Novel Fluorene-Based Copolymers Containing Branched 2-Methyl

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Novel Fluorene-Based Copolymers Containing Branched 2-MethylButyl Substituted Fluorene-Co-Benzothiadiazole Units for Remarkable Optical Gain Enhancement in Green-Yellow Emission Range Zhou Yu, Xiangru Guo, Qi Zhang, Lang Chi, Ting Chen, Ruidong Xia, Longfei Wu, Larry Lüer, and Juan Cabanillas-Gonzalez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01375 • Publication Date (Web): 11 May 2016 Downloaded from http://pubs.acs.org on May 12, 2016

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The Journal of Physical Chemistry

Novel Fluorene-Based Copolymers Containing Branched 2-Methyl-Butyl Substituted Fluorene-CoBenzothiadiazole Units for Remarkable Optical Gain Enhancement in Green-Yellow Emission Range Zhou Yu, § Xiangru Guo, § Qi Zhang, Lang Chi, Ting Chen, Ruidong Xia,* Longfei Wu, Larry Lüer, Juan Cabanillas-Gonzalez* Z. Yu, X. Guo, Q. Zhang, L. Chi, T. Chen, Prof. R. Xia Key Laboratory for Organic Electronics and Information Displays (KLOEID) & 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, E-mail: [email protected], TF: +86 25 8586 6008 L. Wu, Dr L. Lüer, Dr. J. Cabanillas-Gonzalez Madrid Institute for Advanced Studies (IMDEA Nanociencia), Calle Faraday 9, Ciudad Universitaria de Cantoblanco, Spain, E-mail: [email protected], TF: +34 912998784

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ABSTRACT.

We develop novel fluorene-based green-yellow emission copolymers (namely nF1/4F8BT) composed of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and branched 2-methylbutyl substituted fluorene-co-benzothiadiazole units (F1/4BT) in different ratios. Upon blending nF1/4F8BT copolymers with poly(9,9-dioctylfluorene) (PFO), a systematic increase in photoluminescence quantum efficiency (PLQE) is observed as the ratio of the branched monomers increase. Likewise, nF1/4F8BT and PFO:nF1/4F8BT blends exhibit superior optical gain properties respect to F8BT, manifested as a 30% reduction of amplified spontaneous emission (ASE) threshold and 50% increase in optical gain respect to F8BT blends at the same emission wavelength. The optical gain related properties was studied to understand the influence of the branched side chain unit on stimulated emission properties. Femtosecond transient absorption studies confirm exciton-annihilation hindrance in the new copolymers likely caused by interference of branched substituents with molecular packing in films. Branch side-chain substitution of a limited number of monomers is an efficient strategy to boost the optical gain properties of conjugated polymers.

1. Introduction Fluorene-based polymers are drawing increasing attention for optoelectronic applications such as light emitting diodes (LEDs), lasers and optical amplifiers due to their superb optical and electronic properties.1-6 Enhancing charge transport in conjugated polymers without affecting the emitting properties is desirable in order to boost efficiencies in OLEDs and pave the way towards electrically pumped polymer lasers.7,8 Nonetheless, conjugated polymer design strategies aiming to promote simultaneously charge transport and luminescence efficiency


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proved to be elusive until recently. Yap et al.9 adopted a new strategy consisting of developing statistical blue-emitting copolymers of 9,9-dioctylfluorene (F8) and a F8 derivative bearing short side-chain substitution in a low statistical content. Short side-chain sites acted as pinpoints where inter-chain hopping occurred preferentially owing to the associated polymer chains proximity. Owing to the predominance of intra-chain versus inter-chain charge transport in conjugated polymers,10 a low content of such sites is enough to boost charge mobility by two orders of magnitude without altering significantly the emission efficiency. Moreover, a significant improvement in optical gain from optically-pumped laser cavities based on same copolymers was observed and tentatively ascribed to the superior optical confinement respect to PFO homopolymer. Alternative explanations involving photophysical phenomena associated to changes in lateral substituents were not addressed, even though interference between excitons and pinpoint sites may not be negligible on account of the 8-15 nm diffusion length of the former.11-14 Aiming to promote the optical amplifying properties in green-yellow emission range of conjugated polymers, we hereby develop novel copolymers of F8BT and its short branched analogue F1/4BT, nF1/4F8BT copolymers, in low statistical content by Suzuki coupling reaction, (Scheme 1). Here “n” represents the ratio of 9,9-di(2-methyl)butyl (F1/4) in the copolymer. For example, 20F1/4F8BT consisted of 20% 9,9-di(2-methyl)butyl (F1/4), 30% 9,9dioctyl substituted fluorene (F8) units and 50% benzothiadiazole (BT) units. 15F1/4F8BT and 10F1/4F8BT contain 15% and 10% 9,9-di(2-methyl)butyl (F1/4) in the polymer chain, respectively. The ratio of BT was fixed to 50% in our study. F8BT is a widely used green-yellow emissive fluorene-based copolymer.15-19 However, the optical gain coefficient of F8BT is relatively low. It has been previously reported that molecular packing in F8BT films has a


