Photophysical Properties, Self-Assembly Behavior, and Energy

Article pubs.acs.org/JPCC

Photophysical Properties, Self-Assembly Behavior, and Energy Transfer of Porphyrin-Based Functional Nanoparticles Sadananda Mandal,† Santanu Bhattacharyya,† Victor Borovkov,*,‡ and Amitava Patra*,† †

Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, 700 032, India Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

‡

S Supporting Information *

ABSTRACT: This paper focuses on the spectroscopic studies and self-assembly behavior of zinc octaethylporphyrin (ZnOEP) doped semiconducting [poly(N-vinylcarbazole) (PVK)] polymer nanoparticles (NP) using steady state and time-resolved spectroscopy. The bathochromic shift of both Soret (by 12 nm) and Q bands (by 6−8 nm) in the absorption spectra and shortening of the porphyrin lifetime indicate the J-aggregation of porphyrin molecules in the ZnOEP doped PVK NP system. The significant quenching of fluorescence spectrum and the shortening of decay time of the PVK host unambiguously confirm an effective energy transfer (above 90%) from PVK to ZnOEP in the nanoparticles.

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INTRODUCTION Porphyrins and metalloporphyrins are intensively investigated molecules owing to their excellent photophysical, photochemical, electrochemical, structural, and self-assembly properties.1 A very special characteristic of these compounds is their tendency to form diverse self-assemblies of different order and structure, which are of both scientific and technological importance.2−4 Among these (supra)molecular organizations, the J-type (edge-to-edge arrangement) and H-type (face-to-face arrangement) aggregates are of particular interest because of their unique electronic and spectroscopic properties due to their high structural order.5 Specifically, the J aggregation has attracted much attention owing to its potential application as an efficient model for mimicking the chlorophyll light-harvesting antenna complexes with a circle configuration of chromophores in the photosynthesis.6 Accordingly, there are many reports on the porphyrin J aggregation in various aqueous solutions, in mixed solvent and assisted by different types of surfactants and templates.7−11 Besides, Wang et al.12 synthesized the J aggregated zinc octaethylporphyrin (ZnOEP) nanorods to investigate the photoconductivity of ZnOEP, which was one of the first reported examples of porphyrin based organic nanoscaled phototransistors. A new application of Fe(III) ClOEP nanoparticles as the fluorescence-enhanced nucleic acid detection has been demonstrated by Zhai et al.13 All of the cited results clearly demonstrate that porphyrinoids and various porphyrin-based systems are some of the best-suited and versatile chromophoric structures for different light-driven processes in nanomaterials. In connection with this, porphyrin and porphyrin-doped conjugated polymeric systems with special functionalities are also expected to be a very emerging field of research owing to their potential applications in light harvesting, biological sensing, and organic solar cell devices.14−17 Furthermore, incorporation of a second energy/ electron-donating/withdrawing component into nanoparticles may further enhance the spectral and photophysical/photo© 2012 American Chemical Society

chemical features of such nanosystems. In line with this concept, poly(9-vinylcarbazole) (PVK) is one of the most promising candidates18 to serve as an energy-transfer mediated host due to its specific spectral features well-suited for the porphyrin chromophores.19,20 Besides, recently, conjugated polymer NP have attracted considerable attention because of their multifunctional activities in medical imaging, biosensing, drug delivery, photonics, and biophotonics owing to their tunable optical properties, facile synthesis, minor toxicity, and enhanced biocompatibility.21−25 For example, McNeill and coworkers demonstrated the use of conjugated polymer nanoparticles in the energy-transfer-mediated phosphorescence from metalloporphyrin (Pt(II)OEP)-doped polyfluorene NP and its application for biological oxygen sensing.26 Also, Shen et al.27 reported tetraphenylporphyrin doped conjugated polymer, poly[9,9-dibromohexylfluorene-2,7-ylenethylene-alt-1,4-(2,5dimethoxy)phenylene], nanoparticles by using the reprecipitation method to generate singlet oxygen under two-photon excitation. Here, we present the formation of ZnOEP doped conjugated PVK nanoparticles and the aggregation behavior of ZnOEP inside the conjugated polymer matrix in aqueous media. It is observed that the porphyrin molecules are in the J-type aggregated form in PVK matrix, which is comprehensively characterized by steady state and time-resolved spectroscopy. Furthermore, an efficient excited energy transfer from the PVK host as an energy donor to the ZnOEP guest as an energy acceptor is successfully demonstrated that opens up further prospects in designing new porphyrin based materials for application in the efficient light harvesting systems. Received: March 14, 2012 Revised: April 13, 2012 Published: May 3, 2012 11401

