Confined Motion, Excitonic Migration, and Superradiance of Ordered

Article pubs.acs.org/JPCC

Confined Motion, Excitonic Migration, and Superradiance of Ordered Chlorophyll a Assembly Packed in Two Different Polypyrrole Nanostructures Jhimli Sarkar Manna,*,† Debmallya Das,† and Manoj K Mitra Department of Metallurgical and Material Engineering, Jadavpur University, Kolkata 700032, India S Supporting Information *

ABSTRACT: The intermolecular excitonic migration in highly packed environment of light harvesting complex can be regulated by different peptide binding during photosynthesis. Porphyrin based conducting polymer nanostructures might be expected to exhibit similar behavior by introducing disorder through different interactions, thereby facilitating control over excitonic delocalization. Polypyrrole synthesized in the presence of two different chlorophyll a (CHL-a) species [spherical and cylindrical micelles] not only gives rise to two different nanostructures that are spherical and rod shaped (1D), evident from FESEM, but also differ in excitonic interactions, which is evident from fluorescence, UV−vis, and TCSPC experiments. The excitonic delocalization length and dipole−dipole coupling strength are higher in spherical species coupled with strong macrocyclic coordination resulting in enhanced superradiance. The ratio of radiative and nonradiative decay rate is higher in 1D nanostructure, implying that exciton decay rate through nonradiative pathways decreases in this nanohybrid in comparison with spherical counterpart. These facts also corroborate with the greater anisotropy data. The fast and slow anisotropic decay profile of 1D chlorophyll a/polypyrrole (CHL-a/PPY) nanostructure is significantly slower than that of its spherical counterpart [fitted with wobbling and cone model]. The greater value of order parameter in nanosphere, indicating highly oriented distribution, also supports the increased coherence length. These findings suggest that the nanohybrids are promising candidates in solar energy conversion schemes.

■

INTRODUCTION In nature chlorophylls are often self-organized into nanoscale superstructures where sunlight is funneled through light harvesting complexes comprising ∼200 molecules to a reaction center and where energy is transferred via a sequence of quantum mechanical energy-transfer processes across a total distance of ∼20−100 nm with near-unit quantum efficiency.1 Thus, systems containing artificial porphyrin arrays with specific geometry are very likely to mimic the antenna function for its long-range association of π-network and can afford favorable excitonic migration for photovoltaic and organic electronic application. The localized π−π* transitions of the monomer can be delocalized as coherent excited states in aggregates of porphyrins within this system.2,3 Functional connectivity of pigments in the peripheral antenna and the core antenna in the photosystem unit [PSU] is determined by both the degree of overlap of spectral forms and the structural coupling of the components of the PSU, including distance and orientation. Pigment−pigment and pigment−protein interactions are primarily responsible for energy transfer efficiency of as high as 95%. This ideal packing environment comprising a perfect stacking pattern of chromophores, well-supported by a protein scaffold4,5 of light-harvesting complexes, is required for strong excitonic interaction and long-range dipole−dipole interaction.5 Among various strategies employed to prepare porphyrin-based © 2014 American Chemical Society

molecular assemblies, noncovalent self-assembly of molecular units is advantageous because it provides versatility in molecular networking in multidimensional space.6,7a−d As the porphyrin aggregations are formed by many weak interactions, namely, van der Waals, π−π, electrostatic, and hydrogen-bonding interactions, the stacking disorders are prone to occur in porphyrin self-assembly; thus, porphyrin aggregates within a conducting polymer may be in an altered macrocycle conformation, similar to plants where exciton coupling is optimized by regulating peptide conformation. These porphyrin-conducting polymer nanohybrids are relevant for the design of artificial light-harvesting antennas. We have pursued the self-assembly approach to construct CHL-a [chlorophyll a]/PPY [polypyrrole] hybrids where the presence of different self-assembled CHL-a species affects the evolution of two different PPY nanostructures. In contrast with the conventional inorganic semiconductors with rigid band, the carriers of conjugated polymers polyaniline and polypyrrole upon doping or photoexcitation are self-localized and form the nonlinear excitations on the polymer chains, like solitons, polarons, or bipolarons, depending on the ground-state degeneracy.7e Investigations on various photophysical properReceived: November 17, 2013 Revised: March 6, 2014 Published: March 6, 2014 6558

dx.doi.org/10.1021/jp411285z | J. Phys. Chem. C 2014, 118, 6558−6564

The Journal of Physical Chemistry C

Article

Figure 1. FE-SEM (inset TEM) images of (A) CHL-a/PPY nanorod and (B) CHL-a/PPY nanosphere.

