María
del Rosario Benites
a,
Thomas E.
Johnson‡
a,
Steven
Weghorn
a,
Lianhe
Yu
a,
Polisetti Dharma
Rao
a,
James R.
Diers
b,
Sung Ik
Yang
c,
Christine
Kirmaier
c,
David F.
Bocian
*b,
Dewey
Holten
*c and
Jonathan S.
Lindsey
*a
aDepartment of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, USA
bDepartment of Chemistry, University of California, Riverside, California 92521-0403, USA
cDepartment of Chemistry, Washington University, St. Louis, Missouri 63130-4889, USA
First published on 15th November 2001
A convergent synthesis employing porphyrin building blocks has afforded dendrimeric multiporphyrin arrays containing n Zn-porphyrins (n = 4, 8, or 20) and one free base- (Fb-) porphyrin joined via diarylethyne linkers. Size exclusion chromatography was used extensively for purification. The arrays have sufficient solubility in toluene or other solvents for routine handling. With increasing size, the intense near-UV Soret (S0 → S2) absorption band broadens, splits, and red shifts due to inter-porphyrin exciton coupling. In contrast, the weaker visible bands (S0 → S1) remain essentially unchanged in position or width in proceeding from the monomer all the way to the 21-mer; however, the molecular extinction coefficients of the visible bands scale with the number of porphyrins. Similarly, the one-electron oxidation potentials of the porphyrins are virtually unchanged as the arrays get larger. These results are indicative of relatively weak (but significant) electronic coupling between ground states and between the photophysically relevant lowest-excited-singlet states of the diarylethyne-linked porphyrins; thus, the characteristic properties of the individual units are retained as the architectures increase in complexity. Efficient excited-singlet-state energy transfer occurs among the Zn-porphyrins and ultimately to the sole Fb-porphyrin in each of the arrays, with the overall arrival time of energy at the trapping site increasing modestly with the number of Zn-porphyrins = 1 (45 ps), 2 (90 ps), 8 (105 ps), and 20 (220 ps). The overall energy-transfer efficiencies are 98%, 96%, 96%, and 92% in the same series. The ground-state hole-storage properties of the 21-mer (Zn20Fb) were examined. Bulk electrolysis indicates that 21 (or more) electrons can be removed from this array (e.g., one hole resides on each porphyrin) to yield a stable “super-charged” π-cation radical. Taken together, these results indicate that the convergent building-block synthesis approach affords dendrimeric multiporphyrin arrays with favorable properties for light-harvesting and hole storage.
Two basic types of approaches have been taken in the design of multi-chromophoric arrays for light-harvesting applications. Both approaches have favorable characteristics depending on the desired properties, design constraints, and assembly strategies. One approach employs linkers that give very strong electronic interactions between the units.5–11 In the strong coupling limit, such constructs typically exhibit supermolecular properties in that energy/electron delocalization involves wavefunctions that extend over a large number of chromophores. Consequently, such arrays have many characteristics that are much different from those of the isolated pigments, and that can change substantially with the size and architecture of the assembly. The second approach employs linkers that give rather modest couplings between pigments: such couplings are sufficiently weak so that key properties (e.g., ground-state redox potentials, photophysical characteristics of the lowest excited singlet state) designed into the individual pigments are essentially retained in the arrays, yet are of sufficient magnitude to support ultrafast excited-state energy transfer and facile ground-state hole/electron hopping. We have chosen this second strategy and have found that the diarylethyne linker affords the desired inter-pigment couplings, molecular properties, and wide versatility in a molecular building-block approach.2–4,12–23
