Two-Photon Absorption Property of a Conjugated Polymer: Influence

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J. Phys. Chem. B 2010, 114, 22–27

Two-Photon Absorption Property of a Conjugated Polymer: Influence of Solvent and Concentration on Its Property Hongli Wang,†,‡ Zhen Li,† Pin Shao,† Jingui Qin,*,† and Zhen-li Huang*,§ Department of Chemistry and Hubei Key Laboratory on Organic and Polymeric Optoelectronic Materials, Wuhan UniVersity, Wuhan 430072, China, Department of Chemistry, Central China Normal UniVersity, Wuhan, 430079, China, and Britton Chance Center for Biomedical Photonics, Wuhan National Laboratory for Optoelectronics, Department of Biomedical Engineering, Huazhong UniVersity of Science & Technology, Wuhan 430074, China ReceiVed: July 3, 2009; ReVised Manuscript ReceiVed: NoVember 25, 2009

The Suzuki coupling reaction was utilized to prepare target conjugated polymer P1, whose main chain consists of 3,5-dicyanopyridine moieties as electron acceptors and both triphenylamino groups and fluorene moieties as donors. This polymer exhibits excellent solubility in organic solvents and high thermal stability (5% of weight loss at 398 °C). Detailed spectroscopic studies including absorption and fluorescence emission in solvents of different polarity were also conducted. The influence of the solvent and concentration on its two-photon absorption (TPA) property was studied by a two-photon-induced fluorescence (TPIF) method. It was found that the highest two-photon absorption cross section (13 260 GM at a laser wavelength of 940 nm) appeared in 1,2-dichloroethane, a solvent with a moderate polarity. Meanwhile, with the increase of its concentration in chloroform, its TPA cross section value is reduced, implying that its two-photon absorption process in its chloroform solution is not a pure intrachain process. Introduction Recently, two-photon absorption (TPA) materials are of particular interest due to their potential applications, which range from optical limiting1 to three-dimensional (3D) fluorescence microscopy,2 3D micro- and nanofabrication,3 optical data storage,4 etc. Thanks to the enthusiastic efforts of scientists, the effect of varying the electronic properties of the terminal groups and π-bridges in small linear molecules has been studied in detail to obtain molecules with high two-photon absorption activities.5,6 In this field, conjugated polymers are of particular interest due to their extended delocalization of π-electrons along the polymer backbone, which remains a key point in designing materials with enhanced third-order optical nonlinearity.7 Until now, some large electron-delocalized aromatic units are founded to show impressive TPA properties. For example, a large TPA was observed for poly(9,9-didecyl-2,7-diphenylaminofluorene) with a TPA cross section (δ) of more than 10 000 GM.8 An unprecedented high δ value has been reported for a methylsubstituted ladder-type poly(p-phenylene) (m-LPPP), and its backbone exhibits low flexibility, high planarity, and good effective conjugation.9 The two-photon-induced emission of DNA intercalators can be significantly enhanced by fluorescence resonance energy transfer using cationic conjugated polymers, such as poly(9,9-bis(6N,N,N-triethylammonium)hexylfluorenephenylene) (PFP).10 A kind of interesting hyperbranched poly(aroylarylene)11a and linear poly(aroyltriazole)s (PATAs)11b containing triphenylamine units are reported to exhibit large δ values ascribed to their intramolecular charge transfer. Especially, a kind of organometallic dendrimer is reported in the * To whom correspondence should be addressed. Phone and Fax: +8627-68756757 (J.Q.). Phone: +86-27-87792033 (Z.-l.H.). E-mail: jgqin@ whu.edu.cn (J.Q.); [email protected] (Z.-l.H.). † Wuhan University. ‡ Central China Normal University. § Huazhong University of Science & Technology.

