Effect of anionic molecular assembly environments on fluorescence quenching by electron transfer

Radlat Phys Chem Vol 38, No 2, pp 155-164, 1991 lnt J Radtat Appl Instrum, Part C Printed in Great Britain

0146-5724/91 $3 00+0.00 Pergamon Press pie

EFFECT OF ANIONIC MOLECULAR ASSEMBLY ENVIRONMENTS ON FLUORESCENCE Q U E N C H I N G BY ELECTRON TRANSFER M. WOLSZCZAKtand J. K. THOMAS:~ Department of Chemistry, University of Notre Dame, Notre Dame, IN 46556, U S.A (Received for pubhcation 30 November 1990)

Abstract--The quenching of the excited state of [tris(2,2'-bipyridme)ruthemum(II)][Ru(bpy)~ + ] by cationic quenchers [methylviologen, dlquat, copper(II) and thallium(I)] in molecular assemblies such as polyelectrolytes, polysoaps and mlcelles have been investigated by steady-state and time-resolved luminescence techmques. The emission quenching of photoexcited posmvely charged sensltizers was greatly enhanced in aniomc molecular assembhes in comparison to neat water. The quenching m poly(styrenesulfonate)--(PSS) solution Is found to be dynamic in nature In the absence of a quencher, the emission decay *Ru(bpy) 2+ in polyelectrolyte solution is single exponential. Upon addition of cationic quencher the emission decay is clearly nonexponentml The detaded studies showed that emission decay of *Ru(bpy)~ + over the whole range of catiomc quencher concentration, is well described by a general model of dispersed kinetics, in which there is a Gausslan &stribution of the logarithm of the rate constants about some mean. The distribution can be characterized by a mean rate constant,/~, and parameter y--the width of the Gausslan distribution. Laser flash photolysis was used to study the yield of electron transfer as well as the rate of the back reaction Addmon of the polyelectrolyte markedly decreases the efficiency of charge separation m the orgamzed system as compared to the aqueous solution. Steady-state spectroscopic and pulsed laser techniques have been used to investigate electron transfer from excited Ru(bpy)~ + to methylviologen in the PSS film The kinetics of luminescence quenching are consistent with electron-tunnehng mechamsm where the rate constant k (r) has an exponential dependence on the reactant separation &stance, r, with the matrix wbratlon, and is shown to be quite temperature dependent No quenching of *Ru(bpy)] + by methylvlologen was observed in glassy solutions or polymer films at 77 K.

1979; Matsuo et al., 1981; and this field has been recently reviewed by Rabam, 1988), only a few studies have been concerned with the kinetics of quenching of emitting chromophores in polyelectrolyte media (Mlyashita and Matsuda 1984; Chu and Thomas, 1985; Sassoon, 1986). In this paper we investigate the kinetics of the emission decay of an excited probe in the presence of a quencher molecule in polyelectrolyte solutions. Another aspect of this work concerns the quenching o f luminescence of an excited chromophore by electron transfer to a quencher, both o f these are m a PSS [poly(styrenesulfonate)] film. In contrast to a PSS solution where quenching is diffusion controlled, a polymer film is a convenient medium for the study of photo-Induced electron tunneling.

INTRODUCTION

Over the past decade there has been an increased interest m effects of organized assemblies on photochemical reactions (cf. Thomas, J984; Kalyanasundaram, 1982, 1987; Rabam, 1989). Systems such as mlcelles, vesicles, microemulsions, clay colloids, llposomes and polymers provide unique environments to achieve the efficient separation of ion-pair species following redox quenching of an excited state. It is believed that organized assemblies can be used to gain information about natural photosynthesis (Calvin, 1978) or physicochemical properties of blomolecules such as nucleic acids (cf. Gorner. et al., 1988). Several organized assemblies may have particular application in the field of solar energy conversion and storage (Harriman et aL, 1982; Whitten, 1983). Despite the great body of published papers devoted to photochemistry of polyelectrolyte systems (for instance), Meisel et al., 1978; Chen and Thomas,

EXPERIMENTAL

Materials

Trls(2,2'-bipyridme)ruthenium(II) chloride salt (Aldrich) was recrystaUized twice from deionized water (extinction coefficient at 452 n m - 14,600 dm 3 m o l - I c m - l). Methylviologen chloride salt, M V 2+ (Aldrich) was recrystallized twice from

tPermanent address' Institute of Applied Radiation ChemIstry, Technical University of ~V6d$, Wr6blewskIego 15, 93-590 E6d~, Poland. :~To whom correspondence should be addressed 155

