Impact of Redox Stimuli on FerroceneâBuckybowl Complexes

Article pubs.acs.org/Organometallics

Impact of Redox Stimuli on Ferrocene−Buckybowl Complexes: Switchable Optoelectronic and Nonlinear Optical Properties Wen-Yong Wang, Na-Na Ma, Shi-Ling Sun, and Yong-Qing Qiu* Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, Jilin, People’s Republic of China S Supporting Information *

ABSTRACT: A series of redox-active complexes 1−3 (1,1′-diquadrannulenylferrocene, 1,1′-dicorannulenylferrocene, 1,1′-disumanenylferrocene) and their corresponding conformational isomers that are composed of ferrocene donor and various acceptors of different buckybowl (open bowlshaped polyaromatic hydrocarbon) subunits have been investigated by density functional theory. The nature of the redox property makes it possible to develop novel examples of chromophores that are amenable to molecular switches, which can therefore be used in optical devices. The complexes 1−3, with high thermal and chemical stabilities, show π−π stacking interaction between two buckybowl subunits, and support for the presence of significant donor−acceptor interaction was obtained from the employment of the natural bond orbital charge and charge decomposition analysis. Both one-electron oxidation and one- and two-electron reduction have been considered. The results show some important electronic structure changes upon oxidation/reduction that are accompanied by significant differences in the corresponding absorption spectra and second-order nonlinear optical properties. These differences are due to a change in the charge transfer pattern. The redox switch ability suggests that these ferrocene−buckybowl complexes can be viewed as redoxtriggered nonlinear optical switches with one of the complexes having an on/off ratio of 100.2 for the hyperpolarizability values. Thus, our work has helped to establish ferrocene−buckybowl complexes as versatile and fascinating nonlinear optical switching compounds with a promising future.

1. INTRODUCTION Carbon-based nanomaterials with nonplanar π-surfaces, ranging from fullerenes to nanotubes, have been extensively explored in the opticelectric field1,2 because of their extended and delocalized π-electron distribution and rigid skeleton. In a variety of theoretical and experimental investigations,3−6 the curved carbon networks of fullerenes and nanotubes have been modeled by open bowl-shaped polyaromatic hydrocarbon, i.e., buckybowls. The buckybowls are considered to be another group of key materials whose structures lie between planar and spherical carbon network nanomaterials. They retain the structural stabilities of fullerenes while presenting some distinctive electronic structures and physical properties.3,4 The chemistry of buckybowls has received considerable attention in recent years.7−12 The smallest examples of these buckybowls are quadrannulene, corannulene, and sumanene (Scheme 1). Studies on coordination possibilities have given the possibility of a diversified use of buckybowls in coordination chemistry. Buckybowls have multisite coordination possibilities, namely, convex and concave interior polyaromatic faces, as well as edge and rim carbon atoms capped by hydrogen atoms.5 The study of relative preferences of the convex and concave faces of buckybowls for binding metal centers has been a focus of considerable interest in recent years due to its fundamental and practical importance.13,14 A number of complexes with late © XXXX American Chemical Society

Scheme 1. Molecular Structures of (a) Quadrannulene, (b) Corannulene, and (c) Sumanene

transition metals (Ru, Os, Rh, Ir) coordinated to corannulene have been isolated from solution reactions and structurally characterized, which indicates a strong preference of the convex surface of corannulene for coordination.15−19 In contrast, the first selective concave coordination reported by Hirao and coworkers in 2007 has been a breakthrough.9,10,20 As far as the organometallic aspects of such buckybowls are concerned, the redox properties of metal−buckybowls have been scarcely reported, although some electrochemical reports on the Ru(II)corannulene “convex” dication [(η6-C6Me6)Ru(η6-C20H10)]2+ and sumanene Fe(II) “concave” monocation [(η5-C5H5)Fe(η6C21H12)]+ have been investigated as well.10,15 Received: March 4, 2014

A

dx.doi.org/10.1021/om500224g | Organometallics XXXX, XXX, XXX−XXX

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Molecules that reversibly change their chemical and/or physical properties in response to external stimuli such as redox will possess great significance because of their potential versatility in applications related to molecular memory and switches.21−24 In recent years, our group has focused on the study of switching of the second-order nonlinear optical (NLO) responses.25−28 Indeed, the presence of a redox-active metal center within a conjugated system provides excellent opportunities for a reversible switching of the second-order NLO responses. Most of the studies concerning the redoxswitchable molecules have been made on electron-rich ferrocenyl derivatives.29,30 Redox-active ferrocenes (Fc)31,32 are prized owing to the ease in preparation and the presence of low-energy metal-to-ligand or ligand-to-metal charge transfer transitions leading to significant “on/off” switchable NLO behavior. Since its discovery in 1951,33,34 Fc has been recognized as an electron donor moiety30,35 that is suitable for NLO chromophores. On the other hand, Fc, with its unique electrical, magnetic, optical, redox, and crystal properties, as well as its high thermal and chemical stability, is a very useful building block in the construction of various functional materials36−39 and has been used in numerous medicinal and bioorganometallic chemistry applications.40,41 In view of this, Fc-buckybowl complexes are very promising candidates for redox-switchable molecules, because they offer a unique possibility of switching of the NLO properties by reversible oxidation/reduction of the Fc group. 31,42−44 However, to the best of our knowledge, NLO studies based on the reversible redox properties of Fc-buckybowls are relatively lacking at present, especially theoretical works. Buckybowls are known to be an excellent electron acceptors; thus, the introduction of buckybowls to a Fc donor will induce a fascinating charge transfer transition. This indicates that Fcbuckybowls could be a new member of the family of highperformance NLO materials. Continuing our interest in the study of compounds containing a Fc unit,45 Fc-buckybowls such as 1,1′-dicorannulenylferrocene46 (complex 2, ferrocenyl connected to buckybowl corannulene) and 1,1′-disumanenylferrocene47 (complex 3, ferrocenyl connected to buckybowl sumanene) prepared by Topolinski and co-workers have been described here. In the present paper, we also provide 1,1′diquadrannulenylferrocene (complex 1, ferrocenyl connected to buckybowl quadrannulene) as an ideal redox-active organometallic molecule (Figure. 1). Our goal is to systematically investigate a series of redox-switchable Fc-buckybowls that can be utilized for efficient molecular switching based on the inherent chemical stabilities and strong pull−push properties.

