UV-Irradiation-Enhanced Photoluminescence Emission of

Article pubs.acs.org/Macromolecules

UV-Irradiation-Enhanced Photoluminescence Emission of Polyfluorenes Containing Heterocyclic Benzo[c]cinnoline Moieties Hsin-Chung Wu, Jyh-Chien Chen,* and Hong-Ze Lin Department of Materials Science and Engineering, National Taiwan University of Science and Technology, No. 43 Sec. 4 Keelung Rd., Taipei 10607, Taiwan S Supporting Information *

ABSTRACT: Polyfluorenes (PFO) derivatives, containing different ratios of heterocyclic, electron-withdrawing benzo[c]cinnoline moieties, BF10, BF25, and BF50 were synthesized by Suzuki coupling reaction. Photoluminescence (PL) spectra showed red-shifted emission peaks at around 500 nm for dilute polymer solutions and at around 530 nm for film samples. The PL quantum yields of polymer dilute solutions decreased as the ratios of benzo[c]cinnoline moieties in main chain increased. By UV irradiation (352 nm), the PL intensity of dilute polymer solutions was enhanced. The PL emission enhancement was attributed to the shorter conjugated length and the reduced donor−acceptor interaction caused by oxygencontaining groups that were formed on fluorene moieties by UV irradiation. The enhanced emission could be quenched further by the addition of proton donors such as trifluoroacetic acid. The completely alternating polymer BF50 consisting of fluorene and benzo[c]cinnoline moieties in dilute THF solution showed a linear relationship between UV doses and PL quantum yields after UV irradiation. It exhibited good potential for UV-sensing applications.

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INTRODUCTION The applications of organic conjugated polymers for lightemitting, charge-transporting, and sensing materials have been investigated intensively due to their tunable optoelectronic properties.1−7 Among these conjugated polymers, polyfluorenes with wide-band gaps and blue light emission are wellknown for their high quantum yields, good thermal and chemical stability, and good charge-transporting properties. The 9-position of fluorene moieties can be modified by typical organic reactions to improve the solubility or optoelectronic properties of polyfluorenes.1,2 Oxidation is a critical issue especially for light-emitting conjugated polymers because the oxidative products typically behave as fluorescence quenchers.8−17 The oxidative products can be formed during light irradiation or devices operation in air. For polyfluorenes, the formation of fluorenone containing keto defects has been reported, which is resulted from photoor electrooxidative degradation processes. The emission at 540−560 nm in PL and electroluminescence (EL) spectra was attributed to the formation of fluorenone moieties, leading to low PL quantum yields and poor luminescence purities.15−17 In contrast, oxidation enhanced emission was observed in much rarer cases, such as poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene]s (MEH-PPVs) and sulfur-containing polymers. In addition, their emission wavelengths were affected by the amount of oxidant, m-CPBA (m-chloroperbenzoic acid), and reaction time.18−20 To the best of our knowledge, polyfluorene derivatives never exhibited oxidation enhanced emission. © XXXX American Chemical Society

Benzo[c]cinnoline is a planar, nitrogen-containing heterocyclic compound which is physiologically active and can form complexes with metal such as iron.21−23 It has been reported previously that the introduction of benzo[c]cinnoline into poly(1,3,4-oxadiazole)s and MEH-PPVs could lower the HOMO and LUMO energy levels.24,25 The optical properties of benzo[c]cinnoline and benzo[c]cinnoline-containing MEHPPVs were found to be sensitive to solvents.25−28 In addition, benzo[c]cinnoline-containing MEH-PPV exhibited air-stable nchannel OFET characteristics.25 In this study, we report the synthesis of polyfluorenes (PFO) containing different ratios of heterocyclic, electron-withdrawing benzo[c]cinnoline moieties, BF10, BF25, and BF50, by Suzuki coupling reaction. In addition to fundamental characterization, we demonstrate for the first time that PL intensity of PFO derivatives can be enhanced by UV irradiation. We propose that the enhancement in PL intensity can be attributed to the shorter conjugated length and the obstructed donor−acceptor interaction caused by the formation of oxygen-containing groups on fluorene moieties after UV irradiation. The enhanced emission could be quenched irreversibly by the addition of proton donors, such as trifluoroacetic acid. The relationship between UV doses and enhanced quantum yields is also investigated for UV-sensing application. Received: May 22, 2015 Revised: June 19, 2015