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substantial effect on the electron and exciton transport properties due to the large electron localization at the BT unit in the LUMO. According to this scenario, the inter-distance between BT units on neighbor chains becomes a critical parameter which controls charge transport as well as the emission properties. A proof for this is found on the drastic reduction of electron mobility and enhancement in photoluminescence quantum efficiency (PLQE) upon annealing of F8BT films,15,16 characterized by larger BT inter-chain distance respect to pristine films. Likewise, changes in molecular packing induced by side-chain substitution are expected to play an important role on the photophysical properties of F8BT. We demonstrate by means of femtosecond transient absorption spectroscopy that the introduction of branched substituents in low concentration directly influences exciton transport with a concomitant beneficial effect in PLQE and stimulated emission properties. Upon blending nF1/4F8BT copolymers with poly(9,9dioctylfluorene) (PFO), we realize optically-pumped yellow-green gain structures with a 30% reduction of amplified spontaneous emission (ASE) threshold and 50% increase in optical gain respect to F8BT blends. The detailed study has been carried out to understand the correlation between F1/4BT concentration and optical gain performance.

“Scheme 1. The chemical structures of copolymers.”


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2. Experimental Section The copolymers of 9, 9-dialkylfluorene and benzothiadiazole were obtained by Suzuki coupling reaction (see Scheme S1 for details in the supporting information). These copolymers were named as F8BT, F1/4BT and nF1/4F8BT（Scheme 1). Three nF1/4F8BT were synthesized with constant BT ratio of 50% and various F1/4:F8 feed molar ratio: 10F1/4F8BT (F1/4:F8=10:40), 15F1/4F8BT (F1/4:F8=15:35) and 20F1/4F8BT (F1/4:F8=20:30). Molecular weights were determined by gel permeation chromatography in comparison to polystyrene standards. The polymers studied here have similar molecular weights (MW) for meaningful comparisons. The MW, PDI (polydispersity) and PLQE of these materials are shown in Table S1. Spectrosil B (synthetic quartz) substrates were cleaned by successive 15 min sonication in acetone and ethyl alcohol followed by cleaning in oxygen plasma for 10 min (250 W). Films for all materials were spin cast from 15 to 25 mg/ml chloroform solution onto pre-cleaned substrates to obtain 130 ~150 nm thick films. Atomic force microscopy (AFM) was carried out at room temperature using a Bruker Dimension Icon AFM equipped with Scanasyst-Air peak force tapping mode AFM tips from Bruker. Ultraviolet-visible (UV-Vis) spectra were recorded on an UV-3600 Shimadzu UV-VIS-NIR spectrophotometer, while fluorescence spectra were obtained using an RF-5301PC spectrofluoro photometer with a Xenon lamp as light source. The photoluminescence quantum efficiency (PLQE) of the films was measured using an Edinburgh FLSP920 fluorescence spectrophotometer equipped with a Xenon arc lamp (Xe900) using an integrating sphere. The photoluminescence decay curves were measured using an Edinburgh FLSP920 fluorescence


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spectrophotometer equipped with a 375 nm laser (typical pulse width: 55 ps; pulse repetition frequencies: 20 MHz). Transient absorption experiments were carried out with a femtosecond Clark-MXR CPA210 regenerative amplifier delivering 120 fs pulses at 775 nm and 1 KHz repetition rate. The primary beam was split into pump and probe beams. The pump beam was sent to a BBO crystal for second harmonic generation (387.5 nm) and then onto a computer controlled delay line to anticipate it respect to the probe, being finally focused onto the sample (1 mJcm-2). The excitation wavelength employed led to predominant photoexcitation of PFO in blends and to a minor extent, of F8BT and nF1/4F8BT. The probe beam was focused on a sapphire plate to generate a supercontinuum and overlap with the pump spot on the sample. After transmission through the sample, the probe pulses were sent to a prism spectrometer (Entwicklungsburo Stresing GmbH) with a CCD array (256 pixels, VIS-enhanced InGaAs, Hamamatsu Photonics Inc.). For amplified spontaneous emission (ASE) measurements, samples were optically pumped at 390 nm, i.e. close to the absorption peak of PFO.20 Pump excitation was provided by a Qswitched, neodymium ion doped yttrium aluminium garnate [Nd3+: YAG] laser (Continum Surelite II-10) pumped, type-II β-BaB2O4 [BBO], optical parametric oscillator (Panther EX) that delivered 3 ns pulses at a repetition rate of 10 Hz. Calibrated neutral density filters were inserted into the beam path to adjust pulse energy incident on the sample. In ASE measurements, an adjustable slit and a cylindrical lens were combined to create a narrow excitation stripe of 550 µm x 4 mm. The edge emission from samples was collected with a fiber-coupled grating spectrometer (Andor Co.) equipped with a CCD detector (Newton Co.). The gain characteristics of the waveguides were measured with the variable stripe length method. The light intensity


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emitted from the waveguides as a function of excitation stripe length was recorded for a range of different pump energies.