2]. PPY is sp2 hybridized and does not have an unshared electron to react with, thus, it can interact with cluster head

ties of two CHL-a/PPY nanostructures afford the opportunity to elucidate the excitonic delocalization length and superradiance among the supramolecular assemblies of CHL-a, which differ in two different nanoarchitectures, coupled to the different rigidity, dipole−dipole coupling, and degree of inhomogeneity of CHL-a molecular species. Such knowledge based on porphyrin conjugated polymeric nanosystems with special functionalities will enable us to construct chlorophyll based nanomaterials for application in efficient light harvesting system.

■

MATERIALS AND METHODS CHL-a molecules were extracted from fresh spinach leaves and purified according to a previously published method19−21[see Supporting Information]. To synthesize nanohybrids, two kind of aggregated species have been selected absorbing at around 714 nm [named as S714, spherical] and around 740 nm [named as S740, cylindrical]. Polypyrrole nanohybrids have been synthesized in the presence of these two chlorophyll aggregates S714 [concentration of 10−7 M] and S740 [concentration of 10−6 M]. To synthesize three different nanohybrids, distilled pyrrole monomer [2 mM] was added with all of the above CHL-a species. Precooled FeCl3 (2.0−4.0 mM) in 15 mL of deionized water was used as oxidant. The polymerization was allowed for 12 h at 0 °C.

Figure 2. UV−vis spectra of (a) CHL-a micelle and (b) spherical species, with inset showing their corresponding AFM images.

through π-interactions or can be dissolved in CHL-a solution forming a CHL-a/PPY cluster, eventually giving rise to a spherical nanostructure (denoted as PPY1 in figures) after polymerization. The presence of these species was accompanied by a decrease of fluorescence intensity [Figure 3B]. Similarly in the presence of a CHL-a micelle [Figure 2], 1D nanostructure (denoted as PPY2 in figures) of around 50 nm diameter evolved, where PPY/CHL-a polymer can act as a supramolecular template.8,9 The broadening of 460 nm π−π* PPY absorption in UV−vis spectra [(Perkin-Elmer, Lambda 35)] of 1D nanohybrid [Figure 3A] indicates the intimate molecular interaction between the polymer matrix and porphyrins, which originates from a multiplicity of exciton aggregate interactions among a variety of interplanar porphyrin−porphyrin and porphyrin− polymer core geometries, reflecting their altered geometry and different conformational freedom. Absorption bands of these pigment−polymer complexes in π−π* transition region of polypyrrole are broadened in the S740 species; i.e., because of small differences in the environment, each pigment has a slightly different absorption maximum. This effect causes a decrease in exciton delocalization and significant changes in the spectroscopic properties similar to light harvesting complexes. The extent of delocalization depends on the magnitude of the

■

RESULTS AND DISCUSSION Figure 1 shows the FE-SEM [field emission scanning electron microscope (Hitachi, S4800)] and TEM [high resolution transmission electron microscope (JEOL, JEM 2100)] images of CHL-a/PPY nanohybrids formed in the presence of two different CHL-a species, cylindrical micelle (S740) and spherical (S714) micelle. Data unambiguously confirm the formation of two different nanostructures, spherical in species S714 and rod shaped in the presence of S740. The S714 species originate by aggregation of CHL-a molecules in which the phytyl chains are segregated in the inner part, thus fostering the pigments to expose the macrocyclic heads toward the bulk water solvent. The high hydrophobic effect induces the CHL-a molecules to self-organize in order to minimize the repulsive interactions of water−phytyl chains, leading to closed structures, evident from UV−vis [Figure 1] and AFM [Figure 6559



Article

Figure 3. (A) UV−vis spectra, (B) fluorescence spectra, (C) TCSPC, and (D) anisotropy of (a) PPY2 and (b) PPY1, where PPY1 = CHL-a/PPY nanosphere and PPY2 = CHL-a/PPY nanorod.