Figure 1. Optimized geometries of complexes 1−3 calculated at the ωB97XD/6-31G(d,p) (SDD basis set for Fe ion) level in THF solution. corrections. Relative to the previous functionals, such as ωB97X, the new functional is significantly superior for noncovalent interactions and very similar in performance for covalent interactions. In the case of the geometry optimizations, we have adopted the 6-31G(d,p) basis set for C and H atoms and the SDD effective core potential basis set50,51 for the Fe ion. In order to obtain more accurate geometry, the solvent effect has been taken into account in the optimization and modeled using the integral equation formalism version of the polarized continuum model (PCM).52−56 The PCM solvation model was used in its original dielectric formulation (D-PCM) developed by Tomasi and co-workers to calculate the solvation free energies in acetonitrile.57−60 The tetrahydrofuran (THF) solvent was selected in order to reproduce the experimental results since the complexes were soluble in it. Thus, all PCM calculations were performed in THF solution. In density functional theory (DFT), all of the molecular properties are determined by the electron density only. The quality of the DFT results depends on the choice of the exchange−correlation functional. With regard to the calculation of the hyperpolarizability, choosing a proper method is important. The conventional DFT methods have been reported to provoke an overestimation of the hyperpolarizabilities.61−63 Highly correlated coupled-cluster methods (such as CCSD, CCSD(T), or even higher CCSDT and CCSDTQ) are known to be generally reliable for calculating the hyperpolarizabilities of molecular systems. However, the use of such high-level methods is still limited to small- and medium-sized systems due to the high computing cost. Because of the modest accuracy and computational cost, some hybrid DFT methods have been widely used to predict the optoelectronic properties of molecules. It should be stressed that the B3LYP hybrid functional sometimes overestimates the first hyperpolarizabilities of the push−pull molecules. As a check, the long-range corrected functionals CAM-B3LYP and ωB97XD with 6-31+G(d,p) (SDD basis set for the Fe ion) in THF solution were also used for all hyperpolarizability calculations. The first hyperpolarizability β was calculated by analytical third-energy derivatives, which is more efficient and less expensive.64 The total second-order polarizabilities (βtot) for the studied complexes are defined as (eq 1)

2. COMPUTATIONAL DETAILS The geometrical structures of the Fc-buckybowl donor−acceptor complexes 1−3 and their corresponding conformational isomers in their ground states were optimized using the ωB97XD functional as implemented in the Gaussian 09W48 computational chemistry program. The spin-unrestricted ωB97XD functional is adopted for the geometries of their corresponding one-electron-oxidized/reduced species (have one unpaired electron and thus a doublet state) and twoelectron-reduced species (two possible states, the triplet state and singlet state). The calculated square of total spin is quite close to its eigenvalue (Table S1), which indicates that the spin contamination is minor. The ωB97XD functional is the latest functional from HeadGordon and co-worker,49 which includes 100% long-range exact exchange, a small fraction (about 22%) of short-range exact exchange, a modified B97 exchange density functional for short-range interaction, the B97 correlation density functional, and empirical dispersion

βtot = (βx 2 + βy 2 + βz 2)1/2

(1)

where βi is defined as (eq 2)

βi = (1/3) ∑ (βijj + βjji + βjij) j

i , j = {x , y , z} (2)

In order to further explain the second-order NLO behavior for the series of complexes, we employed time-dependent density functional theory (TDDFT) methods to describe their electronic spectra. In recent years, the TDDFT method has emerged as a powerful tool for B


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investigation of electronic transition properties.65−69 Three functionals, i.e., TD-B3LYP, TD-CAM-B3LYP, and TD-ωB97XD, with 631+G(d,p) (SDD basis set for the Fe ion) associated with PCM in THF solution are employed to find a better potential for simulating the excitation energies of the studied complexes. The maximum absorption bands of complex 2 obtained by CAM-B3LYP and ωB97XD (CAM-B3LYP: 224 and 263 nm; ωB97XD: 221 and 261 nm) are shorter than that of experimental data (248 and 292 nm).46 However, predicted data obtained by the B3LYP functional (267 and 293 nm) show a good agreement with the experimental data, which allows accurate assignment of the experimentally observed bands. As a result, the simulated absorption spectrum for each studied complex was calculated by using the TD-B3LYP functional in THF solution. The noncovalent interaction is defined by the reduced density gradient (RDG) (eq 3):70

SRDG = (1/(2(3π 2)1/3 ))((|∇ρ|)/ρ 4/3 )

formation of buckybowls. Further investigations focused on the BLA values of quadrannulenylferrocene, corannulenylferrocene, and sumanenylferrocene (0.0424, 0.0304, and 0.0620 Å, respectively, and see related discussions in the next section) also support this slight dimeric effect. The hybrid molecules of the buckybowl and Fc give rich isomerism. The conformational isomers arise from the curvature of each buckybowl and the rotational Fc moiety. DFT calculations using the ωB97XD/6-31G(d,p) (SDD basis set for the Fe ion) method were carried out for the geometry optimizations of complexes 1−3 and their conformational isomers. The total electronic energies give a similar sequence for each of the Fc-buckybowl complexes and their conformational isomers (Table S3). The nature of the stereochemistry is further investigated by taking six plausible conformers of complex 2 as an example, and the six conformers are presented in Figure 2. Inspection of Figure 2 reveals that the closed form

(3)

where ρ is the electron density of the whole molecule. In the calculations of the noncovalent interaction and natural bond orbital (NBO) charge, ωB97XD at the 6-31+G(d,p) (SDD basis set for the Fe ion) basis set is performed. Charge decomposition analysis (CDA) developed by Franking et al.71,72 was performed as implemented in the Multiwfn 3.2 software.73 The localized orbital locator plots were obtained by employing the Multiwfn software version 3.2. The RDG and electron density difference maps (EDDM) were plotted using VMD 1.9.1.74 The structures of the complexes 1−3 were generated by CYLview 1.0.561 BETA.75

3. RESULTS AND DISCUSSION 3.1. Structural and Bonding Properties. The geometrical structures of complexes 1−3 optimized at the ωB97XD/6-31G(d,p) (SDD basis set for the Fe ion) level in THF solution are depicted schematically in Figure 1. We employed the concept of the average deviation of the bond lengths to test the accuracy and reliability of the theory method. The average deviation (Δr) predicted by the ωB97XD method from the corresponding experimental values is calculated through (eq 4)76,77 Δr =

Figure 2. Complex 2 and its conformational isomers and transitions between them (Cp = cyclopentadienyl).

isomers (complexes 2, 2a, and 2b) show much lower energies than that of open form isomers (complexes 2′, 2a′, and 2b′), supporting the chemical stabilities of the closed form isomers. This stability could be attributed to the overlap of the molecular orbital of two buckybowl monomers. Among these closed form isomers, complex 2 exhibits the lowest relative energy, which can be explained by considering that complex 2 is experimentally observed in the crystal. The stability of complex 2 stems from the weak steric hindrance between two buckybowl monomers with respect to the other closed isomers. Hence, in the following section, only structures 1−3, with high thermal and chemical stabilities will be further discussed, and an extra analysis of the impact of isomerism on other aspects (such as NLO and redox properties) is out of the main focus of this work. The conformational isomers of complexes 1−3 can be interconverted via cyclopentadienyl ring rotation and buckybowl-to-buckybowl inversion. As a prototypical example of these types of Fc-buckybowl systems, complex 2 and its conformational isomers are given in Figure 2 to gain more information on the inversion and rotation processes. To obtain the reasonable position and stable structure of the open conformation complex 2′, we have located all stationary points for rotation about the axis passing through the cyclopentadienyl ring and Fe atom, using the dihedral angle between both buckybowl substituents (θ = 0−180°) as a measure for this rotation by 36°. The computational studies have shown that the open complex 2′ has only one stable position with an eclipsed conformation (θ = 144°), which is also confirmed by the total electronic energies of single-point calculations from 0° to 180° in steps of 36° (Table S4). In addition, we focused on