3. Results and Discussion 3.1 Atomic Force Microscopy (AFM) AFM was conducted to investigate the film morphologies of copolymers. Figure 1 shows the AFM images of pristine F8BT, F1/4BT, and nF1/4F8BT copolymer films as well as their blends in PFO. Comparison of Fig. 1A and 1E shows that pristine F8BT possesses a smooth topography and homogeneous morphology (Fig.1E), in contrast with F1/4BT, perhaps related to the limited solubility of the latter. Film topographies from nF1/4F8BT copolymers display the presence of “black dots”, their density becoming larger with the F1/4 content. The RMS roughness of the samples was 1.75 nm for 20F1/4F8BT, 0.86 nm for 15F1/4F8BT, 0.80 nm for 10F1/4F8BT, 1.80 nm for F1/4BT and 0.37 nm for F8BT, which indicates a roughness increase with F1/4 content in line with the associated decrease in solubility. In particular, 20F1/4F8BT and F1/4BT demonstrate almost the same RMS. In copolymers with F1/4 content above 20F1/4F8BT we observed a detectable reduction in the emission properties concomitant with poor film forming ability. Thus, we kept the proportion of F1/4 units contained in the copolymer below 20 wt. %. Meanwhile, we investigated the morphology of F1/4BT, F8BT and nF1/4F8BT blends in PFO in a ratio of 15 wt. %. Fig. 1 (F) – (J) show high quality films without significant traces for phase separation. Upon PFO blending, the RMS of all PFO:nF1/4F8BT mixtures decreased to 0.32 nm, similar to the RMS found on F8BT films. These different film forming properties have a direct impact on PLQE values as shown later.


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“Figure 1. AFM images (5µm×5µm) of pristine films. (A) 20F1/4F8BT, (B) 15F1/4F8BT, (C) 10F1/4F8BT, (D) F1/4BT, (E) F8BT. Images (F-J) are of the PFO blended films correspond to (A-E).”

3.2 UV-Visible (UV-Vis) Absorption and Photoluminescence (PL) Figure 2(A) shows the normalized absorption and photoluminescence spectrum of F8BT and F1/4BT. Figure 2(B) shows the spectra of 10F1/4F8BT, 15F1/4F8BT, and 20F1/4F8BT. The UV-Vis absorption spectrum of F1/4BT in Fig. 2 (A) shows a slightly blue shift (5 nm, 0.25 eV) compared to the absorption of F8BT film. However, Fig. 2(A) and (B) shows similar PL spectra


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of F8BT and nF1/4F8BT copolymers studied here, no matter whether the substituted side-chain was branched (F1/4) or linear (F5, Figure S3 (A) and (B)). The F1/4:F8 ratio in copolymers has minor consequences in the electronic structure of the same. This is supported by the similar band gap values of nF1/4F8BT and F8BT determined as the difference between the electrochemically measured HOMO and LUMO values (showed in Table S2). Hitherto, the PLQE values in film seem to be deeply influenced by the F1/4 substitution. The PLQE of F8BT (0.32) was higher than the corresponding of F1/4BT (0.14) (Table S1) probably due to the bad film quality of the latter, while the PLQE values of 10F1/4F8BT (0.26), 15F1/4F8BT (0.32) and 20F1/4F8BT (0.37) grow in direct proportion of the F1/4 content in the copolymers. The superior PLQE values observed in blends (Table 1) respect to pristine polymers arises from a combination of improved copolymer solubility and reduced exciton migration to recombination centers in blends. This effect was previously highlighted in blends of PFO and F8BT studied with timeresolved near-field optical microscopy.18 Note here that the PL spectra of 15wt% PFO:F8BT and PFO:nF1/4F8BT blends (showed in Figure S4) resemble the spectra of copolymers without residual PFO emission, suggesting an almost 100% efficient Förster resonance energy transfer (FRET) in both blends. In line with pristine copolymer films, the PLQE of PFO:nF1/4F8BT blends are boosted from 0.34 to 0.52 as the F1/4:F8 ratio raises from 10:40 to 20:30. The PLQE of PFO:20F1/4F8BT blends (0.52) was found to be slightly above the corresponding PFO:F8BT value (0.45). This slightly improvement together with the PLQE increase trend as the F1/4:F8 ratio increases confirm a beneficial effect of butyl-substituted side-chains on the emission properties in blends.


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0.0 400

500

600

0.0 700

Normalized Abs (a.u.)

0.5

0.5

300

1.0（

1.0

B）

1.0

0.5

0.5

0.0 300

400

500

600

Normalized PL (a.u.)