Table 1. TCSPC and Anisotropy Decay Components of All Samples (Excitation at 405 nm) emission decay time

a

anisotropy

sample

emission λ (nm)

τ1 (ns)

a1 (%)

τ2 (ns)

a2 (%)

⟨τ⟩ (ns)

τf (ns)

a1 (%)

τs (ns)

a2 (%)

PPY1 PPY2

650 660

0.375 0.406

86.15 75.86

1.05 2.575

13.85 24.14

0.584 1.857

0.08140 0.44067

62 78.65

0.21552 2.2052

38 21.35

a

PPY1 = CHL-a/PPY nanosphere. PPY2 = CHL-a/PPY nanorod.

porphyrin rings due to site specific inhomogeneity induced by polymeric environment was also invoked to explain the lesser shifting in fluorescence spectrum of 1D causing localization of exciton. Larger shift implies that the porphyrin sphere has a more rigid conformer, which is further supported by TCSPC and anisotropy measurements.12 The time-resolved fluorescence decays of the porphyrin molecules within two nanohybrids are displayed in Figure 3C, and their fitted parameters are tabulated in Table 1. The fluorescence lifetime decreases profoundly in nanohybrids because of the increased conformational heterogeneity.13 The τ values with biexponentially fitted decay curves are given in Table 1. Slow fluorescence decay components of the porphyrin units originate from the more planar porphyrin conformer. The relative amplitude of the fast decay component (τ1) gradually increases from nanorod to nanosphere, implying that spherical nanostructures have the smallest average decay time. This may be due to enhanced nonplanarity of the porphyrin macrocycle originating from different intermolecular coordination.14 This can be the result of static or dynamic disorders introduced via polymeric environment within the macrocycles of the porphyrin species. This enhanced nonplanarity in turn may lead to formation of different excitonic states as a result of polypyrrole-packed pigment interactions. Geometrical homogeneity and packing density of chromophores are significantly important for the effective excitation energy transfer. Polymer environment induces site inhomoge-

pigment−pigment coupling compared to the amount of broadening. The larger the spread in site energies and the amount of broadening, the less delocalized are the excitations.10a Enhanced inhomogeneous broadening of Soret band region in 1D nanohybrid indicates strong site inhomogeneity through polymeric interaction. Inhomogeneity causes an increase of the dipole strength of the lowest state in comparison to the monomer causing localization of electron.10a This is further supported by a lesser delocalization length in 1D nanohybrids calculated from radiative decay rate. We have not found any broadening of spherical species in this region probably because of lesser spreading of energy. However the Q absorption of each CHL-a species (714 nm for S714 and 740 nm for S740) [Figure 3A] is superimposed with a delocalized polaronic peak of PPY in the corresponding nanohybrids, indicating a new transition probability within the polaronic band gap.10b,11 Figure 3B shows the emission spectra [assembled spectrofluorometer with 1000 W xenon source (Spectra Physics 74100)] of CHL-a/PPY nanohybrids in water. The blue shifts from monomeric CHL-a emission (675 nm) and decreased fluorescence intensity in nanohybrids confirm the presence of pigment−pigment interactions related to the exciton delocalization, which are responsible for quenching of the emission signal through intermolecular excitonic interaction. The blue shift is due to electronic interactions among the porphyrin units having enhanced macrocyclic rigidity. Weak coupling between 6560



Article

Table 2. Fluorescence Quantum Yields, Radiative and Nonradiative Rates, and Relative Dipole Strengths of All Samples sample

quantum yield (±3%)

CHL-a PPY1 PPY2 a

0.30 0.20 0.23

a

rad. rate krad(ns−1) (±6%)

nonrad rate knrad (ns−1) (±6%)

krad/knrad

refractive index n

emit dip. strength (D2) (±6%)

0.049 0.34 0.14

0.11 1.36 0.39

0.44 0.25 0.35

1.36 1.92 1.92

1.00 4.65 2.00

Taken from ref 22. PPY1 = CHL-a/PPY nanosphere. PPY2 = CHL-a/PPY nanorod.