∑i ni|riωB97XD − riExp| ∑i ni

(4)

where ni is the number of identical bond lengths, and riωB97XD and riExp are the corresponding bond lengths calculated at the ωB97XD/6-31G(d,p)+SDD level and the experimental values, respectively. The C−C bond deviations (ΔrC−C) of complexes 2 and 3 (the representative complexes 2 and 3 were selected due to the presence of their crystal data) are 0.007 and 0.023 Å, respectively. The Fe−C bond deviations (ΔrFe−C) of complexes 2 and 3 are 0.014 and 0.008 Å, respectively. The small bond deviations indicate that the ωB97XD/6-31G(d,p)+SDD method for geometrical optimizations is qualitatively reliable. The accuracy of the ωB97XD calculations is also precisely supported by the similar bowl depths with respect to the X-ray data (Table S2). The bowl depths of corannulenylferrocene and sumanenylferrocene are virtually identical to that of corannulene78 and sumanene,47 respectively. Upon introduction of a second buckybowl substituent to the molecule, the bowl depth of the upper buckybowl remains almost unchanged and the lower buckybowl is slightly flattened. On the other hand, the dihedral angles (see Figure 1) between Fc and buckybowl subunits for complexes 1−3 are similar to those of the corresponding quadrannulenylferrocene, corannulenylferrocene, and sumanenylferrocene (Figure S1), illustrating the marginal structural changes that occur upon the dimeric C


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Figure 3. Energy scan as a function of the buckybowl substituent inversion for (a) open and (b) closed conformational isomers, respectively.

Figure 4. (a) Cut-plane LOL representations and (b) reduced density gradient (isosurface = 0.500) of complexes 1−3.

simulating the energy variation of open and closed conformation as a function of the buckybowl substituent inversion (Figure 3). It is visualized from Figure 3a that there are two small barriers during the buckybowl-to-buckybowl inversion, leading to easy transitions between open form isomers. However, it can be seen from Figure 3b that a large energy barrier makes the inversion between closed isomers difficult in thermodynamics. These large barriers stem from steric repulsion between buckybowl substituents with respect to the open isomers. The spatial organization of the bonding mechanism in the studied complexes can be easily recognized by the cut-plane localized orbital locator (LOL) representations. The cut-plane representations along the y axis with a distance of 0.0 Å from the xz plane of the LOL for complexes 1−3 are given in Figure 4a. A significant amount of electron density is accumulated between the Fe−C and C−C bonds. This is consistent with the covalent character of the C−C and Fe−C bonds. It is interesting to note that weak interactions exist between the upper and lower buckybowl subunits, as is reflected by the very small LOL values (pale blue). The weak van der Waals interactions could be attributed to the frontier molecular orbitals of buckybowl subunits with conjugated π-electrons overlapping with each other. The weak π−π interactions are clearly visualized in the 3D plots of the reduced density gradient given in Figure 4b. The interaction of complex 1 has a discontinuous wave function overlap with a very weak steric repulsion effect, which illustrates the absence of strong π−π interactions. The interactions of complexes 2 and 3 have a

continuous wave function overlap and an almost dominant van der Waals effect, which is probably due to the better conjugation of coranulenyl and sumanenyl subunits (because the bowl depths of the coranulenyl and sumanenyl subunits are more flat than that of the quadrannulenyl subunit in Table S2). The π−π stacking interaction that arises from dispersion and electrostatic force can be tuned to assemble a molecule into the desired form and, thus, adds further support of the thermal stabilities of these complexes. NBO charge analysis has been performed to interpret the interaction between Fc and the buckybowl subunits. The NBO charges on the Fc subunits are negative, ranging from −0.018 e to −0.030 e (Table 1). Thus, the Fc and buckybowl subunits display electron donor and acceptor character, respectively, and pronounced charge transfer from Fc to buckybowl subunits is Table 1. NBO Charges of All Studied Complexes at the ωB97XD/6-31G(d,p)+SDD Level in THF Solution

Fc quadranulenyl Fc coranulenyl Fc sumanyl D

1+

1

1−

12−

0.780 0.220 2+

−0.018 0.018 2

−0.253 −0.747 2−

−0.318 −1.682 22−

−0.158 −0.842 3−

−0.227 −1.773 32−

0.772 0.228 3+ 0.753 0.247

−0.022 0.022 3 −0.030 0.030

−0.699 −0.301

−0.818 −1.182


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expected. On the other hand, the CDA clearly reveals the charge transfer between the Fc and buckybowls, and the results are compiled in Table 2. The donation d and back-donation b Table 2. CDA Parameters for the Interactions between the Fc and Buckybowl Subunits in Complexes 1−3 Obtained at the ωB97XD/6-31G(d,p) (SDD basis set for Fe ion) Level complex

d

b

d − ba

r

1 2 3

0.309 0.321 0.318

0.229 0.232 0.231

0.080 0.089 0.087

−0.513 −0.573 −0.269

a

The number of net transferred electrons from fragment 1 to 2 due to formation of the corresponding complex orbital.

terms show that the charge donation from the Fc toward the buckybowl subunits occurs in complexes 1−3. The negative values of the charge polarization term, r, indicate that the electrons (0.269−0.573 e) are depleted from the overlap region of the fragment orbitals (FO), thus reflecting an electron repulsive effect. The orbital interaction diagram of a representative complex 2 is given in Figure 5 to illustrate the

Figure 6. Molecular orbitals and energy levels of complexes 1−3.

which is 21%, 38%, and 21% for complexes 1−3, respectively. Hence, it suggests that the Fc subunit may be the oxidation center of complexes 1−3. The Mulliken spin populations of complexes 1+−3+ confirm these qualitative predictions (Figure S2). Most of the spin density in complexes 1+−3+ is located on the Fc subunit, while the rest of the buckybowl subunits carry a small spin density. However, the LUMOs of complexes 1−3 are mainly centralized on the buckybowl subunits. The LUMOs support that the buckybowl subunits will act as the reduced center in the one-electron-reduction process, which is in good agreement with the Mulliken spin density populations of complexes 1−−3−. For two-electron-reduced species, two possible spins, singlet and triplet states, of each complex were considered. DFT results show that the triplet states of all twoelectron-reduced species are more stable than singlet states. The differences of the total energies including the electronic and zero-point-corrected energies are ca. 8.9, 9.9, and 1.8 kcal/ mol−1, respectively. The small differences suggest that it may be a statistical distribution of two possible spin states. Upon calculations of the one-electron-reduced species, complexes 1−−3− (having one unpaired electron and thus a doublet state) show that the αLUMO and βLUMO are still mainly located on the buckybowl subunits, and this indicates that the buckybowl subunits are still the two-electron-reduced center. A perusal of Table 1 reveals that the NBO charges of Fc subunits significantly increase in the one-electron-oxidization process. The charges of Fc subunits of complexes 1+−3+ are close to +1 e (range 0.753−0.780 e), again supporting that the Fc subunit is the one-electron-oxidized center. For one- and two-electron-reduced species, the more negative NBO charges

