Photophysical Probing of Dye Microenvironment, Diffusion Dynamics

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Photophysical Probing of Dye Microenvironment, Diffusion Dynamics and Energy Transfer Suxiao Wang, Alina Thorn, and Gareth Redmond J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11166 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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The Journal of Physical Chemistry

Photophysical Probing of Dye Microenvironment, Diffusion Dynamics and Energy Transfer Suxiao Wang, §,¶, Alina Thorn¶ and Gareth Redmond*,¶ §

Ministry of Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials, College of Chemistry and Chemical Engineering, Hubei University, Wuhan, 430062, China.

¶

School of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland

ACS Paragon Plus Environment

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The Journal of Physical Chemistry 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 objective of this work was to apply the organic dye, Tetraphenylporphrin (TPP), to probe the aggregation state, microviscosity and diffusion dynamics of dye molecules within the interior microenvironment of conjugated polymer poly[(9,9-dioctylfluorenyl-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 Förster 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. 104

cm2 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. SternVolmer 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


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diffusion and Förster 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.


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INTRODUCTION Nanoparticles based on conjugated polymer are emerging as multifunctional nanoscale materials with many useful and interesting properties, which include high fluorescence brightness, excellent photostability, highly efficient energy transfer and sensing capabilities.1 Organic dye doped conjugated polymer nanoparticles (CPNPs) have been investigated in multiple potential applications including light harvesting,2-4 photodynamic therapy photoswiching

7-17

5-6

and

. To date, extensive research has been focused on the energy transfer process

and near infrared emission of organic dye doped CPNPs for bioimaging and photovoltaic devices.18-24 Besides, another group of researches like Sen, S. et al. focused on the location and freedom of organic dyes in the conjugated polymer nanoparticles / nanomicelles.23, 25-27 Patra, A. et al. studied both the energy transfer process and microenvironment / rotational dynamics of organic dyes in CPNPs with different sizes.28-29 In terms of understanding the diffusion dynamics of the dye inside the NPs, a model of fast rotational (wobbling) and slow translational diffusion coupled with the rotational motion of the nanoparticles as a whole has been found when analyzing time resolved anisotropy decays. This has been the best model to describe reorientation dynamics of dye in quasi spherical nanoparticles or nanomicelles. 25, 27 Porphyrins are a group of heterocyclic macrocycle organic compounds which are composed of four modified pyrrole subunits interconnected at their carbon atoms via methane bridges.30 Typically, it has very intense absorption bands in the visible region. These macrocyclic compounds have attracted much attention as they have a wide spectrum of very useful physicochemical and biological properties, such as anion binding, stabilization of metal ions with unusual oxidation states, electron transfer, and facilitate the construction of peculiar supramolecular assemblies.30 Porphyrin as an energy acceptor doped into conjugated polymer


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nanoparticles is designed to be an efficient light harvesting system. Monkman et al. first studied TPP doped PF2/6 thin films and identified that the radiative transfer is negligible but the nonradiative Föster energy transfer is valid in this system.31 McNeill et al. investigated a range of dye-doped poly(9,9-dihexylfluorene) (PDHF) nanoparticles achieving emission color tuning due to a FRET mechanism, with high brightness and good photostability for biological labeling and sensing applications.32 They also used TPP as an energy acceptor when studying poly(9,9dioctylfluorene-2,7-diyl) (PFO) β phase formation in nanoparticles.33 Patra, A. et al. studied the aggregation behavior and FRET process of metal zinc octaethylporphyrin (ZnOEP) in Poly(9vinylcarbazole) (PVK) nanoparticles for potential light-harvesting systems.34 The spectroscopic properties and fluorescence dynamics of isolated molecules are very different with TPP aggregates ， therefore, another very important application for porphyrin is to detect the microenvironment in nanoparticles as an aggregation state probe or polarity probe by using its environmental sensitivity

35-37

Previously, porphyrin aggregates were formed inside conjugated

polymer nanoparticles resulting in quenching and low sensitization ability issues. 34, 38 Compared to the porphyrin aggregates37, 39, isolated dye molecules exhibited higher quantum yield, longer lifetime and smaller fluorescence anisotropy which facilitate intensive applications. Herein, we designed a TPP-doped poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(2,5-p-xylene)] (PFX) nanoparticle system made by the reprecipitation method. The absorption spectra, emission spectra and fluorescence lifetime measurements indicated no TPP aggregates formed inside the core of PFX nanoparticles. The location and degree of freedom were detected by time resolved anisotropy decay measurement and analyzed using a ‘wobbling-in-cone and lateral diffusion’ model. This model yields parameters related to the restricted movement of the probe, as well as the diffusion coefficients for both, its ‘wobbling’ and translational motions.25-27, 40 Therefore, the


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diffusion and rotational times of dye molecules, their degree of freedom within particles and the microviscosity of the nanoparticles observed in the present work, provide an insight into their distribution and local rigidity in PFX nanoparticles. Furthermore, the efficient Förster energy transfer from the PFX host as energy donor to the TPP guest as energy acceptor is successfully demonstrated and opens up further prospects in the design of new porphyrin-based materials for application in light harvesting systems. The combined energy diffusion and Förster energy transfer model was used to simulate the relationship between nanoparticle sizes and energy transfer efficiency, as well as the relationship between dye doping density and energy transfer efficiency. The exciton diffusion length was also estimated. EXPERIMENTAL METHODS Materials. The fluorescent conjugated polymer Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(2,5-pxylene)] (PFX) (ADS145UV) was purchased from American Dye Source, Inc., with a weightaverage molecular weight of 54,000 g/mol. A fluorescent dye tetraphenylporphrin (TPP) (>98 % LC) with a molecular weight of 614.74 g/mol, was purchased from TGI Europe. Anhydrous tetrahydrofuran (THF) was purchased from Sigma Aldrich, Inc. All reagents and solvents were used without further purification. De-ionized water (> 16.1 MΩcm, Milli-Q®, Millipore Corp.) was used for all aqueous solution preparations. The molecular structures of PFX and TPP are depicted in Scheme 1. Preparation of nanoparticles. The dye doped polymer nanoparticles were prepared by the reprecipitation method as follows. PFX powder was first dissolved in anhydrous tetrahydrofuran with one hour continuous stirring (1000 rpm; IKA® RCT basic IKAMAG™ hot plate) to make polymer / THF (P / THF) solutions (0.1 mg mL-1; 100 ppm). The fluorescence dye, TPP, was dissolved in anhydrous tetrahydrofuran with one hour continuous stirring to make dye / THF