that the exciton length in bacteriochlorophyll [BCHL-a850] is around four BCHL-a molecules.15b Many authors have reported delocalization length ranging from a few pigment molecules to the full ring.15c,d Some of this discrepancy may be attributed to the use of different definitions. However, more importantly, different experimental techniques are expected to yield different coherence sizes. For instance, time-resolved and steady-state methods should give different results if the exciton delocalization length is time dependent. The exciton wave functions are linear combinations of molecular states. Since electronic coupling causes molecules to behave coherently, the coherence can be reduced by static and dynamic disorder, which leads to localization. On the other hand, a short and spectrally broad excitation pulse creates a coherent superposition of a number of exciton states, i.e., an excitonic wave packet. Because of interference, the wave packet may have a major part of its amplitude in a very much localized region of the aggregate. At room temperature, the coherence length is about four BCHL-a pigments, in good agreement with previously reported results for a static disorder with a Gaussian profile and a variance of about 210 cm−1.15e−i The coherence length we have found seems longer than that in light harvesting complexes (LHC). The greater coherence length also supports the significance of faster decay component in spherical nanohybrids where excitonic delocalization faces lesser disorder in comparison with the 1D counterpart. Superradiance as well as coupling strength is also higher in the spherical structure and implies that larger excitonic states with strongly coupled molecules with larger coherence length are formed within polymeric environment in the nanosphere. The absorption properties of chlorophylls in LHCs may be tuned over a fairly broad range by structural changes in the surrounding protein environment in photosynthetic architecture. A fascinating piece of evidence for the high degree of optimization of the antenna system is found when looking at the BCHL800 to BCHL850 energy-transfer rate as a function of the size of the fluctuations of the BCHL850 site energies.15j It is worth noting that without disorder in the site energies this energy-transfer rate would be significantly faster. The broad range over which the enhancement occurs testifies not only to the robustness against structural fluctuations but also that the antenna system takes advantage of the fluctuations of the protein environment to fine-tune its energy-transfer properties. The difference in the components of the anisotropic and isotropic decays in nanohybrids strongly suggests the influence of polymer environment over the dipolar coupling strength related to excitonic delocalization. This influence has been mentioned as tuning previously. This is not tuning of selfaggregation. The PPY may induce homogeneity or structural fluctuation via interactions through aromatic residue, resulting in coupling difference like protein does in photosystems. Energy transfer (ET) mechanisms, incoherent versus coherent, are dependent upon (i) the size of the aggregate, (ii) the distances between the interacting pigments, (iii) the relative angles between the transition dipoles of the Qy transitions, and

neity [static disorder] or electron phonon coupling which strongly differs in two different pigment packing and influences the degree of exciton delocalization. The decrease in the fluorescence lifetime also can be attributed to the increase in the number of other possible decay pathways that could arise from the addition of PPY. The spectroscopic observable that is directly related to the exciton delocalization is the so-called superradiance.10a This term means that the radiative rate of a complex of pigments is larger than that of the individual pigments. This phenomenon has been observed in strongly coupled molecular aggregates and has been taken to reflect collective behavior of the pigments within the aggregate. From the quantum yield ϕf1 and the fluorescence lifetime τf1, we measure the radiative decay rate (krad) using the following relation:

k rad =

ϕf1 τf1

This rate can subsequently be related to the dipole strength of the emitting state using the formula of the Einstein coefficient for spontaneous emission k rad = n

16π 3ν 3 2 |μ ⃗ | 3ε0hc 3

(1)

In eq 1, the refractive index n of the bulk matrix is 1.92 for PPY and the vacuum dielectric constant ε0 is of the direct surroundings. Using the radiative lifetime from refs 14b and 14c, we calculate the (vacuum) dipole strength |μ⃗ |2 for superradiance unit of the nanosphere and nanorod, which is 4.65 and 2.00 D2, respectively [Table 2]. The large differences of radiative decay rate between two species can be attributed to the great difference in dipole coupling strength coupled with different specific interactions like hydrogen bonding or the close presence of aromatic residues of the polymeric environment which brings about disorders within systems. These interactions can shift the central absorption of the pigments away from the wavelength of absorption in solution. The dipole−dipole coupling of emitting state as well as superradiance is stronger in the spherical nanohybrid. The observable superradiance is not easily related to the delocalization length, since the emitting (lowest) state does not have to be the exciton state where all the oscillator strength is concentrated. The amount of superradiance, however, sets a lower limit to the number of pigments participating in the emitting state.10a We calculated the coherence length using the radiative decay rate, which is roughly the coherence number times the monomer radiative decay rate. The reduced radiative lifetime is related to exciton coherence of porphyrin species and extends over approximately seven monomers in spherical and approximately three monomers in the 1D nanostructure.15a The disorder, which limits the exciton delocalization length in the 1D nanostructure, is probably much larger because of different polymeric interactions. Available information suggests 6561



Article

Debye relation with the stick boundary condition through following equation.

