1,10-phenanthroline copolymers

Synthetic Metals 159 (2009) 583–588

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Synthesis and lanthanide-sensing behaviour of polyfluorene/1,10-phenanthroline copolymers Joanne Ritchie a , Arvydas Ruseckas b,1 , Pascal André, Christian Münther a , Michael Van Ryssen a , Daniel E. Vize a , Joe A. Crayston a,∗ , Ifor D.W. Samuel b,1 a School of Chemistry, EaStCHEM, University of St. Andrews, St. Andrews, Fife KY16 9ST, United Kingdom b Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St. Andrews, St. Andrews, Fife KY16 9SS, United Kingdom

a r t i c l e

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Article history: Received 16 June 2008 Accepted 28 November 2008 Available online 15 January 2009 Keywords: Polymer Emission Light-emitting diodes Luminescence Fluorene Phenanthroline Terbium Europium

a b s t r a c t Copolymers containing alternating fluorene and 1,10-phenanthroline units, prepared by Suzuki coupling, were found to be capable of binding with Eu3+ and Tb3+ ions in the presence of dibenzoylmethane (dbm) or acetylacetonate (acac) ligands. Photoluminescence studies of the Eu-containing polymer showed a measurable additional longer wavelength emission on complexation compared to the uncomplexed polymer. Narrow linewidth red emission from the metal-centred Eu f states (5 D0 → 7 F2 ) was only observed on raising the concentration of the solution or in the solid state. The lack of intra-chain energy transfer to the Eu atomic levels at low concentration is proposed to be due to the low-spectral overlap between fluorene emission and ligand absorption. The Tb-containing polymer emission evolves into a broad, polymer backbone-based green emission at higher concentrations and in the solid state. Here, the stabilization of phen triplet levels due to conjugation with respect to the Tb atomic levels prevents energy transfer to the Tb metal states. Solution time-resolved emission studies on both polymers showed that after complexation, in addition to the emission from the unmodified polymer, a new long wavelength singlet emission emerges with slightly longer lifetime (0.7–2 ns). © 2009 Elsevier B.V. All rights reserved.

1. Introduction Conjugated polymers with metal binding groups along the polymer backbone offer the prospect of displaying large differences in properties in the presence of a metal ion [1,2]. For example, substantial shifts in the wavelength of light absorption or emission after metal complexation may be observed. This behaviour is of potential application in metal ion sensors and in tuning the emission wavelength of polymer LEDs [3–9]. In the past, conjugated polymers based on chelating ligands have been prepared, but

∗ Corresponding author. Tel.: +44 1334 463826; fax: +44 1334 463808. E-mail addresses: [email protected] (J.A. Crayston), [email protected] (I.D.W. Samuel). 1 Tel.: +44 1334 463103; fax: +44 134 463104. 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.11.023

their solubility is often low. For example, polybipyridinevinylenes, like polybipyridines, are only soluble in formic acid [10], unless metallated or substituted with solubilizing groups [11,12]. Polyfluorenes are known to emit very efficiently under PL or EL conditions with good hole mobilities [13,14]. The large band-gap gives efficient blue emission and the potential to transfer its energy to dopant emitting molecules with lower band-gaps. The solubilising groups in poly(9,9 -dialkylfluorene) materials lie perpendicular to the conjugated chain, so do not hinder planarity. Recently, the versatile Suzuki coupling reaction [15] has been utilized to introduce metal binding groups into fluorene alternating copolymers [16]. For example, using this method, a fluorene-bipyridine copolymer [17,18] a fluorene-quinoline polymer [19], a fluorene-salen polymer [20], a fluorene-carboxylated benzene copolymer [21] and a fluorene-1,10-phenanthroline (fluorene-phen) copolymer, P1 [22] have been reported. The latter polymer showed red-shifted absorption when exposed to metals in solution. The emission was also red-shifted or quenched, depending on the metal ion present.

