Langmuir 2005, 21, 1441-1447
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Oxidative Transformations of Surface-Bound Perylene Maciej Mazur† and G. J. Blanchard*,‡ University of Warsaw, Department of Chemistry, Laboratory of Electrochemistry, 02-093 Warsaw, Pasteura 1, Poland, and Michigan State University, Department of Chemistry, East Lansing, Michigan 48824-1322 Received August 17, 2004. In Final Form: November 19, 2004 We report on the covalent attachment of perylene derivatives to silica and indium-doped tin oxide surfaces. The spectroscopic properties of the immobilized fluorophores are studied by steady-state and time-resolved emission spectroscopy. The redox properties of the molecules are examined by electrochemical methods. It was found that the oxidation of perylene in aqueous medium results in the formation of perylenequinones. The transformation proceeds through a number of steps, with monohydroxyperylene(s) being the intermediate species. The final oxidation products are three isomeric forms of the perylenequinone/ perylenehydroquinone redox couple. Understanding the mechanism of perylene transformation allows estimation of the concentration of molecules on the surface, and comparison of these results to those for pyrene and anthracene derivatives underscores the generality of this type of oxidative degradation for polycyclic aromatic hydrocarbons.
Introduction The self-assembly of monomolecular layers onto surfaces from the solution or vapor phase has evolved to be a versatile technique for the construction of supramolecular assemblies and for controlling surface properties. The surface immobilization of various monomolecular species has been used in areas ranging from molecular electronics and tribology to chemical sensing, and from a more fundamental perspective, self-assembly produces surfaces that are well suited to the detailed characterization of the adsorbed species.1-4 The immobilization of fluorescent probes onto solid interfaces has been examined extensively because of the potential of these species to function as chemical and/or biochemical sensors, optical display materials, and nanoelectronic devices. Polycyclic aromatic hydrocarbons (PAHs) have been the focus of much recent work because of their strong native fluorescence, sensitivity to their local environment, and interesting photophysical and photochemical properties. A number of fluorescent polycyclic aromatic hydrocarbons have been immobilized on a variety of surfaces, including pyrene,5-10 anthracene,8,11,12 and chrysene13 on oxide and metallic substrates. * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Warsaw. ‡ Michigan State University. (1) Sullivan, T. P.; Huck, W. T. S. Eur. J. Org. Chem. 2003, 17. (2) Li, G.; Fudickar, W.; Skupin, M.; Klyszcz, A.; Draeger, C.; Lauer, M.; Fuhrhop, J. H. Angew. Chem., Int. Ed. 2002, 41, 1828. (3) Fendler, J. H. Chem. Mater. 2001, 13, 3196. (4) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (5) Karpovich, D. S.; Blanchard, G. J. Langmuir 1996, 12, 5522. (6) Kelepouris, L.; Krysin´ski, P.; Blanchard, G. J. J. Phys. Chem. B 2003, 107, 4100. (7) Krysin´ski, P.; Blanchard, G. J. Langmuir 2003, 19, 3875. (8) Mazur, M.; Blanchard, G. J. J. Phys. Chem. B 2004, 108, 1038. (9) Gao, L. N.; Fang, Y.; Wen, X. P.; Li, Y. G.; Hu, D. D. J. Phys. Chem. B 2004, 108, 1207. (10) Kamat, P. V.; Barazzouk, S.; Hotchandani, S. Angew. Chem., Int. Ed. 2002, 41, 2764. (11) Fox, M. A.; Li, W. J.; Wooten, M.; McKerrow, A.; Whitesell, J. K. Thin Solid Films 1998, 327, 477. (12) Mitsuishi, M.; Tanuma, T.; Matsui, J.; Chen, J. F.; Miyashita, T. Langmuir 2001, 17, 7449. (13) Dutta, A. K.; Misra, T. N.; Pal, A. J. J. Phys. Chem. 1994, 98, 4365.