dx.doi.org/10.1021/jp302462j | J. Phys. Chem. C 2012, 116, 11401−11407

The Journal of Physical Chemistry C

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MATERIALS PVK [poly(9-vinylcarbazole)] (Aldrich), zinc octaethylporphyrin (Aldrich), distilled tetrahydrofuran (THF) (MERCK), deionized water (MERCK), and dichloromethane (MERCK) were used as received for our synthesis. Scheme 1 shows the molecular structures of ZnOEP and PVK.

photoluminescence spectra were recorded by a Fluoromax-P (HORIBA JOBIN YVON) photoluminescence spectrophotometer. For the time correlated single photon counting (TCSPC) measurements, the samples were excited at 405 nm using a Pico-second diode laser (IBH Nanoled-07) in an IBH Fluorocube apparatus. The typical full width at half-maximum (fwhm) of the system response using a liquid scatter was about 90 ps. The repetition rate was 1 MHz. The fluorescence decays were collected with a Hamamatsu MCP photomultiplier (C487802). The fluorescence decays were analyzed using IBH DAS6 software. For 340 nm excitation, a NANO-LED IBH 340 was used. The following equation was used to analyze the experimental time-resolved fluorescence decays, P(t):29

Scheme 1. Molecular Structures of ZnOEP and PVK

n

P(t ) = b +

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i

EXPERIMENTAL PROCEDURES AND INSTRUMENTATIONS PVK nanoparticles were prepared by the previously reported reprecipitation method.20,28 PVK was properly dissolved in dried THF to maintain the 0.5 mg/mL concentration of PVK. A 500 μL aliquot of this THF solution was rapidly injected into 10 mL of double distilled water under vigorous stirring for 10− 15 min followed by ultrasonication for 15 min. As a result, an aqueous solution of PVK nanoparticles was obtained. Then, to avoid aging of the PVK nanoparticles, THF was evaporated from aqueous solution by partial vacuum evaporation for 1 h followed by filtration through a 0.2 μm filter paper. Finally, we obtained a stable aqueous solution of PVK nanoparticles, which was stable for 3−4 days. The ZnOEP doped PVK nanoparticles were prepared by a similar reprecipitation method as described above. In this particular case, the THF solutions of ZnOEP and PVK were mixed thoroughly to maintain the 0.024 mg/mL (2.5 wt %) concentration of ZnOEP and 0.5 mg/mL concentration of PVK, followed by ultrasonication for 5 min to obtain a clear mixed solution. Then, 500 μL of this THF solution was rapidly injected into double distilled water under vigorous stirring for 10−15 min followed by the standard procedures of ultrasonication, vacuum evaporation, and filtration through a 0.2 μm filter paper. Finally, a solution of ZnOEP doped PVK NP was obtained. The optimal concentration of ZnOEP in PVK matrix for spectral measurements was determined as follows. For this purpose, ZnOEP doped PVK NP were prepared by varying the concentration (from 0.31 to 6.25 wt %) of ZnOEP while keeping the concentration of PVK constant. The emission spectra of all these samples were taken at an excitation wavelength of 405 nm. It was observed that, with the increase of concentration of ZnOEP, the fluorescence intensity of the porphyrin component rises at first and then decreases (Supporting Information, Figure S1). The concentration versus fluorescence intensity plot shows that the fluorescence intensity has a maximum at a concentration of 2.5 wt % of ZnOEP (Supporting Information, Figure S2). Therefore, this concentration was chosen as optimal for all of the spectral experiments. The morphological characters and sizes of PVK NP and ZnOEP encapsulated PVK NP were measured by field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6700F). Room temperature optical absorption spectra were taken by a UV−vis spectrophotometer (SHIMADZU). Room temperature

⎛ t⎞ ⎟ ⎝ τi ⎠

∑ αi exp⎜−

(1)

Here, n is a number of discrete emissive species, b is a baseline correction (“dc” offset), and αi and τi are pre-exponential factors and excited-state fluorescence lifetimes associated with the ith component, respectively. For multiexponential decays, the average lifetime, ⟨τ⟩, was calculated from the following equation:30 n

⟨τ ⟩ =

∑ aiτi i=1

(2)

where ai = αi/∑αi and ai is the contribution of the decay component.

