Alcohol-and water-soluble bis( tpy )quaterthiophenes with phosphonium side groups: new conjugated units for metallo-supramolecular polymers †

Bis( tpy )quaterthiophenes with symmetrically distributed two and four 6-bromohexyl side groups were prepared and modi ﬁ ed by the reaction with triethylphosphine to give the corresponding ionic species. Both ionic and non-ionic bis( tpy )quaterthiophenes (unimers) were assembled with Zn 2+ and Fe 2+ ions to conjugated metallo-supramolecular polymers (MSPs), of which the ionic ones are soluble in alcohols and those derived from tetrasubstituted unimers are soluble even in water. The di ﬀ erences in assembly are speci ﬁ ed between systems with (i) ionic and non-ionic unimers, (ii) Zn 2+ and Fe 2+ ion couplers, and (iii) methanol and water solvents. A substantial decrease in the stability of Fe-MSPs and a surprisingly high red shift of the luminescence band of Zn-MSPs were observed on going from methanol to aqueous solutions.

Chains of conjugated MSPs of this type are composed of molecules of bisterpyridines that are linked via facial and meridian coordination of their tpy end-groups to metal ions such as Ru 2+ , Fe 2+ , Zn 2+ and Co 2+ (generally Mt 2+ ) ions.0][21][22][23][24][25][26][27] The enchained unimers are also quite rigid due to the delocalization of π-electrons.The rigidity of both of these constitutional units together with electrostatic repulsions of the main-chain Mt 2+ cations favor extended conformations of MSP chains that are favorable for inter-chain attraction.Thus the above struc-tural features can be regarded as the main reason for the low solubility of MSPs derived from α,ω-bis(tpy)oligoarylenes.
An increase in the solubility of the discussed conjugated MSPs has been achieved by introducing pendant alkyl groups into unimer units.However, this increase is insufficient. 28urther improvement in the solubility of MSPs can be achieved by introducing cationic pendant groups onto unimer building blocks.The markedly cationic character of MSP chains should reduce the inter-chain attraction and thus make MSPs more soluble mainly in polar solvents such as alcohols or even in water.Such solvents are perhaps the most desired for processing of conjugated MSPs.
The above approach has been recently tested on thiophene and bithiophene with tpy end-groups. 29In the present paper, we report the preparation and basic properties of α,ω-bis(tpy)quaterthiophenes with cationic side groups and related conjugated MSPs with Zn 2+ and Fe 2+ ion couplers.Since the studied MSPs show constitutional dynamics, 30,31 they exist in solutions as short oligomeric chains composed of starting unimers.The term unimer proposed by Ciferri 32 is used in the further text for a building block of MSPs.

Results and discussion
The prepared ionic as well as non-ionic unimers and their abbreviations are shown in Chart 1 together with the numbering of the central block positions and the marking of the rings used in the assignment of NMR spectra.The letter Q denotes the unimers with the quaterthiophene central block and the numbers behind it indicate the positions occupied by hexyl groups (suffix -H) or by hexyl groups capped with a 4-methoxyphenoxy group (suffix -A) or a bromine atom (suffix -Br) or a triethylphosphonium group (suffix -P + ).MSPs are marked with the prefix P Zn ( polymers with Zn 2+ ion couplers) or P Fe (Fe 2+ ion couplers) before the unimer label: for example, P Zn Q27-Br denotes the MSPs formed by the assembly of Zn 2+ ions and unimer Q27-Br that contains 6-bromohexyl groups attached to the quaterthiophene central block at positions 2 and 7; P Fe Q45-P + stands for the MSPs formed from Fe 2+ ions and unimer Q45-P + that contains two 6-(triethylphosphonium)hexyl groups attached to positions 4 and 5 of the central block, etc.

