J. Phys. Chem. B 2003, 107, 5675-5679
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ARTICLES Photoinduced Electron Transfer between Polystyrene and 2,4,6-Triphenylpyrylium Tetrakis(pentafluorophenyl)gallate in Solutions and in Polymer Films‡ Elena Y. Komarova, Kangtai Ren, and Douglas C. Neckers* Center for Photochemical Sciences,† Bowling Green State UniVersity, Bowling Green, Ohio 43403 ReceiVed: July 3, 2002; In Final Form: March 31, 2002
Photoinduced electron transfer between polystyrene and 2,4,6-triphenylpyrylium tetrakis(pentafluorophenyl)gallate (TPPGa) has been studied in dichloromethane solution and in poly(vinyl chloride) film by steady state and time-resolved fluorescence quenching techniques. A dependence of the observed rate constant of fluorescence quenching and the theoretical dependence of the diffusion constant on the molecular weight of the polystyrene were derived. Positive curvature in Stern-Volmer plots for quenching with polystyrene and toluene in solution and negative curvature in the Perrin plot for polystyrene in film was observed. No new absorption or fluorescence bands appeared upon addition of the donor to the dichloromethane solution of TPPGa. Forward and back electron transfer is the possible pathway for stabilization of TPPGa in polystyrene because 100% of TPPGa can be recovered even after prolonged irradiation.
I. Introduction Photoinduced Electron Transfer (PET) in Solutions. Photoinduced electron-transfer reactions are common for numerous organic, inorganic, and biological processes.1 Triphenylpyrylium salts are widely used PET sensitizers. Among the valuable properties of triphenylpyrylium cation as a photooxidizing agent include the selectivity of its excitation (it absorbs in the visible region of the spectrum) and its strongly oxidizing nature in both singlet and triplet excited states.2,3 Fluorescence quenching by means of electron transfer between the excited triphenylpyrylium tetrafluoroborate and several substituted arene derivatives in acetonitrile has been studied using laser flash photolysis by Ramamurthy and co-workers.4 A good correlation of the quenching constant with the free energy change for electron transfer (Marcus’ dependence, normal region) was found confirming the ET quenching pathway. The positively charged pyrylium ring possesses high electron affinity in the ground state and forms ground-state chargetransfer complexes with electron-rich aromatic molecules in nonpolar solvents.5,6 Formation of ground state charge-transfer complexes between substituted arenes such as naphthalene, xylenes, benzene, and others and the pyrylium cation, the stoichiometry of the complexes, and their association constants have been studied extensively in nonpolar solvents.5 A detailed study of the photoinduced electron-transfer reaction between polystyrene and 2,4,6-triphenylpyrylium gallate in dichloromethane solution and in poly(vinyl chloride) film by steady state and time-resolved fluorescence quenching techniques is reported in this work. ‡ This paper is dedicated to Professor Dr. J. W. Neckers on the occasion of his 101st birthday. * To whom correspondence should be addressed. † Contribution #492.
Polystyrene as an Electron Donor for PET Reactions. Polystyrene has been reported to be both a good energy donor as well as electron donor. For example, irradiation of polystyrene in air results in oxidative polymer chain degradation affording carbonyl compounds such as acetophenone as well as diketones and enones.7,8 Almost all aromatic molecules are susceptible to donor-acceptor complex formation in the ground (ground-state charge-transfer complexes) as well as in the excited (exciplex and excimer formation) state and polystyrene is not an exception.9 Formation of an excimer consisting of excited and groundstate monomeric units of polystyrene is responsible for the longer wavelength band of the fluorescence.10-12 Polystyrene was studied as an intra- and intermolecular quencher of the fluorescence of condensed arenes such as pyrene and anthracene. A strong influence of the tacticity of the polystyrene on the quenching rate was found, and isotactic polystyrene was reported to be a much better quencher than atactic polystyrene. The mechanism of the quenching (via energy or electron transfer) was not established.13 In another study undertaken by Mita et al.,14 the inter- and intramolecular quenching rate of the delayed fluorescence of anthracene by polystyrene