A Case Study for Artificial Photosynthesis: Non-Covalent Interactions

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A Case Study for Artificial Photosynthesis: Non-Covalent Interactions between C -Dipyridyl and Zinc Porphyrin Dimer 60

Christina Stangel, Asterios Charisiadis, Galateia E. Zervaki, Vasilis Nikolaou, Georgios Charalambidis, Axel Kahnt, Georgios Rotas, Nikos Tagmatarchis, and Athanassios G. Coutsolelos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11863 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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The Journal of Physical Chemistry

A Case Study for Artificial Photosynthesis: NonCovalent Interactions between C60-Dipyridyl and Zinc Porphyrin Dimer Christina Stangel[a,c], Asterios Charisiadis[a], Galateia E. Zervaki[a], Vasilis Nikolaou[a], Georgios Charalambidis[a], Axel Kahnt*[b], Georgios Rotas,[c] Nikos Tagmatarchis*[c] and Athanassios G. Coutsolelos*[a] [a]

Department of Chemistry, University of Crete, Laboratory of Bioinorganic Chemistry,

Voutes Campus, P.O. Box 2208, 70013 Heraklion, Crete, Greece. E-mail: [email protected] [b]

Department of Chemistry and Pharmacy, Chair of Physical Chemistry I, Friedrich-

Alexander University Erlangen-Nürnberg (FAU), Egerlandstr. 3, 91058, Erlangen, Germany. Email: [email protected] [c]

Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens 11635, Greece.E-mail: [email protected]

Abstract: In this study, a new modified C60 derivative with an oPE/oPPV conjugated bridge bearing two pyridyl groups, has been used in combination with a flexible porphyrin dimer (ZnP2) to construct an electron donor/acceptor hybrid (C60-dipyr•ZnP2). This hybrid is based on metal to ligand coordination between the zinc centers of the porphyrin dimer and the two pyridyl

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groups that oPE/oPPV linker bears. In order to investigate the interactions between the electron donor and acceptor entities, both in the ground state and in the excited states, comprehensive photo-physical assays have been carried out. In particular, both absorption and fluorescence titrations provided evidence for strong interactions between the electron donor and the electron acceptor within the hybrid. A binding constant (Kass) in the order of 5.0 x 105 M−1 has been derived. Furthermore, transient absorption measurements revealed intramolecular electrontransfer from the photoexcited porphyrin dimer (ZnP2) to the fullerene derivative (C60-dipyr), leading to a long-lived charge-separated state with a lifetime of up to 1525 ps.

Introduction In natural photosynthesis, light conversion into chemical energy requires a series of step-wise energy and electron transfer processes followed by sequential electron transfer leading to the formation of charge separated states of considerable lifetimes.1-7 Those processes are inspiration for designing numerous artificial photosynthetic models, which are capable of mimicking the key steps of photosynthesis, aiming to the conversion of light energy into solar fuels and the fabrication of optoelectronic devices.8-12 In an effort of further understanding such processes, a plethora of covalently and non-covalently linked electron donor-acceptor systems have been designed, wherein suitable antenna entities are linked to the electron donor or the electron acceptor providing enhanced communication between the two moieties.13-16 It is well known that self-assembly in nature occurs mainly through non-covalent binding motifs, such as hydrogen bonding, π–π stacking and metal-ligand coordination.17-19 The most noticeable example being the photosynthetic antenna reaction center pigments, which use such intermolecular forces to precisely arrange the donor-acceptor entities, exhibiting multi-step energy and electron transfer processes.20 This revelation has inspired several groups in the


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construction of supramolecular photosynthetic architectures which mimic the photoinduced energy and electron transfer processes.21-22 Macrocycles such as porphyrins and phthalocyanines have been used extensively as electron donors due to their resemblance to the chlorophyll pigment, which is the dominant electron donating unit in natural photosynthetic systems.23-24 Such derivatives are commonly used as both optical and redox active components in electron donor-bridge-acceptor (D-B-A) ensembles. The photophysical and electrochemical properties of these chromophores can be easily tuned by altering their peripheral substitution and/or by the choice of the metal in the center the macrocycle.25-26 On the other hand, fullerenes in general, and C60, in particular, have proven to be really promising candidates as electron acceptors in such systems, due to the great photophysical properties they exhibit.27 More specifically, fullerenes are able to accept up to six electrons and the charge is delocalized over their three-dimensional structure, which stabilizes the unpaired electron resulting in low reorganization energy in electron transfer processes.28-30 The third component of such systems, the π-conjugated bridge, connects the electron donating and accepting units and is responsible for the system’s electronic communication.31-32 Some crucial factors, such as the influence of the length and conformation of those linkers, have been extensively studied in covalently linked D-B-A systems.15, 27, 33-34 However, only a few examples of supramolecular D-B-A systems have been reported in which the aforementioned factors were thoroughly studied.35-36 Some of the most commonly used π-conjugated bridges include oligo(pphenylenevinylene)

(oPPV),36-37

oligo(p-phenyleneethynylenes

(oPPE),38

and

[2,20]

paracyclophaneoligophenylene-vinylene (pCp-oPPV)39 that were proven to be excellent for low attenuation factors (β), high electron transfer rates and strong electronic couplings (V), in electron transfer events.