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(dye / THF) solutions (0.002 mg mL-1; 2 ppm). The pure PFX nanoparticles were synthesized by rapidly injecting 2 mL solution of PFX / THF mixture (20 µg PFX) into 8 mL of de-ionized water in a round bottomed flask while applying sonication for 3 mins. Then a rotary evaporator was used to evaporate THF for 1 h at 10 mbar. 7mL of nanoparticle solution was left in the flask after rotary evaporation. TPP-doped PFX nanoparticles were synthesized as follows. 0.2 mL of 100 ppm PFX / THF stock solution (20 µg PFX) was separately mixed with varying volumes of 2 ppm TPP / THF stock solution: 0.05 mL (0.1 µg TPP), 0.10 mL (0.2 µg TPP), 0.15 mL (0.3 µg TPP), 0.20 mL (0.4 µg TPP), 0.30 mL (0.6 µg TPP), 0.40 µL (0.8 µg TPP) and 0.50 mL (1.0 µg TPP)). THF was added to control the total volume for each sample at 2 mL. The concentrations of TPP were represented by a TPP to PFX weight ratio of 0.5 wt. %, 1.0 wt. %, 1.5 wt. %, 2.0 wt. %, 3.0 wt. %, 4.0 wt. % and 5.0 wt. %, respectively. Solutions were then rapidly injected into 8 mL of de-ionized water in a round bottomed flask while applying sonication for 3 mins. A rotary evaporator was used to evaporate THF for 1 h at 10 mbar, leaving a 7mL of nanoparticle solution after rotary evaporation. Physical Characterization. Dynamic light scattering (DLS) and zeta potential measurements were carried out using a Zetasizer Nano ZS system, (Malvern Instuments, Ltd., UK). DTS Application 5.10 software was employed to analyze the data obtained. Optical Spectroscopy. To minimize artifacts, such as optical scattering, saturation or reabsorption, dilute samples (< 0.1 A.U.) were used to measure absorption and photoluminescence spectra. UV-vis absorption spectra were acquired using a double-beam spectrophotometer (V650; Jasco, Inc.). Water or solvent was used as a reference. Photoluminescence spectra were acquired using a QuantaMaster™ 40, (PTI, Inc.), equipped with a Xe short arc discharge lamp and Czerny-Turner monochromators. Photoluminescence quantum yields were determined using


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the same fluorometer equipped with an integrating sphere (Labsphere, Inc.). A black light illuminator (Gel Logic 200; Kodak, Ltd.) was used when recording true-color emission photographs (Lumix DMC-TZ30; Panasonic, Corp.). Photoluminescence lifetime measurements were performed on a time correlated single photon counting (TCSPC) technique. The measurement system (FluoroCube-01-NL; Jobin Yvon, Horiba Ltd.) was equipped with two semiconductor pulsed light emitting diodes, with emission wavelengths of 370 nm and 409 nm (< 1.2 ns). Fluorescence signal from the samples was collected perpendicularly to the excitation path, passed through a 32 nm band pass filter and detected by a single photon counting module. Time resolved anisotropy measurements were measured using the same measurement system (FluoroCube-01-NL; Jobin Yvon, Horiba Ltd.) with two polarizers in the excitation path and emission path. RESULTS AND DISSCUSSION PFX NPs. PFX nanoparticles were prepared by exposing the polymers to water in the absence of any surfactant stabilizers. The resulting aqueous nanoparticle suspensions were stable and clear (not turbid). Dynamic light scattering (DLS) was employed to measure the hydrodynamic size distributions of these nanoparticles dispersions in water. The measured data indicated that the number average hydrodynamic diameter (dn) of the PFX nanoparticles was ca. 45 nm while the zeta potential was ca. -35 mV at neutral pH; see Figure 1 (a). These negative surface charges which may occur due to the formation of chemical defects at the NP surface during the reprecipitation process, maintain colloidal stability of the NP suspension and inhibit further aggregation.41 Typical intensity-normalized absorption and emission spectra, measured for solutions of PFX in THF and in water dispersions are shown in Figure 1 (b). PFX in THF exhibited an absorption


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spectrum (dashed black line) from 300 nm to 420 nm with a maximum at 350 nm, which is attributed to π-π* transition from the highest occupied molecular orbital (HOMO) to the first excited lowest unoccupied molecular orbital (LUMO) level in the fluorine-benzene polymeric backbones similar with poly(fluorene-alt-phenylene) absorption spectra

42-45

and PFO-co-PX in

the literature.46 Compared with PFX in THF, the absorption spectrum of PFX NPs (solid black lines) exhibited similar behavior without any absorption peak shift observed but a low energy tail appears due to light scattering from NPs in suspension. The emission spectrum (dashed red lines) measured for PFX in THF solution exhibited an intense emission from 350 nm to 550 nm with a maximum vibrionic peak at ca. 410 nm, and a poorly defined vibrionic shoulder at ca. 440 nm, indicating low exciton-phonon coupling.44 The emission spectrum of PFX NPs (solid red lines) exhibited an emission peak at 415 nm with 5 nm red shift compared to PFX in THF; see Figure 1 (b). This red shift might be attributed to a changed polymer chain conformation which presents more planarized / ordered chain segments with higher conjugation length.47 To provide additional benchmark information, typical intensity-normalized absorption, emission and excitation spectra measured for solutions of TPP in THF are shown in Figure 1 (b) as well. The absorption spectrum of porphyrins are well-known, and their bands in different spectral regions are denoted as Q, B, N, L and M.48 Herein, the absorption spectrum (purple line) of TPP in THF exhibited a series of peaks from 300 nm to 750 nm containing one highly intense peak at 416 nm (B band or Soret band), and five additional weak peaks which include one UV region band at 360 nm (M band) and four peaks at the visible range (Q bands)48, see Figure SI.1, supporting information. All of these bands are assigned to π-π* transition from the HOMO levels to the LUMO levels. 49-51 The Soret absorption band is due to S0 → S2 electronic transitions, and


