Chemical physics behind formation of effective and selective non-covalent interaction between fullerenes (C60 and C70) and a designed chiral monoporphyrin in solution

Chemical Physics Letters 646 (2016) 119–124

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Chemical physics behind formation of effective and selective non-covalent interaction between fullerenes (C60 and C70 ) and a designed chiral monoporphyrin in solution Anamika Ray a , Shrabanti Banerjee b , Ajoy K. Bauri c , Sumanta Bhattacharya a,∗ a

Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, India Department of Chemistry, Raja Rammohan Roy Mahavidyalaya, Radhanagore, Hooghly 712 406, India c Bio-organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India b

a r t i c l e

i n f o

Article history: Received 5 November 2015 In final form 23 December 2015 Available online 9 January 2016

a b s t r a c t The present letter extricates the chemical physics behind non-covalent interaction between fullerenes (C60 and C70 ) and a designed chiral monoporphyrin molecule (1) in toluene. Steady state fluorescence studies on complex formation reveal higher binding constant (K) for C70 /1 complex, i.e., K = 16,020 dm3 mol−1 , and very good selectivity of binding, viz., KC70/1 /KC60/1 ∼ 10.0. Time-resolved fluorescence study elicits role of static quenching mechanism behind photoexcited decay of 1* in presence of fullerene. Density functional theoretical calculations in vacuo validates the strong complexation between C70 and 1 and establishes the side-on binding motif of C70 towards 1 during complexation. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Physicochemical insights in electron donor/acceptor models are of immense interest to the researchers for designing and preparing systems having application in the field of light-harvest [1–6]. In this context, porphyrin has been extensively studied owing to its versatile characters, e.g., in mimicking biological systems, possess high chemical and thermal stability, rich optical and electrochemical properties, and the possibility to tune these properties by introducing different substituents at their peripheral positions; these properties are supposed to be the important features for those applications [7–10]. Self-assembled donor/acceptor nanohybrids are considered to be a possible alternative for the covalently linked molecular polyads in order to achieve an increased rate and yield of the charge-separation process and at the same time, prolongation of the lifetime of the charge separated state [11]. In this context, fullerene/porphyrin construction creates noteworthy progress due to their remarkable photoactive, structural and magnetic properties [12,13]. They also find potential application in sensing, switching and catalysis other than electron transfer [3]. Herein, a particularly favourable situation is encountered where close intermolecular contacts and selective

∗ Corresponding author. E-mail address: sum [email protected] (S. Bhattacharya). http://dx.doi.org/10.1016/j.cplett.2015.12.053 0009-2614/© 2015 Elsevier B.V. All rights reserved.

supramolecular interactions prevail in non-polar solvent. Starting from the well established supramolecular approaches in solution [14–20], self-assembled fullerene/porphyrin architecture is presented with a designed chiral monoporphyrin receptor, e.g., 5,15-di(1 -tert-butoxycarbonylamino-2 -phenylethyl) porphyrin (S-isomer) (Scheme 1). Electronic communication between 1 and fullerenes (C60 and C70 ) is studied through various fluorescence spectroscopic techniques. For an in-depth understanding of the physicochemical features of such non-covalently linked system, binding studies followed by geometric structure evaluation of the fullerene/porphyrin systems are reported with the help of suitable quantum chemical calculations. 2. Materials and methods C60 and C70 are purchased from Aldrich, USA. 1 is synthesized according to the method described in Scheme 1. UV–vis spectroscopic grade toluene (Spectrochem, India) has been used as solvent to favour non-covalent interaction between fullerene and 1 and, at the same time, to ensure good solubility and photo-stability of the samples. Steady state fluorescence spectra have been recorded with a Hitachi F-7000 model spectrofluorimeter. Fluorescence decay curves are measured with a HORIBA Jobin Yvon single photon counting set up employing nanoled as excitation source. Fluorescence microscope is measured in a Leica DM1000 model (Germany) instrument. Proton NMR measurements are performed in a Bruker

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O OHC

N

N

+

H

O

O HN

1. TFA, DCM O

2. DDQ

NH

NH

H

H N

1.0 eqivalent

N

HN

HN

1.0 eqivalent Yield~9%

O

O

Scheme 1. Preparation of 5,15-di(1 -tert-butoxycarbonylamino-2 -phenylethyl) porphyrin (S-isomer) (1).

