Photophysical insights into fullerene–porphyrazine supramolecular interactions in solution

Abstract

This communication reports UV-vis, fluorescence and quantum chemical investigations on the supramolecular interactions of a porphyrazine derivative, namely, 2,7,12,17-tetra-tert-butyl-5,10,15,20-tetraaza-21H,23H-porphine (1) with C₆₀ and C₇₀ in toluene and dichlorobenzene. A synergistic combination of rigidity in the host molecule and a shape-selective binding motif give rise to the strongest binding of C₆₀ with porphyrazine.

---

Fullerene-based supramolecular complexes have attracted immense attention in the recent past for the construction of photosynthetic systems and photonic devices.^1–7 From the viewpoint of stability of host–guest complexes of fullerene, various host molecules have been designed in the recent past – for example, calixarenes,^8,9 crown ethers,^10,11 cyclodextrins,^12,13 resorcarenes,^14,15 cyclotriveratrylene,¹⁶ and carbon nanorings.^17,18 Bowl-shaped π-conjugated molecules, for example, sumanene,¹⁹ are promising host molecules because their concave surface closely resembles fullerene segments and can fit precisely to the convex surface of C₆₀. The interaction of corannulene with C₆₀ is observed in a crystal structure, though it is not supposed to be effective in solution.²⁰ Very recently, phosphorus containing chiral molecules have been designed for fullerene recognition based on concave/convex interactions.²¹ However, the complexation between fullerene and porphyrin (Fig. 1(a)) proves that concave/convex rationale is not always necessary to manipulate highly stable complex structure.²² As a result of this, various porphyrin host molecules like diporphyrin,^23–27 phenothiazine-bridged cyclic porphyrin dimer,²⁸ bisporphyrin,²⁹ porphyrin tetramer,³⁰ porphyrin hexamer,³¹ porphyrin box,³² a clicked porphyrin cage,³³ calixarene diporphyrin,³⁴ JAWS porphyrin²³ are designed which exhibits larger value of binding constant (K) with fullerenes in solution compared to concave host molecules listed above. Apart from porphyrin, phthalocyanine (Pc, Fig. 1(b))/fullerene donor/acceptor conjugates have attracted immense attraction in recent past due to their resemblance in the construction of photosynthetic systems and photonic devices.³⁵ The flat π-assembly of Pc shows high binding affinity towards fullerene in solution under the choice of self-assembly protocols.^36–38 Like porphyrin and Pc, porphyrazine (Pz, Fig. 1(c)) and fullerene interactions have also been studied. In 1995, Hoffman et al. reported a fullerene and tetracyanoquinodimethane (TCNQ) donor–acceptor crystals of octakis(dimethylamino)porphyrazine.³⁹ Few years later, the same research group reported the synthesis and structures of two donor–acceptor crystals formed from C₆₀ and metallated porphyrazines.⁴⁰ However, there is no report on binding studies between fullerene and Pz in solution, till date. Herein, we describe spectroscopic and theoretical insights on supramolecular association of 2,7,12,17-tetra-tert-butyl-5,10,15,20-tetraaza-21H,23H-porphine (1, Fig. 1(d)) with C₆₀ and C₇₀ in solvents having varying polarity. The fullerene–Pz association is proposed to be attractive and structure-defining.

Fig. 1 Structure of various tetrapyrrole macrocycles along with 1, namely, (a) porphyrin, (b) phthalocyanine, (c) porphyrazine and (d) 1.

