New Kind of Hyperbranched Conjugated Polymers Containing Alkyl

Article pubs.acs.org/Macromolecules

New Kind of Hyperbranched Conjugated Polymers Containing AlkylModified 2,4,6-Tris(thiophen-2-yl)-1,3,5-triazine Unit for Enhancing Two-Photon Absorption Pengcheng Zhou,† Cheng Zhong,† Xingguo Chen,*,† Jingui Qin,† Inês Mariz,‡ and Ermelinda Maçôas*,‡ †

Hubei Key Laboratory on Organic and Polymeric Optoelectronic Materials, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China ‡ Centro de Química-Física Molecular and Institute of Nanoscience and Nanotechnology (IN), Complexo I, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal S Supporting Information *

ABSTRACT: Two series of hyperbranched polymers based on alkyl-modified 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine central units linked through fluorene bridges of different lengths have been synthesized via Suzuki coupling. The two series of polymers differ in the position of alkyl substitution within the thienyl group, which can be either closer to the triazine core (P0−P10) or to the fluorene bridge (P0′−P10′). Introduction of a hexyl group at one of the β-positions of the thienyl group improves the solubility of the polymers. A good control over the ratio of triazine and fluorene units allows for the systematic study of the polymer composition effects on the electrochemical, linear, and nonlinear photophysical properties. The nonlinear absorption has been shown to have a noticeable promotion with increasing molar ratio of the triazine core, while the emission quantum yield decreases. The position of alkyl substitution within the thienyl group has a significant effect on the twophoton absorption cross section. Substitution at the β-position of the thienyl group closer to the triazine unit favors nonlinear absorption in the P0−P10 series when compared to the P0′−P10′ series. These polymers perform considerably better as nonlinear absorbers than their unsubstituted analogues.

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cells,8 two-photon absorption materials,9 and organic lightemitting diodes.10 To obtain materials with large two-photon absorption (TPA) cross sections, molecules with strong electronic pull−push effect and good molecular coplanarity are necessary. In addition, it has also been proven that electronic correlation effects on multibranched structures can benefit two-photon absorption.11 Because of its relatively high electron affinity, the 1,3,5-triazine unit,12 has been used as electron acceptor in organic solar cells13 and electron transporting or hole blocking materials in OLEDs.14 In addition, the coplanarity of the triazine ring can be used to couple the covalently linked electron donor groups in an extended conjugated network.15,16 Coupling the fluorene unit as an electron donor to the triazine core was shown to result in linear polymers with improved mass-weighted two-photon absorption cross section when compared with the analogous trisubstituted molecule.17 In our previous work, a series of hyperbranched polymers based on 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine and 9,9-di-n-

INTRODUCTION In the past two decades, the intrinsic spatial localization of multiphoton-induced processes provided by their nonlinear dependence on the light intensity has been successfully explored in many applications, such as multiphoton fluorescence microscopy in the biomedical field,1,2 high-density 3D optical data storage,3 and optical power limiting.4 However, even the lowest order multiphoton excitation process (twophoton) is intrinsically much less efficient than the corresponding one-photon-induced transition. In the design of nonlinear functional materials, optimized two-photon absorbers are used as nonlinear near-infrared antenna and combined with active units (e.g., sensors and biologically active units) to perform specific tasks upon demand.5 Conjugated polymers are interesting materials to explore as two-photon absorbers because their extended π-conjugation can promote strong optical nonlinearities.6 Comparing with the linear and dendritic molecules, hyperbranched polymers exhibit advantages for their processability into large scale devices and simple one-pot synthetic access.7 Hyperbranched polymers also show some unique secondary properties, such as low viscosity, high solubility, and disrupted inter- and intramolecular charge transfer, which are advantages in applications in organic solar © 2014 American Chemical Society

