2.1 Ligand synthesis

As haloesters have been successfully employed to initiate ATRP [1], we introduced a bromoester group at the amino electrodonating site of dialkylaminostyryl-2,2′-bipyridyl ligands. Bipyridines b and b′ were obtained in good yields upon esterification of the hydroxy-functionalized 4,4′-dialkylaminostyryl-2,2′-bipyridines [10] a and a′ with 2-bromoisobutyroyl bromide in the presence of pyridine at room temperature (Fig. 1).

These two new ligands were fully characterized by NMR, UV–visible, emission spectroscopy and mass spectrometry, and gave satisfactorily microanalyses. The optical and thermal properties of b and b′ as well as those of the parent ligands 4,4′-diethylaminostyryl-2,2′-bipyridine DEASbpy and 4,4′-dibutylaminostyryl-2,2′-bipyridine DBASbpy are summarized in Table 1. They show a typical intense, structureless and large intraligand charge-transfer band at 385–391 nm, which is blue-shifted by ca. 10 nm as compared to the λ_max of DEASbpy and DBASbpy. This hypsochromic shift can be easily explained by the electron withdrawing effect of the bromoester groups. Likewise, b and b′ display a slightly blue-shifted emission band at 486 and 495 nm, respectively. A more dramatic change was observed by comparing the thermal properties (Td₁₀ determined by TGA) of b/b′ vs. DEASbpy/DBASbpy (Table 1). Whereas the parent ligands are stable at more than 300 °C, the thermal stabilities of b and b′ decrease considerably (T_d10 = 210 °C). To get more insight into this spectacular Td drop down, we heated b in refluxing DMF, which is the common solvent used for complexation to Ruthenium(II). After synthetic workup a new bipyridyl ligand c, corresponding to the loss of two HBr, was characterized by NMR and mass spectrometry (Fig. 2). Thus, the rather weak thermal stability of b (and b′) could be due to this easy dehydrohalogenation process.

2.2 Metalloinitiator synthesis

Two strategies were used for the synthesis of Ruthenium(II), Iron(II) and Zinc(II) complexes 1b–3b, depending on the nature of the metal salt. Sonochemically assisted room temperature reaction of b (3 equiv.) with iron sulfate in acetone/water, followed by an anionic exchange with PF₆^–, yielded 1b as a greenish black microcrystalline powder in 75% yield (Fig. 3). However, this route could not be used to prepare the corresponding ruthenium complex 2b, due to the dehydrobromination process in refluxing DMF.

Thus, 2b was obtained by a second route (Fig. 4) in a two-step synthesis: (i) the deep red complex 2a bearing six hydroxy groups was first prepared in 83% yield upon treatment of RuCl₂(dmso)₄ with 3 equiv. of a, followed by an anionic metathesis; (ii) treatment at room temperature of 2a with an excess of 2-bromoisobutyroyl bromide in THF, in the presence of pyridine, yielded 2b in 70%. The same methodology was employed to prepare the zinc complex 3b which was isolated in 60% yield as an orange-red powder. Complexes 1b–3b were characterized by ¹H and ¹³C NMR spectroscopy and microanalysis (see Section 4) as well as by UV–visible and emission spectroscopy. The UV–visible spectra are very similar to those of the parent complexes (Table 1). The iron compound 1b displays two well separated charge-transfer bands at 456 and 589 nm corresponding to the ILCT and MLCT transitions, respectively. As expected, the ruthenium complex 2b exhibits an overlap of these two transitions at 438 and 501 nm (Fig. 8), whereas the absorption spectrum of zinc complex 3b shows only one ILCT band at 448 nm. Photoluminescence similar to that of the parent tris-bipyridyl metal complexes is observed for 1–3b in diluted dichloromethane solution (Table 1). Iron(II) and Zinc(II) complexes 1b and 3b exhibit a broad, intense structureless emission band at 620 nm assigned to ligand-centered emission with very large Stokes shifts (5827 and 6192 cm^–1, respectively). By contrast, upon excitation at the MLCT or the ILCT wavelength, ruthenium complex 2b features a red-shifted photoluminescence at 713 nm (Table 1). The excitation spectrum overlays the absorption spectrum with two maxima corresponding to the ILCT and MLCT transitions, indicating that the observed emission arises from MLCT triplet states [11].

