Axle Length Effect on Photoinduced Electron Transfer in Triad

Mar 4, 2010 - crown-ether, a short axle with C60 and triphenylamine (TPA) at both ... reported similar rotaxane with cationic longer axle (150-170 ns)...
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Axle Length Effect on Photoinduced Electron Transfer in Triad Rotaxane with Porphyrin, [60]Fullerene, and Triphenylamine Atula S. D. Sandanayaka,†,‡ Hisahiro Sasabe,§ Yasuyuki Araki,† Nobuhiro Kihara,| Yoshio Furusho,⊥ Toshikazu Takata,*,# and Osamu Ito*,† IMRAM, Tohoku UniVersity, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan, Japan AdVanced Institute of Science and Technology, Asahidai, Nomi 923-1292, Japan, Department of Organic DeVice Engineering, Yamagata UniVersity, Yonezawa, Yamagata 992-8510, Japan, Department of Chemistry, Kanagawa UniVersity, Tsuchiya, Hiratsuka 259-1293, Japan, Department of Molecular Design and Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan, and Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan ReceiVed: NoVember 24, 2009; ReVised Manuscript ReceiVed: February 5, 2010

Photoinduced multiple electron-transfer processes of a newly synthesized rotaxane with one acceptor and two donors are studied with the time-resolved fluorescence and absorption methods. In this rotaxane, zinc porphyrin (ZnP) with a crown-ether necklace is employed as a photosensitized electron donor; through the crown-ether, a short axle with C60 and triphenylamine (TPA) at both terminals is penetrating as an electron acceptor and a hole-shift, respectively (abbreviated as (ZnP;C60-(AS)-TPA)Rot). The time-resolved fluorescence and transient absorption measurements reveal that the through-space electron-transfer processes take place via the excited states of the ZnP unit to the spatially arranged C60 moiety, giving the radical ion pair (ZnP•+;C60•--(AS)-TPA)Rot in polar solvents. Consecutively, (ZnP;C60•--(AS)-TPA•+)Rot is also generated by the through-space hole-shift between ZnP and TPA, in addition to the through-bond charge separation via the excited state of the C60 moiety. Both radial ion pairs have lifetimes of 320-420 ns, which are longer than those of the previously reported similar rotaxane with cationic longer axle (150-170 ns). 1. Introduction To establish a sustainable society free from consuming the finite amounts of fossil fuels, chemists can now construct various kinds of photovoltaic devices.1,2 The design and preparation of the supramolecular systems containing various electron donors and electron acceptors that are capable of undergoing photoinduced electron transfer (PET) are most important to build the light-driven devices.3,4 Light-conversion efficiencies of such devices depend on the properties of the initially generated charge-separated (CS) states; the high quantum yields and long lifetimes of the CS states are essential.5 The excited-state properties of fullerene derivatives (C60) incorporated in such molecular systems have attracted much interest in the studies of photosynthesis models, as well as studies directed toward the development of new photodevices.6–8 To overcome the weak visible-light absorbing ability of the C60 derivatives, zinc porphyrins (ZnPs) have been widely used as light harvesting antennae, which act as photosensitized electron donors with respect to the C60 moiety having high electronaccepting ability.9 Thus, the ZnP-C60 connected molecule systems induce energy-transfer (EnT) and electron-transfer (ET) processes between them as basic steps in photosynthetic processes under photoillumination.10–12 In covalently bonded C60-ZnP, further connection of the hole-shift (HS) reagent as * Corresponding authors. E-mail: T.T., [email protected]; O.I.: [email protected]. † Tohoku University. ‡ Japan Advanced Institute of Science and Technology. § Yamagata University. | Kanagawa University. ⊥ Nagoya University. # Tokyo Institute of Technology.

