Submillisecond-lived photoinduced charge separation in inclusion complexes composed of Li + @ C 60 and cyclic porphyrin dimers †

Lithium ion encapsulated [60]fullerene (Li@C60) is included within a free base and nickel complex of a cyclic porphyrin dimer (M-CPDPy, M 1⁄4 H4 and Ni2) to afford supramolecules (Li@C603M-CPDPy) in a polar solvent (benzonitrile) with the association constants of 2.6 10 M 1 and 3.5 10 M , respectively. From the electrochemical analysis, the energies of the charge-separated (CS) states are estimated to be 1.07 eV for Li@C603H4-CPDPy and 1.20 eV for Li @C603Ni2-CPDPy. Both values are lower than the triplet excited energies of the fullerene and porphyrin. Upon the photoexcitation at the Q-band of the porphyrin chromophore of Li@C603H4-CPDPy, electron transfer from the triplet excited state of the free base porphyrin to Li@C60 occurs to produce the CS state. Li @C603Ni2-CPDPy also undergoes photoinduced electron transfer to produce the CS state. The lifetimes of the resulting CS states are 0.50 ms for Li@C603H4-CPDPy and 0.67 ms for Li @C603Ni2-CPDPy. These remarkably long CS lifetimes are the best values ever reported for non-covalent porphyrin-fullerene supramolecules in solution and are attributable to the lower CS energies than the triplet energy of each chromophore.


Introduction
In a natural photosynthetic reaction centre, the multistep electron-transfer reactions occur following the excitation of dimeric chlorophyll to attain the long-lived charge-separated (CS) state. 1 The redox-active components such as chlorophyll, pheophytin and quinones are elegantly located in the protein matrix by non-covalent interactions. 1Extensive efforts have so far been devoted toward the design of electron donor-acceptor composites using covalently and non-covalently linked systems to form the long-lived CS state upon photoexcitation.  The1][22][23][24][25][26][27][28] On the other hand, the porphyrin compounds have very strong absorption bands in the visible region and their photoexcited states are generally good electron donors. 28[15][16][17][18][19][20][21][30][31][32][33][34][35] However, non-covalent binding between highly p-conjugated compounds such as porphyrins and fullerenes is not strong enough in polar solvents which are generally used for studies on photoinduced electron-transfer reactions.24b The energy of CS state should be lower than the triplet excited energy of each component.This is a typical dilemma for the long-lived charge separation in supramolecular donor-acceptor complexes.
In order to solve this problem, we have recently reported the cyclic porphyrin dimers (CPDs) as shown in Scheme 1a. 35eceptor molecules composed of multiple porphyrins are suitable for the inclusion of pristine C 60 and the derivatives.The strong supramolecular binding by p-p interaction was observed to form inclusion complexes. 35,36Unfortunately, the inclusion complex of C 60 and the nickel cyclic porphyrin dimer (C 60 3Ni 2 -CPD Py ) in crystalline state did not show the expected CS state in the time-resolved transient absorption spectra upon photoexcitation because the singlet excited state of the nickel porphyrin immediately gives rise to the triplet excited state by the rapid intersystem crossing, followed by energy transfer to afford the low-energy triplet excited state of C 60 ( 3 C 60 * ). 35The estimated energy level of the CS state (1.98 eV) is higher than that of 3 C 60 * (1.57eV). 35,37In contrast, the corresponding inclusion complex of C 60 and the free-base porphyrin dimer (C 60 3H 4 -CPD Py ) underwent photoinduced electron transfer from the porphyrin to C 60 owing to the lower oxidation potential and the slower intersystem crossing of the free-base porphyrin than those of the nickel complex.However, the lifetime of this CS state was very short probably because its energy level (1.83 eV) is still higher than that of 3 C 60 * . 35The energy limit of a CS state is ca.1.50-1.60eV, which is the triplet excited energies of fullerenes and porphyrins. 199][40][41][42] The higher reduction potential of Li + @C 60 than C 60 makes the energy levels of the resulting CS states lower than the triplet excited energy. 40,41It is expected that the combination of Li + @C 60 and the cyclic porphyrin dimers will make it possible to achieve both efficient formation of supramolecules and long-lived photoinduced charge separation.Thus, we report herein that supramolecular systems composed of cyclic porphyrin dimers and Li + @C 60 afford photoinduced CS states with higher energies and longer lifetimes than those composed of Li + @C 60 and monomeric porphyrins. 41

