Article pubs.acs.org/JPCC
Cyclo[8]pyrrole: An Androgynous Expanded Porphyrin That Acts as Both an Electron Donor and Acceptor in Anion-Bound Supramolecular Electron Donor−Acceptor Complexes Kei Ohkubo,† Kentaro Mase,† Elizabeth Karnas,‡ Jonathan L. Sessler,*,‡ and Shunichi Fukuzumi*,† †
Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan ‡ Department of Chemistry, The University of Texas, Austin, Texas 78712-1224, United States S Supporting Information *
ABSTRACT: Cyclo[8]pyrrole (C8) is an octapyrrolic expanded porphyrin. In its diprotonated form it stabilizes strong supramolecular complexes (association constants of ca. 105 M−1) in benzonitrile with both the zinc porphyrin carboxylate (ZnP) and pyrene carboxylate (Py) anions via a combination of hydrogenbonding and electrostatic interactions. Upon nanosecond laser photoexcitation of the C8−ZnP complex, electron transfer (ET) from the triplet excited state of ZnP to C8 occurs to produce the charge-separated state (C8•−−ZnP•+) in which C8 acts as an electron acceptor as inferred from the characteristic transient absorption spectral features. In this case, the energy of the chargeseparated state C8•−−ZnP•+ (0.6 eV) is lower than the alternative radical ion pair (C8•+−ZnP•−) that would be produced if electron transfer were to occur in the opposite direction. In contrast to what is seen for the C8−ZnP complex, photoexcitation of the C8− Py complex results in electron transfer from the singlet excited state of Py to C8; this produces a charge-separated state (C8•+− Py•−) wherein C8 acts as an electron donor rather than an electron acceptor. The energy of this charge-separated state (C8•+− Py•−; 2.58 eV) is much higher than that of the corresponding alternative charge-separated state, C8•−−Py•+ (1.31 eV). The fact that electron transfer does not occur in the opposite direction to produce this latter alternative charge-separated radical ion pair (C8•−−Py•+) is rationalized in terms of it lying deep in the Marcus inverted region. Because two different directions for photoinduced electron transfer are observed depending on the choice of anion-bound partner, ZnP or Py, we conclude that C8 may act as either an electron donor or acceptor under conditions of photoinduced charge separation. This androgynous character stands in contrast to what is seen for typical porphyrinoids.
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most porphyrin macrocycles.18 A challenge, therefore, is to find a porphyrin or porphyrin analogue that can elicit both functions, namely act as an acceptor and donor under similar electron-transfer conditions. Here, we report one particular expanded porphyrin, namely the diprotonated form of [30]octaphyrin(0.0.0.0.0.0.0.0) (cyclo[8]pyrrole; C8), that, depending on the anionic partner, functions as either an electron donor or electron acceptor within a set of analogous anion-bound noncovalent photoinduced charge-separating complexes. In order to model and understand inter alia the early events of photosynthesis, considerable work has been devoted to the construction of photoinduced electron-transfer systems. Indeed, with this goal in mind, many electron donor−acceptor ensembles have been synthesized by covalent linking of
INTRODUCTION Porphyrins, which contain extensively conjugated two-dimensional π-electron pathways, can undergo efficient electron transfer because the uptake or release of electrons results in minimal structural changes and little solvent reorganization.1−3 Porphyrins form complexes with numerous metal ions and metalloporphyrins. Because the oxidation potentials of metalloporphyrins are usually lower than those of the corresponding free-base porphyrins, these complexes have been used extensively as electron donors, particularly in photoinduced electron-transfer reactions.4−17 On the other hand, protonated porphyrins and metalloporphyrins with electron-withdrawing substituents can act as electron acceptors.18−24 Thus, depending on how they are modified, porphyrins can act as either electron donors or electron acceptors. However, in no case of which we are aware has the same porphyrin derivative been shown to act as both an electron donor and acceptor in the ground state. This lack of duality is ascribable to the large HOMO−LUMO gap, normally larger than 2 eV, present in © 2014 American Chemical Society
Received: June 10, 2014 Revised: July 22, 2014 Published: July 22, 2014 18436
dx.doi.org/10.1021/jp505750e | J. Phys. Chem. C 2014, 118, 18436−18444
The Journal of Physical Chemistry C
