- Email: [email protected]
Photophysical insights behind zinc naphthalocyanine-gold nanoparticle interaction and its effect over supramolecular interaction between zinc napthalocyanine and PyC60 in solution
Journal of Molecular Liquids 232 (2017) 188–194
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Photophysical insights behind zinc naphthalocyanine-gold nanoparticle interaction and its effect over supramolecular interaction between zinc napthalocyanine and PyC60 in solution Anamika Ray, Sumanta Bhattacharya ⁎ Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, India
a r t i c l e
i n f o
Article history: Received 16 November 2016 Received in revised form 31 January 2017 Accepted 2 February 2017 Available online 4 February 2017 Keywords: Zinc naphthalocyanine AuNp Energy transfer Surface binding PyC60 Binding constant
a b s t r a c t This work envisages an efficient energy transfer phenomenon from zinc naphthalocyanine (1) to dodecanethiol functionalized gold nanoparticles (AuNp) having size of 3–6 nm in toluene. Significant enhancement in singlet energy transfer efficiency is observed for 1-AuNp conjugate. Role of AuNp over the non-covalent interaction between 1 and C60 pyrrolidine tris-acid ethyl ester (PyC60) is studied by various spectroscopic tools; considerable reduction in the magnitude of binding constant for 1-PyC60 supramolecule is observed in presence of AuNp. Dynamic light scattering, atomic force microscope and scanning tunneling microscope measurements give clear evidence in favour of the surface binding of 1 to AuNp. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The concept of non-covalent interaction in forming supramolecular structure is promising for having a large impact on nanoscience if adequate nanoscale manipulation methods are used. Gold nanoparticles (AuNp) have unusual optical, electronic and chemical properties, for which researchers are seeking to put to use in a range of new technologies, starting from nanoelectronics to biomedical applications [1,2]. The physical properties like stability and size distribution of AuNp are determined by stabilization mechanism through interacting with organic molecules. In this connection, many different kinds of chromophores have been assembled on surfaces of AuNp, including pyrene [3], porphyrin [4], and fullerene [5]. Among porphyrinoids, phthalocyanines (Pc) have been used in a range of fields not only due to their unique properties such as thermal and chemical stability, high degree of aromaticity, photophysical, photochemical, redox and coordination properties but also because of their tunable electronic character and designable structures [6,7]. On the other hand, fullerenes are also employed as suitable building blocks for the development of multicomponent systems because of their three dimensional structure [8]. As a result of this, combination of fullerene and Pc can be a promising candidate for possible
⁎ Corresponding author. E-mail address: [email protected] (S. Bhattacharya).
http://dx.doi.org/10.1016/j.molliq.2017.02.009 0167-7322/© 2017 Elsevier B.V. All rights reserved.
application in optoelectronics and in organic solar cells [9]. An interesting aspect of the chemistry of fullerenes and Pc's is that they undergo spontaneous self-assembly phenomenon with each other as a result of ground state complexation in solution [10,11]. Accordingly, both covalent and non-covalently connected Pc-fullerene dyads and triads have been designed and studied to understand the mechanistic details behind construction of the artificial photosynthetic reaction centre [12– 14]. In a Pc-fullerene dyad, the role of the Pc is dual: first it is to function as an antenna and the second to act as a donor molecule once photo-excited. Among Pc, zinc naphthalocyanine (ZnNc) has good solubility in many organic solvents and an intense absorption band in near infrared region, in addition to the possible tuning of its optical [15] and electronic properties [16]. For the above mentioned reasons, zinc 2,11,20,29tetra-tert-butyl-2,3-naphthalocyanine (1, Scheme 1) is chosen as a donor molecule for this work. Although 1 possesses excellent electron-donating properties compared to porphyrins and Pc, the studies of ZnNc-based dyads and triads are rare [17–19]. Moreover, there is no such investigation on non-covalent interaction between fullerene and ZnNc in presence of metal nanoparticles in solution, although there are some reports on interaction between fullerene and Pc in presence of Ag- and AuNp in recent past [20,21]. The objective of the present research work is to envisage electronic interaction between 1 and AuNp and to utilize self-assembly protocols to monitor 1-fullerene complexation in presence of AuNp in solution. The motivation behind selecting the PyC60 (Scheme 1) molecule as electron acceptor in our present
A. Ray, S. Bhattacharya / Journal of Molecular Liquids 232 (2017) 188–194
189
Scheme 1. Structures of (a) PyC60 and (b) 1.
studies comes from the work of 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 [22].
software. DLS measurements have been done with Malvern Zeta Seizer instrument of Model No. NANOZS90. All the scattered photons are collected at 90° scattering angle.
2. Materials and methods
3. Results and discussions
1 is purchased from Aldrich, USA having product no. 432210. The acceptor, namely, PyC60, is collected from Aldrich, USA having product no. 709093. The Gold nanoparticles, namely, AuNp, is procured from Sigma (Catalogue no. 54349). UV–Vis spectroscopic grade toluene (Merck, Germany) has been used as solvent to favour non-covalent interaction between fullerene and 1 and, at the same time, to ensure good solubility and photo-stability of the samples. UV–Vis spectral measurements are performed on a Shimadzu UV-2450 model spectrophotometer using quartz cell with 1 cm optical path length. Emission spectra have been recorded with a Hitachi F-4500 model spectrofluorimeter. Fluorescence decay curves are measured with a HORIBA Jobin Yvon Single Photon Counting Setup employing Nanoled as excitation source. Theoretical calculations are performed using SPARTAN’14 Windows version
3.1. UV–Vis absorption studies Absorption spectrophotometric investigations fingerprint new photophysical insight when AuNp solution is added to 1 in toluene. A new broad absorption band is observed around 400 nm (Fig. 1). Redshifted broad absorption bands have been already reported for donor (porphyrin)-acceptor systems [23]. It should be mentioned at this point that AuNp is coated with dodecanethiol and may contain stabilizer which may play role in the observed modification of 1 spectrum. Interesting feature is noted in case of Q-absorption band of 1 resulted from S1 ← S0 transition; significant blue shift (~7 nm) in Q absorption band of 1 (from 769 to 762 nm) in presence of AuNp clearly hints possibility of energy transfer from 1 to AuNp in toluene. All of the above
Fig. 1. UV–Vis spectral feature of 1 (4.75 × 10−6 mol·dm−3, green colour line) in presence of AuNp (red colour line), PyC60 (3.30 × 10−5 mol·dm−3, blue colour line) and PyC60 (3.30 × 10−5 mol·dm−3) + AuNp mixture (brown colour line) recorded in toluene at 298 K against the solvent as reference. In inset of this figure, the shift in Q absorption band along with the absorption spectrum of AuNp (brown colour line) is clearly demonstrated.