Figure 5. Orbital interaction diagram for complex 2 formed upon interaction of the Fc (frag1) and buckybowl (frag2) subunits.

relationship between complex orbitals and FOs more intuitively. In complex 2, the 28th occupied molecular orbital (HOMO−28) is responsible for the covalent interactions between Fc and corannulenyl subunits. It can be seen that the complex orbital HOMO−28 shows σ-type bonding character; therefore we can infer that the nature of the Fc → corannulene electron transfer can be largely interpreted as the mix between the highest occupied fragment orbital (HOFO) of Fc and HOFO−22 of the corannulenyl subunits. Notice that the HOFO−7 of Fc is a π-type orbital; certainly it cannot participate in the σ-type donor−acceptor interaction between Fc and corannulenyl subunits. 3.2. Redox Property. The redox properties of complexes 1−3 were exhaustively studied in THF solution with the aim to deeply investigate the experimentally demonstrated concept of redox reaction. It is well known that the changes of the electronic properties are related to the highest occupied molecular orbitals (HOMO) and affect the lowest unoccupied orbitals (LUMO), thus leading to the change in the redox properties. The HOMOs and LUMOs of the studied complexes are plotted in Figure 6. It is found that the HOMOs in complexes 1−3 formally delocalize over the whole molecules. The MO contribution of the Fc unit on the HOMO is large, E


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signify the existence of the reduced centers in the corresponding buckybowl subunits. Hence, the NBO charge distribution can provide obvious evidence for the redox center. As mentioned above, the reduction centers of complexes 1− 3 are the buckybowl subunits. This means that the one- and two-electron-reduction processes will affect the geometrical structures of the quadranulenyl, coranulenyl, and sumanyl units. To clarify the redox effect for the geometrical structures, we employ the concept of bond length alternation (BLA), which is correlated to the aromaticity index, i.e., the standard deviations in the C−C ring bonds (eq 5):79 BLA =

1 n

1,1′‐disumanylferroceneox (aq) + e− ⎯→ ⎯ 1,1′‐disumanylferrocene red(aq)

The term ΔG represents the free energy of the reduction process in solution, which can be obtained by frequency calculation. The normal hydrogen electrode has been recalculated to be −4.28 V.86 Fc*/Fc*+ (Fc* = decamethylferrocene) that was used as an internal standard is found to be −0.48 V against Fc/Fc+.46 Fc/Fc+ is 0.40 V against the normal hydrogen electrode.87 Thus, the reference electrode Fc*/Fc*+ should be −4.20 V. Combining the Nernst equation (E0 = −ΔG0/nF) with the reference value −4.20 V, the redox potentials Ecal are predicted. Inspection of Table 4 reveals that

∑ (di − dM)2 (5)

i

where n is the total number of C−C bonds, di is the ith bond length, and dM is the mean of the C−C bond length. Thus, when all of the C−C distances are the same as in benzene, the BLA is equal to zero, while if the C−C bonds are not equivalent because their bond order is changed as a consequence of the πelectron localizations, the BLA increases.80 Table 3 lists the

Table 4. Oxidation−Reduction Potentials in THF Calculated at the ωB97XD/6-31G(d,p)+SDD Level

Table 3. Calculated BLA (Å) Values of Buckybowl Subunits for the Studied Complexes complex

BLA

complex

BLA

complex

BLA

1+ 1 1− 12−

0.0419 0.0419 0.0407 0.0388

2+ 2 2− 22−

0.0306 0.0305 0.0279 0.0252

3+ 3 3− 32−

0.0628 0.0627 0.0625 0.0609

a b

1,1′‐diquadranulenylferroceneox (aq) + e− ΔG

(6)

1,1′‐dicorannulenylferroceneox (aq) + e− ⎯→ ⎯ 1,1′‐dicorannulenylferrocene red(aq) ΔG

complex

Eox1 1/2 (V)

Ered1 1/2 (V)

Ered2 1/2 (V)

1 2 3

1.27 0.47 (0.45)a 0.40 (0.43)b

−2.49 −2.45 (−2.05)a −2.73 (−2.47)b

−2.76 −2.71 (−2.56)a −3.27 (−2.91)b

Experimental potentials in THF for complex 2 in ref 46. Experimental potentials in THF for complex 3 in ref 47.

the theoretical predictions are in reasonable agreement with the experiment. Therefore, the theoretical protocol is successful for predicting the experimental redox potentials. We sought to use the associated HOMO and LUMO energies to gain insight into the chemical nature of the shifts in redox potentials. In Table 4, complex 3 shows the smallest oxidation potentials of the relative series, suggesting its easier one-electron removal process. This fact is more clearly evidenced by the significant increase of the HOMO energy level of complex 3 (Figure 6). The oxidation potential of complex 1 is positively shifted with respect to complexes 2 and 3 owing to the relatively electrondeficient Fc unit (NBO charges of −0.018 e, −0.022 e, and −0.030 e for complexes 1−3, respectively) and consequent inhibition in the oxidation of complex 1. On the other hand, the most stabilized LUMOs of complexes 2 and 2− would lead to the one- and two-electron reductions of complex 2 more easily as compared to complexes 1 and 3, in accordance with our theoretical reduction potential predictions. Finally, the redox activity, which is linked to the frontier orbitals involved in the main charge transfer transition, suggests that these complexes may exhibit redox switching of molecular first hyperpolarizabilities.88 3.3. Absorption Spectrum. In order to obtain a more intuitive description of the band assignments of the electronic absorption spectra and the trends in the NLO behaviors of the studied complexes, TD-DFT calculations in THF solution were carried out at the B3LYP/6-31+G(d,p) (SDD basis set for the Fe ion) level. The excited-state transition energies (eV), oscillator strengths (f), and relevant molecular orbitals of all complexes in singlet excited-state transitions are summarized in Table S5. The simulated absorption spectra of complex 1 and its oneelectron-oxidized and one- and two-electron-reduced species along with EDDM for the most intense electronic transitions calculated in the THF solution are depicted schematically in Figure 7, while their 3D contour plots of MOs involved in the electronic transitions are given in Figure S3. The spectra of complex 1 and its one-electron-oxidized and one- and two-

