Hereditary Character of Alkyl-Chain Length Effect on Î²-Phase

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Hereditary Character of Alkyl-Chain Length Effect on #-Phase Conformation from Polydialkylfluorenes to Bulky Polydiarylfluorenes Bin Liu, Jin-Yi Lin, Meng-Na Yu, Bin Li, Linghai Xie, Changjin Ou, Feng Liu, Tao Li, Dan Lu, and Wei Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06330 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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The Journal of Physical Chemistry

Hereditary Character of Alkyl-Chain Length Effect on β-Phase Conformation from Polydialkylfluorenes to Bulky Polydiarylfluorenes Bin Liu,a Jinyi Lin,b Mengna Yu,a Bin Li,a Linghai Xie,*,a Changjin Ou,b Feng Liu,*,c Tao Li,d Dan Lu,d Wei Huang*,a,b a

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. b

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

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

Department of Physics and Astronomy, Shanghai Jiaotong University, Shanghai 200240,

China. d

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

University, 2699 Qianjin Avenue, Changchun, China.

ACS Paragon Plus Environment

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ABSTRACT. Although alkyl chain engineering provides an effective tool for regulation of polymorphic behaviors of conjugated polymers, there is no report on extracting universal rule from complicated structure-property relationships among various molecule systems. Here, a series of bulky polydiarylfluorenes with incremental length of n-alkyl chains (PnDPF, n = 6, 7, 8, 9) were investigated, examining conformation, gelation and crystallinity behavior. By analyzing the UV-vis absorbance and photoluminescence (PL) spectra of PnDPF from single chains to the aggregate state, β-phase was found in P7DPF and P8DPF. P8DPF adopted the largest torsional angle inferring from the degree of red-shift in PL and Raman spectra. By thermal annealing at 220 oC, crystallization process was observed in these four polymers, but β-phase only appeared in P7DPF and P8DPF. Finally, both thermodynamic and kinetic paths were used to illustrate the mechanism of β-phase formation for PnDPF. Our results suggested that the alkyl-chain length effect on PnDPF exhibited the characters inheriting from the corresponding polydialkylfluorenes (PFn, n = 6, 7, 8, 9). By comparative study between PnDPF and PFn, an explicit relationship between the side-chain length and polymorphic behaviors is established for the first time in bulky polymer semiconductors.

INTRODUCTION: The dual role of alkyl side chain engineering has made it increasingly popular for conjugated polymers (CPs) in recent years. In one aspect, it increases solubility that facilitates low-cost solution processing; what’s more, it can be used to optimize the polymorphic behaviors of CPs.1 Many works have revealed that solubility, chain conformations, gelation behaviors, phase separation, film crystallinity and charge transport, and self-assembly characteristics are sensitive to the alkyl chain structures.2-14 For instance, a quantitative model based on thermodynamics was


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proposed recently that the conformation entropy of n-alkanoate chains released in dissolution greatly stabilized the colloidal solution, while the strong chain-chain interdigitation between adjacent particles induced strong aggregation.15,16 Also, manipulating the alkyl-chain length can trigger the variation of polymer chain conformation and yield the maximum degree of molecular backbone coplanarity, which ultimately resulted in the highest quality of molecular packing and optimized hole mobility.17 Despite these discoveries, there are limiting rules to guide molecular design on the alkyl chain except for trial and error. Additionally, most reported structureproperty relationships were demonstrated for non-steric CPs rather than the light-emitting one, and these results are controversial among different molecular structures. Therefore, to discover a universal rule helps efficiently choose the alkyl chain structures when designing CPs. Polyfluorenes (PFs) are a family of wide-bandgap semiconducting materials that are widely used in polymer light-emitting diodes (PLEDs), organic lasers and sensors, with the advantages of deep blue emission, high fluorescent quantum yield, and easy modifications.18 Their long-range and short-range conformations of polymer chains give rise to their polymorphism, offering a versatile library to optimize device performance.19-22 Polymorphism involves networks of complex noncovalent bonding among the polymer chains, related to the conformation change, deformation, steric strains, as well as chain entanglement from solution to film under the nonequilibrium external conditions, this makes the manipulation of PFs polymorphism becomes extremely challenging.23-29 Among the various morphologies of PFs, the β-phase exhibits several merits over the amorphous state, such as efficient light-emission, charge carrier mobility, and amplified spontaneous emission.30-32 The alkyl-chain length effect has been demonstrated in polydialkylfluorenes (PFn, n=6, 7, 8, 9) comprehensively, for example, the octyl chain was the most favorable length to induce β-phase, and different intrachain torsional angles have


