Donor–Acceptor Complexes of (5,10,15,20-Tetra(4-methylphenyl)porphyrinato)cobalt(II) with Fullerenes C60: Self-Assembly, Spectral, Electrochemical and Photophysical Properties

The noncovalent interactions of (5,10,15,20-tetra(4-methylphenyl)porphinato)cobalt(II) (CoTTP) with C60 and 1-N-methyl-2-(pyridin-4-yl)-3,4-fullero[60]pyrrolidine (PyC60) were studied in toluene using absorption and fluorescence titration methods. The self-assembly in the 2:1 complexes (the triads) (C60)2CoTTP and (PyC60)2CoTTP was established. The bonding constants for (C60)2CoTTP and (PyC60)2CoTTP are defined to be (3.47 ± 0.69) × 109 and (1.47 ± 0.28) × 1010 M−2, respectively. 1H NMR, IR spectroscopy, thermogravimetric analysis and cyclic voltammetry data have provided very good support in favor of efficient complex formation in the ground state between fullerenes and CoTTP. PyC60/C60 fluorescence quenching in the PyC60/C60–CoTTP systems was studied and the fluorescence lifetime with various CoTTP additions was determined. The singlet oxygen quantum yield was determined for PyC60 and the intensity decrease in the 1O2 phosphorescence for C60 and PyC60 with the CoTTP addition leading to the low efficiency of intercombination conversion for the formation of the 3C60* triplet excited state was found. Using femtosecond transient absorption measurements in toluene, the photoinduced electron transfer from the CoTTP in the excited singlet state to fullerene moiety was established. Quantum chemical calculations were used for the determination of molecular structure, stability and the HOMO/LUMO energy levels of the triads as well as to predict the localization of frontier orbitals in the triads.


Introduction
Among the wide variety of fullerenes, C 60 is one of the most common and actively studied fullerenes due to its pronounced electron-withdrawing property and great potential for application in many areas of applied science [1][2][3][4][5][6]. C 60 fullerene is known to be a strong singlet oxygen generator, and it has only limited use in photodynamic therapy due to its extremely low solubility in water [7]. However, there are investigations devoted to the study of the effect of aqueous suspensions of C 60 fullerene on microorganisms [8,9]. The modification of the fullerene core by attaching various groups or introducing a metal atom into the carbon cage provides the possibility of fine-tuning the physical and chemical properties of these compounds. The fullerene derivatization increases its solubility in an aqueous solvent and reduces aggregation to possible application in medicinal chemistry as antibacterial/antifungal agents [10][11][12][13]. The transition to trifluoromethyl derivatives of C 60 leads to the increase in the electron affinity compared to initial fullerene and in the ability to form long-lived anion-radical particles [14,15]. The dimerization of fullerenes, namely the formation of pyrrolizidine/cyclobutane-bridged double-caged fullerene derivatives, promotes the charge delocalization over both carbon cages, upshifts energies of frontier molecular orbitals, a narrowed bandgap and a reduced electron-transfer reorganization energy relative to molecular C 60 [16,17]. The endohedral C 60 derivatives are interesting because a metal atom encapsulated into C 60 is able to transfer a part of its spin density to the carbon cage, which ensures long spin relaxation times [18]. The modification of C 60 by pyridyl groups, when the pyridyl nitrogen atom donates its lone pair of electrons to the electron-deficient π-system of fullerene, is used in the control of electron-withdrawing properties (LUMO energy) of fullerene derivatives. Such modification also allows for the design of various donor-acceptor systems, providing the high efficiency of photoinduced separation of charges when irradiated with the visible light [19][20][21][22][23].
One of the strategies for improving the charge-separation process is the formation of covalent and noncovalent supramolecular systems based on fullerenes and electro/photoactive compounds as acceptors and donors, respectively [24]. Porphyrins and their complexes are promising among such compounds due to their structural diversity, photophysical/photochemical properties, broadband spectrum, high extinction coefficients, stability at relatively high temperatures and the presence of a π-conjugated macrocycle [25,26]. These systems offer new opportunities in the preparation of materials that may produce long-lived charge-separated states in high quantum yields.
