Photon Harvesting in Conjugated Polymer-Based Functional

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Photon Harvesting in Conjugated Polymer Based Functional Nanoparticles Bikash Jana, Arnab Ghosh, and Amitava Patra J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01936 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on September 4, 2017

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The Journal of Physical Chemistry Letters

Photon Harvesting in Conjugated Polymer Based Functional Nanoparticles

Bikash Jana, Arnab Ghosh, and Amitava Patra* Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India

*

Corresponding author’s e- mail: [email protected] Phone: (91)-33-2473-4971, Fax: (91)-332473-2805

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ABSTRACT The design of new generation light harvesting systems based on conjugated polymer nanoparticles (PNPs) is an emerging field of research to convert solar energy into renewable energy. In this Perspective, we focus on the understanding of the light harvesting processes like exciton dynamics, energy transfer, antenna effect, charge carrier dynamics and other related processes of conjugated polymer based functional nanomaterials. Spectroscopic investigations unveil the rotational dynamics of the dye molecules inside PNPs and exciton dynamics of the self-assembled structures. A detailed understanding of the cascade energy transfer for white light and singlet oxygen generation in multiple fluorophores containing PNP system by time resolved spectroscopy is highlighted. Finally, ultrafast spectroscopic investigations provide direct insight into the impacts of electron and hole transfer at the interface in the hybrid materials for photocatalysis and photocurrent generation to construct efficient light harvesting systems.

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Nanomaterials based efficient light harvesting systems have aroused increasing attention nowadays for conversion of solar energy to renewable energy.1-6 Nanomaterials are promising antenna materials because of several advantages like high molar extinction coefficient, broad absorption, and high photostability.7-9 Therefore, nanomaterials such as quantum dots (QD's), metal nanoparticles, polymer based nano-materials, heterostructures and inorganic – organic hybrid nanostructures are now used for hydrogen generation, photovoltaic and other light harvesting applications.10-15 Among these nanomaterials, conjugated polymer based nanomaterials are found to be promising antenna materials because of its broad absorption with extraordinary high molar extinction coefficient.16-19 Bending or twisting in extended π-conjugated multichromophoric polymer creates subunits, and each subunit acts as chromophores.20-21 The delocalized π-electrons absorb light which lead to the transition from ground state to excited state and the photophysical properties of conjugated polymer strongly depend on electron delocalization, and inter/and intra molecular interactions.17, 22 Tightly bound Frenkel excitons (electron-hole pair with the binding energy ~ 0.1 -0.4 eV)18 are created in conjugated polymer after photo-excitation where the electronic excitation is coherently delocalized over many chromophoric units in the collapsed conformation (polymer nanoparticle) and an incoherent hopping process is favourable in extended conformation [polymeric solution in tetrahydrofuran (THF)].23-25 The band gap from UV to the near-infrared region can be tuned by suitable selection of the conjugated polymer for potential applications.26 Surprisingly, less attention has been given on developing light harvesting systems based on conjugated polymer nanoparticles (PNP). Piok et al.27 have reported the efficient organic light-emitting diode (OLED) based on conjugated PNP. Scherf and co-workers have demonstrated the enhancement of the external quantum efficiency of polymer nanoparticles based solar cells.28-29 List and co-workers have reported the polymer nanoparticles based polymer light-emitting diode (PLED) and light-emitting electrochemical cells (LECs).30 So far, major emphasis has been given on the conjugated polymer nanoparticles based fluorescent markers for in vivo and in vitro imaging applications.31-32 For light harvesting applications, the governing factors are exciton generation, radiative recombination of exciton, nonradiative relaxation, internal conversion, intersystem crossing, exciton transfer and charge transfer processes. Therefore, the understanding of exciton generation, exciton diffusion, interfacial charge transfer, exciton migration towards electrodes, and the energy transfer are crucial for photocatalysis, and photovoltaic light harvesting applications (Shown in Fig. 1). The exciton diffusion process is an important issue in the optoelectronic devices, and it is known that exciton can diffuse up to ~20 nm to 3 ACS Paragon Plus Environment


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reach the interface or the excitons are directly transferred to the acceptor through Förster resonance energy transfer (FRET) before the dissociation of exciton.18,

