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 (Φf = 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.