Fb Porphyrin Research Articles

Effects of Multiple Pathways on Excited-State Energy Flow in Self-Assembled Wheel-and-Spoke Light-Harvesting Architectures

Static and time-resolved optical measurements are reported for three cyclic hexameric porphyrin arrays and their self-assembled complexes with guest chromophores. The hexameric hosts contain zinc porphyrins and 0, 1, or 2 free base (Fb) porphyrins (denoted Zn(6), Zn(5)Fb, or Zn(4)Fb(2), respectively). The guest is a core-modified (O replacing one of the four N atoms) dipyridyl-substituted Fb porphyrin (DPFbO) that coordinates to zinc porphyrins of a host via pyridyl-zinc dative bonding. Each architecture is designed to have a gradient of excited-state energies for excitation funneling among the weakly coupled constituents of the host to the guest. Energy transfer to the lowest-energy chromophore(s) (coordinated zinc porphyrins or Fb porphyrins) within a hexameric host occurs primarily via a through-bond (TB) mechanism, is rapid ( approximately 40 ps), and is essentially quantitative (>or=98%). Energy transfer from a pyridyl-coordinated zinc porphyrin of the host to the guest in the Zn(6)*DPFbO complex has a yield of approximately 75%, a rate constant of approximately (0.7 ns)(-1), and significant Förster through-space (TS) character. In the case of Zn(5)Fb*DPFbO, which has an additional TS route via the Fb porphyrin with a rate constant of approximately (20 ns)(-1), the yield of energy transfer to the guest is somewhat lower ( approximately 50%) than that for Zn(6)*DPFbO. Complex Zn(4)Fb(2)*DPFbO has an identical TS pathway via the Fb porphyrin plus an additional TS pathway involving the second Fb porphyrin (closer to the guest) with a rate constant of approximately (0.5 ns)(-1). This complex exhibits an energy-transfer yield to the guest that is significantly enhanced over that for Zn(5)Fb*DPFbO and comparable to that for Zn(6)*DPFbO. Collectively, the results for the various arrays suggest designs for similar host-guest complexes that are expected to exhibit much more efficient light harvesting and excitation trapping at the central guest chromophore.

Mechanisms, Pathways, and Dynamics of Excited-State Energy Flow in Self-Assembled Wheel-and-Spoke Light-Harvesting Architectures

Static and time-resolved optical measurements are reported for two cyclic hexameric porphyrin arrays and their self-assembled complexes with guest chromophores. The hexameric hosts contain zinc porphyrins and 0 or 3 free base (Fb) porphyrins (denoted Zn(6) or Zn(3)Fb(3), respectively). The guests are a tripyridyl arene (TP) and a dipyridyl-substituted free base porphyrin (DPFb), each of which coordinates to zinc porphyrins of a host via pyridyl-zinc dative bonding. Each architecture is designed to have an overall gradient of excited-state energies that affords excitation funneling within the host and ultimately to the guest. Collectively, the studies delineate the various pathways, mechanisms, and rate constants of energy flow among the weakly coupled constituents of the host-guest complexes. The pathways include downhill unidirectional energy transfer between adjacent chromophores, bidirectional energy migration between identical chromophores, and energy transfer between nonadjacent chromophores. The energy transfer to the lowest-energy chromophore(s) within the backbone of a hexameric host (Fb porphyrins in Zn(3)Fb(3) or pyridyl-coordinated zinc porphyrins in Zn(6)*TP and Zn(6)*DPFb) proceeds primarily via a through-bond mechanism; the transfer is rapid (approximately 40 ps depending on the array) and essentially quantitative (>or=98%). The energy transfer from a pyridyl-coordinated zinc porphyrin of the host to the Fb porphyrin guest in the Zn(6)*DPFb complex is almost exclusively Förster through-space in nature; this process is much slower ( approximately 1 ns) and has a lower yield (65%). These studies highlight the utility of cyclic architectures for efficient light harvesting and energy transfer to a designated trapping site.

Synthesis of cyclic hexameric porphyrin arrays. Anchors for surface immobilization and columnar self-assembly.