The Pz, namely, 1 and fullerenes (C₆₀ & C₇₀) are obtained from Aldrich, USA and used without any further purifications. The solvents, namely, toluene and 1,2-dichlorobenzene (DCB) are of UV-vis spectroscopic grade and used after proper distillation. Evidence in favor of interactions between fullerenes and 1 first comes from UV-vis spectroscopic measurements. Addition of 1 to C₇₀ in DCB causes appearance of additional absorption peak in the visible region. This observation is a good fingerprint in favor of complexation between fullerenes and 1, leading us to postulate electronic interactions between these two chromophores, in the ground state. Identification of the charge transfer (CT) spectra in the UV-vis spectroscopic experiment of fullerene–1 complexes is quite interesting as this may be the first example to track the CT behaviour in solution for such complexation process. Fig. 2 nicely demonstrates the colour changes for various CT complexes of 1 with different electron acceptors, i.e., o-chloranil, p-chloranil, tetracyanoethylene (TCNE), 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and C₇₀ in comparison to pristine 1 and C₇₀. No significant colour change is noticed for C₆₀–1 complex in DCB. Fig. 3 shows the electronic absorption spectra of 1 and mixtures containing 1 + C₇₀, 1 + TCNE, 1 + o-chloranil and 1 + DDQ, in DCB. Fig. S1 (in ESI‡) shows the electronic absorption spectra of remaining mixture containing 1 with p-chloranil in DCB. It should be mentioned at this point that high dielectric constant value of DCB (ε = 9.98) induces CT character as we failed to detect such absorption bands in toluene (ε = 2.39). Moreover, the solubility of fullerene, i.e., C₆₀ in toluene and DCB are estimated to be 2.8 and 27 mg mL⁻¹, respectively.⁴¹ Therefore, solvent polarity associated with solvophobic effect are very much responsible behind CT interaction between fullerene and porphyrazine measured in DCB.⁴² The CT absorption spectra have been analyzed by fitting to the Gaussian function y = y₀ + [A/w√(π/2)]exp[−2(x − x_c)²/w²], where x and y denote wavelength and absorbance, respectively. The wavelengths at these new absorption maxima (λ_max = x_c) and the corresponding CT transition energies (hν_CT) are summarized in Table 1. CT transition energies of these complexes are related to the vertical electron affinity (E^v_A) of the electron acceptors by the relation, 2C₁ + hν_CT = (1/I^v_D)C₁(C₁ + hν_CT) + {(C₂/I^v_D) + I^v_D}.⁴³ The vertical electron affinities of C₇₀, p-chloranil, TCNE, DDQ and o-chloranil are collected from literature.^44–47 A plot (Fig. S2, in ESI‡) of 2C₁ + hν_CT versus C₁(C₁ + hν_CT) for a given donor and various electron acceptors yields a slope of 1/I^v_D from which the value of 1/I^v_D is determined to be 7.87 eV. Utilizing the values of C₁ and C₂, degrees of CT (α) may be calculated as follows: α = (C₂/2)/[(I^v_D − E^v_A + C₁)² + (C₂/2)].^43,45 The values of α (listed in Table 1) are small and indicate that very little amount of charge transfer occurs in the ground state. Variation of α with the electron affinity of the acceptors is found to be non-linear (Fig. 4). This is quite expected as the model we use here to determine various electronic parameters are based upon interaction of a common donor species with structurally dissimilar electron acceptors. From the CT absorption spectra, we can enumerate the magnitude of the oscillator strength (f). The oscillator strength is estimated using the formula: f = 4.32 × 10⁻⁹∫ε_CTdν, where the term, ∫ε_CTdν denotes the area under the curve of the extinction coefficient of the absorption band in question vs. wavenumber. The observed oscillator strengths of the CT complexes of 1 are summarized in Table 1. The extinction coefficient is related to the transition dipole moment (μ_EN) by the relationship: μ_EN = 0.0952(ε_maxΔν_1/2/Δν)^1/2, where μ_EN is defined as −e∫ψ_ex(∑r_i)ψ_gdτ. Large value of μ_EN (Table 1) for C₇₀–1 complex substantiates the CT character in ground state. Another important physicochemical parameter, namely, resonance energy (R_N),⁴⁸ is estimated for the CT complexes of 1 with different electron acceptors in present work and reported in Table 1. R_N is determined according to the equation: ε_max = 7.7 × 10⁴/{(hν_CT/R_N) − 3.5}. Moderate value of R_N for the C₇₀–1 complex provides very good support in favour of strong electronic interaction between these two species in solution.

Fig. 2 Photographs of uncomplexed (a) 1 (2.3 × 10⁻⁵ M) and (b) C₇₀ (7.0 × 10⁻⁵ M) along with CT complexes of 1 (2.3 × 10⁻⁵ M) in presence of (c) C₇₀ (1.8 × 10⁻⁵ M), (d) TCNE (6.5 × 10⁻⁴ M), (e) DDQ (2.2 × 10⁻⁴ M), (f) p-chloranil (1.3 × 10⁻³ M) and (g) o-chloranil (7.0 × 10⁻⁴ M) in DCB.

Fig. 3 UV-vis spectral variation of (a) 1 (3.45 × 10⁻⁶ M) in presence of (b) C₇₀ (8.80 × 10⁻⁵ M), (c) TCNE (5.70 × 10⁻⁴ M), (d) DDQ (2.40 × 10⁻⁴ M) and (e) o-chloranil (9.60 × 10⁻⁴ M) solution recorded in DCB. CT spectra obtained by recording the spectra against 1 solution as in reference except C₇₀ for which the solvent is used as reference; temp. 298 K.