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the Q-band is due to S0 → S1 electronic transitions.52 The B band splitting depends on the distance between the molecules, the aggregation states and the number of the interacting molecules.53 Therefore, no B band splitting was observed here indicating no interactions between two or more molecules in the porphyrin solution due to aggregates which results in two or more excitonic transitions with high transition moment. The emission spectrum measured for TPP in THF (Figure 1 (b), green line) exhibited an intense visible emission from 420 nm to 850 nm with peaks located at 650 nm and 716 nm. Regarding porphyrin, internal conversion, i.e., S2 → S1, is very fast,52 so the peaks exhibited here correspond to the Q (0- 0) and (0-1) transitions, respectively, from first singlet excited state to the ground state S1 → S0.54 Photoluminescence decay data were also measured using the time-correlated single photon counting technique (under pulsed 370 nm excitation, collecting at 415 nm); see Figure 1 (c). Excited state lifetimes were extracted from the decay traces using commercial software by fitting the measured photoluminescence decay traces with multi exponential functions. PFX solutions in THF shows single exponential decay with a lifetime as 0.53 ns which is consistent with poly(fluorene-alt-phenylene) in solvent lifetime in the literature (0.49 ns in toluene, chlorobenzene and dichloromethane).43 But for PFX NPs, double exponential decays were observed. The residuals show the good quality of the fittings, see Figure SI.2, supporting information. The fast component estimated for PFX NPs was 0.49 ns while the slower component was 0.78 ns. The slower component of the PFX NPs lifetime might be also due to polymer chain conformation changing.55 TPP in PFX NPs. Different concentrations of TPP were doped into PFX polymer nanoparticles prepared using a reprecipitation process. The concentration of PFX polymer was kept the same at 20 µg / 7 ml of nanoparticles; and the concentration of TPP was varied from 0 -


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5 wt. % (0 - 1 µg / 7 mL). As with undoped PFX NPs, the hydrodynamic diameters and zeta potentials of doped NPs was tightly controlled during NP formation; see Figure SI.3 and Figure SI.4, supporting information, respectively. The nanoparticles sizes for PFX / TPP NPs were ca. 45 nm (PDI: ca. 0.2) and the zeta potential were ca. -35 mV, see Figure 2 (d). These negative surface charges provided the sufficient electrostatic repulsion to maintain the colloidal stability and avoid aggregation.41 The absorption and emission spectra (exciting the dominant peak of TPP) of PFX / TPP NPs (2 wt. % TPP doping) and TPP in THF (same concentration with TPP in NPs, grey line) are shown in Figure 2 (a). According to Figure 1, the two absorption peaks at ca. 350 nm and ca. 420 nm are separately assigned to PFX and TPP, respectively. The overlapping of M bands of TPP with PFX in the UV region makes the M band difficult to observe. Comparing the absorption spectrum of TPP in THF (Figure 2 (a), grey line) and TPP in PFX NPs (Figure 2 (a), green line), no absorption peak shift or splitting suggests that TPP doesn’t aggregate within NPs,

56-57

and

also, there is no leakage of TPP outside of the nanoparticles since they are water insoluble. In addition, the absorbance of TPP in nanoparticles (0.027, at ca. 420 nm) is slightly lower than TPP in THF (0.033, at ca. 420 nm) which may be attributed to environmental changes for TPP moving from solvent to NPs phase. The peak positions are summarized in Figure SI.5, supporting information. By comparing the emission spectra of TPP in THF (Figure 2 (a), grey line) and in NPs (Figure 2 (a), green line), no emission peak shift was observed indicating that TPP molecules don’t aggregate inside of the PFX NPs.56 In addition, the slightly drop of the emission intensity is due to the environmental changes of TPP from THF to NPs. The absorption and emission spectra (exciting the dominant peak of TPP) of PFX / TPP NPs with different concentration of TPP dopant are shown in Figure 2 (b). With the increasing of TPP


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concentrations, B and Q absorption bands of TPP in the visible region become progressively more pronounced. And the gradually increase of TPP emission intensity indicates no TPP aggregates formed inside or outside of NPs leading to concentration quenching. According to this, TPP can be proved to be encapsulated into the PFX NPs due to their hydrophobic properties and aggregation free detection in the nanoparticle solution. The absolute absorbance of different concentrations of TPP in PFX NPs (at ca. 420 nm) by taking away the pure PFX NPs absorbance (at ca. 420 nm) were plotted as a function of TPP concentration in Figure 2 (c). With increasing TPP concentration, the absolute absorbance of TPP peak increase linearly with good consistency. Through linear regression analysis of data shown in Figure 2 (c) (black line), the molar extinction coefficient of TPP in PFX NPs (at ca. 420 nm) was determined using the Beer-Lambert law, A = ɛcl, where A is absorbance of the sample, ɛ is molar extinction coefficient (M-1 cm-1), c is concentration (M) of the sample and l is path length (cm), respectively. The molar concentration (mol / L) of TPP in PFX NPs solution was calculated by assuming all TPP molecules have been doped into NPs. This is simply calculated using the mass concentration of TPP (g / L) which is derived from synthesis method, divided by the molecular weight of TPP (614.74 g / mol). A molar extinction coefficient for TPP in PFX NPs was returned as ca. 2.05 × 105 M-1 cm-1 while a value of 3.40 × 105 M-1 cm-1 was returned for TPP in THF using the same approach. This is consistent with literature values for TPP in solvent like ethanol58 , see Figure 2 (c), grey line. The reason for the higher molar extinction coefficient of TPP in THF than in NPs is due to the environmental changes from THF to NPs and this phenomenon provides strong evidence for TPP incorporating into NPs. The values of the photoluminescence quantum yield (PL QY) of TPP in THF and in nanoparticle dispersions were ca. 0.0956, 59 and ca. 0.0560 determined (λexc. = 420 nm) using an


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absolute method, respectively. The PLQY of TPP in THF is higher than in NPs and it is likely due to the environmental changes from solvent to NPs which again demonstrate TPP incorporation into NPs, further illustrating why the emission intensity of TPP in solvent is slightly higher than in NPs. Fluorescence lifetime of TPP in PFX NPs with increasing of TPP concentration was measured by directly exciting at a wavelength of strong TPP absorption (409 nm) and collecting at a wavelength of dominant TPP emission (650 nm), as shown in Figure 3. The fluorescence lifetime of TPP in THF was also measured directly for comparison, as shown in Figure SI.6, supporting information. Excited state lifetimes were extracted from decay traces, fit with a multi exponential function using commercial software; see Figure SI.7, supporting information. Fitting residuals are shown in Figure SI.8, supporting information. For pure PFX NPs, the short component lifetime is ca. 0.27 ns (82 %) and the long component lifetime is ca. 8.13 ns (18 %). By comparing with the lifetime of PFX NPs by exciting and collecting at dominant PFX polymer (λexc. = 370 nm, λcoll. = 415 nm), refer to Figure 1 (c), the short component here was very similar, however, the longer component was much longer. It is consistent with a model that describes the polymer as a continuous distribution of site energies. In this model, each state corresponds to a conjugated polymer segment that is interrupted by chain defects (conformational or chemical). The longer segments having lower energy, so the incoherent hopping of excitations to lower energy states well describe the energy migration. Emission from high energy states (here, 416 nm) should exhibit a faster decay rate due to energy transfer to lower energy chromophores within the system, and the emission from lower energy states (here, 650 nm) should exhibit a slower decay rate (longer lifetime) because of the emission trapping sites.61 This is consistent with observations for other conjugated polymer nanoparticles.62