Received: May 4, 2014 Revised: September 1, 2014 Published: September 16, 2014 6679

dx.doi.org/10.1021/ma500914v | Macromolecules 2014, 47, 6679−6686

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Scheme 1. Synthetic Route for the Polymers P0−P10 and P0′−P10′

routes for some key monomers are shown in Scheme S1 of the Supporting Information. Their structural characterization is also given as in the Supporting Information. M1 and M2 were prepared according to the literature method.19,20 Different synthetic methods for M3 and M3′ were applied, in which M3 was obtained via a typical nucleophilic substitution of Grignard reagent with 1,3,5-trichlorotriazine and M3′ was obtained from the cyclotrimerization of thiophene nitrile derivative. All the polymers exhibited good solubility in common organic solvents (such as tetrahydrofuran, chloroform, and toluene). They were primarily purified by precipitation in methanol and further purified by Soxhlet extraction with acetone and hexane to remove monomers. The polymers recovered from Soxhlet extraction with chloroform were precipitated in methanol and dried under vacuum to get the products as a yellow to green solid. Figure 1 shows the 1H NMR spectra of P0−P10 and P0′− P10′ with different percentages of the alkyl-modified 2,4,6tris(thiophen-2-yl)-1,3,5-triazine core. For P0−P10, the chemical shift of proton around 7.0 ppm belongs to the thienyl group. The chemical shift of the proton around 3.5 ppm can be ascribed to −CH2 unit linked directly to the thienyl group, while the chemical shift around 2.1 ppm represents the protons in −CH2 unit linked directly to the fluorenyl group. Similarly, in P0′−P10′ the chemical shift at 8.2, 2.8, and 2.1 ppm can be attributed to the thienyl group, the −CH2 unit linked directly to the thienyl group, and −CH2 unit linked directly to the fluorenyl group, respectively. As expected, Figure 1 also shows that the relative integral area of the protons in the thiophene unit decreases, while the integral area of the protons in fluorenyl group increases as the percentage of the triazine unit in the polymers decreases. The molecular weights and polydispersity index (PDI) were determined by gel permeation chromatography (GPC) analysis with a polystyrene standard

hexylfluorene were synthesized, and their linear and nonlinear absorption properties have been investigated by controlling the molar fraction of the 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine group.18 This study revealed that increasing the triazine molar fraction had a positive effect on the nonlinear absorption probability. However, due to their poor solubility, the molecular weights of the polymers were relatively low and the molar fraction of triazine could not be increased beyond 0.2. Herein, we introduce a hexyl group into the thienyl group to improve the solubility and increase the relative triazine content of the polymers. The electron donor and acceptor components are carefully tuned to study the influence of the triazine unit on the linear and nonlinear properties of the polymers systematically. The different positions of βsubstitution alkyl chain in the thienyl group are shown to have a noticeable effect on the two-photon absorption cross section. The properties of the alkyl-substituted polymers are discussed in comparison with their analogues without alkyl group in the triazine unit.

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RESULTS AND DISCUSSION Synthesis and Characterization. Scheme 1 shows the synthetic route for the polymers via the Suzuki coupling following an “A2 + B2 + C3” approach. In comparison with the analogous hyperbranched polymers previously reported by our group,18 a hexyl group was introduced to β-position of the thienyl group in 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine core so as to improve the solubility of the hyperbranched polymer. By changing the β-substituted position of hexyl group and adjusting the relative ratios of M1, M2, M3, or M3′, two series of hyperbranched polymers (P0−P10 and P0′−P10′) with different percentages of 2,4,6-tris(thiophen-2-yl)-1,3,5triazine core to fluorene group were obtained. The synthetic 6680


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showed similar irreversible oxidation behavior (see Figures S1 and S2). The electrochemical oxidation starts at around 1.40 V vs Ag/AgCl reference electrode. Calibration against an internal standard, ferrocene (Fc) with the ionization (IP) value of −4.8 eV of the Fc/Fc+ redox system below the vacuum level was performed. The formal potential of Fc was measured to be 0.50 eV against Ag electrode. Therefore, the HOMO levels of copolymers can be calculated by the formula EHOMO/eV = −e(Eox + 4.30). The difference between the HOMO level and the absorption edge band gap (Egopt) was used to estimate the LUMO energy level of the polymers. The data are summarized in Table 1. All of them exhibit low LUMO levels due to the existence of the strong electron-acceptor triazine unit. Linear Photophysical Properties. Figure 2 shows the one-photon absorption and emission spectra of the polymers

