Mediating Reductive Charge Shift Reactions in Electron Transport

Subscriber access provided by LAURENTIAN UNIV

Article

Mediating Reductive Charge Shift Reactions in Electron Transport Chains Maximilian Wolf, Carmen Villegas, Olga Trukhina, Juan Luis Delgado, Tomas Torres, Nazario Martín, Timothy Clark, and Dirk M. Guldi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b08670 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Journal of the American Chemical Society is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

Mediating Reductive Charge Shift Reactions in Electron Transport Chains Maximilian Wolf,1‡ Carmen Villegas,2‡ Olga Trukhina,3,4 Juan Luis Delgado,5,6* Tomás Torres,3,4,7* Nazario Martín,2,4,9* Timothy Clark,8 and Dirk M. Guldi1,9* 1 Department of Chemistry and Pharmacy & Interdisciplinary Center of Molecular Materials (ICMM) Friedrich-AlexanderUniversity Erlangen-Nuremberg Egerlandstr. 3, 91058 Erlangen, Germany. 2 Departamento de Quíımica Orgánica, Facultad de Ciencias Quíımicas, Universidad Complutense de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain. 3 Departamento de Química Orgánica, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain 4 Imdea-Nanoscience, C/ Faraday 9, Campus Cantoblanco, 28049 Madrid, Spain 5 Faculty of Chemistry & POLYMAT, University of the Basque Country UPV/EHU, Avenida de Tolosa 72, 20018 Donostia-San Sebastian, Spain 6 Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain 7 Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain 8 Department of Chemistry and Pharmacy, Computer Chemistry Centre (CCC) Friedrich-Alexander-University ErlangenNuremberg Nägelsbachstraße 25, 91052 Erlangen, Germany. 9 Lead Contacts ABSTRACT: We report the synthesis of a full-fledged family of covalent electron donor-acceptor1-acceptor2 conjugates and their charge-transfer characterization by means of advanced photophysical assays. By virtue of variable excited state energies and electron donor strengths, either Zn(II)Porphyrins or Zn(II)Phthalocyanines were linked to different electron-transport chains featuring pairs of electron accepting fullerenes, that is, C60 and C70. In this way, a fine-tuned redox gradient is established to power a unidirectional, long-range charge transport from the excited-state electron donor via a transient C60•- towards C70•-. This strategy helps minimize energy losses in the reductive, short-range charge shift from C60 to C70. At the forefront of our investigations are excitedstate dynamics deduced from femtosecond transient absorption spectroscopic measurements and subsequent computational deconvolution of the transient absorption spectra. These provide evidence for cascades of short-range charge-transfer processes, including reductive charge shift reactions between the two electron-accepting fullerenes, and for kinetics that are influenced by the nature and length of the respective spacer. Of key importance is the postulate of a mediating state in the charge-shift reaction at weak electronic couplings. Our results point to an intimate relationship between triplet-triplet energy transfer and charge transfer.

INTRODUCTION: In Nature, energy and electron transfer processes following light absorption by the photosynthetic machinery have been perfected.1 To mimic the key steps in natural photosynthesis, that is, efficiently utilizing light to separate charges and, in turn, to generate solar fuels, a great variety of multicomponent artificial photosynthetic systems have been designed and synthesized.2–21 Incentives for designs built around molecular building blocks have been taken from Nature: For example, implementing suitable electrochemical gradients along arrays of multiple entities have enabled the unidirectional transformation of solar energy into electrical energy. A key parameter is the right balance between the reduction/oxidation strengths of the individual building blocks and their spatial arrangements.22 Both factors affect the thermodynamic driving forces for each step and, in turn, the overall efficiency. Notably, a variety of acceptor-donor1-donor2 (AD1D2) or acceptor-donor1-donor2-donor3 (AD1D2D3) types of artificial photosynthetic systems have been reported.23–36

Besides an initial charge transfer, charge shifts take place via oxidative electron transfers from donor1 to donor2 and, eventually, to donor3. DA1A2 types, in which a reductive charge shift from acceptor1 to acceptor2 as part of electron transport chains occurs, are, however, rare.37 Recently, we have linked two chemically modified fullerenes with different electron-accepting strengths, [60]pyrrolidinofullerene and [70]pyrazolinofullerene, to a metalloporphyrin (ZnP) electron donor to afford DA1A2 type ZnP-C60-C70 138,39. Both C60 and C70, are versatile electron acceptors, which are readily available, and which can be functionalized by several methodologies.40–43 The delocalization of charges, together with the rigid and confined structures of C60 and C70, are real assets for stabilizing charge species, such as their one-electron reduced or oxidized forms.44–47 Most important are the small reorganization energies of C60 and C70 in electron-transfer reactions, which have been shown at short electron donor-acceptor dis-

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

tances (0.9 eV)48 and at large electron donor-acceptor distances (0.6 eV) to accelerate charge separation and decelerate charge recombination within multicomponent electron donoracceptor arrays and, in turn, to facilitate charge shift.49–52 In this context, we were able to show in ZnP-C60-C70 1, that the initial charge separation, which yields a ZnP•+-C60•--C70 chargeseparated state with a lifetime of a hundred picoseconds, is followed by a (reductive) charge shift to yield a ZnP•+- C60C70•- charge-separated state. The lifetime of the latter, which is in the range of a hundred nanoseconds, illustrates the longlived nature of the charge-separation process. Such a charge shift from a primary (C60; acceptor1) to a secondary electron acceptor (C70; acceptor2) along an electrochemical gradient retards charge recombination and contrasts with the more common oxidative charge shift between primary, secondary, etc. electron donors. Considering the effectiveness of multicomponent systems, in which photoexcitation triggers a cascade of short-range electron transfers, we report on two sets of novel multicomponent DA1A2s – Figure 1. These DA1A2 types feature electron-transport chains with C60 and C70 of different electron acceptor strengths to create an electrochemical gradient. As a complement to electron accepting C60/C70, both zinc porphyrins (ZnP) and zinc phthalocyanines (ZnPc) were employed. The motivation for our pump-probe investigations was to establish the role of ZnPc or ZnP as light harvester and electron donor to power unidirectional (reductive) charge-shift reactions along a variety of electron-transport chains. In the latter, the C60 to C70 distances were increased systematically starting at van der Waals contacts in 2, where they are likely to form electron traps,53 a phenylene moiety in 3-4, and a diphenylacetylene spacer in 5-6. These molecular spacers separate C60 from C70 and introduce rigidity. In analogy to the electrontransfer cascade in photosynthetic reaction centers, we set our focus on small variances in the reduction potentials of the primary and secondary electron acceptors to minimize energy losses in the formation of the final charge-separated state. Our investigations suggest an intermediate in powering the chargeshift reaction between C60 (pyrrolidino[60]fullerenes) and C70 (pyrazolino[70]fullerenes), especially when their mutual electronic coupling is weak. This mediating process goes hand in hand with triplet-triplet energy transfer (TTEnT) between the ZnP and C70 triplets, which are close in energy..

