Axially Assembled Photosynthetic Antenna-Reaction Center Mimics

Article pubs.acs.org/IC

Axially Assembled Photosynthetic Antenna-Reaction Center Mimics Composed of Boron Dipyrromethenes, Aluminum Porphyrin, and Fullerene Derivatives Anthi Bagaki,†,# Habtom B. Gobeze,‡,# Georgios Charalambidis,† Asterios Charisiadis,† Christina Stangel,†,§ Vasilis Nikolaou,† Anastasios Stergiou,§ Nikos Tagmatarchis,*,§ Francis D’Souza,*,‡ and Athanassios G. Coutsolelos*,† †

Department of Chemistry, University of Crete, Laboratory of Bioinorganic Chemistry, Voutes Campus Heraklion 70013, Crete, Greece ‡ Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States § Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Contantinou Avenue, Athens 11635, Greece S Supporting Information *

ABSTRACT: Sequential photoinduced energy transfer followed by electron transfer leading to the formation of charge separated states in a newly assembled series of supramolecular triads comprised of boron dipyrromethenes (BODIPY or BDP), aluminum porphyrin (AlTPP) and C60 is demonstrated. In the present strategy, the energy donor (BDP) and electron acceptor (C60) were axially positioned to the plane of AlTPP via the central metal. The structural integrity of the newly synthesized compounds and self-assembled systems were fully established using spectral, electrochemical and computational methods. Thermodynamic feasibility of energy transfer from 1BDP* to AlTPP and subsequent electron transfer from 1AlTPP* to generate BDP-AlTPP•+-C60•− charge separated states was derived from free-energy calculations. Occurrence of ultrafast energy transfer from 1BDP* to AlTPP was established from studies involving steady-state and timeresolved emission, as well as femtosecond transient spectroscopic techniques. The BDP-AlTPP•+-C60•− charge separated states persisted for several nanoseconds prior returning to the ground state.

■

INTRODUCTION

functionalization on the periphery and/or by importing different metal ions in the center of the macrocycle ring.12,16 Furthermore, when combined with other redox-active species, especially electron acceptors, such as quinones and fullerenes, they form a wide variety of covalent and noncovalent D−A conjugates and supramolecular assemblies.12,13,17−22 Fullerene derivatives exhibit remarkable electron-accepting properties, since delocalization of the charge over their spherical structure stabilizes the unpaired electron, resulting in low reorganization energies, making these compounds ideal as electron acceptors in such systems.23−27 A third component that can be employed in such supramolecular systems to improve photoinduced charge-transfer reactions concerns a secondary energy/electron donating unit. Boron dipyrromethenes (BDPs) are interesting light-harvesting units, able to play that role.28 They are largely used, since they exhibit large extinction coefficients, high fluorescence quantum yields and reasonably long excited state

It is well established that nature manages to convert solar energy to chemical energy through photosynthesis.1,2 In photosynthetic organisms, solar energy conversion is achieved by photoinduced multistep energy and electron transfer processes between the light harvesting antenna moiety and the reaction center complexes.3−6 During the last few decades, many efforts have been made toward better understanding those principal processes, by exploring synthetically prepared supramolecular multicomponent architectures.7−11 The main challenge in such donor−acceptor (D−A) assemblies is strongly dependent on the ability of the complex to capture useful sunlight and generate highly efficient long-lived chargeseparated states via light-induced sequential electron transfer.12−15 In most of these D−A systems, porphyrin derivatives are used as photosensitizers due to their structural similarities to chlorophyll, their strong absorption in the visible region, and their rich redox chemistry. Another important aspect is that all those optical and redox properties can easily be tuned by © 2017 American Chemical Society

Received: April 27, 2017 Published: August 22, 2017 10268

DOI: 10.1021/acs.inorgchem.7b01050 Inorg. Chem. 2017, 56, 10268−10280

Article

Inorganic Chemistry

Figure 1. Chemical structures of the axial supramolecular triads, BDP-COO-AlTPP·C60-PPV-pyr, BDP-COO-AlTPP·C60-pyr, BDP-O-AlTPP·C60PPV-pyr, and BDP-O-AlTPP·C60-pyr investigated in this study.

lifetimes.29,30 In the case of porphyrin−BDP conjugates, usually good spectral overlap, between the energy donor’s (BDP) emission and the energy acceptor’s (porphyrin) absorption, is observed.31−33 Such features are desirable for efficient intramolecular energy transfer. Therefore, a combination of fullerenes, porphyrins, and BDPs leads to enhanced lightharvesting efficiency and improved conversion of light into the desirable high-energy charge-separation states.34−36 In order for any electron transfer system to function properly, the electronic coupling between the donor and acceptor should be taken into consideration, which depends to a great extent on their spatial arrangement.37 The influencing factors, such as the length and conformation of the bridges, have been studied in numerous systems in which the components are covalently linked via bridges to the periphery of the porphyrin.12,13,37−40 Nonetheless, there are fewer studies of these factors in complexes where the redox-active components are linked axially via the central metal of the ring.18,41−45 However, stepwise electron transfer processes are rarely observed in latter artificial architectures.46,47 The arrangement where the two units (donor and acceptor) are placed on opposite faces of the porphyrin cannot easily be achieved, since in most cases transition metals are being used. In transition metal complexes, axial coordination of only one component is commonly attained, while the attachment of two different axial ligands is rather difficult to control. In this respect, main group element porphyrins have been used in order to address this issue, such as aluminum(III),48,49 tin(IV),50,51 or indium(III) porphyrins.52 Aluminum(III) porphyrins (AlPor) are unique as they are able to form covalent bonds with carboxylic acids46,53−56 or alcohols,48,57−59 leading to five-coordinate metal centers. Furthermore, Al(III) due to its high Lewis acidity can adequately form six-coordinate complexes by axial coordination of Lewis bases (e.g., pyridine, imidazole, etc.,),47,60,61 thus resulting in complexes with a covalently linked ligand on one face and the coordination bond on the opposite face. This particular geometry of AlPor complexes provides an excellent opportunity to study the influence of electronic coupling and reorganization energy on the energy and electron transfer processes in the perpendicular (axial) direction to the porphyrin plane and also prevents