156

M WOLSZCZAKand J K. THOMAS

methanol and dned overnight under vacuum. Aqueous solutions of dehydrated methylviologen showed absorption maxima at 258nm with a molar absorption coefficient at this wavelength of 20,500 dm 3 mol -I cm -l. Diquat &bromide, (6,7dihydrodlpyrido[1,2-a: 2', l'-c]pyrazinium) &bromade, was prepared as described by Homer and Tomhnson (1960), and was recrystallized twice from ethanolwater. Poly(styrenesulfonate) sodium salt (PSS, MW 500,000, Scientific Polymer Product) was purified by dialysis of 4% solutmn against 10 -3 mol dm -3 EDTA followed by dlalysas against water. PA-18 K2 whach is the potassmm salt of a 1:1 copolymer of malmc anhydride and 1-octadecene was prepared m our laboratory (from PA-18, a copolymer derived from 1-octadecene and malmc anhydride, supphed by Chevron/Gulf Co.). Cellophane film (DuPomt Chemmal Co.) was purified as previously described (Wolszczak and Stradowski, 1985) Benzoqmnone (Fisher) was purified by tripple vacuum sublimation. Copper chloride (Aldrich); Sodium dodecyl sulfate, SDS (BHD Chem.), Poly(wnyl alcohol), PVA (Aldrich); poly(wnylpyrrolidone), PVP (Aldrich); 9-methylanthracene (Eastman) and thalhum(I) sulfate (Fisher) were used without further purificatmn

Spectroscopac measurements Luminescence spectra and emission polanzahon studies were carried with a spectrofluorometer (SLM/Ammco, model SPF500) eqmpped wxth a 250 W Xe lamp, which was interfaced to a Zemth Z-238 computer. The intensity of the fluorescence emission ~s measured at parallel Ivy, InH and crossed IVH, Iav position of polarizing filters. Degree of emissmn polanzatmn is given by P=

I w -- Ivn (IHv/IHH)

lvv + IVH(Inv /InH)"

The subscripts denote the orientation of the electric vector of the hght which passes the slits: excitation (first letter) and emissmn second letter). V represents a vertical and H a horizontal orientation. Fluorescence lifetimes were determined with a laser system equipped with PRA LN-1000 mtrogen flow laser (337.1 nm, pulse w i d t h - - I ns, energy per pulse-1 mJ) or PRA LN-100 laser (337.1 nm, 0.12 ns, 70 #J) for excitatmn of the sample. In the case of Ru(bpy)] +, a nitrogen laser was used to pump a dye solutmn m 1 cm path length rectangular cuvette. For blue light excitation (~-,,a, = 4 2 0 n m ) an ethanol solution of the dye LD 420 (Excxtatmn) was used, and for green light excitation (2 = 490 nm) an ethanol solutmn of the dye LD 490 (Excitatmn). In transient absorption measurements a Candela dye laser (kin,x = 490 nm with Coumarln LD 490 dye, pulse width 120ns, energy per pulse 50mJ) was

used as the excitation source The expenmental equipment has been described in detail (Hashimoto and Thomas, 1983; Iu and Thomas 1989). All samples were deoxygenated by bubbling with prepurified mtrogen for 20 man immedmtely before taking the measurements. Poly(styrenesuifonate) films were prepared by slow evaporation of water from a 10% solution of the polymer, Ru(bpy)3z+, and methylvlologen on a plastm mold. The PSS foals were dried under a reduced pressure of about 20 torr at room temperature for 7 days. The films were removed from the molds, cut to saze and placed in a wide neck quartz cell. The cell was connected through a two-way stopcock to the vacuum lane, and after pumping the cell for 2 days the stopcock was closed. The films were carefully respected for evadence of mhomogenmty. The sample films were mounted at a 45 ° angle to the incident exmtatmn hght and the detector, to minimize hght scatter.

RESULTS AND DISCUSSION

(A) Effect of anionic assembhes on the electron transfer quenchmg of the emisston of photoexcited Ru(bpy)~ + by cationtc quenchers The steady-state and time-resolved quenching of *Ru(bpy)~ + by methylviologen (MV 2+ ) in the low region of MV 2+ concentration (2 × 10-5-2 × 10 -3 mol dm -3) m the presence of PSS, PAl 8-K2 and SDS shows that emission quenching reactmn: *Ru(bpy)~ + + MV 2+ ~ Ru(bpy)] + + MV +