In this paper we focus on the covalent immobilization of perylene on oxide surfaces and report on its spectroscopic and electrochemical properties. We are interested in the chromophore perylene not only because of its optical properties and high fluorescence quantum yield but also because it remains unclear if all PAHs are susceptible to similar oxidative degradation effects when bound to interfaces. We have found that the combination of spectroscopic and electrochemical techniques we use provides an effective means for evaluating the chemical reactivity of surface-bound PAHs. Steady-state and timeresolved spectroscopies are sensitive to local environment, molecular motion, and any quenching processes that may operate on these chromophores. Electrochemical measurements provide a direct means of monitoring surface reactions and, in addition to monitoring multiple step processes, these data can be used to estimate surface loading density. From the complementary information provided by both techniques, we are able to understand in a more general way the reactivity of structurally different PAH compounds in the restricted environment(s) formed by their covalent binding to an interface. Experimental Section Chemicals. All chemicals were of the highest quality available commercially: perylenedodecanoic acid (PDA) (Molecular Probes), 3,4,9,10-perylenetetracarboxylic dianhydride (Aldrich), (aminopropyl)triethoxysilane (Aldrich, 99%), triethylamine (Aldrich, 99.5%), 1,3-dicyclohexylcarbodiimide, DCC (Aldrich, 99%), lithium perchlorate (Aldrich, 95+%), sodium hydroxide (Aldrich, ACS grade), sodium sulfate (Fluka, >99%), hydrochloric acid (CCI, ACS grade), sulfuric acid (Aldrich, 99.999%), ammonium persulfate (NH4)2S2O8 (Fluka, >98%), adipoyl chloride (Aldrich, 98%), 4-methylmorpholine (Aldrich, 99%), aminopyrene (Aldrich, 98%), acetonitrile (Aldrich, anhydrous, 99.8%), ethyl acetate (Spectrum Chemical, ACS grade), chloroform (EMD, ACS grade), cyclohexane (Aldrich, 99+%), methanol (Aldrich, 99.0%), and water (Aldrich, HPLC grade). Perylenetetracarboxylic acid14 (PTA) and 3,10-perylenedione15 were prepared according to literature procedures. Steady-State Emission Spectroscopy. Emission and excitation spectra were recorded with a Spex Fluorolog 3 spec(14) Ferrere, S.; Zaban, A.; Gregg, B. A. J. Phys. Chem. B 1997, 101, 4490. (15) Brass, K.; Tengler, E. Chem. Ber. 1931, 64, 1646.
10.1021/la0479509 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/19/2005
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trometer using a 3 nm band-pass for excitation and a 3 nm bandpass for emission collection. Time-Resolved Emission Measurements. The time-correlated single-photon counting (TCSPC) system used to acquire time-resolved spectroscopic data has been described elsewhere,16 and we present a brief overview of the system here. The third harmonic of the output of a mode-locked CW Nd:YAG laser (Coherent Antares 76-S) is used to pump a cavity-dumped dye laser (Coherent 702-2) operated at 420 nm using stilbene 420 laser dye (Exciton Chemical Co.). The output of the dye laser was typically 50 mW average power at 1 MHz repetition rate with ∼5 ps pulses. The 420 nm light incident on the sample is attenuated to provide ca. 1 mW average power at the sample. The detection electronics are characterized by an instrument response function of 35 ps full width at half-maximum (fwhm). The emission collection wavelength and polarization are computer controlled using National Instruments LabVIEW version 7.0 software. Electrochemical Measurements. Electrochemical measurements were made using a computer-controlled Electrochemical Workstation (CH Instruments model 650A), with a small volume three-electrode cell with Pt wire as the counter electrode. All potentials are reported versus a Ag/AgCl/3 M KClaq reference electrode. Substrate Preparation. Quartz slides (NSG Precision Cells, Inc., P/N 10040 nonfluorescent UV fused silica windows) were cleaned by immersion in piranha solution for ca. 20 min. Indiumdoped tin oxide (ITO) films (Bayview Optics, Dover-Foxcroft, ME) deposited