Synthesis and characterization of unimers and polymers
The reference unimers Q and Q27-H were prepared using the Suzuki-Miyaura coupling strategy (Scheme 1) and the conditions applied earlier. 28,29,33Br-unimers were prepared using the strategy shown in Schemes 1 and 2a.The starting monomer 3-[6-(4-methoxyphenoxy)hexyl]thiophene was prepared using the procedure described elsewhere. 34The procedure starting from 3-(6-bromohexyl)thiophene used for the synthesis of ionic unimers with mono-(M) and bithiophene (B) central blocks 29 was not so effective owing to low efficiency of purification of unimers with bromohexyl groups.Connecting tpy end-groups by Suzuki coupling (Scheme 1c) was accompanied by partial dehydrobromination of bromohexyl side groups promoted with tpy end-groups. 29Purification of short unimers could be done easily (for M) or feasibly (for B), but the purification of bis(tpy)quaterthiophenes was almost impossible.
The use of new starting monomers avoided the above difficulties and, in addition, made the isolation of all intermediates as well as A-unimers much easier.The A-unimers were then allowed to react with BBr 3 (Scheme 2a) to give the corresponding Br-unimers with bromohexyl side groups (yield 85-95%), which were finally treated with triethylphosphine to give the corresponding ionic P + -unimers (Scheme 2b).Excess PEt 3 was easily removed by vacuum distillation and its oxide (POEt 3 ) was washed away with toluene and ether.Solid products were isolated by centrifugation (yield of the ionization step was from 75 to 95%).
Solubility in methanol was the first evidence of successful transformation of the Br-into P + -unimers.The NMR spectra of modified unimers accordingly showed a 31 P signal of P +groups at around 39 ppm (38.93 for Q27-P + , 38.44 for Q45-P + , 39.99 for Q2457-P + ), 1 H signals of ethyl groups ( part of P + ) but no signal at 3.4 ppm that is typical of CH 2 -Br groups.Weak 1 H signals at 5.93 ppm and 5.55-5.45ppm were also observed indicating that some side chains contain terminal double bonds formed by dehydrobromination accompanying quaternization of Br-unimers.Complete removal of imperfect molecules from P + -unimers was not successful since they are soluble in alcohols.
TGA analyses of Br-unimers and P + -unimers showed thermal stability up to 205 °C.
Metallo-supramolecular polymers were simply prepared by mixing solutions of a given unimer and zinc(II) or iron(II) perchlorate in the metal ions to a unimer mole ratio of r = 1 (r = [Mt 2+ ]/[unimer]).Br-unimers were assembled in the acetonitrile/chloroform mixed solvent (1/1 by vol.) while P + -unimers were assembled in methanol.
The solubility of the prepared unimers and polymers depends on the substitution of the unimer central block.The unsubstituted unimer Q is soluble in dichloromethane but poorly soluble in chloroform.Br-unimers are highly soluble in solvents such as THF, CHCl 3 , CH 2 Cl 2 , DMSO and the acetonitrile/chloroform mixture, which facilitates their isolation and purification.Unimers with two ionic groups are highly soluble in polar solvents such as methanol, ethanol and DMSO and sparingly soluble in water (complete dissolution to a colloidal solution takes a few weeks).Unimer Q2457-P + carrying four ionic groups is easily soluble in water, which is quite unusual for this type of conjugated structure.Nevertheless, complete dissolution of Q2457-P + to the molecular level takes time on the day scale as can be seen from the time develop-ment of the UV/vis spectrum of its aqueous solution shown in Fig. S1, ESI.† MSPs show similar solubility to the corresponding unimers.