Figure 1. Structures of ZnP-C60-C70s 1, 3, and 5 as well as ZnPcC60-C70s 2, 4, and 6

Page 2 of 11

RESULTS AND DISCUSSION Synthesis: The synthesis of 2 was carried out following a synthetic methodology similar to that used to synthesize 1.38 Copper catalyzed “click-chemistry” of fullerene dimer (C60C70; A1A2) with zinc phthalocyanine (ZnPc) methylazide (7), which was reported for the first time in 54 and synthesized in the current work in a more straightforward procedure described in the Supporting Information, afforded ZnPc-C60-C70 2 in good yield (40%, Scheme 1 SI). The synthesis of both ZnP-C60-C70 3 and 5 and ZnPc-C60-C70 4 and 6 required a strategy based on the preparation of different C60-C70 dimers. To this end, it was necessary to prepare hydrazones 9 and 13 from their corresponding aldehydes 8 and 12. Then, hydrazones 9 and 13 were reacted with NBS and Et3N in ClPh at room temperature in the presence of C70, to give the formyl-protected derivative 10 or 14 as a mixture of regio and locoisomers. Deprotection of aldehydes 10 and 14 was carried out by treatment with TFA at room temperature to yield the formylated derivative 11 or 15 in each case. Finally, the azomethine ylides, which were obtained from 11 or 15 and DL-propargylglycine, were reacted with C60 to yield the corresponding chemically modified C60-C70 dimers (Schemes 2 and 3 SI). Electron donor-acceptor1-acceptor2 conjugates 3-6 were synthesized following the synthetic methodology developed for the synthesis of 1. In particular, 3 / 5 or 4 / 6 were obtained through copper catalyzed “click-chemistry” between ZnP azide 16 or ZnPc azide 7 and C60-C70 alkynes, respectively, in good yields (41-52%, Scheme 4, SI). All new chemical compounds were fully characterized by NMR, FTIR, MS, and HPLC (See SI). The results of, for example, 3 (Figure S10) document the detection of the molecular ion peak [M-] at 2934.7 (calculated molecular weight for C216H89N11O2Zn: 2935.7), and of the peak that corresponds to the loss of C60 [M-C60] at 2212.6, in addition to those of either C60 (720.0 a.m.u) or C70 (840.0 a.m.u). Similar findings are shown in the SI for 4-6. They confirm the successful formation of the electron donor-acceptor1-acceptor2 conjugates 2-6. Steady-state investigations: In terms of ground state characterization, the absorptions of ZnPc-C60-C70s 2, 4, and 6 in THF are similar to those seen for the ZnPc reference (SI, Fig. S21). In particular, a very intense Q-band absorption is observed around 675 nm, which features vibrational fine structure maxima at 645 and 610 nm. The less intense Soret absorption at 350 nm overlaps, however, with both the C60 and C70 absorptions, which induces a virtual blue-shift. Overall, 2, 4, and 6 reveal minor red shifts and broadening relative to the ZnPc reference. Likewise, for ZnP-C60-C70s 3 and 5 in THF, the typical ZnP strong Soret band absorption at 428 nm and two weaker Q-band absorptions at 560 and 600 nm are found in addition to shoulders at 250 and 320 nm, which stem from C60 and C70, respectively (SI, Fig. S22). The long-wavelength absorption maxima are listed in Table S3, SI. As far as the reduced and oxidized states in o-DCB:ACN (4:1 v/v) are concerned, two oxidations at +0.45 and +1.22 V and two reductions at -1.39 and -1.87 V evolve in the ZnPc reference, while the ZnP reference gives rise to two oxidations at +0.52 and +0.90 V and three reductions at -1.62, -1.84 and 2.00 V. The electrochemical features of the C60-C70 electrontransport chains are best described as two sets of reductions. The first set of reductions appears at -0.74, -1.12, -1.48, and -


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.81 V and is in sound agreement with the characteristic reductions of pyrazolino[70]fullerenes. The second set of reductions is associated with pyrrolidino[60]fullerene-centered reductions. These are seen at -0.85, -1.25, -1.61, and -1.96 V. Turning to ZnPc-C60-C70s 2, 4, and 6, two ZnPc oxidations,55 which are seen at +0.45 and +1.22 V, are complemented by reductions of ZnPc at -1.39/-1.87 V, C60 at -0.88/-1.28 /-1.63/2.00 V, and C70 at -0.77/-1.14/-1.50/-1.83 V56. For ZnP-C60C70s 3 and 5, ZnP-centered oxidations are discernable at +0.53 and +0.90 V and reductions due to ZnP at -1.62/-1.84 V, C60 at -0.87/-1.24/-1.61/-1.96 V, and C70 at -0.76/-1.13/-1.48/-1.80 V.57 The fact that the redox processes in the electron donoracceptor1-acceptor2 conjugates differ by no more than 1-2 mV relative to those of the references confirms that they are electronically decoupled from each other due to their spatial separation. Insights into excited states came from fluorescence measurements, where all of the ZnPc-C60-C70s exhibit maxima at 680 nm and the ZnP-C60-C70s at 605/655 nm. In the ZnP and ZnPc references, the fluorescence quantum yields are solvent invariant with values of 0.04 and 0.30, respectively. In stark contrast, the fluorescence in both ZnPc-C60-C70s 2, 4, and 6 and in ZnP-C60-C70s 3 and 5 is strongly solvent dependent and is quenched with quantum yields in the ranges between 0.006 and 0.036 and between 0.004 and 0.006, respectively. None of the ZnPc or ZnP fluorescence patterns, however, are affected by the quenching. Neither do the relative C60 to C70 distances in the different C60-C70 electron transport chains affect the effective fluorescence quenching. Thermodynamic driving forces: The thermodynamic driving forces for charge separation starting from the photoexcited electron donors in their singlet excited states to yield D•+-C60•-C70, for charge shift from D•+-C60•--C70 to afford D•+-C60-C70•-, and for charge recombination evolving from D•+-C60•--C70 or D•+-C60-C70•- to repopulate the singlet ground state were determined using electrochemical and spectroscopic assays – see SI, Table S3. They vary only slightly in 2, 4, and 6, and in 3 and 5 and, in turn, are marginally affected by the varying distances between C60 and C70 in the different electrontransport chains. The small driving forces of close to 0.1 eV for the underlying charge-shift reactions are noteworthy. ZnPc-C60-C70s: In femto- and nanosecond-resolved pumpprobe experiments with ZnPc-C60-C70 2, 4, and 6, argonsaturated benzonitrile solutions were photoexcited with 150 fs laser pulses at 656 nm, which corresponds to the low-energy Q-band absorption of ZnPc. The corresponding ZnPc first singlet excited state reveals transient features in the form of broad absorptions between 400 and 500 nm, 590 and 630 nm maxima, and 610 and 670-680 nm minima. In benzonitrile, these convert rapidly into the characteristics of the oneelectron oxidized state of ZnPc, which develop at 435, 520, and 840 nm58,59, and the 1020 nm fingerprint of the oneelectron reduced state of C6060 – see SI Fig. S23 for differential absorption spectra of the one-electron reduced and oneelectron oxidized forms. All of the aforementioned develop within the first 10 ps (SI, Fig. S24-S26) after the excitation and imply an intramolecular charge separation evolving from the ZnPc singlet excited state and affording the ZnPc•+-C60•-C70 charge-separated state – Table 1.

t / µs

0.025 0.020 0.015 ∆OD / a.u.

Page 3 of 11

0.010 0.005

-0.02 1E-4 2.1E-4 5.8E-4 0.001 0.002 0.005 0.01 0.02 0.05 0.1

0.2 0.5 1 2 5 10 20 50 100 200 400

800

900

0.000 -0.005 -0.010 400

500

600

700

wavelength / nm

Figure 2. Differential absorption changes (visible) obtained upon nanosecond pump-probe experiments (656 nm) of 4 in argon saturated benzonitrile at room temperature with several time delays between 100 ps and 400 µs.