(1)

is enhanced many tames an the presence of anmnic molecular assemblies an comparison to neat water. Quenching of steady-state emission exhabits large devmtmns from Stern-Volmer plots in the presence of a polyelectrolyte, Fig. 1. Under conditions where the M W + concentratmn as low, both the luminescence probe and quencher are bound on the PSS chain, and the fluorescence quenching is efficient (Fag. 1). In this case quenching of excited Ru(bpy) 2+ is completely dynamic in nature. In PSS solution the emission decay of *Ru(bpy) 2+ an the presence of MV 2+ becomes strongly non-exponential (Fig. 2) The emission decays of *Ru(bpy)~ +, for the whole range of MV 2+ concentrations, are well described by a general model for dispersed kinetics. This model was proposed by Albery et al. (1985). In numerous cases an adequate description of kinetics an heterogeneous systems has been achieved (Albery et al., 1985) if one assumed that free energy of actwation of a system can be described by a Gaussian distribution A consequence of at is that the dispersion an the first-order rate constants about a mean rate constant, /7, becomes: In(k) = ln(/~) + 7x

(2)

Effect of aniomc molecular assembly environments

157

,/

Z'n

+

+ \

3O

20

10

~

1

0

""'+"~ x , 00004 00006

00002

I 1 00008 Q001

' 00012

x 00014

.... xI 00016

, 00016

r M V 2 + ] ) mo( dm -3 Flg

1 S t e r n - V o l m e r plots for the l u m i n e s c e n c e q u e n c h i n g o f excited Ru(bpy)~ + b y m e t h y l v l o l o g e n in

water (1) and in aqueous solution contalnmg poly(styrenesulfonate) Polymer concentrations are given in molanty of monomer umts. (2) 0.010, (3) 0 0078, and (4) 0 005 mol dm -3. Comparison of(A) actual and (©) Gaussian simulated steady-state MV2÷ quenching of *Ru(bpy)2+ derived from equatmn (5) and the Ume-resolved luminescence decay fitting parameters (Table I).

The extent of luminescence quenching reaction, can be expressed by an integration of individual firstorder reactions across the normal distribution, exp(-x2), as follows:

j

According to the Gaussian distribution model, I(t) is gaven by the equation (3). The integration over t and x can be easy performed and normalized to the unquenched emission intensity I0 and is given by:

~oo

I(t) _ _

=

7 - ko exp

e x p ( - x 2) e x p [ - E t exp(?x)] dx -~

(3)

I(0)

exp( -- x 2) dx -oo

where

f

~ e x p ( - x : ) d x = ~r1/2 -oo

and I(0) is initial emission intenmty and I(t) is the emission intens]ty at time equals t The observed luminescence decay profile (Fig. 2) is then characterreed by the width of the normal distribution, 7, and a mean quenching rate constant, k. The integration of the numerator of equation (3) can be carried out by using the extended Slmpson's rule as described by Albery et aL (1985). The Gaussian &stribution model was applied to the study of emission of *Ru(bpy)~ + quenching by MV 2+, DQ 2+, Cu 2+ and TI + in the presence of PSS. We have found that the parameter y reaches a constant value (about l) when the concentration of MV 2+ is a higher than 0.0002 ml dm -3 (Table 1). The emission intensity detected m a steady-state measurement is the integral of the time-dependent fluorescence decay curve: I =

I(t) dt. 0

(4)

-

(5)

where k0 is Ru(bpy)2+ excited-state decay rate constant m the PSS solution without quencher (note that m this case 70 = 0). There is a good agreement between the experimental steady-state quenching and the Gausslan simulated quenching from timeresolved data (equat]on 5) for reactions of MW ÷, DQ :+, Cu 2+ and T1+ with *Ru(bpy)~÷ in the PSS solutions. An exemplary comparison of the actual and Gaussian simulated steady-state quenching is shown m Fig. 1. Table 1 Gausman model fit parameters for the quenching of *Ru(bpy)~ + emission by methylvlologen m 0 0078 m o l d m -3 PSS solution [Methylvlologen] (mol-dm -3)

/~ x 10 6(s i)

?

0 0 00005 0 00010 0 00015 0 00020 0 00025 0 00030 0.00035 0 00040 0 00050 0 00060 0 00070 0 00080 0 00090 0 00100

1 17 2 04 3 29 4 66 6 02 7 94 |0 20 12 20 14 60 22 70 32 10 39 40 46 10 53 20 66 70

0000 0 726 0 879 0 967 1 020 1 020 1 050 1 040 0 977 1 040 1 000 1 090 0 987 1 020 1 120

158

M WOLSZCZAKand J K. THOMAS Gausslan distribution decay No 15 4 11-22-1988

~ 400 (B)

ko/s -1 = 4 66 x 10 +6 Io = 5080 y = 0 967

I

40001

(A)

dk/k (1) % = -0581 dk/k (2) % = - 0 5 1 2 dk/k (3) % = - 0 8 5 6

3000

i

x2 dx 2 dump =ter ~F skip

2000 - ~

1000 -

.

