Synthesis and excited-state photodynamics of phenylethyne-linked porphyrin-phthalocyanine dyads

Free base, zinc and magnesium mono-ethynyl phthalocyanines have been prepared as building blocks for constructing phthalocyanine-containing molecular devices. The phthalocyanine building blocks were prepared by the statistical reaction of 4,5-diheptylphthalonitrile and 4-(3-hydroxy-3- methylbut-1-ynyl)phthalonitrile, followed by chromatographic separation and subsequent deprotection. Seven porphyrin-phthalocyanine dyads in various metalation states have been prepared (M¹PM²Pc; M¹, M² = Zn²⁺, Mg²⁺, or 2H⁺). ZnPH₂Pc and ZnPZnPc were synthesized by Pd-coupling reactions of an ethynylphthalocyanine and an iodoporphyrin. Five other dyads (H₂PH₂Pc, MgPH₂Pc, MgPMgPc, H₂PMgPc, ZnPMgPc) were prepared by selective metalation and demetalation reactions starting from ZnPH₂Pc, based on the stability differences of metalloporphyrins and metallophthalocyanines. Transient absorption and static emission experiments indicate the following: (1) Excited singlet-state intramolecular energy transfer from the porphyrin to the phthalocyanine moiety is very fast (≤ 10 ps). (2) The efficiency of the energy-transfer process is very high (typically ≥ 90%), and is greatest in dyads in which competing charge transfer is inhibited on energetic grounds (e.g. >98% for H₂PH₂Pc). (3) Charge transfer involving the excited phthalocyanine and the porphyrin occurs to a limited degree (typically < 10%) depending on the redox characteristics of the chromophores. (4) The desirable strong emission properties of monomeric phthalocyanines are retained in most of the dyads (Φ(f) = 0.37-0.75). This paper establishes the foundation for utilizing phthalocyanines as strong-red absorbers, energy-transfer acceptors, and bright emitters in conjunction with porphyrin-based molecular photonic devices.