Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 566–573

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

Absorption spectrophotometric, fluorescence and quantum chemical investigations on non-covalent interaction between PC70BM and designed diporphyrin in solution Anamika Ray a, Ajoy Bauri b, Sumanta Bhattacharya a,⇑ a b

Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, India Bio-organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 PC70BM/diporphyrin non-covalent

MEP of PC70BM/1 system (end-on orientation of PC70BM) done by ab initio calculation in vacuo.

interaction is established in solution.  Carbazole spacer diporphyrin exhibits largest value of binding constant with PC70BM.  High selectivity of binding is observed in polar solvent.  Ab initio calculations establish size selective orientation during complexation.

a r t i c l e

i n f o

Article history: Received 2 May 2014 Received in revised form 5 June 2014 Accepted 12 June 2014 Available online 21 June 2014 Keywords: PC70BM Diporphyrin Self-assembly Binding constant MEP calculations

a b s t r a c t Present work reports the photophysical insights on supramolecular interaction of a C70 derivative, namely, [6,6]-phenyl C71 butyric acid methyl ester (PC70BM), with two designed diporphyrin molecules having dithiophene (1) and carbazole (2) spacer in toluene and benzonitrile. Both absorption spectrophotometric and steady state fluorescence investigations reveal efficient complexation of PC70BM with 1 and 2 in both toluene and benzonitrile. The magnitude of average value of binding constant, viz., Kav, for the complexes of PC70BM with 1 and 2 in toluene (benzonitrile) are estimated to be 2.185  103 dm3 mol1 (3.215  103 dm3 mol1) and 10.180  103 dm3 mol1 (25.405  103 dm3 mol1), respectively. Selectivity in binding for the complexation process of PC70BM with 1 and 2 is estimated to be 4.6 and 7.90 as observed in toluene and benzonitrile, respectively. The complexation between PC70BM and diporphyrin is well accounted by a theoretical model which takes into account the electronic subsystems of both acceptor and donor. Ab initio calculations in vacuo establish that size selective orientation pattern of PC70BM towards the cavity of diporphyrin dictates the magnitude of binding and electronic structure of the PC70BM/diporphyrin complexes. Ó 2014 Elsevier B.V. All rights reserved.

Introduction

⇑ Corresponding author. Tel.: +91 9433962777; fax: +91 342 2530452. E-mail address: [email protected] (S. Bhattacharya). http://dx.doi.org/10.1016/j.saa.2014.06.089 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

Self-assembled donor–acceptor nanohybrids are considered to be a viable alternative for the covalently linked molecular polyads in order to achieve an increased rate and yield of the

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charge-separation process and at the same time, prolongation of the lifetime of the charge separated state. Nanohybrid systems that combine the favorable features said above, for example, fullerenes and porphyrins as electron acceptors and electron donors, respectively, have received considerably interest in recent past [1–7]. As a consequence, new materials with a wide range of unique and spectacular physicochemical properties have resulted in noteworthy advances in the areas of light induced electron transfer chemistry and solar energy conversion [8–10]. It is mainly the small reorganization energy that fullerenes exhibit in electron transfer reactions, is accountable for a significant breakthrough [11–12]. One of the commonly used strategies to achieve the formation of charge-separated states during photo induced electron and/ energy transfer in fullerene/porphyrin systems involves promoting multistep electron transfer reactions along well-defined redox gradients [13,14]. The most important and unprecedented finding is that electronic communication between the porphyrin donor and fullerene acceptor is even possible through a considerable number of r- and hydrogen bonds. This is expected to open new avenues of approaches for the inexpensive and efficient construction of new prototypes of supramolecular recognition element. The intention of this work is to develop and to analyze novel electron donor–acceptor systems consisting designed diporphyrin molecules and a C70 derivative, namely, [6,6]-phenyl C71 butyric acid methyl ester (PC70BM; Fig. 1). For enhancing the solar light harvest, the soluble C70 derivative, i.e., PC70BM has been used as electron acceptor in recent past [15,16]. The most important character of PC70BM is based upon narrow band gap for enhancing the solar light harvest in the wavelength region of 350–500 nm [17– 19]. Starting from our well established supramolecular approaches in solution [20,21], non-covalent directed recognition motif is chosen to construct self-assembled fullerene/diporphyrin architecture with designed diporphyrin receptor, e.g., diporphyrins having dithiophene (1) and carbazole spacer (2) unit between two monoporphyrins [22,23] (Fig. 1). We anticipate that the introduction of suitable spacer (or group) between two monoporphyrin units would lead to gable type diporphyrin receptors with selective recognition ability towards PC70BM in solvent having varying polarity. UV–vis and steady state fluorescence spectroscopic tools supported by ab initio calculations in vacuo give credence to the relation between molecular complex formation and stability. Fig. 1. Structures of PC70BM, 1 and 2.