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Here we present a study of P1 when complexed to the lanthanide ions Eu3+ and Tb3+ with a diketonate, dibenzoylmethane (dbm) or acetylactonate (acac), as the auxiliary ligand. We also prepared a statistical (random) alternating copolymer P2 with a 1:1 ratio of phen and 1,4-benzene groups, and, for comparison, the benchmark alternating fluorene-phenylene copolymer P3. The choice of the metals is such that their metal-centred 5 D states are close in energy to the likely triplet energy levels of both fluorene and phen excited states. Emission from these states leads to characteristic narrow linewidth emission, which would be attractive from the point of view of the selective sensing of metal ions. In addition, phosphorescent metal complex emitters in LEDs have attracted considerable interest since a variety of purer colours can be emitted [23,24] and the metals are sensitized by both singlet and triplet excitons, increasing the device efficiency significantly [25,26]. The dbm ligand is chosen to absorb strongly in the near-UV region, which should also assist energy transfer to the metalcentred states. The use of polymer deposition rather than vacuum deposition of lanthanide materials for LEDs avoids problems of decomposition at the high temperatures needed to volatilize lanthanide complexes. The covalent attachment of lanthanide complexes to the polymer should give greater resistance to aggregation during the device lifetime than simply doping large band-gap polymers [16]. 2. Experimental 2.1. Synthesis of alternating fluorene-phenanthroline copolymer P1 The reaction was carried out in an oven dried 25 cm3 2neck round-bottomed-flask, flushed with argon. A solution of 3,8-dibromophenanthroline (0.36 g, 1.07 mmol), 2,7-bis-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dihexylfluorene (0.62 g, 1.07 mmol) and Pd(0) (PPh3 )4 (12.3 mg, 0.0107 mmol), were dissolved in a mixture of dried and de-gassed THF (8 cm3 ) and tetraethyl-ammoniumhydroxide (5.33 cm3 ). The solution was refluxed overnight and a second addition of Pd(0) (PPh3 )4 (∼12 mg) was performed. The reaction was carried on for a further 48 h before pouring the solution into methanol (100 cm3 ) and filtering in a Buchner funnel. The precipitate was a light yellow powder (0.42 g, 64%). The product was then washed for 24 h in a Soxhlet apparatus using acetone (30 cm3 ) to remove oligomers and catalyst residues. This gave a yellow/brown solid (0.28 g, 42%) which showed fluores-

