Near-Infrared Photoelectrochemical Conversion via Photoinduced Charge Separation in Supramolecular Complexes of Anionic Phthalocyanines with Li⁺@C₆₀

Two phthalocyanines possessing carboxylate groups ((TBA)₄ H₂ Pc·1 and (TBA)₄ H₂ Pc·2) form 1:2 supramolecular complexes with lithium cation-encapsulated C₆₀ (Li⁺@C₆₀ ) [H₂ Pc·1^4-/(Li⁺@C₆₀ )₂ and H₂ Pc·2^4-/(Li⁺@C₆₀ )₂ ] in a polar mixed solvent. From the UV-vis spectral changes, the binding constants (K) were estimated as ca. 10¹² M^-2. Upon the photoexcitation of constructed supramolecular complexes, photoinduced electron transfer occurred to form the charge-separated (CS) state. The lifetime of the CS state was determined to be 1.2 ms for H₂ Pc·2^4-/(Li⁺@C₆₀ )₂ , which is the longest CS lifetime among the porphyrinoid/fullerene supramolecular complexes. H₂ Pc·1^4-/(Li⁺@C₆₀ )₂ also afforded the long-lived CS state of 1.0 ms. The spin state of the long-lived CS states was determined to be a triplet, as indicated by the EPR signal at g = 4. The reorganization energy (λ) and the electronic coupling term were determined to be λ = 1.70 eV, V = 0.15 cm^-1 from the temperature dependence of the rate constant for the charge recombination of the CS state of H₂ Pc·1^4-/(Li⁺@C₆₀ )₂ . The energy of the CS state (0.49 eV) is much smaller than the reorganization energy, indicating that the back-electron-transfer process is located in the Marcus normal region. The small electronic coupling term results from the spin-forbidden back electron transfer due to the triplet CS state. Supramolecular complexes of anionic zinc phthalocyanines with Li⁺@C₆₀ were also prepared and investigated. The ZnPc·4^4-/Li⁺@C₆₀ supramolecular nanoclusters were assembled on the optically transparent electrode (OTE) of nanostructured SnO₂ (OTE/SnO₂ ) to construct the dye-sensitized solar cell. The IPCE (incident photon-to-photocurrent efficiency) values of OTE/SnO₂ /(ZnPc·4^4-/Li⁺@C₆₀ )_n were much higher than the sum of the two IPCE values of the individual systems OTE/SnO₂ /(Li⁺@C₆₀ )_n and OTE/SnO₂ /(ZnPc·4^4-)_n , covering the near-infrared region.