Introduction

π-Conjugated polymers with fluorescent properties have attracted significant attention in the fields of organic light-emitting diodes1 and plastic solar cells,2 in which, recently, much effort has been devoted to the introduction of metal ions to the backbone to tune photophysical properties by harvesting the triplet state emission.3 However, poor processing properties often prevent device application of these polymers. Recently, fluorescent metallo-supramolecular polymers have become of great interest because their properties, such as solubility or viscosity, are controllable by changing the ligands and metal-ion species. In particular, the importance of metallo-supramolecular polyelectrolytes (MEPE) based on bisterpyridine (BTPY) or its derivatives in tuning optoelectronic properties after linking with conjugated molecules has been well recognized.4 The optoelectronic properties, which largely depend on the light emission efficiency of fluorescent MEPE, can be tuned by the choice of metal ion and the modification of ligands. Using different metal ions5, 6, 7 as templates to assemble organic building blocks into polymer chains through coordination to chelating terpyridyl units is an appealing strategy for the construction of photoluminescent materials.

Recently, we revealed the importance of structural modification of BTPY with electron-donating and -accepting groups substituted at the peripheral position in electrochemical properties of the corresponding Fe(II)-, Co(II)- and Ru(II)-MEPEs and photophysical properties of Ru(II)-MEPEs. After a successful study of the substituent effects of BTPYs with electron-donating methoxy groups for change in the photophysical properties of Ru(II)-MEPEs, we have designed new BTPY with bulky electron-donating triethylene glycol (TEG) chains to investigate the substituent effects on the photophysical properties of Fe(II)-MEPEs. The Fe(II) ion generates stable Fe(II)-MEPEs with a well-defined octahedral coordination geometry, but fluorescent Fe(II)-MEPEs have not been reported so far4, 8, 9, 10, 11 to our knowledge; however, it is well known that Fe(II) ions quench the fluorescence of Fe(II)-MEPEs bearing BTPY9, 12 or 2,6–bis(1′-methylbenzimidazolyl)-pyridine.13 Therefore, we chose Fe(II) ions for the study of substituent effects on fluorescent properties in metallo-supramolecular polymers.

The motivation for this study stems from the necessity for new fluorescent materials with insights from a systematic investigation of the structure–property relationship, which is virtually unknown but is highly desirable for the design of new fluorescent functional materials. In the quest to develop new functional materials, we report (1) the synthesis of novel BTPY ligands with TEG chains as flexible substituents at the peripheral pyridines of BTPY, (2) the formation of Fe(II)-MEPEs during the complexation of new ligands with Fe(II) ions, (3) the fluorescent properties of Fe(II) polymers and (4) the relationship between fluorescent properties and polymer structures.

Experimental procedure

Materials and general experimental details

Unless otherwise noted, all reactions were performed under an inert atmosphere of argon using conical glass vials that can be capped with a septum. All chemicals were of reagent grade and used as received, unless otherwise specified. Fe(OAc)2 was purchased from Sigma-Aldrich (St Louis, MO, USA), and dehydrated ethanol, methanol, acetic acid and dimethylsulfoxide were purchased from Wako (Osaka, Japan). The spectroscopic grade and fluorescence-grade methanol and CHCl3 were purchased from Kanto Chemical (Tokyo, Japan).

Synthesis of 4′-(4-bromo-phenyl)-6{2-[2-(2-methoxy-ethoxy)-ethoxy]-ethox}-[2, 2′; 6′, 2′′] terpyridine (L1)