cence under UV (long wave) radiation in the blue region, in both acetone and THF. 1 H NMR: ı 0.61–0.90 (8H, m, –CH CH (CH ) CH ), 0.92–1.22 2 2 2 3 3 (12H, m, –CH2 CH2 (CH2 )3 CH3 ), 1.80–2.22 (6H, m, –(CH2 )5 CH3 ), 7.67–7.85 (4H, m, fluo/phen-ArH),7.85–7.97 (4H, m, fluo-ArH), 8.40–8.50 (2H, d, phen-H(5,6) ), 9.45–9.54 (2H, d, phen-H(4,7) ). 13 C NMR: ı 152.81, 150.03, 136.70, 133.91, 129.03, 128.56, 127.65, 127.17, 122.33, 121.28, 40.75, 31.87, 30.06, 24.23, 22.97, 14.43. 2.2. Preparation of the mixed phenanthroline-1,4-phenylene fluorene copolymer P2 and fluorene-1,4-phenylene copolymer P3 Copolymer P2 with Ar = 1,4-dibromobenzene or phen in a statistical distribution was also prepared (74% yield) by using a mixture containing a 1:0.5:0.5 ratio of 4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dihexylfluorene:3,8dibromophenanthroline:1,4-dibromobenzene. Again, NMR spectroscopy of copolymer P2 confirmed the feed ratio. The benchmark alternating fluorene-1,4-phenylene copolymer P3 [15] was also prepared with a 1:0:1 feed ratio (58% yield). 2.3. Synthesis of Eu complexed P1 Polymer P1 (0.125 g, 0.245 mmol), dibenzoylmethane (0.165 g, 0.735 mmol) and triethylamine (0.125 g, 1.235 mmol) were dissolved in dried THF (15 ml) under argon. Europium(III) chloride hexahydrate (0.0898 g, 0.245 mmol) was dissolved in methanol (3 ml) and added dropwise to the solution. After stirring for 4 h at 50 ◦ C, the solvent was removed under reduced pressure. The solid was filtered in a Buchner funnel and washed with methanol. The yellow/orange solid (0.27 g, 83%) was dried and showed fluorescence under UV (long wave) radiation in the red region, in both acetone and THF. 1 H NMR: ı 0.31–1.07 (18H, m, –(CH ) CH CH CH CH ), 2.01 2 2 2 2 2 3 (4H, m, –CH2 CH2 (CH2 )3 CH3 ), 2.49 (4H, m, –CH2 (CH2 )4 CH3 ), 5.54–5.95 (6H, m, DBM-ArH), 6.53–6.85 (12H, m, DBM-ArH), 7.0–8.1 (6H, m, fluo-ArH), 8.46 (2H, m, phen-ArH), 10.9–12.1 (4H, m, phen-ArH). CHN microanalysis data was corrected for incomplete combustion. This is a common problem for carbon and nitrogen-rich conjugated polymers [27,28]. Anal. calcd. for 83% combustion (100% combustion figures in parentheses) of ((C37 H38 N2 )(Eu·C45 H33 O6 )0.15 )n : C, 68.80 (82.89); H, 5.67 (6.83); N, 3.67 (4.42). Found: C, 69.31; H, 6.21; N, 3.69. Thermal gravimetric

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analysis (TGA) calcd.: assuming total decompositon to Eu2 O3 , 4.15. Found: Eu, 5 ± 1.0%. 2.4. Synthesis of Tb complexed P1 P1 (0.049 g, 0.096 mmol), dibenzoylmethane (0.0659 g, 0.294 mmol) and triethylamine (0.0496 g, 0.490 mmol) were dissolved in anhydrous THF (7 cm3 ). Under argon gas, terbium(III) chloride hexahydrate (0.0366 g, 0.096 mmol) was dissolved in methanol (1 cm3 ) and added dropwise to the THF solution. The resultant yellow/orange solution was refluxed at 50 ◦ C for 7 h. The solvent was removed under vacuum to produce a yellow/orange solid. This solid was filtered in a Buchner funnel and washed with methanol (0.12 g, 93%). CHN microanalysis data was corrected for incomplete combustion, diagnosed from two separate determinations which gave identical atomic ratios. This is a common problem for carbon and nitrogen-rich conjugated polymers [27,28]. Anal. calcd. for 93% combustion (100% combustion figures in parentheses) of (C37 H38 N2 Tb·C45 H33 O6 )n : C, 68.38 (73.53); H, 4.96 (5.34); N, 1.94 (2.09). Found: C, 68.17; H, 5.03; N, 1.86. TGA calcd: assuming total decompositon to Tb2 O3 , 13.7. Found: Tb, 12.0 ± 1.0%. 2.5. Time-resolved PL technique For time-resolved measurements samples were excited with 100 fs light pulses at a repetition rate of 80 MHz. Photoluminescence (PL) was spectrally resolved using a spectrograph and measured using a synchroscan streak camera with a 3 ps time resolution. Solutions were stirred during measurements. Solid films were prepared by drop-casting the polymer solution onto fused silica substrates and measured in a vacuum of 10−4 mbar. 3. Results and discussion Polymer P1 could be prepared only with relatively lowmolecular weights and large polydispersity as seen from the GPC data (Table 1, data for two prepared batches shown). This is probably caused by the poor solubility of the growing polymer in the reaction medium. Relatively low-molecular weights are typical of conjugated polymers and probably arise from the low solubility which limits the chain-extension reactions. The GPC data may also overerestimate the true molecular weight of these conjugated polymers due to the use of polystyrene standards. On the other hand, the GPC data ignores the higher molecular weight insoluble fraction. The number average molecular weight from GPC corresponds to about 12 phenanthroline units. Data is also provided for the benchmark polymer P3. This polymer was considerably more soluble and allowed the use of the more convenient THF solvent. It also gave higher molecular weights. In an attempt to prepare a more soluble phenanthroline containing polymer, P2 was prepared. However, the GPC data shows that the molecular weight was not significantly improved. The structures of the polymers were studied by 1 H NMR spectroscopy, which confirmed the 1:1 fluorene:phen ratio expected from the feed ratio. In principle, given the low-molecular weights, the NMR could also gave an estimate of the average chain length by end-group analysis. The H2/9 resonance of the phen group appears at ı 9.5 ppm. There was a shoulder on this peak which could be interpreted as due to an end-group peak, but this region is complicated by second order patterns, as shown in related polymers [29]. Fig. 1 shows the UV–visible absorption spectra of P1, P2 and P3 in THF. The two batches of P1 had identical properties. The absorption maximum of P1 (max = 378 nm; ε ∼45,000 mol−1 dm3 cm−1 ) is somewhat red-shifted from the benchmark polymer P3 (Table 1),

