Conformational Gating of Charge Separation in Porphyrin Oligomer

Article pubs.acs.org/JPCC

Conformational Gating of Charge Separation in Porphyrin OligomerFullerene Systems Mélina Gilbert,‡ Louisa J. Esdaile,† Marie Hutin,† Katsutoshi Sawada,† Harry L. Anderson,† and Bo Albinsson*,‡ ‡

Department of Chemical and Biological Engineering/Physical Chemistry, Chalmers University of Technology, 412 96 Göteborg, Sweden † Department of Chemistry, Oxford University, Chemistry Research Laboratory, 12 Mansfield Road, Oxford OX1 3TA, United Kingdom S Supporting Information *

ABSTRACT: The rate of the photoinduced charge-separation in C60-terminated butadiyne-linked porphyrin oligomers Pn (n = 4, 6) is strongly influenced by their molecular conformation. In these systems, the presence of the butadiyne linkers gives rise to a broad distribution of conformations in the ground state, due to an almost barrierless rotation of individual porphyrin units in the oligomer chain. The conformational states of these oligomers, either twisted or planar, could be selected by varying the excitation wavelength, thereby providing different initial excited states for charge separation. Charge separation in the different conformers was followed using both steady-state and 2D timeresolved emission using a streak camera system. Singular value decomposition (SVD) analysis applied on streak camera data provides here a powerful tool to study the conformational dependence of the charge separation in long PnC60 systems. Both the kinetics and spectral changes accompanying charge separation could be analyzed for different populations of conformation. From this analysis we show that, for both systems studied, twisted conformations undergo faster charge separation than planar conformations. This disparity in charge separation rates was ascribed mainly to the difference in driving force for charge separation between twisted and planar conformations. Charge separation was also studied in oligomers PnC60 coordinated to an octadentate ligand T8 that hinders the rotation of porphyrin subunits. The semicircular complexes PnC60-T8 show dramatic changes in their spectral properties, as well as slow excitation wavelength independent rate of charge separation and corresponding low efficiency compared to their linear counterparts. This slow charge separation rate was attributed to fast relaxation to the lowest excited vibronic state and lack of driving force for charge separation in these close to planar semicircular systems; i.e., the template systems behave like “normal” donor−acceptor systems without slow conformational relaxation. This work illustrates how control of conformation can be used to tune the rate of charge separation.

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INTRODUCTION Electron transfer processes play an important role in both the natural photosynthesis and solar energy devices. Over the years, inspired by Nature, a large variety of donor−acceptor model systems has been designed and studied to obtain a better understanding of charge separation processes, and more generally the factors influencing charge transport in molecular systems. Indeed to use molecular systems as components in solar cells or functional electronic devices, one primary requirement is controlling the molecular factors that govern electron transfer.1−7 Conformational gating of the electron- and energytransfer processes has previously been observed in several systems containing flexible molecular bridges. Wasielewski and co-workers have already reported the effect of rotational dynamics in the excited state on charge separation in donorbridge-acceptor systems containing oligo-p-phenylene vinylene bridge molecules.8 In particular, they found that the excited state rotational dynamics induced a conformational gating of the donor−acceptor electronic coupling, and hence affected the charge separation rate constant. Examples of conformational © 2013 American Chemical Society

gating of energy transfer in directly meso−meso-linked zincporphyrin arrays have also been reported by Osuka and coworkers.9 Other molecular systems showing conformational dependence of their spectral and electron transfer properties have previously been reported by the groups of Sundström,10 ̀ and Durrant.11−13 Guldi, Martin, In the present work, we have investigated how conformation influences charge separation in long butadiyne-linked porphyrin oligomer-fullerene systems, PnC60 (n = 4, 6) (Figure 1). In these systems, the butadiyne linkers allow an almost barrierless rotation of individual porphyrin moieties, resulting in a ground state with a wide distribution of rotational conformers that vary with respect to the porphyrin−porphyrin dihedral angle. Previous studies have shown that the photophysical properties displayed by these systems are highly governed by this conformational flexibility.14−16 In particular, the influence of Received: October 3, 2013 Revised: November 25, 2013 Published: November 27, 2013 26482

dx.doi.org/10.1021/jp4098342 | J. Phys. Chem. C 2013, 117, 26482−26492

The Journal of Physical Chemistry C

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Figure 1. (left) Molecular structure of the studied model Pn and the D−A systems PnC60 studied in this work. The aryl substituents (Ar) are 3,5-bis (octyloxy)phenyl groups. (right) Molecular structure of the octadentate ligand T8 used in the formation of semicircular complexes P6C60-T8 and P4C60T8.

complexes PnC60-T8 (n = 4, 6) can be obtained by coordination of the oligomers PnC60 to an octadentate ligand T8 that prevents the rotation of individual porphyrins. Thus, the influence of conformation on charge separation has also been studied in constrained systems.

