Influence of Keto Groups on the Optical, Electronic, and

20076

J. Phys. Chem. C 2008, 112, 20076–20087

Influence of Keto Groups on the Optical, Electronic, and Electroluminescent Properties of Random Fluorenone-Containing Poly(fluorenylene-vinylene)s Roberto Grisorio,† Claudia Piliego,‡,§ Marinella Striccoli,| Pinalysa Cosma,⊥ Paola Fini,| Giuseppe Gigli,‡,# Piero Mastrorilli,† Gian Paolo Suranna,*,† and Cosimo Francesco Nobile† Dipartimento di Ingegneria delle Acque e di Chimica (DIAC), Politecnico di Bari, Via Orabona, 4 I-70125 Bari, Italy; National Nanotechnology Laboratories (NNL) of CNR-INFM, Lecce, Italy; Istituto Superiore UniVersitario di Formazione Interdisciplinare (ISUFI)-sezione Nanoscienze, Lecce, Italy; Istituto per i Processi Chimico Fisici (IPCF) del CNR, Sede di Bari, Bari, Italy; Dipartimento di Chimica dell′UniVersita` di Bari, Campus UniVersitario, Bari, Italy; and Dipartimento di Ingegneria dell′InnoVazione, UniVersita` del Salento, Lecce, Italy ReceiVed: August 1, 2008; ReVised Manuscript ReceiVed: October 10, 2008

This study deals with the effect that the incorporation of 2,7-fluorenone into the polymer backbone of a poly(fluorenylene-vinylene) exerts on its photophysical properties. The synthesis of a series of random poly(fluorenylene-vinylene)s containing fluorenone at 10, 5, 3, 1% mol/mol ratio with respect to fluorene units (PFVK1-4) was achieved by the Suzuki-Heck reaction cascade between potassium vinyl trifluoroborate and the equivalent quantity of the suitable feed of the corresponding dibromoaryl comonomers in different ratios. The polymers were characterized by 1H NMR, IR, TGA, DSC, cyclic voltammetry, and UV-vis as well as stationary and time-resolved PL both in solution and in the solid state. In chloroform solution, PFVK1-4 show an emission originating from the PFV backbone while no fluorenone emission could be detected. Moreover, the presence of fluorenone lowers the efficiency quantum yields that inversely follow the fluorenone/fluorene ratio. Conversely, in the solid state, a complete energy transfer occurs and fluorenone acts as the only luminophor even in PFVK1, containing it only in 1% amount respect to fluorene. Consequently, in the solid state PFVK1-4 are all yellow-orange emitters in the solid state. The yellow-orange fluorescence of the obtained polymers in the solid state was compared to the optical behavior of the monodispersed compound 2,7-distyrylfluorenone (DSF). The analysis of the fluorescence decay pathways of the molecules suggests that, differently from DSF, the low-energy emission in the polymers does not originate from a cofacial interaction between fluorenones units. On the contrary, in addition to the fluorenone emission, complex interactions between the fluorenone luminophors and the poly(fluorenylene-vinylene) matrix have to be taken into account for a rationalization of the photophysical properties of these fluorenone-containing polymers in the solid state. Notwithstanding the presence of carbonyl-containing units, usually considered deleterious for the emission properties of poly(arylene-vinylene)s, PFVK1-4 show potential as emitting layers in yelloworange OLEDs, exhibiting luminances up to 1387 cd/m2 and current efficiencies as high as 0.15 cd/A. Introduction The interest devoted to fluorene as building block for the preparation of electroluminescent organic materials has been uninterrupted. For instance, polyfluorenes1 continue to be studied because of their potentially widespread applicability as active layer in light-emitting diodes (OLEDs), due to their efficient solid-state photonic2 and electronic properties.3 The functionalization of the fluorene methylene bridge with alkyl chains permits to achieve sufficiently processable oligomers and polymers and preserves the luminophors from intermolecular interactions in the solid state. On the other hand, numerous fluorene-based materials suffer from poor spectral stability, * Address correspondence to this author at: Department of Water Engineering and of Chemistry (DIAC), Polytechnic of Bari, Bari, Italy. Tel: +390805963603. Fax: +390805963611. E-mail: [email protected]. † Politecnico di Bari. ‡ National Nanotechnology Laboratories (NNL) of CNR-INFM. § Istituto Superiore Universitario di Formazione Interdisciplinare (ISUFI)sezione Nanoscienze. | Istituto per i Processi Chimico Fisici (IPCF) del CNR. ⊥ Dipartimento di Chimica dell′Universita` di Bari. # Universita` del Salento.

consisting in the appearance, both in the photo- and in the electroluminescence spectra, of a longer wavelength emission band.4 Though it has been ascertained beyond any reasonable doubt that the C-9 oxidation to fluorenone is responsible for this drawback, the physical origin of the undesired emission connected to the fluorenone formation is so far under investigation. Among the explanations for the low energy emission, two mainstream interpretations have been given: the low-energy band is supposed to originate (i) from a charge transfer (CT) π-π* transition of keto-defects located onto the conjugated chain (the so-called “on chain defect” hypothesis)5 or (ii) from fluorenonebased excimers.6 None of these two opposing views on the origin of the spectral instability of fluorene based materials has yet received clear-cut proof. Theoretical calculations have suggested that in fluorenone-containing systems the allowed CT π-π* transition lies energetically above the forbidden n-π* transition.7 This would justify the fact that fluorenone-based systems are usually poor emitters in solution, but not the fact that in the solid state fluorenone-containing molecular materials are good luminophors.8,9 Moreover, the observation that the longer wavelength emission crops up also for polyfluorenes containing

10.1021/jp806879c CCC: $40.75  2008 American Chemical Society Published on Web 11/19/2008

