Rigid, Branched Porphyrin Antennas - American Chemical Society

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Rigid Dendritic-Shaped Porphyrin Antennas: Controlling Cascades of Unidirectional Energy Funneling and Charge Trans-fer Maximilian Wolf, Astrid Herrmann, Andreas Hirsch, and Dirk M. Guldi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b04589 • Publication Date (Web): 27 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017

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Journal of the American Chemical Society

Rigid, Branched Porphyrin Antennas: Control over Cascades of Unidirectional Energy Funneling and Charge Transfer Maximilian Wolf[a], Astrid Herrmann[b], Andreas Hirsch*[b], and Dirk M. Guldi*[a] [a] Friedrich-Alexander-University Erlangen-Nuremberg, Department of Chemistry and Pharmacy, Egerlandstr. 3, 91058 Erlangen, Germany [b] Friedrich-Alexander-University Erlangen-Nuremberg, Department of Chemistry and Pharmacy, Henkestr. 42, 91054 Erlangen, Germany ABSTRACT: Porphyrin arrays, which consist of three peripheral ZnPs and a central H2P – all rigidly linked to each other –serve as light harvesting antennas as well as electron donors and are flexibly coupled to an electron accepting C60 to realize the unidirectional flow of i) excited state energy from the ZnPs at the periphery to the H2P, ii) electrons to C60, and iii) holes to H2P and, subsequently, to ZnP. Dynamics following photoexcitation are elucidated by time resolved transient absorption measurements on the femto-, pico, nano-, and microsecond time scales and are examined by multiwavelength as well as target analyses. Hereby, full control over the charge shift between H2P and ZnP to convert the (ZnP)3-H2P•+-C60•- charge separated state into (ZnP)3•+-H2P-C60•charge separated state is enabled by the solvent polarity: It is deactivated / switched-off in apolar toluene, while in polar benzonitrile it is activated / switched-on. Activating / switching impacts the recovery of the ground state via charge recombination rates, which differ by up to two orders of magnitude. All charge separated states lead to the repopulation of the ground state with dynamics that are placed in the inverted region of the Marcus parabola.

Introduction. The natural photosynthetic apparatus serves as an inspiration for the realization of light harvesting and energy conversion schemes. In essence, energy is harvested quantitatively by an elusive arrangement of chromophores, funneled unidirectionally across numerous pigments, and stored in the form of oxidized chlorophyll and reduced quinones. The sophistication of photosynthesis is the driving force to mimic it in the form of artificial systems. Promising approaches are built around a plethora of molecular electron donor-acceptor systems. To this end, electron donating tetrapyrroles, bodipys, triphenylamines, and thiophenes as well as electron accepting rylenes and quinones have been used extensively next to porphyrins and fullerenes, respectively.1–16 The design, synthesis, and investigations of electron donoracceptor systems based (metallo)porphyrins and fullerenes have trend set the field of photosynthetic mimics. Porphyrins, for example, have evolved as powerful surrogates for chlorophylls. They combine panchromatic absorption through the visible part of the solar spectrum with energetically high lying excited states and ease of oxidation. Zinc porphyrins (ZnP) and free base porphyrins (H2P) are the perfect light harvester / electron donor. Fullerenes are readily reduced and feature small reorganization energies in electron transfer reactions. They have emerged as the electron acceptor of choice not only in molecular electron donor-acceptor systems, but also in organic photovoltaics.

charge recombination. In most porphyrin / fullerene electron donor-acceptor systems, kinetically long-lived and energetically high-lying charge separated states are formed in high quantum yields.17–20 It is typically the first singlet excited state of the porphyrin, from which electron transfer evolves, while only a few examples are known, in which the second singlet excited states powers the electron transfer.21–25 Next to apolar examples of electron donor-acceptor systems, also watersoluble26–30 and dendritic systems are reported.31,32 Key for the design of electron donor-acceptor systems – featuring more than just a single porphyrin and fullerene – is to create energy and redox gradient and to control the unidirectional flow of excited state energies and charges. Linearshaped representatives of a multicomponent system featuring several, but different porphyrins and a fullerene have been reported: ZnP-H2P-C60, Fc-ZnP-H2P-C60, etc.33–39 Star-shaped examples of four ZnPs in combination with one H2P have been realized and showed enhanced antenna effects: (ZnP)3-ZnPH2P-C60.40–42 Nevertheless, to further boost the light harvesting and to channel the absorbed energy to a single fullerene, dendritic-shaped multiporphyrin arrays have evolved as the optimum choice. In a first contribution, however, only branched arrays of ZnPs are used: [(ZnP)3]2-ZnP-C60.43

Integrating porphyrins as well as fullerenes into electron donor-acceptor systems has enabled a sound mechanistic understanding of light harvesting, transduction of excited state energy, charge separation, charge shift, charge transport, and charge recombination. Of particular relevance is the fact that a fast charge separation is typically combined with a slow