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Up to now, there have been various reports in the literature of supramolecular systems where the two entities are axially linked via the metal center of the macrocyclic ring through a single binding site.14, 36, 40-44 Even less examples can be found in which the metal-ligand coordination between the donor-acceptor units takes place via two binding sites,44-45 or even by adopting a sixpoint hydrogen-bonded motif, as reported by Guldi, Hirsch and co-workers.46 Αssemblies that are formed via a “dual-site” axial coordination are more stable, meaning higher binding constants (Kass) than those where the interaction occurs simply through a single binding site.47 Furthermore an increase in the stabilization of the charge separated state was observed in some cases.42 We have previously reported several non-covalent ensembles in which functionalized fullerenes (C60) with oPPV bridges of different length (short and long) were coordinated axially on the metal center of a porphyrin (ZnP) via one pyridyl group.36 These ensembles demonstrated photoinduced electron-transfer processes in which the longer life time of the charge separated state was found 1585 ps and the reorganization energy (λ) 0.74 eV using the short length oPPV linker. In the present study, we extended this approach by using similar length oligo-(pphenyleneethynylene)/oligo-(p-phenylene vinylene) (oPE/oPPV) bridge bearing C60 and a zinc porphyrin dimer (ZnP2)48 (Scheme 1). Formation of such supramolecular ensembles bearing a metal dimer porphyrin on one side and an electron acceptor moiety on the other, is of notable interest as they mimic the components of the bacterial photosynthtetic reaction center. We demonstrated the “dual-site” axial coordination of a C60-dipyr derivative by its two pyridyl groups to the metal centers of a covalently linked zinc porphyrin dimer (ZnP2) donor moiety, forming the desired stable supramolecular C60-dipyr•ZnP2 hybrid. The C60-dipyr (10) (Scheme 2) shows enhanced solubility in polar and non-polar solvents such as THF, o-DCB, dichloromethane and toluene. Porphyrin dimer ZnP2 is composed by two metallated porphyrin


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units which are covalently linked by a 1,3,5-triazine group. This flexible linker enables the rotation of the zinc porphyrins and hence the efficient coordination of ZnP2 to the pyridyl groups of C60-dipyr. The two pyridyl groups in C60-dipyr are essential as N-donor ligands, since they can coordinate to ZnP2 and enable the formation of a novel supramolecular electron D-B-A architecture C60-dipyr•ZnP2. The newly formed complex was characterized in detail by spectroscopic techniques (NMR, MALDI). Finally, the basic electron-transfer phenomena in the D-B-A hybrid were studied by advanced photophysical studies (steady-state and time-resolved absorption and photoluminescence). Results and discussion Synthesis. Scheme 2 illustrates the synthesis of oPE/oPPV aldehyde 7 terminated with two pyridyl groups, and the preparation of the corresponding C60-dipyr conjugates (8 and 10). Specifically, 1,3-bis(bromomethyl)-5-iodobenzene 2 was prepared

from commercially

available1-iodo-3,5-dimethylbenzene 1 in 54% yield according to a Wohl–Ziegler reaction following a modified method.49

Subsequent treatment with an excess of P(OEt)3 under

Michaelis-Arbuzov conditions afforded bis(phosphonate) 3 in 88% yield.50 Then, HornerWadsworth-Emmons (HWE)51 reaction of bis(phosphonate) 3 with commercially available 4pyridinecarboxaldehyde 4 in THF in the presence of t-BuONa gave the E-isomer 5 in 90% yield. This step was followed by a Sonogashira coupling with 4-ethynylbenzaldehyde 6 to give the corresponding oPE/oPPV based aldehyde 7 in 85% yield.