Several distinct routes have been reported for synthesizing multiporphyrin arrays with potential light-harvesting properties. Self-assembly routes have afforded windowpane arrays of Pd-coordinated pyridyl-substituted porphyrins,24 indefinite aggregates of Zn-chlorins bearing hydroxy and keto groups,25 discrete aggregates of pyridyl-coordinated Zn-porphyrins,26 and other diverse architectures.27Polymerization approaches have yielded backbone polymeric arrays of up to 128 porphyrins with direct meso–meso linkages,9 or shorter polymers with oligo(phenylenevinylene),28 phenylethyne,29 or butadiyne linkers.30Stepwise synthesis procedures have afforded the majority of synthetic light-harvesting arrays. The chemistries employed have included (1) Wittig reactions affording stilbene linkers;31 (2) Sonogashira reactions affording diphenylethyne,2,12–23 oligophenylethyne,32 or diethynylethene linkers;33 (3) Glaser reactions affording diphenylbutadiyne linkers;23,34 (4) Heck reactions yielding ethene linkers;35 (5) condensation reactions affording phenylene linkers;36,37 (6) alkylation reactions affording benzoxyphenyl linkers,38 polyalkoxy linkers39 or 1,3,5-triazine units joining aniline groups;40 (7) amide forming reactions,41,42 and (8) a sequence of complementary chemistries including the Sonogashira reaction affording diphenylethyne and other linkers.17,43,44 It is noteworthy that some routes have led to porphyrin dendrimers. In particular, dendrimers have been prepared that are composed of one porphyrin at the core;45 16,41 32,42 or 64,41 porphyrins at the periphery; or up to 21 porphyrins in the framework of the dendrimer.37
We have employed the Sonogashira reaction46 in a building block approach for the modular construction of covalently linked multiporphyrin arrays.2,23 This modular route takes advantage of the availability of porphyrin building blocks with control over the pattern of the substituents at the four meso-positions. The typical substituents are aryl rings that bear solubilizing groups (e.g., mesityl) and/or synthetic handles (e.g., iodo and ethynyl groups) for further elaboration. The porphyrins are used as the free base (Fb) or metal chelate. The Pd-coupling reactions of iodophenyl and ethynylphenyl porphyrin building blocks yield diarylethyne linkages and are performed under mild, non-acidic, non-metalating conditions, which preserve the metalation state of the porphyrin building blocks.47,48 The diarylethyne linker undergoes a modest degree of bending49 and enables free rotation (about the cylindrically symmetric ethyne unit), thereby affording a relatively fixed distance of separation and access to all dihedral angles of adjacent linked porphyrins.50 We have employed this synthetic approach to construct arrays consisting of porphyrins or porphyrins plus accessory pigments in linear,2,13,21 T-shaped,51 star-shaped,12,19 cyclic,16 or square15 architectures. The constructs include light-harvesting arrays, optoelectronic gates, and a variety of dyads and triads to probe the mechanisms of excited-state energy transfer and ground-state hole/electron hopping between porphyrins and between porphyrins and accessory chromophores. Each of our diarylethyne-linked arrays prepared and studied to date has been comprised of six or fewer porphyrins. A selection of such arrays includes dimers (ZnFbU,23 ZnFbP,2 Zn223), molecular squares (cyclo-Zn2Fb2U, cyclo-Zn4U),15 trimers (Zn3, ZnZnFb),2 and star-shaped pentamers (Zn4FbU, Zn5U)12,19 as shown in Charts 1 and 2;52 the properties of these arrays provide the benchmarks for understanding the properties of the new arrays described herein.
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Chart 1 Representative arrays prepared previously.50,52 |
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Chart 2 Representative arrays prepared previously.50,52 |
In this paper, we have extended the building block approach to the synthesis of dendrimeric multiporphyrin arrays containing up to five, eight, nine, or 21 porphyrins. The arrays contain Zn-porphyrins and one (or no) Fb-porphyrin and are joined via diarylethyne linkers. Issues related to synthesis include the size of the array that can be constructed, the solubility of the larger arrays, and the use of size exclusion chromatography (SEC) as a means of purification. The arrays have been characterized by static and time-resolved optical spectroscopy to investigate their light-harvesting and energy-funneling characteristics, and electrochemistry to probe their hole-storage capabilities.
