ARTICLE pubs.acs.org/JPCA
New Photophysical Insights in Noncovalent Interaction between Fulleropyrrolidine and a Series of Zincphthalocyanines Anamika Ray,† Kotni Santhosh,‡ and Sumanta Bhattacharya*,† † ‡
Department of Chemistry, The University of Burdwan, Golapbag, Burdwan - 713 104, India School of Chemistry, University of Hyderabad, Hyderabad, AP - 500 046, India
bS Supporting Information ABSTRACT: The present article reports, for the first time, the photophysical aspects of noncovalent interaction of a fullerene derivative, namely, C60 pyrrolidine tris-acid ethyl ester (PyC60) with a series of zincphthalocyanines, for example, underivatized zincphthalocyanine (1), zinc-1,4,8,11,15,18,22, 25-octabutoxy-29H,31H-phthalocyanine (2), and zinc-2,3,9,10,16,17,23,24-octakis-(octyloxy)-29H, 31H-phthalocyanine (3) in toluene. Ground state electronic interaction of PyC60 with 1, 2 and 3 has been evidenced from the observation of well-defined charge transfer (CT) absorption bands in the visible region. Utilizing the CT transition energy, vertical electron affinity (EAv) of PyC60 is determined. Steady state fluorescence experiment enables us to determine the value of binding constant (K) in the magnitude of 2.60 104 dm3 3 mol1, 2.20 104 dm3 3 mol1, and 1.27 104 dm3 3 mol1 for the noncovalent complexes of PyC60 with 1, 2, and 3, respectively. K values of PyC60ZnPc complexes suggest that PyC60 is incapable of discriminating between 1, 2, and 3 in solution. Lifetime experiment signifies the importance of static quenching phenomenon for our presently investigated supramolecules and it yields larger magnitude of charge separated rate constant for the PyC60-1 species in toluene. Photoinduced energy transfer between PyC60 and ZnPc derivatives, namely, 1, 2, and 3, in toluene, has been evidenced with nanosecond laser photolysis method by observing the transient absorption bands in the visible region; transient absorption studies establish that energy transfer from TPyC60* to the ZnPc occurs predominantly, as confirmed by the consecutive appearance of the triplet states of PyC60. Theoretical calculations at semiempirical level (PM3) evoke the single projection geometric structures for the PyC60-ZnPc systems in vacuo, which also proves that interaction between PyC60 and ZnPc is governed by the electrostatic mechanism rather than dispersive forces associated with ππ interaction.
1. INTRODUCTION Natural photosynthesis processes which rely on highly organized supramolecular assemblies have inspired chemists to mimic many aspects on successful energy transmuting process in laboratory scale. Several molecular and supramolecular systems have been elegantly designed in this connection and studied with an emphasis on generating long-lived charge separated states through a charge migration route.15 Studies on donoracceptor dyads capable of mimicking light-induced electron or energy transfer process(s) are of current interest, mainly due to the development of artificial photosynthetic systems,69 and also to develop molecular optoelectronic logic gates and devices.10 After the initial discovery in 1984,11 the fortuitous contemporary growth of two apparently independent research lines, namely, synthetic fullerene chemistry and supramolecular fullerene photochemistry, has been reciprocally beneficial and contributed to boost activity in both fields. However, the formation of multicomponent “supermolecules” acting as artificial photosynthetic reaction centers represents one of the most active research area in fullerene science for the last 15 years.12 The unique spherically symmetric structure13 and low reorganization energy14 of fullerene can accelerate the forward electron transfer rate as well as slow down the backward electron transfer rate, resulting in long-lived charge separated state during r 2011 American Chemical Society
energy and/electron transfer process(s) in donoracceptor type reaction.15 Covalent linkage of fullerenes C60 and C70 to a number of interesting electro- or photo active species offers new opportunities in the preparation of materials that may produce long-lived charge separated state in high quantum yield.16 However, use of functionalized form of fullerene C60 to undergo noncovalent type of interaction with donor molecules having photoactive and electroactive units would certainly generate some outstanding physicochemical aspects in supramolecular photochemistry of fullerenes. In recent past, a vast majority of donoracceptor systems have been reported comprising fullerenes and porphyrins (metallloporphyrins).1725 However, use of phthalocyanine (Pc), as an electron donor26 in donoracceptor system, offers several advantages over porphyrin which includes exceptional thermal and chemical stability,27 and strong absorption in the visible region capable of being better sensitizer for light energy harvesting application.28 It has been also verified, that presence of central metal ion along with peripheral and nonperipheral substitution strongly influence the photophysical behavior of Pc molecules.29 Received: May 26, 2011 Revised: July 19, 2011 Published: July 20, 2011 9929
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Figure 2. UVvis spectrum of PyC60, 1, 2, and 3 recorded in toluene medium.
Figure 1. Structures of (a) PyC60, (b) 1, (c) 2, and (d) 3.
