Energy Transfer from Polyaniline to Chlorophyll‑a SupramolecularAssembly in NanohybridJhimli Sarkar Manna,* Debmallya Das, and Manoj K. Mitra

Department of Metallurgical and Material Engineering, Jadavpur University, Kolkata-700032, India

*S Supporting Information

ABSTRACT: Polyaniline (PANI)/chlorophyll-a (CHL-a) nanohybrids have beensynthesized using two different oxidants (APS, FeCl3) where the shape of polymericnanostructure is influenced by CHL-a supramolecular arrangement in FeCl3 oxidizedsystem. The presence of stacked CHL-a porphyrin (evident from hypsochromic shift ofQ absorption and shortening of lifetime at CHL-a emission) assists the evolution ofnanorod cluster from PANI nanoflakes connected by 1D nanofibers. The radiative decayrate of CHL-a is found to increase in nanohybrids oxidized via FeCl3 rather than thosevia the APS counterpart as there is a greater amount of CHL-a aggregates present in theformer. This phenomenon indicates energy flux along supramolecular stacking. Thesignificant quenching of the PL spectra and the shortening of the decay time of hostPANI with increasing CHL-a concentration show the energy transfer from PANI toCHL-a is more pronounced in FeCl3 oxidized system, due to shorter donor−acceptor distances. These findings clearly pave theway to architect CHL-a-based functional nanomaterial for effective energy transfer.

■ INTRODUCTION

Porphyrins and metallo-porphyrins with extensive π conjuga-tion are intensively investigated for their excellent photo-physical, photochemical, electrochemical, structural, and self-assembly properties.1 During the fundamental process knownas excitation-energy transfer in photosynthesis, solar energy isharvested by “antenna” molecules, which are well-oriented Mg-porphyrins having the ability to transfer absorbed photons withgreat precession to a distant reaction site. Considerableattention has been paid to porphyrins, metalloporphyrins, andcorresponding porphyrinoid-based nanoparticles due to theirpotential applications in supramolecular chemistry, photo-sensitization, photonics, and various sensor technologies.Nazeeruddin et al. used Zn and Cu complexes of porphyrinsin nanocrystalline dye-sensitized solar cells to convert lightenergy into electrical energy.2 Porphyrinoids and variousporphyrin-based systems are one of the best-suited andversatile chromophoric structures for different light-drivenprocesses. The chlorophylls are a structurally and functionallydistinct group of macrocyclic tetrapyrrole pigments having Mgas the central metal and a long-chain esterifying alcohol.Physically, they are characterized by long-lived excited statesand by intense absorptions in monodisperse solutions. Thespectra range from 330 to 800 nm, extending in aggregates orin vivo to 1020 nm.3 Because of their high absorption and long-lived excited states, chlorophylls are powerful photosensitizers.They strongly absorb light in visible region and transfer theexcitation energy with quantum efficiencies of almost 100%.3

Chlorophylls are often self-organized into nanoscale super-structures where the properties of noncovalently boundassemblies are drastically different from the monomer. Thus,novel functions emerge through ensemble characteristics.

As they are chemically unstable to acids, bases, oxidation, andlight and have a pronounced tendency for aggregation orinteraction with their molecular environments, chlorophyllstabilization in vitro is the greatest challenge to designchlorophyll-conjugated polymer hybrids. We have achievedthis in nanoporous silica and conductive polymeric materi-als.4a,b It is well-known that stable organic materials with a lowband gap are necessary for an enhanced photon harvesting witha long operation time. Conjugated polymers polyaniline andpolypyrrole have an anisotropic, quasi-one-dimensional elec-tronic structure with π electrons coupled to the polymerbackbone via electron−phonon interactions. In contrast withthe conventional inorganic semiconductors with rigid band, thecarriers upon doping or photoexcitation are self-localized andform the nonlinear excitations5,6 on the polymer chains, likesolitons, polarons, or bipolarons, depending on the ground-state degeneracy. Because of their superior and flexibleelectronic, mechanical, and optical properties, they arecandidates in organic photovoltaic (PV) or light emittingdevices that are lightweight and flexible. In the donor−acceptorsystem, poly(9-vinylcarbazole) (PVK) as donor and porphyrinas chromophor acceptor have been investigated thoroughly.7−9