To investigate new architectures for the self-assembly of multiporphyrin arrays, a one-flask synthesis of a shape-persistent cyclic hexameric array of porphyrins was exploited to prepare six derivatives bearing diverse pendant groups. The new arrays contain 6-12 carboxylic acid groups, 12 amidino groups, 6 thiol groups, or 6 thiol groups and 6 carboxylic acid groups in protected form (S-acetylthio, TMS-ethyl, TMS-ethoxycarbonyl). The arrays contain alternating Zn and free base (Fb) porphyrins or all Zn porphyrins. The one-flask synthesis entails a template-directed, Pd-mediated coupling of a p/p'-substituted diethynyl Zn porphyrin and a m/m'-substituted diiodo Fb porphyrin. The porphyrin building blocks (trans-A(2)B(2), trans-AB(2)C) contain the protected pendant groups at nonlinking meso positions. A self-assembled monolayer (SAM) of a Zn(3)Fb(3) cyclic hexamer containing one thiol group on each porphyrin was prepared on a gold electrode and the surface-immobilized architecture was examined electrochemically. Together, the work reported herein provides cyclic hexameric porphyrin arrays for studies of self-assembly in solution or on surfaces.

Weakly coupled molecular photonic wires: synthesis and excited-state energy-transfer dynamics.

Molecular photonic wires, which absorb light and undergo excited-state energy transfer, are of interest as biomimetic models for photosynthetic light-harvesting systems and as molecular devices with potential applications in materials chemistry. We describe the stepwise synthesis of four molecular photonic wires. Each wire consists of an input unit, transmission element, and output unit. The input unit consists of a boron-dipyrrin dye or a perylene-monoimide dye (linked either at the N-imide or the C9 position); the transmission element consists of one or three zinc porphyrins affording short or long wires, respectively; and the output unit consists of a free base (Fb) porphyrin. The components in the arrays are joined in a linear architecture via diarylethyne linkers (an ethynylphenyl linker is attached to the C9-linked perylene). The wires have been examined by static absorption, static fluorescence, and time-resolved absorption spectroscopy. Each wire (with the exception of the C9-linked perylene wire) exhibits a visible absorption spectrum that is the sum of the spectra of the component parts, indicating the relatively weak electronic coupling between the components. Excitation of each wire at the wavelength where the input unit absorbs preferentially (typically 480-520 nm) results in emission almost exclusively from the Fb porphyrin. The static emission and time-resolved data indicate that the overall rate constants and quantum efficiencies for end-to-end (i.e., input to output) energy transfer are as follows: perylene-(N-imide)-linked short wire, (33 ps)(-1) and >99%; perylene-(C9)-linked short wire, (26 ps)(-1) and >99%; boron-dipyrrin-based long wire, (190 ps)(-1) and 81%; perylene-(N-imide)-linked long wire, (175 ps)(-1) and 86%. Collectively, the studies provide valuable insight into the singlet-singlet excited-state energy-transfer properties in weakly coupled molecular photonic wires.

Glaser-mediated synthesis and photophysical characterization of diphenylbutadiyne-linked porphyrin dyads.

The Pd-mediated Glaser coupling of a zinc monoethynyl porphyrin and a magnesium monoethynyl porphyrin affords a mixture of three 4,4'-diphenylbutadiyne-linked dyads comprised of two zinc porphyrins (Zn-pbp-Zn), two magnesium porphyrins (Mg-pbp-Mg), and one metalloporphyrin of each type (Zn-pbp-Mg). The latter is easily isolated due to the greater polarity of the magnesium versus the zinc chelate. Exposure of Zn-pbp-Mg to silica gel results in selective demetalation, affording Zn-pbp-Fb where Fb = free base porphyrin. This synthesis route employs the magnesium porphyrin as a latent form of the Fb porphyrin, thereby avoiding copper insertion during the Glaser reaction, and as a polar entity facilitating separation. The absorption spectrum of Zn-pbp-Mg or Zn-pbp-Fb is the sum of the spectra of the component parts, while in each case the fluorescence spectrum upon illumination of the Zn porphyrin is dominated by emission from the Mg or Fb porphyrin. Time-resolved absorption spectroscopy shows that the energy-transfer rate constants are (11 ps)(-1) and (37 ps)(-1) for Zn-pbp-Mg and Zn-pbp-Fb, respectively, corresponding to energy-transfer quantum yields of 0.995 and 0.983, respectively. The calculated Förster through-space rates are (1900 ps)(-1) and (1100 ps)(-1) for Zn-pbp-Mg and Zn-pbp-Fb, respectively. Accordingly, the through-bond process dominates for both dyads with a through-bond:through-space energy-transfer ratio of > or =97:1. Collectively, the studies show that the 4,4'-diphenylbutadiynyl linker supports fast and efficient energy transfer between Zn and Mg or Fb porphyrins.

