The synthesis and excited-state dynamics are described for zinc-methylenechlorin−porphyrin−pyromellitimide triads, ZC−HP−I and ZC−ZP−I, and related dyads ZC−HP, ZC−ZP, HP−I, and ZP−I, where ZC, HP, ZP, and I indicate a zinc-methylenechlorin, a free-base porphyrin, a zinc-porphyrin, and pyromellitimide, respectively. In the steady-state fluorescence spectra of ZC−HP, ZC−HP−I, ZC−ZP, and ZC−ZP−I, only the emission from ZC is commonly observed, indicating efficient intramolecular singlet−singlet excitation energy transfer from HP or ZP to ZC. ZC−HP−I undergoes a stepwise electron-transfer relay:  1(ZC)*−HP−1 → (ZC)+−(HP)-−I → (ZC)+−HP−(I)- with overall quantum yields of 0.70 and 0.07 in THF and DMF, respectively. In ZC−ZP−I, a pre-formed equilibrium between 1(ZC)*−ZP−I and (ZC)+−(ZP)-−I is followed by a rapid charge-shift reaction to provide a secondary ion-pair state (ZC)+−ZP−(I)- with the overall quantum yield of 0.90 and 0.47 in THF and DMF, respectively. The intermediate ion-pair state (ZC)+−(ZP)-−I is stabilized enough so as to be clearly detected in DMF, while it is slightly higher in energy than 1(ZC)* in THF, rendering its detection difficult.

The synthesis and excited-state dynamics are described for fixed-distance porphyrin–oxochlorin–pyromellitdimide triads (P–C–Im) and related reference compounds. In zinc-oxochlorin–pyromellitdimide (ZnP–Im), the 1(ZnC)* is quenched by the attached Im through intramolecular charge separation (CS) in benzene, THF, and DMF, while the 1(H2C)* in the corresponding free base is not significantly quenched by the Im even in polar DMF. In the steady-state fluorescence emission spectra, only the emission from the 1(C)* is commonly observed, indicating an efficient intramolecular singlet–singlet excitation energy transfer from P to C. Of these, the fluorescence intensities of the 1(H2C)* in ZnP–H2C and ZnP–H2C–Im are significantly reduced in polar DMF solution and this is attributed to the intramolecular CS that gives (ZnP)+–(H2C)−–Im and (ZnP)+–(H2C)−, respectively. The (ZnP)+–(H2C)−–Im ion pair is clearly shown, by picosecond absorption spectroscopy, to be converted into a secondary, longer-lived charge separated state (ZnP)+–H2C–(Im)− via charge-shift reaction in competition with wasteful charge recombination to the ground state. The (ZnP)+–H2C–(Im)− state is formed in 0.09 quantum yield from ZnP–1(H2C)*–Im and has a lifetime of 0.24 μs in DMF.

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1,3-Dioxane-bridged pyropheophorbide–diimide dyads and pyropheophorbide dimer were synthesized by simple acid-catalyzed reactions of methyl pyropheophorbide d with diimide-substituted diols and with pentaerythritol, respectively. 1,3-Dioxane-bridged bacteriopyropheophorbide–diimide was similarly prepared from bacteriopyropheophorbide a. Intramolecular electron-transfer and energy-transfer reactions in these models have been studied by picosecond fluorescence measurements.

5,15-Diaryl-substituted oxochlorins were prepared from osmium tetraoxide oxidation of 5,15-diaryloctaethylporphyrin followed by acid-catalyzed pinacol rearrangement. Optical and electrochemical properties of this macrocyclic ring are studied by measuring the absorption and fluorescence spectra, fluorescence lifetimes, and one-electron oxidation and reduction potentials.

Fucoxanthin-pyropheophorbide dyads, 3c and 4c, and zeaxanthin-pyropheophorbide dyads, 3d and 4d, were prepared as the first example of natural carotenoid-linked pyropheophorbide. Singlet–singlet energy transfer from carotenoid to pyropheophorbide is more efficient in fucoxanthin-pyropheophorbide dyads than the corresponding zeaxanthin-pyropheophorbide dyads, while the singlet excited state of the pyropheophorbide is quenched more strongly in the zeaxanthin-linked molecules. Marked differences in carotenoid-pyropheophorbide singlet excited-state interactions between fucoxanthin and zeaxanthin strongly suggest their different roles in vivo; antenna function for fucoxanthin and energy-dissipation for zeaxanthin. Both carotenoids similarly quench the triplet excited state of the pyropheophorbide through triplet–triplet energy transfer with rates which depend on temperature and the linkage.

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