Spectral and theoretical studies on effective and selective non-covalent interaction between tetrahexylporphyrins and fullerenes

Spectrochimica Acta Part A 68 (2007) 495–503

Spectral and theoretical studies on effective and selective non-covalent interaction between tetrahexylporphyrins and fullerenes Sumanta Bhattacharya a,∗ , Naruto Ujihashi a,b , Shuji Aonuma b , Takahide Kimura a , Naoki Komatsu a,∗ b

a Department of Chemistry, Shiga University of Medical Science, Seta, Otsu 520-2192, Japan Department of Materials Science, Osaka Electro-Communication University, Neyagawa, Osaka 572-8530, Japan

Received 23 October 2006; received in revised form 4 December 2006; accepted 16 December 2006

Abstract The host–guest interaction of zinc(II) 5,10,15,20-tetrahexylporphyrin (Zn-THP) and its free base (H2 -THP) with fullerenes (C60 and C70 ) has been studied in toluene medium. Binding constants (K) for H2 - and Zn-THP complexes of fullerenes were determined by UV–vis, fluorescence and NMR spectroscopic techniques. Large K values of C70 /THP complexes (KC70 ) were obtained in the range of 1.4–2.5 × 104 M−1 , while those of C60 /THP complexes (KC60 ) were smaller (1.0–3.2 × 103 M−1 ). These results show that the KC70 is about 10 times as large as KC60 in both ◦ ◦ THPs (KC70 /KC60 = 10). Enthalpies of formation (Hf ) for various fullerene/THP complexes were estimated by ab initio calculations; Hf for ◦ −1 C60 /H2 -THP, C70 /H2 -THP, C60 /Zn-THP and C70 /Zn-THP complexes are 5.82, 2.80, 2.31 and 1.54 kcal mol , respectively. The trends in Hf support the experimental results of selective complexation of THPs towards C70 over C60 and fullerenes towards Zn-THP over H2 -THP. © 2006 Elsevier B.V. All rights reserved. Keywords: Tetrahexylporphyrins; Fullerenes; Spectral and theoretical investigations; Non-covalent complexes; Binding constants

1. Introduction There is an increasing interest in cooperative effect of noncovalent interactions for the construction of supramolecular architectures [1]. Multiple hydrogen bonds and coordination bonds as well as several other forms of homogeneous and heterogeneous weak interactions can be used to bind components strongly. Several classes of porphyrins and their derivatives have been used for this purpose and in many cases the resulting systems have been served as models for the study of photo-induced energy [2–4] and electron [5–10] transfer processes that mimic the events occurring in natural photosynthesis. Fullerenes are also employed as suitable building blocks for the construction of multi-component systems because of their three-dimensional structure, relatively low reduction potentials and strong electron acceptor properties [11]. In order to understand the nature of dialog between fullerenes and porphyrin chromophores, the topology of the two moieties has been systematically varied and

a wide range of non-covalently linked porphyrin/fullerene systems has been reported [12–22]. However, the introduction of selectivity paradigm into supramolecular chemistry brings about a fundamental change in ways, means and outlook. The ultimate goal is to make a platform for the formation of selective and effective complexes. Therefore, in the pursuit of improved stability and control over the distance and orientation, molecular design of porphyrin hosts that can selectively bind with either C60 or C70 is a formidable task in the field of host–guest chemistry. In the present investigations, we have used zinc(II) 5,10,15,20-tetrahexylporphyrin (Zn-THP) and its free base (H2 THP) (Fig. 1) as host molecules to study their binding affinity towards C60 and C70 . The ␲-stacking of THPs with the surface of fullerenes produced a stable 1:1 complex with large binding constants (K) and high selectivity towards C70 over C60 . Computation quantum chemistry method has provided a good support in interpreting the selectivity towards C70 . 2. Materials and methods

∗

Corresponding authors. Tel.: +81 77 548 2102; fax: +81 77 548 2405. E-mail addresses: [email protected] (S. Bhattacharya), [email protected] (N. Komatsu). 1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.12.018

Both C60 and C70 were purchased from Frontier Carbon Corporation, Japan. Toluene (Spectroscopic grade, Nacalai Tesque) was used as solvent, because it favors non-covalent