Fluorenone-Containing Poly(fluorenylene-vinylene)s an amount of fluorenones as low as 0.1% mol/mol8 casts doubts on the fluorenone excimer hypothesis for the emission in degraded fluorene-based materials, since for these systems the probability of cofacial interaction between two fluorenone units in the polymer film, which would be required for fluorenone excimer formation, is very low. An interplay has also been suggested between keto defects and intermolecular interactions governing the intensity of the low-energy-emission band without necessarily invoking a cofacial arrangement between fluorenone units.10 From the chemical point of view, the mechanisms proposed to explain the fluorenone formation indicate that both the type of the C-9 substitution10,11 and the purity of the fluorene-based monomer12 play a pivotal role in controlling the kinetic of the appearance of the low-energy-emission band. As the undesired longer wavelength fluorescence usually dominates the solidstate emission and is often undetectable in solution, many attempts have been made to preserve the optical properties of the fluorene-based materials by the introduction of bulky substituents onto the polymer backbone, or by the use of crosslinkable moieties, or by spiro-functionalization of the polymer backbone.10b,13 The rationale for this approach, which has been applied to blue-emitting polyfluorenes with some success, was either to limit the energy transfer toward fluorenone or to hamper excimer formation by increasing the steric hindrance around the luminophor. The knowledge generated by the investigations on the presence of fluorenone (deriving from photooxidation or deliberately incorporated) in polyfluorenes has also triggered some recent efforts addressed at exploiting the fluorenone emission for the design of efficient green,8 red,9 or white-emitting materials.14 In the framework of our studies on the origin of the low-energy emission band in polyfluorenes and on the development of a new methodology for the synthesis of random poly(arylene-vinylene)s, we deemed it worthwhile to gain insights into the role exerted by fluorenones on the optical behavior and properties of other polyconjugated backbones, namely the poly(fluorenylene-vinylene). Four new poly(fluorenylene-vinylene-ran-fluorenonylene-vinylene)s were prepared by a Suzuki-Heck reaction cascade15 between 2,7-dibromofluorene and 2,7-dibromofluorenone with an equivalent quantity of potassium vinyl trifluoroborate. The random copolymers have been obtained with fluorenone molar ratios of 10%, 5%, 3%, 1% with respect to fluorene (PFVK1-4, respectively) and their chemicophysical properties have been studied. Since our previous investigations on PFV16 have demonstrated that the fluorene units in a PFV backbone undergo oxidation with greater difficulty than those in polyfluorenes, and intrigued by the solidstate emission properties of PFVK1-4 with respect to those exhibited in solution, we also studied the electroluminescence (EL) of the fluorenone-containing PFVs. Experimental Section All syntheses were carried out under inert nitrogen atmosphere using Schlenk techniques. All solvents were carefully dried and freshly distilled prior to use. All reactants were purchased from Aldrich, Acros, or Fluka and used without further purifications. 2,7-Dibromofluorenone16 and potassium vinyl trifluoroborate16,17 were synthesized according to literature procedures. NMR spectra were recorded at 295 K on a Bruker Avance 400 MHz. UV-vis spectra were recorded on a Kontron Uvikon 942 instrument and fluorescence spectra were obtained on a Varian Cary Eclipse spectrofluorimeter. Quantum yields were calculated by the diluted solution method using 9,10-

J. Phys. Chem. C, Vol. 112, No. 50, 2008 20077 diphenylanthracene (φ ) 0.90 in cyclohexane) as standard. FTIR measurements were recorded on a Bruker Vector 22 spectrophotometer. Gel permeation chromatography (GPC) analyses were carried out on an Agilent Series 1100 instrument equipped with a Pl-gel 5 µm mixed-C column. THF solutions for GPC analysis were eluted at 25 °C at a flow rate of 1.0 mL/min and analyzed using a multiple wave detector. Molecular weights and molecular weight distributions were reported against polystyrene standards. Thermogravimetric analyses (TGA) were carried out with a Perkin-Elmer Pyris TGA 6 thermobalance. Differential scanning calorimetry (DSC) analyses were carried out on a DSC Q200 TA instrument at a scanning rate of 10 °C/min under a nitrogen flow. To investigate the fluorescence decay, the technique of time-correlated single photon counting (TCSPC) was used. The samples (in solution or as film on quartz plate) were excited by a pulsed picosecond (80 ps) laser diode (NanoLED 375 L), operating at a wavelength of 375 nm and a repetition rate of 1 MHz. The obtained emission was focalized on the slit of a double monocromator and detected by a photoncounting photomultiplier module (TBX from Horiba Jobin Yvon). The signal was timed by a time amplitude converter, digitized and sent to a multichannel analyzer. After deconvolution of the instrumental response, the PL decay signal was then analyzed applying a fitting procedure, in order to extract the radiative lifetime. Cyclic voltammetry (CV) measurements were carried out under inert nitrogen atmosphere with an Autolab potentiostat PGSTAT 10 using a three-electrode cell. The CV measurements were carried out in acetonitrile solutions of tetrabutylammonium tetrafluoroborate (0.10 M) at a scan rate of 100 mV/s. An indium-tin oxide (ITO) electrode coated with a thin film of the relevant polymer was used as the working electrode. A Pt wire and a Ag/AgNO3 electrode were utilized as the counter electrode and reference electrode, respectively. All measurements were calibrated against ferrocene, the ionization potential of which is +4.80 eV.18 The evaluation of the HOMO level of the polymers was carried out by measuring the onset of the oxidation potential in the anodic scan. The OLED devices were prepared by spin coating the hole transporting layer, namely poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT-PSS) onto an O2-plasma-treated ITOcoated glass substrate. Subsequently, the active material was spin cast from a chloroform solution (2 mg in 0.3 mL of CHCl3), and finally a calcium (45 nm)/aluminum (150 nm) electrode was deposited by thermal evaporation (10-6 mbar). In the case of the optimized devices, a 10 nm layer of Alq3 was thermally evaporated before Ca/Al deposition. The characterization of the devices was performed at RT in air. 2,7-Dibromo-9,9-bis(2-ethylhexyl)fluorene. Slightly adapting a literature procedure,19 a suspension of 2,7-dibromofluorene (5.00 g, 15.43 mmol) and tetrabutylammonium bromide (1.61 g, 5.00 mmol) in a 50% w/v NaOH aqueous solution (20 mL) was stirred for 15 min at 60 °C. To the red suspension, 2-ethylhexyl bromide (6.18 g, 32.00 mmol) was added, and the mixture kept under stirring overnight. After cooling the solution, the product was extracted with diethyl ether (3 × 75 mL) and the organic layer dried over Na2SO4. After the solvent was removed, the crude product was purified by flash chromatography (SiO2, petroleum ether 40-60 °C) to afford 2,7-dibromo9,9-bis(2-ethylhexyl)fluorene (7.78 g, 92%) as colorless oil. 1 H NMR (400 MHz, CDCl3): δ (ppm) 7.56-7.44 (m, 6H), 2.00-1.90 (m, 4H, C9-CH2-), 1.00-0.64 (m, 22H), 0.59-0.43 (m, 8H, -CH3, C9-CH2-CH-). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) 152.4 (three signals), 139.2 (three signals), 130.1 (three signals), 127.4 (three signals), 121.1, 120.9 (three

20078 J. Phys. Chem. C, Vol. 112, No. 50, 2008

Grisorio et al.