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Scheme 1. Hybrid C60-dipyr•ZnP2. 4-Ethynylbenzaldehyde 6 was prepared from 4-bromobenzaldehyde in two steps using Sonogashira reaction according to reported procedure.52 The 1H NMR spectrum of oPE/oPPV aldehyde confirms the E configuration of the double bonds by a coupling constant of 16.3 Hz for the AB system corresponding to the vinylic protons. O I

I

I

N

I

P(OEt)3, reflux 48h

4

NBS, benzoyl peroxide

t-BuONa, THF, R.T., 2h

CCl4, reflux, 5h

(EtO)2OP

5 N

3

Br

PO(OEt)2

2

Br 1

N

PdCl2(PPh3), CuI, PPh3 DMF/Et3N, 60 oC, 20h

O

O O

6

N

N

CHO 9 C60, sarcosine, Chlorobenzene, 100oC, 3h

H2N

O

N H

O

O O

O

N H

O

C60, Chlorobenzene, 100oC, 2h

N

8

N

N

7

N

N

10

N


6

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Scheme 2. Synthesis of C60-dipyr conjugates. The synthetic procedure for the preparation of C60-dipyr conjugates is based upon the 1,3dipolar cycloaddition of azomethine ylides, generated in situ upon thermal condensation of an aldehyde and a functionalized amino acid to C60.53 In our case treating oPE/oPPV aldehyde 7 with C60 and an excess of sarcosine or N-tert-butoxycarbonyl-2,2’-ethylenedioxybis(ethylamine) 9 in refluxing chlorobenzene gave C60-dipyr 8 in 27% yield and C60-dipyr 10 in 41% yield respectively.

α-Amino

acid

9

was

prepared

in

three

steps

from

2,2-

(ethylenedioxy)bis(ethylamine) according to literature procedures.54-55 Because of the presence of the long ether chain fulleropyrrolidine 10 is quite soluble in common organic solvents such as toluene, THF, dichloromethane, in contrast with conjugate 8 that is more soluble in polar solvents like chlorobenzene. In the 1H NMR spectra for both fullerenes derivatives the fulleropyrrolidine signals appear at around 5 ppm. Furthermore, their structures were confirmed by identifying their molecular ion peaks in the MALDI-TOF mass spectra; [M]+ 1159.21 for 8 and [M+H]+ 1377.35 for 10 respectively. Density Functional Theory (DFT) Calculations. In an effort to explore the molecular structure along with the electronic properties of C60-dipyr•ZnP2, theoretical calculations were performed. The gas-phase optimized structure of C60-dipyr•ZnP2 is presented in Figure 1, while the corresponding coordinates are listed in Table S1. In addition, the optimized structure of ZnP2 is depicted in Figure S1, while the optimized coordinates are provided in Table S2.


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Figure 1. Gas-phase geometry optimized structure of C60-dipyr•ZnP2. Carbon, nitrogen, hydrogen, oxygen and zinc atoms correspond to grey, blue, white, red and green spheres, respectively. From the optimized structure of ZnP2 (Figure S1) we observe that the porphyrin units are slightly twisted against each, in a perpendicular orientation with respect to the triazine frame. A different view perspective of ZnP2 (Figure S2) reveals that the two bridging phenyl groups of the porphyrin entities are almost co-planar the triazine ring. In the case of C60-dipyr•ZnP2 though, the two zinc porphyrin rings were aligned in a V-shape with an angle of 107.7o and their phenyl groups were almost perpendicular to the macrocycle (Figure S3). Furthermore, the phenyl groups attached to the dipyridyl moiety are almost co-planar against each (Figure 1). In Figure S4 we observe that the distance between the zinc metal centers of the porphyrin rings is decreased from 22.7 Å in ZnP2 to 17.1 Å in C60-dipyr•ZnP2. Consequently, the presence of the flexible triazine linker allows the rotation of the porphyrin macrocycles for efficient coordination to the pyridyl group through their zinc atom. Moreover, the distance from the fullerene edge to the zinc atom of the porphyrin macrocycles is almost equal and estimated to be


8

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23.7 Å and 27.8 Å (Figure S5). However, it is known from the literature56 that the torsional potential energy for meso-aryl porphyrins is expected to be shallow and this fact will significantly affect the conformation of the hybrid. As previously suggested in such fullereneporphyrin derivatives,57 there is a dependence between the geometry alterations and the photoinduced electron transfer processes. The frontier molecular orbitals of C60-dipyr•ZnP2 and ZnP2 are depicted in Figure S6 and Figure S7, respectively. In case of C60-dipyr•ZnP2 we observe that the HOMO energy levels are predominantly spread on the porphyrin units. This delocalization on both the zinc porphyrins suggests the existence of intramolecular interactions between the two porphyrin entities. On the other hand, the LUMO energy levels are mainly located over the C60 moiety and partially on the bridging dipyridyl unit. In addition, the electron density distributions and the calculated energy levels of C60-dipyr•ZnP2 and ZnP2 are depicted in Figure S8. Steady-state photophysical characterization. The electronic ground state absorption spectrum of the ZnP dimer (ZnP2) is very similar to that of ZnP.58 In particular, a strong maximum is seen around 429 nm, which is flanked by minor maxima at 557 and 602 nm, which correspond similar as for individual ZnP to the Soret and Q-bands, respectively (Figure S9). Turning to the C60-dipyr•ZnP2 hybrid system first indications for ground state interactions between ZnP2 and the C60-dipyr(10) fullerene derivative came from absorption titration experiments. When measuring the absorption spectra of anisole solutions containing 1 x 10-6 M ZnP2 and 0 to 3.89 x 10-6 M C60-dipyr the intensity of the absorption band at 429 nm decreases with increasing C60-dipyr concentration and red-shifted by 2 nm (Fig. 2a and Figure S10). Also, the Q band at 557 nm is decreased in intensity and is red-shifted by 4 nm. All the above shifts in the porphyrin bands are typical of axial coordination of nitrogen ligands, such as pyridyl groups,