Table 1 CT absorption maximum (λ_CT), CT transition energy (hν_CT), vertical electron affinity of various electron acceptors (E^v_A), degrees of CT (α), oscillator strength (f), transition dipole strength (μ_EN), and resonance energy of interaction (R_N) for the complexes of 1 with various electron acceptors recorded in DCB (temperature 298 K)

System	λCT, nm	hνCT, eV	E^vA, eV	α	f	μEN,D	RN, eV
C70–1	655	1.88	2.59	0.026	0.038	7.37	0.217
o-Chloranil–1	643	1.92	2.87	0.020	0.020	5.33	0.205
p-Chloranil–1	473	2.62	1.37	0.043	0.033	4.28	0.115
DDQ–1	702	1.76	3.27	0.011	0.0028	1.85	0.022
TCNE–1	638	1.94	3.17	0.014	0.0023	1.71	0.024

---

Fig. 4 Variation of E^v_A against α for various CT complexes of 1 recorded in DCB at 298 K.

It is observed that both fullerenes and 1 attract each other spontaneously in toluene and DCB as evidenced from steady state fluorescence investigations; fluorescence intensity of 1 diminishes gradually by the addition of varying concentration of C₆₀ and C₇₀ upon excitation at its corresponding Soret (S₂ ← S₀) absorption peak. The steady state fluorescence titration experiments have been performed at constant concentration of 1. The spectral changes finally reach a plateau, indicating that the fluorescence quenching is induced by the complexation. 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 of the fluorescence intensity of 1 suggests a static quenching event inside the well defined fullerene–1 supramolecular complexes. On the basis of the aforementioned results, we reach the conclusions that, in the fullerene–1 complexes, the fluorescence state of 1 is quenched by the addition of electron-accepting C₆₀ and C₇₀. The binding constant (K) of various fullerene–1 complexes have been determined according to Benesi–Hildebrand (BH) fluorescence type equation⁴⁹ and reported in Table 2. An excellent correlation factor of 0.99 is observed for all the systems studied in the present work. One typical fluorescence quenching curve for C₆₀–1 system in toluene along with fluorescence induced curve and BH plot are shown in Fig. 5. Similar sort of fluorescence spectral representations for C₆₀–1 system in DCB and C₇₀–1 system in both the solvents are demonstrated in Fig. S3–S5, respectively (in ESI‡). Noteworthy is that, ZnPc, a structural analogue of 1, is reported to exhibit ∼3.1 times lower magnitude³⁶ of K than that of 1 during complexation with C₆₀ in toluene. However, both 1 and ZnPc exhibit comparable value of K with C₇₀ as reported in Table 2 (i.e., K_C70–1 = 37 000 M⁻¹) and literature (i.e., K_C70–ZnPc = 25 690 M⁻¹).³⁶ It is interesting to note that the increase in magnitude of K led to increase of the fluorescence quenching efficiency. This can be viewed in terms of the fact that the rigidity of 1 introduces tight fixation of C₆₀ compared to C₇₀ unit in fullerene–1 host–guest complex. Table 2 also suggests that C₆₀ always gives larger magnitude of K with 1 in both toluene and DCB in comparison to C₇₀. This is quite unnatural as structural analogue of Pz, i.e., Znporphyrin^22,50,51 and ZnPc,³⁶ exhibit stronger binding with C₇₀. As we use the Soret absorption band as our source of excitation wavelength in fluorescence experiment, the second excited singlet state of 1 has been deactivated by the following mechanism:

Table 2 Binding constants (K, M⁻¹) determined by steady state fluorescence method, heat of formation (ΔH⁰_f) value estimated in vacuo by HF/3-21G calculations, lifetime of the excited singlet (τ^s) of 1, rate constant of charge-separation of the excited singlet (k^s_CS) and quantum yield of charge-separation for the excited singlet (φ^s_CS) for the complexes of 1 with C₆₀ and C₇₀ recorded in toluene (and DCB) at 298 K

System	K, M^−1	ΔH^0f, kcal mol^−1	τ^s, ns	k^sCS, s^−1	φ^sCS
1	—	—	2.86 (2.135)	—	—
C60–1	73 000 (44 585)	−1.090	2.84 (2.05)	2.40 × 10^6 (1.9 × 10^7)	0.0068 (0.037)
C70–1	37 000 (27 570)	−0.660 (end-on)	2.85 (2.11)	4.2 × 10^5 (5.6 × 10^6)	0.0015 (0.0126)
		−0.100 (side-on)

---

Fig. 5 Steady state fluorescence spectral variation of 1 (2.60 × 10⁻⁶ M) in presence of C₆₀ (7.15 × 10⁻⁶ to 4.60 × 10⁻⁵ M) recorded in toluene at 298 K; the inset of Fig. 5 shows fluorescence induced curve and BH plot. λ_ex = 337 nm; λ_em = 630 nm.