(iv) the extent of static and dynamic disorder. In both the nanohybrid systems, interactions between different polymeric structures and chlorophyll species introduce altered geometry via static and dynamic disorder, which in turn affects the angles between the transition dipoles of the Qy transitions. We calculated the nonradiative decay15k rate of nanohybrids following the equation

k nrad =

τM =

(2)

where ϕf1 is the quantum yield and τf1 is the lifetime of nanohybrids. The faster radiative decay rate [Table 2] in comparison to the nonradiative decay rate in the nanohybrids implies that the exciton decay through other deactivation pathways is pronounced. The slower nonradiative rate in 1D nanohybrid in comparison to the spherical counterpart may be further supported by anisotropy data where rotational motion is more restricted in the nanorod structure. Thus, in the nanostructures other exciton deactivation pathways like formation of triplet state, energy transfer to photochemical trap (ect) may become prominent. According to the model involving wobbling-in-cone and translational diffusion along the surface, the anisotropy decay r(t) of nanohybrids arises as a result of three independent motions: (i) wobbling motion rw(t) of the CHL-a molecules in a cone; (ii) translational motion rt(t) of the CHL-a along the surface of the nanosphere and nanorod; (iii) rotation rM(t) of the nanohybrids as a whole. r(t ) = rw(t )rt(t )rM(t )

τM =

Dr =4

Cr=

τr (ns)

τd (ns)

τm (μs)

Dw (s−1)

PPY1 PPY2

71.77 53.91

0.616 0.469

0.1012 0.5507

0.0814 2.2052

15.9 144

1.647 0.302

(6)

kT (ln p + C⊥r ) πη0L3 0.917 0.05 − 2 P P

kTP 2 A 0πη0L3(1 + C r )

0.677 0.183 − P P2

(7) (8)

(9) (10)

A 0 = 3.84

and aspect ratio p =

L d

Calculated values of τr [rotational diffusion time], τd [translational diffusion time], and the wobbling diffusion coefficient Dw for both the systems are given in Table 3. [Equations are in Supporting Information.] The value of Dw indicating the dynamics of the CHL-a assembly in the restricted nanospace is 1.647 s−1 in the spherical nanohybrid, much higher than that from the nanorod, 0.302 s−1, because of the lesser special restriction of nanosphere. The wobbling angle θ of the CHL-a assembly of 71.77° in the spherical nanohybrid and 53.91° in the nanorod [calculated from equation given in Supporting Information] featuring the CHL-a spherical assembly enjoys greater degree of wobbling freedom. The magnitude of order parameter S, as a measure of the spatial restriction of individual porphyrin, is higher [0.616] in the spherical nanohybrid rather than the nanorod [0.469] and also indicates a highly oriented distribution in the former one and also supports the fact that spherical assembly experiences greater coherence, and it is also the cause of increased relative amplitude of the fast decay component in TCSPC measurement, which may be the cause of increased coherence length.

Table 3. Decay Time Components of All Samples (Excitation at 405 nm)a S

r

where

(4)

θ

(2D

1 + 4D r )

C⊥r = −0.662 +

where rw, rt, and rM are the time constants for wobbling, translation motion of the porphyrin molecules, and overall rotation of the nanohybrids, respectively.16 In accord with this model, the fluorescence anisotropy decay of the CHL-a assembly in nanostructure is a two-exponential function [Figure 3D, Table 3].17

sample

⊥

D⊥r = 3

(3)

⎡ ⎛ t ⎞ ⎛ t ⎞⎤ r(t ) = r0⎢a exp⎜ − ⎟ + (1 − a)exp⎜ − ⎟⎥ ⎢⎣ ⎝ τslow ⎠ ⎝ τfast ⎠⎥⎦

(5)

Here, η is the viscosity of the medium, k and T are the Boltzmann constant and temperature in kelvin, respectively, and rh is the hydrodynamic radius of the polymer nanosphere which is 25 nm. The overall rotational correlation time τM of the nanorod is estimated according to TG theory, through following equations.18

1 − ϕf1 τf1

4πηrh 3 3kT

PPY1 = CHL-a/PPY nanophere. PPY2 = CHL-a/PPY nanorod. θ = wobbling angle. S = order parameter. τr = rotational correlation decay time due to wobbling. τd = translational corelation time along the surface. τm = overall rotational corelation time. Dw = wobbling diffusion constant. a

Here, τslow and τfast are the two reorientation times associated with the slow and fast motions of CHL-a molecules in PPY nanostructure. “a” is the pre-exponential factor which indicates the relative contributions of the slow and fast motions to the decay of the anisotropy. Assuming that the slow and the fast motions are separable, the time constant for the overall rotation of polymer nanospheres can be estimated using the Stokes−Einstein−

■

CONCLUSION The faster decay component of spherical nanohybrids implies enhanced nonplanarity and the stronger macrocyclic coordination in nanosphere. This fact is also reflected in the blue-shifted emission spectra in spherical nanohybrids, implying enhanced macrocyclic rigidity. The polymeric environment induces 6562