Results and discussion
1 Supramolecular formation of cyclic porphyrin dimers with Li + @C 60 Cyclic porphyrin dimers without OC 6 H 13 groups have poor solubility in organic solvents. 35,36Therefore, we have designed and prepared new dimers with four long alkoxy substituents on the meso-phenyl groups (H 4 -, Ni 2 -CPD Py (OC 6 ), Scheme 1) to improve the solubility.The synthetic procedures are shown in Scheme 2. The products were characterized by 1 H-NMR, 13 C-NMR, IR and mass spectroscopies (see the Experimental section and ESI † S1-S14).They are sufficiently soluble in benzonitrile (PhCN) to allow photochemical and electrochemical analysis with Li + @C 60 in solution.
Upon photoexcitation at the Soret band (430 nm) of H 4 -CPD Py (OC 6 ) in PhCN, the uorescence due to the porphyrin is observed at l max ¼ 650 and 717 nm as shown in Fig. 2. Addition of Li + @C 60 to a PhCN solution of H 4 -CPD Py (OC 6 ) induced a noticeable decrease in the uorescence intensity of H 4 -CPD Py (OC 6 ).From the plot of the uorescence intensity change vs. the concentration of Li + @C 60 , the K assoc value was determined to be 1.7 Â 10 5 M À1 , which agrees with the value obtained from the absorption spectral change within an experimental error (vide supra).

Energetics of photoinduced processes
Cyclic voltammograms of Li + @C 60 3H 4 -CPD Py (OC 6 ) and Li + @C 60 3Ni 2 -CPD Py (OC 6 ) are shown in Fig. 3a and b.The comparison with the uncomplexed compounds shows that the cyclic voltammograms consist of the electron oxidation processes of CPD Py (OC 6 ) and the electron reduction process of Li + @C 60 . 43The electrochemical data are summarized in Table 2.The energy of the CS states determined from the potential difference between one-electron reduction and oxidation potentials are 1.07 eV for Li + @C 60 3H 4 -CPD Py (OC 6 ) and 1.20 eV for Li + @C 60 3Ni 2 -CPD Py (OC 6 ).These values are smaller than those of the singlet excited states of cyclic porphyrin dimers (1.90 eV for H 4 -CPD Py (OC 6 ), 35c 1.97 eV for Ni 2 -CPD Py (OC 6 ), 35b and 1.94 eV for Li + @C 60 (ref.40)).Thus, the free energy changes of photoinduced electron transfer to Li + @C 60 via the singlet excited states are negative (exergonic).The energies of the triplet excited states were determined by phosphorescence spectra in a frozen PrCN/EtI (3:1 v/v) glasses at 77 K to be 1.51 eV for H 4 -CPD Py (OC 6 ) and 1.50 eV for Ni 2 -CPD Py (OC 6 ) (Fig. S18 in ESI †).The energy of the triplet excited state of Li + @C 60 ( 3 [Li + @C 60 ] * ; * denotes the excited state) was reported to be 1.53 eV. 39,40The energy level of the resulting CS states are lower than those of the triplet excited states of both Li + @C 60 and the porphyrin dimers.Thus, photoinduced electron transfer from the singlet or triplet excited state of the porphyrin dimers to Li + @C 60 as well as from the porphyrin dimers to the singlet or triplet excited state of Li + @C 60 is energetically possible in the supramolecular complexes to form the CS states.
In contrast to the case of Li + @C 60 , the estimated energy levels of the CS states of C 60 3H 4 -CPD Py (OC 6 ) and C 60 3Ni 2 -CPD Py (OC 6 ) and (1.86 and 1.67 eV) are higher than those of the triplet excited state of C 60 and CPD Py (OC 6 ) as observed in our previous studies, 35 suggesting no formation or short lifetimes of the CS states.

