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Through-space ultrafast photoinduced electron transfer dynamics of a C -encapsulated bisporphyrin covalent organic polyhedron in a low-dielectric medium. 70

Michael Ortiz, Sung Cho, Jens Niklas, Seonah Kim, Oleg G. Poluektov, Wei Zhang, Garry Rumbles, and Jaehong Park J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b00220 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 13, 2017

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Journal of the American Chemical Society

Through-space ultrafast photoinduced electron transfer dynamics of a C70-encapsulated bisporphyrin covalent organic polyhedron in a low-dielectric medium. Michael Ortiz†,‡, Sung Cho#, Jens Niklas¶, Seonah Kim§, Oleg G. Poluektov¶, Wei Zhang‡,*, Garry Rumbles†,‡,,*, Jaehong Park†,* †

Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado 80309, United States # Department of Chemistry, Chonnam National University, Gwangju, 500-757, South Korea ¶ Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § National Bioenergy Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States Renewable and Sustainable Energy Institute, University of Colorado at Boulder, Boulder, Colorado 80309, United States Supporting Information Placeholder ‡

ABSTRACT: Ultrafast photoinduced electron transfer

(PIET) dynamics of a C70-encapsulated bisporphyrin covalent organic polyhedron hybrid (C70@COP-5) is studied in a non-polar toluene medium with fluorescence and transient absorption spectroscopies. This structurally rigid donor (D)–acceptor (A) molecular hybrid offers a new platform featuring conformationally predetermined co-facial D–A orientation with a fixed edge-to-edge separation, REE, (2.8 Å) without the aid of covalent bonds. Sub-ps PIET (τET≤ 0.4 ps) and very slow chargerecombination (τCR≈ 600 ps) dynamics are observed. The origin of these dynamics is discussed in terms of enhanced D–A coupling (V= 675 cm–1) and extremely small reorganization energy (λ≈ 0.18 eV), induced by the intrinsic structural rigidity of the C70@COP-5 complex.

Converting photon energy into chemical and electrical energy plays a key role in not only fundamental photophysics research, such as artificial and natural photosynthesis, but also in optoelectronic applications, such as photovoltaics, and solar fuels.1 The two key steps determining the efficacy of light energy conversion are photoinduced electron transfer (PIET) and charge recombination processes. Unlike artificial photosynthetic systems such as molecular electron donor (D)–acceptor (A) dyads that are surrounded by a high-dielectric medium, condensed-phase organic systems such as organic photovoltaics often suffer from a medium of intrinsic low dielectric constant that essentially increases Coulomb attraction due to poor dielectric screening. In spite of this fact, ultrafast PIET in low-dielectric media of this type is often observed experimentally, which leads to debates about the origin of this ultrafast behavior.1a,2

Here, we report on the PIET and charge recombination dynamics of a bisporphyrin-based covalent organic polyhedron (COP-5) with a C70 fullerene bound inside its cavity in a 1:1 ratio (C70@COP-5). At the interface between COP-5 and C70, the COP-5 serves as an electron donor whereas C70 acts as an electron acceptor. Upon complexation, this structurally rigid D–A, molecular hybrid offers a conformationally predetermined co-facial D–A orientation with a fixed D–A edge-to-edge distance (REE= 2.8 Å) in the absence of any covalent bonding. Such a highly rigid molecular hybrid exhibits ultrafast PIET (τET≤ 0.4 ps) in non-polar toluene. But in addition, a unique long-lived charge-transfer state (τCR≈ 600 ps) is achieved due to the extremely small reorganization energy (λ≈ 0.18 eV) of the D-A complex. Chart 1. (a) Molecular structure of COP-5. (b) A geometry optimized structure of C70@COP-5 with methyl groups instead of hexadecyl chains for simplicity.

Chart 1 and Figure S1 display the molecular structure of COP-5 and C70@COP-5. The individual electronic absorption spectra (EAS) of COP-5 and C70 in toluene are shown in Figure S2. Figure 1a shows the evolution of the EAS that chronicles the titration of COP-5 with C70, while maintaining a constant COP-5 concentration, and related spectroscopic parameters are tabulated in Table S1. A previous study established that COP-5 exhibits a 1:1 binding with C70 with a very high association constant (Kas) of 1.5×108 M–1 in toluene solvent.3 In addition to the previous observations about the red-shift

