J. Phys. Chem. 1994, 98, 12032-12039
12032
Excited-State Electron Transfer from Anthracene and Pyrene Covalently End-Tagged onto Poly(ethy1ene oxide) Jiunn-Shyong Hsiao, Andrew R. Eckert, and S. E. Webber* Department of Chemistry and Biochemistry and Center for Polymer Research, The University of Texas at Austin, Austin, Texas 78712 Received: May 19, 1994; In Final Form: August 28, 1994@
Photoinduced electron transfer has been investigated for poly(ethy1ene oxide) covalently end-tagged with a single anthracene and pyrene moiety (abbreviated as PEO-An and PEO-Py, respectively) both in homogeneous aqueous solution and adsorbed at the interface of water-soluble latexes, referred to as “microspheres” (abbreviated as $3). The electron-acceptor quencher is a zwitterionic viologen, SPV (4,4’-bipyridinyl- 1,l’bis(propane sulfonate)), that becomes anionic upon reduction. Charge separation following singlet- and tripletexcited-state quenching was found for PEO-An and PEO-Py in both environments. The efficiency of charge separation from the triplet state is high (ca. 0.6- 1.O) and is relatively insensitive to the environment because the back electron-transfer reaction is spin-forbidden. For the singlet state, the efficiency of charge separation is more modest (ca. 0.2-0.3) and is sensitive to changes in environment. In all cases the ion pairs have a long lifetime, in excess of 1 ms. Both fluorescence quenching and charge-transfer-complex formation are used to probe the local environment of the aromatic moiety. We conclude that the chromophores of the PEO-An and PEO-Py are protected either in homogeneous solution or adsorbed onto ,US such that formation of “tight” geminate ion pairs in electron-transfer processes is prevented. The adsorption properties of PEOAn and PEO-Py at H20/pS interfaces are also discussed.
Introduction
bonded anthracene to poly(methacry1ic acid) (PMA) could drastically modify the polymer adsorption properties onto H20 Electron transfer from an excited-state species is one of the pS interfaces even with only ca. 0.9 mol % of anthracene moiety most important photochemical processes. The motivation for randomly distributed in the PMA chain sequence. This has studying this process has come partly from the desirability of important implications for the general field of polymer adsorpefficient conversion of photon energy into chemical potential. tion.l0 An accompanying paper” has focused on investigating The general reaction scheme can be represented as adsorption behavior for the same water-soluble poly(ethy1ene oxide) systems described herein. 2s+lD* + A 2S+lID.+, A.-l k,,2D.+ + 2A.In the present paper we study a water-soluble polymer with (la) a precise binding position of the aromatic moiety to provide more conclusive mechanistic results. These polymers are poly(ethylene oxide) covalently end-tagged with anthracene and pyrene (abbreviated as PEO-An and PEO-Py, respectively) with a charge separation quantum yield given by quenched by a zwitterionic viologen, SPV (chemical name: 4,4‘bipyridinyl- 1,1’-bis(propane sulfonate)), in both H20 homogeneous and H20/pS biphasic systems (see Chart 1 for structures). Measurements of triplet-state lifetimes of PEO-An and PEOPy in various PEO-Ar/pS mixtures (Ar represents the aromatic chromophore) are used to characterize the adsorption processes .In the above D represents a general electron donor and A of PEO-Ar at the H20/pS interfaces. An aromatic chrorepresents a general electron acceptor. In recent years studies mophore-SPV charge-transfer (CT) complex was found in all of electron-transfer reactions in microheterogeneous media have cases. The equilibrium constant (KC-) of the CT complex attracted considerable interest.2 The use of a variety of formation for PEO-An or PEO-Py is at least 20-fold smaller organized assemblies such as micelle^,^ microemulsions? than for anthracene- or pyrene-tagged PMA in aqueous solution membra ne^,^ colloids,6 polyelectrolytes, and water-soluble at high pH. The steady-state and lifetime fluorescence quenchpolymer^^^*^^^ has been proposed to elucidate the effect of ing by SPV demonstrate a mixed dynamic and static quenching structural factors on electron-transfer reactions. More recently, mechanism. Both the static quenching component and KCT we have used water-soluble polystyrene latex particles (comdecrease with increased adsorption of polymers at the H20/pS mercially referred to as microspheres, abbreviated as pS interface. Charge separation following from both singlet- and hereafter) as photoredox media to illustrate that the “interfacial triplet-excited states was found for PEO-An and PEO-Py in effect” that results from adsorbing polymers onto pS can enhance the charge-separation efficiencies of adsorbed s y ~ t e m s ? ~ , ~ ~homogeneous aqueous solution and H20/pS biphasic systems. Unlike the corresponding PMA-Ar case, we conclude that even In this earlier work it was noted that addition of a covalently in homogeneous solution the aromatic moiety of PEO-An and PEO-Py systems has sufficient structural protection to prevent @Abstractpublished in Advance ACS Absrracrs, October 15, 1994.