190
A. Ray, S. Bhattacharya / Journal of Molecular Liquids 232 (2017) 188–194
Table 1 Binding constants (K, dm3·mol−1) estimated by UV–Vis and steady state fluorescence measurements for 1-PyC60 complex in absence and presence of AuNp recorded in toluene at 298 K. System
1-PyC60 in absence of AuNp 1-PyC60 in presence of AuNp
K, dm3·mol−1 UV–vis
Fluorescence
24,025 3410
27,270 5260
findings prove that AuNp acts as a very good energy storage material in presence of 1. 1 undergoes spontaneous self-assembly phenomenon with PyC60 in toluene as evidenced from UV–Vis titration experiment (Fig. 2). Detection of isobestic point at 672 nm gives very good evidence in favour of 1:1 complexation between 1 and PyC60. The Q absorption band of 1 suffers gradual decrease in presence of increasing concentration of PyC60 (inset of Fig. 2). The binding constant (K) of 1-PyC60 system is estimated to be 24,025 dm3·mol−1 (Table 1) obtained from Scatchard plot (inset of Fig. 2) [18]. New photophysical insights come out when the UV–vis experiment is performed in presence of AuNp. It is observed upon addition of AuNp to the solution 1, the absorption maximum (λmax) of 1 observed at 769 nm (green colour line, Fig. 1) suffers 8 nm blue shift, i.e., λmax = 762 nm (red colour line, Fig. 1). Moreover, a broad absorption peak is observed at 397 nm due to possible energy transfer from 1 to AuNp. In presence of PyC60, although no such shift in λmax is observed, absorbance value of 1 is reduced to 0.890 (blue colour line, Fig. 1). This phenomenon certainly proves that binding mechanism is the only path behind decrease in the absorbance value of 1. When both PyC60 and AuNp are present in the mixture, reduction in the absorbance value of 1 takes place associated with the blue shift of λmax (brown colour line, Fig. 1). Although there is large peak shift observed for the 1-PyC60 mixture in presence of AuNp (~8 nm) due to possible energy transfer from 1 to AuNp, still we have been able to measure the value of K for 1-PyC60 system in presence of AuNp with the help of Scatchard plot (inset of Fig. 3). Very good isobestic point is observed at 660 nm (Fig. 3); the large blue shift in the isobestic point (i.e., 12 nm) gives clear indication in favour of inhibition in binding between PyC60 and 1 in presence of AuNp. Considerable reduction in the value of K for 1-PyC60 system (see Table 1) in presence of AuNp is marked by high value of selectivity of binding, i.e., K(1-PyC60)/K(1-PyC60-AuNp) = ~ 5.9. UV–vis investigations, therefore, may be treated as fingerprint observation in favour of inhibition in binding between 1 and PyC60 in presence of AuNp. Noteworthy is that, when 1 undergoes complexation with 2-(4′-imidazolylphenyl)-fulleropyrrolidine [18], it exhibits ~2.58 times higher magnitude of K compared to 1-PyC60 system (see Table 1).
Kav, dm3·mol−1
Selectivity in K in presence of AuNp
25,645 4335
~5.9
3.2. Steady state fluorescence studies To study the photo-induced behavior of 1 in absence and presence of AuNp and PyC60, steady-state fluorescence measurements are carried out in toluene. Fluorescence measurements would also reveal the effect of molecular structure on molecular and plasmonic resonances in 1AuNp interaction. Fig. 4 shows the fluorescence spectrum of 1 in absence and presence of AuNp, PyC60 and AuNp + PyC60 mixture. It is clearly seen from Fig. 4 that the fluorescence intensity of uncomplexed 1 suffers sharp quenching in presence of AuNp compared to PyC60; moreover, the fluorescence peak maximum of 1 suffers 2 nm blue shift in presence of AuNp. The fluorescence quenching of 1 in the presence of AuNp may be attributed to energy transfer from photoexcited 1 to AuNp. As expected, the fluorescence intensity of 1 gets maximum quenching when both AuNp and PyC60 are present in the mixture. This energy transfer may be ascribed as surface energy transfer as the photoexcited 1 transfers its energy to AuNp, upon relaxation back to the ground state. This phenomenon generally occurs for AuNp having small diameters [25] and that are larger than for Förster resonance energy transfer [26]. It is also possible that slight aggregation observed for 1 when conjugated to AuNp can result in self-quenching behind decrement in the fluorescence intensity. The prodigious photophysical characteristic observed for metallated naphthalocyanine towards AuNp and its effect over non-covalent binding between 1 and PyC60 in presence of AuNp is further explored by steady state fluorescence titrations. In fluorescence titration, the concentration of 1 is kept fixed at a certain concentration and varying the concentration of PyC60. It has been observed that upon excitation at 340 nm absorption band (S1 ← S0) maxima of 1, a PyC60 concentration dependent decrease in the intensity of the fluorescence maxima of 1 is seen in absence and presence of AuNp as shown in Figs. 1S & 2S, respectively. The decrease in fluorescence intensity of 1 in presence of PyC60 suggests photo-excited decay of the excited singlet state of ZnNc by the addition of electronaccepting fullerene; like UV–Vis investigations, a sharp decrease in the value of K is observed (Table 1). In absence of AuNp, an excellent liner
Fig. 2. UV–Vis spectral variation of 1 (4.20 × 10−6 mol·dm−3) in presence of PyC60 (5.95 × 10−6 to 2.35 × 10−5 mol·dm−3) recorded in toluene at 298 K. The inset of this figure indicates decrease in the absorbance value of the Q-absorption band of 1 in presence of PyC60 along with Scatchard plot of 1-PyC60 system for the estimation of K.
A. Ray, S. Bhattacharya / Journal of Molecular Liquids 232 (2017) 188–194
191
Fig. 3. UV–Vis spectral variation of 1 (4.75 × 10−6 mol·dm−3) in presence of PyC60 (3.15 × 10−6 to 3.30 × 10−5 mol·dm−3) and AuNp recorded in toluene at 298 K. The inset of this figure indicates decrease in the absorbance value of 1-AuNp mixture in presence of PyC60 at 760 nm and Scatchard plot of 1-PyC60 system in presence of AuNp for the estimation of K.
correlation is obtained as evidenced from fluorescence BH plot [24] (inset of Fig. 1S). However, in presence of AuNp, a non-linear curve fitting is associated from which the value of K is determined (Table 1, inset of Fig. 2S). The striking feature of the present investigations is the average selectivity in binding in absence and presence of AuNp which is estimated to be ~5.9 (Table 1). To prove that only binding is associated between 1 and PyC60 not the energy transfer phenomenon, steady state fluorescence experiment is performed at the excitation maximum of PyC60, i.e., 430 nm (resulting from forbidden singlet-singlet transition). It is observed that the fluorescence intensity of PyC60 remains same even in presence of 1 (Fig. 5). This observation supports our rationale behind binding mechanism between 1 and PyC60 rather than energy transfer.
understanding of the surface energy transfer (SET) mechanism between an excited state molecule and a nearby metal particle becomes a field of active research for various applications [28,29]. In a metal nanoparticle, the free conduction electrons of the metal passes near and/perpendicular to the metal surface and as a result of this, they strongly interact with the donor molecules [30]. Therefore, in comparison with the R−6 distance-dependent fluorescence resonance energy transfer process (FRET), the SET process depends on the R−4 distance from the molecule to the surface of the metal, where spectral overlap between donor emission and the acceptor absorption spectrum is not required; here R defines the distance which correlates the energy transfer efficiency. Considering the Fermi Golden rule approximation, the energy transfer rate (kET) is related to the square dependence of the excitonic coupling. The SET rate is given by the following expression (Eq. (1))
3.3. Evidence of energy transfer in present work It is generally believed that metal nanoparticles strongly quench the singlet-excited states of chromophores when attached to nanoparticle surfaces, through an energy-transfer mechanism [27]. Recently, the
4
K SET ¼ ð1=τD Þ ðd0 =dÞ
ð1Þ
where the lifetime of the donor (here 1) in the absence of an acceptor molecule is given by τD, and the distance between 1 and the acceptor (here AuNp) molecule is marked by d. The distance d0, is calculated
Fig. 4. Steady state fluorescence spectral variation of 1 (4.15 × 10−6 mol·dm−3, blue colour line) in presence of AuNp (orange colour line), PyC60 (5.80 × 10−5 mol·dm−3, magenta colour line) and AuNp + PyC60 mixture (5.80 × 10−5 mol·dm−3, green colour line) recorded in toluene at 298 K. λex = 685 nm; λem = 776 nm.
Fig. 5. Steady state fluorescence spectral variation of PyC60 (4.55 × 10−5 mol·dm−3, black colour line) in presence of 1 (4.75 × 10−6 mol·dm−3, blue colour line) recorded in toluene at 298 K. λex = λex = 430 nm; λem = 705 nm.