aggregation, one of the most commonly observed problems in such porphyrin-based assemblies. Over the past few years, various AlPor-based D-AlPor-A systems consisting of threedimensional (3D) fullerene or two-dimensional (2D) naphthalenediimide as an electron acceptor and ferrocene, tetrathiafulvalene, or phenothiazine entities as secondary electron donors have been reported.46,47,62−65 In these systems, the energy and electron transfer reactions were investigated in the axial direction as a function of electronic coupling, orientation, reorganization energy, and the distance between donor and acceptor units. More recently, two AlPor-based dyads (AlPor-Ph-H2Por and AlPor-Ph-AuPor·PF6) were also reported, in which a fluoro-substituted free-base or a gold(III) porphyrin (H2Por or AuPor) were axially bound and functioned as a 2D electron acceptor to study the electron transfer in the axial direction.66,67 Herein, we report the synthesis and characterization of a series of new AlPor-based triads, in which the nature of the secondary donor, the electron acceptor and the bridging moieties are varied in order to study the factors determining the energy and electron transfer processes in such axial triads. Those four studied triads (BDP-COO-AlTPP·C60-PPV-pyr, BDP-COO-AlTPP·C60-pyr, BDP-O-AlTPP·C60-PPV-pyr, and BDP-O-AlTPP·C60-pyr, where pyr = pyridine and PPV = poly(p-phenylenevinylene) are presented in Figure 1. As energy harvesters, two BDP units were employed (BDP-COOH and BDP-OH) bearing a carboxylate or a hydroxyl group, respectively, allowing them to be covalently attached to the aluminum center of the porphyrin, leading to the formation of two new dyads: BDP-COO-AlTPP and BDP-O-AlTPP, as examples of carboxylate and ether linkers. Furthermore, these dyads were converted to triads by coordination of an acceptor that carries a pyridyl moiety. For that purpose we have chosen: (i) a pyridyl-C60 compound and (ii) a functionalized C60 derivative with a PPV bridge bearing a pyridyl unit (C60-pyr and C60-PPV-pyr), both acting as acceptors. As it has already been reported by our group, such C60 derivatives when axially coordinated to a porphyrin allow basic electron-transfer phenomena to be monitored and also lead to stable chargeseparated states.43 Fluorescence titration experiments indicated the presence of excited state interactions between the 10269


Article

Inorganic Chemistry

Scheme 1. Synthesis of Dyads BDP-COO-AlTPP and BDP-O-AlTPP and Reference Compounds Ph-COO-AlTP and Ph-OAlTP

due to the shielding effect from the ring current of the porphyrin macrocycle. For example, the resonances of the phenyl protons a and b (Scheme 1), which appear at 6.95 and 7.12 ppm respectively, in the unbound BDP-OH, are strongly upfield shifted to 2.67 and 5.69 ppm, respectively, in the BDPO-AlTPP dyad. Similarly, all the other signals of BDP-OH (c, d, and e) were shifted from 1.44, 5.98, and 2.55 ppm to 0.64, 5.69, and 2.39 ppm, respectively, after the attachment of the BDP unit to the AlTPP. Likewise, in the carboxylate-bridged BDP-COO-AlTPP dyad, the resonances of the BDP chromophore protons were shifted upfield due to the porphyrin ring current. However, due to the longer distance between the BDP moiety and the porphyrin core, much less pronounced upfield shifts were observed. For instance, the resonances assigned to the phenyl protons f and g (Scheme 1) of the BDPCOOH were shifted from 8.23 and 7.44 ppm to 5.17 and 6.34 ppm, respectively, after the coordination of the BDP-COOH to the aluminum metal center. In the 1H NMR spectra of the reference compounds Ph-COO-AlTPP and Ph-O-AlTPP, similar shielding effects were observed for the protons of the axially linked phenyl substituents. The chemical structures of the prepared triads are presented in Figure 1. The fullerene derivatives employed in this study bear one pyridyl group; however, the distance between the pyridyl group and the fullerene sphere is varied. The formation of the triads is spontaneous and achieved by simply mixing the dyad with the desired fullerene derivative in a noncoordinating solvent. Lewis acid−base interactions between the aluminum metal and the nitrogen of the pyridyl group are responsible for the construction of these supramolecular self-assembled triads. UV−visible absorption and steady-state fluorescence titrations were employed to monitor their formation. However, the asformed self-assembled triads were not stable enough in order to be isolated. UV−Visible Absorption Spectroscopy. The electronic absorption spectra of dyads BDP-COO-AlTPP and BDP-OAlTPP and their corresponding reference compounds were measured in toluene and shown in Figure 2. The absorption

supramolecular triads. Sequential energy transfer followed by electron transfer was predicted from studies involving optical absorption and emission, electrochemistry and computational calculations. Femtosecond transient spectroscopy studies revealed ultrafast singlet−singlet energy transfer from 1BDP* to AlTPP in both BDP-AlTPP dyads (kENT = 109−1011 s−1), and a better efficiency in the case of BDP-O-AlTPP compared to BDP-COO-AlTPP due to distance and orientation factors. The singlet excited state of the porphyrin (1AlTPP*) promoted electron transfer to the axially coordinated C60 derivatives, resulting in the formation of BDP-AlTPP•+-C60•− charge separated states of appreciable lifetimes (100−120 ns).

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RESULTS AND DISCUSSION

Synthesis. The synthesis of BDP-COO-AlTPP and BDPO-AlTPP dyads and their corresponding reference complexes Ph-COO-AlTPP and Ph-O-AlTPP is presented in Scheme 1. In all cases the starting compound was the metalloporphyrin Al(OH)TPP which was prepared according to a literature procedure.54 The two BDP-based chromophores BDP-COOH and BDP-OH were synthesized following the procedures described by Akkaya and co-workers.68,69 The desired dyads BDP-COO-AlTPP and BDP-O-AlTPP were readily prepared by reaction of Al(OH)TPP with the carboxy- or the phenoxylsubstituted BDPs, respectively, in dry toluene at 40 °C. Model compounds Ph-COO-AlTPP and Ph-O-AlTPP were produced in a similar manner by adding 1 equiv of benzoic acid or phenol in Al(OH)TPP, respectively, in dry toluene at 70 °C. The preparation of all these complexes was straightforward in high yields and mild conditions, without requiring of any tedious purification procedures such as column chromatography. The obtained axially substituted compounds were very sensitive to moisture and for that reason were stored under argon, prior to their photophysical studies. The successful formation of the compounds was confirmed by 1H NMR spectroscopy. As expected, after the axial coordination of the BPD units to the Al(OH)TPP porphyrin, the signals of the BDP moiety were shifted to lower frequencies 10270


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Figure 2. UV−visible absorption spectra of the dyads BDP-COO-AlTPP and BDP-O-AlTPP and their corresponding reference compounds PhCOO-AlTP and Ph-O-AlTP as well as BDP-COOH and BDP-OH measured in dry toluene.

