Photophysical insights into supramolecular interaction of a designed bisporphyrin with fullerenes C60 and C70

Spectrochimica Acta Part A 78 (2011) 185–190

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Photophysical insights into supramolecular interaction of a designed bisporphyrin with fullerenes C60 and C70 Debabrata Pal a , Mina Furukawa b , Naoki Komatsu c , Hidemitsu Uno b,∗ , Sumanta Bhattacharya a,∗∗ a b c

Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, West Bengal, India Department of Chemistry and Biology, Graduate School of Science and Engineering, Ehime University, Matsuyama 790 8577, Japan Department of Chemistry, Shiga University of Medical Science, Seta Tsukinowa-cho, Otsu 520 2192, Japan

a r t i c l e

i n f o

Article history: Received 25 June 2010 Accepted 11 September 2010 Keywords: C60 and C70 Designed bisporphyrin UV–vis and steady state fluorescence Binding constants Time resolved emission studies Molecular mechanics calculations

a b s t r a c t The present paper reports the photophysical investigations of a designed bisporphyrin (1), and its supramolecular complexes with C60 and C70 in toluene medium. UV–vis studies reveal appreciable ground state interaction between fullerenes and 1. The stoichiometry of the fullerene complexes of 1 is found to be 1:1. Steady state fluorescence studies elicit quenching of fluorescence of 1 in the presence of fullerenes. The binding constants of the C60 /1 and C70 /1 complexes are estimated to be 3760 and 31,222.5 dm3 mol−1 , respectively. Time resolved emission studies establish relatively long-lived charge separated state for the C70 /1 complex. Molecular mechanics calculations in vacuo evoke the stereoscopic structures of the fullerene/1 complexes and interpret the stability difference between C60 and C70 complexes of 1 in terms of heat of formation values. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Since the discovery of fullerenes in the year 1985 [1], a new domain in supramolecular chemistry is explored. The unique physical and chemical properties of fullerenes, viz. electronic absorption bands expanding throughout the entire UV–vis region [2], strong electron acceptor character [3–5], efficient singlet oxygen sensitizing ability [6] and superconductivity upon doping with alkali metals [7] – make them an attractive component to be incorporated in functional molecular systems in the most versatile fashion [8]. The interaction of electron deficient fullerenes with ␲-electron rich macrocyclic receptors such as calixarenes, crown ethers, cyclodextrins and many others have been studied in the light of ‘host–guest chemistry’, and the forces responsible for effective binding are well characterized for various diversified systems [9]. Among the above mentioned class of representative compounds, porphyrin systems [10] are probably the most explored systems in view of their interaction with fullerenes. The low reorganization energy and the high electron affinity value of fullerenes are presumably be effective behind the attractive association between the curved and flat ␲-surfaces of fullerenes and porphyrins, respectively [11]. Various fullerene/porphyrin systems have been developed

∗ Corresponding author. Fax: +81 89 927 9610. ∗∗ Corresponding author. Fax: +91 342 2530452. E-mail addresses: [email protected] (H. Uno), sum [email protected] (S. Bhattacharya). 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.09.019

where the two electro-active units are bound together either by covalent or non-covalent bonds [12–19]. A variety of covalently linked fullerene/porphyrin dyads of donor–acceptor type, have already been reported in this connection, in which the key feature is that the porphyrin loses its characteristic luminescence property as a result of light induced energy and/or electron transfer to the fullerenes [20]. The above characteristics mimic the natural process of photosynthesis [21]. Photovoltaic cells based on a fullerene/porphyrin dyads have already been shown to exhibit very high quantum yield [22]. The surprisingly close contacts observed between the porphyrin planes and fullerenes in the solid state have generated considerable efforts towards the design of preorganized porphyrin hosts suitable for strong complexation with fullerenes in solution [23]. However in case of fullerene/porphyrin supramolecular assemblies, the origin of the binding force may be hydrogen bonding, dipole–dipole interaction, charge transfer interactions (CT), ␲–␲ stacking, electrostatic interaction or any other effects which may not conjointly taking part as the system may be [24]. However the point of interest is that even though each interaction is weak and easily exchangeable, their accumulation provides thermodynamically stable structures [25]. In case of the monoporphyrin systems, the strength of the interaction between porphyrin and fullerene is affected by peripheral substituents on the porphyrin [26]. However, for the supramolecular association of free-base or metallo-bisporphyrins with fullerenes, the spacer unit between the two monoporphyrin units plays a significant role in the mode of binding, apart from other factors like the peripheral substitution, choice of metal etc. The ultimate goal is