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For TPP doped NPs, the short component (less than 5 %) at ca. 0.12 ns could be attributed to PFX polymer emission at 650 nm. At the longer wavelength, the absorption spectrum of TPP still has an overlapping area with the emission spectrum of PFX, leading to energy transfer from PFX to TPP, therefore, the lifetime of PFX decreased from ca. 0.27 ns (pure NPs) to ca. 0.12 ns (TPP doped NPs). However, the lifetime of PFX (short component) doesn’t vary with different concentrations of TPP doping which is due to the spatial distribution of polymer contributions (too small weight percent, less than 5 %). Considering the fluorescence lifetime of TPP in THF (ca. 9.5 ns) in Figure SI.6 which is consistence with literature

59, 63

, the long component of TPP

doped NPs (ca. 12.0 ns (more than 95 %)) was attributed to TPP molecules in the PFX NPs. The reason for the longer lifetime of TPP in NPs (12.0 ns) than TPP in THF (9.5 ns) is because of the environmental changes from THF to NPs which also provide strong evidence of TPP incorporating into NPs and no TPP aggregates formed (0.5-3 ns of TPP aggregates from literature56, 64). Time resolved fluorescence anisotropy measurements were performed using pulsed excitation at 409 nm and collecting at the dominant TPP (650 nm) for PFX / TPP NPs, see Figure 4 (a). The time resolved emission anisotropy of TPP in organic solvent was also measured at the same wavelength for comparison as shown in Figure 4 (b). All the fitting data is shown in Figure 4 (c). For TPP in THF, the anisotropy decay was fitted with a single exponential function. The rotational correlation time τ was ca. 0.18 while r0 was ca. 0.09, consistent with literature values63. For PFX / TPP NPs, the fluorescence anisotropies were fitted with double exponential function as follows:


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r = −

+ 1 − −

Eq

1 Here, τslow and τfast are the two reorientation times associated with the slow and fast motions of TPP in PFX NPs while a is the pre-exponential factor which indicates the relative contributions of the slow and fast motions to the anisotropy decay. Average reorientation time can be expressed as 〈 !〉 =

#$%&

+ 1 −

'(#

Eq 2

Average reorientation times of 0.25, 0.27, 0.24 and 0.87 ns for 1 %, 2 %, 3 % and 5 % TPP encapsulated in PFX nanoparticles, were returned respectively. Consistent r0 values of 0.38, 0.33, 0.35 and 0.35 were returned for 1 %, 2 %, 3 % and 5 wt. % TPP / PFX NPs. In order to understand the diffusion dynamics, location and degree of freedom of the organic dye TPP within PFX NPs, the ’wobbling-in-cone and lateral diffusion’ model was applied 25-28, 40 This model accounts for slow lateral diffusion (τD) of TPP molecules on the surface of the polymer nanoparticles and fast wobbling motion (τE) of TPP inside polymer nanoparticles which are coupled with the overall rotation of the polymer nanoparticles in aqueous suspensions. This model yields the cone angle (θ0) and wobbling diffusion coefficient (Dw) by assuming that the slow and the fast motions are separable and the above rotational parameters were calculated by applying the following equations: 25-26, 65 )

)

=

)

*

=

+

)

+

)

+

)

,


15

Eq 3


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τfast and τslow are the fast and slow reorientation times of TPP dye molecules. τM is the time constant for the overall rotation of polymer nanoparticles and can be estimated using the StokesEinstein-Debye relation with the stick boundary condition:66 -

=

./0!12 345

Eq 4

Here, η is the viscosity of the medium. k (=1.3805 × 10-23 J K-1) is Boltzmann constant, and T the temperature in Kelvin. rh is the hydrodynamic radius of the polymer nanoparticles (22.5 nm). The τM value was calculated as 7.26 µs for the PFX nanoparticle system through application of Equation 4. As the τM value is in the microsecond region, it has a negligible effect on lateral diffusion. As mentioned earlier, τslow represents the time constant for lateral diffusion and is related to the lateral diffusion constant DL by the relation:

67 =

!18

9+

Eq 5

The DL calculated from Equation 5 are all very similar: 6.34 × 10-4 cm2s-1, 8.04 × 10-4 cm2s-1 and 6.97 × 10-4 cm2 s-1 for 1, 2, and 3 wt. % of TPP doping PFX NPs confined systems, respectively. However, at 5 wt. % TPP doping, a smaller lateral diffusion value of 0.99 × 10-4 cm2 s-1 was achieved, which may be due to molecular crowdedness prohibiting fast lateral diffusion at the NP surface. The reason behind the longer component depolarization process attributed to TPP diffusion on the NP surface is explained as follows. In homogeneous media and in planar membranes, the translational diffusion of the molecule does not reorient the molecule, however, in spherical NPs, the dye molecule is oriented with respect to the interface. For this NP system, the dye molecule is preferentially situated near the interface,67-68 therefore, translational diffusion on the two-dimensional quasi spherical surface of the NPs is possible. Transport of TPP along the spherical surface inevitably reorients the molecule with respect to its initial


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orientation leading to translational diffusion depolarization.25 The translational diffusion coefficient of nanoparticles can be calculated using the Stokes- Einstein equation69 as follows: 6 =

45

9/!1 0

Eq.6

where D (cm2 s-1) is translational diffusion coefficient of the nanoparticles, k (=1.3805 × 10-23 J K-1) is Boltzmann coefficient, T (K) is Kelvin temperature, rh is hydrodynamic radius (here 22.5 nm), η is viscosity of medium. The translational diffusion coefficient of PFX NPs is calculated as 1.09× 10-7 cm2 s-1, which is much smaller than lateral diffusion coefficient of TPP on the surface of NPs, therefore, it has negligible effect of TPP lateral diffusion. Order parameter (S) (restriction parameter) is considered a measure of the spatial restriction of the probe dye molecules, and the value varies from 0 to 1 depending on the level of rotational restriction of dye molecules. The order parameter is related with the amplitude of the slower component of anisotropy decay by the following equation70 :; =

Eq. 7

The order values S are all below 0.3 as 0.14, 0.14, 0.30 and 0.26 for 1, 2, 3 and 5 wt. % of TPP doping PFX NPs systems, respectively, and 0 for TPP in THF solution. The low values for the order parameters indicates TPP molecules are experiencing low restricted rotation in PFX NPs and the degree of rotational freedom is relatively high yet not completely free, like TPP in THF solution, which tumbles freely and whose equilibrium orientational distribution is completely random.25 The order values derived here are low enough compared to other dye probes in micelles in the literature.25, 63 According to the ‘wobbling in a cone’ model, the TPP molecules wobble inside a cone of semiangle θ which depends on the parameter S. The relation between S and θ is as follows: S = 0.5 cos C 1 + cos C