Fig. 1. UV–visible spectra of 2.0 × 10−5 mol dm−3 P1–3 in THF. Corresponding PL spectra (ex = 335 nm) in THF also shown (all at 1 × 10−6 g dm−3 , emission intensity normalized).

presumably due to conjugation with the phen groups. This compares to the phen-dialkoxybenzene copolymer which has max ∼ 380 nm [30]. The spectra of P1 and P2 were remarkably similar, indicating a similar extent of conjugation despite the presence of phenylene groups in P2. The polymers were not particularly soluble in polar solvents such as acetonitrile and acetone–water, and in other solvents the absorption spectrum showed little dependence on solvent polarity, consistent with the excited state having 1 (␲, ␲* ), rather than 1 (n, ␲* ) character. All polymers emit strongly in the blue (Fig. 1, exc = 335 nm, and Table 1), with little dependence on excitation wavelength, though the emission spectra of P1 had to be run at low concentration to avoid concentration quenching. The emission is typical of emission from the 1 (␲, ␲* ) state of polyfluorenes: significantly shifted absorption with clear vibronic structure. Increasing the solvent polarity led to a small bathochromic shift in the main polymer emission (403 nm in toluene to 418 nm in DMSO) which was linearly related to the solvatochromic indices [31] Z and ET N , suggesting that the singlet excited state shows significant charge transfer character (presumably from fluorene to phenanthroline). 3.1. Protonation effects The effect of protonation on the absorption spectra of P1 is to cause substantial bathochromic shifts in both the absorption and emission peaks in THF (Fig. 2) and a quenching of the emission, as observed in other phen polymers [30]. Proton binding to the phen ligand is generally supposed [32] to lower the energy of the phenH+ 1 (␲, ␲* ) by introducing more charge transfer character. The red shift on protonation of the conjugated polymer (to 540 nm) is much greater than in phen itself [33]. From the shift it is possible to calculate [33] the difference in pKa between the ground state and the excited state as 3.5. Assuming the polymer behaves in a similar way to phen itself only the monoprotonated form of phen is emissive [32]. The increasing involvement of the phen groups in the lowest excited state of the polymer may be partly responsible for the quenching of the fluorescence. The lowest excited singlet state of phen itself is 1 (␲, ␲* ), but the 3 (n, ␲* ) state is close in energy which leads to a large rate of intersystem crossing to the triplet and a correspondingly low-quantum yield of fluorescence from the phen chromophore [32]. There is also likely to be greater interaction with the solvent upon protonation, leading to rapid radiationless decay of the excited state.