The use of conjugated polymer nanoparticles in the energy-transfer mediated phosphorescence from metalloporphyrin(Pt(II) octaethylporphyrin)-doped polyfluorene nanoparticleto conjugated polymer and its application to biological oxygensensing have also been demonstrated. Now the challenge is toarchitect a hybrid system using chlorophyll-a (CHL-a) as

Received: February 8, 2013Revised: April 18, 2013Published: April 18, 2013

Article

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photosensitizer, within nanometer interface, having manyelectron acceptors at a time, with high charge mobility.Nanostructured conductive polymer as polyaniline can servethe purpose as an energy-transfer mediated host due to itsspecific spectral features well-suited for a CHL-a absorption,thus incorporating special functionality for future photonic andbiophotonic applications. However, application of porphyrin-based functional nanostructures using nanoscopic environmentis still in the embryonic stage.10−13

Therefore, investigations in this field are necessary for in-depth understanding of future prospects in photonic andbiophotonic applications. To the best of our knowledge, thereis no report regarding the study of energy transfer inpolyaninile (PANI)/chlorophyll-a(CHL-a) nanohybrid, wherePANI (host) as an energy donor and CHL-a (guest) as anenergy acceptor are successfully demonstrated focusing onfurther prospects in designing new porphyrin-based materialsfor application in the efficient light harvesting systems. We havealso studied the effect of oxidant during synthesis over themorphology and energy-transfer efficiency of nanohybrids.

■ MATERIALS AND METHODS

Aniline (MERCK), deionized water (MERCK), and ethanol(MERCK) were used as received. CHL-a was isolated fromspinach according to a previously mentioned method throughcolumn chromatography.14

■ EXPERIMENTAL PROCESS ANDINSTRUMENTATION

PANI nanostructures were prepared by the simple templatefree polymerization method using two different oxidants (APSand FeCl3) with aniline as monomer. The oxidant monomerratio has been maintained at 1:1 and 1:3, respectively.15 Thetypical synthesis process is as follows: 0.2 mL of aniline (2.0mmol) is dissolved in 15 mL of deionized water with magneticstirring at room temperature for 0.5 h, and cooled in an icebath. A solution of precooled APS (2.0−4.0 mmol) in 15 mL ofdeionized water is then added to the above solution. Themixture is allowed to react for 12 h. In APS oxidized polymers,the temperature is maintained around 4 °C. The precipitatesare collected and washed several times with deionized water,methanol, and ether. Finally, the products are dried in a vacuumat room temperature for 24 h. In this approach, the oxidantserves as both oxidant and dopant at the same time because ofprotons (H+) generated from the oxidant during the polymer-ization process. The method not only provides a quantitativeway of controlling the diameter of the PANI nanofibers, butalso simplifies the reagents. During the synthesis of CHL-aentrapped nanohybrids, CHL-a is added to the monomersolution and utrasonicated for 20 min followed by the dropwiseaddition of oxidant in ice bath. After 24 h reaction at respectivetemperature, the dark green products are washed and obtainedby filtration. To understand the quenching process, themeasurement of fluorescence intensity with varying quencher(CHL-a) concentrations (2.79 × 10−6, 5.58 × 10−6, 8.37 × 10−6

mol) per mole of aniline has been chosen.The morphological characters and sizes of PANI and PANI/