SCHEME 1: Synthesis of the Polymers and of the Model Compound DSF

signals), 55.3, 44.3, 34.6, 33.5 (two signals), 28.0 (two signals), 27.0 (two signals), 22.7, 14.1, 10.3 (two signals). Poly[9,9-bis(2-ethylhexyl)-2,7-fluorenylenevinylene-ran2,7-fluorenonylenevinylene] (PFVK1). A solution of 2,7dibromo-9,9-bis(2-ethylhexyl)fluorene (0.548 g, 1.00 mmol), 2,7-dibromofluorenone (34.0 mg, 0.1 mmol), potassium vinyl trifluoroborate (0.147 g, 1.10 mmol), Pd(AcO)2 (11.0 mg, 0.05 mmol), and P(o-Tol)3 (76.0 mg, 0.25 mmol) in triethylamine (0.35 mL, 0.253 g, 2.50 mmol), DMF (2.5 mL), and toluene (2.5 mL) was refluxed overnight. After cooling to room temperature, the solution was poured into methanol (150 mL). The resulting precipitate was filtered, washed with methanol, and dissolved in the minimum amount of chloroform for precipitation in ethanol to give PFVK1 (0.300 g, 69%) as a red powder. 1 H NMR (400 MHz, CD2Cl2): δ (ppm) 7.96-7.45 (br, m, 6.6H, aromatic protons),7.38-7.17 (br, m, 2.2H, vinyl protons), 2.24-1.94 (br, m, 4H, C9-CH2-), 1.09-0.39 (br, m, 30H). FT-IR (KBr): ν (cm-1) 3025, 2956, 2922, 2855, 1719 (CdO), 1597, 1469, 1378, 1034, 959 (trans HCdCH), 821, 752. Poly[9,9-bis(2-ethylhexyl)-2,7-fluorenylenevinylene-ran2,7-fluorenonylenevinylene] (PFVK2). An analogous synthetic procedure was adopted using 2,7-dibromo-9,9-bis(2-ethylhexyl)fluorene (0.548 g, 1.00 mmol), 2,7-dibromofluorenone (17.0 mg, 0.05 mmol) and potassium vinyl trifluoroborate (0.147 g, 1.10 mmol) to afford PFVK2 (0.287 g, 67%) as a reddish solid. 1 H NMR (400 MHz, CD2Cl2): δ (ppm) 7.96-7.45 (br, m, 6.3H, aromatic protons),7.38-7.17 (br, m, 2.1H, vinyl protons), 2.19-1.97 (br, m, 4H, C9-CH2-), 1.05-0.48 (br, m, 30H). FT-IR (KBr): ν (cm-1) 3025, 2956, 2923, 2855, 1718 (CdO), 1598, 1468, 1377, 1088, 960 (trans HCdCH), 821, 753. Poly[9,9-bis(2-ethylhexyl)-2,7-fluorenylenevinylene-ran2,7-fluorenonylenevinylene] (PFVK3). An analogous synthetic procedure was adopted using 2,7-dibromo-9,9-bis(2-ethylhexyl)fluorene (0.548 g, 1.00 mmol), 2,7-dibromofluorenone (10.1 mg, 0.03 mmol), and potassium vinyl trifluoroborate (0.147 g, 1.10 mmol) to afford PFVK3 (0.318 g, 74%) as an orange solid. 1 H NMR (400 MHz, CD2Cl2): δ (ppm) 7.95-7.46 (br, m, 6.2H, aromatic protons),7.34-7.20 (br, m, 2.1H, vinyl protons), 2.21-1.96 (br, m, 4H, C9-CH2-), 1.06-0.46 (br, m, 30H).

FT-IR (KBr): ν (cm1) 3024, 2956, 2923, 2854, 1718 (CdO), 1598, 1467, 1377, 1088, 959 (trans HCdCH), 821, 752. Poly[9,9-bis(2-ethylhexyl)-2,7-fluorenylenevinylene-ran2,7-fluorenonylenevinylene] (PFVK4). An analogous synthetic procedure was adopted using 2,7-dibromo-9,9-bis(2-ethylhexyl)fluorene (0.548 g, 1.00 mmol), 2,7-dibromofluorenone (3.4 mg, 0.01 mmol), and potassium vinyl trifluoroborate (0.147 g, 1.10 mmol) to afford PFVK4 (0.306 g, 73%) as a yellow-orange solid. 1 H NMR (400 MHz, CD2Cl2): δ (ppm) 7.77-7.37 (br, m, 6.0H, aromatic protons),7.31-7.24 (br, m, 2.0H, vinyl protons), 2.15-1.97 (br, m, 4H, C9-CH2-), 1.03-0.49 (br, m, 30H). FT-IR (KBr): ν (cm1) 3024, 2956, 2923, 2855, 1717 (CdO), 1598, 1468, 1378, 1087, 960 (trans HCdCH), 820, 753. 2,7-Distyrylfluorenone (DSF). A mixture of 2,7-dibromofluorenone (0.338 g, 1.00 mmol), styrene (0.219 g, 2.10 mmol), Pd(OAc)2 (45.0 mg, 0.20 mmol), P(o-Tol)3 (0.122 g, 0.40 mmol), DMF (15 mL), and triethylamine (3 mL) was stirred at 90 °C overnight. After the solution was cooled to room temperature, diethyl ether (50 mL) was added. The resulting solution was washed with water (3 × 20 mL) and then dried over Na2SO4. After the solvent was removed, the crude product was purified by column chromatography (SiO2, CH2Cl2) to afford DSF (0.215 g, 56%) as a red solid. Mp ) 270-271.9 °C. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.90 (s, 2H), 7.62-7.47 (m, 8H), 7.45-7.35 (m, 4H), 7.35-7.29 (m, 2H), 7.22 (d, J ) 16.1 Hz, 2H), 7.13 (d, J ) 16.1 Hz, 2H). 13 C{1H} NMR (100 MHz, CDCl3): δ (ppm) 193.8 (CdO), 143.2, 138.5, 136.8, 135.1, 133.3, 129.8, 128.8, 128.1, 127.4, 126.7, 121.6, 120.4. FT-IR (KBr): ν (cm1) 3022, 1715 (CdO), 1590, 1494, 1468, 1447, 978, 954 (trans HCdCH), 832, 746, 691, 529. Results and Discussion Synthesis and Characterization. The preparation of the polymers, described in Scheme 1, was carried out by the Suzuki-Heck cascade polymerization using Pd(AcO)2/P(o-Tol)3 as catalytic system, triethylamine as base, and DMF/toluene as solvent. The comonomers 2,7-dibromo-9,9-bis(2-ethylhexyl)fluorene and 2,7-dibromofluorenone were combined in 100/

Fluorenone-Containing Poly(fluorenylene-vinylene)s

Figure 1.

J. Phys. Chem. C, Vol. 112, No. 50, 2008 20079

1

H NMR spectra of PFVK1-4 in CD2Cl2. Signals in the range 1.2-1.6 ppm are solvent impurities.

