Photophysical Probing of Dye Microenvironment, Diffusion Dynamics, and Energy Transfer

Abstract

The objective of this work was to apply an organic dye, tetraphenylporphrin (TPP), to probe the aggregation state, microviscosity, and diffusion dynamics of dye molecules within the interior microenvironment of conjugated polymer poly [(9,9dioctylfluorenyl-2,7-diyl)-co-(2,5-p-xylene)] nanoparticles (PFX NPs) which should open up further prospects in designing new porphyrin based nanoparticle materials and improve the knowledge of the better design of the nanophotonic devices based on dye-doped polymer NPs. This designed, aggregation-free TPP-doped PFX NPs system, exhibited remarkably high Forster resonance energy transfer (FRET) efficiency, leading to a potential application as a light harvesting system or a loadable drug carrier. To this end, different concentrations of TPP were doped into PFX NPs, prepared using a reprecipitation method. To compare TPP in organic solvent with TPP within the NP microenvironment, a successful doping of TPP into PFX NPs and no TPP aggregation formed inside of the NPs were indicated. Diffusion dynamics, location and degree of freedom of TPP in the PFX NPs were investigated using time-resolved anisotropy measurements fit with a "wobbling-in-cone and lateral diffusion" model. TPP molecules were found to wobble inside the core of NPs at a semiangle of ca. 70°and laterally diffuse on the NP surface with a diffusion coefficient of ca. 10 −4 cm 2 s −1 . The microviscosity of the nanoparticles was calculated from the wobbling model as 0.9−1.2 cP which is higher than the viscosity of THF (0.48 cP). By increasing the concentration of TPP from 0 to 5 wt %, the emission color could be gradually tuned from violet to red. Stern−Volmer analysis indicates that a single TPP molecule can quench 12 PFX polymer chains, and at 5 wt % TPP, PFX emission was almost completely quenched. The emission intensity of TPP in PFX NPs is 70 times higher than in organic solvent with the excitation wavelength at 350 nm which highlights the excellent sensitization properties of this system. The combined energy diffusion and Forster energy transfer model was used to simulate the relationship of nanoparticle sizes and the energy transfer efficiency. Both of the simulation and experimental results indicate that in this system, the FRET efficiency increases with the dye density increasing and they increase gradually for small particles range of 5−20 nm, approaching constant value for particle radii above 30 nm. The exciton diffusion length was estimated as 6.5 nm.

Maximizing Amplified Energy Transfer: Tuning Particle Size and Dye Loading in Conjugated Polymer Nanoparticles

“…In the present work, we will use the terms “FRET” or “energy transfer” to refer to the convolution of processes that delivers excitons to acceptors and “Förster transfer” and “exciton diffusion” to refer to the individual processes. The interplay between Förster transfer and exciton diffusion has been explored extensively in CPNs doped with fluorescent dyes that are not photochemically reactive. − For example, Jiang and McNeill have demonstrated that CPN fluorescence quenching that involves both Förster transfer and exciton diffusion can be 2 to more than 4 times more efficient than Förster transfer alone . Energy transfer from CPN donors is referred to as “amplified FRET” for this reason. , Highly efficient CPN-to-dye FRET has been used to amplify a fluorescence signal for sensing applications, which have been reviewed. , We − and others − have also used FRET from CPNs to nonfluorescent photochromic dyes to modulate the CPNs’ fluorescence.…”

“…The use of CPNs as sensitizers for inefficient photochemical reactions or photophysical processes requires high CPN-to-dye FRET efficiency. The groups of McNeill and Redmond have investigated FRET efficiency in dye-doped CPNs as a function of nanoparticle size. For CPNs with dyes randomly distributed throughout the particles, energy transfer efficiency increases as particle radius increases from 5 to 20 nm before leveling off around 30 nm. , Most of these results come from simulation rather than from the experiment and involve dyes with high extinction coefficients that are excellent FRET acceptors.…”

“…The groups of McNeill and Redmond have investigated FRET efficiency in dye-doped CPNs as a function of nanoparticle size. For CPNs with dyes randomly distributed throughout the particles, energy transfer efficiency increases as particle radius increases from 5 to 20 nm before leveling off around 30 nm. , Most of these results come from simulation rather than from the experiment and involve dyes with high extinction coefficients that are excellent FRET acceptors. Additionally, the relationship between FRET efficiency and CPN size has not been investigated for CPNs in which the dye dopants are located only on the particle surface or for those in which the dyes are reactive.…”

Exciton Diffusion, Antenna Effect, and Quenching Defects in Superficially Dye-Doped Conjugated Polymer Nanoparticles

“…Hence, the simulation sequence starts again from stage iii and continues in this loop (stages iv–viii) until a decay event ( e I , e A , or e Q ) is obtained. It is important to note that codes from previous related works ,,, calculate event probabilities form their corresponding rate constants in a given time step and later determine event outcomes from these probabilities. When none of the events occur in the time step, the exciton is artificially forced to walk.…”

Micelle Heterogeneity from the 2D Kinetics of Solute Rotation

“…In soft, microstructured materialsmicelles, vesicles, gels, star polymers, polymer nanoparticles, and so onthere are intertwined questions of where a solute resides and what local properties it sees. The photophysics of a solute that is also a chromophore are often used to gain information. − Static (0D) measurements, for example, the fluorescence quantum yield or Stokes’ shift, give a spatial average of static properties, such as hydrogen-bond availability or polarity. Time-resolved measurements with one time dimension (1D) give rates that characterize dynamic properties.…”