Reactive & Functional Polymers 32 (1997) 179- 185

REACTIVE 84

FUNCTIONAL POLYMdRS

Luminescence behavior of tris(2,2’-bipyridine)ruthenium(II) and (2,2’-bipyridine)bis(2,2’-bipyridine-4,4’-disulfonate)ruthenate(II) With

cobalt(III)-complex-containing poly(methacrylic acid)

Masahiro Suzuki a, Mutsurni Kimura a, Kenji Hanabusa a, Hirofusa Shirai a,*, Yoshimi Kurimura b

0 Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu University, Ueda, Nagano 386i, Japan b Department of Chemistry Faculty of Science, Ibaraki University, Mito, Ibaraki 310, Japan

Received 1 July 1996; revised version received 9 September 1996; accepted 10 October 1996

Abstract

Luminescence behavior of Ru(bpy): (bpy = 2,2’-bipyridine) and Ru(bpy)(bpds)i- (bpds = 2,2’-bipyridipe-4,4’- disulfonate dianion) in aqueous solution of cobalt(III)-complex-containing poly(methacrylic acid) (Co”‘PMA), @hich is affected by a unique pH-induced conformational transition, was investigated by means of fluorescence spectroscopy. The bell-shaped pH dependence of the luminescence intensity of the photoexcited Ru(bpy):f with a maximum value at pH 5 was observed for Ru(bpy):+ in Co”‘PMA solutions. The maximum wavelength of the luminescence (h,,) also depended on pH and the blue shift was observed at pH 5. The luminescence behavior of the photoexcited Ru(bpy)(bpds)z- was independent of pH in Co”‘PMA solutions. The Ru(bpy):+ species were strongly interacted with CoPMA at pH 5, while the Ru(bpy)(bpds)z- species had no interaction. Stem-Volmer plots for the quenching of Ru(bpy)(bpds):- with Co”‘P

Y A gave

straight lines at various pH. The fact also indicated that the Ru(bpy)(bpds):- species have no interaction with Co”’ MA at any pH. The observed quenching constants significantly depended on pH and sharply decreased in the pH range of 3.5 to 5. It was suggested that the quenching reaction was influenced by the pH-induced conformational transition of Co”‘PMA.

Keywords: Conformational transition; Luminescence quenching; Stem-Volmer plot; Macromolecule-metal complex

1. Introduction

Microheterogeneous media such as micelles, vesicles, microemulsions, and polyelectrolytes [ 1,2] significantly affect the physicochemical properties of molecules. Actually, microhetero- geneous media have been increasingly utilized to control the photochemical reactions [3-6]. It is known that the photoinduced electron-transfer

* Corresponding author.

rate between excited state of Ru(bpy):+ and methylviologen are greatly accelerated in the presence of anionic micelles [7,8] or anionic polyelectrolytes [9-l 11. This acceleration is at- tributed to the concentration effect of the reacting agents in anionic domain by electrostaticaIly at- tractive interaction.

Hydrophobic interaction is also an imflortant factor because it greatly affects the photophysi- cal and photochemical properties of metals com- plexes [ 1 l-141. For example, the luminescence

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180 M. Suzuki er al. /Reactive & Functional Polymers 32 (1997) 179-185

intensity and the lifetime of the excited state of Ru(bpy);+ in aqueous solution are dramatically enhanced by addition of poly(sodium styrene- sulfonate) [11,15,16]. This is attributed to the hydrophobic interaction between Ru(bpy)i+ and PSS as well as electrostatic interaction.

It has been reported that poly(methacrylic acid) (PMA) undergoes a unique pH-induced conformational transition from a compact coil to a compact micelle-like structure, and then an expanded structure with increasing pH [12,17- 20]. In the pH-induced conformational transition, the luminescence intensity and lifetime of the excited sate of Ru(bpy)F show the bell-shaped pH dependence with increasing pH through a maximum value at pH 5, and the luminescence maximum shifts to the shorter wavelength. These are explained on the assumption that Ru(bpy)t+ species are incorporated into the hydrophobic micelle-like domain by the electrostatic and hy- drophobic interactions [l&20]. It has also been reported that Co(III)-complex-containing PMA (Co’uPMA) with a low coordination degree un- dergoes a pH-induced conformational transition similar to the parent PMA [20]. However, the luminescence intensity and the lifetime of the ex- cited state of Ru(bpy):+ are smaller than those of Ru(bpy);+ in PMA solution due to the fact that the Ru(bpy):+ is quenched by the Co@) complex.

