Exciton Dynamics and Formation Mechanism of MEH-PPV Polymer

Subscriber access provided by UNIVERSITY OF CONNECTICUT

Article

Exciton Dynamics and Formation Mechanism of MEH-PPV Polymer -Based Nanostructures Arnab Ghosh, Bikash Jana, Sandipan Chakraborty, Sourav Maiti, Biman Jana, Hirendra N. Ghosh, and Amitava Patra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08336 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

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

The Journal of Physical Chemistry

Exciton Dynamics and Formation Mechanism of MEHPPV Polymer -Based Nanostructures

Arnab Ghosh†, Bikash Jana†, Sandipan Chakraborty$, Sourav Maiti§#, Biman Jana$, Hirendra N. Ghosh§£ and Amitava Patra†*

†

Department of Materials Science, Indian Association for the Cultivation of Science,

Jadavpur, Kolkata-700032, India $

Department of Physical Chemistry, Indian Association for the Cultivation of Science,

Jadavpur, Kolkata-700032, India §

Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085,

India #

Department of Chemistry, Savitribai Phule Pune University, Ganeshkhind, Pune 411007, India

£

Institute of Nano Science and Technology, Mohali, 160062, Punjab (India)

*To whom correspondence should be addressed. E-mail: [email protected] Phone: (91)-33-2473-4971, Fax: (91)-33-2473-2805

1 ACS Paragon Plus Environment


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

ABSTRACT The recent emergence of the conjugated polymer-based nanostructured materials has stimulated a lot of interest in developing light harvesting systems. Here, we describe the formation of nanoparticles from polymer molecules [poly[2-methoxy-5-(2-ethylhexyloxy)1,4-phenylenevinylene] (MEH-PPV)] by adding a non-solvent (water) and understand their collapsing mechanism from extended form by using molecular dynamics simulations. Free energy calculations reveal that the thermodynamically stable state of the polymer in water and 75% (v/v) water/THF mixture is a collapsed state. The red shifting of the absorption band of the collapsed state is found due to change in polarity of solvent. The change in intensity of blue and red emission band with changing the solvent polarity is explained due to change of conformation from extended state to collapsed state of polymer. Ultrafast spectroscopic analysis reveals a systematic decrease of faster component at 554 nm (33 ps to 2 ps), indicating the energy transfer process. The faster component (150 fs) of time resolved anisotropy decay due to the fast depolarization process confirms the interchain energy transfer in collapsed state. The fundamental understanding of photophysics of conjugated polymer nanoparticles should pave for future development of light harvesting systems.


Page 2 of 31

Page 3 of 31

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


1. Introduction: The design of efficient light harvesting systems based on semiconducting polymer is an active area of research nowadays because of their semiconductors like properties, high fluorescence efficiency, and large absorption cross section.1-4 Conjugated polymers are multichromophoric systems where the -electrons are delocalized along the chain, and the transition of delocalized -electron occurs from ground state to excited state after excitation.56

Polymer molecules create charge-neutral molecular excitations (Frenkel excitons) after

photoexcitation. In Frenkel’s exciton model where the electronic excitation in the collapsed state is not confined to a single chromophoric sub-unit, but it is coherently delocalized over many chromophoric units.7 However, the interaction with the environment (internal vibrations, solvent, etc.) is much stronger in extended state (polymer in THF) than the resonant excitation transfer interaction. In such case, the excitation is no longer described by coherent exciton motion but it becomes an incoherent hopping process.7-9 The photophysical properties depend on spatial orientations of the multi-chromophore units and their inter/and intra molecular interactions.10 Kinking and bending along polymer backbone lead to conformational change from extended to collapsed state which leads to the spectral shift in absorption and emission spectra.11 The close contact between chains in collapsed conformation enhances the interchain energy migration process.10 Barbara et al. have demonstrated the significant impacts of interchain interactions and non-Flory polymer conformation on photophysical properties.12 They have reported that the collapsed state with defects is the stable state of MEH-PPV in aqueous solutions. Recently, Salleo et al. have reported the influence of the aggregated polymer structures on the charge transport properties which eventually modify the device performance.13 It is to be mentioned that the impacts of polymer nanoparticles on the charge carrier relaxation and recombination are not well studied which is crucial to design light harvesting systems based on conjugated polymer nanoparticles. So far, primary emphasis is given on the applications of semiconducting polymer nanoparticles for optoelectronics and bio-imaging due to less toxicity for bio-compatibility and photo-stability.14-15 Several studies on dye-doped polymer nanoparticles are reported for light-emitting diodes and photovoltaic devices, and also the energy transfer process from the polymer host to the guest molecule.5, 16-17 Nanoparticle-based LED's is reported for efficient OLED devices than the conjugated polymer films based devices.18 Scherf and co-workers have used polymer nanoparticles based solar cell devices to enhance external quantum 3 ACS Paragon Plus Environment


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

Page 4 of 31

efficiency.19-20 List and co-workers have used polymer nanoparticles for fabrication of PLED and light-emitting electrochemical cells (LECs).21 The importance of the conjugated polymer nanoparticles as efficient fluorescent markers for in vivo and in vitro imaging applications has been demonstrated by Chiu and his co-workers.22-23 However, less emphasis has been given on understating the molecular mechanism of the formation of polymer nanoparticles (collapsed state) from extended polymer. Here, we have given an atomistic insight into the formation of polymer nanoparticles (collapsed ensemble) from the extended polymer by considering both equilibrium simulation and free energy calculations. Taking an analogy from the bio-polymer,24-30 here we have tried to generate nanoparticles of different shapes and order of a conjugated stiff polymer of MEH-PPV in a binary

mixture.