192
A. Ray, S. Bhattacharya / Journal of Molecular Liquids 232 (2017) 188–194 Table 3 Values of quantum yield (Φf), radiative rate constant (kr) and non-radiative rate constant (knr) of 1 in absence and presence of AuNp, PyC60 and PyC60-AuNp systems; Temp. 298 K.⁎, ⁎⁎ System
Φf
⁎kr, s−1
1 1-AuNp 1-PyC60 1-PyC60-AuNp
0.03 3.62 × 10−3 4.10 × 10−3 3.30 × 10−3
1.15 2.40 1.58 2.32
× × × ×
⁎⁎knr, s−1 107 106 106 106
3.65 6.63 3.85 7.12
× × × ×
108 108 108 108
⁎ kr = Φf/τs. ⁎⁎ knr = (1/τs) − Φf/τs = (1/τs) − kr.
Fig. 6. Time-resolved fluorescence decay profile of 1 (6.80 × 10−6 mol·dm−3, blue colour line) in presence of (b) AuNp (orange colour line), (c) PyC60 (4.90 × 10−5 mol·dm−3, magenta colour line) and (d) PyC60 (4.90 × 10−5 mol·dm−3) + AuNp mixture (green colour line) recorded in toluene at 298 K.
of PyC60 + AuNp mixture in solution. Utilizing the value of Φf, we have been able to determine the value of radiative (kr) and non-radiative (knr) rate-constants of 1, 1-AuNp, 1-PyC60 and 1-PyC60-AuNp systems (Table 3). Table 3 clearly demonstrates while there is a marked reduction in the value of kr for 1 in presence of AuNp, viz., ~ 4.8 times, the rate of knr for 1-AuNp system gets ~ 1.8 times enhancement in its value compared to uncomplexed 1. The trend observed for the estimation of Φf of 1 (as discussed earlier) in presence of PyC60 and PyC60 + AuNp mixture maintains very good correlation with the estimated value of kr and knr. These findings certainly prove that non-radiative pathway plays predominant role for the deactivation of the excited singlet state of 1 in presence of AuNp.
using the model given by Eq. (2) 1=4 d0 ¼ 0:225 c3 Φdye =ωdye 2 ω F κ F
ð2Þ
where d0 is the distance at which the dye transfers energy and emits spontaneously, Φdye indicates the quantum efficiency of the dye, ωdye is given as the frequency of the donor electronic transition, ωF is the Fermi frequency, and finally, the Fermi wave vector is given by κF [30]. Using the value of ω = 3.6 × 1015 s−1, ωF = 8.4 × 1015 s−1, κF = 1.2 × 108 cm−1, Φdye = 0.03, and c = 3 × 108 cm−1, the d0 value is estimated to be 240 Å. The value of d0 clearly proves that energy transfer takes place at a much larger distance compared to radiative rate of energy transfer which actually operates at very close distances (i.e., b10 Å) [28,29]. The general form of the quantum efficiency of energy transfer according to SET theory is given by Eq. (3) n o ΦET ¼ 1= 1 þ ðr=r0 Þ4
3.4. Time-resolved fluorescence investigations Apart from steady state fluorescence measurements, we have performed detailed nanosecond time-resolved fluorescence experiment for the investigated supramolecule in absence and presence of AuNp. 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 (τs) of 1 is measured to be 2.65 ns (Fig. 6). The time-resolved fluorescence experiment, therefore, suggests presence of only one species in
ð3Þ
As our rationale is validated according to SET theory, there is no question of getting any overlap between the fluorescence band of 1 and the absorption band of AuNP that we normally observe for FRET. From above discussions, we may surmise that possibility of electron transfer from 1 to AuNp is excluded in our present work due to SET phenomenon. Formidable support in favour of energy transfer also comes from the measurement of quantum yield (Φf) of 1 in absence and presence of AuNp, PyC60 and mixture of PyC60 and AuNp. Utilizing the value of lifetime of the excited singlet state, i.e., τs (see Table 2), it is observed that the Φf value of uncomplexed 1 (i.e., 0.03) suffers considerable reduction in presence of AuNp (Table 3). The extent of reduction is found to be larger in magnitude compared to when only PyC60 is present in solution. The highest decrease in the value of Φf of 1, however, is observed in case
Table 2 τs, ksEnt and φsEnt of 1 and its complexes with PyC60 in absence and presence of AuNp recorded in toluene at 298 K. System
τs, ns
ksEnt, s−1
φsEnt
1 1-AuNp 1-PyC60 1-PyC60-AuNp
2.65 1.50 2.58 1.42
– 2.90 × 108 10.2 × 106 3.27 × 108
– 0.435 0.026 0.464
Fig. 7. DFT/B3LYP/6-31G* optimized geometric structure of 1-PyC60 complex done in vacuo.
A. Ray, S. Bhattacharya / Journal of Molecular Liquids 232 (2017) 188–194
193
Fig. 8. DFT (B3LYP/6-31G*) calculated frontier (a) HOMO and (b) LUMO for the 1-PyC60 complex done in vacuo. The calculations are done using SPARTAN '14 software.
solution. In presence of AuNp, 1-AuNp conjugate shows a bi-exponential decay (Fig. 6) where the first component (with τs1 of 2.187 ns) may be due to the presence of free 1 and the second component (with τs2 of 0.919 ns) is due to the conjugation of the nanoparticles to 1. In contrary to AuNp, PyC60 exhibits a mono-exponential decay suggesting that 1 attached with PyC60 through non-covalent linkage is quenched to the point where it is not fluorescent (Fig. 6). This phenomenon is very much consistent with the trend in τs value of free-base Pc in presence of silver nanoparticles [21] and for Pc-AuNp conjugate reported by Nyokong et al. [25]. Conjugation of 1 with AuNp in presence of PyC60 (Fig. 6), reduces the τs value of 1 to 2.15 and 0.766 ns for 1st and 2nd component, respectively, compared to its original value reported earlier.
Taking into consideration of all the components, we have estimated the τs value for bi-exponential fit [26]. Utilizing the value of τs, singlet energy-transfer rate constants (ksEnt) and quantum yield of energy transfer efficiency (φsEnt) [31] have been enumerated and listed in Table 2. The notable feature is that, 1-PyC60 complex exhibits facile energy transfer efficiency in presence of AuNp as evidenced from large value of φsEnt, i.e., 0.464. The lifetime experiment, also, evokes that 1-PyC60 system exhibits considerable amount of ksEnt in presence of AuNp, i.e., 3.27 × 108 s−1, while it exhibits value of only 10.2 × 106 s−1 in absence of AuNp. Remarkable enhancement in both ksEnt and φsEnt values for 1PyC60 system in presence of AuNp compared to 1-PyC60 in absence of AuNp, i.e., ~32 and 18 times, respectively, provide formidable support in favour of strong electrostatic interaction between 1 and PyC60 in presence of AuNp. These two physicochemical parameters nicely demonstrate that energy transfer is the only path behind quenching of fluorescence of 1 in presence of AuNp. This is because of the fact that order of ksEnt lies in the range of 108 s−1 for 1-AuNp system which is proved to be much below for a process exhibiting electron transfer phenomenon, viz., 1012 s−1 The order of ksEnt strongly supports our rationale that energy transfer is the only path in present case behind deactivation of the photo-excited singlet state of 1 in presence of AuNp [32]. 3.5. Theoretical calculations
Fig. 9. MEP of 1-PyC60 complex done by DFT/B3LYP/6-31G* calculations in vacuo.