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corresponding unit cells in their thin film states.33-37 However, green-band emission of PFs is detrimental to the efficiency and color purity of PLEDs.38-40 Steric groups have been incorporated into PFs to effectively avoid green-band emission, as well as the ladder-type PFs modified with aryl groups.41-43 Whereas, these strategies either impeded the formation of β-phase or were limited by difficult synthesis. According to supramolecular steric hindrance effect (SSH), we obtained β-phase in a polydiarylfluorene recently, which exhibited not only a high fluorescent efficiency and excellent stability by thermal annealing, but also a superior crystallinity among the sterically hindered PFs.44-47 Whether the alkyl-chain length effect on the conformation and crystallization of PFn can be inherited to polydiarylfluorenes? In this contribution, we synthesized a series of bulky polydiarylfluorenes with varying alkyl side chain length to exam this hereditary character. Optical spectroscopy combined with dynamic light scattering (DLS) were applied to investigate the relationship between chain aggregation and photophysical properties both in solution and thin film states. In addition to the PL spectra, the sequence of intrachain torsional angles was further determined by Raman spectra. AFM and GIXD were employed to reveal how the alkyl-chain length influenced the morphological evolution and final crystalline structures of the annealed films. The unambiguous relationship of alkyl chain length, conformation, aggregates, gelation, and crystallinity behaviors was established in this series of wide-bandgap luminescent semiconductors, which further highlighted the crucial role of morphology-oriented molecular design for plastic semiconducting materials. RESULTS AND DISCUSSION


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Figure 1. (a) The chemical structures of PnDPF. (b) The solubility of four polymers in toluene and the corresponding photographs of solutions under sunlight (top), 365 nm UV-light (middle) with the concentration of 0.5 mg/mL, as well as gels (bottom). (c, d) The UV-vis absorbance and PL profiles of PnDPF in toluene solutions with the concentration of 10-5 mg/mL. Synthesis

and

Fundamental

Characteristics

of

PnDPF.

Poly[4-(n-alkoxy)-9,9-

diphenylfluoren-2,7-diyl]-co-[5-(n-alkoxy)-9,9-diphenylfluoren-2,7-diyl] (PnDPF) (Figure 1a) was prepared via Yamamoto-type polymerization according to our previous works.39 The 1H NMR spectra of PnDPF are revealed in Figure S1, all the polymers exhibited the same chemical shift in the aromatic region, and the integral peaks increased with the increasing length of alkyl chain in the high field region. The number-average molecular weight (Mn) and polydispersity index (PDI) of PnDPF are shown in Table 1. These four polymers have comparable molecular weights and PDI, which helps exclude the molecular weight effect on various properties of polyfluorenes. The DSC curves of PnDPF showed that glass transition temperatures (Tg) of


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PnDPF were about 199 oC, 207 oC, 212 oC, and 133 oC (Figure S2-5). Generally, Tg decreases with the increasing length of alkyl chain for the polymers with the same backbone structure. However, PnDPF did not follow this rule. The Tg of P7DPF and P8DPF were lower than P6DPF. This abnormal phenomenon was ascribed to the different chain conformations of these four polymer powders, where P7DPF and P8DPF can adopt the more planar β-phase conformation that were not observed in P6DPF and P9DPF, giving them more rigid structures. This will be discussed in the following parts. Table 1. Summary of the fundamental characteristics of PnDPF.

a

PnDPF

Mn (Da)

PDI

Tg (oC)

UV-vis (nm)a

PL (nm)a

P6DPF

3.47×104

1.79

201

394

429, 453, 484

P7DPF

4.19×104

1.40

207

395, 437

429, 450, 480, 510

P8DPF

6.57×104

1.58

212

395, 442

429, 450, 483

P9DPF

2.68×104

2.01

133

394

429, 453, 484

Measured in dilute toluene solutions of PnDPF.

The Solubility and Gelation Behaviors of PnDPF. The trend in backbone stiffness of PnDPF powders follows the solubility with an order of P7DPF < P8DPF < P6DPF < P9DPF in toluene (Figure 1b). This explained the distinction of solutions by naked eye, where P6DPF and P9DPF were both transparent with weak blue emission, but P7DPF and P8DPF both exhibited green emission under the sunlight. By excitation with 365 nm UV-light, blue emission from P6/9DPF and blue-green emission from P7/8DPF were revealed. As demonstrated before, poly(9,9dioctylfluorene) (PF8) gel occurred in a concentrated solution (5 mg/mL) in a solvent with a suitable solubility parameter such as 1,2-dichloroethane (DCE) (9.8 Cal/cm3)1/2, which suggested that DCE with moderately poor solubility is a relatively effective solvent to induce the formation


