Self-Assembly-Induced Ultrafast Photodriven Charge Separation in

Letter pubs.acs.org/JPCL

Self-Assembly-Induced Ultrafast Photodriven Charge Separation in Perylene-3,4-dicarboximide-Based Hydrogen-Bonded Foldamers Kelly M. Lefler, Dick T. Co, and Michael R. Wasielewski* Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States S Supporting Information *

ABSTRACT: We report the synthesis, self-assembly characteristics, and ultrafast electron transfer dynamics of a perylene-3,4dicarboximide (PMI) covalently linked to an N,N′-bis(3,4,5tridodecyloxyphenyl)melamine electron donor (D) via a biphenyl spacer (PMI-Ph2-D). Synchrotron-based small- and wide-angle Xray scattering (SAXS/WAXS) measurements in methylcyclohexane solution show that PMI-Ph2-D self-assembles into π−π stacked, hydrogen-bonded foldamers consisting of two or three hexameric rings or helices. Ultrafast transient absorption spectroscopy reveals that photoinduced charge separation within these nanostructures occurs by a unique pathway that is emergent in the assembly, whereas electron transfer does not occur in the PMI-Ph2-D monomers in tetrahydrofuran. SECTION: Physical Processes in Nanomaterials and Nanostructures

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and wide-angle X-ray scattering (SAXS/WAXS) experiments showed that a π−π stacked dodecameric assembly is formed in methylcyclohexane (MCH), which consists of either two circular rings or a two-turn helix. These assemblies have substantially shorter through-bond pathways between the PDI and D in the hydrogen-bonded assembly, which increases the rate of photoinduced charge separation from τCS = 15 ± 2 ps in the monomer to 54.7°, making the electronic transition between the ground state and lower energy exciton state allowed.56 The fluorescence spectra of the PMI-Ph2-D monomer in THF and its π−π stacked assembly in MCH are also given in Figure 3. The spectrum of the monomer in THF shows an emission maximum at 575 nm, which is once again red-shifted relative to the 535 nm emission maximum of unsubstituted PMI because of the bay-region substituents.43 By contrast, the fluorescence of PMI-Ph2-D in MCH is completely quenched. The essentially complete fluorescence quenching of the PMIPh2-D assemblies in MCH results from competitive electron transfer (see below). The PMI chromophore in PMI-Ph2-D was selectively photoexcited with 110 fs, 532 nm laser pulses. The spectra in THF show a ground state bleach feature at 520 nm and stimulated emission at 580 nm (Figure 4A). The singlet excited state feature at 690 nm decays with a time constant of τ = 1.7 ± 0.2 ns. Charge separation within PMI-Ph2-D can be monitored by observing the formation of PMI−•, which absorbs at 590 nm for monomeric PMI and shifts to 620−630 nm for π−π stacked PMI derivatives.31,44 We do not observe the formation of

Figure 4. Transient spectra and kinetics of PMI-Ph2-D (1 × 10−4 M) (A) in THF; Inset: Transient kinetics at 690 nm with fit. (B) Transient spectra and kinetics of PMI-Ph2-D (3 × 10−4 M) in MCH. (C) Principal components of the spectra shown in B from SVD. Scattered laser light at 520−540 nm is removed from the spectra.

PMI−• for PMI-Ph2-D monomers in THF. In contrast, the spectra of the PMI-Ph2-D hydrogen-bonded foldamer in MCH show ground-state bleaching at 550 nm and no stimulated emission (Figure 4B). Following photoexcitation, the 1*PMI absorption band appears immediately at 690 nm followed by formation of the π−π stacked PMI−• feature at 630 nm. Singular value decomposition (SVD) of the three-dimensional ΔA versus time and wavelength data set followed by global fitting yields the spectral components illustrated in Figure 4C. The electron transfer reaction: 1*PMI-Ph2-D → PMI−•-Ph2D+• occurs with τCS = 3.4 ± 0.4 ps, as indicated by the formation of the PMI−• spectrum. The D+• spectrum is apparently sufficiently weak in the visible region of the spectrum, so that it is not obvious in the transient spectra. Interestingly, PMI−•-Ph2-D+• decays to ground state with 3801