literature that shows σ2 values in the range 3000-4000 GM by a femtosecond open-aperture Z-scan technique.12 Although a large number of polymers were developed and showed TPA effects, in order to assess the potential use of a TPA molecule in applications, the effect of the molecular environment and solvent polarity should be addressed, since they would influence the intramolecular charge transfer and thus the TPA properties. There are some experimental data about the effect of solvent polarity on the TPA cross sections of small linear molecules, for example, different solvent effects on the two-photon absorption of distyrylbenzene chromophores,13a water-soluble [2.2]paracyclophane chromophores,13b ladder-type oligo-p-phenylene-cored chromophores,13c and the fluorene and carbazole derivatives.13d On the other hand, there are only limited experimental reports regarding molecular environment effects on the TPA properties of polymers, for instance, the laser-powerdependent and concentration-dependent TPA measurement of ladder-type para-phenylene-type polymers, to be found, the measurements show that a clear quadratic dependence of the luminescence intensity on the laser power is observed, proving that the excitation process is a two-photon process, and also, a linear behavior of two-photon luminescence on the concentration of the m-LPPP solution is found, which substantiates that there is no influence of the concentration on the two-photon absorption cross section.9 There is another kind of solvent-dependent TPA measurement of polyfluorene derivative; it was found that this polymer possesses a large TPA cross section in THF, while it shows a decreased δ in toluene and DCM.10b The above observations were tentatively attributed to develop new TPA polymers, while the effects of the surrounding medium on TPA property for polymeric TPA materials have not been fully elucidated. Especially, polymers with new motifs and structures (such as linear polymers with donors and acceptors) can multiply the variety of the TPA materials, and it will be helpful to gain more information about the structure-property relationships.

10.1021/jp906264x  2010 American Chemical Society Published on Web 12/16/2009

Two-Photon Absorption Property of a Conjugated Polymer

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SCHEME 1: Synthetic Route to P1

With this aim in mind, here we would like to report the synthesis and TPA property of conjugated linear P1, which contains a 3,5-dicyanopyridine moiety in its backbones (Scheme 1). In our study, P1 has relatively high TPA cross sections with the laser wavelength varied from 720 to 960 nm. The emphasis of this work has been put on elucidating the influence of local molecular environment (concentration or different solvents) on the TPA property of polymer P1 to establish the structureproperty relationships for its effective two-photon absorption behaviors. We found that the two-photon absorption cross sections of this linear polymer in the four solvents are not proportional to the polarity of the corresponding solvents. On the other hand, with the increase of its concentration in chloroform, the δ values of P1 are reduced, implying that the two-photon absorption process in its chloroform solution is not a pure intrachain process. Experimental Section Measurements. 1H NMR was performed on a MERCURYVX300 spectrophotometer with CDCl3 as a solvent. The IR spectra were recorded on a NICOLET 170SKFT-IR spectrophotometer with KBr pellets in the region 4000-400 cm-1. UV-vis absorption spectra were measured on a Shimadzu12106 recording spectrophotometer. The steady-state fluorescence spectra measurement was performed using a Hitachi 5000 spectrofluorimeter. Differential scanning calorimetry (DSC) was performed with a Pyris 1 DSC instrument under nitrogen at a heating rate of 15 °C/min with gas flow of 20 mL/min. Thermogravimetric analyses (TGA) were performed with a Shimadzu-DT40 instrument at a heating rate of 20 °C/min under argon with gas flow of 50 mL/min. Elemental analysis (EA) was performed on a Calo-Erba elemental analyzer (model 106). The molecular weights were determined by gel permeation chromatography with multiangle static light scattering (GPCMALLS). GPC-LLS measurements were performed on a DAWN DSP multiangle laser photometer with a P100 pump (Thermo Separation Products, San Jose, USA) equipped with TSK-GEL G4000HHR and a differential refractive index detector (RI-150) at 25 °C. The eluent was THF at a flow rate of 1.00 mL min-1. All solutions were filtered with a sand filter and a 0.45 µm filter (PTFE, Puradisc 13 mm Syringe Filters, Whattman, England). Astra software was utilized for data acquisition and analysis. The two-photon-absorption fluorescence experiments were determined using a femtosecond multipass Ti:sapphire amplifier (Spectra-Physics, Mai Tai, pulse width 80 fs, repetition frequency 80 MHz, tunable from 720 to 960 nm) as the irradiation source. Single photon photoluminescence and absorption measurements were conducted in chloroform which was anhydrous grade after further purification and kept in the dark before the measurements, and the solution concentration was 1 × 10-6 M. The fluorescence quantum yield was determined using rhodamine B as the reference by the literature method.14 Materials. 3,5-Dicyano-2,4,6-trimethylpyridine,15 4-N-(pbromophenyl)-N-phenylaminobenzaldehyde (2),16 and 3,5-dicyano-2, 4-di(N-(p-bromophenyl)-N,N-diphenylamino)styryl-6-