Figure 1. 1H NMR spectra of P0−P10 (a) and P0′−P10′ (b).

calibration and THF as an eluent, and the corresponding data are collected in Table 1. Because of the introduction of alkyl group, all the polymers show much higher number-average molecular weights around 10 kDa with molecular weight polydispersity (Mw/Mn) between 2.3 and 4.3. In addition, the thermal stability of the polymers was determined by the thermogravimetric analysis (TGA). As can be seen in Table 1, all the polymers show high decomposition temperature (Td), indicating that they have good thermal stability. Electrochemical Properties. Cyclic voltammetry (CV) was used to investigate the electrochemical properties of the polymers and estimate their HOMO and LUMO energy levels. Because of the same donor and acceptor building blocks, they

Figure 2. UV−vis spectra of P0−P10 in THF: (a) absorption (abs) and emission (em) spectra and (b) the ratio of absorption (circles) and emission (triangle, λexc = 370 nm) of the ICT state to that of the fluorene localized state. The line is used only for guiding the eye.

P0−P10 in THF (see Supporting Information for details on the instrumentation). The evolution of the main spectral features with the polymers composition is also displayed. The absorption spectra of P0−P10 in the lower panel of Figure 2a show a band with maximum around 380 nm in the low triazine content polymers (P5, P6, and P10) and a shoulder at 425 nm. The band at 425 nm increases in intensity with the increment of the triazine content and becomes the

Table 1. Molecular Weights (Mn), Polydispersity (PDI), Decomposition Temperature (Td), and Electrochemical Data of P0− P10 and P0′−P10′ polymer

M1:M2:M3(M3′)

Mn (kDa)

PDI

Td (°C)

Egopt a (eV)

HOMOb (eV)

LUMOc (eV)

P0 P0.5 P1 P2 P4 P5 P6 P10 P0′ P1′ P2′ P5′ P6′ P10′

0:1.5:1 0.5:2:1 1:2.5:1 2:3.5:1 4:5.5:1 5:6.5:1 6:7.5:1 10:11.5:1 0:1.5:1 1:2.5:1 2:3.5:1 5:6.5:1 6:7.5:1 10:11.5:1

4.6 15.3 19.9 7.7 8.6 11.5 10.6 14.1 32.1 22.7 6.4 14.1 12.6 38.0

2.29 3.64 4.29 2.28 2.28 3.01 3.67 3.58 2.66 2.66 1.69 1.90 2.02 2.29

371 398 393 397 357 403 395 389 361 403 394 418 424 413

2.56 2.50 2.52 2.56 2.56 2.56 2.61 2.61 2.68 2.63 2.65 2.64 2.68 2.68

−5.70 −5.80 −5.73 −5.68 −5.69 −5.61 −5.61 −5.64 −5.71 −5.69 −5.80 −5.70 −5.73 −5.70

−3.14 −3.30 −3.21 −3.12 −3.13 −3.05 −3.00 −3.03 −3.03 −3.06 −3.15 −3.06 −3.03 −3.02

Egopt was obtained on the onset absorptions of the polymers. bCalculated by formula EHOMO/eV = −e(Eox + (4.8 − E(Fc/Fc+))). cCalculated by formula ELUMO/eV = Egopt + EHOMO. a

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Table 2. Linear and Nonlinear Properties of P0−P10: One-Photon Absorption (λOPA) and Emission (λOPE) Maxima in Solution/Thin Film, Two-Photon Absorption Maxima (λTPA), Emission Quantum Yield (Φ), Linear Extinction Coefficient (ε), and Two-Photon Absorption Cross Section (σ2)a in Solution polymer P0 P0.5 P1 P2 P4 P5 P6 P10

