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

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.


MALDI-TOF Mass Spectrometry
MALDI-TOF analysis in positive mode for all rotaxanes revealed the expected ion mass peak corresponding to the molecular mass of the rotaxanes lacking the PF 6counter-ion as well as the classical fragmentation pattern observed for [Cu(phen) 2 ] + -based interlocked systems upon ionization. [1][2][3][4] For example, a MALDI-TOF spectrum of rotaxane 2 ( Figure S1) clearly reveals the ion mass peak at m/z 3939.9 (rotaxane 2 -PF 6 ) corresponding to the proposed rotaxane structure. The latter comes along with two extra ion mass peaks both with high intensity.
These peaks correspond to a fragment lacking the fullerene (m/z 3219.4), suggesting rupture of the cyclopropane moiety that links the carbon cage to the phen-macrocycle, and to the thread component coordinated to a Cu ion (m/z 2522.8), indicating further rupture and loss of the phen-macrocycle. It is interesting to note that the ion mass peak at m/z 3219.4 appears as doublets, while the peak at m/z 2522.8 appears as a triplet. The doublets have a difference of m/z 28, while the triplet signal has a difference of m/z 28 and 62. We propose that the extra peaks that appear at m/z 28 units lower in the MALDI spectrum correspond to species that had lost a single N 2 molecule due to fragmentation of the triazole ring upon ionization. 5,6 The species detected at m/z 62 units lower corresponds to the protonated thread fragment lacking the Cu ion. The minor peaks in the MALDI spectrum likely correspond to unusual fragments of the rotaxane structure since increasing the laser power during the MALDI experiments increases the intensity of those minor peaks while significantly decreasing the peak corresponding to rotaxane 2. Similar MALDI spectra were obtained for rotaxanes 1 and 3.

1 H Nuclear Magnetic Resonance -NMR
1 H NMR investigation revealed that rotaxanes 1, 2, and 3 are very flexible structures existing in several interconverting conformations with complex dynamics on the NMR time scale. It is safe to assume that they are driven by secondary interactions between the chromophores. 1, 2, 7-13 Variable temperature 1 H NMR studies on rotaxane 1 in DMF-d 7 showed increasing broadening of the peaks at lower temperatures with no coalescence even at temperatures as low as 233 K. A somewhat cleaner spectrum was obtained at 100 o C, which allowed us to identify key protons of the proposed structure, namely the pyrrolic protons of the porphyrin between 8.92 and 8.86 ppm, the triazole proton at 8.05 ppm, and the ferrocene nuclei 10, 14 at 4.71, 4.33 and 4.01 ppm as well as the protons attached to the phenyl rings linked to positions 2 and 9 of the phen moieties, which appears in the usual upfield region at 7.42, 7.22, 6.18, 6.09 ppm, confirming that the two phen groups are entwined around the Cu(I) template ion. 15 For rotaxanes 2 and 3, which bear phthalocyanines, aggregation is evident in the 1 H NMR spectra even at high temperatures. Although some key features of the proposed rotaxane structures can be safely assigned, the signals are unfortunately too broad for a detailed analysis.

Electrochemistry
To probe the redox properties of the electron donor-acceptor rotaxanes (1-3) as well as the corresponding reference compounds (4-12) square wave voltammetry and differential pulse voltammetry experiments were carried out in dichloromethane (DCM) in the presence of 0.1 M tetrabutylammonium hexafluorophosphate (TBAFPF 6 ) as supporting electrolyte and ferrocene/ferrocenium as internal reference. Table S1 summarizes the electrochemical data with all redox potentials reported in volts (V) relative to the ferrocene couple (Fc/Fc+).
While the C 60 reference 10 was inactive under oxidative conditions, one-electron reductions were observed at -1.06 and -1.44 V resembling the trend found for pristine C 60 . [16][17][18] Due to the partial loss of π-conjugation, 10 reveals a shift towards more negative reductions when compared to pristine C 60 . ZnTPP (11) exhibits one-electron oxidations at +0.28 and +0.62 V. Zn t Bu 4 Pc (12) features only one oxidation at +0.22 eV within the electrochemical window. Catenane 9 reveals a single oxidation at +0.16 V, which correlates with the one-electron oxidation of the copper center, namely [Cu(phen) 2 ] + /[Cu(phen) 2 ] 2+ . 1, 7 C 60 -[Cu(phen) 2 ] + catenane 8 shows two one electron reductions at -1.12 and -1.48 V, corresponding to C 60 centered processes. In addition, the one-electron oxidation of [Cu(phen) 2 ] + evolves at +0.16 V. To this end, the presence of C 60 has no notable impact on the [Cu(phen) 2 ] + oxidation. Reference thread 5 features two oxidations at +0.26 and +0.62 V, corresponding to the oxidation of ZnP. A third oxidation at -0.01 V assigned to the ferrocene oxidation is shifted by 0.01 V to lower potentials compared to the ferrocene reference. In rotaxane 4, three oxidations are discernible. The first oxidation at +0.03 V is assigned to a ferrocene centered process. The second oxidation at +0.28 V is twice in intensity when compared to the one at +0.03 V. Thus, we hypothesize that it corresponds to the coalescence of a [Cu(phen) 2 ] + as well as a ZnP oxidation. 1 Finally, the third oxidation at +0.88 V, which relates to the second oxidation of ZnP, is shifted towards more positive potentials when compared to ZnTPP and reference 11. It is likely that the presence of [Cu(phen) 2 ] + impacts the ZnP oxidation. Thread reference 7 is inactive under reductive conditions. Under oxidative conditions, two oxidations are discernible for 7 at +0.21 and +0.75 V. Considering that the former is broader and more intense, we assume that it is a superimposition of the first ZnP oxidation and the first ZnPc oxidation. The second oxidation is assigned to the second oxidation of ZnP. A similar conclusion is derived for reference 6. Here, the first oxidations of ZnP, ZnPc, and [Cu(phen) 2 ] + cannot be clearly resolved at about +0.20 V, while the second ZnP oxidation sets in at ~0.7 V. No reduction peaks were observed within the electrochemical window of DCM.      2 ] + -C 60 2 (grey), references 6 (dark grey) and 7 (grey)and corresponding fits (red) upon excitation at 403 nm and detection at 600 nm in THF at RT. Right: fluorescence decay of ZnP-ZnPc-[Cu(phen) 2 ] + -C 60 2 (grey), reference compounds 6 (dark grey) and 7 (grey) and corresponding fits (red) upon excitation at 647 nm and detection at 690 nm in THF at RT.  The differential absorption spectra of C 60 10 are known from the literature. 3,5 Upon, for example, 387 nm fsexcitation the singlet excited state is formed immediately after the laser pulse with maxima at 510 and 920 nm.

