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Spectroscopic and theoretical insights on effective and selective non-covalent binding between fullerenes (C60 and C70) and a designed diporphyrin in solution
Accepted Manuscript Spectroscopic and theoretical insights on effective and selective non-covalent binding between fullerenes (C60 and C70) and a designed diporphyrin in solution Sibayan Mukherjee, Ajoy K. Bauri, Sumanta Bhattacharya PII: DOI: Reference:
S1386-1425(13)00727-0 http://dx.doi.org/10.1016/j.saa.2013.06.113 SAA 10734
To appear in:
Spectrochimica Acta Part A: Molecular and Biomo‐ lecular Spectroscopy
Received Date: Revised Date: Accepted Date:
10 April 2013 23 June 2013 28 June 2013
Please cite this article as: S. Mukherjee, A.K. Bauri, S. Bhattacharya, Spectroscopic and theoretical insights on effective and selective non-covalent binding between fullerenes (C60 and C70) and a designed diporphyrin in solution, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2013), doi: http://dx.doi.org/10.1016/j.saa. 2013.06.113
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Spectroscopic and theoretical insights on effective and selective non-covalent binding between fullerenes (C60 and C70) and a designed diporphyrin in solution Sibayan Mukherjee(a), Ajoy K. Bauri(b), and Sumanta Bhattacharya(a)* (a) Department of Chemistry, The University of Burdwan, Golapbag, Burdwan – 713 104, India. (b) Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai – 400 085, India.
Abstract The present work reports the photophysical insights behind effective non-covalent interaction between a designed diporphyrin (1) and fullerenes C60 and C70 in toluene. Ground state electronic interaction between fullerenes and 1 is evidenced from UV-vis measurements in which it is observed that the intensity of the Soret absorption band of 1 is decreased considerably in presence of both C60 and C70. Steady state fluorescence studies reveal efficient quenching of the fluorescence intensity of 1 in presence of fullerenes. Evaluation of binding constants for the fullerene/1 systems evoke that 1 may be employed as an efficient molecular tweezers for C70 in toluene as selectivity in binding is determined to be ~9.4. Time resolved emission studies establish relatively long-lived charge separated state for the C70/1 system in comparison to C60/1. Molecular mechanics calculations by force field method in vacuo interpret well regarding stability difference between C60 and C70 complexes of 1 and give formidable support in favour of side-on orientation motif of C70 towards the plane of 1 during non-covalent complexation process.
Keywords: C60 and C70; diporphyrin; UV-vis, steady state and time resolved and fluorescence measurements; binding constant; MMMF calculations. *Corresponding author; Email: [email protected]; Fax: +91-342-2530452.
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1. Introduction The self-assembly of fullerene-containing components using non-covalent interactions rather than covalent chemistry, has recently generated significant research efforts [1]. This is in view of the fact that fullerenes (C60 and C70) and their derivatives have been used as ultimate electron acceptors owing to their favorable reduction potentials and very small reorganization energies in electron-transfer (ET) and/ energy-transfer (Ent) reactions [2-5]. Photoactive supramolecular systems, in which a donor and an acceptor moieties are non-covalently linked, are particularly appealing either as models for natural photosynthesis or for the conversion of light into electric current [6-8]. These systems also provide novel photochemical properties that inspire the fabrication of diverse supramolecular assemblies having possible application for the construction of light-harvesting systems [9,10]. Since fullerenes are known to manifest unique photo-acceptor properties, their insertion in supramolecular porphyrin complexes can result in the design of new promising photoactive systems. Therefore, the choice of planar porphyrin and curved fullerene molecules as artificial reaction centers helps to reduce the complicated natural mechanism to its basic elements. Excitation of porphyrin with visible light leads to its first singlet excited state, viz., 1*Porphyrin. A rapid intramolecular electron transfer process is then followed follows which
subsequently
yields
a
long-lived
charge-separated
state
[11-13].
Therefore,
supramolecular architectures consisting of porphyrin derivatives (donors) and fullerenes (acceptors) are gaining current impetus in artificial photosynthesis and photovoltaic devices [1418]. Very recently, Jacob et al. have noted both the photoinduced intramolecular ET and EnT processes in two rotaxanes, one containing both zinc porphyrin and C60 fullerene moieties incorporated around the Cu(I) bisphenanthroline core [(ZnP)2−Cu(I)(phen)2−C60] and a second complex lacking the fullerene [(ZnP)2−Cu(I)(phen)2] [19]. An efficient excitation energy transfer from the photoexcited rhenium cluster moiety to the fullerene takes place in a molecular donor– acceptor dyad comprising a hexarhenium cluster core consisting [Re6(µ3-Se)8]2+, and a fullerene
2
moiety which are covalently linked through a pyridine ligand [20]. In recent past, Hasobe et al. have constructed new molecular composites: fullerene-encapsulated porphyrin hexagonal nanorods composed of zinc-meso-tetra(4-pyridyl) porphyrin [ZnP(Py)4] and fullerene derivatives [fullerene-ZnP(Py)4 nanorod] by aiding surfactant: cetyltrimethyl ammonium bromide (CTAB) in a DMF/acetonitrile mixed solvent [21,22]. He has proposed that a bar-shaped structure composed of two different molecules with separated inside and outside layers like above is a very good candidate for the photovoltaic application since these processes are simultaneously and continuously performed within the molecular assembly [23]. In general, fullerenes are considered as excellent electron acceptors in forming electron donor-acceptor or charge transfer (CT) complexes for their ability to accept multiple electrons [24]. The primary component of the fullerene/porphyrin interaction is, therefore, driven by the dispersive forces associated with π-π interaction, which is augmented by weak electrostatic or donor-acceptor stabilization [25]. Fullerene-porphyrin complexes held together by π-π interactions show the distance between flat porphyrin and curved fullerene somewhat to be shorter than the distance expected from stacked porphyrins. For efficient recognition of spherical guests like fullerenes, the recognition sites in the porphyrin receptor should be pre-organized to initiate favorable interaction [26]. However, the synthesis of the above kind of receptors are usually of low efficiency or time consuming [27]. We anticipate that the introduction of suitable spacer (or group) to rationally designed folding scaffolds would lead to gable type of diporphyrin receptor(s) with selective recognition ability. As the design of supramolecular ensembles with high binding constants (K) always remains an important challenge in supramolecular chemistry, therefore, tailoring of diporphyrin with judiciously chosen functionalities plays an important role in controlling or modifying the electronic structure. The important thing is to be considered, here, that the interaction ought to be directional and selective. Keeping in the mind of above rationale, we report the non-covalent interaction of a designed diporphyrin, 1 (Fig. 1S), with fullerenes C60 and C70 in solution using
3
various spectroscopic tools like, UV-vis, steady state and time resolved fluorescence along with suitable quantum chemical calculations in vacuo in the present paper. It is observed that fullerenes undergo both effective and selective complexation phenomenon with 1.