methylpyridine (3)16 were synthesized following the literature. 2,4,6-Trimethylpyridine (98%) and 2,2′-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis(1,3,2-dioxaborinane) (4) were commercially available from Acros Organics. Bromine, piperidine, and n-propyl alcohol were AR grade. The above chemicals and other chemicals were used as received without further purification. Preparation of the Polymer P1 (Scheme 1). Tetrakis(triphenylphosphine)palladium (Pd(PPh3)4) (0.0081 g, 0.00693 mmol) was added to a mixture of compound 3 (0.20 g, 0.238 mmol), compound 4 (0.1197 g, 0.238 mmol), 10% aqueous solution of K2CO3 (2.4 mL), and THF (30 mL). The mixture was refluxed at 60 °C with vigorous stirring for 4 days under an argon atmosphere. After being cooled down to room temperature, the reaction mixture was poured into a solution of 2% HCl and MeOH. Then, the mixture was filtered and the filtrate was poured into a large volume of methanol. The precipitated material was recovered by filtration through a funnel. The resulting solid material was washed for 24 hours using MeOH to remove oligomers and catalyst residues. The polymer P1 was obtained as a red solid with a yield of 0.17 g (45%) after drying under a vacuum. 1H NMR (CDCl3, δ/ppm): 8.01 (d, J ) 14.5 Hz, ArsH), 7.81 (d, J ) 14.5 Hz, sCHdCHs), 7.65 (m, ArsH), 7.46 (b, sCHdCHs and ArsH), 7.31-6.93 (m, ArsH), 2.88 (m, PysCH3), 1.86 (br, CH2), 1.31-1.27 (br, CH2), 0.88 (s, CH2), 0.77 (s, CH3). 13C NMR (CDCl3, 300 MHz, δ/ppm): 151.75, 146.07, 140.56, 132.84, 132.50, 131.39, 129.66, 128.38, 127.50, 126.40, 125.45, 124.45, 122.22, 121.08, 120.10, 119.63, 116.38, 62.05, 55.29, 40.42, 31.49, 29.70, 27.44, 23.75, 22.58, 14.03. FTIR (KBr pellet, cm-1): 2217(CN), 970(CHdCH). Elem. Anal. Calcd for (C73H65N5)n: C, 86.65; H, 5.90; N, 6.92. Found: C, 86.21; H, 6.20; N, 6.73. Results and Discussion Preparation and Structural Characterizations of P1. The synthetic route of P1 is shown in Scheme 1, and compound 3 was prepared by the Knoevenagel reaction between 3,5-dicyano2,4,6-trimethylpyridine (1) and 4-N-(p-bromophenyl)-N-phenylaminobenzaldehyde (2) with piperidine as the catalyst.16 The polymerization reaction was conducted by the well-known palladium-catalyzed Suzuki coupling reaction between 3 and 4 in the presence of a catalytic amount of Pd(PPh3)4 (3 mol %) and Na2CO3 (10% aqueous solution) in THF at 60 °C for 4 days under an argon atmosphere, yielding the red solid P1. The molar ratio between 3 and 4 was 1:1, which generated a linear polymer P1 with the yield of 45%. P1 is soluble in common solvents such as toluene, THF, CH2Cl2, CHCl3, and DMF, and its films with high optical quality can be easily prepared by spin-coating. It is believed that the existence of non-coplanarity of the triphenylamine moiety and the long hexyl groups in the fluorene moiety result in its good solubility.16a The molecular structure and purity of P1 was confirmed by NMR, IR, and UV-vis spectroscopy. As illustrated in its 1H NMR spectrum in CDCl3, it shows a signal at 2.82 ppm which corresponds to the unreacted methyl group; the peaks corresponding to vinylene protons and aromatic protons all appeared around 7.16-8.31 ppm. The absorption around 2212 and 969 cm-1 in their FTIR