10, 100/5, 100/3, and 100/1 molar ratio and reacted with an equivalent amount of potassium vinyltrifluoroborate. The corresponding polymers PFVK1-4 were obtained in 67-74% yields and are readily soluble in common organic solvents such as chloroform, THF, and toluene. The number-average molecular weights (Mn) of the polymers, estimated by GPC, are in the range of 7000-8000 Da with polydispersities (PDI) in the range between 1.6 and 2.0. The copolymer composition was ascertained by comparison of the 1H NMR integrals of selected signals (vide infra). With respect to our previous reports,15b,16 the replacement of dioxane with DMF/toluene led to lower molecular weights, but the polymers obtained were characterized by an almost complete trans configuration of the vinylene units (no 1H NMR signals attributable to cis-disubstituted double bonds or 1,1-diarylvinylenes, which can form after the Heck reaction step of the polymerization,15b were detected in the range 6.5-5.5 ppm). On the other hand, weak signals attributable to the hydrogen atoms of the terminal vinyl groups could be identified in the range of 7.0-5.0 ppm, due to the relatively low molecular weights of PFVK1-4 (Figure 1). The incorporation of the fluorenone units into the polymer backbone was ascertained by the appearance of some peculiar signals both in the 1H NMR and FT-IR spectra (Figures 1 and 2). A signal at δ ∼7.90, attributable to the C-1 hydrogen atoms of fluorenone, appears in the 1H NMR spectra and allows the quantification of fluorenone in our PFV. However, this signal is not observed in PFVK4, in which the fluorenone amount is too low for the 1H NMR detection. In FT-IR spectra, a peak at 1717-1718 cm-1, associated to the vibration stretching of the CdO groups in fluorenones, was recorded for all the polymers. In the case of PFVK1-3 the copolymer composition was ascertained by the comparison of the signal integrals of the methylene hydrogen atoms linked to the CR of the fluorene alkyl

Figure 2. FT-IR spectra of PFVK1-4. The arrows point to the CdO stretching band and the out-of-plane bending of trans-vinylenes.

chains (falling at ca. 2.00 ppm) with those of the aromatic and vinyl hydrogen atoms. The configuration of the vinylene functionality was checked by the appearance of medium FTIR bands attributable to the out-of-plane C-H bending of the trans-vinylene moiety at 959-960 cm-1 (Figure 2). The corresponding 1H NMR signals fall in the region of 7.40-7.20 ppm. The thermal stability of PFVK1-4 was investigated by thermogravimetric analysis (TGA). The weight percent losses of the polymers as a function of the temperature are reported in Figure 3. Increasing the fluorenone amount in the copolymer enhances the thermal stability, as indicated by the observed raise in decomposition temperature at 5% weight loss (Table 1). The thermal characterization of PFVK1-4 was carried out by differential scanning calorimetry (DSC). All samples, subjected


Figure 3. TGA plots of PFVK1-4.

to a heating scan to 250 °C, a cooling scan to room temperature and a reheating scan (all carried out at 10 °C/min under inert atmosphere), showed glass transition temperatures (Tg) better evident in the reheating scan. The midpoint of these events (Table 1) approximately follow the decrease of the fluorenone concentration in the polymers. The lower amount of C-9 alkyl chains going from PFVK4 to PFVK1, reducing the polymer flexibility, accounts for the observed thermal behavior. Optical Features. The absorption and emission behavior of the obtained polymers was studied both in solution and in the solid state. The UV-vis spectra recorded in chloroform are shown in Figure 4A. The absorption profiles of PFVK1-3 are practically superimposable and exhibit an absorption maximum at 421 nm with a shoulder at longer wavelengths, while PFVK4 shows an absorption maximum at 415 nm with a less pronounced low-energy shoulder and is comparable with the absorption spectrum of a PFV.15b,16,20 The presence of fluorenone units in a PFV backbone exerts an influence on the electronic structure of the corresponding polymer, only if present in molar amount higher than 1% with respect to the 9,9-dialkylfluorene units. Furthermore, the introduction of fluorenones in the PFV backbone brings about an additional absorption band, appearing as a tail in the range 500-550 nm, which can be ascribed to the n-π* transition of the fluorenone groups, more evident for PFVK1. Concerning the photoluminescence (PL) behavior, chloroform solutions of ∼0.1 absorbance of the polymers were analyzed by exciting at their absorption maxima. The normalized PL spectra of PFVK1-4 (Figure 4A) show identical emission profiles with maxima at 461 nm together with a vibronic replica at 490 nm, comparable with the solution behavior of PFV.15b,16,20 An important aspect is related to the PL quantum yields of the polymers which are strongly influenced by the amount of fluorenone units. In fact, the quantum yields decrease from 0.85 to 0.22 as the molar amount of fluorenone randomly incorporated in the polymer increases from 1% to 10%. This optical behavior in solution can be explained by the fact that in chloroform the fluorenone contained in PFVK1-4 (acting as acceptors in an intramolecular energy transfer process from the higher band gap segments, constituted by the fluorenylenevinylene units) possess a barely detectable fluorescence.21 The optical features of PFVK1-4 were also analyzed in the solid state, casting films of the polymers on quartz from 5 mg/ mL chloroform solutions (Figure 4B). The absorption maxima of the polymers are slightly blue-shifted with respect to those recorded in solution (414 nm for PFVK1-3 and 404 nm for PFVK4), while the absorption profiles in CHCl3 and in the solid state are similar. Concerning the fluorescence behavior, a strong

Grisorio et al. yellow-orange emission was recorded from a thin film of these polymers with maxima ranging from 581 to 602 nm following the increase in the fluorenone amount. In these spectra, the complete quenching of the blue-green emission was observed by exciting the samples at their absorption maxima. This result is very surprising considering that, even if fluorenone amounts at the detection limit of common polymer characterization techniques are incorporated in the polymer backbone, the corresponding PFV tends to act as a full yellow-orange emitter, while PFV not containing fluorenones is a green emitter (λem ) 504 nm) in the solid state. Intrigued by these results, we deemed it worthwhile to embark on a deeper investigation on the origin of this keto-induced emission band, starting with an analysis and comparison of the optical behavior of PFVK1 in different solvents (CHCl3, CH2Cl2, o-dichlorobenzene, THF, toluene, and CCl4). The absorption spectra of the polymer reveal the independence of the ground state of PFVK1 from solvent effects. On the other hand, the emission profile of the polymer is distinctively influenced by the solvent, as a longer wavelength band in the yellow region appears in THF, toluene and CCl4 (Figure 5). This band is barely detectable in o-dichlorobenzene (ODCB) and, as mentioned before, is completely absent in chloroform and methylene chloride. The optical behavior described can be, in a first interpretation, attributed to the quenching of the fluorescence by the dipole-dipole interactions of the fluorenone units with the polar solvents.5c This optical investigation permitted to identify the solvents in which the longer wavelength band was more pronounced. However, the fluorenones emission in PFVK1-4 is very low, even in less polar solvents as CCl4 or toluene, if one also takes into account that the energy transfer toward fluorenones causes a drop in quantum yield in CHCl3 from PFVK4 (0.85) to PFVK1 (0.22). We also analyzed the fluorescence of PFVK2-4 in toluene, the solvent in which the yellow fluorescence is more exalted. We were able to observe a dependence of the emission intensity at 550 nm from the amount of fluorenone in the polymer (Figure 6). Surprisingly, the yellow emission does not crop up in the case of toluene solutions of PFVK3-4. As in the solid state the emission features of all the polymers are completely dominated by the longer wavelength band, these results highlight the key role of interchain interactions in promoting the intermolecular energy transfer suppressing the blue-green emission and favoring the appearance of the low-energy band in PFVK1-4. In order to gain insight into the solvatochromic behavior of PFVK1, its optical behavior was compared to that exhibited by a monodispersed compound, the structure of which was conceived in order to build a conjugated structure devoid of orthogonal alkyl chain in order to favor intermolecular interactions in the solid state, and embodying at the same time the fluorenone moiety. The structure chosen was 2,7-distyrylfluorenone (DSF). The compound was synthesized by a Heck reaction between 2,7-dibromofluorenone and styrene in 56% yield, and characterized by 1H NMR, 13C{1H} NMR, and FTIR spectroscopy. The normalized absorption and PL spectra of DSF in the above-mentioned solvents are shown in Figure 7, A and B. The UV-vis spectra exhibited a maximum at ∼365 nm associated to the π-π* transition of the π-conjugated molecular backbone, together with a weak absorption band located at ∼470 nm ascribable to the presence of the carbonyl moiety and assigned to the low-strength oscillator n-π* transition. This absorption band is red-shifted with respect to that of fluorenone-containing oligofluorenes22 as consequence