In our present work, a detailed conformational analysis of the individual components as well as the 1-PyC60 complex has been performed in vacuo by DFT/B3LYP/6-31G* calculations. The geometric parameters of the complexes are obtained after complete energy minimization. The strong binding of PyC60 in the cleft of 1 (Fig. 7) is reflected in terms of negative value of heat of formation for this complex estimated in vacuo, i.e., −3.87 kcal·mol−1. The frontier highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) for the investigated supramolecule have been obtained by DFT method for 1-PyC60 system. It is also observed that, HOMO of both 1-PyC60 (− 4.3483 eV) complex corroborates fairly well with that of uncomplexed 1 (−4.3414 eV) while LUMO of this complex, i.e., −3.0314 eV, remains very close to that of PyC60 (−3.1501 eV). The small HOMO-LUMO gap (i.e., 1.3169 eV) gives formidable support in favour of strong electronic interaction between 1 and PyC60. The HOMO and LUMO pictures of 1PyC60 complex are provided in Fig. 8. Molecular electrostatic potential (MEP) maps have also been generated for the investigated supramolecule in present work with the help of DFT calculations and visualized in Fig. 9; MEP provides strong indication in favour of electronic
194
A. Ray, S. Bhattacharya / Journal of Molecular Liquids 232 (2017) 188–194
redistribution in donor (here 1) and acceptor (here PyC60) during noncovalent interaction. It may be mentioned here that validity of molecular orbitals generated by density functional methods is already being recognized [33]. The accuracy of these methods, especially B3LYP, is being well demonstrated by D'Souza [34] et al. and, more recently, on covalently linked C60-Znporphyrin system [12] and fullerene-based photoactive layers for the construction of heteroconjugation solar cells [35]. 3.6. DLS, STM and AFM studies To gain further insight into the intermolecular interaction between 1 and AuNp, we have performed various physical characterization methods like dynamic light scattering (DLS), scanning tunneling microscope (STM) and atomic force microscope (AFM) measurements. It is observed that the addition of AuNp to the solution of 1 increases the size of the nanoparticles in the range of 50–60 nm. All the above findings give clear evidence in favour of the surface binding of 1 to AuNp. This phenomenon also gives strong support in favour of decrease in the value of K for 1-PyC60 complex measured in toluene in presence of AuNp. The photographs obtained from DLS, STM and AFM measurements are depicted in Figs. 3S(a), (b) & (c), respectively. This may be the first example to track the occurrence of complexation between a metallated naphthalocyanine and functionalized fullerene in presence of nanoparticles by various particle size and surface binding analysis. 4. Conclusions In conclusion, it is observed that 1-AuNp conjugate emerges as an illustrious example for exhibiting energy transfer phenomenon in solution. The binding of PyC60 with 1 is affected greatly in presence of AuNp. This study proves that self-assembled 1-AuNp-PyC60 conjugate provides very good perspective for the development of efficient photochromic devices in near future. Acknowledgement AR thanks CSIR, New Delhi for providing Research Associateship to her. Grant-In-Aid through Research Project of Sanction No. 01(2711)/ 13/EMR-II is also greatly acknowledged. Appendix A. Supplementary data Steady state fluorescence titration of 1-PyC60 system in absence and presence of AuNp estimated in toluene, DLS, STM and AFM photographs of 1-AuNp system are given as Figs. 1S–3S, respectively. Figs. 1S–3S are provided as supporting information. Supporting information associated
with this paper is freely available through Internet. Supplementary data associated with this article can be found in the online version, at doi: 10. 1016/j.molliq.2017.02.009.
References [1] T.G.W. Edwardson, K.L. Lau, D. Bousmail, C.J. Serpell, H.F. Sleiman, Nat. Chem. 8 (2016) 162–170. [2] S.C. Bera, K. Sanyal, D. Senapati, P.P. Mishra, J. Phys. Chem. B 120 (2016) 4213–4220. [3] B.I. Ipe, K.G. Thomas, S. Barrazzouk, J. Phys. Chem. B 106 (2002) 18–21. [4] J. Ohyama, Y. Hitomi, Y. Higuchi, M. Shinagawa, H. Mukai, M. Kodera, K. Teramura, T. Shishido, T. Tanaka, Chem. Commun. (2008) 6300–6302. [5] V. Amendola, G. Mattei, C. Cusan, M. Prato, M. Meneghetti, Synth. Met. 155 (2005) 283–286. [6] F. D'Souza, O. Ito, Chem. Commun. (2009) 4913–4928. [7] G. Bottari, G.D. de la Torre, D.M. Guldi, T. Torres, Chem. Rev. 110 (2010) 6768–6816. [8] R. Taylor, R. Lecture Notes on Fullerene Chemistry: A Handbook for Chemists, Imperial College Press, London, 1999. [9] M.–.E. Ragoussi, T. Torre, Chem. Asian J. 9 (2014) 2676–2707. [10] R.F. Enes, J.–.J. Cid, O. Trukhina, A. Hausmann, A. Gouloumis, P. Vázquez, J.A.S. Cavaleiro, A.C. Tomé, D.M. Guldi, T. Torres, Chem. –Eur. J. 18 (2012) 1727–1736. [11] C.B. KC, K. Ohkubo, P.A. Karr, S. Fukuzumi, F. D'Souza, Chem. Commun. 49 (2013) 7614–7616. [12] O. Ito, F. D'Souza, Molecules 17 (2012) 5816–5835. [13] G. Bottari, J.A. Suanzes, O. Trukhina, T. Torres, J. Phys. Chem. Lett. 2 (2011) 905–913. [14] D.M. Guldi, B.M. Illescas, C.M. Atienza, M. Wielopolski, N. Martin, Chem. Soc. Rev. 38 (2009) 1587–1597. [15] I. Dhanya, C.S. Menon, Opt. Electron. Adv. Mater. 4 (2010) 2085–2087. [16] M.E. El-Khouly, Phys. Chem. Chem. Phys. 12 (2010) 12746–12752. [17] E. Maligaspe, T. Kumulainen, H. Lemmetyinen, N.V. Tkachenko, N.K. Subbaiyan, M.E. Zandler, F. D'Souza, J. Phys. Chem. A 114 (2010) 268–277. [18] M.E. El-Khouly, L.M. Rogers, M.E. Zandler, G. Suresh, M. Fujitsuka, O. Ito, F. D'Souza, Chem. Phys. Chem. 4 (2003) 474–481. [19] N. Kobayashi, Coord. Chem. Rev. 227 (2002) 129–152. [20] A. Ray, S. Bhattacharya, Chem. Phys. Lett. 651 (2016) 66–71. [21] A. Ray, S. Bhattacharya, Chem. Phys. Lett. 639 (2015) 183–188. [22] J.L. Sessler, J. Jayawickramarajah, A. Gouloumis, G.D. Pantos, T. Torres, D.M. Guldi, Tetrahedron 62 (2006) 2123–2131. [23] H. Imahori, A. Fujimoto, S. Kang, H. Hotta, K. Yoshida, T. Umeyama, Y. Matano, S. Isoda, M. Isosomppi, N.V. Tkachenko, H. Lemmetyinen, Chem. –Eur. J. 11 (2005) 7265–7275. [24] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703–2707. [25] N. Nombona, E. Antunes, C. Litwinski, T. Nyokong, Dalton Trans. 40 (2011) 11876–11884. [26] B. Valeur, Molecular Fluorescence: Principles and Applications, Wiley-VCH, 2002. [27] S. Kundu, A. Patra, Chem. Rev. 117 (2017) 712–757. [28] R.R. Chance, A. Prock, R. Silbey, In Advances in Chemical Physics, John Wiley & Sons, Inc., 2007 [29] B.N.J. Person, N.D. Lang, Phys. Rev. B 26 (1982) 5409–5415. [30] E. Dulkeith, A.C. Morteani, T. Niedereichholz, T.A. Klar, J. Feldmann, S.A. Levi, F.C.J.M. van Veggel, D.N. Reinhoudt, M. Möller, D.I. Gittins, Phys. Rev. Lett. 89 (2002) 203002. [31] F. D'Souza, P.M. Smith, M.E. Zandler, A.L. McCarty, M. Itou, Y. Araki, O. Ito, J. Am. Chem. Soc. 126 (2004) 7898–7907. [32] A. Ray, K. Santhosh, S. Bhattacharya, J. Phys. Chem. B 116 (2012) 11979–11998. [33] R. Stowasser, R. Hoffmann, J. Am. Chem. Soc. 121 (1999) 3414–3420. [34] 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, O. Ito, J. Phys. Chem. A 106 (2002) 3243–3252. [35] Y. Li, D. Qi, P. Song, F. Ma, Materials 8 (2015) 42–56.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Photophysical insights behind zinc naphthalocyanine-gold nanoparticle interaction and its effect over supramolecular interaction between zinc napthalocyanine and PyC60 in solution Anamika Ray, Sumanta Bhattacharya ⁎ Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, India