Rational syntheses of cyclic hexameric porphyrin arrays for studies of self-assembling light-harvesting systems.

Two new cyclic hexameric arrays of porphyrins have been prepared in a rational, convergent manner. The porphyrins in each cyclic hexamer are joined by diphenylethyne linkers affording a wheel-like array with a diameter of approximately 35 A. One array is comprised of five zinc (Zn) porphyrins and one free base (Fb) porphyrin (cyclo-Zn(5)FbU) while the other is comprised of an alternating sequence of two Zn porphyrins and one Fb porphyrin (cyclo-Zn(2)FbZn(2)FbU). The prior synthesis employed a one-flask template-directed process and afforded alternating Zn and Fb porphyrins or all Zn porphyrins. More diverse metalation patterns are attractive for manipulating the flow of excited-state energy in the arrays. The rational synthesis of each array employed three Pd-mediated coupling reactions with four tetraarylporphyrin building blocks bearing diethynyl, diiodo, bromo/iodo, or iodo/ethynyl groups. The final ring closure yielding the cyclic hexamer was achieved by reaction of a porphyrin pentamer + porphyrin monomer or the joining of two porphyrin trimers. In the presence of a tripyridyl template, the yields of the 5 + 1 and 3 + 3 reactions ranged from 10 to 13%. The 5 + 1 reaction in the absence of the template proceeded in 3.5% yield, thereby establishing the structure-directed contribution to cyclic hexamer formation. The 3 + 3 route relied on successive ethyne + iodo/bromo coupling reactions. One template-directed route to cyclo-Zn(2)FbZn(2)FbU employed a magnesium porphyrin, affording cyclo-Zn(2)FbZn(2)MgU from which magnesium was selectively removed. The arrays exhibit absorption spectra that are nearly the sum of the spectra of the component parts, indicating weak electronic coupling. Fluorescence spectroscopy showed that the quantum yield of energy transfer in toluene at room temperature from the Zn porphyrins to the Fb porphyrin(s) was 60% in cyclo-Zn(5)FbU and 90% in cyclo-Zn(2)FbZn(2)FbU. Two dipyridyl-substituted porphyrins, a Zn tetraarylporphyrin and a Fb oxaporphyrin, have been synthesized for use as guests in the cyclic hexamers, affording self-assembled arrays for light-harvesting studies.

Synthesis and Excited-State Photodynamics in Perylene-Porphyrin Dyads 2. Effects of Porphyrin Metalation State on the Energy-Transfer, Charge-Transfer, and Deactivation Channels

The photophysical properties of two perylene−porphyrin dyads have been examined in detail with the aim of expanding the functional utility of these constructs for molecular optoelectronics applications. The dyads consist of a perylene-bis(imide) dye (PDI) connected to either a magnesium porphyrin (Mg) or a free base porphyrin (Fb) via a diphenylethyne (pep) linker. The photophysical behavior of these two dyads show both similarities and differences to one another and to the dyad containing a zinc porphyrin (Zn) that was examined in the previous paper in this series. In the case of both PDI−pep−Fb and PDI−pep−Mg in toluene, the excited perylene unit (PDI*) decays rapidly (Fb = 2.9 ps; Mg = 2.5 ps) by energy transfer to the porphyrin forming PDI−pep−Por* in relatively high yield (Fb ∼ 85%; Mg ∼ 50%) and hole transfer to the porphyrin forming PDI-−pep−Por+ (Fb ∼ 15%; Mg ∼ 50%). This behavior parallels that observed for PDI−pep−Zn, for which rapid (2.5 ps) decay of PDI* affords PDI−pep−Zn* and PDI-−pep−Por+ with yields of 80% and 20%, respectively. The subsequent behavior of the Fb-containing dyad is distinctly different in two ways from that of the Zn or Mg porphyrin-containing dyads. (1) Charge recombination within PDI-−pep−Fb+ primarily forms PDI−pep−Fb*, thereby complementing the formation of the latter species from PDI*−pep−Fb. (2) PDI−pep−Fb* subsequently decays to the ground state via fluorescence emission with a rate and yield that are nearly identical to those of an isolated Fb porphyrin. In contrast, for both PDI−pep−Mg and PDI−pep−Zn, the predominant decay process for PDI−pep−Por* is electron-transfer yielding PDI-−pep−Por+ (Zn ∼ 80%; Mg >99%). The rapid electron-transfer quenching of PDI−pep−Por* and the nonemissive character of PDI-−pep−Por+ leads to negligible fluorescence from the two metalloporphyrin-containing dyads after photoexcitation. The PDI-−pep−Por+ charge-separated product with Por = Mg or Zn is very long-lived (>10 ns) in toluene but decays much more rapidly (<0.5 ns) in acetonitrile. The differences in the rates of the various charge-transfer and charge-recombination processes of all of the dyads are consistent with a rate versus free-energy-gap profile (based on the relative redox potentials of the porphyrin constituents) that is in qualitative accord with electron-transfer theory. Collectively, the studies reported in this and the previous paper indicate that PDI−pep−Fb has the greatest potential utility in photonics applications wherein light harvesting by an accessory pigment, energy transport to an output chromophore, and emission (or energy transfer to another chromophore) are desired. On the other hand, PDI−pep−Mg (like PDI−pep−Zn) would be most useful as an all-optical gating element in which excited-state energy in an appended chromophore chain can be quenched by the charge-separated state of the perylene-porphyrin dyad, thereby shunting the light output or flow of energy.