TABLE 1: Average Molecular Weights, Thermal and Electrochemical Properties of PFVK1-4 average molecular weights thermal properties PFVK1 PFVK2 PFVK3 PFVK4

a

electrochemical properties b

c

Mn (Da)

D

Td (°C)

Tg (°C)

Eox (V)

HOMO (eV)

Egap (eV)d

LUMO (eV)

7800 7000 7500 8000

1.3 1.3 1.5 1.6

397 352 368 307

95.7 97.1 82.0 75.1

1.09 1.02 1.00 0.90

-5.45 -5.38 -5.36 -5.26

2.19 2.19 2.19 2.27

-3.26 -3.19 -3.17 -2.99

a Temperature at 5% weight loss. b Midpoint of the thermal event. c Onset of the oxidation potential against the Ag/Ag+ couple. d Estimated by the onset of the absorption curve in the solid state.

Figure 5. Normalized PL spectra of PFVK1 in different solvents (λexc ) 420 nm).

Figure 4. Normalized UV-vis and PL spectra of PFVK1-4 in chloroform (A) and in solid state (B) (λexc ) 420 nm). In (A) are reported the relative PL quantum yields (Φ) of PFVK1-4 in CHCl3 solution.

of the increase in conjugation extension due to presence of double bonds in the 2,7-positions of fluorenone. On the contrary, a remarkable solvent effect was observed in the PL behavior of DSF (Figure 7B). Its emission is composed by two bands: the first located at ∼400 nm due to the intrachain singlet excitons of the molecule backbone, and the second band can be attributed either to the CT π-π* transition deriving from the presence of the carbonyl group or to the formation of fluorenone excimers in solution. The peculiarity of this fluorescence band is the dependence of its intensity from the solvent: for example, in carbon tetrachloride, it is ∼40 times more intense than that recorded in chloroform or methylene chloride. In this case, the reasons of this behavior cannot be merely ascribed to the polarity of the solvents, as in THF, possessing a relatively high dielectric constant, the intensity of the low-energy band is rather high. Moreover, a strong influence of the solvent on the maximum λem was observed, passing from 560 nm in CCl4 to 610 nm in CHCl3. In all the tested solvents, the emission quantum yield of DSF is low and does not exceed 18% (in CCl4).

Figure 6. Normalized PL spectra of PFVK1-4 in toluene (λexc ) 420 nm).

In order to investigate the role of aggregation on the intensity of the longer wavelength band, we carried out PL measurements on toluene solutions of DSF (∼0.1 absorbance) at different temperatures. Passing from 20 to 100 °C, an increase of the longer wavelength band intensity with respect to the blue emission was observed as well as a slight blue shift of the emission maxima (Figure 8). The process was completely reversed as the temperature was lowered to 20 °C. The same behavior was also observed in ODCB, in the same temperature range. An opposite effect should be expected if aggregate formation was the sole responsible for the fluorescence: the cleavage of possible DSF-based aggregates by the raise in temperature should have caused a lowering of the emission intensity at ∼560 nm. The increment of the longer wavelength band by raising the temperature can be interpreted by assuming that a fraction of excited molecules release part of their energy by collision with other molecules at their ground state, which, in turn, can emit at longer wavelengths. This collisional energy


Grisorio et al.

Figure 9. Normalized solid-state PL spectra of neat DSF and of a 1%w blend of DSF in PVK (λexc ) 360 nm).

Figure 7. Normalized UV-vis (A) and PL spectra (B) of DSF in different solvents (λexc ) 360 nm).

Figure 8. Normalized PL spectra of DSF in toluene as function of temperature. The arrow indicates the blue shift of the emission maximum (λexc ) 360 nm).

transfer is reasonably enhanced at higher temperature.5b On the other hand, the blue shift of the emission maximum (∼15 nm passing from 20 to 100 °C) indicates more complicated explanations for the origin of the longer wavelength band in DSF. A drop-casting from DSF chloroform solution permitted to obtain a polycrystalline film exhibiting a broad emission remarkably red-shifted with respect to that observed in toluene, THF, or CCl4 solution (Figure 9). This red shift is attributed to the establishment of intermolecular interactions in the neat film of DSF.23 This assumption was corroborated by the optical behavior of DSF dispersed in poly(vinylcarbazole) (PVK, 1%w),

Figure 10. Normalized solid-state PL spectra of PFV and blends of PFV with DSF and PFVK1 (λex ) 400 nm).

shown in Figure 9. This spectrum shows only a broad emission centered at ∼560 nm, considerably blue-shifted with respect to that of the neat film emission, supporting the hypothesis that intermolecular interactions are involved in the fluorescence of pure DSF. In order to shed more light on the optical behavior of fluorenone units, we decided to reproduce the solid-state emission behavior of PFVK4 (containing 1% mol/mol of fluorenone units) by investigating the fluorescence of two blends containing comparable molar amounts of fluorenone, namely, a blend of poly(fluorenylene-vinylene) (PFV,16 Scheme 1) with (i) 1%w of DSF or (ii) 10%w of PFVK1. In the PL spectrum of a PFV/DSF (1%w) blend, the presence of the DSF fluorescence only causes a change in the emission profile (Figure 10) due to the appearance of the more pronounced vibronic peak at ∼550 nm (the fluorescence wavelength of DSF in PVK). The PL spectrum obtained for the PFV/DSF(1%w) blend is completely different from that obtained for PFVK4 in the solid state, showing only a pronounced emission at 577 nm. On the other hand, the optical behavior of PFV/PFVK1(10%w) (containing as well ∼1% mol/mol of fluorenone units in the blend) strictly resembles the emission spectrum of PFVK4 in the solid state, in terms of both profiles and emission wavelength (576 nm). The difference in optical behavior of the two blends can be explained by admitting that PFV, by its alkyl chains, tends to keep the DSF molecules isolated and sufficiently distant from the polymer backbone thus making the energy transfer more difficult. Consequently the DSF fluorescence results barely detectable. On the contrary, in the PFV/PFVK1(10%w) blend