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CT absorption band in inclusion complexes of fullerenes with cyclic dimers of metalloporphyrins [28]. Fig. 2 shows the electronic absorption spectra of H2 - and Zn-THPs in the absence and presence of C70 in toluene medium. It was observed that the intensity of Soret absorption bands of H2 - and Zn-THPs located at 420 and 424 nm, respectively, decreased gradually (see inset) following the gradual addition of C70 solution. Similar phenomenon was observed by Tashiro et al. for their particular C60 /metalloporphyrin system [29]. Similarly, the Soret band intensity of the THPs decreased systematically with the gradual addition of C60 solutions (Fig. 1S). All these phenomena indicate that ground state electronic interaction took place between fullerenes and THPs. The gradual decrease in the absorbance of the Soret band of the THPs was utilized to determine the binding constant (K) for the fullerene/porphyrin systems by Benesi–Hildebrand (BH) equation [30]. In all the cases, very good linear plots were obtained, two such plots being shown in Fig. 3. Values of K for various fullerene/THP complexes are summarized in Table 1. Fig. 1. Structure of M-THP (M = H2 and Zn).

3.2. Fluorescence spectroscopic studies interaction between fullerene and porphyrin, ensures good solubility and photo stability of the samples, and prevents fullerenes from forming clusters [23,24]. H2 - and Zn-THPs were prepared according to the reported method [25]. UV–vis spectral measurements were performed on a Shimadzu UV-3100 model spectrophotometer using quartz cell with 1 cm optical path length. All fluorescence spectral measurements were recorded in a Hitachi F-4500 model fluorescence spectrophotometer. All 1 H NMR spectra were recorded on a JEOL NMR spectrometer at 270 MHz. For NMR measurements, the solvent, toluene-d8, was collected from Cambridge Isotope Laboratories, Inc., USA. All theoretical calculations were computed in ab initio level using SPARTAN ’04 software. 3. Results and discussions 3.1. UV–vis spectroscopic studies Evidence for the fullerene-THP interactions first comes from UV–vis measurements. Addition of a fullerene solution to a toluene solution of H2 - or Zn-THP decreased the absorbance of the Soret band and no additional absorption peaks were observed in the visible region. The former observation is a good fingerprint in favor of complexation between fullerenes and THPs, leading us to postulate electronic interactions between the two chromophores, i.e., fullerene and porphyrin, in the ground state. The latter observation indicates that the interaction is not governed by charge transfer (CT) type transition in ground state. Interestingly, the Soret region was the only transition that seems to be effected more in all the complexes studied. Similar sort of observation was also noticed by Guldi et al. for their particular fullerene/porphyrin ensemble [26]. However, Armaroli et al. reported a broad absorption band at a near-IR region in a face-to-face fullerene/porphyrin system, which was assigned as CT band [27]. Tashiro and Aida also reported similar type of

The photochemical behaviors of fullerene/THP complexes have been investigated by steady-state emission measurements. As far as the emission spectra were concerned, it must be pointed out that in our present investigations fluorescence experiments could be reliably done due to one very favorable circumstance. The large molar extinction coefficients of porphyrins with respect to the fullerenes in the visible UV–vis spectral region allowed us to preferentially excite the THPs, although the porphyrin concentrations were much lower than those of the fullerenes. It was observed that fluorescence of H2 -THP at 667 and 725 nm, upon excitation at 554 nm (i.e., Q-absorption band), was diminished gradually by the addition of varying concentration of C60 or C70 in toluene medium (Fig. 4). Photo-physical studies as well as molecular modeling already proved that, in conformationally flexible porphyrin/fullerene systems, ␲stacking interactions facilitated the through-space interactions between these two chromophores. The interactions were demonstrated by quenching of 1* porphyrin fluorescence through formation of fullerene-excited states (energy transfer) and/or generation of porphyrin•+ –fullerene•− ion-pair states (electron transfer) [31]. In a non-polar medium like toluene, however, energy transfer phenomena dominates over electron transfer in deactivating the photo-excited chromophore, 1* porphyrin, formed in the final instance of the fullerene triplet excited state. Similar sort of rationale was already provided by Yin et al. for their particular cis-2 ,5 -dipyridinylpyrrolidino[3 ,4 :1,2]C60 /zinc(II) tetraphenylporphyrin (TPP) supramolecule [32]. Therefore, the quenching phenomenon in the present case can be ascribed to photo-induced energy transfer from porphyrins to C60 or C70 in the fullerene/THP complexes. Like H2 -THP system, the fluorescence intensity of Zn-THP at 652 nm upon excitation at 554 nm also reduced due to the quenching effect by the gradual addition of fullerene solution (Fig. 2S). In the case of H2 and Zn-THP/C60 complexes, isoemissive points were located at 582 and 580 nm, respectively. The appearance of isoemis-