2.3 Star-shaped polymers

ATRP of MMA was carried out in 1,2-dichlorobenzene at 90 °C using a 2:1:1 ratio of N-propyl-2-pyridylmethanimine [12]: CuBr: 1b, 2b or 3b as initiator (In) and [MMA]: [In] = 260: 1. The resulting polymers 1c–3c (Fig. 5) were obtained as green (Fe), brown (Ru) and orange (Zn) flocculent powders, respectively, after purification by column chromatography on alumina (eluent: CH₂Cl₂) and precipitation in CH₂Cl₂/pentane. Molecular weight characterizations were determined in toluene using GPC equipped with a differential refractive index (DRI) detector. The monomer conversion was analyzed every 30 min and representative data for different reaction times are summarized in Table 2. The rate of polymerization strongly depends on the nature of the metalloinitiator: whereas a conversion of 94% was achieved after 4 h with 3b (Zn), only 50% conversion was reached when 2b (Ru) was used. Kinetic plots of the three polymerizations show a first-order behavior consistent with a living system (Fig. 6). We also observed an induction period of approximately 25 min for the iron and ruthenium complexes. This induction period is frequently observed and is not fully understood. It could be due to the presence of residual oxygen [13]. It is noteworthy that the molecular weight distribution remains quite high and increased throughout the polymerization, especially in the case of 3c where loss of control of the PDI was observed, reaching 3.47 at 94% conversion. At first glance, this could be due to termination reactions, but no higher molecular weight could be observed by GPC. As these results (low molecular weights and high PDI) seemed to be surprising, we carried out the molecular weight analysis of polymer 3c by a GPC equipped with DRI and low angle laser light scattering (LALLS) detectors. The data are summarized in Table 3 and Fig. 7 shows that M_n increases linearly with conversion while the PDI remains consistently low. As can be seen in Table 3, the experimental and theoretical values of M_n correspond well. These results are thus consistent with a living atom transfer radical polymerization.

The UV–visible and emission spectra of these new polymers display absorption and emission bands very similar to those of the corresponding metallo-initiators 1b, 2b and 2b, clearly indicating that the metallo-chromophoric structure is conserved upon polymerization (see Table 1 and Fig. 8 for the ruthenium complex). Thermal characterizations were performed by TGA and DSC (Table 1). They exhibit good thermal stability, with 10% weight losses (T_d10) occurring at ca. 290 °C. It is noteworthy that the thermal stabilities of 1c–3c are increased as compared to those of the corresponding metallo-initiators 1b–3b, which undergo dehydrohalogenation reactions. The glass-transition temperature (T_g) is found at ca. 125 °C, a temperature that is comparable to values reported in the literature for linear [13] and star-shaped [4] PMMA prepared by ATRP.

Finally, thin films were easily obtained by spin-coating a trichloro-1,2-ethane solution of 1c–3c onto glass slides. Scanning electron microscopy (SEM) reveals the formation of very uniform films without any chromophore aggregation, and a thickness varying approximately between 1 and 2 μm (see Fig. 9 for the ruthenium polymer film). Solid state UV–visible spectroscopy (Table 1 and Fig. 8 for the ruthenium polymer film) unambiguously confirmed that the metallic tris(bipyridyl) chromophores remain intact within the film.

4.1 General data

All experiments were carried out using Schlenk vessel under inert atmosphere. NMR spectra were recorded at room temperature on a BRUKER DPX 200 spectrometer. UV–visible spectra were performed on a KONTRON UVIKON 941 in diluted dichloromethane solution (ca. 10^–5 mol l^–1). Solid-state UV–visible spectra were obtained as thin films (prepared by spin coating) on a Perkin Elmer Lambda 2000 spectrophotometer. Routine fluorescence measurements were done using a Photon Technology International spectrophotometer. Infrared spectra were recorded in KBr pellets with a Nicolet FTIR spectrometer. Molar mass distributions were measured using size exclusion chromatography (SEC) at ambient temperature, on a system equipped with a guard column one 3-μm mixed E column (Polymer Laboratories) with DRI detection using tetrahydrofuran as eluent, at a flow rate of 1 ml min^–1. Poly(MMA) standards in the range (6 × 104–200 g mol^–1) were used for specific calibration. A dn/dc = 0.086 for poly(MMA) was used to calculate LALLS molecular weights. High resolution mass spectrometry (HRMS) and elemental analysis were carried out in the ‘Centre de mesures physiques de l'Ouest’ in Rennes. SEM pictures were performed in the ‘Centre de microscopie électronique à balayage et microanalyse’ in Rennes on a Jeol Scanning Electron Microscope JSM-6301F. Tetrahydrofuran was dried and distilled before used over Na/benzophenone, pyridine was just distilled before used. bipyridines a and a′, and complex 3a were synthesized using an already described procedure [10]. N-Propyl-2-pyridylmethanimine was synthesized as previously reported [12] and stored under anhydrous conditions prior to use. Copper(I) bromide (Aldrich, 98%) was purified according to the method of Keller and Wycoff [15]. MMA was obtained from Aldrich and filtered before utilization through a basic alumina column to remove the radical inhibitor. 2-Bromoisobutyroyl bromide, 1,2-dichlorobenzene, zinc acetate dihydrate and iron sulfate heptahydrate were obtained from Acros and used as received.