TABLE 2: Results (τi, Bi, χ2) Obtained from the Analysis of Fluorescence Decays of DSF and PFVK1-2 at Room Temperature in Different Solvents DSF

PFVK1 PFVK2 a

solvent

λem (nm)a

τ1 (ns)b

τ2 (ns)b

τ3 (ns)b

B1 (%)c

B2 (%)c

B3 (%)c

χ2

CHCl3 CH2Cl2 ODCB THF CCl4 toluene toluene toluene

604 589 577 566 565 564 559 559

0.47 1.04 1.22 2.37 2.47 2.43 0.82 0.64

6.8 6.8 2.18 3.34 5.9 5.5 2.6 2.47

44.2 41.7

98.7 96.1 85.9 64.6 91.9 90.9 19.6 34.8

1.3 3.9 14.1 35.4 8.1 9.1 57.5 49.6

22.9 15.6

1.217 1.213 1.104 1.057 1.155 1.208 1.070 1.152

Monitored fluorescence wavelength. b Fluorescence lifetimes extracted by multiexponential fits. c Coefficients of the multiexponential fits.

Figure 11. Fluorescence decays of the longer wavelength emission band of DSF in different solvents (λexc ) 375 nm).

Figure 12. Fluorescence decay of the low energy emission band of DSF and PFVK1-4 in the solid state (λexc ) 375 nm).

the two types of polymer backbones are assumed to be better entangled and consequently, the fluorenone moieties even at lower amounts, can easily accept energy from the PFV host. This conclusion is substantiated by the fact that the PFV/ DSF(1%w) blend was irradiated at 400 nm, ensuring that DSF can only be excited by an energy transfer from the PFV host, since it does not absorb at this wavelength. However, this picture should also take into account that, beyond energy transfer effects, a different local environment around the fluorenones can strongly influence the intrinsic emission efficiency of DSF and PFVK1 in blends with PFV. Time-Resolved Fluorescence Decays. Needing more insights into the mechanisms governing the emission of the fluorenone containing PFVs, we analyzed the fluorescence decay profiles of the longer wavelength band of DSF and PFVK1-4 both in solution and in the solid state. The emission decays of the longer wavelength band of DSF in solution are best fitted by a biexponential function, the time constant of which strictly depended from the solvent (Figure 11 and Table 2). The fluorescence decays of the yellow-orange emission are significantly slower in solvents which enhanced its intensity (vide supra). The biexponential behavior of the decay is characterized by a faster component (ranging from 470 ps in CHCl3 to 2.47 ns in CCl4) and a slower component in the nanosecond range. The contribution of the faster component is always predominant (ranging from 98.7% in CHCl3 to 64.6% in THF) and can be attributed to the CT π-π* transition deriving from the presence of the carbonyl group in the molecule. The dramatic reduction of the lifetime in CHCl3, CH2Cl2, and ODCB should consequently be ascribed to the interactions (either dipole-dipole5c or hydrogen bond type24) between solvent molecules and the carbonyl moiety of DSF, enhancing the rate of the nonradiative decay pathways. The known charge redistribution around the carbonyl group in the excited state7 of the fluorenone-containing

molecules corroborates the observation and also explains the red shift of the emission in these solvents. The presence of the slower component of the fluorescence decay, with a contribution that reaches its maximum value (35.4%) in THF, indicates the complex nature of the optical behavior of DSF in relation with the environment, which might also imply interactions between DSF molecules, notwithstanding the dilution of the sample. The fluorescence decay of the longer wavelength band for the polymers in solution could be recorded for PFVK1-2 at 559 nm in toluene, as reported in Table 2. The analysis of the fitting parameters reveal a dependence of the decay profile from the amount of fluorenone in the polymers. The fluorescence decay could be best fitted using three-exponential functions. The shortest time constant (0.82 and 0.64 ns for PFVK1-2, respectively) can be ascribed to the residual intrachain π-π* singlet emission25 of the PFV backbone and its contribution increases passing from PFVK1 to PFVK2. The intermediate decay constants (2.6 and 2.47 ns for PFVK1-2, respectively) can be attributed to the CT π-π* transition of fluorenones and are very similar to those found for DSF in toluene, CCl4 and THF. The longer decay time (∼40 ns), absent in DSF, can be attributed to other processes involving the keto groups which could be tentatively ascribed to an intramolecular photoinduced hole-electron dissociation and subsequent recombination. Next, we investigated the fluorescence decay behavior of DSF and PFVK1-4 in the solid state. The analysis of Figure 12 reveals that the time-resolved fluorescence decay of DSF is completely different with respect to that exhibited by the polymers PFVK1-4. For DSF, the decay curve can be best interpolated by using a three-exponential function, characterized by longer time constants with respect to PFVK1-4 (Table 3). It can be supposed that in the case of DSF the absence of orthogonal alkyl chains favors the intermolecular packing and


Grisorio et al.

TABLE 3: Results (τi, Bi, χ2) Obtained from the Analysis of Fluorescence Decays of DSF and PFVK1-4 at 295 K in the Solid State DSF PFVK1 PFVK2 PFVK3 PFVK4 a

λem (nm)a

τ1 (ns)b

τ2 (ns)b

τ3 (ns)b

B1 (%)c

B2 (%)c

B3 (%)c

χ2

592 583 578 573 558

3.95 1.02 1.08 1.01 1.11

4.70 2.0 1.9 2.4 3.5

57.7 47.8 33.3 40.8 28

11 55.7 34.3 19.3 39.7

82.9 26.6 55.9 65.7 51.7

6.1 17.7 9.8 14.9 8.6

1.46 1.141 1.239 1.071 1.531

Monitored fluorescence wavelength. b Fluorescence lifetimes extracted by multiexponential fits. c Coefficients of the multiexponential fits.

Figure 13. Comparison of the fluorescence decay of the low-energy emission band of DSF and PFVK1 both in toluene and in the solid state (λexc ) 375 nm).

Figure 14. Anodic scans of PFVK1-4 films on ITO.

