“Umpolung” Photoinduced Charge Separation in an Anion-bound Supramolecular Complex

Abstract

A new supramolecular system, consisting of an expanded porphyrin, cyclo[8]pyrrole (C8) and a pyrene carboxylate (Py) is capable not only of electron transfer upon photoexcitation, but results in “umpolung” or a reversal of the predicted charge-separation behavior in that the higher energy radical ion pair, C8^•+–Py^•− (2.58 eV) is formed as opposed to C8^•−–Py^•+ (1.31 eV). This observation is rationalized in terms of the latter radical ion pair, C8^•−–Py^•+ being in the Marcus inverted region. Transient absorption spectral studies reveal a charge-separated lifetime of 300 µs, which decays to the triplet state of cyclo[8]pyrrole, also an exceedingly long-lived species. The novel features of this non-covalent dyad highlight the potential utility of anion binding in the construction of supramolecular electron transfer systems.

Recent decades have borne witness to an explosion in research devoted to mimicking natural photoinduced electron transfer processes, such as those associated with the early events in photosynthesis.^1,2 Many of the systems reported in this context have relied on the use of covalently tethered electron donors and acceptors. An alternative strategy, wherein the donors and acceptors are linked through non-covalent forces, has also received attention of late.^2a,3 This approach is attractive in terms of its versatility, flexibility in design, and ease of synthesis. While a number of supramolecular motifs have been explored for this purpose, there are only a few examples where anion binding is used to connect the donor and the acceptor;⁴ to the best of our knowledge, none of these involve the use of an expanded porphyrin as the anion binding element.⁵

Here, we report that the combination of diprotonated [30]octaphyrin(0.0.0.0.0.0.0.0) (cyclo[8]pyrrole; C8; Figure 1) and a pyrene carboxylate anion produces a supramolecular ensemble, C8-Py (Py: denoted the pyrene moiety) that, upon photoexcitation, yields a charge-separated (CS) state. This CS state has two unprecedented novel features: One is the unusual direction of electron transfer to form C8^•+–Py^•−, a CS that is higher in energy than the alternative putative CS species, C8^•−–Py^•+, while the other feature is the long CS lifetime of C8^•+–Py^•− (300 µs), a species that decays to a long-lived triplet state (lifetime ≥ 200 ms) before returning to the ground-state.

Figure 1.

(a) Proposed complex formation. (b) UV-vis titration of tetra-n-butylammonium 1-pyrenebutyrate, Py into cyclo[8]pyrrole, C8 at 1.5 × 10⁻⁵ M in MeCN at 298 K. Inset: Curve fit (line) to a 1:1 binding isotherm produced from the change in absorbance at 1155 nm (points).

Cyclo[8]pyrrole, was first reported by us in 2002,⁶ and is a system that has shown promise in a variety of applications, including as a sulfate anion extractant.⁷ The photophysical and electrochemical properties of cyclo[8]pyrrole have been reported previously,^5,8 whereas its anion binding properties are the subject of an ongoing investigation. In preliminary studies, a high affinity has been observed for carboxylate anions, leading us to consider that this particular expanded porphyrin could be used to construct an anion-binding based supramolecular electron transfer system. To test this idea, tetra-n-butylammonium 1-pyrenebutyrate, Py, was chosen as the anionic, electron acceptor “partner”. Figure 1 shows the UV-vis titration between these two species, as well as the resulting binding isotherm. From these spectral changes, an association constant, K_a, of (2.6 ± 0.3) × 10⁵ M⁻¹ could be derived⁹ for the formation of C8–Py.

The two-electron reduction potential of C8 in MeCN is −0.05 V vs SCE, while the oxidation potential is 0.46 V vs SCE.^8a In the presence of one equivalent of Py, both the oxidation and reduction potentials of C8 are decreased slightly to 0.44 and −0.03 V vs SCE, respectively. In the presence of C8, the reduction potential of Py is decreased from −2.18 to −2.14 V vs SCE, while the oxidation potential is increased from 1.24 to 1.28 (cf. Figure S1). Spectral monitoring of the one-electron electrochemical oxidation of cyclo[8]pyrrole, or chemical oxidation using Fe(bpy)₃³⁺ (bpy = 2,2'-bipyridine), reveals the presence of new features with λ_max at 482, 738, and 818 nm (cf. Figures S2, S3).