Reference compounds excited at 420 nm (fs-laser)
ZnTPP 11 was excited with 420 nm fs-laser pulses. In this particular case, transient characteristics, which are formed immediately, include minima at 420, 560, and 600 nm, a maximum at 460 nm, and a broad absorption

References excited at 387 / 420 / 660 nm (fs-laser) and 425 / 670 nm (ns-laser)
Upon exclusive excitation of ZnPc of ZnPc-phen-ZnP thread 7 at 660 nm the differential absorption spectra feature only ZnPc transients, namely its singlet excited state at ~ 800 nm with a lifetime 2.9 ns in THF and its triplet excited state at 490 nm including ground state bleaching at 680 nm. 24,[26][27][28][29][30][31] In contrast, upon excitation of 7 at 420 nm, which matches the ZnP Soret band, the visible part of the spectrum is dominated by the ZnP signature absorptions.

General Information and Materials
NMR spectra were obtained on either a Bruker AVANCE 400 (400 MHz) or an AVANCE 800 (800 MHz) spectrometer using deuterated solvents as the lock. The spectra were collected at 25 o C unless otherwise noted and chemical shifts (δ, ppm) were referenced to residual solvent peak ( 1 H, CDCl 3 at 7.26 ppm; 13 C at 77.2 ppm). In the assignments, the chemical shift (in ppm) is given first, followed, in brackets, by multiplicity (s, singlet; d, Catenane model compounds 8 and 9 were prepared following the conditions reported in our previous work. 31 Compounds 10 17 , 11 32 , 12 33 , 13, 1, 34, 16 8, 9, 32 18 1 , 20 33 and 22 10 were synthesized following literature procedures.

Synthesis
General Procedure for the Synthesis of Rotaxanes. In the reaction flask, the suitable phen-macrocycle derivative ( concentrated to a volume of 5 mL and then stirred for 3 h with a saturated MeOH solution of KPF 6 (10 mL) to effect the anion exchange. The solvents were evaporated under reduced pressure, the remaining insoluble solid was extracted with CH 2 Cl 2 (3 x 50 mL) and filtered through paper. The solvent was evaporated under reduced pressure and the crude product was purified by column chromatography (SiO 2 ) using appropriate CH 2 Cl 2 /CH 3 OH mixture for each case as eluent. The excess alkynyl derivative was eluted first, followed by the corresponding thread compound, whose isolation indicates that some unthreading occurred during rotaxane assembly. The target rotaxane was the third eluted product from the column.

Electrochemical and Photophysical Studies
All solvents used were purchased from commercial suppliers (spectroscopic grade; 99.5 %) and used without further purification. A single-compartment, three electrode cell configuration was used for the square wave voltammetry measurements, using a glassy carbon electrode (3 mm diameter) as a working electrode, a platinum wire as a counter and a silver wire as a reference electrode. All electrochemical measurements were performed with a METROHM FRA 2 µAutolab Type III potentiostat. For the photophysical characterization the samples were placed in fluorimetric cuvettes with different pathways and, when necessary, purged with molecular oxygen or argon. Steady-state UV-vis absorption spectroscopy was performed on a Lambda2 spectrometer (Perkin Elmer).
Steady state fluorescence spectra were carried out at a FluoroMax3 spectrometer (Horiba) in the visible detection range and at a FluoroLog3 spectrometer (Horiba).