BLA values of buckybowl subunits for complexes 1−3 and their corresponding oxidized and reduced species. The BLA values of one-electron-oxidized species increase slightly with respect to the neutral complexes. This means that the oxidation processes have a slight influence on the geometries of the buckybowl subunits and further reveals the Fc oxidized center. In contrast to the one-electron-oxidized species, the one- and two-electron reduction processes act to significantly reduce the BLA values, which could be attributed to the reductions taking place at the buckybowl subunits. The results show that the reduction processes will enhance the molecular conjugation and affect the electron acceptor strength of the buckybowl subunits and thereby switch the second-order NLO properties of the studied complexes. On the basis of that mentioned above, we proceeded to reproduce the electrochemical behaviors of the present studied complexes. The oxidation−reduction potentials can be predicted theoretically in solution.81−85 Here, predications in THF solution were conducted. The redox potentials of complexes 1−3 require the determination of the free energy associated with the process (eq 6−8)

⎯→ ⎯ 1,1′‐diquadranulenylferrocene red(aq)

(8)

ΔG

(7) F


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Figure 7. Absorption spectra of complex 1 and its one-electron-oxidized and one- and two-electron-reduced species along with EDDM for the most intense electronic transitions. Green and yellow colors indicate depletion and accumulation of electron density, respectively.


electron-reduced species can be characterized as one main highenergy transition with large oscillator strength and a series of low-energy transitions with relatively small oscillator strengths. In the absorption spectrum of complex 1, the most intense band of highest energy appearing at 257 nm is due mainly to the electronic transitions from HOMO−10 → LUMO+1 and HOMO → LUMO+11. The low-energy, less intense electronic transition at 315 nm arises mainly from HOMO−3 → LUMO +3 and HOMO−1 → LUMO+4, while the electronic transition at 364 nm arises from HOMO−1 → LUMO. Taking into account all the MOs involved in these electronic transitions, these electronic transitions are assigned mainly as Fc to quadrannulenyl subunit transition character, which is in reasonable agreement with the respective EDDMs. However,

in contrast with neutral complex 1, one-electron-oxidized species 1+ has a wide absorption from 650 to 1000 nm, which is traced to the electronic transition of βHOMO → βLUMO, defined as quadrannulenyl to Fc charge transfer. Two second most intense bands absorbing at 498 and 416 nm are observed. The former is due mainly to βHOMO−4 → βLUMO and the latter is due to βHOMO−2 → βLUMO+1 excitations, respectively, and both of these bands are assigned as quadrannulenyl to Fc charge transfer. Finally, the most intense band absorbing at 315 nm arise from αHOMO−7 → αLUMO and βHOMO−2 → βLUMO+5 and could also be assigned as a quadrannulenyl to Fc charge transfer transition. The quadrannulenyl to Fc transition is due to the electron-deficiency of Fc, as one-electron-oxidized centers exist in the Fc moiety. G


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HOMO−9, HOMO−7, LUMO+1, LUMO+2, and LUMO+3 is mainly located at the corannulenyl subunits (Figure S4). Complex 2 also exhibits a low-energy wide absorption at 408 nm, which is mainly due to HOMO → LUMO,+1 electronic transitions and is assigned as Fc to corannulenyl subunit charge transfer. Two intense electronic transitions of one-electronoxidized species 2+ appear in the visible region absorbing at 384 and 543 nm, being red-shifted with respect to the corresponding electronic transitions in the spectrum of neutral complex 2. The less intense low-energy band of complex 2+ is due mainly to two electronic transitions absorbing at 627 and 543 nm, which are ascribed to βHOMO−2,−4 → βLUMO and αHOMO−5 → αLUMO excitations, respectively. The most intense high-energy band absorbing at 384 nm is associated with αHOMO−4 → αLUMO+1 excitation. According to the relevant EDDM and the shape of the MOs during all electronic transitions of complex 2+, we could assign them as charge transfer from two corannulenyl subunits to Fc, which can be also explained by considering that the Fc moiety is a electrondeficient center in the one-electron-oxidization process. On the other hand, the spectrum of one-electron-reduced species 2− is quite different with respect to that of neutral complex 2; that is, it exhibits one broad near-infrared reflection (NIR) absorption band at 2159 nm and two intense high-energy bands at 863 and 406 nm. The NIR absorption band, a so-called chargeresonance band, is ascribed to the result of negative-charge delocalization of complex 2− over the two corannulenyl subunits,89−93 indicating that complex 2− is in the closed form, where two corannulenyl subunits are cofacially stacked. The charge-resonance bands in the NIR region are explained as the absorptions corresponding to the electronic transitions between two corannulenyl subunits, since the αHOMO and αLUMO,+1,+2 are delocalized over the corannulenyl subunits, which also constitutes additional evidence of the negativecharge delocalization nature of complex 2−. For two-electronreduced species 22−, the calculated absorption spectrum exhibits one broad absorption band absorbing at 1444 nm and one intense high-energy band absorbing at 613 nm along with two shoulders. The less intense low-energy bands are due

Upon one-electron-reduced reaction, the absorption spectrum of the complex 1− shows remarkable red-shifting with respect to the corresponding neutral complex 1. The absorption spectrum of 1− exhibits two intense high-energy electronic transitions along with a low-energy electronic transition of lower intensity. The weak band of low energy is due mainly to two electronic transitions absorbing at 1235 and 711 nm, which are ascribed to αHOMO → αLUMO+3 and αHOMO → αLUMO+6 excitations, respectively. The former is assigned as π−π* charge transfer within the lower quadrannulenyl subunit, while the latter is assigned as a mixed charge transfer from the lower quadrannulenyl to the Fc moiety coupled with some charge transfer to the upper quadrannulenyl. The most intense band of complex 1− at 520 nm arises from αHOMO → αLUMO+12 and βHOMO → βLUMO+2 excitations. Accordingly, the strong electronic transitions could be assigned as a mixed charge transfer from the lower quadrannulenyl to the upper quadrannulenyl subunit and Fc to the lower quadrannulenyl subunit. The calculated absorption spectrum of two-electron-reduced species 12− is similar to that of the one-electron-reduced species 1− although with some differences. The low-energy bands absorbing at 982 and 704 nm are respectively due to the electronic transitions associated with αHOMO−1 → αLUMO+1,+2 and αHOMO−1 → αLUMO +5, while the high-energy bands are ascribed to αHOMO−1 → αLUMO+9 excitation. According to the relevant EDDM and the shape of the MOs during the electronic transitions, we could assign them as π−π* charge transfer from the lower quadrannulenyl to the upper quadrannulenyl. Upon introduction of two corannulenyl subunits to Fc, the most intense electronic transition of complex 2 is red-shifted by 36 nm, absorbing at 293 nm, and is accompanied by a second one, which either appears as a shoulder in the high-energy region (at 267 nm) or widens the main peak (Figure 8). The shoulder can be characterized as a Fc to corannulenyl transition. The most intense transition of complex 2 arises from HOMO−7 → LUMO+3 and HOMO−9 → LUMO+1,+2 excitations and is assigned as interlayer charge transfer between two corannulenyl subunits, since the electronic distribution of H