λOPA(THF/film) (nm) 426/429 426/424 426/422 416/407 415/403 389/387 385/386 386/431

λOPE(THF/film) (nm) 475/478 475/478 474/476 472/475 472/474 472/477 418/418 418/418

λTPAb (nm) 760 720 720 710 710 710 710 710

(738) (727) (720) (694) (694) (647) (640) (640)

ϕ (%)

ε (M−1 cm−1)

σ2c (GM)

σ2/Mn (GM/kDa)

54 44 51 48 64 57 64 71

× × × × × × × ×

5100 17635 20700 5900 6300 6600 (7800) 4800 (6000) 3500 (4900)

1.1 1.2 1.1 0.8 0.7 0.6 (0.7) 0.4 (0.6) 0.2(0.3)

3.2 1.2 1.6 4.5 5.0 7.6 7.6 1.2

5

10 106 106 105 105 105 105 106

a Measured by two-photon induced emission using fluoresceiń in water pH = 11 as a standard. bThe maximum λTPA within our observation window, at which σ2 was measured, is shown together with the wavelength of maximum intensity (in parentheses) extracted from the fit of the experimental data with two Gaussian functions. The fitting parameters are given in the Supporting Information. cThe σ2 values obtained from the fitting procedure are also given in parentheses for P5−P10 where there was a significant different between the measured values and those extracted from the fit. The experimental errors associated with the measured σ2 values are of the order of ±15%.

substituted in the P0′−P10′ set is the one closer to the fluorene unit. This small difference has a noticeable effect on the absorption spectra, shown in the lower panel of Figure 3.

strongest band in the higher triazine mole fraction polymers (P1, P0.5, and P0). This trend is more clearly illustrated in Figure 2b showing the ratio of intensity of the two absorption bands as a function of the triazine molar fraction. The band at 380−390 nm is typical of the π−π* transition of π-conjugated polyfluorene chain.21 The band at 425 nm is attributed to the ICT (intramolecular charge transfer) band described as a delocalized transition with overall charge transfer from the polyfluorene arm to the triazine core.18 The one-photon extinction coefficients (Table 2), determined at the wavelength of maximum absorption, scale linearly with the molecular mass being relatively insensitive to the triazine molar fraction. This means that both the fluorene localized state and the ICT states have similar one-photon absorption cross section. Likewise, in the upper panel of Figure 2a, the fluorescence spectra of the low triazine content polymers (P5, P6, and P10) show a vibrational progression at around 415−450 nm due to emission from an excited state localized in the polyfluorene domains.22 Concomitantly, emission from the ICT state is observed at 475 nm with a shoulder at 500 nm. This dual emission phenomenon is well documented as originating from an incomplete quenching of the fluorene emission by Förster energy transfer.18 The resonant energy transfer is supported by the fact that in P10 the absorption of the ICT state at 425 nm is negligible, and yet about 30% of the observed emission originates from this state. The dual emission is explained in terms of the existence of localized fluorene states (defects) that are unable to transfer their energy to the ICT state on account of a presumably unfavorable orientation. Alkyl substitution of the thienyl group improves the solubility of the polymers with respect to the analogues with no alkyl substitution, allowing for the synthesis of polymers with a higher content of triazine (P0−P1).18 As the triazine content increases, the size of the fluorene chain is reduced and so is the presence of such defect states. Thus, the emission at 415−440 nm is negligible in polymers P0−P1. Indeed, the emission of these polymers follows closely that of the linear alternating polymers with only one fluorene connecting two triazines units.17 For the higher triazine mole fraction polymers, only the ICT band is observed and the shape of the emission spectra is wavelength independent. The P0′−P10′ set of polymers differs from P0−P10 in the position of hexyl group substitution of thienyl group in the 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine core. The β-position

Figure 3. UV−vis absorption (abs) and emission (em, λexc = 370 nm) spectra of P0′−P10′ in THF.