an additional photoactive center prolongs the CS-state lifetimes very much.13–15 Supramolecular approaches using coordination ability of the Zn atom in the ZnP with pyridine/imidazole-functionalized C60 derivatives are also quite useful to achieve the CS process under light illumination.16 In such a case, addition of the third photoactive unit similarly affects the lifetimes of the CS states.17 In the case of rotaxanes containing the C60 and ZnP moieties, the PET processes are influenced by the fluctuating relative position and distance of the electron donor and acceptor.18 The most prominent difference of rotaxanes from other molecule systems is the through-space ET processes between the spatially positioned donor-acceptor in each rotaxane.19 In such C60-ZnP rotaxanes, we showed that the CS process takes place via the excited triplet states (3ZnP* and 3C60*) in addition to the excited singlet states (1ZnP* and 1C60*), depending on the spatial distances between the C60 and ZnP moieties.20 By further addition of the third photoactive unit such as ferrocene21 and aromatic amine to the rotaxanes with two components, it has been revealed that the third component plays an important role as a HS reagent acting via through-space process.22,23 In the present study, we synthesize a triad rotaxane with short neutral axle as shown in Chart 1(abbreviated as (ZnP;C60-(AS)TPA)Rot), in which the ZnP moiety with crown-ether wheel acts as an antenna and an electron donor. Through the wheel, the axle with the C60 and triphenylamine (TPA) moieties at the both ends is penetrating, in which the C60 and TPA moieties act as an electron acceptor and a HS, respectively. As references, previously synthesized triad rotaxane with a cationic long axle ((ZnP;C60-(AL+)-TPA)Rot)23 and dyad neutral rotaxane (ZnP;C60(AS)-Ph)Rot are adopted.24 In the newly synthesized (ZnP;C60(AS)-TPA)Rot, the axle has a neutral amide group, resulting in

10.1021/jp911177q  2010 American Chemical Society Published on Web 03/04/2010

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CHART 1

high mobility of the axle with respect to the wheel. This is advantageous in comparison to the rotaxane with the axle involving the ammonium cation, which may fix the C60 and TPA moieties to the crown-ether wheel with the ZnP moiety, although the axle is slightly long in (ZnP;C60-(AL+)-TPA)Rot (see Chart 1), in which the rate of the CS process was quite slow with low CS efficiency.23 For this newly synthesized (ZnP;C60-(AS)-TPA)Rot, we investigate here the photoinduced multistep ET processes using the time-resolved transient absorption and fluorescence measurements with changing solvent polarity. The goal of this report is to reveal the roles of the shorter neutral axle in (ZnP;C60(AS)-TPA)Rot in the PET processes, as compared with the previous triad rotaxane with the longer cationic axle. 2. Experimental Section 2.1. General Information. For preparative HPLC, a JAICO LC-908 system using columns JAIGEL 1 (φ 20 mm × 600 mm) and JAIGEL 2 (φ 20 mm × 600 mm) was employed. (ZnP;C60(AS)-Ph)Rot and (ZnP;C60-(AS+)-Ph)Rot, TPA-COCl, and CrownZnP were prepared according to the methods described previously.22–24 Other materials and solvents of reagent grade were used without further purification. IR spectra were recorded on a JASCO FT-IR model 230 spectrometer. 1H and 13C NMR measurements were performed on a JEOL JNM-L-400 spectrometer in CDCl3 with tetramethylsilane as an internal reference. MALDI-TOF-MS measurements were performed on a AXIMA-CFR mass spectrometer. Steady-state absorption spectra in the visible and near-IR regions were measured on a JASCO V570 DS spectrometer. 2.2. Synthesis of (ZnP;C60-(AS)-TPA)Rot. To a solution of (ZnP;C60-(AS+)-Ph)Rot (55 mg, 20 µmol) in CD3CN/CDCl3 (0.60 mL/0.3 mL) were added TPA acid chloride (31 µg, 100 µmol) and triethylamine (35 µL, 250 µmol) in the dark. The mixture was stirred at ambient temperature for 1 h. The reaction mixture was evaporated, and the residue was dissolved in CHCl3. The solution was washed with saturated NaHCO3 solution and then brine, dried over anhydrous MgSO4, filtered, and evaporated to dryness. The residue was subjected to silica gel column chromatography (eluent: CHCl3) to afford (ZnP;C60-(AS)TPA)Rot as a black purple solid (17 mg, 40%), and unreacted (ZnP;C60-(AS+)-Ph)Rot (15 mg, 26%) was recovered. 1H NMR (400 MHz, CDCl3, 323 K): δ ) 9.40 (br, 1H), 9.23 (s, 1H),