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of Soret- and Q-bands upon forming C70@COP-5,3 a new absorption band near 700 nm emerges below the optical energy gaps of either COP-5 or C70, evincing the ground-state interaction between COP-5 and C70. Figures 1a inset and 1b highlight the detailed absorption evolution near 700 nm, indicating that the 700 nm absorption originates from C70@COP-5 as this signal becomes saturated at a ~1:1 ratio of [C70]:[COP-5]. From previous literature reports that examined porphyrinfullerenes (C60 or C70), featuring face-to-face interactions4 as well as the frontier orbital calculation results of C70@COP-5, determined using B3LYP approach with a basis set of 6-31G(**) shown in Figure S3, we attribute this sub-optical NIR gap absorption signature to a charge-transfer (CT) transition with a significant extinction coefficient (εCT,694nm= 1,900 M–1 cm–1, FWHM= 1550 cm-1). Porphyrin-fullerene architectures with faceto-face porphyrin-fullerene interactions showed suboptical gap CT absorption and emission features as well as enhanced electronic coupling strength (V≈ 100–300 cm-1) between the donor and the acceptor,4a,4b,4f contrasting covalently bound porphyrin-fullerene complexes that lack this face-to-face type of interaction, reveal only the superposition of the EAS of each entity, and V< 10 cm-1.5

Figure 1. (a) The EAS that chronicle the titration of COP-5 with C70 in toluene. Inset of (a): Enlarged Q-band EAS in (a). (b) Absorbance at 710 nm as a function of C70:COP-5 molar ratio. (c) (green and yellow) Comparative spectra with (gray) Gaussian fits of (green) absorption for C70:COP-5 (molar ratio of 2.0) and (yellow) CT emission for C70:COP-5 (≈10 molar ratio), extracted using a method supplied in the Supporting Information. In ssPL experiments, ~10/1 molar ratio of C70/COP-5 is used to suppress unbound COP-5 emission. (d) (circle) PLE spectra of C70@COP-5 (molar ratio≈10) in toluene, monitored at the noted wavelengths with overlaid EAS of (blue line) COP-5 and (purple line) C70@COP-5 (molar ratio=2.6). Vertical lines show peak positions of the Soret- and Q-bands.

Figure S4 shows steady-state photoluminescence (ssPL) results of C70:COP-5 (1:1 molar ratio) with excitation (λex) at 435 nm, which is one of the isosbestic points in absorption titration experiments, show ~91%

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PL quenching and suggest effective C70 and COP-5 binding and excited-state interactions between COP-5 and C70 to deactivate 1(COP-5)*. The absence of C70 emission result is consistent with the acquired transient absorption spectral data. Beyond a 1:1 molar ratio of C70:COP-5, when the residual COP-5 emission is suppressed (i.e. C70:COP-5≈ 10), a new emission peak at around 820 nm becomes evident as shown in Figures S5b & S5c. Excess C70 beyond a 1:1 molar ratio shifts the equilibrium to favor C70@COP-5 and minimizes residual unbound COP-5. The newly emerging emission spectrum centering at ~820 nm, extracted using a method described in Supporting Information, is shown in Figures 1c and S5c with a Gaussian peak fit, and it exhibits a mirror image to the CT absorption peak with a ~2300 cm-1 Stokes shift. In this regard, this new 820 nm emission peak is ascribed to CT emission of the C70@COP-5 hybrid as a result of radiative charge recombination, leading to transition [(COP-5)•+–C70•–]→[(COP-5)–C70], consistent with previous porphyrin-fullerene supramolecular structures featuring close face-to-face interactions.4a,4b,4f The CT state is estimated to be positioned at ~1.65 eV above the ground-state, using the overlap of the normalized CT absorption and emission spectra. The Gibbs energy driving force for electron transfer (∆GET≈ -0.25 eV) for C70@COP-5 upon exciting the COP-5, determined from the ∆E between the S1-state of COP-5 (=1.90 eV) and CT-state (≈ 1.65 eV) indicates favorable PIET to C70. The PL excitation (PLE) spectra of C70@COP-5 comparatively monitoring at 720 nm and 820 nm in Figure 1d further support that the 820 nm emission originates from C70@COP-5, contrary to the 720 nm emission from residual unbound COP-5. From this CT emission spectrum in Figure 1c, we can determine energetic parameters for the PIET, including the Gibbs energy of charge recombination (∆G°CR) and internal reorganization energy (λi), using a semi-classical Marcus electrontransfer theory, describing emission intensity (I) as a function of photon energy (ν, frequency), as discussed in the Supporting Information and Figure S5d.6 The spectrum-fitting result using (Eq. S2) shown in Figure S5d determines ∆G°CR= -1.62 ± 0.02 eV and λi= 0.13 ± 0.03 eV. This 1.62 eV of ∆G°CR closely matches the 1.65 eV of the CT state above the ground-state, determined earlier from the CT absorption and emission spectra. The decay dynamics of CT emission is further examined using time-resolved PL (TRPL) experiments. Figure 2 displays the PL decay profiles integrated between 790 nm and 840 nm where the CT emission band is evident (Figures 1c & S5c). The TRPL decay signal (purple line) of C70@COP-5 (molar ratio= ~10) is clearly distinguished from that (blue line) for neat COP-5 collected in the same spectral window. The lifetime (τF,CT) of the CT emission is determined to be ~0.60 ns from a singleexponential decay function convoluted with an instrumental response function (IRF). Although the 0.60 ns of