-
0022-3654/94/2098-12032$04.50/0 0 1994 American Chemical Society
Excited-State Electron Transfer from Tagged Polymers
J. Phys. Chem., Vol. 98, No. 46, 1994 12033
CHART 1
H
SPV
PEO-An and PEO-Py
generation of a “tight” geminate ion-pair {D+,k-} in eq la. In all cases the ion-pairs have a long lifetime, in excess of 1 ms. Experimental Section (a) Preparation of Polymers in HzO/pS Biphasic Systems. The poly(ethy1ene oxide) with covalently end-tagged anthracene and pyrene chromophores was prepared and characterized as reported elsewhere.l1 PEO-An and PEO-Py have only one aromatic moiety per polymer and a molecular weight estimated by GPC as ca. 5600 and 6800, respectively. Polystyrene microspheres (abbreviated as pS) are received as an aqueous solution with 2.5 wt % solids, with a hydrodynamic diameter of 59.0 nm (Polysciences), and were used as received. These latexes owe their water stability to surface sulfate groups. Distilled, deionized, filtered water was used as solvent. The original pS solution (25 g/L) was diluted to 1 g L . Subsequently, an aliquot of concentrated PEO-An or PEOPy aqueous solution (ca. 20 g/L) was added to the pS solution to form the desired polymer concentration. Sample solutions of the desired polymer/pS weight ratios were prepared immediately in advance of experiments. (b) Absorption and Fluorescence Quenching Study. UV absorption spectra were recorded on an HP8451A diode array spectrophotometer. Steady-state fluorescence quenching was performed on a Spex Fluorolog 2. This instrument employs a 450 W xenon lamp for excitation and a Hamamatsu R508 photomultiplier tube for emission detection and has been described e l ~ e w h e r e .Fluorescence ~ lifetimes were measured using time-correlated single-photon counting at the Center of Fast Kinetics Research at the University of Texas at Austin. The excitation source was a cavity-dumped Pyridine-1 dye laser, which is frequency-doubled to yield tunable UV light (-345 nm for our work), synchronously pumped by a Coherent Antares Nd:YAG laser mode-locked at ca. 86 MHz. Fluorescence decays were monitored by a Hamamatsu R1564U microchannel plate photomultiplier tube at right angles to the excitation source. A monochromator and cutoff filters were used for the selection of the emission wavelength (415 and 400 nm were chosen for PEO-An and PEO-Py, respectively). SPV was prepared according to published procedures.12 The SPV was then purified by repeated precipitation of SPV from an aqueous solution with methanol until aqueous SPV solutions were neutral. (c) Transient Absorption System. Laser flash photolysis was performed by using a Nz laser system (Laser Photonics Model LN100C) with emission at 337 nm. The laser pulse width was about 15 ns. The laser power at the sample cell position was measured by a Coherent power meter (Model FM) and was kept at ca. 2 mT/cm2. The setup and arrangement of optics and the detection system were described in previous papers.ga-b
The concentrations of pS for H*O/pS studies were kept at 1 g/L so that the overall effect of scattering by the colloidal particles was consistent and negligible in the final data analysis. We found that time-resolved transient absorption spectra acquired under this condition were reproducible and would not change if the pS concentration varied &OS g/L. The sample solution was accommodated in a quartz cell with a monitoring path length of 1 cm. Before each measurement, fresh solutions were prepared and bubbled with dry N2 gas for at least 15 min to remove 0 2 . During measurements a continuous N2 gas flow was maintained inside the capped quartz cell to exclude 0 2 . The extinction coefficients of the triplet state for PEO-An and PEO-Py aqueous solutions have been determined by laser flash photolysis using .the ground-state depletion method of Hadley and Keller (GSD-HK),13aas described p r e v i o ~ s l yThe .~~ triplet-state properties of the anthracene triplet state were assessed at the Center for Fast Kinetics at the University of Texas to take advantage of the 355 nm radiation from a Nd: YAG Q-switched system. The quantum yield of intersystem crossing, his,, was obtained by using the relative actinometry m e t h ~ d . ~With ~ J ~hSc known for a standard (anthracene in benzene with $is, = 0.6713b)and the extinction coefficient obtained by the GSD-HK method for the standard and the sample solution, one can evaluate his, (see eq 7 in ref 9a). The yield of ion-pair formation is determined as in our earlier work.9 For the triplet-state work Y,, (the yield of SPV- per triplet-state moiety) is computed according to eq 9 in ref 9a. For the singlet-state work (Pspv- (yield of ion-pair formation) and $cs (yield of ion-pair per quenching event) are calculated according to eqs 3 and 6 in ref 9c, respectively. In addition, des" (yield of ion-pair in the limit of 100% quenching) is estimated by using a double-reciprocal plot of yield ((Pspve-) and quencher c~ncentration.’~ Results and Discussion (a) Triplet-State Lifetime of PEO-An and PEO-Py as a Function of Adsorption at a pS Interface. In other work” we have demonstrated that a PEO polymer end-tagged with an aromatic chromophore is more effectively adsorbed onto pS in aqueous solution than untagged PEO. The adsorption of these polymers onto pS essentially follow a Langmuir isotherm. We find that the lifetimes of triplet-excited-state species for PEOAr are very sensitive to the transition from homogeneous aqueous solution to the HzO/pS biphasic systems. The triplet decay curves monitored at 426 nm (PEO-3An*) at different PEO-An/pS weight ratios are presented in Figure 1. The calculated lifetime data are listed in Table 1 and plotted as an inset in Figure 1. The triplet-state lifetimes of PEO-An decrease when we raise the PEO-An/pS weight ratios. The lifetime plot (inset in Figure 1) shows two limiting values, which are
12034 J. Phys. Chem., Vol. 98, No. 46, 1994
\
Hsiao et al. did not observe any absorption in the CT region with concentrated SPV alone. The value of the equilibrium constant for CT complexation (KcT) was calculated on the basis of the bulk concentration of the aromatic chromophore (Ar) and quencher (Q). i.e.