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of gels.25 As shown in the Table 2, we also examined the gelation behavior of these four polymers in five solvents with various solubility parameters. Gels occurred in 10 mg/mL P6DPF of o-xylene solution, 5 mg/mL P7DPF of toluene solution at room temperature, and 10 mg/mL P8DPF o-xylene and toluene solutions at -18 oC (the corresponding photos of the gels are shown in inset of Figure 1b from left to right). The formation conditions of these four gels further confirmed that conjugated polymer gels were readily formed in the corresponding solvents with moderate solubility. Moreover, β-phase was not observed in the P6DPF-based gel, demonstrating that the interaction between the solvent and polymers plays a more crucial role than the planar conformation in the capacity of forming a gel. Table 2. Summary of the gelation behaviors of PnDPF.

Solvent

o-xylene

Toluene

THF

Chloroform

DCE

8.8

8.9

9.2

9.5

9.8

P6DPF

Yes

No

No

No

No

P7DPF

Undissolved

Yes

No

No

No

P8DPF

Yes (-18 oC)

Yes (-18 oC)

No

No

No

P9DPF

No

No

No

No

No

σ (Cal/cm3)1/2

The concentrations of all the solutions are 10 mg/mL, σ: solubility parameter. Photophysical Properties of PnDPF Solutions. As discussed above, toluene gave moderate solubility for P8DPF, with a small amount of β-phase observed in solution. Here, we address how the length of alkyl chains influences the photophysical properties of PnDPF. Firstly, dilute solutions with concentration of 10-5 mg/mL in toluene were prepared for investigating the


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photophysical properties of the single-chain-like state. In addition to the max absorbance peak at 395 nm, another sharp peak at ~440 nm was observed in each UV-vis spectrum of P7DPF and P8DPF (Figure 1c), which was assigned to the β-phase conformation of PnDPF according to our former studies. The emission peaks (Figure 1d) at 430 nm, 453 nm, and 483 nm for P6DPF and P9DPF, are assigned to 0-0, 0-1, and 0-2 π*-π transition of delocalized backbone of the typical amorphous polyfluorene chains. As expected, obvious red-shifts are observed in the spectra of P7DPF and P8DPF, where the emission peaks of 453 nm, 483 nm, and 512 nm became more intensive and sharper due to the appearance of β-phase. Therefore, we make an initial proposition that the suitable alkyl-chain lengths of PnDPF for maintaining the planar β-phase conformation are -OC7H15 and -OC8H17. This result is very similar to that of PFn, where -OC8H17 is the most favourable length.2 Additionally, β-phase content of P7DPF is a little more than that of P8DPF according to the intensity of UV-vis absorbance peaks at ~440 nm. This is attributed to the discrepancy of their solubility in toluene solutions, where a stronger chain collapse in P7DPF results in a more intense interaction of the chains. Unexpectedly, we also find several nanometres variation between P7DPF (435 nm) and P8DPF (441 nm) according to the UV-vis spectra, which reflects that the torsional angle of P8DPF β-phase is larger than that of P7DPF, which we confirm later in this study with Raman spectroscopy.


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Figure 2. DLS of PnDPF toluene solutions with a concentration of 0.5 mg/mL. Dynamic Light Scattering Study of PnDPF Solutions. We applied DLS technique to confirm chain aggregation behaviours of PnDPF in toluene solutions with a concentration of 0.5 mg/mL. Figure 2 shows the correlation function (g2(t)-1) and the corresponding hydrodynamic radius distribution (Rh) of PnDPF in toluene solutions at 25 °C. We see two types of relaxation time, one for P6DPF and P9DPF (defined as fast mode), and the other P7DPF and P8DPF (defined as slow mode) respectively. Firstly, the correlation function of P6DPF suggests a short relaxation time, which infers a comparable small-scale segment in solution. In combination with the average Rh of ~10 nm, we attributed it to the translational diffusion of individual P6DPF chains. Meanwhile, P9DPF exhibited a faster mode compared to P6DPF with a smaller average Rh of ~8 nm. This shorter relaxation time can be attributed to a better solubility of P9DPF that further reduced the rigidity of the backbones as well as the persistence length. Additionally, we can see fast mode with the average Rh lower than 1 nm in P6DPF and P9DPF solutions, which were attributed to the motion of the flexible side chains of PnDPF.48 The intensity correlation function is enhanced significantly in solutions of P7DPF and P8DPF. Here the correlation functions are