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to Zinc porphyrins and confirm the formation of the supramolecular dyad C60-dipyr•ZnP2. A binding constant of 5.0 x 105 M−1 was calculated from the underlying concentration vs. absorption change relationships for C60-dipyr•ZnP2 in anisole (Fig. 2b), which is over an order of magnitude higher than that reported for fullerene - porphyrin hybrids in which one binding site was used for the metal to ligand coordination.47,59 Time-resolved photophysical characterization. Investigating the excited state properties of ZnP2, the dimer exhibits strong fluorescence between 580 nm and 720 nm with maxima at 611 and 662 nm (Figure S11). A fluorescence quantum yield of 0.056 in toluene and 0.055 in anisole (using H2TPP and ZnTPP as references with fluorescence quantum yields of 0.11 and 0.03 in toluene) was determined.60 Initial insights into electron donor-acceptor interactions in the C60-dipyr•ZnP2 hybrid arose from fluorescence assays. Fluorescence titrations revealed a decrease of the ZnP2 centered fluorescence with increasing C60-dipyr concentration (Fig. 2c and Fig. 2d). From the fluorescence titration experiments a quenching of the ZnP2 centered fluorescence quantum yield in the C60-dipyr•ZnP2 hybrid in anisole from 0.055 (no C60-dipyr present) to 0.022 (1 x 10-6 M ZnP2 and 3.9 x 10-6 M C60-dipyr) was seen. From this titration experiment, a binding constant of 2.0 x 105 M−1 was obtained for the C60-dipyr•ZnP2 hybrid. The observed fluorescence quenching in the hybrid is prompting to an additional decay channel stating from the first excited singlet state of ZnP2 in the C60-dipyr•ZnP2 hybrid system – electron transfer or energy transfer.


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Figure 2. (a) Steady-state absorption spectra, of anisole solutions containing 1.0 x 10-6 M ZnP2 and 0 – 4.0 x 10-6 M C60-dipyr. (b) Plot of the change in absorption intensity at 429 nm as a ratio of A/Ao versus the concentration of C60-dipyr in anisole. The solid line shows the curve fit obtained by non-linear least-squares analysis used to determine the binding constant. (c) Fluorescence spectra, of anisole solutions containing 1.0 x 10-6 M ZnP2 and 0 – 4.0 x 10-6 M C60-dipyr upon photoexcitation at 560 nm. (d) Plot of the change in fluorescence intensity at 610 nm as a ratio of I/Io versus the concentration of C60-dipyr in anisole. The solid line shows the curve fit obtained by non-linear least-squares analysis used to determine the binding constant.


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Transient absorption measurements based on femtosecond laser photolysis provided insight into the formation and fate of the excited states, in particular the S1-SN transitions. Solutions of ZnP2 in anisole were photo-excited with femtosecond laser pulses with a wavelength of 420 nm. The transient absorption spectra (Fig. 3a) show after the laser pulse one major maximum at 460 nm accompanied by transient absorption maxima at 575, 620, 700 and 790 nm, flanked by transient absorption minima at 555, 600 and 655 nm (Fig. 3a). These transient absorptions are dedicated to the S1-SN transitions, and are decaying (Fig. 3b) with a lifetime of 2100 ps, giving rise to a new set of transient absorptions with a strong maximum at 465 nm, flanked by minor maxima at 580 and 800 nm, accompanied by transient minima at 555 nm and 595 nm. This new transient absorption is assigned to the corresponding triplet manifold.


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Figure 3. (a) Femtosecond transient absorption spectra of ZnP2 in argon saturated anisole upon photoexcitation at 420 nm; 1 ps (black), 10 ps (red), 100 ps (green), 1000 ps (blue) and 5500 ps (cyan) after excitation at 420 nm. (b) Corresponding time absorption profiles at 460 nm (black) and 570 nm (red). Transient absorption spectroscopy based on femtosecond pump–probe experiments shed light on the notion of an electron-transfer deactivation. For C60-dipyr•ZnP2 in anisole, the singlet excited state is formed immediately after the laser pulse, revealing a broad transient absorption with maxima at 460, 575, 630, 700 and 785 nm, accompanied by minima at 565 and 605 nm mirroring the ground-state absorption of the C60-dipyr•ZnP2 hybrid.