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Chart 3 Monomeric porphyrin building blocks. |
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Scheme 1 Synthesis of tetrameric arrays.50 |
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Scheme 2 Synthesis of the asymmetric Zn4Fb pentamer.50 |
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Fig. 1 SEC traces for the synthesis of Zn8Fb using 1000, 500, and 100 Å columns in series. (A) Reaction mixture before the addition of Pd-catalyst. (B) Reaction mixture after 19 h. (C) Purified Zn8Fb. |
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Scheme 3 Synthesis of nonameric arrays.50 |
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Scheme 4 Synthesis of pentameric porphyrin building blocks.50 |
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Fig. 2 SEC traces for the synthesis of Zn20Fb using a 103 Å column in series with a guard column. (A) Reaction mixture before the addition of Pd catalyst. (B) Reaction mixture after 3.5 h. (C) Reaction mixture after 22 h. (D) Purified Zn20Fb. |
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Scheme 5 Synthesis of Zn20Fb and Zn21.50 |
The 21-mer could not be purified by the standard methods of column chromatography. After investigating a variety of separation methods (see Electronic Supplementary Information†), we turned to the use of semi-preparative SEC–HPLC columns. The optimum set of conditions for separation of the 21-mer on analytical SEC–HPLC was one 103 Å column or two 103 Å columns in series, a flow rate of 0.8 mL min−1, and THF as the mobile phase, but only 50–100 µg could be partially purified in each run. The use of a preparative HPLC–SEC (103 Å) column allowed larger sample injections (∼2 mg). A flow rate of 3.0 mL min−1 offered a good compromise between peak separation and run time [retention time (Zn20Fb) = 22.1 min], affording Zn20Fb as a broad peak with some tailing of porphyrins for over 15 min. Fractions (0.5–1.0 mL) were cut and impure fractions were rechromatographed. A total of 5–7 HPLC–SEC separations (for 5–8 mg of material) were carried out to yield pure Zn20Fb (1.9 mg, 19%). Analytical SEC of the purified sample gave a single sharp peak (Fig. 2D). Satisfactory absorption and MALDI-MS data were obtained but a well resolved 1H NMR spectrum was not obtained. Metalation of Zn20Fb with Zn(OAc)2·2H2O gave the all-zinc 21-mer Zn21.
In summary, the convergent synthetic approach affords diarylethyne-linked arrays in sufficient quantities for spectroscopic studies. The yields and ease of purification were satisfactory for the tetramer and pentamers (Zn3Fb-TMS, 61%; Zn5-TMS, 66%; asymmetric Zn4Fb, 81% yield) but the larger arrays were obtained in lower yield following extensive chromatography (Zn8Fb, 20%; Zn20Fb, 19% yield). We are currently working to develop refined approaches to obtain multiporphyrin arrays. It is noteworthy that a dendrimeric array similar in architecture to that of Zn20Fb, but incorporating 20 nickel porphyrins and one Fb-porphyrins with p-phenylene linkers, has been prepared via successive porphyrin-forming reactions.37
The availability of a family of arrays of increasing size but otherwise similar architectures prompted the comparison of the chromatographic mobilities with analytical SEC columns of different pore size. The elution patterns of a family of seven arrays containing 1–5, 9 and 21 porphyrins on four different analytical SEC columns (100, 500, 103, 104 Å) are shown in Fig. 3. All arrays elute according to molecular size. The 500 Å or 103 Å column provides superior separation for all members of this set of arrays. The metalation state does not influence the retention time of the porphyrin array in SEC, as Fb-porphyrins and metalloporphyrins with the same structure eluted with nearly identical retention times.
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Fig. 3 Analytical SEC elution profiles of porphyrin arrays using columns of different pore sizes. The labels “Zn4” and “Zn5” refer to Zn4-TMS and Zn5-TMS, respectively. |
For the characterizations performed herein, intact, singly ionized molecules were observed for each of the arrays. Mass measurements agreed with the expected theoretical values within 0.3% accuracy. The matrices 4-hydroxy-α-cyano-cinnamic acid (4-HCCA) and ferulic acid (trans-4-hydroxy-3-methoxycinnamic acid), in saturated solutions of CHCl3 or toluene, were investigated for the analysis of the arrays comprised of 9 or 21 porphyrins. The best results (based on the signal intensity of the molecule ion peak under the same experimental conditions) were obtained using ferulic acid in toluene as matrix, followed closely by 4-HCAA in toluene. The spectrum with ferulic acid afforded a stronger molecule ion peak and less fragmentation (see Electronic Supplementary Information†).