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substantial configurational rearrangements, which place ZnP and C60 in close proximity to each other. The charge separated state lifetimes are as short as 100 ps in polar benzonitrile and 500 ps in non-polar toluene. In the current contribution, we have overcome the deficiencies stemming from flexible linkages between the different porphyrins. We have replaced them with rigid linkages. A rigid triple bond binding motif locks the porphyrin in the (ZnP)3H2P-C60 conjugates into spatially defined positions relative to each other, similar to what has been documented for the natural photosynthetic apparatus. In turn, it defines the settings for the unidirectional transduction of energy and charges – along covalent bonds rather than hydrogen-bonds – and assists in affording long-lived charge separated states. In particular, we designed and synthesized a family of (ZnP)3-H2P-C60 electron donor-acceptor conjugates featuring ethyl chains (1), hexadecyl chains (2), first-generation Newkome-type dendrons (3), and second-generation Newkome-type dendrons (4) – Scheme 1. The design for the different (ZnP)3-H2P-C60 conjugates and a possible flux of energy and charges are summarized in Figure 1. Figure 1. Design of (ZnP)3-H2P-C60 conjugates, in which three peripheral ZnPs and a central H2P are, on one hand, rigidly linked to each other and, on the other hand, flexibly linked to C60. The cascade of i) energy (En), ii) electron (e-), and iii) hole (h+) transfers is highlighted.

O

O

O

N

N

1

O

O

N

O

R=

R= O

O

N

O

6

O

O

O

NH N

N

N HN

3

R

4 O

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R=

O

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R=

O

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O O

O

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N

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O O

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N

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O NH

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O N H

O

O O

O

NH

N H

O O

O

O

O O

O

Zn O

O O

O

O

O O

O

O

Scheme 1. Structures of ZnP3-H2P-C60 conjugates 1-4.

O

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I

N Zn

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O O

O

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O O

O

We have recently reported on a perfectly branched, dendriticshaped multiporphyrin array: (ZnP)3-H2P-C60. Flexible linkages were employed to connect the individual constituents, that is, three ZnPs, one H2P, and one fullerene. A unidirectional energy transfer and a fast charge separation afforded, nevertheless, (ZnP)3•+-H2P-C60•- charge separated state.32 In light of the structural flexibility, charge separation and charge recombination proceed exclusively through-space rather than through-bond. Through-space interactions are difficult to be controlled. For example, a nearly microsecond lived charge separated state was only observed in highly viscous media such as Triton X-100 or an Agar matrix. Investigations in organic media such as toluene, THF, and benzonitrile trigger

O

2

Zn N

N N Zn N N

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O

O

O

O

N N Zn N N

O O

7 O

O O

N N

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O

N Zn N

O

O O

O

O

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N Zn N N

NH N

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N HN

O O

I

NH N

N

N Zn N N

NH N

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O

1 O

O

O O

I N HN

O

5

O

N N Zn N N

O

O

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O

O

O

Scheme 2. Outline of the synthetic route towards (ZnP)3-H2P-C60 conjugate 1.

Results and Discussion


N N Zn N N

O

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O O O

N HN

9 8

I

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O

O O

O

Zinc porphyrin 8 was prepared following published procedures.49 The product was purified by column chromatography with CH2Cl2 and toluene/THF (30:1 v/v) as eluents. Free base porphyrin 5 and zinc porphyrin 8 were coupled in a threefold copper-free Sonogashira reaction furnishing (ZnP)3H2P array 9 – Scheme S2 of the SI. Here, copper-free reaction conditions were essential to avoid metalation of the free base porphyrin, which would result in a reversion of the internal redox gradient. Pd2(dba)3 x CHCl3 and AsPh3 served as catalyst and co-catalyst, respectively. (ZnP)3-H2P array 9 was transformed into the corresponding alcohol 10 using lithium aluminum hydride (LAH) in THF at 15 °C, purified by column chromatography with toluene/THF (15:1 v/v), and multiple precipitations from n-pentane. The required porphyrin ethyl malonate 11 was synthesized in a substitution reaction from porphyrin alcohol 10, commercially available ethyl malonyl chloride, and pyridine in cold CH2Cl2. The crude product was purified by column chromatography with toluene/THF (10:1 v/v) – Scheme S3 of the SI In the final step, cyclopropanation of C60 was carried out under modified Bingel reaction conditions. In particular, C60 was dissolved in anhydrous toluene under the exclusion of light. Porphyrin malonate 11, CBr4, and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were added successively – Scheme S4 of the SI. The crude product was purified by column chromatography with toluene/THF (15:1 v/v) and was obtained in yields of up to 28%. 1 was fully characterized by 1HNMR-, 13C-NMR-, IR-, UV/Vis spectroscopy, and mass spectrometry (MALDI-TOF and ESI). Although aggregation interfered with the interpretation of the NMR spectra, high resolution mass spectra support the structural assignments. Synthesis of porphyrin-fullerene conjugates 2-4. Detailed descriptions on the syntheses of conjugates 2-4 (Scheme S17 of the SI), featuring hexadecyl, dendritic G1, and dendritic G2 functionalities, respectively, are given in Schemes S1-S16 of the SI. Steady state absorption and emission. The absorption spectra of (ZnP)3-H2P array 11 and (ZnP)3-H2P-C60 conjugates 1-4 are best described as the superimposition of the individual constituents. In 11 and 1-4, ZnP and H2P give rise to one, very intense Soret-band absorption around 430 nm as well as four, weak Q-band absorptions at 515, 555, 595, and 650 nm with an intensity ratio of approximately 5:10:5:1. Minor redshifts stem from the influence of the diphenylacetylene bridges. In 1-4, the same ZnP and H2P absorptions are seen next to a weak absorbance in the 300 to 400 nm region, which relate