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DOI: 10.1021/acs.macromol.5b01108 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Synthetic Route of Polymers and the Chemical Structure of FBF

Table 1. Molecular Weights and Thermal Properties of Polymers polymer

Mna (g mol−1)

PDIa

yield (%)

BZCb (%) theoretical

BZCc (%) actual

Tdd (°C)

Tge (°C)

char yieldf (wt %)

PFO BF10 BF25 BF50

10 000 13 000 13 000 10 000

2.13 1.72 1.56 1.81

72 90 75 97

0 10 25 50

0 11 30 50

430 430 433 441

58 75 106 202

45 49 57 58

a Soluble part obtained from APC system using THF as solvent and calibrated with polystyrene standards. bTheoretical contents of benzo[c]cinnoline moieties calculated from feed ratios. cActual contents of benzo[c]cinnoline moieties calculated from 1H NMR spectra. dMeasured by TGA at a heating rate of 20 °C min−1 in nitrogen. eMeasured by DSC at a heating rate of 10 °C min−1 in nitrogen. fResidual weight percentage at 800 °C in nitrogen.

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2H), 7.88 (d, J = 7.8 Hz, 2H), 7.86−7.82 (m, 4H), 7.78 (d, J = 6.6 Hz, 2H), 7.42−7.33 (m, 6H), 2.12−2.01 (m, 8H), 1.26−1.00 (m, 40H), 0.79 (t, J = 7.2 Hz, 12H), 0.72 (m, 8H). Synthesis of Polymers. PFO. To a 100 mL, round-bottomed, three-necked flask, equipped with a condenser and a nitrogen inlet were added compound 2 (0.273 g, 0.50 mmol), compound 3 (0.321 g, 0.50 mmol), Pd(PPh3)4 (0.023 g, 0.02 mmol), Cs2CO3 (1.153 g, 3.54 mmol), and DMF/toluene (3/1, 50 mL). The reaction mixture was heated at 120 °C for 48 h under a nitrogen atmosphere. After being cooled to room temperature, the reaction mixture was poured into methanol. The precipitate was collected and washed with water and then methanol. The product was further purified using Soxhlet extraction in acetone to afford 0.278 g (72% yield) of yellow solid. BF10. The polymer was synthesized following the same procedure as PFO with compound 1 (0.034 g, 0.10 mmol), compound 2 (0.219 g, 0.40 mmol), and compound 3 (0.321 g, 0.50 mmol) to afford 0.344 g (90% yield) of yellow solid. BF25. The polymer was synthesized following the same procedure as PFO with compound 1 (0.085 g, 0.25 mmol), compound 2 (0.137 g, 0.25 mmol), and compound 3 (0.321 g, 0.50 mmol) to afford 0.229 g (75% yield) of yellow solid. BF50. The polymer was synthesized following the same procedure as PFO with compound 1 (0.168 g, 0.50 mmol) and compound 3 (0.321 g, 0.50 mmol) to afford 0.275 g (97% yield) of yellow solid.

EXPERIMENTAL SECTION

Materials. 3,8-Dibromobenzo[c]cinnoline (1), benzo[c]cinnoline, 2-bromo-9,9-dioctylfluorene, 2,7-dibromo-9,9-dioctylfluorene (2), 2(9,9-dioctyl-9H-fluoren-2yl)-4,4,5,5-tetramethyl[1,3,2]dioxaborolane, and 2,7-di(9,9-dioctyl-9H-fluoren-2yl)-4,4,5,5-tetramethyl[1,3,2]dioxaborolane (3) were prepared following the literature procedures.24,29 Tetrabutylammonium perchlorate (TBAP) was recrystallized twice in ethyl acetate and dried at 120 °C under reduced pressure overnight prior to use. All of the other reagents and solvents used in this research were purchased from commercial companies and used as received unless specified otherwise. Synthesis of Model Compound FBF. To a 100 mL, roundbottomed, three-necked flask equipped with a condenser and a nitrogen inlet were added 2-(9,9-dioctyl-9H-fluoren-2yl)-4,4,5,5-tetramethyl[1,3,2]dioxaborolane (0.47 g, 0.91 mmol), 3,8-dibromobenzo[c]cinnoline (0.15 g, 0.45 mmol), Pd(PPh3)4 (0.02 g, 0.018 mmol), Cs2CO3 (1.78 g, 5.46 mmol), and DMF/toluene (3/1, 45 mL). The reaction mixture was heated at 120 °C for 48 h under a nitrogen atmosphere. After being cooled to room temperature, the reaction mixture was filtered with Celite. The clear filtrate was collected, washed with brine and water, and dried over anhydrous MgSO4. The solvent was evaporated. The crude solid was purified by column chromatography using ethyl acetate/hexane (1/9) as the eluent to afford 0.24 g (56% yield) of yellow solid. 1H NMR (CDCl3, 600 MHz, δ, ppm): 9.09 (s, 2H), 8.71 (d, J = 8.4 Hz, 2H), 8.31 (d, J = 8.4 Hz, B