Irrespectively of the triazine content, all the polymers in this set show a single broadened absorption band around 390 nm. This band comprises contributions from both fluorene localized states and ICT states. The change in the relative contribution of the states manifests itself as a red-shift of the absorption maxima with increasing triazine mole fraction (lower panel of Figure 3). Concomitantly, the fluorescence peaks assigned to emission localized in the fluorene (415−440 nm) decrease in intensity relatively to the ICT band (475 nm), as previously observed for the P0−P10 set of polymers. The higher energy of the ICT state in the P0′−P10′ set with respect to the P0−P10 set may be caused by an enhanced steric hindrance between the alkyl group of the thienyl unit and the adjacent fluorene unit, which increases the dihedral angle between the two rings and reduces the conjugation between the 2,4,6-tris(thiophen-2-yl)1,3,5-triazine core and the polyfluorene chain. Electronic structure calculations where performed in model monomers in order to confirm this interpretation and also provide more structural information for these two series of polymers. The optimized geometries of 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine with an ethyl group at different β-position of thiophene group are shown in Figure S3, in which the hexyl group is replaced by the ethyl group to simplify the calculations. As can be seen, the 6682


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may result from the nature of hyperbranched polymers; their fluffy structure can prevent strong intermolecular interactions. The band shape in thin films is broader than in solution due to the typical conformational disorder of the solid phase. In the emission spectra of the thin films, the peaks localized in the fluorene are absent (see P10 and P10′ in Figure 4), and the ICT emission appears red-shifted in the high triazine content polymers (see P0 and P0′ in Figure 4). The missing fluorene emission is due to self-absorption and resonant energy transfer between polymer chains in the solid phase. The red-shift of the ICT emission is also associated with resonant energy transfer, and it affects in particular the high triazine content polymers due to the extended planarity and increased rigidity associated with the planar triazine core. Note that this core has been previously used to prepare crystalline organic polymer frameworks, thus forming a well-defined solid arrangement with extended planar sheets connected with each other by intermolecular interactions.23 An efficient energy relaxation between different polymer sites promoted by the close proximity between different polymers sheets leads to a redshift in the emission spectra. The quantum yields for the two sets of polymers (Tables 2 and 3) are plotted as a function of the triazine molar fraction in Figure S4. In general, the emission quantum yields of the polymers containing alkyl-substituted thiophene rings (40− 70%) are lower than those of the analogous unsubstitued polymers reported earlier (60−90%).17 The observed trend is opposite in the substituted versus unsubstituted polymers. For molar fractions up to 0.20, an increasing molar fraction of the triazine core leads to an increase of the quantum yield in the unsubstituted polymers and a decrease in the substituted ones. For higher triazine molar fractions, the quantum yield is quite insensitive to the polymer composition. In the alkyl-substituted polymers, the flexible hexyl group in the thiophene ring seem to contribute to increase the nonradiative decay, thus reducing the emission quantum yield with increasing molar fraction of the 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine unit. It is noteworthy that this effect is quite subtle in the P0′−P10′ set, where the quantum yield changes by less that 8% (from 54% in P1′ to 62% in P6′). In the P0−P10 series the systematic variation of the quantum yield is more significant, going from 74% in P10 to about 50% in the high triazine content polymers (P0−P2). Since P10 and P0 are dominated by either fluorene localized states or ICT states, respectively, we can conclude that the quantum yields of the latter states (ΦICT) are around 50% and those of the former (ΦFL) approach 70%. Judging by the similarity between the quantum yields of the P0−P10 and

coplanarity of ethyl-modified 2,4,6-tris(thiophen-2-yl)-1,3,5triazine core with respect to the adjacent fluorene groups in the P0−P10 set is the same as that in unsubstituted 2,4,6tris(thiophen-2-yl)-1,3,5-triazine core (angle of 26° between the triazine−thienyl rings and the fluorene ring), indicating that introducing a hexyl group at a β-position of thienyl group close to the triazine core does not affect the steric hindrance between the 1,3,5-triazine core and the fluorene chain. Conversely, a large dihedral angle (about 48°) between the thienyl ring and the fluorene ring is observed in the P0′−P10′ set with introduction of the ethyl group close to fluorene unit. This indicates that the coplanarity of the conjugated backbone is severely compromised in the P0′−P10′, resulting in a less favorable geometry for the ICT. As an illustrative example of the solid state effects on the photophysics, Figure 4 shows the absorption and emission