186

D. Pal et al. / Spectrochimica Acta Part A 78 (2011) 185–190

Fig. 1. Structure of 1.

to make a platform for the formation of selective and effective complexes with fullerenes. Therefore, in the pursuit of improved stability and control over the distance and orientation, molecular design of bisporphyrin hosts that can selectively bind with either C60 or C70 becomes a challenging task in order to illuminate the understanding of the flexibility criterion necessary for cooperative interaction in the field of host–guest chemistry. In our present paper, we report the photophysical properties on complexation behavior of a designed Zn2 –bisporphyrin (1, Fig. 1) with fullerenes C60 and C70 in toluene medium. UV–vis, steady state and time resolved fluorescence spectroscopic studies reveal that 1 serves as an effective and selective molecular tweezers towards C70 . Theoretical calculations in molecular mechanics level have provided very good support in favor of above findings. 2. Experimental Both C60 and C70 are collected from Aldrich, USA. Toluene (Spectroscopic grade, Merck) has been used as solvent. 1 has been obtained as a gift item from Ehime University [27]. UV–vis spectra are recorded in a Shimadzu UV-1601 model spectrophotometer fitted with a Peltier controlled thermo bath using a quartz cell of 1 cm optical path length. All steady state fluorescence spectral measurements are done in a Hitachi F-4500 model fluorescence spectrophotometer. Fluorescence decay curves are measured with a HORIBA Jobin Yvon Single Photon Counting Set up employing Nanoled as excitation source. Theoretical calculations are done using SPARTAN’06 software. 3. Results and discussion 3.1. UV–vis absorption studies Evidence in favor of ground state interaction between fullerenes and 1 first comes from UV–vis spectral measurements of 1 in toluene against the solvent as reference. The UV–vis absorption spectra of 1 in toluene shows one broad Soret absorption band, i.e., B band, at 402 nm and two Q absorption bands at 531 and 569 nm (Fig. 2). Generally in case of metalloporphyrin, the appearance of B and Q absorption bands arise from configuration interaction of four orbitals, the nearly degenerate pair a1u , a2u , highest occupied molecular orbital (HOMO) and the doubly degenerate e1g , lowest unoccupied molecular orbital (LUMO), respectively [28]. The Soret band corresponds to the transition to the second excited singlet state S2 and Q-bands corresponding to the vibronic sequence of the transition to the lowest excited singlet state S1 . However, in case of diporphyrin, splitting of Bx and By components of the Soret absorption band is caused by the decrease in the symmetry which is reflected by the appearance of two sharp peaks in the visible region. This phenomenon can be qualitatively accounted by exci-

Fig. 2. (a) UV–vis absorption spectra of (i) 1 (2.735 × 10−6 mol dm−3 ) recorded against the solvent as reference and set (ii)–(x) indicates UV–vis absorption spectra of 1 (2.735 × 10−6 mol dm−3 ) in the presence of C60 (1.920 × 10−5 –1.730 × 10−4 mol dm−3 ) recorded against the pristine C60 solution as reference; (b) UV–vis absorption spectra of 1 (2.735 × 10−6 mol dm−3 ) recorded against the solvent as reference and set (ii)–(vii) indicates UV–vis absorption spectra of 1 (2.735 × 10−6 mol dm−3 ) in the presence of C70 (2.143 × 10−5 –1.5 × 10−4 mol dm−3 ) recorded against the pristine C70 solution as reference.

ton coupling originated from the Coulombic interactions between the transition dipole moments of the two porphyrin subunits [29]. In case of 1, however, overlapping of the two Soret absorption peaks effectuated the appearance of a broad peak at 402 nm. In the UV–vis titration experiment, it is observed that gradual addition of a C60 solution to a toluene solution of 1 decreases the absorbance of the Soret absorption band and red-shifted it from 402 to 403 nm (inset of Fig. 2a). The appearance of the isobestic point at 413 nm provides very good support in favor of molecular complexation between C60 and 1. However, the effect is seen to be more pronounced in case of C70 /1 complex (Fig. 2(b)) causing the red-shift of the Soret absorption band to 7 nm (i.e., 409 nm) from its original position observed at 402 nm with the appearance of the isobestic point at 413 nm (inset of Fig. 2(b)). However, no additional absorption peaks are observed in the visible region, indicating that the interaction is not controlled by CT type transition. It should be mentioned at this point that neither of the two Q bands experience any shift upon addition of either C60 or C70 solution to the toluene solution of 1 (Fig. 2(a) and (b)). The larger extent of red shift of the Soret absorption band exemplifies the strong interaction of 1 with C70 compared to C60 . In C70 /1 complex, isosbestic point has been located at 413 nm, giving good support of 1:1 complexation between these two species [17]. The stoichiometry for both the C60 /– and C70 /–