'

~

= 275 = 0155 = 00556 = 800 = "1 O0

~

0

L

0

400

800

1200

1600

2000

Time / ns

Fig 2 (A) Emtssmn decay of *Ru(bpy)]+ in 0.005moldm -3 PSS solution In the presence of 0.0003moldm-3MV2+ Smooth hnes represent the best fit according to "Gaussmn" klneUcs (B) Unweighted residue for the kinetics fit

It is important to note that the emission of *Ru(bpy)] + is simdarly quenched by diquat as by methylviologen Analysis of the decay curves in the PSS system with diquat, shows that they obey "Gausslan" kinetics, and the parameter y is about 10% larger than that observed for MV 2+. This indicates that the binding process of both quenchers to PSS polyelectrolyte is very similar. Spectroscopic measurements using inert salts support this conclusion (Wolszczak and Thomas, in preparation). It is pertinent to note that kinetic models such as, double exponential kinetics (Chu and Thomas, 1987), the Poisson kinetics model (Thomas, 1984), or Sassoon's model (Sassoon, 1986) do not fit our experimental results The continuum model for diffusion controlled reactions (based upon time-dependent rate constants) were apphed by Nemzek and Ware to fluorescence quenching (Nemzek and Ware, 1975). We found that the prediction from this model I ( t ) = I o ( - a t - bx~tt ) also fits our experimental data. However, the parameters a and b should be linear functions of quencher concentration. This was not the case, and the fit was worse than for the "Gaussian" model. The last conclusion was also valid when we used the time dependent rate constant k ( t ) given by the relation' k ( t ) = B t ~- ~ were B and 0 < :t < 1 are constants. The phenomenological approach to dispersive kinetics given by Pionka (1988) interprets the parameter ~t m terms of an activaUon energy distribution. However, the densities of the activation energy distributions for the few elementary reactions studied are slightly different from "Gaussmn" character; however in

certain case, a model of dlsperswe kineUcs with timedependent reactivity leads to a "Gausslan" one The quenching of *Ru(bpy)] + luminescence by MV 2+ in the presence of polysoap (PA 18-K2, concentration: 0.0067moldm -3) also obeys "Gausslan" kinetics. The efficiency of quenching is not as large as that in PSS solution (Fig. 3) Although Gaussian kinetics give a good fit to the data, a Polsson model also fits at low concentrations of MV 2+ These data give an aggregation number for PA18-K2 of 13 (number of monomer units in the polymer rnlcelle). This value is two times smaller than that found by Chu and Thomas (1987) for the same polysoap. This disagreement is caused by properties of methylvlologen, as we found an aggregation number for PA18K2 of 25 (the same as before was found by Chu and Thomas) using 9-methylanthracene instead of methylvlologen. An explanation is that MV 2+ is not totally bound to the polysoap (MV 2+ is distributed between bound and unbounded states). The Interactions Ru(bpy)~+--PSS and MV 2 + PSS are strong. The interaction between MV 2+ and PSS was studied by observation of the charge-transfer complex formation (red shift of the absorption band of MV 2+ after PSS addition). The MV :÷ is able to interact with the phenyl ring (probably with two rings) forming complexes with polyelectrolyte (Wolszczak and Thomas, in preparation) A restriction of the diffusion of both reagents is necessary to observe "Gausslan" kineucs in the presence of a polyelectrolyte. It is noted that the quenching of *Ru(bpy)] + luminescence by neutral quenchers or quenching of neutral probes by charged

Effect of anionic molecular assembly environments

159

12

11 10

~ 60

9

C.

o

8 -

-

801

'

S

• lO/t 2

. : / /

2o

f~:/

o°.,r

,

o o2

/

L

o o.

.,oo0n o 5 --

j

/+

40

o

o,

.,o,, /:.