■

greater site specific inhomogeneity in the 1D system in comparison with the spherical nanohybrid, thus restricting the delocalization length, and brings about significant changes like inhomogeneous broadening of absorption spectra. The dipole strength of the emitting state in packed pigment, which is 2.00 and 4.65 in 1D and spherical nanohybrid, respectively, indicates the presence of collective excitations in these systems. Superradiance, which is the increase of the radiative rate caused by the effect that in an exciton manifold one specific exciton transition can gather much more dipole strength than that of the monomer, is much greater in the spherical nanostructure. The decreased radiated lifetime implies that excitonic delocalization is profound [Nc = 7] with lesser inhomogeneity in spherical species. The fast and slow anisotropic decay profiles ascribed to wobbling motion, lateral diffusion of CHL-a molecules in the nanostructure, and the rotation of the nanohybrids as a whole vary significantly in spherical and 1D nanoarchitectures (Scheme 1). The higher

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +919831566632. Author Contributions †

J.S.M. and D.D. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

The authors acknowledge the technical assistance of Subrata Das of Indian Association Cultivation of Science, Kolkata 700032, India, for TCSPC and anisotropy characterization. J.S.M. thanks the CSIR for awarding a fellowship. Financial support from University Grant Commission under BSRMeritorious 2011−2012 to D.D. is also gratefully acknowledged.

Scheme 1. Schemetic Representation of Spherical and 1D Complex

(1) Scholes, G. D. Quantum-Coherent Electronic Energy Transfer: Did Nature Think of It First? J. Phys. Chem. Lett. 2010, 1, 2−8. (2) McHale, J. L Hierarchal Light-Harvesting Aggregates and Their Potential for Solar Energy Applications. J. Phys. Chem. Lett. 2012, 3, 587−597. (3) Satake, A.; Kobuke, Y. Artificial Photosynthetic Systems: Assemblies of Slipped Cofacial Porphyrins and Phthalocyanines Showing Strong Electronic Coupling. Org. Biomol. Chem. 2007, 5, 1679. (4) Huber, R. A. Structural Basis of Light Energy and Electron Transfer in Biology. Angew.Chem., Int. Ed. 1989, 28, 848. (5) Verma, S.; Ghosh, H. N. Exciton Energy and Charge Transfer in Porphyrin Aggregate/Semiconductor (TiO2) Composites. J. Phys. Chem. Lett. 2012, 3, 1877−1884. (6) (a) Takei, F.; Yanai, K.; Onitsuka, K.; Takahashi, S. Screw-SenseSelective Polymerization of Aryl Isocyanides Initiated by a Pd−Pt μEthynediyl Dinuclear Complex: A Novel Method for the Synthesis of Single-Handed Helical Poly(isocyanide)s with the Block Copolymerization Technique. Chem.Eur. J. 2000, 6, 983. (b) Takei, F.; Onitsuka, K.; Kobayashi, N.; Takahashi, S. Precise Synthesis and Properties of Poly(isocyanide)s Bearing Porphyrins as a Pendant Group. Chem. Lett. 2000, 29, 914. (c) Takei, F.; Hayashi, H.; Kobayashi, N.; Takahashi, S. Helical Chiral Polyisocyanides Possessing Porphyrin Pendants: Determination of Helicity by Exciton-Coupled Circular Dichroism. Angew.Chem., Int. Ed. 2001, 40, 4092. (d) Takei, F.; Nakamura, S.; Onitsuka, K.; Ishida, A.; Tojo, S.; Majima, T.; Takahashi, S. Preparation and Photochemical Properties of Polyisocyanides with Regularly Arranged Porphyrin Pendants. Chem. Lett. 2003, 32, 506. (7) (a) Solladié, N.; Hamel, A.; Gross, M. Synthesis of Multiporphyrinic α-Polypeptides: Towards the Study of the Migration of an Excited State for the Mimicking of the Natural Light Harvesting Device. Tetrahedron Lett. 2000, 41, 6075. (b) Solladié, N.; Hamel, A.; Gross, M. Towards Multiporphyrinic α-Helices with a Polypeptidic Backbone as System Endowed with Light Harvesting Capabilities. Chirality 2001, 13, 736. (c) Aubert, N.; Troiani, V.; Gross, M.; Solladié, N. Novel Porphyrinic Peptides with Assigned Sequence of Metallatedchromophores, a Further Step towards Redox Switches. Tetrahedron Lett. 2002, 43, 8405. (d) de Witte, P. A. J.; Castriciano, M.; Cornelissen, J. J. L. M.; Scolaro, L. M.; Nolte, R. J. M.; Rowan, A. E. Helical Polymer-Anchored PorphyrinNanorods. Chem.Eur. J. 2003, 9, 1775. (e) Manna, J. S.; Das, D.; Mitra, M. K. Energy Transfer from Polyaniline to Chlorophyll-a Supramolecular Assembly in Nanohybrid. J. Phys. Chem. C 2013, 117, 9573−9580.