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larger in intensity and shifted to longer times, and the average Rh is sharply increased to ~100 nm. From this result, we assert that polymer chains of P7DPF and P8DPF physically cross-linked into aggregates due to the comparable low solubility, also leading to the adoption of the rigid planar β-phase conformation. The DLS results reveal the strong aggregate state of P7DPF and P8DPF chains in toluene solutions, while P6DPF and P9DPF both exhibit the single chain behavior at this concentration. Solutions were then investigated by UV-vis and PL spectroscopy to accurately obtain detailed spectral information. We cannot see obvious difference in the UVvis absorbance spectra between 0.5 mg/mL solutions (Figure S6) and previously shown dilute solutions (Figure 1). With regard to PL spectra (Figure S7), a slight enhancement of the emission intensity from β-phase was observed in the PL spectra of P7DPF and P8PDF. We attribute this to the more efficient energy transfer from high energy emission (amorphous) to the β-phase in more concentrated solutions.


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Figure 3. (a) Low temperature PL spectra of PnDPF. (b) Independent analysis of P7DPF PL profile. Four solutions concentrations are all 10-5 mg/mL, the spectra were obtained after reaching equilibrium at -196 oC. Photophysical Properties of PnDPF Solutions under Low Temperature. As previously reported, the transition temperature between disordered and ordered conformation of PF8 was 261 K in dilute methylcyclohexane solution, where the ordered segments (β-phase) coexisted with less ordered domains (amorphous conformation) along the same chains.2 Furthermore, when the ordered domains were present, they acted as efficient energy traps and the fluorescence from the disordered regions was quenched. In order to examine the conformation behavior of PnDPF at low temperature, we tested the PL and PLE (emission at 485 nm) spectra of PnDPF in


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dilute toluene solutions with a concentration of 10-5 mg/mL. Owing to the comparably low solubility of PnDPF at such a low temperature, sharp peaks appeared at 410 nm in the PLE spectra of P6DPF and P9DPF resulting from strong chain aggregation, but no additional peak appeared at ~440 nm (Figure S8). With respect to P7DPF and P8DPF, not only the comparably high intensity at 410 nm occurred, but also the additional peak at ~440 nm, which apparently demonstrated the existence of β-phase. Moreover, a 6 nm discrepancy was also found in the PLE β-phase peak of P7DPF (436 nm) and P8DPF (442 nm). Because of lower vibrational motion in the molecule at low temperature, the peaks of the PL spectra of PnDPF become much sharper than those at room temperature. As shown in Figure 3a, excited state transitions of 0-0, 0-1, and 0-2 were easily distinguished, which produced 425, 453, and 485 nm emission for P6DPF and P9DPF. Complex PL profiles are seen for P7DPF and P8DPF, where there are two chromophores co-existing. This also occurred in the room temperature solutions (Figure 1d and S8). Without doubt, the whole emission consists of luminescence from amorphous and β-phases, and is divided into two parts according to excited state transition types: 425, 453, and 485 nm from the amorphous segments, and 456, 488, and 525 nm from the planar segments, where the β-phase emission profile was obtained by subtracting the pure amorphous emission from the whole mixed emission (Figure 3b). In the situation of PF8 and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), complete energy transfer can fully quench the emission from the high energy sites (amorphous state), leading to a purely low energy type profile.2,49 Here, we attribute the co-existence of the two types of chromophores to the steric hindrance effect on the bulky polyfluorenes. The energy transfer route is blocked by the bulky side groups, where the steric hindrance weakens the dipole-dipole interaction of intrachain segments (Förster energy transfer). It was also


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demonstrated that the dual amplified spontaneous emission (ASE) phenomenon reported in our previous work mainly resulted from a low energy transfer efficiency from amorphous to β-phase segments.45 Moreover, almost no enhancement of emission from β-phase was observed at low temperature compared with the PL spectra at room temperature (Figure 1d). Consequently, we infer that low temperature makes no contribution to conformational transition for PnDPF, where the driving force induced by aggregation at such a low temperature (~77 K) was not strong enough to further induce more planar segments.

Figure 4. UV-vis (a) and PL (b) spectra of PnDPF films drop-casted from toluene solutions, UVvis (c) and PL (d) spectra of PnDPF spin-coated films after annealing. Photophysical Properties of PnDPF Drop-Casting Films and Annealed Films. Previously, we proposed that the planar β-phase conformation resulted from the balance of steric hindrance repulsion from bulky groups at the 9-position and van der Waals’ forces from the alkyl chains.