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TABLE 1: Photoproperties of P1 in Different Solventsa λabsmaxb λspfmax(Φ)c λtpfmaxd δmax(GM)e

toluene

THF

chloroform

1,2-dichloroethane

DMF

476 557 (70%) 560 1579

478 595 (51%) 595 1615

480 593 (62%) 598 f

484 605 (47%) 606 13 260

489 621 (42%) 625 9822

a Solvent polarity increases from left to right: toluene (0.099), THF (0.207), chloroform (0.259), 1,2-dichloroethane (0.327), DMF (0.386). Normalized empirical parameters of solvent polarity, ETN, are given in parentheses.17 b λabsmax is the peak wavelength of one-photon absorption spectra. c λspfmax is the peak wavelength of one-photon fluorescence emission spectra, and the Φ values in the brackets are the fluorescence quantum yields. d λtpfmax is the peak wavelength of two-photon absorption upconverted fluorescence. e δmax is the TPA cross section value measured at 940 nm, which is given in GM with fluorescein in H2O (pH ≈ 13) used as the reference, 1 GM (Go¨ppert-Mayer) ) 10-50 cm4 s photon-1. The systematic error for δ measurements was estimated to be 40%. f Not obtained.

Figure 1. UV-vis absorption spectra of P1 in toluene (a), THF (b), chloroform (c), 1,2-dichloroethane (d), and DMF (e).

Figure 2. One-photon photoluminescence spectra of P1 in toluene (a), THF (b), chloroform (c), 1,2-dichloroethane (d), and DMF (e).

spectra is due to the CN stretching and out-of-plane bending motions of trans-vinylene, respectively. The weight-average molecular weight (Mw) of P1 was 10.2 × 104 Da, and its polydispersity was 1.14 (determined by GPC analysis). In order to study its thermal behavior, thermogravimetric analyses (TGA) on P1 were carried out. The TGA experiment of P1 indicated that it had good thermal stability with the 5% weight loss temperature at 398 °C in argon; a weight loss of 30% was subsequently observed between 500 and 700 °C, leaving a residue corresponding to 60% of the original sample weight. Repeated DSC scans with controlled heating and cooling were performed on P1, and revealed a glass transition, Tg, onset at 150 °C and a midpoint at 151 °C. The DSC analysis indicated that P1 was amorphous, in which no melting or crystallization transitions were detected between -40 and 300 °C. Optical Properties of P1. The one-photon absorption and fluorescence emission maxima of P1 in toluene, THF, chloroform, 1,2-dichloroethane, and DMF are summarized in Table 1, and its linear absorption spectra in these solvents are shown in Figure 1. The absorption spectra of P1 in different solvents are all at the same concentration of 10-5 M, which are characterized by two kinds of bands, one near 300-400 nm (divided into two peaks) and the other at around 400-500 nm. The former band can be assigned to the π-π* transition, whereas the latter, the lower-energy band, which is more intense, is largely of charge transfer character.16a The charge transfer occurs between 3,5-dicyanopyridine, a strong electron acceptor, and two donors (triphenylamino group and 9,9-dihexylfluorene group). The positions of the main peaks in the absorption spectra in toluene, THF, and chloroform, λabsmax, appeared at almost the same wavelengths. Compared with the absorption spectrum