conformation on the spectral properties of the butadiyne-linked porphyrin dimer P 2 was investigated and two distinct spectroscopic species were identified, namely, a planar and twisted conformer, that could be selectively excited. The conformational dependence of charge separation was also studied in the fullerene-appended porphyrin dimer P2C60 and we found evidence of conformational gating of the electronic coupling. This resulted in an electron transfer rate for twisted conformations of P2C60 to be about 4 times higher than for planar conformers.15 More recently, the temperature dependence of the charge separation and recombination in PnC60 (n = 1−4, 6) systems has been investigated. For the entire series of PnC60 (n = 1−4, 6) systems, the charge separation reaction could be modeled by a combination of direct charge separation and migration of the excitation energy along the oligomer chain followed by charge separation, as the excitation energy could be localized on a porphyrin unit either far from or close to the fullerene acceptor.16 The energy migration step was found to be controlled by the temperature-dependent conformational dynamics of the long oligomers and to be the limiting step for a high quantum yield of charge separation. The aim of this study is different: we are not primarily interested in the conformational dependence of energy migration, but rather in the conformational dependence of the charge separation step and whether the inherent conformational flexibility of these systems could be used to control the charge separation rate. For these long oligomers, we demonstrate how the driving force for electron transfer can be tuned either by selective excitation of different conformers or by ligand coordination. Although the ground-state potential energy of these systems is quite insensitive to rotation of the porphyrins, the first excited state is known to depend strongly on the dihedral angles with a distinct minimum for the fully planar conformations.14 Thus, tuning the excitation wavelength makes it possible to prepare different initial excited states in term of conformations (twisted/planar) that, at least partially, charge separate together with C60 before conformational relaxation is completed. Several studies have also shown that the conformational distribution of porphyrin oligomers can be restrained and controlled by supramolecular assembly, via coordination of axial ligands to the zinc atoms to form circular constructs, planar assemblies, and ladder complexes.17−22 In this work, semicircular

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EXPERIMENTAL SECTION Materials. The measurements were done in either freshly distilled 2-methyltetrahydrofuran (2-MTHF) or a mixture of solvents dichloromethane/toluene (60:40 in vol) with a small percentage of pyridine added to prevent aggregation of the porphyrin oligomers. For the semicircular complexes PnC60-T8 (n = 4, 6), it was necessary to use a mixture of dichloromethane and toluene (60:40 in vol) due to the absence of electron transfer in pure toluene. The octadentate ligand T8 could be dissolved in toluene, and was then added to the solution of porphyrin oligomers dissolved in dichloromethane with 0.1% pyridine added. Synthesis of the compounds PnC60 (n = 4, 6) and the octapyridyl ligand T8 have already been reported elsewhere.16,20,23,24 Temperature Studies. For investigating the temperature dependence of the charge separation process, a temperature controlled cryostat (Oxford Instrument) cooled with liquid nitrogen was used. Steady-State Absorption and Emission Spectroscopy. Ground-state absorption spectra were measured on a Cary 5000 UV−vis spectrophotometer. The spectra were recorded between 350 and 1000 nm with a spectral bandwidth of 0.5 nm. The sample was contained in a 1 cm quartz cuvette. Fully corrected emission spectra were recorded on a Spex Fluorolog 3 spectrofluorimeter equipped with a Xenon lamp. Emission spectra were recorded by exciting the sample in the wavelength range 730−840 nm and by collecting emission between 600 and 1000 nm. The spectral bandwidth for the emission and excitation monochromators was between 3 and 6 nm to get a good signalto-noise ratio. For all excitation wavelengths, emission spectra were averaged over 10 scans and were measured in two steps by recording emission below and above the excitation wavelength with a gap of 20 nm around the excitation wavelength to avoid distortion due to scattering of the excitation light. 26483



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Figure 2. (top) Ground-state absorption spectra of the linear systems: (a) P4C60 (red) and (b) P6C60 (red) in 2-MTHF with 1% pyridine added. (bottom) Ground-state absorption spectra of the semicircular complexes: (c) P4C60-T8 (red) and (d) P6C60-T8 (red) in DCM/Toluene (60/40) with 0.1% pyridine added. The absorption spectra of the reference systems, Pn and Pn-T8, are plotted in gray. All measurements were done at room temperature.

Time-Resolved Fluorescence Spectroscopy. Time-resolved emission measurements were carried out using a streak camera system. Excitation pulses (1−2 ps broad) with a repetition rate of 82 MHz were generated by a Tsunami Ti:sapphire laser (Spectra-Physics) that was pumped by a Millenia Pro X laser (Spectra-Physics). The Tsunami output was tuned to generate excitation wavelengths in the range 730−840 nm. A beam attenuator was used to reduce the excitation pulse energy hitting the sample to ca. 0.1 nJ for all measurements. The emitted photons were passed through a spectrograph (Acton SP2300, Princeton Instruments) and finally recorded by a streak camera (C5680, Hamamatsu) with a synchroscan unit (M5675, Hamamatsu). Negligible changes in the absorption spectra of the samples were observed following fluorescence lifetime measurements confirming sample integrity. The acquisition mode used was photon counting with approximately 10 to 20 million photons recorded per image. Singular value decomposition (SVD) analysis was applied on the series of fluorescence decays from the emission wavelength range to extract the most significant rate constants and fluorescence lifetimes (see Supporting Information for details). The SVD analysis was performed using an in-house built Matlab script. Thus for each excitation wavelength, the fluorescence decay curves could be fitted globally over the whole emission wavelength range from ∼745 nm to ∼865 nm. For the semicircular complexes PnC60-T8 (n = 4, 6), due to their low fluorescence quantum yield and absence of emission wavelength dependence, time-resolved fluorescence measurements were carried out using time-correlated single photon counting (TCSPC) at selected emission wavelengths. As described in the previous paragraph, the excitation was provided

by a tsunami Ti:sapphire laser (Spectra-Physics) pumped by a Millenia Pro X (Spectra-Physics). But instead of a streak camera, the emitted photons were collected by a thermoelectrically cooled microchannel plate photomultiplier tube (R3809U-66, Hamamatsu) with a spectral bandwidth for the emission set to 6 nm. The signal was digitalized using a multichannel analyzer with 512 channels (SPC-300, Edinburgh Analytical Instruments), and in order to get good statistics, at least 10 000 counts in the top channel were recorded. The decay was then fitted to monoexponential expressions by the program FluoFit Pro v 4 (PicoQuant GMBH) after deconvolution of the data with the instrument response function.