All arrays containing four, five or nine porphyrins exhibited a similar fragmentation pattern with peaks separated by approximately 728 and 904 Da from the parent ion, corresponding to the loss of a peripheral trimesityl Zn-porphyrin and a trimesityl Zn-porphyrin containing a diphenylethyne linker, respectively, from the array. Some information was also obtained concerning the composition of the HMWM. The LD-MS data indicate that SEC fractions containing HMWM consisted of mixtures of materials in which one to 8–9 additional porphyrins are attached to a structure of mass corresponding to that of the desired array.
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Fig. 4 Absorption spectra of Zn-porphyrin arrays in toluene at room temperature, showing the strong near-UV Soret B band(s) and the weaker visible Q bands. |
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Fig. 5 Absorption spectra of the Zn-porphyrin arrays shown in Fig. 4, but focusing on the visible, Q-band region. |
Compound | Soret (B) region | Visible (Q) region | ||||||
---|---|---|---|---|---|---|---|---|
λ max/nm | ε/M−1 cm−1 × 105 | λ max/nm | ε/M−1 cm−1 × 105 | λ max/nm | ε/M−1 cm−1 × 104 | λ max/nm | ε/M−1 cm−1 × 104 | |
ZnTPP | 423 | 5.2 | 549 | 2.2 | 588 | 0.34 | ||
Zn2 | 426 | 6.8 | 550 | 4.3 | 588 | 0.72 | ||
Zn3 | 423 | 8.9 | 431 | 8.9 | 550 | 6.9 | 590 | 1.4 |
Zn4-TMS | 424 | 14 | 431 | 14 | 550 | 11 | 591 | 2.2 |
Zn5-TMS | 423 | 14 | 432 | 14 | 550 | 11 | 590 | 2.2 |
Zn8Fb | 423 | 17 | 432 | 18 | 550 | 15 | 590 | 3.6 |
Zn20Fb | 423 | 49 | 433 | 52 | 550 | 42 | 590 | 8.9 |
As the arrays progressively increase in size, some characteristics of the absorption spectra change appreciably while others do not. In the former category are the characteristics of the Soret (S0 → S2) absorption. Fig. 4 shows that the Soret band red shifts and splits (6–8 nm from Zn3 to Zn21) with increasing number of porphyrins in the array. These effects are well understood in terms of the pronounced exciton interactions between the large transition dipoles (reflected in ε ∼ 5 × 105 M−1 cm−1) of the Soret transitions of adjacent porphyrins, as we have described for star-shaped pentameric and smaller multiporphyrin arrays.19,20
On the other hand, the positions and widths of the Q bands (500–600 nm) remain virtually unchanged with increasing array size, except for a small red shift in the features (Fig. 5). The small spectral shift with increasing array size can be understood in terms of factors that include (1) the environmental effect of each Zn-porphyrin being surrounded by more porphyrins and less solvent than in smaller arrays, and (2) effects of relatively small interactions between the weak Q-transitions of adjacent porphyrins (for which the exciton splittings are expected to be less than the energy-widths of the bands58). It is also noteworthy that the molar decadic extinction coefficients (ε) of the Q bands scale approximately linearly with the number of Zn-porphyrins in an array. For example, for the Q(1,0) transition, Zn5-TMS exhibits ε = 110000 M−1 cm−1 which is 5 times that of ZnTPP (ε = 22
000 M−1 cm−1)
(Table 1).