The

MEH-PPV

(poly[2-methoxy-5-(29-ethylhexyl)oxy-1,4-

phenylenevinylene]) polymer is an attractive molecular scaffold where almost 140 different local "quasi-chromophores" exist along the polymer chain and each of the chromophoric units has a characteristic length of 10-17 units.12 From the coarse-grained simulation, it is demonstrated that chemical defects along the polymer chain induce conformational diversity of the polymer which is responsible for the observed extraordinary photophysical properties.12 The polymer is soluble in tetrahydrofuran (THF) whereas the solubility is very poor in water. Here, we investigate the self-assembly of MEH-PPV in water/THF binary mixture with the combined aid of theoretical and experimental tools. Molecular mechanism of the tunable size and shape of the self-assembled polymer and its dependency on water/THF composition has been probed with the aid of equilibrium and free energy simulation. Intricate examination and characterization of excited states using the femtosecond fluorescence up-conversion and transient absorption spectroscopy have been carried out to investigate the exciton dynamics and inter/intra energy transfer of the extended and collapsed states. 2. EXPERIMENTAL SECTION: Materials and Reagents: Tetrahydrofuran (THF) [MECRK] and de-ionized water (MERCK) were used as received. Synthesis of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]

(MEH-PPV)

was

prepared from 2,5-bis(chloromethyl)-methoxy-4-(2-ethylhexyloxy)benzene according to the reported procedure.31 The polymer showed weight average molecular weight of 330,000 g/mol and number average molecular weight of 122,000 g/mol with a polydispersity index of 4 ACS Paragon Plus Environment

Page 5 of 31

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


2.7 in gel-permeation chromatography with the monodisperse polystyrene standard. Then, MEH-PPV was dissolved in THF to produce a stock solution of 1mg/ml. Sample preparation To prepare the sample for studying steady-state and time-resolved spectroscopy, we follow the following procedure. At first, 50 µL of the stock solution was taken in all the sets. Then 4950 µL, 4450 µL, 3950 µL, 1200 µL spectroscopic graded THF was added to those sets. Finally, to have desired set of the solution, 0 µL, 500 µL, 1000 µL and 3750 µL of HPLC water was slowly drop wise added to the stock solution, so that final volume in each set was 5 mL. Sets were then kept for two hours before record any optical spectra. Accordingly, we obtained sets where water contents were 0%, 10%, 20%, 75% respectively. Characterization Morphological studies were performed in field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6700F). UV-Visible absorption spectra were obtained with Shimadzu, UV-2450. The emission spectra were recorded in a fluoro Max-P (Horiba JobinYvon) luminescence spectrophotometer. Deconvolution of emission spectra was done using software origin version 6.0 with the spectrum in wavelength and fitting the peak profile with Gaussian functions. Ultrafast spectroscopic data (decay traces and time-resolved anisotropy)

were

investigated

using

a

Femtosecond

fluorescence

upconversion

spectrophotometer with a Halcyone ultrafast setup. The sample was excited at 400 nm wavelength, using an 800 nm femtosecond (fs) pump laser (140 fs pulse width, 80 MHz repetition rate) with the laser pulse (4.4 W) from a Ti: sapphire oscillator (Chameleon, Coherent). This is coupled with a second harmonic generator (by BBO type I crystal). The emission wavelength (554 nm and 595 nm) and the gate pulse of the fundamental beam (800 nm) are upconverted using a nonlinear crystal (BBO type II). The FWHM of the instrument response function is about 288 fs. The femtosecond time-resolved decay data were fitted using Surface Xplorer 4 fitting software. Wave for transient absorption (TA) measurement, the samples were excited with 400 nm pump beam (~120 fs), and the resulting excited state dynamics was probed in the entire visible range. The pump power was kept low enough to avoid complications arising from multiexcitons. A great care has been taken to avoid sample degradation and multiexcitonic effects by keeping the fluence ~ 0.5 μJ. Thus, photo-oxidation is not expected at this low fluence. The polymeric moieties in the aggregated form are quite stable in non-solvent water.32 Briefly, the pump pulse was derived from frequency doubling of 800 nm laser obtained from Ti-Sapphire laser oscillator (Tissa 50, CDP, Moscow, Russia) and amplified in a multi-pass amplifier pumped by a 20W DPSS laser (Jade-II, Thales Laser, 5 ACS Paragon Plus Environment