of P1 in toluene, the λabsmax of P1 in 1,2-dichloroethane and DMF showed a bathochromic shift of 8 or 13 nm, respectively, with increasing polarity of solvents. A similar bathochromic shift also happened in 1,2-dichloroethane and in DMF, and varied over the range from 484 nm (in 1,2-dichloroethane) to 489 nm (in DMF). One-photon emission spectra of P1 in different solvents, obtained upon excitation at the maximum of the absorption band, show the λspfmax values of emission shift from 557 nm in toluene to 621 nm in DMF (shown in Table 1 and Figure 2) (all fluorescence spectra obtained for this series were independent of the excitation wavelength λexmax in the entire absorption region). With the same trend as observed in its UV-visible absorption, except for in THF and in chloroform, P1 displays a strong emission solvatochromism with increasing solvent polarity. This solvatochromic behavior is typical for compounds with an internal charge transfer upon excitation and has been fully documented for numerous compounds containing donoracceptor monomeric unit structure.18 In addition, the above solvatochromic behavior is accompanied by a decrease of the fluorescence quantum yield (shown in Table 1), Φ, which was determined by exciting at the absorption maximum, λspfmax, for each dye, when the polarity of the solvent increases. The origin of this quenching phenomenon could be attributed to the lowered energies of the intramolecular charge transfer (ICT) states18c but still remains unclear and its amplitude rather unpredictable.19 The relatively good values of Φ for P1 in different solvents, relative to rhodamine B in ethanol,14 make it suitable for using two-photon excited fluorescence to estimate its TPA cross sections. The solid-state luminescence of a spin-cast thin film of P1 is also measured, along with the emission spectra in chloroform. As expected for a neat sample, the emission

Two-Photon Absorption Property of a Conjugated Polymer

Figure 3. Two-photon-absorption-induced fluorescence emission spectra of P1 in toluene (a), THF (b), chloroform (c), 1,2-dichloroethane (d), and DMF (e).

spectrum was red-shifted but maintained the general spectral features of the solution-phase spectrum, which exhibited maxima of emission in film at 610 nm, with no shoulders appearing. Two-Photon Absorption Properties of P1. The δ values of P1 were determined by the two-photon-induced fluorescence (TPIF) technique.5a,20 The experiment was conducted by using a femtosecond Ti:sapphire laser as the irradiation source. The beam of the pump source was focused by an f ) 15 cm lens, and the solution sample in a fluorimeter cuvette (four optically clear windows) was placed at a fixed distance of ∼15.5 cm from the focusing lens. An aqueous solution of fluorescein (C ) 2.79 × 10-4 M, pH ≈ 13) was selected as the reference in our experiment; its δ values at different wavelengths have been determined by Webb et al.,20b,c and it is a common standard in δ measurements.21 In this study, the TPA cross sections of P1 were calculated on the basis of the repeating monomeric units (as shown in Scheme 1) of the polymer, and the solution concentration of the repeating monomeric units of P1 in all different solvents was 1.01 × 10-5 M. The two-photon-induced fluorescence signal was collected at the same detection wavelength for both the reference and sample compounds. The polymer studied was stable under the experimental conditions described here, and no bleaching of the solutions was observed at the end of the measurement. Here, a strong tunable pump beam excites the polymers via TPA and the total integrated fluorescence is monitored as a function of input frequency. The two-photon fluorescence is almost quadratically dependent on the pump irradiance, confirming that the TPA process is responsible for the observed up-converted fluorescence emission. Typical two-photon absorption upconverted fluorescence signatures for P1 in different solvents, such as toluene, THF, chloroform, 1,2-dichloroethane, and DMF, are included in Figure 3. P1 exhibits a major change in emission color on changing of the solvent polarity, the same bathochromic shift as above with the polarity of the solvents increased. As shown in Figure 3, green emission was found for P1 in the low-polarity solvent (toluene), while red emission was observed in a high-polarity solvent, DMF, varied from 560 to 625 nm. Furthermore, compared with its corresponding one-photon fluorescence, some peaks of two-photon excited fluorescence (λtpfmax in toluene, chloroform, and DMF) have an obvious red-shift. From Table 1, one may find that the TPA values for P1 in toluene, THF, 1,2-dichloroethane, and DMF (other than chloroform) at 940 nm are 1579, 1615, 13 260, and 9822 GM,

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Figure 4. Two-photon-induced fluorescence excitation spectra of P1 in toluene (a), THF (b), 1,2-dichloroethane (c), and DMF(d).

