Single-Component Oligomer Nanoparticle-Based Size-Dependent

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Single-Component Oligomer Nanoparticle Based Size-Dependent Dual Emission Modulation Wei Liu, Mingliang Sun, Liangmin Yu, Xin Song, Feng Li, Chun-Sing Lee, and Vellaisamy A. L. Roy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12272 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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The Journal of Physical Chemistry

Single-Component Oligomer Nanoparticle Based Size-Dependent Dual Emission Modulation Wei Liu1, Mingliang Sun1,2,3 *, Liangmin Yu 4, Xin Song2, Feng Li5, Chun-Sing Lee1, Vellaisamy A.L. Roy1* 1. Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong SAR, P.R.China 2. School of Material Science and Engineering, Ocean University of China, Qingdao 266100, P.R.China 3. Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P.R.China 4. Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, P.R.China 5 Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Engineering, Qingdao University of Science & Technology, 53 Zhengzhou Road, Qingdao 266042, P.R.China AUTHOR INFORMATION *Corresponding Authors Mingliang Sun; [email protected] Vellaisamy A.L. Roy; [email protected]

Abstract Multi-chromophoric oligomers offer a versatile platform for nanoparticle multi-color fluorescent modulation. A donor-acceptor-donor (D-A-D) type oligomer (DDBTD), with blue emitting antenna and red emitting core, is chosen to assemble into fluorescent colloidal nanoparticles (FCNs) using a nano-precipitation method. By modulating the DDBTD concentrations in good solvent, the DDBTD nanoparticles (NPs) with average diameters ranging from sub-10 to 300 nm are obtained by the nano-precipitation process in aqueous solution. Interestingly, multi-color

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photo-luminescence (PL) can be realized from bright blue (~ 440 nm) to rose red (~ 630 nm) based on FCNs size control. The size-dependent PL is originated from the aggregation-enhanced fluorescence resonance energy transfer (FRET) from diphenyl-aminofluorenyl antenna unit (blue emitter) to benzothiadiazole based core (red emitter). Furthermore, the lifetime measurement of the FCNs in excited state shows a size-dependent behavior, which confirms that the sizedependent multi-color PL modulation is adjusted by FRET in the nanoscale oligomer. This work highlights the potential of the single-component multi-chromophoric oligomer FCNs for luminescent modulation applications.

1. Introduction Fluorescent colloidal nanoparticles (FCNs) have received intensive research interest for their unique fluorescent properties, which is different from isolated atoms/molecules and bulk materials counterparts.1 Typically, a wide photoluminescence (PL) spectrum from pure blue to bright red can be modulated by tuning inorganic FCNs sizes.2-3 This correlation between PL modulation and particle size within rational range originates from the quantum confinement effect due to the narrow radii of the Frenkel exciton.4 Similarly, FCNs from organic molecules with tunable sizes also show modulated PL emission, with intrinsically different opto-electronic properties from those of inorganic counterparts.5-6 The opto-electronic properties of organic FCNs can be modulated by flexible chemical modifications or self-assembly of chromophores, making organic FCNs an active research subject in light-emitting applications such as luminescent solar concentrators, optical displays and sensing tools featured by easy solution process.7-9


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Organic FCNs based on conjugated chromophoric aggregates, possessing versatile optoelectronic properties, have been developed for various luminescent application in recent years. For example, perylene-based aggregates show both redshifted absorption and PL emission concomitant with the increased sizes, representing the first discovery on the size-dependent photo-physical properties of organic FCNs.10 This discovery has inspired great efforts in chromophore system design for improving FCN performance such as higher light-harvesting and multiple luminescence colors. Unfortunately, adverse aggregation effects, i.e. formation of Haggregate, on luminescent properties such as blue-shifted absorption and/or weakened PL emission are common, making the design of molecular chromophores a challenging work.11-12 Alternatively, an energy gradient multi-chromophoric system capable of the fluorescence resonance energy transfer (FRET) process offers numerous opportunities for the development of multi-color FCNs. Typical approaches involve mixing of molecular dyes,13-14 blending of polymers,15-17 encapsulation of molecular dyes into polymers18-19 to enable FRET process for FCNs-based luminescent modulation. By adjusting FRET process, simultaneous chromophores emitting can be achieved to cover a wide color spectrum using various emissive energy acceptors.20 In this perspective, the multi-chromophoric system possessing different color emitters emerges as a promising FCN candidate for luminescent modulation. However, phase segregation within chemically different chromophores blends usually results in ill-defined luminescent properties due to the uncontrolled FRET process.14, 21 Covalently linked multi-chromophoric oligomers with intramolecular energy gradient present an effective strategy to avoid undesirable phase segregation usually occurred in multichromophores blends system. Moreover, the oligomers have a well-defined chemical structure, definite molecular weight and high purity over conjugated polymers,22 making them an interesting