2.0E6

H2P

1.6E6

ZnP 1 11

-1

We started our synthetic part with the synthesis of a porphyrin-fullerene conjugate featuring an ethyl end group. To this end, free base porphyrin 5 – Scheme S1 of the SI –was synthesized via a statistical condensation of 4-iodobenzaldehyde 6, methyl-4-formylbenzoate 7, and pyrrole with a Lewis acid catalyst, following the reaction conditions developed by Lindsey et al.44–48 Oxidation of the porphyrin was achieved using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The product was purified by column chromatography with toluene/CH2Cl2 (20:1 v/v) and precipitated from n-pentane.

to that of C60. As far as the extinction coefficients of 1-4 are concerned, they are close to the linear combinations of the constituent’ extinction coefficients, leading to values larger than 1.5 x 106 Mcm-1 in, for example, the Soret-band region. In summary, the four porphyrins lack in the (ZnP)3-H2P array 11 and in the (ZnP)3-H2P-C60 conjugates 1-4 any appreciable ground-state interactions and act as individual porphyrin constituents – Figure 2. Please note that the different C60 functionalization in 1-4 lacks any notable impact on any spectroscopic features. Consequently, the data presentation is in the following concentrated on 1.

-1

Synthesis of porphyrin-fullerene conjugate 1. A brief outline of the synthetic route is shown in Scheme 1, while detailed synthetic schemes are gathered in the SI.

ε / l mol cm

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


1.2E6

8.0E5

4.0E5

0.0 300

400

500

600

700

500

600

700

Wavelength / nm

Figure 2. Absorption spectra of ZnP reference, H2P reference, (ZnP)3-H2P array 11, and (ZnP)3-H2P-C60 conjugate 1 at room temperature in THF.

Turning to steady state fluorescence, a comparison of (ZnP)3H2P array 11 with a 3:1 mixture of a ZnP reference and a H2P reference documents a highly efficient and unidirectional energy transfer from the peripheral ZnPs to the central H2P – Figure 3. In 11, the strongly quenched, short-wavelength ZnP fluorescence evolves next to the unquenched, long-wavelength H2P fluorescence. In the 3:1 mixture of ZnP and H2P, both porphyrins fluoresce with the same intensity. Taking the relative intensities of the short-wavelength ZnP fluorescence, the energy transfer efficiency is estimated to be > 95% with an underlying rate constant of > 2 x 1010 s-1. Importantly, the overall H2P fluorescence quantum yield of the ZnP3-H2P array 11 (0.09) is close to that of H2P (0.11).

1.0 ZnP:H2P 3:1

norm. Emission / a.u.

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1 11

0.8

0.6

0.4

0.2

0.0 550

600

650

700

750

800

Wavelength / nm

Figure 3. Normalized fluorescence spectra of a 3:1 mixture of ZnP and H2P, (ZnP)3-H2P array 11, and (ZnP)3-H2P-C60 conjugate 1 upon 550 nm excitation, demonstrating efficient ZnP to H2P



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energy transfer and charge separation between photoexcited H2P and C60.

In the (ZnP)3-H2P-C60 conjugates 1-4, the presence of C60 induces a nearly quantitative quenching of the porphyrin fluorescence with ZnP and H2P fluorescence quantum yields of 0.007 and 0.005, respectively, in non-polar toluene and polar benzonitrile. This finding prompts to an additional excited state deactivation in 1-4 in the form of an energy or electron transfer to C60. Thermodynamic driving forces. Osteryoung-square wave voltammetric measurements were performed with (ZnP)3-H2P array 11 as well as (ZnP)3-H2P-C60 conjugates 1-4 to determine the redox properties and the energies of the charge separated states. Deoxygenated ortho-dichlorobenzene (o-DCB) and 0.2 M tetra-butyl ammonium perchlorate (TBAClO4) were used as solvent and electrolyte, respectively. For (ZnP)3-H2P array 11, the square wave voltammograms reveal two oxidations as well as two reductions within the potential window between -2.0 and 0.6 V vs. Fc/Fc+. To this end, processes at -1.82 and +0.17 V are attributed to the first reduction and oxidation of the three peripheral ZnPs, respectively, while those at -1.52 and +0.44 V correspond to the first reduction and oxidation of the central H2P, respectively. For (ZnP)3-H2P-C60 conjugate 1, all porphyrin-centered reductions and oxidations are seen to shift slightly to more positive potentials by about 0.1 V. In addition, the first reduction of C60 appears at -1.10 V vs Fc/Fc+. Notable is, however, that the second, third, and fourth reductions of C60 are masked by the ZnP- and H2P-centered reductions. By combining the results from electrochemistry with those from steady state spectroscopy we estimated the thermodynamic driving forces in o-DCB for energy transfer to afford the ZnP singlet excited state (-0.15 eV) as well as charge separation to afford the (ZnP)3-H2P•+-C60•- charge separated state (0.25 eV), charge shift to afford the (ZnP)3•+-H2P-C60•- charge separated state (-0.27 eV), and charge recombination to afford the ground state (-1.38 eV). Please note the lack of any appreciable differences in the driving forces for 1, 2, 3, and 4. Transient absorption spectroscopy. Considering a ZnP to H2P ratio of 3:1 and different Soret-band absorbances, we estimate that 430 nm photoexcitation of either (ZnP)3-H2P array 11 or (ZnP)3-H2P-C60 conjugates 1-4 generates 90% ZnP-centered and 10% H2P-centered excited states. Multiple excitations of single porphyrin arrays are ruled out by the low pulse energies of 90%). The third species is the 10 ns lasting H2P first singlet excited state. An intense additional bleaching at 520 nm is seen in the corresponding SAS. Its dominating fate is intersystem crossing and yields the long-lived H2P triplet excited state, which is the fourth and final species. It decays within tens of µs and features a slightly red-shifted maximum at 460 nm and a new shoulder at 780 nm. Next, we turned to pump probe experiments with the (ZnP)3H2P-C60 conjugates 1-4 in toluene and benzonitrile. In the following, we have focused the discussion of our experimental findings on 1 and on developing a unified model applicable for all (ZnP)3-H2P-C60 conjugates, that is, 1-4. A summary of the results is given in Tables 1 and 2.

