Polyfluorenes (PFs) Single Chain Conformation, Î² Conformation and

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Polyfluorenes (PFs) Single Chain Conformation, # Conformation and Its Stability and Chain Aggregation by Side Chain Length Change in the Solution Dynamic Process Bin Liu, Tao Li, Hao Zhang, Tengning Ma, Jiaxuan Ren, Bo Liu, Bin Liu, Jin-Yi Lin, Mengna Yu, Linghai Xie, and Dan Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03504 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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The Journal of Physical Chemistry

Polyfluorenes (PFs) Single Chain Conformation, β Conformation and Its Stability and Chain Aggregation by Side Chain Length Change in the Solution Dynamic Process

Bin Liu ,a Tao Li ,a Hao Zhang ,a Tengning Ma ,a Jiaxuan Ren ,a Bo Liu, a Bin Liu ,b Jinyi Lin ,c Mengna Yu ,b Linghai Xie ,b Dan Lu*a

a

State Key Laboratory of Supramolecular Structure and Materials, College of

Chemistry, Jilin University, 2699 Qianjin Avenue, Changchun, 130012, China b

Centre for Molecular Systems and Organic Devices (CMSOD), Key Laboratory for

Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing, 210023, China c

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials

(IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing 211816, China

ACS Paragon Plus Environment

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

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Abstract: Effects of alkyl side chains length with different carbon atom number, called

as

poly(9,9di-hexlyfluorene)

poly(9,9-dioctylfluorene)

(PF8),

(PF6),

poly(9,9-diheptylfluorene)

poly(9,9-dinonylfluorene)

(PF9)

(PF7), and

poly(9,9-didecylfluorene) (PF10), on the Polyfluorenes (PFs) single chain conformation, β conformation and its stability and chain aggregation in the solution dynamic process were systematically investigated by Dynamic/Static Light Scattering (DLS/SLS), UV-vis absorption spectra, photoluminescence (PL) spectra and Scanning Electron Microscope (SEM). β conformation was the low-energy chain conformation and its characteristic peak was at 437nm, 427nm and 428nm in the UV-vis spectrum of PF8, PF9 and PF10, respectively. It was interestingly found that the shape parameters (Rg/Rh) (i.e. ratio of radius of gyration (Rg) and hydrodynamic radius (Rh)) of PFs single chains in toluene solution showed odd-even property with the increase of side chain length, which revealed PFs chains with even carbon atom were more rigid than those with odd carbon atom. The highest contents of β conformation were all around 42% in PF8, PF9 and PF10 toluene/ethanol mixed solutions. But，PF8 was the easiest to form β conformation, PF9 followed and PF10 was the last. It was firstly found that the β conformation formation and content were strongly connected to the chain packing density but no aggregation size. High chain packing density was more advantageous to β conformation formation, it had been well proved by static fractal dimension (df) reflecting the compactness of chain aggregation (i.e. chain packing density) and the chain self-similarity and calculated by exponential law from SLS. Besides, it was also found that the β conformation content could be 1


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stabilized at the maximum value range (42%) in the high ethanol content (80%) and independent of the side chain length even after placing for 21 days. While in lower ethanol content (30% and 40%)，the β conformation contents could also be stabilized in two different time stages. The conclusions are significant to understand deeply the solution dynamic process of film-forming based on condensed matter physics of conjugated polymer to well control its condensed matter structure to achieve photoelectric devices with high carrier mobility, stability and efficiency.

2



Introduction The conjugated polymers have been widely applied for photoelectronic devices, such as polymer lighting-emitting diodes (PLED), photovoltaics, organic electric pump laser, etc.1-3 Polyfluorenes (PFs) as a model of blue light conjugated polymer has been widely studied, but it is more important as model materials called hairy-rod molecules. The condensed matter structure of optoelectronic thin films can directly connect with devices performance.4,5 Especially, the optoelectronic behaviour of conjugated polymer films with doped PFs has been significantly improved.6,7 The optoelectronic devices are usually fabricated by solution-processing methods, for example, spin-coating film, etc.8,9 but conjugated polymer chains aggregated easily even in common organic solvents due to the strong π–π interchain stacking.10 However, it has been well proved that condensed state structure in solution could be directly inherited to film.11-13 So, it is crucial to explore the solution dynamic process, such as change of the chain aggregation size and shape, chain packing density in precursor solution in film-forming dynamic process, which is very important to control the condensed state structure of photoelectric thin film. As we know, chain condensed state structures of the conjugated polymer originate from the single chain arranging and packing.14-16 Essentially, the conformation transformation of single chain can directly influence on chain arranging and packing up to form condensed state structure by impacting conjugated polymer’s electronic structure in turn,17,18 so single chain conformation can dominate the 3


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formation and evolution of condensed state structure in the film-forming process. Further, single chain conformation can affect the interchain chromophores, which is related to the color stability and decide the manner of the chains packing up to charge carrier transport. PFs is of characteristic of polymorphism, and the different phase state critically depends on the single chain conformation.