500 MHz instrument. Theoretical calculations are performed using SPARTAN’14 V1.1.0 Windows version software. por5,15-di(1 -tert-butoxycarbonylamino-2 -phenylethyl) phyrin (S-isomer) (1) is prepared by the procedure starting from corresponding S-phenyl alaninal (Scheme 1); the typical procedure for the preparation of 1 is described below. The reaction is conducted with a solution of dipyrromethane and S-phenylalaninal (1:1 equivalent) in dry dichloromethane (DCM) in presence of catalytic amount of trifluoroacetic acid at room temperature in presence of argon atmosphere for 48 h under dark condition and the solution is stirred magnetically maintaining the speed of 800 rpm. The reaction product is then treated with 2,3dichloro-5,6-dicyano-p-benzoquinone (1.8 equiv.) under stirring condition for additional 3 h. The reaction is then quenched by the addition of triethylamine and poured into water followed by usual work up principle. The product is finally extracted with DCM to obtain the final crude product. The crude product is then purified by column chromatography over silica (280–400 mesh) followed by neutral alumina (eluting with chloroform). The product is characterized by UV–vis, NMR and HRMS. Spectral data: max (CHCl3 ): 406, 504, 537, 576 and 630 nm; 1 H NMR (CDCl3 , 200 MHz) 10.20 (s, 2H, meso-H), 9.57 (4H, ␤-H), 9.38 (4H, ␤-H), 7.37 (2H, phenyl), 6.80–7.00 (m, 8H, phenyl), 6.25 (2H, methine), 4.33 (benzylic), 1.39 (s, 18 H tert butyl), 0.85 (s, 2H, >NH), -2.86 (s, 2H, pyrrol H), MALDI-TOF mass (m/z) 746.6 (found), 748.4 (calcd.) (see supporting information).

3. Results and discussions 3.1. Steady state and time resolved fluorescence studies To study the photo-induced behaviour for C60 /1 and C70 /1 supramolecular complexes and the recognition motif of fullerenes towards 1, steady-state fluorescence measurements are carried out in toluene. The simple mixing of the individual components, i.e., fullerene and 1, leads to a superstructure for which we can expect that the highly fluorescent singlet excited state of 1, viz., 1* , is quenched by an inter-complex energy transfer to fullerene. It has been reported earlier that charge separation can occur from the excited singlet state of the porphyrin to the fullerene in fullerene/porphyrin hybrid system(s) [21,22]. Photophysical studies prove that in case of conformationally flexible dyads comprising fullerenes and macrocyclic receptor molecules, like porphyrin, ␲-stacking interactions are facilitated due to through-space interactions between these two chromophores [23]. This has been demonstrated by quenching of 1* Por fluorescence and formation of fullerene-excited states (by energy transfer) or generation of fullerene.− /Por.+ ion-pair states (by electron transfer) [24]. Evidence in favour of the deactivation of the excited singlet state of 1 comes from the steady state fluorescence titration experiment performed in toluene; in fluorescence titration, the concentration of fullerene is varied maintaining a fixed concentration of 1 in toluene. It has been observed that upon excitation of 1 at its Soret absorption band (S2 ← S0 ) maxima, a C60 and/C70 concentration dependent

Figure 1. Steady state fluorescence spectral variation of 1 (1.70 × 10−6 mol dm−3 ) in presence of C70 done in toluene; the concentration of C70 in C70 /1 mixture varies from 1.85 × 10−6 to 4.85 × 10−5 mol dm−3 ; inset of this figure shows SV type plot of the same system estimated in toluene. ex = 407 nm; em = 637 nm.

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Table 1 Binding constants (K, dm3 mol−1 ), selectivity of binding (KC70-1 /KC60-1 ), heat of formation (Hf 0 , kcal mol−1 ) values obtained from DFT calculations in vacuo and solvent reorganization energies (Rs , eV) of the C60 /1 and C70 /1 systems estimated in toluene. Temp. 298 K. System

K dm3 mol−1

1 C60 -1 C70 -1 (side-on) C70 -1 (end-on)

– 1580 16,020 –

KC70-1 /KC60-1

∼10.0 – –

Hf 0 kcal mol−1

Rs eV

– −1.12 −44.50 −44.05

– −0.65 −0.86 −0.24

Figure 4. 1 H NMR chemical shift of –NH protons of 1 (1.0 × 10−4 mol dm−3 ) in (a) absence and in presence of (b) C60 (1.0 × 10−4 mol dm−3 ) and (c) C70 (1.0 × 10−4 mol dm−3 ) recorded in toluene-d8 .