In the present paper, luminescence behavior of the photoexcited Ru(bpy)(bpds)i- in aqueous solution of PMA and Co”‘PMA was investigated in connection with that of Ru(bpy)i+.

2. Experimental

2.1. Materials

Poly(methacrylic acid) (PMA) having an av- erage molecular weight of 370000, determined by viscosity measurement, was used. Tris(2,2’- bipyridine)ruthenium(II) dichloride hexahydrate (Ru(bpy)3Cl2 - 6H2O) was prepared and purified according to the literature [21]. Disodium 2,2’- bipyridine-4,4’-disulfonate (Nsz(bpds)) and di-

sodium (2,2’-bipyridine)bis(2,2’-bipyridine-4,4’- disulfonate)ruthenate(II) (Naz[Ru(bpy)(bpds)z]) were obtained by the method described elsewhere [22]. The cis-[Co”‘(en)2(OH2)]-containing PMA, where ‘en’ is ethylenediamine, was synthesized by the method reported in the literature [20]. The cis-[Co1n(en)2(OH2)]-containing PMA hereafter is abbreviated to Co’nPMA.

2.2. Measurements

Luminescence spectra were recorded on a Hi- tachi 650-60 fluorescence spectrophotometer by using 1 cm x 1 cm quartz cell. The excitation wavelength for Ru(bpy)F and Ru(bpy)(bpds$ were 453 nm and 470 nm, which corresponded to its absorption maxima. Sample solutions for Ru(II)/CoPMA were adjusted to 2.0 x 10e3 mol dme3 based on the unit concentration of Co@) residue. The concentration of Ru(I1) complexes was 2.0 x 1 0m5 mol dmm3. The ionic strength was adjusted to 0.1 by NaCl. The pH of the solutions was adjusted by addition of dilute hydrochloric acid or sodium hydroxide dissolved in redistilled water. All measurements were carried out under an argon atmosphere at 25°C.

3. Results and discussion

The chemical structures of CoPMA and Ru(bpy)(bpds)i- are shown in Fig. 1.

3.1. pH ej%ect

Both luminescence intensity and maximum wavelength of the photoexcited Ru(bpy)(bpds);-, referred to as *Ru(bpy)(bpds&-, in aqueous so- lution were constant in the pH range of 2 to 9. It indicates that four sulfonic acid groups in the Ru(bpy)(bpds)i- are deprotonated in this pH range. In other words, the Ru(bpy)(bpds$ has bivalent negative charge in this pH range.

The pH dependence of the relative lu- minescence intensity Z/lo and the maximum wavelength of the luminescence spectra of *Ru(bpy)(bpds);- in PMA solution are shown

M. Suzuki et al. /Reactive & Functional Polymers 32 (1997) 179-185 181

: ethylenediamine

CoPMA Fig. 1. Chemical structure of Ru(bpy)(bpds$ and CoPMA.

1.5 I MO T 640 1 Qu “02 O” 4 I

0.5 I 600 123456789

PH Fig. 2. Dependence of relative luminescence intensity (I/lo) and maximum wavelength (J.,,,,,) of *Ru(bpy)(bpds)z- on pH in PMA solution.

in Fig. 2. Here I and IO are the luminescence intensity of *Ru(bpy)(bpds)i- in solution with and without PMA, respectively. The lumines- cence intensity and the maximum wavelength of *Ru(bpy)(bpds$ were constant regardless of pH, and almost the same as those in the solu- tion without PMA. In the previous paper [ 181, the luminescence intensity of the photoexcited Ru(bpy);+, referred to as *Ru(bpy)i+, showed the bell-shaped pH dependence with the max- imum value at pH 5 and the maximum wave- length shifted from 609 nm to 595 nm at pH 5. PMA undergoes a pH-induced conformation al transition from a compact coil structure at low pH to an expanded one above the pH 6 through a compact micelle-like one at pH 5 [ 17,181. The *Ru(bpy)F sp ecies have no interaction with the polymer of the compact coil structure at low pH, while it is electrostatically interacted with the

expanded polymer at high pH. the luminescence behavior of the *Ru(bpy)i+ species for both the compact and expanded structure is similq to that in the solution without PMA. At near pH 5, how- ever, the *Ru(bpy)t species are incorporated and restricted into the hydrophobic mice/lie-like structure of PMA, leading to the enhadcement of the luminescence intensity and the bltie shift of the maximum wavelength. The Ru(bfly)i+ is incorporated into the hydrophobic rnicetlle-like structure by both electrostatic and hydr+phobic interactions; the Ru(bpy)t+ approaches the poly- mer by electrostatic attraction, and then is incor- porated into the hydrophobic micelle-like domain by hydrophobic interaction.