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Table 1 Polymer molecular weight data (GPC, polystyrene calibration). Polymer

Solvent

P1 (#1) P1 (#2) P2 P3

CHCl3 CHCl3 CHCl3 THF

Mn 6,580 4,731 4,750 11,200

Mw 21,700 8,227 26,500 18,500

PD

DP

max /nm

εmax /mol−1 dm3 cm−1

em /nm

3.3 1.74 5.6 1.7

12.9 9.2 10.3 27.4

378 377 376 368

45,500 45,000 52,600 50,100

411; 430(sh) 411; 430 (sh) 410; 431(sh) 409; 430(sh)

Symbol = / refers to batch number. PD = polydispersity Mw /Mn ; DP = degree of polymerization (averaged repeat unit used for P2).

3.2. Lanthanide complexes As in the case of protonation, the effect of a metal on the absorption spectra of P1 and P2 is to cause a bathochromic shift in the absorption spectrum [22,34]. When a metal such as Zn2+ is bound a longer wavelength emission appears. Although in polybipyridines such shifts were attributed to metal-induced planarization of the polymer [17], this cannot operate on the already-planar phen ligand.

Complexation of Eu(dbm)3 to P1 was carried out in a similar way to that reported by Yasuda et al. using the related polymer, P4 [30]. The NMR spectrum of the resulting Eu complex to P1 displayed the characteristic contact shifts induced by the paramagnetic lanthanide ion and showed that all but the bridgehead phen signals have been shifted to the 11–12 ppm region. The molecular complex, [Eu(phen)(dbm)3 ] also shows phen signals that are shifted downfield in this region [35]. The question of the stoichiometry of complexation is settled by comparing the integrations of the dbm signals (5.5–7 ppm) which emerges as phen:dbm = 1:3, indicating 100% complexation of the phen groups. Further, the NMR spectrum did not seem to indicate the presence of remaining signals due to uncomplexed phen or dbm groups. The UV–vis spectrum of the Eu-complexed polymer was now dominated by a peak at 356 nm, characteristic of the ␲ → ␲* transition of the dbm ligand. The main absorption band of the backbone polymer now appears

Fig. 2. Absorption and photoluminescence spectra of P1 after the addition of HCl in THF (ex = 335 nm). Both absorbance and emission intensities are normalised to unity.

as a shoulder on the low-energy side. Unfortunately, attemps to prepare another batch of this polymer led to significantly lower degrees of Eu incorporation. Thermal gravimetric analysis (TGA) and microanalysis (particularly the C:N ratio) of the new batch confirmed that in this case only ca. 15% of the phen groups had an attached Eu(dbm)3 group. The UV–vis spectrum shows a much weaker 356 nm band in this sample and the absorption maximum is closer to that of P1 itself. The reaction was also carried out using Tb(dbm)3 as the complexing moiety and the resulting material was slightly soluble in the NMR solvent, CDCl3 . Thermal gravimetric analysis (TGA) and microanalysis (C:N ratio) confirmed that all of the phen groups were coordinated to Tb. In addition, laser-ablation ICPMS also confirmed the full coordination of all of the phen groups to Tb. The 1 H NMR spectrum of the polymer was very broad, due to paramagnetism of the Tb3+ ion, and was weak due to the low solubility of the polymer. Nevertheless, weak signals could be discerned in the range indicated by the Eu polymer (i.e. at 6–7 ppm for coordinated dbm and 7–11 for coordinated phen). 3.3. Emission spectroscopy of the Eu and Tb adducts In dilute solution the Eu-coordinated polymer showed both strong emission, characteristic of uncomplexed P1, and a weak, narrow, metal-centred red emission near 600 nm (Fig. 3) due to the 5 D0 → 7 F2 transition. The line pattern was very similar to that expected for the Eu(phen)(dbm)3 coordination environment [36]. Sensitisation of the Eu emission is thought to arise from Dexter energy transfer to the lanthanide ion via the phen and dbm triplet states, which are presumably produced more efficiently in metal complexes through metal-promoted spin-orbit coupling. The efficiency of the Dexter energy transfer process from the ligand to the metal depends on the overlap of the triplet energy levels with the metal levels. Literature data suggest that both phen and dbm triplet levels are similar in energy to the Eu 5 D0 lowest excited state [36].