respectively. Here, the cross sections (given in GM) are plotted versus the sum of the energies of the two photons involved in the nondegenerate process (this energy has been converted to wavelength for reference). As shown in the TPA spectra (Figure 4), the TPA cross section values at 940 nm for P1 were found to not be monotonically dependent on the polarity of the solvents. This finding is in agreement with recent theoretical predications of TPA cross section values of organic chromophores in solvents.13c,22 Solvents with different polarities may lead to different geometric distortions of monomers, thus resulting in a large variation of TPA cross section values. The TPA cross section values of P1 in 1,2-dichloroethane are found to be the highest, whereas those values in toluene are the lowest. The TPA cross section values for P1 in THF are quite similar to those in toluene; while comparing the TPA cross section values for P1 in DMF with those in 1,2-dichloroform, one may find that relative large TPA cross section values are found in the latter. Thus, the results confirm that the nature of solvents used would play an important part in the outcome of the measured TPA cross section. The possible reason for the above phenomena may be related with the ICT of P1. The magnitudes of TPA cross sections depend on the degree of ICT upon excitation;5b then, since solvent polarity influences the magnitude of ICT, it is expected to change TPA cross section in D-π-D and D-π-A chromophores,13 and P1 with D-π-A monomeric units. However, the reasons for the above solvent effect on TPA polymers are quite complicated. Many factors, such as the described factors as solute-solvent interactions, changes in the monomer geometry, and aggregation,13 can all affect the ICT states. On the other hand, despite the fact that the above TPA values clearly indicate a kind of strong solvent dependency on the nonlinearity of P1, there a relative increase of the TPA cross section as the concentration of polymer is reduced also exists. To examine the nature of the high two-photon absorption cross section of P1, we performed concentration-dependent measurements of the two-photon absorption to investigate the influence of a possible interchain interaction within the P1 solution (Figure 5). We measured concentrations with molarities ranging from 1 × 10-6 to 5 × 10-5 M in chloroform at 800 nm laser wavelength and found an inverse proportion behavior of twophoton luminescence on the concentration of the P1 solution, which substantiates that there is influence of the concentration on its two-photon absorption cross section (Figure 5). In the high concentration of P1, the strong absorption in the first few

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Wang et al. cence properties make P1 a good candidate for two-photonbased applications, such as optical power limiting or two-photonpumped (TPP) upconversion lasers, the aspects of which are currently under investigation. References and Notes

Figure 5. Concentration-dependent measurements of two-photon luminescence in P1 (in chloroform, at 800 nm laser wavelength).

layers of molecules maybe prevents the other absorption process from occurring in the rest of the sample. With decreasing concentration, the number of photons available per molecule increases and the weaker absorption process sets in.8 Therefore, the result may come from the aggregation in its solution, so that, in the P1-chloroform solution, the two-photon absorption process is not a pure intrachain process, not like the paraphenylene-type polymer (m-LPPP) in toluene.8 The above conclusion is brief and incomplete; further work is in process to elucidate the results. Conclusions In conclusion, a linear conjugated polymer P1 has been synthesized by the Suzuki coupling reaction. This polymer has shown not only good solubility in common organic solvents and high thermal stability but also intense emission. Determined in solvents with different polarities (toluene, THF, chloroform, 1,2-dichloroethane, and DMF), P1 displays strong absorption and emission solvatochromism with increasing solvent polarity. P1 also exhibited a very large two-photon absorption cross section, with the maximal value up to 13 260 GM in 1,2dichloroethane at 940 nm laser wavelength under femtosecond excitation. The TPA spectra, determined in the concentration range from 1 × 10-6 to 5 × 10-5 M in chloroform at 800 nm laser wavelength, stated the TPA cross sections measured at low concentration were noticeably larger than the values measured at high concentration. Concentration-dependent measurements show that the two-photon process of P1 in solution is not a purely intrachain process. Analysis of the experimental data shed light on understanding some molecular structure/ nonlinear optical property relationships for this polymer. The results suggest that the TPA property of the conjugated polymer consisting of electron-donating and electron-withdrawing groups can be influenced by the surrounding environment. In the case of P1, the highest two-photon absorption cross section value appears not in DMF or toluene but in 1,2-dichloroethane, a solvent with a moderate polarity, though this D-π-A type polymer exhibits a normal solvatochromism. On the other hand, its TPA cross sections are in inverse proportion to the concentration, implying that its two-photon absorption process is not a pure intrachain process in its solution. The synthetic efficiency, high photochemical and thermal stability, high two-photon absorptivity, and desirable lumines-

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