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Here, we explore the conformational evolution during solvent evaporation of the drop-casting procedure, where the chains interaction underwent a comparable mild process. As shown in Figure 4a, we see a high intensity UV-vis absorbance peak at ~440 nm for P7DPF and P8DPF, but no equivalent peak for P6DPF and P9DPF. Furthermore, we see a dramatic discrepancy in the PL spectra of the four polymers (Figure 4b), where P7DPF and P8DPF exhibit the transition peaks at 454 nm, 483 nm, and 515 nm. The max peak at 483 nm is attributed to the relatively large thickness of the films by drop-casting, which results in a reduction of the 0-0 transition possibly due to self-absorption. Unexpectedly, the PL profile of P6DPF shows every similar red shift to that of β-phase, whose vibrational transition peaks of 0-0, 0-1, and 0-2 are located at 452 nm, 476 nm, and 505 nm. Combining the widened UV-vis profile with bathochromic PL spectrum of P6DPF, we propose that P6DPF chains may adopt a more planar conformation after drop-casting, which is similar to γ-phase of PF8, with a smaller torsional angle (~155o) than the critical value for forming β-phase (~165o).50 Moreover, the transition vibrational energy level difference in P6DPF is smaller than those of P7DPF and P8DPF β-phase. It is also worth mentioning that there is a 6 nm discrepancy in the β-phase absorption peak of P7DPF (435 nm) and P8DPF (441 nm), which further confirmed the observation in the UV-vis and PLE profiles discussed above. Additionally, we investigated the effect of solvent solubility parameter on the formation of β-phase by drop-casting films from the five solvents mentioned above. The results are shown in Figure S9 for P8DPF, where the UV-vis peak at 441 nm is seen only for toluene, demonstrating that β-phase segments are preferentially formed in solvents with relatively low solubility. P8DPF has previously demonstrated a conformational transition in film by thermal annealing treatment.45 Here, we further explore the influence of alkyl chain length by annealing pristine


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spin-coated films at 220 oC. As shown in Figure 4c, no trace of β-phase was found in P6DPF and P9DPF according to the UV-vis profiles. Conversely, sharp absorbance peaks appeared after thermal treatment for P7DPF and P8DPF, at 435 nm and 441 nm respectively. Furthermore, an additional peak appeared at 410 nm in each UV-vis spectra of P7DPF and P8DPF, assigned to the 0-1 vibronic absorption transition of the β-phase. Combining with the results of low temperature solutions and drop-casting films experiments discussed above, we propose that chain aggregation can be a necessary but not sufficient condition for the formation of β-phase. It means β-phase in PnDPF can form in corresponding aggregates, but only when the length of alkyl chains is suitable, van der Waals’ forces can planarize the conformation. Herein, -OC7H15 and OC8H17 are found to be the optimized length for the formation of β-phase in PnDPF. With respect to the PL spectra (Figure 4d), dramatic red-shift (∆ = 16 nm) predictably emerged in P7DPF and P8DPF with no change for P9DPF. Similar to that in drop-casting film, slight broadening in the UV-vis spectrum and a uniform 10 nm red-shift in PL profile of P6DPF were detected after thermal treatment, 6 nm less than β-phase of P7DPF and P8DPF. From the results discussed above, we consider that the driving force for inducing β-phase in PnDPF is chain aggregation, based on the balance of attractive and repulsive force coming from the steric hindrance groups and alkyl chains.


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Figure 5. (a) Raman spectra of the PnDPF series before (red lines) and after annealing (blue lines) at 220 oC. (b) Relative height of 1304 cm-1 to 1600 cm-1 of PnDPF. (c) Soft-shift value of PnDPF at 1600 cm-1 after annealing treatment. Raman Spectra for Determining the Conformations of PnDPF. Raman spectroscopy has previously been demonstrated as a sensitive tool to reveal the conformation of CPs. In our previous study, the backbone planarity of P8DPF was evaluated by assessing the relative 1304 cm-1:1600 cm-1 peak intensity ratio, as well as the soft-shift of the 1600 cm-1 peak to lower wavenumbers.45 Here, by comparing the variation of two states (amorphous in pristine films and β-phase in annealed films), we can easily distinguish the conformational evolution between the torsional angles of the two states. As shown in Figure 5a, an obvious enhancement in the region from 1280 cm-1 to 1350 cm-1 appeared in P7DPF and P8DPF after annealing at 220 °C, which was ascribed to planarization of the backbone. On the contrary, negligible variation is seen for