D. Pal et al. / Spectrochimica Acta Part A 78 (2011) 185–190

187

Table 1 singlet singlet ) and quantum yield of the charge separation at singlet state (CS ), binding constants (K, dm3 mol−1 ), average K (Kav ) and selectivity in Rates of charge separation (kCS K, i.e., KC70 /KC60 , of the complexes of C60 and C70 with 1 recorded in toluene medium at 298 K. singlet

CS

1.10 × 108 6.20 × 109

C60 /1 C70 /1

Method UV–vis 4800 ± 440 25580 ± 1250

0.083 0.835

complexes of 1 have been confirmed by Jobs plot of continuous variation method. One typical Jobs plot for the C70 /1 system is shown in Fig. 3. The most important observation in the present investigation is that the Soret absorption bands get affected more than the Q-absorption bands, which is already observed by Guldi et al. [30] for their particular fullerene/porphyrin systems. Therefore, the gradual decrease in the absorbance value of the Soret band of 1 is utilized to determine the binding constant (K) of the fullerene/1 complexes employing Benesi–Hildebrand (BH) plot [31]. In all the cases, very good linear plots are obtained as shown in Fig. 4, and the determined binding constants are summarized in Table 1. 3.2. Steady state fluorescence studies The photo induced behavior of the complexes of C60 and C70 with 1 has been investigated by steady-state emission measurements, which also substantiates the binding of 1 with fullerenes. The large molar extinction coefficient value of 1 with respect to the fullerenes in the UV–vis spectral region allows us to preferentially excite 1, keeping the porphyrin concentration at much lower level compared to fullerenes. In toluene solution, upon excitation at 402 nm, 1 exhibits two emission bands at 572 and 625 nm corresponding to (0,0) and (0,1) transitions, respectively. According to Gouterman et al. [28], the (0,0) band results from a transition between the ground state and the lowest exited state, and the (0,1) band is related to the transition between the lowest exited state and the vibronic state involving the most active vibrations. The fluorescence spectral changes of 1 upon addition of C60 and C70 solutions are shown in Figs. 5(a) and 1S(a), respectively. As the concentrations of fullerenes are increased, the emission intensity of 1 is reduced. The titration is performed at a constant concentration of 1. This finding indicates that there is some relaxation pathway from the excited singlet state of the porphyrin to the fullerene in toluene. It is already reported that charge separation can also occur from the excited singlet state of the porphyrin to the C60 in toluene medium [32]. Competing

Kav , (dm3 mol−1 )

KC70 /KC60

3760 31,222

8.3

Fluorescence 2720 ± 135 36865 ± 1800

between the energy and electron transfer processes is a universal phenomenon in fullerene/donor molecule complexes [33], solvent dependent photo physical behavior is a typical phenomenon of the most fullerene/porphyrin dyads studied to date [34]. Photophysical studies as well as theoretical calculations already prove that, in conformationally flexible fullerene/porphyrin systems, ␲-stacking ability facilitates the through-space interactions between these two chromophores. This type of interaction is clearly demonstrated by quenching of 1 porphyrin* fluorescence through formation of fullerene-excited states (energy transfer) and/or generation of fullerene−• /porphyrin+• ion-pair states (electron transfer) [35]. In a non-polar medium like toluene, generally, energy transfer dominates over the electron transfer in deactivating the photoexcited chromophore (here 1 porphyrin* ). Similar sort of rationale is already provided by Yin et al. for their particular cis-2 ,5 dipyridinylpyrrolidino[3 ,4 :1,2]C60 /zinc(II)tetraphenylporphyrin supramolecule [36]. However, very recently it is reported that, for C60 /zincporphyrin (ZnP) dyad system, a new band corresponding

a -9

3.00x10 -3 2

(s−1 )