By contrast, the Ru(bpy)(bpds)i- species have no interaction with the polymer at any pH be- cause the electrostatic repulsion between the Ru(bpy)(bpds)z- having negative charge land the negative charges of the polymer hinders their ap- proach each other. Even if the Ru(bpy)(bpds);- species should approach the polymer doMain, it would have no interaction with the hydrqphobic micelle-like domain due to the ionic chariacter of the sulfonated bipyridine ligand.

Fig. 3 shows the pH dependefice of Z/lo and the maximum waveleneth of the *Ru(bpy)(bpds$- in Co”‘PMh. The pH dependence were similar to @at for Ru(bpy)(bpds&/PMA, however, the lpmines- cence intensity was smaller than that in PMA so- lution. This indicates that the *Ru(bpy)(bpds$ species are quenched by Co(II1) uomplex on CoPMA. Later, it will be codsidered

182 M. Suzuki et al. /Reactive & Functional Polymers 32 (1997) 179-185

0.5 - 600 123456769

PH Fig. 3. Dependence of relative luminescence intensity (Z/lo) and maximum wavelength (h,-,& of *Ru(bpy)(bpds$ on pH in CoPMA solution.

in detail with respect to the quenching of *Ru(bpy)(bpds&- by Co”‘PMA.

3.2. Quenching studies

The interaction of the *Ru(II) complexes with Co”‘PMA was investigated by the electron- transfer quenching between them. The *Ru(II) complexes are quenched by Co(III) complex as expressed by Eqs. 1 to 3:

Ru(I1) ; *Ru(II) (1)

*Ru(II) + Co”‘PMA 3 Ru(II1) + Co”PMA (2)

Ru(III) + Co”PMA + Ru(II) + Co”‘PMA (3)

It has been reported that Co”‘PMA causes the pH-induced conformational transition similar to pMA, and the environment in the vicinity of the Co(II1) complex significantly changes with the conformational transition [20]. The CoPMA has two kinds of Co@)-complex species: one is reactive species exposing to aqueous phase, the other is unreactive species shielded and protected by polymer backbone. The latter cannot react with any chemical species. Therefore, in these quenching studies, it is necessary to use the con- centration of the former Co(II1) complex, but not

the total concentration of Co(II1) complex. For- tunately, the degree of reactive Co(II1) complex on CoPMA at various pH has been reported [20], so that the concentration of the reactive Co(III) complex was able to be calculated from the total Co(II1) concentration. Since CoPMA had absorp- tion at the excitation wavelength of the Ru(I1) complexes, the luminescence intensity was cor- rected for absorption of the incident light by the Co(III) complexes using Eq. 4 [23]:

(4)

where (IO/&s is the observed value; Ad and A, are the absorbances for the Ru(II) complex and Co(II1) complex, respectively. The data were analyzed by the Stem-Volmer equation 5:

10

0 r = 1 + &q)obs ’ TO * [co(III)Ir (5)

CO*

where (k&&s is the observed quenching con- stant, to is the luminescence lifetime of Ru(II) complexes in solution without the quencher, and [Co(III)], represents the concentration of reactive Co(II1) complex.

Fig. 4 shows Stern-Volmer plots for the quenching of *Ru(bpy)z+ with Co”‘PMA at pH 2, 5, and 8. The Stem-Volmer plot at pH 2 was linear, and the (kq)&s calculated from the slope

2

0.5 0 1 2 3

[Co(lll)] / 10” M

Fig. 4. Stem-Volmers plot for quenching of *Ru(bpy):+ by CoPMA at pH 2 (o), 5 (0). and 8 (A).