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P9DPF, whereas there is a small increase for P6DPF. We plot the relative height of the 1304 cm1

peak for the four polymers in Figure 5b, and infer that the relative intensity at 1304 cm-1 is

directly proportional to the backbone planarity, with an order of P8DPF > P7DPF > P6DPF > P9DPF, and follows the trend in β-phase content for P7DPF and P8DPF with P8DPF > P7DPF. As reported previously, enhanced planarity of the backbone increases the delocalization of πelectrons, which gives rise to the soft-shift at ~1600 cm-1. Hence, the trend in torsional angle of the backbone for PnDPF was further proved by the soft-shift at ~1600 cm-1, where the sequence of shift values is the same as that of the relative intensity at 1304 cm-1 (Figure 5c). The full Raman profiles of the four polymers are provided in Figure S10-13. UV-vis and PL spectra indicate no trace of a conformational transition can be found between the pristine and annealed films of P9DPF, supported by an absence of an enhancement in the 1304 cm-1 and soft-shift in the 1600 cm-1 vibrational modes in the Raman spectra. As discussed previously for Figure 4, although we do not see characteristic β-phase features in the UV-vis and PL of annealed P6DPF, we attribute the 10 nm red-shift in PL spectra to an increasingly planar conformation after annealing, which is further supported by the soft-shift of the 1600 cm-1 Raman mode. However, the variation of torsional angle in P7DPF and P8DPF due to annealing is more obvious from their larger soft-shift at 1600 cm-1. We found a several nanometer difference in the β-phase peak between the two polymers in their UV-vis profiles as discussed above. By analyzing the softshift in 1600 cm-1 in Raman spectra, we confirm that this discrepancy derives from the difference between the torsional angles of P8DPF and P7DPF, with the former exhibiting a larger value. In actual fact, the β-phase absorption peak of PF8 also red shifted several nanometers compared to those of PF7 and PF9.2 We propose that only once the torsional angle has overcome a threshold value can a well-defined β-phase conformation be generated, evidenced by an additional narrow


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absorption peak at ~440 nm, due to the lower energy of the planarized segments within the bulky film. On this occasion, we have observed a different degree of electronic delocalization along the backbone of the β-phase segments for P7DPF and P8DPF, demonstrating the sensitivity of the planarized conformation to alkyl side chain length.

Figure 6. AFM height images of PnDPF spin-coated films before (a, b, c and d) and after (e, f, g and h) thermally annealing under 220 oC. Atomic Force Microscopy (AFM) Study on the Morphology of PnDPF. Apparent aggregation of PnDPF can be judged from the UV-vis profiles, where there was an additional peak appearing at 410 nm. To shed light on the difference in the crystallinity and microstructure


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of the annealed films of PnDPF, we explored the detailed morphology variation of these four polymers before and after thermal annealing by AFM. As shown in Figure 6a, b, c and d, all the pristine films exhibited smooth surfaces with root mean square roughnesses of 0.521 nm for P6DPF, 0.369 nm for P7DPF, 0.6 nm for P8DPF, and 0.242 nm for P9DPF. After annealing at 220 oC, changes in the apparent nanostructures appeared in the surface of the films. The annealed films exhibited roughness of 0.581 nm for P6DPF, 0.696 nm for P7DPF, 1.81 nm for P8DPF, and 1.4 nm for P9DPF, respectively. As reported in our previous work, crystalline β-phase microstructure can be induced by chain aggregation in the film by thermal annealing, where the nanostructures on the surface were assigned to the aggregates of the β-phase. As shown in Figure 6e, f, g, and h, the density of the granular domains on the surface follows the sequence P6DPF > P7DPF > P8DPF > P9DPF, which corresponds to the crystallinity estimated from GIXD patterns discussed below.


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Figure 7. (a, b, c and d) 2D-GIXD diffractograms of PnDPF after annealing at 220 oC for 30 min and the corresponding pristine films (insets), (e, f, g and h) the corresponding 1D profiles of the 2D-GIXD diffractograms. Two Dimension Grazing Incidence X-ray Diffraction (2D-GIXD) Study of PnDPF. Usually, introducing bulky groups into conjugated polymer brings about higher Tg and thermal stability, but gives rise to amorphous polymer chains arrangement with low mobility, in which there are abundant sites for excitons to emit radiatively. Previously, we made a breakthrough and obtained the crystalline structure of β-phase P8DPF by annealing the film at 220 oC, which was unprecedented for bulky polyfluorenes.45 In this report, we have further studied how the crystalline structures were altered by changing the alkyl-chain length of PnDPF. Firstly, we examined the XRD profiles of these four polymer powders and found that P7/8DPF exhibited more diffraction peaks than P6/9DPF (Figure S14). Then, we verified their crystallinity in the annealed films by 2D-GIXD. As shown in the insets of Figure 7a, b, c and d, we can see all the diffractograms from pristine films exhibit broad diffractions with q ≈ 0.8 Å-1 and 1.4 Å-1 along the out-of-plane direction, which confirm the amorphous nature of PnDPF chains in the pristine films. On the contrary, four annealed films all exhibited apparent diffraction patterns showed in Figure 7a, b, c and d, which revealed the strong tendency for thermally induced crystallization for our polydiarylfluorenes due to the effect of supramolecular steric hindrance. Because the unit cells of these four annealed films are not fully understood, we did not mark the Miller indices in the diffraction patterns. As discussed above, only P7DPF and P8DPF exhibited β-phase in these four crystal films, we proposed that it is the crystallization that induced conformational transition, rather than the conformational transition leading to crystallization. Although P7DPF and P8DPF have different alkyl-chain lengths, they both exhibit the same diffraction patterns