Fig. 3. Absorption and photoluminescence spectra of a dilute solution of 100% Euand 100% Tb complexed P1 in THF (Ex = 350 nm). The corresponding spectra of P1 are shown for comparison.

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3.4. Time-resolved spectroscopy of the Eu and Tb polymers

Fig. 4. Thin film PL of P1, Eu-P1, and Tb-P1 (emission intensity normalized).

We believe that energy transfer from the fluorene to the phen or dbm ligands is very inefficient for at least two reasons. First, the spectral overlap between the fluorene emission and the phen or dbm ligands is poor, despite the small but significant bathochromic shift in the polymer absorption band onset after complexation. Secondly, the triplet energy level of the phen-based ligand will tend to be lower in the conjugated polymer than for phen itself, and will not be high enough to transfer energy to the metal. Similar inefficiency of sensitisation in solution has been observed in other europium polymeric systems [21,37,38]. The line emission near 600 nm was very sensitive to concentration, and increased rapidly in relative intensity with concentration. The emission dominated the thin film solid-state spectrum (Fig. 4). In concentrated solution and in the solid-state intermolecular energy transfer is much more likely, leading to efficient quenching of the polymer backbone emission, and subsequent energy transfer to lower energy states [13,39,40]. In the case of the Tb-containing polymer no Tb-based narrow line emission was observed in dilute solution, and the PL spectrum was almost identical to that of the uncomplexed polymer. Increasing the concentration of the solution led to an apparent shift to green emission (max 528 nm) at high concentrations and in the solid state (Fig. 4). We assign this emission to the regions of the polymer with coordinated phen groups. The 528 nm fluorescence is not observed for the uncomplexed polymer since the emission is only shifted to this region by metal complexation. Such a large shift is observed for other metal ions such as Zn2+ , and after protonation, as noted above [33]. So in the solid state the excited phen groups emit weakly or are converted to triplet states, but these triplet states do not have sufficiently high energy to transfer energy to the lowest Tb excited state (5 D4 ). This is somewhat unexpected as phen is usually an excellent sensitizer for Tb-centred emision in complexes. However, as mentioned above for the Eu polymer, the phen levels, in particular the triplet level, are shifted to lower energy in the P1 complex than in molecular complexes due not only to the metal coordination but also to the increased conjugation in the polymer [33]. In some Tb complexes it is observed that changing the diketone ligand to one with a higher energy triplet can lead to energy transfer to the Tb metal centre [37,41,42]. Accordingly, we prepared the Tb(acac)3 (acac = acetyl acetonate or 2,4-pentanedionate) analogue of the polymer, which now had a prominent absorption band due to the acac ligand at 270 nm, and would therefore be expected to have a much higher triplet energy than dbm. But again this polymer showed only the broad green luminescence band when excited at either 270 or 335 nm. This suggests that energy is transferred from the peripheral acac ligands to the polymer backbone.