The supramolecular interactions of the fullerenes with the porphyrins have been extensively studied in recent decades [27,28]. However, there is little data on the interaction of the non-functionalized fullerene C 60 with metalloporphyrins (MPs). The C 60 complexes with Zn(II), Co(II) and Fe(III) octaethylporphyrins (OEPs) bonding due to the van der Waals interactions were obtained by M.M. Olmstead et al. in 1999 [29]. Their structures, established by X-ray diffraction, contained one C 60 molecule surrounded by two metalloporphyrin molecules in the case of ZnOEP and CoOEP, but one metalloporphyrin molecule in the case of ClFeOEP [29]. In the work of [30], molecular complexes of fullerene C 60 with Mn(II), Co(II), Cu(II), Zn(II) and Fe(III) tetraphenylporphyrins (TPPs) were synthesized and studied by ESR, IR, UV-vis and X-ray photoelectron spectroscopy. The formation of molecular complexes with fullerenes affects the ESR spectra of MPs lowering the spin state in the case of MnTPP or changing the constant of hyperfine interaction in the case of CoTPP and CuTPP [30]. Additionally, C 60 -containing complexes based on jaw-like Pd, Zn, Cu, Co, Fe and Mn bis(metalloporphyrins) were described in the work of [31]. In 2007, Aida et al. reported that iridium(III) porphyrin cyclic dimer bonds C 60 to form the 1:1 complex [32]. In 2017, Yamada et al. have synthesized iridium porphyrin bearing peripheral alkyl chains, which forms not only the 1:1 complex but also the 2:1 one in toluene [33]. The electronic interactions between zinc(II) porphyrin containing the reactive amino groups and C 60 were established by steady state fluorescence, time correlated single photon counting measurements and DFT calculations [34]. These new data point to the consideration of metalloporphyrin-C 60 systems as the basis for numerous photovoltaic devices.
The self-assembly of the cobalt(II) porphyrin (CoP) coordination triads (where the triad is the molecule with various combinations of electron donor and acceptor) with fullero [60]pyrrolidines have been reported in our early works [35][36][37][38], while the one with non-functionalized fullerene C 60 have not been explored to date. We report here about the ability of cobalt(II) porphyrin bearing peripheral 4-methylphenyl groups to bond both C 60 and PyC 60 , thus forming the 2:1 coordination complexes. The self-assembly in cobalt(II) porphyrin-C 60 /PyC 60 systems in toluene was studied by the absorption and fluorescence titration methods. The spectral properties of triads in the ground and excited states are observed and discussed in this article.

Results and Discussion
The UV-vis and fluorescence spectroscopy are suitable methods to study the supramolecular interaction of metalloporphyrins upon bonding fullerenes [39][40][41]. Both methods allow one to easily determine the stability constants of the CoTTP-C 60 /PyC 60 triads. The spectrophotometric methods are attractive due to the large changes in the CoPs UV-vis spectra when a non-covalent bond is formed along the axial axis causes. Figure 1 shows the scheme of the CoTTP reaction with PyC 60 and the corresponding UV-vis spectrum transformations during the titration of the CoTTP solution in toluene by PyC 60 . The time-dependent titration at τ = 0 and τ = ∞ [42] allowed us to describe both rapid/slow equilibriums and the rate of the second-stage reaction. The fast reversible (PyC 60 )CoTTP formation (at τ = 0) is characterized by the gradual decrease in the intensity at 414 nm in the CoTTP spectrum. The (PyC 60 ) 2 CoTTP formation occurs over time (within 100-1100 s depending on the PyC 60 concentration, see Figure S1 in Supplementary Materials). It is accompanied by the gradual decrease in the absorption at 414 nm together with the appearance of the new band at 437 nm. The analysis of the logI-logC PyC60 dependence slope has confirmed one PyC 60 molecule bonding at the first and second stage ( Figure S2, Supplementary Materials). The step constants, K 1 and K 2 , were calculated by the equation for the three-component equilibrium system (see Equation (S1) in Supplementary Materials) and were found to be (2.3 ± 0.3) × 10 4 and (6.4 ± 0.9) × 10 5 M −1 , respectively. The stability constant of the (PyC 60 ) 2 CoTTP triad, β = K 1 × K 2 is (1.47 ± 0.28) ×10 10 M −2 . The rate of forward reactions (k 2obs ) in the slow equilibrium was measured (Table S1, Supplementary Materials). Using the data in Table S1 and Figure S3, the first order with respect to CoTTP and PyC 60 , respectively, are determined ( Figure S3, Supplementary Materials). The rate constant k 2 found from Figure S3 data is (62.3 ± 5.1) M s −1 . The rate constant k −2 equal k 2 /K 2 is 9.7 × 10 −5 M 2 s −1 .