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because of the β-phase conformation induced by thermal annealing. As for P6DPF and P9DPF annealed films, we see completely different diffractograms, resulting from the differences in conformation and alkyl-chain length. Intuitively, with the same main chain structures and the only difference being one additional or one less methyl group in the side chain, the same conformation resulted the same unit cell for P7DPF and P8DPF, but different conformations lead to different unit cells for P6DPF and P7DPF, P8DPF and P9DPF. Here, the sensitivity of chain conformation to alkyl chain length and the resulting influence on crystal structure is again illustrated. Therefore, chain conformation played a more crucial role than alkyl-chain length in terms of determining the crystalline structure. This result is consistent with that of PFn (n = 6, 7, 8, 9), α phase of PFn all show very similar crystalline structures, but α, β, and γ phase of PF8 with three chain conformations (Cα ~ 135o, Cβ ~ 165o, Cγ ~ 155o) exhibit three different crystalline structures (three kinds of unit cells).34 As shown by Figure 7e, f, g and h, each of the crystalline films exhibited a diffraction spot at ~0.83 Å-1 that was attributed to interbackbone scattering. It also demonstrated the edge-on arrangement of the polymer chains in these four crystalline films. According to the full width at half maximum of the diffraction peaks at ~0.83 Å-1, we estimate the degree of crystallinity within the bulky films follows the order of P6DPF > P7DPF > P8DPF > P9DPF. Therefore, a short alkyl chain would produce a higher degree of crystallinity for PnDPF. We note that the diffraction peak at ~1.5 Å-1 along the in-plane direction only appears in annealed P7DPF and P8DPF films, arising from the periodicity of monomers along the backbone in the (001) direction, a diffraction feature unique to the planarized β-phase conformation.33,34 This series of polydiarylfluorenes which exhibit varying degrees of crystallinity, represent a remarkable exception in the class of bulky conjugated polymers,


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demonstrating for the first time that steric hindrance is not destined to impede crystallinity in CPs.51

Figure 8. Schematic illustration of the conformational transition via thermodynamic path (crystallization) and kinetic path (steric hindrance combined with van der Waals force). Proposed mechanism of β-phase formation. As discussed above, we found β-phase formed only in aggregates and films of P7DPF and P8DPF, but not in P6DPF and P9DPF. Therefore, on the premise of appropriate chemical structures, crystallization induced interchain interactions provide the essential driving forces for the conformational transition to form β-phase. Based on the results discussed in this contribution, we propose the mechanism of β-phase formation both from the view of thermodynamic path and kinetic path (Figure 8). Thermodynamic path: when the thin film was thermally treated with 220o, the conformational transition and crystallization did occur spontaneously at this temperature. According to the thermodynamic equation of Gibbs free energy change: ∆G = ∆H – T∆S, ∆G is negative at this time. Meanwhile, ∆S is also negative during crystallization, therefore, ∆H must be negative, which is an exothermic process. As


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shown in Figure 8, the conformational transition from amorphous state to β-phase needs to overcome the activation energy that is an endothermic process. Consequently, thermal annealing plays a positive role in conformational transition. As we know, β-phase is a metastable state, why did it only appear in P7DPF and P8DPF with the same thermodynamically favorable environment? At this point, chemical structures should be considered in the view of kinetic process. In order to be able to describe the β-phase formation mechanism, we examined the single crystal structure of the P8DPF monomer, which provided indirect evidence for revealing the interactions of P8DPF polymer chains within thin films. As shown in Figure S15, steric hindrance of the phenyl in the 9-position is found between adjacent monomers in the packing diagram of the single crystal, which inhibits the π-π interaction of the π-electron plane of the fluorene backbone. Moreover, the alkyl chains are interspersed in the space created by the steric hindrance of phenyls. After polymerization, the steric hindrance and van der Waals interactions propagate to the corresponding polymer. Kinetic path: when treated with thermal annealing, crystalline process provides polymer chains with enough energy to rearrange and cross the threshold of the conformational transition. On this occasion, the repulsive steric hindrance of the phenyl groups in 9-position provides enough space for intrachain angle rotation. After that, the alkyl chains that interspersed in the space created by phenyl groups, with attractive van der Waals’ forces (the interdigitated alkyl chains holding the backbones together in the thin films), are the key determinant of whether the conformational transition is able to occur.2 We believe that the formation of crystalline chains are kinetically favored over β-phase chains for high temperature annealing, only P7DPF and P8DPF with a suitable alkyl-chain length are able to achieve the metastable β-phase conformation, and such a state is derived from the balance between steric hindrance force and van der Waals’ force among the crystalline domains.