Figure 4. UV−vis absorption and emission spectra of P0, P10 (a) and P0′, P10′ (b) in solution and thin film.

spectra of P0, P10, P0′, and P10′ in thin films in comparison with the spectra in solution. For all the polymers in the two series, the absorption maxima in the thin films exhibit little difference from those in dilute solutions (Tables 2 and 3). This

Table 3. Linear and Nonlinear Properties of P0′−P10′: One-Photon Absorption (λOPA) and Emission (λOPE) Maxima in Solution/Film, Two-Photon Absorption Maxima (λTPA), Emission Quantum Yield (Φ), Linear Extinction Coefficient (ε), and Two-Photon Absorption Cross Section (σ2)a in Solution polymer

λOPA(THF/film) (nm)

λOPE (nm)

λTPAb (nm)

ϕ (%)

P0′ P1′ P2′ P5′ P6′ P10′

396/393 393/391 390/391 387/387 387/388 390/379

472 472 472 473 418 419

687 683 677 669 669 675

57 54 56 61 62 62

ε (M−1 cm−1)

σ2 (GM)

σ2/Mn (GM/kDa)

× × × × × ×

15400 9600 2600 3700 3300 6000

0.48 0.42 0.40 0.26 0.27 0.16

2.6 1.8 5.2 1.4 1.4 3.8

106 106 105 106 106 106

a Measured by two-photon induced emission at 710 nm using fluoresceiń in water at pH = 11 as a standard. bλTPA found by fitting the experimental data with a single Gaussian function. The fitting parameters are given in the Supporting Information. The cross-section values obtained from the fit are equal to those measured at 710 nm, within our experimental error (±15%).

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functions gives TPA maxima at 694 and ∼180 nm full width at half-maximum (fwhm). This band appears broader in P2 and P4 than the corresponding band of P0−P1 (fwhm ∼130 nm), a trend that is consistent with the OPA spectra. The fitting parameters are given as Supporting Information. A free fit of the experimental data of P5 is still giving consistent results (λTPA at 647 nm and fwhm of 311 nm), with the λTPA maxima shifted by 131 nm from twice the λOPA maxima. The nonlinear cross-section data of P6 and P10 could only be fitted by constraining the TPA maxima to 640 nm, corresponding to a blue-shift of 130 nm from twice the OPA maxima. For P0−P4, the differences between the measured TPA maxima and the values obtained from the fit are less than 5%, which is within our experimental error. For P5−P10, the fitting procedure gave 20−40% higher values, shown in parentheses in Table 2. The cross-section data measured for polymer P0′ were fitted without constrains by a single Gaussian function with a maximum at 687 nm and a fwhm of 186 nm (Figure 6). This maximum is blue-shifted with respect to twice the OPA maximum by 105 nm. The rest of the polymers in the set could only be fitted with constrained Gaussian functions by fixing their maxima to values that are also shifted by ∼100 nm from their corresponding doubled OPA maxima. As illustrated in Figure 6, for all the polymers in the P0′−P10′ series, the differences between the measured TPA maxima and the values obtained from the fit are less than 5%, which is within our experimental error. The nonlinear emission spectra (TPE) of the low triazine content polymers excited at 710 nm are significantly different from their linear emission counterpart (OPE) excited at 370 nm. More precisely, the vibronic progression at 400−450 nm due to emission localized in the fluorene is considerably reduced. As illustrated in Figure 7, this effect is of particular

P0′−P10′ (Tables 2 and 3 and Figure S4), the site of alkyl substitution of the thiophene ring does not have a significant effect on the emission quantum yield. Two-Photon Absorption (TPA) Properties. The twophoton absorption cross sections of the hyperbranched polymers were measured by a two-photon excited fluorescence technique in the 710−990 nm region (details of the setup are given as Supporting Information). Figures 5 and 6 show the

Figure 5. TPA (a) and OPA (b) spectra for P0−P10. The straight lines in the TPA spectra are fits of the experimental data with a sum of two Gaussian functions.