UV-vis spectral changes observed during CoTTP bonding with C 60 are similar to the ones for the CoTTP-PyC 60 system. The formation of a supramolecular complex between CoTTP and C 60 is accompanied by the Soret band shift from 414 nm to 432 nm.
Job's plot points to the CoTTP-C 60 bonding stoichiometry of 1:2 ( Figure 2). The use the time-dependent titration method for the CoTTP-C 60 system allowed us to also identify two stages (the fast equilibrium and slow two-way reaction of C 60 coordination, Figure S4, Supplementary Materials) and to determine the thermodynamic stability constants, which were found to be K 1 = (5.02 ± 0.94) × 10 4 M −1 , K 2 = (6.63 ± 0.98) × 10 4 M −1 and β = (3.47 ± 0.69) × 10 9 M −2 . The stability constants (the formation ones) of (PyC 60 ) 2 CoTTP and (C 60 ) 2 CoTTP were also determined with the help of steady state fluorescence measurements in toluene. Since cobalt(II) porphyrins were non-fluorescent [43], and C 60 [44] as well as their derivatives [45] were fluorescent with a low fluorescence quantum yield (3.2 × 10 −4 for C 60 ), we studied the fluorescence of C 60 /PyC 60 with various additions of CoTTP at thermostating solutions for 20 min (τ = ∞). The fluorescence quantum yield of PyC 60 determined in the toluene is 5.3 × 10 −4 . It was established that the C 60 /PyC 60 fluorescence upon excitation at 450 nm is quenched following the gradual addition of CoTTP. The steady state fluorescence titration experiments for the PyC 60 -CoTTP and C 60 -CoTTP systems recorded in toluene are demonstrated in Figure 3a,b, respectively. The process of C 60 /PyC 60 fluorescence quenching with CoTTP was investigated using the Stern-Volmer equation. The results are given in Figure 3c According to the literature data [46], the fluorescence lifetime of C 60 varies from 1.17 to 1.3 ns. As the results of our measurements, the fluorescence lifetime of C 60 in toluene was determined to be 1.38 ns. C 60 exhibits bi-exponential decay which is in agreement with the works of [47,48]. PyC 60 also exhibits bi-exponential decay with the fluorescence lifetime of 1.61 ns.
The time-resolved fluorescence spectroscopy results for the C 60 , PyC 60 , C 60 -CoTTP and PyC 60 -CoTTP solutions in toluene are shown in Table S2  Another feature of fullerenes is their ability to generate singlet molecular oxygen ( 1 O 2 ) with high quantum yields [49], which we have used to confirm the formation of supramolecular systems with the charge-separation state [50]. The 1 O 2 quantum yield for C 60 is often equated to 1 [51,52]. Given these data, the quantum yield of singlet oxygen of PyC 60 was determined and found to be 0.7 (see Section 3.4). This value is consistent with the higher triplet energies and the lower quantum yields of triplet and singlet oxygen for functionalized C 60 [53]. The intensity of the 1 O 2 phosphorescence decreases when the addition of CoTTP is increased. Emission spectra of the 1 O 2 obtained by the photoirradiation (λ = 453 nm) of the C 60 , PyC 60 , C 60 -CoTTP and PyC 60 -CoTTP solutions are shown in Figure 4. The phosphorescence decay of singlet oxygen formed by the sensitization with PyC 60 and PyC 60 -CoTTP is represented in Table S3, Supplementary Materials. The intensity of the 1 O 2 phosphorescence for C 60 and PyC 60 decreases with the maximum CoTTP addition by 3.5 and almost 6 times, respectively. This fact additionally confirms the realization of photoinduced electron transfer (PET) in (PyC 60 ) 2 CoTTP/(C 60 ) 2 CoTTP from the donor CoTTP fragment to the acceptor fullerene-containing fragment, since this process possibly leads to the low efficiency of intercombination conversion for the formation of the 3 C 60 * triplet excited state.