ratio of radiative vs nonradiative decay and the restricted rotational motion in 1D nanohybrid also support the fact that the nonradiative decay pathway of exciton is more pronounced in the spherical nanohybrid because of greater rotational freedom. The higher magnitude of wobbling diffusion coefficient signifies the greater degree of wobbling freedom of the packed pigment in the nanosphere. The greater magnitude of order parameter in the spherical nanohybrid, indicating highly oriented distribution, also supports the increased coherence length of the spherical nanohybrid. From these results it is evident that the electronic coupling in packed pigment can be tuned by introducing disorder through polymeric interaction similar to antenna complexes where exciton delocalization is limited by disorder introduced through pigment protein interaction. In our system the excitonic motion or coherent energy transfer is more prominent in spherical nanostructure, which is evident from radiative rate. These findings suggest that CHL-a/PPY nanohybrids have the potential of being promising in the framework of mimicking natural light harvesting arrays.

■

Article

ASSOCIATED CONTENT

S Supporting Information *

Experimental methodology, synthesis of CHL-a species, and TCSPC measurements. This material is available free of charge via the Internet at http://pubs.acs.org. 6563



Article

(8) Liu, J.; Wan, M. Synthesis, Characterization and Electrical Properties of Microtubules of Polypyrrole Synthesized by a TemplateFree Method. J. Mater. Chem. 2001, 11, 404−407. (9) Umetsu, M.; Wang, Z.-Y.; Kobayashi, M.; Nozawa, T. Interaction of Photosynthetic Pigments with Various Organic Solvents Magnetic Circular Dichroism Approach and Application to Chlorosomes. Biochim. Biophys. Acta 1999, 1410, 19−31. (10) (a) Palacios, M. A.; de Weerd, F. L.; Ihalainen, J. A.; van Grondelle, R.; van Amerongen, H. Superradiance and Exciton (De)Localization in Light-Harvesting Complex II from Green Plants? J. Phys. Chem. B 2002, 106, 5782−5787. (b) Fujitsuka, M.; Okada, A.; Tojo, S.; Takei, F.; Onitsuka, K.; Takahashi, S.; Majima, T. Rapid Exciton Migration and Fluorescent Energy Transfer in Helical Polyisocyanides with Regularly Arranged Porphyrin Pendants. J. Phys. Chem. B 2004, 108, 11935−11941. (11) Kim, P.; Lim, J. M.; Yoon, M. C.; Aimi, J.; Aida, T.; Tsuda, A.; Kim, D. Excitation Energy Migration Processes in Self-Assembled Porphyrin Boxes Constructed by Conjugated Porphyrin Dimers. J. Phys. Chem. B 2010, 114, 9157−9164. (12) Yoon, M. C.; Cho, S.; Kim, P.; Hori, T.; Aratani, N.; Osuka, A.; Kim, D. Structural Dependence on Excitation Energy Migration Processes in Artificial Light Harvesting Cyclic Zinc(II) Porphyrin Arrays. J. Phys. Chem. B 2009, 113, 15074−15082. (13) Hwang, I. W.; Kamada, T.; Ahn, T. K.; Ko, D. M.; Nakamura, T.; Tsuda, A.; Osuka, A.; Kim, D. Porphyrin Boxes Constructed by Homochiral Self-Sorting Assembly: Optical Separation, Exciton Coupling, and Efficient Excitation Energy Migration. J. Am. Chem. Soc. 2004, 126, 16187−98. (14) (a) Aratani, N.; Kim, D.; Osuka, A. Discrete Cyclic Porphyrin Arrays as Artificial Light-Harvesting Antenna. Acc. Chem. Res. 2009, 42, 1922−1934. (b) Monshouwer, R.; Abrahamsson, M.; van Mourik, F.; van Grondelle, R. Superradiance and Exciton Delocalization in Bacterial Photosynthetic Light-Harvesting Systems. J. Phys. Chem. B 1997, 101, 7241−7248. (c) Connolly, J. S.; Samuel, E. B.; Janzen, A. F. Effect of Solvent on the Fluorescence Property of Bacteriochlorophyll a. Photochem. Photobiol. 1982, 36, 565. (15) (a) Kim, D.; Osuka, A. Photophysical Properties of Directly Linked Linear Porphyrin Arrays. J. Phys. Chem. A 2003, 107, 8791− 8816. (b) Dahlbom, M.; Pullerits, T.; Mukamel, S.; Sundstrom, V. Exciton Delocalization in the B850 Light-Harvesting Complex: Comparison of Different Measures. J. Phys. Chem. B 2001, 105, 5515−5524. (c) Nagarajan, V.; Alden, R. G.; Williams, J. C.; Parson, W. W. Ultrafast Exciton Relaxation in the B850 Antenna Complex of Rhodobacter Sphaeroides. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 13774. (d) Leupold, D.; Stiel, H.; Teuchner, K.; Nowak, F.; Sandner, W.; Ucker, B.; Scheer, H. Size Enhancement of Transition Dipoles to Oneand Two-Exciton Bands in a Photosynthetic Antenna. Phys. Rev. Lett. 1996, 77, 4675. (e) Kü hn, O.; Sundströ m, V. Pump−Probe Spectroscopy of Dissipative Energy Transfer Dynamics in Photosynthetic Antenna Complexes: A Density Matrix Approach. J. Chem. Phys. 1997, 107, 4154. (f) Ray, J.; Makri, N. Short-Range Coherence in the Energy Transfer of Photosynthetic Light-Harvesting Systems. J. Phys. Chem. A 1999, 103, 9417. (g) Novoderezkhin, V.; Monshouwer, R.; van Grondelle, R. Exciton (De)Localization in the LH2 Antenna of Rhodobacter Sphaeroides As Revealed by Relative Difference Absorption Measurements of the LH2 Antenna and the B820 Subunit. J. Phys. Chem. B 1999, 103, 10540. (h) Meier, T.; Zhao, Y.; Chernyak, V.; Mukamel, S. Polarons, Localization, and Excitonic Coherence in Superradiance of Biological Antenna Complexes. J. Chem. Phys. 1997, 107, 3876. (i) Bakalis, L. D.; Knoester, J. Pump−Probe Spectroscopy and the Exciton Delocalization Length in Molecular Aggregates. J. Phys. Chem. B 1999, 103, 6620. (j) Sumi, H. Structural Strategies in the Antenna System of Photosynthesis on the Basis of QuantumMechanical Coherenc among Pigments. J. Lumin. 2000, 87−89, 71− 76. (k) Mandal, S.; Bhattacharyya, S.; Borovkov, V.; Patra, A. Porphyrin-Based Functional Nanoparticles: Conformational and Photophysical Properties of Bis-Porphyrin and Bis-Porphyrin Encapsulated Polymer Nanoparticles. J. Phys. Chem. C 2011, 115, 24029−24036.