Vibrational spectra of unimers and polymers
The IR spectra of unimers show each the bands of stretching modes (ν CC , ν CN ) of tpy end-groups (1500-1620 cm −1 ), the quaterthiophene central block (1370-1500 cm −1 ), aromatic ν CH modes (3000-3100 cm −1 ) and the main out-of-plane modes (ρ CH ) of aromatic moieties (790 and 658 cm −1 ) at nearly identical positions (Fig. S2, ESI †).Differences due to different substitutions of the central blocks are mainly seen in the fingerprint region from 850 to 1350 cm −1 .The presence of hexyl chains is mainly observable in the C-H stretching region (2800-3000 cm −1 ).The most intensive bands characteristic of vibrations of hexyl groups in the fingerprint region (1466 and 1379 cm −1 ) are overlapped by the ring stretching modes of the quaterthiophene central blocks.Only a small shift of the band maximum from 1459 cm −1 (for Q) to 1466 cm −1 (for Q27-H) and a new shoulder at 1384 cm −1 for Q27-H are observable.The broad spectral band at 3400 cm −1 observed for all P + -unimers is due to the presence of hydrogen bonded water molecules in these unimers.
The off-resonance Raman spectra of unimers show strong stretching bands of quaterthiophene blocks but weak bands of tpy end-groups.Their spectral patterns reflect differences in the substitution of quaterthiophene blocks (Fig. S3, ESI †).The Raman spectra of Zn-polymers were disturbed by strong fluorescence but spectra of non-fluorescent Fe-polymers were well measurable.The bands characteristic of tpy groups 35 occur at 1610 cm −1 (ν s ), 1290 cm −1 (δ ip ) and 1038 cm −1 (breathing mode) while the bands of quaterthiophene blocks [36][37][38] occur in the region 1380-1520 cm −1 (Fig. S4a-S7a, ESI †).Deconvolution of the latter band using the OMNIC software gave robust results, showing that this band is composed of at least five bands (Fig. S4b-S7b, ESI †) whose intensities depend on the positions of side groups.The band at 1472 cm −1 should also be attributed to transitions in tpy groups. 35