The interactions between CoTTP and C 60 /PyC 60 in the ground states are confirmed by spectral methods such as mass spectrometry and IR/ 1 H NMR spectroscopy. MALDI mass spectrometry is a rapid qualitative test for fullerene bonding by porphyrin [31]. Using the dithronol matrix, (PyC 60 60 ] + ) ( Figure 5). No molecular ion peak of (C 60 ) 2 CoTTP was observed due to the dissociation of the noncovalent complex in the conditions of the mass spectral experiments.  According to IR spectral studies, the position and intensity of the pyrrole ring stretching vibrations of CoTTP remain practically unchanged in the IR spectra of (PyC 60 ) 2 CoTTP and (C 60 ) 2 CoTTP, pointing to the Co atom location in the macrocycle plane [54,55]. This is related to the signals at 1684 cm −1 , ν (C=N), 1452 cm −1 , 1404 cm −1 , 1377 cm −1 ν(C-H) pyr in the CoTTP spectrum that is listed in Experimental Section. The planar coordination center was also confirmed by both theoretical calculations (see next section) and the data from the work of [42] on the example of (2,3,7,8,12,18-hexamethyl,13,17-diethyl,5-(2-pyridyl)porphinato)cobalt(II) and its triads with PyC 60 . Along with the signals of the porphyrin macrocycle, there are signals of coordinated PyC 60 in the IR spectrum of (PyC 60 ) 2 CoTTP (Table S4, Supplementary Materials). The vibration signals of the C 60 skeleton at 527 cm −1 , 574 cm −1 and 1428 cm −1 as well as signals of pyridyl/pyrrolidinyl fragments of the fullerene core at 1245 cm −1 , 1281 cm −1 and in the 400-710 cm −1 range ( Figure 6, Table S4, Supplementary Materials) are observed. These signals are shifted by 1-14 cm −1 relative to uncoordinated PyC 60 [36,42]. The signal at 456 cm −1 related to ν(Co-N) is considerably broadened; the ν(Co-N PyC60 ) signal appears at 463 cm −1 as the shoulder ( Figure 6). The IR (C 60 ) 2 CoTTP spectrum is the sum of the C 60 [56] and CoTTP spectra, suggesting the weak interaction between C 60 and CoTTP in the ground state.  The thermal stability of (PyC 60 ) 2 CoTTP/(C 60 ) 2 CoTTP studied in comparison with the its precursors by the TGA method demonstrates the high parameters of most porphyrins [57,58]. Figure S7a, Supplementary Materials, shows the TG curve and its first derivative (DTG) for CoTTP. The weight loss of 0.85% between 25 • C and 479 • C is connected with the desorption of solvent molecules rather than the metalloporphyrin degradation. The elimination of the tolyl groups and the degradation of macrocycle ring manifest itself in the form of the weight loss of 12% in the temperature range of 479-509 • C with the major weight loss at 504 • C and of 19.65% between 719 • C and 750 • C with the maximum value at 738 • C, respectively. C 60 has also displayed the high thermal stability, exhibiting the decomposition process in the range of 827-937 • C ( Figure S7b, Supplementary Materials). The TGA results for PyC 60 ( Figure S7c, Supplementary Materials) show the start of the compound decomposition from around 374 • C to 454 • C, i.e., at a considerably lower temperature than the one for pure C 60 . The main drop in the PyC 60 mass corresponding to the fullerene cage decomposition occurs between 807 • C and 904 • C; the peak of the derivative on the weight loss is observed at 883 • C.