17

Eq. 8


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In this ‘wobbling in cone‘ system, the dye molecules can be considered as freely rotating if θ value is calculated as 90°, like derived from TPP in THF. The θ values calculated for NPs here are all close to 90° as 76.7°, 76.7°, 65.0° and 67.5° for 1, 2, 3 and 5 wt. % of TPP doping PFX NPs confined systems, respectively, which corresponds to relatively low rotational restriction. The diffusion coefficient for wobbling motion Dw for three different systems is calculated by using the following equation:71 )

6D = E)FG8 H I *

JKG 8 L)MJKGL8 ;JKGLF)

NOP

)MQ%#L ;

+

)FQ%#L ;

R+

)FQ%#L ;.

（6 + 8UVWC − UVW ; C −

12UVW 3 C − 7UVW . C）Z Eq.9 The diffusion coefficient for wobbling motion of TPP in THF solution is 13.90 × 108 s-1. But all of the Dw values are smaller than TPP in THF within the range of (7.49-9.95) × 108 s-1, indicating that TPP in NPs is almost as free as in a liquid of a higher viscosity than THF.63 These values for the model (wobbling-in-cone and lateral diffusion) are summarized in Table 1 and are consistent with other data for dyes incorporated into conjugated polymer nanoparticles.28 By applying this model, the diffusion dynamics, location and freedom of TPP dye molecules in the PFX NPs were well investigated. But we have to be clear that this analysis is only for diffusion dynamics of TPP which induced fluorescence depolarization, so it may not include all motion dynamics of TPP in the CPNPs. The microviscosity inside the nanoparticles detected by a probe is a complex function of the nature of the polymer chains, structure of the probe and the position of the probe molecules inside the micelles and can be related to the wobbling diffusion coefficient of dye probe in NPs. The microviscosity of PFX NPs can be roughly estimate by applying the StokesEinstein equation72-73:


18

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6[ = \T/8πη 3

Eq.10

where a is the radius of the probe, ηis the microviscosity of the nanoparticles. η was calculated as 0.9-1.2 cP which is higher than the viscosity of THF (0.48 cP) which is consistent with the wobbling diffusion coefficient of TPP in NPs and in THF. PFX / TPP NPs –FRET Our dye doped nanoparticle system has two components; photoactive energy donor and accepter, therefore, an efficient energy transfer process is expected. It is clear that the emission spectrum of PFX NPs exhibited good overlap with the absorption spectrum of the TPP / THF, as required for efficient energy transfer via the Förster mechanism, see Figure 5 (a).72 The fluorescence spectra of the TPP doped PFX NPs with increasing TPP concentration under 350 nm excitation wavelength are shown in Figure 5 (b). Emission from the PFX polymer host noticeably decreased at a dye content of as little as 0.5 wt. %, and was almost completely quenched at 5 wt. %, whereas the red emission of the dye dopant became progressively more intense, reaching a maximum at 2.0 wt. %. Above 2.0 wt. %, the TPP emission intensity began to decrease and level off which is likely due to FRET saturation74, see Figure 5 (c). The true-colour cuvette emission photographs of 0-5 wt. % TPP doping PFX NPs under a UV light illuminator (365 nm) are shown in Figure 5 (f). The pure PFX NPs was dominated by near UV color (415 nm emission) while 5 wt. % TPP doping NPs was dominated by red color (650 nm, 716 nm) emission. With increasing TPP concentrations, the color varies from violet to red which indicates that apparent emission colour tuning has been achieved. The data is consistent with the Förster mechanism of resonance energy transfer whereby sufficient spectral overlap exists between the host PFX emission spectrum and the excitation spectrum of the TPP dopants. According to Förster theory72, the Förster radius is defined as the donor-acceptor distance at which the rate


19


Page 20 of 59

constant for energy transfer is equal to the total decay rate constant of the donor in the absence of the acceptor. The Förster radius and overlap integral are calculated as 5.2 nm and 1.55 × 1013 M-1 cm-1 nm4, respectively, based on the spectral overlap in Figure 5 (a) which indicate that the distance between TPP and PFX is within 5.2 nm (smaller than the size of the NPs) and provides further evidence of successful doping of TPP within PFX NPs. The number of the host PFX quenched by single acceptor molecules was calculated using the Stern-Volmer equation:22, 28, 32 ab a

= 1 + cGd EeH

Eq.

11 Here, F0 and F are fluorescence intensity due to PFX in the absence and presence of TPP (data extracted from Figure 5 (b) at 415 nm), respectively. Ksv is the Stern-Volmer quenching constant, and [A] is the concentration of the acceptor (TPP) molecules. Here, [A] is expressed as the molecular fraction of the quencher (TPP), and Ksv represents the number of host molecules quenched by a single acceptor molecule. McNeill et al. reported that 3-9 polymer molecules are quenched by a single dye molecule, depending upon the nature of the dye and found that 8.9 molecules were quenched for each TPP molecule in PDHF NPs.32 The Stern-Volmer plot here (see Figure 5 (d)) suggests that about 12 polymer molecules were quenched by one single TPP dye molecule which is higher than the PDHF / TPP NPs system from McNeill et al.32. This super quenching phonomenon is due to energy migration along the polymer backbone while the quencher molecule is able to trap almost all the excitons along the polymer backbone.75 At 2 wt. % of TPP doping NPs, under direct excitation at PFX (350 nm), the 650 nm emission peak intensity of TPP is 14 × 106 cps, as shown in Figure 5 (e) (green line), which is four times higher than direct excitation at TPP (420 nm) (only 3.5 × 106 cps, as shown in Figure 2 (d) green


