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

• Published in: The journal of physical chemistry. B 2015-06, Vol.119 (24), p.7690-7697
• Main Authors: Kawashima, Yuki, Ohkubo, Kei, Blas-Ferrando, Vicente Manuel, Sakai, Hayato, Font-Sanchis, Enrique, Ortíz, Javier, Fernández-Lázaro, Fernando, Hasobe, Taku, Sastre-Santos, Ángela, Fukuzumi, Shunichi
• Format: Article
• Language: English
• Subjects: Anions - chemistry; Electrochemical Techniques; Electrodes; Fullerenes - chemistry; Indoles - chemistry; Infrared Rays; Lithium - chemistry; Molecular Structure; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry; Photochemical Processes
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp5123163

Two phthalocyanines possessing carboxylate groups ((TBA)4H2Pc·1 and (TBA)4H2Pc·2) form 1:2 supramolecular complexes with lithium cation-encapsulated C60 (Li+@C60) [H2Pc·1 4–/(Li+@C60)2 and H2Pc·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 H2Pc·2 4–/(Li+@C60)2, which is the longest CS lifetime among the porphyrinoid/fullerene supramolecular complexes. H2Pc·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 H2Pc·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.