The decomposition of triads shown in Figure 8 is the multi-stage process which includes the degradation of both the porphyrin macrocycle and the fullerene core. In the case of (C 60 ) 2 CoTTP, the peaks of the derivative on the weight loss at 494 • C/738 • C and 823 • C correspond to the thermal decomposition of CoTTP and C 60 , respectively (Figure 8a). The thermal behavior of (PyC 60 ) 2 CoTTP (Figure 8b) was similar. The peaks of the derivatives on the weight loss for both triads are observed at temperatures lower than that of their individual components. Thus, the results of the TGA experiment suggest the direct bonding between CoTTP and C 60 /PyC 60 in the high stable triads.  Figure 9. There are two reversible peaks in the positive potential range for CoTTP. The first and the second oxidation/reduction correspond to the central metal atom and the macrocycle, respectively [59]. The cyclic voltammograms of C 60 and PyC 60 in CH 2 Cl 2 are characterized by three pairs of peaks, which is typical of fulleropyrrolidines [60,61]. The redox potentials for all these compounds are given in Table S5, Supplementary Materials. The CV of (C 60 ) 2 CoTTP and (PyC 60 ) 2 CoTTP are essentially the sum of the CoTTP and C 60 /PyC 60 peaks. Three well-pronounced reversible redox waves corresponding to the sequential addition of electrons to the fullerene unit are observed in the negative potential window. Two reversible redox waves in the positive potential window correspond to the central metal atom and the macrocycle of CoTTP. However, these peak potentials are shifted relative to ones for the individual components (Table S5, Supplementary Materials), which indicates the interaction between the π system of CoTTP and C 60 /PyC 60 and the redistribution of the electron density within the triad.
The DFT calculations have demonstrated the higher stability of (PyC 60 ) 2 CoTTP compared with (C 60 ) 2 CoTTP. The bonding energy in (PyC 60 ) 2 CoTTP and (C 60 ) 2 CoTTP calculated as the difference between the total energy of triad minus the sum of the energies of CoTTP and C 60 /PyC 60 is 253 kJ·mol −1 and 293 kJ·mol −1 , respectively. The coordination of CoTTP is accompanied by a distortion of the macrocycle and its deviation from planarity by 7.31 • (Figure 10). The coordination center has an octahedral geometry with average values of Co-N macrocycle and Co-N PyC60 bond lengths of 2.002 Å and 2.382 Å, respectively ( Figure S8a, Supplementary Materials). The additional (PyC 60 ) 2 CoTTP stabilization is facilitated by both π-π CoTTP-PyC 60 stacking and the peripheral tolyl CH 3 -PyC 60 ·π interactions.  In contrast to donor-acceptor bonding, the effect of CoTTP-C 60 π-π stacking in the macrocycle geometry, with a slight distortion of 1.47 • , is softer. The average length of the Co-N macrocycle bonds is 2.004 Å ( Figure S8b, Supplementary Materials); the distance between the centers of the CoTTP and C 60 closest contacts is 2.686 Å. In addition, one can note the presence of the peripheral tolyl CH 3 -C 60 ·π contacts. The characteristic feature of the considered triads is the charge transfer from the porphyrin macrocycle to the acceptor fullerene moieties. This is clearly seen as the decrease in negative charge of the CoTTP nitrogen atoms in (PyC 60 ) 2 CoTTP and (C 60 ) 2 CoTTP ( Figure S9, Supplementary Materials). At the same time, the pyrrolidine moiety in (PyC 60 ) 2 CoTTP and the fullerene fragment in (C 60 ) 2 CoTTP acts as the donors (the latter due to the interactions of the carbon atoms of the electron-saturated [6,6] double bonds with the complexing agent atom), transferring the electron density to the metal atom. This is confirmed by the CoTTP central atom positive charge decrease in (PyC 60 ) 2 CoTTP and (C 60 ) 2 CoTTP ( Figure S9, Supplementary Materials). The axial coordination does not lead to the cobalt atom displacement from the macrocycle plane.