20

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line), and also, it is 70 times higher than TPP in THF with the same concentration and same excitation condition (0.2 × 106 cps), see Figure 5(e), grey lines. Therefore, this doped nanoparticle system successfully achieved efficient sensitization with energy transfer from PFX to TPP. The excitation spectra were carefully measured by collecting at a wavelength of dominant PFX emission (415 nm). With increasing TPP concentration, the intensity of the excitation spectra decreased at wavelengths of strong PFX excitation, consistent with progressive PFX emission quenching, as shown in Figure 6 (a). At a wavelength of dominant TPP emission (650 nm), the excitation intensity increased at wavelengths of strong PFX excitation (415 nm) before dropping, for increasing TPP concentration from 0 wt. % to 2.0 wt. %, as shown in Figure 6 (b). This intensity dropping is possible due to the FRET saturation.74 In this regard, direct and selective excitation of TPP gave an increasing photoluminescence signal indicating no TPP aggregates formed (e.g., at 420 nm), that might cause self-quenching. These observations were consistent with progressive FRET-based sensitization of acceptor emission in blended NPs. The different excitation intensity as a function of TPP doping concentrations are summarized at Figure 6 (c). Fluorescence lifetime was measured by exciting at a wavelength of strong PFX absorption (370 nm) and collecting at a wavelength of dominant PFX emission (415 nm) with increasing TPP concentrations as shown in Figure 7 (a). Excited state lifetimes were extracted from decay traces using commercial software by fitting the measured photoluminescence decay traces with double exponential functions; the average lifetime were estimated by amplitude weight average; see Figure 7 (c). PL lifetime of PFX gradually decreased with increasing TPP concentration (fast component decrease from 0.49 ns to 0.09 ns, slow component decrease from 0.78 ns to 0.56 ns) due to non-radiative FRET-based sensitization of acceptor emission in doped NPs. The average


21


lifetime of PFX decreased as a function of TPP concentration as shown in the plot in Figure 7 (b). Residuals of lifetime fits are shown in Figure SI. 7, Supporting information. The emission spectra, excitation spectra and fluorescence lifetime together provide strong evidence of Fӧrster energy transfer occurring in the PFX / TPP blended nanoparticles system. To compare the energy transfer of PFX / TPP in NPs with PFX / TPP in THF bulk solution, 20 µg of PFX was mixed with different concentrations of TPP in THF solution. The absorbance at 416 nm increased with increasing of TPP concentration, as seen in Figure SI.10 (a). The emission spectra were measured by exciting at 350 nm but neither PFX emission quenching nor TPP emission intensity increases were observed, indicating no FRET occurred in this bulk solution system, as seen in Figure SI.10 (b). With the concentration of TPP increasing, the excitation intensity of PFX doesn’t change by collecting at the dominant PFX wavelength (415 nm), as seen in Figure SI.10 (c). Exciting each system at 370 nm and collecting at 415 nm showed that the fluorescence lifetime of the mixed bulk solution exhibited a similar result as with PFX in THF on its own (ca. 0.5 ns), as seen in Figure SI.10 (d). All of the measurements indicate no FRET happening in this bulk solution system, as expected, due to the prohibitively large distance between donor and acceptor molecules. The photoluminescence quantum yield of PFX / TPP NPs by exciting the dominant polymer at 370 nm were 0.66, 0.22 and 0.03 for 0, 0.5 and 3.0 wt. % TPP, respectively; see Table 2. The photoluminescence radiative (kr) and nonradiative (knr) rate constants were estimated by combining the photoluminescence quantum yield (ϕ), where ϕ = kr / (kr + knr), and the average fluorescence lifetime (τ), where τ = (kr + knr )−1, kr = ϕ / τ and knr=(1- ϕ) / τ .62 Values for the radiative rate constant of 0, 0.5 and 3.0 wt. % TPP / PFX NPs estimated in this manner are 1.22 × 109 s−1, 0.88 × 109 s−1 and 0.63 × 109 s−1 respectively, while the non-radiative rate constant of 0,


22

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0.5 and 3.0 wt. % TPP / PFX NPs estimated in this manner were 0.25 × 109 s−1, 3.12 × 109 s−1 and 8.08 × 109 s−1 respectively, see Table 2. The collective photoluminescence quantum yield decrease, radiative rate decreases and non-radiative rate increase with TPP concentration increase all illustrate the non-radiative quenching of PFX by a FRET process. The Fӧrster energy transfer efficiency is typically measured using the relative fluorescence intensity of the donor, in the absence (FD) and presence (FDA) of an acceptor:72 E = 1 − ghi /gh

Eq.

12 The Fӧrster energy transfer efficiency can also be calculated from the lifetimes under these respective conditions: E=1−

hi / h

Eq.

13 By applying the equations, the energy transfer efficiency was calculated in Table 2. With increasing TPP concentration, the energy transfer efficiency calculated from the lifetime (increased from 54% to 78%) were similar with the efficiency calculated from emission intensity (increased from 60% to 97%). With only 3 wt. % of TPP, high energy transfer efficiency was achieved. Very similar results were observed by McNeil et al. for TPP- doped PDHF NPs.32 While the Förster mechanism was considered to be the primary energy transfer mechanism, other mechanisms, such as Dexter transfer,76 could not be ruled out due to the apparent close proximity of the host polymer and guest polymer. The relationship between energy transfer efficiency and the sizes of NPs were simulated by using the combination of Exciton diffusion model and Forster energy transfer model.32 The simulation algorithm is described as follows. The simulation code was written as a set of C++


23


Page 24 of 59

scripts. The Exciton diffusion model is based on the 3D random walk on a discrete cubic lattice. TPP dyes are distributed randomly within the nanoparticle, represented by a sphere. An initial population of excitons is also distributed randomly within the sphere. The exciton is given an initial random position within the NP. After a time interval of duration ∆t, the exciton moves a single step of length ɛ in a random direction, subject to the constraints imposed by the geometry. The average number of steps N for the exciton to travel a distance equal to the exciton diffusion length LD is given by N = ( LD / ɛ). Values for ɛ between 0.05 and 0.5 yield very similar energy transfer efficiency suggesting little sensitivity to this parameter and 0.1 was used here. The time step size ∆t is related to the fluorescence lifetime of PFX NPs, τD with absence of acceptor (0.52 ns for PFX NPs here) by N∆t= τD. At each step, the overall energy transfer rate constant k’ET is calculated based on the position of the exciton and positions of the acceptors according to Förster energy transfer model in the following equation: o

o

)

q

\ j k5 = ∑n p ck5,n = ∑n p b 9 q +

r

Eq.