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

France) in a β-barium borate (BBO) crystal at 1KHz repetition rate. The probe white light continuum (from 450 to 850 nm) was generated by focusing a portion of the 800 nm light on a sapphire window. Details of Computational modeling All the simulations were performed using GROMACS 4.5.533-35 Packages with the GROMOS 53a636 united atom force field. SPC/E37 water model was used throughout all the modeling studies. Modeling and parameterization of the poly[2-methoxy-5-(29-ethylhexyl)oxy-1,4phenylenevinylene] (MEH-PPV) polymer The polymer, MEH-PPV, was build according to the GROMOS 53a6 force field. All the atoms were considered explicitly, except for non-polar hydrogen. Non-polar hydrogen atoms were merged with the bound carbon atom. Due to the large size of the polymer, we considered the polymer as a 10-mer. Initial conformation of the polymer in extended conformation was built. Parameters of the polymer according to the GROMOS 53A6 force field were obtained using automated topology builder software (ATB).38-39 The molecule was optimized using AM1-BCC semi-empirical force field and charges were calculated using the same level of theory. Parameter of tetrahydrofuran (THF) and preparation of water/THF binary mixtures United-atom parameters of the THF were obtained from the ATB according to the GROMOS53a6 force field. THF parameters were obtained from density functional theory (DFT) calculations using the 6-31G* basis set and B3LYP functional. THF parameters were then validated considering two aspects. First one is to reproduce the experimentally known density of THF and the second one is the proper mixing of the THF/water binary mixtures. Initially, a simulation box of 52 × 52 × 52 Å3 dimensions containing 5000 THF molecules was prepared, and energy minimized using 500 steps of steepest-descent algorithm. Then the system was equilibrated for 10 ns molecular dynamics simulation in NPT ensemble at 298 K. Temperature was maintained using an external bath with a coupling constant of 0.1 ps using a v-rescale algorithm. The pressure was kept constant (1 bar) by using isotropic ParrinelloRahman barostat with the time-constant set to 2 ps. Electrostatic interactions were calculated using particle mesh Ewald summation method. From the simulation, the calculated density of THF averaged over last 2 ns was 851.91 kg/m3. However, the experimental density of THF is 889 kg/m3. Simultaneously, we prepared two systems containing the polymer in 10% and 75% (v/v) water/THF binary mixtures, respectively. Both the systems were then optimized using 500 steps of a steepest-descent algorithm. Each of the minimized systems was then 6 ACS Paragon Plus Environment

Page 6 of 31

Page 7 of 31

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


equilibrated using 10 ns position restrained dynamics in the isothermal-isobaric (NPT) ensemble at 298 K by adding restraining forces on the polymer while both the water and THF molecules were allowed to move freely. During the simulation, the homogeneity of the mixture is completely disturbed, and THF segregated into several micro-domains in the simulation box. To improve the homogeneity of the binary mixture of water/THF mixtures, we increased the charges on the oxygen atom of THF and the molecule was made charge neutral by adjusting the charges on the carbon atoms of the THF directly engaged in bonded connection with the oxygen. This new charge distribution of THF improves the density, obtained from equilibrium simulation of a box containing THF molecules. The improved calculated density of THF was 865 kg/m3 averaged over last 2 ns of simulation, in close agreement with the experimental density data. Also, this improved charge model of THF increased the homogeneity of both the binary mixtures. During the position, restrained equilibrium simulation of both the binary mixtures at 298 K in NPT ensemble, the mixtures remain homogeneous without any noticeable segregated micro-domain formation by THF. Details of the final THF parameter are now provided in supporting information (Figure S1) and were considered further. Equilibrium and metadynamics simulations of the polymer in THF, water and water/THF binary mixtures Finally, four different systems were considered where the polymer was immersed in a simulation box containing THF, water, 10% and 75 % (v/v) water/THF mixtures, respectively. The size of the simulation box was 100 × 70 × 70 Å3 for polymer in water and 75% (v/v) water/THF binary mixture, 97 × 68 × 68 Å3 and 85 × 60 × 60 Å3 in THF and 10% (v/v) water/THF binary mixture, respectively. Each of the pre-equilibrated systems, as explained in the earlier segment was then used to perform equilibrium molecular dynamics simulation in NPT ensemble at 298 K. Temperature was maintained using an external bath with a coupling constant of 0.1 ps using a v-rescale algorithm. The pressure was kept constant (1 bar) by using isotropic Parrinello-Rahman barostat with the time-constant set to 2 ps. Electrostatic interactions were calculated using particle mesh Ewald summation method. Trajectories were saved in the interval of 10 ps. To explore the conformational landscape of the polymer in water, THF, 10% and 75% (v/v) water/THF mixture, 1-D well-tempered metadynamics simulations40 were performed using the radius of gyration (Rg, nm) as order parameter using the PLUMED 1.3 plugin41 for GROMACS 4.5. The Gaussian functions of the initial height of 0.4 kJ/mol and sigma value 7 ACS Paragon Plus Environment


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

of 0.1 nm were considered for metadynamics simulation. However, we used the welltempered version of metadynamics where the added Gaussians are progressively changed with the simulation time and adjusted according to the nature of the potential well where it samples. Gaussians were added in every 2 ps. Well-tempered metadynamics simulations were performed at a simulation temperature of 300 K with a bias factor of 10 for all the simulations. Self-assembly of the polymer in water/THF binary mixtures To probe the large supra-molecular structure formation by the polymer due to self-assembly, equilibrium molecular dynamics simulations were used. The process was modeled based on the self-assembly of eight polymers. Initially, the eight polymer chains were randomly oriented and inserted in a box of ~ 137 × 146 × 117 Å3 dimensions. Two different simulation boxes were prepared where the eight polymers chains are solvated in 10% and 75% (v/v) water/THF mixtures, respectively. Each system was the equilibrated using 1 ns position restrained dynamics in NPT ensemble at 298 K where all the polymer chains were restrained. Each system was then subjected to unrestrained equilibrium molecular dynamics simulation. All the production runs were performed in NPT ensemble at 298 K. Details of the simulation parameters and protocols are similar to the equilibrium simulation of the monomer in the binary mixture, mentioned above. 3. RESULTS and DISCUSSION 3.1 Atomistic insight into the formation of different morphologies of MEH-PPV nanostructures Self-assembly of the conjugated polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene]

(MEH-PPV) is investigated upon gradual addition of water to the

polymeric solution in THF. There are several studies42-43 which focus on the simple aggregate formation upon co-solvent variation. Appropriate observations on intermediate during the formation of aggregates can lead us to give an atomistic insight into the formation of collapsed state from an extended state which in turn could give us a pictorial view of the mechanism of nanoparticle formation from pristine polymer chain as mentioned above. The structure of each monomeric unit of the polymer is shown in Figure 1A. It is constituted with an aromatic backbone and asymmetric alkoxy side chain which enables us to select a wide range of co-solvent to observe solvent induced morphological variation. Figure 1 shows the FE-SEM images of nanostructured materials made of MEH-PPV polymer when different 8 ACS Paragon Plus Environment