20, 33

The longer

exciton diffusion length is found to be desirable to enhance light-harvesting performance in solar cells, whereas the exciton diffusion is undesirable in the case of light emitting diodes (LED)7, 34 where the injection of holes and electrons from opposite electrodes can generate excitons to get emission through radiative recombination.35 To improve the efficiency of polymer based LED, low band gap dopant is introduced into high band gap polymer host for efficient energy transfer.36 Design of dye doped conjugated nanoparticles is a strategy for harvesting light energy and bio-imaging applications.26, 37-38 Generally, PNPs are synthesized by rapid collapsing of the extended polymer chains after adding bad solvent into polymer solution in good solvent.39 It is expected that the spatial orientations of the chromophoric units and inter/ intra molecular interactions are modified during the formation of PNPs which eventually control the excited state dynamics and the intrachain or interchain energy transfer.22 The optical absorption bands (π-π* transition) of conjugated PNPs are found to be blue or red shifted, depending on the nature of polymer and their packing.39-43 The blue shifting in absorption band is observed for poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), poly[{9,9-dioctyl-2,7-divinylene-fluorenylenealt-co-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene}] (PFPV) nanoparticles which is due to decrease in the conjugation length and the bending or kinking of the polymer backbone. However, in case of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV), poly(3-hexylthiophene-2,5-diyl) (P3HT), and poly(3-octylthiophene-2,5-diyl) (P3OT) nanoparticles, the red shifted optical absorption bands (π-π* transition) are evident due to increase in the interchain interactions during chain collapsing.44-45 The exciton relaxation of nanoparticles is faster than a polymeric solution because of the efficient energy transfer (inter energy transfer) between adjacent chromophores. Spectroscopic properties and different applications of different polymer nanoparticles are given in Table 1. For photovoltaic and solar cell applications, photocurrent generation occurs due to the charge separation and the charge migration of photogenerated electrons and holes toward opposite electrodes. The free charge carrier generation at the interface of heterostructure can be produced by the change of energy level offset which is a governing factor for photocurrent generation in photo-driven devices.18, 33, 46 Two important processes occur at the interface: (a) photo-generated exciton can dissociate through the electron transfer from polymer to the acceptor, and (b) direct excitation of acceptor materials (mainly inorganic materials) or excitation of both the organic and inorganic counterparts can lead to generate free charge carriers by hole transfer from 4 ACS Paragon Plus Environment

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inorganic materials to the polymer. Thus, the photoinduced charge transfer process could be either electron transfer from lowest unoccupied molecular orbital (LUMO) of π-conjugated polymer to the conduction band of inorganic materials or hole transfer process from the valence band of QD to highest occupied molecular orbital (HOMO) of π-conjugated polymer (Fig. 1). In this Perspective, the fundamental processes of the light harvesting processes like exciton dynamics, energy transfer, charge carrier dynamics and other related processes of conjugated polymer based functional nanomaterials are discussed for future development of light harvesting systems.

Path 1 Energy Transfer

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Figure 1: A scheme for polymer nanoparticles based light harvesting systems by using energy transfer and charge transfer processes. Table 1: Spectroscopic properties of different polymer nanoparticles Conjugated PNP

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PFO PF PDHF PPE PFPV F8BT MEHPPV P3HT PVK P3DDUT

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Live cells imaging Energy transfer Energy transfer Live cells imaging Live cells imaging Live cells imaging Live cells imaging Organic Photovoltaic Light harvesting Spectroscopic properties

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PFO: Poly(9,9-dioctylfluorenyl-2,7-diyl); PF: Poly(9,9-dihexylfluorenyl-2,7-diyl); PDHF: Poly(9,9-dihexylfluorenyl-2,7diyl); PPE: Poly(2,5-di(3´,7´-dimethyloctyl)phenylene-1,4-ethynylene; PFPV: Poly[{9,9-dioctyl-2,7-divinylenefluorenylene-alt-co-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene}]; F8BT: Poly-(9,9-dioctylfluorene-2,7-diyl-cobenzothiadiazole); MEHPPV: Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]; P3HT: Poly(3-hexylthiophene2,5-diyl); PVK: Poly(9-vinylcarbazole); P3DDUT: Poly(3-[2-(N-dodecylcarbamoyloxy)ethyl]-thiophene-2,5-diyl).



