A tightly coupled linear array of perylene, bis(porphyrin), and phthalocyanine units that functions as a photoinduced energy-transfer cascade

We have prepared a linear array of chromophores consisting of a perylene in put unit, a bis(free base porphyrin) transmission unit, and a free base pht halocyanine output unit for studies in artificial photosynthesis and molecu lar photonics. The synthesis involved four stages: (1) a rational synthesis of trans-AB(2)C-porphyrin building Mocks each bearing one meso-unsubstitut ed position, (2) oxidative, meso,meso coupling of the zinc-porphyrin monome rs to afford a bis(zinc porphyrin) bearing one phthalonitrile group and one iodophenyl group, (3) preparation of;a bis(porphyrin)-phthalocyanine array via a mixed cyclization involving the bis(free base porphyrin) and 4-tert- butylphthalonitrile, and (4) Pd-mediated coupling of an ethynylperylene to afford a perylene-bis(porphyrin)- phthalocyanine linear array. The perylene -bis(porphyrin)-phthalocyanine array absorbs strongly across the visible sp ectrum. Excitation at 490 nm, where the perylene absorbs preferentially, re sults in fluorescence almost exclusively from the phthalocyanine (Phi(f) = 0.78). The excited phthalocyanine forms with time constants of 2 ps (90%) a nd 13 ps (10%). The observed time constants resemble those of corresponding phenylethyne-linked dyads, including a perylene-porphyrin (less than or eq ual to 0.5 ps) and a porphyrin-phthalocyanine (1.1 ps (70%) and 8 ps (30%)) . The perylene-bis(porphyrin)- phthalocyanine architecture exhibits efficie nt light-harvesting properties and rapid funneling of energy in a cascade f rom perylene to bis(porphyrin) to phthalocyanine.