PEO-An
PEO-Ar
OD (a.u.:
'
!O
I
60
'
io
1
'
1
'
140
1
180
'
22d
Time (microseconds) Figure 1. Time-dependent optical density of transient absorption at 426 nm (single-exponential decay fit is also shown), all scaled to the same maximum, for PEO-MpS weight ratios indicated. Inset: plot of the triplet lifetime vs PEO-AdpS weight ratio. TABLE 1: Results of Triplet-State Lifetime for PEO-An as a Function of PEO-AdpS Weight Ratios [PEO-An],glL @SI, glL PEO-AdpS (glg) t,ps 0.6 1.2 0.6 0.6 0.3
0.0 1.o 1.0 1.5 1.5
m
1.2
0.6 0.4 0.2
45 103 145 200 219
Time-dependent decay curves were obtained by exciting at 337 nm, monitoring at 426 nm, and fitting to AOD(r) = e-'F attributed to polymers absorbed at the H20/pS interface with the An group protected from H20 (PEO-An/@ 5 0.4) or in the H20 bulk phase (PEO-An/@ 1 1.2), respectively. It is critical to the later analysis to understand the conditions under which all polymers are adsorbed onto the pS. We also measured the triplet lifetime decay curves of PEO-3Py* systems (monitored at 418 nm). We find that the triplet-state lifetime of PEOPy increases immediately from 104 ps for homogeneous solution to 208 ps for H20/pS biphasic systems (over the PEO-Py/pS weight ratio range from 0.2 to 1.2), respectively (data not plotted). This suggests that all the PEO-Py chromophores are in a modified environment when exposed to the pS. We assume that complete localization of PEO-Ar at the interface of H20/ pS biphasic systems is achieved by keeping the PEO-An/pS weight ratio 5 0.4 and PEO-Py/pS 5 0.2 on the basis of the above results and our other related w0rk.l' (b) Charge-Transfer Complexation with SPV. It is wellknown that methyl viologen (MV2+) can form charge-transfer (CT) complexes with aromatic hydrocarbons. l6 In previous work, we also found that SPV formed CT complexes with poly(methacrylic acid)-bound anthracenesd and pyrene8b but to a much smaller extent (at least 10-fold) than MV2+. We have proposed that the formation of CT complexes as well as weakly binding complexes will generate "tight" geminate ion-pairs following electron-transfer8a which cannot form long-lived charge-separation ion-pairs." For the PEO-Ar polymers we have observed formation of CT complexes in all cases only at a relatively high concentration of SPV (see Figure 2 for an example). All absorption spectra demonstrated isosbestic points, so we assume we have a single CT complex formation. We
+ Q KCT +.PEO-ArQ
(3)
The changes in the optical density as a function of added quencher can be fit to the equilibrium in eq 3 by either the Nash18 modification of the Benesi-Hildebrand method or a direct computer least-squares fit. The KCTvalues obtained and presented in Table 2 demonstrate the relative facility of CT complexation for different systems. The values of KCTfor the present systems decrease systematically as the polymer is adsorbed onto the pS. We also note that the KCTvalues for the PEO-Ar systems are much smaller than was found for high-pH aqueous solution for the corresponding PMA-Ar polymer but larger than for the collapsed PMA-Ar polymer at low pH (see Table 2). It is reasonable that the local environment of PEOAr would be similar to the latter case since neither is charged. This will also be the conclusion from the fluorescence studies reported next. (c) Fluorescence Quenching by SPV. Fluorescence quenching data are important because they help elucidate the structural factors that govem the charge-separation process (Le. donoracceptor proximity, steric hindrance, etc.). It is also necessary to investigate fluorescence quenching to establish the experimental requirements for both triplet-state and singlet-state electron-transfer quenching reactions. For triplet-state studies we must limit the amount of SPV added to the solution such that negligible singlet-state quenching occurs. For the singletstate studies we need to add sufficient SPV such that there is only minimal contribution from the triplet-excited-state species. The steady-state and lifetime Stem-Volmer (SV) plots of PEO-An and PEO-Py as a function of PEO-An/pS ratios are shown in Figures 3 and 4, respectively. We have measured steady-state fluorescence intensity (I)and fluorescence lifetime (z) at different SPV quencher concentrations and have fitted the data to the following empirical equations:
IAZ = 1 + A[SPV] + B[SPV]*
(4)
+
(5)
(tO)/(t) = 1 K,,'[SPV]
where IdI and ( t ~ ) /are ( t )the ratios of the unquenched to quenched steady-state fluorescence intensity and average lifetime, respectively . In eq 5 (z) is the average lifetime of the singlet state derived from a two- or three-exponential fit of the decay curve ((t)= CLlaizi). A quadratic form is used for IdI because this quantity often exhibits upward or downward curvature. The lifetime SV quenching constant, Ksvt, is equal to kqt0, the product of a bimolecular quenching constant and unperturbed lifetime.19 We found that all quenching data fit well to eq 4 or 5. To estimate the fraction of static quenching, one can assume that all static quenching originates with a ground-state complex formed between chromophore and quencher (PEO-Ar*Q) and obtain the apparent equilibrium constant, K,, by fitting to the following equation:8b,8d PEO-Ar
+Q
Kq
PEO-Ar*Q
(6)
In eq 6 the Kq value is derived from the IdI SV plot and is proportional to the fraction of static quenching. Note that the
J. Phys. Chem., Vol. 98, No. 46, 1994 12035
Excited-State Electron Transfer from Tagged Polymers 7.0
L1
I
I
I
OD 5.0
3.0
0.5
1.0 0.000 2.5
0.004 0.008 [SPVI, M
0.000 I
I
1.4
0.004 0.008 [SPVI, M
I
I
0.0 360
400
1 (nm) Figure 2. Absorption spectra of PEO-An in aqueous solution (0.6 g/L) upon addition of SPV. Concentrations of SPV are, from top to bottom, 0, 33, 61, 86, 108, and 127 mM.