19,20

Among all chain

conformations, β conformation has been highly concerned because of its more planar backbone conformation and extended π-conjugation length, which lead to form ordered chain structure and then enhance the carrier mobility and efficiency of the photoelectron device.19,21 Therefore, regulating and controlling the single chain conformation is essential to not only control conjugated polymer condensed state but also enhance the device performance fundamentally.22 However, relevant research on single chain conformation of conjugate polymer in solution dynamic process is scarce till now. The side chain engineering of the conjugated polymers including the side chain length, the branching point position and bulkiness23-25 can strongly impact on chain conformation transformation, molecular backbone coplanarity, the quality of molecular packing and hole mobility,26 further, interchain interactions and π-planar distance which can contribute to the excimers formation in the excited state.27 PFs β conformation formation strongly dependent on the structure of side chain. A. P. Monkman had found only the PFs with linear side chain could form β conformation,28,29 Besides, the change of side chains length will directly take effect on the solution dynamic process. PFs phase structures can be controllably modified by 4



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choice of solvent and subsequent thermal treatment as well as side chain change.30-33 In previous work, we had successfully induced the ordered β conformation in solution by

adding

poor

solvent

(ethanol)

with

different

percemtage

into

the

poly(9,9-dioctylfluorene) (PF8) good solvent(toluene). M. Knaapila found the sheets structure of poly(9,9di-hexlyfluorene) (PF6), poly(9,9-diheptylfluorene) (PF7), PF8 and poly(9,9-dinonylfluorene) (PF9) in methyl cyclohexane (MCH) solution after heating–cooling cycle, and the structural characteristics showed an odd-even dependence on the side chain. But, the concentration range they used was relatively higher (10 mg/mL - 50 mg/mL) and the solutions were gel-like, only part of poly(9,9-didecylfluorene) (PF10) chains were assembled into sheet-like structure.34 D. W. Bright observed that PF7/MCH, PF8/MCH, PF9/MCH and PF10/MCH dilute solution system contained the conformational isomer Cβ in subambient temperature with concentration 7 µg/mL.35 However, detailed information about the influence of liner side chain on the single chain conformation and the mechanism of β conformation formation is still not clearly, and so far, little attention has been devoted to the stability of β conformation. In this research, we chose PF6, PF7, PF8, PF9 and PF10 to focus on the PFs single chain conformation, β conformation and its stability and chain aggregation by side chain length change in the solution dynamic process with regulating the ethanol percentage in toluene solution. PFs single chain shape in dilute toluene solution was investigated; effect of alkyl side chain length on β conformation and its stability was explored; chain aggregation size, packing density and mechanism of β conformation 5


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formation was revealed in the solution dynamic process from single chain to condensed state formation. The stability of β conformation was probed. More details will be discussed below.

Chart 1. Chemical Structure of Studied PFs R = CxH(2x+1); x = 6, 7, 8, 9 and 10 for PF6, PF7, PF8, PF9, and PF10, respectively.

2. Experiment Section

2.1. Materials and Samples Preparation. PFs with different side chain length were supplied by Prof. Linghai Xie research group, the Nanjing University of Posts and Telecommunications. The related parameters were got by Gel Permeation Chromatography (GPC), which was performed using a LC-20AD instrument with tetrahydrofuran as an eluent and polystyrene as the standard sample, the results were shown in Table 1. We selected toluene and ethanol as good and poor solvent, respectively. All samples were dissolved in toluene by stirring at 75 °C firstly for half hour to equilibrium and then cooled down to room temperature for hours. The ethanol with different percentage were used as poor solvent to induce β conformation formation,

6



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the final solutions concentration was all 0.05 mg/mL. The solvents were produced by Beijing Chemical Company, China. They were all chromatographically pure.

Table 1. Molecular Weight and Polydispersity Index of the Samples samples

PF6

PF7

PF8

PF9

PF10

molecular weight(Mw) (g.mol-1)

30287

23904

70460

43539

48600

polydispersity index (PDI)

2.15

1.85

1.17

1.48

1.47

2.2. UV-vis Measurement. The measurement of UV-vis absorption spectra was performed with Shimadzu UV-3000 spectrophotometer to prove the existence of β conformation. The measurements were conducted after placing the solutions 10 minutes for the equilibrium, and quartz cells of 1cm thickness were used for the measurement. 2.3. Photoluminescence (PL) Spectra. PL spectra were taken with a fluorescence spectro-photometer (Shimadzu RF-5301PC) equipped with a xenon lamp as excitation source. The measurements were recorded in a range from 400nm to 600nm with excitation wavelength at 384 nm at room temperature. 2.4. The Calculation Method of β Conformation Content. The calculation method of the PFs β conformation content had been reported in our previous researches,36-39 and it has been presented in Supporting Information.

7


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2.5. Light Scattering (LS) Measurements. An ALV/CGS-3 (made in Germany) light scattering spectrometer equipped with an ALV/LSE-7004 multiple-τ digital correlator was used in the LS measurement. The JDS-Uniphase solid-state He-Ne laser with the output power of ca. 22 mW at the operating wavelength of 632.8 nm was used as the light source. The LS cell was held in a thermostat filled with purified and dust-free toluene. The aggregation structure with polymer nonlinear property can be assessed by means

of

Static

Light

Scattering

(SLS).

In

the

measurement,

the

average scattered light intensity I(q) could be described as a function of the scattering wave vector q=4π⁄λsin(θ⁄2), where θ is the scattering angle and λ is the wavelength, we set the accessible scattering angles range from 50° to 150°，so the range of q was in 12.6 µm-1 < q < 28.7 µm-1. The collected light intensity I(q) depends mainly on the particle form factor P(q) and the aggregate structure factor S(q), they could connect followed by the formula：

I (q) = K P(q) S(q)

(1)

Here, P(q) contains information on the optical properties of the individual particle, whereas S(q) quantifies mass correlations within the aggregation, K depends on the optical properties of the light scattering device. In stable systems without any spatial correlation between the monomer positions, S(q) ≈ 1. In this case, P(q) can be determined easily from direct measurements of the scattered light intensity, since I(q)

∝P(q). For an aggregated system, however, the structure factor S(q) is directly related to the fractal dimension of the clusters formation, df quantifies how compact the 8



fractal aggregates are, meanwhile, q was in the range of Ragg > q-1 > Rg, where Ragg and Rg are the radius of gyration for the aggregations and single chain, respectively.

