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Shihai Yan,* Sang Joo Lee,* Kyoung Chul Êî,* Sunwoo Kang,* and Jin Yong Lee**
Department of Chemistry, Institute of Basic Science, Sungkyunkwan University, Suwon, 440-746, Korea, and Supercomputing Center, Korea Institute of Science and Technology Information, 52, Eoeun-dong, Yuseong, Daejeon, Korea 305-806
Received: June 29, 2008; Revised Manuscript Received: October 21, 2008
A recent study (J. Phys. Chem. B 2006,110, 5337) proposed that fullerene
—porphyrin—fullerene triad (C60PC60) could be utilized as a photoinduced switch due to the difference in electron transfer directionality for the cis and trans conformer. It is found that the rotational barrier between the SS and AA conformers of C60PC60 is about 3—5 kcal/mol, which can be facilely controllable in experiment. The rotational energy barrier for the anion system is slightly higher than the neutral C60PC60, whereas the total energy of the neutral triad is decreased by about 60 kcal/mol upon electron attachment. The corresponding reorganization energy is very small. These results reveal that the C60PC60 is a potential candidate for electrochemical machinery and energy storage material.
A variety of short-range electron transfer (ET) and energy transfer take place in the photosynthetic reaction center. Understanding of the photochemical processes and mechanisms involved in photosynthesis at the molecular level is significant for the design of artificial photosynthetic systems that mimic the natural photosystems where the photoinduced ET and charge separation occur. The natural photosystems are very complicated; thus, it is of great importance to design a suitable simpler synthetic system which can efficiently convert solar energy into useful chemical energy. The designed synthetic systems are commonly composed of chromophores with electron acceptors or donors and covalent linkage between the redox active moieties. Over the past decades, a number of efficient artificial systems that show the enhanced ET rates in the aimed directions and the reduced ET rates in the opposite directions have been synthesized.1-8
Porphyrins and fullerenes, the attractive molecular components for the design and application of molecular electronic devices owing to their unique structures and rich photoelectric properties, have been extensively employed in building molecular polyads to elaborate the photochemical processes. In general, fullerenes are considered as excellent electron acceptors in charge transfer complexes for their ability to accept multiple electrons,9-11 but at a local level, a specific fullerene bond may donate electron density to the positive center of a metallopor-phyrin.12 The photoinduced energy and charge transfer processes involving fullerene have been investigated experimentally.13-19 The primary component of the porphyrin-fullerene (PC60) interaction is driven by the dispersive forces associated with π-π interaction, which is augmented by weak electrostatic or donor-acceptor stabilization. Fluorescence spectroscopy illustrates that the metal to ligand charge transfer (MLCT) decay kinetics in fullerene is accelerated by the activation of the intramolecular ET pathway.20 The binding of fullerides (C60-) to metalloporphyrin is stronger than the binding of fullerene to
metalloporphyrin.21 The structures and the spectroscopic properties related to ET in nonbonded22 and covalently bonded23 PC60 dyads have been studied employing the time dependent density functional theory (TDDFT). Fullerene can also act as an electron donor in a donor-acceptor dyad to attain the long-lived (23 ( 4 ms) charge-separated state by complexation with scandium ion.24 The determined driving force (1.48 eV) of the intramolecular back ET process is larger than the reorganization energy (0.67 eV).
The first fullerene contained triad is carotene-porphyrin-fullerene;25 since then, the triads involving both porphyrin andfullerene have been extensively investigated.15,17,26-31 The fullerene-porphyrin-fullerene (C60PC60) triad, in which two fullerene units are coordinatively linked to the metallopor-phyrin, exhibits strong π-π interactions between the central metal ion of Sn(IV) porphyrin and the fullerene moieties.32 This is the first metal involving triad example linked by axial coordination of two fullerene units to a metalloporphyrin, and it has attracted much attention since its synthesis. In this triad, the Sn-N distances are similar to those of reported Sn(IV) porphyrins, while the Sn-O distances are 2.116 Å, a little longer than those of other carboxylato Sn(IV) porphyrins.33,34 We have compared the conformational stability of cis and trans isomers and revealed their electrochemical properties employing an ab initio approach.35 C60PC60 is expected to be a potential candidate for electrochemical machinery by setting up proper experimental equipment to adopt the trans/cis conformer in the neutral/anion compound based on the calculated relative stabilities of the two conformers. However, to realize the conformational change in experiment, the rotational barrier should be adequate (not too small and not too large). If the barrier is too small (less than 1 kcal/mol), the conformational change is too fast to control, and if too large (more than 20 kcal/mol), the conformational change hardly occurs.36-42 Therefore, the explorations on the potential energy surface (PES) of C60PC60 revolving around the axial C-C bond in the neutral and anion states are attractive and significant. This is the predominant motivation for us to carry out the present study.