Metal Pc (MPc) complexes are traditionally used as dye and pigments.30 More recently, they have been used as the photoconducting agent in photocopiers, because of their easy synthesis, high stability and presence of intense π f π* transitions in the visible region.31 MPc also finds its application in photodynamic therapy as photosensitizer.32 Among all of the MPc derivatives, zincphthalocyanine (ZnPc) has received special attention due to its intense fluorescence and singlet oxygen producing property33 which allow their use in the detection and treatment of tumors.34 Although the first report on fullerene-Pc array dates back to late 1995, when these two redox-active moieties are covalently linked following different synthetic strategies to form a fullerene-Pc hybrid,35 noncovalent donoracceptor ensembles comprising fullerene and Pc still remains challenging because of the synthetic difficulties associated in building hostguest assemblies.36 Some investigations where Pc units are linked covalently with derivatized fullerene have been reported in recent past.37 Very recently, we have substantiated the role of electrostatic interaction behind the formation of stable fullerene-Pc noncovalent assembly by choosing very simple unsubstituted free-base, that is, H2- and ZnPc molecules in toluene.38 We have also established that stabilization of charge-recombined state takes place in case of C70-ZnPc system when C70 approaches the plane of ZnPc in side-on interaction motif.39 However, the central interest of our present investigations is to find out the new photophysical features originated in fullerene-Pc noncovalent interaction employing C60 pyrrolidine tris-acid ethyl ester (PyC60, Figure 1). The motivation behind selecting the PyC60 molecule as electron acceptor in our present studies comes from the work of Sessler et al. in which they have employed fulleropyrroline bearing a guanosine moiety as a recognition motif for the construction of Pc-C60 dyad system.40 The basic intention of the present work, therefore, is to find out the possibility of ground state noncovalent interaction between PyC60 and a series of ZnPc in solution. Various spectroscopic tools like absorption spectrophotometric, steady state fluorescence, timeresolved fluorescence and transient absorption studies are employed to find out the noncovalent insights persists between PyC60 and the said ZnPc molecules, that is, underivatized zincphthalocyanine
(1, Figure 1) zinc-1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (2, Figure 1), and zinc-2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine (3, Figure 1) in toluene. Quantum chemical calculations at semi empirical level of theory using third parametric method (PM3) are employed to find out the stability differences between various PyC60-ZnPc complexes. We anticipate that PyC60 molecule may impart some novel photophysical characteristics which may produce some additional information in fullerene-phthalocyanine noncovalent interactions.
2. MATERIALS AND METHODS Compounds 1, 2, 3, and PyC60 are purchased from SigmaAldrich, U.S.A., and are used without further purification. UVvis spectroscopic grade toluene (Merck, Germany) is used as solvent to favor the intermolecular interaction between PyC60 and ZnPc, as well as to provide good solubility and photostability of the samples. UVvis spectral measurements have been performed on a Shimadzu UV-2450 model spectrophotometer using quartz cell with 1 cm optical path length. Steady state emission spectra are recorded with a Hitachi F-4500 model fluorescence spectrophotometer. Fluorescence decay experiments have been measured with a HORIBA Jobin Yvon singlephoton counting setup employing Nanoled as excitation source. For the transient absorption spectra in the visible region, a photomultiplier tube has been used as a detector for the continuous Xe-monitor light (150 W). PM3 calculations in vacuo are performed using SPARTAN ’06 Windows version software. 3. RESULT AND DISCUSSIONS 3.1. Observation of Charge Transfer (CT) Absorption Bands. It is already well established that MPcs are characterized
by their electronic absorption with high extinction coefficients in the visible region, namely, Q-band, and weaker absorption in the lower wavelength region, that is, Soret absorption band (B band), resulting from S1 r S0 and S2 r S0 transitions, respectively.41 Figure 2 demonstrates the absorption spectral feature of 1, 2, 3, and PyC60 in toluene. In the case of 3, it is observed that the electron donating substituents at the β position increases the electron density in the Pc ring which causes 6 nm red shift of the Q-band (at 677 nm) compared to 1 (at 671 nm). In case of 2, the Q-band appears at 740 nm along with a longer wavelength peak 9930
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Figure 3. UVvis absorption spectrum of (i) PyC60 (3.36 105 mol 3 dm3) in toluene along with the CT absorption spectra of the mixtures of (ii) PyC60 (3.36 105 mol 3 dm3) + 1 (3.80 104 mol 3 dm3), (iii) PyC60 (7.20 105 mol 3 dm3) + 3 (1.40 104 mol 3 dm3) and (iv) PyC60 (5.35 105 mol 3 dm3) + 2 (4.65 106 mol 3 dm3) in toluene measured against the pristine donor solution.
Figure 4. Gaussian analysis for the CT band of PyC60-1 system.
around 805 nm. The eight octabutoxy group at the R position of ZnPc ring shifts the Q-band at longer wavelength region.42 For PyC60 unit (Figure 3(i)), a weak absorption at 430 nm is observed which is the characteristic band of the [6,6] mono adduct formation of C60.43 Evidence in favor of ground state interaction between PyC60 and ZnPc, that is, 1, 2, and 3, first comes from UVvis spectroscopic measurement in toluene. Figure 3 shows the electronic absorption spectra of toluene solution of PyC60 with 1, 2, and 3. Spectra of the above solutions have been recorded against the pristine donor solution as reference. Mixture of PyC60 and various ZnPc results the formation of additional absorption band(s) in the visible region. The newly appeared bands do not exhibit any resemblance to the spectral characteristics of the both parent uncomplexed ZnPc and PyC60. In Figure 3(ii)(iv), the CT peaks are identified to be the peaks in the region of 470530 nm, because they normally