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single-component material for multi-color FCNs. As a result, strong π-stacking interactions are enabled to realize ultrafast directional intramolecular FRET to enhance the photo-response of energy acceptor segment.23-24 This leads to the single emission spectra from energy acceptor segment that usually fails the application for dual fluorescent labelling.25 Therefore, the selection of energy gradient chromophores capable of simultaneous light-emitting is important for luminescent modulation in FCNs. Since an energy-donating chromophore is needed, diphenylaminofluorenyl (DPAF) has shown highly fluorescent property and is herein used as a light-harvesting antenna unit.24 By covalently attaching the DPAF with an emissive acceptor, the face-to-face π-stacking conformation of the fluorene moieties can facilitate the charge transport via the intramolecular stacked orbitals.20 Among emissive acceptors 4,7-dithiophen-[2,1,3]benzothiadiazole (DBT) is used as an energy acceptor in the D-A molecular system for solar cell application due to the capabilities in modulation of energy gap, optical properties and charge separation.26-29 Based on a simple molecular system with intramolecular energy gradient, the above chromophores are covalently linked into a single-component conjugated oligomer. In the nanoscale form, the oligomer based FCN can yield tunable PL emission by adjusting FRET process to control the intra- and interchain exciton diffusion and excitation energy transferring to the emissive acceptor DBT. In this study, a symmetric D-A type molecular system by one DPAF end-capped DBT, termed as DDBTD oligomer, is used to develop multicolor FCNs in aqueous dispersion. Sub 10-300 nm FCNs are obtained using the nano-precipitation method by easy controlling the DDBTD concentration, showing distinctively different PL emission colors from bright blue to rose red through the orange light emitter DBT.30-31 Although DBT containing FCNs have seen increasing reports in recent years, the size-dependent modulation of dual PL emissions in nanoscale singlecomponent oligomer have not been reported to date.


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2. EXPERIMENTAL METHOD 2.1 Materials The detailed DDBTD synthesis route (in supporting information) refers to the reported procedure by Sun et al.29 The DDBTD solid film was formed by drying up drops of DDBTD tetrahydrofuran (THF) (2.5×10-1 mg ml-1) solution on ITO glass substrate. The DDBTD NPs were prepared by the nano-precipitation approach. Typically, the DDBTD in tetrahydrofuran (THF) solutions with different concentrations (2.5×10-1, 5×10-2, 1×10-2, and 5×10-4 mg ml-1) were prepared and labeled as the VH, H, M, and L solution respectively. 0.5 ml each solution was injected to 4.5 ml deionized water (DI-water) under strong stirring for overnight at room temperature to obtain suspensions. The obtained four samples are labeled as VH-S, H-S, M-S, and L-S. The L-S, M-S sample were filtered by 0.2 µm nanopore membrane filter. The suspensions were each further diluted by 2, 10, 50 and 100 times in DI-water and are labeled as D2, D10, D50, and D100 respectively.

2.2 Instrumentation UV-vis absorption spectrum was measured with a Shimadzu 1700 spectrophotometer. PL spectra were taken by using Perkin-Elmer LS 50B luminescence spectrometer. Surface electric zetapotential and size distribution (hydrodynamic diameters) were measured by dynamic light scattering (DLS) using a Malvern Instruments Zetasizer Nano ZS machine. PL lifetime decay curves were recorded on an Edinburgh Instrument FLS920 spectrometer and the decay of the PL intensity (I) with time (t) was fitted by exponential function. Transmission electron microscopy (TEM) images were taken with a Philips CM200 FEG TEM operated at 200 kV. A drop of each



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sample was trickled on a piece of carbon-coated copper grid. The sample was air-dried under ambient conditions before being placed into the TEM specimen holder. Theoretical calculations were carried out by a Gaussian 09 program package (in supporting information).