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Fig. 2. (a) UV–vis absorption spectra of THP (8.175 × 10−6 mol dm−3 ) recorded against the solvent as reference and set (ii)–(vii) indicates UV–vis absorption spectra of THP (8.175 × 10−6 mol dm−3 ) in the presence of C70 (1.650 × 10−5 to 6.60 × 10−5 mol dm−3 ) recorded against the pristine C70 solution as reference; (b) UV–vis absorption spectra of Zn-THP (3.702 × 10−6 mol dm−3 ) recorded against the solvent as reference and set (ii)–(viii) indicates UV–vis absorption spectra of Zn-THP (3.702 × 10−6 mol dm−3 ) in the presence of C70 (3.085 × 10−6 to 6.170 × 10−5 mol dm−3 ) recorded against the pristine C70 solution as reference.

sive points provided a good support in favor of 1:1 complex formation between these two chromophores. The binding constants of all the fullerene/THP complexes were determined by Stern–Volmer (SV) equation [33]. In the present investigations, steady-state fluorescence quenching studies afforded a linear SV

plot, which was explained by fluorescence of porphyrin being quenched only by diffusion as opposed to static mechanism [33]. Excellent SV plots having correlation coefficient of more than 0.98 were obtained for each of the fullerene/porphyrin systems. Typical SV plots of H2 - and Zn-THP/C60 systems are shown in

Table 1 Binding constants (K, dm3 mol−1 ) of H2 - and Zn-THP complexes for C60 (KC60 ) and C70 (KC70 ) and their ratios Host

UV–vis 10−3

H2 -THP Zn-THP

× KC60

2.6 ± 0.1 3.4 ± 0.3

Temperature = 298 K.

Fluorescence 10−4

× KC70

1.8 ± 0.2 1.4 ± 0.03

KC70 /KC60

10−3

6.8 4.1

1.7 ± 0.1 2.2 ± 0.1

× KC60

NMR 10−4

× KC70

1.5 ± 0.05 2.5 ± 0.1

KC70 /KC60

10−3 × KC60

10−4 × KC70

KC70 /KC60

8.5 11

1.0 ± 0.2 2.7 ± 0.2

1.4 ± 0.01 1.7 ± 0.1

14 6.1

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Fig. 3. BH plots of (a) C60 /H2 -THP and (b) C70 /H2 -THP systems at 298 K.

Fig. 5. SV plots of (a) C60 /H2 -THP and (b) C60 /Zn-THP systems.

Fig. 5(a and b), respectively. The K values determined from SV plot show good agreement with those determined by UV–vis method (Table 1). 3.3. 1 H NMR spectroscopic studies

Fig. 4. Fluorescence quenching experiment of H2 -THP (4.10 × 10−5 mol dm−3 ) in the presence of (a) C60 ((i) 0.0 to (vii) 3.90 × 10−4 mol dm−3 ) and (b) C70 ((i) 0.0 to (vii) 11.13 × 10−5 mol dm−3 ) in toluene medium.

Due to simplicity of 1 H NMR spectra, the complexation of H2 - and Zn-THPs with fullerenes can be very easily monitored spectroscopically. In NMR spectrometric titration experiments, the concentration of the fullerene solution was varied maintaining a constant concentration of the THP. In the case of fullerene/H2 -THP systems, it was observed that the –NH– protons of H2 -THP molecule, which appeared at −1.961 ppm in the absence of fullerene, shifted to upfield with the gradual addition of C60 or C70 solution. This can be ascribed to the ring current effect of the fullerene on porphyrin [34]. The maximum of 0.019 and 0.018 ppm shifts were observed for the H2 -THP complexes of C60 (Fig. 6) and C70 (Fig. 3S), respectively. The greater upfield shift observed for the pyrrolic protons in both C60 and C70 complexes compared to other protons of porphyrin macrocycle suggests that the fullerene is located just above the center of the porphyrin macrocycle in the complex. The peaks of other protons at ␤-positions suffered very little shift and almost settled at 9.483 ppm. Similar sort of small but significant