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Macromolecules

Figure 1. 1H NMR spectra of polymers and model compound FBF in CDCl3.

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RESULTS AND DISCUSSION Synthesis and Characterization. The monomers 3,8dibromobenzo[c]cinnoline (1), 2,7-dibromo-9,9-dioctylfluorene (2), and 2,7-di(9,9-dioctyl-9H-fluoren-2yl)-4,4,5,5tetramethyl[1,3,2]dioxaborolane (3) were prepared following literature procedures.24,29 As shown in Scheme 1, compounds 1, 2, and 3 were fed by the ratios of 0:1:1, 0.2:0.8:1, 0.5:0.5:1, and 1:0:1. PFO, BF10, BF25, and BF50 were synthesized by the Suzuki coupling reaction. The monomers and polymers were identified by 1H NMR spectra as shown in Figures S1−S7 (Supporting Information). Simultaneously, model compound, FBF, was also synthesized by the similar procedures. Based on monomer feed ratios, BF50 is a complete alternating polymer containing benzo[c]cinnoline (BZC) and fluorene (FL) moieties, while BF10 and BF25 contain both BZC/FL alternating segments and FL block segments. The numberaverage molecular weights (Mn) were from 10 000 to 13 000 g mol−1 measured by the APC system (more information is provided in the Supporting Information) in THF solvent as shown in Table 1. Figure 1 shows the 1H NMR spectra of polymers and model compound FBF. For PFO homopolymer, the protons appeared at 7.87−7.58 ppm (region A in Figure 1). For BF50, containing purely BZC/FL alternating segments, the protons of benzo[c]cinnoline moieties were observed at 9.14, 8.74, and 8.34 ppm (region C), and the peaks at 8.05−7.87 ppm (region B) were assigned to fluorene moieties. For BF10 and BF25, the peaks were observed in all three regions, indicating that these

two polymers contain both BZC/FL alternating segments (regions B and C) and FL block segments (region A). The protons of fluorene moieties in BZC/FL alternating segments appeared in the more deshielded positions than those in FL block segments. It can be attributed to the adjacent electronwithdrawing benzo[c]cinnoline moieties in BZC/FL alternating segments. It can also be observed that the peak intensity of region A decreased as the content of benzo[c]cinnoline moieties increased and vanished completely when no FL block segments can be formed in the case of BF50. These assignments were further confirmed by the 1H NMR (Figure 1) and 1H 1H COSY spectra (Figure S8) of FBF. In addition, the contents of benzo[c]cinnoline moieties in the main chain of BF10, BF25, and BF50 were calculated from their 1H NMR spectra to be 11%, 30%, and 50%, respectively. These values are slightly higher than those calculated from monomer feed ratios. It indicated that in this condition of Suzuki coupling reaction the dibromide of benzo[c]cinnoline was more reactive than the dibromide of fluorene due to the stronger electron-withdrawing nature of benzo[c]cinnoline.30 Figure S9 shows the TGA thermograms of PFO, BF10, BF25, and BF50. The decomposition temperatures at 5% weight loss (Td) were all higher than 430 °C, exhibiting good thermal stability (Table 1). The second-heating DSC curves are shown in the inset of Figure S9. The glass transition temperatures of PFO, BF10, BF25, and BF50 were from 58 to 202 °C. The Tds and Tgs of these polymers both increased with the contents of benzo[c]cinnoline moieties. This means C


