Reactive & Functional Polymers 40 (1999) 241–248www.elsevier.com/ locate / react

NMF effect on photosensitized charge separation using partiallyquaternized poly(1-vinylimidazole)-bound ruthenium(II)

complexes1Masahiro Suzuki , Naoki Yokoyama, Mutsumi Kimura, Kenji Hanabusa,

*Hirofusa ShiraiDepartment of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu University, Ueda, Nagano 386, Japan

Received 6 February 1998; received in revised form 16 March 1998; accepted 10 April 1998

Abstract

The solvent effects on the photosensitized charge separation using partially quaternized poly(1-vinylimidazole)-boundruthenium(II) complexes (RuQPIm), in which bis(2,29-bipyridine)ruthenium(II) complexes are coordinated to the imidazolylresidues on the poly(1-vinylimidazole) partially quaternized by 1-bromohexadecane and the degree of quaternization is 19(RuQPIm-19) and 44 (RuQPIm-44) molar percentage, have been investigated in methanol and methanol–NMF (NMF:N-methylformamide). These systems consist of RuQPIm-19 and RuQPIm-44 as photosensitizers, 1,19-dimethyl-4,49-

21 21bipyridinium dication (MV ) and 1,19-didodecyl-4,49-bipyridinium dication (C V ) as electron acceptors, and tri-12

ethanolamine (TEOA) as a sacrificial donor. The addition of NMF mainly affect the forward and back reactions in the chargeseparation reactions. In RuQPIm-19 systems, the reaction proceeds through an interactive process, which is through the

21 21viologen having p–p interaction for MV and van der Waals interaction for C V . In contrast, the reaction proceeds1221 21through a direct process by MV having no interaction and through an interactive process by C V undergoing van der12

21 ?1Waals interaction with the polymer for RuQPIm-44 systems. For MV , the rates of MV formation increased, although thequenching efficiency decreased with increasing NMF content for RuQPIm-19 and RuQPIm-44. These results are attributed to

?1 ?1stabilization of MV species by p–p interaction for RuQPIm-19 and steric repulsion between MV species and21 ?1RuQPIm-44; namely, the restriction of the back reactions by these effects. In contrast, for C V , the rates of C V12 12

formation decreased and the quenching efficiency increased with the addition of NMF. These are attributed to that the21 ?1addition of NMF increases the van der Waals interaction of the C V with these polymers and the diffusion of the C V12 12

species into the bulk solution; namely, the back reaction is accelerated. Furthermore, it is suggested that the conformationalchanges in these polymers contribute to the charge separation reaction. 1999 Elsevier Science B.V. All rights reserved.

Keywords: Photosensitized charge separation; Poly(1-vinylimidazole); Ruthenium(II) complex; Methylviologen; Solvent effect

1. Introduction significantly affected by solvents, temperature,and distance between reaction centers [1–3].

Photoinduced electron transfer reactions are Although the kinetics of many electron transfersare diffusion-limiting, the diffusional compo-nent in photoinduced electron transfer reactions*Corresponding author.

1 can be neglected by attachment of the donor andResearch Fellow of the Japan Society for the Promotion ofScience (JSPS Research Fellow). acceptor. Therefore, such systems have been

1381-5148/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PI I : S1381-5148( 98 )00039-X

242 M. Suzuki et al. / Reactive & Functional Polymers 40 (1999) 241 –248

frequently applied to investigations of photo- CO., Ltd., and was purified by recrystallizationinduced electron transfer reactions [4–6]. from methanol. 4,49-bipyridine and 1-bromo-

Ruthenium complexes are among the most dodecane, which were commercially availablewidely studied organometallic molecules in guaranteed reagents, were used without furtherrecent years because of their variety of attrac- purification. 2,29-Azobisisobutyronitrile (AIBN)tive functions based on luminescence properties was recrystallized from methanol at 308C. 1,19-with a long excited state lifetime, redox prop- Didodecyl-4,49-bipyridinium bromide