In the differential absorption spectra of (ZnP)3-H2P array 11 we note upon 430 nm photoexcitation in toluene maxima at 450, 535, 575, 620, and 690 nm as well as minima at 425, 520,


0.05

time delay / ps 1.5 20 200 1000 5000

0.04

∆ 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


0.03 0.003

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10

Time delay / µs

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0.006636 0.005545 0.004455

1

0.003364 0.002273

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0.001182 9.091E-05 -1.000E-03

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1E-3 0.000

1E-4

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Figure 4. Differential absorption changes (visible and nearinfrared) obtained upon femtosecond pump probe experiments (430 nm) of 1 in argon saturated toluene at room temperature with several time delays between 1.5 and 5000 ps – see Figure Legend for details. Please note the detector change and the laser fundamental at 775 nm.

In toluene, singlet excited state features of ZnP and H2P are discernible immediately after 430 nm excitation similar to what has been seen with (ZnP)3-H2P array 11 – Figure 4.50 It is on the time scale of 15-20 ps that a unidirectional energy transfer induces deactivation of the ZnP singlet excited state and activation of the H2P singlet excited state. The rate constant for the energy transfer is 5 x 1010 s-1. Unlike in (ZnP)3H2P array 11, the H2P singlet excited state decay is in 1 faster (3.1 x 109 s-1) than the intersystem crossing (9 x 107 s-1). To this end, maxima at 450, 535, 575, 620, and 690 nm as well as minima at 520, 555, 595, and 650 nm emerge in the visible range accompanied by a maximum in the 1020 – 1040 nm in the near-infrared range. Importantly, the features in the visible range are a perfect match of the fingerprints known for the one-electron oxidized form of H2P, while those in the nearinfrared range resemble those of the one-electron reduced form of C60. In other words, the H2P singlet excited state transforms via an intramolecular charge separation into the (ZnP)3-H2P•+-C60•- charge separated state. The fate of the (ZnP)3-H2P•+-C60•- charge separated state is charge recombination. Products of the charge recombination are in toluene the C60 and the H2P triplet excited states with their fingerprint absorptions at 700 and 440-500 / 780 nm, respectively. Overall, the decay of (ZnP)3-H2P•+-C60•- charge separated state coincides with the growth of the two triplet excited states with an underlying rate constant of 3 x 108 s-1. Complementary nanosecond experiments further confirmed the aforementioned conclusions – Figure 5. In particular, the C60 and the H2P triplet excited states are seen to evolve from the charge recombination of the (ZnP)3-H2P•+-C60•- charge separated state. Notable is, however, that the C60 triplet excited state is 0.2 eV higher in energy that the corresponding H2P triplet excited state. This 0.2 eV driving force powers the 3 x 108 s-1 triplet-triplet energy transfer from C60 to H2P and leads to a 40% intensification of the H2P triplet excited state absorption features at 440-500 / 760 nm. From this point on, it is only the H2P triplet excited state features that decay slowly with 3 x 105 s-1 to reinstate the ground state.

Figure 5. Differential absorption changes (visible and nearinfrared) obtained upon nanosecond pump probe experiments (430 nm) of 1 in argon saturated toluene at room temperature with time delays between 100 ps and 400 µs. Please note the retector change at 915 nm and the probe fundamental at around 1064 nm.

Target analyses of the results on the femto- to nanosecond time scales (see SI for SAS and fits – Figures S8-S11) required a “6 species” model57 as sketched in Figure 6. 1 **(ZnP)

3-(H2P)-(C 60)

i) 1 *(ZnP)

3-(H2P)-(C 60)

i i) i)

(ZnP) 3-1 *(H2P)-(C 60)

iii)

(ZnP) 3-(H2P) •+-(C60)•-

iv) iv)

(ZnP) 3-(H2P)- 3*(C60) v) (ZnP) 3-3*(H2P)-(C 60)

vi)

(ZnP) 3-(H 2P)-(C 60)

Figure 6. Energy diagram summarizing the excited state interactions in conjugates 1-4 in toluene: i) an interconversion (IC) from higher (second) singlet excited states of ZnP and H2P to yield the lowest (first) singlet excited states of ZnP and H2P, ii) a singlet excited state energy transfer (EnT) to yield the H2P singlet excited state, iii) a charge separation (CS) to yield the (ZnP)3-H2P•+-C60•charge separated state, iv) a charge recombination (CR) to populate the C60 / H2P triplet excited states, v) a triplet excited state energy transfer (EnT) to generate the H2P triplet excited states, and vi) a recovery of the ground state (GSR).