Materials and methods PC70BM is purchased from Aldrich, USA. 1 and 2 have been synthesized according to the method described in literature [22,23]. UV–vis spectroscopic grade toluene and benzonitrile (Merck, Germany) have been used as solvent to favor non-covalent interaction between fullerene and diporphyrn and, at the same time, to ensure good solubility and photo-stability of the samples. UV–vis spectral measurements are performed on a Shimadzu UV2450 model spectrophotometer. Fluorescence spectra have been recorded with a Hitachi F-7000 model spectrofluorimeter. Theoretical calculations are performed in a Pentium IV computer using SPARTAN’06 V1.1.0 Windows version software. Results and discussions UV–vis absorption studies The extensively conjugated aromatic chromophoric system of the diporphyrins, namely, 1 and 2, generate intense bands in their absorption spectra. The stronger and the most well-resolved absorption bands of 1 and 2 have been detected in the visible region (ranging from 350 to 600 nm (Fig. 2(a)) and popularly

known as Soret (or B) and Q peaks. For metalloporphyrin, generation of Q and B bands follow the orbital model proposed by Gouterman et al. [24,25]. B and Q absorption bands in metalloporphyrin correspond to the vibronic sequence of the transition from the ground singlet S0 to 2nd (S2) and 1st excited singlet states (S1), i.e., S2 S0 and S1 S0, respectively. The position of the B and Q bands of the diporpyrins are listed in Table 1. Table 1 clearly demonstrates with increasing the solvent polarity from toluene (e = 2.4) to benzonitrile (e = 25.2), considerable amount of red shift of the absorption bands takes place for both 1 and 2. PC70BM shows main absorption peaks at 375, 407 and 463 nm in toluene, which suffers very little peak shift as observed in benzonitrile (Fig. 2(b)). Evidence in favor of ground state complexation phenomenon between PC70BM and 1 comes from UV–vis absorption spectrophotometric titration measurements. Addition of varying concentration of PC70BM (in toluene and benzonitrile) [26,27] to 1 (fixed concentration) produces considerable change in the absorbance value of the uncomplexed diporphyrin solution (i.e., A0) at its B absorption band. A systematic relative decrease in the absorbance value of 1 (DA) in presence of PC70BM takes place in PC70BM/1 mixture with increasing concentration of acceptor solution (Fig. 3(a) and (b) for PC70BM/1 mixture in toluene and benzonitrile, respectively). Here, DA = A0 – A, and A signifies the

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H¼

N max  X

i

dPC70BM rPC70BM d2 r2x;i x



 1  3 cos2 hi =1 r3i

ð1Þ

i¼1

where dPC70BM and di[2] are dipole moments of the corresponding transitions in PC70BM and the ith 2 molecule, rx and rx,i2 are the corresponding Pauly matrices, ri is the distance between the PC70 BM and 2 molecules, and e1 in Eq. (1) is the high-frequency dielectric constant. The reconstruction of the resulting spectrum, taking into account Eq. (1), is determined by mixing of the states of the PC70BM molecule and the surrounding 2 molecule.

n o1=2 Ei ¼ ðEPC70BM þ E2 Þ=2  ½ðEPC70BM  E2 Þ=22 þ jVi j2

ð2Þ

For one PC70BM/2 pair, Eq. (1) gives Eq. (2), where EPC70BM and E2 are the energies of dipole transitions of PC70BM and 2, respectively, and Vi = [dPC70BMdi2 – (1–3 cos2 hi)]e11 ri3 is the matrix element of the state mixing. The final expression has the form



Fig. 2. (a) UV–vis absorption spectrum of 1 in toluene (magenta color line, [1] = 1.60  106 mol dm3), 1 in benzonitrile (blue color line, [1] = 1.45  106 mol dm3), 2 in toluene (green color line, [2] = 1.70  106 mol dm3) and 2 in benzonitrile (dark brown color, [2] = 1.00  106 mol dm3), recorded against the solvent as reference; (b) UV–vis absorption spectrum of PC70BM recorded in (i) toluene (4.95  106 mol dm3) and (ii) benzonitrile (5.70  106 mol dm3) against the solvent as reference. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 Position of B and Q absorption bands of 1 and 2 recorded in toluene and benzonitrile. Temp. 298 K. Compound

1 2

Absorption peak at B band (nm)

Absorption peak at Q band (nm)