21erties, excited state reactivities and relatively (laurylviologen: C V ) was prepared by heat-12

high chemical and thermal stability. Such ing a N,N-dimethylformamide solution of 1-unique features make the ruthenium(II) complex bromododecane and 4,49-bipyridine and wasdoped materials quite attractive as optical sen- purified by recrystallization from methanol.sors [7–10], photocatalysts [11], photoelec- Poly(1-vinylimidazole) (PIm) was prepared bytrodes for solar cells [12,13], and nonlinear free-radical polymerization of freshly distilledoptical materials [14]. In particular, the 1-vinylimidazole with AIBN as an initiator intris(2,29-bipyridine)ruthenium(II) complex, ethanol at 708C for 12 h under a nitrogen

21 atmosphere. The PIm was purified by reprecipi-Ru(bpy) , has been widely employed as a3

tation twice from methanol into acetone. Thephotosensitizer in a variety of energy andaverage molecular weight was determined byelectron transfer reactions [5,6,15,16].viscometry [28] and estimated to be aboutPoly(1-vinylimidazole) and poly(4-vin-100 000. The PIm-bound ruthenium(II) com-ylpyridine)-bound ruthenium(II) complexesplex (RuPIm) and partially quaternized poly(1-have been investigated, and application of thesevinylimdazole)-bound ruthenium(II) complexespolymers to catalytic and photofunctional ma-(RuQPIm) were prepared by a previously de-terials has been suggested [17–23]. We havescribed method [21,24,25].reported on the quenching [24], the photosensit-

ized charge separation [25], and the photoin-duced hydrogen generation [26,27] using par- 2.2. Measurementstially quaternized poly(1-vinylimidazole)-bound

UV–VIS spectra were recorded on a JASCOruthenium(II) complexes (RuQPIm). These re-V-570 UV/VIS/NIR spectrophotometer.sults indicated that the polymer photosensitizersLuminescence spectra of the photoexcited RuQ-have excellent photosensitizing ability. Further-PIm were recorded on a Hitachi 650-10S fluo-more, non-covalent bondings such as van derrescence spectrophotometer. A JASCO FTIRWaals and electrostatic interactions played an7300 Fourier-transform spectrophotometer wasimportant role in these reactions. In the presentused for the measurements of Fourier-transformwork, the solvent effects on the photosensitizedinfrared (FTIR) spectra.charge separation reaction using RuQPIms as

The photosensitized charge separation re-polymer photosensitizers were investigated inactions were carried out in solutions containingorder to obtain the detailed information on non-

25 232.5 3 10 mol dm ruthenium(II) complexcovalent interactions.23 23residue, 5.0 3 10 mol dm viologen, and 0.1

23mol dm triethanolamine (TEOA). A spectro-scopic cell (1 3 1 cm) containing RuQPIm,2. Experimentalviologen, and TEOA in methanol or methanol–NMF was irradiated with a 300 W slide projec-2.1. Materialstor (ELMO Co., Ltd., S-300) equipped with a

N-Methylformamide (NMF) and methanol UV cut filter (l . 440 nm). The absorbance ofwere purified by distillation. 1,19-dimethyl-4,49- the solution at 605 nm, which corresponds tobipyridinium chloride was purchased from TCI the absorption maximum of the viologen radical

M. Suzuki et al. / Reactive & Functional Polymers 40 (1999) 241 –248 243

3 21 21(e 5 13 400 dm mol cm ) [29], was605

monitored as a function of irradiation time. Theinitial rate of viologen radical formation wascalculated from the initial linear portion in theplotting of the concentration of the viologenradical versus irradiation time. The quenchingefficiency was calculated from the luminescence

23 23intensity at [viologen] 5 5.0 3 10 mol dmand defined as [(I 2 I) /I ], where I and I were0 0 0

the luminescence intensities in the absence andScheme 1.in the presence of the viologen.

hvII II→Ru QPIm *Ru QPIm (1)←3. Results and discussion

D

II 21 III ?1The chemical structure of RuQPIm and the *Ru QPIm 1 V → Ru QPIm 1 V (2)degree of coordination and quaternization areshown in Fig. 1. III ?1 II 21Ru QPIm 1 V → Ru QPIm 1 V (3)