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Fig. 6. Variation of NH proton signals of H2 -THP (2.895 × 10−3 mol dm−3 ) in the presence of (a) 0.0 mol dm−3 to (j) 1.85 × 10−4 mol dm−3 of C60 .

mutual upfield ring current-induced shifts were detected in the 2 -TPP and C60 in toluene solution [35]. In the case of fullerene/Zn-THP complexes, the alkyl proton of Zn-THP, which appeared at 1.601 ppm in uncomplexed state got perturbed after addition of fullerene solutions and shifted to 1.614 and 1.618 ppm in the presence of C60 and C70 , respectively. Binding constants of fullerene/THP complexes were determined by NMR analogue of BH equation [30]. In all the fullerene/THP complexes, excellent linear plots were obtained with the present data. Two typical NMR plots for Zn-THP/C60 and /C70 are shown in Fig. 7. 1 H NMR plots of H2 THP/C60 and /C70 systems are also provided in Fig. 4S. The K values determined by NMR method agree fairly well with those obtained by optical absorption and luminescence measurements (Table 1). 1 H NMR spectrum of the mixture of H

pared to H2 -THP in most cases. The closest distance between ˚ the fullerene C atom and Zn metal was measured to be 2.89 A, ˚ We slightly shorter than sum of van der Waals radii (3.09 A). assume that a direct interaction between the Zn metal of the porphyrin molecule and the surface of fullerene may lead to

3.4. Binding constants Binding constants (K) of various fullerene/THP complexes are summarized in Table 1. It is observed that both THPs undergo appreciable amount of complexation with both C60 and C70 . The binding constants of THPs with C60 (K = 2600 and 3400 dm3 mol−1 ) are comparable to those of the previously reported thiacalixarene bisporphyrin (K = 2340 dm3 mol−1 ) [36] and somewhat lower than those of the C60 complexes of zinc(II) resorcarene (K = 5010 dm3 mol−1 ) [37] and terphenyl porphyrin tetramer (K = 5800 dm3 mol−1 ) [38]. The binding of fullerenes to Zn-THP was comparable to that reported for JAWS porphyrin (K = 1950 dm3 mol−1 ) and, somewhat surprisingly, stronger than a number of other first row transition metalloporphyrins (K = 815 dm3 mol−1 ) [16]. However, cyclic dimers of Zn-porphyrins (K = 1.1 × 105 dm3 mol−1 ) [39] and porphyrin hexamer (K = 1.4 × 108 dm3 mol−3 ) [40] show much higher binding affinity towards C60 . Table 1 also reveals that Zn-THP gave somewhat higher K values with fullerenes com-

Fig. 7. NMR BH plots of (a) C60 /Zn-THP and (b) C70 /Zn-THP systems.