A）


1.0（


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0.0 700

Wavelength (nm)

Wavelength (nm)

“Figure 2. Normalized absorbance and PL spectra of (A) F8BT (filled squares) and F1/4BT(open circles) and (B) 10F1/4F8BT (open triangles), 15F1/4F8BT (open squares), and 20F1/4F8BT (filled squares).”

3.3 Photoluminescence decay and PL lifetime PL lifetimes of copolymers (Table 1) were obtained from bi-exponential fits of the pristine and blend PL decays (see Figure S6 in Supporting Information) and compared in order to trace for structural changes in copolymer films directly.21 In agreement with PLQE values, the PL lifetimes of pristine nF1/4F8BT and PFO:nF1/4F8BT blends increase with the F1/4:F8 ratio, (Table 1). Furthermore, all compounds exhibit a PL lifetime enhancement upon copolymer dilution in PFO matrix respect to pristine films, an effect ascribed to partial hindrance of diffusion-assisted recombination at quenching centers.22 We note that the effect of dilution on PL lifetime is more significant in nF1/4F8BT copolymers than in F8BT. These differences could be attributed to the already mentioned limited solubility of nF1/4F8BT copolymers affecting the PLQE and PL lifetimes of pristine films.


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“Table 1. Fluorescence Decay Lifetimes of all the copolymers in Solid Films.” Materials

Pristine copolymers a) (ns)

τ c) Copolymers blended in PFO τ c) b) (ns) (ns) (ns)

τ1(A1)

τ2(A2)

τ1(A1)

τ2(A2)

F8BT

1.28(85.97%)

2.89(14.03%) 1.50

1.73(86.34%)

2.96(13.66%) 1.89

20F1/4F8BT

0.88(89.88%)

2.72(10.12%) 1.06

1.18(58.75%)

2.01(41.25%) 1.52

15F1/4F8BT

0.69(89.44%)

2.58(10.56%) 0.89

1.19(67.82%)

2.14(32.18%) 1.49

10F1/4F8BT

0.54(85.14%)

2.43(14.86%) 0.82

1.07(82.45%)

2.55(17.55%) 1.33

F1/4BT

0.51(83.48%)

2.40(16.52%) 0.85

0.86(65.53%)

2.29(34.47%) 1.35

a)

PL decay lifetime of pristine films. b) PL decay lifetime in blend films. c) τ=A1*τ1+A2*τ2 3.4 Amplified spontaneous emission and optical gain The ASE spectra of PFO:F8BT and PFO:20F1/4F8BT compared with their corresponding UV-

Vis absorption and PL spectra are shown in Figure 3(a). (ASE spectra of 10F1/4F8BT, 15F1/4F8BT and 20F1/4F8BT blends are displayed in Figure S4 in Supporting Information). The ASE peaks are centered at 549 nm, and 551~552 nm for PFO:F8BT and PFO:nF1/4F8BT films respectively, (Table 2). Figure 3(b) displays the ASE peak which develops and outweighs the emission spectra rapidly, as the pulse energy increases. The full width at half maximum (FWHM) values remain between 13~15 nm (~ 0.07 eV). In order to quantify the efficiency of ASE, we calculated the ASE threshold (Eth) values as the photoexcitation energy at which FWHM drops to half of the PL linewidth.


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“Figure 3. (a) Absorbance (solid lines), Normalized PL (dash lines) and ASE (filled lines) spectrum of F8BT (above) and 20F1/4F8BT (below) blend films, (b) ASE spectra of a PFO:20F1/4F8BT film as a function of fluence. (c) Edge emitted output intensity and FWHM values (inset), as a function of the pump energy, for PFO:F8BT (filled squares) and PFO:20F1/4F8BT (open triangles) blend films (15 wt.%), (d) gain coefficients as a function of excitation energy of PFO:F8BT (filled squares) and PFO:20F1/4F8BT (open circles) blend films.”

Accordingly, Eth values of 95 nJ pulse-1, 138 nJ pulse-1, 112 nJ pulse-1 and 62 nJ pulse-1 were obtained for PFO:F8BT, PFO:10F1/4F8BT, PFO:15F1/4F8BT, and PFO:20F1/4F8BT respectively, (Table 1). Thus, a 30% reduction of the Eth value is found in PFO:20F1/4F8BT


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compared to PFO:F8BT. Figure 3(c) depicts the emitted output intensity and FWHM values (inset) of PFO:F8BT and PFO:20F1/4F8BT films as a function of pump energy. The larger slope observed in the latter points once more to highly efficient optical gain in these blends. To further elucidate the gain characteristics of copolymer blends, we measured the optical gain coefficients in films with the variable stripe length method. Net gain coefficients of PFO:F8BT and PFO:20F1/4F8BT films as a function of pump energies are shown in Figure 3(d). The maximum gain coefficients of PFO:F8BT and PFO:20F1/4F8BT films are ~22 cm-1 and ~32 cm1