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RESULT AND DISCUSSION 1. Structural Studies. To obtain the corresponding structural and morphological characteristics, the FE-SEM images of ZnOEP doped PVK NP were taken (Figure 1). These images clearly indicate the formation of spherical nanoparticles, and the size distribution curve (Figure 1B, inset) shows that the average diameter of ZnOEP doped PVK nanoparticle is about 70 nm. This value is the same as found for the pure PVK nanoparticles (Supporting Information, Figure S3), indicating that the size of polymer nanoparticles remains essentially unchanged after the encapsulation of ZnOEP. The reason behind such invariability is clearly explained in our recently published paper and is based upon the concept that the concentration of ZnOEP in the stock THF solution is much lower than that of PVK, thus resulting in PVK serving as a sizedeterminant component of the whole system.20 A general mechanism of nanoparticle formation is as follows: during the injection of THF solution of PVK into water (a poor solvent for the PVK molecules) under stirring conditions, the solution is divided into many droplets by strong shearing force. Simultaneously, THF molecules quickly diffuse into water to expose the PVK molecules to bulk water. Since water is a poor solvent for hydrophobic conjugated polymer, the PVK molecules become aggregated to form the corresponding nanoparticles to avoid contact with water. 2. Steady-State and Time Resolved Spectroscopy. In general, porphyrins and metalloporphyrins feature two welldefined absorption regions, a so-called high-energy B (also known as Soret) and low-energy Q bands. The Soret absorption band corresponds to S0 → S2 electronic transitions, while Q bands correspond to S0 → S1 electronic transitions. Both the B and Q bands arise from π−π* electronic transitions.31 When (supra)molecular aggregations [“H” (face11402



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Figure 2. Absorption spectra of (a) ZnOEP in DCM, (b) ZnOEP encapsulated in PVK NP, and (c) pure PVK NP.

can be ruled out because no spectral changes occurred in the DCM solution of ZnOEP in the presence of PVK [Supporting Information, Figure S5 (A)]. However, some porphyrin molecules may still exist in the nonaggregated state due to a relatively low concentration of ZnOEP inside NP. As a result, the observed red shifts of absorption bands are averaged and rather moderate in comparison to some other porphyrin based systems being predominantly in the J-type structural arrangement.36 This assumption is further ascertained by the timeresolved results (see below). The absorption maximum at 346 nm is the characteristic of PVK nanoparticles.20,37 Figure 3 shows the emission spectra of pure PVK NP; ZnOEP encapsulated PVK NP and ZnOEP in DCM solvent at

Figure 3. Fluorescence spectra of (a) ZnOEP in DCM, (b) ZnOEP encapsulated in PVK NP, and (c) pure PVK NP at an excitation wavelength of 340 nm.

Figure 1. FE-SEM images of ZnOEP doped PVK NP.

to-face) or “J” (edge-to-edge) type] occur, both S1 and S2 excitation states are split into the lower- and higher-level energy excited states.32 According to the exciton coupling model,33 the higher excited states are transitionally allowed in the case of H aggregation, whereas lower excited states are transitionally allowed for J aggregation. As a result, hypsochromic and bathochromic shifts of absorption spectra in the corresponding H and J aggregates occur (with respect to the monomer absorption). For example, Gao et al. reported the J aggregation of 5,15-di[4-(5-acetylsulfanylpentyloxy)phenyl]porphyrin derivatives with the corresponding red shift in absorption spectra.34 In our case, ZnOEP in DCM solvent shows a very sharp Soret band at 400 nm and Q bands at 530 and 567 nm (Figure 2), thus indicating a pure monomeric form.35 However, the bathochromic shifting of both Soret (by 12 nm) and Q bands (by 6−8 nm) are observed when ZnOEP is encapsulated inside PVK NP. The observed steady-state spectral changes suggest that, in the case of ZnOEP doped PVK NP, the ZnOEP molecules form the J-aggregated state due to the increment of local concentration of ZnOEP inside the PVK matrix. It is observed that the OD value increases nonlinearly with increasing concentration of ZnOEP molecules inside the PVK matrix (Supporting Information, Figure S4), which further confirms the formation of aggregation. The PVK ligation inside NP, as another possible reason of the corresponding red shifts,