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enhancement in binding constant. Similar sort of interaction was also observed by Boyd et al. [35] in Zn-TPP/C70 complex. The average selectivity of C70 over C60 is found to be ∼8.4 (Table 1). The present selectivity is found to be comparable to that of bridged calix[5]arene (∼10.2) [41], and even larger than those of azacalix[m]arene[n]pyridine (∼1.9) [42], cyclotriveratrylenophane (∼2.5) [43] and calixarene bisporphyrin (∼4.3) [36] hosts. However, the present selectivity is lower than those observed for carbon nanoring (∼16) [44], cyclic dimers of Znporphyrins (∼25.5) [39] and H2 -diporphyrin (∼32) [45]. Such simple and flat molecules as THPs are found to exhibit relatively high selectivity to C70 . Practical applications of such high selectivity was observed in the preferential precipitation of C70 over C60 with host molecule like p-halohomooxacalix[3]arenes [46]. It is well known that the stability of the supramolecular complexes depends upon attractive interactions between host and guest, and on solvation of the binding partners [47]. In the present investigations, since the host and guest were separately solvated in solution, the desolvation of the host and guest was requisite in the association process. However, the process of desolvation is energetically an uphill task and, hence, an association takes place only when the energy gain between the host and guest interaction exceeds this unfavorable energy. From the trends in the K values of the fullerene/THP complexes, we may infer that the extent of solvation and desolvation of the binding partners might play an important role in forming weak or strong supramolecular complexes. Haino et al. [48] showed that the stability of the complex increases as the solubility of fullerene in a solvent decreases, since less energy is required for the desolvation of fullerene which must necessarily precede its complexation with a host molecule. This would be one of the reasons for the higher K values of the C70 complexes in toluene, since the solubility of C60 is higher in toluene (4.1 mg mL−1 ) than that of C70 (2.0 mg mL−1 ) [49]. 3.5. Computational studies In order to gain insight into the preferred molecular geometry ◦ and to measure the heat of formation (Hf ), a detailed conformational analysis of the individual components as well as the fullerene/THP adducts were performed in vacuo. The geometric parameters of the complexes were obtained after complete energy minimization. Figs. 8 and 5S show the geometric structures of the Zn- and H2 -THP/fullerene complexes, respectively, optimized by ab initio calculation. In all the structures reported here, the four hexyl groups of the THPs are arranged so that they can form octopus-like embrace to the fullerene surface, facilitating a close face-to-face fullerene/porphyrin contact. For the Zn-THP complexes, the Zn–N distance of the axial coordination ˚ which was almost similar to bond was computed to be 1.933 A, the bond distances of the three other Zn–N bonds of the Zn-THP macrocycle. It was also observed that the Zn metal was pulled out of the porphyrin plane after complex formation with fullerenes. Such a square pyramidal geometry is well known for metalloporphyrin [50]. The center-to-center distance, i.e., the distance between the central zinc metal and the center of the C60 and ˚ respecC70 spheroids were computed to be 6.55 and 6.75 A,

tively. The most important feature of all these structures is their unexpectedly close fullerene/porphyrin contact, which was manifested in the C H· · ·␲ interaction between the fullerene carbon atom and the H atom of the hexyl group of Zn-THP receptor. As shown in Fig. 8b, C70 is centered over the porphyrin; electron rich 6:6 ring-juncture C C bonds are in close approach to the H atom of the hexyl unit of Zn-THP in the range between 2.768 ˚ These values accord excellently well with the earand 2.943 A. lier reported values of fullerene/metalloporphyrin complexes by some other workers [35]. In the case of C60 /Zn-THP complex, ˚ however, the estimated C–H distances was found to be 3.220 A (Fig. 8a). The larger distance in the latter case was due to weaker interaction between porphyrin and C60 which was reflected in the trend of the binding constants in Table 1, i.e., KC70 > KC60 . These findings indicate that C70 /Zn-THP complex is stabilized more compared to C60 /Zn-THP complex in terms of C H· · ·␲ interactions. The above structural analysis indicates that the porphyrin and fullerene ␲-electrons in the presently investigated fullerene/porphyrin complexes would strongly interact owing to their close proximity. In our present investigations, the existence of the intermolecular interactions between the porphyrin macrocycle and fullerene moiety was also evidenced by the results obtained on frontier highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) using high level HF/3-21G* method. The majority of the orbital distributions of HOMO and HOMO − 1 were found to be located on THP moiety with a small orbital coefficient on the axial hexyl part. On the other hand, all the orbital distributions of the LUMO and LUMO + 1 were positioned in fullerene entity. The absence of HOMO on fullerene and LUMO on the porphyrin macrocycle suggests weak or no CT interactions between fullerene and porphyrin in ground state. The present findings accord superbly with our previous findings obtained from UV–vis experiments as discussed above. EHOMO and ELUMO of all the fullerene complexes along with the individual components are given in Table 2. Moreover, the regions of high negative electrostatic potential of the Zn-THP (the four N atoms) were facing the regions of strong positive electrostatic potential of the C70 (the centers of hexagons and pentagons) and vice versa (6:6 bond is high negative electrostatic potential and Zn is high positive electrostatic potential) [51,52]. The same reasoning can be done in terms of high charge density and low regions. In this particular system, therefore, the electrostatic interaction eventually determined the absorption geometry of the Zn-THP with respect to C70 . Some pictures of HOMOs and LUMOs of the C70 /H2 -THP complexes are visualized in Fig. 9. Fig. 9 also reveals that LUMO + 6 and HOMO are of porphyrin character and correspond to the frontier orbital according to the Gouterman four-orbital model for porphyrin [53]. Frontier molecular orbitals of C60 /Zn-THP, C70 /Zn-THP and C60 /H2 -THP systems are shown in Figs. 6S–8S, respectively. The impinging feature of the present investigations is that C70 binds more strongly to THPs than C60 (KC70 > KC60 ). Other than the electrostatic mechanisms discussed above, the attractive interactions between C70 and porphyrins is also driven by the dispersive forces associated with ␲–␲ interactions. Detailed