I (q) ∝ S (q) ∝ q-df

(2)

The normalized scattering intensity autocorrelation function g2(t) was measured in Dynamic Light Scattering (DLS), where t is the decay time. g2(t) is related to the normalized first-order electric field time correlation function g1(t), and they could establish the Siegert relation as g2(t) = 1 + β| g1(t)|2. g1(t) is the Laplace transformation of the decay time distribution function A(t) of the decay rate t, and the

A(t) can be required using the CONTIN analysis. 40,41 2.6. Scanning Electron Microscope (SEM). The SEM measurements were carried out on JEOL JSM 6700F at an accelerating voltage of 5 kV to conduct the morphology research.

3. Results and Discussion

3.1. Odd-Even Effect of Single Chain Conformation in Dilute Solution. Here, it

should be emphasized the shape parameter Rg/Rh can reflect the molecular chain shape in dilute solution, where Rg represents the z-average mean square radius of gyration, Rh is the hydrodynamic radius, and Rg/Rh is sensitive to the macromolecular shape which relies on the chain conformation but no the molar mass.42,43. Especially, the ratio of Rg/Rh is always used to describe the topological structure and extension state of polymer single chain in solution.37 9


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According to the relevant reports, the Rg/Rh value is 1.78 for the flexible chain in good solvent. So, it indicates the degree of chain rigidity, bigger the value is, more extended chain conformation showed in dilute solution.43,44 We used the SLS/DLS to explore the influence of PFs with different side chain length on the value of Rg/Rh. From Table 2, it was found that all samples indeed dissolved into single chains owing to the Rh value less than 10nm,45 and it was inferred that all samples with single chains adopted the extended chain conformation since the Rg/Rh values were all higher than 1.78. Interestingly, it was noticed that the Rg/Rh values presented odd-even characteristic with the carbon atom number of the side chains change shown in Figure 1. Obviously, when the Rg/Rh values were all in 2.7 for PF7 and PF9, respectively, while the values were all in 2.9 for PF6, PF8 and PF10, respectively, the results indicated that the PFs chains with even carbon atom in side chains were more rigid than those with odd carbon atom. Therefore, chains conformation of PF6, PF8 and PF10 were more extended than those of PF7 and PF9.

Table 2. PFs Properties in Toluene Dilute Solution. samples

Rg/nm

Rh /nm

Rg/Rh

PF6

21.8

7.5

2.9

PF7

23.4

8.8

2.7

PF8

25.3

8.7

2.9

PF9

23.7

8.8

2.7

PF10

26.9

9.2

2.9

10



3.1 3.0 Rg/Rh

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

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2.9

2.9

2.9

2.9

2.8 2.7

2.7

2.7

2.6 2.5

6

7 8 9 the carbon number

10

Figure 1. The trend of Rg/Rh value with the side chain length.

Odd-even effect is early found in fatty acid for researching the relationship between the structure and basic physicochemical properties.46 Recently, the conclusions have been used to instruct synthesis of the new photo-electronic materials.47-49 In general, the alkyl chains present the tilting zigzag carbon chains, and the carbon number in the side chain can influence the titling angle. Specifically, the final carbons are trans to adjacent carbons in the even-chain series, while those are cis in side chains with odd carbons, and all end-group planes with even carbon atom in side chains are parallel, while for chains with odd carbon atom, their adjacent end-group planes were different so that the extension direction of the side chains, the side chains symmetry, the packing formation of chains were also different.50-52 In this research, the odd-even

characteristic was showed in single chain conformation, it revealed the change of chain rigidity degree with the carbon number of the side chains, it is significative to enrich the understanding of conjugated polymer chain odd-even properties. 11


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3.2. Effect of Side Chain Length on β Conformation and Content in the Dynamic Process of Chain Condensed State Formation. PF8 has been attracting researchers’ attention is due to its special β conformation with characteristic peak at 437nm in UV-vis absorption spectrum. With the β conformation, the PF8 main chains adopted the more planar conformation, which enhance chain aggregation orderness. The conjugated polymer with ordered chain structure will be very favourable to the charge carrier mobility as well as the quantity of both triplet state exciton and polaron till PL efficiency. Therefore, controlling the orderness of chain conformation is crucial to enhance the photoelectronic devices efficiency fundamentally. To further explore effect of side chain length on both PFs β conformation and its content in the solution dynamic process of chain condensed state formation, we chose mixed solvent of toluene and ethanol with different ratio to investigate the variation of β conformation contents for all samples. The normalized UV-vis absorption spectra with the samples (PF6, PF7, PF8, PF9 and PF10) were shown in Figure 2a, b, c, d, e, respectively, and the dependence of β conformation contents on the side chain length were shown in Figure 2f. The results were summarized in Table 3, the other detailed information was shown in Table 4. As well known, the characteristic peak of PF8 β conformation is at 437nm,53 and it was found from the Figure 2c the UV-vis absorption spectra of PF8 showed a sharp peak at 437nm, meanwhile, PF9 at 427nm (in Figure 2d) and PF10 at 428nm (in 12



Figure 2e) also showed shoulder peak which were all attributed to the β conformation characteristic peak. However, it was noteworthy that the initial ethanol contents of PF8, PF9, PF10 were obvious different when the β conformation firstly appeared, i.e., the ethanol content was in 20% to PF8 solution (in Table 4), while the ethanol contents were in 40% and 50% to PF9 and PF10 solutions, respectively. So, we could draw the conclusion that the β conformation formation was more easily to PF8 solution than that to PF9 and PF10, in other words, PF8 was of distinct advantage to form the β conformation. Besides, it was found that β conformation content could reach the maximum around 42% when the ethanol content exceeded 60% to PF8, 70% to PF9 and PF10 solutions (in Table 3), respectively. Meanwhile, it was noticed the main peak was red shifted as shown in Figure 2c, 2d, 2e, respectively when β conformation content reached the maximum, and the maximum was stable and independent on the ethanol content as shown in Figure 2f.