Mechanisms of Excited-State Energy-Transfer Gating in Linear versus Branched Multiporphyrin Arrays

We have investigated electrochemical switching of excited-state electronic energy migration in two optoelectronic gates with different architectures. Each gate consists of diarylethyne-linked subunits: a boron-dipyrrin (BDPY) input unit, a Zn-porphyrin transmission unit, a free-base-porphyrin (Fb-porphyrin) output unit, and a Mg-porphyrin redox-switched site connected either to the Fb porphyrin (linear gate) or to the Zn porphyrin (branched, T gate). Both the linear and branched architectures show Fb-porphyrin emission when the Mg porphyrin is neutral and nearly complete quenching when the Mg porphyrin is oxidized to the π-cation radical. To determine the mechanism of gating, we undertook a systematic photophysical study of the gates and their dyad and triad components in neutral and oxidized forms, using static and time-resolved optical spectroscopy. Two types of photoinduced energy-transfer (and/or charge-transfer) processes are involved in gate operation: transfer between adjacent subunits and transfer between nonadjacent subunits. All of the individual energy-transfer steps that funnel input light energy to the fluorescent output element in the neutral systems are highly efficient, occurring primarily by a through-bond mechanism. Similarly efficient energy-transfer processes occur between the BDPY and the Zn and Fb porphyrins in the oxidized systems, but are followed by rapid and efficient energy/charge transfer to the redox-switched site and consequent nonradiative deactivation. Energy/charge transfer between nonadjacent porphyrins, which occurs principally by superexchange, is crucial to the operation of the T gate. Collectively, our studies elucidate the photophysics of gating and afford great flexibility and control in the design of more elaborate arrays for molecular photonics applications.

Design and Synthesis of Porphyrin-Based Optoelectronic Gates

Two porphyrin-based optoelectronic gates and several prototypical redox-switching components of gates have been synthesized for studies in molecular photonics. Linear and T-shaped molecular optoelectronic gates contain a boron-dipyrrin (BDPY) dye as the input unit, a zinc (Zn) porphyrin as the transmission unit, a free base (Fb) porphyrin as the output unit, and a magnesium (Mg) porphyrin as the redox-switching unit. The linear gate and T gate were synthesized using a molecular building block approach. In the linear gate synthesis, a BDPY−Zn porphyrin dyad was coupled with a Fb porphyrin−Mg porphyrin dimer. The synthesis of the T gate utilized a Zn porphyrin bearing four different meso substituents: mesityl, 4-iodophenyl, 4-[2-(trimethylsilyl)ethynyl]phenyl, and 4-[2-triisopropyl)ethynyl]phenyl. Attachment of the three different groups to the Zn porphyrin was accomplished using successive Pd-mediated coupling reactions in the following sequence: Fb porphyrin (output unit), BDPY dye (input unit), and Mg porphyrin (redox-switching unit). Both the linear gate and T gate syntheses introduce the Mg porphyrin at the final step to minimize demetalation of the Mg porphyrin. Refinements to various components of these gates were investigated through the preparation of a ferrocene−porphyrin, a ferrocene−phthalocyanine, and a ferrocene−porphyrin−phthalocyanine. A dyad motif for studies of optically based redox switching was prepared that contains a derivative of Ru(bpy)3X2 coupled to a porphyrin. From these and related studies have emerged a number of design considerations for the development of refined optoelectronic gates.