2. Experimental section Both C60 and C70 have been collected from Aldrich, USA. Toluene (spectroscopic grade, Merck) is used as solvent, because it is sufficiently apolar to favor non-covalent interaction between fullerene and porphyrin and, at the same time, ensures good solubility and photo-stability of the samples. The receptor, 1, is obtained as a gift from Dr. A. K. Bauri, Bio-organic Division, Bhabha Atomic Research Centre, India. UV–Vis spectral measurements are done in a Shimadzu UV-2450 spectrophotometer using quartz cell with 1 cm optical path length. Steady state fluorescence spectral measurements have been recorded in a Hitachi F-7000 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 Windows Version software.
3. Results and discussions 3.1. UV-Vis investigations The ground state absorption spectrum of 1 (Fig. 1(a)) displays one broad Soret absorption band (λmax = 410 nm) corresponding to the transition to the second excited singlet state S2, viz., S2← S0. Due to the presence of two porphyrin moieties, diporphyrins generally exhibit broad Soret absorption bands. This feature of the Soret absorption band may be qualitatively accounted in terms of excitation coupling that is originating from the Coulombic interactions between the transition dipole moments of the two monoporphyrin subunits in diporphyrin [28]. As for the Q absorption bands, 1 shows four absorption bands at 464, 501, 537 and 575 nm (inset of Fig. 1(a)) 4
corresponding to the vibronic sequence of the transition to the lowest excited singlet state S1, i.e., S1← S0. Evidence in favor of ground state electronic interaction between fullerenes and 1 first comes from the UV–Vis titration experiment. It is observed that gradual addition of a C60 and/ C70 solution to a toluene solution of 1 decreases the absorbance of the Soret absorption band (inset of Fig. 1(b)). However, no additional absorption peaks are observed in the visible region. The former observation extends a good support in favor of ground state non-covalent interaction between fullerenes and 1 in solution. The latter observation indicates that the interaction is not controlled by CT type transition. The interesting observation of the UV-Vis experiment is the larger extent of decrease in the absorbance value of 1 in presence of C70 in comparison to C60. This spectroscopic observation may be quantified in terms of greater amount of interaction between C70 and 1 compared to what we observe in case of C60/1 complexation process.
3.2. Fluorescence investigations Steady state. The photo induced behavior of the complexes of C60 and C70 with the diporphyrin 1 has been investigated by steady-state fluorescence measurements. It is observed that the fluorescence intensity of 1 at 580 and 635 nm upon excitation at Soret-absorption band diminishes gradually during titration with C60 (Fig. 2S(a)) and C70 (Fig. 2(a)) in toluene. This indicates that there is a relaxation pathway from the excited singlet state of the porphyrin to that of the fullerene in toluene. The spectral changes finally reach a plateau, indicating that the fluorescence quenching is induced by the complexation with C60 and C70 (inset of Figs. 2S(a) and 2(a), respectively). Although 1 exhibits fluorescence quenching upon the addition of fullerenes, the quenching efficiency is found to be much higher in case of C70 than that of C60. It is already reported that charge-separation can also occur from the excited singlet state of the porphyrin to the C60 in toluene medium [29]. Competing between the energy and electron transfer processes is a universal phenomenon in donor molecule-fullerene complexes [30]. Solvent dependent photo
5
physical behavior is a typical phenomenon of the most porphyrin/fullerene dyads studied to date [31]. Photophysical studies already prove that in conformationally flexible fullerene/porphyrin dyads, π-stacking interactions facilitate the through-space interactions between these two chromophores which is demonstrated by quenching of 1*porphyrin fluorescence and formation of fullerene-excited states (by energy transfer) or generation of porphyrin+/fullerene- ion-pair states (by electron transfer) [32]. However, in non-polar solvent, energy transfer generally dominates (over the electron transfer process) the photo physical behavior in deactivating the photo excited chromophore 1*porphyrin of fullerene/porphyrin dyad. Similar sort of rationale has already provided by Yin et al. for their particular cis-20,50-dipyridinylpyrrolidino[30,40:1,2]C60zinc tetraphenylporphyrin supramolecule [33]. Therefore, the quenching phenomenon can be ascribed to photo-induced energy transfer from porphyrins to fullerenes in the supramolecular complexes of C60 and C70 with 1. As ground state complexation between fullerenes and 1 is evidenced from observation of decrease in the absorbance value of the Soret absorption band of 1 in presence of fullerenes (as discussed in section 3.1), let us consider the formation of a nonfluorescent 1:1 complex according to the following scheme: fullerene + 1 ⇔ fullerene/1
(1)
The fluorescence intensity of the uncomplexed solution containing 1 decreases upon addition of fullerenes. Using the relation of binding constant (K) we obtain, K = [fullerene/1] / [fullerene][1]
(2)
Imposing the mass conservation law, we can write [1]0 = [1] + [fullerene/1]
(3)
where [1]0, [1] and [fullerene/1] are the initial concentrations of 1, 1 in presence of fullerenes, and fullerene/1 complex, respectively. Eq. (3) can be rearranged as [1]0/[1] = 1 + [fullerene/1]/[1]
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(4)
Using the value of K in place of [fullerene/1]/[1] from Eq. (2), we may write the Eq. (4) as follows [1]0/[1] = 1 + K [fullerene]
(5)
Considering the fact that fluorescence intensities are proportional to the concentrations, Eq. (5) is expressed as F0/F = 1 + K [fullerene]
(6)
where, F0 is the fluorescence intensity of 1 in absence of fullerenes and F is the fluorescence intensity of 1 in presence of quencher (i.e., fullerenes). In our present investigations, steady-state fluorescence quenching studies afforded excellent linear plots (Figs. 2(b) & 2S(b)), which is explained by fluorescence of 1 is being quenched only by static mechanism, as opposed to diffusional quenching process. Binding constants obtained from above plots for the C60/1 and C70/1 systems are estimated to be 2,600 and 24,520 dm3⋅mol-1, respectively (Table 1). It is interesting to note that the increase in magnitude of binding constant led to increase of the fluorescence quenching efficiency. Although the details are not clear at this point, the welldefined structure of 1 affording tight fixation of C70 should give rise to correct host–guest orientation. This phenomenon certainly ascribes that complexation takes place between 1 and fullerenes in solution.
