Extensive efforts have so far been devoted to mimic light harvesting and charge separation processes in natural photosynthesis.¹ Multiporphyrin arrays have been employed as light-harvesting units. Light-harvesting and charge-separation units have been combined by coordination bonds between metalloporphyrins and electron acceptor moieties bearing Lewis base ligands. Thus, metal centres have been required for construction of supramolecular complexes between porphyrins acting as light harvesting units and electron acceptors containing Lewis base ligands for coordination to the metal centers. As such there has been only few report on supramolecular complexes of free base porphyrin arrays and electron acceptors without Lewis base ligands, in which the binding is rather weak. We report herein construction of supramolecular complexes of   free base porphyrin polypeptides (P(H₂P)_n: n = 4, 8) with lithium ion-encapsulated C₆₀ (Li⁺@C₆₀)^2,3 (Figure 1), in which binding is much stronger than the caes of C₆₀ in benzonitirle (PhCN). The photodynaimcs were studied by femtosecond and nanosecond laser-induced transient absorption and fluorescence lifetime measurements.⁴

    Upon mixing a PhCN solution of Li⁺@C₆₀ with that of P(H₂P)₈, the intensity of the Soret band decreased with increasing concentration of Li⁺@C₆₀. A Soret band was slightly red-shifted from 425 to 427 nm by the addition of Li⁺@C₆₀ to the solution. From the net absorption change at 425 nm in which the absorption due to Li⁺@C₆₀ was subtracted, a linear Benesi-Hildebrand plot was obtained, indiating that each porphyrin unit of P(H₂P)₈ forms a 1:1 supramolecular charge-transfer complex with Li⁺@C₆₀ independently with approximately the same binding constant of the unit of M^–1. The binding constant at 298 K was determined to be 2.1 ´ 10⁴ M^–1, which is significantly larger than that of C₆₀ (5.3 ´ 10³ M^–1). The stronger binding of Li⁺@C₆₀ as compared with C₆₀ may result from the stronger elecron acceptor ability of Li⁺@C₆₀, which 

facilitates the charge-transfer interaction as reported for the stronger charge-transfer binding of Li⁺@C₆₀ with corranulerene. Similary the binding constant of the supramolecular complex of P(H₂P)₄ was determined from the spectral titration to be 6.2 × 10³ M^–1. In the case of P(H₂P)₂ and P(H₂P)₁, howerver, the spectral change was too small to be able to determine the binding constants accurately. Thus, multiporphyris may facilitate charge-transfer interactions through encapsulation of Li⁺@C₆₀ by multiple porphyrins.⁴

       Nanosecond laser-induced transient absorption spectra of P(H₂P)₈ with Li⁺@C₆₀ (40 mM) at the excitation wavelength of 532 nm showed the transient absorption band at 730 nm due to the triplet excited state of Li⁺@C₆₀ (³Li⁺@C₆₀^*) observed together with the absorption band at 1035 nm due to Li⁺@C₆₀^•–. The decay of the absorption at 730 nm coincides with the appearance of Li⁺@C₆₀^•–. Thus, electron transfer from P(H₂P)₈ to ³Li⁺@C₆₀^* occurs to produce the triplet charge-separate (CS) state of P(H₂P)₈^•+ and Li⁺@C₆₀^•–. The transient absorption due to P(H₂P)₈^•+ is overlapped with that of ³Li⁺@C₆₀^* in the 600-700nm region. The photoexcitation of 532 nm resulted in the excitation of Li⁺@C₆₀ leading to the formation of ³Li⁺@C₆₀^* via intersystem crossing.  The decay of the absorbance at 1035 nm due to Li⁺@C₆₀^•– obeyed first-order kinetics with the lifetime of 210 ms. Thus, back electron transfer from Li⁺@C₆₀^•– to P(H₂P)₈^•+ occurs in the supramolecular complex. The lifetime is long because of the spin-forbidden back electron transfer in the triplet CS state.⁴

References

[1]    Fukuzumi, S.; Ohkubo, K.; Suenobu, T. Acc. Chem. Res. 2014, 47, 1455.

[2]   Kawashima, Y.; Ohkubo, K.; Fukuzumi, S. Chem. Asian J. 2015, 10, 44. (Review)

[3]   (a) N. Solladié , A. Hamel and M. Gross, Tetrahedron Lett., 2000, 41, 6075; (b) F. Aziat, R. Rein, J. Peon, E. Rivera and N. Solladié, J. Porphyrins Phthalocyanines, 2008, 12, 1232

[4]   Ohkubo, K.; Hasegawa, T.; Rein, R.; Solladié, N.; Fukuzumi, S. Chem. Commun. 2015, 51. 17517.