On the timescale of up to 100 µs, the differential absorption spectra give rise to kinetics that suggest the involvement of multiple species during the excited-state deactivation. At first glance, we note that the features of the ZnPc triplet excited state at 480 nm grow in as early as the deactivation of the ZnPc•+-C60•--C70 charge-separated state sets in, but they decay again within a few µs. Throughout the growth and decay of the ZnPc triplet excited state, the one-electron oxidized state of ZnPc is, however, persistent. From this observation, we conclude that the two states coexist. In 2, 4, and 6, the characteristic 1020 nm absorption of the one-electron reduced state of C60 is seen to disappear within a few nanoseconds, while the oneelectron oxidized state of ZnPc is stable in 2 (SI, Fig. S27) and 4 (Figure 2) for tens of nanoseconds and for hundreds of nanoseconds in 6 (SI, Fig. S28). Implicit in these findings is the evolution of a ZnPc•+-C60•--C70 to ZnPc•+-C60-C70•- charge shift. To this end, detection of the one-electron reduced state of C70 is based on analyzing the spectroelectrochemical data regarding the oxidation of the electron donating ZnPc and the reduction of the electron accepting C60 and C70. Owing to the fact that the one-electron reduced state of C70 absorbs more strongly than that of C60 at short wavelengths, we expect a relative absorption increase in the 400 to 600 nm spectral region upon charge shift from ZnPc•+-C60•--C70 to ZnPc•+-C60-C70•- (compare SI, Fig. S23).61 This is, indeed, the case, when comparing the species-associated spectra (SAS) of the ZnPc•+-C60•--C70 and ZnPc•+-C60-C70•- charge-separated states, as shown, for example, for 6 – Figure 5. The according increase in relative absorption serves as evidence for the charge-shift reaction. It is only on a timescale exceeding 1 µs that the ZnPc triplet excited state is totally absent and only features of the one-electron oxidized state of ZnPc are discernable. In other words, the initial intramolecular charge separation starting from the ZnPc singlet excited state is followed by an intermolecular charge separation developing from the ZnPc triplet excited state. Its bimolecular nature was confirmed in concentration-dependent assays (see SI, Fig S29). Overall, the quantum yield of approximately 8% for the transient ZnPc triplet excited state is much lower than the 20% found for 2 or 4 and the 60% found for the ZnPc reference.62 Based on the aforementioned observations, we postulated the kinetic model shown in Figure 3 and implemented it in the global target analysis63 by means of Global and Target Analy-



0.015 0.010 0.005 0.000 -0.005

0.9 0.8

relative concentration

SAS1 SAS2 SAS3 SAS4

0.020

SAS

sis (GloTarAn)64/TIMP65. It involves, besides the aforementioned intra- and intermolecular charge-separation processes, the transient formation of the ZnPc triplet excited state and the corresponding deactivations to the ground state. Results from the fitting based on this model are best for 6 (Figure 4), while the kinetics seen for 2 and 4 (SI, Figs. S30, S31) lead to some residuals, which are, however, minor.66 Here, we consider that the aforementioned rotational degrees of freedom may lead to distributions of rate constants rather than single rate constants for each step of the electron transfer, namely charge separation, charge shift, and charge recombination.

0.7 0.6 0.5 0.4 0.3 0.2 0.1

-0.010 400

500

600

700

800

900

0.0 1E-4

1E-3

0.01

wavelength / nm

0.1

1

10

100

time / µs

Figure 5. Left: Deconvoluted transient absorption spectra of ZnPc•+-C60•--C70 (black), ZnPc•+-C60-C70•- (red), 3ZnPc-C60-C70 (grey), and ZnPc•+-C60-C70 + ZnP-(C60-C70)•- (blue) for 6 as obtained by global target analysis (visible). Right: Evolution of the population of the involved states.

A final note concerns the ZnPc triplet excited-state lifetimes. They are in ZnPc-C60-C70 not only limited by collision with another ZnPc-C60-C70, but also the presence of residual oxygen. For example, reference experiments with oxygensaturated benzonitrile led to lifetimes of only 200 ns and, in turn, to a complete shutdown of the intermolecular chargetransfer channel. Table 1: Lifetimes of charge-separated states in 2, 4, and 6 in benzonitrile.

Figure 3. Kinetic model used to fit/deconvolute the excited-state surfaces of 2, 4, and 6 in benzonitrile via Global and Target Analysis (GloTarAn). Pathways of charge separation, shift, and recombination are highlighted by double-lined arrows and bolded transients.

∆OD / a.u.

0.015

residuals

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 11

440 nm 530 nm 840 nm fit fit fit

0.010

0.005

0.000 5.0E-4 0.0 -5.0E-4 1E-4

1E-3

0.01

0.1

1

10

100

time / µs

Figure 4. Time absorption profiles of the spectra obtained upon nanosecond pump-probe experiments (656 nm) of 6 in argon saturated benzonitrile at room temperature at 440 nm (black), 530 nm (red), and 840 nm (grey) illustrating the excited state decay and corresponding fits.

The spectral features of the SAS as they were derived from the global target analysis are in sound agreement with the proposed kinetic model of Figure 3 and, in turn, corroborate the nature of the transient species – Figure 5. In Table 1, we have listed the lifetimes of the individually formed chargeseparated states obtained from the target analysis.

τCS / ps •+

•-

2

4

6

2.7

2.6

3.4

τ (ZnPc -C60 -C70) / ns

1.3

2.4

0.7

τ (ZnPc•+-C60-C70•-) / ns

40

58

199

τ (3*ZnPc-C60-C70) / µs

13

11

8

τ (ZnPc-C60-C70)•+ + (ZnPc-C60C70)•- / µs

>100

>100

>100

Charge shift efficiency

0.20

0.25

0.15

ZnP-C60-C70s: In the case of ZnP-C60-C70s 3 and 5, 430 nm laser pulses were used for photoexcitation in the femto- and nanosecond pump-probe experiments as they match the intense Soret-band absorption of ZnP. Again, all measurements were performed in benzonitrile. Following 430 nm photoexcitation, the instantaneously formed second singlet excited state of ZnP converts within 1-2 ps 67,68 to the corresponding first singlet excited state of ZnP. Characteristics of the latter include maxima at 460, 580, and 620 nm and minima at 430, 560, and 600 nm. A comparison with ZnPc-C60-C70s 2, 4, and 6 shows that charge separation in ZnP-C60-C70s 3 and 5 evolving from the ZnP singlet excited state rather than from the ZnPc singlet excited state is slowed down due to slightly larger electron donor-acceptor distances – 1.5 nm between ZnPc and C60 in 2, 4, and 6 versus 1.9 nm between ZnP and C60 in 3 and 5. In ZnP•+-C60•--C70, the presence of one-electron oxidized form of ZnP is corroborated by its 415 and 650 nm characteristics,69,70 while the 1020 nm absorption of the one-electron reduced form of C60 documents the involvement of C60 in the charge separation – SI, Figures S32, S33. For 3 and 5, the time constants are 8 and 37 ps, respectively – Table 2. In succession to the ZnP•+-C60•--C70 formation, a number of transient species develop as the ZnP•+-C60•--C70 charge-separated state starts to decay. For example, the characteristic 1020 nm absorption of the one-electron reduced state of C60 is seen to disappear in 3 within a few nanoseconds (SI, Fig. S34), while the one-electron oxidized state of ZnP is stable for tens of nanoseconds (SI, Fig. S35). This goes hand-in-hand with a