Fluorescence lifetimes. The time-resolved fluorescence spectral feature of the fullerene/1 system tracks the steady state measurements. The fluorescence decay–time profiles of the investigated complexes of 1 (monitored at 580 nm using 462 nm LASER diode) shows an enhanced decay rate as compared to uncomplexed 1. In toluene, all of the investigated species, viz., 1 (Fig. 3S(a)), C60/1 (Fig. 3S(b)) and C70/1 complexes (Fig. 3S(c)), reveal bi-exponential decay. Quenching of the fluorescence lifetime is observed for the investigated supramolecules; the C70/1 system clearly shows a higher efficiency of quenching than that of C60/1 complex (Fig.
7
3S). The life time of the excited singlet state of 1, i.e., τs, along with the value estimated in presence of fullerenes C60 and C70, are reported in Table 2. 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 the rate constant of charge separation (kCSsinglet) and quantum yield of charge separation (φCSsinglet) in an usual manner employed for the intermolecular energy-transfer process and the estimated data are provided in Table 2. An examination of Table 2 reveals the following: (i) the experimentally determined values of kCSsinglet and φCSsinglet are found to be higher for C70/1 complex as compared to the C60/1 complex. This observation is consistent with the close proximity of the diporphyrin molecule 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 C70) [34], values of both the terms, namely, kCSsinglet and φCSsinglet, increase; and (iii) compared with the literature value of the kCSsinglet and φCSsinglet for various fullerene/zincporphyrin dyads done in dichlorobenzene and benzonitrile medium [35], our investigated supramolecules show much lower value (10 to 100 times) 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 electron transfer phenomenon.
3.3.
Binding
constants
and
theoretical
calculations
Table 1 lists the vales of K for the C60/1 and C70/1 systems in toluene. It is observed that C70/1 exhibits larger value of K in comparison to C60/1. The most important finding of the present investigations is the selectivity in binding between C70 and C60, i.e., KC70/KC60. KC70/KC60 is estimated to be 9.4 in our present investigations. The high selectivity ratio suggests that 1 may 8
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 the diporphyrin. Primarily, the attractive interactions between C70 and the diporphyrin is driven by the presence of dispersive forces associated with π–π interactions. The most concrete evidence of the above statement is illustrated by the side-on rather than end-on binding of C70 with the diporphyrin. 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 diporphyrin receptor. Thus, in the case of C70/1 complex, the side-on interaction of C70 with 1 generates heat of formation (∆Hf ) value of -358.80 kJ⋅mol−1, whereas ∆Hf is determined to be -349.70 kJ⋅mol−1 for the similar system at its end-on orientation. As a result of this, C70/1 system gains ~ 9 kJ⋅mol−1 of additional stabilization energy when it approaches the cavity of 1 in side-on orientation motif rather than at its end-on orientation pattern. It is already well established that the 6:6 ring-juncture bond of the C70, rather than 6:5 ring-juncture bond, lies closest to the porphyrin plane [36,37] as the 6:6 “double” bonds of C70 are more electron-rich than 6:5 “single” bonds. Therefore, 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 π–πinteractions. MMMF optimized single projection geometric structures for all the fullerene/1 complexes are visualized in Fig. 4S.
9
Conclusions From the foregoing discussions, we can surmise the key points of our present investigations as follows: (1) The designed gable type diporphyrin, namely 1, forms effective and selective ground state non-covalent complexes with both C60 and with C70 in toluene. Magnitude of K value suggests that 1 may be employed as a selective molecular tweezers for C70. (2) Absence of CT absorption bands in the spectrophotometric investigations strongly suggests that the interaction between fullerenes and 1 is dominated by dispersive force associated with interactions. (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. (4) Heat of formation values indicates that side-on interaction of C70 with the plane of diporphyrin is very much favoured over its end-on binding motif. (5) Finally, the designed gable type diporphyrin, i.e., 1, may be employed as a potential candidate for the construction of supramolecular recognition and crystal packing elements in near future.
Acknowledgement S.M. thanks CSIR, New Delhi for providing financial assistance to him through Senior Research Fellowship. SB acknowledges DST, New Delhi for providing financial assistance through the project no. SR/S1/PC-39/2011.
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Supporting information available Structure of 1, steady state fluorescence quenching titration plot of 1 in presence of C60 in toluene, fluorescence induced curve for C60/1 system in toluene and SV type fluorescence plot of C60/1 system in toluene, time-resolved fluorescence spectrum of 1 in uncomplexed and complexed form and finally, single projection geometric structures of fullerene/1 complexes done by MMMF calculations in vacuo are provided as Figs. 1S-4S, respectively. Figs. 1S-4S are given as supporting information. Supporting information for this article is available free of charge via the Internet.