The analysis of the frontier molecular orbitals (FMOs) of (PyC 60 ) 2 CoTTP and (C 60 ) 2 CoTTP has shown that HOMO and LUMO in (PyC 60 ) 2 CoTTP are localized on CoTTP/the pyridine moiety nitrogen atom and on both PyC 60 fullerene moieties, respectively ( Figure 11). In (C 60 ) 2 CoTTP, HOMO is localized on both CoTTP and the carbon atoms of the electronsaturated [6,6] double bonds while LUMO is localized on both C 60 moieties (Figure 11). Such localization of FMOs in the triads may indicate the mechanism of the implementation PET from the donor CoTTP moiety to the acceptor fullerene-containing moiety. The triad HOMO-LUMO gaps obtained from the DFT calculations ( Figure 11) are consistent with the ones estimated from the oxidation and reduction potentials as difference between the first CoTTP oxidation and the first fullerene reduction potentials [60]. The HOMO-LUMO gap is 1.19 V and 1.45 V for (C 60 ) 2 CoTTP and (PyC 60 ) 2 CoTTP, respectively. The information about the excited state of CoTTP and its linear spectra dynamic obtained by the transient absorption measurements using a femtosecond pump-supercontinuum probe setup is as follows: as shown in Section 3.2, the CoTTP Soret band at 414 nm displays an almost 15 times higher molar extinction coefficient than the Q band at 529 nm. The negative signal at 416 nm related to ground-state bleaching and dominating the spectral range of transient absorption spectra was excluded from the studied spectral window for greater visibility and greater significance of the positive signals of in femtosecond transient absorption (FTA) spectra. The CoTTP FTA spectra recorded after the 435 nm laser pulse excitation were collected between 430 nm and 780 nm in different time windows ( Figure S10, Supplementary Materials). The CoTTP FTA spectrum displays the characteristic spectral shape of porphyrin singlet excited states [39]. There is the broad intense absorption band with the maximum at 443 nm as well as the Q-band at 527 nm in the CoTTP singlet excited state spectrum, which bleaches at the respective groundstate absorption band ( Figure S10, Supplementary Materials). The excitation wavelength 435 nm used in the present work corresponds to the transition from S0 to the vibrationally excited level of the S2 state of porphyrins [32]. The analysis of transient absorbance kinetics at 443 nm at short delays has revealed the fast rise component with a time constant of only 0.11 ps, which can be presumably attributed to the S2→S1 internal conversion ( Figure S11, Supplementary Materials). Thus, this time constant can be interpreted as the lifetime of the S2 excited state (τ 1 ). This is in agreement with the data reported earlier for other Co(II) porphyrins [19,55]. The τ 1 values of 0.08 ps, 0.17 ps and 0.099 ps were found for the toluene solutions of (5,10,15,20-tetraphenylporphinato)cobalt(II), (5,10,15,20tetra(4-trifluoromethylphenyl)porphinato)cobalt(II) [62] and meso-carbazole substituted cobalt(II)porphyrins, respectively [26]. The singlet excited state (S1) lifetime (τ 2 ) of CoTTP was calculated from the monoexponential fit to the decay profile at 443 nm and was found to be 5.95 ps ( Figure S11, Supplementary Materials).
For the spectral range of our instrument setup from 400 nm to 850 nm, it is not possible to detect a similar peak. That is why the charge-separated states formed in the triads are confirmed in our work by the changing CoTTP •+ bands and S2 and S1 excited state lifetimes. As shown in Figure 12, the positive signal at 456 nm shifted by 13 nm compared to that of CoTTP is observed after the excitation of the (PyC 60 ) 2 CoTTP. This signal, displayed at 486 nm in the case of (C 60 ) 2 CoTTP, shifts much more ( Figure 12). The time profiles of the CoTTP •+ bands are represented in Figure 13. The difference within the same time observation window (100 ps) can be seen between the two triads studied. The time constants of the (PyC 60 ) 2 CoTTP and (C 60 ) 2 CoTTP singlet excited state, τ 2 , extracted from the CoTTP •+ decay are 7.28 ps and 14.53 ps, respectively. The analysis of the τ 2 values shows a not too long lifetime for the radical ion-pairs of the triads, CoTTP •+ :PyC 60 •− /C 60 •− , in toluene; however, these results indicate PET from CoTTP to bonded PyC 60 /C 60 . The excited state lifetime can be changed by the variation of solvent properties. The chargeseparated state lifetime of triad based on CoTTP and phenyl imidazole functionalized fullero [60]pyrrolidine was found at about 1.61 ns in o-dichlorobenzene [43].