When the visible light was irradiated on a21 21solution containing RuQPIm, MV or C V ,12 III IIRu QPIm 1 TEOA → Ru QPIm 1 (TEOA)oxand TEOA under an argon atmosphere, forma-

?1 ?1tion of the viologen radical (MV and C V ) (4)12

was observed. The photosensitized charge sepa-21ration was achieved as illustrated in Scheme 1: 3.1. RuQPIm-19 /MV system

21 ?1where V and V are alkylviologens and theirradicals. Oxidation of the ruthenium(II) com- Fig. 2 shows the NMF effects on the initial

?1plex, which is generated by visible light irradia- rates of MV formation and quenching ef-tion, is accomplished by electron transfer to ficiency [30] for RuQPIm-19 and RuQPIm-44viologen. The singly oxidized ruthenium(III) systems. For the RuQPIm-19 systems, the ratescomplex is then scavenged to its original state, increased with increasing NMF content, al-the ruthenium(II) complex, by a sacrificial though the quenching efficiency decreased. Inelectron donor, TEOA, which is an irreversible contrast, the rates slightly increased up to anelectron donor. Consequently, the viologen radi- NMF content of ca. 40% and then quicklycal species were accumulated. This series of increased for RuQPIm-44. As mentioned above,reactions are shown as expressed by Eqs. (1)– the photosensitized charge separation reactions(4).

?1Fig. 2. NMF effects on the initial rates of MV formation andFig. 1. Chemical structure of RuQPIm. quenching efficiency for RuQPIm-19 (d) and RuQPIm-44 (j).

244 M. Suzuki et al. / Reactive & Functional Polymers 40 (1999) 241 –248

proceed according to Eqs. (1)–(4). Eq. (1), [25,30]. Here, we define the charge separationcorresponding to the photoexciting process, is reaction through the dynamic forward reactionbarely affected by the NMF because the as a direct process. The inter- and/or intra-luminescence spectra do not change. Under a polymer van der Waals interaction betweenlarge excess of TEOA for the ruthenium(II) alkyl side chains on the RuQPIm-44 is in-complex residues, Eq. (4) (scavenge reaction) is creased by addition of NMF, and the density ofalso independent. Therefore, the reactions the alkyl side chains in the vicinity of thewould be affected by Eq. (2) [quenching re- ruthenium(II) complex residue increases. Theaction (forward reaction)] and Eq. (3) (back forward reaction is restricted by the steric

21reaction). The MV species undergo weak p– hindrance, although it is accelerated by a de-p interaction with the RuQPIm-19, and the crease in electrostatic repulsion. The stericforward reaction proceeds through only a static hindrance also restricts the back reaction. Theprocess which is electron transfer reaction from restriction of the forward reaction is counterbal-the photoexcited ruthenium(II) complex residue anced by the restriction of the backward re-

21to the MV species undergoing weak p–p action, consequently leading to a slight increase?1interaction with the RuQPIm-19. Because the in the rates of MV formation. At high NMF

observed Stern–Volmer plot well fits the theo- content, the rate is increased by the restrictionretical curve using the modified Stern–Volmer of the back reaction because the forward re-plot obtained from which the quenching re- action is independent of the NMF content.action occurs through only a static quenching Compared with the RuQPIm-19 system, the

?1process [30,31]. Here, we define the charge NMF effect on the rate of MV formation andseparation reaction through the static process as quenching efficiency is small. This fact can bean interactive process. With the addition of explained by the conformation of these poly-NMF having high polarity, the p–p interaction mers and the contribution of electrostatic effect.becomes strong, and the electrostatic repulsion Although the electrostatic effect on the reaction

21between the RuQPIm-19 and MV decreases. induced by addition of NMF is small, theThe strong p–p interaction restricts the static conformation change (shrinking) readily occurs

21quenching because the number of the MV because the positive charges and alkyl sidespecies which participates in the reaction de- chains on the RuQPIm-19 are relatively small.crease due to the decrease in the movement of In contrast, the large conformational change in