0.000
0.004
0.008
0.000
0.004
0.008
[SPVI, M
WVI, M
Figure 4. Steady-state (Zdl) and lifetime ((ro)/(r))quenching plots for PEO-Py as a function of PEO-Py/pS weight ratios.
PEO-An, pS (0.6)
1 0.00
different patterns and trends in the quenching effectiveness and degree of curvature in SV plots between the PEO-An and PEOPy systems. We will discuss these systems separately as follows: PEO-An Systems. In homogeneous aqueous solution a mixed static-dynamic mechanism of quenching is observed (see Table 2 and Figure 3). The modest upward curvature (see Table 2, B term) of the steady-state SV curve combined with the linear lifetime quenching is evidence for the formation of a weakly binding complex between the anthracene moiety of PEO-An and SPV. Since KCTand Kq agree within experimental error, one may surmise that it is the CT complex that accounts for the static quenching process in eq 6. For PEO-AdpS we observe that the quenching efficiency as measured by IdI gradually drops and the upward curvature of steady-state SV curve diminishes as the PEO-AdpS weight ratios decrease. The loss of diffusion of a PEO-An polymer that is associated with the pS would account for a decrease in quenching efficiency by a factor of ca. 2. The diminished upward curvature (which becomes negative for PEO-An, pS(0.2)) in the steady-state SV curve implies that the anthracene moiety of PEO-An is partially protected from complexation or quenching by SPV. However, since the lifetime quenching is less efficient than steady-state quenching, there remains some static or hindered diffusional quenching component even for the case of the sample denoted
:E\!iEi 0.04 0.06 WVI, M
0.02
PEO-An,
0.00
0.02
0.04
WVI,
0.06
M
PEO-An, CIS(0.2)
FS (0.4)
3
1
0.00
0.02
0.04
0.06
0.00
0.02
0.04
0.06
M [SPVI, M Figure 3. Steady-state (I&) and lifetime ( ( t ~ ) / ( rquenching )) plots for PEO-An as a function of P E O - M p S weight ratios. [SPVI,
PEO-ArQ, CT complex, and PEO-Ar*Q, a statically quenched complex, are not necessarily identical.* Values of the quenching parameters Kq, Ksv', kq, A, and B as a function of PEO-Ar/pS weight ratios are collected in Table 2. By comparing these quantities (also see Figures 3 and 4), we note that there are
TABLE 2: Values of Quenching (Kq, KSV,k,, A, and B ) and CT Complexing (KcT) Parameters for Polymer-Bound Anthracene and Pyrene with SPV as Electron-Acceptor Quencher KCT,
Kq,
polymer
pH
system8
M-'
M-'
(r)! ns
S-1
k,, M-I s-'
PMA-An' PMA-An PMA-An PMA-An PEO-An PEO-An PEO-An PEO-An PMA-~Pyf PMA-VPY PEO-Py PEO-Py PEO-Py PEO-Py
2.8 11 4.0 11
HzO Hz0 pS(1.0) pS(1.0) Hz0 pS(0.6) pS(0.4) ps (0.2) H20 H20
-0 190 NA NA 9 4 4 2d -0 630 26 23 17 2
24 365 7 29 7 6 5 e -0 80 8 e e e
11.7 11.0 10.6 11.2 4.1 4.0 4.0 3.9 151 100 143 100 93 89
24 59 NA 38" 15 10 9 6 270 330 338 148 89 51
2.1 x 5.4 x NA 3.4 x 3.7 x 2.4 x 2.4 x 1.4 x 1.8 x 3.3 x 2.4 x 1.5 x 9.6 x 5.7 x
4.0 11
H20 pS(0.4) pS(0.2) pS(O.1)
Ksv', M-'
109 109 1090 109 109 109 109 109 109 109 109 lo8 108
A
52 230 70 98 68 52 48 19 380 790 570 370 240 40
B 3.6 x 4.6 x 1.3 x 1.8 x 5.7 x 3.5 x 4.2 x -5.7 x -0 1.5 x -0 -1.6 x -1.1 x -7.8 x
ref lo2 104 lo2 103 102 102 lo2 10 105
8d 8d 9c 9c a
a a U
8b 8b a
104 104 lo2
a a
a
a This work. * (r) = ~ & a , r for , multiexponential decay. PMA-Ar = poly[(methacxylic acid)-co-(2-(9-anth1yl)ethylmethacrylate)]. This value is measured for pS(O.1). e The K, value is meaningless (negative) when the Id1 SV plot has a downward curvature. f PMA-vPy = poly[(methacrylic acid)-co-(2-ethylpyrene)]. g p S ( x ) represents the polymer in the presence of the pS with a weight ratio of PEO-Ar to pS of x.