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Macromolecules

−5.97 eV, ELUMO = −3.46 eV). This is consistent with our previous observation that the introduction of heterocyclic benzo[c]cinnoline would lower the HOMO and LUMO energy levels of conjugated polymers.24,25 Compared with polyfluorene derivatives containing different heterocyclic acceptors such as quinoxaline, 2,1,3-benzothiadiazole, thieno[3,4-b]-pyrazine, 1,8naphthyridine, and diazafluorene, BF50 showed the lowest HOMO and LUMO energy levels as shown in Figure 2.32−34 It indicated that benzo[c]cinnoline moieties, among these heterocycles mentioned above, exhibited the strongest electron affinity and the best oxidation stability when incorporated into conjugated polymers. Optical Properties. Figure 3 presents the UV−vis and PL spectra of model compound FBF and polymers in THF

that the thermal properties were improved by the introduction of benzo[c]cinnoline moieties. The qualitative solubility was determined by using 0.001 g of polymer in 1 mL of various solvents, including N-methyl-2pyrrolidinone (NMP), dimethylacetamide (DMAc), chloroform (CF), and tetrahydrofuran (THF). All of the polymers were soluble in CF and THF and partially soluble in NMP and DMAc at room temperature. These polymers showed better solubility in nonpolar solvents than in polar solvents due to the nonpolar alkyl side chains of fluorene moieties. Electrochemical Properties. The electrochemical properties were characterized by CV measurements. The cyclic voltammograms of PFO, BF10, BF25, and BF50 in film type are shown in Figure S10. Irreversible anodic and cathodic scans were observed for all polymers in this research. The oxidation and reduction onset potentials of theses polymers were from 1.02 to 1.26 V and from −1.25 to −1.62 V, respectively. The HOMO and LUMO energy levels were calculated from the onset potentials of oxidation and reduction, respectively. The absolute energy level of ferrocene/ferrocenium (Fc/Fc+) is assumed to be 4.8 eV below the vacuum level. The external Fc/ Fc+ redox standard E1/2 was 0.09 V, which was determined versus Ag/Ag+ in acetonitrile by our CV measuring system.31 The results are summarized in Table 2. The calculated HOMO Table 2. Electrochemical Properties of Polymers polymer

Eonsetox a (V)

Eonsetred a (V)

EHOMOb (eV)

ELUMOc (eV)

Egec d (eV)

Egopt e (eV)

PFO BF10 BF25 BF50

1.02 1.14 1.20 1.26

−1.62 −1.47 −1.45 −1.25

−5.73 −5.85 −5.91 −5.97

−3.09 −3.24 −3.26 −3.46

2.64 2.61 2.65 2.51

2.77 2.86 2.78 2.69

Figure 3. UV−vis absorption spectra and PL emission spectra of FBF and polymers in THF (2 × 10−5 M), excited by λmaxUV.

Measured by cyclic voltammetry. EHOMO = −(Eonset + 4.80 − 0.09) (eV). cELUMO = −(Eonsetred + 4.80 − 0.09) (eV). dEgec = IP(Eonsetox) − EA(Eonsetred) (eV). eCalculated by the equation 1240/λonset. a

b

ox

solution. The maximum absorption wavelengths (λmaxUV) and emission peaks (λmaxPL) of polymers in THF solution were at 370−382 nm and 415−500 nm, respectively, as shown in Table 3. The slightly blue-shifted absorption wavelengths could be observed upon increasing the contents of benzo[c]cinnoline moieties in polymers. In the PL spectra of polymers, PFO showed typical vibronic features at around 450 nm as reported previously in the literature.35,36 The PL intensity of emission

and LUMO energy levels of PFO, BF10, BF25, and BF50 were from −5.73 to −5.97 eV and from −3.09 to −3.46 eV, respectively. Among these polymers, BF50 containing the highest content of benzo[c]cinnoline moieties exhibited the most low-lying HOMO and LUMO energy levels (EHOMO =

Figure 2. Energy gap diagram of polyfluorene derivatives with various electron acceptors. D


Article

Macromolecules Table 3. Optical Properties of Polymers film

solutiona (nm) polymer FBF PFO BF10 BF25 BF50

λmax

UV

343 382 382 370 371

λmaxPL

fwhm

ΦPLb (%)