Toluene

Benzonitrile

Toluene

Benzonitrile

408 412

412 416

535, 570 542, 578

542, 578 545, 585

absorbance value of 1 in presence of PC70BM. The binding constant (K) of the PC70BM/1 complex in toluene and benzonitrile medium have been evaluated by constructing a modified Benesi–Hildebrand plot [28,29] as shown in inset of Fig. 3. The values of K for PC70BM/1 complex are estimated to be 1,785 and 2,010 dm3 mol1, respectively (Table 2). In case of PC70BM/2 complexation process, however, significant increase in the value of K is observed in both toluene and benzonitrile. Like previous case, the relative decrease in the absorbance value of 2 at its B absorption band is utilized (Fig. 4) to construct modified Benesi–Hildebrand plot [28,29]. Excellent linear correlations are obtained with the present data (correlation factor 0.99). The modified BH plots for the PC70BM/ 2 system in toluene and benzonitrile medium are shown in inset of Fig. 4(a) and (b), respectively. Theoretical model in favor of electric dipole–dipole interaction between PC70BM and diporphyrin Consider the interaction of PC70BM and 2. The interaction between the dipole–dipole transitions of PC70BM and 2 can be represented in the form

¼ ð0Þ  ðjV i jN1=2 Þ=2

ð3Þ

where V is the amplitude of the non-diagonal flip–flop dipole– dipole matrix element for PC70BM and of the dipole transitions in neighboring 2 molecules, and N is the number of neighboring 2 molecules. Such a dependence of the absorption band edge is valid only under the condition N < Nthr, where Nthr is the maximum number of 2 molecules that can take part in the dipole–dipole flip–flop interaction with PC70BM. A further increase in the concentration of 2 does not increase the number of these molecules in the nearest environment of PC70BM. Typical sigmoidal curve showing the relative variation of absorbance against concentration of PC70BM for PC70BM/2 systems are shown in Fig. 5. Fig. 5 shows that the dependence is saturated when the concentration of PC70BM exceeds 4.0  105 mol dm3 concentration, in agreement with the theory. The saturation curve of PC70BM/1 system in toluene and benzonitrile are shown in Fig. 6(a) and (b), respectively. Steady state fluorescence studies To study the photo-induced behavior in PC70BM/1 and PC70BM/ 2 supramolecular complexes and the recognition motif of PC70BM towards 1 and 2, steady-state fluorescence measurements are carried out in toluene. From above discussions, we may infer that other than p-stacking effect, electrostatic interactions play vital role with a trend towards formation of well-directed and oriented self-assembly of PC70BM/diporphyrin complexes. The simple mixing of the individual components, i.e., PC70BM and diporphyrin, leads to a novel superstructure, for which we can expect that the highly fluorescent singlet excited state of 1 and 2, i.e., 1 and 2, are quenched by an inter-complex energy and/electron transfer to fullerene forming fullerened. It has been reported earlier that charge separation can occur from the excited singlet state of the porphyrin to fullerene in fullerene/porphyrin hybrid system [30– 34]. Photophysical studies prove that in case of conformationally flexible dyads comprising fullerenes and macrocyclic receptor molecules, like porphyrin, p-stacking interactions are facilitated due to through-space interactions between these two chromophores. 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) [35,36]. In present work, the steady state experiment is performed after excitation at B band of 1 and 2 in toluene and benzonitrile. The emission peak maxima and the corresponding Stokes Shift in this connection are tabulated in Table 3. Evidence in favor of the deactivation of the excited singlet state of 1 and 2 obtains from the steady state fluorescence titration experiment; in fluorescence titration, the concentration of either 1 or 2 is kept fixed at certain concentration and varying the concentration of

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2

0

2

1.6

-50

1.6

-100

1.2

0.8

-20

-30

0

75000

150000 225000 -1 1/[PC 70BM], dm3.mol

0.4

500

600

20000

0.4 0

400

-15

-25

0.8

-150

0 300

A0/ΔA

A0 /ΔA

1.2

-10

Abs.

Abs.

0 -5

700

40000

60000

80000 100000 120000

1/[PC70 BM], dm3.mol -1

300

350

400

450

Wavelength, nm

500 550 600 Wavelength, nm

650

700

750

800

(a)

(a) 1

0

2 -4

-20

0.8 -12

0.4

-16

0.2

-20 40000

A0/ΔA

1.5

Abs.

A0 /ΔA

Abs.

-8

0.6

-40 -60

1

-80 60000

80000

100000

0.5

120000

0

5

1x10

0 300

350

400

450

500

550

600

650

5

2x10

5

3x10

5

4x10

5

5x10

-1

1/[PC70BM], dm3.mol

1/[PC70BM], dm 3 .mol-1

0 300

700

350

400

Wavelength, nm

450

500

550

600

650

700

750

Wavelength, nm

(b)

(b)

Fig. 4. (a) Spectrophotometric titration for the determination of K of PC70BM/2 complex in toluene. In UV–vis titration experiment, concentration of 2 is kept fixed at 1.70  106 mol dm3 throughout the titration and PC70BM varies in the range of 8.80  106 mol dm3 to 3.45  105 mol dm3; the inset of Fig. 4(a) demonstrates the modified BH plot of PC70BM/2 system in toluene. (b) Spectrophotometric titration for the determination of K of PC70BM/2 complex in benzonitrile. In UV–vis titration experiment, concentration of 2 is kept fixed at 1.0  106 mol dm3 throughout the titration and PC70BM varies in the range of 2.30  106 mol dm3 to 4.32  105 mol dm3; the inset of Fig. 4(b) demonstrates the modified BH plot of PC70BM/2 system in benzonitrile.