In Figure 7, the populations and the decays of the all species are summarized for toluene as solvent. Like in (ZnP)3-H2P array 11, deactivation of the higher (second) singlet excited states and population of the lowest (first) singlet excited states of ZnP and H2P go hand-in-hand. Correspondingly, these are the first and second species. These interconversions proceed with about 1 ps and are followed by the ZnP to H2P energy transfer. The energy transfer product – third species – transforms within about 300 ps via charge separation into the (ZnP)3-H2P•+-C60•- charge separated state – fourth species – rather than via intersystem crossing into the H2P triplet excited state. 3.3 ns is the lifetime of the (ZnP)3-H2P•+-C60•- charge separated state. However, neither the (ZnP)3•+-H2P-C60•charge separated state nor the ground state is formed in the deactivation of the (ZnP)3-H2P•+-C60•- charge separated state. Instead, the triplet excited states of C60 and H2P evolve as fifth


Journal of the American Chemical Society and sixth species, respectively. These interconvert to those of the H2P triplet excited state exclusively. Target analyses of the complementary nanosecond results reveal that the species after 8 ns is the H2P triplet excited state, which then decays to the ground state with approximately 30 µs. A summary of the corresponding lifetimes of the associated states / species is given in Table 1. 1**

relative concentration

1.0

(ZnP)3-(H2P)-(C60)

1*

(ZnP)3-(H2P)-(C60)

(ZnP)3-1*(H2P)-(C60)

0.8

(ZnP)3-(H2P).+-(C60).(ZnP)3-(H2P)-3*(C60)

0.6

3*

(ZnP)3- (H2P)-(C60)

0.4 0.2 0.0 -13 -12 -11 -10 -9 10 10 10 10 10

-8 10 t/s

-7

10

10

-6

-5

10

-4

10

Figure 7. Relative population of the transient species used for the target analysis model – see Figure Legend for details. Please note that in the plot results from analyses of both fs-ns and ns-µs timescale experiments are combined. Table 1. Lifetimes of species / processes involved in the excited state deactivation of conjugates 1-4 upon 430 nm excitation in toluene derived from target analyses.a τ

1

2

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4

ICZnP (i)a

0.87 ps

0.80 ps

0.87 ps

0.81 ps

EnTZnP-H2P (ii)a

14 ps

13 ps

12 ps

18 ps

CS (iii)a

325 ps

400 ps

272 ps

325 ps

ICH2P (i)a

a

CR (iv)

2.3 ns

2.8 ns

2.5 ns

2.5 ns

EnTC60-H2P (v)a

8.4 ns

7.9 ns

5.9 ns

5.4 ns

GSRb (vi)a

28 µs

29 µs

31 µs

32 µs

a

b

3

the processes are sketched in Figure 6; the H2P lifetimes are limited by triplet-triplet energy transfer with molecular oxygen, triplet-triplet annihilation, and deactivation with ground state H2P.

0.05

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∆ OD / a.u.

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Legend for details. Please note the detector change and the laser fundamental at 775 nm.

In polar benzonitrile, minima at 520 and 650 nm as well as at 555 and 595 nm evolve in the differential absorption spectra for 1 with the conclusion of the 430 nm photoexcitation – Figure 8. These features attest the formation of the H2P and ZnP singlet excited states, respectively.50 Like in toluene, the ZnP singlet excited state populates the H2P singlet excited state via an energy transfer. But, it reacts at an apparent rate as fast as its formation with 5 x 1010 s-1. In essence, the ZnP singlet excited state features convert directly to the oneelectron oxidized form of H2P and the one-electron reduced form of C60. These are confirmed through fingerprint absorption maxima at 450, 535, 575, 620, and 690 nm and minima at 520, 555, 595, and 650 nm as well as a maximum in the 1020 – 1040 nm region. Next, the absorptions of the one-electron oxidized form of H2P in the visible range are replaced by those of ZnP, with a fingerprint absorption in the 400 to 420 nm region (see Figure S12, SI).25,58 At first glance, we postulate a transformation of the (ZnP)3-H2P•+-C60•- charge separated state into the (ZnP)3•+-H2P-C60•- charge separated state. A closer look reveals, however, that the growth of the 415 nm absorption is biexponential in nature – Figures S13 and S14 of the SI: We infer two contributions to the population of the (ZnP)3•+H2P-C60•- charge separated state. Firstly, it is a charge shift, which furnishes the formation of the (ZnP)3•+-H2P-C60•- charge separated state with 5 x 109 s-1. Secondly, it is a charge separation between the singlet excited state of ZnP and C60 with ~4 x 1010 s-1. Charge separation between the singlet excited state of ZnP and C60 is only, however, feasible when two important criteria are met. The first requirement is that ZnP and C60 are in close proximity to each other – vide infra. This is enabled by the structural flexibility of the C60 tether. A second requirement is a sufficient thermodynamic driving force. As a matter of fact the driving force for the charge separation between ZnP and C60 is appreciably larger than that for the ZnP to H2P energy transfer. For example, in conformations, where C60 is placed at similar proximity relative to ZnP, charge separation was shown to be as fast as 2 x 1010 s-1and to compete effectively with the energy transfer between ZnP and H2P. 59,60 Throughout the charge shift, the signature absorption stemming from the one-electron reduced form of C60 at 1030 nm reduces slightly in intensity. From an estimated 20% change we conclude that the charge shift with 5 x 109 s-1 outperforms the charge recombination with 1 x 109 s-1. The resulting (ZnP)3•+-H2P-C60•- charge separated state decays in a biphasic manner with rate constants that differ by over one order of magnitude, namely 1.2 x 108 versus 6.5 x 106 s-1 – Figure S9 of the SI. The implication of our findings is that two distinctly different (ZnP)3•+-H2P-C60•- charge separated states are present. When considering the central H2P, the two ZnPs, which are in close proximity, cis-position relative to C60, are likely to be involved in a fast, through-space charge recombination. In contrast, the ZnP in distant proximity, trans-position is subject to a slow, through-bond charge recombination.