13


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1.0 0.8

Normolized absorption (a.u.)

1.2

0.6 0.4 0.2 0.0

c) 1.4 Normolized absorption (a.u.)

b) 1.4

ethanol 0% ethanol 10% ethanol 30% ethanol 50% ethanol 60% ethanol 70% ethanol 80%

PF6

350

400 450 500 550 Wavelength (nm)

600

1.2 1.0 0.8 0.6 437nm

0.4 0.2 0.0

350

400 450 500 550 Wavelength (nm)

600

e) 1.4 427nm

1.0 0.8

ethanol 0% ethanol 10% ethanol 30% ethanol 40% ethanol 50% ethanol 60% ethanol 70% ethanol 80%

0.6 0.4 0.2 0.0

1.0

437nm

0.8


0.6 0.4 0.2 0.0

350

400 450 500 550 Wavelength (nm)

1.2

PF9

1.0

427nm

0.8

600


0.6 0.4 0.2 0.0

350

400 450 500 550 Wavelength (nm)

600

f) 100 PF10

the content of beta phase (%)

1.2

PF7

1.2

d) 1.4

ethanol 0% ethanol 10% ethanol 20% ethanol 30% ethanol 40% ethanol 50% ethanol 60% ethanol 70% ethanol 80%

PF8



a) 1.4


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


350

400 450 500 550 Wavelength (nm)

600

PF8 PF9 PF10

80 60 0

10

20

30

42%

40 20 0 -20

0

20 40 60 80 the content of ethanol (%)

Figure 2. (a), (b), (c), (d), (e): Normalized UV-vis absorption spectra of PF6,

PF7, PF8, PF9 and PF10 solutions with different ethanol percentage from 0% to 80%. (f): The dependence of β conformation content on the side chain length with different ethanol percentage from 0% to 80%. The concentration of all samples was all in 0.05 mg/mL.

14



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Table 3. β Conformation Contents in PFs Solutions with Different Ethanol Percentage poor solvent

ethanol

percentage 10% 20% 30% 40% 50% 60% 70% 80% β conformation contents 0 0.5 1.8 27 35 41 42 41 (in PF8 solution) β conformation contents 0 0 0 20 35 39 42 42 (in PF9 solution) β conformation contents 0 0 0 0 35 40 42 43 (in PF10 solution) Note：The concentration of all samples was all in 0.05 mg/mL.

Table 4. UV-vis Spectral Characteristics of PFs

feature peak of the β conformation no

ethanol content a

ethanol content b

0%

0%

no

0%

0%

PF8

437nm

20%

60%

PF9

427nm

40%

70%

samples PF6 PF7

PF10 428nm 50% 70% Note: a: the ethanol content when β conformation appeared firstly; b: the ethanol content when β conformation content increased up to the maximum (42%). The concentration of all samples was all in 0.05 mg/mL.

However, it was also seen that there were only small shoulders around 437nm in PF7 solutions as shown in Figure 2b. To further explore the characteristics of the shoulder peaks, we carried out PL spectrum measure, which was more sensitive than UV-vis absorption spectrum. From Figure 3, it was found that the PF7 characteristic peaks were at 434nm, 461nm and 493nm. We knew that the PL spectrum characteristic peaks of α conformation were at 422 nm (strong, 0–0 band), 447 nm (moderate, 0–1 band), and 470 nm (weak, 0–2 band), but PL spectrum characteristic peaks of β conformation were at 439 nm (0–0 band), 467 nm (0–1band), and 496 nm

15


Page 17 of 43

(0–2 band).54,55 It had been evidenced from the PL spectrum that there was an excitation peak at 438nm and followed absorption peak existence at 437 nm. So, it was considered the characteristic peaks showed in PF7, as aforesaid at 434nm, 461nm and 493nm, were neither α conformation nor β conformation characteristic peaks but transition state conformations. However, there were no absorption peaks around 437nm in PF6, and PF6 main peaks was just broadened and had a little red-shift, that is, the PF6 chain conformation couldn’t transform into β conformation, it was only an increase of chain conjugated degree. 600

PF7

434nm

ethanol 50% ethanol 60% ethanol 70% ethanol 80%

500 PL.Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400 461nm

300 200

493nm

100 0 400 420 440 460 480 500 520 540 Wavelength (nm)

Figure 3. The PL spectra of PF7 solutions with different ethanol percentage from 50% to 80%. The concentration of all samples was all in 0.0005 mg/mL.

As mentioned above, it was considered β conformation formation and contents were strongly affected by side chain length in the dynamic process of chain condensed state formation with ethanol percentage change. The β conformation formation in PF8 solution was more easily than that in PF9 and PF10 solutions. But,

16



there were transition state conformations between α and β conformation in the PF7 solutions, and PF6 showed only α conformation and chain conjugated degree increase.