A similar sort of experimental observations have already been reported by Yin et al.⁵² and Guldi et al.⁵³ We are very much thankful to one of the learned reviewers for his/her comments, as it creates the opportunity to find out the value of one important photophysical parameter, namely, the Stern–Volmer constants (K_SV) for the C₆₀–1 and C₇₀–1 systems in steady state fluorescence titration experiments. If we make a plot of F₀/F (where F₀ and F are the steady state fluorescence intensity of 1 in absence and presence of fullerenes, respectively) vs. concentration of the quencher (here fullerene) at dilute concentration range of the quencher, excellent linear plots are obtained for all the systems studied in present work. However, at higher concentrations of quencher, the plots generate an upward curvature. From the slope of such plots, K_SV may be enumerated. The values of K_SV for C₆₀–1 and C₇₀–1 systems in toluene (and DCB) are estimated to be 69 620 M⁻¹ (56 600 M⁻¹) and 65 410 M⁻¹ (51 200 M⁻¹), respectively. Typical fluorescence spectral variation for determination of SV plot for C₆₀–1 system in DCB is shown Fig. 6. SV plots for C₆₀–1 system in toluene and C₇₀–1 system in both the solvents are provided as Fig. S6–S8, respectively (in ESI‡). The bimolecular quenching constant, k_q, calculated from the linear segment of the SV plots revealed that the k_q values are 3 order of magnitude higher than that expected for diffusion controlled processes in DCB and toluene, respectively. The linear nature of SV plots at dilute concentration range of quencher along with order of magnitude of k_q suggest that static type quenching is the main process for quenching of fluorescence of 1 in presence of fullerenes.^54,55 The K_SV values measured in this manuscript fall into the range between pure diffusion controlled quenching and hydrogen bonding facilitated quenching in solutions where fullerene is a fluorescent quencher as suggested by one of the learned reviewer.^56–58 The above results, therefore, provide enough evidences in favour of the complexation between fullerene and 1 in both toluene and DCB.

Fig. 6 Steady state fluorescence spectral variation of 1 (2.6 × 10⁻⁶ M) in presence of C₆₀ (1.0 × 10⁻⁶ to 1.4 × 10⁻⁵ M) recorded in DCB at 298 K; the inset of Fig. 6 shows SV plot. λ_ex = 340 nm; λ_em = 632 nm.

Another interesting aspect of the present investigations is that 1 did not exhibit significant selectivity in binding toward C₆₀ or C₇₀. The selectivities of C₆₀ over C₇₀ are found to be ∼2.0 and 1.6 in toluene and DCB, respectively. Although, the present selectivity ratio of porphyrazine is comparable to that of azacalix[m]arene[n]pyridine (∼1.94),⁵⁹ it is much lower than those of calixarene diporphyrin (∼4.3),³⁴ cyclic dimers of Zn–porphyrins (∼25.5)⁶⁰ and corresponding H₂–diporphyrin (∼32)⁶¹ complexes.

In our present work, a detailed conformational analysis of the individual components as well as the fullerene–1 complexes have been performed in vacuo by HF/3-21G calculations. The geometric parameters of the complexes are obtained after complete energy minimization. To limit the number of basis functions, we have replaced tert-butyl group with methyl in the structure of 1. The strongest binding of C₆₀ towards 1 is reflected in terms of larger negative value of heat of formation (ΔH_f°) for this complex compared to C₇₀–1 estimated in vacuo (Table 2). Comparing the relative stability of C₆₀–1 and C₇₀–1 complexes in terms of ΔH_f° value (listed in Table 2), it is revealed that C₇₀–1 complex is supposed to be less stable in side-on orientation of C₇₀ (Fig. S9, in ESI‡) compared to its end-on orientation (Fig. 7(b)) by 0.560 kcal mol⁻¹. We anticipate that the contact area of 1 with C₆₀ (Fig. 7(a)) would not differ much with respect to that of C₇₀. Estimation of the surface area of the C₇₀ complexes of 1 (side-on) and 1 (end-on) are estimated to be 857.32 and 857.23, Ų, respectively, which is very much close to that of C₆₀–1 complex, i.e., 836.12 Ų. Thus, unlike fullerene–porphyrin interaction,²² computations of the preferred conformations of fullerene–1 complexes confirm that van der Waals attractive forces play negligible role in present case. The frontier highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) for the investigated supramolecules have been obtained by DFT/B3LYP/6-31G* method and are demonstrated in Fig. 8 and S10 (in ESI‡) for C₆₀–1 and C₇₀–1 systems, respectively. In the studied systems, the electron distribution of HOMO and LUMO is exclusively found to be located on the 1 entity and fullerene spheroid, respectively. These results suggest weak charge-transfer interaction between 1 and fullerene in the ground state and at the same time, prove the existence of charge-transfer transition from 1 entity to fullerene spheroid. It is also observed that, HOMO of both C₆₀–1 (−5.5945 eV) and C₇₀–1 (−5.5964 eV) complexes corroborate fairly well with that of uncomplexed 1 (−5.6013 eV) while LUMO of these complexes, i.e., −3.0820 eV and −3.0912 eV lie very close to that of C₆₀ (−3.1469 eV) and C₇₀ (−3.1427 eV), respectively. The small HOMO–LUMO gap gives formidable support in favour of weak CT from 1 to fullerene as evidenced from low values of α tabulated in Table 1. Molecular electrostatic potential (MEP) maps have also been generated for the investigated supramolecules in present work with the help of DFT calculations and visualized in Fig. 9. Fig. 9 gives strong indication 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.⁶² The accuracy of these methods, especially B3LYP, is being well demonstrated by D'Souza⁵⁴ and Schaefer et al.⁶³ and, more recently, on covalently linked C₆₀–Znporphyrin system⁶⁴ and fullerene-based photoactive layers for the construction of heteroconjugation solar cells.⁶⁵

