SOLUBLE SYNTHETIC MULTIPORPHYRIN ARRAYS .2. PHOTODYNAMICS OF ENERGY-TRANSFER PROCESSES

Soluble ethyne-linked tetraarylporphyrin arrays that mimic natural lig ht-harvesting complexes by absorbing light and directing excited-state energy have been investigated by static and time-resolved absorption and fluorescence spectroscopies. Of particular interest is the role of the diarylethyne linkers in mediating energy transfer. The major conc lusions from this study, which is Limited to the examination of arrays containing Zn and free-base (Fb) porphyrins, include the following: ( 1) Singlet excited-state energy transfer from the Zn porphyrin to the FD porphyrin is extremely efficient (95-99%). Competitive electron-tra nsfer reactions are not observed. (2) The rate of energy transfer is s lowed up to 4-fold by the addition of groups to the linker that limit the ability of the linker and porphyrin to adopt geometries tending to ward coplanarity. Thus, the mechanism of energy transfer predominantly involves through-bond communication via the linker. Consistent with t his notion, the measured lifetimes of the Zn porphyrin in the dimers a t room temperature yield energy-transfer rates ((88 ps)(-1) <k(trans) (24 ps)(-1)) that are significantly faster than those predicted by the Forster (through-space) mechanism ((720 ps)(-1)). Nevertheless, the e lectronic communication is weak and the individual porphyrins appear t o retain their intrinsic radiative and non-radiative rates upon incorp oration into the arrays. (3) Transient absorption data indicate that t he energy-transfer rate between two isoenergetic Zn porphyrins in a li near trimeric array terminated by a Fb porphyrin is (52+/-19 ps)(-1) i n toluene at room temperature, while the time-resolved fluorescence da ta suggest that it may be significantly faster. Accordingly, incorpora tion of multiple isoenergetic pigments in extended linear or two-dimen sional arrays will permit efficient overall energy transfer. (4) Mediu m effects, including variations in solvent polarity, temperature, visc osity, and axial solvent ligation, only very weakly alter (less than o r equal to 2.5-fold) the energy-transfer rates. However, the Fb porphy rin fluorescence in the Zn-Fb dimers is quenched in the polar solvent dimethyl sulfoxide (but not in toluene, castor oil, or acetone), which is attributed to charge-transfer with the neighboring Zn porphyrin fo llowing energy transfer. Collectively, the studies demonstrate that ex tended multiporphyrin arrays can be designed in a rational manner with predictable photophysical features and efficient light-harvesting pro perties through use of the diarylethyne-linked porphyrin motif.