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Dynamics of dye molecules inside polymer nanoparticles Dye doped conjugated polymer nanoparticles are promising alternative luminescent nanomaterials because of several advantages i.e. brighter luminescence, better photostability, low toxicity and low blinking behavior. The brightness and photo stability of the dye doped PNP are evident due to modification of radiative relaxation by circumventing the concentration quenching.51,

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Considering the importance of dye encapsulated PNPs, the

fundamental understanding of the rotational motion, wobbling motion, and lateral diffusion of the dye molecules, local concentration of dye molecule, brightness, photo stability, and biocompatibility are important for developing efficient fluorescent probe for bio-imaging applications.55-58 Furthermore, the correlation of particle brightness with the size of the PNPs as well as population of the dye molecules inside the PNPs is illustrated. Fluorescence correlation spectroscopic (FCS) investigation has been done to provide a quantitative analysis of particle size, brightness, and concentration of dye molecules inside the PNP (Fig 2A).54 It is evident from FCS study that the concentration of dye molecule influences the brightness which increases with increasing the concentration of dye. The brightness of encapsulated dye is one order of magnitude higher than free dye in solution because of the enhancement of the radiative decay rate of the confined dye. Again, the photo stability of the dye doped PNP is found to be 15 fold enhancements which are beneficial for a bio-imaging probe. The rotational relaxation behavior of dye doped polymer nanoparticles has been investigated by using time resolved anisotropy spectroscopy. The rotation of dye molecule is restricted inside the polymer nanoparticles due to confinement effect. Recent findings reveal that the particle size of PNP plays an important role on the rotational motion, wobbling motion, and lateral diffusion of the dye molecules.51 Time resolved anisotropy decay analysis reveals that the average rotational time increases from 0.23 ns to 0.62 ns with increasing the size (10 nm to 80 nm) of PNPs (Fig. 2B) because the matrix rigidity and micro-viscosity are increased with increasing the size of the nanoparticles. The rotational dynamics of dye depending on size and concentration of dye is evident.

The lateral diffusion constant

increases with increasing the size and the diffusion coefficient for wobbling motion of dye molecules decreases with increasing the size of PNPs.59-60 The heterogeneous distributions of the dye molecules inside PNP is found to be increased with increasing the population of the dye molecules and enhancing the possibility of energy hopping between different dyes molecules inside PNP. Such fundamental knowledge is very useful to develop efficient fluorescent probe for bio-imaging applications.


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Design of self-assembled structures of conjugated organic molecules is a challenging area for development of light harvesting systems because of their rapid exciton energy migration and enhanced charge transfer dynamics.8, 61 Conjugated polymer nanoparticles act as an efficient host matrix for energy migration to develop efficient light harvesting. The efficient energy transfer from PNP (host) to bis-porphyrin (guest) opens up new possibilities to design efficient light harvesting systems. The J-aggregated porphyrin molecules doped conjugated polymer nanoparticle is designed to mimic the chlorophyll-based light-harvesting system62 where the conformation of the bis-porphyrin molecule is tuned by encapsulation into polymer nanoparticles. A strong intermolecular π- electronic coupling occurs in the J aggregation which enhances the coherent electronic delocalization.63 Again, H-aggregated fiber and flake nanostructures of quaterthiophene (QTH) molecules with changing the solvent polarity have demonstrated for photon harvesting applications.64 Structural analysis reveals that fiber structure is crystalline in nature whereas flake structure is found to be amorphous. Spectroscopic investigations reveal that the nature of aggregation has an impact on photoswitching, thermo-responsive photoluminescence properties, and the relaxation dynamics of the self-assembled structures, such knowledge is beneficial for the developing of organicelectronics.65-66 Moreover, the reversible transformation from fiber to flake with changing solvent polarity opens up new possibility to construct tunable light harvesting systems (shown in Fig. 2C). Exciton dynamics of the excited state of aggregated structures is modified with changing the morphology from fiber to flakes (Fig. 2D) which suggests that such functional materials are promising for photo-driven devices. More emphasis should be given on fundamental understanding on inter/intra energy transfer by ultrafast spectroscopy.


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Figure 2: (A) Particle brightness (black ball) and number of dye molecules per particle (red ball) with varying the concentration of dye inside the PVK PNP 54, (B) Time resolved anisotropic decay curves of C153 dye encapsulated in 10 nm (a) and 80 nm (b) PVK PNPs. Adapted with permission from ref 51, (C) Schematic representation of reversible transformation of QTH fiber and flakes nanostructures, and (D) Decay curves of QTH in THF (a), QTH fiber (b) and QTH flakes (c). Adapted with permission from ref 64. Photo-induced energy transfer In natural photosynthesis, solar light is absorbed by the antenna or pigment molecules and the absorbed light energy thereafter is funnelled to the reaction centre through multistep processes.9, 67-68 The large effective cross-section of antenna materials is essential for light harvesting system to increase the probability of capturing photons. In general, the process of light harvesting is based on energy transfer between chromophores.69-70 In case of dye doped conjugated PNPs host, conjugated polymer acts as efficient energy donor (antenna) which efficiently transfers its energy towards the encapsulated dye molecules. The most advantage of host-guest intra-particle FRET process is the large Stokes shift which diminishes the probability of re-absorption. Furthermore, one can easily tune the emission color by simply varying the concentration of acceptor molecules with respect to donor polymer. This type of energy transfer phenomenon is generally non-radiative energy migration process based on fundamental FRET process. Generally, the point dipole approximation is valid for the larger distance between the interacting chromophores than the size of the excitons. However, line dipole approximation is being considered for the conjugated polymer based energy transfer 8 ACS Paragon Plus Environment

