[2]Catenanes decorated with porphyrin and [60]fullerene groups: design, convergent synthesis, and photoinduced processes

Abstract

A new class of [2]catenanes containing zinc(II)-porphyrin (ZnP) and/or [60]fullerene (C(60)) as appended groups has been prepared. A complete description of the convergent synthetic approach based on Cu(I) template methodology and "click" 1,3-dipolar cycloaddition chemistry is described. This new electron donor-acceptor catenane family has been subjected to extensive spectroscopic, computational, electrochemical and photophysical studies. (1)H NMR spectroscopy and computational analysis have revealed that the ZnP-C(60)-[2]catenane adopts an extended conformation with the chromophores as far as possible from each other. A detailed photophysical investigation has revealed that upon irradiation the ZnP singlet excited state initially transfers energy to the (phenanthroline)(2)-Cu(I) complex core, producing a metal-to-ligand charge transfer (MLCT) excited state, which in turn transfers an electron to the C(60) group, generating the ZnP-[Cu(phen)(2)](2+)-C(60)(*-) charge-separated state. A further charge shift from the [Cu(phen)(2)](2+) complex to the ZnP subunit, competitive with decay to the ground state, leads to the isoenergetic long distance ZnP(*+)-[Cu(phen)(2)](+)-C(60)(*-) charge-separated radical pair state, which slowly decays back to the ground state on the microsecond time scale. The slow rate of back-electron transfer indicates that in this interlocked system, as in previously studied covalently linked ZnP-C(60) hybrid materials, this process occurs in the Marcus-inverted region.

Figure 1

Structures of the new porphyrin-C₆₀-[2]catenates and the model [2]catenates used in the electrochemical and photophysical investigations.

Figure 2

Porphyrin and C₆₀ reference compounds used in the electrochemical and photophysical investigations.

Figure 3

MALDI-TOF mass spectrum of catenate 1 (positive mode, α-cyano-4-hydroxycinnamic acid (CCA) matrix). The absence of peaks between the molecular ion peak and the peaks corresponding to the individual macrocycles is characteristic of catenane structures., = Cu(I) ion; =Zn(II)-porphyrin and = C₆₀.

Figure 4

¹H NMR spectra (800 MHz, CD₃CN, 298 K) of catenate 1. The red peaks correspond to the protons of the zinc-porphyrin moiety. The peaks in green are assigned to the CH₂ protons adjacent to the triazole ring and the proton on the triazole ring itself, while the black and the blue peaks are assigned to the protons of the [Cu(phen)₂]⁺ complex core and the oligo(ethyleneglycol) linkers, respectively.

Figure 5

Molecular model of catenate 1. For clarity, the hydrogen atoms have been removed from the structure.

Figure 6

Fluorescence spectra at 298 K of: A) [Cu(phen)₂]⁺ catenate 2 in DCM, excitation at 320 nm; B) ZnP-[Cu(phen)₂]⁺ catenate 3 in DCM, excitation at 320 nm; C) [Cu(phen)₂]⁺-C₆₀ catenate 4 in DCM, excitation at 320 nm and D) ZnP-[Cu(phen)₂]⁺-C₆₀ catenate 1, red in DCM and black in PhCN, excitation at 420 nm.

Figure 7

Left: transient absorption spectra (visible and near-infrared) registered upon femtosecond flash photolysis (387 nm, 220 mJ) of [Cu(phen)₂]⁺ catenate 2 in benzonitrile with time delay of 3000 ps at room temperature. Right: time-absorption profiles of the spectra shown on the left at 550 nm, monitoring the MLCT excited state.

Figure 8

Left: transient absorption spectra (visible and near-infrared) registered upon femtosecond flash photolysis (420 nm, 150 mJ) of ZnP-[Cu(phen)₂]⁺ catenate 3 in benzonitrile with time delays between 0 and 3000 ps at room temperature – arrow indicates the evolution of the differential changes. Right: time-absorption profiles of the spectra on the left at 466 nm, monitoring the energy transfer process.

Figure 9

Left: transient absorption spectra (visible and near-infrared) registered upon femtosecond flash photolysis (387 nm, 220 mJ) of [Cu(phen)₂]⁺-C₆₀ catenate 4 in DCM with time delays between 0 and 1000 ps at room temperature; the arrow indicates the evolution of the differential changes. Right: time-absorption profiles of the spectra on the left at 930 nm, monitoring the dynamics of charge separation.

Figure 10

Left: transient absorption spectra (visible and near-infrared) observed upon femtosecond flash photolysis (420 nm, 150 mJ) of ZnP-Cu⁺-C₆₀ catenate 1 in benzonitrile with time delays between 0 and 3000 ps at room temperature. Right: time-absorption profiles of the spectra on the left at 460 nm, monitoring the kinetics of energy transfer.

Figure 11

Left: transient absorption spectra (visible and near-infrared) observed upon nanosecond flash photolysis (355 nm) of ZnP-Cu⁺-C₆₀ catenate 1 in benzonitrile with a time delay of 100 ns at room temperature. Right: time-absorption profiles of the spectrum on the left at 1040 nm, monitoring the charge recombination process.

Figure 12

A) Schematic energy level diagrams and decay pathways for catenate 3 upon excitation at 420 nm, and B) catenate 4 upon excitation at 387 or 355 nm.

Figure 13

Schematic energy level diagrams, proposed decay pathways and rate constants for ZnP-Cu⁺-C₆₀ catenate 1 upon excitation at 420 nm. k_EnT = energy transfer rate, k_ET = electron transfer rate, k_CR = charge recombination rate.

Chart 1

General approach to the synthesis of porphyrin-fullerene interlocked molecules using Cu(I) template synthesis and the CuAAC reaction.

Scheme 1

Precursors and synthetic route for preparation of catenate 1. a) BOP-Cl, Et₃N, CH₂Cl₂, rt, 12 h, 45 % yield; b) C₆₀, DBU, I₂, toluene, rt, 24 h, 55 % yield; c) TsCl, Et₃N, CH₂Cl₂, 0°C for 4 h and rt for 20 h, 75 % yield; d) NaN₃, DMF, 80°C, 24 h, 93 % yield; e) [Cu(CH₃CN)₄][PF₆], CH₂Cl₂/CH₃CN, rt, 3 h, quantitative; f) BF₃OEt₂, PPh₄Cl, CH₂Cl₂, rt, 1 h and then 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ), rt, 12 h, 9 % yield; g) Zn(OAc)₂, CH₂Cl₂/CH₃OH, reflux, 6 h, quantitative; h) TBAF, THF, rt, 30 min, quantitative; i) CuI, sodium ascorbate, SBP, DBU, H₂O/EtOH, rt, 12 h, 57 % yield.

Scheme 2

Precursors and synthetic route for preparation of [2]catenate model compounds. a) [Cu(CH₃CN)₄][PF₆], CH₂Cl₂/CH₃CN, rt, 3 h, quantitative; b) CuI, sodium ascorbate, SBP, DBU, H₂O/EtOH, rt, 12 h.