a r t i c l e
i n f o
Article history: Received 16 November 2016 Received in revised form 31 January 2017 Accepted 2 February 2017 Available online 4 February 2017 Keywords: Zinc naphthalocyanine AuNp Energy transfer Surface binding PyC60 Binding constant
a b s t r a c t This work envisages an efficient energy transfer phenomenon from zinc naphthalocyanine (1) to dodecanethiol functionalized gold nanoparticles (AuNp) having size of 3–6 nm in toluene. Significant enhancement in singlet energy transfer efficiency is observed for 1-AuNp conjugate. Role of AuNp over the non-covalent interaction between 1 and C60 pyrrolidine tris-acid ethyl ester (PyC60) is studied by various spectroscopic tools; considerable reduction in the magnitude of binding constant for 1-PyC60 supramolecule is observed in presence of AuNp. Dynamic light scattering, atomic force microscope and scanning tunneling microscope measurements give clear evidence in favour of the surface binding of 1 to AuNp. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The concept of non-covalent interaction in forming supramolecular structure is promising for having a large impact on nanoscience if adequate nanoscale manipulation methods are used. Gold nanoparticles (AuNp) have unusual optical, electronic and chemical properties, for which researchers are seeking to put to use in a range of new technologies, starting from nanoelectronics to biomedical applications [1,2]. The physical properties like stability and size distribution of AuNp are determined by stabilization mechanism through interacting with organic molecules. In this connection, many different kinds of chromophores have been assembled on surfaces of AuNp, including pyrene [3], porphyrin [4], and fullerene [5]. Among porphyrinoids, phthalocyanines (Pc) have been used in a range of fields not only due to their unique properties such as thermal and chemical stability, high degree of aromaticity, photophysical, photochemical, redox and coordination properties but also because of their tunable electronic character and designable structures [6,7]. On the other hand, fullerenes are also employed as suitable building blocks for the development of multicomponent systems because of their three dimensional structure [8]. As a result of this, combination of fullerene and Pc can be a promising candidate for possible
⁎ Corresponding author. E-mail address: [email protected] (S. Bhattacharya).
http://dx.doi.org/10.1016/j.molliq.2017.02.009 0167-7322/© 2017 Elsevier B.V. All rights reserved.
application in optoelectronics and in organic solar cells [9]. An interesting aspect of the chemistry of fullerenes and Pc's is that they undergo spontaneous self-assembly phenomenon with each other as a result of ground state complexation in solution [10,11]. Accordingly, both covalent and non-covalently connected Pc-fullerene dyads and triads have been designed and studied to understand the mechanistic details behind construction of the artificial photosynthetic reaction centre [12– 14]. In a Pc-fullerene dyad, the role of the Pc is dual: first it is to function as an antenna and the second to act as a donor molecule once photo-excited. Among Pc, zinc naphthalocyanine (ZnNc) has good solubility in many organic solvents and an intense absorption band in near infrared region, in addition to the possible tuning of its optical [15] and electronic properties [16]. For the above mentioned reasons, zinc 2,11,20,29tetra-tert-butyl-2,3-naphthalocyanine (1, Scheme 1) is chosen as a donor molecule for this work. Although 1 possesses excellent electron-donating properties compared to porphyrins and Pc, the studies of ZnNc-based dyads and triads are rare [17–19]. Moreover, there is no such investigation on non-covalent interaction between fullerene and ZnNc in presence of metal nanoparticles in solution, although there are some reports on interaction between fullerene and Pc in presence of Ag- and AuNp in recent past [20,21]. The objective of the present research work is to envisage electronic interaction between 1 and AuNp and to utilize self-assembly protocols to monitor 1-fullerene complexation in presence of AuNp in solution. The motivation behind selecting the PyC60 (Scheme 1) molecule as electron acceptor in our present
A. Ray, S. Bhattacharya / Journal of Molecular Liquids 232 (2017) 188–194
189
Scheme 1. Structures of (a) PyC60 and (b) 1.
studies comes from the work of 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 [22].
software. DLS measurements have been done with Malvern Zeta Seizer instrument of Model No. NANOZS90. All the scattered photons are collected at 90° scattering angle.
2. Materials and methods
3. Results and discussions
1 is purchased from Aldrich, USA having product no. 432210. The acceptor, namely, PyC60, is collected from Aldrich, USA having product no. 709093. The Gold nanoparticles, namely, AuNp, is procured from Sigma (Catalogue no. 54349). UV–Vis spectroscopic grade toluene (Merck, Germany) has been used as solvent to favour non-covalent interaction between fullerene and 1 and, at the same time, to ensure good solubility and photo-stability of the samples. UV–Vis spectral measurements are performed on a Shimadzu UV-2450 model spectrophotometer using quartz cell with 1 cm optical path length. Emission spectra have been recorded with a Hitachi F-4500 model spectrofluorimeter. Fluorescence decay curves are measured with a HORIBA Jobin Yvon Single Photon Counting Setup employing Nanoled as excitation source. Theoretical calculations are performed using SPARTAN’14 Windows version
3.1. UV–Vis absorption studies Absorption spectrophotometric investigations fingerprint new photophysical insight when AuNp solution is added to 1 in toluene. A new broad absorption band is observed around 400 nm (Fig. 1). Redshifted broad absorption bands have been already reported for donor (porphyrin)-acceptor systems [23]. It should be mentioned at this point that AuNp is coated with dodecanethiol and may contain stabilizer which may play role in the observed modification of 1 spectrum. Interesting feature is noted in case of Q-absorption band of 1 resulted from S1 ← S0 transition; significant blue shift (~7 nm) in Q absorption band of 1 (from 769 to 762 nm) in presence of AuNp clearly hints possibility of energy transfer from 1 to AuNp in toluene. All of the above
Fig. 1. UV–Vis spectral feature of 1 (4.75 × 10−6 mol·dm−3, green colour line) in presence of AuNp (red colour line), PyC60 (3.30 × 10−5 mol·dm−3, blue colour line) and PyC60 (3.30 × 10−5 mol·dm−3) + AuNp mixture (brown colour line) recorded in toluene at 298 K against the solvent as reference. In inset of this figure, the shift in Q absorption band along with the absorption spectrum of AuNp (brown colour line) is clearly demonstrated.