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CONCLUSIONS We have investigated a series of bulky polydiarylfluorenes with varying alkyl chain length (PnDPF, n=6, 7, 8, 9), in order to further clarify the intrinsic influence of chain length on molecular conformation and subsequent material properties, examining the generality of the structure-property relationships previously studied in PFn. The effect of chain length on chain conformation, aggregation, gelation behavior, and film crystallinity was determined. It was revealed that the gelation behaviors were more determined by solvent solubility, which governs the long-range conformation of polymer chains, rather than the torsional angle that controlled the short-range conformations. Photophysical properties of PnDPF in various environments revealed that the driving force for inducing β-phase is chain aggregation, whilst the ideal balance of repulsive forces coming from sterically hindered phenyl groups and attractive forces from the alkyl side chains necessary for β-phase formation was readily achieved in the aggregates of P7DPF and P8DPF. Moreover, the degree of backbone planarity in PnDPF obeyed the order: P8DPF > P7DPF > P6DPF > P9DPF by evaluating the soft-shift at ~1600 cm-1 from Raman spectra. Increasing crystallinity with decreasing alkyl-chain length was demonstrated in the films after annealing at 220 oC, where the sensitivity of the intrachain dihedral angle to alkyl chain length was revealed in the crystalline structures measured by GIXD. Also, crystallization acted as a driving force to induce conformational transition. Finally, thermodynamic and kinetic processes were proposed to illustrate how the supramolecular steric hindrance effect induces the formation of β-phase in PnDPF during crystallization (n = 7 and 8 only). In summary, the alkylchain length effect of PFn on molecular conformation and crystallization has been revealed to pass on to a series of bulky polydiarylfluorenes. Our results set a platform for guiding future


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morphology-oriented molecular design, for the purposes of improving the device performance of solution processible light-emitting conjugated polymers. ASSOCIATED CONTENT Supporting Information. Experimental methods, synthesis, and characterization of PnDPF; CCDC numbers for monomer of P8DPF is 1554546. 1H NMR of PnDPF (Figure S1); DSC Curve of PnDPF (Figure S2-5); UV-vis spectra of PnDPF toluene solutions with the concentration of 0.5 mg/mL (Figure S6); PL spectra of PnDPF toluene solutions with the concentration of 0.5 mg/mL (Figure S7); The PLE spectra of four polymers measured at -196 oC with the concentration of 10-5 mg/mL (Figure S8); UV-vis spectra of P8DPF films drop-cast from various solvents with different solubility (Figure S9); Raman spectra of PnDPF drop-casting films (Figure S10-13); 1D XRD profiles of PnDPF powders (Figure S14); Packing diagram of the P8DPF monomer single crystal (Figure S15). AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected]; [email protected]. ORCID Bin Liu: 0000-0002-5240-2509 Jin-Yi Lin: 0000-0002-2607-3633 Ling-Hai Xie: 0000-0001-6294-5833 Wei Huang: 0000-0001-7004-6408 Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (U1301243, 21504041), Doctoral Fund of Ministry of Education of China (20133223110007), Excellent science and technology innovation team of Jiangsu Higher Education Institutions (2013), the Six Peak Talents Foundation of Jiangsu Province (XCL-CXTD-009), Natural Science Foundation of Jiangsu Province (BM2012010), Synergetic Innovation Center for Organic Electronics and Information Displays, Program for Postgraduates Research Innovation in University of Jiangsu Province (KYLX15_0849), Priority Academic Program Development of Jiangsu Higher Education Institutions, PAPD (YX03002), Natural Science of the Education Committee of Jiangsu Province (15KJB430019), China Postdoctoral Science Foundation (2015M580419), Jiangsu Planned Projects for Postdoctoral Research Funds (1501019B). REFERENCES 1.

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Hereditary Character of Alkyl-Chain Length Effect on Î²-Phase

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