[C60][1]/ΔAbs, (mol.dm )

singlet

kCS

System

-9

2.25x10

-9

1.50x10

-10

7.50x10

0.00 -5

0.0

5.0x10

-4

1.0x10

-4

1.5x10

-4

2.0x10

-4

2.5x10

-3

[C60], mol.dm 0.18 -9

1.0x10

b

0.16 -10

-3 2

8.0x10 [C70][1]/ΔAbs, (mol.dm )

ΔAbsorbance

0.14 0.12 0.10 0.08 0.06

-10

6.0x10

-10

4.0x10

-10

2.0x10

0.04

0.0

0.02 0.0

0.2

0.4

0.6

0.8

Mole fraction of C70 Fig. 3. Jobs plot of continuous variation for C70 /1 system.

1.0

0.0

-5

5.0x10

-4

1.0x10

-4

1.5x10

-3

[C70], mol.dm

Fig. 4. BH plot for (a) C60 /1 (b) C70 /1 complex recorded in toluene medium at 298 K.

188

D. Pal et al. / Spectrochimica Acta Part A 78 (2011) 185–190

(i)

7000

Realtive fluorescence intensity

a

6000

Fluorescence intensit y

(ii) – (x) 5000

4000

15

10

5

3000 2.0x10

4.0x10

6.0x10

[C ], mole.dm 2000

1000

0 550

560

570

580

590

600

610

620

630

640

650

wavelength (nm)

b Relative fluorescence intensity

15

10

5

0 0.0

4

4

6.0x10

3.0x10

3

4

9.0x10

-1

1/[C60], dm .mol

Fig. 5. (a) Fluorescence quenching of 1 (2.735 × 10−6 mol·dm−3 ) by C60 (from (i) 0 to (xi) 6.94 × 10−5 mol dm−3 ) recorded in toluene medium at 298 K (inset, plot of relative fluorescence intensity vs. [C60 ] for 1 at 298 K); (b) fluorescence BH plot of C60 /1 system recorded in toluene medium at 298 K.

to C60 fluorescence develops at 710 nm in addition to ZnP emission at 600 and 650 nm [37]. This phenomenon indicates that rapid intramolecular transduction of singlet excitation energy takes place from the excited ZnP to fullerene. In our present investigations, however, the fullerene fluorescence at 710 nm is not observed, although the fullerene/1 complexes exhibit the characteristic emission of 1 at 572 and 625 nm. This finding suggests that singlet energy transduction to give 1 fullerene* -1 followed by generation of 3 fullerene* -1 (by intersystem crossing) is not operative in our present investigations. The spectral changes finally reach a plateau, indicating that the fluorescence quenching is induced by the complexation with C60 and C70 (inset of Figs. 5(a) and 1S(a), respectively). Although 1 exhibits fluorescence quenching upon the addition of fullerenes, the quenching efficiency of C70 is much higher than that of C60 . The binding constants are evaluated according to a modified Benesi–Hildebrand equation [31] (see Eq. (1)):

In Eq. (1), F0 and F are the fluorescence intensity of 1 without and with the fullerenes, respectively, and [fullerene] indicates the molar concentration of fullerene; A is a constant associated with the difference in the emission quantum yield of the complexed and uncomplexed porphyrin. By plotting relative fluorescence intensity, viz., F0 /(F0 − F) vs. 1/([fullerene]), excellent linear plots have been obtained for both the C60 /– and C70 /– complexes of 1 (Figs. 5(a) and 1S(b) respectively). The values of K are given in Table 1. Trend in the K values for the fullerene/1 complexes suggests that tight fixation of C70 takes place in the well defined structure of 1 giving rise to correct host–guest orientation. K values determined by fluorescence method corroborates fairly well with those obtained from the UV–vis absorption studies (see Section 3.1 and Table 1).

F0 1 = + F0 − F A

The time-resolved fluorescence spectral features of the fullerene/1 track the steady state measurements. The fluorescence

1  KA

1 fullerene



(1)

3.3. Time-resolved fluorescence studies

D. Pal et al. / Spectrochimica Acta Part A 78 (2011) 185–190

189

Fig. 6. Time resolved fluorescence spectrum of 1 (9.50 × 10−6 mol dm−3 ) in the (a) absence and presence of (b) C60 (6.75 × 10−5 mol dm−3 ) and (c) C70 (9.40 × 10−5 mol dm−3 ).