Structural Control of Photoinduced Energy Transfer between Adjacent and Distant Sites in Multiporphyrin Arrays

A family of diphenylethyne-linked porphyrin dimers and trimers has been prepared via a building block approach for studies of energy-transfer processes. The dimers contain Mg and Zn porphyrins (MgZnU); the trimers contain an additional free base porphyrin (MgZnFbU). In both the dimers and trimers, sites of attachment to the Mg porphyrin (at the meso- or β-position) and diphenylethyne linker (at the para- or meta-positions) were varied, producing four Mg porphyrin−Zn porphyrin arrangements with the following linker configurations: meso-p/p-meso, meso-m/p-meso, β-p/p-meso, and β-m/p-meso. All four trimers employ a meso-p/p-meso Zn porphyrin−Fb porphyrin connection. The ground- and excited-state properties of the porphyrin dimers and trimers have been examined using static and time-resolved optical techniques. The rate of energy transfer from the photoexcited Zn porphyrin to the Mg porphyrin decreases according to the following trend: meso-p/p-meso (9 ps)-1 > β-p/p-meso (14 ps)-1 > meso-m/p-meso (19 ps)-1 > β-m/p-meso (27 ps)-1. In each compound, energy transfer between adjacent porphyrins occurs through a linker-mediated through-bond process. The rate of energy transfer between Zn and Fb porphyrins is constant in each trimer ((24 ps)-1). Energy transfer from the photoexcited Zn porphyrin branches to the adjacent Fb and Mg porphyrins, with nearly one-half to three-fourths proceeding to the Mg porphyrin (depending on the linker). Energy transfer from the excited Mg porphyrin to the nonadjacent Fb porphyrin occurs more slowly, with a rate that follows the same trend in linker architecture and porphyrin connection site: meso-p/p-meso (173 ps)-1 > β-p/p-meso (225 ps)-1 > meso-m/p-meso (320 ps)-1 > β-m/p-meso (385 ps)-1. The rate of transfer between nonadjacent Mg and Fb porphyrins does not change significantly with temperature, indicating a superexchange mechanism utilizing orbitals/states on the intervening Zn porphyrin. Energy transfer between nonadjacent sites may prove useful in directing energy flow in multiporphyrin arrays and related molecular photonic devices.

Template-Directed Synthesis, Excited-State Photodynamics, and Electronic Communication in a Hexameric Wheel of Porphyrins

To investigate new architectures for molecular photonics applications, a shape-persistent cyclic hexameric architecture (cyclo-Zn3Fb3U-p/m) has been prepared that is comprised of three free base (Fb) porphyrins and three zinc porphyrins linked at the meso-positions via diphenylethyne units. The synthesis involves the Pd-mediated coupling of a p/p-substituted diethynyl Zn porphyrin and a m/m-substituted diiodo Fb porphyrin, forming p/m-substituted diphenylethyne linkages. The isolated yield of cyclo-Zn3Fb3U-p/m is 5.3% in the presence of a tripyridyl template. The array has C3v symmetry, 108 atoms in the shortest path, and a face-to-face distance of ∼35 Å across the cavity. The excited-state lifetime of the Zn porphyrin in cyclo-Zn3Fb3U-p/m is 17 ps, giving a rate of energy transfer to each adjacent Fb porphyrin of ktrans = (34 ps)-1 and a quantum efficiency of Φtrans = 99.2%. This rate is comparable to that in a dimer (ZnFbU-p/m) having an identical linker, but slower than that of a p/p-linked ZnFb dimer, which has ktrans = (24 ps)-1. At ambient temperatures, the hole/electron hopping rate in [cyclo-Zn6U-p/m]+ is comparable to or faster than the EPR time scale (∼4 MHz). The hole/electron hopping rate in [cyclo-Zn6U-p/m]+ appears to be more than 2-fold larger than for [Zn2U-p/m]+; [Zn2U-p/m]+ has a rate at least 10-fold slower than for the p/p-linked dimer [Zn2U]+. Both excited-state energy transfer and ground-state hole/electron hopping proceed via through-bond mechanisms mediated by the diphenylethyne linker. The origin of the slightly slower energy-transfer rate, and substantially slower ground-state hole/electron hopping rate, in the p/m-linked arrays versus the p/p-linked analogues, is attributed primarily to the larger electron density of the frontier molecular orbitals at the p- versus m-position of the phenyl ring in the diphenylethyne linker. Collectively, these results indicate that the site of attachment of the porphyrin to the linker could be used to direct energy and/or hole/electron flow in a controlled manner among porphyrins in diverse 3-dimensional (linear, cyclic, tubular) architectures.