Figure 2. Fluorescence decay profile of uncomplexed 1 (1.70 × 10−6 mol dm−3 ), and 1 (1.70 × 10−6 mol dm−3 ) in presence of C60 (3.5 × 10−5 mol dm−3 ) and C70 (3.5 × 10−5 mol dm−3 ) solution of toluene; in this, magenta, yellow, purple, black lines and green colour lines represent uncomplexed 1 in toluene, C60 + 1, C70 + 1, instrument response function and fit to decay, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

decrease in the intensity of the fluorescence maxima of 1 is seen as shown in Figure 1S and Figure 1, respectively. The decrease in fluorescence intensity of 1 in presence of both C60 and C70 suggests photo-excited decay of the excited singlet state of 1 by the addition of electron-accepting fullerenes. The steady state fluorescence investigation initiates the opportunity to find out the value of one important photophysical parameter, namely, the binding constant (K) for the C60 /1 and C70 /1 systems in toluene. K values of C60 /1 and C70 /1 systems are evaluated with the help of an equation [25]

assuming the formation of a ground-state non-fluorescent complex and the values of K reported in Table 1. Excellent linear plots are resulted which are shown in insets of Figure 1S and Figure 1, respectively. Apart from steady state fluorescence measurements, we have performed detailed nanosecond time-resolved fluorescence experiment for the fullerene/1 systems in toluene using 374 nm laser pulse. The experiment has been carried out at a fixed concentration of 1. The time-resolved fluorescence measurement shows mono-exponential decay for the uncomplexed 1; lifetime value of the excited singlet excited state ( s ) of 1 is measured to be 9.0 ns (Figure 2) in toluene; no significant change in the value of  s is observed for C60 (9.05 ns, Figure 2) and C70 (9.03 ns, Figure 2). The lifetime experiment, therefore, suggests presence of static quenching mechanism in present investigations. Since the value of  s does not change much in the presence of the quencher, there is no question of observing the lifetime value in another time range like the picosecond or femtosecond region. This important photophysical observation establishes that diffusion controlled mechanism is not operative behind quenching of the fluorescence intensity of 1 in presence of C60 and C70 in toluene. Scanning the emission feature in longer wavelength regions (700–800 nm), a weak emission band is detected at 750 nm corresponding to the singlet emission of C60 in toluene (marked as black colour in Figure 3(a)). The fluorescence

Figure 3. (a) Steady state fluorescence spectral variation of C60 (2.0 × 10−5 mol dm−3 , black colour line) in presence of 1 (1.7 × 10−5 mol dm−3 , blue colour line) recorded in toluene at 298 K; ex = 335 nm; em = 750 nm; (b) steady state fluorescence spectral variation of C70 (2.0 × 10−5 mol dm−3 , black colour line) in presence of 1 (1.7 × 10−5 mol dm−3 , blue colour line) recorded in toluene at 298 K; ex = 335 nm; em = 750 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Figure 5. DFT optimized geometric structures of (a) C60 /1, (b) C70 /1 (side-on orientation of C70 ) and (c) C70 /1 (end-on orientation of C70 ) done by B3LYP calculations using 6-31G* basis set in vacuo.

intensity of this band for a fixed concentration of C60 is found to be remain almost same as that obtained in presence of 1 (blue colour spectrum, Figure 3(a)). This result indicates that there is a relaxation pathway from the excited singlet state of 1 to that of the C60 in toluene. Similar sort of observations have been made for C70 /1 system recorded in toluene (Figure 3(b)) studied in present work.

3.2. Fluorescence microscope measurements The decrease in fluorescence intensity of 1 in presence of both C60 and C70 gets tremendous support from the fluorescence microscope experiments. It is observed that 1 exhibits intense yellow fluorescence after exposure of 1.5 s (Figure 2S(a)). Following the

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addition of fullerenes, fluorescence intensity of 1 is reduced considerably and it is now exhibiting wan and salmon colour fluorescence in case of C60 (Figure 2S(b)) and C70 (Figure 2S(c)) solution, respectively, with much larger exposure time, i.e., 5.4 s. In case of C70 /1 system, the fluorescence intensity is reduced significantly compared to C60 /1 system with the same exposure time supporting the substantial complexation in case of such system.

3.3. Proton NMR investigations A 1 H NMR spectroscopic study of 1 in the absence or presence of C60 and C70 is performed in toluene-d8 . As shown in Figure 4, the chemical shift of internal pyrrolic protons of 1 appearing at −2.85 ppm, suffers 0.05 and 0.1 ppm up-field shifts upon addition of toluene–d8 solution of C60 and C70 , respectively, as compared with that of the pristine 1. Since the up-field shift of the internal pyrrolic protons following the addition of fullerenes may be derived from an electron deficiency on the porphyrin moiety of 1, the NMR study unambiguously implies that effective intermolecular interactions persist between 1 and fullerenes in solution. Larger extent of chemical shift of the –NH protons of 1 in presence of C70 corroborates fairly well with our reported K value of C70 /1 complex estimated in toluene (Table 1).