has been investigated. The influence of molecular weight (degree of polymerization, DP) on the rate of quenching in the case of intermolecular interactions has been derived. The quenching rate constant is approximately proportional to DP-0.79. The influence of the molecular weight of polystyrene on the fluorescence quenching of TPPGa has been investigated in dichloromethane solutions in the present work. Previous Work on TPPGa. TPPGa was originally synthesized because it was expected that it would be a reactive visible light cationic photoinitiator. Though this is the case, the poor thermal stability of the salt proved to be a disadvantage. As a result, TPPGa was microencapsulated in polystyrene to improve its thermal stability and to enhance the shelf life of ready-to-
10.1021/jp026446e CCC: $25.00 © 2003 American Chemical Society Published on Web 05/28/2003
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react photoinitiator-monomer mixtures. The microcapsules we were able to synthesize contained 15% (w) of the initiator, and microencapsulation did, in fact, render TPPGa thermally as well as photochemically stable. Thus, we pursued several photochemical processes of polystryrene encapsulated TPPGa. The following work was conducted to elucidate the interactions between the core material of the capsule (TPPGa) with the wall material (polystyrene) in order to deduce the probable reasons for its photostability. II. Experimental Section Materials. All of the solvents and polystyrene were purchased from Aldrich Chemical Co. and used as received unless noted otherwise. Polystyrene standards were purchased from Polysciences Inc. The preparation of 2,4,6-triphenylpyrylium tetrakis(pentafluorophenyl)gallate was reported elsewhere.15a Instruments. UV-vis absorption spectra were recorded on a Hewlett-Packard 8452A diode array UV-vis spectrometer. Fluorescence and excitation spectra were taken with SPEX Fluorolog 2 spectrometer. Cyclic voltammetry was performed with a BAS 100A potentiostat equipped with a platinum disk electrode and platinum wire as an auxiliary electrode. Fluorescence lifetimes were measured with a single photon counting spectrofluorimeter from Edinburgh Analytical Instruments (FL/ FS 900). The excitation source was a nanosecond flash lamp operating under an atmosphere of H2 gas (0.50-0.55 bar, 0.7 nm fwhm, 40 kHz repetition rate). The samples were excited at 455 ( 2 nm. The 500 ( 2 nm emission was collected at a right angle. The luminescence was measured with a Peltier-cooled (-25 °C), R955 red sensitive photomultiplier tube. The data were analyzed by iterative convolution of the luminescence decay profile with the instrument response function using software provided by Edinburgh as well as with Microcal Origin software. Steady State and Time-Resolved Fluorescence Quenching Experiments in Solution. Dichloromethane was washed with concentrated sulfuric acid to destroy the amylene stabilizer and next distilled over phosphorus pentoxide. Polystyrenes were purified by dissolution in tetrahydrofuran followed by precipitation in methanol. The absorption at 420 nm of triphenylpyrylium gallate was ≈0.1 in all experiments. TPPGa solutions were purged with argon. Molar concentrations of polystyrene in dichloromethane in these experiments are monomeric unit molar concentrations. Nanosecond Flash Photolysis Experiments. Nanosecond flash photolysis experiments were carried out using the third harmonic of a Q-switched Nd:YAG laser (Continuum, YG660) as the excitation source.16 Pulse widths were ca. 7 ns. The transient species were monitored at a 90° angle with respect to the laser beam using a 150 W xenon arc lamp and a Hamamatsu % R928 photomultiplier tube. A Lecroy 9450 digital oscilloscope (350 MHz band-pass) was employed to convert the signal to digital form. The sample solutions were contained in 1 cm2 cuvettes that were degassed continuously with argon during the experiments. Polymer Film Preparation. Solution cast polymer films containing 15% (w) of TPPGa were prepared as follows: mixtures of polystyrene and poly(vinyl chloride) (1% (w) of the polymer mixture in dichloromethane) were dissolved in dichloromethane containing TPPGa. Films for fluorescence intensity measurements were spin-coated. III. Results and Discussion TPPGA Fluorescence Quenching in Dichloromethane Solution. The fluorescence of 15% TPPGa in poly(vinyl
Figure 1. Fluorescence of 15% TPPGa in polystyrene and poly(vinyl chloride).