Table 1. UV−Visible Absorption and Steady-State Fluorescence Data of Investigated Compounds fluorescence λem/nm

absorption λmax/nm (ε/mM−1 cm−1)

compound Ph-COO-AlTP Ph-O-AlTP BDP-COOH BDP-OH BDP-COO-AlTPP BDP-O-AlTPP a

415 415 503 503 415 415

(421.3), (441.3), (52.6) (50.6) (500.4), (484.4),

λex = 485

547 (17.1) 547 (17.0)

501 (54.7), 547 (17.5) 500 (51.9), 546 (18.1)

522 517 522, 596, 646 516, 593, 644

λex = 546

Φf

τ, ns

596, 645 595, 645

0.081 0.077 0.36 0.72 0.010,a 0.081b 0.070,a 0.074b

8.16 7.65 3.39 3.40a 6.98b 5.71b

596, 645 595, 644

Quantum yield and lifetime of the BDP chromophore. bQuantum yield and lifetime of the porphyrin.

accompanied by one or more isosbestic points, as shown for BDP-O-AlTPP binding to C60-pyr in Figure 3 (see Figures

maxima and molar absorption coefficients (ε) of the Soret and Q bands are summarized in Table 1. The two reference compounds (Ph-COO-AlTP and Ph-O-AlTP) exhibit typical aluminum porphyrin absorption features, namely, an intense Soret band at 415 nm and one moderately intense Q-band at 547 nm. On the other hand, BPD-based chromophores (BDPCOOH and BDP-OH) showed a characteristic absorption signal at 503 nm, corresponding to the first singlet excited state transition π−π*. As expected, dyads BDP-COO-AlTPP and BDP-O-AlTPP display a combination of absorption features from their two constituent chromophores. The intense peak at ca. 500 nm originates from the BDP part, while the strong absorption at 415 nm and the weaker transition at ca. 546 nm and 600 are due to the porphyrin-based Soret and Q-band, respectively. In both dyads the band positions and the molar extinction coefficients remain similar to their pristine species. Overall, the absorption studies suggest weak or even no interactions between the porphyrin and the axial coordinated BDP chromophores in the ground state. Regarding the BDP-O-AlTPP·C 60 -PPV-pyr, BDP-OAlTPP·C60-pyr, BDP-COO-AlTPP·C60-PPV-pyr, and BDPCOO-AlTPP·C60-pyr hybrids, information on the supramolecular complexation and ground state interactions between the porphyrin dyads and the fullerene derivatives was obtained by absorption titrations. To this end, variable amounts of toluene solutions containing various concentrations of C60PPV-pyr and/or C60-pyr were added to solutions containing BDP-O-AlTPP and/or BDP-COO-AlTPP. It is worth mentioning that during the titration experiment the porphyrin concentration remained constant. Substantial changes were discerned in the absorption spectra as a result of complex formation.43 These involved reduction in Soret peak intensity

Figure 3. Steady-state absorption spectra of BDP-COO-AlTPP upon increasing additions of C60-PPV-pyr in toluene. The figure inset is a Benesi−Hildebrand plot constructed for evaluating the binding constant.

S1−S3 in the Supporting Information for spectral changes associated with axial fullerene binding to other porphyrins). The presence of isosbestic points indicated the existence of only one equilibrium process. The binding constants were evaluated by using the spectral data according to Benesi− Hildebrand method70 (see figure inset for plots). The determined binding constant, K, was found to be 7.57 × 103 M−1 for BDP-O-AlTPP·C60-pyr, 5.04 × 103 M−1 for BDP-OAlTPP·C60-PPV-pyr, 5.2 × 103 M−1 for BDP-COO-AlTPP· C60-pyr, and 2.27 × 103 M−1 for BDP-COO-AlTPP·C60-PPV10271


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Inorganic Chemistry

Figure 4. Room temperature emission spectra of isoabsorbing (A = 0.1) toluene solutions of (left) BDP-COO-AlTPP and BDP-COOH and (right) BDP-O-AlTPP and BDP-OH. The excitation wavelength was at 485 nm where only the BDP chromophore absorbs.

the fluorescence titrations revealed a decrease of the porphyrin centered fluorescence with a maximum at 645 nm, while increasing the fullerene derivatives concentration in toluene solutions (Figure 5). Corresponding titrations for the other

pyr formation. The magnitude of K values is consistent with Rpyr (R = donor or acceptor) binding to either zinc porphyrin or aluminum porphyrin.45 Within the studied systems, the binding constants were slightly higher for BDP-O-AlTPP compared to BDP-COO-AlTPP and were slightly higher for C60-pyr binding over C60-PPV-pyr binding. Fluorescence Spectroscopy. Steady-state fluorescence measurements were performed in toluene solutions at ambient temperature, in order to investigate possible intramolecular interactions from the excited state. The spectra of the dyads (BDP-COO-AlTPP and BDP-O-AlTPP) and their corresponding reference compounds are presented in Figures 4 and S4. A summary of the measured spectroscopic data is listed in Table 1. The emission spectra of BDP-COOH and BDP-OH, upon excitation at 485 nm, showed the expected strong emission signals at 522 and 517 nm, respectively, corresponding to the radiative decay of the BDP-based singlet excited state. Excitation of the two reference compounds Ph-COO-AlTPP and Ph-O-AlTPP at 546 nm results in the appearance of typical fluorescence bands at ca. 595 and 645 nm, which are characteristic for AlTPP. Irradiation of the BDP-COO-AlTPP dyad at 485 nm, corresponding to selective excitation of the BDP unit, results in emission from the AlTPP chromophore at 596 and 646 nm, as well as residual fluorescence from the BDP group at 522 nm. The BDP emission is strongly quenched as shown in Figure 4, where the fluorescence spectra of isoabsorbing solutions of BDP-COO-AlTPP and BDPCOOH are compared. Similar findings were observed in the hydroxy-substituted dyad BDP-O-AlTPP, where the fluorescence of the BDP-OH unit is also quenched after coordination with the Al-metal center of the porphyrin. The observed reduction of the BDP-based emission can be explained on the basis of photoinduced energy transfer from the 1BDP* to the lower lying singlet excited state of AlTPP and/or to the existence of new nonradiative pathways from the 1BDP* to the ground state. Moreover, the excitation spectra of the two dyads monitoring at the low-energy emission of the porphyrin moiety at 660 nm show the absorption features corresponding to the porphyrin unit along with a clear BDP absorption feature at ca. 504 nm (Figure S5). All these results suggest occurrence of photoinduced energy transfer from the 1BDP* to the lower lying singlet excited state of the AlTPP unit. First indications for excited state interactions between the supramolecular triads came from fluorescence titration experiments. In the case of BDP-COO-AlTPP·C60-PPV-pyr hybrid, when excited at 546 nm to match the absorption of the Q-band,