Excited-State Energy Transfer and Ground-State Hole/Electron Hopping in p-Phenylene-Linked Porphyrin Dimers

The ground- and excited-state properties of a series of p-phenylene-linked porphyrin dimers have been examined using a variety of static and time-resolved spectroscopic techniques. The dimers consist of a zinc porphyrin and a free base (Fb) porphyrin (ZnFbΦ), two zinc porphyrins (Zn2Φ), or two Fb porphyrins (Fb2Φ). In each array, the porphyrins are joined by the p-phenylene linker at one meso position, with the nonlinking meso positions bearing mesityl groups. Three analogous dimers in which the mesityl groups are replaced with pentafluorophenyl groups (F30ZnFbΦ, F30Zn2Φ, and F30Fb2Φ) were also synthesized and characterized. The excited-state energy-transfer rate from the photoexcited Zn porphyrin to the Fb porphyrin is (3.5 ps)-1 for ZnFbΦ and (10 ps)-1 for F30ZnFbΦ. The quantum yields of excited-state energy transfer are ≥99% for both complexes. The energy-transfer rates in the p-phenylene-linked dimers are considerably faster than those observed for the analogous dimers containing a diphenylethyne linker ((24 ps)-1, ZnFbU; (240 ps)-1, F30ZnFbU). At these distances, both through bond and through space contributions to the electronic coupling are important. The faster energy-transfer rates in the p-phenylene- versus diarylethyne-linked dimers are attributed to enhanced electronic coupling between the porphyrins in the former dimers arising primarily from the shorter inter-porphyrin separation. The electronic coupling in the p-phenylene-linked dimers is sufficient to support ultrafast energy transfer in both ZnFbΦ and F30ZnFbΦ, but is not so large as to significantly perturb the redox or inherent lowest excited-state photophysical properties of the porphyrin constituents. Electronic perturbations resulting from fluorination have little effect on the energy-transfer rates in the p-phenylene-linked dimers, but the rates of room-temperature ground-state hole/electron hopping processes in the corresponding monocation radicals of the bis-Zn analogues of the p-phenylene-linked dimers (≥(0.05 μs)-1, [Zn2Φ]+; ≤(2.5 μs)-1, [F30Zn2Φ]+) are significantly influenced by the fluorination-induced changes in the electronic structure. Collectively, these characteristics make these constructs attractive candidates for incorporation into extended multi-porphyrin arrays for a variety of molecular photonics applications.

Synthesis and Excited-State Photodynamics of a Molecular Square Containing Four Mutually Coplanar Porphyrins

To examine the effect of restricted porphyrin−porphyrin rotation on energy transfer in diphenylethyne-linked porphyrin arrays, a square macrocyclic array of four porphyrins has been prepared that locks the porphyrins in a mutually coplanar architecture. The palladium-mediated coupling of Zn(II) 5,10-dimesityl-15,20-bis(4-ethynylphenyl)porphyrin and 5,10-dimesityl-15,20-bis(4-iodophenyl)porphyrin afforded the molecular square (cyclo-Zn2Fb2U) with zinc (Zn) and free base (Fb) porphyrins on alternating corners. The yield of cyclo-Zn2Fb2U is relatively insensitive to concentration with reactants at 5−0.5 mM but declines significantly at ≤0.05 mM. Transient absorption data from cyclo-Zn2Fb2U in toluene at room temperature indicate that the rate of energy transfer from the photoexcited Zn porphyrin to a neighboring Fb porphyrin is (26 ps)-1. A ZnFb porphyrin (ZnFbU) with an identical diphenylethyne linker but no constraints on rotation exhibits an identical energy-transfer rate. Resonance Raman spectroscopy sho...

Effects of Orbital Ordering on Electronic Communication in Multiporphyrin Arrays

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.

Soluble Synthetic Multiporphyrin Arrays. 2. Photodynamics of Energy-Transfer Processes

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 < ktrans < (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.