Figure 2. Photoinduced electron transfer from polystyrene to 1[TPPGa]*.
chloride) and in polystyrene films is shown in Figure 1. The intensity of fluorescence is much lower in the case of the polystyrene film. The fluorescence maximum is also shifted to the red, from 465 nm in poly(vinyl chloride) and dichloromethane to 510 nm in polystyrene and in toluene. The reduced fluorescence in the polystyrene film is attributed to electrontransfer quenching of the excited state of the pyrylium salt by the polystyrene because decreased fluorescence intensities were also noticed in dichloromethane solution both in the presence of polystyrene and of toluene. Singlet-singlet energy transfer quenching processes between TPPGa and polystyrene are not feasible thermodynamically in that the singlet energy of TPPGa is 65 kcal/mol and the singlet energy of polystyrene is 106 kcal/ mol17 making ∆G for the process 41 kcal/mol. A negative change of -30 kcal/mol (calculated using the Rehm-Weller equation) in the standard free energy suggests a photoinduced electron-transfer quenching mechanism (Figure 2). The oxidation potential of polystyrene in dichloromethane is 1.9 and 2.1 V (vs SCE) for toluene. A linear Stern-Volmer dependence (eq 1 has been obtained only at low concentrations of quencher (polystyrene or toluene). At concentrations of quencher higher than 0.05 M (molar monomer unit concentration), positive curvature in SternVolmer plots have been observed in all of the cases. A typical Stern-Volmer plot is shown in Figure 3.
Φ0 τ0 ) ) 1 + kqτ0[Q] Φ τ
(1)
[Stern-Volmer formulation18 where Φ and Φ0 are the luminescence (fluorescence) quantum yields in the presence and in the absence of quencher; τ and τ0 are the fluorescence lifetimes in the presence and in the absence of quencher, kq is the rate constant of the quenching process, and [Q] the molar concentration of quencher.]
PET between Polystyrene and TPPGa
J. Phys. Chem. B, Vol. 107, No. 24, 2003 5677 TABLE 1: Dependence of the Observed Quenching Constant (kob) on the Molecular Weight (MW) of Quencher (Polystyrene (PST)), with kd as the Calculated Diffusion Rate Constant
Figure 3. Steady-state quenching of TPPGa fluorescence with toluene in dichloromethane solution.
Figure 4. Time-resolved TPPGa fluorescence quenching in dichloromethane solution.
The Stern-Volmer equation describes dynamic, diffusioncontrolled quenching. Static (associational) quenching presumes close proximity of donor and acceptor (ground state complexes, chemically associated pairs, bilayers), and this quenching is instantaneous upon excitation thus having no discernible effect on the lifetime of the excited molecules. Associational quenching does, however, influence the number of excited molecules and, therefore, the intensity and the quantum yield of luminescence. The best way to distinguish between the two types of quenching is by combining lifetime measurements with the steady state (intensity measurement) experiments. Systems that exhibit both types of quenching obey equation 2,19
Φ0 ) 1 + (Ksv + βKas)[Q] + βKsvKas[Q]2 Φ
(2)
where Kas is equilibrium constant of complex (associated pair) formation between the donor and acceptor, Ksv is a SternVolmer constant (Ksv ) τ0kq, τ0 ) 3 ns), and β ) 1 for optically dilute solutions. Time-resolved quenching experiments resulted in linear Stern-Volmer relationships for TPPGa- polystyrene pair as well as for TPPGa-toluene (Figure 4). Because no new peaks appeared in the absorption spectrum or in the fluorescence spectrum of TPPGa upon addition of excess of donor (polystyrene or toluene), the association constant of the complex can be derived by combining time-resolved and steady-state intensity measurements (eq 2). Plotting (I0/I - 1)/ [Q] versus [Q] gives (Ksv + Kas) as the intercept and KsvKas as the slope of the graph. Ksv can be calculated from time-resolved Stern-Volmer experiments. Kas is equal to 3.5 L mol-1 for the toluene-TPPGa complex and 2.5-3 L mol-1 for complex formation between polystyrene and TPPGa. Kas has been confirmed from the polynomial fit of the observed SternVolmer dependence in steady-state experiments as well. In comparison, the association constants for the ground-state
MW of PST, amu
Ksv
92 450 4000 20000 50000 90000 160000 280000 1000000
22 12 4.5 3.3 2.1 2.15 2.2 2 1.2
kob × 10-9 L/(mol s) kd × 10-9 L/(mol s) 7 4.1 1.43 1.06 0.67 0.68 0.7 0.63 0.38
12 8.5 3 1 0.6 0.4 0.25 0.2 0.06