190
A. Ray, S. Bhattacharya / Journal of Molecular Liquids 232 (2017) 188–194
Table 1 Binding constants (K, dm3·mol−1) estimated by UV–Vis and steady state fluorescence measurements for 1-PyC60 complex in absence and presence of AuNp recorded in toluene at 298 K. System
1-PyC60 in absence of AuNp 1-PyC60 in presence of AuNp
K, dm3·mol−1 UV–vis
Fluorescence
24,025 3410
27,270 5260
findings prove that AuNp acts as a very good energy storage material in presence of 1. 1 undergoes spontaneous self-assembly phenomenon with PyC60 in toluene as evidenced from UV–Vis titration experiment (Fig. 2). Detection of isobestic point at 672 nm gives very good evidence in favour of 1:1 complexation between 1 and PyC60. The Q absorption band of 1 suffers gradual decrease in presence of increasing concentration of PyC60 (inset of Fig. 2). The binding constant (K) of 1-PyC60 system is estimated to be 24,025 dm3·mol−1 (Table 1) obtained from Scatchard plot (inset of Fig. 2) [18]. New photophysical insights come out when the UV–vis experiment is performed in presence of AuNp. It is observed upon addition of AuNp to the solution 1, the absorption maximum (λmax) of 1 observed at 769 nm (green colour line, Fig. 1) suffers 8 nm blue shift, i.e., λmax = 762 nm (red colour line, Fig. 1). Moreover, a broad absorption peak is observed at 397 nm due to possible energy transfer from 1 to AuNp. In presence of PyC60, although no such shift in λmax is observed, absorbance value of 1 is reduced to 0.890 (blue colour line, Fig. 1). This phenomenon certainly proves that binding mechanism is the only path behind decrease in the absorbance value of 1. When both PyC60 and AuNp are present in the mixture, reduction in the absorbance value of 1 takes place associated with the blue shift of λmax (brown colour line, Fig. 1). Although there is large peak shift observed for the 1-PyC60 mixture in presence of AuNp (~8 nm) due to possible energy transfer from 1 to AuNp, still we have been able to measure the value of K for 1-PyC60 system in presence of AuNp with the help of Scatchard plot (inset of Fig. 3). Very good isobestic point is observed at 660 nm (Fig. 3); the large blue shift in the isobestic point (i.e., 12 nm) gives clear indication in favour of inhibition in binding between PyC60 and 1 in presence of AuNp. Considerable reduction in the value of K for 1-PyC60 system (see Table 1) in presence of AuNp is marked by high value of selectivity of binding, i.e., K(1-PyC60)/K(1-PyC60-AuNp) = ~ 5.9. UV–vis investigations, therefore, may be treated as fingerprint observation in favour of inhibition in binding between 1 and PyC60 in presence of AuNp. Noteworthy is that, when 1 undergoes complexation with 2-(4′-imidazolylphenyl)-fulleropyrrolidine [18], it exhibits ~2.58 times higher magnitude of K compared to 1-PyC60 system (see Table 1).
Kav, dm3·mol−1
Selectivity in K in presence of AuNp
25,645 4335
~5.9
3.2. Steady state fluorescence studies To study the photo-induced behavior of 1 in absence and presence of AuNp and PyC60, steady-state fluorescence measurements are carried out in toluene. Fluorescence measurements would also reveal the effect of molecular structure on molecular and plasmonic resonances in 1AuNp interaction. Fig. 4 shows the fluorescence spectrum of 1 in absence and presence of AuNp, PyC60 and AuNp + PyC60 mixture. It is clearly seen from Fig. 4 that the fluorescence intensity of uncomplexed 1 suffers sharp quenching in presence of AuNp compared to PyC60; moreover, the fluorescence peak maximum of 1 suffers 2 nm blue shift in presence of AuNp. The fluorescence quenching of 1 in the presence of AuNp may be attributed to energy transfer from photoexcited 1 to AuNp. As expected, the fluorescence intensity of 1 gets maximum quenching when both AuNp and PyC60 are present in the mixture. This energy transfer may be ascribed as surface energy transfer as the photoexcited 1 transfers its energy to AuNp, upon relaxation back to the ground state. This phenomenon generally occurs for AuNp having small diameters [25] and that are larger than for Förster resonance energy transfer [26]. It is also possible that slight aggregation observed for 1 when conjugated to AuNp can result in self-quenching behind decrement in the fluorescence intensity. The prodigious photophysical characteristic observed for metallated naphthalocyanine towards AuNp and its effect over non-covalent binding between 1 and PyC60 in presence of AuNp is further explored by steady state fluorescence titrations. In fluorescence titration, the concentration of 1 is kept fixed at a certain concentration and varying the concentration of PyC60. It has been observed that upon excitation at 340 nm absorption band (S1 ← S0) maxima of 1, a PyC60 concentration dependent decrease in the intensity of the fluorescence maxima of 1 is seen in absence and presence of AuNp as shown in Figs. 1S & 2S, respectively. The decrease in fluorescence intensity of 1 in presence of PyC60 suggests photo-excited decay of the excited singlet state of ZnNc by the addition of electronaccepting fullerene; like UV–Vis investigations, a sharp decrease in the value of K is observed (Table 1). In absence of AuNp, an excellent liner
Fig. 2. UV–Vis spectral variation of 1 (4.20 × 10−6 mol·dm−3) in presence of PyC60 (5.95 × 10−6 to 2.35 × 10−5 mol·dm−3) recorded in toluene at 298 K. The inset of this figure indicates decrease in the absorbance value of the Q-absorption band of 1 in presence of PyC60 along with Scatchard plot of 1-PyC60 system for the estimation of K.
A. Ray, S. Bhattacharya / Journal of Molecular Liquids 232 (2017) 188–194
191
Fig. 3. UV–Vis spectral variation of 1 (4.75 × 10−6 mol·dm−3) in presence of PyC60 (3.15 × 10−6 to 3.30 × 10−5 mol·dm−3) and AuNp recorded in toluene at 298 K. The inset of this figure indicates decrease in the absorbance value of 1-AuNp mixture in presence of PyC60 at 760 nm and Scatchard plot of 1-PyC60 system in presence of AuNp for the estimation of K.
correlation is obtained as evidenced from fluorescence BH plot [24] (inset of Fig. 1S). However, in presence of AuNp, a non-linear curve fitting is associated from which the value of K is determined (Table 1, inset of Fig. 2S). The striking feature of the present investigations is the average selectivity in binding in absence and presence of AuNp which is estimated to be ~5.9 (Table 1). To prove that only binding is associated between 1 and PyC60 not the energy transfer phenomenon, steady state fluorescence experiment is performed at the excitation maximum of PyC60, i.e., 430 nm (resulting from forbidden singlet-singlet transition). It is observed that the fluorescence intensity of PyC60 remains same even in presence of 1 (Fig. 5). This observation supports our rationale behind binding mechanism between 1 and PyC60 rather than energy transfer.
understanding of the surface energy transfer (SET) mechanism between an excited state molecule and a nearby metal particle becomes a field of active research for various applications [28,29]. In a metal nanoparticle, the free conduction electrons of the metal passes near and/perpendicular to the metal surface and as a result of this, they strongly interact with the donor molecules [30]. Therefore, in comparison with the R−6 distance-dependent fluorescence resonance energy transfer process (FRET), the SET process depends on the R−4 distance from the molecule to the surface of the metal, where spectral overlap between donor emission and the acceptor absorption spectrum is not required; here R defines the distance which correlates the energy transfer efficiency. Considering the Fermi Golden rule approximation, the energy transfer rate (kET) is related to the square dependence of the excitonic coupling. The SET rate is given by the following expression (Eq. (1))
3.3. Evidence of energy transfer in present work It is generally believed that metal nanoparticles strongly quench the singlet-excited states of chromophores when attached to nanoparticle surfaces, through an energy-transfer mechanism [27]. Recently, the
4
K SET ¼ ð1=τD Þ ðd0 =dÞ
ð1Þ
where the lifetime of the donor (here 1) in the absence of an acceptor molecule is given by τD, and the distance between 1 and the acceptor (here AuNp) molecule is marked by d. The distance d0, is calculated
Fig. 4. Steady state fluorescence spectral variation of 1 (4.15 × 10−6 mol·dm−3, blue colour line) in presence of AuNp (orange colour line), PyC60 (5.80 × 10−5 mol·dm−3, magenta colour line) and AuNp + PyC60 mixture (5.80 × 10−5 mol·dm−3, green colour line) recorded in toluene at 298 K. λex = 685 nm; λem = 776 nm.
Fig. 5. Steady state fluorescence spectral variation of PyC60 (4.55 × 10−5 mol·dm−3, black colour line) in presence of 1 (4.75 × 10−6 mol·dm−3, blue colour line) recorded in toluene at 298 K. λex = λex = 430 nm; λem = 705 nm.