12036 J. Phys. Chem., Vol. 98, No. 46, 1994 “PEO-An, pS(0.2)”, which represents the most complete adsorption onto the pS. PEO-Py Systems. In H20 homogeneous solution a mixed static-dynamic mechanism of quenching is observed (see Table 2 and Figure 4). The lack of upward curvature of the steadystate SV curve combined with the similar lifetime quenching curve indicates that the pyrene moiety of PEO-Py complexes with SPV to a lesser extent than is the case for PEO-An (cf. Figure 3). In PEO-PyIpS solutions the quenching efficiency drops with added pS and downward curvature is evident as the PEO-Py/pS weight ratios decrease. Negative curvature in these quenching curves is normally interpreted as indicative of a fraction of the chromophores being inaccessible to the quencher. When the PEO-Py/pS weight ratio has decreased to 0.1, we observe that the ZdZ and (zo)/(z) SV curves overlap almost perfectly (see Figure 4). The simplest interpretation of this data is that all quenching is dynamic (Le. diffusive) when all PEOPy is adsorbed onto the pS, although the approach of the SPV to the Py moiety must be strongly hindered (note the trends in k, values in Table 2). The general differences under similar experimental conditions between PEO-An and PEO-Py systems are as follows: (a) PEOAn systems have the higher quenching constants than PEO-Py systems when adsorbed onto pS by a factor of ca. 2; (b) PEOAn systems exhibit a higher fraction of static quenching than PEO-Py (cf. K, and B ) ; (c) PEO-Py systems are more strongly adsorbed onto the H20/pS interface than PEO-An systems (see earlier discussion). There are a few factors which may contribute to these differences. First of all, the compositions of the two polymers are somewhat different with respect to molecular weight and chromophore tagging percent. Secondly, the hydrophobicity for PEO-Py is slightly higher than PEO-An which strengthens hydrophobic-hydrophobic attraction between chromophore moiety and polystyrene entity of pS.” In order to envision the actual structure around the photoactive chromophore, it is helpful to compare the quenching parameters k,, A, and B of PEO-Ar systems as a group to those of PMAAr. We note that PEO-An in both H20 homogeneous and H20/ pS biphasic systems resembles PMA-An in H20 homogeneous solution at low pH (see also the earlier discussion in section b). This implies that the compact “globular” structure of the PMA coil around the chromophore is similar to the PEO-An local structures. For PEO-Py there is also a strong similarity between low-pH PMA-Py and PEO-Py in homogeneous solution, but when PEO-Py is adsorbed onto the pS, the drop in quenching efficiency is much more drastic than for PEO-An. (d) Photoinduced Electron-Transfer Quenching by SPV. Both singlet- and triplet-state electron-transfer experiments used 337 nm excitation at ca. 2 ml/cm2, which is chosen to minimize the occurrence of direct photoionization.20 Extinction Coefficient. The extinction coefficients of T-T absorption for PEO-3A* and PEO-3Py* in H20 homogeneous solution are determined using the GSD-HD method13aand listed in Table 3 (see Experimental Section). Note that these extinction coefficients are significantly reduced from the values for unsubstituted anthracene or pyrene in organic solution. If the true extinction coefficients are higher than assumed in our calculation, then the calculated yield of ion-pairs would be higher. Because of the turbidity of the pS solution, the GSDHD method cannot be applied accurately. Therefore, we use the same extinction coefficient values for PEO-Ar in H20/pS biphasic systems with the assumption that no change in extinction coefficient will arise from modifications in the local environment of the aromatic moiety. For the extinction coef-
Hsiao et al. TABLE 3: Extinction Coefficients at the Wavelength of Maximum Absorption for PEO-3A*and PEO-3Py* polymer
system
PEO-An PEO-Py
H20
Hz0
A,,,,
nm
426 417
dm3 mol-’ cm-’ 26 000 f 22OOb 21 000 f 3000‘
Values are weighted means, and the errors are standard derivations of the mean. Four concentrations were used for measurement: 0.9, 0.6, 0.5, and 0.4 tlL, respectively. For 3An* in ref 13b, I,,, = 430 nm, cmax = 61 000 f 5670 dm3 mol-’ cm-’. A factor of 1.8 was required to correct 6430 34 000 dm3 mol-I cm-’, which we obtained for anthracene in benzene at the CFKR facility. Three concentrations were used for measurement: 0.2, 0.1, and 0.07 g/L, respectively. For 3Py* in ref 13b, I,,, = 415 nm, E,,, = 36 000 f 14 700 dm3 mol-’ cm-I, which agrees with €415 -48 000 dm3 mol-’ cm-’, which we obtained for pyrene in benzene at our lab.
-
TABLE 4: Yields of Intersystem Crossing and Charge Separation of Triplet-State Electron-Transfer Quenching as a Function of Environment for PEO-An and PEO-Py [SPVI, mM 0.16
1 - IIIO“ #is? PEO-AdH20(0.6 g/L) 0.01 0.95
0.76
0.16
PEO-AdpS(0.4) 0.01 0.91
0.60
0.08
PEO-PylH20(0.2 g L ) 0.04 0.53
0.97
0.08
PEO-PyIflS(0.2) 0.02 0.33
0.85
yc:
1 - I/Zo is the fractional fluorescence quenching which is kept as low as possible for triplet-state studies. #,sC was determined using eq 7 in ref 9a. Y,, was determined using eq 9 in ref 9a.