Wavelength / nm

Figure 8. Differential absorption changes (visible and nearinfrared) obtained upon femtosecond pump probe experiments (430 nm) of 1 in argon saturated benzonitrile at room temperature with several time delays between 1.5 and 5000 ps – see Figure


0.01500 0.01355

100

0.01209 0.01064

10

Time delay / µs

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


0.009182 0.007727 0.006273

1

0.004818 0.003364

0.1

0.001909 4.545E-04 -1.000E-03

0.01 1E-3 1E-4 400

600

800

1000

1200

Wavelength / nm

Figure 9. Differential absorption changes (visible and nearinfrared) obtained upon nanosecond pump probe experiments (430 nm) of 1 in argon saturated benzonitrile at room temperature with time delays between 100 ps and 400 µs. Please note the detector change at 915 nm and the probe fundamental at around 1064 nm.

Notable, a small fraction of H2P triplet excited state is present throughout the nano- and microsecond time scales – Figures 9 - 11. It is only observable after the decay of the much stronger absorptions of the (ZnP)3•+-H2P-C60•- charge separated states features has come to a full end. Despite a >90% porphyrin fluorescence quenching, a minor ISC contribution is the origin for the H2P triplet excited state.61

Once more, the first species corresponds to the higher (second) singlet excited states, which interconvert within 1 ps, and, which are superseded by the second species, namely the lower (first) singlet excited states. A competition between two deactivation pathways for the ZnP singlet excited state, which is the second species, leads to a branching. As such, the third and fourth species, which exhibit the spectral characteristics of (ZnP)3-H2P•+-C60•- and (ZnP)3•+-H2P-C60•- charge separated states, develop simultaneously within ca. 15 ps. Interestingly, the product of energy transfer, that is, the singlet excited state of H2P, is undetectable as charge separation in benzonitrile is at least as fast as energy transfer. The (ZnP)3-H2P•+-C60•charge separated state lives for only about 200 ps, as it charge shifts to afford the (ZnP)3•+-H2P-C60•- charge separated state. On the nanosecond timescale, the (ZnP)3•+-H2P-C60•- charge separated state decays with 8 and 150 ns, that is, a short- and a long-lived component as fourth and fifth species, respectively. These reveal, however, identical absorption characteristics. From this point on, only the characteristics of the long-lived H2P triplet excited state are discernable as the sixth and final species. Figure 11 summarizes the aforementioned by displaying the populations and decays of the involved species for benzonitrile. 1**

1.0

relative concentration

Page 7 of 11

The observations from multiwavelength analysis of the femtosecond-resolved experiments were again corroborated by target analyses as well as by the resulting SAS and fits – Figures S13-S16 of the SI. A “6 species” model was used to fit the experimental data – Figure 10.62

1 **(ZnP)

3-(H2P)-(C 60)

(ZnP)3-(H2P)-(C60)

(ZnP)3-(H2P)-(C60)

(ZnP)3-(H2P).+-(C60).-

0.8

.+

.-

.+

.-

(ZnP)3 -(H2P)-(C60) cis (ZnP)3 -(H2P)-(C60) trans

0.6

(ZnP)3-(H2P)3*-(C60)

0.4 0.2 0.0 10-13 10-12 10-11 10-10 10-9

10-8 t/s

10-7

10-6

10-5

10-4

Figure 11. Relative population of the transient species used for the target analysis model – see Figure Legend for details. Please note that in the plot results from analyses of both fs-ns and ns-µs timescale experiments are combined.

3-(H2P)-(C 60)

i) 1 *(ZnP)

1*

i i) i)

(ZnP)3-1 *(H2P)-(C 60) iii) (ZnP) 3-(H2P) •+-(C60)•iv)

(ZnP) 3-3*(H2P)-(C 60)

For 2-4, analogous analyses are included as Figures S17-S19 of the SI. Table 2 summarizes the lifetimes of the associated states / species based on femtosecond and nanosecond resolved pump-probe experiments in benzonitrile.

(ZnP)3•+-(H2P)-(C 60)•v)

vii)

vi)

(ZnP) 3-(H 2P)-(C 60)

Figure 10. Energy diagram summarizing the excited state interactions in conjugates 1-4 in benzonitrile: i) an interconversion (IC) from higher (second) singlet excited states of ZnP and H2P to yield the lowest (first) singlet excited states of ZnP and H2P, ii) a singlet excited state energy transfer (EnT) to yield the H2P singlet excited state, iii) a charge separation (CS) to yield the (ZnP)3H2P•+-C60•- charge separated state and part of the cis-(ZnP)3•+H2P-C60•- charge separated state, iv) a charge shift (CShift) to populate the cis- and trans-(ZnP)3•+-H2P-C60•- charge separated states, v) a charge recombination from the cis-(ZnP)3•+-H2P-C60•charge separated state to the ground state, vi) a charge recombination from the trans-(ZnP)3•+-H2P-C60•- charge separated state to the ground state, and vii) recovery of the ground state (GSR) from remaining triplet excited H2P.