charge-transfer complex for anthracene and pyrene with 4-acetyl2,6-diphenylpyrylium perchlorate are 2 and 4 L mol-1 respectively.5 Influence of Molecular Weight of Polystyrene on Fluorescence Quenching Rate. The solvent, as well as the environment for a reaction, has an important influence on the direction, rate, and efficiency of electron-transfer reactions. Understanding the origin of the effect of the solvent helps control the outcome of the reactions in that the nature of the medium influences both the thermodynamics and kinetics of the reaction. The viscosity of the solvent is an important factor for the kinetics of PET. It has been shown that diffusion can limit the rate of the electron transfer in solution. The limiting kd for a certain solvent can be calculated using the Debye expression:20,21
kd )
8RT 300 η
(3)
where η is a viscosity of the solvent (poise) and R and T have their usual meanings. Though a linear relationship between kd as the limiting step in the electron-transfer process and 1/η are not always confirmed experimentally due to many other factors that influence the process, eq 3 allows one to estimate an approximate range of rates where diffusion can limit the electron-transfer rate in a certain solvent.18,22 Dependence of the quenching rate constant on the molecular weight of polystyrene has been observed. An increase in molecular weight results in a decrease in the observed rate constant for both types of experiments (time-resolved and steady state). The rate constants of quenching observed in time-resolved experiments with different molecular weight polystyrenes are given in Table 1. Toluene was chosen as the model for the monomeric unit of polystyrene. As shown above, viscosity has a dramatic influence on quenching. The excited molecule and the quencher must diffuse together for electron transfer to occur. If the diffusion of the polymer is the limiting step of the reaction the rate constants should be in the range of 105-106 l/(mol s), which is the order of an average rate constant for diffusion controlled reaction between two polystyrene molecules, making it impossible to quench TPPGa excited singlet14 during its lifetime (lifetime of the fluorescence in the absence of quencher: τfl ) 3 ns in dichloromethane15). The rate constants obtained in the experiments above are much higher (>107, Table 1). Therefore, an increase in the molecular weight of polystyrene slows the diffusion of TPPGa because it changes the viscosity of the medium. Equation 4 shows the dependence of the intrinsic viscosity of a polymer solution on molecular weight.23,24
[η] ) KMa
(4)
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Figure 5. Dependence of the observed (calculated) rate constant on the molecular weight of the quencher. (Dash line and empty circles, calculated diffusion constant; solid line and squares, quenching rate constant observed experimentally.)
where K (mL/g) and a are constants derived experimentally for the certain polymer-solvent pair and [η] is an intrinsic viscosity of polymer solution. The coefficients a as well as K have been determined experimentally for many widely used polymers. a is equal to 0.73 for polystyrene in several solvents including dichloromethane,25 K ) 11× 10-3 mL/g. The intrinsic viscosity can be calculated (eq 5) for dilute solutions (c f 0) only
η -1 η0 [η] ) lim cf0 C
(5)
where η is the viscosity of the polymer solution, ηo is the viscosity of the pure solvent, and C is the concentration of the polymer in solution (%, (w)). The concentrations of the polymers in the quenching experiments do not exceed 0.5% (w). Substituting the viscosity term (η) in the Debye equation (3) with the expressions from eqs 4 and 5 gives eq 6, which allows one to estimate the limiting diffusion constant
kd )
8RT 3000(η0 + η0CKM0.73)
(6)
The rate constants for diffusion calculated (using the derived eq 6) are shown in Table 1. Plotting the calculated rate constant versus molecular weight gives a hyperbola that is close to that observed experimentally (Figure 5). The results obtained by Mita et al. fit into the derived eq 6 because the observed rate constant was proportional to DP-0.79. The degree of polymerization (DP) is directly derived from the molecular weight of the polymer.14 Nanosecond Flash Photolysis of TPPGa. Laser flash photolysis studies were conducted in dichloromethane in the presence of different concentrations of quenchers (toluene, polystyrene). A peak at 480 nm was observed in the absence of quenchers in deoxygenated solutions of triphenylpyrylium gallate after a 355 nm laser pulse. This peak can be assigned to triplet-triplet absorption of the triphenylpyrylium cation in accordance with previous observations.26,27 The lifetime of the transient at 480 nm is 10 µs and it is similar to the lifetime of the phosphorescence of TPP+.28 A strong peak at 550 nm was observed in the presence of either quencher. This peak was assigned to the pyranyl radical (the reduced form of triphenylpyrylium cation) in accord with preceding studies.28,3 The appearance of the pyranyl radical peak confirms the proposed electron-transfer mechanism in the TPPGa fluorescence quenching with polystyrene and toluene. The concentration of pyranyl