Time-resolved spectroscopy was carried out on the 15% Eudoped polymer and the 100% Tb-doped polymers. PL spectra in pristine and 15% Eu-containing polymers in chloroform solution at an early time after excitation (integrated from 0 to 0.5 ns) show a main peak at 418 nm (Fig. 5) and further vibronic peaks at 440 and 470 nm. The PL lifetime (the decay to 1/e of the value of the initial intensity) in the 400–440 nm window is 0.6 ns in both polymers (see insets in Fig. 5), which agrees very well with a reported value of 0.5 ns in a fluorene-phenylene copolymer in solution. Based on spectral shape and lifetime, this emission is safely assigned to the conjugated fluorene segments. On a longer time scale (time window of 1–2 ns), the red-shifted emission with a maximum at about 490 nm dominates the PL spectrum in the Eu-containing polymer and shows a lifetime of ∼2 ns in the spectral windows of 560–640 and 640–720 nm. Pristine polymer also shows some red-shifted emission in the time interval of 1–2 ns, but it is much weaker and the lifetime is shorter (0.7 and 0.9 ns in the 460–490 and 490–530 nm windows, respectively) than when Eu is complexed. The PL spectrum in the 100% Tb-containing polymer with a maximum at 490 nm (Fig. 6) is substantially red-shifted as compared to the early PL spectra in Fig. 5 and shows a longer lifetime. Thus, it can be attributed to the Tb-coordinated phenanthroline segments. The shape of the PL spectrum changes very little with time, and the PL decay in the 450–490 nm window is dominated by a component with a time constant of 1.2 ns, whereas in the 490–530 nm window the dominant lifetime is 1.5 ns. The lifetime variation and broad emission spectrum indicate a large inhomogeneity of optical transitions in the Tb-coordinated segments. The instantaneous rise

Fig. 5. Time-resolved PL spectra of a pristine polymer (a) and of the ca. 15% Eucontaining polymer (b) for the 0.1 mg/ml solutions in chloroform and in solid films (c) taken after excitation with 375 nm photons. Insets show PL kinetics for the indicated spectral windows. Film PL spectra (c) were taken for the time interval of 0–0.05 ns.

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Acknowledgements We thank EPSRC for a studentship for JR, and SHEFC for funding of the Organic Semiconductor Centre. We thank Sylvia Williamson for help with the analytical data. References

Fig. 6. PL spectra in the ca. 100% Tb-containing polymer in chloroform solution. PL decays are shown in the inset. Excitation wavelength: 375 nm.

of the PL in the 490–530 nm window suggests that Tb-containing segments are excited directly with 375 nm photons or populated by very rapid (within 10 ps) excitation transfer from fluorene units. A weak emission from the fluorene units with a maximum at ∼418 nm is observed in the Tb-containing polymer and shows the same lifetime of 0.6 ns as in the pristine and Eu-containing polymers. In contrast to the Eu polymer, which showed a finite rise time of the PL in the 640–720 nm window, much stronger and instantaneous emission was observed from the Tb-containing polymer. This suggests that some Eu-coordinated segments are excited by energy transfer from fluorenes, which would be consistent with less extensive complex formation with Eu than with Tb, reflecting the doping levels used (∼15% and ∼100%, respectively). These experiments were repeated in THF solution, which showed similar results, but with weaker and less long-lived emission from the longer wavelength component. This may reflect conformational changes in this solvent which affect the rate of energy transfer, or possible partial decomplexation in this solvent. In solid films, the PL decays much faster, on a time scale of 10–20 ps (not shown), which indicates very rapid excitation transfer to quenching sites, such as inter-chain aggregates and oxidised units. Strong aggregation in these polymers is not surprising because no side groups were attached to the polymer backbone, which could keep conjugated chains apart. The difference in PL spectra of pristine and Eu-containing polymers in solid films is not significant (Fig. 5(c)); thus, we cannot draw reliable conclusions about the excitation of Eu-containing segments in films. 4. Conclusions The fluorene-phen alternating copolymer, P1, is capable of binding to Eu3+ and Tb3+ ions to give adducts in which all of the phen groups are coordinated. Up to 100% doping levels are possible, but the control of doping level is difficult. For both metal-doped polymers in dilute solution there was no evidence for energy transfer along the polymer chain to the metal f-orbital centred emission. However, sensitisation of the Eu metal emission was observed at higher concentrations and in thin films. The Tb-containing polymer shows only polymer backbone based emission, but there is a strongly red-shifted component compared to the uncomplexed polymer. Time-resolved studies suggest that this red-shifted emission evolves rapidly from a new singlet state of the phen moiety which appears on complexation, and is more prominent at higher metal doping levels. Once control over the synthesis of polymers with different doping levels has been established, further studies will be aimed at mapping the precise relationship between the doping level and the photophysical properties of the polymers.

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