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τF,CT is similar to the 0.65 ns of C70 τF, PLE results in Figure 1d as well as the integrated emission spectrum for C70@COP-5 (molar ratio= ~10) shown in Figure S6c exclude the contribution from emission of excess C70 due to its fluorescence spectral position of around 660 nm, poor extinction coefficient at the excitation wavelength (ε(C70,435nm)= 14,000 M–1cm–1), and low fluorescence quantum yield (8.5×10-4).7

Figure 2. TRPL decay signals of (blue) COP-5 and (purple) C70@COP-5 (~10 molar ratio) in toluene, integrated between 790 nm and 840 nm with λex= 435 nm. The singleexponential fitting with τF,CT= 0.60 ns result for C70@COP5 is shown as a violet line.

Figure 3. (a) Contour plot of transient spectroscopic results and (b) representative transient absorption spectra of C70@COP-5 (1.1 molar ratio) in toluene with λex= 400 nm. In (b), (gray dotted line) inverted ground-state electronic absorption spectrum is shown for comparison. (c) Illustrative summary depicting photoinduced ultrafast electrontransfer and charge recombination processes of C70@COP5 with photoexcitation of COP-5 in toluene. †)The T1-state of COP-5 is assumed to be similar to that of H2TPP from Ref. 11.

Excited-state dynamics of C70@COP-5 is further examined using femtosecond and nanosecond transient absorption (fsTA and nsTA) spectroscopic experiments, and the detailed excited-state dynamics of the reference COP-5 are discussed in the Supporting Information and Figures S7, S8, and S10.8 Figure 3 displays representative fsTA spectroscopic results of C70@COP-5 in toluene (C70:COP-5 molar ratio= 1.1) and those of reference COP-5 in toluene solvent are shown in Figure S7 upon λex= 400 nm. TA spectroscopic results of C70@COP-5 shown in Figure 3 upon photoexciting the COP-5 moiety (λex= 400 nm) clearly contrast to that of the reference COP-5 in Figure S7. Upon photoexcitation, the TA spec-

trum obtained at tdelay= 1.1 ps (blue in Figure 3b) exhibits porphyrin Q-bands ground-state bleaching (GSB) as well as a strong induced absorption (IA) at ~480 nm, a broad IA band spanning from 600 to 800 nm due to freebase porphyrin radical cation9, and a ~1385 nm IA band, corresponding to C70 radical anion species,10 as established from the electronic absorption peak of chemical or electrochemical studies. The red-shifted Q-band GSB signatures at 525, 564, 597, and 656 nm of C70@COP-5 hybrids, contrasting to GSB signatures of unbound COP5, demonstrate dominant depleted COP-5 ground-state in C70@COP-5 hybrids. The absence of COP-5 stimulated emission signatures, showing up for reference COP-5 at 652 nm and 726 nm, suggests that the S1-state of COP-5 in C70@COP-5 even at tdelay= 1.1 ps is already depleted. These TA spectroscopic results indicate photoinduced electron transfer [1(COP-5)*–C70]→[(COP5)•+–C70•–] on a sub-ps time scale. The TA spectrum of C70@COP-5 obtained at tdelay= ~5.2 ns (purple in Figure 3b) clearly contrasts to that at tdelay = 1.1 ps, and it shows that the NIR IA beyond 950 nm completely disappears and initial IA features in the visible range are replaced by IA at ~480 nm, ~650 nm, and ~780 nm. These IA features persist to microsecond time domain with only intensity decrease (Figure S8a), and correspond to the T1-state, localized on the COP-5 entity supported by time-resolved EPR measurements in the Supporting Information (Figures S8 & S9).4b Therefore, in non-polar toluene solvent, we conclude that the charge recombination produces [3(COP-5)*–C70] as well as radiative recombination to [1(COP-5)–C70], as summarized in Figure 3c,11 while considering the substantially low quantum yield of CT emission (ΦCT < 10-4) and negligible ground-state recovery within 5 ns, [3(COP-5)*–C70] is a dominant charge recombination product in toluene solvent. Multiwavelength global fitting analysis determines the time constants of electron transfer (τET), charge recombination (τCR), and triplet-state relaxation (τT) to be τET≤ 0.4 ps, τCR= 647 ps, and τT= 110 µs, respectively, shown in the Supporting Information (Figures S8c and S10a). This ultrafast PIET time constant agrees with our TRPL quenching experiment showing static quenching or quenching on a time-scale less than 15 ps in Figures S6a and S6b. Note that ~0.6 ns of τCR agrees with the lifetime (τF,CT= 0.6 ns) of the CT emission in Figure 2. The ≤0.4 ps τET for C70@COP-5 in non-polar toluene is particularly interesting, considering the ∆GET of -0.25 eV. In our comparative TA studies of C70@COP-5 in a more polar benzonitrile solvent (PhCN, εr= 25.9) in Figure S11, C70@COP-5 exhibits similar ultrafast PIET, [1(COP-5)*–C70]→[(COP-5)•+–C70•–] with τET≤ 0.4 ps, and a charge-recombination process of [(COP-5)•+–C70•– ] that occurs with τCR= 307 ps, although the charge recombination of [(COP-5)•+–C70•–] in PhCN results in bypassing the [3(COP-5)*–C70] triplet state and recovering the [1(COP-5)–C70] ground state. Note that ∆GET in