dx.doi.org/10.1021/jz3018946 | J. Phys. Chem. Lett. 2012, 3, 3798−3805

The Journal of Physical Chemistry Letters

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decay is most likely a consequence of a small VDA for this reaction. By contrast, charge separation involving PMI-Ph2 -D incorporated into the hydrogen-bonded foldamer in MCH is very fast. The value of ΔGCS for charge separation in MCH should be more positive than that in THF, although accurate estimates of S using the Weller formalism,60 which is based on dielectric continuum theory and assumes spherical ions, are difficult in nonpolar solvents. Even if one assumes that the ionic radii of both PMI−• and D+• are half the donor−acceptor distance, i.e., rD = rA = 9.5 Å when r = 19 Å through the Ph2 spacer, or rD = rA = 4.5 Å when r = 9 Å through the hydrogenbonded pathway, the Weller treatment predicts that ΔGCS = 0 and 0.11 eV in MCH, respectively. Adding to the difficulty in estimating ΔGCS is the fact that π−π stacking of the PMI molecules places a substantial fraction of the PMI molecules in a dielectric environment that is more like benzene than MCH; thus making the effective static dielectric constant as well as the polarizability of the “solvent” higher. As shown earlier, charge separation in the analogous PDI-PhD hydrogen-bonded foldamer occurs much more rapidly (τCS < 1 ps) than in the monomer (τCS = 15 ± 2 ps).28 This was attributed to the shorter hydrogen-bonded pathway between the D donor and PDI acceptor. Similarly, in the PMI-Ph2-D foldamer, one oxygen atom of the PMI is separated from an amine nitrogen atom of D by only two bonds, one of which is a hydrogen bond, so that the through-bond pathway is very short. This results in VDA for charge separation being sufficiently large to produce a fast rate (τCS = 3.4 ± 0.4 ps). Perhaps the most interesting feature of the electron transfer dynamics of PMI-Ph2-D hydrogen-bonded foldamer is its biexponential charge recombination. The analogous PDI-Ph-D hydrogen-bonded foldamer also displays biexponential charge recombination (τCR = 4.2 ± 0.5 ps (60%) and τCR = 164 ± 45 ps (40%)),28 where the amplitude ratio of the short/long components is the same within experimental error as that of the PMI-Ph2-D assembly. If the dominant structure of the PMI-Ph2-D hydrogenbonded foldamer consists of either three π−π stacked rings or a three-turn helix, approximately one-third of the PMI chromophores will be sandwiched by partner PMI chromophores that provide a more benzene-like solvent environment for the PMI chromophores that are in the middle of the sandwich. This should result in a somewhat more polar environment than MCH, so that the energy of PM−•-Ph2-D+• should be lowered. Since charge recombination is in the Marcus inverted region of the rate versus free energy dependence,58,59 making ΔGCR more positive should result in an increase in charge recombination rate for one-third of the PMI-Ph2-D molecules. However, this is contrary to the experimental observation that one-third of the PMI−•-Ph2-D+• ion pairs have the slower charge recombination rate. On the other hand, if the PMI-Ph2-D hydrogen-bonded foldamer is principally a two-turn helix, the two PMI chromophores at the ends of the dodecameric oligomer, which are hydrogen bonded to only one other PMI chromophore, π−π stack with PMI chromophores that are hydrogen-bonded at both ends, so that the resulting four PMI molecules comprising this pair of π−π stacked dimers are structurally and/or electronically different than the remaining eight PMI molecules. These remaining eight PMI molecules form four π−π stacked dimers in which each PMI is hydrogen bonded at both ends to a partner PMI. This results in two