3. Results and Discussion 3.1. Spectroscopic properties of DDBTD The DDBTD molecule is synthesized according to the reported procedure29, and the molecular structure of DDBTD is show in Figure 1a. The photographic oligomer solution and thin film under UV lamp is show in Figure 1b, and the DDBTD THF solution shows strong PL emission. The UV-vis absorption and PL emission spectra of the DDBTD THF solution and the thin film are shown in Figure 1c. For the solution UV-vis absorption spectrum, 382 nm peak is attributable to the π-π* transition of diphenylaminofluorenyl (DPAF) unit, which is similar with the polyfluorene absorption at 382 nm.32 The other peak at 526 nm, tailing off to 621 nm, can be assigned to the charge transfer complex absorption originated from the acceptor 4,7-dithiophen[2,1,3]benzothiadiazole (DBT) in conjugation with the donor DPAF.29, 33 The UV-vis absorption spectrum in solution is similar with that in thin film which shows weak redshift, suggesting that the π-stacking in DDBTD oligomer lead to minor variation in the absorption properties. The PL spectrum of DDBTD in dilute solution and thin film shows a different pattern. The solution PL emission shows dual peaks appearing at 440 nm and 661 nm, while for the thin film only single emission peak at 702 nm is shown. This phenomenon indicates that the excitation energy is almost completely transferred from DPAF unit to DBT core in solid thin-film, leading to the low-energy emission in the deep red region, while the energy transfer is incomplete in solution. 6 ACS Paragon Plus Environment

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3.2. DDBTD NPs in suspension To reveal the difference in the aggregates in the four suspensions, TEM observation and statistical size distributions by DLS were adopted and shown in Figure 2. From the TEM images, aggregations (~100 particles) from the four suspensions are observed to be spherical nanoparticles (NPs) with respective statistical sizes of 7.3±1.8 nm, 68.5±7.6 nm, 157±18.2 nm, and 236±17.4 nm. The NPs are in amorphous character typically shown in Figure S2. The hydrodynamic diameters (~1011 particles) by DLS are measured to be ca. 17.59 nm, 65.63 nm, 184.1 nm and 249.2 nm with narrow size distribution. The differences in the sizes measured by DLS from those in TEM images are due to the swelling effect of the NPs in aqueous dispersion, and the different statistical errors by the two methods of measurement.34-35 Clearly, the NPs sizes in the nano-precipitation process show the dependence on the oligomer concentrations. Using the diluted solutions, the small size (~ca.10 nm) for L-S can be achieved through minute stacking volume of the oligomers. With the increased concentrations, the NPs sizes become larger (~32 times) because of the increased DDBTD aggregates possibility. Therefore, the increased NPs sizes determined by TEM and DLS agree with the varied sizes of aggregates in aqueous suspensions.

3.3. Spectroscopic properties of the DDBTD NPs Stable NPs dispersion based on the self-assembly of the oligomer enables direct access to spectroscopic evaluation to understand the FRET process. Zeta-potential measurement (in Figure S1) shows that the surface electric potentials values are lower than -29.5 mV at pH value ca. 7.4, indicating stable aqueous dispersion of NPs with negatively charged surface. The spectroscopic properties of the suspensions are evaluated by normalized UV–vis absorption and PL emission



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spectra, shown in Figure 3. The characteristic absorption of DPAF and DBT unit can be found at 380 nm and 531 nm respectively. However, the DBT featured absorption in L-S is indiscernible, this is most probably due to the low oligomer concentration and the absorption broadening by water scattering.36 The spectral broadening of the four suspensions compared with that of the molecularly dissolved DDBTD in THF, is due to the differently solvated oligomer chains by different solvents. This suggests the aggregation behavior of the oligomer in the aqueous suspension. The UV-vis absorption spectra show no change after 90 days on shelf storing at room temperature. In the PL emission spectra, well-defined peaks at 433 with vibronic shoulder and ~ 630 nm can be found and assigned to the emission band of the disordered glassy fluorene and DBT respectively, indicating the contribution of different chromophores to the emission spectrum. With the FCN sizes increase, the emission peak intensity ratio (I433/I630 for L-S is 36.27, M-S 2.99, H-S 1.75 and VH-S 0.42) across the four suspensions shows a marked reduction combined with the progressively enhanced emission at ~ 630 nm (~29 times). The variation in the peak intensity ratio featured by the gradual emission quenching at 433 nm evidences the efficient but not complete FRET from the excited DPAF unit to the DBT emissive segment. This promoted FRET process can be associated with the strengthened inter (or intra) molecular interaction by mounting molecular stacking volumes in the aggregation process, leading to an increased molecular packing efficiency and molecular interaction. By this change, the broadened and slightly blue-shifted emission (~ 7 nm) spectrum of the DDBTD FCNs is attributed to the aggregation-induced inter (or intra) molecular interaction differences in the FCNs with a twisted and torsional conjugated backbone.37 Therefore, the L-S weak emission at ~ 630 nm is resulted from the weak inter (or intra) molecular interaction inhibited excitation energy transfer. Overall,