Figure 6. Perrin plot for TPPGa fluorescence quenching in poly(vinyl chloride) film
radicals produced by reduction of TPPGa singlet with both toluene and polystyrene was proportional to the concentration of toluene in solution though the pyranyl radical lifetime was independent of the toluene concentration. The lifetime of 550 nm transient decreased with the increase in polystyrene concentration, though no clear dependence can be derived. The rate of back electron transfer is slower at higher polystyrene concentrations. Fluorescence Quenching in Polymer Films. In recent years, the study of photoinduced electron or energy transfer in polymer matrixes has been carried out from the viewpoint of theory and its potential applications. For instance, electron transfer is an efficient way for quenching the excited states of dye chromophores. Therefore, this can be a functional pathway for stabilization of the dyes in polymer films as well as an image formation mechanism. Though electron transfer has been extensively studied in solution, most studies in solids were carried out at low temperatures in frozen glasses. There are comparatively few examples of such studies in the polymer films. The greater inhomogeneity of a polymer film is the most important difference from a frozen glass solution.29-31 The Perrin formulation
ln
( )
Φ° ) VN[Q] Φq
(7)
where Φ° is the quantum yield of emission of the excited acceptor in the absence of quencher, Φq is the quantum yield of the emission of the excited acceptor in the presence of donor, N is Avogadro’s number, V is the volume of the quenching sphere (in cm3), and [Q] is molar concentration of the quencher (mol/mL) is typically applied to study electron and energy transfer processes in rigid matrixes. Steady-state quenching experiments in polymer films resulted in nonlinear (negative curvature) Perrin plots (Figure 6). An increase in polystyrene concentration in the poly(vinyl chloride) film results in a longer wavelength band in the TPPGa fluorescence spectrum. In 100% polystyrene film, the shorter (465 nm) wavelength fluorescence band disappears completely. The excitation spectra of TPPGa did not show the appearance of the longer band upon an increase in polystyrene concentration in PVC, though a longer wavelength shoulder appeared in 100% polystyrene film. The absorption maximum of TPPGa in the polystyrene film (450 nm) is shifted to the red in comparison to that in the PVC film (420 nm). The negative curvature of the Perrin plot confirms the above discussion on the possibility of ground-state charge-transfer complex formation. IV. Conclusions Photoinduced electron transfer between polystyrene and TPPGa has been studied in dichloromethane solution and in a
PET between Polystyrene and TPPGa poly(vinyl chloride) film by steady state and time-resolved fluorescence quenching techniques. The dependence of the observed rate constant of the fluorescence quenching on the molecular weight of polystyrene has been explained by the influence of the viscosity change in dilute solutions with the increase in the molecular weight of the polymer. A theoretical dependence of the diffusion constant on the molecular weight of the polystyrene has been derived. Positive curvature in SternVolmer plots for quenching with polystyrene and toluene in solution and negative curvature in the Perrin plot for polystyrene in film was assigned to ground-state charge-transfer complex formation. Though no new absorption, as well as fluorescence, bands appear upon addition of the donor to the dichloromethane solution of TPPGa, the association constant of the complex between TPPGa and donors (polystyrene and toluene) in dichloromethane has been calculated. The reduced form of triphenylpyrylium cation (pyranyl radical) was observed during nanosecond flash photolysis experiments that supports the electron-transfer mechanism of the fluorescence quenching. Acknowledgment. We thank the National Science Foundation Division of Materials Research (DMR 9803006) for financial support of this work. References and Notes (1) Photoinduced Electron Transfer. Parts A-D; Fox, M. A., Chanon, M., Eds; Elsevier: Amsterdam, 1988. (2) Miranda, M. A.; Garcı´a, H. Chem. ReV. 1994, 94, 1063-1089. (3) Wintgens, V.; Pouliquen, J.; Kossanyi, J New J. Chemistry 1986, 10, 345-350. (4) Ramamurthy, P.; Jayanthi, S. S J. Phys. Chem. A 1997, 101, 20162022. (5) Wintgens, V.; Pouliquen, J.; Simalty, S.; Kossanyi, J. J. Photochemistry 1984, 26, 131-140. (6) Dinculescu, A.; Koutrakis, H. N.; Balaban, A. T. ReV. Roum. Chim. 1979, 24, 439.
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