Fig. 3. (a) Spectrophotometric titration for the determination of K of PC70BM/1 complex in toluene. In UV–vis titration experiment, concentration of 1 is kept fixed at 1.75  106 mol dm3 throughout the titration and PC70BM varies in the range of 4.65  106 mol dm3 to 4.35  105 mol dm3; the inset of Fig. 3(a) demonstrates the modified BH plot of PC70BM/1 system in toluene. (b) Spectrophotometric titration for the determination of K of PC70BM/1 complex in benzonitrile. In UV–vis titration experiment, concentration of 1 is kept fixed at 1.35  106 mol dm3 throughout the titration and PC70BM varies in the range of 8.85  106 mol dm3 to 2.10  105 mol dm3; the inset of Fig. 3(b) demonstrates the modified BH plot of PC70BM/1 system in benzonitrile.

Solvent reorganization energy (RS) for the PC70BM/1 and PC70BM/2 complexes

PC70BM in toluene and benzonitrile. It has been observed that upon excitation at B band maxima of the diporphyrins, a PC70BM concentration dependent decrease in the intensity of the fluorescence maxima of either 1 (and/ 2) is seen in toluene and benzonitrile; the quenching experiments of 1 (and 2) in presence of PC70BM in toluene and benzonitrile are shown in Fig. 7 (and Fig. 1S). It should be mentioned at this point that a purely diffusion-driven process is ruled out, on the basis of the applied fullerene concentration. The decrease in fluorescence intensity of 1 (and 2) in presence of PC70BM suggests a static quenching event inside the well-defined PC70BM/1 and PC70BM/2 supramolecular complexes. On the basis of the aforementioned results, we reach the conclusion that the excited singlet state of 1 (and 2) is quenched by the addition of electron-accepting PC70BM. K values of the PC70BM/diporphyrin complexes are evaluated according to a modified BH equation [28,37]. Excellent linear plots are obtained in all the cases studied and they are demonstrated in inset of Figs. 7 and Fig. 1S. K values are listed in Table 2.

The effect of solvent over electronic coupling phenomenon between PC70BM and the diporphyrins, namely, 1 and 2, can be better understood by the estimation of solvent reorganization energy (Rs) for the PC70BM/1 and PC70BM/2 complexes. The total reorganization energy, in general, is a sum of the two terms, i.e., inner-sphere reorganization energy (solvent-independent) R0 [38], and outersphere reorganization energy (solvent-dependent) Rs. In case of fullerene, as far as Rs contribution is concerned, this is believed to be small as well. 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 PC70BM/1 and PC70BM/2 complexes have been estimated by applying the dielectric continuum model developed by Hauke et al. [39] with the following parameters: radius of donor, R1 = 7.531 Å; R2 = 6.780 Å; radius of acceptor (RPC70BM) = 4.268 Å;

Table 2 Binding constant (K) determined from UV–vis and fluorescence titration experiment in toluene and benzonitrile medium along with the average value of K (Kav) and selectivity in binding, i.e., X = Kav(PC70BM/2)/Kav(PC70BM/1), for the complexes of PC70BM with 1 and 2: Temp. 298 K. System

K (dm3 mol1) Toluene

PC70BM/1 PC70BM/2

Benzonitrile

KUV–vis

Kfluorescence

Kav

X

KUV–vis

Kfluorescence

Kav

X

1785 12,100

2585 8260

2185 10,180

4.65

2010 25,880

4420 24,930

3215 25,405

7.90

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0.08

0.030

0.07

0.025

0.06

0.020

ΔAbs.

Δ Abs.

570

0.05

0.04

0.015

0.010

0.03 0.005 0.02 0.000 0.0

-5

1.0x10

-5

2.0x10

-5

3.0x10

-5

4.0x10

-5

5.0x10

-5

0.0

6.0x10

1.0x10

-5

2.0x10

-5

3.0x10

-5

4.0x10

-5

-3

[PC70BM], mol.dm

[PC70 BM], mol.dm-3

(a)

(a) 0.030

0.08

0.025 0.06

Δ Abs.

Δ Abs.