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by increasing FCNs sizes, the FRET process can be promoted because of the enhanced inter (or intra) molecular interaction, achieving the size-dependent modulation of dual PL emission. Clearly, the obtained four suspensions are transparent at ambient light, while they show distinctive luminescence colors from bright blue to rose red under 365 nm UV light illumination, as shown in Figure 4a and 4b. The corresponding PL spectrum is estimated by CIE (Commission internationale de l'éclairage) 1931 color space coordinates, which can be fitted by the equation Y=0.66X-0.018 (0.17 < X < 0.42), shown in Figure 4c. Consequently, a wide emission range from bright blue to rose red can be achieved by FRET within the single-component DDBTD FCNs aqueous suspensions.

3.4. Theoretical calculation To understand the dual PL emission behavior of the DDBTD, quantum-chemical calculation (density functional theory (DFT), B3LYP/6-31G) was performed for the optimized molecular geometry of the DDBTD and the orbital electron distributions of the calculated highest occupied molecular orbital (HOMO) / lowest unoccupied molecular orbital (LUMO) levels, shown in Figure 5. The alkyl chains were replaced by methyl groups to speed up the computation. The molecular conformation in the top view shows a small dihedral angle of ~1°within the DBT core, together with a dihedral angle of 25°between thiophene and the flanking DPAF antenna, while in the front view a large dihedral angle over 41° is shown within the DPAF antenna. The nonplanar twist conformation between the DPAF antenna and the DBT core may yield a limited intramolecular π-orbital overlap38, resulting in distorted molecular stackings in the aggregation state and hampered energy transfer.39 From the electron density distribution, a delocalized HOMO (-4.67 eV) is equally distributed over the entire molecular structure, which is responsible 9 ACS Paragon Plus Environment


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for the absorption at the molecular ground state. While the LUMO energy level (-2.58 eV) is almost localized on the DBT core, indicating the electron cloud transfer from the DPAF antenna to the DBT core.40 Therefore, the theoretical calculations support the dual emission/absorption behavior of the DDBTD.

3.5. Inter (or intra)-nanoparticle interaction To clarify whether the tunable PL emissions are arising from changes in the DDBTD FCNs concentrations or internal structures, control experiments by diluting the FCNs suspensions were carried out. By this work, the concentrations can be changed while the FCNs internal structures are intact.41 The four suspensions samples VH-S, H-S, M-S, and L-S were diluted by 2, 10, 50 and 100 times to obtained each diluted suspension sample. As a result, simultaneous decreased trend on dual PL emission did not change upon diluting the concentration up to 100 times, and more clarified by the normalized counterparts are shown in Figure S3, confirming the color tuning not result from the variations in FCNs concentrations. The concentration-independency on the ratio-metric dual PL emission intensity suggests that the PL emission modulation result from intra-nanoparticle interactions. Furthermore, the increased FCN sizes combined with emission peaks evolution can be correlated with the controlled FRET from the DPAF unit to the acceptor DBT as the emissive segment.

3.6. PL quantum yields The effect of DDBTD FCN size on PL quantum yield (QY) was evaluated by using PL standard Quinine sulfate aqueous solution for L-S, M-S and Rhodamine B in methanol for VH-S, H-S at room temperature. The PL QYs are calculated according to the equation and standard QYs of 10 ACS Paragon Plus Environment

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0.1M Quinine sulfate aqueous solution (0.52) and Rhodamine B methanol (0.70) summarized by Brouwer et al,42 shown in Table S1. The PL QY of L-S with small size is ~0.23, while that of VH-S decreased to ~0.11. The FCNs QYs variation shows its dependence on the sizes, suggesting the effect of the intra-particle FRET process on the emissive DBT segment present in the FCNs.