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process.18,

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emission spectrum of donor with the absorption spectrum of acceptor. The efficiency of the energy transfer depends strongly on the distance of separation between donor and acceptor molecules. In case of dye encapsulated conjugated PNPs, irrespective of dipole-dipole interactions, the overall host-guest energy transfer phenomena is influenced by exciton energy diffusion process throughout the conjugated PNP matrix. The energy diffusion is found to enhance the FRET process in fluorescent dye doped conjugated PNPs. Modified Stern-Volmer data reveals that single dye molecule is quenched by multiple polymer chains having multiple chromophoric units.26,

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process from poly(9-vinylcarbazole) (PVK) conjugated PNPs to encapsulated coumarin dye molecules and their enhanced photostability are reported. Steady state spectroscopic study suggests that there are no such ground state electronic interactions between the host-guest molecules.51,

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energy transfer (>90%) from host PNP to dopant dye molecules. A large enhancement of decay time of encapsulated coumarin 153 (C153) dye molecules further confirms the energy transfer process. Beyond the single fluorophore doped system, the multistep cascade energy transfer process generates white light in multiple dye doped conjugated PNPs. In the previous report, both green emitting C153 and red emitting nile red (NR) dye molecules are simultaneously doped inside PVK PNPs.52 The rate of energy transfer process in each and individual step is investigated and the rise time of the acceptor molecule confirms the efficient energy transfer process (Figs. 3A and 3B). Excitation of PVK PNPs lead to a 180 ps rise time in the emission decay of C153 in dye doped PVK NP indicating the energy transfer from the PVK to C153. The rise time for the C153 emission is disappeared and the overall relaxation dynamics of C153 becomes faster in nile red (NR) and C153 co-doped PVK NP system.52 Interestingly, 360 ps rise time in the emission decay of NR with excitation of PVK confirms the energy transfer from the PVK to NR. However, co-doping of C153 with NR inside PVK exhibits a much faster 140 ps rise time in NR emission which signifies the enhancement of energy transfer process by cascade energy transfer from PVK to NR via C153. For this multi-chromophoric system, the possible energy transfer steps are : (a) direct energy transfer from host PVK to C153, (b) direct energy transfer process from host PVK to NR, (c) FRET between two dopant dye molecules (C153 to NR) inside polymer nanoparticle, and finally, (d) overall cascade energy transfer process from PVK to NR dye through C153 (Fig. 3C). Finally, the tunable emission from blue to red with bright white light emission are 9 ACS Paragon Plus Environment