0.008 0.006 0.004

0.2 0.5 1 2 5 10 20 50 100 200 400

415 480 1017 fit fit fit

0.010 0.008 0.006 0.004 0.002 0.000 6.0E-4 0.0 -6.0E-4 1E-4

1E-3

0.01

SAS1 SAS2 SAS3 SAS4 SAS5

0.6 0.4 0.2 0.0 -0.2 400

500

600

700

wavelength / nm

-0.002 400

500

600

700

800

900

1000

1100

wavelength / nm

Figure 6. Differential absorption changes (visible) obtained upon nanosecond pump-probe experiments (430 nm) of 5 in argon saturated benzonitrile at room temperature with several time delays between 100 ps and 400 µs. 1

*ZnP-C 60-C70

3

ZnP

*ZnP-C 60-C70

-C60•--C70

•+

ZnP •+-C60-C70•X

ZnP •+-C60-C70

+

ZnP-C 60-C70•-

ZnP-C 60-C70

Figure 7. Kinetic model used to fit / deconvolute the excited state surfaces of 5 in benzonitrile via GloTarAn. Pathways of charge

10

100

Therefore, fitting the kinetics recorded for 5 requires the expansion of the kinetic model by an additional state “X” (Figure 7), which mediates the ZnP•+-C60•--C70 to ZnP•+-C60-C70•- charge shift. The fitting results upon applying the revised model are satisfying and lack residual signals above the noise level Figure 8.

0.8

0.000

1

Figure 8. Time absorption profiles of the spectra obtained upon nanosecond pump-probe experiments (430 nm) of 5 in argon saturated benzonitrile at room temperature at 415 nm (black), 480 nm (red), and 1017 nm (grey) illustrating the excited state decay and corresponding fits.

1.0

0.002

0.1

time / µs

normalized SAS

-0.02 1E-4 2.1E-4 5.8E-4 0.001 0.002 0.005 0.01 0.02 0.05 0.1

0.010

0.012

800

900

0.8

relative concentration

t / µs

0.012

separation, shift, and recombination are highlighted by doublelined arrows and bolded transients.

∆ OD / a.u.

ZnP•+-C60•--C70 to ZnP•+-C60-C70•- charge shift. Within the time window of a few tens of nanoseconds to a few microseconds, the only detectable species is the ZnP triplet excited state. Based on the steady state fluorescence quenching – see section steady-state investigations and Table S3, SI – we derived triplet quantum yields of 0.08 and 0.11 for 3 and 5, respectively.71 Beyond that, it is replaced by the features of the oneelectron oxidized state of ZnP. Considering that the associated kinetics depend on the effective concentrations, we conclude the occurrence of an intermolecular charge separation starting from the ZnP triplet excited state. For 3, a kinetic model (SI, Fig. S36) was implemented via GloTarAn that resembles the one used for ZnPc-C60-C70s 2, 4, and 6 – vide supra. Quite different is the case of 5 (Figure 6): The one-electron oxidized form of ZnP and the one-electron reduced form of C60 decay simultaneously with a time constant shorter than one nanosecond, the spectral features of the ZnP triplet excited state remain. Nevertheless, within the next 50 nanoseconds the one-electron oxidized form of ZnP grows back in and coexists with the ZnP triplet excited state. Its decay, which happens with close to 190 nanoseconds, is linked to the decay of ZnP•+C60-C70•- charge-separated state. Later on, on the microsecond time scale, all of the residual ZnP triplet excited-state signatures are seen to convert to that of the charge-separated state via a diffusion controlled bimolecular process as postulated for ZnPc-C60-C70s 2, 4, and 6 and ZnP-C60-C70 3.

∆OD / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


residuals

Page 5 of 11

0.6

0.4

0.2

0.0 1E-4

1E-3

0.01

0.1

1

10

100

time / µs

Figure 9. Left: Deconvoluted, normalized transient absorption spectra72 of ZnP•+-C60•--C70 (black), intermediate X (red), 3ZnPC60-C70 (blue), ZnP•+-C60-C70•- (grey), and ZnP•+-C60-C70 + (ZnPC60-C70)•- (green) for 5 as obtained by global target analysis (visible and near-infrared). Right: Evolution of the population of the involved states.

A minor fraction of ZnP•+-C60•--C70 directly undergoes the charge shift, resulting in the formation of ZnP•+-C60-C70•-, while the dominant pathway of deactivation is the population of the intermediate state “X”. “X” is described by the SAS and time profile colored red in Figure 9 (individual plots of all 5 SAS may be found in the SI, Figure S37). Its transient features comprise broad absorptions between 450 and 550 nm and in the 650 to 750 nm region. As such, it combines features of the ZnP, C60, and C70 triplet excited states – see Figure S38 of the SI. We wish to point out that higher lying states, such as the second singlet excited state of ZnP, play no significant role in the deactivation pathway, since reference experiments with 550 nm excitation to populate exclusively the first singlet excited state, lead to exactly the same kinetics. Table 2: Lifetimes of charge-separated states in 3 and 5 in benzonitrile.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

τCS / ps •+

•-

3

5

8

37

τ (ZnP -C60 -C70) / ns

2

0.7

τ (X) / ns

a

36

τ (ZnP•+-C60-C70•-) / ns

52.8

187.8

τ (3*ZnP-C60-C70) / µs

14

35

τ (ZnP-C60-C70)•+ + (ZnP-C60C70)•- / µs

>100

>100

Charge shift efficiency

0.35

0.15

a

not detected in case of 3

To gather support for the triplet-mediated electron transfer intermediate “X”, we explored nanosecond-resolved transient absorption experiments following 355 nm pulses with variable magnetic field strengths: 0, 0.20, 0.26, 0.36, and 1.0 T (Figures S39 – S41). Considering a time resolution of about 10 ns, the transient state “X” in ZnP-C60-C70 5 is seen right after the conclusion of the photoexcitation. At zero magnetic field, global analyses point to a conversion of the transient state “X” to ZnP•+-C60-C70•- with a time constant of 30 ns (Figure S39). Upon increasing the magnetic field to 0.20 and 0.26 T, the corresponding values are 27.6 and 26.9 ns, respectively. Interestingly, the time constants upon further increasing the magnetic field strength to 0.36 T and, finally, 1 T are 27.4 and 29.0 ns, respectively.73 These changes stem from resonance conditions for the conversion at around 0.25 T, at which the conversion is accelerated by destabilizing the intermediate state “X”. ZnPc-C60-C70s versus ZnP-C60-C70s: We probed in our photophysical assays light harvesters/electron donors (ZnPc or ZnP) to power unidirectional charge-shift reactions along a variety of electron-transport chains (C60-C70). Importantly, the electron-transport chains were designed to separate the oneelectron oxidized donor (ZnPc or ZnP) from the one-electron reduced acceptor (C70) in terms of overall distance without, however, compromising the energy of the charge-separated state. From the excited-state dynamics following photoexcitation of ZnPc-C60-C70s 2, 4, and 6 and ZnP-C60-C70s 3 and 5, we infer a rather complex interplay of structural and energetic factors that determine the individual channels of the excitedstate deactivation. With focus on the charge-shift reaction along the electrontransport chains, that is, the ZnPc•+-C60•--C70 to ZnPc•+-C60-C70•or ZnP•+-C60•--C70 to ZnP•+-C60-C70•- transformation, we conclude that 2, 3, 4, and 6 exhibit similar pathways. Those for 5 differ appreciably. Overall, the relative positioning of the ZnPc triplet excited state at 1.1 eV versus the ZnP triplet excited state at 1.5 eV makes the difference. For example, in ZnPc-C60-C70s 2, 4, and 6 it is approximately 1.1 eV and, in turn, slightly lower in energy than the ZnPc•+-C60•--C70 charge-separated state at around 1.2 eV. Consequently, population of the ZnPc triplet excited state outcompetes the charge-shift reaction and leads to poor charge-shift efficiencies, which are approximately 20 ± 5%. Hereby, the efficiency found for 2, in which C60 and C70 are directly linked to each other, is not the highest in the series of homologues presented. Van der Waals-contacts/dimers between C60 and C70 are highly probable in 2 and as we have shown recently,53 lead to a thermodynamic trap state in the form of a dimer upon