8.97-8.91 (m, 8H), 8.67 (br, 1H), 8.09-7.50 (m, 18H), 7.34-6.76 (m, 24H), 6.06 (br, 2H), 4.37-3.04 (m, 32H), 2.25 (s, 6H), 1.57-1.44 (m, 54H) ppm. IR (NaCl): 2961, 1714, 1591, 1506, 1456, 1265, 1126, 797, 754, 673, 663, 527 cm-1. UV-vis λmax/nm (ε/M-1 cm-1 in CHCl3): 426 (510 000), 552 (20 000), 593 (7000). MALDI-TOF-MS (matrix; dithranol): m/z 2881.8 (calcd for C198H147N7O12Zn ) 2878.0). Anal. Calcd for C198H147N7O12Zn · (CHCl3)0.5: C, 81.05; H, 5.05; N, 3.33. Found: C, 81.02; H, 4.80; N, 3.18. 2.3. Spectral Measurements. Steady-state fluorescence spectra were measured on a Shimadzu RF-5300PC spectrofluorophotometer. The time-resolved fluorescence spectra were measured by single-photon counting method using the second harmonic generation (SHG, 410 nm) of a Ti:sapphire laser [Spectra-physics, Tsunami 3950-L2S, 1.5 ps full width at halfmaximum (fwhm)] and a streak scope (Hamamatsu Photonics, C4334-01) equipped with a polychromator as excitation source and detector, respectively.25 Nanosecond transient absorption measurements were carried out using the SHG (532 nm) of an Nd:YAG laser (Spectra Physics, Quanta-Ray GCR-130, fwhm 6 ns) as the excitation source. For the transient absorption spectra in the near-IR region (600-1600 nm), the monitoring light from a pulsed Xe lamp was detected with a Ge-avalanche photodiode (Hamamatsu Photonics, B2834). Photoinduced events in micro- and millisecond time regions were estimated using a continuous Xe lamp (150 W) and an InGaAs-PIN photodiode (Hamamatsu Photonics, G5125-10) as the probe light and detector, respectively. The details of the transient absorption measurements were described elsewhere.25 All the samples in a quartz cell (1 × 1 cm) were deaerated by bubbling argon through the solution for 15 min. 2.4. Electrochemical Measurements. The cyclic voltammetry measurements were performed on a BAS CV-50 W electrochemical analyzer in deaerated PhCN solution containing 0.10 M Bu4NPF6 as a supporting electrolyte at 298 K (100 mV s-1). The glassy carbon working electrode was polished with BAS polishing alumina suspension and rinsed with acetone before use. The counter electrode was a platinum wire. The measured potentials were recorded using Fc/Fc+ electrode as reference electrode.

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SCHEME 1

3. Results and Discussion 3.1. Design and Preparation of (ZnP;C60-(AS)-TPA)Rot. (ZnP;C60-(AS)-TPA)Rot was synthesized by N-acylation of the ammonium cation of the axle in (ZnP;C60-(AS+)-Ph)Rot24 with triphenylamine acid chloride (TPA-COCl) in the presence of Et3N (Scheme 1). The characterization of (ZnP;C60-(AS)TPA)Rot was established on the basis of MALDI-TOF-MS, 1H NMR, IR, and elemental analyses as described in the Experimental Section (section 2.4). 1H NMR spectral data are consistent with the expected structure for (ZnP;C60-(AS)TPA)Rot, in which the ZnP and C60 moieties are mechanically placed in the molecular system independently connected to the wheel and axle, respectively.

3.2. Molecular Structure Calculations. There appeared many energy minima in the molecular structures for (ZnP;C60(AS)-TPA)Rot, when the optimized structures were calculated by the MD (MM3 level) force field in the MacroModel package,26 suggesting the fluctuation of the structures within 1-2 Å (Figure 1a,b), although these optimized structures keep the penetrating interlocked structure (Figure 1c). From 1H NMR spectra, the C60 moiety of (ZnP;C60-(AS)TPA)Rot can be considered to locate spatially near the ZnP moiety, supporting the calculated structures shown in Figure 1c, in which the TPA moiety is positioned between the ZnP and C60 moiety at the almost the same distance. The averages of the center-to-center distances (RCCav) between C60-TPA, ZnP-

Figure 1. Fluctuations of RCC values of (ZnP;C60-(AS)-TPA)Rot calculated with the MD method: (a) RCC values with simulation time and (b) probability distribution of RCC values; (c) representations of multiple optimized structures of (ZnP;C60-(AS)-TPA)Rot and (ZnP;C60-(AS)-Ph)Rot (blue, N; red, O; gray, C; white, H).