Fig. 7 Ab initio (HF/3-21G) optimized geometric structures of (a) C₆₀–1 complexes and (b) C₇₀–1 (end-on orientation of C₇₀) done in vacuo. The calculations are done using SPARTAN '06 software.

Fig. 8 DFT (B3LYP/6-31G*) calculated frontier (a) HOMO and (b) LUMO for the C₆₀–1 complex done in vacuo. The calculations are done using SPARTAN '14 software.

Fig. 9 MEP maps of (a) C₆₀–1 and (b) C₇₀–1 (end-on orientation of C₇₀) systems done by DFT (B3LYP/6-31G*) calculations in vacuo. The calculations are done using SPARTAN '14 software.

Evidence in favor of end-on orientation of C₇₀ toward the plane of the 1 gets strong experimental support from variable temperature ¹³C NMR experiment of C₆₀–1 (Fig. 10) and C₇₀–1 systems (Fig. 11) recorded in toluene-d₈ medium. The ¹³C NMR spectra of C₆₀ and C₇₀ shown in Fig. 10 and 11, respectively, are not decoupled. Similar to the case of C₆₀, the ¹³C NMR signals due to C₇₀ in C₇₀–1 complex are upfield-shifted from those of uncomplexed C₇₀. Since such an upfield shift is more pronounced for the pole carbon signals than for those located at the equator, C₇₀ appears to adopt an end-on conformation with respect to 1 moiety in order to maximize the host–guest interaction. Both theoretical and experimental observations reveal that such a binding motif of C₇₀ toward 1 takes place to maximize the electrostatic interactions. As shown in Fig. 10, ¹³C-enriched C₆₀ and 1 in Tol-d₈ shows a single resonance for C₆₀ at 298 K due to time-averaging of bound and unbound C₆₀. Upon cooling, this peak gradually broadens and at 243 K, it appears as separate singlet signal corresponding to bound C₆₀ (140.3 ppm) and unbound C₆₀ (142.8 ppm). The upfield shifts of the fullerene in the bound states are the result of ring currents effects of the porphyrazine π-system on the fullerene. Similar sort of observations is noticed by Boyd et al. for calix[4]arene-linked bisporphyrin host for fullerene.⁶⁶

Fig. 10 ¹³C NMR spectrum of C₆₀ in presence of 1 at different temperatures.

Fig. 11 ¹³C NMR spectrum of C₇₀ in presence of 1 at different temperatures.

Apart from steady state fluorescence measurements, we have performed detailed nanosecond time-resolved fluorescence experiment for the fullerene–1 systems in toluene and DCB 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 uncomplexed 1; lifetime value of the singlet excited state (τ^s) of 1 is measured to be 2.86 (Fig. S11 in ESI‡) and 2.135 ns (Fig. 12(a)) in toluene and DCB, respectively. When the experiment is carried out in toluene, no significant change in the value of τ^s is observed for C₆₀ (2.84 ns, Fig. S11 in ESI‡) and C₇₀ (2.85 ns, Fig. S11 in ESI‡). However, slight reduction in τ^s value is observed for 1 in presence of C₇₀ (2.11 ns, Fig. 12(b)) and C₆₀ (2.05 ns, Fig. 12(c)) measured in DCB. Utilizing the value of τ^s in DCB, rate constant of charge-separation of the excited singlet (k^s_CS) and quantum yield of charge-separation for the excited singlet (φ^s_CS)^67–69 have been evaluated and listed in Table 2. The most fascinating thing is that in DCB, C₆₀–1 complex exhibits little charge-separation as φ^s_CS value is estimated to be 0.037. The lifetime experiment, therefore, suggests enhancement in electrostatic interaction between 1 and fullerene in solvent having high dielectric constant value. To validate the presence of the static quenching mechanism in our present investigations, we have performed detailed nanosecond time-resolved fluorescence measurements for fullerene–1 complexes in both toluene and DCB. The titration experiment is carried out at a fixed concentration of 1 and a variable concentration of C₆₀ and C₇₀. It is observed that, upon the gradual addition of fullerenes C₆₀ and C₇₀ in toluene (and DCB), the value of τ^s suffers little change, and mono-exponential decay is followed. Plot of τ^s/τ vs. concentration of quencher resulting from time-resolved titration experiment for C₆₀–1 and C₇₀–1 systems recorded in DCB are shown in Fig. S12(a) & S12(b) respectively (in ESI‡); here, τ^s and τ denote the lifetime value of 1 in absence and presence of quencher, respectively. Since the value of τ^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 both toluene and DCB. Similar sort of observations is noticed when the experiments are performed in toluene. Scanning the emission feature in longer wavelength regions (700–800 nm), it is revealed a weak emission band at 763 nm corresponding to the singlet emission of C₆₀ in DCB (marked as red colour in Fig. 13). The fluorescence intensity of this band for a fixed concentration of C₆₀ is found to remain almost same as that obtained in presence of 1. This result indicates that in DCB, there is a relaxation pathway from the excited singlet state of 1 to that of the C₆₀. Similar sort of observations have been made for C₆₀–1 system in toluene (Fig. S13‡) studied in present work.