Optical spectra of unimers and polymers
The solution UV/vis absorption spectra of unimers (ESI, Fig. S8a †) show: (i) a flat band at 280-284 nm mainly contributed by transitions in tpy end groups, and (ii) a band with the apex at a wavelength λ A from 381 nm (Q2457-P + ) to 441 nm (Q) belonging to transitions from HOMO that is spread over thiophene rings and central rings of tpy groups. 28,33The value of λ A (see Table 1) decreases (i) with increasing distortion of the quaterthiophene central block (see Table 2), and (ii) on going from the non-ionic (-Br, -H) to ionic unimer (-P + ) of the same type.Absorption maxima in the spectra of unimer thin films (ESI, Fig. S8b †) are not in such good correlation with the chain distortion, which reflects the importance of the molecular packing effect or the electronic effect of substituents.An exceptionally high λ A of Q2457-Br thin films (500 ± 10 nm) was obtained.The fact that Q2457-P + shows much lower λ A can be ascribed to the bulkiness of P + Et 3 groups and the effect of bromine counterions.
In the spectra of Zn-polymers, the absorption band of transitions involving quaterthiophene blocks is red shifted by about 35-65 nm compared to its position λ A for the unimer in the solution spectra and by about 20-75 nm in thin films (Table 1).The only but great exception is the spectrum of P Zn Q2457-Br thin films that surprisingly shows a blue shift of λ A of about −70 nm, which is obviously due to the exceptionally high value of λ A of the unimer Q2457-Br.
The spectra of Fe-polymers contain a new band belonging to transitions in the metal-to-ligand charge transfer (MLCT) complex which is typical of tpy-Fe-tpy linkages 35,39 (see Table 1 and Fig. S8c and S8d in the ESI †).In Fe-polymers, this band is significantly contributed by transitions involving neighboring oligothiophene blocks. 29uminescence spectra of unimers in solutions (ESI, Fig. S9a †) show higher similarity than their UV/vis spectra (λ F around 550 nm; lowered values of about 535 nm are actually given by different band shapes).This is obviously due to the fast transition of excited unimer molecules to nearly coplanar conformations with quinoidal rings, from which the light emission takes place. 36Minor differences are nevertheless seen: unimers with less distorted chains (Q, Q27-H, Q27-Br and Q27-P + ) show a better resolved vibrational structure than unimers with more distorted chains (Q45-Br, Q45-P + , Q2457-Br and Q2457-P + ).The luminescence spectra of unimers and Znpolymer thin films are shown in ESI, Fig. S9b-d † and the band wavelengths are summarized in Table 1, and the luminescence lifetimes are presented in ESI, Table S1.† Fe-polymers do not show luminescence. 28,29,36Assembly of unimers to metallo-supramolecular polymers in solutions The assembly in solutions was monitored by UV/vis and luminescence spectroscopy, viscometry and size exclusion chromatography (SEC).A chloroform/acetonitrile mixed solvent (1/1 by volume) was used for Br-unimers while methanol and water were used for P + -unimers.For spectroscopic studies, a set of solutions of a constant unimer concentration (2 × 10 −5 M) and a stepwise increasing metal ions to unimer mole ratio (r from 0 to 3) was prepared for each Mt 2+ /unimer system and solutions were allowed to equilibrate for 24 hours before monitoring the spectra.The SEC and viscometric measurements were done with solutions of the concentration of 5 × 10 −4 M. Spectral changes accompanying the assembly of unimers with Mt 2+ ions showed three stages differing in the development trend, similarly to the related systems studied recently. 21,25,28,29,40The UV/vis spectra obtained for systems of composition ratios r from 0 to ca. 0.5 (the first stage of assembly) showed up to three isosbestic points (see examples in Fig. 1a and 2a and a complete set of the spectra in ESI, Fig. S10 and S11 †), which indicates the transformation of the unimer species into another well-defined species.Regarding the stoichiometry, the new species should be a dimer species unimer-Mt 2+ -unimer.
The spectra obtained for systems with ratios r from ca. 0.6 to 1 also show isosbestic points but at different wavelengths (Fig. 1b and 2b).This indicates that the systems entered the second stage of assembly in which longer polymer chains are formed.As can be seen from ESI, Fig. S10 and S11, † the absorption bands characteristic of free unimers and dimers disappear while the band of enchained unimer units fully develops in the case of systems with non-ionic Br-unimers.These systems then enter the third stage of assembly (r > 1), where spectral changes are quite low and can be attributed to the end-capping of polymer chains with the metal ions and partial depolymerization of the polymer chains to the shorter also end-capped ones (Fig. 1c and 2c).The reaction of (tpy) 2 Zn 2+ species with Zn 2+ ions giving two (tpy)Zn 2+ species has been reported for mono-as well as bis(tpy) species. 18,29,41,42The reaction of (tpy) 2 Fe 2+ species with Fe 2+ ions giving two (tpy)Fe 2+ species was reported only for metallosupramolecular polymers. 18,29,43he spectral changes in the second stage of assembly of ionic P + -unimers are less progressive than those in the case of Br-unimers or even incomplete, which indicates lowered thermodynamic stability (i.e., stability constants) of ionic polymers in methanol.The lowered stability of ionic Fe-polymers is also seen from the changes in the position and intensity of the MLCT band (at around 595 nm) that is not fully developed at the ratio r ≅ 1 (Fig. 2b).The UV/vis spectral patterns indicate that the ionic polymers acquire their maximum length at the ratios r of about 1.5 or higher in methanol solutions.
The changes in luminescence spectra accompanying the assembly of ionic unimers with Zn 2+ ions are shown in Fig. 3.
The complexation is manifested by the disappearance of the unimer emission band and the creation of a new band red shifted by about 130 nm.Unlike the case of shorter ionic polymers derived from bis(tpy) mono-and bithiophenes, 29 the new emission band is much less intense than the band of free unimers.A similar luminescence attenuation is also exhibited by systems with non-ionic polymers.This shows that the prolongation of the unimer central oligothiophene block increases the efficiency of non-radiative paths of the decay of excited states in Zn-polymers.
Unlike the systems with Zn 2+ ions, those with Fe 2+ ions show a monotonous luminescence quenching with increasing ratios r up to ca. 0.6, at which the luminescence disappears (for example see ESI, Fig. S12 †).This behaviour, which is exhibited by other systems with bis(tpy)Fe 2+ species, is attributed to the fact that the lowest excited state of bis(tpy)Fe 2+ species, the d-d triplet state, is close to the ground state. 44As the d-d triplet state easily depletes higher excited states and potential phosphorescence from the d-d state is spin forbid- den, its decay by non-radiative transitions is unambiguously preferred in accord with the energy gap law. 8,45he molar mass distribution of Br-polymers in CHCl 3 / CH 3 CN (1 : 1) solutions was examined using an SEC system equipped with a diode-array UV/vis detector (DAD).(Analysis of ionic polymers failed owing to the strong adsorption of their chains inside SEC columns.)Mixed solutions of a Brunimer (0.5 mM) and Zn 2+ or Fe 2+ ions (r from 0 to 2.0) equilibrated for one day were injected into the SEC system.The SEC records of systems with Zn 2+ ions showed nothing but the peak of free unimers, which proves rapid dissociation of the Zn-polymer chains upon multifold dilution of their solution inside SEC columns.In contrast, the systems with Fe 2+ ions provided SEC records typical of covalent polymers (Fig. 4 and ESI, S13 †), which demonstrates very slow constitutional dynamics of Fe-polymers in the used solvent.Similar results were recently obtained for MSPs of shorter bis(tpy)thiophenes. 29ell resolved SEC records were obtained only for systems with a composition ratio r < 1 (Fig. 4).Systems with r ≥ 1 gave poorly resolved SEC records, in which the area under the elution peak decreased with increasing value of r.This indicates retention of longer chains in SEC columns.The detained polymer chains, obviously end-capped with Fe 2+ ions, had to be additionally washed out of the columns with 2,2′-bipyridine.The UV/vis spectral pattern of SEC fractions showed a perfect development with the elution time (t el ): a pattern typical of long polymer chains was observed for the first eluted SEC fractions while that typical of the dimers for the last fraction (Fig. 5a and ESI, Fig. S14a †).Differences are also seen when comparing the spectra of fractions of dimers formed in systems of different compositions (Fig. 5b and ESI, Fig. S14b †).These differences can be attributed to the endcapping of their molecules with Fe 2+ ions.However, these differences are substantially smaller than those observed for the Fe-polymers formed from unimers with mono-and bithiophene central blocks. 29he presence of higher fractions in solutions containing a stoichiometric lack of Fe 2+ ions (r = 0.2 and 0.5) can be explained by the transiently locally increased concentration of the ions and unimers during mixing of their solutions.The formation of polymer chains is most likely a kinetically con-  trolled process which, on mixing twenty five times more concentrated solutions (0.5 mM instead of 0.02 mM), shall be ca.625 times accelerated.Thus it can give rise to a significant number of longer chains that do not dissociate during the SEC analysis thanks to their slow constitutional dynamics.Thus the degree of polymerization, X, of Fe-polymer chains in solution could be estimated.If the peak eluted at t el = 1460 s (see Fig. 4) is ascribed to dimers, the peak with t el = 1305 s to trimers, and so on, one can still resolve the peak of heptamers at t el = 1114 s.Calculations based on this peak assignment provide the weight-average degree of polymerization equal to ca. 7 for Fe-polymer in the solution with r ≅ 1 (Table 3).In addition, it is seen from Fig. 4 that the stoichiometric excess of Fe 2+ ions in solution results in the formation of shorter chains.The latter is supported by the results of viscometric measurements, which also indicate shortening of the polymer chains in the presence of excess Fe 2+ ions (ESI, Fig. S15 †).