Page 6 of 11

one-electron reduction. The charge shift efficiency in 4 is higher. As such, it seems that in 4 the C60 to C70 distance in the electron transport chain is well balanced to power the charge shift: Firstly, no thermodynamic trap state/dimer is formed, and secondly, the electronic coupling between C60 and C70 is still sufficient. The latter argument is supported by the lower charge-shift efficiency in 6 due to an even larger C60 to C70 distance, which causes an even weaker electronic coupling. The situation for ZnP-C60-C70s 3 and 5 is different: The ZnP triplet excited state (1.5 eV) is above that of the ZnP•+-C60•-C70 charge-separated state with an energy of around 1.3 eV. Efficiencies as high as 35% in, for example, 3 are now only limited by the overall weak driving force of approximately 0.1 eV for the charge-shift reaction, but, in contrast to ZnPc-C60C70s 2, 4, and 6, no longer by the triplet excited state energy. We have determined for 5 a transient species “X” that mediates the ZnP•+-C60•--C70 to ZnP•+-C60-C70•- charge shift, is susceptible to magnetic field effects and displays signs of resonance in the proximity of 0.25 T. Its nature appears ambivalent; it exhibits the triplet excited-state fingerprints of ZnP, C60, and C70. The energy of the ZnP triplet excited state is pivotal for a weakly coupled C60-C70 electron transfer chain. It is within resonance of the ZnP•+-C60•--C70 charge-separated state, but lies above it experimentally but below it in the calculations described below, which make charge-transfer states too unstable. Coupling to this triplet is thermodynamically feasible without fully populating it but it, together with the lowestlying triplet centered on C70 can mediate between the ZnP•+C60•--C70 and ZnP•+-C60-C70•- charge-separated states. This mediation is likely to be necessary because each of the alternative charge-transfer states will be the more stable in its own fully relaxed solvent environment. This means that considerable reorganization of the solvent is needed before the two can interconvert. This solvent reorganization could typically be caused by a transition to a relatively non-polar state. Neither of the two charge-transfer states would then be stabilized preferentially, so that they would be close in energy, as they are in the solvent environment of the ground state. 3 features a more strongly coupled C60-C70 electron transfer chain that reduces the necessity for a mediating step. We, however, cannot rule out its presence, but its detection would be limited by a very short lifetime and masked by the formation of the ZnP•+-C60-C70•- charge-separated state.74 In contrast, the lower ZnPc triplet excited state energy in 6 promotes the effective coupling to the ZnPc•+-C60•--C70 charge-separated state, but cancels, at the same time, the mediation between the two charge-separated states. Instead, it serves as the main deactivation channel. With focus on the lifetimes of the ZnPc•+-C60-C70•- and ZnP•+-C60-C70•- charge-separated states, several trends are noted. First, the charge recombination slows-down as a function of ZnP to C70 / ZnPc to C70 distance. Second, ZnP-C60C70s reveal comparable lifetimes to ZnPc-C60-C70s at comparable electron donor-acceptor distances. Third, the largest difference evolves between 4 and 6 as well as between 3 and 5, while the difference between 2 and 4 is rather insignificant. Finally, the ZnPc•+-C60-C70•- and ZnP•+-C60-C70•- chargeseparated state lifetimes are longer by at least one order of magnitude than those found for ZnPc•+-C60•--C70 and ZnP•+C60•--C70 charge-separated states. THEORY AND MODELING


Page 7 of 11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The results described above require the existence of an intermediate state “X”, which was investigated using MNDO semiempirical configuration interaction (CI) calculations with single excitations only and an active orbital window of the highest 64 occupied and the lowest 64 virtual orbitals.75 Benzonitrile as solvent was simulated using the published polarizable continuum model,76,77 using the in-house version of the VAMP program.78 As the energies of the conformations investigated were similar for 5 and 6 and the calculated spectra and excited states showed little dependences on conformation, we discuss the anti-conformer of ZnPc-C60-C70, 6 (see CI Calculations; the optimized geometries are given in Table S4 of the SI). The intense Q-band is found at 660 nm in the calculations and a number of moderately intense C60, C70, and ZnPc absorptions between 360 and 363 nm. The calculated absorption spectra are shown in Tables S5 and S6 of the Supporting Information. The state energies with fully relaxed solvent environment for each state provide a likely explanation for the experimental observations. The MNDO configuration-interaction calculations show that the two lowest-lying triplet excited states, that is, T1 and T2, for anti-ZnPc-C60-C70 6 are localized on ZnPc and are found at 1.15 and 1.41 eV, respectively. T3, in contrast, is a local C70 triplet excited state at 1.47 eV. The situation in anti-ZnP-C60-C70 5 is different: T1 and T3 are centered on ZnP at 1.31 and 1.53 eV, respectively, whereas the intervening T2 is centered on C70 at 1.42 eV. The lowest pair of singlet and triplet ZnPc•+-C60-C70•- charge-separated states is found at 2.66 eV (2.29 eV for anti-6) and the lowest pair of ZnPc•+-C60•-–C70 states at 2.80 eV (2.22 eV for anti-6). The charge-separated states featuring the one-electron reduced form of C70 are more strongly stabilized in solution because of their large (155-160 Debye) dipole moments compared to the analogous one-electron reduced form of C60 (80-85 Debye), due to the shorter electron donor-acceptor distance in the latter. The difference in behavior between 5 and 6 can be traced back to the energies of the low-lying triplet states in fully relaxed benzonitrile. As outlined above, the lowest-lying ZnPand C70-centered triplet excited states are comparable in energy. Triplet-triplet energy transfer (TTEnT) is generally treated as two consecutive electron-transfer steps, which it is not particularly effective at long range. However, in the case of the electron-transport chains, a relatively low-lying C60 triplet excited state lies geometrically between the low-lying ZnP and C70 triplet excited states. This means that C60 triplet excited state, which lies at approximately at the same energy or slightly higher than that of C70, can act as a mediator for the longrange TTEnT, much like superexchange in electron transfer. This suggests that the 3ZnP to 3C70 and the 3C70 to 3ZnP TTEnT-processes in 5 should be quite fast, setting up an equilibrium between them. 3C70, in turn, can interconvert to the ZnP•+-C60-C70•- charge-separated state by two short-range and possibly concerted electron transfer steps:

Figure 10. Schematic diagram of the triplet-mediated electron transfer from ZnP to C70 in 5. Mediating states are shown in square brackets. The ZnP+.-C60-.-C70 state first relaxes to 3ZnPC60-C70, which exists in a fast TTEnT equilibrium with ZnP-C603 C70 mediated by ZnP-3C60-C70. In contrast to ZnP•+-C60•--C70, ZnP•+-C60•--3C70 can undergo fast conversion to ZnP•+-C60-C70•-.