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RESULTS AND DISCUSSION Ground-State Characterization. Figure 2 shows the ground state absorption spectra of both donor−acceptor systems P4C60 and P6C60 together with the absorption spectra of the reference systems Pn (n = 4, 6). The fullerene substituent does not have substantial absorption in the displayed region and interacts very little with the ground state of the porphyrin oligomers. The spectra of the D−A systems and their respective references are consequently almost identical. All spectra show a strong Soret band with a maximum at 465 nm and a Q-band that extends from 650 nm to ca. 850 nm. A recent study on the dimer system P2 has shown that, in the Q-band, twisted conformations predominantly absorb at short wavelength (∼670 nm) while more planar conformers absorb at long wavelength (∼740 nm).14,15 This was also supported by time-dependent DFT (TDDFT: B3LYP/6-31G*) calculations of the electronic transitions for the different conformers. As expected, the energy 26484



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of the dihedral angle for the dimer are expected to also prevail for longer oligomers. In other words, for long oligomers, the Q-band is expected to be sensitive to the dihedral angle with planar conformations predominantly absorbing at long wavelength and twisted conformers at shorter wavelength. In Figure 2a and b, the arrows indicate the excitation wavelengths chosen in the Q-band to excite different populations of conformers of P4C60 and P6C60. When the octadentate ligand T8 is added to a solution of P4C60 and P6C60, 1:1 semicircular complexes PnC60-T8 are formed in which rotation of individual porphyrin units is prevented (see Figure 2). Figure 2c and d shows the ground-state absorption spectra of, respectively, P4C60-T8 and P6C60-T8 along with the spectra of the respective reference systems Pn-T8. All spectra present similar features with a new red-shifted peak growing in and a more structured Q-band as a result of the increased rigidity and planarity of the systems. The octadentate ligand T8 has absorption bands in the region from 300 to 650 nm and the final spectra above 650 nm are exclusively that of the oligomer part of the complexes P4C60-T8 and P6C60-T8. As an example, Figure 4 shows in more detail the spectral changes occurring upon titration of P6 with the octadentate ligand T8. Similar spectral changes were observed for P4, P4C60, and P6C60 upon titration with the octadentate ligand T8 (see Figures S1− S3 in the SI). Photoinduced Charge Separation Process. Steady-state emission measurements gave general information on the charge separation process in PnC60 systems and its quantum yield. As reported previously, quenching of the emission intensity of Pn in the presence of C60 can be directly related to the charge separation reaction and formation of the biradical Pn+C60−.15,16 Figure 5 shows the charge separation quantum yields, respectively, for (a) P6C60 and (b) P4C60 as a function of the excitation wavelength at 300 K calculated from steady-state and time-resolved measurements. At some excitation wavelengths, the two calculation methods give quite different values of the charge separation quantum yield. The origin of this difference will be discussed below in connection with the suggested model for charge separation. Nevertheless, both systems P4C60 and P6C60 show efficient charge separation at all excitation wavelengths. For the longer system P6C60, the shape of the emission spectra did not vary significantly with the excitation wavelength at 300 K (Figure S4 in the SI). However, the

of the electronic transitions varied significantly with the dihedral angle. Increasing the dihedral angle between the two porphyrin moieties from 0° (planar) to 90° (twisted) resulted essentially in a blue-shift of the Q-band. Figure 3 shows the position of the

Figure 3. (top) Calculated electronic spectra for the planar conformer of P1 (black), P2 (red), P3 (green), P4 (blue), and P6 (cyan). (TDDFT: B3LYP/6-31G*) (bottom) Ground-state absorption spectra of P1 (black), P2 (red), P3 (green), P4 (blue), and P6 (cyan).

lowest electronic transitions in the Q-band spectral region for the planar conformer of the entire series of Pn (n = 1−4, 6) systems estimated by time-dependent DFT. For all oligomers Pn (n = 1− 4, 6), a single electronic transition dominates in the longwavelength region of the Q-band irrespectively of the size. Also, as the oligomers get longer, the electronic transition shifts to longer wavelengths due to the increased π-conjugation. Although an increasing number of rotational conformers exist for the longer oligomers PnC60 (n = 4−6), the experimentally observed behavior and the calculations of electronic transitions as function

Figure 4. (left) The Q-band region of P6. A solution of P6 titrated by the octadentate ligand T8 in DCM/Toluene (60/40) with 0.1% pyridine added. The total concentration of P6 was approximately 0.4 μM. Inset: concentration of P6-T8 complex formed vs concentration of template T8 added with fitted binding curve yielding Kb = 9.7 × 107 M−1. (right) Molecular structure of the semicircular complex P6-T8. 26485



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Figure 5. (top) Quantum yield for charge separation as a function of the excitation wavelength based on steady-state (blue) and time-resolved fluorescence (black) measurements for (a) P4C60 and (b) P6C60 in 2-MTHF with 1% pyridine added at 300 K. (bottom) Quantum yield for charge separation as function of the excitation wavelength based on steady-state (blue) and time-resolved fluorescence (black) measurements for (c) P4C60-T8 and (d) P6C60-T8 in DCM/Toluene (60/40) with 0.1% pyridine added at 300 K. The values obtained from steady-state measurements were calculated as 1 − If(PnC60)/If(Pn), where the If values are the integrated fluorescence intensities from samples of PnC60 and Pn with equal absorbance at the excitation wavelength. The Q-band regions of the normalized absorption spectra are shown in gray solid lines. The normalized emission spectra represented in gray dashed lines were recorded using the excitation wavelengths 765 nm for the tetramer systems (P4C60 and P4C60-T8) and 813 nm for the hexamer systems (P6C60 and P6C60-T8).