0.020

0.015

0.04

0.010 0.02 0.005

0.000

0.00 0.0

1.0x10

-5

2.0x10

-5

3.0x10

-5

4.0x10

-5

-3

[PC70 BM], mol.dm

(b)

0.0

1.0x10-5

2.0x10-5

3.0x10-5

-3

[PC70 BM], mol. dm

(b)

Fig. 5. Variation of absorbance vs. concentration of PC70BM for PC70BM/2 system done in (a) benzonitrile and (b) toluene medium.

Fig. 6. Variation of absorbance vs. concentration of PC70BM for PC70BM/1 system recorded in (a) toluene and (b) benzonitrile medium.

donor–acceptor separation (RPC70BM/1(2)): RPC70BM/1 = 4.52 Å, RPC70BM/2 = 4.79 Å; solvent dielectric constant (es): etoluene = 2.4 and ebenzonitrile = 25.2; solvent dielectric constant for electrochemical measurements, eR = 9.93. Values of Rs for the above supramolecular systems are tabulated in Table 4. It is to be mentioned here, that the solvent reorganization energies obtained in the present investigation do not corroborate well with that observed for quinone/porphyrin system [40]. The discrepancy in the value of Rs for quinone/porphyrin and fullerene/porphyrin systems may be due to the subtle structural change in the supramolecular complex which exert a large influence upon the photo-induced electron and/ energy transfer process. Table 4 reveals that Rs value of the PC70BM/diporphyrin complexes become more positive in benzonitrile compared to toluene. This trend is quite consistent with those observed for total reorganization energy reported by D’Souza et al. [41]. The increasing value of Rs for PC70BM/1 complex compared to PC70BM/2 is consistent with an increased donor–acceptor separation distance in former case due to the presence of dithiophene spacer.

that 2 undergoes appreciable amount of complexation with PC70 BM in toluene and benzonitrile. Due to solvophobic effect, PC70BM forms strong complexes with both 1 and 2 in benzonitrile compared to toluene [42,43]. The selectivity of binding, i.e., KPC70BM/2 over KPC70BM/1 is estimated to be very high, i.e., 4.7 and 7.9 in toluene and benzonitrile, respectively. The selectivity in binding even exhibits 3 times larger value compared to well known designed diporphyrin molecules like cyclic porphyrin dimer (3.1) [44] and calixarene bisporphyrin ( 3.2) [45]. The remarkable decrease in K value of PC70BM/1 system in comparison to PC70 BM/2 as well as very large KPC70BM/2/KPC70BM/1 selectivity ratio (Table 2) can be ascribed by the different conformational motif of PC70BM towards 1 and 2 in solution. Quantum chemical calculations may provide some light for the elucidations of geometric and electronic structures as well as association energies of the PC70BM/1 and PC70BM/2 ensembles for different orientations of C70 derivative. We have performed explicit ab initio calculations using Slater type of orbitals (STO) at 3-21G basis set for above purposes; STO/3-21G basis set is used as they are capable of precisely predicting the optimized geometric structures with a far-less expensive treatment of electron correlation. The heat of formation (DHf°) values for the PC70BM/1 and PC70BM/2 complexes are estimated from the difference between the total

Binding constants and computational calculations Binding constants for the complexes of 1 and 2 with PC70BM in toluene and benzonitrile are summarized in Table 2. It is observed

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Table 3 Excitation and emission peak maxima for 1 and 2 recorded in toluene and benzonitrile along with Stokes shift observed in two different solvent. Temp. 298 K. Compound

Excitation maxima (nm)

1 2

Stokes shift (cm1)

Emission maxima (nm)

Toluene

Benzonitrile

Toluene

Benzonitrile

Toluene

Benzonitrile

408 412

412 416

575 (& 625) 585 (& 635)

585 (& 645) 590 (& 640)

7100 (8500) 7210 (8560)

7210 (8800) 7100 (8430)

Table 4 Heat of formation values (DHf0) of the PC70BM/1 and PC70BM/2 complexes done in vacuo along with solvent reorganization energies (Rs) of the complexes of PC70BM with 1 and 2 estimated in toluene and benzonitrile. Temp. 298 K. System

DHf° (kcal mol1)

Rs (eV) Toluene

Benzonitrile

PC70BM/1 PC70BM/2

16.63 (side-on) 12.24 (end-on) 2.53 (side-on) 1.14 (end-on)

0.9289 1.027

0.3305 0.3470

5

F0/(F0-F)

Fluorescence Intensity, a.u

6

2000 1500 1000

4 3 2 1 0 4

0.0

2.0x10

500 0 500

4

4.0x10

4

4

6.0x10 3

8.0x10

-1

1/[PC70BM], dm .mol

550

600

650

700

750

800

Wavelength, nm

(a) 5

F0/(F0-F)