λmaxUV

λonsetUV

λmaxPL

500 417 415 417 500

101 38 38 89 87

0.4 89.8 12.7 0.6 0.3

379 388 397 381

448 434 446 461

438 523 529 533

Measured in THF with a concentration of 2 × 10−5 M. bThese values were estimated using quinine sulfate (dissolved in 1 N H2SO4(aq), assuming ΦPL of 0.546) as a standard. a

peaks at around 500 nm was enhanced as the contents of benzo[c]cinnoline moieties increased. BF50 showed a large Stokes shift of 129 nm. It indicated there was significant difference in the structures of its ground and excited states.37,38 The emission peak of model compound, FBF, was also at 500 nm, which was comparable with that of BF50. The emission peak at 500 nm could be thus assigned as the indicator of PL emission from BZC/FL alternating segments due to the completely alternating structures of FBF and BF50. In addition to emission peaks at 410−460 nm attributed to FL block segments, BF10 and BF25 also exhibited emission shoulder or peak at 500 nm (Figure 3) resulted from their BZC/FL alternating segments. The quantum yields decreased (Table 3) upon increasing electron-withdrawing benzo[c]cinnoline moieties due to the charge transfer interaction. The decrease in quantum yields was also observed for polyfluorene derivatives containing various electron-withdrawing moieties.35,36,39 The optical properties of polymers in solid states were also investigated. The results are shown in Figure S11 and summarized in the Table 3. The absorption edges extended, and the emission peaks (at 530 nm) were also red-shifted, compared with those of solution samples. UV-Irradiation-Enhanced PL Emission. Figure 4 shows the UV−vis and PL spectra of BF50 in THF (2 × 10−5 M) after exposure to UV irradiation. In Figure 4a, the maximum absorption wavelengths (λmaxUV) were blue-shifted after UV irradiation. In Figure 4b, at the beginning, the enhanced PL intensity of the peaks at around 500 nm was observed. After longer irradiation time, the shorter wavelength peaks at around 445 nm were enhanced and became dominant peaks eventually. The PL quantum yields of BF50 in THF (2 × 10−5 M) after different exposure time are summarized in Table 4. The PL quantum yields of BF50 increased from 0.3% to 7.2% after irradiation by UV irradiation (352 nm) for 90 min. Model compound FBF exhibited similar PL behavior after UV irradiation as shown in Figure S12. The UV-irradiationenhanced PL emission could be also observed in the cases of BF10 and BF25 as well (Figure S13b,c). On the other hand, no obvious change in PL intensity could be observed after PFO in THF was subjected to UV irradiation (Figure S13a). The UVirradiation-enhanced PL emission was affected by solvents and polymer concentrations. The enhanced emission was observed when THF, NMP, and toluene were used as solvents, while no perceivable change in PL intensity can be detected in polymer/ CHCl 3 solution (Figure S13d−f). The UV-irradiationenhanced PL emission can also be observed for BF50 in THF at a concentration of 2 × 10−4 M (Figure S14). On the other hand, when polymer concentration reached 2 × 10−3 M or in film state, PL intensity decreased after UV irradiation (Figure S15). It indicated that intermolecular interaction or

Figure 4. (a) UV−vis absorption and (b) PL emission spectra of BF50 in THF at 2 × 10−5 M with different UV irradiation time.

Table 4. PL Properties of BF50 in THF on Exposure to UV Irradiation optical property

0 (min)a

10 (min)

30 (min)

50 (min)

70 (min)

90 (min)

λmaxPL ΦPLb (%)

500 0.3

507 4.0

507 5.1

502 5.0

449 6.6

447 7.2

a

Exposure time to UV lamp (352 nm). bUsing quinine sulfate (dissolved in 1 N H2SO4(aq), assuming a value of ΦPL of 0.546) as the standard.

aggregation effect could prohibit the UV-irradiation-enhanced PL emission. Figure 5 shows the photographs of polymers and model compound FBF under hand-held UV light (365 nm) after UV irradiation (352 nm) for different periods of time. The UV-irradiation-enhanced PL emission of BF25 and BF50 with different exposure time could be observed obviously by the naked eyes. In addition, the process of UV-irradiation-enhanced PL emission was irreversible even if the solutions were treated by visible-light irradiation or heating. In order to investigate the UV-irradiation-enhanced PL emission, the FT-IR spectra of the original and irradiated BF50 and FBF were characterized. To prepare the irradiated samples, BF50 and FBF were dissolved in THF at 2 × 10−5 M. After exposed to UV irradiation for 90 min, the solutions were concentrated. The irradiated products were then dissolved in a E


