CHLOROPHYLL PORPHYRIN HETERODIMERS WITH ORTHOGONAL PI-SYSTEMS - SOLVENT POLARITY DEPENDENT PHOTOPHYSICS

The synthesis and properties of a series of fixed-distance chlorophyll-porphyrin molecules are described. These molecules consist of a methyl pyrochlorophyllide a moiety which is directly bonded at its 2-position to the 5-position of a 2,8,12,18-tetraethyl-3,7,13,17-tetramethyl-15-(p-tolyl)porphyrin. Steric hindrance between adjacent substituents rigidly positions the x systems of both macrocycles perpendicular to one another. The macrocycles were selectively metalated with zinc to give the four possible derivatives, HCHP, ZCHP, HCZP, and ZCZP, where, Z, C, and P denote free base, Zn derivative, chlorin, and porphyrin, respectively. The lowest excited singlet states of HCHP and ZCHP, which are localized on HC and ZC, respectively, exhibit lifetimes and fluorescence quantum yields that are solvent polarity independent and do not differ significantly from those of chlorophyll itself. ZCZP, however, displays a lowest excited singlet state lifetime and fluorescence quantum yield that are strongly solvent polarity dependent. The fluorescence lifetime is 3.4 ns in toluene and decreases to 119 ps in butyronitrile. Since the fluorescence quantum yield of ZCZP is proportional to its fluorescence lifetime, nonradiative decay is strongly enhanced for ZCZP in polar media. However, picosecond transient absorption measurements show no evidence for a distinct ion-pair intermediate. This is consistent with the fact that the energy levels of hypothetical charge-separated states within ZCZP, estimated from redox potentials, lie above that of S, and are therefore virtual states. HCZP also displays distinctive solvent dependent photophysics. Once again, nonradiative decay is strongly enhanced as solvent polarity increases. However, in HCZP picosecond transient absorption measurements show clear evidence of ion-pair formation in butyronitrile in 4 ps. The ion pair recombines in 43 ps. HCZP is the only compound in the series for which redox potentials in polar solvents predict that an ion-pair state, HC-ZP+, lies below S, and is therefore a real ion-pair state. Thus, the influence of both virtual and real charge-transfer states on donor-acceptor photophysics can be directly observed in similar molecules. © 1990, American Chemical Society. All rights reserved.