Dry hexane (40 ml) was placed into a 200 ml two-neck flask containing NaH (0.10 g, 4.16 mmol) and stirred under a nitrogen gas atmosphere. After 2 min, the hexane was removed and dried under vacuum, followed by the addition of dry tetrahydrofuran (30 ml). A solution of TEG monomethyl ether (3) (0.30 g, 1.82 mmol) in dry tetrahydrofuran (10 ml) was added and the solution was refluxed for 1 h. The solution of 6-bromo-4′-(4-bromo-phenyl)-[2,2′;6′,2′′] terpyridine (1)14 (0.75 g, 1.62 mmol) in dry tetrahydrofuran (100 ml) was added slowly. The reaction mixture was refluxed and the course of the reaction was monitored by thin-layer chromatography. The reaction was quenched with saturated NH4Cl, followed by extraction and neutralization with CH3COOH. The solvent was evaporated under vacuum, and the viscous liquid was purified by column chromatography on silica gel using (CH2Cl2/hexane: 1/1) and (CH2Cl2/EtOAc: 9.5/0.5) as eluents to yield a colorless viscous liquid L1 (0.87 g, 87%), which was solidified after cooling. 1H nuclear magnetic resonance (NMR) (300 MHz, CDCl3) δ=8.64–8.61 (m, 3H, H7,9,15), 8.57 (d, 1H, J=1.5 Hz, H12), 8.24 (d, 1H, J=7.5 Hz, H4), 7.84 (ddd, 1H, J1= J2=7.6 Hz, J3=1.8 Hz, H14), 7.76–7.62 (m, 5H, H3, 17, 18), 7.35 (m, 1H, H13), 6.85 (d, 1H, J=8.25 Hz, H2), 4.65 (t, 2H, J=4.38 Hz, tpyOCH2), 3.95 (t, 2H, J=5.1 Hz, tpyOCH2CH2), 3.79–3.64 (m, 6H, TEG backbone), 3.54 (t, 2H, J=2.7 Hz, CH2OCH3), 3.33 (s, 3H, OCH3); 13C NMR (75 MHz, CDCl3) δ=162.99, 155.97, 155.93, 153.17, 149.04, 148.88, 139.37, 137.67, 136.87, 132.12, 128.80, 123.88, 123.34, 121.34, 118.39, 118.36, 114.17, 111.68, 71.88, 70.67, 70.63, 70.53, 69.70, 65.02, 59.01; HRMS (LCMS-IT-TOF-MS): found m/z: 571.965 [M+Na]+; C28H28NaN3O4Br requires 571.960.

General procedure for one-pot synthesis of BTPY

A diboronic compound (4 or 5, 0.5 equiv), K2CO3 (3.0 equiv) and PdCl2(PPh3)2 (5 mol %) were sequentially added to a dimethylsulfoxide solution of mono-terpyridine (L1 or 4′-(4-bromo-phenyl)-[2,2′;6′,2′′]terpyridine (2)). The solution was degassed and stirred at 80–120°C under an argon atmosphere. The course of the reaction was monitored by thin-layer chromatography. Catalysts were removed by filtration and washed thoroughly with CHCl3 after the reaction mixture was cooled to room temperature. The solvent was evaporated under vacuum and then precipitated into H2O. The solid compounds were filtered and washed with H2O, followed by washing with MeOH. BTPYs L2, L3 and L4 were dried and purified by column chromatography on activated basic alumina (CH2Cl2-EtOAc as eluents), followed by use of a gel permeation chromatography (LC-9104, Recycling Preparative HPLC, Japan Analytical (Tokyo, Japan)) column using CHCl3 as an eluent (yields: 28–38%).

BTPY L2 (0.041 g, 29%) is a light-yellow viscous liquid: 1H NMR (300 MHz, CDCl3) δ=8.72–8.56 (m, 8H, H7, 7′, 9, 9′, 12, 12, 15, 15′), 8.25 (d, 2H, J=7.6 Hz, H4, 4′), 7.78-7.69 (ddd, 2H, J1= J2=7.8 Hz, J3=3.0 Hz, H13, 13′), 7.67–7.63 (m, 14H, H14, 14′, 17, 17′, 18, 18′, tpyAr-C6H4-), 7.37–7.34 (m, 2H, H3, 3′) 6.85 (d, 2H, J=8.25 Hz, H2, 2′), 4.65 (t, 2H, J=4.8 Hz, tpyOCH2), 3.95 (t, 2H, J=4.8 Hz, tpyOCH2CH2), 3.79–3.64 (m, 12H, TEG backbone), 3.54 (t, 4H, J=2.7 Hz, CH2OCH3), 3.33 (s, 6H, OCH3); 13C NMR (75 MHz, CDCl3) δ=163.03, 156.22, 155.87, 153.40, 149.56, 149.10, 141.14, 139.62, 139.43, 137.75, 136.91, 127.77, 127.58, 123.86, 121.40, 118.58, 114.21, 11.65, 71.91, 70.72, 70.68, 70.58, 69.79, 65.09, 59.09; HRMS: found m/z: 1039.425 [M+Na]+; C62H60NaN6O8 requires 1039.423.