Page 8 of 31

Page 9 of 31

percentages of water were added to the solution of the polymer in tetrahydrofuran (THF). All microscopic investigation is carried out after two hours of growth time. To start with, we added 10% (v/v) water to a THF solution of MEH-PPV; where a hyperbranched, onedimensional nanofiber is observed (Figure 1B). The length of the fiber is in the range of micrometers where the width of the fiber is around 10 nm to 12 nm. With this amount of added water (10%), the striking observation is that the fibers start to coil up. These phenomena can be ascribed due to a decrease of polymer solubility in the presence of water which causes the formation of toroid structure. Interestingly, with a further addition of water (20 % water content) to the polymer in THF, polymer chains are collapsed into spherical nano-structure with the average size of 50 nm (Figure 1C). Finally, in 75% (v/v) water/THF mixture, particles are strongly aggregated into a more compact state with a characteristic size of these particles reduces to an average size of 20 nm with more pronounced aggregation

(B)

(A)

(C)

In THF

10 % water 5

10

15

Intensity

(Figure 1D).

Intensity

20 % water

20

40

Width of Fiber (nm)

60

80

100

Particle Size (nm)

75 % water

(D)

Intensity

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


10 15 20 25 30 Particle Size (nm)

Figure 1. (A) Molecular structure of MEH-PPV. FE-SEM images of samples when 10 % (B), 20 % (C), 75 % (D) water content was present in THF solution of MEH-PPV. (inset in figure (B) shows width distribution of nano-fiber and inset in figures (C) and (D) show particle size distribution) We consider an atomistic description of the polymer, i.e. united atom model where all the atoms are considered explicitly, except for non-polar hydrogen. Non-polar hydrogen 9 ACS Paragon Plus Environment


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

Page 10 of 31

atoms are merged with the bound carbon atom. As the experimentally observed large size of the polymer is practically intractable using molecular dynamics, we consider the polymer as a decamer. Recently, Wang et al.44, demonstrated that an average conjugation length of 7 monomers of the MEH-PPV polymer is sufficient enough to capture the conformational plasticity of the polymer. Most interestingly, strong correlation is observed between the population distribution of conformation and chromophoric units in first excited state. We perform the well-tempered metadynamics simulations to construct a reduced 1D free-energy surface using Rg as order parameter and results are shown in Figure 2A. Simultaneously, we performed equilibrium simulations, and the conformational ensemble of the polymer sampled during the simulation timescale are analyzed in terms of root mean square deviation (RMSD), a radius of gyration (Rg) and the end to end distance (Figure S2). End to end distance is computed from the center of mass (COM) distance between the benzene moieties of the polymer from the first and last monomeric unit of the decamer.

Figure 2. (A) 1-D free energy profiles of the polymer in pure water (red), pure THF (black), 10% (v/v) water/THF mixture (yellow) and 75% (v/v) water/THF mixture (green) obtained from well-tempered metadynamics simulation are shown. Structures of the polymer corresponding to different minima is shown in stick representation. (B) Structures of the 10 ACS Paragon Plus Environment

Page 11 of 31

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


polymer corresponding to the free energy minimum in 10% (v/v) and 75% (v/v) water/THF mixture are shown. Two equally probable structures of the polymer in 75% (v/v) water/THF mixture, evident from the free energy profile, are shown as BI and BII. The structure of the polymer is shown as the cyan stick. Hydration layer is rendered in transparent surface mode. THF is shown as brown surface whereas water is shown in ice-blue color. Evident from the free energy simulation (Figure 2A), the thermodynamically stable state of the polymer in water is a collapsed state with a radius of gyration of ~ 0.82 nm. The result is in agreement with the equilibrium simulation where during the simulation time scale the polymer collapses to a state with characteristic Rg varies between 0.8-0.85 nm (Figure S2A). Interestingly, the free energy profile of the polymer in pure THF and 10% (v/v) water/THF mixture is very similar which again highly complements with the equilibrium simulation data. The thermodynamically stable state of the polymer in both the solutions is in extended conformation with Rg of 1.91 nm. Also, evident from the free energy profile, in both the solutions, another extended conformation with a characteristic Rg of 1.7 nm is also highly probable. In a case of pure THF, the free energy barrier between the two states with Rg of 1.9 and 1.7 nm is ~4 kJ/mol, while the barrier in the 10% (v/v) water/THF is ~ 3 kJ/mol. Thus, it is expected that the polymer is in equilibrium between several extended conformations in THF and 10% (v/v) water/THF mixture. During equilibrium simulation, we have started the simulation from the extended state, and during the simulation timescale, the polymer remains in the extended conformation in both THF and 10% (v/v) water/THF mixture (Figures S2B and S2C). The collapsed state is rarely observed during the simulation. Only a few transitions from the extended state to a bent-like conformation, characterized by reduced Rg and end-to-end distance, are observed. An interesting observation is that in THF and 10 % water/THF mixture, there is a minimum at ~ Rg of 1.17 nm in the free energy profile. Structural insights reveal that the polymer adopts toroid like structure. Interestingly, our SEM result also shows similar toroid like structure formation in 10 % water/THF mixture. In 75% (v/v) water/THF mixture, the thermodynamically stable state of the polymer is a highly collapsed state with a charecteristic Rg of 0.9 nm, the structure of the polymer is very similar to the free energy minimum of the polymer in pure water. Interestingly, there is another stable minima of the polymer with characteristic Rg of 1.12 (nm) with a barrier of 5.2 kJ/mol. The extended state of the polymer is 5 kJ/mol higher that the free energy minimum with a energy barrier of 16 kJ/mol. Therefore, the most populated state is collapsed-like state of the polymer. Equilibrium simulation also reveals that the polymer is conformationally 11 ACS Paragon Plus Environment