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obtained by manipulating the relative concentration of dyes (Fig. 3D). Tuning of emission wavelength and white light generation are achieved by the cascaded energy transfer which will open up new possibilities to design single nanoparticle based white light OLEDs. Furthermore, singlet oxygen generation by multistep energy transfer process is reported in dye doped PNPs.75-76 Considering the importance, we have demonstrated the singlet oxygen generation by multistep energy migration process from C153 doped PVK PNPs to the surface attached Rose Bengal (RB) dye molecules.77 Positive charged surface functionalized C153 doped PVK polymer NP is attached with Rose Bengal dye molecules by using electrostatic attraction. Steady state and time resolved spectroscopic data confirm the cascade energy transfer from PVK nanoparticle to surface attached RB dye molecules through encapsulated dye molecules followed by singlet oxygen generation. Dopant dye C153 is being used as energy ladder which rapidly funnelled out the energy from multichromophoric PVK NP to RB in this system. Again, the intersystem crossing from lower excited singlet state (S1) of RB to the triplet state (T1) occurs by a nonradiative relaxation process and finally T1 state of RB nonradiative transfer its energy to the triplet oxygen (3O2) for conversion of singlet oxygen (1O2) which exhibits emission band at 1270 nm. Singlet oxygen quantum yield was further verified by photo-degradation of 2chlorophenol in Deuterium oxide (D2O) medium. This newly developed dye doped conjugated PNPs act as an efficient benign system for efficient photosensitization as well as singlet oxygen generation by multistep energy transfer process for photodynamic therapy applications. Again, hybrid of QDs - dye doped PNPs have been designed for light harvesting system where QDs act as antenna material for visible light absorption.78 The efficient energy transfer is found from the surface attached QDs to the randomly distributed dye molecules inside the PNPs after excitation at QDs. In another strategy, hybrid nano-composite has been designed by electrostatic attachment of positively charged poly[9,9-bis(3′-((N,N-dimethyl)-Nethylammonium)propyl)-2,7-fluorene-alt-1,4-phenylene]dibromide (PDFD) polymer with thioglycolic acid capped negatively charged cadmium telluride (CdTe) QDs for efficient energy transfer.79-80 Furthermore, laser emission property from dye doped electro spinning polymer nanofibers is reported previously.81 Therefore, the controlled fabrication technique of fluorophore doped PNPs should pave the way for future development of artificial light harvesting systems. For light harvesting applications, it is very much important to measure the antenna effect (AE) of a donor-acceptor system. Measurement of AE is described the 10 ACS Paragon Plus Environment

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energy transfer efficiency in terms of light harvesting properties of the donor to acceptor which is measured by the increase in acceptor emission intensity with the addition of donor in terms of donor excitation.82-84 Recently, self assembled QTH molecules encapsulated into dye doped PNPs has been designed efficient light absorbing antenna material (Fig 3E).85 The efficient energy transfer process is possible by the coexistence of donor and acceptor in the same nanoparticle to experience very close distance between donor to acceptor. Here, the measured energy transfer efficiency is nicely matched with the energy transfer efficiency measured using FRET theory. At the donor to acceptor ratio of 0.008, AE is found to be 11.94 which further increases to 31 at donor to acceptor ratio of 0.82 by the insertion of C153 in this system which is very promising for light harvesting system.86 In multi cascade energy transfer, AE mainly depends on the energy transfer efficiency in each step, the ratio of the extinction coefficient of the donor to acceptor at their excitation wavelength and the ratio of donor to acceptor. Assembly of coumarin doped PVK nanoparticles - nile red loaded BSA protein scaffold has been designed for efficient light harvesting system.86 In such case, NR dyes are encapsulated into specific hydrophobic pocket of BSA protein. Decay kinetics of PVK in presence of NR loaded BSA is significantly reduced from 3.69 ns to 0.32 ns (Fig. 3F) and a very fast 150 ps rise time in NR emission decay confirms the energy transfer from PVK to NR via C153 (Fig. 3G). 91.3% energy transfer efficiency was obtained by considering cascade resonance energy transfer from PVK to NR via C153. The effective molar extinction coefficient of the acceptor (NR) is increased to 80.93 x 104 M-1 cm-1 which is unprecedented. Another strategy based on carbon-dots (C-dots) - MEH-PPV PNP hybrid system is designed for efficient energy transfer where the energy transfer efficiency is tuned from 66% to 89% with changing the crystallinity of C-dots. The capping ligand of C-dots influences the energy transfer efficiency because the distance between donor and acceptor varies with changing the chain length.87

Thus, polymer based hybrid materials are found to be promising for

developing efficient light harvesting systems. In principle, the energy transfer in conjugated polymer facilitates the OLED application with tunable color. In an earlier report, 2.5% electroluminescent quantum efficiency is obtained by using a hole‐transport layer of amorphous diamine and luminescent layer of a host material, 8‐hydroxyquinoline aluminium.88 Recently, white organic lightemitting diodes (WOLEDs) are gaining significant importance for lighting applications89 and Park et al. have already demonstrated the bright white LED application using PNPs.90