BTPY L3 (0.020 g, 38%) is a light-yellow solid: 1H NMR (300 MHz, CDCl3) δ=8.81 (d, 4H, H1, 15), 8.76–8.66 (m, 8H, H4, 7, 9, 12), 8.06-7.99 (ddd, 4H, J1= J2=9.0 Hz, J3=4.0 Hz, H3, 12), 7.97–7.65 (m, 12H, H17, 18, TPY-Ph-TPY), 7.45-7.35 (m, 4H, H2, 14); 13C NMR (75 MHz, CDCl3) δ=156.4, 156.1, 149.7, 149.25, 141.3, 137.8, 136.9, 128.1, 128.0, 124.02, 121.6; MALDI-MS (%): calculated m/z : 692.27, observed m/z : 692.4 (100).

BTPY L4 (0.020 g, 38%) is a light-yellow viscous liquid: 1H NMR (300 MHz, CDCl3) δ=8.75–8.66 (m, 8H, H7, 9, 12, 15), 8.27 (d, 2H, J=7.2 Hz, H4), 7.99–7.73 (m, 12H, H3, 13, 17, 18), 7.26 (m, 2H, H14) 6.85 (d, 2H, J=8.25 Hz, H2), 4.68 (t, 4H, J=6.0 Hz, tpyOCH2), 3.95 (t, 4H, J=4.2 Hz, tpyOCH2CH2), 3.77–3.64 (m, 12H, TEG backbone), 3.54 (t, 4H, J=6.0 Hz, CH2OCH3), 3.34 (s, 6H, OCH3); 13C NMR (75 MHz, CDCl3) δ=163.06, 156.25, 155.93, 153.43, 149.57, 149.12, 140.98, 139.43, 138.02, 136.92, 127.83, 123.87, 121.42, 118.63, 114.22, 111.68; HRMS: found m/z: 963.405 [M+Na]+; C56H56NaN6O8 requires 963.407.

General procedure for the preparation of Fe(II)-based MEPEs (FeL2-MEPE, FeL3-MEPE and FeL4-MEPE)

Equimolar amounts of the ligand and Fe(OAc)2 were refluxed in argon-saturated CH3COOH (5 ml solvent per mg of ligand) for 24 h. The reaction solution was cooled to room temperature and filtered to remove a small amount of insoluble residue. The filtrate was moved to a Petri dish, and the solvent was evaporated slowly to dryness. The brittle film was collected and dried further in vacuum overnight to yield the corresponding Fe(II)-MEPEs (>92%).

Results and Discussion

We have designed and synthesized a new TPY ligand (L1) containing a TEG chain at the ortho-position of the peripheral pyridine ring of 6-bromo-4′-(4-bromo-phenyl)-[2,2′;6′,2′′] terpyridine (1)14 in good chemical yield by means of chemoselective nucleophilic substitution (Scheme 1).15 Only one peripheral pyridine ring of TPY was substituted to prevent steric crowding in the coordination sphere. The synthon of L1 opens the door for the synthesis of new BTPY ligands, L2 and L4, using one-pot Suzuki-type dimerization of mono-terpyridines. For comparative study of the properties of MEPEs, L3 without TEG chains is also synthesized, using the above methodology, from 4′-(4-bromo-phenyl)-[2,2′;6′,2′′] terpyridine (2).14 All of the ligands, L1–L4, were purified by alumina column chromatography, followed by preparative gel permeation chromatography, and were characterized by NMR and mass spectroscopy techniques.

Scheme 1
figure 1

Synthesis of L1–L4.

Metal-ion-induced self-assembly of BTPYs was carried out by refluxing equimolar amounts of the ligand and Fe(OAc)2 in acetic acid under an argon atmosphere, followed by slow evaporation of the solvent and drying in vacuum. The color of the solution turns purple during complexation, indicating the formation of the corresponding Fe(II)-MEPEs (Figure 1).

Figure 1
figure 2

The structures of (a) bisterpyridine ligands (L2, L3 and L4) and (b) the corresponding Fe(II)-MEPEs (FeL2-MEPE, FeL3-MEPE and FeL4-MEPE) and the pictures of fluorescence in each compound.

Ultraviolet-visible and fluorescent spectra of ligands (L2, L3 and L4) and the corresponding Fe(II)-MEPEs are shown in Figure 2, and the data are summarized in Table 1. Ultraviolet-visible spectra of the ligands show a red shift up to 12 nm when the spacer length from two phenylene (L4, λ=308 nm) to three phenylene units (L2, λ=320 nm) changes, owing to the elongation of the π-conjugation along the chains. Absorption spectra show a change with an increase in ɛmax of these ligands from two to three phenylene units, which is ascribable to intraligand ligand-centered transitions. In contrast, Fe(II)-MEPEs (FeL2-MEPE, λ=571 nm; and FeL4-MEPE, λ=570 nm) show an almost constant metal-to-ligand charge transfer (MLCT) band, except for an increase in ɛmax, when the spacer increases from two to three phenylene groups, attributed possibly to the decoupling of the two terpyridine moieties by interposed phenylene units. However, the introduction of flexible, electron-donating TEG units at the peripheral position of BTPY (L2) shows an increase in ɛmax, compared with normal BTPY (L3). Similarly, their corresponding Fe(II)-MEPEs show a change in ɛmax, except to the MLCT band. The obtained results clearly follow the reported result,16 which deals with the effects of electron-rich and electron-deficient groups on the absorption properties.