Figures 9 and S38 (SI) suggest that “X” is in fact an equilibrium mixture of ZnP and C70 triplet excited states (possibly also with the local 3C60 state). The C70 triplet excited state can, in turn, convert readily to the ZnP•+-C60-C70•- charge-separated state. Overall, TTEnT-processes accelerate the formation of the final charge-separated state. Solvent reorganization in polar solvents plays a major role in making this mechanism favorable: The ZnP•+-C60•--C70 to ZnP•+-C60-C70•- charge-shift process involves two specifically solvated charge-separated states, so that the activation energy will be substantial. The TTEnT-process, on the other hand, only involves non-polar states: it will therefore not be impeded by large solventreorganization barriers. The final ZnP-C60-3C70 to ZnP•+-C60C70•- transformation can be mediated by ZnP•+-C60•--3C70, as suggested in Figure 10. CONCLUSIONS We have designed a series of electron donor-acceptor1acceptor2 conjugates – ZnPc-C60-C70 and ZnP-C60-C70 – around ZnPc or ZnP as excited-state electron donor as well as C60 and C70 as primary electron acceptor1 and secondary electron acceptor2, respectively. A cascade of charge transfer and (reductive) charge shift reactions separates charges, that is, electrons and holes, in the corresponding ZnPc•+-C60-C70•- and ZnP•+-C60C70•- by up to 27 Å. Key in the electron shift mediation, when the electronic coupling is weak, is the energy of the lowest triplet excited state. For example, relative to the energy of the initial charge-separated state, an energetically lower lying ZnPc triplet excited state bottlenecks the electron shift and limits the efficiency to 25%, while an energetically higher lying ZnP triplet excited state allows for efficiencies of up to 35%. The spectroscopic nature of the mediating state is between that of the ZnP triplet excited state and the C60 and C70 triplet excited states, so that “X” is suggested to be an equilibrium mixture of low-lying triplet states. Triplet-triplet energy transfer mediated by the geometrically intermediate C60 allows energy to be transferred to C70 without incurring significant barriers caused by solvent reorganization. ZnPc-C60-3C70 to ZnPc•+-C60-C70•- conversion is suggested to be mediated by states, in which the C60 is charged and C70 is in its triplet excited state configuration. EXPERIMENTAL PROCEDURES



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Femtosecond time resolved transient absorption spectroscopy: The laser source was a Clark MXR CPA2110 Ti:Sapphire amplifier with a pulsed output of 775 nm at 1 kHz and pulse width of 150 fs. Time resolved transient absorption spectra with 150 fs resolution and time delays from 0 to 5500 ps were acquired using Ultrafast Systems HELIOS Femtosecond Transient Absorption Spectrometer. Visible white light (~400-770 nm) was generated by focusing a fraction of the fundamental 775 nm output onto a 2 mm sapphire disk; for the (near) IR (780-1500 nm), a 1 cm sapphire was used. Excitation pulses of 430, 656, or 676 nm wavelength were generated by a NOPA (with subsequent frequency doubling in case of 430 nm); bandpass filters with ±5 or ±10 nm were used to ensure low spectral width and to exclude 775 and 387 nm photons. Ultrafast Systems EOS Sub-Nanosecond Transient Absorption Spectrometer was employed to measure transient absorption spectra with time delays of ~1 ns to 400 µs with 1 ns time resolution. White light (~370 to >1600 nm) was generated by a built-in photonic crystal fiber supercontinuum laser source with a fundamental of 1064 nm at 2 kHz output frequency and pulse width of approximately 1 ns. Samples were dissolved in benzonitrile at concentrations between 2-5·10-5 M and deoxygenated by flushing with Argon for 15 minutes in quartz glass cuvettes of 2 mm thickness. Steady state UV-vis absorption and emission spectroscopy: Absorption spectra between 300 and 900 nm were measured with a Perkin-Elmer Lambda2 dual beam absorption spectrometer with a scan rate of 600 nm/min and a resolution of 0.5 nm. Sample solutions with increasing concentrations between 10-8 and 10-6 M were titrated into 1 x 1 cm quartz glass cuvettes. For determination of fluorescence quantum yields by the comparative method, the OD at the wavelength of excitation and beyond was kept below 0.1. Emission spectra between 400 and 850 nm were recorded with a Horiba Fluoromax with a resolution of 0.5 nm and excitation / detection spectral bandwidth of 2 nm. Spectroelectrochemistry: Solutions of analytes were prepared in o-DCB with 0.2 M of tetra-tBu-ammonium perchlorate electrolyte. Spectra were recorded with a Varian/Agilent Cary 5000 between 300 and 1600 nm. A home-built threeneck cell was used to contain the three electrode setup with platinum gauze as working electrode, Pt wire as counter electrode and Ag wire as pseudo-reference electrode. Argon was used to flush the cell before (20 min) and during measurements to remove oxygen. Potentials were provided by a Metrohm Autolab PGSTAT101, controlled via Metrohm Autolab Nova 1.10 software. Square-wave-voltammetry: Osteryoung voltammograms were recorded on a potentiostat/galvanostat Autolab equipped with PGSTAT30. The redox potentials of the triads were measured at room temperature using a three-electrode cell, comprising a platinum wire counter electrode, a glassy carbon working electrode, and an Ag/AgNO3 reference electrode Bu4NClO4 0.1 M was used as electrolyte and a mixture of oDCB / MeCN 4: 1 as solvent. NMR: NMR spectra (1H, 13C) were recorded at room temperature on Bruker DPX 300 MHz. Data are listed in parts per million (ppm) and are reported relative to tetramethylsilane,

Page 8 of 11

residual solvent peaks being used as internal standard (CHCl3, 1H: 7.26 ppm, 13C: 77.36 ppm, CDCl3). MS: MALDI and High resolution MALDI mass spectrometry measurements were performed at the Unidad de Espectrometría de Masas of Universidad Complutense de Madrid and Universidad Autónoma de Madrid. HPLC: HPLC analysis was performed on an Agilent 1100, equipped with a Cosmosil 5-PYE Waters (4.6 x 250 mm) column, and a diode-array detection system. The specific solvents and retention times for each compound can be found on the SI. CI Calculations: All calculations used the MNDO Hamiltonian75 in configuration interaction calculations that used all single and double excitations (CISD) within an active window of 10 occupied and 10 virtual orbitals. The benzonitrile solvent was simulated using the published SCRF model76,77 without the dispersion correct76. The geometries of ZnPc-C60-C70 6 and ZnP-C60-C70 5 were first optimized in conformations in which C60 and C70 were situated both syn and anti to each other. As the energies of the two conformations were similar in each case and the calculated spectra and excited states showed little dependence on conformation, we discuss the anti-conformer of ZnPc-C60-C70. All calculations used the in-house development version of the VAMP program.78

ASSOCIATED CONTENT Supplemental Information includes detailed synthesis and analyses of the products by NMR, MS, and HPLC, electrochemical results, and additional steady state and time-resolved absorption spectra.

AUTHOR INFORMATION Corresponding Author *Correspondence: [email protected]; [email protected]; [email protected]; [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENT This work was partially supported by the European Research Council (ERC-320441-Chirallcarbon), MINECO of Spain (Grant Numbers CTQ-2014-52045-R, CTQ-2014-52869-P, MAT201347192-C3-2-R and CTQ2015-70921), Comunidad de Madrid (FOTOCARBON Project S2013/MIT-2841) and Basque Government (PC2015-1-03 (16-79)). J.L.D. acknowledges Ikerbasque, the Basque Foundation for Science, for an “Ikerbasque Research Fellow” contract, Polymat Foundation and MINECO of Spain for IEDI-2015-00666 grant and Iberdrola Foundation for financial support. Work in Erlangen was supported by the “Solar Energy goes Hybrid” (SolTech) initiative of the Bavarian Ministry for Science, Culture and Education and by the Deutsche Forschungsgemeinschaft as part of SFB953 “Synthetic Carbon Allotropes”.