Figure 5. Fluorescence spectra of toluene solutions containing 1.0 × 10−6 M BDP-COO-AlTPP upon increasing addition of C60-PPV-pyr, photoexcitation at 546 nm.

triads are provided in the Supporting Information: BDP-COOAlTPP·C60-pyr (Figure S6), BDP-O-AlTPP·C60-PPV-pyr (Figure S7), and BDP-O-AlTPP·C60-pyr (Figure S8). Computational Studies. Theoretical calculations using B3LYP/6-31G* basis set were carried out in order to clarify the electronic properties and visualize the optimized structures of all supramolecular triads. The gas phase optimized structures of BDP-COO-AlTPP·C60-PPV-pyr, BDP-COO-AlTPP·C60-pyr, BDP-O-AlTPP·C60-PPV-pyr, and BDP-O-AlTPP·C60-pyr are presented in Figure 6, while their corresponding gas-phase optimized coordinates are listed in Tables S1−S4, respectively (see Supporting Information). Noticeably, in all supramolecular assemblies the porphyrin macrocycle adopts a similar planar scaffold since the central Al atom is coplanar with the four pyrrole rings of the porphyrin. In addition, the phenyl rings of the porphyrin as well as the axial phenyl rings from the BDP and C60 moiety are also perpendicular to the porphyrin plane. The essential alteration in the geometry of the supramolecular assemblies is the different nature of the bond (phenolate or carboxylate) formed via axial the substitution by each BDP entity. More specifically, in the AlTPP phenolate complexes the DBP moiety lies closer to the porphyrin plane in 10272


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Inorganic Chemistry

Figure 6. B3LYP/6-31G* optimized structures in the gas phase of BDP-COO-AlTPP·C60-PPV-pyr, BDP-COO-AlTPP·C60-pyr, BDP-O-AlTPP· C60-PPV-pyr, and BDP-O-AlTPP·C60-pyr. Carbon, nitrogen, hydrogen, oxygen, fluoro, aluminum, and boron atoms correspond to gray, blue, white, red, light blue, pink, and yellow spheres, respectively.

The electron density distributions and the corresponding energies of the frontier molecular orbitals (FMOs) of BDPCOO-AlTPP·C60-PPV-pyr, BDP-COO-AlTPP·C60-pyr, BDPO-AlTPP·C60-PPV-pyr, and BDP-O-AlTPP·C60-pyr are displayed in Figures S10−S13, respectively. In all supramolecular assemblies, the LUMO energy levels are predominantly spread on the C60 unit. However, the HOMO energy levels are mainly spread on the porphyrin ring, with some additional contributions on the bridging phenyl unit that connects AlTPP with BDP. The above distributions imply that in each supramolecular assembly charge separated state would be BDPAlTPP•+-C60•−. The HOMO−LUMO gaps of all supramolecular compounds were calculated and found to be similar to the corresponding values obtained from electrochemistry measurements and are presented in Table S5. Electrochemical Studies. The dyads BDP-COO-AlTPP and BDP-O-AlTPP, along with their four corresponding triads and the control compounds, were electrochemically studied using cyclic (CV) and differential pulse voltammetry (DPV) in o-dichlorobenzene (DCB) solutions containing 0.10 M (nBu)4NClO4 as the supporting electrolyte. The measured redox potentials are listed in Table 3, while representative voltammograms are shown in Figure 7. The cyclic voltammograms of the control compounds, Ph-O-AlTPP and Ph-COO-AlTPP, revealed their first reversible oxidation and first reversible reduction processes at 0.38 and −1.69 V and at 0.38 and −1.70 V (versus Fc/Fc+), respectively. In the DPVs of the dyads BDP-O-AlTPP and BDP-COO-AlTPP, while the first reversible oxidation was due to AlTPP, the second oxidation was attributed to the BDP moiety. The covalent bond between the two constituent chromophores (BDP and AlTPP) had little effect in the redox potential of the porphyrin as it can be observed from the tabulated data. In order to investigate the change in the redox potential of the dyads after coordination with C60-PPV-pyr or C60-pyr, electrochemical studies were performed in 1:1 mixtures of each both dyads with the corresponding functionalized fullerenes. While a slight anodic shift is observed in the first oxidation of BDP-O-AlTPP, about 40 mV, there was no change in the first oxidation of BDPCOO-AlTPP after addition of either C60-PPV-pyr or C60-pyr. On the reduction side, the first and second reduction potentials for the complex BDP-O-AlTPP·C60-PPV-pyr appeared at −1.17 V and −1.56 V, while for BDP-COO-AlTPP·C60-PPV-