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[17] S. Fukuzumi, T. Kojima, J. Mater. Chem. 18 (2008) 1427–1439. [18] T. Hasobe, Phys. Chem. Chem. Phys. 12 (2010) 44–57. [19] M. Jakob, A. Berg, R. Rubin, H. Levano, K. Li, D. I. Schuster, J. Phys. Chem. A 113 (2009) 5846–5854. [20] A. Kahnt, L. –P. Heiniger, S. –X. Liu, X. Tu, Z. Zheng, A. Hauser, S. Decurtins, D. M. Guldi, Chem. Phys. Chem. 11 (2010) 651–658. [21] A. S. D. Sandanayaka, T. Murakami, T. Hasobe, J. Phys. Chem. C 113 (2009) 18369– 18378. [22] T. Hasobe, A. S. D. Sandanayaka, T. Wada, Y. Araki, Chem. Commun. (2008) 3372–3374. [23] T. Hasobe, Phys. Chem. Chem. Phys. 14 (2012) 15975–15987. [24] R. C. Haddon, L. E. Brus, K. Raghavachari, Chem. Phys. Lett. 125 (1986) 459–464. [25] P. D. W. Boyd, C. A. Reed, Acc. Chem. Res. 38 (2005) 235–242. [26] G. W. Gokel, W. M. Leevy, M. E. Weber, Chem. Rev. 104 (2004) 2723–2750. [27] E. Weber, J. L. Toner, I. Goldberg, F. Vo¨gtle, D. A. Laidler, J. F. Stoddart, R. A. Bartsch, C. L. Liotta, Crown Ethers and Analogues. Wiley: New York. (1989) p. 558. [28] V. B. Borovkov, G. A. Hembury, Y. Inoue, Acc. Chem. Res. 37 (2004) 449–459. [29] H. Imahori, Y. Sakata, Eur. J. Org. Chem. (1999) 2445–2457. [30] I. B. Martini, B. Ma, T. Da Ros, R. Helgeson, F. Wudl, B.J. Schwartz, Chem. Phys. Lett. 327 (2000) 253–262, and the references cited there in. [31] 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– 11600. [32] D. Gust, T. A. Moore, A. L. Moore, D. Kuciauskas, P. A. Liddell, B. D. Halbert, J. Photochem. Photobiol. B 43 (1998) 209–216. [33] G. Yin, D. Xu, Z. Xu, Chem. Phys. Lett. 365 (2002) 232–236.
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Table 1. Binding constants (K) and the selectivity in binding (KC70/KC60) of the fullerene/1 systems determined in toluene along with the heat of formation (∆Hf0) values for such complexes done by MMMF calculations in vacuo.
System
K, (dm3⋅mol-1)*
C60/1
2,600 ± 55
C70/1 (side-on orientation of C70) C70/1 (end-on orientation of C70)
24,520 ± 556
∆Hf0, KC70/KC60
(KJ⋅mole-1) -65.35
9.4
-358.80 -349.70
*Temp. 298K. Table 2. Fluorescence lifetimes (τs), rate constant of charge separation (kCSsinglet) and quantum yield of charge separation (ΦCSsinglet) for the designed diporphyrin 1 in absence and presence of C60 and C70 recorded in toluene. Temp. 298K.
1 a
108 kCSsinglet (sec-1)a
τs (ns)
Diporphyrin
singlet Φ CS
b
Blank
C60
C70
C60
C70
C60
C70
1.17
1.01
0.93
1.4
2.2
0.14
0.20
kCSsinglet = (1/τf)sample – (1/τf)ref
b
singlet Φ = [(1/τf)sample – (1/τf)ref] (1/τf)sample CS
15
Figure captions: (1) (a) UV-Vis absorption spectrum of uncomplexed 1 (2.28 × 10-6 mol⋅dm-3) recorded in toleuen against the solvent as reference; the inset of Fig. 2(a) clearly shows the existence o Q absorption bnads in 1; (b) UV-Vis absorption spectra of the mixture of 1 (2.28 × 10-6 mol⋅dm-3) + C60 (8.0 × 10-5 mol.dm-3) and mixture of 1 (2.28 × 10-6 mol⋅dm-3) + C70 (8.0 × 10-5 mol⋅dm-3) recorded in toluene against the pristine toluene solution of fullerene as in reference. (2) (a) Fluorescence quenching of 1 (2.75 × 10-6 mol⋅dm-3) by C70 (from (i) (4.0 ×10-6 mol⋅dm-3) to (x) (4.0 ×10-5 mol⋅dm-3) recorded in toluene and in inset, plot of relative fluorescence intensity vs. [C70] for the C70/1 system; (b) plot of relative fluorescence intensity (viz., F0/F) vs. [C70] for the C70/1 system in toluene.
16
(a)
(b) Fig. 1
17
16 14
120
Relative Fluorescence Intensity
1
100 80
C70 + 1
60
12 10 8 6 4 2 0.0
40
-5
1.0x10
-5
2.0x10
-5
3.0x10 -3
[C70], mol.dm
20 0 520
570
620
670
720
Emission wavelength, nm (a) 2.5
2.0
1.5 F0/F
Fluorescence intensity, a.u.
140
1.0
0.5
0.0
0.0
1.0x10
-5
2.0x10
-5
[C 70 ], mol.dm
(b) Fig. 2 18
3.0x10 -3
-5
4.0x10
-5
-5
4.0x10
Graphical Abstract:
16 14
120
1
Relative Fluorescence Intensity
Fluorescence intensity, a.u.
140
100 80
C70 + 1
60
12 10 8 6 4 2 0.0
40
-5
1.0x10
-5
2.0x10
-5
3.0x10 -3
[C70], mol.dm
20 0 520
570
620
670
Emission wavelength, nm
19
720
-5
4.0x10
Scientific Highlights: Fullerene/diporphyrin (1) ground state complexation in solution. Quenching of fluorescence intensity of 1 takes place in presence of fullerenes. Time-resolved emission study reveals static quenching mechanism. MMMF calculations explore orientation pattern of C70 during complexation with 1.