The observations on the Q bands reflect the fact that the linker-mediated inter-porphyrin interactions in the photophysically most relevant excited electronic state (S1) are relatively small. Furthermore, the results obtained here show that this is true even when considering the composite interactions that exist in an array with 21 porphyrins. The consequences include the following: (1) the inherent excited-state decay properties of the isolated porphyrin (microscopic rate constants for fluorescence, internal conversion, intersystem crossing) are retained for each porphyrin in the large arrays, and can be used as a benchmark for assessing additional processes that may occur in the arrays, such as excited-state energy transfer; (2) properties designed into the individual chromophores or smaller units will be retained (and need not be re-evaluated) as the architectures become increasingly larger and complex. We have previously noted these findings and expectations regarding the S1 excited state from detailed photophysical studies on smaller arrays,15,16,19,20 and find them to be manifest in the very large assemblies prepared and studied here. (Analogous conclusions regarding weak, but significant, inter-porphyrin electronic communication in the ground electronic state are drawn from the similarity in the redox properties of arrays with increasing size, as is described below.) It is also noteworthy that the increasing extinction coefficients of the Q-bands with increasing number of porphyrins in the array (Figures 4 and 5; Table 1) afford larger optical cross sections that enhance the light-harvesting function of the extended architectures relative to the smaller architectures.
The absorption spectra of the arrays containing a central Fb-porphyrin and either eight Zn-porphyrins (Zn8Fb) or 20 Zn-porphyrins (Zn20Fb) are shown in Fig. 6. These spectra are dominated by the spectral characteristics of the multiple Zn-porphyrins and are thus very similar to those of the all-Zn-porphyrin analogs Zn9 and Zn21 (Figures 4 and 5). The Soret band of the Fb-porphyrin is buried under the intense Soret absorption of the Zn-porphyrins; similarly, the Fb-porphyrin QY(0,0) and QX(1,0) bands at ∼550 and ∼590 nm are buried under the Zn-porphyrin Q(1,0) and Q(0,0) bands at these wavelengths. Close inspection of the visible-region spectra of Zn8Fb and Zn20Fb shows the presence (with the appropriate relative intensity) of the Fb-porphyrin QY(1,0) band at 515 nm, and particularly the QX(0,0) band at ∼650 nm that is well removed from any Zn-porphyrin features (see Electronic Supplementary Information†).
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Fig. 6 Absorption spectra of Zn8Fb and Zn20Fb in toluene at room temperature. The insets show emission spectra in toluene using λex = 550 nm. |
The fluorescence spectra of the Zn8Fb and Zn20Fb arrays are dominated by emission from the central, single Fb-porphyrin component, even when the Zn-porphyrins are predominantly excited at 550 nm (Fig. 6, insets). These emission features are ascribable to the Fb-porphyrin QX(0,0) and QX(0,1) bands at ∼650 and ∼720 nm. The spectra of these large arrays also show a small amount of fluorescence from the large Zn-porphyrin pool, as is indicated by Q(0,0) Zn-porphyrin emission at ∼600 nm. Integration of the total emission (Zn- plus Fb-porphyrins from 565 to 800 nm) using predominant Zn-porphyrin excitation at 550 nm gives a total emission yield of 0.084 for Zn20Fb and 0.050 for Zn8Fb in toluene at room temperature. These values can be compared with the emission yields of Φf = 0.033 for ZnTPP and Φf = 0.11 for FbTPP.57,59 The observations that the total emission yields in the large arrays are less than that for FbTPP and that some residual Zn-porphyrin emission is observed both indicate that energy transfer to the central Fb-porphyrin is high, but not quantitative. However, it is difficult to draw quantitative conclusions of the energy-transfer efficiency from these emission studies alone (and the value for Zn8Fb seems anomalously low; vide infra). This is so because of (1) the technical difficulties associated with making detailed comparisons (which arise because of the large differences in relative absorption intensities in the large arrays and reference monomers), (2) the presence of emission from two types of porphyrins in the arrays, and (3) the effect on the apparent yields of only a minor Zn-porphyrin impurity in the large assemblies. Thus, estimates for the efficiency of energy transfer from the Zn-porphyrin light-harvesting components to the central Fb-porphyrin in Zn20Fb and ZnF8Fb are best made from the time-resolved absorption studies described below. These latter results are generally in good agreement with the emission data described above.