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Fig. 8. Geometric structures of the complexes of Zn-THP with (a) C60 and (b) C70 optimized in ab initio geometry.

discussions in this matter were already provided in one of our ◦ recent work [54]. Hf values for various fullerene/porphyrin complexes are summarized in Table 2. Estimation of the surface areas for the complexes of C60 /H2 THP, C60 /Zn-THP, C70 /H2 -THP and C70 /Zn-THP were found ˚ 2 , respectively. to be 1179.21, 1156.68, 1207.28 and 1210.56 A

These results also manifest that interaction between C70 and THPs is favored over C60 . It is also observed that the alignment of C70 with the plane of H2 -THP (Fig. 9) and Zn-THP (Fig. 8b) ◦ was found to be side-on as revealed from Hf for the C70 /THPs complexes in two different orientations (Table 2). Such motif of C70 towards THPs took place in order to maximize van der Waals

Table 2 ◦ Theoretically determined enthalpies of formation (Hf ) and HOMO–LUMO energies for fullerene/THP complexes Host

H2 -THP Zn-THP

◦

Hf (kcal mol−1 )

Ehomo (eV)

Elumo (eV)

C60

C70 (side-on)

C70 (end-on)

C60

C70

C60

C70

5.82 2.80

2.31 1.54

5.19 2.26

−6.54 −6.05

−6.51 −6.06

−0.74 −0.72

−0.85 −0.91

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Fig. 9. HOMOs and LUMOs of C70 /H2 -THP complex at different electronic states; e.g., (a) HOMO, (b) HOMO − 1, (c) LUMO, (d) LUMO + 1 and (e) LUMO + 6.

interaction and greater ␲–␲ contact area of the porphyrin with the equatorial region of C70 . C70 can be viewed as two hemispheres of C60 with insertion of an equatorial belt of graphite like hexagons, creating a flattening in the equator. This flattening allows a greater fullerene/porphyrin contact. However, in endon orientation (Fig. 9S), C70 cannot enjoy the above favorable ◦ scenario which resulted in lower value of Hf .

somewhat strongly with fullerenes than H2 -THP, implying direct interaction of Zn metal with the surface of fullerenes to stabilize ◦ the complexes. Trends in the Hf value also support experimental findings of selective complexation of THPs towards C70 over C60 and fullerene towards Zn-THP over H2 -THP.

4. Conclusions

Spectra of UV–vis spectrophotometric titration experiments of C60 with H2 -THP and Zn-THP, fluorescence quenching experiments of Zn-THP in presence of fullerenes, NMR chemical shift experiments of C70 /H2 -THP complex, NMR BH plot of C60 / and C70 /Zn-THP systems, ab initio optimized geometric struc-

Both H2 - and Zn-THPs showed large binding constants for fullerenes and high selectivity towards C70 over C60 though they are flat and simple. It is also observed that Zn-THP binds