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Figure 3: Decay curves of (A) C153 doped PVK NP (1) and C153 and NR dye co-doped PVK NP (2), (B) NR dye doped PVK NP (1) and C153 and NR dye co-doped PVK NP (2), (C) A scheme of multistep energy transfer processes from PVK nanoparticles to C153 and NR dyes. Adapted with permission from ref 52. (D) Photographs of tuning of emission band and quantum yield of water dispersed C153, NR doped PVK NPs at different weight ratios of dyes. Adapted with permission from ref 52 (E) Schematic representation of the energy transfer process from self-assembled QTH to NR dye doped PMMA nanoparticle. Adapted with permission from ref 85. (F) Decay curves of PVK NPs in the absence (a) and presence (b) of NR. Adapted with permission from ref 86. (G) Decay curves of (a) NR encapsulated BSA and (b) complex with PVK NP. Adapted with permission from ref 86. Interfacial charge transfer: Photo-induced electron transfer: Recent advances and challenging issues on interfacial charge transfer and energy transfer of inorganic-organic hybrid materials are discussed for the development of efficient light harvesting. According to classical Marcus theory, the change of free energy in electron transfer process is ~ 1-1.5 eV.91 Based on this theory; the energy transfer is feasible when the free energy of intramolecular coordinate matches with the product. Then, the electron is tunnelled through, and the final state is organized to thermal equilibrium by relaxation. Electron transfer mainly depends on the suitable energy level alignment of the counterpart and there should be much overlap between wave function of two materials,92 i.e. molecular 12 ACS Paragon Plus Environment

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orbital of two materials (electron donor and electron acceptor) must decouple to prevent the back electron transfer.93 Inorganic-organic hybrid of P3HT PNPs and Au nanoparticles (NPs) is designed for electron transfer process because the HOMO and LUMO position of regiorandom P3HT is -5.5 eV and -3.2 eV, respectively and the work function of Au NPs is -5.1 eV. A strategy is used to form hybrid material by electrostatic attraction of positively charged PNP with negative charged Au nanoparticles to enhance charge separation at the interface by electron transfer from PNP to Au NP. A gradual bathochromic shift along with the enhancement of absorption bands of P3HT PNPs is observed with the attachment of Au NPs. Decay dynamics of the donor in the presence of acceptor reveal the shortening of decay time and the appearance of faster decay component confirms the electron transfer from PNP to Au NP (Fig 4A). Then, photocatalysis performance of this inorganic-organic hybrid of P3HT PNPs and Au nanoparticles has been investigated under visible light irradiation. The analysis reveals that OH· radical is responsible for the photocatalysis of dye which is formed due to efficient electron transfer from the P3HT PNPs towards Au NPs.44 The rate of photocatalyis enhances from 55% to 90.6% for pure P3HT PNPs to P3HT PNPs/Au NPs hybrid system, respectively (Fig. 4B). Polymer–graphene nanocomposites are potential for photocatalysis and photovoltaic light harvesting applications because of efficient charge separation in presence of reduced graphene oxide (r-GO) and photoinduced electron transfer. Considering the importance of this composite, hybrid of r-GO-MEH-PPV PNP is designed by an addition of r-GO to the surface modified positively charged MEH-PPV PNPs. Based on the energy level alignment of MEH-PPV PNP and r-GO, photo excited electron is favorable from the PNP towards rGO.45 The bleaching kinetics (Fig. 4C) of the ground state is investigated by using transient absorption spectroscopy (TAS), and the charge separation is confirmed by the faster bleach recovery (510 nm) along with the appearance of broad bleach (635 nm) in the presence of rGO (lower panel of Fig. 4C). Ultrafast fluorescence spectroscopic study confirms the electron transfer from MEH-PPV PNP towards r-GO by reducing the average decay time from 22.8 ps to 10.3 ps (Fig. 4D). Upon light irradiation on the r-GO-MEH-PPV PNP nanocomposites, the enhancement of the photocurrent was observed due to the greater extent of charge separation. The fast electron transfer from conjugated polymer nanoparticle to r-GO leads to enhancement of photocurrent under visible light illumination which opens up the new possibility to develop solar light harvesting devices. Another way to probe the electron transfer process is the observation of polaron formation.93 After electron transfer from polymer or hole transfer towards polymer creates a positive charge on polymer backbone 13 ACS Paragon Plus Environment


which is called polaron.94 This positive charge assists the structural rearrangement of the polymer backbone95 and polaron signal is evident at near infrared (NIR) region due to photo induced electron transfer from MEH-PPV to the electron acceptor material.96 The fundamental understanding of the electron transfer process in the hybrid systems is important to design efficient photovoltaic or other applications.97-98 (B)

