Light harvestingzinc naphthalocyanine–perylenediimide supramolecular dyads: long-lived charge-separated states in nonpolar media

Abstract

Photoinduced electron-transfer dynamics of self-assembled donor–acceptor dyads formed by axial coordination of zinc naphthalocyanine, ZnNc, and perylenediimide (PDI) bearing either pyridine (py) or imidazole (im) coordinating ligands were investigated. The PDIim unit was functionalized with tert-octylphenoxy groups at the bay positions, which avoid aggregation providing solubility, to examine the effect of the bulky substituents at the bay positions on the rates of electron-transfer reactions. The combination between zinc naphthalocyanine and perylenediimide entities absorbs light over a wide region of the visible and near infrared (NIR) spectrum. The binding constants of the self-assembled ZnNc:PDIpy (1) and ZnNc:PDIim (2) in toluene were found to be 2.40 × 10⁴ and 1.10 × 10⁵ M⁻¹, respectively, from the steady-state absorption and emission measurements, indicating formation of moderately stable complexes. The geometric and electronic calculations by using an ab initio B3LYP/6-311G method showed the majority of the highest occupied frontier molecular orbital (HOMO) on the zinc naphthalocyanine entity, while the lowest unoccupied molecular orbital (LUMO) was on the perylenediimide entities, suggesting that the charge-separated states of the supramolecular dyads are ZnNc˙⁺:PDI˙⁻. The electrochemical results suggest the exothermic charge-separation process via the singlet states of both ZnNc and PDI entities in nonpolar toluene. Upon coordination of perylenediimide to ZnNc, the main quenching pathway involved charge separation via the singlet-excited states of ZnNc and PDIs. Clear evidence of the intramolecular electron transfer from the singlet-excited state of ZnNc to PDI within the supramolecular dyads in toluene was monitored by the femtosecond laser photolysis by observing the characteristic absorption band of the PDI radical anion (PDI˙⁻) and the ZnNc radical cation (ZnNc˙⁺) in the visible and NIR regions. The rate constants of charge-separation (k_CS) processes of the self-assembled dyads 1 and 2 were determined to be 4.05 × 10¹⁰ and 1.20 × 10⁹ s⁻¹, respectively. The rate constant of charge recombination (k_CR) and the lifetime of charge-separated states (τ_CS) of dyad 1 were determined to be 2.34 × 10⁸ s⁻¹ and 4.30 ns, respectively. Interestingly, a slower charge recombination (2.20 × 10⁷ s⁻¹) and a longer lifetime of the charge separated state (45 ns) were observed in dyad 2 in nonpolar toluene by utilizing the nanosecond transient measurements. The absorption in a wide section of the solar spectrum and the high charge-separation/charge-recombination ratio suggest the usefulness of the self-assembled zinc naphthalocyanine–perylenediimide dyads as good photosynthetic models.