been observed by Anderson and co-workers in circular complexes c-P6-T6 with T6 a hexadentate ligand.22,23,25 Upon binding to T6 the fluorescence quantum yield for P6 dropped from ϕf (P6) = 0.08 to ϕf (c-P6-T6) = 0.0012.22 This was interpreted as a consequence of symmetry-related constraints on the electronic transitions that arise when changing the geometry of the linear hexamer to a cyclic hexamer.23 Here, both systems P4C60 and P6C60 form more opened structures with T8 and hence the symmetry-related selection rule relaxes. Thus, less significant drop of the fluorescence quantum yields is observed in P4C60-T8 and P6C60-T8 complexes. Charge separation quantum yields of both P4C60-T8 (Figure 5c) and P6C60-T8 (Figure 5d) were also calculated as function of the excitation wavelength from steady-state and time-resolved measurements. Both semicircular complexes PnC60-T8 (n = 4, 6) showed wavelength independent and much lower charge separation efficiencies than their linear counterparts. This was also reflected in the wavelength independent shape of the emission (see Figures S8 and S9 in the SI). The absence of dependency of the charge separation efficiency with the excitation wavelength can be explained by the increased rigidity and planarity of the systems in the presence of the ligand. Similar to when exciting more planar conformations (at long wavelength in the Q-band), the quantum yield for charge separation decreases when forming semicircular complexes. The low charge separation efficiency shown by the template systems could not be assigned to the change in solvent polarity, but rather to planarization induced by coordination to the template. Charge separation quantum yields of the linear system P6C60 are similar in DCM/Toluene (60/40) and 2-MTHF clearly showing that the two solvents have minor influence on the charge separation in this case (see Figure S10 in the SI). Thus, steady-state measurements provide a first evidence of the conformational

quantum yields of charge separation varied systematically with the excitation wavelength and more particularly decreased with increasing excitation wavelength, i.e., as more planar conformations are excited. At low temperature (180 K) a similar trend was observed for P6C60 and the variation in charge separation efficiency was also accompanied by a change in the shape of the emission with the excitation wavelength (Figures S5−S6 in the SI). Excitations at 730 and 780 nm result in broader and blueshifted emission, indicative of emission from an increasing number of nonrelaxed conformers. For the shorter system P4C60, excitation at short wavelength leads also to a broader and blueshifted emission from nonrelaxed conformers (Figure S7 in the SI). However, the charge separation efficiency shows less dependence on the excitation wavelength than P6C60, which can be attributed to the narrower distribution of conformers. From previous studies it is known that, in the excited state, the planar conformation is stabilized and twisted conformations tend to planarize before emission if the conditions (temperature and/ or viscosity) allow it. Thus, here the observed change in the emission shape confirms that excitation of long oligomers PnC60 (n = 4, 6) at short wavelengths in the Q-band mainly populates twisted excited states, while long wavelength excitation produces more planar excited conformations. Emission spectra of the semicircular D−A systems PnC60-T8 (n = 4, 6) and their respective reference compounds were also recorded as function of the excitation wavelength (Figures S8− S9 in the SI). These semicircular complexes showed strongly redshifted onsets of their absorption and emission (40 nm) and very small Stokes shifts (Δλ ∼10 nm), along with lower fluorescence quantum yields than the linear systems. As an example for P6, upon binding to T8 its fluorescence quantum yield dropped from ϕf (P6) = 0.08 to ϕf (P6-T8) = 0.028. Similar effects have already 26486



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Figure 6. 2D streak camera emission images of P6C60 at 300 K in 2-MTHF with 1% pyridine excited at 730 nm (top left), 780 nm (top right), 820 nm (bottom left), and 840 nm (bottom right).

Figure 7. (left) 2D streak camera images of the emission of P6C60 at 300 K in 2-MTHF with 1% pyridine excited at 780 nm. (right) Extracted fluorescence decays of P6C60 at two emission wavelengths 788 and 817 nm (marked with white dotted lines on the 2D image).

recorded with the same excitation wavelengths (Figures S11− S14 in the SI). For both P6 and P6C60, the shapes of the emission vary significantly with the excitation wavelength. The behavior observed in steady-state is more evident here with the presence of fast fluorescence decays when exciting twisted conformers (λexc = 730 nm and λexc = 780 nm) and slower fluorescence decays dominating the emission when exciting predominantly planar conformers (λexc = 820 nm and λexc = 840 nm). Similar

dependence of the electron transfer rates and show that planar conformations are less prone to undergo charge separation. 2D-time-resolved fluorescence measurements were performed to get a more detailed understanding of the influence of conformation on the kinetics of the charge separation process. 2D streak camera images of the emission of P6C60 at 300 K excited at 730 nm, 780 nm, 820 nm, and 840 nm are shown in Figure 6. As reference measurements, 2D images of P6 were also 26487