Electron Transfer in Triad Rotaxane

Figure 2. Frontier orbitals of (ZnP;C60-(AS)-TPA)Rot calculated with DFT B3LYP/3-21G(*) methods.

C60, and ZnP-TPA for (ZnP;C60-(AS)-TPA)Rot were evaluated to be 10.2, 11.3, and 9.2 Å from Figure 1b. This indicates that (ZnP;C60-(AS)-TPA)Rot affords a good example to investigate role of the TPA moiety such as a hole-shift, keeping the relative position of ZnP-C60. Compared with the results previously reported (ZnP;C60-(AL+)-TPA)Rot with a longer axle with cationic center,23 the RCCav value of C60-TPA in (ZnP;C60-(AS)TPA)Rot is short, whereas those of C60-ZnP and ZnP-TPA are long by ca. 1-2 Å. For (ZnP;C60-(AS)-Ph)Rot,24 the RCCav value between ZnP-C60 (11.2 Å) is almost the same as that of (ZnP;C60-(AS)-TPA)Rot. Figure 2 shows the electron densities of the HOMO, HOMO-1, and LUMO of (ZnP;C60-(AS)-TPA)Rot calculated by the DFT-B3LYP/3-21G(*) method.27 The electron density of the LUMO is mainly localized on the C60 spheroid, whereas the HOMO and HOMO-1 were found to be localized on the ZnP moiety and TPA moiety, respectively. These results suggest that the stable CS states are (ZnP•+;C60•--(AS)-TPA)Rot and (ZnP;C60•--(AS)-TPA•+)Rot. Although the calculated energy levels of such huge molecules are not always reliable enough, the energy gap between the HOMO and HOMO-1 is smaller than the HOMO-LUMO gap in the nonpolar medium, suggesting that both radical ion pairs (RPIs) possibly have similar energy levels in a polar medium, as observed in the electrochemical measurements. 3.3. Steady-State Absorption Studies. The absorption spectrum of (ZnP;C60-(AS)-TPA)Rot is shown in Figure 3 with reference compounds such as Crown-ZnP and C60-TPA in toluene (these molecular structures are shown in Supporting Information).23 The Soret and Q-bands of the ZnP moiety in (ZnP;C60-(AS)-TPA)Rot appeared at 427 and 552-592 nm, respectively. The absorption bands of the C60 moiety appeared at 705 nm and in the shorter wavelength region than 350 nm. The absorption spectrum of the triad rotaxane is almost a superimposition of those of references. These absorption spectral features of (ZnP;C60-(AS)-TPA)Rot do not change with varying solvent polarity. In the visible spectral region, the absorption spectrum of (ZnP;C60-(AS)-Ph)Rot is similar to that of (ZnP;C60(AS)-TPA)Rot. These absorption spectral studies confirmed that there is no appreciable electronic interaction between the C60 moiety and the ZnP moiety or the TPA moiety in their ground states. The absorption in the longer wavelength region than 705 nm is weaker than that of the previously reported (ZnP;C60(AL+)-TPA)Rot,23 in which broad weak absorption due to the charge-transfer interaction with the C60 moiety extends to 800 nm.

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Figure 3. Steady-state absorption spectra of (ZnP;C60-(AS)-TPA)Rot, C60-TPA, and Crown-ZnP in toluene. The concentrations are common at 0.005 mM. Inset: spectra magnified by 10 times in the longer wavelength region than 450 nm.

