Multistep energy and electron transfer processes in novel rotaxane donor-acceptor hybrids generating microsecond-lived charge separated states

Abstract

A new set of [Cu(phen)₂]⁺ based rotaxanes, featuring [60]-fullerene as an electron acceptor and a variety of electron donating moieties, namely zinc porphyrin (ZnP), zinc phthalocyanine (ZnPc) and ferrocene (Fc), has been synthesized and fully characterized with respect to electrochemical and photophysical properties. The assembly of the rotaxanes has been achieved using a slight variation of our previously reported synthetic strategy that combines the Cu(i)-catalyzed azide-alkyne cycloaddition reaction (the "click" or CuAAC reaction) with Sauvage's metal-template protocol. To underline our results, complementary model rotaxanes and catenanes have been prepared using the same strategy and their electrochemistry and photo-induced processes have been investigated. Insights into excited state interactions have been afforded from steady state and time resolved emission spectroscopy as well as transient absorption spectroscopy. It has been found that photo-excitation of the present rotaxanes triggers a cascade of multi-step energy and electron transfer events that ultimately leads to remarkably long-lived charge separated states featuring one-electron reduced C₆₀ radical anion (C₆₀˙^-) and either one-electron oxidized porphyrin (ZnP˙⁺) or one-electron oxidized ferrocene (Fc˙⁺) with lifetimes up to 61 microseconds. In addition, shorter-lived charge separated states involving one-electron oxidized copper complexes ([Cu(phen)₂]²⁺ (τ < 100 ns)), one-electron oxidized zinc phthalocyanine (ZnPc˙⁺; τ = 380-560 ns), or ZnP˙⁺ (τ = 2.3-8.4 μs), and C₆₀˙^- have been identified as intermediates during the sequence. Detailed energy diagrams illustrate the sequence and rate constants of the photophysical events occurring with the mechanically-linked chromophores. This work pioneers the exploration of mechanically-linked systems as platforms to position three distinct chromophores, which are able to absorb light over a very wide range of the visible region, triggering a cascade of short-range energy and electron transfer processes to afford long-lived charge separated states.

Fig. 1. [Cu(phen)₂]⁺–C₆₀ based rotaxanes (1–3) stoppered by three different combinations of electron donors investigated in the present work. Ar = 3,5-di-tert-butylphenyl.

Fig. 2. Rotaxane and catenane models as well as reference compounds used to investigate the photophysical processes of the target rotaxanes.

Fig. 3. UV/Vis absorption spectra of Fc–ZnP–[Cu(phen)₂]⁺–C₆₀ 1 (red), ZnP–ZnPc–[Cu(phen)₂]⁺–C₆₀ 2 (olive) and Fc–ZnPc–[Cu(phen)₂]⁺–C₆₀ 3 (blue) in PhCN.

Fig. 4. Top: Emission spectrum of rotaxane 2 (olive) and ZnTPP 11 (magenta) in THF upon excitation at 420 nm (OD = 0.084); bottom: excitation spectrum of rotaxane 2 in THF for emission at 760 nm.

Fig. 5. Top: Transient absorption spectrum (visible and near-infrared) registered upon femtosecond flash photolysis (420 nm, 150 nJ) of Fc–ZnP–[Cu(phen)₂]⁺–C₆₀ rotaxane 1 in THF with time delays between 0 (black) and 7.5 ns (wine) at room temperature. Inset: zoom into the near infrared region. Bottom: Time absorption profiles of 1 (dark grey), 4 (grey), and 5 (black) at 455 nm upon excitation with a 420 nm laser pulse, monitoring the decay of the ZnP singlet excited state.

Fig. 6. Left: Differential absorption spectra (visible and near infrared) registered upon nanosecond flash photolysis (425 nm, 5 mJ) of Fc–ZnP–[Cu(phen)₂]⁺–C₆₀ rotaxane 1 under aerobic conditions in THF with time delays between 200 ns (purple) and 15 μs (wine) at room temperature. Right: Differential absorption spectra (near infrared) registered upon nanosecond flash photolysis (425 nm, 5 mJ) of Fc–ZnP–[Cu(phen)₂]⁺–C₆₀ rotaxane 1 under aerobic conditions in THF with time delays between 1 (green) and 5 μs (red) at room temperature.