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

Page 12 of 31

dynamic in this binary mixture composition and can accept for different collapsed-like states with Rg varies from 1.3-0.9 nm during the simulation timescale (Figure S 2D). Both equilibrium simulation and free energy calculations reveal that there are two distinct conformational ensembles of the polymer and their population distribution is strongly dependent on the composition of the binary mixtures. Structure and hydration properties of the polymer in extended and collapsed states are shown in Figure 2B. In THF and also in 10% (v/v) water/THF binary mixture, the polymer adopts flexible extended form. It is noteworthy that the polymer is a conjugated stiff polymer; therefore, it preferentially takes extended form where only lateral flexibility is allowed. Solvation shell of the polymer mostly constitutes by the THF in 10% (v/v) water/THF mixture (Figure 2B). The polymer is highly hydrophobic, and therefore water molecules within the first solvation shell are drastically low. Interestingly, in 75% water/THF mixture, the minimum with characteristic Rg of ~ 0.9 nm is in highly collapsed form almost similar with the structure of the polymer in pure water. The aromatic ring from each monomer constitutes the outer surface of the collapsed state whereas the long alkoxy side chains are packed inside the core (Figure 2 BI). Another conformation of the polymer in 75% (v/v) water/THF mixture with characteristic Rg of 1.17 nm is in highly bent conformation (U-shaped). Each monomer aromatic ring consists of the outer surface while the alkoxy chains are packed inside (Figure 2BII). Interestingly, in both the conformations of the polymer, the solvation layer is exclusively constituted by THF and water content is very less. Hydrophobicity primarily dictates the structure of the polymer. In THF and 10% (v/v) water/THF mixture, the extended form is solvated by THF. However, the amount of THF is very less with 75% (v/v) water/THF mixture and there are not enough THF to solvate the extended form. Therefore, the effective surface area is substantially reduced in the collapsed spherical form which is solvated by the available THF molecules. The effective surface area controls the conformational plasticity of the polymer in water/THF binary mixtures. It is noteworthy that this type of long distance conformational order is common in biological polymers, particularly single stranded DNA and RNA. Long distance intra and inter molecular association leads to super-secondary structure formation by nucleic acids45-47, nano-rod and nano-wire formation48 by circular DNA, also the long distance association is the molecular basis of DNA origami49-50. We also investigate the process of nanostructure formation due to self-aggregation of the polymers in 10% and 75% (v/v) water/THF binary mixtures by simulating eight copies of the decamer polymeric fragments using equilibrium simulation. Figure 3A summarizes the process of self-assembly of the polymer in 10% and 75% (v/v) water/THF mixture. 12 ACS Paragon Plus Environment

Page 13 of 31

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


Figure 3. (A) Self-aggregation of the polymer in 10% and 75% (v/v) water/THF mixture with progression of simulation. The polymer is rendered in cyan CPK mode. (B) (I) The structure of the large spherical self-assembly of the MEH-PPV polymer in 75% (v/v) water/THF binary mixture obtained after 100 ns equilibrium simulation is shown. Each of the polymers is rendered as cyan sticks. (II) Solvation structure of the large spherical selfassembly is shown. Polymers are rendered as the cyan stick. THF appears as brown surface whereas water is shown in ice-blue color. In 10% water/THF binary mixture, each polymer remains in extended form which shows lateral flexibility and the process of self-assembly starts ~ 10 ns (Fig. 3A). All the polymer molecules are highly stiff and they do not form spherical self-assembly. Several long extended self-aggregated assemblies are observed during the simulation timescale. Formation of such elongated self-aggregated assembly is observed at ~ 18 ns. However, the polymer remains dynamic in nature therefore rapidly associated and dissociates to form self-assembly of different orders. The structure of the self-assembled polymer is evident at ~ 50 ns. Whereas, eight copies the polymer aggregates into a largely branched fibril-like structure is observed at ~ 90 ns (Fig. 3A). Interestingly, our simulation reveals that the mostly extended fibril-like structure is obtained in 10% (v/v) water/THF mixture not the collapsed selfaggregated structure during the simulation which is highly consistent with the SEM result. 13 ACS Paragon Plus Environment