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S1386-1425(13)00727-0 http://dx.doi.org/10.1016/j.saa.2013.06.113 SAA 10734
To appear in:
Spectrochimica Acta Part A: Molecular and Biomo‐ lecular Spectroscopy
Received Date: Revised Date: Accepted Date:
10 April 2013 23 June 2013 28 June 2013
Please cite this article as: S. Mukherjee, A.K. Bauri, S. Bhattacharya, Spectroscopic and theoretical insights on effective and selective non-covalent binding between fullerenes (C60 and C70) and a designed diporphyrin in solution, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2013), doi: http://dx.doi.org/10.1016/j.saa. 2013.06.113
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Spectroscopic and theoretical insights on effective and selective non-covalent binding between fullerenes (C60 and C70) and a designed diporphyrin in solution Sibayan Mukherjee(a), Ajoy K. Bauri(b), and Sumanta Bhattacharya(a)* (a) Department of Chemistry, The University of Burdwan, Golapbag, Burdwan – 713 104, India. (b) Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai – 400 085, India.
Abstract The present work reports the photophysical insights behind effective non-covalent interaction between a designed diporphyrin (1) and fullerenes C60 and C70 in toluene. Ground state electronic interaction between fullerenes and 1 is evidenced from UV-vis measurements in which it is observed that the intensity of the Soret absorption band of 1 is decreased considerably in presence of both C60 and C70. Steady state fluorescence studies reveal efficient quenching of the fluorescence intensity of 1 in presence of fullerenes. Evaluation of binding constants for the fullerene/1 systems evoke that 1 may be employed as an efficient molecular tweezers for C70 in toluene as selectivity in binding is determined to be ~9.4. Time resolved emission studies establish relatively long-lived charge separated state for the C70/1 system in comparison to C60/1. Molecular mechanics calculations by force field method in vacuo interpret well regarding stability difference between C60 and C70 complexes of 1 and give formidable support in favour of side-on orientation motif of C70 towards the plane of 1 during non-covalent complexation process.
Keywords: C60 and C70; diporphyrin; UV-vis, steady state and time resolved and fluorescence measurements; binding constant; MMMF calculations. *Corresponding author; Email: [email protected]; Fax: +91-342-2530452.
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1. Introduction The self-assembly of fullerene-containing components using non-covalent interactions rather than covalent chemistry, has recently generated significant research efforts [1]. This is in view of the fact that fullerenes (C60 and C70) and their derivatives have been used as ultimate electron acceptors owing to their favorable reduction potentials and very small reorganization energies in electron-transfer (ET) and/ energy-transfer (Ent) reactions [2-5]. Photoactive supramolecular systems, in which a donor and an acceptor moieties are non-covalently linked, are particularly appealing either as models for natural photosynthesis or for the conversion of light into electric current [6-8]. These systems also provide novel photochemical properties that inspire the fabrication of diverse supramolecular assemblies having possible application for the construction of light-harvesting systems [9,10]. Since fullerenes are known to manifest unique photo-acceptor properties, their insertion in supramolecular porphyrin complexes can result in the design of new promising photoactive systems. Therefore, the choice of planar porphyrin and curved fullerene molecules as artificial reaction centers helps to reduce the complicated natural mechanism to its basic elements. Excitation of porphyrin with visible light leads to its first singlet excited state, viz., 1*Porphyrin. A rapid intramolecular electron transfer process is then followed follows which
subsequently
yields
a
long-lived
charge-separated
state
[11-13].
Therefore,
supramolecular architectures consisting of porphyrin derivatives (donors) and fullerenes (acceptors) are gaining current impetus in artificial photosynthesis and photovoltaic devices [1418]. Very recently, Jacob et al. have noted both the photoinduced intramolecular ET and EnT processes in two rotaxanes, one containing both zinc porphyrin and C60 fullerene moieties incorporated around the Cu(I) bisphenanthroline core [(ZnP)2−Cu(I)(phen)2−C60] and a second complex lacking the fullerene [(ZnP)2−Cu(I)(phen)2] [19]. An efficient excitation energy transfer from the photoexcited rhenium cluster moiety to the fullerene takes place in a molecular donor– acceptor dyad comprising a hexarhenium cluster core consisting [Re6(µ3-Se)8]2+, and a fullerene
2
moiety which are covalently linked through a pyridine ligand [20]. In recent past, Hasobe et al. have constructed new molecular composites: fullerene-encapsulated porphyrin hexagonal nanorods composed of zinc-meso-tetra(4-pyridyl) porphyrin [ZnP(Py)4] and fullerene derivatives [fullerene-ZnP(Py)4 nanorod] by aiding surfactant: cetyltrimethyl ammonium bromide (CTAB) in a DMF/acetonitrile mixed solvent [21,22]. He has proposed that a bar-shaped structure composed of two different molecules with separated inside and outside layers like above is a very good candidate for the photovoltaic application since these processes are simultaneously and continuously performed within the molecular assembly [23]. In general, fullerenes are considered as excellent electron acceptors in forming electron donor-acceptor or charge transfer (CT) complexes for their ability to accept multiple electrons [24]. The primary component of the fullerene/porphyrin interaction is, therefore, driven by the dispersive forces associated with π-π interaction, which is augmented by weak electrostatic or donor-acceptor stabilization [25]. Fullerene-porphyrin complexes held together by π-π interactions show the distance between flat porphyrin and curved fullerene somewhat to be shorter than the distance expected from stacked porphyrins. For efficient recognition of spherical guests like fullerenes, the recognition sites in the porphyrin receptor should be pre-organized to initiate favorable interaction [26]. However, the synthesis of the above kind of receptors are usually of low efficiency or time consuming [27]. We anticipate that the introduction of suitable spacer (or group) to rationally designed folding scaffolds would lead to gable type of diporphyrin receptor(s) with selective recognition ability. As the design of supramolecular ensembles with high binding constants (K) always remains an important challenge in supramolecular chemistry, therefore, tailoring of diporphyrin with judiciously chosen functionalities plays an important role in controlling or modifying the electronic structure. The important thing is to be considered, here, that the interaction ought to be directional and selective. Keeping in the mind of above rationale, we report the non-covalent interaction of a designed diporphyrin, 1 (Fig. 1S), with fullerenes C60 and C70 in solution using
3
various spectroscopic tools like, UV-vis, steady state and time resolved fluorescence along with suitable quantum chemical calculations in vacuo in the present paper. It is observed that fullerenes undergo both effective and selective complexation phenomenon with 1.