Fig. 7. Left: Differential absorption spectra (visible) registered upon nanosecond flash photolysis (425 nm, 5 mJ) of Fc–ZnP–[Cu(phen)₂]⁺–C₆₀ rotaxane 1 under argon atmosphere in THF with time delays between 8 μs (purple) and 400 μs (wine) at room temperature. Right: Time absorption profile of the spectra on the left at 460 nm, monitoring the decay of the ZnP triplet excited state.

Fig. 8. Schematic energy level diagrams, proposed decay pathways, and rate constants for Fc–ZnP–[Cu(phen)₂]⁺–C₆₀ rotaxane 1 upon excitation at 425 nm. k _EnT = energy transfer rate; k _CR = charge recombination rate contant.

Fig. 9. Transient absorption spectrum (visible and near-infrared) registered upon femtosecond flash photolysis (387 nm, 200 nJ) of ZnP–ZnPc–Cu(phen)₂–C₆₀ rotaxane 2 in PhCN with time delays between 0 (black) and 7.5 ns (wine) at room temperature. Inset: enhanced spectra in near infrared region.

Fig. 10. Left: Differential absorption spectra (visible and near infrared) registered upon nanosecond flash photolysis (670 nm, 5 mJ) of ZnP–ZnPc–[Cu(phen)₂]⁺–C₆₀ rotaxane 2 under aerobic conditions in THF with time delays between 90 ns (blue) and 1.0 μs (wine) at room temperature. Inset: zoom into the near infrared region with time delays between 65 ns (purple) and 1.0 μs (wine). Right: Differential absorption spectra (visible and near infrared) registered upon nanosecond flash photolysis (425 nm, 5 mJ) of ZnP–ZnPc–[Cu(phen)₂]⁺–C₆₀ rotaxane 2 under aerobic conditions in THF with time delays between 120 ns (blue) and 4.0 μs (wine) at room temperature. Inset: expanded spectra in the near infrared region with time delays between 23 ns (purple) and 400 ns (green).

Fig. 11. Time absorption profiles at 1010 nm of ZnP–ZnPc–[Cu(phen)₂]⁺–C₆₀ rotaxane 2 in THF at room temperature under aerobic conditions upon excitation at 670 nm (left) and 425 nm (center and right), monitoring the charge recombination.

Fig. 12. Schematic energy level diagrams, proposed decay pathways, and rate constants for ZnP–ZnPc–[Cu(phen)₂]⁺–C₆₀ rotaxane 2 upon excitation at 425 nm and 670 nm. k _EnT = energy transfer rate k _CR = charge recombination rate constant.

Fig. 13. Transient absorption spectrum (visible and near-infrared) registered upon femtosecond flash photolysis (660 nm, 150 nJ) of Fc–ZnPc–[Cu(phen)₂]⁺–C₆₀ rotaxane 3 in THF with time delays between 0 (black) and 5.8 ns (wine) at room temperature. Inset: zoom into the near infrared region.

Fig. 14. Left: Differential absorption spectra (visible and near infrared) registered upon nanosecond flash photolysis (670 nm, 5 mJ) of Fc–ZnPc–[Cu(phen)₂]⁺–C₆₀ rotaxane 3 under aerobic conditions in THF with time delays between 9 ns (purple) and 2.0 μs (wine) at room temperature. Inset: zoom into the near infrared region after 200 ns. Right: Differential absorption spectra (near infrared) registered upon nanosecond flash photolysis (670 nm, 5 mJ) of Fc–ZnPc–[Cu(phen)₂]⁺–C₆₀ rotaxane 3 under aerobic conditions in THF with time delays between 9 (purple) and 50 ns (green) at room temperature.

Fig. 15. Time absorption profiles of Fc–ZnPc–[Cu(phen)₂]⁺–C₆₀ rotaxane 3 upon 670 nm excitation in THF at room temperature under aerobic conditions at 830 nm (left) and 1010 nm (center and right), monitoring the charge recombination.

Fig. 16. Schematic energy level diagrams, proposed decay pathways, and rate constants for Fc–ZnPc–[Cu(phen)₂]⁺–C₆₀ rotaxane 3 upon excitation at 670 nm. k _EnT = energy transfer rate k _CR = charge recombination rate constant. Energy levels not to scale.