3.3. Chain Aggregations Size, Packing Density and β Conformation in the Solution Dynamic Process. In the above part, we had explored the effect of side chain length on both β conformation formation and content, but we still didn’t know the relationship between chain aggregation behaviours (such as chain aggregation size, packing density) and β conformation by the side chain length change in the solution dynamic process of condensed state formation. However, it is significative to deeply understand the formation mechanism of β conformation and the condensed state structure in the solution dynamic process. It is considered that chain condensed state formation is a process from isolated single chains to chain aggregations containing many chain entanglement and interpenetration in solution. The single chain conformation is complex since a polymer chain consists of lots of structure units. Although polymer chains are quite long, but they are of self-similarity, fractal characteristics, and the scaling property that other condensed matter does not have. Therefore, we can use the exponential law by SLS to clearly describe the nonlinear characteristic and laws of the chain aggregation in the solution dynamic process from single to the chain condensed state formation.56 Here, we used DLS/SLS to characterize the chain aggregation behaviours including chain size and packing density, which can be described by the static fractal dimension (df) using SLS. The scaling property of polymer chain aggregation can reflect the 17


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compactness of chain aggregation (i.e. chain packing density) and the chain self-similarity,39 the df value is bigger, the chain aggregations are more compact. Figure 4 showed the PFs aggregation behaviours in solutions with 30% ethanol. It was found that Rh distributions in the PF8, PF9 and PF10 solution were almost uniform，all distributed with four different size levels, while PF6 distributed in three different size levels and PF7 distributed in two different size levels, respectively. The biggest Rh valves gradually increased with the side chain length increase. It indicated that chain aggregation size was gradually increased with the increase of side chain length. It was noticed from Figure 4 that there were the smallest Rh about 5nm in PF9 and PF10 solutions, respectively, which were similar to PF8 solution in which the smallest Rh was 5nm and it may be attributed to the single chains collapse of PF8 since the worse solubleness weakened the interaction between PF8 chains and solvent molecules, meanwhile, strengthened the interaction between interchain and intrachain interaction of PF8.57 The bigger aggregation with high Rh value was considered to be mainly caused by the interchain interaction.22 According to the results given in the Table 3, there were no β conformation formation in PF9 and PF10 solutions with 30% ethanol, nevertheless, it was found the sizes of chain aggregations of the PF9 and PF10 were obvious bigger than those of PF8. So, it was considered that chain aggregation was not a necessary condition to β conformation formation.

18



b) 1.2

PF6

1.2

PF6 PF7 PF8 PF9 PF10

ethanol 30%

1.0 0.8

0.6 0.3 0.0 1.2 PF7

0.9

f(Rh)

a)

f(Rh)

0.9

0.6

0.6 0.3 0.0 1.2 PF8

f(Rh)

0.9

0.4 0.2

0.6 0.3 0.0 1.2 PF9

0.9

0.0 -2

10

-1

10

0

10

1

2

3

10 10 10 Log Time(ms)

4

10

5

10

f(Rh)

Normalized Correlation Intensity Function g2(t)-1

0.6 0.3 0.0 1.2 PF10

0.9

f(Rh)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 43

0.6 0.3 0.0 -1

10

0

10

1

10

2

10

3

10 Rh(nm)

4

10

5

10

Figure 4. (a) The normalized intensity correlation function of PFs solutions. (b) The corresponding Rh distribution of PFs, the concentration of all samples was all in 0.05 mg/mL, the ethanol proportion was in 30%.

To deeply explore the fact, the percentage of poor solvent (ethanol) was increased to 40% (shown in Figure 5). It was found there were obvious and visible particles in PF6 solution because of worse solubleness, which might damage the LS instrument, so we could only characterize the other samples, such as PF7, PF8, PF9 and PF10, respectively. From Figure 5, it was seen that the Rh distributions in all samples from PF7 to PF10 were almost uniform, they were all around 100nm, 1000nm and up to 105 to 106 nm with the increase of side chain length. However, the aggregation size of PF10 was the biggest up to 106 nm but still no β conformation appeared (see Table 3)， the phenomenon was the same to that of solutions with 30% ethanol. Therefore, it was 19


Page 21 of 43

again proved that there was no necessary relationship between the aggregation size and the β conformation formation.

f(Rh)

b) 1.2 1.0 a) PF7 PF8 PF9 PF10

1.2 ethanol 40%

f(Rh)

1.0 0.8 0.6

f(Rh)

0.4 0.2 0.0 -1

10

0

10

1

10

2

10

3

10

Log Time(ms)

4

10

5

10

f(Rh)

2 -1 Normalized Correlation Intensity Function g (t)

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


PF7

0.8 0.6 0.4 0.2 0.0 1.2 1.0 0.8 0.6 0.4 0.2 0.0 1.2 1.0 0.8 0.6 0.4 0.2 0.0 1.2 1.0 0.8 0.6 0.4 0.2 0.0

PF8

PF9

PF10

0

10

1

10

2

10

3

4

10 10 Rh(nm)

5

10

6

10

Figure 5. (a) The normalized intensity correlation function of PFs solutions. (b) The corresponding Rh distribution of PFs, the concentration of all samples was all 0.05 mg/mL, the ethanol proportion was in 40%.