Our observation of charge separation in toluene, benzonitrile, and DMF followed by charge shift in benzonitrile and DMF is sound agreement with the thermodynamic driving forces determined by means of the continuum model63 as described in the literature.64 The earlier process is exergonic in all solvents with values reaching from -0.28 in toluene to -1.31 eV in DMF. The latter is, however, only exergonic in benzonitrile with -0.25 eV and DMF with -0.26 eV. Table 2. Lifetimes of species / processes involved in the excited state deactivation of conjugates 1-4 upon 430 nm excitation in benzonitrile derived from target analysis.a τ

1

2

3

4

ICZnP (i)a

0.81 ps

0.96 ps

0.98 ps

0.53 ps

13 ps

11 ps

11 ps

14 ps

ICH2P (i)a EnTZnP-H2P (ii)a CSZnP-C60 (iii)a


Journal of the American Chemical Society CSH2P-C60 (iii)a

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

CRH2P-C60 (iv)a

268 ps

174 ps

147 ps

189 ps

CRcisZnP-C60 (v)a

8.3 ns

7.2 ns

6.6 ns

8.3 ns

CRtransZnP-C60 (vi)a

155 ns

139 ns

149 ns

157 ns

GSRb (vii)a

33 µs

90 µs

40 µs

53 µs

CShiftH2P-ZnP (iv)a

a

the processes are sketched in Figure 10; b the lifetimes are limited by triplet-triplet energy transfer with molecular oxygen, triplet-triplet annihilation, and deactivation with ground state H2P.

Insights into the nature the two different charge separated states came from transient absorption measurements conducted in a temperature range from 300 to 220 K. Both lifetimes increased upon lowering the temperature. A closer look reveals that the longer lifetime increased by a factor of 5, that is, from 130 ns at 300 K to 630 ns at 220 K, while the factor is 2 for the shorter lifetime, that is, from 7 ns at 300 K to 14 ns at 220 K. The corresponding activation barriers – Figure S20 of the SI – are different and values of 41 and 124 meV confirm the through-space and through-bond mechanisms, respectively, for the charge recombination. In the context of the earlier, the two ZnPs, which are in cis-position relative to C60, are involved and undergo charge recombination. In the context of the latter, it is the ZnP, which is in trans-position relative to C60, that is subject to charge recombination. Additional insights into the charge recombination mechanism came from nanosecond-resolved transient absorption measurements with 1 in highly polar N,N-dimethylformamide (DMF). In DMF, 1 is subject to the same fundamental excited state deactivation pathways as discussed for benzonitrile – vide supra. As such, the cis and trans (ZnP)3•+-H2P-C60•charge separated states deactivate with 5.3 and 100 ns, respectively – Figure S21 of the SI. Overall, shorter (ZnP)3•+-H2PC60•- charge separated state lifetimes in DMF than in benzonitrile are indicative for charge recombination dynamics in the inverted region of the Marcus parabola. Importantly, the weak and strong solvent dependences are in sound agreement with through-space and through-bond mechanisms, respectively, established in previous investigations. 33–39,59,60,65 Conclusions Conjugates of rigid, antenna-like (ZnP)3H2P arrays and C60 are synthesized and investigated. Particular attention is placed on the influence that the solvent polarity exerts on the pathways of excited state deactivation as investigated by means of femtosecond and nanosecond resolved transient absorption spectra. First, in reference experiments with (ZnP)3-H2P 11, the unidirectional funneling of singlet excited state energy from the peripheral ZnPs to the central H2P is corroborated. Next, in experiments with (ZnP)3-H2P-C60 1-4 the involvement of the peripheral ZnPs in charge shift processes is found to be either “switched on” or “switched” off in highly polar or non-polar environments, respectively. For example, in non-polar environments – Figure 6 – charge separation is restricted to a process between H2P and C60 and charge recombination is limited to a few nanoseconds. Deactivation from the energy-rich (ZnP)3-H2P•+-C60•- charge separated state (1.65 eV) proceeds

Page 8 of 11

stepwise via population of the lower lying triplet excited states of C60 (1.5 eV) and, subsequently, that of H2P (1.4 eV). As such, the triplet excited state of H2P is the final product of a cascade of charge and energy transfer reactions. Fundamentally different dynamics are found in highly polar solvents like benzonitrile – Figure 10: The singlet excited state of ZnP is still found to transfer most of its energy to H2P. But, it also undergoes a direct electron transfer to C60. Rate limiting for the (ZnP)3-H2P•+-C60•- charge separated state formation is the energy transfer from ZnP to H2P. Once formed, it quantitatively deactivates within a few 100 ps by means of a charge shift to form the (ZnP)3•+-H2P-C60•- charge separated state. For the latter, a short- and a long-lived species, which differ between 5-8 and 140-160 ns, are registered at room temperature. At 220 K, the corresponding lifetimes are 14 and 630 ns. A rationale for observing two different (ZnP)3•+-H2P-C60•- deactivations is given by the unique structure of the (ZnP)3-H2P-C60 conjugates. When considering the central H2P, the two ZnPs, which are in cis-position relative to C60, are at a distance of about 1 nm. They are likely to power a fast, through-space charge recombination. In contrast, the ZnP in trans-position has a 3 nm distance to C60 and drives a slow, through-bond charge recombination. Important for the through-space versus through-bond charge recombination is the rather flexible tether, which links C60 to the central H2P. As a result, the oneelectron oxidized form of ZnP and the one-electron reduced form of C60 are either separated by 1 or 3 nm, depending on the charge shift from the initial (ZnP)3-H2P•+-C60•- charge separated state to the final (ZnP)3•+-H2P-C60•- charge separated state. We are currently focusing on, for example, reversing the metallation and employing different (metallo)porphyrins in the antenna-like porphyrin arrays.