2. Experimental section Both C60 and C70 have been collected from Aldrich, USA. Toluene (spectroscopic grade, Merck) is used as solvent, because it is sufficiently apolar to favor non-covalent interaction between fullerene and porphyrin and, at the same time, ensures good solubility and photo-stability of the samples. The receptor, 1, is obtained as a gift from Dr. A. K. Bauri, Bio-organic Division, Bhabha Atomic Research Centre, India. UV–Vis spectral measurements are done in a Shimadzu UV-2450 spectrophotometer using quartz cell with 1 cm optical path length. Steady state fluorescence spectral measurements have been recorded in a Hitachi F-7000 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 Windows Version software.
3. Results and discussions 3.1. UV-Vis investigations The ground state absorption spectrum of 1 (Fig. 1(a)) displays one broad Soret absorption band (λmax = 410 nm) corresponding to the transition to the second excited singlet state S2, viz., S2← S0. Due to the presence of two porphyrin moieties, diporphyrins generally exhibit broad Soret absorption bands. This feature of the Soret absorption band may be qualitatively accounted in terms of excitation coupling that is originating from the Coulombic interactions between the transition dipole moments of the two monoporphyrin subunits in diporphyrin [28]. As for the Q absorption bands, 1 shows four absorption bands at 464, 501, 537 and 575 nm (inset of Fig. 1(a)) 4
corresponding to the vibronic sequence of the transition to the lowest excited singlet state S1, i.e., S1← S0. Evidence in favor of ground state electronic interaction between fullerenes and 1 first comes from the UV–Vis titration experiment. It is observed that gradual addition of a C60 and/ C70 solution to a toluene solution of 1 decreases the absorbance of the Soret absorption band (inset of Fig. 1(b)). However, no additional absorption peaks are observed in the visible region. The former observation extends a good support in favor of ground state non-covalent interaction between fullerenes and 1 in solution. The latter observation indicates that the interaction is not controlled by CT type transition. The interesting observation of the UV-Vis experiment is the larger extent of decrease in the absorbance value of 1 in presence of C70 in comparison to C60. This spectroscopic observation may be quantified in terms of greater amount of interaction between C70 and 1 compared to what we observe in case of C60/1 complexation process.
3.2. Fluorescence investigations Steady state. The photo induced behavior of the complexes of C60 and C70 with the diporphyrin 1 has been investigated by steady-state fluorescence measurements. It is observed that the fluorescence intensity of 1 at 580 and 635 nm upon excitation at Soret-absorption band diminishes gradually during titration with C60 (Fig. 2S(a)) and C70 (Fig. 2(a)) in toluene. This indicates that there is a relaxation pathway from the excited singlet state of the porphyrin to that of the fullerene in toluene. The spectral changes finally reach a plateau, indicating that the fluorescence quenching is induced by the complexation with C60 and C70 (inset of Figs. 2S(a) and 2(a), respectively). Although 1 exhibits fluorescence quenching upon the addition of fullerenes, the quenching efficiency is found to be much higher in case of C70 than that of C60. It is already reported that charge-separation can also occur from the excited singlet state of the porphyrin to the C60 in toluene medium [29]. Competing between the energy and electron transfer processes is a universal phenomenon in donor molecule-fullerene complexes [30]. Solvent dependent photo
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physical behavior is a typical phenomenon of the most porphyrin/fullerene dyads studied to date [31]. Photophysical studies already prove that in conformationally flexible fullerene/porphyrin dyads, π-stacking interactions facilitate the through-space interactions between these two chromophores which is demonstrated by quenching of 1*porphyrin fluorescence and formation of fullerene-excited states (by energy transfer) or generation of porphyrin+/fullerene- ion-pair states (by electron transfer) [32]. However, in non-polar solvent, energy transfer generally dominates (over the electron transfer process) the photo physical behavior in deactivating the photo excited chromophore 1*porphyrin of fullerene/porphyrin dyad. Similar sort of rationale has already provided by Yin et al. for their particular cis-20,50-dipyridinylpyrrolidino[30,40:1,2]C60zinc tetraphenylporphyrin supramolecule [33]. Therefore, the quenching phenomenon can be ascribed to photo-induced energy transfer from porphyrins to fullerenes in the supramolecular complexes of C60 and C70 with 1. As ground state complexation between fullerenes and 1 is evidenced from observation of decrease in the absorbance value of the Soret absorption band of 1 in presence of fullerenes (as discussed in section 3.1), let us consider the formation of a nonfluorescent 1:1 complex according to the following scheme: fullerene + 1 ⇔ fullerene/1
(1)
The fluorescence intensity of the uncomplexed solution containing 1 decreases upon addition of fullerenes. Using the relation of binding constant (K) we obtain, K = [fullerene/1] / [fullerene][1]
(2)
Imposing the mass conservation law, we can write [1]0 = [1] + [fullerene/1]
(3)
where [1]0, [1] and [fullerene/1] are the initial concentrations of 1, 1 in presence of fullerenes, and fullerene/1 complex, respectively. Eq. (3) can be rearranged as [1]0/[1] = 1 + [fullerene/1]/[1]
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(4)
Using the value of K in place of [fullerene/1]/[1] from Eq. (2), we may write the Eq. (4) as follows [1]0/[1] = 1 + K [fullerene]
(5)
Considering the fact that fluorescence intensities are proportional to the concentrations, Eq. (5) is expressed as F0/F = 1 + K [fullerene]
(6)
where, F0 is the fluorescence intensity of 1 in absence of fullerenes and F is the fluorescence intensity of 1 in presence of quencher (i.e., fullerenes). In our present investigations, steady-state fluorescence quenching studies afforded excellent linear plots (Figs. 2(b) & 2S(b)), which is explained by fluorescence of 1 is being quenched only by static mechanism, as opposed to diffusional quenching process. Binding constants obtained from above plots for the C60/1 and C70/1 systems are estimated to be 2,600 and 24,520 dm3⋅mol-1, respectively (Table 1). It is interesting to note that the increase in magnitude of binding constant led to increase of the fluorescence quenching efficiency. Although the details are not clear at this point, the welldefined structure of 1 affording tight fixation of C70 should give rise to correct host–guest orientation. This phenomenon certainly ascribes that complexation takes place between 1 and fullerenes in solution.