on the same quartz slides were used for spectroscopic measurements, and ITO films on glass substrates (Delta Technologies) were used for electrochemical measurements. We used silica substrates because of their favorable optical properties and ITO substrates because of their conductive properties, allowing electrochemical measurements to be made. Previous work has indicated that silica and ITO surfaces are sufficiently similar to allow comparisons to be made between data sets for the two substrates.8,19 Bonding 3-Perylenedodecanoic Acid and Perylenetetracarboxylic Acid to Quartz and ITO. Silica and ITO substrates were reacted with 3-aminopropyltriethoxysilane (0.3 mL) in dry acetonitrile (10 mL), using triethylamine (0.3 mL) as a catalyst, under reduced pressure for 2 h. The silane-modified substrate was removed from solution, rinsed with acetonitrile, and dried. The terminal amino groups were reacted with the carboxylic acid by exposing the substrates to a solution containing either 3-perylenedodecanoic or perylenetetracarboxylic acid (5 mM) and DCC (10 mM) in ethyl acetate for 1.5 h. The reacted substrates were removed from the reaction vessel, washed with ethyl acetate, and dried under a stream of nitrogen. Bonding Aminopyrene to Quartz. The procedure of covalent attachment of pyrene is described elsewhere.8 Preparation of Thin Films of 3,10-Perylenedione on ITO. 3,10-Perylenedione thin (physisorbed) films were prepared on ITO by immersing the substrates into a ca. 0.1 M solution of the compound in ethyl acetate, followed by removal from the solution and air-drying. The resulting modified electrodes were characterized electrochemically in aqueous solution containing 0.5 M electrolyte (H2SO4).
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Figure 1. Excitation (emission wavelength, 480 nm) and emission (excitation wavelength, 415 nm) spectra in air of PDA covalently attached to quartz.
Figure 2. Fluorescence decays of PDA: (a) solution phase (hexane), (b) covalently bound to quartz (in air). Inset: Fluorescence decays of PDA attached to quartz; the sample is immersed in various solvents.
Emission Spectroscopy of Surface-Bound Perylene. The primary goal of this work is to understand the chemical reactivity of substituted perylene chromophores bound covalently to oxide surfaces. As noted above, the combination of spectroscopic and electrochemical techniques provide substantial information on the intermediate and final oxidation reaction products for this family of chromophores. We are interested in understanding the extent to which PAH oxidative degradation chemistry can be predicted for this family of compounds and have chosen
perylene because its spectroscopic properties and reactivity differ significantly from those of other PAHs such as pyrene. To attach fluorophore molecules covalently, we first selfassembled aminopropyltriethoxysilane on the silica and ITO surfaces and then reacted the terminal amino groups of the silanes with carboxylic acid groups of PDA. This reaction sequence produced an adlayer of chromophores on the oxide surfaces. Covalent chromophore attachment was confirmed by steady-state emission and excitation spectroscopy. We show in Figure 1 the excitation and emission spectra of perylenedodecanoic acid bound covalently to silica. These spectra have the mirrorlike symmetry that is characteristic of perylene and substituted perylenes. The band positions are similar to those of solution phase 1-methylperylene,17,18 with absorption band maxima at 395, 417, and 442 nm and emission band maxima at 451, 480, and 515 nm. These spectral data for silica-bound PDA are the same as those for the chromophore bound to ITO (not shown). The presence of charge carriers in the ITO substrate does not influence the steadystate emission response of perylene significantly. Time-domain spectroscopic data (Figure 2) reveal that the fluorescence lifetime of PDA is significantly shorter
(16) Dewitt, L.; Blanchard, G. J.; Legoff, E.; Benz, M. E.; Liao, J. H.; Kanatzidis, M. G. J. Am. Chem. Soc. 1993, 115, 12158.