have the longest wavelength among the peaks different from those obtained from the spectra of the components. One important point to note that even though PyC60 and ZnPc undergo CT interaction with great ease, one may still argue that the way they approach to each other is dictated by packing consideration than by the special affinity we have highlighted above. The choice of toluene as solvent undoubtedly promotes the aggregation of the hydrophobic fullerene and ZnPc entities. The demonstration of PyC60-ZnPc binding in toluene is therefore considered to be a spontaneous attraction. The CT absorption spectra are√analyzed by fitting to the Gaussian function y = y0 + [A/(w (π/2)] exp[2(x xc)2/w2], where x and y denote wavenumber and molar extinction coefficient, respectively. One typical Gaussian analysis plot is shown in Figure 4. The wavelengths at these new absorption maxima (λmax = xc) and the corresponding transition energies (hνCT) are summarized in Table 1. The Gaussian analysis fitting is done in accordance with the method developed by I.R. Gould et al.44 One important point to mention here is that Gaussian analysis of a curve generally gives a decent result near the maxima of the curve spread over a very small region. For this reason, although the errors in the center of the CT spectra for the complexes of PyC60 with various electron donors are very small, there are appreciable errors in the y0 value. 3.2. Determination of Vertical Electron Affinity (EAv) of PyC60. For ground state complex formation between neutral donor and acceptor, a CT band corresponds to the absorption of quanta for the transfer of an electron from donor to acceptor moiety. hνCT
Dδþ 3 3 3 A δ sf Dð1 δÞþ 3 3 3 A ð1 δÞ According to Mulliken’s theory,45 the relationship between energy of charge transfer (hνCT) and vertical ionization potential 9931
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Table 1. CT Absorption Maxima (λmax), CT Transition Energies (hνCT), Degrees of Charge Transfer (r), Oscillator Strengths (f), Transition Dipole Moments (μEN), Resonance Energies (RN), Electronic Coupling Elements (V), and Solvent Reorganization Energies (RS) for the Complexes of PyC60 with 1, 2, and 3 in Toluene along with the Vertical Electron Affinity (EAV) of PyC60 Measured in Solution system
λCT (nm)
hνCT (eV)
R (%)
f
μEN (D)
RN (eV)
V (cm1)
RS (eV)
EAV of PyC60 (eV)
PyC60-1
475
2.61
0.796
0.0042
2.15
0.024
257.44
0.820
0.315
PyC60-2
460
2.70
0.855
0.0133
2.07
0.032
818.33
0.763
PyC60-3
520
2.38
0.779
0.0111
3.08
0.035
PyC60 molecule is determined to be 0.315 (Table 1). The above plot is demonstrated in Figure 5. 3.3. Degrees of CT (r) for the PyC60 Complexes of ZnPc. In Mulliken two state model,45 the ground-state (ψg) and excitedstate (ψex) wave functions of the CT complexes are described by a linear combination of dative ψ(D0, A0) and ionic ψ(D+, A) states √ √ ψg ¼ f ð1 RÞgψðD0 , A 0 Þ þ ð RÞψðDþ , A Þ ð5Þ √ √ ψex ¼ f ð1 RÞgψðDþ , A Þ ð RÞψðD0 , A 0 Þ
ð6Þ
where R is defined as the degree of CT. The function ψ(D , A) differs from ψ(D0, A0) by the promotion of an electron from the donor to the acceptor. R is given by45 +
Figure 5. Plot of 2IDv hνCT vs IDv (IDv hνCT) for various PyC60ZnPc systems measured in toluene.
(IDV)
of donor molecule can be expressed as
hνCT ¼ ID v C1 þ C2 =ðID v C1 Þ
ð1Þ
Here, C1 ¼ EA v þ G0 þ G1
ð2Þ
where EAv is the vertical electron affinity of the acceptor; G0 is the sum of several energy terms (like dipoledipole, van der Walls interaction, etc.) in the “no-bond” state; and G1 is the sum of a number of energy terms in the “dative” state. In most cases, G0 is small and can be neglected, whereas G1 is largely the electrostatic energy of attraction between D+ and A. The term C2 in eq 1 is related to the resonance energy of interaction between the “no-bond” and “dative” forms in the ground and excited states and for a given acceptor it may be supposed to be constant.45 Rearranging eq 1, we get 2ID V hνCT ¼ ð1=C1 ÞID V ðID V hνCT Þ þ ðC1 þ C2 =C1 Þ
ð3Þ
Using the observed CT transition energy and theoretically determined IDV values for three donors (Table 1S), we have calculated the following data, 2ID V hνCT ¼ ð0:225 ( 0:01124ÞID V ðID V hνCT Þ
þ ð4:7796 ( 0:1775Þ
ð4Þ
From eq 4, C1 is estimated to be 4.43. Neglecting G0 and taking the typical DA distance in π-type EDA complexes to be 3.5 Å, the major contributing term G1 (= e2/4πεor) is estimated to be 4.13 eV. When the value of G1 in eq 2 is used, the value of EAv of
R ¼ ðC2 =2Þ=½ðID v EA v þ C1 Þ2 þ ðC2 =2Þ
ð7Þ
The values of R (calculated by using eq 7 and given in Table 1) indicate that appreciable amount of ground state CT takes place between PyC60 and 2 in toluene. The magnitude of R indicates that the CT complexes formed between PyC60 and ZnPc are almost neutral in ground state. 3.4. Determination of Oscillator (f) and Transition Dipole Strengths (μEN). From the CT absorption spectra, we can enumerate the magnitude of the oscillator strength. The oscillator strength, f, is estimated using the formula Z ð8Þ f ¼ 4:32 109 εCT dν R where the term, ( εCTdν) is the area under the curve of the extinction coefficient of the absorption band in question vs frequency. To a first approximation f ¼ 4:32 109 εmax Δν1=2
ð9Þ
where εmax is the maximum extinction coefficient of the band and Δν1/2 is the half-width, that is, the width of the band at half the maximum extinction. The observed oscillator strengths of the CT complexes of PyC60 are summarized in Table 1. Table 1 clearly proves that the PyC60-1 system exhibits a very low value of f in comparison to other systems. From this observation, we may anticipate strong binding between these two components during intermolecular interaction. It is worth mentioning that we need a proper calculation of oscillator strengths of PyC60-ZnPc CT complexes. This is because oscillator strength is very much sensitive to the molecular configuration and the electron charge distribution in CT complex. In the PyC60-1, PyC60-2, and PyC603 complexes, we could not use a simple model assuming a charge localized at a certain cite of fullerene sphere, because π-bonds of the fullerenes are directed radially with a node on the molecular cage. 9932
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The extinction coefficient is related to the transition dipole moment by μEN ¼ 0:0952½εmax Δν1=2 =Δν1=2