Photoinduced charge separation
The photodynamics of these inclusion complexes was investigated by the transient absorption spectra measured in PhCN by the use of femtosecond and nanosecond laser ash photolysis.The time-resolved transient absorption spectra of Li + @C 60 3H 4 -CPD Py (OC 6 ) measured by femtosecond laser ash photolysis (l ex ¼ 420 nm) in the time range from 1 ps to 3000 ps (Fig. 4a), which showed little difference from the transient spectra of only H 4 -CPD Py (OC 6 ) (Fig. S19a  )] * ) was determined from the absorption change at 630 nm (inset of Fig. 4a) to be 1.0 Â 10 9 s À1 , which is slightly larger than the value of intersystem crossing of 1 [H 4 -CPD Py (OC 6 )] * without Li + @C 60 (k ISC ¼ 8.0 Â 10 8 s À1 ) (Fig. S19a in ESI †).0][41] Thus, no electron transfer from 1 44 The intersystem crossing process was not detected by use of our femtosecond laser system (fwhm ¼ 130 fs).The k ISC value to 3 [Ni 2 -CPD Py (OC 6 )] * is much larger than that to 3 [H 4 -CPD Py (OC 6 )] * because of the heavy atom effect of Ni. 3 [N 2 -CPD Py (OC 6 )] * decayed to the ground state with the rate constant of k T ¼ 5.3 Â 10 9 s À1 .
0][41] This clearly indicates the occurrence of electron transfer from 3 [H 4 -CPD Py (OC 6 )] * to Li + @C 60 to produce the CS state (Li + @C 60 _ À 3H 4 -CPD Py (OC 6 )_ + ).The rate of electron transfer from 3 [H 4 -CPD Py (OC 6 )] * to Li + @C 60 was too fast to detect in the time scale of the nanosecond laser ash photolysis experiments (k ET > 10 7 s À1 ). 45The absorbance at 1035 nm due to Li + @C 60 _ À in the CS state decayed obeying rst-order kinetics with the same slope irrespective of the difference in the laser intensity.This clearly indicates that the decay of the CS state occurs via intrasupramolecular back electron transfer rather than a bimolecular reaction.The CS lifetime was determined from the slope of the rst-order plots in Fig. 5b to be 0.50 ms.
Similarly nanosecond laser excitation of Li + @C 60 3Ni 2 -CPD Py (OC 6 ) at 520 nm also results in formation of the CS state (Li + @C 60 _ À 3Ni 2 -CPD Py (OC 6 )_ + ) as shown in Fig. 6a, where  )_ + and Li + @C 60 _ À were observed.In this case, however electron transfer occurs from Ni 2 -CPD Py (OC 6 ) to the triplet excited state of Li + @C 60 ( 3 [Li + @C 60 ] * ) rather than from 3 [Ni 2 -CPD Py (OC 6 )] * to Li + @C 60 as indicated by the disappearance of the absorption band at 750 nm due to 3 [Li + @C 60 ] * , accompanied by the appearance of the absorption band at 1035 nm due to Li + @C 60 _ À (Fig. 6b).Photoexcitation at 520 nm where Li + @C 60 has absorption results in the formation of 1 [Li + @C 60 ] * , which is converted to 3 [Li + @C 60 ] * via the intersystem crossing.The rate constant of electron transfer from Ni 2 -CPD Py (OC 6 ) to 3 [Li + @C 60 ] * to produce the CS state was determined from the rise in the absorbance at 1035 nm due to Li + @C 60 _ À to be 5.7 Â 10 7 s À1 .The CS lifetime was determined from the slope of the rst-order plots in Fig. 6c to be 0.67 ms, which is the longest value ever reported for non-covalent porphyrin-fullerene supramolecules in solution. 41,46The quantum yields of the CS states were estimated to be 0.32 for Li + @C 60 3H 4 -CPD Py (OC 6 ) and 0.13 for Li + @C 60 3Ni 2 -CPD Py (OC 6 ) by means of the comparative method with the absorption intensities of the CS states (Li + @C 60 _ À : P (1035 nm) ¼ 7300 M À1 cm À1 23,39 When Li + @C 60 was replaced by pristine C 60 , the transient absorption spectra of both C 60 3H 4 -CPD Py (OC 6 ) and C 60 3Ni 2 -CPD Py (OC 6 ) measured by nanosecond laser ash photolysis in PhCN exhibited only 740 nm bands for the triplet excited state of C 60 with no transient absorption bands due to CPD Py (OC 6 )_ + or C 60 _ À (Fig. S20 in ESI †).Thus, no CS states were produced as predicted by their higher energy levels than those of the triplet excited states of CPD Py (OC 6 ) and C 60 .