biexponential kinetics, indicating that there are two distinct populations of PMI−•-Ph2-D+•. The relative amplitudes of the two decay components are τCR = 25 ± 2 ps (60%) and τCR = 261 ± 30 ps (40%). None of the kinetic components are laser intensity dependent, ruling out the assignment of any of these kinetic components to singlet−singlet annihilation.57 Photoinduced electron transfer occurs only in the PMI-Ph2-D hydrogen-bonded foldamer. According to electron transfer theory,58,59 this is typically a consequence of differences in ΔGCS between the monomer and the assembly, and/or a significant difference in the electronic coupling matrix element (VDA) for the reaction. Focusing on ΔGCS, the one-electron oxidation and reduction potentials of D and PMI within PMIPh2-D in THF are 1.10 V and −1.01 V vs SCE, respectively (see Supporting Information). From the energy-averaged absorption and emission maxima of PMI, we estimate that the PMI lowest excited singlet state energy is 2.27 eV. The free energy of reaction for photoinduced electron transfer is ΔG = EOX − E RED − ES −

eo2 +S rεS

(2)

where eo is the electronic charge, r is the distance between the ions, εS is the static dielectric constant of the solvent (εS = 6.9 for THF and 2.0 for MCH), and S is the difference between the ion pair energy in polar THF, in which EOX and ERED are measured, and in a nonpolar solvent, such as MCH, in which the redox potentials cannot be readily measured. Since the electron transfer reaction: 1*PMI-Ph2-D → PMI−•-Ph2-D+• for the monomer in THF can only occur via the relatively long (r = 19 Å) though-bond biphenyl pathway (Figure 5), e2o/rεS ≅ 0, and since THF is polar, S ≅ 0, so that ΔGCS = −0.16 eV. Given that ΔGCS < 0, the fact that VDA is exponentially distance dependent and r = 19 Å, the inability of intramolecular electron transfer within 1*PMI-Ph2-D to compete with excited state

Figure 5. Covalent and hydrogen-bonded molecular pathways for electron transfer in PMI-Ph2-D. 3802



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populations (1/3, 2/3) of π−π stacked PMI molecules having different environments, which may change the reduction potential of the PMI resulting in biexponential charge recombination.61 Using this model, a three-turn helix would have two populations (1/9, 8/9), which would also be reflected in the amplitudes of the biexponential kinetic components. On the basis of this analysis, if the PMI-Ph2-D supramolecular assembly consists of two π−π stacked hexameric rings, then, to a first approximation, all six pairs of π−π stacked PMI dimers are equivalent, so that biexponential charge recombination kinetics are not expected. Thus, within experimental error, the amplitudes of the two charge recombination kinetic components favor the two-turn helix model. By contrast, the charge separation reaction does not exhibit biexponential kinetics, which may indicate that the observed fast kinetics (τCS = 3.4 ± 0.4 ps) reflect rate-limiting vibrational and/or solvent relaxation dynamics in this system. The rapid charge separation dynamics preclude large amplitude motions of the structure being coupled to this process, while the slower charge recombination dynamics may involve some structural relaxation. In summary, SAXS/WAXS studies have shown that the PMIPh2-D monomer can form hydrogen-bonded foldamers in MCH, where a combination of the π−π stacking ability of the PMI chromophore and the multipoint hydrogen bonding drives the assembly. Ultrafast transient absorption spectroscopy shows that electron transfer is an emergent property of the assemblies, even in a nonpolar solvent, while the monomers show no evidence of intramolecular electron transfer. This work shows that the photophysical behavior of PMI chromophores can be controlled through the design of self-assembling structures in a manner similar to the more widely studied PDI derivatives, and provides potential new avenues toward PMI-based photofunctional materials.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization of 2−4, 6, and 8−10; description of the SAXS/WAXS, femtosecond transient absorption, and electrochemical measurements; and additional modeling of the SAXS/WAXS data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001059.

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