4.1.1 4,4′-Bis{N-ethyl-N-[2-(2-bromoisobutyroylester)ethyl]aminostyryl}-[2,2′]-bipyridine (b)

A solution of 2-bromoisobutyroylbromide (1.27 ml, 10 mmol) in THF (15 ml) was added dropwise to a solution of the hydroxy functionalized ligand a (1.83 g, 3.4 mmol) and pyridine (0.7 ml, 8.5 mmol) in THF (50 ml). After being stirred overnight at room temperature, the reaction mixture was poured into aqueous HCl (40 ml, 4 N). The organic layer was separated and the aqueous layer was extracted with CH₂Cl₂ (3 × 100 ml). The combined organic layers were washed with an aqueous solution of NaOH (3 × 50 ml, 1 M), dried over MgSO₄, filtered and the solvents were removed under vacuum. The product was purified by precipitation from ether/pentane (1:2) to afford a brown powder (2 g, 70%).

¹H NMR (CDCl₃) δ ppm: 8.69 (d, J = 5.1 Hz, 2H, H₆), 8.56 (s, 2H, H₃), 7.6–7.3 (m, 8H, H₈, H₅, H₁₀), 6.89 (d, J = 16.2 Hz, 2H, H₇), 6.79 (d, J = 8.9 Hz, 4H, H₁₁), 4.42 (t, J = 6.1 Hz, 4H, H₁₄), 3.75 (t, J = 6.1 Hz, 4H, H₁₃), 3.49 (q, J = 7 Hz 4H, H_13′), 1.98 (s, 12H, H₁₅), 1.22 (t, J = 7 Hz, 6H, H_14′). ¹³C NMR (CDCl₃) δ ppm: 172.2 (C₁₅), 156.5 (C₂), 149.5 (C₆), 148.3 (C₁₂), 147.3 (C₄), 134.0 (C₈), 129.1 (C₁₀), 124.9 (C₉), 121.8 (C₇), 121.0 (C₅), 118.3 (C₃), 112.3 (C₁₁), 63.7 (C₁₄), 55.9 (C₁₆), 48.7 (C₁₃), 45.7 (C_13′), 31.2 (C₁₇), 12.7 (C_14′). UV–visible (CH₂Cl₂): λ_max = 385 nm, ε_max = 55,000 l mol^–1 cm^–1. Emission (CH₂Cl₂): λ_em = 486 nm. Anal. Calc. (found) for C₄₂H₄₈N₄O₄Br₂·1.5CH₂Cl₂: C, 54.36 (54.70); H, 5.45 (5.50); N, 5.84 (6.27). HRMS (FAB) calc. for C₄₂H₄₉N₄O₄Br₂ [M + H]⁺: 833.2105, found: 833.2057 uma.

4.1.3 Dehydrohalogenation of b

In a Schlenk flask, bipyridine b (0.05 g; 0.059 mmol) was dissolved in anhydrous DMF (5 ml). The mixture was then refluxed for 15 h. The solvent was then evaporated under vacuum. The product was washed first with pentane and finally with ether to afford c as a brown powder (0.032 g, 80%).

¹H NMR (CDCl₃) δ ppm: 8.7 (d broad, 4H, H₆, H₃); 7.6–7.4 (m, 8H, H₈, H₅, H₁₀); 7.02 (d, J = 16.6 Hz, 2H, H₇); 6.83 (d, J = 8.8 Hz, 4H, H₁₁); 6.14 (s large, 2H, H₁₈); 5.64 (s large, 2H, H₁₈); 3.65–3.60 (m, 4H, H₁₃); 3.55–3.45 (m, 4H, H_13′); 1.98 (s, 12H, H₁₇); 1.26 (t, J = 6.97 Hz, 6H, H_14′). MS (FAB) calc. for C₄₂H₄₉N₄O₄Br₂ [M + H]⁺: 671, found: 671.

4.2 General procedure for the polymerization

Methyl methacrylate was polymerized using 1b, 2b and 3b (50 mg, 1.7 × 10^–2 mmol) as initiator (MMA/CuBr/n-propyl-2-pyridylmethanimine/initiator = 260:1:2:1). In a Schlenk tube, the initiator, MMA and CuBr(I) were dissolved into 2 ml of 1,2-dichlorobenzene and the solution was deoxygenated by three freeze-pump-thaw cycles. N-Propyl-2-pyridylmethanimine was added under argon and the solution was immediately immerged in an oil bath maintained at 90 °C. Samples were taken periodically using degassed syringes for conversion and molecular weight analysis. The copper was removed from the samples by passing through a column of neutral alumina plug (eluent: CH₂Cl₂), further precipitation in dichloromethane/pentane afforded the pure polymer.