M. Suzuki et al. /Reactive & Functional Polymers 32 (I 997) 179-185 I 8:s

of its straight line was about 4.47 x lo9 M-’ s-l, The lo/Z at pH 5 and 8 was constant regardless of the Co(III) concentration. This indicates that the *Ru(bpy)z+ species interacting strongly with the polymer backbone, were quenched. In fact, *Ru(bpy);+ species in PMA have hydrophobic interaction at pH 5 and electrostatically attractive interaction at pH 8 with the polymer backbone [l&19], so that the present quenching reactions at pH 5 and 8 would be regarded as intra-polymer processes. This assumption is supported by the fact that the luminescence intensity was con- stant even when the Co(II1) concentration in- creased. The analytical method for kinetics of intra-polymer electron transfer was reported in our previous paper [24].

At pH 8, the *Ru(bpy):+ species were not appreciably quenched by Co”‘PMA. This result can be explained as follows. Co”‘PMA takes an expanded structure because most of the carboxy groups on Co”‘PMA dissociates. It is unlikely that *Ru(bpy)z+ species are electrostatically in- teracted with carboxylate anion in the vicinity of Co(II1) complex on Co”‘PMA having low de- gree of coordination. In other words, *Ru(bpy)t+ species would not approach to Co(II1) complexes in the region where electron-transfer quench- ing between *Ru(bpy)t+ and Co(II1) complex occurs. Consequently, it becomes difficult to quench *Ru(bpy):+ with Co(II1) complex.

On the other hand, the lo/Z for quenching of *Ru(bpy);+ at pH 5 was below 1. The lu- minescence intensity at pH 5 may be governed by two factors: one is an enhancement effect for the increasing of the luminescence intensity, the other is the quenching effect which makes the luminescence intensity decrease. As men- tioned above, the *Ru(bpy)i+ species are incor- porated into the hydrophobic micelle-like domain on Co”‘PMA at pH 5, therefore, the enhancement effect works strongly. Since the luminescence in- tensity of *Ru(bpy): in Co”‘PMA solution is clearly lower than that in PMA solution, the quenching effect also works. Further, the lumi- nescence intensity of *Ru(bpy)z+ in Co”‘PMA is larger than that in the solution without Co”‘PMA.

0.5 - 0123456

[Co(lll)] / 1O-3 M

Fig. 5. Stem-Volmer plots for quenching of *Ru(bpy)i(bpds)z- by CoPMA at pH 2 (o), 5 (O), and 8 (A).

These led us to conclude that the enhancement effect rather than the quenching effect would be predominant for the luminescence intensity in quenching at pH 5. Actually, the lo/Z of the Stem-Volmer plot at pH 5 is small as shown in Fig. 4.

Stem- Volmer plots for electron-transfer quenching of *Ru(bpy)(bpds)- with Co”‘PMA at pH 2,5. and 8 is shown in Fig. 5. The obtained straight lines strongly support that *Ru(bpy)(bpds)- species have no interaction with polymer backbone at any pH. The (ks)& for Ru(bpy)(bpds)z-/Cot”PMA and Ru(bpy)i+/Co”‘PMA, calculated from the slope of the straight line in each pH, are summarized in Table 1. The (kq),bs for Ru(bpy)(bpds)i-/Co”‘PMA at pH 2 is about three times as less as that for Ru(bpy):+/Co’r’PMA. The result may be best interpreted by the

Table 1 Observed quenching constants ((ks),~,/109 M-’ s-’ ) for quenching of Ru(I1) complexes by Co”tPMA at various pH

PH Ru(bpy):+ Ru(bpy)(bpds):’

2.00 4.47 zt 0.03 I .23 + 0.02 3.00 I .26 * 0.01 3.53 1.11 hO.04 5.01 0.36 zt 0.02 5.25 0.14 * 0.03 6.01 0.16 f 0.02 7.02 0.15 f 0.01 8.02 =0 0.15 f 0.04

184 ht. Suzuki et al./Reactive & Functional Polymers 32 (1997) 179-185

123458789 PH

Fig. 6. Plots of the logarithm of the quenching constants versus pH for *Ru(bpy)(bpds);-/CoPMA.

steric hindrance of the polymer backbone with Ru(bpy)(bpds)z- having bpds of bulky ligand, and the long distance between Ru(I1) of reactive cen- ters and Co(III) complexes. The relationship be- tween the log@,) and pH, based on the data of Table 1, is plotted in Fig. 6.