decay-time profiles of the investigated complexes of 1 (monitored at 402 nm using 462 nm LASER diode) show an enhanced decay rate as compared to uncomplexed 1. In toluene, all the investigated species, viz.,1, C60 /1 and C70 /1 complexes along with uncomplexed 1 show biexponential decay. Substantial quenching of the fluorescence lifetime ( 0 ) of 1 is observed for the investigated complexes (). The C70 /1 system clearly shows a higher efficiency of quenching than that of C60 /1 complex (Fig. 6). While the insufficient polarity of toluene prevents an appreciable stabilization of the radical pair, we can assume that the quenching is due to energy transfer from the singlet excited state of porphyrin to fullerene. In our present investigations, we have determined singlet the rates of charge separation (kCS ) and quantum yield of the singlet

charge separation at singlet state (CS ) in an usual manner employed for the intermolecular energy and/electron transfer process, and the data are tabulated in Table 1. Inspection of Table 1 singlet singlet in terms of kCS and CS reveal the following important singlet

features: (i) the experimentally determined values of kCS

and

singlet CS are found to the C60 /1 complex.

be higher for C70 /1 complex as compared to This observation is in conformity with the close proximity of the designed tweezer 1 and fullerene entity in the C70 /1 complex as obtained from the magnitude of K value of such complex; (ii) in agreement with the steady state emission results, time-resolved fluorescence experiment suggests that with the increasing value of electron affinity of the acceptor (here singlet singlet C70 ), both the kCS and CS value increase; (iii) compared singlet

singlet

with the literature value of the kCS and CS for various fullerene/zinc–porphyrin dyads done in 1,2-dichlorobenzene and benzonitrile medium [38–40], our investigated supramolecules exhibit much lower value in toluene. This observation proves that at least in toluene, where there is a weaker overlap between the porphyrin fluorescence and the fullerene absorption, singlet–singlet energy transfer dominates over the electron transfer phenomenon. 3.4. Binding constants and theoretical calculations Table 1 reports the K values of various fullerene/1 complexes. It is observed that for all the complexes studied, C70 exhibits larger

Fig. 7. MMMF optimized geometric structures of (a) C60 /1, (b) C70 /1 (side-on) and (c) C70 /1 (end-on) complexes done in vacuo.

value of K in comparison to C60 . The most important finding of the present investigations is the selectivity ratio in terms of K value between C70 and C60 , i.e., KC70 /KC60 . KC70 /KC60 is estimated to be 8.3 in our present investigations. The high selectivity ratio suggests that 1 may discriminate C60 from C70 in solution. Such a stabilization of the C70 /1 supramolecular complexes can be attributed due to the presence of additional intermolecular interaction between the two graphitic 6:6 planes of C70 and the monoporphyrin subunits in

190

D. Pal et al. / Spectrochimica Acta Part A 78 (2011) 185–190

the bisporphyrin. Primarily, the attractive interactions between C70 and the bisporphyrin is driven by the presence of dispersive forces associated with ␲–␲ interactions. The most concrete evidence of the above statement is illustrated by the side-on rather than endon binding of C70 with the bisporphyrin. Molecular mechanics force field (MMMF) calculations well reproduce the above feature regarding orientation of bound guest (here C60 and C70 ) with the plane of the bisporphyrin receptors. Thus, in the case of C70 /1 complex, the side-on interaction of C70 with 1 generates enthalpies of formation (Hf0 ) value of −111.0 kJ mol−1 , whereas Hf0 is determined to be −103.0 kJ mol−1 in its end-on orientation. Thus, C70 /1 complex gains ∼8.0 kJ mol−1 of extra stabilization energy when it approaches the cavity of 1 in side-on manner rather than in its end-on orientation. It is already well established that the 6:6 ringjuncture bond of the C70 , rather than 6:5 ring-juncture bond, lies closest to the porphyrin plane as the 6:6 “double” bonds of C70 are more electron-rich than 6:5 “single” bonds [41,42]. Thus, the equatorial face of C70 is centered over the electropositive center of the porphyrin plane which can be viewed as an enhancement in van der Waals interaction due to availability of greater surface area favoring strong ␲–␲ interactions. It should be noted at this point that C60 /1 complex exhibits much lower value of Hf0 , viz., −95.0 kJ mol−1 compared to C70 /1 complex in both of its orientation. Stereoscopic structures for all the fullerene/bisporphyrin complexes are visualized in Fig. 7. 4. Conclusions From the foregoing discussions, we can surmise the key points of our present investigations as follows: (1) The designed bisporphyrin, namely 1, forms effective and selective ground state non-covalent complexes with both C60 and C70 in toluene medium. Magnitude of K value suggests that 1 may be employed as a selective receptor molecule for C70 . (2) Steady state quenching experiment reveals static quenching is the key factor behind the decrease in fluorescence intensity of uncomplexed 1 in presence of fullerenes. (3) Time resolved fluorescence measurements establish efficient charge-separation for the fullerene/1 complexes. Order of kCS value establishes that energy transfer takes place from the excited singlet state of porphyrin to the fullerene in toluene medium. (4) Heat of formation values indicates side-on interaction of C70 molecule with the flat ␲-belt of 1. (5) Finally, we can say that the fullerene/1 “ensembles” may be of potential use for the construction of further supramolecular organizations by cooperating with other supramolecular recognition and crystal packing elements as well as for encapsulation of nanotubes having small diameter in near future. Acknowledgements DP thanks The University of Burdwan for providing basic research facility to him. SB wishes to record sincere gratitude to UGC, New Delhi, for providing financial assistance to him through the project of Ref. No. F. No. 34-336\2008 (SR). Appendix A. Supplementary data Steady state fluorescence quenching of 1 with C70 along with fluorescence induced curve and plot of relative fluorescence intensity vs. concentration of C70 for C70 /1 system are provided.