comparison to the carboxylate ones. As presented in Figure S9, in the phenolate derivatives (BDP-O-AlTPP·C60-PPV-pyr and BDP-O-AlTPP·C60-pyr) the formed angles between the BDP unit and the porphyrin plane are 141° in both cases. On the other hand though, the relevant angles formed in the carboxylate derivatives are 173° and 176° for BDP-COOAlTPP·C60-PPV-pyr and BDP-COO-AlTPP·C60-pyr, respectively. Other important structural parameters that were examined are the Al−O−C angle along with the Al−O distance. In detail, the Al−O−C angle in the carboxylate derivatives (BDP-COO-AlTPP·C60-PPV-pyr and BDP-COOAlTPP·C60-pyr) is ∼135°, while the Al−O distance is 1.88 Å. However, in the AlPorphyrin phenolate complexes (BDP-OAlTPP·C60-PPV-pyr and BDP-O-AlTPP·C60-pyr) the Al−O− C angle is ∼140°, while the Al−O distance is 1.84 Å. We have also calculated the distance between the AlTPP and the C60 unit, as well as the distance between the BDP and the C60 unit. All calculated distances between the components in the supramolecular assemblies are listed in Table 2. The BDP Table 2. Optimized Distances between (a) AlTPP and C60, (b) BDP and C60 compound BDP-COO-AlTPP· C60-PPV-pyr BDP-COO-AlTPP· C60-pyr BDP-O-AlTPP·C60PPV-pyr BDP-O-AlTPP·C60pyr

distance between AlTPP and C60 (Å)

distance between BDP and C60 (Å)

13.07

17.24

6.73

10.66

12.65

15.27

7.04

9.96

entity is about 5−6 Å away in the case of carboxylate bound BDP-COO-AlTPP compared to phenolate bound BDP-OAlTPP. This distance and orientation suggest that the phenolate derivatives promote more efficient energy transfer from 1BDP* to AlTPP, a fact that is in agreement with the femtosecond transient spectroscopy results (vide supra). Interestingly, between C60-pyr and C60-PPV-pyr binding to AlP, there is about 5−6 Å increase in the distance in the latter system due to the PPV spacer. Under such conditions, the rate of electron transfer from 1AlP* to C60-PPV-pyr would be slower compared to that from 1AlP* to C60-pyr. 10273


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Table 3. Redox Potentials, Free-Energy Changes for Charge Separation (ΔGCS) and Recombination (ΔGCR) for the Investigated Donor−Acceptor Conjugates in o-DCBa compound

2nd Oxc

BDP Ph-O-AlTPP Ph-COO-AlTPP C60-PPV-pyr BDP-O-AlTPP BDP-COO-AlTPP BDP-O-AlTPP·C60-PPV-pyr BDP-COO-AlTPP·C60-PPV-pyr BDP-O-AlTPP·C60-pyr BDP-COO-AlTPP·C60-pyr

0.74 0.69

1st Oxb

1st Redd

2nd Redb

3rd Redc

−ΔGCR, eV

−ΔGCS, eV

1.59 1.55 1.57 1.55

0.55 0.59 0.57 0.60

−1.74 −1.69 −1.70

0.38 0.38 −1.17

0.71 0.81 0.69 0.71 0.69 0.71

0.38 0.39 0.42 0.38 0.41 0.39

−1.17 −1.17 −1.16 −1.16

−1.70 −1.67 −1.72 −1.58 −1.73 −1.74

−1.81 −2.03 −1.82 −2.01 −1.83 −2.00

a Only the first oxidation and first reduction potentials of each redox active entities are listed. bAlTPP centered first oxidation and first reduction potentials. cBDP centered first oxidation and first reduction potentials. dC60 centered first reduction reductions.

−ΔGCS = ΔE0 − 0 − ( −ΔGCR )

where E1/2(D•+/D) is the potential for the first oxidation process of the donor, AlTPP, E1/2(A/A•−) is the potential for the first reduction process of the acceptor, C60. The magnitude of both ΔGCR and ΔGCS values, listed in Table 3, suggest thermodynamic feasibility of formation of BDP-AlTPP•+-C60•− from the 1AlTPP* as the electron transfer product in the present series of triads. Femtosecond Transient Absorption Studies. First, an energy transfer from the singlet excited state of the BDP unit (1BDP*) to the AlTPP in the dyads was probed, by selective excitation of the BDP entity by a 500 nm laser (100 fs pulse width). The control pristine BDP compounds revealed instantaneous formation of their singlet excited states with the appearance of a negative peak at 508 nm as a result of ground state bleaching and stimulated emission (Figure 8a). The recovery of this peak was rather slow (Figure 8d(i)), as it can be expected for the relatively long-lived 1BDP* with an excited state lifetime of 3.4 ns. Interestingly, for both dyads (BDP-O-AlTPP and BDP-COO-AlTPP), transient spectral features demonstrating singlet−singlet energy transfer were observed. More specifically, in the case of BDP-O-AlTPP, the recovery of the instantaneously formed 1BDP*, revealed new positive peaks at 450 nm related to transitions originating from the energy transfer product (singlet excited state of AlTPP). In addition, a negative peak at 550 nm as well as two negative peaks at 590 and 650 nm were observed, corresponding to ground state bleaching and stimulated emission of 1AlTPP*, respectively. At higher delay times, conversion of 1AlTPP* to 3 AlTPP* was also observed with a new peak emerging in the 480 nm region. As expected, the recovery of the 1BDP* peak in the dyad was much faster (Figure 8d(ii)) due to the occurrence of competitive energy transfer. Comparable transient spectral features were observed for the BDP-COO-AlTPP dyad upon selective excitation of BDP as shown in Figure 8c. The rapid recovery of the 1BDP* peak at 510 nm was accompanied by a positive peak at 450 nm, and negative peaks at 550, 592, and 650 nm due to photochemical events originating from 1 AlTPP*. The time constants for the 1BDP* recovery for the BDP-O-AlTPP dyad was 1.07 and 410 ps, while that for BDPCOO-AlTPP was 2.33 and 1013 ps (biexponential fits perhaps due to the presence of two geometric isomers). Using these time constants along with the singlet lifetime of 1BDP*, we were able to calculate the rate of energy transfer, kENT. The determined values were found 9.3 × 1011 s−1 and 2.1 × 109 s−1

Figure 7. DPVs of the indicated compounds in DCB containing 0.1 M (n-Bu)4NClO4 supporting electrolyte. Scan rate = 5 mV/s, pulse width = 0.25 s, pulse height = 0.025 V. The asterisk indicates ferrocene oxidation used as an internal standard.

pyr they emerged at −1.17 V and −1.58 V. On the basis of the redox potential of the control compounds, the first oxidation potential in the hybrid triads is in both cases attributed to the AlTPP moiety, while the first reduction is credited to the C60PPV-pyr. This observation is well in agreement with the computational studies where for all four supramolecular assemblies the HOMO energy levels are mainly spread on the AlTPP ring, and the LUMO orbital is exclusively located on the C60 unit. Using the above redox potential data and the energy of the lowest excited state of the porphyrin (1AlTPP*) (E0,0 = 2.14 eV), the thermodynamic driving forces for the charge recombination (−ΔGCR) and charge separation (−ΔGCS), concerning the BDP-AlTPP•+-C60•− charge separated states, were estimated using eqs 1 and 2 by ignoring any contributions from solvation. −ΔGCR = E1/2(D•+ /D) − E1/2(A/A•−)

(2)

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Figure 8. Femtosecond transient spectra at the indicated delay times of (a) BDP, (b) BDP-O-AlTPP, and (c) BDP-COO-AlTPP in Ar-degassed toluene (500 nm of 100 fs laser pulses). (d) The time profile of the 508 nm peak.