3.7. Resonance energy transfer process To further understand the energy transfer associated with the nature of the excited state between the donor and the acceptor, the PL lifetimes of the DDBTD FCNs in different sizes were measured. The PL lifetime decays for the four NPs suspensions monitored at 435 nm and 630 nm as PL emission maxima of the energy donor and acceptor respectively, are shown in Figure 6. The PL decay profiles can be well fitted by exponential function, and the weighted mean lifetimes (τm) were calculated for comparison. Size-dependent behavior can be found from the changes in lifetimes, which are summarized in Table S2. There are two components in the lifetime segments with independent PL emission, indicating two relaxation pathways in their PL decays. The PL decay monitored at 435 nm shows significant τm reduction from 3.207±0.187 ns in L-S to 0.979±0.040 ns in VH-S. While the decay at 630 nm has the τm of 4.076±0.062 ns in LS, slightly extended to 4.637±0.014 ns in VH-S. The donor lifetime decays become faster when the FCN sizes increase, indicating the occurring of efficient FRET as a nonradiative decay path for the donor DPAF.43 On the other hand, the slight extension of lifetimes monitored at 630 nm suggests that the acceptor BDT is sensitized by the neighboring excited donor DPAF.44 It is obvious that the evolution of the ratio-metric PL emission intensity is concomitant with the varied FCNs sizes and their surface areas. This size-dependent dual PL emission of this organic



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FCNs can be correlated with several reasons. One is the particle lattice softening induced by the torsional and twisted conjugated backbone, leading to the incomplete FRET process because of the weakened intramolecular interaction.10 Also, with the increased local concentration of emissive segments in larger size FCNs, the electronic coupling between the energy donor and acceptor becomes stronger that extends the energy transfer dimensionality within the FCNs. 45 Again, because energy transfer rate (

) is sensitively dependent on the donor-acceptor distance

(R) according to the numerical relationship:46

(

is the natural lifetime of the donor DPAF without acceptors24,

is the Förster radius), the

improved contact and π-stacking of the chromophores in larger FCNs promote the excitation energy transfer to emissive acceptors spatially close to excited donors.13 Under these conditions, resonance energy transfer from the excited energy donor DPAF to energy acceptor DBT becomes more efficient when the sizes of FCNs increase. Thus, the size-dependent dual PL emission modulation originates from the increased intermolecular interaction with the increase in the FCNs size. In this case, successful tuning the FCN sizes is an important aspect that influences the energy transfer process for luminescence color modulation, which has heavy impact on solution-processed organic nanomaterials for opto-electronic device engineering.47

4. Conclusion 10-300 nm stable nanoparticles aqueous dispersion was achieved by nano-precipitation method from one fluorene and benzothiadiazole based oligomer. The size of nanoparticles increased with the increasing concentrations of the oligomer in tetrahydrofuran through the nano-precipitation process. Bright blue to rose red luminescence color with 10-20% quantum yield can be achieved 12 ACS Paragon Plus Environment

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by fluorescence resonance energy transfer between wide and narrow band gap segments in one oligomer conjugated chain. For this first time, we show that the size of the nanoscale singlecomponent oligomer particles has a clear impact on excitation energy transfer, achieving dual PL emission modulation from bright blue to rose red light-emitting.

ASSOCIATED CONTENT

Supporting Information. The synthetic procedure of DDBTD oligomer. Characterization data for DDBTD NPs (zeta-potentials, TEM image, PL emission, PL QL data and PL lifetime decay data). This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author

* Email: [email protected]; [email protected] Notes

The authors declare no competing financial interests. ACKNOWLEDGMENT

The authors are deeply grateful to the National Natural Science Foundation of China (21274134) and the Hong Kong Scholar Program (XJ2012042, 2012T50630) for financial support. Mingliang Sun thanks Prof. Yong Cao at South China University of Technology for providing the DDBTD material in this work.

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Figure 1. (a) the DDBTD molecular structure. (b) DDBTD in THF under 365 nm UV light (left) and solid thin-film under ambient light (middle) and 365 nm UV light (right). (c) Normalized UV-vis absorption and PL emission spectra of DDBTD in THF solution and in solid film (excited by 330 nm).



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Figure 2. TEM images of DDBTD nanoparticles in L-S (a), M-S (c), H-S (e) and VH-S (g); and corresponding DLS size distribution profiles of L-S (b), M-S (d), H-S (f) and VH-S (h) in aqueous suspensions.


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Figure 3. Normalized UV-vis absorption (dotted line) and PL emission spectra (solid line) of the obtained four NPs aqueous suspensions.



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Figure 4. Photographic four aqueous suspensions in cuvette under ambient light (a) and 365 nm UV light (b); corresponding CIE color coordinates (c).


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Figure 5. DFT-B3LYP/6-31G calculated ground-state geometry (top view, front view) with torsional angles and frontier orbital map for the DDBTD.



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Figure 6. Lifetime decay behavior of the PL emission for L-S (in square fitted by solid line) and VH-S (in circle fitted by dash line) monitored at 445 nm (a) and 630 nm (b).


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TOC Graphic


Single-Component Oligomer Nanoparticle-Based Size-Dependent

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