Transient absorption spectra were recorded after photoexcitation of C8 and Py alone (cf. Figure S4), and in a 1:1 ratio in MeCN (cf. Figure 2). Based on the absorption maxima there is clear evidence for formation of C8^•+ (but NOT C8^•−). Such an observation is consistent with photoinduced electron transfer taking place from C8 to the singlet excited state of Py (¹Py*) to produce the CS state, C8^•+–Py^•−; this happens even though the energy of C8^•+–Py^•− (2.58 eV) is higher than that of C8^•−–Py^•+ (1.31 eV). The transient absorption due to Py^•− appears at 480 nm and is overlapped with features due to C8^+•. The CS state decays to the triplet excited state (³C8*) rather than to the ground state, as inferred from the fact that the transient spectrum of the ensemble recorded at 900 µs in Figure 2 agrees with that observed for ³C8* produced following photoexcitation of C8 alone (Figure S4).

Figure 2.

Transient absorption spectra of C8 and Py, in a 1:1 ratio at 5.0 × 10⁻⁵ M in deaerated MeCN at 298 K taken at 10 µs (△) and 900 µs (●) after laser photoexcitation (355 nm, 25 mJ per pulse).

Fitting the decay at both 470 and 740 nm to a single exponential results in a charge recombination rate (k_cr) of (3.3 ± 0.1) × 10³ s⁻¹, a value that corresponds to a lifetime for the charge separated state (τ_cr) of 300 µs (Figure 3). Changing the laser power has no affect on the rates, as underscored by the invariance of the first-order plots included in Figure 3. Such findings provide support for the proposal that intramolecular electron transfer takes place within the supramolecular complex. As expected, the use of pyrene-1-butyric acid, instead of the base (i.e., Py) in conjunction with C8 under otherwise identical conditions, yielded none of the peaks characteristic of C8^•+ following photoirradiation.

Figure 3.

Decay profiles at (a) 470 nm and (b) 740 nm obtained from the transient absorption spectra of Figure 2. Inset: First-order plots of the spectral changes at 740 nm.

The formation of C8^•+ following photoexcitation in the presence of Py is particularly interesting given the fact that pyrene typically acts as a good electron donor.¹⁰ The charge reversal seen in the present “umpolung” system has also been reported for electron transfer from electron donors to ¹Py*.¹¹ In the specific case of C8-Py, the proposed path is similar, namely excitation of a 1:1 solution of C8 and Py at 355 nm affords first the singlet excited state of pyrene, ¹Py* (3.46 eV); this is then followed by electron transfer from C8 to form the CS state, C8^•+–Py^•− (Figure 2). The CS rate constant was determined to be 5.1 × 10⁶ s⁻¹ from the decrease in the fluorescence lifetime of C8–Py (140 ns) as compared to that of the reference pyrene (480 ns); see Figure S5. The slow CS was confirmed by femtosecond laser flash photolysis measurements, where little change was observed in the transient absorption due to ¹Py* in C8–Py (see Figure S6). That this pathway is followed, rather than an alternative one leading to the formation of C8^•−–Py^•+, is rationalized by the Marcus theory of electron transfer, which predicts that the charge separation to produce the lower energy CS state (C8^•−–Py^•+), with a larger driving force of 2.15 eV, is deeply in the Marcus inverted region; this makes the CS rate much slower than the CS rate required to produce the high energy CS state C8^•+–Py^•−, which lies in the Marcus normal region and which is characterized by a driving force of 0.88 eV.¹² After initial photoinduced charge separation, recombination occurs via decay to the triplet excited state, ³C8*,¹³ which then returns to the ground-state extremely slowly, in a time exceeding 200 ms (cf. Figure S8). This mode of recombination is also rationalized by the slower back electron transfer to the ground state as compared to the triplet excited state in the Marcus inverted region.^12–15

In summary, we have shown that anion binding to an appropriately chosen expanded porphyrin, namely C8, can be used to create a non-covalently linked dyad capable of electron transfer. In the present system, which is predicated on the use of an anionic pyrene derivative, photoexcitation produces a CS state of C8^•+–Py^•− that is higher in energy than the alternative CS, C8^•−–Py^•+. This high energy CS state decays with a lifetime of 300 µs to give a long-lived triplet state. Formation of such a high energy and long-lived CS state justifies further studies with C8 in conjunction with other potential electron acceptors, as well as efforts to create more elaborate ensembles wherein anion recognition is used in combination with other non-covalent interactions to link together multiple donor and acceptor subunits. Work along these lines is in progress.