Figure 9. Femtosecond transient spectra at the indicated delay times of (a) BDP-O-AlTPP, (b) BDP-COO-AlTPP, (c) BDP-O-AlTPP·C60-pyr, (d) BDP-O-AlTPP·C60-PPV-pyr, and (e) BDP-COO-AlTPP·C60-pyr in Ar-degassed toluene (400 nm of 100 fs laser pulses). (f) The decay time profile of the 1228 nm peak of 1AlTPP* of (i) BDP-O-AlTPP (green), (ii) BDP-O-AlTPP·C60-pyr (magenta), and (iii) BDP-O-AlTPP·C60-PPVpyr (blue). The decay profile of C60•− at 1016 nm of the corresponding triads is shown in their respective figure insets.

for BDP-O-AlTPP, and 4.3 × 1011 s−1 and 0.67 × 109 s−1 for BDP-COO-AlTPP revealing ultrafast energy transfer events.

Next, photoinduced electron transfer from 1AlTPP* to the coordinated C60 derivatives was investigated in the reported 10275


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Inorganic Chemistry triads. Here, the samples were excited at 400 nm where mainly the AlTPP moiety absorbs. Figure 9, panels a and b display the transient absorption spectra at the indicated delay times of BDP-O-AlTPP and BDP-COO-AlTPP dyads, respectively. In agreement with our previous report on femtosecond transient spectra of AlTPP,61 immediately after excitation, positive peaks at 450, 576, 610, 678, and 1228 nm were revealed, which are attributed to the generated 1AlTPP* entity. The 1228 nm peak has been ascribed to the singlet−singlet transition assigned to the 1AlTPP*.61 Apart from these positive peaks, negative peaks emerged at 546 nm and at 590 and 646 nm resulting from the ground state bleaching and the stimulated emission of 1 AlTPP*, respectively. As shown in the figure insets, the decay of the 1228 nm peak was rather slow consistent with the longer lifetimes of 1AlTPP* (7−8 ns) (see Table 1). It is also worth mentioning that in the 500 nm range a negative peak was also depicted corresponding to the ground state bleaching of 1 BDP*, suggesting part of the BDP unit was simultaneously excited. However, this peak is short-lived (only a few picoseconds) due to the singlet−singlet energy transfer phenomenon that occurs, leading to the population of the 1 AlTPP*. Coordination of either C60-pyr or C60-PPV-pyr to the AlTPP center of the dyad revealed additional spectral changes, supporting occurrence of photoinduced electron transfer from the 1AlTPP* to C60, as presented in Figure 9c−e. Key observations include the signature peak of the one electron reduced fullerene radical anion (C60•−) at 1016 nm and the relatively faster decay/recovery of the transient peaks corresponding to 1AlTPP* corroborating the hypothesis of an electron transfer process from the 1AlTPP* to the C60 derivatives. The transient peak corresponding to the one electron oxidized porphyrin (AlTPP•+) expected in the 620− 650 nm range was merged with the other transient peaks of 1 AlTPP*. The decay time profiles of the 1228 nm peak of the BDP-O-AlTPP dyad as well as its corresponding supramolecular triads, with either C60-pyr or C60-PPV-pyr, are displayed in Figure 9f. Faster decays were recorded in the case of both triads, with a slightly faster decay for the BDP-OAlTPP·C60-pyr complex due to the closer spatial proximity of C60 and AlTPP. However, the decay of this peak for the BDPO-AlTPP-based triads lasted over 3 ns suggesting relatively slow electron transfer. This was also the case of BDP-COOAlTPP derived triads. Under these conditions, the estimated rates were ∼108−109 s−1 and slightly higher for the supramolecular triads containing C60-pyr compared to those with the C60-PPV-pyr derivative. An attempt was also made to estimate the rate of charge recombination by monitoring the decay of C60•− at 1016 nm and these profiles are included in Figure 9c− e. In all the studied complexes the decay lasted over 3 ns, suggesting persistence of the charge separated state. Nanosecond transient spectra were also recorded to estimate the lifetime of all final charge separated states, even though the signals corresponding to C60•− were too weak for accurate rate determination. Similar spectral observations were also made for the BDP-COO-AlTPP·C60-PPV-pyr triad. From the slope of the C60•− decay plot in Figure 9, it is safe to say that the radical ion-pair persists approximately 100−120 ns in these supramolecular triads. All the different consecutive photochemical events detected for the four supramolecular triads reported in this study are presented in the form of an energy level diagram in Figure 10.

Figure 10. Energy level diagram depicting different photochemical process in BDP-AlTPP·C60 triads.

For the construction of this diagram, optical spectral data and free-energy values from Table 3 were utilized. Selective excitation of the BDP moiety, in the dyads and triads resulted in successful formation of 1BDP*. Both fluorescence (steadystate and time-resolved) and transient spectral studies revealed efficient energy transfer from 1BDP* to the energetically lowlying AlTPP. An electron transfer from 1BDP* to C60 to yield BDP•+-AlTPP-C60•− could be ruled out due to distance considerations and competing singlet−singlet energy transfer process involving AlTPP. The 1AlTPP* formed either as a product of energy transfer or by direct excitation could undergo intersystem crossing to populate the 3AlTPP* as the transient spectra of the dyads revealed. In the presence of C60, charge separation from 1AlTPP* to yield BDP-AlTPP•+-C60•− charge separated states has been monitored in all supramolecular assemblies. Nonetheless, the measured rates of electron transfer were found to be modest and increased distance between the AlTPP and C60 led to decreased rates. The lack of transient peak in the 700 nm range at the latter delay times corresponding to 3C60* implies the direct charge recombination of the radical ion-pair to the ground state.