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Macromolecules

hydroxyl, carbonyl, and peroxide groups can be identified as shown in Figure 6. It indicated that oxidation would occur during UV irradiation. According to the reported oxidation mechanism, hydroxyl and peroxide groups would form during oxidation.40 In addition, the absorption peak at 1726 cm−1 (1723 cm−1 for FBF) is similar to that of fluorenone moieties in polyfluorenes.11,15 Some other carbonyl analogues that were reported when fluorenes were oxidized are shown in Table S1. The peaks assigned to the carbonyl groups connected to sixmembered rings were reported to be at 1645−1675 cm−1.41−43 From Figure 6, the presence of carbonyl groups attached to sixmembered rings cannot be excluded. It has been reported that the fluorescence of conjugated polymers would be quenched due to photobleaching after UV irradiation. Some of these cases involved chain scission, degradation, and oxidation and formed products such as carbonyl groups, endoperoxide, and dioxetanes.8,11,13,40,44−47 In our case, chain scission is excluded because the molecular weights of BF50 barely changed even under UV irradiation up to 60 min as shown in Table S2. For polyfluorenes, it has been proved that keto-containing fluorenone moieties can be observed for both 9-monoalkylated and 9,9-dialkylated polyfluorenes due to photo- or electro-oxidative degradation.15 The emission of green light (540−560 nm) and lower quantum yields were observed in PL spectra or EL spectra due to the formation of fluorenone moieties in polyfluorenes.16,17,35,48 No enhanced PL emission has ever been reported for polyfluorene derivatives by oxidation. However, some sulfur-containing conjugated polymers and MEH-PPVs showed fluorescence enhancement and higher quantum yields due to oxidation when they were treated by oxidants such as m-chloroperbenzoic acid or hydrogen peroxide.18−20 The sulfone, sulfoxide, and epoxide groups that formed by oxidation were identified by IR spectra. According to these literatures, the enhanced fluorescence was attributed to the decreased effective conjugated length, the reduced probability of nonradiative decay, or the increased fluorescence lifetime due to the formation of these groups. In our case, the oxidation of FBF and BF50 after UV irradiation was confirmed by the presence of oxygen-containing groups, which were identified from IR spectra. The more blueshifted absorption peaks were also observed as shown in Figure 3. It indicated that the effective conjugated length was reduced due to the formation of oxygen-containing groups, not UVirradiation induced chain scission. It could be speculated that the decreased effective conjugated length of BF50 and FBF was one of the reasons that resulted in enhanced PL emission. In addition, for BF50 and FBF, fluorene and benzo[c]cinnoline moieties are the electron donors and acceptors, respectively. Conjugated polymers with donor−acceptor structures show decreased quantum yields due to donor−acceptor interaction.35,36,39 However, the formation of oxygen-containing groups such as fluorenones by UV irradiation might decrease the electron-donating nature of fluorene moieties, leading to less interaction with electron-withdrawing benzo[c]cinnoline moieties and thus enhanced PL emission. A control experiment, in which no discernible UV-irradiation-enhanced PL emission was observed when BF50 was UV-irradiated under nitrogen as shown in Figure S16, further confirms our assumption. To the best of our knowledge, this is the first example of polyfluorene derivatives with UV-irradiation-enhanced PL emission. In order to investigate the effect of protonation on the enhanced PL emission, trifluoroacetic acid (TFA, pKa = 0.52), acetic acid (pKa = 4.76), and triethylamine (TEA, pKa = 10.75)

Figure 5. Photograph of polymers and FBF in dilute THF solutions (2 × 10−5 M) under hand-held UV light (365 nm) after UV irradiation (352 nm) for different time.

small amount of CHCl3 and drop-cast on KBr tablets. Solvents were evaporated before FT-IR measurements. Figure 6 shows the FT-IR spectra. Some oxygen-containing groups such as

Figure 6. Infrared spectra of (a) pristine BF50 and UV-irradiated BF50 and (b) pristine FBF and UV-irradiated FBF. F


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Macromolecules

Figure 7. (a) PL emission spectra of UV-irradiated (30 min) BF50 in THF (2 × 10−5 M) with different amounts of TFA. (b) Photograph of pristine BF50 in THF, UV-irradiated BF50 in THF, and TFA-containing (0.88 M) UV-irradiated BF50 in THF under hand-held UV light (365 nm). (c) The proposed model of ON−OFF behavior.