3.4. Electrochemical Studies. The electrochemical properties of (ZnP;C60-(AS)-TPA)Rot and references have been measured in PhCN (see Supporting Information, Figure S1); in the region 0.28-0.30 V vs Fc/Fc+, the oxidation potentials (EOx) of the TPA and ZnP moieties are overlapping. The first reduction potential (ERed) ) -1.05 V is due to the C60 moiety. From the electrochemical data for the components, it is confirmed that there is no appreciable electronic interaction between the C60 and the ZnP or TPA moieties in the ground states under the electrochemical conditions. Compared with the energy for (ZnP;C60-(AL+)-TPA)Rot, the EOx of the TPA moiety is slightly different, suggesting that the TPA moiety may be placed at different circumstances. The free-energy changes for the CS process (∆GCS) and charge-recombination (CR) process (∆GCR) were calculated by the Weller equation.28 As listed in Table 1 for (ZnP;C60-(AS)TPA)Rot, the ∆GCS values via 1ZnP* (∆GSCSI) with respect to C60 are negative in polar solvents, predicting that the CS process producing (ZnP•+;C60•--(AS)-TPA)Rot is possible. The ∆GCS values via 1C60* (∆GCSII) with respect to ZnP and TPA are also negative in polar solvents, expecting that the CS processes producing (ZnP•+;C60•--(AS)-TPA)Rot and (ZnP;C60•--(AS)TPA•+)Rot are possible. In nonpolar toluene, on the other hand, the ∆GSCSI and ∆GSCSII are positive, predicting that the above CS processes do not occur. For (ZnP;C60-(AS)-Ph)Rot, similar ∆GCR and ∆GCS values were evaluated.24 Compared with results for (ZnP;C60-(AL+)-TPA)Rot,23 the ∆GCS values in Table 1 are more negative, whereas the ∆GCR vales are less negative. 3.5. Fluorescence Studies. The steady-state fluorescence spectra of (ZnP;C60-(AS)-TPA)Rot and the reference compound were measured with excitation at 550 nm, which selectively excites the ZnP moiety. The fluorescence peaks at 600 and 650 nm in Figure 4 are attributed to the ZnP moiety. The fluorescence intensities of the ZnP moiety in (ZnP;C60-(AS)-TPA)Rot were weaker than those of Crown-ZnP, indicating that the quenching of the 1ZnP* moiety is induced by the C60 moiety. In toluene, a new fluorescence peak of the C60 moiety clearly appeared at 715 nm, suggesting that EnT from the 1ZnP* moiety to the C60 moiety takes place. In polar solvents, on the other hand, the fluorescence peak of 1C60* was not observed, as shown in Figure 4. Compared with the case for (ZnP;C60-(AL+)TPA)Rot,23 the degree of quenching of the ZnP fluorescence is large for (ZnP;C60-(AS)-TPA)Rot.

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TABLE 1: Free-Energy Changes of Charge Separation (∆GSCSI) via 1ZnP* and (∆GSCSII) via 1C60*, Charge Recombination (∆GCR) of Radical Ion Pair rotaxane

solvent

∆GSCSI/eVa via 1ZnP* vs C60

∆GSCSII/eVa via 1C60* vs ZnP (or TPA)

∆GCRI/eV (∆GCRII/eV)b

(ZnP;C60-(AS)-TPA)Rot

DMF PhCN THF TN

-0.79 -0.77 -0.64 (0.2)c

-0.51 -0.48 -0.31 (0.2)c

-1.23 -1.27 -1.40 -2.0c

a -∆GCR ) -e(Eox + Ered) - ∆GS; ∆GSCS ) E0,0 - ∆GCR; E0,0(1ZnP*) ) 2.10 eV, E0,0(1C60*) ) 1.75 eV, ∆GSCSI and ∆GSCSII are CS from ZnP* and 1C60*, respectively. ∆GS ) e2/(4πε0))[(1/(2R+) + 1/(2R-) - 1/RCC)/εS - (1/(2R+) + 1/(2R-))/εR], where R+ and R- are radii of cation and anion; εS and εR are dielectric constants of solvents used for photophysical studies and for measuring the redox potentials, respectively. b ∆GCRI for (ZnP•+;C60•--(AS)-TPA)Rot is the same as ∆GCRII for (ZnP;C60•--(AS)-TPA•+)Rot in each solvent. c ∆G values in toluene (TN) include considerable error. 1

Figure 4. Steady-state fluorescence spectra of (ZnP;C60-(AS)-TPA)Rot (0.1 mM) in PhCN and toluene; λex ) 550 nm.