TABLE 4: EL Properties of PFVK1-4 CIE coord Lmax max curr ext (x, y)a (cd/m2, V) eff (cd/A, V) eff (%)a

polymer PFVK1 PFVK1 PFVK2 PFVK2 PFVK3 PFVK3 PFVK4 PFVK4

(type (type (type (type (type (type (type (type

I) II) I) II) I) II) I) II)

1144, 20 1387, 16 1318, 18 1365, 14 915, 18 1328, 14 494, 16 858, 14

0.06, 12 0.08, 12 0.10, 18 0.10, 12 0.11, 10 0.15, 10 0.05, 16 0.11, 10

0.026 0.043 0.042 0.043 0.047 0.065 0.023 0.042

0.54, 0.44 0.54, 0.44 0.54, 0.44 0.52, 0.45 0.50, 0.46 0.50, 0.46 0.46, 0.46 0.45, 0.46

a

At 12 V. Type I: (ITO/PEDOT-PSS/PFVK1-4/Ca/Al. Type II: (ITO/PEDOT-PSS/PFVK1-4/Alq3/Ca/Al.

the excimer formation through cofacial interaction between the fluorenones, which would be responsible for the overall increase in decay time, as well as for the red shift of the emission maximum. Also for PFVK1-4, a three-exponential function was employed for the best fitting of their PL decays, but differently

from DSF, the found τ2 (1.9-3.5 ns, Table 3) can be ascribed to the CT π-π* transition of the fluorenone units, as these values are very similar to those observed in toluene for PFVK1-2. The presence of a faster component is noteworthy, ranging from 1.01 to 1.11 ns interplaying with the other emissive processes leading to the observed difference in the decay profiles of PFVK1-4. Notwithstanding the increasing amount of fluorenone should favor local intermolecular interaction in the solid state between fluorenones, attributed to the more planar structure of this unit, the PL decay behavior of the polymers is not in accordance with a cofacial arrangement of fluorenones that we suppose might be operative in DSF. Rather, intermolecular interactions between the planar fluorenone-containing segments and the PFV backbone should be taken into account for justifying the faster decay of the low energy emission for the polymers, which is evident from the comparison of the fluorescence decays of PFVK1 and DSF in the solid state. The observation of this very different behavior is substantiated by the fact that the fluorescence decays of the above-mentioned molecules are very similar in toluene solution (Figure 13). On the other hand, the faster PL decay of PFVK1-2 with respect to PFVK3-4 could be interpreted by the increased number of nonradiative decay pathways deriving from fluorenone interactions with the local environment. It can be reasonably supposed that these interactions can also have an influence on the reciprocal position9 of the n-π* and of the CT π-π* energy levels, affecting the emission efficiency of PFVK1-4. Electrochemical Properties of PFVK1-4. To investigate how the incorporation of different amount of fluorenones has an influence on the electronic structure of the fluorenylenevinylene copolymers, we analyzed polymer films of PFVK1-4 by cyclic voltammetry (CV) measurements, a technique especially useful for the comparison of ionization potentials of polymers with a similar primary structure. From the CV scans reported in Figure 14, an irreversible oxidative behavior is evident for PFVK1-4, due to the formation of unstable hole-charged states for all the polymers. The anodic event is very similar in the case of PFVK1-3, suggesting the similarity of their electronic structures. The ionization potentials derived from the onset of the oxidation (reported in Table 1). are higher (of ∼0.1-0.2 eV) than those found for PFVK4 (IP ) 5.26 eV) and this effect can be ascribed to the higher amount of electron-withdrawing groups in PFVK1-3, exerting an influence on the HOMO of the corresponding materials. As a general trend, the ionization potential increases with the amount of fluorenone incorporated in the polymer. No processes could be recorded in the reduction scans for PFVK1-4, revealing that electrons are not easily introduced in the polymer bulk, even if 10% of electronwithdrawing fluorenones are incorporated in the PFV backbone. Electroluminescence properties. We investigated the electroluminescence (EL) features of PFVK1-4 by using these compounds as active layers in diodes with ITO/PEDOT-PSS/ PFVK1-4/Ca/Al configuration (type I). The relevant devices



Figure 15. EL spectra of the polymers at different bias.

exhibited a turn-on voltage of 6 V. The EL spectra of the polymers show a similar yellow-orange band which dominates the emission (Figure 15). However, while PFVK1-2 only show the yellow orange emission, in the case of PFVK3-4 a substantial difference with respect to their solid state PL spectra is evident: a blue-green emission appears, which is relatively pronounced with respect to the longer wavelength component. Such behavior is reasonably explained by admitting that in the electroluminescent emission the charge recombination, and therefore the exciton formation, can also involve the higher energy segments. This means that for PFVK3-4 the energy transfer efficiency toward the fluorenone moieties is more difficult, due to the low fluorenone percentage. The EL profiles of the OLED devices based on PFVK1-4 do not change by increasing the applied bias (Figure 15), an important requisite of optical stability for OLED applications. As a matter of fact, only a slight enhancement of the blue-green emission with respect to the yellow-orange band could be observed for PFVK3-4, containing the lowest amounts of fluorenone. Notwithstanding our devices were measured in air and without encapsulation, there is no evidence of the formation of fluorenone during device operations. In fact, if fluorenone formation occurred in our OLEDs based on PFVK3-4 (that showed a component of the electroluminescence spectrum falling in the blue-green region ascribable to the PFV backbone) we would have observed its decrease with an increase of driving voltage, as reported for polyfluorenes.5b The opposite behavior observed (i.e., the decrease of the Iorange/Iblue ratio) is in our opinion a hint that the oxidation of fluorene units to fluorenone is not a favored process in PFVK1-4 polymers during the device operation although we cannot exclude other degradation pathways involving the double bonds.

Notwithstanding the presence of keto defects in poly(arylenevinylene)s is known to exert a deleterious effect on the electroluminescent properties,26 the introduction of fluorenones enhanced the performances of our materials. In single-layer configuration, the polymers exhibited very good performances and reached 1318 cd/m2 and 0.10 cd/A at 18 V in the case of PFVK2 (Figure 16). In any case, the luminance and the current efficiency of these polymers are considerably higher with respect to those of a PFV previously prepared by the same synthetic protocol.16 This behavior derives from a better charge recombination efficiency due to the fluorenones incorporated onto the polymer backbone, acting both as electron accepting moieties, and as radiative decay sites. An improvement of the device performances was achieved by the insertion of a 10 nm layer of Alq3 between the cathode and the polymer (ITO/PEDOT-PSS/PFVK1-4/Alq3/Ca/Al configuration, type II) allowing a more balanced carrier transport within the OLED. The EL spectra of these devices at 12 V are shown in Figure 17. For all devices, an increase of the current density was observed due to the higher contribution of electrons to the total current (Figure 16A). As consequence of this better balanced charge transport, the luminances of type II devices are higher, thanks to the enhancement of the charge recombination at the same bias. In particular, the best device performance improvement was obtained for PFVK3 (1328 cd/m2 at 14 V, 0.15 cd/A at 10 V). Conclusions Poly(fluorenylene-vinylene)s containing different amount of fluorenone (10-1% mol/mol with respect to the fluorene units, PFVK1-4) were obtained by the Suzuki-Heck cascade reac-


Grisorio et al.

Figure 17. EL spectra of PFVK1-4 for type II-devices (at 12 V).