ð10Þ R where Δν = ν at εmax and μEN is defined as e Ψex∑iriΨgdτ. The transition dipole strengths (μEN) for the complexes of PyC60 with various electron donors are given in Table 1. It has been observed that the value of μEN for the PyC60-3 complex is somewhat higher compared to PyC60-1 and PyC60-2 complexes. This trend in μEN is in conformity with the fact that PyC60-3 system forms loose molecular complex than those of PyC60-1 and PyC60-2 systems which is corroborated fairly well with the binding constant (K) values of the respective complexes discussed later in section 3.8. 3.5. Determination of Resonance Energy (RN). Briegleb and Czekalla46 theoretically derives the relation εmax ¼ 7:7 104 =½hνCT =jRN j 3:5
ð11Þ
where εmax is the molar extinction coefficient of the complex at the maximum of the CT absorption, hνCT is the CT transition energy, and RN is the resonance energy of the complex in the ground state. RN values for the complexes of PyC60 with various ZnPc molecules are summarized in Table 1. 3.6. Determination of Electronic Coupling Element (V) and Solvent Reorganization Energy (RS) for PyC60-ZnPc Complexes. The electronic coupling element (V) is related to the extent of overlap between the appropriate donor or host and acceptor or guest orbitals, and scale the dependency of free energy of electron transfer (i.e., driving force) on such rate constant. The electronic interaction between PyC60 and various ZnPc in PyC60-1, PyC60-2, and PyC60-3 systems used mix electronic character and induces electron transfer (ET). It also creates an opportunity for the estimation of electronic basis inducing dipole-allowed optical ET with the magnitude of the perturbation dictating the intensities of the inter valence transfer bands. This assumption implies that V between PyC60 and ZnPc is small or moderate assuming an ET process in terms of Marcus formalism.47 For the investigated supramolcules in the present investigations, values of V are estimated from the absorption maxima of PyC60-ZnPc complexes applying model provided by Verhoeven et al.48 Values of V for the PyC60-1 and PyC60-2 systems are listed in Table 1. Estimated electronic coupling elements obtained for the above systems are very much comparable to the other donoracceptor and hostguest systems found in literature.49 V values determined in the present investigation were significantly larger than typical values for the solventseparated radicalion pair (∼12 cm1),49 indicating that PyC60 and ZnPc are in close contact to form a contact radical ion pair. There are some recent observations of large solvent dependence of energy transfer rate constants and electronic coupling elements in electron donoracceptor systems where the donor and acceptor were in close proximity. Zimmt et al. reported the large solvent dependent electronic coupling matrix element for their C-clamp-shaped molecule.50 They propose that solvent mediated super exchange coupling phenomenon is the main contributing factor behind such higher V values. Therefore, the effect of solvent over V is likely to play an important role in the present systems. For this reason, solvent reorganization energies (Rs) were calculated for the PyC60 complexes of ZnPc. The total reorganization energy, R, is a sum of the two terms, that is, inner-sphere reorganization energy (solvent-independent), R0,
and outer-sphere reorganization energy (solvent-dependent), Rs. In the case of fullerene, contributions from R0, which are related to the differences in nuclear configurations between an initial and a final state, are very small, ∼4.3 105 eV.51 This observation implies that the structure of C60 in the ground and excited states is very similar, which is related with 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 required little energy for the adjustment of an excited or reduced state to the new solvent environment. In the present investigation, Rs for the complexes of PyC60 with 1 and 2 were estimated applying the dielectric continuum model developed by Hauke et al.52 Values of Rs for the PyC60-1 and PyC60-2 systems are given in Table 1. It should be mentioned at this point that solvent reorganization energies obtained in the present investigations do not corroborate well with those observed for porphyrin-quinone system found in literature.53 The discrepancy in the value of Rs for porphyrin-quinone and PyC60-ZnPc systems may be due to the subtle structural change in the hostguest complex, which exerts a large influence upon the photo induced electron and energy transfer process. 3.7. Theoretical Model in Favor of Electric DipoleDipole Interaction Between PyC60 and ZnPc. Consider the interaction between PyC60 and ZnPc. The interaction between the dipole dipole transitions for the above-mentioned molecules can be represented in the form H ¼
Nmax
i σx, i ZnPc Þð1 3 cos2 θi Þ=∈∞ ri 3 ∑ ðdPyC60σx PyC60 dZnPc
i¼1
ð12Þ i
where dPyC60 and d [ZnPc] are the dipole moments of the corresponding transitions in PyC60 and the ith number of ZnPc molecule, σx and σx,iZnPc are the corresponding Pauly matrices, ri is the distance between PyC60 and ZnPc molecules, and ∈∞ in eq 12 is the high-frequency dielectric constant. The reconstruction of the resulting spectrum, taking into account eq 12, is determined by mixing of the states of the derivatized fullerene molecule and the surrounding ZnPc molecule. E( i ¼ ðEPyC60 þ EZnPc Þ=2 ( f½ðEPyC60 EZnPc Þ=22 þ jVi j2 g1=2
ð13Þ
For one PyC60/ZnPc pair, eq 12 gives eq 13, where EPyC60 and EZnPc are the energies of dipole transitions of PyC60 and ZnPc, respectively, and Vi = [dPyC60diZnPc (1 3 cos2θi)]∈∞1ri3 is the matrix element of the state mixing. The final expression has the form ∈ ¼ ∈ ð0Þ ðjVi jN 1=2 Þ=2
ð14Þ
where V is the amplitude of the nondiagonal flip-flop dipole dipole matrix element for PyC60 and of the dipole transitions in neighboring ZnPc molecules and N is the number of neighboring ZnPc molecules. Such a dependence of the absorption band edge is valid only under the condition N < Nthr, where Nthr is the maximum number of PyC60 molecules that can take part in the dipoledipole flip-flop interaction with ZnPc. A further increase in the concentration of PyC60 does not increase the number of these molecules in the nearest environment of ZnPc. Figure 6 9933
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Table 2. Binding Constants (K), Rate Constant of Charge Separation (kCS), and Quantum Yield for Charge Separation (ΦCS) and Enthalpies of Formation (ΔHf0) for Various PyC60-ZnPc Systems Recorded in Toluene Medium
Figure 6. Variation of absorbance vs concentration of PyC60 for PyC60-1 system in toluene.