Fig. 7 shows representative absorption difference spectra for Zn8Fb in toluene at room temperature. The spectrum at 1 ps contains the features expected for Zn*, the most prominent of which is the bleaching of the Q(1,0) ground-state absorption band at ∼550 nm. Other Zn* features include the Q(2,0) bleaching at 510 nm, Q(0,0) bleaching and stimulated emission at ∼590 nm, and Q(0,1) stimulated emission at 650 nm. The Zn* features have largely decayed by 1.5 ns, and characteristics of Fb* have developed, as a result of energy funneling to the central porphyrin of the Zn8Fb array (Scheme 3). The most notable signatures of the overall Zn*Fb → ZnFb* energy-transfer process are the decay of Zn-porphyrin bleaching at 550 nm and growth of the Fb-porphyrin QY(1,0) bleaching at 515 nm (along with partial decay of the broad transient absorption extending from 450 to past 700 nm). Other Fb* features include QY(0,0) and QX(1,0) ground-state absorption bands at 550 and 590 nm, together with QX(0,0) bleaching and stimulated emission at 650 nm. Some of these features have similar positions to Zn* features, but have different intensity ratios as expected from the static optical spectra. In addition to the dominant contribution of Fb* to the 1.5 ns spectrum, there also appears to be some small residual Zn* contribution that most likely arises from monomeric pigments formed by minor decomposition.
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Fig. 7 Time-resolved absorption spectra for Zn8Fb in toluene at room temperature, obtained using 130 fs excitation flashes at 550 nm. The inset shows a kinetic profile at 515 nm and a fit, giving an effective Zn* lifetime in the array of 105 ps (data before and during the flash and between 500 ps and 3.5 ns not shown for clarity). |
A representative time profile at 515 nm is shown in the inset to Fig. 7 (with data before and during the flash, and between 500 ps and 3.5 ns, not shown for clarity). The kinetic profiles at this and other wavelengths (e.g., 460 and 550 nm) are dominated by a component having a time constant of 105 ± 15 ps, obtained by averaging values from these regions on repeated samples. As is discussed below, this value represents the effective Zn* lifetime in the array, which decays essentially quantitatively via transfer of energy to the Fb-component. Inclusion of an additional, smaller amplitude component having a time constant of several nanoseconds gives the best fits, and this component is consistent with the presence of a small amount of monomeric Zn* resulting from minor decomposition. The presence of this kinetic component and the minor decomposition product does not affect the interpretation of spectral and kinetic data associated with the decay of Zn* or energy flow in the arrays. It is noteworthy that a fast component having a time constant of ∼10 ps is observed with increasing amplitude as the photon flux is progressively increased over that used for the measurements depicted in Fig. 7. This component is ascribed to Zn*Zn* → Zn*Zn excited-state annihilation involving multiple Zn* produced in a given array at high (but not low) excitation intensities.60 It is also noteworthy that the dominant 105 ps component (due to overall Zn*Fb → ZnFb* energy transfer) is unchanged within experimental error as the excitation intensity is varied.
Similar transient absorption studies were performed for the larger array Zn20Fb in toluene at room temperature. A time constant of 220 ± 40 ps is obtained from measurements at several wavelengths (e.g., 515 and 550 nm) on multiple samples using different excitation intensities. A fast component having a time constant of ∼7 ps is found at high, but not low, excitation intensities.