General Information
Organic solvents, chromatographic materials and fullerene (C 60 ) were obtained from Aldrich Chemicals and Acros Organics and used as received without further purification. Meso-tetra(4-tolylphenyl)porphyrin was obtained from PorphyChem.
The UV-vis spectra were measured on an Agilent 8454 UV-visible spectrophotometer in toluene. The stationary fluorescence, emission spectra of singlet molecular oxygen and time-resolved fluorescence measurements were carried out by means of a high-performance fluorescence lifetime and steady-state spectrometer FluoTime 300 with a laser 450 nm, LDH-P-C-450, (PicoQuant, Berlin, Germany) as an excitation source. 1 H NMR spectra were recorded on a Bruker Avance III-500 NMR spectrometer in CDCl 3 at room temperature. Chemical shifts (δ) were referenced to the residual solvent peak (CDCl 3 , 7.26 ppm). IR spectra were obtained using a VERTEX 80v spectrometer in KBr and CsBr pellets. The solid samples of the triads for the IR measurements were prepared by the vacuum removal of the solvent. Mass spectra were performed on an Axima Confidence (Shimadzu Biotech, a 337 nm nitrogen laser) spectrometer. Mass spectra were accumulated using reflectron positive ion mode for MALDI-TOF MS.
Thermal analysis (TG/DTG) of the complexes was carried out in argon using a Netzsch TG 209 F1 microthermal balance. The rate of sample heating, the sample weights and the temperature range were 10 • C/min, 3.5-4.7 mg and 25-920 • C, respectively. The resolution of microthermal balances is 1 × 10 −4 mg.
The experiments of cyclic voltammetry (dichloromethane, 0.1 M (n-Bu) 4 NClO 4 ) were performed on a Solartron SI 1260 using the three-electrode system (a platinum button, a platinum wire and a saturated calomel electrode (SCE) as the working, counter and reference electrode, respectively). Cyclic voltammograms were measured at the potential sweep rate of 1000 mV·s −1 . All solutions were purged from the dioxygen prior to electrochemical measurements using an argon gas.

Thermodynamics and Kinetics
The two-way reaction of CoTTP with C 60 and PyC 60 in toluene was spectrophotometrically studied at 298 K using the time-dependent titration method in which the thermodynamic and kinetic reaction parameters are obtained in one experimental series (see Supplementary Materials for details).

Fluorescence Spectroscopy
The fluorescence spectra of C 60 /PyC 60 and its complexes with CoTTP (after excitation 450 nm (LDH-P-C-450)) were measured in toluene in quartz cuvettes (10 mm × 10 mm) on FluoTime 300 PicoQuant spectrofluorimeter. The self-assembly in the systems CoTTP-C 60 /PyC 60 were studied using the method of the molar ratios. The molar series of the solutions with constant concentrations of C 60 /PyC 60 and varying concentrations of CoTTP were prepared by adding CoTTP solution to the C 60 (6.51 × 10 −5 M) and PyC 60 (6.55 × 10 −5 M) solutions in toluene. To evaluate the efficiency of the C 60 /PyC 60 fluorescence quenching, the Stern-Volmer constants (K SV ) were determined using the equation: where I 0 and I are the fluorescence intensity of C 60 /PyC 60 in the absence and presence of CoTTP, respectively.
The CoTTP-C 60 /PyC 60 bonding constants (K BH ) were estimated using modified Benesi-Hildebrand equation: where I 0 , I x and I max are the fluorescence intensities of C 60 /PyC 60 in the absence of CoTTP, at an intermediate C 60 /PyC 60 concentration and at the concentration of complete interaction, respectively.
The PyC 60 fluorescence quantum yield was determined in the toluene at room temperature using C 60 in toluene as a fluorescence standard (Φ st , C60 = 3.2×10 −4 ) [44]. The value of Φ F was calculated using the equation: where Φ F is the fluorescence quantum yield, Φ std -the fluorescence quantum yield of the standard, S-the integrated fluorescence intensity (the area under spectrum), A-the absorbance at the excitation wavelength and n-the refractive index of the solvents used. The fluorescence decay curves were measured and the fluorescence lifetimes were obtained by the reconvolution of the decay curves using the EasyTau 2 software package (PicoQuant). The biexponential fluorescence decay model was applied for C 60 , PyC 60 and C 60 -CoTTP/PyC 60 -CoTTP systems in toluene. The instrument response function (IRF) of the system was measured with the stray light signal of the dilute colloidal silica suspension (LUDOX ® ).