192
A. Ray, S. Bhattacharya / Journal of Molecular Liquids 232 (2017) 188–194 Table 3 Values of quantum yield (Φf), radiative rate constant (kr) and non-radiative rate constant (knr) of 1 in absence and presence of AuNp, PyC60 and PyC60-AuNp systems; Temp. 298 K.⁎, ⁎⁎ System
Φf
⁎kr, s−1
1 1-AuNp 1-PyC60 1-PyC60-AuNp
0.03 3.62 × 10−3 4.10 × 10−3 3.30 × 10−3
1.15 2.40 1.58 2.32
× × × ×
⁎⁎knr, s−1 107 106 106 106
3.65 6.63 3.85 7.12
× × × ×
108 108 108 108
⁎ kr = Φf/τs. ⁎⁎ knr = (1/τs) − Φf/τs = (1/τs) − kr.
Fig. 6. Time-resolved fluorescence decay profile of 1 (6.80 × 10−6 mol·dm−3, blue colour line) in presence of (b) AuNp (orange colour line), (c) PyC60 (4.90 × 10−5 mol·dm−3, magenta colour line) and (d) PyC60 (4.90 × 10−5 mol·dm−3) + AuNp mixture (green colour line) recorded in toluene at 298 K.
of PyC60 + AuNp mixture in solution. Utilizing the value of Φf, we have been able to determine the value of radiative (kr) and non-radiative (knr) rate-constants of 1, 1-AuNp, 1-PyC60 and 1-PyC60-AuNp systems (Table 3). Table 3 clearly demonstrates while there is a marked reduction in the value of kr for 1 in presence of AuNp, viz., ~ 4.8 times, the rate of knr for 1-AuNp system gets ~ 1.8 times enhancement in its value compared to uncomplexed 1. The trend observed for the estimation of Φf of 1 (as discussed earlier) in presence of PyC60 and PyC60 + AuNp mixture maintains very good correlation with the estimated value of kr and knr. These findings certainly prove that non-radiative pathway plays predominant role for the deactivation of the excited singlet state of 1 in presence of AuNp.
using the model given by Eq. (2) 1=4 d0 ¼ 0:225 c3 Φdye =ωdye 2 ω F κ F
ð2Þ
where d0 is the distance at which the dye transfers energy and emits spontaneously, Φdye indicates the quantum efficiency of the dye, ωdye is given as the frequency of the donor electronic transition, ωF is the Fermi frequency, and finally, the Fermi wave vector is given by κF [30]. Using the value of ω = 3.6 × 1015 s−1, ωF = 8.4 × 1015 s−1, κF = 1.2 × 108 cm−1, Φdye = 0.03, and c = 3 × 108 cm−1, the d0 value is estimated to be 240 Å. The value of d0 clearly proves that energy transfer takes place at a much larger distance compared to radiative rate of energy transfer which actually operates at very close distances (i.e., b10 Å) [28,29]. The general form of the quantum efficiency of energy transfer according to SET theory is given by Eq. (3) n o ΦET ¼ 1= 1 þ ðr=r0 Þ4
3.4. Time-resolved fluorescence investigations Apart from steady state fluorescence measurements, we have performed detailed nanosecond time-resolved fluorescence experiment for the investigated supramolecule in absence and presence of AuNp. 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 (τs) of 1 is measured to be 2.65 ns (Fig. 6). The time-resolved fluorescence experiment, therefore, suggests presence of only one species in
ð3Þ
As our rationale is validated according to SET theory, there is no question of getting any overlap between the fluorescence band of 1 and the absorption band of AuNP that we normally observe for FRET. From above discussions, we may surmise that possibility of electron transfer from 1 to AuNp is excluded in our present work due to SET phenomenon. Formidable support in favour of energy transfer also comes from the measurement of quantum yield (Φf) of 1 in absence and presence of AuNp, PyC60 and mixture of PyC60 and AuNp. Utilizing the value of lifetime of the excited singlet state, i.e., τs (see Table 2), it is observed that the Φf value of uncomplexed 1 (i.e., 0.03) suffers considerable reduction in presence of AuNp (Table 3). The extent of reduction is found to be larger in magnitude compared to when only PyC60 is present in solution. The highest decrease in the value of Φf of 1, however, is observed in case
Table 2 τs, ksEnt and φsEnt of 1 and its complexes with PyC60 in absence and presence of AuNp recorded in toluene at 298 K. System
τs, ns
ksEnt, s−1
φsEnt
1 1-AuNp 1-PyC60 1-PyC60-AuNp
2.65 1.50 2.58 1.42
– 2.90 × 108 10.2 × 106 3.27 × 108
– 0.435 0.026 0.464
Fig. 7. DFT/B3LYP/6-31G* optimized geometric structure of 1-PyC60 complex done in vacuo.
A. Ray, S. Bhattacharya / Journal of Molecular Liquids 232 (2017) 188–194
193
Fig. 8. DFT (B3LYP/6-31G*) calculated frontier (a) HOMO and (b) LUMO for the 1-PyC60 complex done in vacuo. The calculations are done using SPARTAN '14 software.
solution. In presence of AuNp, 1-AuNp conjugate shows a bi-exponential decay (Fig. 6) where the first component (with τs1 of 2.187 ns) may be due to the presence of free 1 and the second component (with τs2 of 0.919 ns) is due to the conjugation of the nanoparticles to 1. In contrary to AuNp, PyC60 exhibits a mono-exponential decay suggesting that 1 attached with PyC60 through non-covalent linkage is quenched to the point where it is not fluorescent (Fig. 6). This phenomenon is very much consistent with the trend in τs value of free-base Pc in presence of silver nanoparticles [21] and for Pc-AuNp conjugate reported by Nyokong et al. [25]. Conjugation of 1 with AuNp in presence of PyC60 (Fig. 6), reduces the τs value of 1 to 2.15 and 0.766 ns for 1st and 2nd component, respectively, compared to its original value reported earlier.
Taking into consideration of all the components, we have estimated the τs value for bi-exponential fit [26]. Utilizing the value of τs, singlet energy-transfer rate constants (ksEnt) and quantum yield of energy transfer efficiency (φsEnt) [31] have been enumerated and listed in Table 2. The notable feature is that, 1-PyC60 complex exhibits facile energy transfer efficiency in presence of AuNp as evidenced from large value of φsEnt, i.e., 0.464. The lifetime experiment, also, evokes that 1-PyC60 system exhibits considerable amount of ksEnt in presence of AuNp, i.e., 3.27 × 108 s−1, while it exhibits value of only 10.2 × 106 s−1 in absence of AuNp. Remarkable enhancement in both ksEnt and φsEnt values for 1PyC60 system in presence of AuNp compared to 1-PyC60 in absence of AuNp, i.e., ~32 and 18 times, respectively, provide formidable support in favour of strong electrostatic interaction between 1 and PyC60 in presence of AuNp. These two physicochemical parameters nicely demonstrate that energy transfer is the only path behind quenching of fluorescence of 1 in presence of AuNp. This is because of the fact that order of ksEnt lies in the range of 108 s−1 for 1-AuNp system which is proved to be much below for a process exhibiting electron transfer phenomenon, viz., 1012 s−1 The order of ksEnt strongly supports our rationale that energy transfer is the only path in present case behind deactivation of the photo-excited singlet state of 1 in presence of AuNp [32]. 3.5. Theoretical calculations
Fig. 9. MEP of 1-PyC60 complex done by DFT/B3LYP/6-31G* calculations in vacuo.