Figure 2
figure 3

(a) Absorption and (b) emission spectra of ligands (L2, L3 and L4; c=1 × 10−5M, l=10 mm) and Fe(II)-MEPEs (FeL2-MEPE, FeL3-MEPE and FeL4-MEPE; c=1.5 × 10−5M, l=10 mm) in dichloromethane and methanol solvents, respectively.

Table 1 Photophysical properties of BTPYs and Fe(II)-MEPEs

The photoluminescence properties of ligands and Fe(II)-MEPEs are summarized in Table 1. A strong excitation band for ligands L2 (λ=337 nm) and L3 (λ=338 nm) was observed when they were excited at their corresponding emission band. Ligands L2 (λ=387 nm) and L3 (λ=384 nm) show a broad photoluminescence peak, whereas ligand L4 shows sharp excitation (λ=325 nm) and emission (λ=371 nm) bands. Ligand L2 shows the highest absolute fluorescence quantum yield (Φabs=0.82), compared with ligands L3 (Φabs=0.78) and L4 (Φabs=0.68). The increase in quantum yield is caused by both extended conjugation of the spacer and the electron-donating effect of TEG chains at the ortho-position of the ligand. Ligands L2 and L3 show a dark blue luminescence color, whereas ligand L4 is colorless at the 365 nm excitation. In addition, the resultant Fe(II)-MEPEs, FeL2-MEPE (bright green), FeL3-MEPE (dark green) and FeL4-MEPE (blue), show a change in emission color with respect to substituents and the length of the spacer (Figure 1). The emission spectra of FeL2-MEPE and FeL3-MEPE show a band at 489 and 500 nm, respectively (Figure 2b). However, the emission band of FeL4-MEPE, which has one less phenylene unit as spacer, seems to be at a shorter wavelength (443 nm) than those of FeL2-MEPE and FeL3-MEPE. The difference in wavelength is caused by the difference in the length of π-conjugation. Moreover, in the emission spectra of FeL2-MEPE and FeL3-MEPE, a shoulder band at 600 nm is observed, which can be assigned as an emission band arising from the excitation of the MLCT of the polymers. However, in the emission spectrum of FeL4-MEPE, such a shoulder band does not exist because of weak MLCT absorption of the polymer. Interestingly, we observed that FeL4-MEPE shows the highest absolute quantum yield (Φabs=0.22), compared with FeL2-MEPE (Φabs=0.11). FeL3-MEPE shows the highest quenching of photoluminescence properties and an absolute quantum yield of just 4% (Φabs=0.04), which is nearly threefold less than that of the substituted TEG chain-based FeL2-MEPE. The observed result for FeL3-MEPE is similar to the reported results12, 13 for Fe(II) quenching, which only describes the spacer modifications at the 4-position of TPY. The enhancement of emission in FeL2-MEPE and FeL4-MEPE may be caused by the charge transfer occurring within the monomer between the electron-rich TEG chain and the metal-coordinated, electron-deficient BTPY moiety,17 but the possibility of shielding18 fluorescent MEPEs with ortho-position-substituted TEG chains cannot be ruled out and may be responsible for the resultant properties.

To understand emission properties in depth, decay profiles of the luminescence properties of ligands and Fe(II)-MEPEs were measured by the time-correlated single-photon counting technique, using a picosecond diode as the exciter. The unsubstituted ligand L3 shows a longer lifetime (τ=0.9 ns) than the corresponding substituted ligands L2 (τ=0.7 ns) and L4 (τ=0.8 ns). Possibly, a substituent at the peripheral position of the pyridine ring prohibits effective π-conjugation of the aromatic rings of the free ligand.19 FeL3-MEPE shows the longest lifetime (τ=2.9 ns), compared with FeL2-MEPE (τ=2.3 ns) and FeL4-MEPE (τ=2.2 ns), and is demonstrated in Table 1 and Figure 3. These results are in good agreement with our recently reported results17 that reveal that Fe(II)-MEPEs modified by either electron-donating or -accepting groups show shorter lifetimes than unsubstituted Fe(II)-MEPEs. Moreover, there was a lesser effect of the spacer unit on lifetime.