From Figure 6, it was amazingly found the df, which revealed chain aggregation compactness and chain self-similarity, was gradually reduced with the increase of side chain length, the values changed from 2.70, 1.24 to 0.77, respectively for PF8, PF9 and PF10 in solutions with 30% ethanol (in Figure 6a), and the df changed from 2.79, 2.50 to 1.99 in solutions with 30% ethanol，respectively (in Figure 6b). To illustrate clearly, the df results were summarized in Table 5. Above mentioned results indicated chain packing density in PF8 solutions was the most compact whether the ethanol 20



percentage was 30% or 40%. But, in PF8 solutions, although aggregation size were the smallest, but its β conformation content was higher. However, in PF10 solution, it was obvious found that the aggregation size increased up to 106, but no β conformation was found. So, it was inferred that the higher chain packing density in aggregation was conducive to β conformation formation but no larger size aggregation. This is a new understanding of β conformation formation.

a) 8.5

b) 10.5

8.0

10.0 LogI(q)

df(PF8)=2.70

LogI(q)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 43

7.5 df(PF9)=1.24

7.0

df(PF9) =2.50

9.0 8.5

6.5 df(PF10)=0.77

6.0

df(PF8)=2.79

9.5

1.1

1.2

1.3 1.4 Logq(µm-1)

df(PF10)=1.99

8.0 1.5

1.1

1.2

1.3 1.4 Logq(µm-1)

1.5

Figure 6. Plot of log I(q) versus log q for the aggregation. (a) The percentage of ethanol was 30% in ethanol/toluene solution, (b) The percentage of ethanol was 40% in ethanol/toluene solution. The concentration of all samples was in 0.05 mg/mL.

Table 5. The Static Fractal Dimension (df) of PFs Solutions

df

static fractal dimension percentage (ethanol)

30%

40%

PF8

2.70

2.79

PF9

1.24

2.50

PF10 0.77 1.99 Note：The concentration of all samples was all in 0.05 mg/mL. 21


Page 23 of 43 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


3.4. The Mechanism of β Conformation Formation by Side Chain Length. It has been well known that the mechanism of β conformation formation to only PF8 solution

system

was

induced

by

both intramolecular and

intermolecular

interactions.21,58 However, in this research, it was found that the essence of β conformation formation was directly related to the balance between the chain flexibility and side chain length, i.e., the more flexible and with shorter side chain was more favorable to form β conformation. Only in PF8, PF9 and PF10 solution, the molecular chain could transform into β conformation, thus the interchain interaction also played an important role during of β conformation formation. PFs chains can carry out self-organization by chain arranging, packing and aggregating till up forming condensed state structure under action of external field (solvent) in the solution dynamic process from single chain to condensed state. Poor solvent (e.g. ethanol) was thought to be an external field, which could provide energy including the stronger repulsive force between solvent molecules and PFs chains (i.e. intermolecular force) as well as the attraction between the PFs intrachain units (i.e. intramolecular force). With the increase of poor solvent (ethanol), the chain packing density became higher, aforesaid interactions would be stronger so that the energy of side chain interaction could overcome the steric hindrance of PFs main chain planarizartion to form β conformation. Hence, there is direct relationship between the aggregation packing density and conformation transformation, and the more compact aggregation was the more advantageous to the conformation transformation.

22



3.5. Effect of Side Chain Length on Stability of β Conformation and Aggregation Morphology. β conformation is a kind of local ordered chain structure with more planar backbone conformation and extended π-conjugation length, which can enhance the carrier mobility of PFs materials,11,59 color stability and efficiency of the photoelectron device. Although much research has been done on the PFs β conformation, however, no studies have investigated on β conformation stability up to now. Therefore, the research about the effect of side chain length on β conformation stability is very necessary to actual application for photoelectric devices. PF8, PF9 and PF10 could form β conformation from Figure 2c, 2d and 2e in dilute solution, while PF6 and PF7 only showed conjugated length increase from Figure 2a and Figure 2b. To quantificationally explore the stability of β conformation, we chose all solution samples with 80% ethanol (shown in Figure 7). Subsequently, put all the solution samples for different time. As for PF6, short side chain reduced the interaction between PF6 chains and solvent molecules, but increased the interchain interaction, phase separation occurred in solutions with high ethanol percentage, so PF6 didn’t suit for the experiment. As we know, when the concentration is under critical contact concentration (c*), polymer chains were in the rigid single chain dispersing in the solution; when the concentration increased, polymer chains began to contact each other, tangle and form interpenetrating network structure composed of fiber and banded structure due to the interchain interaction. Finally, sheet-like structure appeared, that is, the aggregation size increased and aggregation structure changed with concentration.60,61 It was found 23


Page 24 of 43

Page 25 of 43 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


from the Figure 8 that the aggregations morphology changed from band to sheet-like and the size increased gradually in PF8 (Figure 8a) and PF9 (Figure 8b) solutions with 80% ethanol after placing 5, 10 and 15 days, and the aggregation morphologies of PF7 (Figure 8c) and PF10 (Figure 8d) were also similar with placing time increase. It indicated that the PFs chains had underwent the self-organization during the solution dynamic process derived by the interchain interaction force. However, it was surprised to find that from the normalized UV-vis absorption spectrums shown in Figure 7a, the peaks intensity at 437nm for PF8 solutions and at 427nm for PF9 solutions (Figure 7c) and at 428nm for PF10 solutions (Figure 7e) were all changeless with placing time, and it was even interestingly seen from Figure 7b, 7d, 7f, that the contents of β conformation didn’t also change with placing time in all PFs solutions. Results from figure 7 indicated that once the β conformation content reached the maximum (42%), it was very stable and independent on the side chain length and time. These results proved again that the formation of β conformation wasn’t related with the aggregation size. From the Figure 7g, it was seen that the intensity of shoulder peaks around 437nm didn’t change in PF7 solutions, but the main peaks was widened, which meant the chain conjugated degree increased. In the above part of this research, we had proved that the formation of β conformation was associated with the chain packing density but no aggregation size. So, it was speculated that the reason of the β conformation stability was the aggregation packing density no change all the time. Specifically, in the beginning of the dynamic process, β conformation content reached to the maximum, 24



simultaneously, the chain packing density also reached the maximum, but the aggregation size was relative small at this time; Gradually, the most compact but small size aggregations associated with each other into large size aggregations whose packing density didn’t change and kept the most compact in the solution, so we considered that the reason for the stability was due to only aggregation packing density but no aggregation size with time.