Figure 16. Current density (A), luminance (B), and current efficiency (C) plots of PFVK1-4 for type I and type II devices.

tion, aiming at studying the effect of the presence of the fluorenone luminophor on the photophysical properties of the poly(fluorenylene-vinylene) macromolecular backbone. The polymers were prepared in good yields and exhibited moderate molecular weights. Very good thermal stability and glass transition temperatures dependent on the copolymer composition were deduced by thermal analysis. While in chloroform all the fluorenone-containing polymers are predominantly blue-green emitters like PFV, in the solid state even a 1% fluorenone drastically shifts the polymer emission wavelength completely in the yellow-orange region. Moreover, also in toluene the fluorenone emission is barely detectable. The analysis of

fluorescence decays in toluene solution indicates that the lowenergy band, due to the presence of fluorenone, can be ascribed to the CT π-π* transition, while in the solid state another faster process was also observed, presumably triggered by local interactions of fluorenones with the PFV matrix, activating nonradiative decay pathways. In order to investigate the nature of these interactions involving fluorenone in the solid state, we analyzed the optical features of the planar model system 2,7distyrylfluorenone (DSF). In the solid state, its emission was strongly red-shifted with respect to a dispersion of DSF in PVK, highlighting the role of intermolecular interactions for this model compound in neat films. The analysis of the time-resolved fluorescence decays in the solid state gives further hints that the mechanism of emission is profoundly different in DSF (which is more prone to excimer formation in the solid state, due to the absence of orthogonal alkyl chains) and in PFVK1-4, where complex local intermolecular interactions, between the planar fluorenone-containing segments and the PFV backbone, lead to a significantly faster fluorescence decay with respect to DSF. Moreover, the differences in optical behavior between DSF and PFVK1 in blends with PFV clearly indicate the pivotal role of intermolecular interactions, governing both the energy transfer processes and the emission efficiency of fluorenone-based molecules. OLEDs of ITO/PEDOT-PSS/PFVK1-4/Ca/Al configuration emitted in the yellow-orange region with efficiencies considerably higher with respect to a PFV homopolymer (1318 cd/m2 and 0.10 cd/A at 18 V in the case of PFVK2). In the case of PFVK3-4 a residual blue-green emission is observed due to incomplete energy transfer to fluorenones, with the ratio between the two emissions being independent of the bias. We improved the device performances by inserting 10 nm Alq3 layer between the cathode and the polymer and we obtained an increase in current densities, as well as higher luminances and current efficiencies. In particular, the best figures of merit were obtained for PFVK1 (1387 cd/m2 at 16 V) in terms of luminances and PFVK3 (0.15 cd/A at 10 V) in terms of current efficiencies. The reported results highlight the potentialities of fluorenone moieties as yellow-orange emitting units in poly(arylene-vinylene)s. References and Notes (1) (a) Leclerc, M. J. Polym. Sci. A. Polym Chem 2001, 39, 2867– 2873. (b) Bernius, M. T.; Inbasekaran, M.; O′Brien, J.; Wu, W. AdV. Mater. 2000, 12, 1737–1750. (c) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402–428. (d) Grimsdale, A. C.; Mu¨llen, K. AdV. Polym. Sci. 2006, 199, 1–82. (e) Grimsdale, A. C.; Mu¨llen, K. Macromol. Rapid Commun. 2007, 28, 1676–1702.

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J. Phys. Chem. C, Vol. 112, No. 50, 2008 20087 (14) Gong, X.; Moses, D.; Heeger, A. J.; Xiao, S. J. Phys. Chem. B 2004, 108, 8601–8605. (15) (a) Grisorio, R.; Suranna, G. P.; Mastrorilli, P.; Nobile, C. F. Org. Lett. 2007, 9, 3149–3152. (b) Grisorio, R.; Mastrorilli, P.; Nobile, C. F.; Romanazzi, G.; Suranna, G. P. Tetrahedron Lett. 2005, 46, 2555–2558. (16) Grisorio, R.; Mastrorilli, P.; Nobile, C. F.; Romanazzi, G.; Suranna, G. P.; Gigli, G.; Piliego, C.; Ciccarella, G.; Cosma, P.; Acierno, D.; Amendola, E. Macromolecules 2007, 40, 4865–4873. (17) (a) Molander, G. A.; Rivero, M. R. Org. Lett. 2002, 4, 107–109. (b) Darses, S.; Michaud, G.; Geneˆt, J. P. Eur. J. Org. Chem. 1999, 1875– 1883. (18) De Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F. Synth. Met. 1997, 87, 53–59. (19) Weber, S. K.; Galbrecht, F.; Scherf, U. Org. Lett. 2006, 8, 4039– 4041. (20) (a) Anuragudom, P.; Newaz, S. S.; Phanichphant, S.; Lee, T. R. Macromolecules 2006, 39, 3494–3499. (b) Mikroyannidis, J. A.; Yu, Y.J.; Lee, S.-H.; Jin, J.-I. J. Polym. Sci. Part A: Polym. Chem. 2006, 44, 4494–4507. (c) Jin, S. H.; Kang, S. Y.; Kim, M. Y.; Chan, Y. U.; Kim, J. Y.; Lee, K.; Gal, Y. S. Macromolecules 2003, 36, 3841–3847. (d) Jin, S. H.; Park, H. J.; Kim, J. Y.; Lee, K.; Lee, S. P.; Moon, D. K.; Lee, H. J.; Gal, Y. S. Macromolecules 2002, 35, 7532–7534. (21) The interactions between fluorenones in the polymer with chloroform can rapidly deactivate the excited systems. The simultaneous quenching of the blue-green emission and the appearance of the yellow-orange one was observed only in a higher molecular weight PFV containing a higher amount of fluorenone (ref 16). (22) (a) Chi, C.; Im, C.; Enkelmann, V.; Ziegler, A.; Lieser, G.; Wegner, G. Chem. Eur. J. 2005, 11, 6833–6845. (b) Li, J.; Li, M.; Bo, Z. Chem. Eur. J. 2005, 11, 6930–6936. (23) Odoi, M. Y.; Hammer, N. I.; Rathnayake, H. P.; Lathi, P. M.; Barnes, M. D. ChemPhysChem 2007, 8, 1481–1486. (b) Rathnayake, H. P.; Cirpan, A.; Karasz, F. E.; Odoi, M. Y.; Hammer, N. I.; Barnes, M. D.; Lahti, P. M. Chem. Mater. 2007, 19, 3265–3270. (24) (a) Matsushita, Y.; Ichimura, T.; Hikida, T. Chem. Phys. Lett. 2002, 360, 65–71. (b) Biczo´k, L.; Be´rces, T.; Lischitz, H. J. Am. Chem. Soc. 1997, 119, 11071–11077. (25) (a) Dias, F. B.; Knaapila, M.; Monkman, A. P.; Burrows, H. D. Macromolecules 2006, 39, 1598–1606. (b) Dias, F. B.; Maiti, M.; Hintschich, S. I.; Monkman, A. P. J. Chem. Phys. 2005, 112, 0540904. (26) Papadimitrakopoulos, F.; Konstadinidis, K.; Miller, T. M.; Opila, R.; Chandross, E. A.; Galvin, M. E. Chem. Mater. 1994, 6, 1563–1568.

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