Figure 3
figure 4

Transient photoluminescence decay curve of Fe(II)-MEPEs (FeL2-MEPE, FeL3-MEPE and FeL4-MEPE) in methanol on excitation at 371 nm (c=3.5 × 10−5M, l=10 mm).

We further investigated the effect of the concentration of Fe(II)-MEPE and the solvent on fluorescent properties. Figure 4a shows the change in emission intensity with an increase in concentration of FeL2-MEPE (c=0.1–10.0 × 10−5M). Emission intensity increases up to 3.5 × 10−5M, and thereafter it gradually decreases with increasing concentration. The quantum yield also decreased from 0.11 to 0.02 (with respect to anthracene in ethanol solvent) when the concentration changed from 3.5 × 10−5 to 10 × 10−5M. The insets of Figure 4a show a linear increase in MLCT band with respect to increase in concentration.

Figure 4
figure 5

(a) Change in emission intensity with respect to the concentration of FeL2-MEPE (c=0.1–10 × 10−5M) in methanol solvent, and (b) emission spectra of FeL2-MEPE in different solvents (c=3.5 × 10−5M). The inset (a) shows the relationship between absorbance in the MLCT band (λ=571 nm) and concentration, and (b) absorbance in the MLCT band in different organic solvents.

The fluorescent properties of FeL2-MEPE show a change in different solvents (Figure 4b). FeL2-MEPE shows a high emission intensity in pure methanol, compared with an acetic acid–methanol (9:1) mixture. However, the fluorescence is completely quenched in a water–methanol (9:1) mixture. These results are in good agreement with previously reported results that deal with the quenching of fluorescence in water or acidic media.9, 20 Interestingly, in ethylene glycol, FeL2-MEPE shows a twofold increase in emission intensity, compared with pure methanol. We also measured the absorption spectra to confirm the existence of FeL2-MEPE in these solvents by comparing their MLCT absorption (Figure 4b, inset). The obtained result is in good agreement with reported results, which explains that viscosity is an important parameter for fluorescent enhancement because of the increased solubility of the compound.20 In addition to this, the possibility of an interaction between TEG chains and the ethylene glycol solvent cannot be ruled out as a possible cause of the enhancement of fluorescent properties.

The molecular weights and electrochemical properties of Fe(II)-MEPEs are summarized in Table 2. The molecular weights of Fe(II)-MEPEs are determined by the SEC-viscometry-RALLs method. The high-molecular weight of Fe(II)-MEPEs shows the complex formation of TPY units with an Fe(II) ion, even though the TPY units have bulky TEG chains at the ortho-position of the peripheral pyridine ring. The electrochemical properties of ligands and Fe(II)-MEPEs were investigated by cyclic voltammetry. No redox peaks are observed for ligands, but Fe(II)-MEPEs show a reversible wave based on the redox between Fe(II) and Fe(III). The redox potential (E1/2), HOMO and band gap of Fe(II)-MEPEs are summarized in Table 2. FeL2-MEPE and FeL4-MEPE, the ligands of which are modified by electron-donating TEG chains, display a redox potential at 0.74 V, about 50 mV more negative than the unsubstituted FeL3-MEPE. By contrast, no significant effect is observed by varying the spacer, as a comparison between FeL2-MEPE and FeL4-MEPE. These results are in good agreement with previously reported results.14 In addition, the introduction of substituted TEG chain into BTPY ligands does not have a significant effect on the band gap of their corresponding MEPEs.

Table 2 Molecular weight and electro-chemical properties of Fe(II)-MEPEs

Conclusion

Novel fluorescent Fe(II)-MEPEs were synthesized through complexation of Fe(II) ions with new BTPYS containing flexible TEG chains at the ortho-position of peripheral pyridine rings. The introduction of TEG chains to the ligand resulted in an enhancement of fluorescence of the corresponding polymers: FeL2-MEPE with TEG chains retains nearly threefold higher fluorescence quantum yield than FeL3-MEPE without TEG chains. As a proof of principle, we have studied their photophysical and electrochemical properties, taking into account the effect of the substituent at the peripheral pyridine unit and the length of the spacer. This study confirms that the controlled design of ligands may help to modulate the optoelectronic properties of a given chromosphere through metallo-supramolecular polymerization.