(17) Jiang, Y.; Blanchard, G. J. J. Phys. Chem. 1994, 98, 9417. (18) Jiang, Y.; Blanchard, G. J. J. Phys. Chem. 1995, 99, 7904.
Results and Discussion
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Table 1. Lifetimes of Perylene Immobilized on Quartz in Various Solvents in Which the Substrate Is Immersed solvent
τ1 (ns)
τ2 (ns)
chloroform cyclohexane ethyl acetate methanol water
0.26 ( 0.01 0.19 ( 0.01 0.46 ( 0.02 0.13 ( 0.01 0.35 ( 0.02
1.44 ( 0.05 1.49 ( 0.04 1.98 ( 0.12 1.00 ( 0.02 1.63 ( 0.06
when immobilized on SiOx than it is in the solution phase. Although the origin of this behavior is not clear at this stage, it seems this is a general phenomenon, as we observed lifetime shortening previously for other fluorophores, e.g., pyrene19 or dansyl20 chromophores, when bound to a silica substrate. While an explanation of this phenomenon at this point is speculative, it is possible that the formation of chromophore aggregates could play a role in the observed lifetime reduction or, less likely, that the presence of a spatial gradient in the dielectric response of the system along the interface normal could give rise to the lifetime diminution. For the PDA chromophore, the time-resolved transients appear similar, whether oxygen is present or not (measurements performed in air or in a N2 atmosphere), and there is little dependence of the measured lifetime on the solvent in which the substrate is immersed (Figure 2 inset, and Table 1). There appears to be no significant dependence of these lifetime data on local environment, a result that is consistent with the behavior of perylene in solution. The dominant effect we observe is the reduction in lifetime associated with attachment of the chromophore to the substrate. Electrochemistry of PDA in Aqueous Acidic Medium. We are interested in the evaluation of the surface loading density of the covalently attached chromophores. Measuring surface loading density can be performed electrochemically on conducting substrates such as ITO, but before this determination can be made, the mechanism of perylene redox transformation needs to be established. Consecutive cyclic voltammograms of PDA attached to ITO, recorded in sulfuric acid solution, are shown in Figure 3. In the first scan, we observe an irreversible peak at 0.92 V. This peak decreases rapidly in subsequent cycles, suggesting that oxidized PDA molecules are transformed into some new species. This indication is confirmed by the appearance of new redox peak pairs at 0.55, 0.34, 0.23, and 0.19 V. The signals at 0.55 V diminish with additional cycling, indicating this feature is associated with an intermediate species. The other signals, at 0.34, 0.23, and 0.19 V (deconvoluted anodic peaks of these signals are presented in the inset to Figure 3), are stable indicating they are the final products of electrochemical oxidation. The irreversible feature at 0.92 V is assigned to the oxidation of the perylene moiety. We believe the oxidation results in the formation of reactive radical cations which undergo subsequent reactions. We have reported previously on the oxidation reactions characteristic of pyrene. We have found that the oxidation of pyrene leads to radical cations, which further react with water molecules resulting in formation of quinone/hydroquinone derivatives.8,19 The experimental data presented here indicate that the oxidative reaction chemistry of PDA is qualitatively similar to that seen for pyrene. If that is indeed the case, following initial oxidation, the perylene radical cations react with water to produce hydroxyl and quinone derivatives. The appearance of new redox signals at 0.34, 0.23, (19) Mazur, M.; Blanchard, G. J. Bioelectrochemistry, accepted. (20) Bakiamoh, S. B.; Blanchard, G. J. Unpublished results.