Singlet molecular oxygen was detected directly during 1 O 2 luminescence spectrum recording in the range from 1200 nm to 1350 nm for C 60 , PyC 60 and C 60 -CoTTP/PyC 60 -CoTTP systems in toluene. The PyC 60 luminescence signal of singlet oxygen was measured and compared with the similar C 60 signal for which Φ ∆ 1 O 2 value in toluene is known [51]. The value was calculated using the equation: where S is the area of singlet oxygen emitted spectrum, A is the absorbance of the solution at the excitation wavelength and τ is the singlet oxygen lifetime of PyC 60 or C 60 . The symbol reported as subscript (std) refers to the C 60 standard sample.

Femtosecond Laser Photolysis Setup
Femtosecond transient absorption spectra were measured at 20 • C on a pump and light supercontinuum probe setup with a 3.3 fs to 1 ps delay step and a 1 nm wavelength step.
The spectra were subjected to correction procedure [64][65][66]. The details of the transient spectra measurement were described in Supplementary Materials. The analysis of the characteristic times (τ) of the transient absorption spectra was carried out using the kinetic simulation based on the singular value decomposition (SVD) of the obtained data matrix. This approach involves the consideration of the entire array of the experimental values ∆A(λ, t). Exponential processing of the obtained data at the wavelength of the excited state is used to determine the lifetime of the excited state.

DFT Calculations
The DFT calculations were carried out for (PyC 60 ) 2 CoTTP and (C 60 ) 2 CoTTP using the Gaussian 16, Revision C.01 program package [67]. The B3LYP functional [68] and the def2-SVP [69] basis set on all atoms (the def2-ECP is automatically assigned to cobalt atoms) were used for the geometric optimization of the triads. Influence of intramolecular dispersion interactions on molecular structure was explored using the dispersion corrections introduced by S. Grimme (D3) [70]. The geometric optimization of the triads was carried out without limitation in symmetry. Harmonic vibrational frequencies were calculated at the same theory level in order to characterize the stationary points as true minima, representing the equilibrium structures on the potential energy surfaces. ChemCraft 1.8 [71] (the graphical program for visualization of quantum chemistry computations, https://chemcraftprog.com) was used for analyses of results and molecular graphics.

Conclusions
(5,10,15,20-tetra(4-methylphenyl)porphinato)cobalt(II) (CoTTP) in toluene bonds fullerene (C 60 ) and 1-N-methyl-2-(pyridin-4-yl)-3,4-fullero [60]pyrrolidine (PyC 60 ) in the two-step two-ways processes end in the noncovalent 1: 2 π-π complex and coordination complex, respectively. The bonding constants are (3.47 ± 0.69) ×10 9 M −2 and (1.47 ± 0.28) ×10 10 M −2 , respectively, according to the methods of electron absorption spectroscopy. The fluorescence titration gives the close values of the constants with an approximately 1.3 times higher standard deviation. 1 H NMR, IR, thermogravimetric analysis and cyclic voltammetry studies of CoTTP, fullerenes and their complexes provide very good support in favor of the effective ground-state complexation between fullerenes and CoTTP. The quenching of the PyC 60 /C 60 fluorescence and the decreasing intensity of the singlet molecular oxygen phosphorescence parallel to the gradual increase in the CoTTP additions during the steadystate emission experiments have confirmed the photoinduced electron transfer (PET) in the triads, (PyC 60 ) 2 CoTTP/(C 60 ) 2 CoTTP. The PyC 60 /C 60 fluorescence lifetime and the exited-state characteristics were obtained using fluorescence and femtosecond transient absorption. DFT calculations used to predict the localization of the frontier orbitals and to determine the HOMO/LUMO energy levels have also confirmed the PET from the singlet excited CoTTP to the fullerene moiety in the triads. These results are of particular importance in the development of the porphyrin-fullerene systems, having potential for energy-harvesting applications.