Experimental Section 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 nm wavelength were generated by a NOPA with subsequent frequency doubling; a bandpass filter with ±5 nm was 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. Temperature dependent measurements were performed with an Oxford Instruments Optistat DN2 liquid nitrogen cooled


Page 9 of 11

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cryostat fit to our transient absorption setup by home-made holders. ZnP3-H2P-C60 conjugates 1-4 were dissolved in solvents of strongly differing polarity, i.e. toluene (Tol) and benzonitrile (BN). Sample concentration was 1·10-5 M in all cases, solutions were deoxygenated by flushing with Argon (15 min). Excitation was performed with 150 fs / 430 nm laser pulses, corresponding to the Soret absorption bands (S0-S2 transitions) of both ZnP and, to a lesser extent, H2P. 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 three neck 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 o-DCB / MeCN 4: 1 as solvent.

ASSOCIATED CONTENT Supporting Information. 16 reaction schemes detailing synthesis of the investigated compounds. Synthesis experimental section. 21 supplemental figures (reference FS-TAS experiments, global target analysis results and fits, spectro-electochemistry, Arrhenius plot). AUTHOR INFORMATION

Corresponding Author * [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.

Funding Sources We thank the Deutsche Forschungsgemeinschaft DFG (SFB 953 Synthetic Carbon Allotropes) and the State of Bavaria (Soltec) for financial support.

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(50) Contributions from second singlet excited states and interconversions to the corresponding first singlet excited states are acknowledged, but only considered in the global target analyses. (51) Van Stokkum, I. H. M.; Larsen, D. S.; Van Grondelle, R. Biochim. Biophys. Acta - Bioenerg. 2004, 1657 (2–3), 82. (52) Mullen, K. M.; Stokkum, I. H. M. van. J. Stat. Softw. 2007, 18 (3). (53) Snellenburg, J. J.; Laptenok, S. P.; Seger, R.; Mullen, K. M.; Stokkum, I. H. M. van. J. Stat. Softw. 2012, 49 (3). (54) 3 species were needed to model the femto- / nanosecond timescale and 2 species for the nano- / microsecond timescale, the H2P singlet excited state is present in both Models. (55) Liu, X.; Tripathy, U.; Bhosale, S. V.; Langford, S. J.; Steer, R. P. J. Phys. Chem. A 2008, 112 (38), 8986. (56) Differentiation between ZnP and H2P higher (second) excited states is hampered by their extremely short lifetime. (57) 4 species were needed to model the femto- / nanosecond timescale and 3 species for the nano- / microsecond timescale, the (ZnP)3H2P•+-C60•- charge separated state is present in both models. (58) Okhrimenko, A. N.; Gusev, A. V.; Rodgers, M. A. J. J. Phys. Chem. A 2005, 109 (34), 7653. (59) Kuciauskas, D.; Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. a; Gust, D.; Drovetskaya, T.; Reed, C. a; Boyd, P. D. W. J. Phys. Chem. 1996, 100 (39), 15926. (60) Tkachenko, N. V.; Lemmetyinen, H.; Sonoda, J.; Ohkubo, K.; Sato, T.; Imahori, H.; Fukuzumi, S. J. Phys. Chem. A 2003, 107 (42), 8834. (61) It is formed via intrinsic H2P singlet excited state deactivation (9 x 107 s-1), which competes with the initial charge separation. The triplet quantum yields are, therefore, less than 1-2%. . (62) 4 species were needed to model the femto- / nanosecond timescale and 4 species for the nano- / microsecond timescale, (ZnP)3-H2P•+C60•- and cis-(ZnP)3•+-H2P-C60•- are present in both models. . (63) -∆GET° = ∆E0-0 - (-∆GBET); -∆GBET°= E1/2(D·+/D) -

E1/2(A/A·-) + ∆GS; ∆GS = (e2/(4πε0))[(1/(2R+) + 1/(2R-) - 1/RD-A)(1/εs) - (1/(2R+) + 1/(2R-))(1/εs)]

(64) Guldi, D. M.; Luo, C.; Prato, M.; Troisi, A.; Zerbetto, F.; Scheloske, M.; Dietel, E.; Bauer, W.; Hirsch, A. J. Am. Chem. Soc. 2001, 123 (37), 9166. (65) Meaningful analyses in medium polar solvents such as THF and chlorobenzene are hampered by the direct formation of the (ZnP)3•+H2P-C60•- charge separated state.


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Rigid, Branched Porphyrin Antennas - American Chemical Society

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