ficient of SPV- at 602 nm we used the literature value of 13 700 M-1 cm-l 21 Transient Absorption. Comparing the intensity of the T-T absorption spectrum of PEO-An and PEO-Py in both H20 homogeneous and H20/pS biphasic systems with carefully purified anthracene in benzene (&, = 0.6713b),we are able to estimate & for our systems (see Table 4) using the relative actinometry m e t h ~ d . ~We ~ . ’note ~ that there is a ca. 65% drop in intersystem crossing yield for PEO-Py in going from homogeneous to biphasic systems. We do not have a good explanation for this and attribute it to “environmental effects”. The spectral shape of the T-T absorption for the PEO-An system is essentially identical to anthracene in benzene. The relative intensity of the PEO-Py T-T absorption at 418-530 nm increases by a factor of ca. 1.43 compared to pyrene in benzene (see Figure 5 ) . The pyrene T-T absorption spectrum is known to be sensitive to s01vent.l~~The decay curves of T-T absorption for PEO-Ar systems are very sensitive to structural variables (see Figure 1 for PEO-An, Table 1, and the detailed discussion that was given in section a). Charge Separation Following Triplet-Excited-StateQuenching. No more than ca. 4% of PEO-’Ar* species are quenched at the SPV concentration used for triplet-state quenching (see Table 4). We have studied the triplet-excited-state electrontransfer reaction for PEO-An and PEO-Py in both H20 homogeneous and H20/pS biphasic systems. Our time-resolved transient absorption spectra obtained from these systems reveal a typical electron-transfer p r o c e s ~ . ~ We initially detect the excited-state species PEO-3Ar*immediately after the laser pulse, followed by a loss of PEO-3Ar* and a growing-in of radical ion-pairs (PEO-An’+ and SPV-, respectively) at later time (see Figure 6). We do not resolve the PEO-An’+ absorption spectrum because the absorption spectrum of the anthryl cation radical from PEO-An systems underlies the long-wavelength portion of the SPV- spectrum.22 The electron-transfer kinetics
J. Phys. Chem., Vol. 98, No. 46, 1994 12037
Excited-State Electron Transfer from Tagged Polymers
PEO-Py
SPV' (a.u.) ODI
h
20
\
OD 10'3
Py'
455 nm +
30 50 70 90 Time (microseconds)
10
0 350
450
550
650
750
(nm) 380
480
580
680
(nm) Figure 5. Time-resolved transient absorption spectra for PEO-Py (0.2 g/L) in HzO homogeneous solution (spectrum with higher OD at 418 nm) and pyrene in benzene at 2 ps delay. These spectra are normalized at 530 nm for the ease of comparison in relative intensity at different peaks (see text).
370
770
Figure 6. Time-resolved transient absorption spectra for PEO-An (0.6 g/L) in H20 homogeneous solution upon addition of 0.16 mM SPV at the time delay indicated. Inset: time-dependent optical density at 432 nm for PEO-An (0.6 g/L) in HzO homogeneous solution without SPV and with added SPV concentration at 0.16 mM, respectively. can be further c o n f i i e d by obtaining the time-dependent optical density at the appropriate wavelength: PEO-3An* (432 nm), PEO-3Py* (418 nm), PEO-Py'+ (455 nm), SPV- (602 nm), and PEO-An'+ (715 nm), re~pectively.~~ For example, in the insert of Figure 6, the more rapid decay of time-dependent optical density upon addition of SPV for PEO-3An* in H20 homogeneous solution illustrates that PEO-3An* is quenched by SPV. Given that this triplet-excited-state quenching is accompanied by the formation of SPV-, we can conclude that electron-transfer quenching does occur for these systems. However because of the severely overlapping transient spectra, we did not carry out a quantitative comparison of the rate of 3Ar* decay and SPVbuildup. We note that the relatively rapid lifetime decay for PEO-Py'+ (see inset of Figure 7) is similar to that for poly(styrene-alt-maleic a~id-co-4-vinylpyrene)~~ and 1-pyrenebu-
Figure 7. Time-resolved transient absorption spectra for PEO-Py (0.2 g/L) in H20 homogeneous solution upon addition of 2.7 mM SPV at the time delay indicated. Inset: time-dependent optical density at 455 and 602 nm for PEO-Py (0.2 g/L) in H20 homogeneous solution with SPV concentration at 2.7 mM.
w+
tyric acid.gb We have previously suggested that the species can react with nearby COOH groups in a photo Kolbe reaction," but there must be some other cause for this fast decay, at least in the present case, since the PEO polymer has no COOH groups. The yields of SPV- per PEO-3Ar*produced in the laser flash (Yes) for PEO-An both in H20 homogeneous and H20/pS biphasic systems are calculated by using eq 9 in ref 9a and collected in Table 4. Ycs is equivalent to the quantum yield of charge separation (&) if all triplet excited states are quenched by an electron-transfer process. For PEO-An polymer systems, the Y,, value is calculated to be 0.76 and 0.60 for homogeneous and biphasic systems, respectively. This difference is modest (125%) and is probably attributed to the drop of quenching efficiency in going from homogeneous to biphasic systems (see section c). We cannot obtain an accurate & value unless an accurate determination of the residual unquenched tripletexcited-state concentration can be made. The severe overlap in the transient spectra is a major obstacle for making a reliable assessment of this quantity. The Y,, values are likely to underestimate & for a system with diminished quenching efficiency. For PEO-Py polymer systems, the Y,, value is calculated to be 0.97 and 0.85 for homogeneous and biphasic systems, respectively. In spite of the difficulty in obtaining accurate values, we estimate that the & values are comparable to one another for both PEO-Ar systems in homogeneous solution and adsorbed onto $3. In all cases the ion-pairs have a lifetime well in excess of 1 ms. In our previous work9 we have found that & for electrontransfer quenching of the triplet excited state is relatively insensitive to the environment. The present triplet-state study is consistent with that finding. In the next subsection, we will discuss the structural effects on charge separation following singlet excited-state quenching for these PEO-AI systems. Charge Separation Following Singlet-Excited-State Quenching. More than 50% of PEO-'Ar* species are quenched at the lowest SPV concentration used (see Table 5) for singlet-state quenching. In the absence of 0 2 the transient absorption spectrum contains contributions from SPV- and PEO-Af+ that are identical to those for triplet-state quenching. The viologen (V) radical ions are known to undergo electron transfer to oxygen, while the anthryl cation remains unaffected:
12038 J. Phys. Chem., Vol. 98, No. 46, 1994
v'-
+ 0, - v + 02'
Hsiao et al.