an excitation wavelength of 340 nm that corresponds to the direct excitation of the PVK chromophoric moiety. It is clear from this figure that, after the encapsulation of porphyrin inside PVK NP, the fluorescence intensity of PVK drastically quenches (down to 82%) at the PVK emission band (465− 490 nm), while the fluorescence intensity of porphyrin increases (in 1.5-fold) at the porphyrin emission band (575 nm). This phenomenon is apparently associated with the Fröster energy transfer from PVK to ZnOEP that is further confirmed by time-resolved spectroscopy (as discussed below). Upon excitation of ZnOEP in DCM solvent at 405 nm, that corresponds to the porphyrin B absorption the fluorescence spectra exhibit two emission bands at 570 nm (Q*X00) and 623 nm (Q*X01) (Figure 4), which are the characteristic peaks of the ZnOEP chromophore in DCM solvent and in good agreement with the previously reported data.38 In the case of ZnOEP encapsulated PVK NP, the emission peaks are observed at 575 nm (Q*X00) and 630 nm (Q*X01). A small red-shifted emission (by 5 and 7 nm, correspondingly) and lower fluorescence intensity of ZnOEP encapsulated PVK NP in comparison to ZnOEP in DCM are apparently due to the J aggregation (as discussed above) induced by intermolecular porphyrin− porphyrin (such as π−π/hydrophobic) interactions. In the case of ZnOEP in DCM, the emission bands are at 570 and 623 nm regardless of the excitation wavelength, whether it is 405 11403



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fluorescence spectra are taken at different excitation and emission wavelengths. At first, the PVK component was selectively excited at 340 nm. Thus, Figure 5 shows the time-

Figure 4. Fluorescence spectra of (a) ZnOEP in DCM and (b) ZnOEP encapsulated in PVK NP at an excitation wavelength of 405 nm.

nm (B-band region) or 531 nm (Q-band region) [Supporting Information, Figure S6 (A)]. Similarly, the ZnOEP doped PVK NP system exhibited the emission bands at 575 and 630 nm for both excitation wavelengths of 405 and 537 nm [Supporting Information, Figure S6 (B)]. Therefore, the nature of emission spectra is similar for these two systems regardless of whether the B-band or Q-band is excited. This photophysical feature corroborates the result observed for the free base porphyrin emission as reported by Akins et al.39 It was also found that the corresponding absorption and excitation spectra of ZnOEP in DCM as well as ZnOEP inside PVK NP are matched well (Supporting Information, Figure S7). Therefore, we can conclude that, in the case of ZnOEP in DCM, the emission of ZnOEP occurs from the lowest singlet excited electronic state (S1) associated with the Q-band. The intense Q (0, 0) band at 570 nm is attributed to the S1−S0 transition, while the low intensity second Q (0, 1) emission band at 623 nm corresponds to the electronic transition to a vibrationally excited state of the S0 level (Scheme 2). On the other hand, in

Figure 5. Time-resolved fluorescence spectra of (a) pure PVK NP (emission: 390 nm) and (b−d) ZnOEP encapsulated in PVK NP (emission: 630, 575, and 365 nm, respectively) at an excitation wavelength of 340 nm.

resolved fluorescence spectra of pure PVK NP and ZnOEP encapsulated PVK NP at an excitation wavelength of 340 nm and at different emission wavelengths. The decay curve of pure PVK NP (curve a) is well-fitted by multiexponential kinetics having an average lifetime of 1.56 ns, which is in good agreement with our previously reported data.20 In the case of ZnOEP (2.5 wt %) encapsulated PVK NP, at an emission wavelength of 365 nm, the decay curve is fitted by multiexponential kinetics having an average lifetime of 0.04 ns. This drastic shortening of lifetime value corresponding to the PVK component also indicates the energy transfer from PVK to ZnOEP. Further, in the case of ZnOEP in DCM, at an excitation wavelength of 340 nm, the decay curves for both emission wavelengths of 570 and 623 nm are fitted monoexponentially with the corresponding decay times of 1.49 and 1.53 ns (Figure 6). However, in the ZnOEP doped

Scheme 2. Absorption and Emission Features of ZnOEP and Energy Transfer from PVK to ZnOEP (IC = Internal Conversion, ET = Energy Transfer)

Figure 6. Time-resolved fluorescence spectra of ZnOEP in DCM: (a) emission: 570 nm, (b) emission: 623 nm (excitation wavelength of 340 nm).