Fluorescence Intensity, a.u

6

900

600

4 3 2 1 0

300

0.0

4

2.0x10

4

4.0x10

4

6.0x10 3

4

8.0x10

-1

1/[PC70BM], dm .mol

0 500

550

600

650

700

750

800

Wavelength, nm

(b) Fig. 7. (a) Steady state fluorescence spectral variation of 1 (3.35  106 mol dm3) in presence of PC70BM done in toluene : the concentration of PC70BM in PC70BM/1 mixture varies from 1.30  105 mol dm3 to 3.75  105 mol dm3, Fluorescence BH plot of the same system is shown in inset of (a). (b) Steady state fluorescence spectral variation of 1 (2.35  106 mol dm3) in presence of PC70BM done in benzonitrile: the concentration of PC70BM in PC70BM/1 mixture varies from 1.30  105 to 4.35  105 mol dm3, Fluorescence BH plot of the same system is shown in inset of (b).

energy of the supramolecular complex and the sum of the individual host and guest entities separated from the optimized structure (single-point calculation). The estimated DHf° values are reported in Table 4. Table 4 reveals that PC70BM approaches the plane of diporphyrin 1 in end-on binding motif rather than side-on as DHf° gains 4.0 kcal mol1 stabilization energy in former orientation. The intermolecular interaction between PC70BM and 1 is driven by the dispersive forces associated with p–p interactions [46]. In case of PC70BM/2 complexation process, however, the fullerene derivative favors side-on orientation pattern

Fig. 8. Single projection geometric structures of (a) PC70BM/1 (end-on orientation of PC70BM) and (b) PC70BM/2 (side-on orientation of PC70BM) systems done by HF/ 3-21G calculations in vacuo.

compared to its traditional end-on binding motif. The experimental data provided in Tables 3 and 4 support the idea that the fundamentally strong p–p interaction is augmented by electrostatic or donor–acceptor type stabilization [47]. The frequently observed alignment of a fullerene 6:6 ring juncture ‘‘double’’ bond with a trans N. . .N vector has already been rationalized on electrostatic grounds [46,47]. This is consistent with the prevailing view of porphyrin dimers where dispersion forces create the fundamental attraction and electrostatic forces control the mutual orientation. The new physical insight of the present work is that while strong dispersive forces associated with p–p interaction dominates the extent of binding between PC70BM and 2, electrostatic interaction

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of PC70BM (Fig. 9(b)). Interestingly, in the PC70BM/1 complex (Fig. 9(c)), the original blue–green color of the uncomplexed PC70BM is changed to green, and red color of 1 changed to reddish-yellow, indicating strong possibility of electron transfer between these two chromophores upon photo-excitation. It should be mentioned at this point, that electrostatic interaction is one of the important components (and the most important one), which can contribute to the stabilization of the complexes through the van der Waals interaction since both components, viz., fullerenes and 1, are not charged (the very small differences in electronic density distribution over the neutral molecule cannot provide strong electrostatic interaction between porphyrin and fullerene). Together with electrostatic interaction other types of PC70BM/diporphyrin interaction like polarization interaction, p–p interaction, d–p interaction, etc. between neutral molecules also play vital role in stabilizing the complex. Conclusions In conclusions, PC70BM undergoes effective complexation with two designed diporphyrins, namely, 1 and 2. The selectivity of binding is observed to be moderate to large as estimated in non-polar and polar solvent, respectively. Absence of charge transfer absorption band in UV–vis study gives very good support in favor of dispersive forces associated with strong p–p interaction between fullerene and diporphyrin in present work. Ab initio calculations provide very good reasoning behind size-selective interaction between PC70BM and diporphyrin during complexation as well as shade light into the electronic structure of the said complexes in vacuo. Such information would be helpful for better understanding the fundamental properties of electron transfer on donor–acceptor type supramolecular assembly for the development of artificial photosynthetic systems. Fig. 9. MEPs of (a) 1, (b) PC70BM and (c) PC70BM/1 system (end-on orientation of PC70BM) done by ab initio calculation in vacuo.

plays very vital role during PC70BM/1 complexation process. It is already proved that possibility of electrostatic interactions in fullerene/phthalocyanine systems are estimated to be much more higher compared to dispersive forces associated with p–p interactions [48,49]. The negative DHf° value in Table 4 suggests formation of strong supramolecular complex between PC70BM and 2 compared to PC70BM and 1. The trend in DHf° value correlates excellently well with the trend in K of the respective complexes. Typical geometric structures of the PC70BM/1 and PC70BM/2 systems done by ab initio calculations are shown in Fig. 8(a) and (b), respectively. Electrostatic interactions originating from the electron density at surface of the PC70BM and 1 in PC70BM/1 complex are supposed to play a vital role in the interaction between PC70BM and 1. Molecular electrostatic potential (MEP) maps have been generated for PC70BM and 1, and PC70BM/1 systems to visualize the electrostatic interactions (Fig. 9). The MEP for 1 (Fig. 9(a)) shows negative electrostatic potential (shown in red1) on the porphyrin ring (mostly located on the nitrogen atoms) and blue color inside two monoporphyrin units indicating presence of strong electrostatic origin within the porphyrin core. The MEP for the PC70BM system represents two different scenarios. Firstly, the C70 portion of the derivatized fullerene is seen to be blue-green indicating positive electrostatic potential. However, presence of negative electrostatic potential may be observed over the N atom in the functionalized part