1
(7)
Unlike the triplet-state case, we are now able to examine the PEO-An'+ spectrum25and account for its contribution from the overlapped region by carrying out electron-transfer quenching in the presence of 0 2 . 9 c All the shortest time spectra are mainly composed of a broad band at ca. 600 nm and a sharp band at ca. 400 nm from SPV- for PEO-Py systems, and another sharp band at ca. 455 nm from PEO-Py'+ is also discernible. The time-dependent OD of SPV- for PEO-An in H20 homogeneous solution monitored at 602 nm wavelength is shown in Figure 8 with SPV concentrations of 0.16 and 20.1 mM for triplet- and singlet-state work, respectively. One does not observe a timeresolved buildup of OD of SPV- on the microsecond time scale for the singlet, unlike the grow-in observed for the triplet state. The SPV- derived from either the singlet- or triplet-state quenching persists into the millisecond time scale. We have collected the quantum yields of charge separation for PEO-'An* and PEO-'Py* in H20 homogeneous and H20/ pS biphasic systems in Table 5 as a function of SPV concentration, including the value extrapolated to infinite SPV concentration ($,,"). The &" values of our systems are consistently smaller than the individual & value obtained at finite concentrations of added SPV. The fraction of charge separation from residual triplet states can be estimated from the product
In this equation (Z/Zo)& is the fraction of singlet states that survive quenching and undergo intersystem crossing. Y,, is the upper limit of the yield of ion-pairs per triplet state formed. &ST decreases as the singlet state is quenched and cp,, - &ST = 4:,. This latter quantity decreases smoothly to &" as the SPV concentration increases, except for PEO-PypS(0.4) for which &s x &,- for d l SPV concentrations26 Thus, except for this latter case, there seems to be a process other than the triplet contribution that diminishes $ :, at higher SPV concentrations. We suggest that this is a result of the formation of a higher fraction of tight-binding chromophore-SPV pairs with reduced charge-separation efficiency. For PEO-An polymer systems &," values are calculated to be 0.29 and 0.32 for homogeneous and biphasic systems, which are identical within experimental error. For PEO-Py polymer systems, the #," value is calculated at 0.28 and 0.16 for homogeneous and biphasic systems, respectively. Thus, for all these polymers in homogeneous solution there is significant singlet-state charge separation, and association with the pS does not enhance charge separation. This is consistent with the conclusion that the local structure of the polymer around PEO-Ar is similar to PMA-Ar at low pH. Summary Covalently end-tagged poly(ethy1ene oxide) with a single anthracene or pyrene moiety (PEO-Ar) provides a means to dissolve a hydrophobic chromophore in water with a precise location of the aromatic moiety on the polymer chain, unlike random copolymer^.^ We have reported elsewhere that a single aromatic moiety can drastically modify the amphiphilic character of PEO and have characterized the equilibria between PEO-Ar in the aqueous bulk phase and at H20/pS interfaces.'l In earlier work we have used pS as photoredox media to illustrate that the HzO/polymer interface can enhance charge-separation efficiencies of adsorbed systems.9c In the present paper, we have found that the aromatic moiety of PEO-An and PEO-Py is protected from close association with SPV by either the coiled
I
OD (a.u.)
'1
87
27
I
I
I
0
100
200
Time (microseconds) Figure 8. Time-dependent optical density at 602 nm for PEO-An (0.6 g/L) in H2O homogeneous solution with added SPV concentration at 0.16 and 20.1 mh4, respectively.
TABLE 5: Quantum Yield of Charge Separation of Singlet-State Electron-Transfer Quenching as a Function of Environment for PEO-An and PEO-Pv
20.1 30.7 45.8
PEO-An/H20(0.6 s/L) 0.55 0.44 0.80 0.69 0.40 0.57 0.80 0.33 0.41 (0.29)
0.33 0.22 0.14
0.47 0.35 0.27
20.0 35.8 55.3
0.53 0.69 0.80
PEO-AdpS(0.4) 0.44 0.83 0.44 0.64 0.33 0.41 (0.32)
0.26 0.17 0.11
0.57 0.47 0.30
2.7 5.0 7.4
0.57 0.70 0.81
PEO-Py/H20(0.2 g/L) 0.32 0.56 0.30 0.43 0.29 0.36 (0.28)
0.22 0.11 0.10
0.34 0.32 0.26
3.7 5.3 10.3
0.53 0.60 0.68
PEO-PyIpS(0.4) 0.16 0.30 0.16 0.27 0.16 0.24(0.16)
0.13 0.11 0.09
0.17 0.16 0.15
&v- was determined using eq 3 in ref 9c. & was determined using eq 6 in ref 9c. The limiting quantum uield of SPV- (&-) is calculated using a double reciprocal plot of yield ( & p ~ ' - ) vs quencher concentration and is indicated in parentheses (see ref 15). See eq in the text.
PEO chain or the H20/pS interface. Both types of structural protection can effectively retard the generation of "tight" geminate ion-pairs from a photoinduced electron-transfer reaction. The singlet-state limiting yields for charge separation range from 0.16 for PEO-Py/pS to 0.32 for PEO-An/@ In all cases the ion-pairs have a long lifetime (> 1 ms). Perhaps the most surprising result is that significant singlet-state charge separation can occur for PEO-Ar in homogeneous solution. This illustrates that the concept of "hydrophobic protection" of chromphores by polymers is subtle and imperfectly understood. Acknowledgment. The support of this work by the Department of Energy, Division of Chemical Sciences (Grant DEFG03-93ER114337), is gratefully acknowledged. The Center for Fast Kinetics is supported jointly by the Biomedical Research Technology Program of the Division of Research Resources of the National Institutes of Health (RR008806) and the University of Texas at Austin.