Conclusions
Li + @C 60 is included within a free base and nickel complex of a cyclic porphyrin dimer to give stable supramolecules in benzonitrile.From the electrochemical analysis, the energies of the expected CS states are estimated to be lower than the triplet excited energies of the fullerene and porphyrin.Li + @C 60 3H 4 -CPD Py (OC 6 ) undergoes photoinduced electron transfer from the triplet excited state of the porphyrin to Li + @C 60 to afford the CS state.Li + @C 60 3Ni 2 -CPD Py (OC 6 ) also undergoes photoinduced electron transfer from the nickel porphyrin to the triplet excited state of Li + @C 60 with a rate constant of 5.7 Â 10 7 s À1 .The lifetimes of the resulting CS states are 0.50 ms for Li + @C 60 3H 4 -CPD Py (OC 6 ) and 0.67 ms for Li + @C 60 3Ni 2 -CPD Py (OC 6 ).These CS lifetimes are the longest values ever reported for non-covalent porphyrin-fullerene supramolecules in solution and are attributed to the lower CS energies than the triplet energy of each chromophore.

Materials
Reagents and solvents of best grade available were purchased from commercial suppliers and were used without further purication unless otherwise noted.Lithium ion-encapsulated C 60 (Li + @C 60 PF 6 À : 96%) was obtained from Daiichi Jitsugyo Co.
Ltd, Japan.N,N-Dimethylformamide (DMF) was puried by distillation from CaH 2 under reduced pressure.Benzonitrile (PhCN) was puried by distillation from P 2 O 5 under reduced pressure aer being stirred with K 2 CO 3 overnight.Dry tetrahydrofuran (THF) was obtained by distillation from Na and benzophenone under N 2 atmosphere.Dry triethylamine (Et 3 N) was obtained by distillation from CaH 2 under N 2 atmosphere, aer being stirred with KOH overnight.

NMR and mass measurements
Nuclear magnetic resonance (NMR) spectra were recorded on a JEOL JNM-ECS400 (400 MHz for 1 H), JEOL JNM-ECA500 (500 MHz for 1 H), or Bruker AVANCE III 600 (151 MHz for 13 C) spectrometer.Chemical shis were reported as d values in ppm relative to tetramethylsilane.High-resolution fast atom bombardment mass spectra (HR-FAB-MS) were measured with 3-nitrobenzyl alcohol (NBA) as a matrix and recorded on a JEOL LMS-HX-110 spectrometer.

UV-vis and IR absorption measurements
Ultraviolet-visible (UV-vis) absorption and infrared (IR) spectra were recorded on Shimadzu UV-3100PC and BIO RAD FTS6000 spectrophotometers, respectively.

Emission measurements
Fluorescence spectra were measured on a Horiba FluoroMax-4 spectrouorophotometer with a quartz cuvette (path length ¼ 10 mm) at 298 K. Phosphorescence spectra were measured on a Horiba Fluorolog s3 spectrophotometer with a quartz tube (i.d.¼ 4 mm) at 77 K.

Electrochemical measurements
Electrochemical measurements were performed on a ALS630B electrochemical analyzer in deaerated PhCN containing 0. light continuum covering the visible region from l ¼ 410 nm to 800 nm was generated via self-phase modulation.A variable neutral density lter, an optical aperture, and a pair of polarizer were inserted in the path in order to generate stable white light continuum.Prior to generating the probe continuum, the laser pulse was fed to a delay line that provides an experimental time window of 3.2 ns with a maximum step resolution of 7 fs.In our experiments, a wavelength at l ¼ 393 nm of SHG output was irradiated at the sample cell with a spot size of 1 mm diameter where it was merged with the white probe pulse in a close angle (<10 ).The probe beam aer passing through the 2 mm sample cell was focused on a ber optic cable that was connected to a CMOS spectrograph for recording the time-resolved spectra (l ¼ 410-800 nm).Typically, 1500 excitation pulses were averaged for 3 seconds to obtain the transient spectrum at a set delay time.