Fig. 12 Time-resolved fluorescence decay profile of (a) 1 (3.0 × 10⁻⁶ M) in presence of (b) C₇₀ (4.30 × 10⁻⁵ M) and (c) C₆₀ (4.05 × 10⁻⁵ M) recorded in DCB at 298 K. Black and green colour lines represent instrument response function and fit to decay, respectively.

Fig. 13 Steady state fluorescence spectral variation of C₆₀ (2.0 × 10⁻⁵ M, red colour line) in presence of 1 (2.5 × 10⁻⁶ M, blue colour line) recorded in DCB at 298 K; λ_ex = 340 nm; λ_em = 763 nm.

In conclusion, 1 has been demonstrated to be good shape-selective host molecule for fullerene guests. The fullerene–1 supramolecular recognition element may find potential application to construct discrete host–guest complexes in near future. Further studies on photophysical characterization and single crystal X-ray analysis of the complexes of porphyrazine with various fullerene derivatives are in progress.

Acknowledgements

Grant-In-Aid through Research Award Sanction no. F. 30-17/2012 (SA-II) supported by UGC, New Delhi is greatly acknowledged.

Notes and references

1. M. E. El-Khouly, S. Fukuzumi and F. D'Souza, ChemPhysChem, 2014, 15, 30.
2. S. Fukuzumi, K. Ohkubo, F. D'Souza and J. L. Sessler, Chem. Commun., 2012, 9801.
3. M. E. El-Khouly, C. A. Wijesinghe, M. E. El-Khouly, V. N. Nesterov, M. E. Zandler, S. Fukuzumi and F. D'Souza, Chem.–Eur. J., 2012, 18, 13844.
4. R. Chakrabarty, P. S. Mukherjee and P. J. Stang, Chem. Rev., 2011, 111, 6810.
5. G. Bottari, G. de la Torre, D. M. Guldi and T. Torres, Chem. Rev., 2010, 110, 6768.
6. D. Gonzalez-Rodriguez and G. Bottari, J. Porphyrins Phthalocyanines, 2009, 13, 624.
7. M. Alvaro, P. Atienzar, P. Cruz, J. L. Delgado, V. Troiani, H. Garcia, F. Langa, A. Palkar and L. Echegoyen, J. Am. Chem. Soc., 2006, 128, 6626.
8. S. X. Fa, L. S. Wang, D. X. Wang, L. Zhao and M. X. Wang, J. Org. Chem., 2014, 79, 3559.
9. S. Shinkai and A. Ikeda, Gazz. Chim. Ital., 1997, 127, 657.
10. F. D'Souza, E. Maligaspe, A. S. D. Sandanayaka, N. K. Subbaiyan, P. A. Karr, T. Hasobe and O. Ito, J. Phys. Chem. A, 2010, 114, 10951.
11. B. M. Illescas, J. Santos, M. C. Díaz, N. Martín, C. M. Atienza and D. M. Guldi, Eur. J. Org. Chem., 2007, 2007, 5027.
12. W. Zhang, X. Gong, C. Liu, Y. Piao, Y. Sun and G. Diao, J. Mater. Chem. B, 2014, 2, 5107.
13. T. Braun, Fullerene Sci. Technol., 1997, 5, 615.
14. O. D. Fox, M. G. B. Drew, E. J. S. Wilkinson and P. D. Beer, Chem. Commun., 2000, 391.
15. F. C. Tucci, D. M. Rudkevich and J. Rebek, J. Org. Chem., 1999, 64, 4555.
16. F. Yang, Q. Chen, Q.-Y. Cheng, C.-G. Yan and B.-H. Han, J. Org. Chem., 2012, 77, 971.
17. T. Kawase, K. Tanaka, N. Shiono, Y. Seirai and M. Oda, Angew. Chem., Int. Ed., 2004, 43, 1722.
18. T. Kawase, N. Fujiwara, M. Tsutumi, M. Oda, Y. Maeda, T. Wakahara and T. Akasaka, Angew. Chem., Int. Ed., 2004, 43, 5060.
19. H. Sakurai, T. Daiko and T. Hirao, Science, 2003, 301, 1878.
20. L. T. Scott, M. M. Hashemi and M. S. Bratcher, J. Am. Chem. Soc., 1992, 114, 1920.
21. M. Yamamura, T. Saito and T. Nabeshima, J. Am. Chem. Soc., 2014, 136, 14299.
22. P. D. W. Boyd and C. A. Reed, Acc. Chem. Res., 2005, 38, 235.
23. D. Sun, F. S. Tham, C. A. Reed, L. Chaker, M. Burgess and P. D. W. Boyd, J. Am. Chem. Soc., 2000, 122, 10704.
24. A. Ouchi, K. Tashiro, K. Yamaguchi, T. Tsuchiya, T. Akasaka and T. Aida, Angew. Chem., Int. Ed., 2006, 45, 3542.
25. Y. Shoji, K. Tashiro and T. Aida, J. Am. Chem. Soc., 2004, 126, 6570.
26. S. Bhattacharya, T. Shimawaki, X. Peng, A. Ashokkumar, S. Aonuma, T. Kimura and N. Komatsu, Chem. Phys. Lett., 2006, 430, 435.
27. S. Bhattacharya, K. Tominaga, T. Kimura, H. Uno and N. Komatsu, Chem. Phys. Lett., 2007, 433, 395.
28. K. Sakaguchi, T. Kamimura, H. Uno, S. Mori, S. Ozako, H. Nobukuni, M. Ishida and F. Tani, J. Org. Chem., 2014, 79, 2980.
29. H. Uno, M. Furukawa, A. Fujimoto, H. Uoyama, H. Watanabe, T. Okujima, H. Yamada, S. Mori, M. Kuramoto, T. Iwamura, N. Hatae, F. Tani and N. Komatsu, J. Porphyrins Phthalocyanines, 2011, 15, 951.