1 For interpretation of color in Fig. 9, the reader is referred to the web version of this article.

Acknowledgments SB thanks Department of Science & Technology, New Delhi for providing financial assistance through research project of Ref. No. SR/S1/PC-39/2011. Appendix A. Supplementary material Steady state fluorescence spectral variation of 2 in presence of PC70BM in toluene and benzonitrile is given as Fig. 1S. Fig. 1S is provided as supporting information. This material is available free of charge via the Internet. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.saa.2014.06.089. References [1] I. Natori, S. Natori, A. Kanasashi, K. Tsuchiya, K. Ogino, J. Polym. Sci., Part B: Polym. Phys. 66 (2013) 368–375. [2] M. Li, S. Ishihara, Q. Ji, M. Akada, J.P. Hill, K. Ariga, Sci. Tech. Adv. Mater. 13 (2012) 053001. [3] J.P. Hill, M.E. El-Khouly, R. Charvet, N.K. Subbaiyan, K. Ariga, S. Fukuzumi, F. D’Souza, Chem. Commun. 46 (2010) 7933–7935. [4] F. Wessendorf, B. Grimm, D.M. Guldi, A. Hirsch, J. Am. Chem. Soc. 132 (2010) 10786–10795. [5] M.E. El-Khouly, D.K. Ju, K.-Y. Kay, F. D’Souza, S. Fukuzumi, Chem. Eur. J. 16 (2010) 6193–6202. [6] A. Mateo-Alonso, C. Ehli, D.M. Guldi, M. Prato, J. Am. Chem. Soc. 130 (2008) 14938–14939. [7] K. Ohkubo, H. Kotani, J. Shao, Z. Ou, K.M. Kadish, G. Li, R.K. Pandey, M. Fujitsuka, O. Ito, H. Imahori, S. Fukuzumi, Angew. Chem. Int. Ed. 43 (2004) 853–856. [8] G.F. Moore, M. Hambourger, M. Gervaldo, O.G. Poluektov, T. Rajh, D. Gust, T.A. Moore, A.L. Moore, J. Am. Chem. Soc. 130 (2008) 10466–10467. [9] R.F. Kelley, S.J. Lee, T.M. Wilson, Y.I. Nakamura, D.M. Tiede, A. Osuka, J.T. Hupp, M.R. Wasielewski, J. Am. Chem. Soc. 130 (2008) 4277–4284.

A. Ray et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 566–573 [10] A.C. Rizzi, M.V. Gastel, P.A. Liddell, R.E. Palacios, G.F. Moore, G. Kodis, A.L. Moore, T.A. Moore, D. Gust, S.E. Braslavsky, J. Phys. Chem. A 112 (2008) 4215– 4223. [11] D.M. Guldi, S. Fukuzumi, in: D.M. Guldi, N. Martin (Eds.), Fullerenes: From Synthesis to Optoelectronic Properties, Kluwer: Dordrecht, The Netherlands, 2003, pp. 237–265. [12] L. Echegoyen, L.E. Echegoyen, Acc. Chem. Res. 31 (1998) 593–601. [13] M.E. El-Khouly, Y. Araki, O. Ito, S. Gadde, M.E. Zandler, F. D’Souza, J. Porphyrins Phthalocyanines 10 (2006) 1156–1164. [14] G.de la. Torre, G. Bottari, M. Sekita, A. Hausmann, D.M. Guldi, T. Torres, Chem. Soc. Rev. 42 (2013) 8049–8105. [15] F.G. Brunetti, R. Kumar, F. Wudl, J. Mater. Chem. 20 (2010) 2934–2948. [16] S.H. Park, A. Roy, S. Beaupré, S. Cho, N. Coates, J.S. Moon, D. Moses, M. Leclerc, K. Lee, A.J. Heeger, Nat. Photonics 3 (2009) 297–302. [17] N. Blouin, A. Michaud, M.A. Leclerc, Adv. Mater. 19 (2007) 2295–2300. [18] C.-P. Chen, S.-H. Chan, T.-C. Chao, C. Ting, B.-T. Ko, J. Am. Chem. Soc. 130 (2008) 12828–12833. [19] Y. He, G. Zhao, B. Beng, Y. Li, Adv. Func. Mater. 20 (2010) 3383–3389. [20] A. Ray, K. Santhosh, S. Chattopadhyay, A. Samanta, S. Bhattacharya, J. Phys. Chem. A 114 (2010) 5544–5550. [21] D. Pal, D. Goswami, S.K. Nayak, S. Chattopadhyay, S. Bhattacharya, J. Phys. Chem. A 114 (2010) 6776–6786. [22] S. Mukherjee, S. Banerjee, A.K. Bauri, S. Bhattacharya, J. Mol. Struct. 1004 (2011) 13–25. [23] S. Mukherjee, A.K. Bauri, S. Bhattacharya, J. Mol. Struct. 965 (2010) 101–108. [24] M. Gouterman, in: D. Dolphin (Ed.), The Porphyrins: Part A. Physical Chemistry, Academic Press, New York, 1978. [25] A.J. McHugh, M. Gouterman, C. Weiss, Theoret. Chim. Acta 24 (1987) 246. [26] F. Oswald, M.E. El-Khouly, Y. Araki, O. Ito, F. Langa, J. Phys. Chem. B 111 (2007) 4335–4341. [27] M.E. El-Khouly, M. Fujitsuka, O. Ito, J. Porphyrins Phthalocyanines 4 (2000) 713–721. [28] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703–2707. [29] C.K.C. Bikram, N.K. Subbaiyan, F. D’Souza, J. Phys. Chem. C 116 (2012) 11964– 11972.