1.0 0.8


1.2

b)

1day 3days 10days 16days 21days

PF8 ethanol 80%

437nm

0.6 0.4 0.2 0.0 350

400 450 500 550 Wavelength(nm)

c) 1.4 1.2

600

1.0 427nm

0.8 0.6 0.4 0.2 0.0 350


60 42%

40 20 0 0

5

10 15 time (day)

20

25

d)


PF9 ethanol 80%

80 PF8 ethanol 80%

600



a) 1.4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 43

80 PF9 ethanol 80% 60 42%

40 20 0 0

25


5

10 15 time (day)

20

25

e)1.4 Normolized absorption (a.u.)

1.2 1.0 0.8

f)


PF10 ethanol 80%

428nm

0.6 0.4 0.2 0.0 350


600

80 PF10 ethanol 80% 60 42%

40 20 0 0

5

10 15 time (day)

20

25

g)1.4 Normolized absorption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 27 of 43

1.2

PF7 ethanol 80%

1.0 0.8


0.6 0.4 0.2 0.0 350


600

Figure 7. (a), (c), (e), (g): Normalized UV-vis absorption spectrums of PF7, PF8, PF9 and PF10 solution with 80% content ethanol. (b), (d), (f): The contents of β conformation as a function of time. All samples concentration was all in 0.05 mg/mL.

a) 5days

200nm

10days

15days

200nm

200nm

26



Page 28 of 43

b) 5days

10days

200nm

200nm

15days

200nm

c) 10days

5days

200nm

200nm

15days

25days

1 μm

1 μm

d)

Figure 8. (a) SEM images of PF8 solution and (b) PF9 solution after placing 5, 10 and 15 days. (c) SEM images of PF7 solution after placing 5, 10 days and (d) PF10 solution after placing 15, 25 days. All samples concentrations were all 0.05 mg/mL with 80% ethanol.

As we know, thin film used for photoelectric devices need a smooth and flat surface, 27


Page 29 of 43 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


hence, it is necessary that PFs is well dissolved in the precursor solution and can’t form too large aggregation, which means that the percentage of poor solvent in precursor solution can’t be too high otherwise large aggregation will appear so that the thin film can’t realize a smooth and flat surface. Therefore, the balance between percentage of poor solvent and high β conformation is very important to practical application. Based on this consideration, we further investigated the relationship between stability of PFs β conformation and its content in the mixed solutions with lower percentage ethanol. It was interestingly found from UV-vis absorption spectrum shown in Figure 9 where the characteristic peaks intensity of β conformation (Figure 9a, 9b, 9c) or the β conformation contents (Table 6) all stabilized in two stages to PF8, PF9 and PF10 solution with 30%, 40% and 40% ethanol, respectively. From Table 6, it could be clearly seen that the β conformation contents firstly stabilized at 1.1%, 17.0% and 18.1%, respectively, within placing 5 days for the PF8, PF9, PF10, and the β conformation contents still was stable at 3.2%, 27.2% and 30.0%, respectively, after placing 10 days. The reason for the stability was explored by PL spectra shown in Figure 9 d and Figure 9e, it was found that the peak intensity at 438nm (0-1 band) increased until reached the highest value after placing 14 days, and then started to decrease, but the trend of the peak intensity at 414 nm (0-0 band)35 was opposite. However, it's worth noting that the peak intensity at 438nm in PF10 didn’t change all the time (Figure 9f). The reason was further explored as follows. It’s well known that β conformation could enhance the carrier mobility and efficiency of the photoelectron device due to the more order chain structure. 19,21 In 28



previous work, it has been proved that the ratio of peak intensity of the 0-0 to 0-1 peak in PL spectrum could be consider as an effective probe for disorder, which attributes to the 0-0 emission peak in molecular aggregates, and its enhancement indicates the disorder increases.62 By this ratio, we further research the dependence of the ratio of PFs β conformation peak intensity on the time. From Table 7, it was found that the ratios of the 0-0 to 0-1 peak decreased within 14 days to PF8 and PF9 and then almost didn’t change, meanwhile, the ratios to PF10 almost didn’t change all the time. It indicated that aggregation orderness was first increased with placing time, then was in stable. The mechanism was speculated to be that the small size chain aggregation was combined with each other and formed larger aggregation size, in which the aggregation packing density increased quickly, then only the chain segments within aggregation could slightly rearranged themselves to ordered chain structure with placing time, so β conformation contents wasn’t been changed and was stabilized after placing 14 days.

a) 1.4 1.2

d) 1.4

1day 5days 10days 14days 20days 26days

PF8 ethanol 30%

1.0 0.8

Normalized PL.Intensity (a.u.)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 43

0.3

0.6 0.2

0.4 437nm

0.1

0.2 0.0

0.0 300 350 400 450 500 550 600 Wavelength(nm)