Figure 7. (a) Steady state fluorescence spectral variation of 1 (4.20 106 mol 3 dm3) in the presence of PyC60 in toluene medium; the concentrations of PyC60 from top to bottom are as follows: 0, 3.17 106, 6.35 106, 9.53 106, 1.27 105, 1.60 105, 1.90 105, 2.22 105, 2.54 105, 2.86 105, and 3.18 10 5 mol 3 dm3; plot of relative fluorescence intensity vs concentration of PyC60 is shown in the inset of (a); (b) fluorescence BH plot for PyC60-1 system.
shows that the dependences are saturated when the concentration of ZnPc exceeds 2.92 105 mol 3 dm3, in case of PyC60-1 complex, in agreement with the theory. This mechanism, thus, allows us to explain the formation of the CT complexes in the systems under study. 3.8. Fluorescence Spectroscopic Studies. As far as the emission spectra are concerned, it must be pointed out that in our present investigations fluorescence experiments could be reliably done due to one very favorable circumstance. The large molar extinction coefficients of all the ZnPc molecules with
system
K, dm3 3 mol1
kCS, sec1
ΦCS
ΔHf0,kJ 3 mol1
PyC60-1
26011
1.7 106
0.0060
5.182
PyC60-2
21975
1.0 106
0.0032
6.361
PyC60-3
12730
1.4 106
0.0024
—
respect to PyC60 in the visible UVvis spectral region allowed us to preferentially excite the ZnPc, although the ZnPc concentration is fixed at lower concentration than that of the fullerene. It is observed that fluorescence of all the ZnPc molecules diminish gradually by the addition of varying concentration of PyC60 in toluene medium, upon excitation at their corresponding Soret absorption peak. The steady state fluorescence spectral changes of 1, 2, and 3 upon addition of PyC60 solution are shown in Figures 7(a), 1S(a), and 1S(b), respectively. The emission peak of the ZnPc under consideration corroborate fairly well with the reported literature value of various other phthalocyanines.54,55 It is already reported that light-induced energy or electron transfer reactions takes place in self-assembled supramolecular zinc porphyrin/ZnPc and fullerene bearing donoracceptor systems.56 Competing between the energy and electron transfer process is a universal phenomenon in a donorfullerene molecular complex, and solvent-dependent photophysical behavior is a typical phenomena of the most supramolecular fullerene systems studied to date.5759 Usually energy transfer dominates over the photophysical behavior in nonpolar solvent in deactivating the photo excited chromophore *ZnPc. In present investigations, therefore, the quenching phenomenon may be ascribed to the photoinduced energy transfer from the ZnPc to fullerene in the PyC60-1, PyC602, and PyC60-3 supramolecular complexes. The steady state fluorescence titration experiments have been performed at constant concentrations of ZnPc. As the concentration of PyC60 is increased, the emission intensity of ZnPc has been reduced. The spectral changes finally reach a plateau, indicating that the fluorescence quenching is induced by the complexation (inset of Figures 7(a), 1S(a), and 1S(b), respectively). It should be noted at this point that the quenching efficiency is higher in the case of the PyC60-1 system, which indicates strong intermolecular interaction between these two components. The binding constant (K) of the PyC60-ZnPc systems is evaluated according to a modified BenesiHildebrand (BH) equation60 (see eq 15): F0 =ðF0 FÞ ¼ ð1=AÞ þ fð1=KAÞð1=½PyC60 Þg
ð15Þ
In eq 15, F0 and F are the fluorescence intensity of ZnPc without and with the PyC60, respectively, and [PyC60] indicates the molar concentration of fullerene; A is a constant associated with the difference in the emission quantum yield of the complexed and uncomplexed ZnPc. By plotting F0/(F0 F) (relative fluorescence intensity) versus 1/([PyC60]), K values are evaluated for various PyC60-ZnPc complexes (see Table 2). One typical BH fluorescence plot of the PyC60-1 system is shown in Figure 7(b). The BH plots of the complexes of PyC60-2 and PyC60-3 systems are provided as Figures 2S(a) and 2S(b), respectively. In a nonpolar medium like toluene, generally, energy transfer phenomena dominates over electron transfer in deactivating the photoexcited chromophore, 1ZnPc*, formed in the final instance of the fullerene triplet excited state. Detailed mechanism regarding the 9934
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Figure 9. Stereoscopic structures of (a) PyC60-1 and (b) PyC60-2 systems obtained by semiempirical PM3 calculations in vacuo.
Figure 8. Time-resolved decay profile for (i) uncomplexed 1 and the (ii) PyC60 + 1 mixture in toluene medium.