Fluorescence lifetimes. The time-resolved fluorescence spectral feature of the fullerene/1 system tracks the steady state measurements. The fluorescence decay–time profiles of the investigated complexes of 1 (monitored at 580 nm using 462 nm LASER diode) shows an enhanced decay rate as compared to uncomplexed 1. In toluene, all of the investigated species, viz., 1 (Fig. 3S(a)), C60/1 (Fig. 3S(b)) and C70/1 complexes (Fig. 3S(c)), reveal bi-exponential decay. Quenching of the fluorescence lifetime is observed for the investigated supramolecules; the C70/1 system clearly shows a higher efficiency of quenching than that of C60/1 complex (Fig.
7
3S). The life time of the excited singlet state of 1, i.e., τs, along with the value estimated in presence of fullerenes C60 and C70, are reported in Table 2. 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 the rate constant of charge separation (kCSsinglet) and quantum yield of charge separation (φCSsinglet) in an usual manner employed for the intermolecular energy-transfer process and the estimated data are provided in Table 2. An examination of Table 2 reveals the following: (i) the experimentally determined values of kCSsinglet and φCSsinglet are found to be higher for C70/1 complex as compared to the C60/1 complex. This observation is consistent with the close proximity of the diporphyrin molecule 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 C70) [34], values of both the terms, namely, kCSsinglet and φCSsinglet, increase; and (iii) compared with the literature value of the kCSsinglet and φCSsinglet for various fullerene/zincporphyrin dyads done in dichlorobenzene and benzonitrile medium [35], our investigated supramolecules show much lower value (10 to 100 times) 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 electron transfer phenomenon.
3.3.
Binding
constants
and
theoretical
calculations
Table 1 lists the vales of K for the C60/1 and C70/1 systems in toluene. It is observed that C70/1 exhibits larger value of K in comparison to C60/1. The most important finding of the present investigations is the selectivity in binding between C70 and C60, i.e., KC70/KC60. KC70/KC60 is estimated to be 9.4 in our present investigations. The high selectivity ratio suggests that 1 may 8
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 the diporphyrin. Primarily, the attractive interactions between C70 and the diporphyrin is driven by the presence of dispersive forces associated with π–π interactions. The most concrete evidence of the above statement is illustrated by the side-on rather than end-on binding of C70 with the diporphyrin. 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 diporphyrin receptor. Thus, in the case of C70/1 complex, the side-on interaction of C70 with 1 generates heat of formation (∆Hf ) value of -358.80 kJ⋅mol−1, whereas ∆Hf is determined to be -349.70 kJ⋅mol−1 for the similar system at its end-on orientation. As a result of this, C70/1 system gains ~ 9 kJ⋅mol−1 of additional stabilization energy when it approaches the cavity of 1 in side-on orientation motif rather than at its end-on orientation pattern. It is already well established that the 6:6 ring-juncture bond of the C70, rather than 6:5 ring-juncture bond, lies closest to the porphyrin plane [36,37] as the 6:6 “double” bonds of C70 are more electron-rich than 6:5 “single” bonds. Therefore, 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 π–πinteractions. MMMF optimized single projection geometric structures for all the fullerene/1 complexes are visualized in Fig. 4S.
9
Conclusions From the foregoing discussions, we can surmise the key points of our present investigations as follows: (1) The designed gable type diporphyrin, namely 1, forms effective and selective ground state non-covalent complexes with both C60 and with C70 in toluene. Magnitude of K value suggests that 1 may be employed as a selective molecular tweezers for C70. (2) Absence of CT absorption bands in the spectrophotometric investigations strongly suggests that the interaction between fullerenes and 1 is dominated by dispersive force associated with interactions. (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. (4) Heat of formation values indicates that side-on interaction of C70 with the plane of diporphyrin is very much favoured over its end-on binding motif. (5) Finally, the designed gable type diporphyrin, i.e., 1, may be employed as a potential candidate for the construction of supramolecular recognition and crystal packing elements in near future.
Acknowledgement S.M. thanks CSIR, New Delhi for providing financial assistance to him through Senior Research Fellowship. SB acknowledges DST, New Delhi for providing financial assistance through the project no. SR/S1/PC-39/2011.
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Supporting information available Structure of 1, steady state fluorescence quenching titration plot of 1 in presence of C60 in toluene, fluorescence induced curve for C60/1 system in toluene and SV type fluorescence plot of C60/1 system in toluene, time-resolved fluorescence spectrum of 1 in uncomplexed and complexed form and finally, single projection geometric structures of fullerene/1 complexes done by MMMF calculations in vacuo are provided as Figs. 1S-4S, respectively. Figs. 1S-4S are given as supporting information. Supporting information for this article is available free of charge via the Internet.