PF8 ethanol 30%

1.2 414nm(0-0)

1.0

438nm(0-1)

0.8

1days 5days 10days 14days 20days 26days

0.6 0.4 0.2 0.0 400

450 500 550 Wavelength (nm)

29


600

1.2

f) 1.4


PF10 ethanol 40%

1.0 0.8

1.2


414nm(0-0)

0.8

428nm

0.6

PF10 ethanol 40%

1.0

0.6

0.4

438nm(0-1)

0.4

0.2

0.2

0.0

0.0

300 350 400 450 500 550 600 Wavelength(nm)

b) 1.4 1.2 1.0 0.8 0.6

400

e) 1.4


PF9 ethanol 40%



c) 1.4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 31 of 43

427nm

0.4 0.2 0.0 300 350 400 450 500 550 600 Wavelength(nm)

450 500 550 Wavelength(nm)

PF9 ethanol 40%


1.2 414nm(0-0)

1.0 0.8

600

438nm(0-1)

0.6 0.4 0.2 0.0 400

450 500 550 Wavelength(nm)

600

Figure 9. (a), (b), (c): Normalized UV-vis absorption spectrums of PF8, PF9 and PF10 solution with 30%, 40% and 40% ethanol; solution concentrations were all 0.05 mg/mL. (d), (e), (f): The PL emission spectra of PF8, PF9 and PF10 solutions with 30%, 40% and 40% content ethanol; solution concentrations were all in 0.0005 mg/mL.

Table 6. The Dependence of PFs β Conformation Contents on the Time Samples PF8 solution with 30% ethanol PF9 solution with 40% ethanol PF10 solution with 40% ethanol

β-conformation contents within 5 days

β-conformation contents after 10 days

1.1%

3.2%

17.0%

27.2%

18.1%

30.0%

30



Page 32 of 43

Note：The concentration of all samples was all in 0.0005 mg/mL.

Table 7. Dependence of the Ratio of PFs β Conformation Peak Intensity on the Time time

1days

5days

10days

14days

20days

26days

࡭૙ି૙ ൗ࡭ ૙ି૚

Samples

PF8 solution with 0.98 0.90 0.87 0.85 1.56 1.58 30% ethanol PF9 solution with 1.81 1.81 1.55 0.85 1.78 1.81 40% ethanol PF10 solution with 1.84 1.82 1.79 1.81 1.82 1.80 40% ethanol Note：The concentration of all samples was all in 0.0005 mg/mL.

4. Conclusions

PFs single chain conformation, β conformation and its stability and aggregation by side chain length change in the solution dynamic process were revealed. PFs single chains showed odd-even property with the increase of side chain length, chains with even number of carbons were more rigid than those with odd number. β conformation could exist in PF8, PF9 and PF10 solutions, and its highest contents were all stabilized at around 42%. However, PF8 was the easiest to form β conformation, and it was first found the β conformation formation and content was strongly connected to the chain packing density but no aggregation size. High chain packing density was more advantageous to β conformation formation. This is a new understanding to the mechanism of β conformation formation. More importantly, the β conformation contents were very stable even after placing for 21 days once it reached the maximum 31


Page 33 of 43 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


around 42% in 80% content ethanol regardless of the aggregation size and the placing time. While ethanol content lowered to 30% and 40%, respectively, the β conformation contents could also stabilize in two different stages. The conclusions are significant to understand deeply the solution dynamic process of the photoelectric thin film based on condensed matter physics of conjugated polymer to well control its condensed matter structure to achieve photoelectric devices with high carrier mobility, stability and efficiency.

Supporting Information The detailed calculation method for the β conformation content in the PF8 solution and films.

Author Information Corresponding Author *E-mail [email protected] Phone: +86-136-2079-2963

Notes The authors declare no competing financial interest.

Acknowledgements

This work is supported by grants from the National Natural Science Foundation of China (91333103) and (21574053). 32



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Table of Contents Imagine Polyfluorenes (PFs) Single Chain Conformation, β Conformation and Its Stability and Aggregation by Side Chain Length Change in the Solution Dynamic Process Bin Liu ,a Tao Li,a Hao Zhang,a Tengning Ma,a Jiaxuan Ren,a Bin Liu,b Jinyi Lin,c Linghai Xie,b Dan Lu*a

8 .5

3 .1 O d d -e v e n

e ffe c t o f s id e

d f: th e

c h a in

3 .0

p a c k in g

d e n s ity

8 .0 2 .9

2 .9

2 .9

2 .8

LogI(q)

2 .9

d f

(( P F 8 )) )) ((

= 2 .7 0

7 .5 d f

( P F 9 )) ))

= 1 .2 4

7 .0

2 .7

2 .7

2 .7

6 .5

2 .6 2 .5

6 .0

6

7 th e

8 c a r b o n


Rg//Rh

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


Main chain rigidity Side chain steric effect

9 n u m b e r

1 0

d f

1 .1

1 .2

(( P F 1 0 )) )) ((

= 0 .7 7

1 .3 L o g q (( µµ m

- 1 ))

1 .4

1 0 0 th e

s t a b ilit y

P F 8 P F 9 P F 1 0

o f ββ c o n f o r m a t i o n

8 0 6 0

4 2 %

4 0 2 0 0 0

5

1 0 tim e

1 5

2 0

2 5

(( d a y ))

β conformation formation

Chain packing density

42


1 .5

Polyfluorenes (PFs) Single Chain Conformation, Î² Conformation and

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