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These transient absorption decays with a lifetime of 16 ps (29 ps in toluene and 11 ps in oDCB, Figures S12 and S13) giving rise to a new set of transient absorptions with maxima at 460, 585, 720, 795 and 1020 nm and transient minima at 565 and 605 nm (Figure 4). The transient absorption around 1020 nm is a perfect match for the transient absorption of the one electron reduced fullerene radical anion61 corroborating the hypothesis of an electron transfer reaction from the first excited singlet state of ZnP2 to the C60-dipyr. This allows for assigning the transient absorption with maxima at 460, 585, 720 and 795 nm to the one electron oxidized ZnP2●+ and supports the finding of the formation of a charge separated state (C60●-dipyr•ZnP2●+) in the hybrid system upon photoexcitation of the ZnP2 moiety. The transient absorptions of the charge separated state decay with a lifetime of 1300 ps in anisole (1525 ps in toluene and 710 ps in o-DCB, Figures S9 and S10) giving rise to a new set of transient absorptions with a maximum at 465 nm, flanked by minor maxima at 580 and 800 nm, accompanied by transient minima at 565 nm and 605 nm. This transient absorption matches the transient absorption of the ZnP2 triplet excited state and corroborates that the charge separated state decay into the triplet excited state of the ZnP2 moiety. The finding that the lifetime of the charge separated state strongly depends on the solvent polarity, is well in line with the Marcus theory of electron transfer.62-63 As the solvent polarity decreases, the charge-recombination dynamics are pushed deeper into the Marcus inverted region resulting in an increased charge separated state lifetime.


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Figure 4. (a) Femtosecond transient absorption spectra of C60-dipyr•ZnP2 (5 x 10-5 M C60-dipyr and 5 x 10-6 M ZnP2) in argon saturated anisole upon photoexcitaion at 420 nm; 1 ps (black), 10 ps (red), 100 ps (green), 1000 ps (blue) and 5500 ps(cyan) after excitation at 420 nm. (b) Corresponding time absorption profile at 1020 nm. The solid line shows the fit of the transient absorption time profile. Furthermore, the analogous hybrid C60-dipyr (8)•ZnP2 using the fullerene derivative 8 has also been studied by transient absorption spectroscopy. Nevertheless, the results are quite similar in comparison with C60-dipyr•ZnP2 hybrid and we just list the life time of the charge separated


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state in the same solvents. The transient absorptions of the charge separated state decay with a lifetime of 1470 ps in toluene, 1279 ps in anisole and 731 ps in o-DCB. The photochemical procedures occurring in C60-dipyr•ZnP2 hybrid in Toluene are summarized in an energy-level diagram (Figure 5). The energy level of the single excited state of the porphyrin was calculated from the intersection of the normalized absorption and emission spectra. Furthermore, by using the redox potentials of ZnP2 and C60 derivative we were able to estimate the energy of the charge separated state. Specifically, we used the first oxidation of the ZnP2 (0.37 V)48 and the first reduction of similar substituted C60 compounds (-1.24 V).36

Figure 5. Simplified energy-level diagram showing the photochemical events occurring in C60dipyr·ZnP2 in toluene. Solid arrows represent major photochemical events and dashed arrows show minor photochemical events. Conclusions We have successfully prepared a supramolecular D-B-A hybrid (C60-dipyr•ZnP2) based on two metal to ligand coordination between a C60 derivative featuring two pyridyl groups functionalized with oPE/oPPV bridges, and an electron donating porphyrin dimer (ZnP2). The successful complex formation was confirmed by absorption and fluorescence titration