Assembly of Q2457-P + in water
The water-soluble unimer Q2457-P + has been assembled with metal ions also in aqueous solutions.Since molecular dissolution of this unimer in water takes a long time a month-old solution of Q2457-P + was used in these experiments.As can be seen from Fig. 6, the optical spectral changes accompanying the assembly in water substantially differ from those observed for assembly in methanol.
(i) The absorption maxima of Q2457-P + (λ A = 400 nm) and its Zn-polymer (r = 2, λ A = 462 nm) as well as the luminescence maximum of the unimer (λ F = 555 nm) are red shifted by about ca.20 nm compared to their positions in methanol solutions, which indicates that the free as well as enchained unimer species acquire more planar conformations in water than in methanol.This can be attributed to the substantial increase in the solvent permittivity, which, in accord with the Coulomb law, reduces repulsive ionic interactions among neighbouring P + Et 3 groups as well as their attractive interactions with counterions.
(ii) The luminescence emission band observed for Znpolymer (λ F = 720 nm) is enormously red shifted (about 168 nm) compared to the band for methanol solution.The Stokes shift for P Zn Q2457-P + in water (7750 cm −1 ) is much higher than the shift in methanol (4650 cm −1 , Table 1), which proves the much higher extent of conformational relaxation of excited states in aqueous compared to methanol solutions.
(iii) The UV/vis spectra for assembly of Q2457-P + with Zn 2+ ions show a single set of isosbestic points and a fluent course of changes up to r = 2. Luminescence spectra indicate the presence of free unimers in solution with r equal to at least 1.5.These features consistently indicate a lowered stability and increased constitutional dynamics of P Zn Q2457-P + in aqueous solutions.
(iv) Surprisingly, in accord with the last mentioned observations, the UV/vis spectra for assembly of Q2457-P + with Fe 2+ ions show small changes and a weak MLCT band and the luminescence spectra show emission even at the composition ratio r = 3.It should be stressed here that no new emission band occurs; only reluctant luminescence quenching with increasing r is observed.The observed spectral changes indicate that the chains of a highly ionic Fe-polymer are, in aqueous solution, less stable than the chains of its Zn-counterpart.7][48][49] The reduced stability of the ionic Fe-polymer in water is obviously associated with the inhibition of the MLCT process by water.
The solvent dependence of the MLCT is well known. 50,51