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PhCN is determined as -0.71 eV (see Eq. S1). This fsTA study reveals that in spite of the huge difference in the dielectric constant of toluene and PhCN, τET and τCR do not change significantly. From the CT absorption of C70@COP-5 in Figure 1a, the electronic coupling strength between the initial and final states (V) is calculated to be ~675 cm-1, using (Eq. S3). This coupling strength V of C70@COP-5 is substantially larger than those (V≈ 100–300 cm-1) of porphyrinfullerene supramolecular structures, featuring face-toface interactions found in literature, can be ascribed due to the structural rigidity from the cage architecture. With a semi-classical Marcus electron-transfer theory as (Eq. S4), kET in toluene is determined to be 2.1×1014 s-1 (τET= 5 fs), which agrees with our ultrafast PIET behavior from COP-5* to C70 even in a nonpolar solvent, and this ultrafast PIET is induced by enhanced donor-acceptor coupling (V) realized by a rigid three-dimensional cage structure. Note that the discrepancy between calculated τET, and experimental τET is possibly due to our limited IRF (~400 fs) of our fsTA spectrometry system upon λex=400 nm, and the additional S2→S1 relaxation of photoexcited porphyrin entity.12 Furthermore, this covalent organic polyhedron D-A hybrid structure results in small reorganization energy (λ= λS+λi), eventually leading to longer τCR (~0.6 ns), and higher τCR/τET ratio (≥ 1,500). Therefore, such molecular structural engineering provides a promising route to overcome dielectric disadvantage by modulating D-A coupling strength and reorganization energy. This PIET dynamics in C70@COP-5 contrasts to previous porphyrin-linked fullerene dyads, which also feature face-to-face interactions; however, they exhibit two-step PIET dynamics, where the first step reveals exciplex formation and the second step reveals either a charge-separated state or energy transfer to the singlet-state of fullerene with distinct time constants (τExciplex, τET, and τEnT).13 However, in the C70@COP-5 hybrid structure here, no such behaviors are observed and only a single PIET and charge recombination is observed. In conclusion, we show that a noncovalently bound C70-encapsulated COP-5 hybrid structure exhibits an ultrafast PIET time constant (τET≤ 0.4 ps) even in nonpolar toluene solvent with a near-unity electron transfer yield. This ultrafast ET process in toluene is accelerated by significant electronic coupling (V= 675 cm-1) between the porphyrin and C70 entities overcoming the dielectric disadvantage. Also, the rigid D-A hybrid structure features an extremely small reorganization energy (λ= ~0.18 eV) that subsequently delays τCR. Ultrafast PIET behaviors in a low-dielectric medium have been an interesting and puzzling subject in artificial photosynthesis as well as organic optoelectronics. This work shows that enhanced donor-acceptor coupling and small reorganization energy by minimizing structural flexibility represent a promising strategy to achieve high τCR/τET ratio.

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ASSOCIATED CONTENT Supporting Information. Experimental methods, additional data and discussion of steady-state, time-resolved, and EPR spectroscopic results, computational details, spectroscopic and electrochemical parameters.

AUTHOR INFORMATION Corresponding Author

*[email protected], *[email protected], *[email protected].

ACKNOWLEDGMENT This work was supported by the Solar Photochemistry Program of the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, under Contract Number DEAC36–08GO28308 to NREL, and Contract DE-AC0206CH11357 at Argonne National Laboratory (JN and OGP) The synthesis of COP-5 was supported by an NSF grant to WZ (DMR-1055705).

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(12) The comparative fsTA results of C70@COP-5 in toluene upon λex=550 nm with ~100 fs of IRF also show the IRF limited τET≤ 0.1 ps. (13) Kesti, T. J.; Tkachenko, N. V.; Vehmanen, V.; Yamada, H.; Imahori, H.; Fukuzumi, S.; Lemmetyinen, H. J. Am. Chem. Soc. 2002, 124, 8067-8077.



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