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.saa.2010.09.019. References [1] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature 318 (1985) 162; E.A. Rohlfing, D.M. Cox, A. Kaldor, J. Chem. Phys. 81 (1984) 3322. [2] R. Taylor, Lecture Notes in Fullerene Chemistry, Imperial College Press, London, 1996. [3] P.M. Allemand, A. Koch, F. Wuld, Y. Rubin, F. Diederich, M.M. Alvarez, S.J. Anz, R.L. Whetten, J. Am. Chem. Soc. 113 (1991) 1050. [4] D.K. Dubois, M. Kadish, S. Flanagan, R.E. Haufler, L.P.F. Chibante, L.J. Wilson, J. Am. Chem. Soc. 113 (1991) 4364. [5] J. Luo, L.-M. Peng, Z.Q. Xue, J.L. Wua, J. Chem. Phys. 120 (2004) 7998. [6] J.W. Arbogast, A.P. Darmanyan, C.S. Foote, J. Phys. Chem. 95 (1991) 11. [7] R.C. Haddon, L.F. Schneemeyer, J.V. Waszczak, S.H. Glarum, R. Tycko, G. Dabbagh, A.R. Kortan, M.A. Muller, A.M. Mujsce, M.J. Rosseinsky, S.M. Zahurak, A.V. Makhija, F.A. Thiel, M.J. Raghavachari, E. Cockayne, V. Elserr, Nature 350 (1991) 46. [8] F. Diederich, M.G. López, Chem. Soc. Rev. 28 (1999) 263. [9] D.M. Guldi, N. Martín, J. Mater. Chem. 12 (2002) 1978. [10] K.M. Kadish, K.M. Smith, R. Guilard, The Porphyrin Handbook, Academic Press, San Diego, 1999; H. Jintoku, T. Sagawa, T. Sawada, M. Takafuji, H. Ihara, Org. Biomol. Chem. (2010), doi:10.1039/b920058d. [11] H. Imahori, K. Hagiwara, T. Akiyama, M. Aoki, S. Taniguchi, T. Okada, M. Shirakawa, Y. Sakata, Chem. Phys. Lett. 263 (1996) 545. [12] K.C. Hwang, D. Mauzerall, J. Am. Chem. Soc. 114 (1992) 9705. [13] P.A. Liddell, J.P. Sunida, A.N. McPherson, L. Noss, G.R. Seely, K.N. Clark, A.L. Moore, T.A. Moore, D. Gust, Photochem. Photobiol. 60 (1994) 537. [14] D.M. Guldi, I. Zilbermann, G.A. Anderson, K. Kordatos, M. Prato, R. Tafuro, L. Valli, J. Mater. Chem. 14 (2004) 303. [15] S. Yagi, I. Yonekura, M. Awakura, M. Ezoe, T. Takagishi, Chem. Commun. (2001) 557. [16] H. Imahori, J.-C. Liu, K. Hosomizu, T. Sato, Y. Mori, H. Hotta, Y. Matano, Y. Araki, O. Ito, N. Maruyama, S. Fujita, Chem. Commun. (2004) 2066. [17] T. Hasobe, P.V. Kamat, M.A. Absalom, Y. Kashiwagi, J. Sly, M.J. Crossley, K. Hosomizu, H. Imahori, S. Fukuzumi, J. Phys. Chem. B 108 (2004) 12865. [18] H. Imahori, S. Fukuzumi, Adv. Funct. Mater. 