14 where NA is the number of dye molecules per particle (for 0.5 wt. % and 3 wt. % doping, the number of TPP molecules per particles were estimated as 234 and 1400, respectively), Ro is the Forster radius calculated from Förster theory (5.2 nm here). Rj is the distance between the exciton and the jth dye molecules, extracted from the exciton diffusion model simulation. The probability of energy transfer and decay during the time step are calculated as p=1-exp(-k’ET∆t) and p=1-exp(-∆t/ τD), respectively. Comparison of generated random numbers against the probabilities of the two processes is used to determine if the exciton has undergone decay or transfer during the time step, ending the trajectory. If not, the exciton trajectory continues to the next step. Each trajectory is allowed to eventually terminate in either energy transfer or decay in


24

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the total time scale τD. Thousands of exciton trajectories were calculated, and the energy transfer efficiency was determined by counting the number of trajectories that terminate in energy transfer relative to the total number of trajectories. To explore the relationship of nanoparticle sizes with energy transfer efficiency, for each nanoparticle, the number of dye molecules per unit volume was fixed at a value of 0.0049 per nm3 (corresponding to 0.5 wt. % TPP doping in PFX NPs with radius at 22.5 nm) and 0.0290 per nm3 (corresponding to 3.0 wt. % TPP doping in PFX NPs with radius at 22.5 nm). The simulation results were shown in Figure 8. The energy transfer efficiency increases gradually for small particles (5-20 nm), approaching constant value for particle radii above 30 nm for both of the two dye densities. The reason is that for smaller particles, the dopant molecules are more likely to be located close to the surface due to the higher surface to volume ratio. The TPP molecules diffusing near the surface have a smaller effective quenching volume as compared to those farther from the surface (closer to the centre), leading to lower quenching efficiency.32 The energy transfer efficiency of NPs system with 0.0049 / nm3 TPP density range from 0.45 to 0.63 (Figure 8 (a)) and 0.94 to 0.99 for NPs system with 0.029 / nm3 TPP density (Figure 8 (b)), which indicate that the FRET efficiency increases with increasing dye density. To compare with our experimental results in Table 2, the energy transfer efficiency calculated from emission intensity for 0.5 wt. % and 3 wt. % TPP doping in NPs with 22.5 nm radius were 0.600 and0.974 (red dots in Figure 8), respectively, which strongly agrees with the simulation results. Comparing calculated energy transfer efficiencies for a range of diffusion length values to experimental results (0.5 wt. % TPP doping, 22.5 nm radius) yields an estimated exciton diffusion length of ca. 6.5 nm which is consistent with reported values for similar materials.32, 77 CONCLUSION


25


In this work, TPP dye molecules were successfully transferred from organic solvent to water dispersion in conjugated polymer nanoparticles with high emission intensity. The successful doping of TPP into PFX NPs without any TPP aggregation formed in the NPs has been shown by a series of photophysical measurements. TPP was illustrated to wobbling within the core of NPs at a semiangle ca. 70° and laterally diffuse on the surface of NPs with a diffusion coefficient ca. 10-4 cm2 s-1 by using time resolved anisotropy measurements. 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).

Hence, this work contributes through investigating the

microviscosity, dynamics, location and freedom studies of TPP in CPNPs without dye aggregate formation. Also, with increasing of TPP concentration, emission color tuning, super quenching of the polymer (one TPP molecule quench 12 polymer chains based on Stern-Volmer plot) and good sensitization of TPP molecules were observed through the FRET process. The PFX NPs provide a constrained microenvironment for TPP diffusion, thus, efficient FRET can be observed in the nanoparticles system but not in bulk solution system due to the strict donor and acceptor distance requirement for FRET. The combined energy diffusion and Förster energy transfer model was used to estimate the relationship between nanoparticle sizes and energy transfer efficiency. Both of the simulation and experimental results indicate that in this system, FRET efficiency increases with the dye doping density increasing and they increase gradually for small particles (5-20 nm), approaching a constant value for particle radii above 30 nm. The exciton diffusion length was estimated as 6.5 nm. In another words, the FRET efficiency for the PFX/TPP NPs is precisely predictable by correlating nanoparticle sizes and exciton diffusion length, but not for bulk solution system. To conclude, TPP has been demonstrated to probe the aggregation state, microviscosity and diffusion dynamics of organic dyes microenvironment in


26

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the conjugated polymer nanoparticle which should open up further prospects in designing new porphyrin based nanoparticle materials, improving knowledge towards better design of nanophotonic devices based on dye-doped polymer NPs. This aggregation-free TPP doped PFX NPs system exhibits remarkably high energy transfer efficiency which opens up further potential applications as a light harvesting system. ASSOCIATED CONTENT Supporting Information. Data sets presenting lifetime fitting residuals, hydrodynamic diameters distributions, zeta potential distributions measured for TPP doped and undoped PFX NPs; lifetime fitting of TPP/THF and photophysical characterization of PFX/TPP in bulk THF solution. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Tel.: + 353 1 716 2881; E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ABBREVIATIONS Circa (ca.), dynamic light scattering (DLS), Förster resonance energy transfer (FRET), highest occupied molecular orbitals (HOMO), light emitting diode (LED), lowest unoccupied molecular


27


orbital

(LUMO),

nanoparticles

(NPs),

poly(9,9-dihexylfluorene)

Page 28 of 59

(PDHF),

poly(9,9-

dioctylfluorene-2,7-diyl) (PFO), photoluminescence quantum yield (PL QY), poly[(9,9dioctylfluorenyl-2,7-diyl)-co-(2,5-p-xylene)]

(PFX),

Poly(9-vinylcarbazole)

(PVK),

tetraphenylporphrin (TPP), tetrahydrofuran (THF), time correlated single photon counting (TCSPC), zink octaethylporphyrin (ZnOEP). ACKNOWLEDGEMENT S.W gratefully acknowledges the financial support by China Scholarship Council.


28

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Scheme 1. Molecular structures of the key materials employed in this study.


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Figure 1. (a) Number-average hydrodynamic diameter distributions of PFX NPs. Inset: Zeta potential distributions of PFX NPs. (b) Intensity-normalized absorption spectra of PFX in THF solution (dashed black line), PFX NPs (solid black line) and TPP in THF solution (solid purple line). And intensity-normalized emission spectra of PFX in THF solution (dashed red line), PFX NPs (solid red line) and TPP in THF solution (solid green line). (c) Fluorescence lifetime of the PFX in THF (black dots), PFX NPs (red dots) and instrument response function (grey dots).


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Figure 2. (a) Absorption and emission spectra of PFX / TPP NPs (2 wt. % TPP doping) (green lines) and TPP / THF (grey lines). (b) Absorption spectra (dashed dot lines) and emission spectra (solid lines, λexc. = 420 nm) of PFX / TPP NPs with different TPP doping concentrations. (c) Grey line: linear fitting of absolute absorbance of TPP in THF solution as a function of TPP concentrations. Black line: linear fitting of absolute absorbance of TPP in NPs, by taking away the undoped PFX NPs absorbance, as a function of TPP concentrations. (d) Hydrodynamic size (black squares) and zeta potential (red dots) data measured for PFX / TPP nanoparticles as a function of TPP concentrations.


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Figure 3. Fluorescence lifetime decay curves (λexc. = 409 nm, λcoll. = 650 nm) of PFX / TPP NPs and instrument response function (black dots).