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

Page 14 of 31

However, the thermodynamically stable state of the polymer in 75% (v/v) water/THF mixture is a collapsed state. With the progression of the simulation, there is formation of small critical spherical aggregated nuclei with highly packed core ~ 3.2 ns (Figure 3A). These small nuclei then start to merge to form large spherical self-assembled assembly. Structure of such an event is shown in Figure 3A during the 4-6 ns timescale which ultimately results two large spherical self-aggregated assemblies. These two aggregates then start to amalgamate into a large spherical self-assembly at ~ 38 ns. With further progression of the simulation these two aggregates approaches close to each other and ultimately merged into a single large aggregates and it remains stable throughout the simulation (40-100 ns). Rabani et al.51 have reported nanoparticle self-assembly using a coarse-grained model under drying condition. They found that the energetic cost of moving a nanoparticle into the surrounding solvent condition is comparable to kBT. In short timescale limit, the morphology of the nanostructure is strongly dependent on the several environmental factors like disc coverage, solvents. However, in long timescale limit, the structural heterogeneity of nano-particle asymptotically converges to a single nanostructure in all boundary conditions which strongly correlate with our observations. It is noteworthy that the number and dimension of the initial spheroids like self-assembly can change upon increasing water content as well as polymer concentrations. However, all the polymer ultimately converges to a single spherical assembly in long enough timescale. In our simulation, we observed the formation of large spherical nano-particle at ~ 40 ns with six polymer chains and in 75% (v/v) water/THF composition. These timescale can vary with varying polymer size, concentration and water content of the binary mixture composition. Structure of the large spherical polymer self-assembly obtained after 100 ns of equilibrium simulation is shown in Figure 3BI. All the polymers are collapsed into a spheroid. The aromatic rings of each polymer are on the outer surface of the spheroid while the alkoxy side-chains are tightly packed inside the spheroids. Very few THF molecules penetrate within the tightly packed core while no water molecules are present inside the spheroid, indicating the core of the large spherical assembly is mostly dehydrated. An interesting observation is that THF preferentially solvates the large spherical self-assembly and very fewer water molecules are within the first solvation shell (Figure 3BII). 3.2 Steady state spectroscopy The free energy data reveals that the thermodynamically stable state of the polymer shifts from the extended state to a collapsed state with increasing addition of water. We further investigate the effect of conformational changes on the absorption profile of the 14 ACS Paragon Plus Environment

Page 15 of 31

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


polymer. We have considered the extended and collapsed state corresponding to the free energy minimum, optimized using an AM1 semi-empirical method and calculated the absorption spectra using ZINDO/S method. Interestingly, the absorption from the collapsed state is red shifted (Figure 4A). Pure MEH-PPV in THF medium shows a broad absorption peak at around 503 nm (Figure 4B) which attributes to the π-π* transition.31, 52 The observed absorption characteristic resembles well with the benzoid-like polymer ground state and the quinoid-like excited state.53 With the addition of water to the THF solution of MEH-PPV, the peak maximum is red shifted from 503 nm to 507 nm, 512 nm, 525 nm for 10%, 20 %, 75 % water/THF mixture, respectively (Figure 4B) which is consistent with the theoretical data. This red shifting in absorption band is explained by changing the polarity because the excited state is more stable in polar solvent.53 Solvatochromic spectral shift occurs due to change of permanent dipole moment with changing the solvent. It is reported that polar alkoxy backbone substitutions in MEH-PPV molecule are the source of the effective dipole54 which causes the spectral shifting. Scherf and co-workers have reported that the effective dipoles lead to solvatochromic spectral shift in MEH-PPV and LPPP where the polarity changes with changing the solvent.55 Hoffmann et al.56 have studied the origin of inhomogeneous broadening of electronic transition in π-conjugated polymer and proposed that the dominant contribution is torsional intra-chain disorder for MEH-PPV polymer in fluid. In extended form, the polymer is almost planner therefore the torsional contribution is marginal. However in collapsed form of the polymer, the induce conjugation barrier results in exciton confinement. Therefore, the energetic inhomogeneity in reduced effective conjugation length leads to broadening of electronic transitions.



4.0 3.5

(A)

(B)

Extended Collapsed

a (0% H2O)

a

0.6

3.0

Absorbance

Epsilon

2.5 2.0 1.5 1.0

0.4

d (75% H2O)

d

0.2

0.5 0.0

0.0 340

360

380

400

420

440

460

400

480

450

(C)

a, (0% H2O)

a

7

1.6

d, (75% H2O) 7

3.0x10

PL Intensity (a.u)

6.0x10

550

600

650

(D)

a, (0% H2O)

a

d, (75% H2O) 0.8

d

d

0.0 550

500

Wavelength (nm)

Excitation energy (nm)

PL Intensity (a.u)

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

Page 16 of 31

600

650

700

750

0.0 550

Wavelength (nm)

600

650

700

Wavelength (nm)

Figure 4. (A) Theoretical excitation spectra calculated using the ZINDO/S method for collapsed and extended state of the polymer. (B) Recorded UV-visible absorption spectra (C) Recorded

photoluminescence

spectra

(D)

Normalized

(at

595

nm

emission)

photoluminescence spectra of MEH-PPV in THF when 0% (a), 10% (b), 20% (c), 75 % (d) water content was present in the solution. Systematic investigation of photoluminescence spectra with progressive addition of water to the polymeric solution in THF is recorded and shown in Figure 4C. Steady-state fluorescence spectroscopy using the excitation wavelength (λexc = 500 nm) of MEH-PPV in THF shows an emission peak maxima of the MEH-PPV in THF at around 554 nm along with a shoulder at 593 nm. After addition of 10% water to this MEH-PPV solution, the intensity of the peak at 554 nm decreases, and the intensity of the peak at 593 nm increases along with a prominent hump at 640 nm. After addition of 20% water to the polymeric solution in THF, the peak intensity at 554 nm further decreases, and the intensity of the peak at 593 nm increases along with a more intense hump at 640 nm. Interestingly, after addition of 75% water to THF solution of pristine polymer, the peak at 554 nm is diminished, and the intensity of the peak at 593 nm along with the hump at 640 nm is highly intense. Figure 4D depicts the normalized emission spectra normalized at 595 nm. The deconvoluted emission spectra show three vibronic bands 551 nm (0-0), 590 nm (0-1) and 638 nm (0-2) for different chromophoric units 16 ACS Paragon Plus Environment