PVK system, at emission wavelengths of 575 and 630 nm (both emissions are of the porphyrin component), the decay curves are well-fitted by a biexponential decay. In the case of an emission wavelength of 575 nm, the components are 1.44 ns (93.7%) and 3.63 ns (6.3%) with the average lifetime being 1.57 ns. For an emission wavelength of 630 nm, the corresponding components are 1.23 ns (83%) and 3.52 ns (17%) and the average lifetime is 1.60 ns. As clearly seen from these data (Table 1), the decay times of ZnOEP in the doped system at emission wavelengths of 575 and 630 nm are slightly increased with respect to the ZnOEP in the DCM system. This increment of decay time is apparently due to the energy transfer from PVK to ZnOEP.40 However, the enhancement of decay time is very small (5%) with respect to the decrement of lifetime of PVK (above 90%). It indicates the aggregation (Jaggregation) behavior of ZnOEP inside the PVK matrix,41,42

the case of ZnOEP doped in PVK, the emission of ZnOEP occurs from the S1a state, which is associated with the lowest split energy level of excited state upon the J-aggregation. Therefore, as in the case of ZnOEP in DCM, the intense band at 575 nm arises from the S1a−S0 transition, while the low intense band at 630 nm is a result of the transition from S1a to a vibrationally excited state of S0 level (Scheme 2). All possible absorption and emission pathways of ZnOEP in DCM and doped in PVK NP are shown in Scheme 2. In order to gain further information on the photophysical properties of porphyrin systems studied, the time-resolved 11404



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Table 1. Decay Time of ZnOEP (2.5 wt %) inside PVK NP and DCM at Different Emission Wavelengths (Excitation at 340 nm) system ZnOEP ZnOEP ZnOEP ZnOEP

doped PVK NP doped PVK NP in DCM in DCM

emission (nm) 575 630 570 623

τ1 (ns) (a1)

τ2 (ns) (a2)

1.44 1.23 1.49 1.53

3.63 (0.063) 3.52 (0.17)

(0.937) (0.83) (1.0) (1.0)

which was also supported by the steady state fluorescence spectroscopy. To confirm further the aggregation behavior of ZnOEP in the PVK matrix, the ZnOEP molecules are selectively excited at an excitation wavelength of 405 nm. Figure 7 shows the time-resolved fluorescence spectra of ZnOEP in DCM and ZnOEP encapsulated PVK NP at an

τ3 (ns) (a3)

⟨τ⟩ (ns)

χ2

1.57 1.60 1.49 1.53

1.21 1.23 1.24 1.26

5,10,15-trisphenyl-20-(3,4-dihydroxybenzene)porphyrin. 11 Thus, it can be concluded that a certain (relatively small) number of porphyrin molecules is still in the nonaggregated state, while the majority is in the J aggregated state inside the PVK matrix. 3. Photoinduced Energy Transfer Studies. Since our nanoparticle system is comprised of photoactive energy-donor and energy-acceptor components, the photoinduced energy transfer process is highly expected. Indeed, it is clearly seen that there is a perfect overlap between the absorption spectrum of ZnOEP and the emission spectrum of PVK (Supporting Information, Figure S8), thus indicating a possibility of energy transfer from the energy donor PVK host to the energy acceptor ZnOEP guest in the ZnOEP doped PVK NP system. In general, fluorescence resonance energy transfer (FRET) is a process involving the radiation-less (non-radiative) transfer of energy from a “donor” fluorophore to an appropriate “acceptor” counterpart. This process arises from the dipole−dipole interactions and strongly depends upon the center-to-center distance of corresponding energy-donor and acceptor. According to the Förster theory,43 the rate of the energy transfer for an isolated single donor−acceptor pair separated by a distance r is given by the following equation:

Figure 7. Time-resolved fluorescence spectra [(A) ZnOEP in DCM (a, emission: 570 nm; b, emission: 630 nm), (B) ZnOEP encapsulated in PVK NP (a, emission: 575 nm; b, emission: 630 nm)] at an excitation wavelength of 405 nm.