CHL hybrids were measured by a field emission scanningelectron microscope (Hitachi, S4800) and a high resolutiontransmission electron microscope (JEOL, JEM 2100),respectively. Optical measurements were carried out by Fouriertransform infrared spectroscopy (Shimadzu, 8400S) and a UV−

vis spectrophotometer (Perkin-Elmer, Lambda35), and photo-luminescence spectra were recorded by an assembledspectrofluorometer with 1000 W xenon source (Spectraphysics, 74100). For time correlated single photon counting(TCSPC) measurements, the samples were excited at 405 nmusing a picosecond diode laser (IBH Nanoled-07) in an IBHFluorocube apparatus. Typical full width at half-maximum(fwhm) of the system response using a liquid scatter was about90 ps. The repetition rate was 1 MHz. The fluorescence decayswere collected with a Hamamatsu MCP photomultiplier(C487802). The fluorescence decays were analyzed usingIBH DAS6 software. The following equation was used toanalyze the experimental time-resolved fluorescence decays:

∑ ατ

= + −⎡⎣⎢

⎤⎦⎥P t b

t( ) exp

i

n

ii (1)

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

τττ

⟨ ⟩ =∑∑

=

=

a

ain

i i

in

i i

12

1 (2)

ai is the contribution of the decay component.

■ RESULTS AND DISCUSSIONStructural and Conformational Study. Figure 1 shows

the FE-SEM images of bare PANI nanotubes and CHL-a

incorporated nanohybrids oxidized via APS (A,B) and FeCl3(C,D), respectively. The diameter of APS oxidized barepolymer (Figure 1A) is larger (100 nm) than that of theFeCl3 oxidized counterpart (Figure 1C), in which, along withnanofiber of 20 nm diameter, nanoflakes are found. Theprobable explanation may be that as the reaction rate of FeCl3oxidation is slower due to lower reaction temperature (0 °C)and lesser oxidation potential of FeCl3, monomers find enough

Figure 1. FE-SEM images of (A) APS oxidized bare PANI, (B) APSoxidized PANI/CHL-a nanohybrid, (C) FeCl3 oxidized PANI/CHL-a,and (D) FeCl3 oxidized bare PANI.

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passage of time to use ice as the template for synthesis. Ice inreaction mixture (at lower temp 0 °C) expels the solutes, whichare dispersed homogeneously in water associated with theforming ice phase, and entraps them within the directedchannels between the ice crystals.17 The piled up ice crystalsoffer a lamellar and textured template to define structurednanoflakes. Polymerization of aniline takes place in the frozenice phase as well as a small amount of water associated with icephase leading to the growth of finer interconnected nanofibersgrowing from stacked polymeric cluster. By this strategy, PANIflakes connected by 1D nanofibers can be obtained.The size of the micelles (the soft templates) is independent

of the redox potential of the oxidants. However, the redoxpotential of the oxidants (APS 2.05 eV, FeCl3 0.77 eV) willaffect the polymerization or elongation process, which controlsthe growth of the nanofibers. This means that higher redoxpotential leads to higher polymerization or elongation rate,resulting in a larger diameter of the nanofibers for a givenpolymerization time.18,19 This might be the reason for whichthe diameter increased with increasing redox potential of theoxidants.Figure 1B unambiguously confirms that the size of bare

polymer nanofiber remains essentially unchanged after theencapsulation of CHL-a in APS oxidized polymer.16 Yet inFeCl3 oxidized nanohybrids, morphology changes frominterconnected nanofiber like structure (Figure 1C) withnanoflakes to small nanorod clusters (Figure 1D) with diameteraround 20−30 nm, which is much less as compared to former100 nm. Aggregated CHL-a stacked in water-rich region (due

to amphiphilicity) can act as soft template for the formation ofsmall PANI nanorod clusters of around 30−50 nm diameter.The diameter complements efficiently with the size of H-stacked porphyrin.20

UV−Vis Spectroscopy. In nanohybrids, aggregation (bothJ and H) of porphyrin certainly indicates some amount ofstructural deformation from monomer. Signature around 600nm (Figure 2A and B) in FeCl3 oxidized polymer indicates thepresence of blue-shifted aggregates. An intimate molecularinteraction between the polymer matrix and porphyrins resultsin moderate to major structural deformations of the porphyrinframeworks, which is reflected in the hypsochromicaly shiftedabsorption spectra in nanohybrids.21