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[17] S. Fukuzumi, T. Kojima, J. Mater. Chem. 18 (2008) 1427–1439. [18] T. Hasobe, Phys. Chem. Chem. Phys. 12 (2010) 44–57. [19] M. Jakob, A. Berg, R. Rubin, H. Levano, K. Li, D. I. Schuster, J. Phys. Chem. A 113 (2009) 5846–5854. [20] A. Kahnt, L. –P. Heiniger, S. –X. Liu, X. Tu, Z. Zheng, A. Hauser, S. Decurtins, D. M. Guldi, Chem. Phys. Chem. 11 (2010) 651–658. [21] A. S. D. Sandanayaka, T. Murakami, T. Hasobe, J. Phys. Chem. C 113 (2009) 18369– 18378. [22] T. Hasobe, A. S. D. Sandanayaka, T. Wada, Y. Araki, Chem. Commun. (2008) 3372–3374. [23] T. Hasobe, Phys. Chem. Chem. Phys. 14 (2012) 15975–15987. [24] R. C. Haddon, L. E. Brus, K. Raghavachari, Chem. Phys. Lett. 125 (1986) 459–464. [25] P. D. W. Boyd, C. A. Reed, Acc. Chem. Res. 38 (2005) 235–242. [26] G. W. Gokel, W. M. Leevy, M. E. Weber, Chem. Rev. 104 (2004) 2723–2750. [27] E. Weber, J. L. Toner, I. Goldberg, F. Vo¨gtle, D. A. Laidler, J. F. Stoddart, R. A. Bartsch, C. L. Liotta, Crown Ethers and Analogues. Wiley: New York. (1989) p. 558. [28] V. B. Borovkov, G. A. Hembury, Y. Inoue, Acc. Chem. Res. 37 (2004) 449–459. [29] H. Imahori, Y. Sakata, Eur. J. Org. Chem. (1999) 2445–2457. [30] I. B. Martini, B. Ma, T. Da Ros, R. Helgeson, F. Wudl, B.J. Schwartz, Chem. Phys. Lett. 327 (2000) 253–262, and the references cited there in. [31] 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– 11600. [32] D. Gust, T. A. Moore, A. L. Moore, D. Kuciauskas, P. A. Liddell, B. D. Halbert, J. Photochem. Photobiol. B 43 (1998) 209–216. [33] G. Yin, D. Xu, Z. Xu, Chem. Phys. Lett. 365 (2002) 232–236.
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Table 1. Binding constants (K) and the selectivity in binding (KC70/KC60) of the fullerene/1 systems determined in toluene along with the heat of formation (∆Hf0) values for such complexes done by MMMF calculations in vacuo.
System
K, (dm3⋅mol-1)*
C60/1
2,600 ± 55
C70/1 (side-on orientation of C70) C70/1 (end-on orientation of C70)
24,520 ± 556
∆Hf0, KC70/KC60
(KJ⋅mole-1) -65.35
9.4
-358.80 -349.70
*Temp. 298K. Table 2. Fluorescence lifetimes (τs), rate constant of charge separation (kCSsinglet) and quantum yield of charge separation (ΦCSsinglet) for the designed diporphyrin 1 in absence and presence of C60 and C70 recorded in toluene. Temp. 298K.
1 a
108 kCSsinglet (sec-1)a
τs (ns)
Diporphyrin
singlet Φ CS
b
Blank
C60
C70
C60
C70
C60
C70
1.17
1.01
0.93
1.4
2.2
0.14
0.20
kCSsinglet = (1/τf)sample – (1/τf)ref
b
singlet Φ = [(1/τf)sample – (1/τf)ref] (1/τf)sample CS
15
Figure captions: (1) (a) UV-Vis absorption spectrum of uncomplexed 1 (2.28 × 10-6 mol⋅dm-3) recorded in toleuen against the solvent as reference; the inset of Fig. 2(a) clearly shows the existence o Q absorption bnads in 1; (b) UV-Vis absorption spectra of the mixture of 1 (2.28 × 10-6 mol⋅dm-3) + C60 (8.0 × 10-5 mol.dm-3) and mixture of 1 (2.28 × 10-6 mol⋅dm-3) + C70 (8.0 × 10-5 mol⋅dm-3) recorded in toluene against the pristine toluene solution of fullerene as in reference. (2) (a) Fluorescence quenching of 1 (2.75 × 10-6 mol⋅dm-3) by C70 (from (i) (4.0 ×10-6 mol⋅dm-3) to (x) (4.0 ×10-5 mol⋅dm-3) recorded in toluene and in inset, plot of relative fluorescence intensity vs. [C70] for the C70/1 system; (b) plot of relative fluorescence intensity (viz., F0/F) vs. [C70] for the C70/1 system in toluene.
16
(a)
(b) Fig. 1
17
16 14
120
Relative Fluorescence Intensity
1
100 80
C70 + 1
60
12 10 8 6 4 2 0.0
40
-5
1.0x10
-5
2.0x10
-5
3.0x10 -3
[C70], mol.dm
20 0 520
570
620
670
720
Emission wavelength, nm (a) 2.5
2.0
1.5 F0/F
Fluorescence intensity, a.u.
140
1.0
0.5
0.0
0.0
1.0x10
-5
2.0x10
-5
[C 70 ], mol.dm
(b) Fig. 2 18
3.0x10 -3
-5
4.0x10
-5
-5
4.0x10
Graphical Abstract:
16 14
120
1
Relative Fluorescence Intensity
Fluorescence intensity, a.u.
140
100 80
C70 + 1
60
12 10 8 6 4 2 0.0
40
-5
1.0x10
-5
2.0x10
-5
3.0x10 -3
[C70], mol.dm
20 0 520
570
620
670
Emission wavelength, nm
19
720
-5
4.0x10
Scientific Highlights: Fullerene/diporphyrin (1) ground state complexation in solution. Quenching of fluorescence intensity of 1 takes place in presence of fullerenes. Time-resolved emission study reveals static quenching mechanism. MMMF calculations explore orientation pattern of C70 during complexation with 1.
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