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Table 5. First Hyperpolarizability Tensor Components βx and βz (10−30 esu) along with the Corresponding Total First Hyperpolarizabilities βtot (10−30 esu) Computed at the CAM-B3LYP/6-31+G(d,p) (SDD basis set for Fe ion) Level of Theory system 1 +

βx βz βtot

system 2 −

1

1

1

−7.9 −27.8 29.4

−3.2 25.1 27.1

91.9 −134.3 163.5

2−

1

69.2 −106.2 128.1

+

system 3 −

2

2

2

−9.1 −45.1 46.2

−5.3 29.9 30.5

674.7 −112.1 692.4

mainly to the electronic transitions from αHOMO−1 → αLUMO,+1. From inspection of Figure 8 and Figure S4, the electronic transitions at 1444 nm could be assigned as π−π* charge transfer from the lower corannulenyl to the upper corannulenyl subunit. The most intense high-energy band is due mainly to two electronic transitions absorbing at 895 and 613 nm. These electronic transitions are ascribed to αHOMO,−1 → αLUMO+2 and αHOMO−1 → αLUMO+8, +9 excitations, respectively, and are assigned as corannulenyl to Fc with some π−π* transition character. Complex 3, however, exhibits a remarkably different absorption spectrum with respect to those obtained for complexes 1 and 2. The computed absorption spectrum of 3 shows only one intense electronic transitions, at 283 nm, which is due mainly to HOMO−4,3,2 → LUMO+5. The band could be assigned as Fc to sumanenyl charge transfer, as it is accompanied by electron density depletion from the Fc moiety and electron density increase on the sumanenyl subunits (Figure 9). In the case of one-electron-oxidized species 3+, the absorption spectrum shows a remarkably different behavior and is strongly shifted to the red, absorbing at 340, 543, and 747 nm. The main feature of the low-energy region of the spectrum of complex 3+ is the existence of two less intense bands. The low-energy, less intense bands at 747 and 543 nm can be respectively described as βHOMO,−3 → βLUMO and αHOMO−3 → αLUMO excitations. The shapes of the MOs suggest that these transitions can be characterized as sumanenyl to Fc charge transfer (Figure S5). Complex 3+ also exhibits a high-energy, most intense absorption at 340 nm. This band is accompanied by a second one, which appears as a shoulder. The former is due to the transitions αHOMO → αLUMO+2 and βHOMO−3,−5 → βLUMO+1 and can be described as sumanenyl to Fc charge transfer. Upon one-electron-reduction reaction, the absorption spectrum of complex 3− shows three intense electronic transitions, at 479, 682, and 1069 nm. The band of low energy is due mainly to αHOMO → αLUMO+3, +4,+5 excitations, while the middle-energy and high-energy bands arise from βHOMO → βLUMO+2,+3,+4 and αHOMO−1 → αLUMO+2, respectively. All these bands could be assigned as charge transfer from two sumanenyl subunits to Fc according to the relevant EDDM and the shapes of the MOs. Next, the absorption spectrum of two-electronreduced species 32− exhibits one broad NIR absorption band absorbing at 2321 nm and three intense high-energy bands. The broad absorption band is ascribed to the result of negativecharge delocalization of complex 32− over the two sumanenyl subunits, which is similar to that of complex 2−. The chargeresonance band is ascribed to αHOMO → αLUMO,+3,+5 excitations and could be assigned as π−π* charge transfer since electron density is transferred between two sumanenyl subunits. The most intense band, at 643 nm, is mainly due to the electronic transitions associated with αHOMO → αLUMO +20 and βHOMO → βLUMO+3 excitations, while the second intense band at 832 nm is ascribed to αHOMO−1 → αLUMO

2

2−

68.4 −138.4 164.3

3

3

3−

32−

−19.9 −66.1 69.0

−3.0 21.7 22.4

30.8 483.2 484.3

−208.1 2222.6 2244.4

+

+4,+5 excitations. The former and the latter bands are all assigned as Fc to sumanenyl subunit charge transfer by inspecting the relevant shapes of the MOs. 3.4. Second-Order NLO Switch Induced by Redox Stimuli. The absorption spectra of the novel Fc-buckybowl donor−acceptor complexes are strongly sensitive to the redox states. This encouraged us to investigate the possibility of the redox switching of the nonlinear optical response and toward the realization of a molecular optical switch.30,88,94 In order to emphasize the switching behavior of the first hyperpolarizability, we list the total first hyperpolarizabilities (βtot) along with the first hyperpolarizability tensor components (βx and βz) of complexes 1−3 and their corresponding one-electronoxidized and one- and two-electron-reduced species calculated at the B3LYP (Table S6), ωB97XD (Table S6), and CAMB3LYP (Table 5) levels of theory. As seen, the βz values of complexes 1−3 and their corresponding one-electron-oxidized species dominate the second-order NLO responses, which indicates that the main charge transfer is along the z direction. For one- and two-electron-reduced species, however, the contribution of βx values is significantly increased. This means that another charge transfer along the x direction is expected, which is in reasonable agreement with the π−π* transition from the lower buckybowl to the upper buckybowl subunits as discussed in the absorption spectrum section for reduced species. It is worth noting that the calculated βtot values are functional-dependent, but they display similar trends. Among the three methods, the B3LYP method presents the largest βtot values. The overestimation of the hyperpolarizabilities is expected due to the incorrect electric field dependence of the “response part” of the exchange−correlation potential, which lacks a linear term counteracting the applied electric field.63 Further, one can easily see that the βtot values obtained with the two remaining long-range correction functionals (CAM-B3LYP and ωB97XD) are obviously smaller than those of B3LYP. It is important to remark on the long-range corrected functionals, which introduce a growing fraction of exact exchange when the distance increases. Several works have pointed out that DFT functionals with a long-range functional not only remove the overestimation of hyperpolarizabilities by traditional DFT but also provide semiquantitative accuracy with a reasonable computational cost.95−97 The CAM-B3LYP functional,98 which adds a long-range correction using the Coulombattenuating method and includes 19% and 65% short- and long-range HF exchange with μ = 0.3, is suitable to predict the molecular NLO properties of large systems, because its result is very similar to the desirable coupled cluster methods.97 Therefore, we take only the CAM-B3LYP functional as an example to shed light on the changes in the first hyperpolarizabilities of all studied complexes. The use of very flexible and polarized basis sets is expected to provide reasonable results for the hyperpolarizability calculations.99−103 Then, we turn our attention to the basis set I