, respectively, i.e. a 48% increase in net gain is found in the latter. Note that these differences

can be hardly ascribed to changes in optical confinement and waveguide properties upon F1/4 substitution owing to the low F8BT and 20F1/4F8BT contents in blends, (15wt%). Thereby, the different optical gain properties upon F1/4 substitution must stem from photophysical changes. “Table 2. Summary of key optical parameters.” PLQE ±10%)

a)

0.45

549

14

95

1.44

a)

0.52

552

13

62

0.94

a)

0.40

552

15

112

1.70

a)

0.34

551

14

138

2.09

a)

0.30

549

12

308

4.67

a)

0.35

552

13

233

3.53

b)

0.32

570

10

726

11.02

b)

0.30

570

11

1150

17.45

PFO:F8BT PFO:20F1/4F8BT PFO:15F1/4F8BT PFO:10F1/4F8BT PFO:F1/4BT PFO:20F5F8BT F8BTc):F1/4BT F8BTc)

a)

( λ@ASE (nm)

ASE Pth ASE FWHM (± Eth 1, nm) ( ± 7%, nJ ( ± 7%, pulse-1) kW/cm2)

Materials

films pumped at 390 nm. b) films pumped at 450 nm. c) material purchased.


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In order to shed more light into the improved gain properties upon F1/4 substitution two controlled experiments were conducted. Firstly, we compared the ASE of F8BT (1.15 µJ pulse-1) with the corresponding of a 15 wt.% blend of F1/4BT in F8BT. The UV absorbance, PL and ASE properties of purchased F8BT film and a 15% (w.t.) F1/4BT:F8BT blend film are shown in Figure S5. The blend showed a reduced ASE threshold of 0.726 µJ pulse-1 (Table2), confirming that the branched butyl substituents indeed play an important role on improving the laser performance. Second, we synthesized a copolymer (namely 20F5F8BT) analogue to 20F1/4F8BT but replacing the branched methyl-butyl substituents of F1/4 by linear alkyl side chains of similar length (F5). ASE measurements performed on this batch blended with PFO (Figure S4) showed much higher ASE thresholds (233 nJ pulse-1) compared to the ASE thresholds of PFO:20F1/4F8BT blends shown in Fig.3 and Table 2 (e. g., 62 nJ pulse-1). Thus, the superior gain properties of PFO:20F1/4F8BT blends seem to be related to the bulky nature of the F1/4 substitution. In summary, it appears clear that steric effects induced by bulky side-chain substituents promote outstanding optical gain properties on these copolymer blends. Quantum chemical calculations on F8 – BT - F8, F1/4 – BT – F8 and F1/4 – BT – F1/4 complexes in the ground state were carried via spin-restricted DFT calculations at the B3LYP/6-31G(d) level of theory using Gaussian 09 package.23,24 , (see Figure S7 in Supporting Information). Results confirm that branch-substitution leads to a slightly less planar backbone conformation in F1/4BT-F8 and F1/4 – BT – F1/4 respect to F8-BT-F8 monomers. The differences in dihedral angles are however of low magnitude (1º - 2º at most) so that a direct correlation between this effect and molecular packing in solid state cannot be established.


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3.5 Excited state dynamics of pristine polymers and blends Femtosecond TA experiments were conducted in pristine polymers and blends. TA spectra at 0.5 and 7 ps pump-probe delay on PFO, F8BT, 20F1/4F8BT, PFO:F8BT and PFO:20F1/4F8BT blends are displayed in Figure 4. Dashed curves are global spectral fits according to a multiband model (for details, see supplementary material). The TA spectrum of PFO (Figure 4a) is dominated by photoinduced absorption (PA) showing a maximum outside of our spectral range (< 1.9 eV (654 nm)) and a shoulder at 2.1 eV (592 nm). Towards the high energy end of our spectral range, we find negative TA, caused by stimulated emission (SE). The crossover from PA to SE shifts from 2.5 eV (497 nm) at 0.5 ps towards 2.6 eV (478 nm) at 7 ps. On the same time scale, the shoulder at 2.1 eV (592 nm) grows in strength and attains a local maximum at 2.2 eV (565 nm). This band has been previously associated to PFO polaron-pairs.25,26 Note that this PA band is located in the F8BT SE spectral region, and as discussed next, it jeopardizes optical gain in PFO:F8BT blends. The TA spectra of pristine F8BT and 20F1/4F8BT (Figure 4b and c, respectively) are rather similar and dominated by an SE band at 2.2 eV (565 nm), PA towards the lower energy end of the detected spectral window, and another band of negative TA towards the high energy end, caused by ground state photobleach (PB). Since the relative strength of these spectral features depends neither on pump-probe delay time or pump intensity, (Figure S15-S16), we discard the presence of polaron-pairs and fit the spectra with a spectral model of a single state, namely singlet excitons, being responsible for PA, SE and PB bands. Note that the minor discrepancies found in the TA spectra at t = 7 ps are caused by a slight red shift of SE. In order to keep the number of fitting parameters low, we did not include spectral shifts into our fitting models.