decay of photoexcited ZnPc in presence of PyC60 will be discussed latter using transient absorption studies. It is interesting to note that the increase in magnitude of the binding constant led to an increase in the fluorescence quenching efficiency. This can be viewed in terms of the fact that the rigidity of the ZnPc unit brings tight fixation of PyC60 unit in a particular PyC60-ZnPc noncovalent complex. 3.9. Time-Resolved Fluorescence Studies. Apart from steady state fluorescence measurements, we have performed detailed nanosecond time-resolved fluorescence experiments for all the PyC60-ZnPc systems in toluene medium. The experiment has been carried out at a fixed concentration of 1, 2 and 3 and a variable concentration of PyC60. The time-resolved fluorescence of all the ZnPc molecules, namely, 1, 2, and 3, reveals a monoexponential decay with a lifetime of 3.57, 1.68, and 3.21 ns, respectively (Figures 8(i), 3S(a)(i), and 3S(b)(i), respectively). It is observed that, upon the gradual addition of fullerenes, there is practically no change in the lifetime compared to the uncomplexed ZnPc and monoexponential decay is maintained (Figures 8(ii), 3S(a)(ii), and 3S(b)(ii), respectively). Lifetimes of 1, 2, and 3 in the presence of PyC60 are determined to be 3.55, 1.68, and 3.20 ns, respectively. From the lifetime experiment, the rate constant for charge separation (kCS) is calculated using the equation kCS ¼ ð1=τÞcomplex ð1=τÞref
ð16Þ
where τcomplex and τref are the lifetime of the PyC60-ZnPc complexes and uncomplexed ZnPc, respectively. The quantum yield of the charge separated state (ΦCS) is determined according to the equation ΦCS ¼ ½ð1=τÞcomplex ð1=τÞref =ð1=τÞcomplex
ð17Þ
The estimated values of kCS and ΦCS are summarized in Table 2. Table 2 indicates efficient charge separations for all the ZnPc complexes of PyC60 in toluene. Similar sort of phenomenon is already observed for other fullerenephthalocyanine supramolecular complexes.61,62 The values of both kCS and ΦCS is estimated to be higher for PyC60-1 complex compared to PyC60-2 and PyC60-3 complexes. This is quite expected as the value of K is much higher in the case of the PyC60-1 complex. We may anticipate that, apart from electrostatic attraction, a
formidable extent of ππ interactions may arise between PyC60 and 1, which is quite unnatural in the case of fullereneZnPc interactions where predominantly electrostatic interaction control the binding motif between these supramolecular entities.63 Therefore, it would be highly beneficial if quantum chemical calculations may invoke some light on the geometrical arrangement of both PyC60 and ZnPc molecules in PyC60-ZnPc systems. 3.10. Binding Constants. The binding constants (K) of all the PyC60-ZnPc complexes determined by steady state fluorescence measurements are summarized in Table 2. It shows that the underivatized form of ZnPc, that is, 1, undergoes a considerable amount of complexation with PyC60 compared to 2 and 3. The present trend in the K value corroborates fairly well with the earlier reported K value of C60-ZnPc and C70-ZnPc systems.64 Another interesting aspect of the present investigations is that all the ZnPc molecules do not exhibit any sort of selectivity toward PyC60 like free-base Pc and ZnPc toward C60 and C70.64 From the above results we may conclude that, in spite of a huge similarity in structure of zincporphyrin and ZnPc, a remarkable decrease in selectivity is observed in the case of the latter. 3.11. Theoretical Calculations. To gain insight into the preferred molecular geometry and to measure the heat of formation (ΔHf0), a detailed conformational analysis of the individual components as well as the PyC60-ZnPc complexes have been performed in vacuo by semiempirical PM3 calculations. The geometric parameters of the complexes are obtained after complete energy minimization at the molecular mechanics at the force field level of theory. Figure 9(a) & 9(b) shows the stereoscopic structures for the PyC60 complexes of 1 and 2, respectively. Due to the presence of eight numbers of long chain n-octakis(octyloxy) group in case of 3, we are unable to proceed the quantum chemical calculations for the PyC60-3 supramolecule which requires too many basis sets. ΔHf0 values for the PyC60-1 and PyC60-2 complexes are given in Table 2 which gives full support in favor of strong binding between PyC60 and 1 as we highlight the discussion on the K value of such complexes in sections 3.8 and 3.10. In our present investigations, the existence of the electrostatic interactions between the phthalocyanine macrocycle (i.e., ZnPc) and fullerene moiety (i.e., PyC60) is also evidenced by the results obtained on frontier highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) using PM3 method. Positions of HOMO on the ZnPc moiety and LUMO on PyC60 unit certainly indicate that they act as donor and acceptor molecules, respectively, during formation of donoracceptor type complex. The most interesting feature of the theoretical investigations, however, comes from the orbital distributions at higher excited state, namely, HOMO 2, HOMO 3, and so on, for the PyC60-ZnPc complexes. It is observed for 9935
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Figure 11. MESP of (a) 1, (b) 2, (c) PyC60, (d) PyC60-1, and (e) PyC60-2 complexes obtained by semiempirical/PM3 calculations.
Figure 10. HOMOs of the PyC60-1 complex at different electronic states; e.g., (a) HOMO, (b) HOMO 1, (c) HOMO 2, (d) HOMO 3, (e) HOMO 4, (f) HOMO 5, and (g) HOMO 6 done by semiempirical/PM3 calculations.
PyC60-1 complex, while majority of orbital distributions are positioned on the fullerene moiety at HOMO 2 state, in case of HOMO 3, the same aspect is solely located on donor unit. From HOMO 4 onward, the orbital distributions are located on the fullerene moiety. Similar features are observed when we look into detail regarding orbital distributions in terms of LUMO of the PyC60-1 complex. While LUMO electronic state is situated on the fullerene unit of PyC60 molecule, the majority of the orbital distribution of LUMO + 1 is located on the donor unit PyC60-1 system. However, LUMO + 2 and LUMO + 3 states are complete observed in fullerene moiety of the PyC60-1 system. Again, LUMO + 4 and LUMO + 5 states are predominantly situated on the donor, that is, a ZnPc unit of a PyC60-1 system. Finally, LUMO + 6 is positioned on the fullerene unit of the PyC60-1 system, which clearly indicates a considerable amount of electronic redistribution in such a noncovalently linked supramolecule. The above remarkable features, therefore, envisage the role of CT phenomenon during intermolecular interaction between the PyC60-1 system.
Figure 12. (a) Triplettriplet absorption spectra of various ZnPc derivatives in toluene monitored after 1 μs time delay and (b) decay curves for (1) 1 at 470 nm, (2) 3 at 520 nm, and (3) 2 at 630 nm.