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done with a streak camera giving simultaneous monitoring of all emission wavelengths. This dramatically increases the information content compared to single wavelength detection. In an attempt to benefit from the increased information content we developed a singular value decomposition (SVD; see the Supporting Information) method combined with a decay model based on structural relaxation of the twisted conformers into more planar conformers. Planarization of the P4 and P6 donors occur in the 100−500 ps range and therefore compete with the fluorescence decay (600−800 ps). In the PnC60 compounds charge separation occurs on a wide range of timescales, and in order to get meaningful information from fitting the fluorescence decays, all the rate constants related to the planarization and natural decays were fixed to the values found from fitting the decays of the model compounds Pn. In Figure 8, a three state model describes the planarization of the porphyrin oligomer with Pn#, Pn¤, and Pn* corresponding to a twisted, intermediately twisted, and planarized oligomer in the excited state, respectively. This was needed to accurately fit the decays of the model systems when excited at short wavelengths (730 and 780 nm populating the most distorted conformers). For excitation of predominantly planar conformers (Pn¤ and Pn*), only two conformational states were necessary to accurately fit the decays. The extracted rate constants (relaxation rates: k1, k2; and natural decay rates: k#, k¤, and k*) were fixed in the fit of the decays for PnC60 excited at the same wavelength. The fluorescence decay rates (i.e., radiative rate + nonradiative rate in absence of planarization) were only weakly dependent on the conformational states, and almost equally good fits were obtained with the assumption k# = k¤ = k*. As an example, Figure 9 shows the results of the SVD analysis for a sample of P6C60 excited at 730 nm. Three conformational populations contribute to the fluorescence emission in this case (Figure 9c): the first population (in blue) shows a broad emission but dominates the emission at short wavelength, while the third population (in red) emits mostly at higher wavelengths with a maximum at 830 nm. This gradual red-shift in emission wavelength with increasing time is common for all systems studied for both the reference and donor−acceptor systems showing that conformational relaxation of the porphyrin oligomers interplay with the charge separation process. The extracted fluorescence decays of these three species (Figure 9d) show, in agreement with our model, that the first population of excited state can be assigned to twisted conformers and decays monoexponentially with a short lifetime, while the third population is populated by two consecutive relaxations of the twisted conformations, and corresponds to more planar conformations. When exciting mostly planar conformations (λexc = 820 nm and λexc = 840 nm), two populations contribute to the emission (Figures S16−S17 in the SI) both with an emission centered at ca. 825−830 nm, suggesting that relaxation is almost negligible in this case and charge separation occurs only from close to planar conformations. From the time and spectral components extracted from the SVD analysis, 2D images of the emission obtained for the different excitation wavelengths could be reconstructed for both reference systems Pn and D−A systems PnC60 (Figures S11−S33 in the SI). Comparison of the measured (Figure 9a) and reconstructed (Figure 9b) 2D images shows that a reasonably good fit was obtained using the SVD analysis. Similar results were obtained for the SVD analysis of the emission from P4C60 and P4 (see Figures S18−S25 in the SI). Table 2 gives the rate constants for the charge-separation processes as function of the excitation wavelength for P6C60 and P4C60 at 300 K obtained from the optimization procedure. For

effects were observed for the shorter systems P4 and P4C60 when varying the excitation wavelength between 730 and 810 nm (Figures S18−S25 in the SI). The fluorescence decays of the Pn and PnC60 systems are moderately complex, and depend on both excitation and emission wavelengths. As an example, Figure 7 compares the extracted fluorescence decays of P6C60 excited at 780 nm and reveals a short lifetime at short emission wavelength (e.g., 788 nm), while longer lifetimes dominate at longer emission wavelengths (e.g., 817 nm). The 2D streak camera images, in contrast to single wavelength detection, provide additional information on the combination of processes occurring in the excited state. In particular, the shape of the emission suggests a competition in the excited state for twisted conformations between the relaxation (via planarization) and the charge separation processes. In an attempt to slow down the relaxation of twisted conformers in the excited state and observe almost exclusively the charge separation process, samples of P6 and P6C60 were also cooled down to 180 K. The 2D streak camera images of both P6 and P6C60 recorded at 180 K show fluorescence decays again that depend on both emission and excitation wavelength (Figures S26−S33 in the SI). Despite the low temperature, a moderately complex behavior was still observed with both relaxation and charge separation processes slowed down. However, ligand coordination provides a simpler model system to investigate the influence of conformation on the charge separation. By locking the rotation of individual porphyrin moieties, no relaxation process was observed in the excited state, and hence the emission of both model Pn-T8 and D−A systems PnC60-T8 showed no dependency on the emission wavelength (Figure S34 in the SI). The fluorescence decays could be measured using single-wavelength detection while varying the excitation wavelength (Figures S35−S36 in the SI). Due to the increased rigidity and planarity of the systems, the fluorescence decays of Pn-T8 and PnC60-T8 did not depend on the excitation wavelength and could be fitted to simple monoexponential decays. Table 1 provides the fitted lifetimes and the calculated Table 1. Fluorescence Lifetimes τf, Rate Constants for Charge Separation kCS, and Quantum Yield for Charge Separation ϕCS for PnC60-T8 Complexes Excited at 735 nm τf(Pn-T8)/ps a

tetramer system hexamer systemb

499 430

τf(PnC60-T8)/ps

kCSc/s−1

ϕCSd

398 374

5.1 × 10 3.5 × 108

0.20 0.13

8

The lifetimes for P4-T8 and P4C60-T8 were obtained from fitting the fluorescence decays measured using TCSPC. bThe lifetimes for P6-T8 and P6C60-T8 were obtained from fitting the fluorescence decays measured using a streak camera system. cThe charge separation rate constant kCS was determined using the formula kCS = 1/τf (PnC60-T8) − 1/τf (Pn-T8). dThe quantum yield for charge separation ϕCS was calculated as ϕCS = kCS · τf (PnC60-T8). a

rate constants and efficiencies for charge separation. In the presence of the electron acceptor, the fluorescence lifetimes are slightly shorter than for Pn-T8, and hence slow rate for charge separation and corresponding low charge separation quantum yields in agreement with steady-state emission results. Model for Charge Separation. To reach a more complete understanding of the complex interplay between structural relaxation and charge separation in the linear systems requires that the excitation energy is varied while monitoring multiple emission wavelengths. As presented above, measurements were 26488



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Figure 8. Kinetic model used to describe structural relaxation and charge separation in the studied systems. Left: Model Pn systems. Right: Donor− acceptor PnC60 systems. The different rate constants involved are explained in the text.