The florescence time profiles of (ZnP;C60-(AS)-TPA)Rot were measured using a time-correlated single-photon-counting apparatus with excitation at 400 nm (Figure 5) to perform the further quantitative analyses of the events on the excited singlet states. The fluorescence lifetimes (τf(ZnP)) were evaluated by curve-fitting the 625 nm fluorescence decay with a single exponential function (Figure 5a), as summarized in Table 2. The τf(ZnP) values of (ZnP;C60-(AS)-TPA)Rot are shorter than that of Crown-ZnP in both polar and nonpolar solvents, supporting the 1ZnP*-fluorescence quenching in the steady-state measurements. The kSq(ZnP) values for (ZnP;C60-(AS)-TPA)Rot were estimated as (0.9-1.4) × 109 s-1, which are similar to those of (ZnP;C60-(AS)-Ph)Rot; however, these kSq(ZnP) values are larger than those of (ZnP;C60-(AL+)-TPA)Rot by more than 10 times. A similar tendency was observed for the ΦSq(ZnP) values, in agreement with the extent of the steady-state fluorescence quenching.23 The kqS(ZnP) values for (ZnP;C60-(AS)-TPA)Rot increase in the order DMF > PhCN > THF (Table 2), suggesting that the contribution of the CS process increases with the polar solvents. In toluene, kqS(ZnP) can be ascribed as kSEnT from 1 ZnP* to C60, generating 1C60*, since the C60-fluorescence peak was observed in the steady-state measurements (Figure 4). In polar solvents, the absence of the C60-fluorescence peak in the steady-state measurements (Figure 4) suggests that the CS process predominantly takes place; thus, kqS(ZnP) is put to be nearly equal to kSCS via 1ZnP* (kSCSI). In Figure 5b, the fluorescence time profiles at 720 nm are shown, from which the fluorescence lifetimes (τf(C60)) were evaluated, as listed in Table 3; the τf(C60) value in toluene (1350 ps) is the same as ref-C60, indicating no CS via 1C60*. In polar solvents, the τf(C60) values are as short as 150-200 ps, suggesting that the CS process takes place via 1C60*; thus, the

Figure 5. Fluorescence time profiles of (ZnP;C60-(AS)-TPA)Rot observed with 400 nm excitation laser-light (2 ps pulse width): (a) monitored at 625 nm; (b) monitored at 720-800 nm.

rate constant and quantum yields for the CS process via 1C60* were evaluated and denoted as kSCSII and ΦSCSII, respectively. The kSCSII values are in the range (4-6) × 109 s-1 and the ΦSCSII values are in the range 0.85-0.90. Since a similar trend was observed for (ZnP;C60-(AS)-Ph)Rot,24 it is confirmed that the CS process via 1C60* occurs mainly vs the ZnP moiety for (ZnP;C60-(AS)-TPA)Rot. However, such a process was not observed for (ZnP;C60-(AL+)-TPA)Rot,23 suggesting that this process becomes difficult because of the longer axle. It is noticeable that the kSCSII values are larger than the kSCSI values by a factor of ca. 5, indicating that the CS process via the 1C60* moiety is faster than that via the 1ZnP* moiety in both (ZnP;C60-(AS)-TPA)Rot and (ZnP;C60-(AS)-Ph)Rot. Because of efficient CS process via 1C60*, the steady-state C60fluorescence intensity disappeared at 700-720 in PhCN (Figure 4). 3.6. Transient Absorption Studies. Transient absorption spectra of (ZnP;C60-(AS)-TPA)Rot were measured with 550 nm

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TABLE 2: Fluorescence Lifetimes (τf(ZnP)), Fluorescence Quenching Rate Constants (kSq(ZnP), and Quantum Yields (ΦSq(ZnP)) via 1ZnP* of (ZnP;C60-(AS)-TPA)Rot, (ZnP;C60-(AL+)-TPA)Rot, and (ZnP;C60-(AS)-Ph)Rot rotaxanes (ZnP;C60-(AS)-TPA)Rot

(ZnP;C60-(AL+)-TPA)Rotb (ZnP;C60-(AS)-Ph)Rotc

solvent τf(ZnP)/ps kSq(ZnP)a/s-1 ΦSq(ZnP)a DMF PhCN THF TN DMF PhCN DMF PhCN

530 590 670 700 1730 1780 740 780

1.4 × 109 1.2 × 109 1.0 × 109 0.9 × 109