Table 3. Decay Time and Monitoring Wavelength of the ZnPc Derivatives Measured by Transient Absorption Studies in Toluene sample
monitoring wavelength, nm
decay time, μs
1
470
55
2
630
30
3
520
60
Some pictures of HOMOs and LUMOs of the PyC60-1 at its various electronic states are visualized in Figures 10 and 4S, 9936
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The Journal of Physical Chemistry A respectively. Similar features are observed in the case of the PyC602 complex (see Figures 5S and 6S, respectively). Moreover, molecular electrostatic potential (MESP) map calculation reveals that the regions of high negative electrostatic potential of the zincphthalocyanine (the four N atoms, Figure 11(a) & 11(b)) are facing the regions of strong positive electrostatic potential of the PyC60 molecule (Figure 11(c), the centers of hexagons and pentagons) and vice versa (6:6 bond is high negative electrostatic potential and Zn is high positive electrostatic potential; Figure 11(d) & 11(e) for PyC60-1 and PyC60-2 complexes, respectively).
Figure 13. (a) Triplettriplet absorption spectra of PyC60 in toluene obtained after 1, 10, 20, and 30 μs time intervals and (b) decay profile of PyC60 molecule recorded in toluene at 700 nm after 20 μs delay time.
Figure 14. PyC60 sensitized triplettriplet absorption spectral features of 1 in (a) increasing and (b) decreasing order and (c) decay plot of same system observed at 470 nm wavelength.
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3.11. Transient Absorption Studies. The steady state UVvisible spectra of PyC60 and all the ZnPc, namely, 1, 2, and 3, are already shown in Figure 2. Figure 2 reveals that 1, 2 and 3 do not have any appreciable absorption intensity at 532 nm. For this reason, we have predominantly excited PyC60 molecule by 532 nm laser light in our present investigations. Triplettriplet absorption spectra of various ZnPc derivatives, namely, 1, 2, and 3, are shown in Figure 12(a) after 1 μs. Figure 12(b) demonstrates the corresponding decay curve of 1, 2, and 3 monitored at 470, 520, and 630 nm, respectively. The triplet state decay time of the ZnPc derivatives are tabulated in Table 3. Triplettriplet absorption spectral feature of PyC60 in toluene is depicted in Figure 13(a). Triplet state decay of PyC60 molecule monitored at 700 nm is observed in Figure 13(b). Decay is found to be monoexponential and decay time is estimated to be 20 μs. Transient absorption spectra obtained by 532 nm laser light exposure on PyC60 in the presence of 1 in toluene are shown in Figure 15(a) & 15(b). The absorption band appears at 470 nm is attributed to the formation of T PyC60*. However, we fail to detect any absorption bands due to the formation of 1•+ and PyC60•. From the above observations, we may infer that photo induced energy transfer phenomenon via T PyC60* from 1 is confirmed in our present investigations. Figure 14(c) indicates the time profile decay plot at 470 nm. From Figure 14(c) it is clear that the negative absorbance at shorter wavelength region (in comparison to 470 nm) may be due to the depletion of 1, because the decay of TPyC60* is accelerated on addition of 1. Because the decay of TPyC60* is accelerated on addition of 1, this is clearly indicative of the fact that reaction other than electron transfer is taking place for our presently investigated PyC60-1 complex. The decay time at 470 and 700 nm are estimated to be 50 and 8 μs, respectively, while the rise time at 470 nm is determined to be 5 μs (Figure 15(a)). From Figure 15(b), the daughtermother relationship in energy transfer is clearly established. Figures 7S(a) and 7S(b) exhibit the PyC60 sensitized triplettriplet absorption spectra of molecule 3 in rising and decay mode, respectively. The absorption band appears at 505 nm is ascribed to be of T3*.65 The fast rise in the absorption intensity at 505 nm is attributed to be the absorption band of TPyC60* having considerable absorption intensity in this region. Contribution from the direct excitation of 3 to the formation of T3* is small; the initial absorbance of 0.01 due to the formation of T3* is observed in the absence of PyC60 after 1 μs. The decay time profile plot of PyC60 sensitized triplet triplet absorption spectra of 3 monitored at 520 nm in toluene is demonstrated inFigure 7S(c). The decay times at 520 and 730 nm are estimated to be 80 and 10 μs, respectively, while the rise time at 520 nm is determined to be 8.5 μs (Figure 8S(a)).
Figure 15. (a) PyC60 sensitized triplettriplet absorption spectrum of 1 obtained after 1 and 15 μs delay time and (b) daughtermother relationship feature of PyC60-1 system with decay at 470 and 700 nm. 9937
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Figure 8S(b) shows the decay time profile plot of the PyC60 sensitized triplettriplet absorption spectra of 3 in toluene after 1 and 15 μs delay time. Similar spectral features are observed for the PyC60-2 system, which are depicted in Figure 7S. For the PyC60-2 system, decay is monitored at 630 nm. Rise time and decay time for this particular system are determined to be 3 and 40 μs, respectively. Considering all the above findings, the observed reactions may be illustrated as Scheme 1 for PyC60-1, PyC60-2, and PyC60-3 systems. Thus, in toluene, energy transfers from TPyC60* to ZnPc derivatives, like 1, 2, and 3, take place without electron transfer.