Figure 9. (a) Normalized 2D streak camera emission image of P6C60 excited at 730 nm at 300 K in 2-MTHF with 1% pyridine. (b) Reconstructed 2D emission image of P6C60 excited at 730 nm. This image was built from the data obtained in the fitting procedure. (c) Spectral components and (d) decays of the three species contributing to the fluorescence emission. The color code used in (c) and (d) is the same, i.e., the fluorescence decay in blue corresponds to the species with the blue emission spectrum, and so forth.

the charge separation rate constant from nonrelaxed states k#CS is at least fifteen times larger than kCS * , the charge separation rate after relaxation. Also, direct excitation of more planar conformers (λexc = 840 nm) results in very slow charge separation with k*CS = 0.9 × 109 s−1. The shorter oligomer P4C60 shows less variation of the charge separation rate constant with the excitation wavelength. This can be explained by the narrower conforma-

clarity, only the charge separation rate constants are reported in Table 2. The natural decay rates (k#, k¤, k*) and relaxation rates (k1, k2) are listed in Tables S1−S2 in the SI. In general, independently of the excitation wavelength and the size of the oligomer, the charge separation occurs predominantly from nonrelaxed states. When exciting predominantly twisted conformations (λexc = 730 nm and λexc = 780 nm for P6C60), 26489



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Table 2. Wavelength Dependence of the Charge Separation Rate Constants for P4C60 and P6C60 at 300 Ka P4C60

P6C60

λexc/ nm

k#CS/109 s−1

k¤CS/109 s−1

kCS * /109 s−1

λexc/ nm

k#CS/109 s−1

k¤CS/109 s−1

kCS * /109 s−1

730 760 790 810

13.1 14.9 -

2.7 2.7 7.4 6.0

0.9 0.0 1.5 0.6

730 780 820 840

8.6 8.8 -

0.0 2.8 3.3 0.0

0.3 0.5 0.5 0.9

a Estimated average errors in the fitted rate constants are below 0.5 ×109 s−1.

tional distribution as well as a larger driving force for charge separation (vide inf ra). From the optimized rate constants, the fluorescence lifetimes of the different intermediate excited states could be calculated and are provided for both D−A systems and their respective reference compounds in the Supporting Information (Tables S3−S5). In general, the shortest lifetimes were much shorter than those of the systems without the electron acceptor. The long lifetimes were of the same order of magnitude for all excitation wavelengths and for P6C60 close to the lifetime of the reference system P6. To explain the differences observed in charge separation rates, the factors that influence the rate for electron transfer were analyzed. The charge-separation rate constant is expected to follow the Marcus equation and depends on several parameters: the electronic coupling (V), the reorganization energy (λ), and the thermodynamic driving force for the charge-separation process (ΔG0). The driving forces for charge separation between Pn and C60 are quite small and varies between approximately −0.6 and −0.2 eV for n = 1 to n = 6.26 This most certainly means that the charge separation process is well in the Marcus normal region and thus the rates, based only on driving force differences, would be expected to decrease for the longer oligomers. As argued above and from previous studies, it is known that twisted conformations of the donor Pn lie at higher energy in the excited state,14,15 and hence a larger driving force is expected for direct charge separation from these states. This explains the faster charge-separation rate k#CS observed for twisted conformers in P6C60 and P4C60. Because of a small driving force, the charge separation from more planar conformations is very slow. Thus, the variation of the charge separation rate constant with the excited conformations can, at least in part, be attributed to the driving force between the excited state and the charge-separated state that varies with respect to the excited population of conformers. Figure 10 illustrates the variation of driving force for charge separation to occur with respect to the excited conformation for the entire series PnC60 (n = 1−4, 6). Naturally, other factors like the electronic coupling and the reorganization energy affect the charge separation rate. In the present work, all experiments were done at room temperature; thus, it is difficult to predict how the electronic coupling and the reorganization energy for the charge separation vary with respect to the excited conformation. To assess both the electronic coupling and the reorganization energy will require a detailed analysis of the temperature dependence of the charge separation in planar and twisted conformers, which is beyond the scope of this paper. Nevertheless, in a recent study on the entire series PnC60 (n = 1− 4, 6), the temperature dependence of the charge separation was analyzed when exciting a broad distribution of conformers and both parameters, reorganization energy and electronic coupling, could be extracted.16 This work revealed that the reorganization

Figure 10. Energy diagram for the charge separation process estimated from spectroscopic data. The distribution of conformers gets broader as the length of the oligomer increases. The methods used to determine the energies of the excited and charge separated states are described in the SI.

energy is quite insensitive to the length of the oligomers, i.e., the distribution of conformations. Thus, from the previously reported values of the reorganization energy, the activation energy, ΔG‡ = (ΔG0 + λ)2/4λ, for the charge separation reaction can be estimated for twisted conformations and more planar conformations. For both systems P4C60 and P6C60, the difference in the activation energy barrier between twisted and planar conformers is quite substantial. For P4C60, the activation energy varies from 0.038 eV for twisted conformers to 0.085 eV for more planar conformations. This difference in activation energy is even more pronounced in the longer oligomer P6C60 with ΔG‡twisted = 0.029 eV and ΔG‡planar = 0.093 eV. In comparison for the shorter oligomer P2C60, a previous work showed that the activation energy barriers for twisted and planar conformers were approximately equal (ΔG‡twisted = 0.018 eV and ΔG‡planar = 0.015 eV) due to small difference in driving force.15 The faster charge separation observed in twisted conformations of P2C60 was mainly due to a larger electronic coupling.15 Here, for longer oligomers, as shown in Figure 10, the distribution of conformers is broadened when the length of the oligomer increases. Because of the increased π-conjugation in long oligomers, the planar conformers are more stabilized in the excited state and the driving force for charge separation decreases resulting in a higher activation energy for charge separation from these conformers. Thus, for the long systems PnC60 (n = 4, 6), the driving force difference for charge separation from twisted conformations and planar conformations is more pronounced than for shorter oligomers PnC60 (n ≤ 3) and gives rise to the difference in charge separation rates observed. In a previous study of the series of C60 appended porphyrin oligomers, a slightly different picture emerged.16 In this study, excitation into the Soret band (495 nm) led to a broad range of conformers being excited. The heterogeneous and temperature dependent decays of the excited porphyrin oligomers were interpreted with a model in which charge separation was preceded by an energy migration process.16 In this model, upon excitation, two populations of conformers could be distinguished: active and inactive conformers, respectively. While for the active conformer the excitation was located close to the C60 and direct charge separation could occur, in the inactive conformer excitation energy was initially located far away from the donor and needed to migrate before charge separation. 26490