Conclusions
The synthesis strategy developed here enables preparation of bis(tpy)quaterthiophenes with two or four side groups symmetrically distributed along the quaterthiophene central block.
The modification of side groups enabled preparation of ionic unimers that are soluble in green solvents such as alcohols or even in water.Optical spectral patterns of dissolved unimers and the corresponding polymers depend primarily on the distribution of side groups along the quaterthiophene central block and only secondarily on the nature of side-chain-capping groups.The effect of the latter is more apparent in the solid state spectra since the capping groups significantly influence molecular packing.
During the assembly of unimers with metal ions the development of UV/vis spectra with increasing ratio r conclusively indicates that the P + -unimers assemble with metal ions less readily than the Br-unimers.Besides, in water, Q2457-P + assembles with Zn 2+ ions considerably less progressively (with rising r) than in methanol and, with Fe 2+ ions, still much less readily, showing only a very weak MLCT band but significant luminescence of the free unimer even at the ratio r = 3.The solvation effect is thus obvious.A red shift of the luminescence band of P Zn Q2457-P + by about ca.170 nm on going from methanol to aqueous solution is observed.Such a big shift indicates much higher conformational freedom of P Zn Q2457-P + chains in aqueous compared to methanol solutions.

Fig. 1
Fig. 1 Changes in UV/vis spectra accompanying the titration of ionic unimers with Zn 2+ ions.Initial unimer concentration 2 × 10 −5 M in methanol, room temperature.Column (a) shows the first stage of assembly, column (b) shows the second stage and column (c) shows the third stage; see the text.

Fig. 2
Fig. 2 Changes in UV/vis spectra accompanying the titration of ionic unimers with Fe 2+ ions.Initial unimer concentration 2 × 10 −5 M in methanol, room temperature.Column (a) shows the first stage of assembly, column (b) shows the second stage and column (c) shows the third stage; see the text.

Fig. 5
Fig. 5 The UV/vis DAD spectra of SEC fractions of the Fe 2+ /Q45-Br system (r = 0.8) eluted at different elution times t el (a) and comparison of the spectra of the last fraction (dimers; t el = 1470 s) of systems of composition r from 0.2 to 1.0 (b).

Table 2
Calculated geometry of the unimers; δ BC ⋯δ DD' are dihedral angles between the planes of neighbouring main-chain rings given in subscript (for ring labels see Chart 1) a Values are not available for the first 720 hours.

Table 3
Number-average (X n ) and weight-average (X w ) degrees of polymerization and dispersity index (Đ) of P Fe Q45-Br in solution calculated from SEC records