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CONCLUSION The newly constructed supramolecular triads comprised of BDP, AlTPP, and C60 revealed several interesting features. First, the metal axial positioning resulted in setting the energy donor (BDP) and the electron acceptor (C60) at the opposite ends of the porphyrin plane. Moreover, it was possible to predict sequential energy transfer followed by electron transfer from studies involving optical absorption and emission, electrochemistry and computational studies. Additionally, through femtosecond transient spectroscopy studies, we were able to detect ultrafast singlet−singlet energy transfer from 1BDP* to AlTPP in both BDP-AlTPP dyads (kENT = 109−1011 s−1), accompanied by a better efficiency for BDP-O-AlTPP compared to BDP-COO-AlTPP due to distance and orientation factors. The measured energy transfer rates for the current axially positioned BDP-AlTPP dyads are comparable to those reported for BDP-zinc porphyrin dyads of similar distances (variation within 1 order of magnitude), 10276


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Photonics Inc., (Bozeman, MT), while the rest of the output was used for generation of a white light continuum. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. Data analysis was performed using Surface Xplorer software supplied by Ultrafast Systems. All measurements were conducted in Ar-degassed solutions at 298 K. Synthesis. AlPorphyrin (Al(OH)TPP),54 BDP-COOH,68 and BDP-OH69 were prepared according to the literature procedures. (4,4-Difluoro-8-[4-hydroxyphenyl]-1,3,5,7-tetramethyl-4-bora3a,4a-diaza-s-ndacenato)-[5,10,15,20-tetrakis(phenyl)porphyrinato]aluminum(III) (BDP-O-AlTPP). BDP-OH (20 mg, 0.06 mmol) and Al(OH)TPP (40 mg, 0.06 mmol), were heated at 40 °C in dry toluene (20 mL) under nitrogen atmosphere for 1 h. The reaction mixture was left to cool, and the solvent was evaporated. Dry hexane (10 mL) was added to the reaction flask and the solid residue was filtered under anaerobic conditions (yield 52 mg, 87%). UV/vis (toluene) λmax, nm (ε, mM−1 cm−1) 415 (484.4), 500 (51.9), 546 (18.1). 1H NMR (CDCl3, 300 MHz): δ 9.05 (s, 8H), 8.17 (d, 3JH,H = 5.7 Hz, 8H), 7.77 (m, 12H), 5.69 (m, 4H), 2.67 (d, 3JH,H = 8.3 Hz, 2H), 2.39 (s, 6H), 0.64 (s, 6H). MS (MALDI-TOF): m/z 978.365 [M]+, calculated 978.3610 for C63H46AlBF2N6O. Elemental analysis calcd (%) for C63H46AlBF2N6O: C, 77.30; H, 4.74; N, 8.59; Found: C, 77.34; H, 4.80; N, 8.52. (Phenolato)-[5,10,15,20-tetrakis(phenyl)-porphyrinato]aluminum(III) (Ph-O-AlTPP). Phenol (5.6 mg, 0.06 mmol) and Al(OH)TPP (40 mg, 0.06 mmol) were heated overnight at 70 °C in dry toluene (12 mL) under nitrogen atmosphere. The reaction mixture was left to cool, and the solvent was evaporated. To the reaction flask dry hexane (10 mL) was added and the solid residue was filtered under anaerobic conditions (yield 37 mg, 83%). UV/vis (toluene) λmax, nm (ε, mM−1 cm−1) 415 (441.3), 547 (17.0). 1H NMR (CDCl3, 300 MHz): δ 9.03 (s, 8H), 8.14 (d, 3JH,H = 5.9 Hz, 8H), 7.76 (m, 12H), 5.82 (m, 3H), 2.43 (m, 2H). MS (MALDI-TOF): m/z 732.225 [M]+, calculated 732.2470 for C50H33AlN4O. Elemental analysis calcd (%) for C50H33AlN4O: C, 81.95; H, 4.54; N, 7.65; Found: C, 81.90; H, 4.58; N, 7.68. (4,4-Difluoro-8-[4-carboxyphenyl]-1,3,5,7-tetramethyl-4-bora3a,4a-diaza-s-indacenato)-[5,10,15,20-tetrakis(phenyl)porphyrinato]aluminum(III) (BDP-COO-AlTPP). BDP-COOH (22 mg, 0.06 mmol) and Al(OH)TPP (40 mg, 0.06 mmol) were heated at 40 °C in dry toluene (20 mL) under nitrogen atmosphere for 1 h. The reaction mixture was left to cool, and the solvent was evaporated. To the reaction flask dry hexane (10 mL) was added and the solid residue was filtered under anaerobic conditions (yield 52 mg, 85%). UV/vis (toluene) λmax, nm (ε, mM−1 cm−1) 415 (500.4), 501 (54.7), 547 (17.5). 1H NMR (CDCl3, 300 MHz): δ 9.07 (s, 8H), 8.15 (d, 3 JH,H = 6.5 Hz, 8H), 7.75 (m, 12H), 6.34 (d, 3JH,H = 8.0 Hz, 2H), 5.77 (s, 2H), 5.17 (d, 3JH,H = 8.0 Hz, 2H), 2.43 (s, 6H), 0.73 (s, 6H). MS (MALDI-TOF): m/z 1006.358 [M]+, calculated 1006.3559 for C 6 4 H 4 6 A l B F 2 N 6 O 2 . E l e m e n t a l a n a l y s i s c a lc d ( % ) f o r C64H46AlBF2N6O2: C, 76.34; H, 4.60; N, 8.35; Found: C, 76.40; H, 4.64; N, 8.30. (Benzoato)-[5,10,15,20-tetrakis(phenyl)-porphyrinato]Al(III) (PhCOO-AlTPP). Benzoic acid (7.3 mg, 0.06 mmol) and Al(OH)TPP (40 mg, 0.06 mmol) were heated overnight at 70 °C in dry toluene (12 mL) under nitrogen atmosphere. The reaction mixture was left to cool, and the solvent was evaporated. Dry hexane (10 mL) was added to the reaction flask and the solid residue was filtered under anaerobic conditions (yield 43 mg, 93%). UV/vis (toluene) λmax, nm (ε, mM−1 cm−1) 415 (421.3), 547 (17.1). 1H NMR (CDCl3, 300 MHz): δ 9.07 (s, 8H), 8.16 (d, 3JH,H = 5.8 Hz, 8H), 7.74 (m, 12H), 6.75 (t, 3JH,H = 7.4 Hz, 1H), 7.56 (t, 3JH,H = 7.6 Hz, 2H), 5.15 (d, 3JH,H = 7.3 Hz, 2H). MS (MALDI-TOF): m/z 760.243 [M]+, calculated 760.2419 for C51H33AlN4O2. Elemental analysis calcd (%) for C51H33AlN4O2: C, 80.51; H, 4.37; N, 7.36; Found: C, 80.46; H, 4.40; N, 7.39.