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experiments, from which a binding constant of 5.0 x 105 M-1 in anisole was derived. This binding constant is more than one order of magnitude larger than usually obtained for similar hybrid systems with only one coordination binding site,47 showing the benefit of the use of two pyridyl groups for binding C60 to ZnP derivatives, instead of the commonly used one coordination binding for the formation of supramolecular ZnP-C60 hybrids. Transient absorption measurements corroborate strong electronic communications between the ZnP2 and the C60 in the excited state. The excited state interactions lead to a very efficient formation of the radical ion pair state, that is, the one electron oxidized ZnP2 radical cation and the one electron reduced C60 radical anion (C60●—-dipyr•ZnP2●+). The formation of the charge separated state is very rapid for such a supramolecular system with a linker of this length containing an oPE unit. The rate constants for the charge separation are 3.4 x 1010 s-1 in toluene, 6.3 x 1010 s-1 in anisole and 9.1 x 1010 s-1 in o-DCB. This provides solid evidence that two pyridyl groups as coordinative linker are promoting the forward electron transfer. The lifetime of the charge separated state was found to be in the nanosecond range, 1.5 ns in toluene, 1.3 ns in anisole and 0.7 ns in o-DCB. The finding that the lifetime of the charge separated state is even slightly longer than the ones obtained in similar systems44,59 with only a single pyridyl coordination site is a strong indication, that two pyridyl groups are not accelerating the back-electron transfer. In other words, two pyridyl groups coordinated to the ZnP2 dimer are promoting the charge separation, but are not shortening the lifetime of the charge separated state. With regard to the strong association constant, the efficient charge separation and the slow charge recombination, supramolecular assemblies containing bridges with two pyridyl binding


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sites to strongly interact with electron donors, such as ZnP2, will be a guideline for the design of the next generation of supramolecular electron donor-acceptor systems. Experimental section General Methods. Reagents and solvents were purchased as reagent grade and used without further purification. All solvents were dried by the appropriate techniques.64 The following chemicals were prepared according to literature procedures: 4-ethynylbenzaldehyde 6,65 N-tertbutoxycarbonyl-2,2’-ethylenedioxybis(ethylamine)

9,54-55

dimer

ZnP2.48

Thin

layer

chromatography was performed on silica gel 60 F254 plates. Chromatography was carried out on SiO2 (silica gel 60, SDS, 70–230 mesh ASTM). 1,3-Bis(bromomethyl)-5-iodobenzene 2: To a stirred solution of 1-iodo-3,5-dimethylbenzene 1 (2.0 g, 8.62 mmol) and benzoyl peroxide (0.42 g, 1.72 mmol), in CCl4 (40 mL), NBS (4.6 g, 25.8 mmol) was added and the mixture was refluxed for 5 hours. Then, the excess of NBS was removed by filtration in Büchner funnel and the solvent was evaporated under reduced pressure. The residue was purified by column chromatography (SiO2, petroleum ether) to obtain 2 as white solid (1.8 g, 54%). The spectroscopic data are consistent with the literature.49 Tetraethyl (5-iodo-1,3-phenylene)bis(methylene)diphosphonate 3: A mixture of 2 (1.0 g, 2.56 mmol) and triethylphosphite (4.4 mL, 25.7 mmol) was heated at 150 °C for 48h. After cooling to room temperature, the excess of triethylphosphite was distilled under reduced pressure at 70 °C. Product 3 was collected in pure form, without purification, as a colorless solid after being dried under vacuum (1.1 g, 88%). The spectroscopic data are consistent with the literature.50 4,4'-(1E,1'E)-2,2'-(5-Iodo-1,3-phenylene)bis(ethene-2,1-diyl)dipyridine5: t-BuONa (0.57 g, 5.95 mmol) was added to a solution of 4-pyridinecarboxaldehyde 4 (0.21 g, 1.98 mmol) and (5-


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iodo-1,3-phenylene)bis(methylene)diphosphonate 3 (0.50 g, 0.99 mmol) in dry THF, under N2, at room temperature. The resulting solution was stirred for 2 h. The reaction was then quenched by the addition of dilute aqueous HCl solution (0.5 M) followed by distillation of the solvents. The solid residue was dissolved in CHCl3, and then the resulting solution washed with aqueous NaCl solution, dried (Na2SO4), filtered and distilled to dryness. Column chromatography (SiO2, CH2Cl2 / MeOH, 95: 5) gave 5 as a white solid (0.36 g, 90 %). 1H NMR (300 MHz, CDCl3): δ 7.05 (AB, J = 16.3 Hz, 2H), 7.21 (AB, J = 16.3 Hz, 2H), 7.37 (d, J = 5.5 Hz, 4H), 7.60 (s, 1H), 7.84 (s, 2H), 8.61 (d, J = 5.3 Hz, 4H). MS (MALDI-TOF): m/z 410.03 [M]+. 4-((3,5-Bis((E)-2-(pyridin-4-yl)vinyl)phenyl)ethynyl) benzaldehyde 7: A solution of 4ethynylbenzaldehyde 6 (71 mg, 0.55 mmol), iodine derivative 5 (150 mg, 0.37 mmol) and molecular sieves (4 Å) in anhydrous DMF/Et3N (1:1, 12 mL) was heated at 60 °C, under N2. Then, PdCl2(PPh3) (13 mg, 0.018 mmol), PPh3 (9.6 mg, 0.037 mmoL) and CuI (3.5 mg, 0.018 mmol) were added and the mixture was stirred for 20 h at 60 °C. After removal of the solvents under reduced pressure, chloroform was added and the molecular sieves were filtered off. The solution was washed with saturated NH4Cl solution, dried (Na2SO4), filtered and then chloroform evaporated under reduced pressure. Column chromatography (SiO2, CH2Cl2 / MeOH, 95: 5) gave oPE/oPPV aldehyde 7 in pure form (128 mg, 85%). 1H NMR (500 MHz, CDCl3): δ 7.13 (AB, J = 16.3 Hz, 2H), 7.32 (AB, J = 16.3 Hz, 2H), 7.40 (d, J = 5.9 Hz, 4H), 7.67 (s, 1H), 7.72 (m, 4H), 7.91 (d, J = 8.3 Hz, 2H), 8.63 (d, J = 4.8 Hz, 4H), 10.05 (s, 1H). 13C NMR (125 MHz, CDCl3): 191.5, 150.4, 144.2, 137.3, 135.8, 132.3, 131.8, 130.1, 129.8, 129.2, 127.9, 126.3, 123.9, 121.1, 92.5, 89.4. MS (MALDI-TOF): m/z 412.16 [M]+. C60-dipyr 8: A mixture of oPE/oPPV aldehyde 7 (15 mg, 0.036 mmol), C60 (26 mg, 0.036 mmol) and sarcosine (32 mg, 0.36 mmol) in chlorobenzene (24 mL) was heated at 100 °C for 3