Page 9 of 11

Journal of the American Chemical Society 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

27 28

REFERENCES

29 30

1

2 3

4 5

6

7 8 9

10

11

12

13

14

15

16 17

18 19 20 21 22

23 24

25

E. Romero, R. Augulis, V. I. Novoderezhkin, M. Ferretti, J. Thieme, D. Zigmantas, R. van Grondelle, Nat. Phys. 2014, 10, 676. N. Martín, L. Sánchez, M. Á. Herranz, B. Illescas, D. M. Guldi, Acc. Chem. Res. 2007, 40, 1015. P.-A. Bouit, F. Spänig, G. Kuzmanich, E. Krokos, C. Oelsner, M. A. Garcia-Garibay, J. L. Delgado, N. Martín, D. M. Guldi, Chem. - A Eur. J. 2010, 16, 9638. C. Villegas, E. Krokos, P.-A. Bouit, J. L. Delgado, D. M. Guldi, N. Martín, Energy Environ. Sci. 2011, 4, 679. P.-A. Bouit, D. Rauh, S. Neugebauer, J. L. Delgado, E. Di Piazza, S. Rigaut, O. Maury, C. Andraud, V. Dyakonov, N. Martin, Org. Lett. 2009, 11, 4806. C. R. Nieto, J. Guilleme, C. Villegas, J. L. Delgado, D. González-Rodríguez, N. Martin, T. Torres, D. M. Guldi, J. Mater. Chem. 2011, 21, 15914. J. L. Delgado, N. Martín, P. de la Cruz, F. Langa, Chem. Soc. Rev. 2011, 40, 5232. F. G. Brunetti, J. L. López, C. Atienza, N. Martín, J. Mater. Chem. 2012, 22, 4188. J. D. Megiatto, A. Antoniuk-Pablant, B. D. Sherman, G. Kodis, M. Gervaldo, T. A. Moore, A. L. Moore, D. Gust, Proc. Natl. Acad. Sci. 2012, 109, 15578. M. Wielopolski, A. Molina-Ontoria, C. Schubert, J. T. Margraf, E. Krokos, J. Kirschner, A. Gouloumis, T. Clark, D. M. Guldi, N. Martín, J. Am. Chem. Soc. 2013, 135, 10372. Y.-S. Huang, X. Yang, E. Schwartz, L. P. Lu, S. Albert-Seifried, C. E. Finlayson, M. Koepf, H. J. Kitto, B. Ulgut, M. B. J. Otten, J. J. L. M. Cornelissen, R. J. M. Nolte, A. E. Rowan, R. H. Friend, J. Phys. Chem. B 2011, 115, 1590. G. Kodis, Y. Terazono, P. A. Liddell, J. Andréasson, V. Garg, M. Hambourger, T. A. Moore, A. L. Moore, D. Gust, J. Am. Chem. Soc. 2006, 128, 1818. Y. Terazono, G. Kodis, K. Bhushan, J. Zaks, C. Madden, A. L. Moore, T. A. Moore, G. R. Fleming, D. Gust, J. Am. Chem. Soc. 2011, 133, 2916. M. Kloz, S. Pillai, G. Kodis, D. Gust, T. A. Moore, A. L. Moore, R. van Grondelle, J. T. M. Kennis, J. Am. Chem. Soc. 2011, 133, 7007. V. Garg, G. Kodis, M. Chachisvilis, M. Hambourger, A. L. Moore, T. A. Moore, D. Gust, J. Am. Chem. Soc. 2011, 133, 2944. V. Garg, G. Kodis, P. A. Liddell, Y. Terazono, T. A. Moore, A. L. Moore, D. Gust, J. Phys. Chem. B 2013, 117, 11299. M. T. Huynh, S. J. Mora, M. Villalba, M. E. Tejeda-Ferrari, P. A. Liddell, B. R. Cherry, A.-L. Teillout, C. W. Machan, C. P. Kubiak, D. Gust, T. A. Moore, S. Hammes-Schiffer, A. L. Moore, ACS Cent. Sci. 2017, 3, 372. M. W. Kanan, D. G. Nocera, Science (80-. ). 2008, 321, 1072. C. Liu, B. C. Colón, M. Ziesack, P. A. Silver, D. G. Nocera, Science 2016, 352, 1210. L. Hammarström, L. Sun, B. Åkermark, S. Styring, Biochim. Biophys. Acta - Bioenerg. 1998, 1365, 193. M.-T. Zhang, T. Irebo, O. Johansson, L. Hammarström, J. Am. Chem. Soc. 2011, 133, 13224. F. D’Souza, C. A. Wijesinghe, M. E. El-Khouly, J. Hudson, M. Niemi, H. Lemmetyinen, N. V. Tkachenko, M. E. Zandler, S. Fukuzumi, Phys. Chem. Chem. Phys. 2011, 13, 18168. S.-H. Lee, A. G. Larsen, K. Ohkubo, Z.-L. Cai, J. R. Reimers, S. Fukuzumi, M. J. Crossley, Chem. Sci. 2012, 3, 257. A. Molina-Ontoria, M. Wielopolski, J. Gebhardt, A. Gouloumis, T. Clark, D. M. Guldi, N. Martín, J. Am. Chem. Soc. 2011, 133, 2370. B. M. Illescas, J. Santos, M. Wielopolski, C. M. Atienza, N. Martín, D. M. Guldi, Chem. Commun. 2009, 5374.