15


Wavelength (nm)

Wavelength (nm) 478

518

20 (a)

565

478

621

20

0.5 ps 7 ps

10 0

PFO

(b)

0

(d)

0

PFO:F8BT

-10 20

-5

F8BT

-10 5

621

(c)

(e)

3

5

565

518

10

10 ∆Absorbance

3

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“Figure 4. Transient absorption spectra at 0.5 ps (blue) and 7 ps delay (red) of (a) PFO, (b) F8BT, (c) 20F1/4F8BT, (d) PFO:F8BT and (e) PFO:20F1/4F8BT. Pump excitation intensity was 400 nJ. Dotted lines stand for the spectra at same delays obtained with global fit analysis.”

Figure 4d depicts the TA spectra of the PFO:F8BT blend. At 0.5 ps, the spectrum is dominated by PFO excitons and polaron-pairs. A small amount of F8BT excitons are present, (and accounted in our global fit analysis), causing a redshift of the isosbestic point from 2.5 (497 nm) to 2.4 eV (518 nm) in the 0.5 ps spectra. After t = 7 ps, both singlets and polarons in PFO have strongly decayed, and the spectral signatures from F8BT excitons have roughly doubled in intensity, a clear sign of energy transfer from PFO towards F8BT. However at 2.2 eV (565 nm), F8BT SE is overcome by PFO polaron-pair PA, so no gain can be expected in this blend at this pump intensity. In Figure 4e, we show the TA spectra of the PFO:20F1/4F8BT blends. At 0.5 ps, the TA spectra of PFO:20F1/4F8BT and PFO:F8BT are very similar, because prior to any


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transfer or annihilation events, the TA spectra are expected to reflect the composition of the primary photoexcitations, which are akin for both blends as inferred from the similar ground state absorption spectra. However a drastic difference arises at t=7 ps delay. In contrast with PFO:F8BT blends, PFO:20F1/4F8BT blends exhibit gain at 2.2 eV due to larger predominance of 20F1/4F8BT SE over the remaining PFO polaron absorption. This observation is in agreement with the already shown high ASE performance of PFO:20F1/4F8BT respect to PFO:F8BT blends. In order to get further insights into the underlying elementary relaxation pathways we show in Figure 5 time traces at 2.2 eV under different excitation pump intensities. The probe energy is located at the maximum of the SE band in F8BT and 20F1/4F8BT, but contains also contributions from polarons in PFO. In pure PFO (Figure 5a), the TA kinetics at 2.2 eV are governed exclusively by polaron dynamics, as inferred from to the singlet exciton and polaronpair cross-sections obtained with global fit analysis (Figure S14). We resolve a two step polaron decay, with a fast process around 1-2 ps and a slower one on the 10 ps timescale, in agreement with previous findings.27,28 Comparison between the singlet exciton dynamics in pristine F8BT and 20F1/4F8BT films (Figure 5b and c, respectively), confirm that exciton decay is much faster in the former. In fact, all the time traces in F8BT collapse into a same decay curve after only t = 3 ps. This unitary long-term behavior is typical for a bimolecular reaction between identical partners (A+A → B) and was explained in terms of ultrafast exciton annihilation in previous reports.29-31 In 20F1/4F8BT, this behavior occurs only after 50 ps, from which a milder rate of exciton-exciton annihilation is inferred in this copolymer respect to F8BT. Note that this distinct annihilation behavior already constitutes a direct indication that exciton transport is significantly hindered on 20F1/4F8BT respect to F8BT. The reason is that the exciton-exciton annihilation


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rate is given by −ߚ(ܰ௦ (‫))ݐ‬ଶ where ܰ௦ (‫ )ݐ‬and β stand respectively for the time-dependent exciton population density and the exciton-exciton annihilation coefficient, a parameter which is intrinsic to the polymer film and reflects the tendency of excitons to diffuse in the bulk. Assuming similar exciton photogeneration conditions in both polymers (same excitation densities and similar absorbance values at 387 nm), the lower annihilation coefficients inferred in 20F1/4F8BT highlight how minor modifications of the molecular structure can profoundly alter the microstructure with an impact on exciton transport, in agreement with previous reports.32 In Figure 5d we show TA dynamics at 2.2 eV in PFO:F8BT. At the lowest pump intensities, we find a crossover from net PA (caused by PFO polarons) to net gain (by excitons in F8BT) in about 1.5 ps. At the higher intensities, we still find a dynamics on a 1 ps time scale, but net gain is no longer reached. Finally, Figure 5e shows the TA dynamics in PFO:20F1/4F8BT blends. Here, even the time trace at 400 nJ reaches net gain after about 4 ps, whilst the corresponding PFO:F8BT kinetics showed net PA at all times. At 600 nJ, net gain is no longer reached. The time at which crossover occurs is typically a bit longer (2-4 ps) in PFO:20F1/4F8BT than in PFO:F8BT.