(16) Sen, S.; Sukul, D.; Dutta, P.; Bhattacharyya, K. Fluorescence Anisotropy Decay in Polymer−Surfactant Aggregates. J. Phys. Chem. A 2001, 105, 7495−7500. (17) Chen, W. C. W.; Nie, S. Quantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detection. Science 1998, 281, 2016−2018. (18) Ortega, A.; García de la Torre, J. Hydrodynamic Properties of Rod Like and Disk Like Particles in Dilute Solution. J. Chem. Phys. 2003, 119, 9914−9919. (19) Chauvet, J. P.; Viovy, R.; Santus, R.; Land, E. J. One-Electron Oxidation of Photosynthetic Pigments in Micelles. Bacteriochlorophyll a, Chlorophyll a, Chlorophyll b, and Pheophytin a. J. Phys. Chem. 1981, 85, 3449. (20) Chow, H. C.; Serlin, R.; Strouse, C. E. The Crystal and Molecular Structure and Absolute Configuration of Ethyl Chlorophyllide a Dihydrate. A Model for the Different Spectral Forms of Chlorophyll a. J. Am. Chem. Soc. 1975, 97, 7230. (21) Bowman, M. K.; Michalski, T. J.; Tyson, R. L.; Worcestert, D. L.; Katz, J. J. Electron Spin Resonance of Charge Carriers in Chlorophyll a/Water Micelles. Proc. Nati. Acad. Sci. U.S.A. 1988, 85, 1498−1502. (22) Weber, G.; Teale, F. W. J. Determination of the Absolute Quantum Yield of Fluorescent Solutions. Trans. Faraday Soc. 1957, 53, 646.

6564


Confined Motion, Excitonic Migration, and Superradiance of Ordered

Recommend Documents