Figure 6. TPA (a) and OPA (b) spectra for P0′−P10′. The straight lines in the TPA spectra are fits of the experimental data with one Gaussian function.

one-photon absorption (OPA) and two-photon absorption (TPA) of P0−P10 and P0′−P10′. The corresponding data are summarized in Tables 2 and 3. For most of the polymers, the TPA maxima appear on the edge of the lower limit of our observation window (710 nm). Typically, the TPA absorption maxima of substituted 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine polymers are associated with higher energy states and appear shifted to the blue with respect to twice the OPA wavelength.17,18 Accordingly, the TPA maxima of P0−P1 (760, 720, and 720 nm, respectively) are blue-shifted by 90−130 nm with respect to twice the OPA maxima (852 nm). A weaker band is observed at 850−870 nm, which corresponds to nearly twice the OPA maxima. Assuming a similar blue-shift of the stronger band for P2 and P4, their TPA maxima are expected on the edge of our observation window. Indeed, a free fit of the experimental data collected for P2 and P4 with two Gaussian

Figure 7. OPE and TPE of P10′ (a) and P10 (b) illustrating the difference between linear and nonlinear emission observed for the polymer with higher triazine molar fraction when excited at 710 nm.

relevance for P10 and P10′. The discrepancy between the TPE and OPE spectra is due to the existence of two distinct states in the polymer (fluorene localized states and ICT states), with a relative absorption cross section that depends not only on the excitation energy but also on the nature of the excitation process. Within our observation window, the TPA of the fluorene localized states is quite weak. The two-photon induced transition to an excited state localized in the fluorene bridge is expected to have a maximum well below our observation window, in the 540−625 nm, depending on the length of the 6684


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mild electron donor via σ−π hyperconjugation, strengthening the π-electron density and the ICT effect. In addition, the position of the hexyl group within the thienyl unit has a great impact on the nonlinear absorbance. In the P0′−P10′ series, substitution of the β-position next to the fluorene ring is detrimental for the nonlinear absorption due to the reduced coplanarity of its backbone (Figure S3). This reduces the extent of conjugation within the polymer and the efficiency of the intramolecular charge transfer between the fluorene and the triazine. Thus, in the P0′−P10′ set, the positive effect on the TPA due to the mild electron donor nature of the hexyl group is partially canceled out by the negative effect on the coplanarity, resulting in an overall modest increase of the TPA with respect to the unsubstituted polymers. In the P0− P10 set the hexyl substitution of the thiophene ring has almost no influence on the coplanarity of conjugated backbone (Figure S3), and the prevailing effect is the positive mild electron donor effect. For all the polymers series, the mass-weighted TPA cross section increases with increasing molar fraction of the triazine unit (Figure 8 and last column in Tables 2 and 3). In the hexylsubstituted polymers, the evolution of the TPA cross sections with the triazine molar fraction is best fitted by exponential functions. Increasing the triazine molar fraction beyond 0.2 will not have a strong impact on the TPA cross section. The TPA cross section does not scale linearly with the one-photon absorption cross section (Tables 2 and 3). They both increase with the molecular mass, but while the former is critically dependent on the polymer composition the latter is relatively insensitive to it.