31 32 33

34 35

36

37 38 39 40 41 42 43 44

45 46 47 48 49 50 51 52 53

54

55

56 57

58

59 60 61

S. Schlundt, G. Kuzmanich, F. Spänig, G. De Miguel Rojas, C. Kovacs, M. A. Garcia-Garibay, D. M. Guldi, A. Hirsch, Chem. A Eur. J. 2009, 15, 12223. G. N. Lim, E. Maligaspe, M. E. Zandler, F. D’Souza, Chem. - A Eur. J. 2014, 20, 17089. C. B. KC, G. N. Lim, F. D’Souza, Chem. - An Asian J. 2016, 11, 1246. M. R. Wasielewski, S. M. Dyar, A. L. Smeigh, S. D. Karlen, R. M. Young, Chem. Phys. Lett. 2015, 629, 23. M. Wolffs, F. J. M. Hoeben, E. H. A. Beckers, A. P. H. J. Schenning, E. W. Meijer, J. Am. Chem. Soc. 2005, 127, 13484. J. L. Bahr, D. Kuciauskas, P. A. Liddell, A. L. Moore, T. A. Moore, D. Gust, Photochem. Photobiol. 2000, 72, 598. P. A. Liddell, G. Kodis, L. de la Garza, J. L. Bahr, A. L. Moore, T. A. Moore, D. Gust, Helv. Chim. Acta 2001, 84, 2765. S.-C. Hung, S. Lin, A. N. Macpherson, J. M. DeGraziano, P. K. Kerrigan, P. A. Liddell, A. L. Moore, T. A. Moore, D. Gust, J. Photochem. Photobiol. A Chem. 1994, 77, 207. J. Henderson, S. D. Glover, B. J. Lear, D. Walker, J. R. Winkler, H. B. Gray, C. P. Kubiak, J. Phys. Chem. B 2015, 119, 7473. G. Kodis, P. A. Liddell, L. de la Garza, P. C. Clausen, J. S. Lindsey, A. L. Moore, T. A. Moore, D. Gust, J. Phys. Chem. A 2002, 106, 2036. S. L. Gould, G. Kodis, R. E. Palacios, L. de la Garza, A. Brune, D. Gust, T. A. Moore, A. L. Moore, J. Phys. Chem. B 2004, 108, 10566. P. K. Poddutoori, L. P. Bregles, G. N. Lim, P. Boland, R. G. Kerr, F. D’Souza, Inorg. Chem. 2015, 54, 8482. C. Villegas, J. L. Delgado, P.-A. Bouit, B. Grimm, W. Seitz, N. Martín, D. M. Guldi, Chem. Sci. 2011, 2, 1677. C. Villegas, M. Wolf, D. Joly, J. L. Delgado, D. M. Guldi, N. Martín, Org. Lett. 2015, 17, 5056. F. N. Diederich, Pure Appl. Chem. 1997, 69, 395. J. Averdung, G. Torres-Garcia, H. Luftmann, I. Schlachter, J. Mattay, Fuller. Sci. Technol. 1996, 4, 633. M. U. Winters, J. Kärnbratt, M. Eng, C. J. Wilson, H. L. Anderson, B. Albinsson, J. Phys. Chem. C 2007, 111, 7192. L. Y. Chiang, R. B. Upasani, J. W. Swirczewski, J. Am. Chem. Soc. 1992, 114, 10154. Fullerenes: From Synthesis to Optoelectronic Properties, ed. by Dirk M. Guldi, Nazario Martin, Springer Netherlands, Dordrecht, 2002. Fullerenes, ed. by Fernando Langa De La Puente, Jean-Francois Nierengarten, Royal Society of Chemistry, Cambridge, 2011. N. Martín, Chem. Commun. 2006, 2093. D. M. Guldi, M. Prato, Acc. Chem. Res. 2000, 33, 695. M. M. Waskasi, G. Kodis, A. L. Moore, T. A. Moore, D. Gust, D. V. Matyushov, J. Am. Chem. Soc. 2016, 138, 9251. M. R. Wasielewski, Chem. Rev. 1992, 92, 435. D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2001, 34, 40. G. Accorsi, N. Armaroli, J. Phys. Chem. C 2010, 114, 1385. D. M. Guldi, G. M. A. Rahman, V. Sgobba, C. Ehli, Chem. Soc. Rev. 2006, 35, 471. T. E. Shubina, D. I. Sharapa, C. Schubert, D. Zahn, M. Halik, P. A. Keller, S. G. Pyne, S. Jennepalli, D. M. Guldi, T. Clark, J. Am. Chem. Soc. 2014, 136, 10890. I. López-Duarte, M. V. Martínez-Díaz, E. Schwartz, M. Koepf, P. H. J. Kouwer, A. E. Rowan, R. J. M. Nolte, T. Torres, Chempluschem 2012, 77, 700. The values given here represent the average for each oxidation or reduction process across the families of ZnPc or ZnP conjugates, respectively. The ZnPc reductions at -1.39 and -1.87 V overlap, however, with those of C60 and C70 between -1.31 and -1.89 V. Please note that the ZnP reductions overlap, on one hand, with the third and fourth reduction of C60 and, on the other hand, with the fourth reduction of C70. M. Quintiliani, A. Kahnt, T. Wölfle, W. Hieringer, P. Vázquez, A. Görling, D. M. Guldi, T. Torres, Chem. - A Eur. J. 2008, 14, 3765. M. E. El-Khouly, O. Ito, P. M. Smith, F. D’Souza, J. Photochem. Photobiol. C Photochem. Rev. 2004, 5, 79. C. a Reed, R. D. Bolskar, Chem. Rev. 2000, 100, 1075. The one-electron reduced state of C70 features a NIR marker at



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

62

63 64 65 66

67 68

around 1300 nm, whose detection would corroborate the assignment of a D·+-C60-C70·- charge-separated state. Firstly, its molar absorbance is, however, an order of magnitude weaker than that of the NIR marker of the one-electron reduced state of C60 at 1020 nm (see Bolskar and Reed, Chem. Rev. 2000, 100, 1075). Secondly, the charge-shift quantum yields are less than unity. Thirdly, our instrumental setup has a rather poor signalto-noise ratio in this part of the NIR spectrum. Altogether, the aformetioned renders the detection of the 1300 nm feature of the one-electron reduced state of C70 impossible. Notably, multiple degrees of freedom of rotation are present in each system, therefore intramolecular conformations of the conjugates induce uncertainty of intramolecular distances between donor and the two different acceptors. Molecular dynamics of the ground state and excited state species (and possible differences) are unknown. Consequences for the charge separation, charge shift, and recombination steps are conceivable, but unattributable. I. H. M. Van Stokkum, D. S. Larsen, R. Van Grondelle, Biochim. Biophys. Acta - Bioenerg. 2004, 1657, 82. J. J. Snellenburg, S. P. Laptenok, R. Seger, K. M. Mullen, I. H. M. van Stokkum, J. Stat. Softw. 2012, 49. K. M. Mullen, I. H. M. van Stokkum, J. Stat. Softw. 2007, 18. The global fits regarding the ns-µs time range do not account for the charge separation process, which surpasses the time resolution of the experimental setup. Charge separation dynamics were derived from sub-ps to 6 ns measurements with 150 fs resolution. T. Kesti, N. Tkachenko, H. Yamada, H. Imahori, S. Fukuzumi, H. Lemmetyinen, Photochem. Photobiol. Sci. 2003, 2, 251. H. Z. Yu, J. S. Baskin, A. H. Zewail, J. Phys. Chem. A 2002,

69 70 71

72 73

74

75 76 77 78

Page 10 of 11

106, 9845. A. N. Okhrimenko, A. V. Gusev, M. A. J. Rodgers, J. Phys. Chem. A 2005, 109, 7653. D. Villamaina, M. M. A. Kelson, S. V. Bhosale, E. Vauthey, Phys. Chem. Chem. Phys. 2014, 16, 5188. In contrast with the ZnPc systems, the triplet state of ZnP is exclusively populated during the decay of the S1 excited state, due to the relative rates of the competing processes ISC, fluorescence, and charge separation. Population of the ZnP triplet from ZnP•+-C60•--C70 or ZnP•+-C60-C70•- would be an endergonic process and is therefore ruled out. SAS were divided by their respective maximum absorption to clarify differences and similarities, respectively. Independent of the magnetic field strength, ZnP•+-C60-C70•decays with 200-300 ns, before only the ZnP triplet excited state, which is formed by ZnP’s intrinsic ISC in competition with the initial CS, remains for tens of µs. The only rationale for the fast and slow decay of the ZnP•+-C60•-C70 charge separated states in 5 and 3, respectively, via either the indirect or the direct routes is based on structural flexibility. M. J. S. Dewar, W. Thiel, J. Am. Chem. Soc. 1977, 99, 4899. G. Rauhut, T. Clark, T. Steinke, J. Am. Chem. Soc. 1993, 115, 9174. P. Gedeck, S. Schneider, J. Photochem. Photobiol. A Chem. 1997, 105, 165. T. Clark, A. Alex, B. Beck, F. Burkhardt, J. Chandrasekhar, P. Gedeck, A. Horn, M. Hutter, B. Martin, P. O. Dral, G. Rauhut, W. Sauer, T. Schindler, T. Steinke, VAMP 11.0, Erlangen, 2011.


Page 11 of 11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents


11

Mediating Reductive Charge Shift Reactions in Electron Transport

Recommend Documents