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400 ps (47%) components (Fig. 5C). The hole transfer process from CdTe to PNP in the hybrid system reduces the recombination dynamics which is reflected in the intermediate component.106 In addition to the charge separation and the charge migration towards the respective electrodes, the charge recombination process is also important process for the photo current generation. Cappel et al. have developed inorganic-organic hybrid by using cadmium sulfide (CdS) QDs and P3HT polymer and the charge is generated by the individual excitation of CdS and P3HT.107 The P3HT polaron signal (~700-100 nm) is found for both the excitations of CdS QDs and polymer which is generated due to the electron transfer from P3HT or via hole transfer from CdS QDs. Based on decay kinetics of exciton and polaron after excitation at polymer, the formation of polaron beyond 1 ps of the excitation is not directly related to the decay of exciton and the charge concentration (Fig. 5D). Thus, the very fast geminate recombination process at the interface causes the depletion of the polaron generation. However, after excitation at CdS QDs, polaron generation is continuous within few nanoseconds due to higher decay time of QDs.107 More importantly, ultrafast hole 16 ACS Paragon Plus Environment

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transfer occurs at the interface of the hybrid and the hole transfer is found to be slower due to exciton diffusion from the core to the interface. Recent findings reveal that the nongeminate recombination becomes more favourable for highly intense excitation source.107 It is reported that the charge separation through hole transfer process improves the efficiency of photocurrent generation.108 Therefore, in order to achieve better photocurrent generation in the hybrid systems, the hole transfer from the inorganic counterpart is more superior than the electron transfer. Based on charge separation in poly(2,3-bis(2-(hexyldecyl)quinoxaline-5,8diyl-alt-N-(2-hexyldecyl)dithieno[3,2-b:2′,3′-d]pyrrole) (PDTPQx-HD) polymer/ lead sulfide (PbS) QD hybrid, the external quantum efficiency is improved by 15%.109 The relative yield of long lived polaron formation through the charge separation at the interface after excitation at polymer and PbS QDs is investigated. Analysis reveals that the infrared (IR) photo excitation of PbS QDs produces low yield of long lived polaron formation. An external quantum efficiency of 4 % is reported in polymer blend nanoparticles containing poly(9,9dioctylfluorene-2,7-diyl-co-bis-N,N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4phenylenediamine) (PFB) and poly-(9,9-dioctylfluorene-2,7-diyl-co-benzothiadiazole) (F8BT).28-29 Organic – inorganic hybrid composed of polymer nanowires and inorganic QDs has potential for satisfactory power conversion efficiency (PCE) of 4.1%.110-111 Zhicai and co-workers have showed the efficiency of 9.2% in polymer based bulk heterojunction solar cells (PSCs) using an inverted structure.112 The hole transfer rates and polaron formation through the charge separation at the excitation in organic and inorganic counterpart is a subject of future research. Therefore, the fundamental understanding of hole transfer mechanism is crucial to

Energy (eV) vs vacuum

(A)

-3.0 -3.5

Normalized Counts

pave us to design efficient light harvesting materials. -2.83

e

e

-3.54

-3.45

-4.0 -4.5 -5.0

-5.28

-5.5

h

-6.0 -6.5

-6.12

-5.61

h

2.1 nm CdTe QDs

MEHPPV PNP

3.8 nm CdTe QDs

(B)

0.9

(a)

0.6 0.3 0.0

(b) 0

5

-0.3

(a)

-0.6

(b)

-0.9 0

5

10

15

20

10

15

20

Delay Time (ps)

(D)

(C)

0.0

∆A (m O.D.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25

Delay Time (ps)


25


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Figure 5: (A) Schematic representation of band alignment of different sizes of CdTe QDs and MEH-PPV PNPs for hole transfer process, (B) Femtosecond fluorescence decays of small sized CdTe QDs (a) and hybrid (b); (C) bleach recovery kinetics of large sized CdTe QDs (a) and hybrid (b). Adapted with permission from ref 106 and (D) schematic representation of electron transfer for the excitation of polymer and hole transfer for the excitation of QDs. Adapted with permission from ref 107. Table 2: Overview of conjugated polymer based nanoparticle for different applications PNP

Applications

References

Rhodamine B doped F8BT

Temperature sensing

113

PF-DBT5 doped F8BT

In vivo tumour targeting

114-115

Platinum(II) Octaethyl porphine doped PFO

Oxygen sensor for biological imaging

116

PFPV doped PF

Efficient energy transfer

48

C153 doped PVK

Singlet oxygen generation

51, 77

NR doped PVK

Enhancement of particle brightness

54

C153,NR co-doped PVK

White light generation

52

PFB blend with F8BT

Organic solar cell

28

P3HT PNP-Au NP hybrid

Photocatalytic efficiency

44

C153 doped PVK- NR doped BSA hybrid

White light emission

86

MEH-PPV PNP/CdTe QD hybrid

Tuning of hole transfer rate

106

MEH-PPV/r-GO composite

Photocurrent generation

45

m-LPPP

OLED device fabrication

27

PF-DBT5: Poly(9,9-dioctylfluorene)-co-(4,7-di-2-thienyl-2,1,3-benzothiadiazole); m-LPPP: Methyl-substituted ladder type poly(paraphenylene).

