Understanding electronic communication among the constituents in multicomponent macromolecular architectures is essential for the rational design of molecular devices for photonic, electronic, or optoelectronic applications. This Account describes studies aimed at understanding the mechanisms of electronic communication in porphyrin-based architectures that undergo excited-state energy migration and ground-state hole/electron hopping. Porphyrins are ideal building blocks for such constructs owing to their attractive and versatile physical properties and amenability to synthetic control. These properties have permitted the creation of covalently linked multiporphyrin arrays wherein the rates of excited-state energy migration and ground-state hole/electron hopping can be tuned over a wide range.

Boron-dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-t-butylphenyl) group show low fluorescence yields (∼0.06) and short excited-singlet-state lifetimes (∼500 ps), and decay primarily (>90%) by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (∼0.9) and long excited-state lifetimes (∼6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of ∼60°with respect to the dipyrrin framework whereas the angle is ∼80°for mesityl or o-tolyl groups. The calculated potential energy surface for the phenylsubstituted complex indicates that the excited state has a second, lower energy minimum in which the non-hindered aryl ring rotates closer to the mean plane of the dipyrrin, which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a Supporting Information Available: Theoretical analysis of the excited-state surfaces and Franck-Condon-active modes for selected compounds, static absorption and emission spectra, time-resolved absorption and emission spectra, and ORTEP diagrams of the structures. Crystallographic data is available as CIF files. This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public Access

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.

The rational design of molecular photonic devices requires a thorough understanding of all factors affecting electronic communication among the various constituents. To explore how electronic factors mediate both excited- and ground-state electronic communication in multiporphyrin arrays, we have conducted a detailed static spectroscopic (absorption, fluorescence, resonance Raman, electron paramagnetic resonance), time-resolved spectroscopic (absorption, fluorescence), and electrochemical (cyclic and square-wave voltammetry, coulometry) study of tetraarylporphyrin dimers. The complexes investigated include both zinc-free base (ZnFb) and bis-Zn dimers in which the porphyrin constituents are linked via diphenylethyne groups at the meso positions. Comparison of dimeric arrays containing pentafluorophenyl groups at all nonlinking meso positions (F30ZnFbU and F30Zn2U) with nonfluorinated analogs (ZnFbU and Zn2U) directly probes the effects of electronic factors on intradimer communication. The major findings of the study are as follows:  (1) Energy transfer from the photoexcited Zn porphyrin to the Fb porphyrin is the predominant excited-state reaction in F30ZnFbU, as is also the case for ZnFbU. Energy transfer primarily proceeds via a through-bond process mediated by the diarylethyne linker. Remarkably, the energy-transfer rate is 10 times slower in F30ZnFbU ((240 ps)-1) than in ZnFbU ((24 ps)-1), despite the fact that each has the same diphenylethyne linker. The attenuated energy-transfer rate in the former dimer is attributed to reduced Q-excited-state electronic coupling between the Zn and Fb porphyrins. (2) The rate of hole/electron hopping in the monooxidized bis-Zn complex, [F30Zn2U]+, is ∼10-fold slower than that for [Zn2U]+. The slower hole/electron hopping rate in the former dimer reflects strongly attenuated ground-state electronic coupling. The large attenuation in excited- and ground-state electronic communication observed for the fluorine-containing dimers is attributed to a diminution in the electron-exchange matrix elements that stems from stabilization of the a2u porphyrin orbital combined with changes in the electron-density distribution in this orbital. Stabilization of the porphyrin a2u orbital results in a switch in the HOMO from a2u in ZnFbU to a1u in F30ZnFbU. This orbital reversal diminishes the electron density at the peripheral positions where the linker is appended. Collectively, our studies clarify the origin of the different energy-transfer rates observed among various multiporphyrin arrays and exemplify the interconnected critical roles of a1u/a2u orbital ordering and linker position in the design of efficient molecular photonic devices.

Light-harvesting arrays containing one, two, or eight boron-dipyrrin (BDPY) pigments and one porphyrin (free base or Zn chelate) have been synthesized using a modular building block approach. The reaction of pyrrole and 4-(BDPY)benzaldehyde or 3,5-bis(BDPY)benzaldehyde, prepared by Pd-mediated ethynylation with the corresponding iodo-benzaldehydes, affords the desired BDPY-porphyrin array in yields of 10-58%. The arrays are soluble in organic solvents and have been characterized by static and timeresolved absorption and fluorescence spectroscopy. The blue-green BDPY absorption complements spectral coverage of the porphyrin chromophores and rivals the intensity of the porphyrin Soret band when eight BDPY accessory pigments are present. Efficient energy transfer from the BDPY pigment(s) to the porphyrin (free base or Zn-chelate) is observed in arrays containing one or two (>90%) or eight (>85%) accessory pigments per porphyrin. Biphasic excited-state decay behavior is exhibited by the BDPY pigments in isolated form and in the arrays. The time constants are ∼15 and ∼500 ps in the reference compounds (both reflecting deactivation to the ground state) and ∼2 and ∼20 ps in the arrays (both primarily reflecting energy transfer to the porphyrin). The longer-lived kinetic component comprises ∼70% of the decay in each case. Ab initio calculations suggest that the two kinetic components are associated with two energetically accessible excited-state conformers involving the boron-dipyrrin unit and the 5-aryl ring (which is integral to the linker in the arrays). The calculations and experimental results indicate that the two excited-state conformers differ from one another in structure (the planarity of the boron-dipyrrin unit and its orientation with respect to the 5-aryl ring), electronic composition (especially the electron density on the 5-aryl group of the boron-dipyrrin unit), radiative and nonradiative coupling to the ground state, and the rate of energy transfer to the porphyrin constituent in the arrays. The high energy-transfer efficiencies together with favorable light-absorption and chemical properties exhibited by the boron-dipyrrin pigments make them amenable for use in porphyrin-based arrays for molecular photonics applications.

If molecular components are to be used as functional elements in place of the semiconductor-based devices present in conventional microcircuitry, they must compete with semiconductors under the extreme conditions required for processing and operating a practical device. Herein, we demonstrate that porphyrin-based molecules bound to Si(100), which exhibit redox behavior useful for information storage, can meet this challenge. These molecular media in an inert atmosphere are stable under extremes of temperature (400 degrees C) for extended periods (approaching 1 hour) and do not degrade under large numbers of read-write cycles (10(12)).

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