For comparison, the time constant for decay of the Zn bleaching and formation of the Fb bleaching for the ZnZnFb triad (Chart 2) is 90 ± 10 ps in toluene at room temperature (average of values in the two regions obtained here and previously20); this triad exhibits a fast component with a time constant of ∼3 ps at high excitation intensities. For the simpler ZnFbP dyad (Chart 1), the kinetic profiles in both the Zn- and Fb-porphyrin bleaching regions give a time constant of 45 ± 4 ps (average of values found here and previously20); the dyad shows no shorter-lived component at high excitation intensities because excitation of two Zn-porphyrins and consequent excited-state annihilation is not possible in this case. ZnFbP has the same methyl-group steric hindrance on the diarylethyne linker that is present between the Fb-porphyrin core and the adjacent Zn-porphyrins in Zn8Fb and Zn20Fb (Schemes 3 and 5). The related dyad ZnFbU, which has an unhindered linker,61 exhibits a shorter time constant of 24 ± 2 ps.20
The dominant time constant represents the average Zn* lifetime in each of the arrays; this time constant is independent of excitation intensity. When compared with the much longer ∼2.4 ns Zn* lifetime of monomeric Zn-porphyrins, it is also seen that these values are the same within error as the calculated average time constant for energy to arrive from the Zn-porphyrin pool to the Fb-porphyrin core.62 For each array, the energy-transfer time constant represents an average value that includes a range of overall transfer times to the core unit depending on which Zn-porphyrin is initially excited. In particular, the time depends on the distance of the initially excited Zn* to the core and thus, the number of transfer steps between Zn-porphyrins (which includes reversible back-and-forth transfers) before the final irreversible transfer to the central Fb-porphyrin. The data described above show that this overall transfer time to the core unit increases with increasing size of the array in the following order: ZnFbP (45 ps) < ZnZnFb (90 ps) < Zn8Fb (105 ps) < Zn20Fb (220 ps). Following established analyses,19,20,44,53 the lifetimes can be used to obtain (average) yields of energy transfer from the Zn-porphyrin(s) to the central Fb-porphyrin via the formula Φtrans = 1 − τA/τM, where τA is the time constant measured in the array and τM = 2.4 ns for the reference monomer. Thus, the overall energy-transfer efficiencies are ZnFbP (98%) < ZnZnFb (96%) < Zn8Fb (96%) < Zn20Fb (92%). The latter value is in reasonable agreement with the fluorescence quantum yield measured for this compound.
The trends and relative values of the overall transfer times for energy trapping at the core porphyrin are consistent with the structures of the various arrays. In particular, the minimal number of transfer steps between Zn-porphyrins (ignoring reversible back-and-forth hops) on a path directly to the central Fb-porphyrin will be, depending on which Zn-porphyrin is excited, as follows: either 2, 1, or zero for Zn20Fb; either 1 or 0 for Zn8Fb; either 1or 0 for ZnZnFb; and 0 for ZnFbP. Thus, it is reasonable that the smallest increment in transfer time between members of this series occurs between ZnZnFb (105 ps; Chart 2) and Zn8Fb (90 ps; Scheme 3); in both arrays the most peripheral Zn-porphyrin is only one Zn-porphyrin removed from the central Fb-porphyrin. The modestly longer arrival time at the core for Zn8Fb versus ZnZnFb can be explained in terms of the different topologies of the two arrays. In particular, for Zn8Fb more reversible transfers (hops) between Zn-porphyrins are possible owing to the branched architecture in each of the two Zn4 arms.
The above arguments also imply that the overall transfer time for Zn20Fb should be the longest among the arrays, as is observed. For this large array, energy-transfer processes can originate in the four most peripheral Zn-porphyrins, which are much farther from the Fb-porphyrin than are the two most peripheral Zn-porphyrins in Zn8Fb. Hence, more back-and-forth transfers can occur between Zn-porphyrins in Zn20Fb. Likewise, the fastest transfer times are expected for the ZnFbP (and ZnFbU) dyad because the sole Zn-porphyrin excited-state donor has no choice but to transfer energy to the adjacent Fb-porphyrin acceptor.