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sensitivity of the reported values. We calculated the βtot values at the CAM-B3LYP level of theory using the SDD basis set for the Fe ion and the 6-31+G(d,p) basis set (Table 5) and its extension 6-31+G(3df,3pd) (Table S7), respectively, for nonmetal atoms. The results show that the 6-31+G(d,p) basis set and the larger 6-31+G(3df,3pd) basis set are in close agreement. This demonstrates that the NLO properties are almost converged with respect to the basis set size. As a result, 6-31+G(d,p) contains a d Gaussian-type function (GTF) with an exponent of 0.8 on the heavy atom, and a p-GTF with an exponent of 1.1 on the H atom is appropriate for the hyperpolarizability calculations. Inspection of Table 5 reveals that the computed βtot values in series increase as complex 3 (22.4 × 10−30 esu) < complex 1 (27.1 × 10−30 esu) < complex 2 (30.5 × 10−30 esu) according to CAM-B3LYP calculations, but the differences between them are not substantial. This indicates that introduction of the buckybowl subunits into the Fc segment cannot effectively enhance the second-order NLO response. It is worth noting that the βtot values of complexes 1−3 are comparable to those of our previous ferrocenyl chromophores45 and other ferrocenyl nonlinear optical chromophores that also have experimental data.35,104,105 Compared with the neutral complexes 1−3, a slight enhancement of βtot values of oneelectron-oxidized species is observed. The βtot values of complexes 1+−3+ increase to 29.4 × 10−30, 46.2 × 10−30, and 69.0 × 10−30 esu, respectively, which are 1.1, 1.5, and 3.1 times as large as that of corresponding neutral complexes. On the other hand, the changes observed in the βtot values of reduced species with respect to the neutral complexes 1−3 are quite significant. According to the CAM-B3LYP functional, the βtot values of complexes 1−−3− are calculated to be 163.5 × 10−30, 629.4 × 10−30, and 484.3 × 10−30 esu, which are 6.0, 22.7, and 21.5 times as large as that of corresponding neutral complexes. Similarly, the βtot values of complexes 12− (128.1 × 10−30 esu), 22− (164.3 × 10−30 esu), and 32− (2244.4 × 10−30 esu) are 4.7, 5.4, and 100.2 times as large as that of the corresponding neutral complexes. The βtot value of complex 32− seems to be unreasonable. In fact, the value appears to be largely overestimated because it arises from the unexpected and unrealistic low-energy charge transfer transitions (vide supra). To ensure that the result is reliable, the first hyperpolarizabilities have also been tested by the TDDFT sumover-state (SOS) method at the B3LYP functional level, within the framework of the SOS perturbation theory.106−108 One can see in Table S6 that the trends of hyperpolarizability calculations are in accordance with the law reported by the analytic derivatives method above. We took a further step into several main excited states that contribute to the βvec value in order to elucidate the origin of nonlinear second-order responses of these complexes. The convergent behavior of βvec values of representative complexes 2 and their oxidized and reduced species is plotted in Figure 10, while the remaining complexes are given in Figures S6 and S7. Inspection of Figure 10 reveals that the excited state S45, which can be assigned to the interlayer charge transfer transitions between two corannulenyl subunits, has a dominant contribution to the βvec value of complex 2. For the oxidized complex 2+, the excited states S4 have a large contribution to the βvec value, and the dominant contribution to the βvec value can be assigned as charge transfer from two corannulenyl subunits to Fc. For the reduced complexes 2− and 22−, the dominant contribution to the βvec value can be assigned to S3 and to S4 and S7,

Figure 10. Convergent behavior of βvec values of complex 2 and its one-electron-oxidized and one- and two-electron-reduced species dependent on the first 80 states.

respectively. The excited states S3 and S4 can be assigned as the charge transfer from the lower corannulenyl to the upper corannulenyl subunit, while the S7 state is equal to the charge transfer from two corannulenyl subunits to Fc. These transition characters can provide information on the way redox processes influence the intramolecular donor or acceptor character, which accordingly leads to the variations in the computed βvec values. In an effort to elucidate the controlling factors of βtot values, Oudar and Chemla109,110 have proposed a simple link between the first hyperpolarizability and a low-lying charge transfer transition through a two-level expression from the complex SOS expression. According to the two-level model, the βtot value is proportional to the difference between the dipole moments of the ground state and the crucial excited state (ΔE) and the oscillator strength ( f), but inversely proportional to the cube of the transition energy (ΔE). The studied complexes in this work possess the same general skeleton, and thus there is not much difference in the ΔE value. In this way, the relevant lowest optically allowed excited state with substantial oscillator strength will generate a large increase in the static hyperpolarizabilities. However, the similar oscillator strengths of the studied complexes are not enough to decide the βtot value in comparison with the cube of the transition energy. Thus, the transition energy is a decisive factor for determining the βtot value. The transition energies show a descending trend for complexes 1−3, 3 (4.38 eV) > 1 (3.41 eV) > 2 (3.04 eV), which is inversely proportional to the βtot values of complexes 1−3. In addition to this, the descending trend of transition energies explains the gradual increase of βtot values upon redox stimuli. For example, the order of the transition energies of complexes 1−3 and their corresponding one-electron-oxidized and one- and two-electron-reduced species is 1 (3.41 eV) > 1+ (1.58 eV) > 12− (1.26 eV) > 1− (1.19 eV), 2 (3.04 eV) > 2+ (1.98 eV) > 22− (0.86 eV) > 2− (0.57 eV), and 3 (4.38 eV) > 3+ (1.66 eV) > 3− (1.16 eV) > 32− (0.53 eV), which is also in accordance with the trend of the enhanced βtot values. The redox properties encouraged us to probe the secondorder NLO switching. We assume that an increase of the first hyperpolarizability caused by the redox mean is related to switching “ON”, and a decrease is related to switching “OFF”. The difference of the βtot values between the “ON” and “OFF” state must be obvious to obtain the second-order NLO switching. A more significant effect on first hyperpolarizabilities J


