Presented at the 66th Southeastern Regional Meeting of the American Chemical Society, Nashville, TN, October 16-19, 2014; Paper 233 Conjugated polymer nanoparticles (CPNs) represent a versatile class of materials, well-suited for photovoltaic and light-emitting diode technologies, as well as biological imaging applications. We have employed various steady-state and time-resolved spectroscopic methods along with Monte Carlo modeling approaches in order to study these materials. Dye-doping and solvent-induced swelling methods were utilized to quantify the distance scales and rates of exciton transport in CPNs for improvement in device applications, as well as to design better fluorescent probes for imaging applications. Dye-doping methods allowed for quantification of the exciton diffusion length, accounting for intrinsic defects within the polymer structure. Solvent-induced swelling methods coupled with time-resolved anisotropy make probing the rate of exciton energy transfer between chromophores possible. Results suggest that exciton transfer rates increase 10x to 60x in the nanoparticle state, compared to free polymer in solution, which has implications for use of specific polymers for device applications and imaging applications.

1. Probing Exciton
Transport in
Conjugated Polymer
Nanoparticles
Department of Chemistry
Louis C. Groff II
Xiaoli Wang
Yifei Jiang
Jason D. McNeill
0000-0002-6357-0508

2. Outline
• Background
– Conjugated Polymers & Nanoparticles
– CPN Photophysics
• Dye-Doped PFBT CPNs
– Red-shifted Emission
– Complex Fluorescence Lifetime
Kinetics
– Modeling to Determine Intrinsic LD
• Solvent-Induced Swelling of CPNs
– Fluorescence Quantum Yield/Spectral
Shifting
– Picosecond Decay Kinetics
– Fluorescence Anisotropy Decay
– Lattice Model
– Quantum Yield/Mobility Tradeoff
• Summary
• Acknowledgements
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3. Conjugated Polymers – Basic Photophysics
3
• Plastic, organic semiconductors
– Useful for low cost, flexible
photovoltaics, LEDs and solar
cells
• Repurposing CPs to make
nanoparticles
– Have many useful optical
properties
– Allow for probing of relevant
photophysics for device
applications
Dennler, G., Sariciftci, N.S., Proceedings of the IEEE 2005, 93 (8), 1429.
Yim, K., Kim, J. S. et al., Adv. Func. Mater. 2008, 18, 1012.
Scholes, G., Rumbles, G., Nature Mater. 2006, 5, 683.
Background

4. Conjugated Polymer Nanoparticles (CPNs)
• Optical Properties
– Extended π-conjugated
chains
• HOMO/LUMO gap in UV/Vis
– High chromophore density
• Increased by nanoparticle
formation
– Highly fluorescent
– Efficient Förster energy
transfer hosts/dopants
• Red-shifting, sensors
4
Background
Tian, Z. Y., Yu, J., Wu, C., Szymanski, C., and McNeill, J.D. Nanoscale, 2010, 2, 1999.

5. CPN Properties and Structure
Background
5
conjugated
polymer
nanoparticle
conjugated
polymer film
conjugated
polymer in
solution
• Properties of conjugated polymers depend critically on
nanoscale structure, heterogeneity, inter/intrachain
interactions and other nm scale processes
– Energy transfer and charge transfer
• CPNs comprised of one or more collapsed chains
– Nanoscale, disordered, multichromophoric system
– Studies of single CPNs provide unique perspective on
nanoscale phenomena

6. Excited States in Conjugated Polymers &
CPNs: Frenkel Excitons
• Chromophore
– Individual chromophores
are ~2-8 monomer units in
length
• Molecular/Frenkel Exciton
– Delocalized excitation
among several strongly
coupled chromophores
• Coupling of transition
dipoles
– Exciton can diffuse along or
between chains or to
dopant molecules (FRET)
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Background
Scholes, G., Rumbles, G., Nature Mater. 2006, 5, 683.

7. CPN Photophysics
• Many complex
phenomena inherent
in CPNs
– Heterogeneity in
exciton lifetimes
– exciton
diffusion/multiple
energy transfer
– long-range coherent
exciton transport (low
T, high chromophore
coupling strength)
7
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
1

6
6
0
)(
R
R
kkk nrrET 
Background

8. CPN Photophysics
• Many complex
phenomena inherent in
CPNs
– hole polaron
photogeneration/
quenching
– quenching by defects
– emission saturation
– photoswitching/blinking
8
Background
Yu, J., McNeill, J. D. et al., Nano Lett. 2012, 12, 1300.
Dias, F. B., Knaapila, M., and Monkman, A. P., Macromolecules, 2006, 39, 1598.
Fluorenone Defects in Polyfluorenes