14 (2004) 525. [19] H. Imahori, M. Kimura, K. Hosomizu, S. Fukuzumi, J. Photochem. Photobiol. A: Chem. 166 (2004) 57. [20] H. Imahori, N.V. Tkachenko, V. Vehmanen, K. Tamaki, H. Lemmetyinen, Y. Sakata, S. Fukuzumi, J. Phys. Chem. A 105 (2001) 1750. [21] H. Imahori, Org. Biomol. Chem. 10 (2004) 1425. [22] T. Konishi, A. Ikeda, S. Shinkai, Tetrahedron 61 (2005) 4881. [23] A. Ouchi, K. Tashiro, K. Yamaguchi, T. Tsuchiya, T. Akasaka, T. Aida, Angew. Chem. Int. Ed. 45 (2006) 3542. [24] V. Chukharev, N.V. Tkachenko, A. Efimov, D.M. Guldi, A. Hirsch, M. Scheloske, H. Lemmetyinen, J. Phys. Chem. B 108 (2004) 16377. [25] A. Satake, Y. Kobuke, Tetrahedron 61 (2005) 13. [26] S. Bhattacharya, S. Chattopadhyay, S.K. Nayak, S. Bhattacharya (Banerjee), M. Banerjee, Spectrochim. Acta A 68 (2007) 427. [27] Bisporphyrin 1 is prepared according to the modified route as reported in the literature see: H. Uno, A. Masumoto, N. Ono, J. Am. Chem. Soc. 125 (2003) 12082, The detailed procedure will be reported elsewhere. [28] M. Gouterman, G.H. Wagniere, L.C. Synder, J. Mol. Spectrosc. 11 (1963) 108. [29] N. Aratani, A. Osuka, Y.H. Kim, D.H. Jeong, D. Kim, Angew. Chem. Int. Ed. 39 (2000) 1458. [30] D.M. Guldi, T.D. Ros, P. Braiuca, M. Prato, Photochem. Photobiol. Sci. 2 (2003) 1067. [31] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703. [32] H. Imahori, Y. Sakata, Eur. J. Org. Chem. (1999) 2445. [33] I.B. Martini, B. Ma, T.D. Ros, R. Helgeson, F. Wudl, B.J. Schwartz, Chem. Phys. Lett. 327 (2000) 253. [34] D.I. Schuster, P. Cheng, S.R. Wilson, V. Prokhorenko, M. Katterle, A.R. Holzwarth, S.E. Braslavsky, G. Klihm, R.M. Williams, C. Luo, J. Am. Chem. Soc. 121 (1999) 11599. [35] D. Gust, T.A. Moore, A.L. Moore, D. Kuciauskas, P.A. Liddell, B.D. Halbert, J. Photochem. Photobiol. B 43 (1998) 209. [36] G. Yin, D. Xu, Z. Xu, Chem. Phys. Lett. 365 (2002) 232. [37] S.A. Vail, D.I. Schuster, D.M. Guldi, M. Isosomppi, N. Tkachenko, H. Lemmetyinen, X. Chen, J.Z.H. Zhang, J. Phys. Chem. B 110 (2006) 14155. [38] T.D.M. Bell, T.A. Smith, K.P. Ghiggino, M.G. Ranasinghe, M.J. Shephard, M.N.P. Row, Chem. Phys. Lett. 268 (1997) 223. [39] J. Ikemoto, K. Takimiya, Y. Aso, T. Otsubo, M. Fujitsuka, O. Ito, Org. Lett. 4 (2002) 309. [40] T. Otsubo, Y. Aso, K. Takimiya, Pure Appl. Chem. 77 (2005) 2003. [41] D. Sun, F.S. Tham, C.A. Reed, P.D.W. Boyd, PNAS 99 (2002) 5088. [42] D. Sun, F.S. Tham, C.A. Reed, L. Chaker, M. Burges, P.D.W. Boyd, J. Am. Chem. Soc. 122 (2000) 10704.