Taking the trans conformation optimized in our previous paper35 as the initial point, the PESs of the neutral and anion C60PC60 triad are scanned on the OC-CH dihedral angle, where oxygen is axially linked to the center Sn(IV) of metallopor-phyrin, using the nonlocal density function of Becke’s three parameters employing the Lee-Yang-Parr functional (B3LYP) at the 3-21G* basis set level. The conformation optimizations are performed for stationary points employing the ab initio Hartree-Fock (HF) theory approach and the 3-21G basis set with the OC-CH dihedral angle fixed. The B3LYP/3-21G* level single point energy calculations are carried out for the optimized geometry structures. All of the calculations were performed using a suite of Gaussian 03 programs.43 The accuracy of these methods has been demonstrated by Schaefer and co-workers on electron affinities of aromatic compounds44,45 and fullerene-based covalently linked dyads and triads;46 furthermore, this approach has been successfully applied on the C60PC60 system in our previous work.
To study the PES of C60PC60 revolving around the axial C-C bond, we take the rotational reaction coordinates as shown in Scheme 1. In mechanism a, the terminal C60 group rotates around the axial C-C bond with the central metal coupled porphyrin and CO2 group fixed. Meanwhile, the CO2 radical shifts following the rotation of spherical C60 in mechanism b, and the situation of two oxygen atoms is interchanged during the isomerization process. The case of mechanism b is more complicated as compared to mechanism a, whereas it is facile to inspect whether the positions of the two O atoms are interchanged or not in experiment with the isotope-labeling technique. NBO analyses demonstrate that the Sn-O interactions are very strong (∼200 kcal/mol at the HF/3-21G level). As compared with the energy needed for the fullerene rotation around the axial C-C bond, too much energy is necessary for mechanism b. Therefore, only mechanism a is considered in detail here.
Considering the OC-CH dihedral angle, there are two possible conformations in each fullerenoacetato group, anti and syn. Totally, there are four isomers for the C60PC60 triad, one syn-syn (SS), two syn-anti (SA), and one anti-anti (AA) structure. In addition, two SA isomers are equivalent by symmetry. Thus, there are three different isomers, NSS/ASS, NSA/ASA, and NAA/AAA, for the neutral/anion triad. Two isomerization processes from SS to SA and from SA to AA for neutral and anion triads were explored to get a comprehensive understanding on the isomerization mechanism.
In Figure 1, plots a and b represent the PESs of the rotational isomerization reaction between SS and SA for the neutral and anion triads, respectively; graphs c and d describe the corresponding isomerization PESs for SA and AA of the neutral and anion systems. It is clear that the isomerization reactions between SS and SA can take place through two different pathways (backward, -180° f 0°; forward, 0° f 180°), the energy barriers of which are less than 8 kcal/mol. The conditions are similar for both the neutral and anion systems. For the neutral system, it is easier to fulfill the transformation through the forward pathway (∼6.1 kcal/mol), while, for the anion system, the energy barrier of the backward channel (∼6.5 kcal/mol) is lower than the forward one (∼7.9 kcal/mol). It should be noted that these rotational barriers can be lowered because these PESs are scanned without geometry relaxation, even though without geometry relaxation the barrier is not too high to be overcome to achieve the reversible conformational transitions. Therefore, our title compound could be a potential candidate for electrochemical machinery.
On the other hand, the rotational barrier of the backward pathway for the isomerization between NSA and NAA is about 6.0 kcal/mol, whereas the barrier of the forward channel is about 2 times higher (Figure 1c). Therefore, the isomerization reaction between NSA and NAA prefers the backward pathway to the forward one. For the anion system, as seen from Figure 1d, the
energy barrier of the backward pathway is higher than 50.0 kcal/ mol; therefore, it is impossible for the system to fulfill the isomerization through this channel. The other pathway for the isomerization between ASA and AAA can be accomplished facilely due to its low barrier, about 4.5 kcal/mol.
As proposed in our previous work,35 the anionic compound may have the SA conformer from the thermodynamic point of view, while the neutral compound may be manipulated depending on the experimental conditions. In fact, the thermodynami-cally slightly unstable (our calculation results) SS configuration has been observed in experimental conditions.32 If only the experimental equipment which can adopt the SS conformer in neutral conditions and adopt the AA configuration in anion situations is set up, our title triad could be a potential candidate for molecular devices.
The above-mentioned rotational barriers are upper bounds, and should be lowered by geometry relaxation. To get further information about the conformational transitions along with the rotation of fullerene around the axial C-C bond, the stationary points (the maxima and the minima) are all optimized with the OC-CH dihedral angle fixed employing the HF/3-21G method, and the single point calculations are carried out at the B3LYP/ 3-21G* level. The structural parameters are collected in Table 1. For NSS, the conformation synthesized uniquely, our calculated structural parameters are in good agreement with the experimental data. This also demonstrates the reliability of our approach. The other point drawn from this table is that both the rotational isomerization and one-electron addition influence predominantly the coupling of central Sn(IV) and terminal C60 spheroids, as noted in R(Sn-C60) and R(PC60) in Table 1, while
the effect on intramolecular parameters of porphyrin or C60 is very weak. During the conformational transitions from SS to AA through SA, the fullerenes depart from the central metal-loporphyrin significantly (∼1.5 Å) for both neutral and anion systems, as is reflected by the variations of R(Sn-C60) and R(PC60).