k T(r ) =

1 ⎛ R0 ⎞ ⎜ ⎟ τD ⎝ r ⎠

6

(3)

where τD is the lifetime of the donor in the absence of the acceptor and R0 is known as the Förster distance, the distance at which the transfer rate kT(r) is equal to the decay rate of the donor in absence of the acceptor. The Förster distance (R0) is defined as

excitation wavelength of 405 nm. In the case of ZnOEP in DCM, at emission wavelengths of 570 and 623 nm, the decay curves are fitted monoexponentially with lifetimes of 1.49 and 1.50 ns, respectively (Table 2). On the other hand, at emission wavelengths of 575 and 630 nm, the decay curves of ZnOEP encapsulated PVK NP are fitted by biexponential and triexponential decays, respectively. The corresponding components are 0.44 ns (52%) and 1.39 ns (48%), and the average lifetime is 0.89 ns for the emission wavelength of 575 nm. In the case of an emission wavelength of 630 nm, the components are 0.20 ns (38.8%), 1.17 ns (57.2%), and 3.68 ns (4%) with the average lifetime being 0.89 ns. After the encapsulation of ZnOEP in PVK NP, the lifetime of the ZnOEP component decreases but the decrement is rather moderate. Indeed, in the case of perfect J-aggregation, the lifetime of aggregated molecules is much shorter than that of its monomeric form. For example, Verma et al. reported a lifetime of 9 and 0.64 ns for the corresponding monomeric and J-aggregated forms of

R 06 =

9000(In10)κ 2ϕD 128π 5Nn 4

J (λ )

(4)

where ϕD is the quantum yield of donor in the absence of acceptor, N is Avogadro’s number, n is the refractive index of medium, and J(λ) is the spectral overlap integral which is defined as J (λ ) =

∫0

∞

FD(λ)εA (λ)λ 4 dλ

(5)

where FD(λ) is the normalized emission spectrum of donor, εA(λ) is the absorption coefficient of acceptor at the wavelength λ (in nm), and κ2 is the orientation factor of two dipoles interacting. The value of κ2 depends on the relative orientation of the donor and acceptor dipoles. For randomly oriented

Table 2. Decay Time of ZnOEP (2.5 wt %) in DCM and inside PVK NP at Different Emissions (Excitation at 405 nm) system ZnOEP ZnOEP ZnOEP ZnOEP

in DCM in DCM inside PVK NP inside PVK NP

emission (nm) 570 623 575 630

τ1 (ns) (a1) 1.49 1.50 0.44 0.20

τ2 (ns) (a2)

(1.0) (1.0) (0.52) (0.388)

1.39(0.48) 1.17 (0.572) 11405

τ3 (ns) (a3)

⟨τ⟩ (ns)

χ2

3.677 (0.040)

1.49 1.50 0.89 0.89

1.14 1.30 1.13 1.27



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dipoles, κ2 = 2/3, and it varies between 0 and 4 for the cases of orthogonal and parallel dipoles, respectively. For our donor− acceptor system, the calculated overlap integral is found to be 1.23 × 1016 M−1 cm−1 nm4 and the calculated Förster distance is 49.0 Å. Figure 8A shows the fluorescence spectra of pure PVK NP (a) and ZnOEP doped PVK NP (b → d) upon varying the

90, and above 90% for 0.31, 1.15, and 2.5 wt % ZnOEP doped PVK NP, respectively. The extremely high efficiency of energy transfer compared to photoluminescence quenching is apparently not only due to the dipole−dipole interaction between the donor and acceptor but also due to excitonic energy diffusion throughout the polymer chain, which may sufficiently increase the energy transfer effectiveness.41,45 For reference purposes, the steady state fluorescence and timeresolved properties of PVK in DCM solvent in the absence and in the presence of ZnOEP, following the concentrations of PVK and ZnOEP used in the aqueous nanoparticle systems, are determined. The fluorescence intensity quenching of PVK is just within the range 4−12% for the concentration of ZnOEP of 0.31−2.5 wt % (Supporting Information, Figure S9). The time-resolved fluorescence decay measurement shows that the decay time of PVK in DCM remains constant after the addition of ZnOEP (Supporting Information, Figure S10 and Table S1). Therefore, it can be plausibly concluded that almost no energy transfer occurs from PVK to ZnOEP in the DCM solvent.

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CONCLUSIONS In summary, this work unambiguously shows that ZnOEP molecules tended to form J-type aggregates inside the conjugated PVK matrix in aqueous media. While there is no detectable photoinduced energy transfer from PVK to ZnOEP in an organic solvent, the energy transfer occurs from the PVK host to the ZnOEP guest in the ZnOEP doped PVK NP system in aqueous media with more than 90% efficiency. This remarkably high efficiency of energy transfer in the porphyrin doped polymer nanoparticles opens up further prospects in potential applications as light-harvesting systems and other photodriven devices.