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electron donors and acceptors.4−17 An alternative strategy, wherein the donors and acceptors are held together in close proximity through noncovalent forces, has recently received considerable attention because of its versatility, flexibility in design, and the relatively reduced synthetic burden.25−32 Anion binding has provided a particularly convenient way to construct supramolecular complexes of electron donors and acceptors; these systems generally rely on an appropriate choice of anions and anion receptors.30 For some time, expanded porphyrins have been appreciated as acting as anion receptors in their protonated forms.33 To test whether such systems could function as both an electron acceptor and electron donor in noncovalent supramolecular electron-transfer ensembles, we sought a system with a low HOMO−LUMO gap. Cyclo[8]pyrrole (C8), a system first reported by us in 2002,34 has a low one-electron oxidation potential (0.52 V vs SCE) in acetonitrile (MeCN), as well as a high one-electron reduction potential (−0.08 V vs SCE). This results in very small HOMO−LUMO gap (0.60 eV).35 In addition, in its normal, stable, diprotonated form, C8 acts as an anion receptor in the presence of appropriately chosen anions. In the case of carboxylate anions, the binding affinities are large enough to stabilize complex formation. Moreover, the binding dynamics (particularly the off rate) are slow enough to allow photoinduced charge separation to occur within an appropriately designed supramolecular complex. 36 In a recent communication, we reported photoinduced charge separation within a supramolecular anion-bound complex of C8 with pyrene carboxylate anion (Py). In this system, the C8 acts as an electron donor rather than an electron acceptor.37 Because of the small HOMO−LUMO gap of C8, we reasoned that this particular expanded porphyrin might be able to act not only as an electron donor but also as an electron acceptor. As detailed below, C8 is indeed capable of acting as both an electron donor and acceptor under conditions of photoinduced charge separation within anion-bound supramolecular complexes. It acts as an electron acceptor when a zinc porphyrin carboxylate (ZnP) is used as the bound anion (and electron donor). In contrast, it acts as an electron donor when (Py) is used as the bound anionic acceptor. These findings are fully consistent with the Marcus electron-transfer theory. They are summarized schematically in Figure 1 and discussed further below.
Article
EXPERIMENTAL SECTION
Materials and Methods. Cyclo[8]pyrrole (C8) was synthesized according to known literature procedures,35 as was zinc porphyrin carboxylate (ZnP).38 Tetra-n-butylammonium 1-pyrenebutyrate (Py) was obtained by stirring 1pyrenebutyric acid with 1 equiv of tetra-n-butylammonium hydroxide (TBAOH) in MeOH. The resulting TBA-salt was then dried under vacuum with heating for a period of at least 12 h before use. Titrations were carried out by adding known quantities of a solution of ZnP at high concentration into a solution of C8. The solution of ZnP used to effect the titration contained the same concentration of C8 as used in the titration so to avoid need to account for dilution effects during the titration. The data were then fit to a 1:1 binding equation. Electrochemical Measurements. Measurements of cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed using an ALS 630B electrochemical analyzer in deaerated acetonitrile (MeCN) containing 0.10 M tetra-nbutylammonium perchlorate (TBAP) as a supporting electrolyte at 298 K. A conventional three-electrode cell was used with a platinum working electrode and a platinum wire as a counter electrode. The measured potentials were recorded with respect to the Ag/AgNO3 (1.0 × 10−2 M). The Eox and Ered values (vs Ag/AgNO3) are converted to those vs SCE by adding 0.29 V.39 All electrochemical measurements were carried out under an Ar atmosphere. Photophysical Measurements. Femtosecond transient absorption spectroscopy experiments were conducted using an ultrafast source: Integra-C (Quantronix Corp.), an optical parametric amplifier: TOPAS (Light Conversion Ltd.), and a commercially available optical detection system: Helios provided by Ultrafast Systems LLC. The sources for the pump and probe pulses were derived from the fundamental output of Integra-C (780 nm, 2 μJ/pulse and fwhm = 130 fs) at a repetition rate of 1 kHz. Seventy-five percent of the fundamental output of the laser was introduced into the TOPAS, which has optical frequency mixers resulting in tunable range from 285 to 1660 nm, while the rest of the output was used for white light generation. Prior to generating the probe continuum, a variable neutral density filter was inserted in the light path in order to generate a stable continuum. The laser pulse was then fed into a delay line that provides an experimental time window of 3.2 ns with the highest step resolution of 7 fs. In our experiments, a wavelength of 430 nm from the TOPAS, which is the fourth harmonic of signal or idler pulses, was chosen as the pump beam. As this TOPAS output consists of not only the desirable wavelength but also unnecessary wavelengths, the latter were deviated away from the experimental setup using a wedge prism with a wedge angle of 18°. The desirable beam was focused on the sample cell with a spot size of 1 mm diameter where it was merged with the white probe pulse at a close angle (106 s−1) but too small to be determined by femtosecond laser flash photolysis (