One of the primary objectives in preparing progressively larger arrays was to increase the light-harvesting ability of the architectures. The new Zn8Fb and Zn20Fb structures have achieved this goal. The enhanced light-harvesting ability with increasing size of the array is clearly reflected in the static optical spectra, which show the dramatically increasing extinction of the overlapping S0 → S1 absorption bands of the Zn-porphyrin light-harvesting chromophores relative to the single Fb-porphyrin energy trap. The light-harvesting ability of the arrays is also reflected in the static emission and time-resolved absorption characteristics, which reveal rapid energy transfer to the central unit. It is interesting to note that in going from the ZnZnFb triad to the Zn8Fb nonamer, there is a four-fold increase in the light-gathering ability (due to the increased number of Zn-porphyrins), but the average overall time for arrival of energy at the Fb-porphyrin core increases only modestly (from 90 to 105 ps). Again, these factors can be rationalized in terms of the structures of the triad and nonamer. However, more detailed considerations of the photodynamics in these two arrays (and Zn20Fb) must also take into account the fact that significant electronic communication between nonadjacent porphyrins can occur via a superexchange mediated process involving an intervening porphyrin.21 The communication between distant sites is expected to enhance the rates and efficiencies of energy transfer from the pool of light-harvesting pigments to the central porphyrin core.
The voltammetric characteristics of Zn20Fb are similar to those we have measured for other multiporphyrin arrays.3,4,19 In particular, the 21-mer exhibits two reversible oxidation waves (E½(1) ∼0.5 V; E½(2) ∼0.9 V; versusAg/Ag+ in benzonitrile at room temperature; Fc/Fc+, E½ = 0.11 V). These waves correspond to the first and second oxidations of the porphyrin ring. The E½ values for the 20 weakly interacting Zn-porphyrins are very similar and cannot be resolved. In principle, the oxidation waves for the Fb-porphyrin should be resolvable (because the E½ values for this component are higher than those of the Zn- porphyrins3); however, the waves for the single Fb-porphyrin are completely masked by the waves from the large number of Zn-porphyrins. The redox potentials observed for the porphyrins in the 21-mer are essentially the same as those found previously in star-shaped pentameric and smaller arrays and in the isolated chromophores. This observation is indicative of relatively weak ground-state interactions between the constituent porphyrins in the 21-mer, as has been found previously for smaller arrays.3,4,19
Bulk electrolysis studies were carried out on Zn20Fb to investigate the hole-storage capabilities of this array. The progress of the bulk oxidation was monitored optically as shown in Fig. 8 which depicts the 500–1000 nm region of the absorption spectrum (the region in which porphyrin π-cation radicals exhibit the most characteristic spectral features3). As more electron equivalents are removed from the array, the broad, near-infrared optical signatures of a porphyrin π-cation become more pronounced. When 21 electron equivalents have been removed, the Q-band features characteristic of the neutral porphyrin have completely disappeared and only features of a porphyrin π-cation radical remain. These spectral characteristics indicate that the equivalent of one hole resides on each of the 21 porphyrins in the array. The (Zn20Fb)21+ species was relatively stable and exhibited no evidence of decomposition. The bulk-oxidized array was subsequently reduced with the recovery of ≥95% of the neutral material. The “super-charged” cation was also quite soluble and showed no evidence of precipitation. Attempts were not made to remove additional electrons from the array; however, the stability of (Zn20Fb)21+, along with our previous observation that up to eight holes can be stored in star-shaped pentameric arrays (Zn4FbU, Zn5U, others),3,19 suggest that even more holes could be reversibly stored in the 21-mer.
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Fig. 8 Spectroelectrochemistry of (Zn20Fb)n+ in benzonitrile (n = 0, 2, 6, 12, 16, 19, and 21). The number on each spectrum represents the value of n. |
Footnotes |
† Electronic supplementary information (ESI) available: a description of multiphoton effects at high excitation intensities; the complete Experimental section including descriptions of the syntheses of the arrays; SEC data, 1H NMR spectra, and mass spectra for all new porphyrins and multiporphyrin arrays; a description of exploratory studies in the purification of Zn20Fb; data from a comparative study of analytical SEC conditions; representative mass spectral data under different conditions; and representative absorption and fluorescence spectra of the arrays. See http://www.rsc.org/suppdata/jm/b1/b105108n/ |
‡ Contribution from the Department of Chemistry, Carnegie Mellon University. Current address: Department of Chemistry, University of Georgia, Athens, GA 30602, USA. |
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