In our present work, a detailed conformational analysis of the individual components as well as the 1-PyC60 complex has been performed in vacuo by DFT/B3LYP/6-31G* calculations. The geometric parameters of the complexes are obtained after complete energy minimization. The strong binding of PyC60 in the cleft of 1 (Fig. 7) is reflected in terms of negative value of heat of formation for this complex estimated in vacuo, i.e., −3.87 kcal·mol−1. The frontier highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) for the investigated supramolecule have been obtained by DFT method for 1-PyC60 system. It is also observed that, HOMO of both 1-PyC60 (− 4.3483 eV) complex corroborates fairly well with that of uncomplexed 1 (−4.3414 eV) while LUMO of this complex, i.e., −3.0314 eV, remains very close to that of PyC60 (−3.1501 eV). The small HOMO-LUMO gap (i.e., 1.3169 eV) gives formidable support in favour of strong electronic interaction between 1 and PyC60. The HOMO and LUMO pictures of 1PyC60 complex are provided in Fig. 8. Molecular electrostatic potential (MEP) maps have also been generated for the investigated supramolecule in present work with the help of DFT calculations and visualized in Fig. 9; MEP provides strong indication in favour of electronic
194
A. Ray, S. Bhattacharya / Journal of Molecular Liquids 232 (2017) 188–194
redistribution in donor (here 1) and acceptor (here PyC60) during noncovalent interaction. It may be mentioned here that validity of molecular orbitals generated by density functional methods is already being recognized [33]. The accuracy of these methods, especially B3LYP, is being well demonstrated by D'Souza [34] et al. and, more recently, on covalently linked C60-Znporphyrin system [12] and fullerene-based photoactive layers for the construction of heteroconjugation solar cells [35]. 3.6. DLS, STM and AFM studies To gain further insight into the intermolecular interaction between 1 and AuNp, we have performed various physical characterization methods like dynamic light scattering (DLS), scanning tunneling microscope (STM) and atomic force microscope (AFM) measurements. It is observed that the addition of AuNp to the solution of 1 increases the size of the nanoparticles in the range of 50–60 nm. All the above findings give clear evidence in favour of the surface binding of 1 to AuNp. This phenomenon also gives strong support in favour of decrease in the value of K for 1-PyC60 complex measured in toluene in presence of AuNp. The photographs obtained from DLS, STM and AFM measurements are depicted in Figs. 3S(a), (b) & (c), respectively. This may be the first example to track the occurrence of complexation between a metallated naphthalocyanine and functionalized fullerene in presence of nanoparticles by various particle size and surface binding analysis. 4. Conclusions In conclusion, it is observed that 1-AuNp conjugate emerges as an illustrious example for exhibiting energy transfer phenomenon in solution. The binding of PyC60 with 1 is affected greatly in presence of AuNp. This study proves that self-assembled 1-AuNp-PyC60 conjugate provides very good perspective for the development of efficient photochromic devices in near future. Acknowledgement AR thanks CSIR, New Delhi for providing Research Associateship to her. Grant-In-Aid through Research Project of Sanction No. 01(2711)/ 13/EMR-II is also greatly acknowledged. Appendix A. Supplementary data Steady state fluorescence titration of 1-PyC60 system in absence and presence of AuNp estimated in toluene, DLS, STM and AFM photographs of 1-AuNp system are given as Figs. 1S–3S, respectively. Figs. 1S–3S are provided as supporting information. Supporting information associated
with this paper is freely available through Internet. Supplementary data associated with this article can be found in the online version, at doi: 10. 1016/j.molliq.2017.02.009.
References [1] T.G.W. Edwardson, K.L. Lau, D. Bousmail, C.J. Serpell, H.F. Sleiman, Nat. Chem. 8 (2016) 162–170. [2] S.C. Bera, K. Sanyal, D. Senapati, P.P. Mishra, J. Phys. Chem. B 120 (2016) 4213–4220. [3] B.I. Ipe, K.G. Thomas, S. Barrazzouk, J. Phys. Chem. B 106 (2002) 18–21. [4] J. Ohyama, Y. Hitomi, Y. Higuchi, M. Shinagawa, H. Mukai, M. Kodera, K. Teramura, T. Shishido, T. Tanaka, Chem. Commun. (2008) 6300–6302. [5] V. Amendola, G. Mattei, C. Cusan, M. Prato, M. Meneghetti, Synth. Met. 155 (2005) 283–286. [6] F. D'Souza, O. Ito, Chem. Commun. (2009) 4913–4928. [7] G. Bottari, G.D. de la Torre, D.M. Guldi, T. Torres, Chem. Rev. 110 (2010) 6768–6816. [8] R. Taylor, R. Lecture Notes on Fullerene Chemistry: A Handbook for Chemists, Imperial College Press, London, 1999. [9] M.–.E. Ragoussi, T. Torre, Chem. Asian J. 9 (2014) 2676–2707. [10] R.F. Enes, J.–.J. Cid, O. Trukhina, A. Hausmann, A. Gouloumis, P. Vázquez, J.A.S. Cavaleiro, A.C. Tomé, D.M. Guldi, T. Torres, Chem. –Eur. J. 18 (2012) 1727–1736. [11] C.B. KC, K. Ohkubo, P.A. Karr, S. Fukuzumi, F. D'Souza, Chem. Commun. 49 (2013) 7614–7616. [12] O. Ito, F. D'Souza, Molecules 17 (2012) 5816–5835. [13] G. Bottari, J.A. Suanzes, O. Trukhina, T. Torres, J. Phys. Chem. Lett. 2 (2011) 905–913. [14] D.M. Guldi, B.M. Illescas, C.M. Atienza, M. Wielopolski, N. Martin, Chem. Soc. Rev. 38 (2009) 1587–1597. [15] I. Dhanya, C.S. Menon, Opt. Electron. Adv. Mater. 4 (2010) 2085–2087. [16] M.E. El-Khouly, Phys. Chem. Chem. Phys. 12 (2010) 12746–12752. [17] E. Maligaspe, T. Kumulainen, H. Lemmetyinen, N.V. Tkachenko, N.K. Subbaiyan, M.E. Zandler, F. D'Souza, J. Phys. Chem. A 114 (2010) 268–277. [18] M.E. El-Khouly, L.M. Rogers, M.E. Zandler, G. Suresh, M. Fujitsuka, O. Ito, F. D'Souza, Chem. Phys. Chem. 4 (2003) 474–481. [19] N. Kobayashi, Coord. Chem. Rev. 227 (2002) 129–152. [20] A. Ray, S. Bhattacharya, Chem. Phys. Lett. 651 (2016) 66–71. [21] A. Ray, S. Bhattacharya, Chem. Phys. Lett. 639 (2015) 183–188. [22] J.L. Sessler, J. Jayawickramarajah, A. Gouloumis, G.D. Pantos, T. Torres, D.M. Guldi, Tetrahedron 62 (2006) 2123–2131. [23] H. Imahori, A. Fujimoto, S. Kang, H. Hotta, K. Yoshida, T. Umeyama, Y. Matano, S. Isoda, M. Isosomppi, N.V. Tkachenko, H. Lemmetyinen, Chem. –Eur. J. 11 (2005) 7265–7275. [24] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703–2707. [25] N. Nombona, E. Antunes, C. Litwinski, T. Nyokong, Dalton Trans. 40 (2011) 11876–11884. [26] B. Valeur, Molecular Fluorescence: Principles and Applications, Wiley-VCH, 2002. [27] S. Kundu, A. Patra, Chem. Rev. 117 (2017) 712–757. [28] R.R. Chance, A. Prock, R. Silbey, In Advances in Chemical Physics, John Wiley & Sons, Inc., 2007 [29] B.N.J. Person, N.D. Lang, Phys. Rev. B 26 (1982) 5409–5415. [30] E. Dulkeith, A.C. Morteani, T. Niedereichholz, T.A. Klar, J. Feldmann, S.A. Levi, F.C.J.M. van Veggel, D.N. Reinhoudt, M. Möller, D.I. Gittins, Phys. Rev. Lett. 89 (2002) 203002. [31] F. D'Souza, P.M. Smith, M.E. Zandler, A.L. McCarty, M. Itou, Y. Araki, O. Ito, J. Am. Chem. Soc. 126 (2004) 7898–7907. [32] A. Ray, K. Santhosh, S. Bhattacharya, J. Phys. Chem. B 116 (2012) 11979–11998. [33] R. Stowasser, R. Hoffmann, J. Am. Chem. Soc. 121 (1999) 3414–3420. [34] 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, O. Ito, J. Phys. Chem. A 106 (2002) 3243–3252. [35] Y. Li, D. Qi, P. Song, F. Ma, Materials 8 (2015) 42–56.