21 ?1the MV species. On the other hand, MV RuQPIm-44 having many positive charges (thespecies would be more stabilized by p–p quaternized imidazolium residues) and alkylinteraction because the charge is localized: side chains barely occurs due to electrostaticnamely, the back reaction is restricted. Conse- repulsion and steric hindrance, while thequently, the addition of NMF restricts the electrostatic repulsion for the reaction is veryinteractive process, while it accelerates the large. These facts suggest that the charge sepa-direct process, leading to an increase in the rate ration is significantly affected by the conforma-

?1of MV formation. tional change (shrinking); the shrinking of theRuQPIm-19 significantly restricts the interactive

21 process.3.2. RuQPIm-44 /MV system

21The NMF effect appears at high NMF con- 3.3. RuQPIm-19 /C V system12tent. The forward reaction mainly takes placethrough only a dynamic process because the Fig. 3 shows the solvent effects on the initial

21 ?1MV species have no interaction with the rates of C V formation and quenching ef-12

RuQPIm-44, being supported by the fact that ficiency. For RuQPIm-19, the rate decreasedthe Stern–Volmer plot shows a straight line with increasing NMF content, while the quench-

M. Suzuki et al. / Reactive & Functional Polymers 40 (1999) 241 –248 245

charge separation reaction proceeds through theinteractive process. In photoreduction of theviologens with long alkyl groups using RuQPImfilms [32], we reported that only the viologenadsorbing on the polymer film surface throughthe van der Waals interaction participated in thereaction, but not the free viologen species:namely, the direct process hardly took place.Considering this fact, in this system, the direct

?1Fig. 3. NMF effects on the initial rates of C V formation and12 process would barely occur. Furthermore, thequenching efficiency for RuQPIm-19 (d) and RuQPIm-44 (j).viscosity measurement indicates that the vis-cosity of the RuQPIm-44 is significantly de-

21ing efficiency was constant up to ca. 40% NMF creased by addition of the C V [30]. Since12

content and then increased. For RuQPIm-44, the the large conformational change in therate decreased and the quenching efficiency RuQPIm-44 such as shrinking does not occurincreased with the addition of NMF. The charge due to electrostatic repulsion between theseparation reaction in the systems is also domi- quaternized imidazolium residues, the result letsnated by the forward and back reactions. The us to conclude that the RuQPIm-44 is partially

2121 bridged through the C V [30]. With increas-C V species have van der Waals interaction 1212

ing the NMF content, because the van der Waalswith the alkyl side chains on the RuQPIm-19,interaction becomes strong and electrostaticbut hardly p–p interaction due to steric hin-

21repulsion between the C V and polymer isdrance. Since the forward reaction proceeds 12

weakened, further bridging of the RuQPIm-44through the static process, the charge separation21through the C V and the concentration ofoccurs through the interactive process. This 12

21C V species into the polymer domain takeinteractive process is effective, compared with 1221 place; therefore, the quenching efficiency in-the interactive process mentioned in the MV

creases. In addition, an increase in the interac-systems which is induced by p–p interaction.tive process also accelerates the back reaction,The p–p interaction restricts the degree of

21 21 consequently, leading to the decrease in the ratefreedom of MV , while the C V having12?1of C V formation. Compared with thevan der Waals interaction can move to some 12

RuQPIm-19 systems, the NMF effect on theextent due to flexibility of the long alkyl groups.?1rates of C V formation is small, and that onWith the addition of NMF having high polarity, 12

the quenching efficiency is large. This is attribu-the interactive process increases because the van21 ted to the fact that the reaction processes in theder Waals interaction between the C V and12

forward and back reactions are varied by athe RuQPIm-19 becomes strong. The increase inconformational change in these polymers; thethe van der Waals interaction brings about the

?1 RuQPIm-19 shrinks, while the RuQPIm-44 isrestriction of diffusion of the C V species12

partially bridged. The shrinking of the RuQPIm-formed into the bulk solution. Furthermore, the19 with increasing NMF content causes anvan der Waals interaction does not contribute to