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Figure 4. Time-resolved anisotropy decay curves (λexc. = 409 nm, λcoll. = 650 nm) of (a) PFX / TPP NPs and (b) TPP / THF (wine dots). (c) Time resolved anisotropy decay fittings of PFX / TPP NPs and TPP / THF.


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Table 1. Values of the parameters for the model (wobbling-in-cone + translational diffusion) derived from the anisotropy decays of TPP in THF and PFX / TPP NPs.


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Figure 5. (a) Intensity normalized emission spectrum of PFX NPs and absorption spectra of TPP in THF. (b) Fluorescence spectra of PFX / TPP NPs under 350 nm excitation. (c) Plots of 415 nm emission intensities of the host polymer PFX (black line) and 650 nm emission intensities of the dopant dye TPP (red line) as a function of TPP concentration. (d) Fluorescence quenching of PFX donor versus molar fraction of TPP quenchers in doped polymer nanoparticles. The squares define the experimental results. The red solid line is plot fitted by Stern-Volmer equation. (e) Emission spectra of TPP / THF (grey line, λexc. = 350 nm), TPP / Toluene (grey line, λexc. = 350 nm) and PFX / TPP NPs (2 wt. % TPP doping) (green line, λexc. = 420 nm). (f) True-colour cuvette emission photographs of 0-5 wt. % (blue-red) TPP doping PFX NPs under a UV light illuminator (365 nm).


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Figure 6. Excitation spectra of PFX / TPP NPs of (a) λcoll. = 415 nm and (b) λcoll. = 650 nm. (c) Excitation intensity as a function of TPP doping concentration.


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Figure 7. (a) Fluorescence lifetime of PFX / TPP NPs (λexc. = 370 nm, λcoll. = 415 nm) (b) The average lifetime as a function of TPP doping concentration. (c) Summary of the fittings.


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Table 2. Summary of fluorescence lifetime, photoluminescence quantum yield, radiative rate, non-radiative rate and energy transfer efficiency (calculated from the relative fluorescence intensity and lifetime) of PFX / TPP NPs.


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Figure 8. Simulation results of nanoparticle size dependent Forster energy transfer efficiency by combing Exciton diffusion model and Forster energy transfer model. The number of TPP molecules per unit volume is (a) 0.0049 / nm3 (b) 0.0290 / nm3


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Table of Contents Graphic


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83x87mm (300 x 300 DPI)


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Scheme 1. Molecular structures of the key materials employed in this study. 46x26mm (300 x 300 DPI)



Figure 1. (a) Number-average hydrodynamic diameter distributions of PFX NPs. Inset: Zeta potential distributions of PFX NPs. (b) Intensity-normalized absorption spectra of PFX in THF solution (dashed black line), PFX NPs (solid black line) and TPP in THF solution (solid purple line). And intensity-normalized emission spectra of PFX in THF solution (dashed red line), PFX NPs (solid red line) and TPP in THF solution (solid green line). (c) Fluorescence lifetime of the PFX in THF (black dots), PFX NPs (red dots) and instrument response function (grey dots). 145x262mm (300 x 300 DPI)


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Figure 2. (a) Absorption and emission spectra of PFX / TPP NPs (2 wt. % TPP doping) (green lines) and TPP / THF (grey lines). (b) Absorption spectra (dashed dot lines) and emission spectra (solid lines, λexc. = 420 nm) of PFX / TPP NPs with different TPP doping concentrations. (c) Grey line: linear fitting of absolute absorbance of TPP in THF solution as a function of TPP concentrations. Black line: linear fitting of absolute absorbance of TPP in NPs, by taking away the undoped PFX NPs absorbance, as a function of TPP concentrations. (d) Hydrodynamic size (black squares) and zeta potential (red dots) data measured for PFX / TPP nanoparticles as a function of TPP concentrations. 54x37mm (300 x 300 DPI)



Figure 3. Fluorescence lifetime decay curves (λexc. = 409 nm, λcoll. = 650 nm) of PFX / TPP NPs and instrument response function (black dots). 56x39mm (300 x 300 DPI)


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Figure 4. Time-resolved anisotropy decay curves (λexc. = 409 nm, λcoll. = 650 nm) of (a) PFX / TPP NPs and (b) TPP / THF (wine dots). (c) Time resolved anisotropy decay fittings of PFX / TPP NPs and TPP / THF. 100x63mm (300 x 300 DPI)



Table 1. Values of the parameters for the model (wobbling-in-cone + translational diffusion) derived from the anisotropy decays of TPP in THF and PFX / TPP NPs. 30x11mm (300 x 300 DPI)


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Figure 5. (a) Intensity normalized emission spectrum of PFX NPs and absorption spectra of TPP in THF. (b) Fluorescence spectra of PFX / TPP NPs under 350 nm excitation. (c) Plots of 415 nm emission intensities of the host polymer PFX (black line) and 650 nm emission intensities of the dopant dye TPP (red line) as a function of TPP concentration. (d) Fluorescence quenching of PFX donor versus molar fraction of TPP quenchers in doped polymer nanoparticles. The squares define the experimental results. The red solid line is plot fitted by Stern-Volmer equation. (e) Emission spectra of TPP / THF (grey line, λexc. = 350 nm), TPP / Toluene (grey line, λexc. = 350 nm) and PFX / TPP NPs (2 wt. % TPP doping) (green line, λexc. = 420 nm). (f) True-colour cuvette emission photographs of 0-5 wt. % (blue-red) TPP doping PFX NPs under a UV light illuminator (365 nm). 78x38mm (300 x 300 DPI)



Figure 6. Excitation spectra of PFX / TPP NPs of (a) λcoll. = 415 nm and (b) λcoll. = 650 nm. (c) Excitation intensity as a function of TPP doping concentration. 157x309mm (300 x 300 DPI)


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Figure 7. (a) Fluorescence lifetime of PFX / TPP NPs (λexc. = 370 nm, λcoll. = 415 nm) (b) The average lifetime as a function of TPP doping concentration. (c) Summary of the fittings. 144x259mm (300 x 300 DPI)



Table 2. Summary of fluorescence lifetime, photoluminescence quantum yield, radiative rate, non-radiative rate and energy transfer efficiency (calculated from the relative fluorescence intensity and lifetime) of PFX / TPP NPs. 19x4mm (300 x 300 DPI)


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Figure 8. Simulation results of nanoparticle size dependent Forster energy transfer efficiency by combing Exciton diffusion model and Forster energy transfer model. The number of TPP molecules per unit volume is (a) 0.0049 / nm3 (b) 0.0290 / nm3 118x175mm (300 x 300 DPI)


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