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statistical reasons. For the template system P6C60-T8, a very good agreement between steady-state and time-resolved emission is observed (Figure 5d). The shorter template system P4C60-T8 shows a less good agreement between the two ways of calculating the charge separation efficiency, in particular when exciting at short wavelength (Figure 5c). The larger quantum yield of charge separation measured in steady-state can be explained by a small amount of linear P4C60 present in the sample, for which charge separation is also more efficient.

Although energy migration along the porphyrin oligomers most definitively plays an important role upon Soret band excitation, when exciting into the Q-band (as in this study) a narrower selection of conformers is populated. This, together with decay analysis of the complete emission spectrum, gives rise to an even more complex picture in which the conformers with low excitation energies instead show charge separation rates that are limited by the available driving force. In summary, depending on how the excited state of the longer porphyrin oligomer (tetramer or hexamer) is prepared we either see a charge separation that is rate limited by energy migration to the active conformer or charge separation that is rate limited by lack of driving force (cf. Figure 10). Finally, additional strong evidence that the driving force is the determining factor for charge separation in long oligomers PnC60 (n = 4, 6) comes from time-resolved measurements performed on the semicircular complexes PnC60-T8. As mentioned previously, the PnC60-T8 complexes show a slow rate of charge separation compared to the free systems PnC60. Again, due to the almost planar conformation that the porphyrin oligomer adopts, the excited state of these PnC60-T8 complexes is estimated to lie very close to the charge-separated state energy level, resulting in a very small driving force for charge separation and consequently a low quantum yield for the formation of the charge-separated state Pn+C60−. Quantum Yield for Charge Separation. Figure 5 compares the quantum yields for charge separation obtained from steady-state measurements and from the fitted lifetimes or the extracted rate constants (Tables 2 and S1 in the SI) for the PnC60-T8 and PnC60 systems, respectively. According to the kinetic model and rate constants presented in Figure 8, the quantum yield for charge separation for PnC60 systems can be calculated as φCS =

kcs# #

k + k1 +

kcs#

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CONCLUSION From this study of the conformational dependence of charge separation in long porphyrin oligomer-fullerene systems PnC60 (n = 4, 6), there are several conclusions that we wish to emphasize: 1. Charge separation from the excited donor occurs primarily from unrelaxed conformations, independently of the oligomer size. 2. The charge separation rate is significantly faster from twisted conformations of the donor Pn than from the planar conformer. This is explained partly by the higher energy stored in twisted excited states, resulting in a larger driving force for charge separation. 3. The results show that the conformational state of the porphyrin chain exerts a significant control on the charge separation, which leads to conformational gating of charge separation. 4. The concept of conformational gating was also explored in semicircular complexes PnC60-T8 in which rotational motion of the individual porphyrin unit was restricted by the use of the octadentate ligand T8. As expected, these template systems behave like “normal” donor−acceptor systems and did not show any conformational gating (i.e., no excitation wavelength dependence) of the charge separation. Moreover, the lack of driving force for charge separation in these close to planar structures combined with fast vibrational relaxation result in a slow rate of charge separation and corresponding low efficiency compared to the linear oligomers PnC60. 5. Finally, these results illustrate the importance of taking conformational dynamics into account when analyzing electron transfer processes in extended donor−acceptor systems.

⎛ ⎞ k1 ⎟ + ⎜⎜ # #⎟ ⎝ k + k1 + kcs ⎠

⎞ ⎛ ⎞ ⎛ kcs¤ k1 ⎜ ⎟⎟ ×⎜ ¤ + ⎟ ⎜ ¤ ⎝ k + k 2 + kcs ⎠ ⎝ k # + k1 + kcs# ⎠ ⎛ ⎞ ⎛ kcs* ⎞ k2 ⎟ ×⎜ ¤ ¤⎟ × ⎜ * ⎝ k + k 2 + kcs ⎠ ⎝ k + kcs* ⎠

(1)

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As mentioned previously, particularly for the longer system P6C60 (Figure 5b), the two ways of calculating the quantum yields for charge separation show some inconsistencies. In the suggested charge separation model, the structural relaxation of the twisted conformers into more planar conformers and the return to the ground state are considered as the only competing pathways with the charge separation. In reality, as discussed above, the fraction of active and inactive conformers for charge separation should also be taken into account in order to get a complete picture.16 Here, although 2D-time-resolved fluorescence measurements allow a detailed analysis of the fluorescence decays for the complete emission spectrum, the data did not permit us to extract the fraction of inactive/active conformers. And for more clarity, all excited conformers were considered active, i.e., the excitation energy was assumed to be transferred close to the C60 independently of the excited conformation. This assumption may explain some of the inconsistencies observed between steady-state and time-resolved measurements, particularly for the longer oligomer P6C60 for which the fraction of inactive conformers is expected to be larger than for P4C60 due to