30. Y. Kubo, A. Sugasaki, M. Ikeda, K. Sugiyasu, K. Sonoda, A. Ikeda, M. Takeuchi and S. Shinkai, Org. Lett., 2002, 4, 925.
31. M. Ayabe, A. Ikeda, Y. Kubo, M. Takeuchi and S. Shinkai, Angew. Chem., Int. Ed., 2002, 41, 2790.
32. D. M. Guldi, T. Da Ros, P. Braiuca, M. Prato and E. Alessio, J. Mater. Chem., 2002, 12, 2001.
33. J. Zhang, X. Zheng, R. Jiang, Y. Yu, Y. Li, H. Liu, Q. Li, Z. Shuai and Y. Li, RSC Adv., 2014, 4, 27389.
34. M.-X. Wang, X.-H. Zhang and Q.-Y. Zheng, Angew. Chem., Int. Ed., 2004, 43, 838.
35. G. Bottari, J. A. Suanzes, O. Trukhina and T. Torres, J. Phys. Chem. Lett., 2011, 2, 905.
36. A. Ray, D. Goswami, S. Chattopadhyay and S. Bhattacharya, J. Phys. Chem. A, 2008, 112, 11627.
37. R. F. Enes, J.-J. Cid, O. Trukhina, A. Hausmann, A. Gouloumis, P. Vázquez, J. A. S. Cavaleiro, A. C. Tomé, D. M. Guldi and T. Torres, Chem.–Eur. J., 2012, 18, 1727.
38. C. B. KC, K. Ohkubo, P. A. Karr, S. Fukuzumi and F. D'Souza, Chem. Commun., 2013, 49, 7614.
39. D. M. Eichhorn, S. Yang, W. Jarrell, T. F. Baumann, L. S. Beall, A. J. P. White, D. J. Williams, A. G. M. Barrett and B. M. Hoffman, J. Chem. Soc., Chem. Commun., 1995, 1703.
40. D. H. Hochmuth, S. L. J. Michel, A. J. P. White, D. J. Williams, A. G. M. Barrett and B. M. Hoffman, Eur. J. Inorg. Chem., 2000, 593.
41. R. S. Ruoff, D. S. Tse, R. Malhotra and D. C. Lorents, J. Phys. Chem. A, 1993, 97, 3379.
42. I. S. Molina, M. Ince, G. Bottari, C. G. Clasessens, M. V. Martinez-Diaz and T. Torres, Turk. J. Chem., 2014, 38, 1006.
43. R. S. Mulliken, J. Am. Chem. Soc., 1952, 74, 811.
44. S. Bhattacharya, M. Banerjee and A. K. Mukherjee, Spectrochim. Acta, Part A, 2001, 57, 1463.
45. R. Foster, Organic Charge Transfer Complexes, Academic Press, New York, 1969, ch. 3 and 13.
46. M. E. Peover, J. Chem. Soc., 1962, 4540.
47. S. Bhattacharya, M. Banerjee and A. K. Mukherjee, Spectrochim. Acta, Part A, 2001, 57, 2409.
48. G. Briegleb and J. Czekalla, Z. Phys. Chem., 1960, 24, 37.
49. H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 1949, 71, 2703.
50. S. Mukherjee, A. K. Bauri and S. Bhattacharya, Chem. Phys. Lett., 2010, 500, 128.
51. A. Ray, A. Bauri and S. Bhattacharya, Spectrochim. Acta, Part A, 2015, 134, 566.
52. G. Yin, D. Xu and Z. Xu, Chem. Phys. Lett., 2002, 365, 232.
53. D. M. Guldi, T. Da Ros, P. Braiuca and M. Prato, Photochem. Photobiol. Sci., 2003, 2, 1067.
54. F. D'Souza, G. R. Deviprasad, M. E. Zandler, V. T. Hoang, A. Klykov, M. V. Stipdonk, A. Perera, M. E. El-Khouly, M. Fuzitsuka and O. Ito, J. Phys. Chem. A, 2002, 106, 3243.
55. F. D'Spuza and O. Ito, Chem. Commun., 2009, 4913.
56. K. Campbell, A. Zappas, U. Bunz, Y. S. Thio and D. G. Bucknall, J. Photochem. Photobiol., A, 2012, 249, 41.
57. A. Kathiravan, Synth. Met., 2014, 194, 77.
58. F. Li, K. G. Yager, N. M. Dawson, J. Yang, K. J. Malloy and Y. Qin, Macromolecules, 2013, 46, 9021.
59. C. F. Van Nostrum and R. J. M. Nolte, Chem. Commun., 1996, 2385.
60. M. Dudic, P. Lhoták, I. Stibor, H. Petrıícková and K. Lang, New J. Chem., 2004, 28, 85.
61. Y. Shoji, K. Tashiro and T. Aida, J. Am. Chem. Soc., 2004, 126, 6570.
62. R. Stowasser and R. Hoffmann, J. Am. Chem. Soc., 1999, 121, 3414.
63. J. C. Rienstra-Kiracofe, C. J. Barden, S. T. Brow and H. F. Schaefer, J. Phys. Chem. A, 2001, 105, 524.
64. O. Ito and F. D'Souza, Molecules, 2012, 17, 5816.
65. Y. Li, D. Qi, P. Song and F. Ma, Materials, 2015, 8, 42.
66. A. Hosseini, S. Taylor, G. Accorsi, N. Armaroli, C. A. Reed and P. D. W. Boyd, J. Am. Chem. Soc., 2006, 128, 15903.
67. F. D'Souza, R. Chitta, S. Gadde, M. E. Zandler, A. L. McCarty, A. S. D. Sandanayaka, Y. Araki and O. Ito, Chem.–Eur. J., 2005, 11, 4416.
68. A. Ray, K. Santhosh, S. Chattopadhyay, A. Samanta and S. Bhattacharya, J. Phys. Chem. A, 2010, 114, 5544.
69. A. Ray, K. Santhosh and S. Bhattacharya, J. Phys. Chem. B, 2012, 116, 11979.

