Soluble ethyne-linked tetraarylporphyrin arrays that mimic natural light-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 conclusions 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 Fb porphyrin is extremely efficient (95−99%). Competitive electron-transfer reactions are not observed. (2) The rate of energy transfer is slowed 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 toward coplanarity. Thus, the mechanism of energy transfer predominantly involves through-bond communication via the linker. Consistent with this notion, the measured lifetimes of the Zn porphyrin in the dimers at room temperature yield energy-transfer rates ((88 ps)-1 < k trans < (24 ps)-1) that are significantly faster than those predicted by the Förster (through-space) mechanism ((720 ps)-1). Nevertheless, the electronic communication is weak and the individual porphyrins appear to retain their intrinsic radiative and non-radiative rates upon incorporation into the arrays. (3) Transient absorption data indicate that the energy-transfer rate between two isoenergetic Zn porphyrins in a linear trimeric array terminated by a Fb porphyrin is (52 ± 19 ps)-1 in toluene at room temperature, while the time-resolved fluorescence data suggest that it may be significantly faster. Accordingly, incorporation of multiple isoenergetic pigments in extended linear or two-dimensional arrays will permit efficient overall energy transfer. (4) Medium effects, including variations in solvent polarity, temperature, viscosity, and axial solvent ligation, only very weakly alter (≤2.5-fold) the energy-transfer rates. However, the Fb porphyrin 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 following energy transfer. Collectively, the studies demonstrate that extended multiporphyrin arrays can be designed in a rational manner with predictable photophysical features and efficient light-harvesting properties through use of the diarylethyne-linked porphyrin motif.
Excited-state energy migration among a collection of pigments forms the basis for natural light-harvesting processes and synthetic molecular photonic devices. The rational design of efficient energy-transfer devices requires the ability to analyze the expected performance characteristics of target molecular architectures comprised of various pigments. Toward that goal, we present a general tool for modeling the kinetics of energy migration in weakly coupled multipigment arrays. A matrix-formulated eigenvalue/eigenvector approach has been implemented, using empirical data from a small set of prototypical molecules, to predict the quantum efficiency (QE) of energy migration in a variety of arrays as a function of rate, competitive processes, and architecture. Trends in the results point to useful design strategies including the following: (1) The QE for energy transfer to a terminal acceptor upon random excitation within a linear array of isoenergetic pigments decreases rapidly as the length of the array is increased. (2) Increasing the rate of transfer and/or the lifetime of the competitive deactivation processes significantly improves QE. (3) Qualitatively similar results are obtained in simulations of linear molecular photonic wires in which excitation and trapping occur at opposite ends of the array. (4) Branched and cyclic array architectures exhibit higher QEs than linear architectures with equal numbers of pigments. (5) Dramatic improvements in QE are achieved when energy transfer is directed by a progressive downward cascade in excited-state energy. (6) The most effective light-harvesting architectures are those where isolated pools of donors each have independent paths directly to the terminal acceptor. Collectively, these results provide valuable insight into the types of molecular designs that are expected to exhibit high efficiency in overall energy transfer.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.