Supporting information

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ture of fullerene/THP complexes, frontier molecular orbitals of C60 /Zn-THP, C70 /Zn-THP and C60 /H2 -THP systems, and ab initio optimized geometric structures of C70 /H2 -THP and /Zn-THP complexes in end-on orientation are provided as Figs. 1S–9S, respectively, as supplementary materials. These materials are available free of charge via the internet in the online edition of Spectrochimica Acta Part A. Acknowledgements SB acknowledges Japan Society of Promotion of Science for providing post-doctoral fellowship to him (ID No. P05389). He also wishes to record his gratitude to Council of Scientific and Industrial Research, New Delhi for providing him necessary leave to continue his post-doctoral research work. This work was supported by Grant-In-Aid (No. 17-05389) from Ministry of Education, Culture, Sports, Science and Technology, Japan. We also thank the learned reviewers and Professor James R. Durig for making valuable comments. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.saa.2006.12.018. References [1] L.J. Prins, D.N. Reinhoudt, P. Timmerman, Angew. Chem. Int. Ed. 40 (2001) 2426. [2] R.A. Haycock, A. Yartsev, U. Michelsen, V. S¨undstrom, C.A. Hunter, Angew. Chem. Int. Ed. 39 (2000) 3616. [3] H. Shinmori, T. Kajiwara, A. Osuka, Tetrahedron Lett. 42 (2001) 3617. [4] Y. Kuroda, K. Sugou, K. Sasaki, J. Am. Chem. Soc. 122 (2000) 7833. [5] H. Imahori, Org. Biomol. Chem. 2 (2004) 1425. [6] H. Imahori, M.E. El-Khouly, M. Fuzitsuka, O. Ito, Y. Sakata, S. Fukuzumi, J. Phys. Chem. A 105 (2001) 325. [7] D.M. Guldi, Pure Appl. Chem. 75 (2003) 1069. [8] L. Flamigni, M.R. Johnston, New J. Chem. 25 (2001) 1368. [9] F. D’Souza, P.M. Smith, M.E. Zandler, A.L. McCarty, M. Itou, Y. Araki, O. Ito, J. Am. Chem. Soc. 126 (2004) 7898. [10] F. D’Souza, O. Ito, Coord. Chem. Rev. 249 (2005) 1410. [11] D.M. Guldi, Chem. Soc. Rev. 31 (2002) 22. [12] F. D’Souza, N.P. Rath, G.R. Deviprasad, M.E. Zandler, Chem. Commun. (2001) 267. [13] F. D’Souza, R. Chitta, S. Gadde, M.E. Zandler, A.L. McCarty, P.A. Karr, A.S.D. Sandanayaka, Y. Araki, O. Ito, J. Phys. Chem. B 110 (2006) 5905. [14] M.E. El-Khouly, O. Ito, P.M. Smith, F. D’Souza, J. Photochem. Photobiol. C 5 (2004) 79. [15] D.V. Konarev, R.N. Lyubovskaya, G. Zerza, M.C. Scharber, N.S. Sariciftci, Mol. Cryst. Liq. Cryst. 427 (2005) 315. [16] D. Sun, F.S. Tham, C.A. Reed, L. Chaker, M. Burgess, P.D.W. Boyd, J. Am. Chem. Soc. 122 (2000) 10704. [17] F. D’Souza, R. Chitta, S. Gadde, M.E. Zandler, A.L. McCarty, A.S.D. Sandanayaka, Y. Araki, O. Ito, Chem. Eur. J. 11 (2005) 4416. [18] F. Diederich, M. Gomez-Lopez, Chem. Soc. Rev. 28 (1999) 263.