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“Figure 5. TA kinetics at 2.2 eV (565 nm) upon excitation with 100 nJ (black), 200 nJ (red), 400 nJ (green) and 600 nJ (blue) of (a) PFO, (b) F8BT, (c) F1/4F8BT, (d) PFO:F8BT, and (e) PFO:F1/4F8BT. Bold lines depict the kinetics predicted with global fit analysis.”

At first sight, it seems a contradiction that PFO:20F1/4F8BT displays net gain at higher intensities even though cross-over occurs on a longer time scale, the latter fact suggesting a


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slightly lower energy transfer rate than in PFO:F8BT. In order to understand this behavior, we performed global fit analysis of all measured TA spectra with a spectral model including singlet excitons in PFO, F8BT and 20F1/4F8BT, as well as PFO polarons. Remarkably, all TA spectra of pristine and blend films were reproduced at all times and pump intensities upon taking into account these four contributions, (selected fits presented in Figure 4 and Figure S14-18 in Supporting Information). From the global fits, we obtained relative time-resolved concentrations for these species. In Figure 6, we show only the dynamics of singlet excitons in PFO, F8BT and 20F1/4F8BT in the various samples as a function of pump intensity. Bimolecular exciton annihilation is most evident in pristine PFO, F8BT, and 20F1/4F8BT (Figure 6(a), (b) and (c) respectively), reflected as a strongly intensity dependent exciton lifetime. At a pump intensity of 600 nJ, the annihilation-limited exciton lifetime is 800 fs, 8 ps, and 50 ps for PFO, F8BT, and 20F1/4F8BT, respectively. These data indicate that 20F1/4F8BT can sustain a higher exciton concentration than F8BT without strong annihilation losses, a positive feature for lasing materials. In blends, it is expected that PFO excitons should decay even faster than in the pristine films, because of the presence of the FRET associated quenching channel.33 This behavior is indeed found at low intensities: at 100 nJ, the PFO exciton lifetime in pristine PFO is around 3 ps, (Figure 6a), while it is 2 ps in PFO:20F1/4F8BT (Figure 6d) and below 1 ps in PFO:F8BT, (Figures 6e). At high intensities however, the opposite behavior is found: at 600 nJ, the PFO exciton lifetime in pristine PFO is 800 fs, while it is 1.5 ps in PFO:F8BT and 2 ps in PFO:20F1/4F8BT. We conclude that the exciton annihilation rate in the PFO phase is significantly modified upon polymer blending: exciton annihilation in PFO is significantly reduced in PFO:F8BT whilst it seems to be completely inhibited in PFO:20F1/4F8BT blends on account of the intensity independence of the PFO exciton lifetimes.


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“Figure 6. Relative time-dependent concentration of (a) singlets in PFO, (b) singlets in F8BT, (c) singlets in 20F1/4F8BT, (d) F8BT singlets in PFO:F8BT, (e) PFO singlets in PFO:F8BT, (f) F8BT singlets in PFO:20F1/4F8BT and (g) PFO singlets in PFO:20F1/4F8BT blends at 100 (red), 200 (green), 400 (blue) and 600 nJ (purple) excitation intensities extracted from global fit analysis.”


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These findings are corroborated by observation of the relative concentration of excitons in the guest phase: in PFO:F8BT, the highest F8BT exciton density is actually reached upon pumping at 400 nJ (Figure 6d), while at 600 nJ, exciton transfer and exciton annihilation occur on the same time scale, reducing the yield of exciton transfer towards F8BT. In contrast, 20F1/4F8BT excitons reach the highest densities upon pumping at 600 nJ (Figure 6f), indicating that even at the highest excitation fluences, exciton transfer is not severely harmed by exciton annihilation. In summary, the analysis of TA spectra has shown that the reason for the lower ASE threshold in PFO:20F1/4F8BT compared to PFO:F8BT is twofold: on one hand, at comparable exciton densities the 20F1/4F8BT phase provides a longer exciton lifetime than the F8BT phase, due to reduced exciton annihilation. On the other hand, the presence of 20F1/4F8BT in the blends disturbs the morphology of PFO more strongly than F8BT, such that in the former, exciton annihilation in the PFO phase is so strongly retarded that it is outperformed by exciton transfer into the polymer guest phase.

4. Conclusion We investigated the optical gain properties of a novel green-yellow copolymer nF1/4F8BT and its blend with PFO in detail. The large ASE threshold values in PFO:F8BT blends arising from competition between F8BT SE and strong PFO polaron-pair PA, can be reduced by 30% and the net gain increased by 50% in copolymers with an F1/4:F8 ratio of 2:3. We ascribe these results to changes in molecular packing induced by the branched F1/4 substitution causing strong interference with exciton transport and consequently partially hindering exciton annihilation. Our


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results highlight the influence of subtle variations in polymer side-chain substitution on exciton transport and the early photophysics of excited states, leading to a pronounced impact in population inversion and the optical gain properties. The development of copolymers bearing a low concentration of monomers with modified side-chains is an attractive approach for the design of efficient polymer lasers.