4. CONCLUSIONS Various important findings, as discussed in sections 3.1—3.11, may be summarized as follows: For the first time, a fullerene derivative having possible application in organic photovoltaics, for example, PyC60, is shown to form ground state CT interaction with a series of zincphthalocyanines, namely, 1, 2, and 3, in toluene; degrees of CT indicate low percentage of charge transfer in the ground state. Vertical electron affinity value of PyC60 is determined in solution. Magnitude of degrees of CT reveals that only 0.79 to 0.85% electron transfer takes place and it suggest that the PyC60-ZnPc complexes are of neutral character in ground state. Utilizing the CT transition energies, various important physicochemical parameters like degrees of CT, oscillator strength, transition dipole moment and resonance energies are estimated. Efficient quenching of fluorescence intensity of all the ZnPc derivatives in presence of PyC60 takes place in our present investigations. Binding constants of the PyC60-ZnPc complexes are estimated from the fluorescence quenching experiment; the increase in magnitude of binding constant led to increase in the fluorescence quenching efficiency. Lifetime measurements of ZnPc derivatives in the absence and presence of PyC60 establish the presence of static quenching mechanism. PM3 calculations well reproduce the geometry and binding pattern of PyC60 toward 1 and 2 in vacuo during the formation of PyC60-ZnPc supramolecular complexes. Photoinduced energy transfer via TPyC60* from 1, 2, and 3 is confirmed by observing the transient absorption spectra in toluene. Finally, we may infer that the foregoing spectroscopic and theoretical studies on PyC60-ZnPc model systems may be of immense interest for interpreting various photophysical and physicochemical parameters of PyC60-ZnPc hybrid systems. ’ ASSOCIATED CONTENT
bS
Supporting Information. Vertical ionization potential of 1, 2, and 3, the steady state fluorescence spectral changes of 2 and 3 upon addition of PyC60 solution, BH fluorescence plots for PyC60-2 and PyC60-3 systems estimated in toluene, timeresolved fluorescence decay profile for uncomplexed 2 and PyC60 + 2 recorded in toluene medium, time-resolved fluorescence decay profile for uncomplexed 3 and PyC60 + 3 recorded in toluene medium, LUMOs of PyC60-1 complex at different electronic states obtained by semiempirical/PM3 calculations, various electronic states of PyC60-2 complex in terms of HOMO
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and HOMO n, where n = 16 obtained by semiempirical/PM3 calculations, various electronic states of PyC60-2 complex in terms of LUMO and LUMO + n, where n = 16 obtained by semiempirical/PM3 calculations, PyC60 sensitized triplet triplet absorption spectral features of 3 in increasing and decreasing order, decay plot of PyC60-3 system observed at 520 nm wavelength, PyC60 sensitized triplettriplet absorption spectrum of 3 obtained after 1 and 15 μs delay time, daughter mother relationship feature of PyC60-1 system with decay at 520 and 730 nm, PyC60 sensitized triplet-triplet absorption spectral feature of 2 in increasing and decreasing mode and decay plot of PyC60-2 system at 630 nm are given as Table 1S and Figures 1S9S, respectively. Table 1S and Figures 1S9S are provided. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Fax: +91-342-2530452. E-mail: sum_9974@rediffmail.com.
’ ACKNOWLEDGMENT A.R. thanks The University of Burdwan, Burdwan, India, for providing a senior research fellowship to her. Financial assistance provided by the Department of Science and Technology, New Delhi, through the FAST TRACK Project of Ref. No. SR/FTP/ CS-22/2007 is also gratefully acknowledged. The authors also wish to record their sincere gratitude to Prof. Anunay Samanta, School of Chemistry, University of Hyderabad, India, for his helpful co-operations in this work. ’ REFERENCES (1) D’Souza, F.; Chitta, R.; Gadde, S.; Islam, D.-M. S.; Schumacher, A. L.; Zandler, M. E.; Araki, Y.; Ito, O. J. Phys. Chem. B 2006, 110, 25240–25250. (2) D’Souza, F.; Chitta, R.; Ohkubo, K.; Tasior, M.; Subbaiyan, N. K.; Zandler, M. E.; Rogacki, M. K.; Gryko, D. T.; Fukuzumi, S. J. Am. Chem. Soc. 2008, 130, 14263–14272. (3) Yasuyuki, A; Ito, O. J. Photochem. Photobiol., C 2008, 9, 93–110. (4) Gust, D.; Moore, T. A.; Moore, A. L. J. Photochem. Photobiol., B 2000, 58, 63–71. (5) Lemmetyinen, H.; Tkachenko, N. V.; Efimov, A.; Niemi, M. Phys. Chem. Chem. Phys. 2011, 13, 397–412. (6) (a) Gust, D.; Moore, T. A. In The Porphyrin Handbook; Kadish, K. M., Smith, K., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 8, pp 153190. (b) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40–48. (c) Gust, D.; Moore, T. A.; Moore, A. L. Chem. Commun. 2006, 1169–1178. (d) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2009, 42, 1890–1898. (7) (a) Wasielewski, M. R. Chem. Rev. 1992, 92, 435–461. (b) Lewis, F. D.; Letsinger, R. L.; Wasielewski, M. R. Acc. Chem. Res. 2001, 34, 159–170. (c) Lewis, N. S. Science 2007, 315, 798–801. (d) Wasielewski, M. R. Acc. Chem. Res. 2009, 42, 1910–1921. (8) (a) Osuga, A.; Mataga, N.; Okada, T. Pure Appl. Chem. 1997, 69, 797–802. (b) Bolton, J. R. In Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part D, pp 303393. (c) Kurreck, H.; Huber, M. Angew. Chem., Int. Ed. 1995, 34, 849–866. (d) Verhoeven, J. W. Adv. Chem. Phys. 1999, 106, 603–644. (e) Flamigni, L.; Barigelletti, F.; Armaroli, N.; Collin, J.-P.; Dixon, I. M.; Sauvage, J.-P.; Williams, J. A. G. Coord. Chem. Rev. 1999, 190192, 671–682. (f) Diederich, F.; Gomez-Lopez, M. Chem. Rev. Soc. 1999, 28, 263–277. (g) Verhoeven, J. W. J. Photochem. Photobiol., C 2006, 7, 40–60. 9938
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