Figure 8. (A) Fluorescence spectra and (B) time-resolved fluorescence spectra [a, pure PVK NP; b−d, ZnOEP doped PVK NP (b, 0.31 wt %; c, 1.15 wt %; d, 2.5 wt %)] at an excitation wavelength of 340 nm.

concentration of ZnOEP in the system and excitation at 340 nm (corresponding to the absorption of PVK component). The enhancement of quenching efficiency of PVK is observed with increasing concentration of ZnOEP. Hence, the PL quenching efficiencies of PVK are found to be 46, 69, and 82% for the concentrations of ZnOEP 0.31 wt, 1.15, and 2.5 wt %, respectively. To understand the energy transfer mechanism using the FRET method, the corresponding fluorescence decay times of PVK NP with and without ZnOEP are measured. Figure 8B shows the corresponding decay profiles of pure PVK NP and ZnOEP doped PVK NP upon varying the concentration of ZnOEP at an excitation wavelength of 340 nm. The average lifetimes of pure PVK NP and 2.5 wt % ZnOEP doped PVK NP are mentioned in the earlier section. The faster component of 2.5 wt % ZnOEP doped PVK NP is 0.037 ns which is beyond our instrumental resolution. Therefore, quantitative estimation of the energy transfer at this point should be erroneous. To overcome this problem, we have used the decay time data of PVK in the presence of a lower concentration (less than 2.5 wt %) of ZnOEP. The calculated average lifetimes of 0.31 and 1.15 wt % ZnOEP doped PVK NP are 0.22 and 0.15 ns, respectively. The subsequent decrease of decay time of PVK in the presence of ZnOEP molecules (0.31−2.5 wt %) unambiguously confirms the photoinduced host−guest energy transfer from the PVK energy-donor to the ZnOEP energy-acceptor (Scheme 2). The energy transfer efficiency can be calculated by using the following equation:44 τ ϕET = 1 − DA τD (6)

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ASSOCIATED CONTENT

* Supporting Information S

Table S1 presents the fluorescence decay parameters of PVK without and with ZnOEP in DCM. Figure S1 shows the fluorescence spectra of ZnOEP doped PVK NP at an excitation wavelength of 405 nm and upon increasing the ZnOEP concentration. Figure S2 shows the PL intensity vs concentration plot of ZnOEP in PVK matrix. Figure S3 shows the FESEM image of pure PVK NP. Figure S4 shows the optical density vs concentration plot of ZnOEP in PVK matrix. Figure S5 shows (A) the absorption spectra and (B) emission spectra of ZnOEP without and with PVK in DCM. Figure S6 shows (A) the fluorescence emission spectra of ZnOEP in DCM upon excitation at (a) the B-band region (405 nm) and (b) the Qband region (531 nm) and (B) the fluorescence emission spectra of ZnOEP inside PVK NP upon excitation at (a) the Bband region (405 nm) and (b) the Q-band region (537 nm). Figure S7 shows the ZnOEP in DCM (A) and inside PVK (B) absorption (a) and excitation (b) spectra. Figure S8 shows the overlap normalized spectra: (a) absorption spectrum of ZnOEP and (b) emission spectrum of PVK. Figure S9 shows the fluorescence spectra of PVK without and with ZnOEP in DCM (blue, pure PVK; red, PVK+0.31 wt % ZnOEP; magenta, PVK +0.62 wt % ZnOEP; navy, PVK+1.15 wt % ZnOEP; olive, PVK +2.5 wt % ZnOEP) at an excitation wavelength of 340 nm. Figure S10 shows the time-resolved spectra of PVK without and with ZnOEP in DCM (magenta, pure PVK; red, PVK+0.31 wt % ZnOEP; blue, PVK+2.5 wt % ZnOEP) at an excitation wavelength of 340 nm. This material is available free of charge via the Internet at http://pubs.acs.org.

where τDA and τD are the average decay times of ZnOEP doped PVK NP and pure PVK NP, respectively. The calculated energy transfer efficiencies from PVK to ZnOEP are found to be 86, 11406



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.P.); [email protected]. jp (V.B.). Phone: (91)-33-2473-4971 (A.P.); (81)-6-6879-4128 (V.B.). Fax: (91)-33-2473-280 (A.P.); (81)-6-6879-7923 (V.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The CSIR is gratefully acknowledged for financial support. S.M. and S.B. thank CSIR for awarding fellowships. Mr. Subrata Das is acknowledged for the technical assistance.

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