suggesting axial positioning of BDP energy donor of appropriate distance and orientation is equally feasible to observe efficient excitation transfer.71,72 Furthermore, the 1 AlTPP* formed either by energy transfer or by direct excitation, promoted electron transfer to the coordinated C60 unit resulting into the formation of BDP-AlTPP•+-C60•− charge separated states of appreciable lifetimes. Finally, the anticipated sequential energy transfer followed by electron transfer (one photon to two major events), that is, mimicry of photosynthetic “antenna-reaction center” events, was successfully accomplished in the four reported supramolecular triads.

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EXPERIMENTAL SECTION

General. Buckminsterfullerene, C60, (+99.95%) was from SES Research, (Houston, TX). All the reagents were from Aldrich Chemicals (Milwaukee, WI), while the bulk solvents utilized in the syntheses were from Fischer Chemicals. Tetra-n-butylammonium perchlorate, (n-Bu4N)ClO4, used in electrochemical studies was from Fluka Chemicals. Synthesis of C60-pyr73 and C60-PPV-pyr43 are given elsewhere. The C60-based derivatives were purified by HPLC on a Recycling Preparative HPLC instrument LC9101 (Japan Analytical Instruments Co. Ltd.), using a Cosmosil BuckyPrep 20 × 250 mm preparative column, with toluene as eluent at a flow rate 10 mL/min. All reactions were carried out in an atmosphere of nitrogen by using conventional Schlenk techniques. Toluene was dried over 4 Å molecular sieves for 48 h. 1 H NMR and 13C NMR spectra were recorded on Bruker AVANCE III-500 MHz and Bruker DPX-300 MHz spectrometers, as solutions in CDCl3 by using the solvent peak as the internal standard (1H, 7.26 ppm; 13C, 77.16 ppm). Mass spectra were recorded on a Bruker ultrafleXtreme MALDI-TOF/TOF spectrometer using trans-2-[3-(4tert-butylphenyl)-2-methyl-2-propenylidene] malononitrile as matrix. Absorption spectra were recorded on a Shimadzu UV-1700 PharmaSpec instrument. Steady-state emission spectra were obtained using a JASCO FP-6500 fluorescence spectrophotometer. The fluorescence lifetimes were evaluated by using a Horiba Yvon Nanolog coupled with time-correlated single photon counting with nanoLED excitation sources. A right angle detection method was used. Cyclic and differential pulse voltammograms were recorded on an EG&G model 263A electrochemical analyzer using a three electrode system. A platinum button electrode was used as the working electrode. A platinum wire served as the counter electrode and an Ag/ AgCl electrode was used as the reference electrode. Ferrocene/ ferrocenium redox couple was used as an internal standard. All the solutions were purged prior to electrochemical and spectral measurements using nitrogen gas. Density functional theory (DFT)74 calculations were performed to predict the molecular structures along with the electronic properties of all the supramolecular assemblies, using GAUSSIAN 03 program suite.75 Theoretical calculations were carried out at the B3LYP/631G(d)76,77 level of theory using LANL2DZ basis set for Al atoms and the 6-31G(d) for lighter atoms. Computed structures and molecular orbitals were visualized and analyzed by ChemCraft software.78 For describing the solvent effect of o-dichlorobenzene, Tomasi’s polarized continuum model (PCM)79 was used with standard dielectric contestant ε = 9.99. Femtosecond transient absorption spectroscopy experiments were performed using an Ultrafast Femtosecond Laser Source (Libra) by Coherent incorporating diode-pumped, mode locked Ti:sapphire laser (Vitesse) and diode-pumped intra cavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of 1.10 W. For optical detection, a Helios transient absorption spectrometer coupled with femtosecond harmonics generator both provided by Ultrafast Systems LLC was used. The source for the pump and probe pulses were derived from the fundamental output of the Libra (Compressed output 1.10 W, pulse width 100 fs) at a repetition rate of 1 kHz. 95% of the fundamental output of the laser was introduced into a TOPASPrime-OPA system with 290−2600 nm tuning range from Altos 10277


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01050. Absorption spectra, emission spectra, fluorescence spectra, gas phase geometry optimized structures, Frontier molecular orbitals, Coordinates of gas phase geometry optimized structure, DFT calculated properties, 1H NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.G.C.). *E-mail: [email protected] (F.D.). *E-mail: [email protected] (N.T.). ORCID

Francis D’Souza: 0000-0003-3815-8949 Athanassios G. Coutsolelos: 0000-0001-5682-2968 Author Contributions #

A.B. and H.B.G. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been financed from the European Commission (FP7-REGPOT-2008-1, Project BIOSOLENUTI No. 229927), which is greatly acknowledged. A.G.C. acknowledges IKY fellowships of excellence for postgraduate studies in Greece-Research programs of excellence IKY Siemens. C.S. acknowledges IKY fellowships of excellence for postdoctoral studies in Greece-Siemens program. Also this research has been cofinanced by the European Union (European Social Fund− ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF)-Research Funding Program: Heraklitos II. Finally, the COST Action CM1202 PERSPECT-H2O and the Special Research account of the University of Crete are also acknowledged. Support by the USNational Science Foundation (Grant No. 1401188 to F.D.) is acknowledged.

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REFERENCES

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Axially Assembled Photosynthetic Antenna-Reaction Center Mimics

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