9. CPN Photophysics
• Want a model that can explain these complex
phenomena
– Exciton Diffusion Energy Transfer Model
• Want to understand the underlying rate
processes in CPNs as well as the length and
time scales of nanoscale events in CPNs
– Aid in optimizing CPNs for imaging applications
and photovoltaic/OLED applications
– Utilize a variety of time-resolved techniques to
address these issues from multiple angles
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Background

10. Exciton Decay Processes in CPNs
• Photon absorption/exciton
generation
• Exciton decays by an array of
pathways
– Radiative/nonradiative decay
• Energy transfer to lower energy
chromophores (homotransfer/exciton
diffusion)
– Polaron photogeneration/recombination/quenching
• Polaron transfer/diffusion
– Quenching by defects
– Energy transfer to dyes
• Hypothesized to reduce polaron generation (lower kct)
• Competitive energy transfer between hole polarons/dyes
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Background

11. Red-shifted CPNs via Förster Resonance
Energy Transfer
PFBT – Host Polymer
• Very photostable (~109 photons
emitted before photobleaching)
• High quantum yield (0.66 in THF)
• Broad emission spectrum
Perylene Red – Nonpolar dye dopant
• High quantum yield (0.98 in DCM)
• Laser dye – highly photostable
• Excellent spectral overlap with PFBT
• Red-shifted emission spectrum
Dye-Doped PFBT CPNs
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Groff, L. C., Wang, X., McNeill, J. D., J. Phys. Chem. C 2013, 117, 25748.

12. Red-shifted, High Quantum
Yield Dye-doped CPNs
• Efficient quenching of PFBT
emission by perylene red
– 86% quenching at 2%
perylene red
• Quenching obeys Stern-
Volmer equation
• Perylene red emission red-
shifts with increasing
doping
• Aggregation quenching of
acceptor at higher
concentrations
– Dynamic Quenching
– Likely due to dye dimers in
CPNs (J-aggregates)
][10
fK
F
F
SV
Dye-Doped PFBT CPNs
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Stern-Volmer Equation
Groff, L. C., Wang, X., McNeill, J. D., J. Phys. Chem. C 2013, 117, 25748.

13. Picosecond Fluorescence
Lifetimes of Dye-doped CPNs
• Mean exciton lifetimes reduced by doping (dynamic quenching)
– PFBT in THF fits well to single exponential (3 ns lifetime)
– CPNs fit best to bi-exponential and stretched exponential decays
– Decreasing trend in b as doping increases
• Decreasing trend in b indicates a broadening distribution of exciton lifetimes
as doping increases
– Agrees qualitatively with physical picture of exciton multiple energy transfer
– Range of exciton transfer distances prior to emission yields broad distribution of
exciton lifetimes
Dye-Doped PFBT CPNs
13
Groff, L. C., Wang, X., McNeill, J. D., J. Phys. Chem. C 2013, 117, 25748.
))/(exp()( 0
b
tAtI 
KWW/Stretched
Exponential Function

14. Continuum Modeling
of Dye-Doped PFBT Fluorescence
• Average over many excitons, particles, quencher positions
– 3000 excitons per particle, 50 particles per data point
– Poisson distribution of defects and quenchers
– Exciton diffusion length LD = 12 nm, Förster radius R0 = 4 nm
• Approximate match to experimental TCSPC fitting results
– At 0% dopant, 2.3 defect quenchers (dye equivalents) present
– b reduced by implementing Poisson statistics
– Ignoring quenching by defects results in LD 2x lower, lower quality of agreement
Dye-Doped PFBT CPNs
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Groff, L. C., Wang, X., McNeill, J. D., J. Phys. Chem. C 2013, 117, 25748.

15. Solvent-Induced Swelling of CPNs
Solvent-Induced Swelling
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conjugated
polymer
nanoparticle
conjugated
polymer film
conjugated
polymer in
solution
• Use solvent-induced swelling methods to probe the changes in
nanoparticle photophysics
– Probe rates of multiple energy transfer
– Assess reversibility of fluorescence quenching
• Rate of exciton motion hypothesized to be amplified for CPNs
– Densely packed chromophores
– Access to more nearest neighbor chromophores in CPNs vs. polymer
in solution
– Probe via fluorescence anisotropy decay (FAD) and modeling
Rapid mixing with
water, THF Removal
THF Addition

16. Fluorescence Quantum
Yield and Spectral Shifting
• Increase in F as %THF
increases
– ~92% quenching at 0% THF
– Fully reversible
• Normalized spectra show
progressive blue shifting of
emission peak
– Due to chromophore coupling
strength
– ~5 nm for PFBT
– ~40 nm for MEH-PPV
• MEH-PPV spectra suggest
coexistence of CPNs/polymer
at moderate %THF
– Narrow range, 60%-80% THF
– THF spectrum recovered at
higher % THF
Solvent-Induced Swelling
16