---

Footnotes

† The authors wish to dedicate this work to Late Mr S. B. Banerjee for his constant encouragement in pursuing research work.

‡ Electronic supplementary information (ESI) available: CT analysis of p-chloranil–1 complex in DCB (Fig. S1), plot for determination of I^v_D of 1 (Fig. S2), steady state fluorescence spectra, induced curves and BH plots for the determination of the binding constants of 1 with C₆₀ and C₇₀ in different solvents (Fig. S3–S5), steady state fluorescence spectral variation for the determinations of SV constants for C₆₀–1 complex in toluene (Fig. S6) and C₇₀–1 complex in toluene (Fig. S7) and DCB (Fig. S8), optimized structure of C₇₀–1 complex at side-on orientation of C₇₀ (Fig. S9), B3LYP/6-31G* optimized electronic, i.e., HOMO and LUMO, structure of C₇₀–1 system in end-on orientation of C₇₀ (Fig. S10), decay profile of 1 in absence and presence of C₆₀ and C₇₀ recorded in toluene (Fig. S11), plot of τ^s/τ vs. concentration of quencher for C₆₀–1 and C₇₀–1 systems measured in DCB (Fig. S12), and finally, fluorescence spectral variation of C₆₀ in presence of 1 recorded in toluene (Fig. S13). See DOI: 10.1039/c5ra02003d

---