quenching effect can be attributed to the aggregate formation resulted from the intra- or intermolecular interaction (i.e., hydrogen bonding) among the protonated sites.33,34,49,50 The proposed model is shown in Figure 7c. For UV-responsive materials, diacetylenes have been employed as colorimetric UV sensors due to their unique color change by UV irradiation.51 Since polyfluorenes containing benzo[c]cinnoline moieties exhibited UV-irradiation-enhanced PL emission, the relationship between UV doses and quantum yields were further investigated. UV intensity (μW cm−2) was modulated by tuning the potentials and currents of the power supply. The UV intensity was quantified simultaneously by UV light meter. UV doses were calculated from UV intensity and irradiation time. Five BF50 solutions (2

were chosen and added into UV-irradiated (30 min) BF50 in dilute THF solutions (2 × 10−5 M). The PL spectra are shown in Figure 7a, Figure S17, and Figure S18, respectively. The enhanced PL emission can be quenched when proton donors, such as TFA and acetic acid, were added. Especially, TFA with a very low pKa exhibited the best quenching effect. On the other hand, no quenching effect can be observed when TEA (pKa = 10.75) was added. Figure 7b shows PL enhancement ON−OFF behavior of BF50 in THF by UV irradiation and the addition of TFA. It can be concluded that the stronger the proton donor, the more prominent the quenching effect. It was assumed that protonation might occur on the oxygen-containing groups of fluorene moieties formed by UV irradiation. It is likely that hydrogen bonding can also be formed after protonation. The G


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Macromolecules × 10−5 M in THF) were prepared and irradiated for 10 min by UV lamp with different intensity at fixed distances (15.0 cm). The quantum yields were then measured after irradiation. Figure 8a shows their PL spectra. The results are summarized

and UV doses. It indicated that the dilute THF solution of BF50 could be potentially exploited as UV sensors. To the best of our knowledge, this is the first case that polyfluorene derivatives in dilute THF solutions exhibited UV-irradiationenhanced PL emission and showed linear relationship between quantum yields and UV irradiation doses.

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CONCLUSIONS Polyfluorene derivatives containing benzo[c]cinnoline moieties have been successfully synthesized by the Suzuki coupling reaction. These novel polymers in dilute THF solution exhibited UV-irradiation-enhanced PL emission. To the best of our knowledge, this is the first case of polyfluorene derivatives exhibiting UV-irradiation-enhanced PL emission. The oxygen-containing groups formed on fluorene moieties after UV irradiation were responsible for the enhanced emission due to the shorter effective conjugated length and the obstructed donor−acceptor interaction. The enhanced emission could be quenched by the addition of proton donors. A linear relationship between quantum yields and UV doses was also found. It indicated that the dilute THF solutions of polyfluorene derivatives containing benzo[c]cinnoline moieties could be potentially exploited as UV sensors.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1−S18; Tables S1 and S2. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01108.

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AUTHOR INFORMATION

Corresponding Author

*Tel +886-2-27376526; Fax +886-2-27376544; e-mail jcchen@ mail.ntust.edu.tw (J.-C.C.). Notes

The authors declare no competing financial interest.

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Figure 8. (a) PL emission spectra and (b) quantum yields of BF50 in THF solutions (2 × 10−5 M) with different UV doses.

in Table 5. The quantum yields increased from 0.3% without irradiation to 1.5% with a UV dose of 517.2−517.8 mJ cm−2. Figure 8b shows the linear relationship between quantum yields Table 5. Quantum Yields of BF50 in THF solutions with Different UV Dosesa condition

potentialb (V)

currentb (A)

UV intensityc (μW cm−2)

UV dosed (mJ cm−2)

ΦPLe (%)

BF50f A B C D E

12.8 14.9 18.0 21.0 23.9

0.49 0.41 0.52 0.51 0.51

52−53 363−364 654−656 764−766 862−863

31.2−31.8 217.8−218.4 392.4−393.6 458.4−459.6 517.2−517.8

0.3 0.4 1.0 1.2 1.3 1.5

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BF50 in THF with a concentration of 2 × 10−5 M. bPower was modulated by tuning the potentials and the currents. cMeasured by UV light meter. dCalculated from UV intensity and irradiated time (10 min). eThese values were estimated using quinine sulfate (dissolved in 1 N H2SO4(aq), assuming ΦPL= 0.546) as a standard. fNot irradiated. a

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UV-Irradiation-Enhanced Photoluminescence Emission of

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