503

[19] D.M. Guldi, M. Prato, Acc. Chem. Res. 33 (2000) 695. [20] D.M. Guldi, C. Luo, T. DaRos, M. Prato, F. Dietel, A. Hirsch, Chem. Commun. (2000) 375. [21] M.M. Olmstead, D.A. Costa, K. Maitra, B.C. Noll, S.L. Phillips, P.M. Van Calcar, A.L. Balch, J. Am. Chem. Soc. 121 (1999) 7090. [22] D.I. Schuster, P.D. Jarowski, A.N. Krischner, S.R. Wilson, J. Mater. Chem. 12 (2002), 2041 and the references cited there in. [23] Y.P. Sun, C. Bunker, Nature (Lond.) 365 (1993) 398. [24] R. Alargova, S. Deguchi, K. Tsujii, J. Am. Chem. Soc. 123 (2001) 10460. [25] M.J. Crossley, P. Thordarson, J.P. Bannerman, P.J. Maynard, J. Porphyrins Phthalocyanines 2 (1998) 511. [26] D.M. Guldi, T. Da Ros, P. Braiuca, M. Prato, Photochem. Photobiol. Sci. 2 (2003) 1067. [27] N. Armaroli, G. Marconi, L. Echegoyen, J.-P. Bourgeois, F. Diederich, Chem. Eur. J. 6 (2000) 1629. [28] K. Tashiro, T. Aida, J. Incl. Phenom. Macrocyclic Chem. 41 (2001) 215. [29] K. Tashiro, T. Aida, J.-Y. Zheng, K. Kinbara, K. Saigo, S. Sakamoto, K. Yamaguchi, J. Am. Chem. Soc. 121 (1999) 9477. [30] H.A. Benesi, J.h. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703. [31] D. Gust, T.A. Moore, A.L. Moore, D. Kuciauskas, P.A. Liddell, B.D. Halbert, J. Photochem. Photobiol. B 43 (1998) 209. [32] G. Yin, D. Xu, Z. Xu, Chem. Phys. Lett. 365 (2002) 232. [33] V.O. Stern, M. Volmer, Phys. Z 20 (1919) 183. [34] D. Sun, F.S. Tham, C.A. Reed, L. Chaker, P.D.W. Boyd, J. Am. Chem. Soc. 124 (2002) 6604. [35] P.D.W. Boyd, M.C. Hodgson, C.E.F. Rickard, A.G. Oliver, L. Chaker, P.J. Brothers, R.D. Bolskar, F.S. Tham, C.A. Reed, J. Am. Chem. Soc. 121 (1999) 10487. [36] M. Dudi, P. Lhot´ak, I. Stibor, H. Pet´ıkov´a, K. Lang, New J. Chem. 28 (2004) 85. [37] O.D. Fox, M.G.B. Drew, E.J.S. Wilkinson, P.D. Beer, Chem. Commun. (2000) 391. [38] Y. Kubo, A. Sugasaki, M. Ikeda, K. Sugiyasu, K. Sonoda, A. Ikeda, M. Takeuchi, S. Shinkai, Org. Lett. 4 (2002) 925. [39] Y. Shoji, K. Tashiro, T. Aida, J. Am. Chem. Soc. 126 (2004) 6570. [40] M. Ayabe, A. Ikeda, Y. Kubo, M. Takeuchi, S. Shinkai, Angew. Chem. Int. Ed. 41 (2002) 2790. [41] T. Haino, M. Yanase, Y. Fukazawa, Angew. Chem. Int. Ed. 37 (1998) 997. [42] M.-X. Wang, X.-H. Zhang, Q.-Y. Zheng, Angew. Chem. Int. Ed. 43 (2004) 838. [43] H. Matsubara, S. Oguri, K. Asano, K. Yamamoto, Chem. Lett. (1999) 431. [44] T. Kawase, K. Tanaka, Y. Seirai, N. Shiono, M. Oda, Angew. Chem. Int. Ed. 42 (2003) 5597. [45] J.-Y. Zheng, K. Tashiro, Y. Hirabayashi, K. Kinbara, K. Saigo, T. Aida, S. Sakamoto, K. Yamaguchi, Angew. Chem. Int. Ed. 40 (2001) 857. [46] N. Komatsu, Org. Biomol. Chem. 1 (2003) 204. [47] S. Mizyed, P.R. Tremaine, P.E. Georghiou, J. Chem. Soc., Perkin Trans. 2 (2001) 3. [48] T. Haino, M. Yanase, Y. Fukuzawa, Angew. Chem. Int. Ed. Engl. 36 (1997) 259. [49] X. Zhou, Z. Gu, Y. Wu, Y. Sun, Z. Jin, Y. Xiong, B. Sun, Y. Wu, H. Fu, J. Wang, Carbon 32 (1994) 935. [50] J.L. Hoard, Science (Washington, DC) 174 (1971) 1295. [51] D.V. Konarev, I.S. Neretin, Y.L. Slovokhotov, E.I. Yudanova, N.V. Drichko, Y.M. Shul’ga, B.P. Tarasov, L.L. Gumanov, A.S. Batsanov, J.A.K. Howard, R.N. Lyubovskaya, Chem. Eur. J. 7 (2001) 2605. [52] Y.-Bo. Wang, Z. Lin, J. Am. Chem. Soc. 125 (2003) 6072. [53] M. Gouterman, G.H. Wagniere, L.C. Synder, J. Mol. Spectrosc. 11 (1963) 108. [54] S. Bhattacharya, T. Shimawaki, X. Peng, A. Ashokkumar, S. Aonuma, T. Kimura, N. Komatsu, Chem. Phys. Lett. 430 (2006) 435.