?1 increase in the interactive process, leading to thestabilization of the C V . Consequently, the12?1decrease in the rates of C V formation. Theback reaction is accelerated and the rate of the 12

21?1 RuQPIm-44 is partially bridged by the C VC V formation decreases. 1212

species, which brings about an increase in the21 interactive process. Furthermore, a difference in3.4. RuQPIm-44 /C V system12

the NMF dependence of the quenching ef-21The C V species undergo only van der ficiency between both systems would be attribu-12

Waals interaction with the RuQPIm-44, and the ted to the fact that the conformation differs.

246 M. Suzuki et al. / Reactive & Functional Polymers 40 (1999) 241 –248

21Compared with the MV systems, the NMF Waals interaction, but barely p–p interaction.effect on the charge separation reaction clearly The photosensitized charge separation reaction

21differs: the quenching efficiency decreases and through the interactive process by MV under-?1the rates of MV formation increase for going the p–p interaction is more ineffective

21RuQPIm-19, while the factors show inverse than that by C V undergoing the van der12

behavior for RuQPIm-44 with increasing NMF Waals interaction because both the forward and21content. The MV species undergo p–p inter- back reactions take place readily. Addition of

21 NMF brings about p–p and van der Waalsaction, and the C V species undergo van der12

Fig. 4. Tentative illustration of the reaction mechanism for photosensitized charge separation.

M. Suzuki et al. / Reactive & Functional Polymers 40 (1999) 241 –248 247

?1interactions, and the interactive process through C V formation decreases. Furthermore, addi-12

the p–p interaction is restricted in the tion of NMF would vary the polarity of the21RuQPIm-19/MV system, while the interac- media and the conformation of these polymers.

tive process through the van der Waals inter- Based on these results, we propose the tentative21action occurs in the RuQPIm-19/C V sys- reaction mechanism as shown in Fig. 4.12

tem. Consequently, the rates increase and de- The NMF effect on the charge separation21 21crease for MV and C V , respectively. On reaction depends significantly on the degree of12

the other hand, the decrease in electrostatic quaternization. This is attributed to differentrepulsion increases the direct process for the interactions of the viologens with these poly-

21 21RuQPIm-44/MV system, while it accelerates mers and polymer conformations: the MVthe bridging of the RuQPIm-44 through the species undergo p–p interaction with the

21C V and the interactive process. Therefore, RuQPIm-19, but no interaction with the1221 21the rates increase and decrease for MV and RuQPIm-44, and the C V species undergo12

21C V , respectively. the van der Waals interaction with these poly-12

mers. In addition, the RuQPIm-19 shrinks by21addition of MV and the RuQPIm-44 is par-

21tially bridged by C V . These results suggest4. Conclusions 12

that such non-covalent interactions are veryWe reveal the NMF effects on the photo- important factor controlling the photosensitized

sensitized charge separation using RuQPIms. In charge separation reaction.the present systems, the forward reaction andthe back reaction significantly affect the charge

21separation reaction. In the MV systems, the Referencesphotosensitized charge separation reactionproceeds through the interactive process by the [1] G.J. Kavarnos, N.J. Turro, Chem. Rev. 86 (1986) 401.

21 [2] M.D. Newton, N. Sutin, Annu. Rev. Phys. Chem. 35 (1984)MV species having p–p interaction for21 437.RuQPIm-19 and the direct process by the MV

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[9] B.D. MacCraith, C. McDonagh, G. O’Keefe, A.K. McEvoy,der Waals interaction becomes strong: conse-?1 T. Butler, F.R. Sheridan, Proc. SPIE-Int. Soc. Opt. Eng.quently, the rate of C V formation decreases12 2288(Sol-Gel Optics III) (1994) 518.

with increasing NMF content. For the RuQPIm- [10] P. Kiernan, C. McDonagh, B.D. MacCraith, K. Mongey, J.2144/C V system, the polymer is partially Sol-Gel Sci. Technol. 2 (1994) 513.12

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