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(3) Wu, Y. T.; Siegel, J. S. Chem. Rev. 2006, 106, 4843. (4) Sakurai, H.; Daiko, T.; Hirao, T. Science 2003, 301, 1878. (5) Filatov, A. S.; Petrukhina, M. A. Coord. Chem. Rev. 2010, 254, 2234. (6) Tsefrikas, V. M.; Scott, L. T. Chem. Rev. 2006, 106, 4868. (7) Faust, R. Angew. Chem., Int. Ed. Engl. 1995, 34, 1429. (8) Sakurai, H.; Daiko, T.; Sakane, H.; Amaya, T.; Hirao, T. J. Am. Chem. Soc. 2005, 127, 11580. (9) Amaya, T.; Sakane, H.; Hirao, T. Angew. Chem., Int. Ed. 2007, 46, 8376. (10) Zanello, P.; Fedi, S.; de Biani, F. F.; Giorgi, G.; Amaya, T.; Sakane, H.; et al. Dalton Trans. 2009, 9192. (11) Green, J. R.; Dunbar, R. C. J. Phys. Chem. A 2011, 115, 4968. (12) Filatov, A. S.; Sumner, N. J.; Spisak, S. N.; Zabula, A. V.; Rogachev, A. Y.; Petrukhina, M. A. Chem.Eur. J. 2012, 18, 15753. (13) Spisak, S. N.; Zabula, A. V.; Filatov, A. S.; Rogachev, A. Y.; Petrukhina, M. A. Angew. Chem., Int. Ed. 2011, 50, 8090. (14) Seiders, T. J.; Baldridge, K. K.; O’Connor, J. M.; Siegel, J. S. J. Am. Chem. Soc. 1997, 119, 4781. (15) Angelici, R. J.; Zhu, B.; Fedi, S.; Laschi, F.; Zanello, P. Inorg. Chem. 2007, 46, 10901. (16) Vecchi, P. A.; Alvarez, C. M.; Ellern, A.; Angelici, R. J.; Sygula, A.; Sygula, R.; et al. Angew. Chem., Int. Ed. 2004, 43, 4497. (17) Siegel, J. S.; Baldridge, K. K.; Linden, A.; Dorta, R. J. Am. Chem. Soc. 2006, 128, 10644. (18) Zhu, B.; Ellern, A.; Sygula, A.; Sygula, R.; Angelici, R. J. Organometallics 2007, 26, 1721. (19) Bandera, D.; Baldridge, K. K.; Linden, A.; Dorta, R.; Siegel, J. S. Angew. Chem., Int. Ed. 2011, 50, 865. (20) Sakane, H.; Amaya, T.; Moriuchi, T.; Hirao, T. Angew. Chem., Int. Ed. 2009, 48, 1640. (21) Flood, A. H.; Wong, E. W.; Stoddart, J. F. Chem. Phys. 2006, 324, 280. (22) Al-Kaysi, R. O.; Bourdelande, J. L.; Gallardo, I.; Guirado, G.; Hernando, J. Chem.−Eur. J. 2007, 13, 7066. (23) Zhao, H. B.; Qiu, Y. Q.; Liu, C. G.; Sun, S. L.; Liu, Y.; Wang, R. S. J. Organomet. Chem. 2010, 695, 2251. (24) Mendes, P. J.; Silva, T. J. L.; Garcia, M. H.; Ramalho, J. P. P.; Carvalho, A. J. P. J. Chem. Inf. Model. 2012, 52, 1970. (25) Zhang, M. Y.; Wang, C. H.; Wang, W. Y.; Ma, N. N.; Sun, S. L.; Qiu, Y. Q. J. Phys. Chem. A 2013, 117, 12497. (26) Wang, W. Y.; Du, X. F.; Ma, N. N.; Sun, S. L.; Qiu, Y. Q. J. Mol. Model. 2013, 19, 1779. (27) Ma, N. N.; Liu, C. G.; Qiu, Y. Q.; Sun, S. L.; Su, Z. M. J. Comput. Chem. 2012, 33, 211. (28) Sun, X. X.; Ma, N. N.; Li, X. J.; Sun, S. L.; Xie, H. M.; Qiu, Y. Q. J. Mol. Graph. Model. 2012, 38, 248. (29) Asselberghs, I.; Clays, K.; Persoons, A.; Ward, M. D.; McCleverty, J. J. Mater. Chem. 2004, 14, 2831. (30) Kaur, P.; Kaur, M.; Depotter, G.; Van Cleuvenbergen, S.; Asselberghs, I.; Clays, K.; et al. J. Mater. Chem. 2012, 22, 10597. (31) Sporer, C.; Ratera, I.; Ruiz-Molina, D.; Zhao, Y.; Vidal-Gancedo, J.; Wurst, K.; et al. Angew. Chem., Int. Ed. 2004, 43, 5266. (32) Malaun, M.; Kowallick, R.; McDonagh, A. M.; Marcaccio, M.; Paul, R. L.; Asselberghs, I.; et al. J. Chem. Soc., Dalton Trans. 2001, 3025. (33) Kealy, T. J.; Pauson, P. L. Nat. Nanotechnol. 1951, 168, 2. (34) Wilkinson, G.; Rosenblum, M.; Whiting, M. C.; Woodward, R. B. J. Am. Chem. Soc. 1952, 74, 2125. (35) Liao, Y.; Eichinger, B. E.; Firestone, K. A.; Haller, M.; Luo, J.; Kaminsky, W.; et al. J. Am. Chem. Soc. 2005, 127, 2758. (36) Amer, W.; Wang, L.; Amin, A.; Ma, L.; Yu, H. J. Inorg. Organomet. Polym. 2010, 20, 605−15. (37) Debroy, P.; Roy, S. Coord. Chem. Rev. 2007, 251, 203. (38) Whittell, G. R.; Manners, I. Adv. Mater. 2007, 19, 3439. (39) Horikoshi, R.; Mochida, T. Eur. J. Inorg. Chem. 2010, 2010, 5355. (40) van Staveren, D. R.; Metzler-Nolte, N. Chem. Rev. 2004, 104, 5931.

is observed upon one- and two-electron reduction, while the one-electron oxidation gives a moderate enhancement of the first hyperpolarizabilities. Therefore, the NLO switching is more effective using one- and two-electron-reduction reaction stimuli. We suggest that Fc-buckybowl complexes are promising in highly efficient NLO switching.

4. CONCLUSIONS A series of novel push−pull molecules with a Fc donor and varied buckybowl subunits were investigated by density functional theory explicitly taking the PCM approach in THF solution into account. These donor−acceptor materials show better thermal stabilities owing to the π−π stacking interaction between the upper and lower subunits, which are able to force a molecule into the desired form. Correlating the electrochemical calculations carried out for the Fc-buckybowl complexes, it was established that the electron density at the Fc and buckybowl subunits is tunable, and such complexes are therefore novel examples of chromophores that are amenable to redox switching. Noteworthy, the absorption bands of the Fcbuckybowl complexes show redox dependence (proving it to be due to the respectively tunable electron density at the Fc and buckybowl subunits) upon one-electron-oxidization and oneand two-electron-reduction reactions. According to the SOS description, any modification of the absorption spectrum of a molecule would contribute to the modification of the first hyperpolarizability.111 This suggests that these redox-active complexes can be applicable for redox-triggered NLO switches with different hyperpolarizability values in each of the two redox states as well as high on/off ratios. The highest on/off ratio is 100.2, resulting in an intensity ratio for the second-order NLO effect.

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ASSOCIATED CONTENT

S Supporting Information *

Figures and tables giving the optimized geometries, Mulliken spin populations, molecular orbitals, TDDFT results, and full details of first hyperpolarizability values calculated by SOS and different DFT methods, and a text file of all computed molecular Cartesian coordinates in .xyz format for convenient visualization. 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 We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21173035). We are also thankful to Prof. Dr. Kan at the School of Chemistry and Chemical Engineering of Huaiyin Normal University for useful discussions on isomerism calculations.

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REFERENCES

(1) Lee, S. W.; Yabuuchi, N.; Gallant, B. M.; Chen, S.; Kim, B. S.; Hammond, P. T.; et al. Nat. Nanotechnol. 2010, 5, 531. (2) Lahiri, I.; Oh, S. W.; Hwang, J. Y.; Cho, S.; Sun, Y. K.; Banerjee, R.; et al. ACS Nano 2010, 4, 3440. K


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