K) of metallacage 4; (d) ESI-TOF-MS spectra of metallacage 4. (d–f) Crystal structure of metallacage 4. Hydrogen atoms, triethylphosphine units, counterions, and solvent molecules were omitted for clarity.

Figure 3

ESI-TOF-MS spectra of (a) 4⊃G₅ and (b) 4⊃G₆; partial ¹H NMR spectra (600 MHz, CD₃CN, 298 K) of (c) G₅, (d) 4⊃G₅, (e) 4, (f) 4⊃G₆, and (g) G₆. [Host] = [Guest] = 1.00 mM. Fluorescence spectra of metallacage 4 at a fixed concentration upon the addition of (h) G₅ and (j) G₆ in CH₃CN/CHCl₃ (ν/ν = 9/1); nonlinear fitting curves of the emission intensity at 655 and 715 nm of metallacage 4 versus the concentrations of (i) G₅ and (k) G₆; and (l) plots of the logarithms of the association constants versus the number of π electrons on PAHs in CH₃CN/CHCl₃ (ν/ν = 9/1).

Figure 4

Crystal structures of (a, b) 4⊃G₁, (c, d) 4⊃G₂, (e, f) 4⊃G₃, (g, h) 4⊃G₄, (i, j, m) 4⊃G₅, and (k, l, n) 4⊃G₆. Hydrogen atoms, triethylphosphine units, counterions, and solvent molecules were omitted for clarity.

Figure 5

Partial ¹H NMR spectra (600 MHz, CD₃CN, 295 K) of (a) G₁, (b) 4⊃G₁, and (c) 4⊃G₁ upon photoirradiation for 10 min and (d) epidioxyanthracene G₁–O₂. (e) Chemical structures of anthracene derivatives tested in the reversible controlled release study. Partial ¹H NMR spectra (600 MHz, CD₃CN, 295 K) of (f) 4, (g) 4⊃G₁₁, and (h) 4⊃G₁₁ upon photoirradiation for 20 min, (i) and heating at 80°C for 30 min. (j, k) Fatigue cycles for the reversible host–guest system characterized by fluorescence spectroscopy. [Host] = [Guest] = 10.00 M, λ_ex = 405 nm.