Another potential factor that can contribute to the blue shiftof the spectral peaks of CHL-a moiety in nanohybrid is thepossible variation in the electronic charge delocalization withinthe porphyrin macrocycle under polymeric environments. TheHOMO energy gap can be lowered through decreasing theelectron density to the porphyrins macrocycle by an electron-withdrawing group.22 The quinoid species in the PANI chaincan serve the purpose by lowering the HOMO energy state andincreasing the HOMO−LUMO gap. The decreased electrondensity may further assist the stabilization of porphyrin stackingby restricting distortion. This is further supported by FTIRspectra of FeCl3 oxidized (Figure 2C) nanohybrids wherequinoid vibration at 1572 cm−1 is less intense as compared tothe APS counterpart (Figure 2D).The large blue shift may be the cumulative effect of all of

these interactions. The presence of various aggregates can be

Figure 2. UV−vis spectra of APS oxidized (A), and FeCl3 (B) oxidized samples. FTIR spectra of FeCl3 oxidized (C) and APS oxidized (D) samples,respectively.

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further supported by the decay curve obtained from TCSPCdata. The enhanced π-overlap between polymer and adjacentCHL-a molecules may result in redistribution of transitionmoment generating new transition probability within thepolaronic band gap. The signature is enhanced intensity ofthe delocalized polaronic peak in UV−vis spectroscopy ofnanohybrids.In APS oxidized nanohybrids (Figure 2A), along with peak

broadening in the π−π* transition region, a prominentmonomeric 666 nm porphyrin peak of Q absorption andsmall H aggregate peak at 612 nm are evident. The broadeningof the Soret band in all CHL-a incorporated samples originatesfrom a multiplicity of excitonic aggregate interactions, whicharise from a variety of inter planar porphyrin−porphyrin andporphyrin−polymer core geometries. The presence of a large

counterion in larger diameter nanotube and faster reaction ratemay be the probable explanation of lesser interaction in theAPS oxidized nanohybrid.

Steady-State and Time-Resolved Spectroscopy. Figure3A and B shows the emission spectra of pure polyanilinenanostructures and PANI/CHL-a nanohybrids for bothoxidants in DMF in the same environment as thecorresponding UV−vis spectra with 405 nm excitation. Figure3A and B, insets, shows the change in PL intensity with CHL-aconcentrations for APS and FeCl3, respectively. All dataindicate enhancement of quenching efficiency of PANI withincreasing concentration of CHL-a. The PL quenchingefficiencies of PANI in FeCl3 oxidized nanohybrids are foundto be 32%, 70%, and 90%, whereas in APS oxidizednanohybrids they are found to be 13%, 20%, and 22% for

Figure 3. In all spectra, bare PANI are denoted as (a), and nanohybrids with 2.79 × 10−6, 5.58 × 10−6, and 8.37 × 10−6 mol of CHL-a loading aredenoted as (b), (c), and (d), respectively. (A) PL emission intensity of APS oxidized bare PANI decreases with increasing CHL-a concentration. (B)Emission spectrum of FeCl3 oxidized samples where the decrease in emission intensity of bare PANI with CHL-a concentration is more pronouncedthan APS counterpart. Insets show the decrease in PL intensity of PANI with increasing CHL concentrations. (C) Decay curve of FeCl3 oxidizedsamples. (D) Decay curve of APS oxidized samples.