19


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Page 20 of 34

h. After cooling, the reaction mixture was transferred onto the top of a silica gel column and unreacted C60 (8.1 mg, 30%) eluted with chlorobenzene and collected. Then further elution with a mixture of chlorobenzene and methanol (99:1) gave the product 8 (25 mg, 27%) as a brown solid. 1H NMR (300 MHz, C2S/C6D6, 1:2 v/v): δ 2.76 (s, 3H), 4.20 (d, J = 9.3 1H), 4.89 (m, 2H), 6.94 (d, J = 16.3 Hz, 2H), 7.15 (m, 6H), 7.45 (s, 5H), 7.70 (m, 2H), 8.40 (m, 4H). 13C NMR (75 MHz, C2S/C6D6, 1:2 v/v): δ 156.62, 156.59, 155.40, 154.36, 153.89, 153.37, 151.03, 147.99, 147.22, 147.04, 146.93, 146.83, 146.67, 146.65, 146.17, 146.15, 146.03, 145.97, 145.96, 143.96, 143.46, 143.35, 142.89, 142.77, 141.02, 140.98, 140.75, 140.34, 139.75, 138.37, 138.33, 138.20, 137.94, 137.67, 137.23, 136.89, 136.71, 136.68, 136.65, 136.51, 136.43, 132.85, 131.98, 130.60, 130.05, 128.66, 126.13, 125.48, 124.08, 121.32, 91.5, 90.78, 83.93, 77.76, 70.80, 69.62, 40.82. MS (MALDI-TOF): m/z 1159.21 [M]+. C60-dipyr 10: A mixture of oPE/oPPV aldehyde 7 (16 mg, 0.039 mmol), C60 (28 mg, 0.039 mmol) and α-amino acid 9 (24 mg, 0.078 mmol) in chlorobenzene (25 mL) was heated at 100 °C for 2 h. After cooling, the reaction mixture was transferred onto the top of a silica gel column and unreacted C60 (5.5 mg, 30%) eluted with chlorobenzene and collected. Then further elution with a mixture of dichlomethane and methanol (97:3) gave the product 10 (21 mg, 41%) as a brown solid. 1H NMR (500 MHz, CDCl3): δ 1.44 (s, 9H), 2.92 (m, 1H), 3.35 (m, 2H), 3.47 (m, 1H), 3.62 (m, 2H), 3.74 (m, 2H), 3.80 (m, 2H), 4.04 (m, 2H), 4.32 (d, J = 9.5 Hz, 1H), 5.00 (sb, 1H), 5.21 (m, 2H), 7.09 (d, J = 16.3 Hz, 2H), 7.29 (d, J = 16.3 Hz, 2H), 7.40 (d, J = 5.2 Hz, 4H), 7.63 (m, 5H), 7.85 (sb, 2H), 8.61 (s, 4H).