I

ooo,

/

/

~

II

/

L

~

" I

I

Vol

I

i:i:l PA_18 K2

4

2

0

0002

0004 rMV2+],mot

0006

drff 3

Fig. 3. Stern-Volmer plots of steady-state emlsmon intensity of *Ru(bpy) 2+ against methylviologen concentration in 0.0067 mol dm-3 PA 18-K2 solution (2) and in bulk water (1). Insert. steady-state (l) and

time-resolved (2) Stern-Volmer plots of *Ru(bpy)2+ emission vs the concentration of quencher m 0 016 mol dm -s SDS solution.

quenchers in the presence of PSS, does not follow "Gaussian" kinetics (Wolszczak and Thomas, in preparation). The Stern-Volmer plots of quenching of *Ru(bpy)] + by MV 2÷ m sodium dodecyl sulfate mlcellular solutions show upward curvature (see Fig. 3--this figure also demonstrates Stern-Volmer plots in the presence of PA 18-K2). The time--resolved studies give exponential behavior for the emission decay of *Ru(bpy)] + at all concentrations of MV 2÷ (exponential kinetics have been also observed by Rodgers and Becker, 1980). The quenching takes place by surface diffusion on the mlcelle of the quencher and excited probe. "'Gaussian" kineUcs observed in poly(styrenesulfonate) solution can be regarded as the superposltion of many exponential decays with a "Gaussian" distribution of activation energies in the elementary electron transfer step. The distribution in reactivity of methylvlologen or diquat molecules with excited Ru(bpy) 2÷ on the PSS strand is caused by different interactions of MV 2+, (DQ 2÷) with the polymer releasing of MV 2+, (DQ 2÷) from the trap (complex between quencher and monomer units in which both electrostatic and hydrophobic interacUons are involved) allowing quencher diffusion along the PSS chain. Therefore, the observed kinetics of emission decay can be attributed to a dispersive distribution of activation energies for release of the quencher from the trap. The transfer from trap to excited probe can be regarded as a multistep diffusion process with a single hopping event determined by the energy required for quencher release from the trap.

The possibility of long-range electron quenching m PSS solution can be ruled out on the basis of inactive electron tunneling from excited *Ru(bpy)32÷ to MV 2÷ m the PSS film (see later Discussion). Steady-state and transient decay methods were used simultaneously to measure the luminescence quenching of *Ru(bpy)2÷ by Cu 2+. A strong enhancement of the quenching efficiency has been observed in PSS solution. We have found that the "Gausslan" model is appropriate to analyze of Cu 2+ quenching of PSS-bound excited probe. A low value (<0.6) of the ? parameter shows that dispersion in the first-order rate constants for reaction of*Ru(bpy)~ ÷ with Cu 2÷ in PSS solution is small. In our case we found a value of 0.45. This suggests that interaction between Cu 2+ and poly(styrenesulfonate) is mainly nonspecific, l e. electrostatic in nature. Our emission studies show that about 70% of the total stoichiometnc concentration of sulfate groups of the polymer are involved m ion-binding. TI ÷ quenching of *Ru(bpy)] ÷ in the presence of PSS is a purely nondispersive process. In the "Gaussian" kinetic model the dispersion of activation energy of the system is described by parameter y. Results obtained from "Gausslan" kinetics apphed here for luminescence studies allows us to interpret the parameter V m terms of interactions between the quencher and the polyelectrolyte chain. When 7 is high, site specific, electrostatic and hydrophobic forces play important role in the binding process (for example dlquat and methylviologen). A low value of the parameter Vmeans that the quencher

160

M. WOLSZCZAKand J. K. THOMAS

ts bound only territorially vm electrostatic forces to the PSS strand as in the case of Cu 2~ or T1 + cations.

phase. At the same time, the degree of quenching of Ru(bpy)~ + by 0.0025 mol dm -3 MV 2+ in PSS solution is about 98% [measured as [ 1 - ( I / I o ) x 100%] whereas the degree of quenching in water is 39%. For a quenching of 84% the yield of MV +" in water was about 10-times higher than that m PSS solution (at the same degree of quenching). It ~s pertinent to note that at high concentrations o f MV 2+ some charge did escape back reaction and was observed as a long-lived product. Our laser pulse photolysis of the system: Ru(bpy)~+/benzoquinone in water and PSS solution has shown that there ~s practically no effect of polyelectrolyte m increasing the yield of charge separaUon

(B) Laser flash photolysis experlments In aqueous solution the quenching of exc,ted Ru(bpy) 2+ by methylviologen is followed by a fast back reactmn: Ru(bpy)~ + + M V +" ~ Ru(hpy)~ + + MV 2÷.