Table 1. Decay Time Components of All Samplesa

sample emission λ (nm) τ1 (ns) a1 τ2 (ns) a2 τ3 (ns) a3 ⟨τ⟩ (ns) E% χ2

AUD 420 0.819 0.754 0.427 0.0778 1.575 0.1682 1.023 1.19AD1 420 0.696 0.5644 0.396 0.0894 1.332 0.3462 1.013 0.97 1.12AD2 420 0.688 0.0686 1.293 0.2626 0.818 0.6688 0.944 7.72 1.17AD3 420 0.044 0.0074 0.406 0.8794 1.432 0.1132 0.725 29.13 1.13AD3 660 0.030 0.2894 2.293 0.0632 4.541 0.6474 4.423 1.17

aExcitation at 405 nm. AUD = APS oxidized bare PANI; AD1, AD2, and AD3 = APS oxidized nanohybrids with 2.79 × 10−6, 5.58 × 10−6, and 8.37× 10−6 mol of CHL-a loading, respectively.

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CHL-a concentrations of 2.79 × 10−6, 5.58 × 10−6, and 8.37 ×10−6 mol, respectively. These may be due to strongerinteractions in the shorter nanospace of FeCl3 oxidized hybrid.This quenching phenomenon is associated with the Forsterenergy transfer from PANI to CHL-a, which is furtherconfirmed by time-resolved spectroscopy (Figure 3C and D).The decay curves of pure PANI nanostructures (Figure3C(a),D(a)] are well-fitted by multiexponential kinetics havingan average lifetime of 1.023 and 1.087 ns, respectively, in bothof the systems. The decay curve of PANI/CHL-a oxidized viaAPS at an emission wavelength of 420 nm (Figure 3C(d)) isfitted by multiexponential kinetics having an average lifetime of0.296 ns in FeCl3 oxidized and 0.725 ns for highest CHL-aconcentration. The decay time data of polyaniline with varyingCHL-a concentrations are given in Table 1 for APS oxidizedand Table 2 for FeCl3 oxidized samples, and in Figure 3C(a)−(c) for FeCl3, and Figure 3D(a)−(c) for APS oxidizednanohybrids, respectively. Results show that the values oflifetimes corresponding to the PANI component graduallydecrease with CHL-a concentrations in nanohybrids. They aremuch shorter in FeCl3 oxidized nanohybrids than in APSoxidized counterpart. This phenomenon indicates that theenergy transfer from PANI to CHL-a is more pronounced inFeCl3 oxidized materials. It is also observed that at an emissionwavelength of 660 nm (CHL-a emission), the decay curves(Figure 4) are well-fitted by biexponential decay for FeCl3synthesized nanohybrids (Figure 4c) with highest CHL-aconcentration. The decay components are τ1 1.912 ns (16.29%)and τ2 0.348 ns (83.71%) with average lifetime of 1.156 ns,which is t times less than monomeric CHL-a, with the average

lifetime calculated for bare CHL-a in DMF as 4.474 ns (Figure4a). The emission decay envelope in APS synthesizednanohybrid for highest CHL-a concentration (Figure 4(b)comprises the emission decays of the different types ofaggregates. It is triexponential where monomeric CHL-acontributes significantly to the overall decay kinetics. Thedecay components are τ1 0.030 ns (28.94%), τ2 2.293 ns(6.32%), and τ3 4.541 ns (64.74%), with an average lifetime of4.423 ns (Table 1), which is very close to monomeric CHL-a invitro.23 It may be postulated that FeCl3 oxidized polymer asevident from biexponential decay contains two different CHL-asupramolecular arrangement, where monomeric CHL-a(evident from UV−vis spectroscopy) along with aggregates isalso present in triexponential decay of APS oxidized polymer.Emission lifetime of CHL-a within FeCl3 oxidized nano-