Page 17 of 31

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


(Figure S3 and Table S1) in extended polymer. Emission at short wavelength (554 nm) and long wavelength (595 nm) can be attributed to extended fibril and collapsed self-assembled state, respectively. With a systematic addition of water, the intensity of the peak at 554 nm decreases and the enhanced emission at the red end reveals the energy transfer from blue end to red end. The spectral properties57-58 of the polymer is known to change with addition of cosolvent content because of the changing the conformation.42,

59

Grey and co-workers have

confirmed two emissive conformers (red and blue) of MEH-PPV by single molecular spectroscopy.60 Peteanu and co-workers have reported the formation of a “core-shell” model in a single aggregate which consist of “monomer-like” or weakly associated oligomers and “aggregate-like” or more densely packed oligomers depending on changing the chain length and precipitation condition.59 They also showed that the emission spectra of aggregates (formed using nonsolvent and good solvent) exhibit a vibronic pattern where the intensity ratio depends strongly on the ratio of good to poor solvent.61-62 The reversible photoswitching of absorption and emission properties with varying dielectric solvent medium is important for practical applications.63 Interestingly, the photo-switching of absorption and emission band of MEH-PPV with changing the solvent content is observed, indicating the reversible transformation of extended form and collapsed state of the polymer. The normalized UV-vis

absorption

and photoluminescence spectra

of the reversible

transformation with changing the solvent content are shown in supporting information (Figure S4). At 75 % water content, the normalized absorption maximum centered at 518 nm, and emission peak maxima are at 593 nm along with a hump at 640 nm. With further addition of THF (20 % water content), the absorption maxima shifts towards 512 nm and emission peak maxima are at 593 nm. The intensity of the peak at 554 nm becomes prominent. Most importantly, after a further increment of THF content, the absorption spectrum shifts towards higher energy region with a peak maximum at around 507 nm, and the emission peak maxima at 554 nm along with two humps at 593 nm and 640 nm are observed. A dynamic equilibrium between the extended and the collapsed state is shifted with a variation of water content. 3.3 Ultrafast time-resolved decay dynamics Ultrafast decay dynamics of self-assembled polymer with varying degree of orientation are studied using ultrafast spectroscopy where all the samples are excited at 400 nm, and the decay profiles are collected at 554 nm (Figure 5A). For MEH-PPV in THF ultrafast decay at 554 nm is fitted with a bi-exponential function with the decay times of 33.38 ps (2%) and 210 ps (98%). The contribution of faster decay component increases from 34% to 38% for 10% to 20% water content, respectively. Finally, the contribution of faster 17 ACS Paragon Plus Environment


component reaches at maximum (62%) with a characteristic decay time of 2.06 ps and the slower decay time of 47.42 ps (38%) when 75% water was added. Details of all the decay parameters are given in Table S2. At monitoring wavelength of 554 nm, with the gradual addition of water, the contribution of faster decay component is increased due to rapid funneling of absorbed energy to red edge.64-65 (A)

1.0

1.0

Normalized Counts

Normalized Counts

a 0.8

0.6

0.4

d

0.0

0

34

a

10

20

30

40

0.6 0.4

d

0.0

50

0

Decay Time (ps)

(C)

(B)

0.8

0.2

0.2

10

0.4

20 30 40 Decay Time (ps)

50

(D)

554 nm 595 nm

a

0.3

32

Anisotopy

Faster Component (ps)

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

Page 18 of 31

6 4

0.2

b

0.1

2 0 0

10

20

74

76

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Delay Time (ps)

% of Water Addition

Figure 5. Ultrafast decay curves with 400 nm excitation and emission collected (A) at 554 nm and (B) at 595 nm of MEH-PPV in THF when 0% (a), 10% (b), 20% (c), 75 % (d) water content was present in the solution. (C) Change in the faster component of decay with the gradual addition of water to monitoring wavelength 554 nm and 595 nm. (D) Time-resolved anisotropy decay curves of (a) extended state (0% water) and (b) collapsed state (75 % water) of MEH-PPV polymer. To clarify the hypothesis, we measure the decay kinetics of all the samples at 595 nm wavelength (Figure 5B). The emission band at 595 nm is assigned to the aggregated domain. All the decay curves are fitted bi-exponentially, and the decay parameters along with their contribution are given in Table S3. Here, the faster decay component increases from 1.89 ps to 5.72 ps with increasing percentage of water addition. It is seen that the slower decay 18 ACS Paragon Plus Environment

Page 19 of 31

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


component decreases from 210 ps to 47.42 ps with increasing water content up to 75%. This shortening in decay component is due to energy transfer. Bredas and co-workers have already reported that close contacts between chains favor interchain transport over intrachain as efficient channel for excitation energy transfer.10, 66 The electron/energy transfer governs by the faster decay component and a systematic decrease of faster part at 554 nm (33 ps to 2 ps) and increase at 595 nm (2 ps to 6 ps) (Figure 5C and Tables S2 and S3) indicate the interchain energy transfer process from higher energy chromophores (554 nm) to lower energy chromophores (595 nm). Similarly, Bagchi and co-workers have also demonstrated the possibility of the energy transfer from high energy segments to low energy segments in πconjugated system.67 3.4 Ultrafast time-resolved anisotropy Ultrafast anisotropic study gives insight into the inter/intra energy transfer processes. The intrachain energy migration requires the reorientation of the chromophoric unit along polymer chain and it exhibits comparatively slower depolarization for necessary motions of chromophores.11 In collapsed conformation, the distance between segments is short enough to funnel excitation energy through space which favors the interchain energy transfer. Sundstorm and co-workers have reported the depolarization in conjugated MEH-PPV (sub100 fs) is faster than the partially conjugated polymer (>1 fs), indicating the rapid change in exciton delocalization in the fully conjugated MEH-PPV due to structural relaxation.68-69It is seen that the conformation of polymer is determined from the energy transfer which was investigated by using anisotropy from TTAD (two times anisotropy decay), 2DPE (twodimensional photon echo), 3PES (three-pulse photon echo shift) and many other photon echo spectroscopic studies.11, 66 All the samples are excited at 400 nm, and emission collected at 554 nm and 595 nm for extended and collapsed form for time-resolved anisotropy measurement (figure 5D a). In the case of extended form, after initial excitation, slow depolarization occurs with a combination of 790 fs (82%) and 160 fs (18%) (Figure 5D). The slower part (790 fs) arises due to rotational diffusion of the extended form which is the major part for depolarization. In the collapsed state, it is clear that the excitation with polarized light leads to emission with depolarization in a very short time scale (figure 5D b) with a combination of 150 fs (77%) and 1380 fs (23%). This slower component (1380 fs) arises due to the experience of the more restricted domain of the chromophore inside the collapsed form. It is seen from the anisotropy measurement that the component with 150 fs (77%) is due to the fast depolarization process because of the interchain energy transfer in collapsed 19 ACS Paragon Plus Environment