This report highlights the important issues and challenges of conjugated polymer nanoparticles for the development of light harvesting systems. Applications of conjugated polymer based nanoparticles (Table 2) are still in the embryonic stage, fundamental investigations on energy transfer and charge transfer processes are important for development of efficient light harvesting systems. The photophysical properties of PNPs are modified due to change of the spatial orientations of the multi-chromophore units and inter/intra molecular interactions. Fluorophore doped conjugated PNPs are established to be alternative next generation luminescence source due to brighter luminescence properties, easy solution based synthetic procedures, greater photo stability, and extremely non-toxic behavior. Considering 18 ACS Paragon Plus Environment

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the advantages of polymer nanoparticles, sunscreen based polymer nanomaterials would be potential for healthcare applications.117 Emphasis has been given on the role of the particle size and population of the dye molecules inside the PNPs on the rotational dynamics and particle brightness. Time resolved photoluminescence spectroscopic analysis on the cascade energy transfer for white light and singlet oxygen generation in multiple fluorophores containing PNP systems is given. The impacts of exciton dynamics and diffusion length of PNPs for the energy conversion should be investigated. Fundamental understanding on the charge transfer process of the inorganic-organic hybrid system using ultrafast spectroscopy is required for improving the device performance. Significant attention should be given on the development of hybrid systems i.e. graphene oxide (GO) - polymer NP, QDs – polymer NP, porphyrin – PNP, etc. for new challenging light-harvesting systems for efficient charge separation which is the governing factor for photovoltaic and photocatalysis applications. The design of nanosystems based on triplet exciton energy transfer from conjugated polymer to QDs may be another strategy for making efficient light-harvesting systems. The understanding of inter/intra chain energy transfer of the self-assembled/aggregated nanostructures by using ultrafast spectroscopy is beneficial for photon harvesting system.

Notes The authors declare no competing financial interests.

Biographies Bikash Jana obtained his B. Sc Honours in chemistry from Sundarban Mahavidyalaya, University of Calcutta (Kolkata, India) in 2010 and his M. Sc in chemistry with a specialization in physical chemistry from the University of Calcutta in 2012. He is currently pursuing Ph.D. under the supervision of Prof. Amitava Patra in the Department of Materials Science at the Indian Association for the Cultivation of Science (Kolkata, India). His research interest includes photophysical studies of π-conjugated polymer nanomaterials for lightharvesting applications. Arnab Ghosh earned his B. Sc Honours in chemistry from Ramakrishna Mission Vidyamandira, University of Calcutta (Kolkata, India) in 2013. He obtained his M. Sc (under integrated Ph. D program) in chemistry from Indian Association for the Cultivation of Science (Kolkata, India) in 2015. Currently, he is pursuing his Ph. D under the supervision of Prof. Amitava Patra in the Department of Materials Science at the Indian Association for the



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Cultivation of Science. His research interest includes excited state dynamics of π-conjugated polymer based hybrid nanomaterials for light-harvesting applications. Amitava Patra is a senior professor and Head of the Department of Materials Science at the Indian Association for the Cultivation of Science (Kolkata, India). He is a fellow of the Indian Academy of Sciences (FASc), the National Academy of Sciences (FNASc), India, and the Royal Society of Chemistry (FRSC). He is a recipient of the DAE-SRC Outstanding Investigator Award, A. V. Rama Rao Foundation Prize in Chemistry,the C. N. R. Rao National Prize for Chemical Research, AsiaNANO 2010 Award, CRSI Bronze Medal, MRSI Medal, and Ramanujan Fellowship. He was an advisory board member of Nanoscale, and the Journal of Physical Chemistry. He is an author or co-author of more than 189 scientific papers, five book chapters, and 2 Indian patents. His research interests include excited state dynamics, photo-induced energy transfer, and charge transfer of QD, Au nanoparticles, polymer- and porphyrin based light harvesting antenna materials, and up and down converted doped nanomaterials. ACKNOWLEDGMENTS "DAE-SRC Outstanding Investigator Award," and TRC (DST) are gratefully acknowledged for financial support. BJ and AG thank CSIR for awarding fellowship.


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