Supporting Information. Details on Synthetic Protocols, Polymer Structural Characterization, UV-VIS spectra and Global- Fit Analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *[email protected], TF: +34 912998784 *[email protected], TF: +86 25 8586 6008

Author Contributions § Z. Yu and X. Guo are co-first authors with equal contribution to this work.


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ACKNOWLEDGMENT We sincerely thank Dr. Juan Song for valuable discussions. We thank the National Natural Science Foundation of China (Grants 61376023 and 61136003), the National Key Basic Research Program of China (973 Program, 2015CB932203), the Priority Academic Program Development Fund of Jiangsu Higher Education Institutions (PAPD) and the Natural Science Foundation of Nanjing University of Posts and Telecommunications (NUPTSF Grants NY212013, NY213044) for financial support. L.W and Q.Z acknowledge the China Scholarship Council for a PhD studentship (Project number: 201206230083, 201408320176). JC-G thanks the Spanish Ministry of Economy and Competitiveness (MAT2014-57652-C2-1-R and PCIN2015-169-C02-01) and the Madrid regional government under grant S2013/MIT-3007 (MAD2D project) for research funding. Z. Yu and X. Guo contributed equally to this work. REFERENCES (1) Lu, H. H.; Liu, C. Y.; Chang, C. H.; Chen, S. A. Self-Dopant Formation in Poly(9,9-di-noctylfluorene) Via a Dipping Method for Efficient and Stable Pure-Blue Electroluminescence, Adv. Mater. 2007, 19, 2574-2579. (2) Wu, W.; Inbasekaran, M.; Hudack, M.; Welsh, D.; Yu, W.; Cheng, Y.; Wang, C.; Kram, S.; Tacey, M.; Bernius, M. et al. Recent Development of Polyfluorene-Based RGB materials for Light Emitting Diodes, Microelectron. J. 2004, 35, 343-348. (3) Niu, Q.; Zhang, Q.; Xu, W.; Jiang, Y.; Xia, R.; Bradley, D. D. C.; Li, D.; Wen, X.; Solution-Processed Anthracene-Based Molecular Glasses As Stable Blue-Light-Emission Laser Gain Media, Org. Electron. 2015, 18, 95-100.


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Figure 1. “AFM images (5μm×5μm) of pristine films. (A) 20F1/4F8BT, (B) 15F1/4F8BT, (C) 10F1/4F8BT, (D) F1/4BT, (E) F8BT. Images (F-J) are of the PFO blended films correspond to (AE).” ACS Paragon Plus Environment


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Figure 2. “Normalized absorbance and PL spectra of (A) F8BT (filled squares) and F1/4BT (open circles) and (B) 10F1/4F8BT (open triangles), 15F1/4F8BT (open squares), and 20F1/4F8BT (filled squares).” ACS Paragon Plus Environment

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Figure 3. “(a) Absorbance, Normalized PL and ASE spectrum for F8BT (above) and 20F1/4F8BT (below) blend films, (b) ASE narrowing process of a PFO:20F1/4F8BT film as pump energy increase. (c) Edge emitted output intensity and FWHM values (inset), as a function of the pump energy, for PFO:F8BT (filled squares) and PFO:20F1/4F8BT (open triangles) blend films (15 wt.%), (d) gain coefficients as a function of excitation energy ofACS film waveguide for PFO:F8BT (filled squares) and Paragon Plus Environment PFO:20F1/4F8BT (open circles) blend films.”


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Figure 4. “Transient absorption spectra at 0.5 ps (blue) and 7 ps delay (red) of (a) PFO, (b) F8BT, (c) 20F1/4F8BT, (d) PFO:F8BT and (e) PFO:20F1/4F8BT. Pump excitation intensity was 400 nJ. Dotted lines stand for the spectra at same delays obtained with global fit analysis.” ACS Paragon Plus Environment

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Figure 5. “TA kinetics at 2.2 eV (565 nm) upon excitation with 100 nJ (black), 200 nJ (red), 400 nJ (green) and 600 nJ (blue) of (a) PFO, (b) F8BT, (c) F1/4F8BT, (d) PFO:F8BT, and (e) PFO:F1/4F8BT. Bold lines depict the kinetics predicted with global fit analysis.” ACS Paragon Plus Environment


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Figure 6. “Relative time-dependent concentration of (a) singlets in PFO, (b) singlets in F8BT, (c) singlets in 20F1/4F8BT, (d) F8BT singlets in PFO:F8BT, (e) PFO singlets in PFO:F8BT, (f) F8BT singlets in PFO:20F1/4F8BT and (g) PFO singlets in PFO:20F1/4F8BT blends at 100 (red), 200 (green), 400 (blue) and 600 nJ (purple) excitation intensities extracted from global fit analysis.” ACS Paragon Plus Environment

Novel Fluorene-Based Copolymers Containing Branched 2-Methyl

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