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

Page 20 of 31

state. The higher initial anisotropy (0.4) in collapsed state arises due to the experience of a more restricted environment by the chromophores than extended form. It is already discussed in the section above that the rapid interchain energy transport is faster than intrachain which is consistent with previous work.10 3.5 Femtosecond transient absorption spectroscopy The distribution of charge carriers in the excited state and their relaxation and recombination dynamics of extended state to the collapsed coiled state have been studied by Transient absorption spectroscopic (TAS). Samples were excited with 400 nm pump beam, and the excited state dynamics are monitored in the whole visible region through a broadband probe beam. Correlation of absorption and emission spectra with TA spectra of extended form (0% water), collapsed form (75% water) are given in figures 6A and 6D. In the extended form (0% water), the TA spectra (Figure 6) shows an intense bleach ~535 nm (B1), a low-intensity bleach ~595 nm (B2) with a small hump ~650 nm (B3). The B1 feature had contribution both from the ground state bleach and stimulated emission (SE). The bleach signals of B2 and B3 attribute to the SE because there is a broad shoulder ~595 nm and a hump ~650 nm in the emission spectrum is observed (as illustrated in Figure 6).70 Surprisingly, B1 does not resemble with the steady state absorption spectra, indicating that all the excited polymer molecules are not involved in this bleaching process. It is evident that the extended phase (emitting on the shorter wavelength side) does not contribute to the TA signal whereas the TA spectra are mostly dominated by the coiled phase (emitting on the longer wavelength side).71 The B1 and B2 bleach signals are slightly red-shifted to 540 nm and 600 nm, respectively as the water content increases (10%) which are consistent with absorption and theoretical data. Interestingly, the relative intensity of the B3 bleach signal increases as the aggregation starts to take effect for 10% water. This attributes to rising in emission intensity on the longer wavelength side due to aggregation, leading to increasing in the SE signal.


Page 21 of 31

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


Figure 6. (A) Steady state absorption and emission for 0% water (a, a') and 75% water (b,b') corroborated with the transient absorption spectra of MEH-PPV in THF when 0% (B), 10% (C), 75 % (D) water content was present in the solution after 400 nm photo-excitation. The TA spectra drastically change when the amount of water increases to 75%. The B1 bleach signal completely disappears with a formation of a broad bleach signal, suggesting the B1 contributes from SE. The emission is completely quenched at ~550 nm in 75% water. Therefore, the contribution of SE to B1 gets completely quenched. The broad bleach signal



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

Page 22 of 31

results mostly from the SE as the ground state absorption is very less after 550 nm and insignificant after 600 nm. In line with the theoretical investigation and steady state study, as the polymer stable state transforms from extended to coiled state due to significant water induced aggregation, the B1 feature (resulting from an extended state) diminishes whereas B2 and B3 features (arising from the coiled state) dominate the TA spectra. The broadness in the TA spectra for 75% is due to polar (protic) environment in presence of water. It is seen from simulation study that the hydrophobic polymer is rapidly collapsed into a highly packed spherical supramolecular structure in 75% (v/v) water/THF mixture with reduced surface area (Figure 3A) where the aromatic rings are on the outer surface and the alkoxy side-chains are inside the spheroids. Therefore, no water molecules are present inside the spheroid and very few THF molecules are within the tightly packed core. Peteanu and co-workers have also reported that aggregated polymers can form a core/shell like structure where the core and shell are made of aggregated polymeric chains and monomer-like polymers, respectively.32, 61 Interestingly, for OPPV13 aggregates, the emission lifetime at the wavelength of monomer origin is same as that of isolated monomer, whereas the longer wavelength emission bands have significantly shorter lifetime due to aggregated chain.61 Similarly, Egelhaaf and coworkers have investigated the photoexcited state dynamics with different chain length of vinylene unit by using femtosecond pump-probe experiments of nPV thin films.72 Dynamics of bleach recovery with multi-exponential fitting parameters with increasing amount of water provided in the Supporting Information (Figure S5 and Table S4-S6). In all cases, the fitting was multi-exponential as each exponential can represent a distinct process of exciton relaxation-recombination within the polymer chain. The multi-exponential behavior can also result from the broad distribution of polymer configuration as the theoretical modeling suggests (Figure 2A) a wide distribution of possible configurations. Interestingly, for the SE (B2 and B3) the initial bi-exponential recovery becomes tri-exponential with the emergence of a fast (

Exciton Dynamics and Formation Mechanism of MEH-PPV Polymer

Recommend Documents