13

C NMR (125 MHz, CDCl3): δ 156.34, 155.97,

154.05, 153.13, 153.01, 149.81, 147.35, 147.32, 146.64, 146.40, 146.32, 146.24, 146.22, 146.15, 146.13, 146.07, 145.97, 145.73, 145.55, 145.54, 145.46, 145.39, 145.37, 145.30, 145.24, 145.17, 144.75, 144.60, 144.45, 144.36, 143.20, 143.03, 142.73, 142.61, 142.58, 142.27, 142.25, 142.19,


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142.16, 142.10, 142.07, 142.04, 141.97, 141.87, 141.81, 141.72, 141.56, 140.25, 140.20, 139.90, 139.51, 138.04, 136.97, 136.95, 136.46, 135.97, 135.64, 132.18, 132.03, 129.97, 129.60, 127.43, 125.80, 124.47, 122.90, 121.08, 90.17, 89.17, 82.12, 79.30, 76.22, 70.60, 70.44, 69.21, 67.69, 52.19, 40.45, 31.60, 30.95, 28.48, 22.67, 14.14. MS (MALDI-TOF): m/z 1377.34 [M+H]+. NMR spectroscopy.1H NMR and 13C NMR spectra were recorded on Bruker AMX-500 MHz and Bruker DPX-300 MHz spectrometers as solutions in deuterated solvents by using the solvent peak as the internal standard. Mass spectrometry. Mass spectra were performed on a Bruker ultrafleXtreme MALDITOF/TOF

spectrometer

using

trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]

malononitrile as matrix. UV/vis absorption spectroscopy. Steady state absorption spectra were obtained using a Perkin Elmer Lambda 2 UV/vis two-beam spectrophotometer with a slit width of 2 nm and a scan rate of 480 nm/min. A quartz glass cuvette of 10x10 mm was used. Emission spectroscopy. Steady state emission was recorded using a Horiba Jobin Yvon FluoroMax-3 spectrometer using a slit width of 2 nm for excitation and emission and an integration time of 0.2 s. A quartz glass cuvette of 10x10 mm was used. All spectra were corrected for the instrument response. For excitation wavelength below 450 nm a cut off filter (435 nm) was inserted. Fs-transient absorption spectroscopy. Femtosecond transient absorption studies were performed with laser pulses (1 kHz, 150 fs pulse width) from an amplified Ti/sapphire laser system (Model CPA 2110, Clark-MXR Inc.; output 775 nm). For an excitation wavelength of 420 nm, a nonlinear optical parametric amplifier (NOPA) was used to generate ultra-short tunable visible light pulses out of the pump pulses. The transient absorption pump probe


21


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spectrometer (TAPPS) is referred to as a two-beam setup, in which the pump pulse is used as excitation source for transient species and the delay of the probe pulse is exactly controlled by an optical delay rail. As the probe (white-light continuum), a small fraction of pulses stemming from the CPA laser system was focused by a 50 mm lens into a 2 mm thick sapphire disc. The transient spectra were recorded using fresh argon-saturated solutions in each laser excitation. All experiments were performed at 298 K in a 2 mm quartz cuvette. Density Functional Theory (DFT) Calculations. Density functional theory calculations (DFT)66 were carried out at the B3LYP/6-31G(d)67-68 level of theory using Gaussian 03 program suite.69 For the geometry optimizations, the LANL2DZ basis set was used for Zn atoms and the 6-31G(d) basis sets for lighter atoms. The optimized minimum-energy structures were verified as stationary points on the potential energy surface by vibrational frequency analysis calculation. All the computed structures and molecular orbitals of C60-dipyr·ZnP2 and ZnP2 were modeled using ChemCraft software.70 ASSOCIATED CONTENT Supporting Information. DFT calculations corresponding to the zinc-porphyrin dyad and the hybrid; Absorption and emission spectra of the porphyrin dyad; 1H and

13

C NMR spectra (Figures S14-S20) of the

fullerene derivatives and femtosecond transient absorption spectra of the dyads in toluene and oDCB. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] , [email protected], [email protected]


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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 European Commission's Seventh Framework Programme (FP7/2007-2013), Special Research Account of the University of Crete, implemented under the "ARISTEIA II". Deutsche Forschungsgemeinschaft (DFG). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the European Commission's Seventh Framework Programme (FP7/2007-2013)

under

grant

agreement

n.°

229927

(FP7-REGPOT-2008-1,

Project

BIOSOLENUTI) and Special Research Account of the University of Crete, implemented under the "ARISTEIA II" Action of the "OPERATIONAL PROGRAMME EDUCATION AND LIFELONG LEARNING" and is co-funded by the European Social Fund (ESF) and National Resources. IKY fellowships of excellence for post-graduate studies in Greece–Research programs of excellence IKY–Siemens. We would like to thank Dr. Dirian from the FriedrichAlexander-Universität Erlangen-Nürnberg, Erlangen, Germany, for the support during the electrochemical measurements. AK gratefully acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG) via Grant KA 3491/2-1.


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Dendritic

Fluorophores

Containing

Both

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and

Phenyleneethynylene Moieties. J. Org. Chem. 2003, 68, 832-838.


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