Excited-State Electron Transfer from Tagged Polymers
J. Phys. Chem., Vol. 98, No. 46, 1994 12039
References and Notes (1) See the plenary lectures of the Sixth International Conference on Photochemical Conversion and Storage of Solar Energy, Paris, 1986, In New J. Chem. 1987, 11. (2) (a) Fox, M. A., Chanon, M., Eds. Photoinduced Electron Transfer; Elsevier Science Publishers: Amsterdam, 1989. (b) Gratzel, M., Kalyanasundaram, K., Eds. Kinetics and Catalysis in Microheterogeneous Systems; Marcel Dekker: New York, 1990. (3) Thomas, J. K. The Chemistry of Excitation at Interfaces; ACS Monograph 181; American Chemical Society: Washington, DC, 1984. (4) At&, S. S.; Thomas, J. K. J. Am. Chem. SOC.1982, 104, 5868. (5) (a) Fendler, J. Membrane Mimetic Chemistry; Academic: New York, 1983. (b) Fendler, J. H. J. Phys. Chem. 1985, 89, 2730. (c) Hurst, J. K. In ref 2b, pp 183-222. (6) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic: New York, 1987. (7) (a) For a review see: Rabani, J. In ref 2a, Part B, Section 3.4. (b) For a recent review of amuhiuhilic uolvmers see: Morishima. Y. Prop. Polym. Sci. 1990, 15, 949. (8) (a) Delaire, J. A,; Rodgers, M. A,; Webber. S. E. J. Phvs. Chem. 1984, 88, 6219. (b) Stramel, RrD.; Nguyen, C.; Webber, S. E.IRodgers, M. A. J. J. Phys. Chem. 1988,92,2934. (c) Stramel, R. D.; Webber, S. E.; Rodgers, M. A. J. J. Phys. Chem. 1988, 92, 6625. (d) Stramel, R. D.; Webber, S. E.; Rodgers, M. A. J. J. Phys. Chem. 1989, 93, 1928. (e) Chattejee, P. K.; Kamioka, K.; Batteas, J. D.; Webber, S. E. J. Phys. Chem. 1991, 95, 961. (9) (a) Hsiao, J.-S.; Webber, S. E. J. Phys. Chem. 1992, 96, 2892. (b) Hsiao, J.-S.; Webber, S. E. J. Phys. Chem. 1993,97,8289. (c) Hsiao, J.-S.; Webber, S. E. J. Phys. Chem. 1993, 97, 8296. (10) (a) Pditt, G. D., Rochester, C. H., Eds. Adsorption from Solution at the SoliiYLiquid Interface; Academic Press: New York, 1983. (b) Sanchez, I., Ed. Physics of Polymer Surfaces and Interfaces; Manning Publication: Greenwich, CT, 1992. (c) Milner, S. T. Macromolecules 1992, 25, 5487. (11) Eckert, A. R.; Hsiao, J.-S.; Webber, S. E. Preceeding paper.
-
(12) Brugger, P. A.; Gratzel, M.; Guart, T.; McLendon, G. J. Phys. Chem. 1982, 86, 944. (13) (a) Hadley, S. G.; Keller, R. A. J. Phys. Chem. 1969, 73,4351. (b) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ret Data 1986,15, 12. (c) Ledger, M. B.; Salmon, G. A. J. Chem. Soc., Faraday Trans. 2 1976, 76, 883. (14) Amand, B.; Bensasson, R. Chem. Phys. Len. 1975, 34, 44. (15) Stramel, R. D.; Thomas, J. K. J. Chem. SOC.,Faraday Trans. 2, 1986, 82, 799. (16) White, B. G. Trans. Faraday SOC.1969, 65, 2000. (17) Mataga, N.; Shioyama, H.; Kawda, Y. J. Phys. Chem. 1987, 91, 314. (18) Nash, C. P. J. Phys. Chem. 1960, 64, 950. (19) Note that in homogeneous solution with simple diffusional quenching kq derived from A in eq 4 should be equal to that derived from K s v r / r ~ . This is not the case here. (20) At this laser output power, we observed no trace of PEO-A'+, but it was never possible to totally eliminate PEO-Py'+ (also see ref 9a). (21) Watanabe,T.; Honda, K. J. Phys. Chem. 1982,86,2617. The values given are for MV+ (methyl viologen) and have been used for SPV(Willner, I.; Yang, J.; Laane, C.; Otvos, J.; Calvin, M. J. Phys. Chem. 1981, 85, 3277). (22) The optical density at 602 nm from An'+ was neglected in our estimates of the ion-pair yield in order to be comparable to similar data we have obtained for PMA-A systems (see ref 9). (23) All these wavelengths suffer from overlap of the different species (SPV- is the least affected by this artifact) such that a detailed kinetic analysis of the kinetic curves is not very revealing. (24) (a) Kraeutler, B.; Bard, A. J. J. Am. Chem. SOC.1977, 99, 7729. (b) Kraeutler, B.; Bard, A. J. Nouv. J. Chim. 1979,3,31. (c) Jaeger, C. D.; Bard, A. J. J. Phys. Chem. 1979, 83, 3146. (25) Shida, T.; Iwata, S. J. Am. Chem. SOC. 1973, 95, 3473. (26) We note that PEO-Py displayed the most complete "hydrophobic protection" by the pS on the basis of the degree of static quenching and the k, values (see Table 2).