The relative energies of the optimized conformations are represented in Figure 2. As we supposed, the complex is stabilized with the geometrical relaxation. In our previous paper,35 the relative energies of the optimized conformations between SA and SS were 1.38 and 3.29 kcal/mol for the neutral and anion C60PC60. However, these are 1.58 and 1.21 kcal/mol in this study. In this study, the optimizations are performed with the OC-CH dihedral angle fixed because our main concern is to investigate the rotational barrier, while, in the previous study, the SA and SS conformers are all fully optimized. Our emphasis is to represent the applicability of the title complex (C60PC60) in molecular electrochemical machinery through the investigation on conformational transitions along with the rotation of fullerene around the axial C-C bond. We found that the rotational barrier is in the controllable range, and this may not be much distorted with higher calculational levels. The following three conclusions can be drawn from Figure 2. First, the energy of C60PC60 decreases by about 60 kcal/mol upon electron attachment. This demonstrates that the anion triad is more stable than neutral C60PC60. Furthermore, the excess electron populates predominantly on two C60 groups equally due to the degeneracy for the LUMO and LUMO+1 orbitals of NSS, NTS1, and NAA triads. For NSA and NTS2, their LUMO and LUMO+1 orbitals are not degenerate. Therefore, the additional electrons are distributed on the C60 group unequally (Supporting Information). This is in agreement with our previous proposal: the photoin-duced electron transfer pathway is unidirectional from porphyrin to syn-fullerene for the SA conformation, while it is bidirectional from porphyrin to both fullerene moieties for the SS structure.35 The vertical electron detachment energies of anion triads (ASS, 59.29; ASA, 58.82; AAA, 58.40 kcal/mol) are almost equal to the adiabatic electron attachment energy of the corresponding neutral triads, demonstrating that the reorganization energy of the system is very small. With the above analyses, it can be proposed that our title complex has strong thermal energy storage capability (∼60 kcal/mol is stored upon electron capture). Second, compared with the neutral system, the energy barriers of the isomerization processes become slightly larger upon addition of an excess electron for both forward and
backward rotations. Similar to the neutral system, the isomer-ization process should be easy to accomplish because the barriers are all below 5.38 kcal/mol. The isomerization reaction can be very quick if only the reaction barrier is low; therefore, it is conceivable that the axially covalently bonded C60PC60 triad can be employed as ultrafast optical switches. Third, for both neutral and anion triad systems, the AA conformation is lower than SA in energy, and the SS isomer is the highest among them. While only the SS conformation is synthesized in the crystallization experiment,32 it seems possible to realize the other conformers in experiment because the energy differences and the rotational barriers are not large. It is not surprising that the energetically unfavorable conformer in the gas phase calculation, NSS, can be crystallized in experiment because many factors affect the crystallization. A similar phenomenon has been observed in the di(thio)urea-substituted calixarene system for the steric requirements of the hydrogen bonding.48,49 Here, we propose that the intermolecular π-π coupling and C-H· · ·π interaction of neighboring SS molecules lower the energy of the system in the solid state to some degree; the steric requirements (such as the distance between the Sn(IV) ion and the inner center of fullerene spheroid, Table 1) may play a significant role in leading to the observation in experiment of SS conformation, while not the SA or AA structure. In other words, the structure synthesized in experiment may not correspond to the most stable configuration when the intermolecular interaction is strong enough; contrarily, it may be the conformation with the smallest size.
The rotational barrier between the SS, SA, and AA conformers of C60PC60 is about 3-5 kcal/mol. It is significantly lower as compared with the barrier of biaryl analogues (20.6 kcal/mol),50 which corresponds to a half-life of ∼1.9 min at 20 °C. Therefore, the isomerization reaction between the SS, SA, and AA conform-ers of C60PC60 is fast enough for the molecular-switch applications. However, the obtained difference in the energy barriers appears to result in discrepancies in reaction rates that are smaller than a factor of 10 (compare exp(-Ea/kT)). This small difference may reflect the reversibility of our title complex. The reversibility is a necessary feature for real molecular switch. Actually, such a weak signal can be enlarged by some other devices in practice for the observation. Therefore, our title compound could be a potential candidate for electrochemical machinery.
In summary, the rotational isomerization reaction can be accomplished easily for neutral and anion C60PC60 triad, which should be a potential candidate for both photoelectric switch and thermal energy storage material. The energy decrease upon electron attachment demonstrates the stability of anion triad, whereas the activation energy barrier is slightly elevated simultaneously. The reorganization energy of the title triad is small on electron attachment. The coupling of central Sn(IV) and terminal fullerene spheroid varies distinctly with the rotational isomerization, while the excess electron integration only brings weak geometry variations to the neutral system. The intermolecular interactions play a significant role in determination of the system stability in the solid state.
This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (R01-2008-000-10653-0). This work was also supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (R01-2007-012-03002-0).
Optimized geometry structures and frontier molecular orbitals of the neutral and anion fullerene-porphyrin-fullerene triads. This material is available free of charge via the Internet at http://pubs.acs.org.
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