17. Fluorescence Decay Kinetics
of Swelled CPNs
• Exciton lifetimes increase with increasing THF volume fraction
– PFBT in THF fits well to single exponential (3 ns lifetime)
– All others fit best to bi-exponential and stretched exponential decays
– b increases with increasing THF, except PFBT in 80% THF
• Increasing trend in b indicates a narrowing distribution of exciton lifetimes
as THF fraction increases
– Multiple energy transfer is FRET mediated (R-6 dependence)
– As CPN structure swells, neighboring chromophores become less accessible,
reducing exciton mobility.
Solvent-Induced Swelling
17

18. Picosecond Fluorescence Anisotropy Decay
• Measure fluorescence polarization loss as a function of time
– Collect intensity decays at 0°, 90°, 55° polarizations relative to excitation source
– Rotational diffusion does not affect polarization loss
– Polarization loss due to multiple energy transfer of excitons
• Intricate fitting analysis to extract anisotropy parameters
– Reconvolution fitting of polarized intensity decays
– Determine limiting anisotropy r0 and correlation time c from fit results
– r0 gives information about transition dipole alignment at t0
– Able to measure rate constant ket from correlation time, c
Solvent-Induced Swelling
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Cross, A. J., Fleming, G. R., Biophys. J. 1984, 46, 45.

19. Picosecond Fluorescence Anisotropy Decay
• Multiple energy transfer rate constant amplified in CPNs
– For moderately swelled PFBT CPNs (above) ket ~3x1010 s-1
– For PFBT in THF, ket ~1x109 s-1
• Hypothesized to be due to quantity of nearest neighbor
chromophores to move between
– 6 neighbors in CPNs, allow for motion along or between chains
– 2 neighbors in linear polymer, motion is restricted along chain
• Modeling allows for comparison to basic physical picture
– Can measure energy transfer rate constants via modeling
– See if quenching behavior is well-described with model
Solvent-Induced Swelling
19

20. Discrete Lattice Modeling
of Nanoparticle Swelling
• Model Details
– Cubic lattice
– Exciton transfer probability
proportional to ratio of
chromophore densities
– Initial lattice spacing
determined from chromophore
densities at ~1 nm
• Defects/quenchers added to
unswelled lattice until FCPN,
CPN reproduced
– Rq = 4 nm
– ~10 defects per CPN for both
polymers
• Quenching efficiency h
defined via fluorescence
quantum yield
Solvent-Induced Swelling
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)(1exp polys FFh
2
/2 npet xtDp 
43/4
)/( snp xxf 
Multiple energy transfer probability
Defined from Random Walk
As lattice spacing increases,
pet reduced by f 4/3
Experimental quenching efficiency
Förster, T. Ann. Phys. 1948, 437, 55-75
Bardeen, C. J. Annu. Rev. Phys. Chem. 2014, 65, 127-148.

21. Lattice Modeling Results
• Model reproduces
experimental h,  well
– For low to moderate %THF
– b reproduced well for PFBT
• Model divergence ascribed
to coexistence of
CPN/polymer at moderate
to high THF concentrations
• CPN energy transfer rate
constant calculated from
pet
– 10-60x higher than polymer
ET rate constants
– Agrees with experimental
anisotropy fit results
Solvent-Induced Swelling
21

22. Fluorescence Quantum
Yield/Exciton Mobility Tradeoff
• ket is ~2x higher for MEH-PPV CPNs compared to
PFBT CPNs
– Due to stronger transition dipole coupling in MEH-PPV
• F is ~4x higher for PFBT CPNs compared to MEH-
PPV CPNs
– Weaker coupling/more isolated chromophores
• Increased exciton motion efficiently funnels excitons
to defect quenchers
– Reduction in lifetime
– Reduction in F
– Difficulty in determining LD, limited by defect concentration
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Solvent-Induced Swelling

23. Summary
• Successfully prepared redshifted PFBT CPNs with
high F
• Determined LD ~12 nm for PFBT
• Successfully used solvent-induced swelling to
probe rates of exciton mobility
• Multiple energy transfer amplified 10x-60x in
CPNs versus polymer in solution
• Tradeoff between mobility and F provides insight
into device/imaging applications
• Can exploit disorder to tune CPNs for
aforementioned applications
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24. Acknowledgments
• Dr. Jason McNeill
• McNeill Group
– Xiaoli Wang
– Yifei Jiang
• Clemson University Department of Chemistry
• National Science Foundation
– Grants CHE-1058885, and CHE-1412694
• You for your time and patience!
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