(6)

The back reaction in water, monitored at 605 nm (visible peak of methylwologen cation radical absorption band) showed a 2nd order decay with a rate constant 3.8 x 10 9 d m 3 m o l - l s -l. F o r secondorder processes rate constants were obtained by assuming equal concentrations of reactants and the extinction coefficient of M V +' at 605 nm equal to 13,700 dm 3 m o l - l c m - t (Wolszczak and Stradowski, 1989) The yield of charge separation, measured as the absorbance of methylwologen cation radical, m the aqueous solution was proportional to the degree of *Ru(bpy)~ + emissmn quenching In contrast to aqueous solution, the PSS system showed deviatmn from the second order kinetms for the back reaction (6). The decay profiles of M V +" m PSS solution are complex, but in general the hfetime and yield of methylvlologen cation radical are lower than in aqueous solution. The time profiles of transient absorbance of M V + at 605 nm for reaction (6) in aqueous and PSS systems containing the same concentration of methylviologen are shown m Fig. 4. In the case of 0.005 mol dm -3 PSS soluUon, the yield of M V +' is four times lower than that in the water

2nd

order decoy

RU MV woter 0004

-0004 0040

N2

(C) *Ru(bpy)2/ quenchmg by methylvlologen in PSS films Ru(bpy) 2+ and methylvlologen are quite soluble in PSS films, and there was no indication of crystalhzat,on at concentratmn of M W + as high as 0 . 1 7 9 m o l d m -3 (some precipitation was observed after about 2 months of the storage of the sample). It xs important to note that the lifetime of *Ru(bpy)3z+ is independent of concentratmn below 0 . 0 0 1 3 m o l d m -3, and obeys first-order kinetics (k0=6.5 10~s ~, to= 1.54/~s). At higher concentrations of Ru(bpy)3z+ the emission exhibited double exponential decay. The minor short-lived emission had a life-time of 250 ns, while the long-lived emission was dependent on probe concentration and longer than 1/~s In the

No 5 ( 6 0 5 n m ) 02-14-1988 (B)

•

i

L

i

= 00586

dk2'/s- 1 = - 1 0 4 x i 0 dA,

+4

=-000107

0 0075

0 050

x2 dx 2 dump Lter #" skip

0020

o

= 5 12 x l O - 8 =-575x10 -8 = 00185 =.5.00 = 100

0 0025

0 010

I 0

= 2 7 8 x 1 0 +5

A~

i

(A)

g

k2'/s-1

100

I 200 Time

I BOO

I 400

o 500

/p.s

Fig. 4. Shape companson plot between transmnt absorption due to the &sappearance of methylvlologen cation radical vxa reacUon with Ru(bpy)~ + following pulse laser lrra&ation of a solution containing 0.005 tool dm -3 PSS (1) and aqueous solution (2). Both samples containing 3 × 10-5 mol Ru(bpy)32+ and 0.0025 moldm -3 i V 2+. Kinetic analysis of the data from aqueous solution based on second-order kinetics. (B) Residue for the second-order kinetics

Effect of anionic molecular assembly environments present study the Ru(bpy) j+ concentration m PSS film was 0.0013 tool dm -3, and the luminescence decayed exponentially. Nonexponential decay at high prove concentration is caused by quenching of excited *Ru(bpy) 2+ by ground-state Ru(bpy)3:+. The emission spectrum of *Ru(bpy)32+ in poly(styrenesulfonate) film shows a small blue spectral shift, and a slight sharpening of the spectrum with respect to aqueous solution. These emission changes can be attributed to the increased ngi&ty of the probe environment in the films. Similar phenomenon has been observed m other sohd films (Milosavljevic and Thomas, 1983; Wolszczak and Stradowskl, 1985). A comparison was made between the properties of *Ru(bpy) 2+ in PSS and other matrixes e g. cellophane, poly(vinyl alcohol), PVA, and poly(vinylpyrrolidone), PVP. The emission spectrum of *Ru(bpy)J + in PSS is blue shifted with respect to that one m cellophane and is very similar to the spectra in PVP and PVA. The degree of emission polarization P of *Ru(bpy) 2+ film at 77 K is 0.19, and similar to that m PVA (0.205), PVP (0.21) or LiC1 (0.19) glass but appreciably higher than in cellophane (0 16). At room temperature the degree of emission polarization P in PSS film is 0.185 and that in cellophane is 0.11. The lifetime of excited *Ru(bpy)~ + in PSS film (1.54/~s) is similar to data in PVA and PVP, but longer than that in cellophane, 1.06 # s (all data for samples under vacuum). These data and absorption stu&es show that a PSS film is a ngid matrix. The addition of methylviologen to a PSS films containing Ru(bpy)~ + leads to an exponential decrease of yield of excited probe luminescence with the quencher concentration (Fig. 5), and shortening of luminescence lifetime (Fig. 6) The experimental data

161

are explained in terms of an electron transfer mechanism. The nonexponential decay of *Ru(bpy) j+ in the presence of an electron acceptors m solid state has been recently (Milosavljevic and Thomas, 1983, 1985, 1986; Guarr and McLendon, 1985; Guarr et aL, 1985) interpreted m terms of electron tunnelling. We have attempted to fit our experimental decay data in a similar way The model proposed by Miller et al. (1982), assumes that the rate constant of tunneling depends exponentmlly on the separation of reactants and can be approximated by equation (7):

k(r) = v e x p ( - r / a ) .