hybrids (Figure 4c) is 3 orders of magnitude smaller than thoseof monomeric CHL-a24 due to aggregation related annihilation.Singlet−singlet annihilation of excited CHL-a molecules inphotosynthetic antennae systems, where several CHL-amolecules may be coupled by excitonic interactions, accountsfor the decrease in the emission probability. Biomolecularsinglet−singlet annihilation is a well-known phenomenon,which occurs when two or more singlet excitations appearsimultaneously within the same spatial domain;25−28 suchannihilation arises from dipolar interactions between twoexcited pigment molecules. This interaction induces a non-radiative transition in which one of the excited moleculesrelaxes to the ground state and the other is excited to a higher-energy state. Subsequently, the latter excited state rapidlyreturns to the lowest excited state, thus annihilating one of thetwo original excitations. It is appealing to correlate the observeddata with the presence of a greater amount of aggregated CHL-a in FeCl3 oxidized system. These CHL-a aggregates havebeneficial effects on exciton migration dynamics along stackedCHL-a, as the supramolecular packing is expected to be crucialwith regard to energy-transfer characteristics even though theprobability of excitonic migration through polymer chaincannot be completely excluded.29

Photo-Induced Energy-Transfer Studies. It is clearlyseen that there is a perfect overlap (Figure 5) between theabsorption spectrum of CHL-a and the emission spectrum ofPANI, thus indicating a possibility of energy transfer from theenergy donor PANI host to the energy acceptor CHL-a guest inthe nanohybrid systems. In general, fluorescence resonanceenergy transfer (FRET) is a process involving the radiationless(nonradiative) transfer of energy from a “donor” fluorophore toan appropriate “acceptor” counterpart. This process arises fromthe dipole−dipole interactions and strongly depends upon thecenter-to-center distance of corresponding energy-donor andacceptor. According to the Forster theory,30 the rate of the

Table 2. Decay Time Components of All Samplesa

sample emission λ (nm) τ1 (ns) a1 τ2 (ns) a2 τ3 (ns) a3 ⟨τ⟩ (ns) E% χ2

FUD 420 0.368 0.696 0.022 0.6820 1.594 0.2484 1.087 1.19FD1 420 0.264 0.0474 1.006 0.3403 0.632 0.6123 0.798 26.58 1.13FD2 420 0.130 0.0069 2.198 0.0128 0.123 0.9733 0.515 52.62 1.17FD3 420 0.412 0.0017 2.313 0.0010 0.017 0.9973 0.296 72.76 1.02FD3 660 1.912 0.1629 0.348 0.8371 1.156 1.16CHL-a 675 3.458 0.1359 4.594 0.8641 4.474 1.12

aExcitation at 405 nm. FUD = FeCl3 oxidized bare PANI; FD1, FD2, and FD3 = FeCl3 oxidized nanohybrids with 2.79 × 10−6, 5.58 × 10−6, and 8.37× 10−6 mol of CHL-a loading, respectively; CHL-a = bare chlorophyll-a.

Figure 4. Decay curve at 660 nm of (a) APS oxidized, (b) FeCl3oxidized PANI/CHL-a nanohybrid with highest CHL-a concentration,and (c) bare CHL-a in DMF.

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energy transfer for an isolated single donor−acceptor pairseparated by a distance r is given by the following equation:

τ= ⎜ ⎟⎛

⎝⎞⎠k r

Rr

( )1

TD

06

(3)

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

ϕπ

λ=Rk

NnJ

9000(ln 10)

128( )0

62

D5 4 (4)

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

∫λ λ ε λ λ= λ∞

J F( ) ( ) ( ) d0

D A4

(5)

where FD(λ) is the normalized emission spectrum of donor,εA(λ) is the absorption coefficient of acceptor at the wavelengthλ (in nm), and κ2 is the orientation factor of two dipolesinteracting. The value of κ2 depends on the relative orientationof the donor and acceptor dipoles. For randomly orienteddipoles, κ2 = 2/3, and it varies between 0 and 4 for the cases oforthogonal and parallel dipoles, respectively. For our donor−acceptor system, the calculated overlap integral is found to be3.204 × 1015 M−1 cm−1 nm4, and the calculated Forster distanceis 52.77 Å (Table 3). The difference in donor−acceptordistance between APS oxidized (61.19 Å) and FeCl3 oxidizednanohybrids (44.79 Å) clearly indicates the greater energy

transfer rate in the later one, which is 2.46 ns−1 in FeCl3oxidized and 0.32 ns−1 in APS oxidized nanohybrids (Table 3).The subsequent decrease of decay time of PANI in the

presence of different concentrations of CHL-a molecules shownin Figure 3C (in FeCl3 oxidant) and in Figure 3D (in APSoxidant) confirms the photo-induced host−guest energytransfer from the PANI energy-donor to the CHL-a energy-acceptor. The energy transfer efficiency has been calculated byusing the following equation:

ϕττ

= −1ETDA

D (6)

where τDA and τD are the average decay times of CHL-a/PANInanohybrids and pure PANI, respectively. The effect of CHL-aloading on energy transfer efficiency has been given in Table 1(APS) and Table 2 (FeCl3). The calculated energy transferefficiencies from PANI to CHL-a are found to be 29.13% and72.76% for APS and FeCl3 oxidized nanohybrids, respectively,with 8.37 × 10−6 M CHL-a loading. The extremely highefficiency of energy transfer related to photoluminescencequenching is not only due to the dipole−dipole interactionbetween the donor and acceptor but also due to excitonicenergy diffusion throughout the polymer chain, which maysufficiently increase the energy transfer effectiveness. AJablonski diagram representing the energy transfer processfrom polyaniline to H-assembly of CHL-a has been given inScheme 1.

■ CONCLUSIONSThis work intended the implementation of CHL-a supra-molecular arrangement inside the conjugated PANI matrixwhere energy transfer from the PANI host to the CHL-a guestin nano rod oxidized via FeCl3 occurs with more than 70%efficiency (at 8.37 × 10−6 mol of CHL-a concentration). Thisphenomenon may be related to the smaller donor−acceptor

Figure 5. Overlap between the absorption spectrum of (a) CHL-a and(b) emission spectrum of PANI.

Table 3. Data of the Donor−Acceptor System for AllSamplesa

sample R0 (Å) r (Å) kT(r) (ns−1)

AD 52.77 61.19 0.32FD 52.77 44.79 2.46

aAD = APS oxidized nanohybrid with 8.37 × 10−6 mol of CHL-aloading; FD = FeCl3 oxidized nanohybrid with 8.37 × 10−6 mol ofCHL-a loading.

Scheme 1. Jablonoski Diagram of Energy Transfer fromPANI to CHL-aa

aET = energy transfer. (1) CHL-a monomer excitation, (2) vibrationalrelaxation of excited CHL-a monomer, (3) emission of CHL-amonomer, (4) S1 orbital splitting in H aggregated CHL-a, (5)excitation of H aggregated CHL-a, (6) vibrational relaxation of Haggregated CHL-a, (7) emission of H aggregated CHL-a, (8) energytransfer from PANI to H aggregated CHL-a, and (9) PANI donorexcitation.

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distance in comparison with the APS counterpart (29.13%energy transfer efficiency). The gradual enhancement of PLquenching efficiency of PANI is observed with CHL-a loading.The lifetime measurement at CHL-a emission band signifiesenergy flux along supramolecular stacking, which differssignificantly in both the system due to the probable differencesof supramolecular architectures and host−guest distances inpolymeric system. Also, energy diffusion through polymericchain cannot be completely excluded. The remarkably highefficiency of energy transfer in the PANI/CHL-a nanostructureopens their probable application in light-harvesting systems andother photo-driven devices.

■ ASSOCIATED CONTENT*S Supporting InformationUV−vis spectroscopic and aggregation studies. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: +919831566632. E-mail: jhimlisarkar0@gmail.com.Author ContributionsJhimli Sarkar Manna and Debmallya Das have contributedequally in this manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge the help rendered by Mr. Subrata Das ofIndian Association Cultivation of Science, Kolkata 700032,India, for the technical assistance of TCSPC characterization.J.S.M. thanks CSIR for a fellowship. Financial support from theUniversity Grant Commission under BSR-Meritorious 2011−2012 scheme to D.D. is also gratefully acknowledged.

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