fluorene exciton.24 The bifluorene molecule has a TPA maximum at 540 nm (σ2 = 55 GM) that shifts to longer wavelengths (625 nm) in poly(dihexylfluorene) due to an extension of the conjugation length in the polymer.24,25 The coexistence of these two states complicates estimation of the TPA cross section for polymers where both states have important contributions. In that case eq 1 (see Supporting Information) should be modified as follows: σ2(λ) = x FLσ2,FL(λ) + (1 − x FL)σ2,ICT(λ) ⎛ x FLF2,FL(λ) (1 − x FL)F2,ICT(λ) ⎞ ⎟⎟(Cn)p−1 = ⎜⎜ + φFL φICT ⎝ ⎠ ⎛ φCnσ2(λ) ⎞ ×⎜ ⎟ F2 ⎠ref ⎝

(I)

where the FL subscript is relative to the fluorene localized states and xFL is the relative concentration of this states. For the P0′− P10′ series, the fact that φFL ∼ φICT allows us to revert back to the simplified eq 1. However, in the P0−P10 series the φFL and φICT are significantly different. In this case, the contribution from the fluorene localized states was neglected, and the TPA cross section of ICT states alone was determined. This is an acceptable simplification because in both series the contribution from the fluorene localized states up to 710 nm is quite small (see Figure 7). The largest contribution is expected for P10 (P10′), where emission of these states accounts for about 10% of the total TPE. This contribution is reduced to 3 and 2% for P6 (P6′) and P5 (P5′), respectively. Thus, in the calculation of the TPA cross section of ICT states the quantum yield of the ICT state extracted from the average quantum yield found for the high triazine content polymers was considered (φICT ∼ 50%), and only the overall TPE of the ICT states was considered. Mass-weighted TPA cross sections of the different series of hyperbranched polymers are displayed in Figure 8 as a function of triazine molar fraction. Both series of hexyl-substituted polymers perform considerably better as nonlinear absorbers than the unsubstituted polymers.18 The hexyl group works as a

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CONCLUSIONS Two series of hyperbranched polymers based on alkyl-modified 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine and fluorene units have been synthesized via Suzuki coupling. By introducing the alkyl group at the thienyl group in 2,4,6-tris(thiophen-2-yl)-1,3,5triazine unit, polymers with high moleculer weight and enhanced two-photon absorption (TPA) cross section were obtained. The ratio of triazine and fluorene units was carefully tuned to study the influence of the polymer composition on the linear and nonlinear properties of the polymers. The two series of polymers were compared with their analogues with no alkyl group in 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine unit. Both series of hexyl-substituted polymers were shown to perform considerably better as nonlinear absorbers than the unsubstituted polymers. The position of the hexyl group within the thienyl unit had a great impact on the nonlinear absorbance. Substitution of the β-position closer to the triazine core favors the nonlinear absorption. The position of the alkyl substitution and the control of the donor and acceptor molar fraction in an appropriate range are demonstrated to be crucial to improve the photophysical properties of the polymeric materials.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 8. Mass-weighted TPA cross section for the different series of hyperbranched polymers as a function of triazine molar fraction (black squares: no substitution; red circles: P0−P10 hexyl substituted on the triazine side; blue diamonds: P0′−P10′ hexyl substituted on the fluorene side) showing the effect of hexyl substitution of the thienyl ring. Error bars of ±15%, typically associated with the experimental method, are depicted.

Experimental details of the synthesis of the monomers and polymers, details of the electrochemical properties, the optimized geometries, the quantum yields of the polymers, and parameters used to fit the experimental data. This material is available free of charge via the Internet at http://pubs.acs.org. 6685


Macromolecules

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Article

(25) Tsiminis, G.; Ruseckas, A.; Samuel, I. D W; Turnbull, G. A. Appl. Phys. Lett. 2009, 94 (25), 253304−253304-3.

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (X.C.). *E-mail [email protected] (E.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (Nos. 51173138 and 20972122) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20100141110010) for financial support. E.M. and I.M. acknowledge financial support from Fundaçaõ para a Ciência e a Tecnologia (PTDC/CTM-POL/114367/2009 and the postdoc grant SFRH/BPD/75782/2011).

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