The frequency factor v is proportional to a Franck-Condon weighted density of states, which depends on the exothermicity of the electron transfer reaction. In expression (7), r is the separation between redox partners and a is the overlap integral for the electron wave functions. If the luminescence is quenched by electron tunneling from the excited probe [*Ru(bpy)32+ ] to the quencher molecule (MV 2+), then an exact mathematical model for the decay of luminescence can be derived for randomly &stributed donor and acceptor molecules. The kinetics follow the general form (8), were P(t) is the time-dependent factor due to the competitive electron transfer reaction and radiative decay of emission of excited probe with luminescence lifetime to •

I(t) = IoP(t ) e x p ( - t/to).

+

+

0

+

l

-0070 0 700

0 525 -

P(t ) = exp{ - B[Q ] g(vt ) }.

o

o 350

-

/"

0175[--

/

+

// l

l

K A

= 376 = -000775

dK

=380x10 -4 =-379x10 -5

I dA

X2

=267x10

dX 2

=-108x10

dump iter #

= 000206 = 500 = I O0

skip

-4 -9

+ I 004

I 008 [MV2+'I,

I 012 moL

I 0 16

02

dm -3

Fig 5 Perrm plot of steady state quenching of *Ru(bpy)] + by methylviologen lmmoblhzed m poly(styrenesulfonate) matrix under vacuum. RPC 38/2-~E

(8)

The surviving faction, P(t), of donor at time t is described by the equation:

0070 "o

(7)

(9)

162

M WOLSZCZAKand J. K THOMAS 1 O0

-

0 75

c

-

0 50

025

0

I

1

I

2 Time

I

I

3

4

5

(microseconds)

Fig 6 Time dependent emission decay of *Ru(bpy) 2+ monitored at 620nm m the presence of methylvlologen m various concentrations. (1) 0, (2) 0.018; (3) 0045, (4) 0.089, (5) 0 116 and (6) 0.178 mol dm- 3 Average PSS film thickness was about 150 # m Ru(bpy)] + concentration was about 0.00133 tool dm -3 All samples excited at 490 nm Smooth hnes represent the best fit based on tunneling model

In this expression, [Q ] is the q u e n c h e r c o n c e n t r a t i o n , B is coetliclent a n d g(vt) is a f u n c t i o n derived by T a c h l y a a n d M o z u m d e r (1974) a n d m a y be expressed m the following form:

g(vt) = ln3(vt) + hlln2(vt) + h21n(vt ) + h 3. (10) The final e q u a t i o n for the v a r i a t i o n of the emission intensity with time, which is based o n the tunneling m e c h a n i s m , is described by:

I = l o e x p { - k o t - A [ln3(vt) + 1.732 × In2(vt)+ 5 9341n(vt)+ 5.445]}

(11)

where A is a factor p r o p o r t i o n a l to the q u e n c h e r concentration:

A = [(Ro/a) 3 C01 -~ [Q ]

q u e n c h e r c o n c e n t r a t i o n calculated from the slope is 0 0266 tool d m 3. The radius of the active sphere R 0 can be determ i n e d according to the e q u a t i o n R0 = [3 x 1027/(4rrNCo)]l/aA

(13)

where N is A v o g a d r o ' s c o n s t a n t In o u r case R0 is equal to 11 4 4 7 A Figure 7 shows p a r a m e t e r A plotted vs methylvlologen c o n c e n t r a t i o n . It can be observed t h a t A depends h n e a r y o n the q u e n c h e r c o n c e n t r a t i o n as required by theory (equation t2). F r o m the slope (0.00968) a n d the k n o w n values of R0 a n d Co, the 1/a value is given as 0.637. Finally, the rate c o n s t a n t of electron tunneling from excited Ru(bpy)32÷ to methylviologen in PSS film is:

(12)

k(r) = 1 62 × 109 e x p ( - 0 . 6 3 7 r) s -1 R0 IS a critical transfer distance where electron transfer occurs at the same rate as the s p o n t a n e o u s deactivation o f electron d o n o r at the Perrin Critical q u e n c h e r c o n c e n t r a t i o n Co. Table 2 shows the p a r a m e t e r s A, k0 a n d v o b t a i n e d by the fitting time-resolved experimental data to e q u a t i o n (11) T h e n a t u r e of electron tunneling, the fast fall-off in decay rate with distance, implies t h a t the q u e n c h i n g m e c h a n i s m m a y be a p p r o x i m a t e d by the P e r r m static model (Perrin, 1932). In this model, a plot o f ln(Io/l) vs [Q] should be linear a n d yield a slope I/Co F~gure 5 shows t h a t this is the case, a n d the critical