573

[30] H. Imahori, M.E. El-Khouly, M. Fujitsuka, O. Ito, Y. Sakata, S. Fukuzumi, J. Phys. Chem. A 105 (2001) 325–332. [31] Y. Kawashima, K. Ohkubo, M. Kentaro, S. Fukuzumi, J. Phys. Chem. C 117 (2013) 21166–21177. [32] M.E. El-Khouly, O. Ito, P.M. Smith, F. D’Souza, J. Photochem. Photobiol. C 5 (2004) 79–104. [33] J. Iehl, M. Vartanian, M. Holler, J. –F. Nierengarten, B. Delavaux-Nicot, J. –M. Strub, A.V. Dorsselaer, Y. Wu, J. Mohanraj, K. Yoosaf, N. Armaroli, J. Mater. Chem. 21 (2011) 1562–1573. [34] H. Imahori, S. Fukuzumi, Adv. Funct. Mater. 14 (2004) 525–536. [35] D.I. Schuster, K. Li, D.M. Guldi, A. Palkar, L. Echegoyen, C. Stanisky, R.J. Cross, M. Niemi, N.V. Tkachenko, H. Lemmetyinen, J. Am. Chem. Soc. 129 (2007) 15973– 15982. [36] F. D’Souza, O. Ito, Molecules 17 (2012) 5816–5835. [37] A. Ray, S. Bhattacharya, RSC. Adv. 4 (2014) 10648–10651. [38] R.A. Marcus, Angew. Chem. Int. Ed. 32 (1993) 1111–1121. [39] F. Hauke, A. Hirsch, S.G. Liu, L. Echegoyen, A. Swartz, C. Luo, D.M. Guldi, Chem. Phys. Chem. 3 (2002) 195–205. [40] D.A. Williamson, E.B. Bowler, Inorg. Chim. Acta 297 (2000) 47–65. [41] F. D’Souza, R. Chitta, K. Ohkubo, M. Tasior, N.K. Subbaiyan, M.E. Zandler, M.K. Rogacki, D.T. Gryko, S. Fukuzumi, J. Am. Chem. Soc. 130 (2008) 14263–14272. [42] S. Mizyed, P.E. Georghiou, M. Ashram, J. Chem. Soc. Perkin Trans. 2 (2000) 277– 280. [43] D.J. Cram, J.M. Cram, Container Molecules and Their Guests, Royal Society of Chemistry, Cambridge, 1994. [44] J.-Y. Zheng, K. Tashiro, Y. Hirabayashi, K. Kinbara, K. Saigo, T. Aida, S. Sakamoto, K. Yamaguchi, Angew. Chem. Int. Ed. 40 (2001) 1858–1861. [45] M. Dudic, P. Lhota´ k, I. Stibor, H. Petrı´ckova´, K. Lang, New J. Chem. 28 (2004) 85–90. [46] P.D.W. Boyd, C.A. Reed, Acc. Chem. Res. 38 (2005) 235–242. [47] Y.B. Wang, Z.Y. Lin, J. Am. Chem. Soc. 125 (2003) 6072–6073. [48] A. Ray, D. Goswami, S. Chattopadhyay, S. Bhattacharya, J. Phys. Chem. A 112 (2008) 11627–11640. [49] A., Ray, K. Santhosh, S. Bhattacharya, J. Phys. Chem. B 116 (2012) 11979– 11998.