ASSOCIATED CONTENT

* Supporting Information S

Titration of P4 and P4C60 with the octadentate template T8, Titration of P6C60 with the octadentate template T8, Emission spectra of P6 and P6C60 at 300 and 180 K, Emission spectra of P4 and P4C60 at 300 K, Emission spectra of P4-T8 and P4C60-T8, Emission spectra of P6-T8 and P6C60-T8, Quantum yield for charge separation of P6C60 in DCM/Toluene (60/40), Singular Value Decomposition (SVD), Wavelength dependence of the rate constants related to the planarization and natural decays of both studied systems P4 and P6 at 300 K, Wavelength dependence of the rate constants related to the planarization, natural decays and charge separation of P6C60 at 180 K, Extracted fluorescence lifetimes of the reference systems P4 and P6 at 300 K, Extracted fluorescence lifetimes of the P6 and P6C60 at 180 K, Analysis of 2D streak camera images of the emission of P6 and P6C60 at 300 K, Analysis of 2D streak camera images of the emission of P4 and P4C60 at 300 K, Analysis of 2D streak camera 26491



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(14) Winters, M. U.; Kärnbratt, J.; Eng, M.; Wilson, C. J.; Anderson, H. L.; Albinsson, B. Photophysics of a Butadiyne-Linked Porphyrin Dimer: Influence of Conformational Flexibility in the Ground and First Singlet Excited State. J. Phys. Chem. C 2007, 111 (19), 7192−7199. (15) Winters, M. U.; Kärnbratt, J.; Blades, H. E.; Wilson, C. J.; Frampton, M. J.; Anderson, H. L.; Albinsson, B. Control of Electron Transfer in a Conjugated Porphyrin Dimer by Selective Excitation of Planar and Perpendicular Conformers. Chem.Eur. J. 2007, 13 (26), 7385−7394. (16) Kahnt, A.; Kärnbratt, J.; Esdaile, L. J.; Hutin, M.; Sawada, K.; Anderson, H. L.; Albinsson, B. Temperature Dependence of Charge Separation and Recombination in Porphyrin Oligomer−Fullerene Donor−Acceptor Systems. J. Am. Chem. Soc. 2011, 133 (25), 9863− 9871. (17) Anderson, H. L. Conjugated Porphyrin Ladders. Inorg. Chem. 1994, 33 (5), 972−981. (18) Taylor, P. N.; Anderson, H. L. Cooperative Self-Assembly of Double-Strand Conjugated Porphyrin Ladders. J. Am. Chem. Soc. 1999, 121 (49), 11538−11545. (19) Hoffmann, M.; Kärnbratt, J.; Chang, M.-H.; Herz, L. M.; Albinsson, B.; Anderson, H. L. Enhanced π Conjugation around a Porphyrin[6] Nanoring. Angew. Chem., Int. Ed. 2008, 120 (27), 5071− 5074. (20) Hoffmann, M.; Wilson, C. J.; Odell, B.; Anderson, H. L. Template-Directed Synthesis of a π-Conjugated Porphyrin Nanoring. Angew. Chem.., Int. Ed. 2007, 46 (17), 3122−3125. (21) O’Sullivan, M. C.; Sprafke, J. K.; Kondratuk, D. V.; Rinfray, C.; Claridge, T. D. W.; Saywell, A.; Blunt, M. O.; O’Shea, J. N.; Beton, P. H.; Malfois, M.; et al. Vernier Templating and Synthesis of a 12-Porphyrin Nano-Ring. Nature 2011, 469 (7328), 72−75. (22) Sprafke, J. K.; Kondratuk, D. V.; Wykes, M.; Thompson, A. L.; Hoffmann, M.; Drevinskas, R.; Chen, W.-H.; Yong, C. K.; Kärnbratt, J.; Bullock, J. E.; et al. Belt-Shaped π-Systems: Relating Geometry to Electronic Structure in a Six-Porphyrin Nanoring. J. Am. Chem. Soc. 2011, 133 (43), 17262−17273. (23) Hoffmann, M.; Kärnbratt, J.; Chang, M.-H.; Herz, L. M.; Albinsson, B.; Anderson, H. L. Enhanced π Conjugation around a Porphyrin[6] Nanoring. Angew. Chem., Int. Ed. 2008, 47 (27), 4993− 4996. (24) Rathore, R.; Burns, C. L.; Deselnicu, M. I. Multiple-Electron Transfer in a Single Step. Design and Synthesis of Highly Charged Cation-Radical Salts. Org. Lett. 2001, 3 (18), 2887−2890. (25) Chang, M.-H.; Hoffmann, M.; Anderson, H. L.; Herz, L. M. Dynamics of Excited-State Conformational Relaxation and Electronic Delocalization in Conjugated Porphyrin Oligomers. J. Am. Chem. Soc. 2008, 130 (31), 10171−10178. (26) Winters, M. U.; Dahlstedt, E.; Blades, H. E.; Wilson, C. J.; Frampton, M. J.; Anderson, H. L.; Albinsson, B. Probing the Efficiency of Electron Transfer through Porphyrin-Based Molecular Wires. J. Am. Chem. Soc. 2007, 129 (14), 4291−4297.

images of P6 and P6C60 at 180 K, Fluorescence decays of P6-T8 and P6C60-T8, Fluorescence decays of P4-T8 and P4C60-T8, Determination of the excited and charge transfer state energies of PnC60 systems. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Joakim Kärnbratt is acknowledged for the time-dependent DFT calculations performed on the planar conformers of the porphyrin oligomer systems. This work was supported by the Swedish Research Council (VR), the Nanoscience Area of Advance at Chalmers University of Technology, the Engineering and Physical Sciences Research Council (EPSRC), and the Swiss National Science Foundation. We thank the EPSRC Mass Spectrometry Service (Swansea) for mass spectra.

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Conformational Gating of Charge Separation in Porphyrin Oligomer

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