3.4. Computational studies To visualize the molecular geometry and electronic structure of the fullerene/1 complexes, a detailed conformational analysis of the individual components as well as the complexes have been performed in vacuo by DFT/B3LYP/6-31G* calculations. The geometric parameters of the complexes are obtained after complete energy minimization. The strongest binding of C70 towards 1 is reflected in terms of larger negative value of heat of formation (Hf 0 ) for this complex compared to that of C60 /1 system (Table 1, Figure 5); it is revealed that C70 /1 complex is supposed to be much more stable compared to that of C60 /1 system by 43.38 kcal mol−1 . Estimation of the surface area (S) and volume (V) of the C70 /1 and C60 /1 systems are estimated to be 1280 A˚ 2 , 1510 A˚ 3 , 1227 A˚ 2 and 1417 A˚ 3 , respectively. The above phenomenon confirms that van der Waals attractive forces play a major role in the present case [26]. DFT generated frontier highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for C60 /1 and C70 /1 complexes are shown in Figures 3S and 4S, respectively. Obviously, the LUMO and LUMO + 1 are located on the fullerene cage, while the HOMO and HOMO − 1 are dominantly located on the porphyrin macrocycle (Figures 3S and 4S). It is also observed that the HOMO of both C60 /1 (−5.3500 eV) and C70 /1 (−5.3361 eV) complexes corroborate fairly well with that of uncomplexed 1 (−5.3323 eV) while the LUMO of these complexes, i.e., −3.1023 eV and −3.1646 eV, lie very close to that of C60 (−3.1469 eV) and C70 (−3.1427 eV), respectively. The small HOMO-LUMO gap suggests weak or no charge-transfer interactions between 1 and fullerene in the ground state. Molecular electrostatic potential (MEP) maps have been generated for the investigated supramolecules in present work with the help of DFT calculations and visualized in Figure 5S. Figure 5S, therefore, gives strong support in favour of electronic redistribution in donor (here 1) and acceptor (here fullerene) during non-covalent interaction. It may be mentioned here that validity of molecular orbitals generated by density functional methods is already being recognized [27]. The accuracy of these methods, especially B3LYP, is being well demonstrated by D’Souza et al. [28] and, more recently, on covalently linked C60 –Znporphyrin system [29] and fullerenebased photoactive layers for the construction of heteroconjugation solar cells [30].

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3.5. Determination of solvent reorganization energy (Rs ) for fullerene/1 complexes The effect of solvent over electronic coupling between fullerenes and 1 is likely to play an important role in the present systems. For this reason, solvent reorganization energies (Rs ) are calculated for the investigated supramolecules. The total reorganization energy, R, is a sum of the two terms, i.e., inner-sphere reorganization energy (solvent-independent) R0 , and the outer-sphere reorganization energy (solvent-dependent), i.e., Rs . In case of fullerene, contributions from R0 , which is related to the differences in nuclear configurations between an initial and a final state, is estimated to be very small, ∼4.3 × 10−5 eV [31]. This observation implies that the structure of C60 and C70 in the ground and excited states remains very similar, which relates the rigidity of these spherical carbon structures. As far as Rs contribution is concerned, this is also believed to be small as well. Thus the symmetrical shape and large size of the fullerene framework requires little energy for the adjustment of an excited or reduced state to the new solvent environment. In the present investigations, Rs of C60 /1 and C70 /1 systems have been estimated applying the dielectric continuum model developed by Hauke et al. [32]. The value of Rs for the C60 /and C70 /1 systems is given in Table 1. It should be mentioned at this point that the solvent reorganization energies of C60 /1 and C70 /1 systems obtained in the present investigations do not corroborate well with those observed for porphyrin/quinone systems found in literature [32]. The discrepancy in the value of Rs for porphyrin/quinone and fullerene/1 systems may be due to the subtle structural change in the host–guest complex which exerts a large influence upon the photo induced electron and/energy transfer process. The more negative value of Rs for the C70 /1 system (in side-on orientation of C70 ) compared to that of C60 /1 system provides very good support in favour of formidable interaction between C70 and 1. 4. Conclusions In conclusions, effective and selective interaction between fullerenes (C60 and C70 ) and a chiral monoporphyrin, viz., 1, is established in the present work. The photoexcited decay of 1 in presence of fullerenes is primarily governed by static quenching mechanism. The side-on binding motif of C70 towards 1 is established from the DFT calculations. Such information would be helpful for the better understanding of the fundamental properties on donor–acceptor type supramolecular assembly in essence of the development of fullerene/chiral monoporphyrin system. Acknowledgments S. Bhattacharya thanks Department of Science & Technology, New Delhi for providing financial assistance through research project of Ref. No. SR/S1/PC-39/2011. S. Banerjee acknowledges UGC, New Delhi for providing financial assistant through Major Research Project of F. No. 41-307/2012 (SR). AR acknowledges CSIR, New Delhi for providing her financial assistance through research associateship in the project having Ref. No. 01(2711)/13/EMR-II. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2015.12.053. References [1] S. Vijayaraghavan, D. écija, W. Auwärter, S. Joshi, K. Seufert, Ari P. Seitsonen, K. Tashiro, J.V. Barth, Nano Lett. 12 (2012) 4077.

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