The reaction rate sharply decreased in the pH range of 3.5 to 5.5. Co”‘PMA undergoes the pH- induced conformational transition as expressed by Eq. 6:

-H+ -H+ CC + CM T= E

+H+ +H+ (6) low pH pH=5 high pH

where Cc, CM, and E represent a compact coil structure, a compact micelle-like one, and an ex- panded one, respectively [20]. The reaction rate was independent of pH in the low pH range below 3.5 where the polymer backbone took Cc. The re- action rate was also independent of pH in the high pH range above 5.5 where the structure of poly- mer was E. The kqobs in the Cc is about 8 times as large as that in the E. This is attributed to the elec- trostatic repulsion between the Ru(bpy)(bpds)z- species and Co”‘PMA at high pH. Further, it is necessary to consider the contribution of the lo- cal ionic strength in polymer domain for each conformation, because electron transfer between Ru(bpy)(bpds$ and Co(II1) complex occurs in the polymer domain. Generally, it is known that the ionic strength in heterogeneous media such as polyelectrolytes differs from that in bulk solu- tion. Though the ionic strength in the solution is

adjusted to 0.1 in the present experiments, they are different from each other in polymer domains of Cc, CM, and E. The ionic strength in polymer domain for Cc may be similar to that in bulk so- lution, because the electrostatic field hardly exists in the neighborhood of polymer chain whose car- boxy groups are almost undissociated. However, the strong electrostatic field is formed in polymer domain of E which possesses many dissociated carboxy groups; thus, the local ionic strength in the polymer domain of E is larger than that in bulk solution. An increase of the ionic strength in polymer domain of reaction field may reduce the electrostatic attraction between reactive centers, leading to a decrease of reaction rate.

On the other hand, the reaction rate sharply decreased in the pH range of 4.0 to 5.5, where the polymer conformation changed from the com- pact coil structure to the compact micelle-like one. When the compact micelle-like structure is formed by hydrophobic interaction between methyl groups, the exposure of most of the car- boxy groups to the bulk aqueous phase happens, then the negative charge density in polymer do- main increases. Therefore, it is suggested that both the increase in the negative charge density and the steric repulsion in the polymer domain cause the decrease of the reaction rate in the pH range of 4 to 5.5. In other words, the reaction rate is reduced by the electrostatic repulsion between the polymer domain having negative charge and the Ru(bpy)(bpds)i- species, and the steric hin- drance of the polymer backbone. As described above, an increase of the local ionic strength in polymer domain is also one of the factors.

In conclusion, the luminescence behavior of Ru(bpy)(bpds$ in PMA and Co”‘PMA solution was independent of pH. The *Ru(bpy)(bpds)z- species had no interaction with the polymer chain at any pH. In the quenching experiments, the Stern-Volmer plot for Ru(bpy)T/Co”‘PMA was linear below pH 2 and the observed quench- ing constant was 4.47 x lo9 M-’ s-i, while it was nonlinear at pH 5 and 8. It is sug- gested that *Ru(bpy): species were interacted with the polymer backbone by hydrophobic

M. Suzuki et al. /Reactive & Functional Polymers 32 (1997) 179-185 185

interaction at pH 5 and by electrostatic in- teraction at pH 8. The linear Stem-Volmer plots for the quenching of *Ru(bpy)(bpds)i- by Co”‘PMA were obtained, supporting the presumption that the *Ru(bpy)(bpds)i- species were hardly interacted with the polymer back- bone at any pH. The quenching constant for Ru(bpy)(bpds)i-/Co”‘PMA at pH 2 was 3 times less as that for Ru(bpy):+. This is attributed to the steric hindrance of the polymer backbone with Ru(bpy)(bpds)i- having bpds of bulky lig- and. The observed quenching constants obtained from Stem-Volmer plots at various pH signifi- cantly depended on pH: the observed quenching constants were constant below pH 3.5 and above pH 5.5, and sharply decreased in the pH range of 4.0 to 5.5. Further, the observed quenching constant below pH 3.5 was about 8 times as large as that above pH 5.5. The pH dependence was in- terpreted by the pH-induced conformational tran- sition of Co”‘PMA as follows:

(1) As the negative charge density in the poly- mer domain increases with the increase of pH accompanied by the conformational transition, electrostatic repulsion between Ru(bpy)(bpds);- and polymer domain increases.

(2) When the conformational transition from the compact coil to the compact micelle-like structure occurs, most of the carboxy and car- boxylate groups on Co”‘PMA are exposed to aqueous phase, then the polymer domain be- comes bulky. Consequently, the reaction rate decreases by the steric hindrance between Ru(bpy)(bpds)z- and polymer chain.Unlinked BIB’s List:[13,14]

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