Table 2 Parameters obtained from the fitting of methylvlologen quenching of *Ru(bpy)~+ luminescence with electron-tunnehng model for PSS films under vacuum [MV 2+] (mol d m -3)

k o× 10 s (s -~ )

A x 10 3

v × 10 s (s-])

0 00179 0 0446 0 0669 0 1160 0 1320 0 1786

65 65 65 65 65 65 65

0 0 14 0 409 0 635 1 030 1 330 1 757

265 23 0 16 2 8 02 999 13 5

(14)

Effect of amomc molecular assembly enwronments

163

1 80e-4

"o

C D

= 000968 = - 5 0 1 x 10 . 6

dC

= 1 28x10 -4 = - 5 0 6 x l O -5

I~ -1 80e-4

o OOl8o

-f

dD

0 00135

900e-4

X2

=442

dX 2

• - 4 9 4 x 10 - 9

xlO -9

dump =00556

/"

450e-4

.~+

I 0036

)ter#

/

sk=p

I 0 072

I

I

0108

0 144

=20 =1 0

018

CMV2+],mot dm-3

Ftg. 7 Plot of parameter A, obtained by fitting luminescencedecay to the tunneling model, against MV2+ concentration m PSS films.

Recently, reports of photolnduced electron transfer m rigid solutzons (Khalrutinov et aL, 1975; Miller et aL, 1982, Hashlmoto and Thomas, 1983; Guarr et al, 1983; Harriman, 1984, Gasyna et al., 1985; Stradowski and Wolszczak, 1987), polymer films (Milosavljevic and Thomas, 1983, 1985, 1986; Guarr et aL, 1985; Murtagh and Thomas, 1987; Bitting et al., 1988) or zeolites (Iu and Thomas, 1989) have appeared. The values of the frequency factor v and damping factor a obtained from these results, depend on the properties of the system (strong sensitivity to the nature of redox partners, hosts matrices, exoenerg~ty of electron transfer process, temperature etc., Mikkelsen, 1987, Mllosavljevlc and Thomas, 1986; Guarr and McLendon, 1985) The comparison of v and a parameters for various systems ]s thus very d~fficult. Fortunately, a comparison can be made for electron tunnehng from excited Ru(bpy)32÷ to methylvlologen in a few matrices. This reaction has been studied m a cellophane (Milosavljevic and Thomas, 1985, 1986; Stradowskl and Wolszczak, 1987), rigid solut]ons (Guarr et al., 1983; McGulre and McLendon, 1986) and m a PSS film It can be noticed that quenching efficiency measured by steady-state studies lS larger m cellophane (Perrm critical d]stance R = 20 A, M]losavljevlc and Thomas, 1985) than m PSS fod (R = 11 4/~) and in rigid glycerol (at - 1 2 ° C , R = 10 9 ~; Guarr et al., 1983). The thermodynam]c driving force for electron transfer is weak, about 0.37 eV (McGmre and McLendon, 1986) and freezing the sample might make the reaction inefficient (the quenching disappears at 77 K i n several matnces). In our stud]es, we have found that there lS no quenching of *Ru(bpy)32÷ by methylwologen in PSS and PVA films at 77 K.

The effect of temperature on photomduced electron tunnehng is well described in terms of a nonadiabatic multlphonon nonradiatwe decay process according to the small-polaron descript]on of Jortner (Milosavljevlc and Thomas, 1986) The rate constant for electron transfer reaction from photoexcited Ru(bpy)2÷ to methyiviologen as a function of reactant separation m cellophane was gwen by: k(r) --- 101°e x p ( - 0.46r)

(Mllosavljevlc and Thomas, 1985). (15) The enhancement of the rate of electron tunneling by about one order of magmtude in a cellophane with respect to PSS film may be explained by the lower rigidity of the cellophane sample (see above), where the effective coupling between electromc states and molecular-type vibrations modes of medium is stronger. Acknowledgements--We are grateful to National Science Foundation for support of this work M. Wolszczak (M. W.) also wants to thank Dr K Koike and Dr K.-K. Iu for a helpful discussion and writing some computer programs. M.W. is grateful to Professor R S Becker from Department of Chemistry, University of Houston, for this sUmulatlng dlscusslon We thank Dr D-Y. Chu for the preparahon of PA18-K2

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164

M. WOLSZCZAK and J. K THOMAS

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