Figure 3. Consecutive cyclic voltammograms for PDA covalently bound to ITO. The curves were recorded in aqueous 0.5 M H2SO4; sweep rate, 0.1 V/s. Inset: Deconvoluted voltammetric signal (oxidized PDA covalently bound to ITO) indicating the existence of three redox peaks (the baseline is subtracted).
and 0.19 V can be assigned to formation of these new species. The formal potentials of these species are within the range of potentials characteristic for quinonones/ hydroquinones.21 Perylene is known to be oxidized by strong oxidation agents to form 3,10-perylenedione.15,22 Perylene can be also photooxidized to a mixture of products, mainly 3,10-perylenedione and 1,12-perylenedione.23,24 We show in Figure 4 the cyclic voltammogram of the 3,10-perylenedione/3,10-perylenediol redox couple. The formal potential for this redox couple is 0.24 V, very close to the 0.23 V PDA oxidation product we observe experimentally. We believe that one of the products of perylene oxidation is the 3,10-dione/diol. We can confirm formation of quinone/hydroquinone derivatives indirectly through additional electrochemical experiments. By measuring the dependence of the anodic and cathodic peak potentials (for the signals at 0.23 V) versus the log of the voltammetric sweep rate (Figure 5a), we can estimate the number of electrons exchanged during the redox reaction, based on Laviron’s theory25 and solving the set of eqs 1:
RT ) 32.1 mV RnF RT ) 22.8 mV (1 - R)nF
(1)
where R is the gas constant, T is temperature in K, R is the cathodic transfer coefficient, n is the number of electrons, and F is the Faraday constant. We calculated R ) 0.78 and the number of electrons, n ) 1.89, fully consistent with the formation of quinone/hydroquinone perylene derivatives, since the redox reactions of diones/ diols are two-electron processes. In another experiment, we can determine the ratio of the number of electrons to the number of protons exchanged during the redox reaction. By measuring the pH-dependence of the formal (21) Moriconi, E. J.; Rakoczy, B.; O’Connor, W. F. J. 1962, 27, 2772. (22) Zinke, A.; Uterkreuter, E. Monatsh. Chem. 1919, (23) White, B.; Nowakowska, M.; Guillet, J. E. J. Photobiol., A 1989, 50, 147. (24) Burke, N. A. D.; Templin, M.; Guillet, J. E. J. Photobiol., A 1996, 100, 93. (25) Laviron, E. J. Electroanal. Chem. 1979, 101, 19.
Org. Chem. 40, 405. Photochem. Photochem.
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Figure 4. Cyclic voltammogram of 3,10-perylenedione physisorbed on ITO (recorded in aqueous 0.5 M H2SO4). Figure 6. Excitation (emission wavelength, 520 nm) and emission (excitation wavelength, 440 nm) spectra in air of PTA covalently attached to quartz. Inset: Emission spectra of PTA attached to quartz; the sample is immersed in water or aqueous 0.5 M H2SO4.
Figure 5. (a) Dependence of the anodic and cathodic peak potentials versus logarithm of sweep rate for the voltammetric redox signals at 0.24 V (oxidation product of surface-bound perylene). The numbers are the slopes of the linear parts of the dependencies. (b) Dependence of the formal potential of the redox couple (oxidation product of surface-bound PDA) versus pH (H2SO4/Na2SO4; total concentration, 0.5 M).
potential of the redox species (calculated as the average of anodic and cathodic peak potentials), we can determine the ratio of the numbers of electrons and protons from the slope of the linear relationship (Figure 5b).26,27 For our system the slope of this dependence is ca. 45 mV/pH, relatively close to the theoretical value of 59 mV/pH, and may be indicative of a 1:1 e-/H+ ratio for this reaction. The deviations from the theoretical Nernstian slope can be the effect of the change of liquid junction potential at the reference electrode/investigated solution interface influencing the overall electrode potential. Thus, the real (26) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonquist, T.; Kankare, J. Langmuir 1998, 14, 1705. (27) Mazur, M.; Krysinski, P. Langmuir 2001, 17, 7093.
value may be in fact much closer to 59 mV/pH. From these data we infer that the redox reaction is a two-electron, two-proton process, supporting the hypothesis that the oxidation of PDA yields quinone/hydroquinone derivatives. The other less intense signals seen on voltammograms (Figure 3) at 0.34 and 0.19 V are likely other isomers of the perylenedione/diol couple. For perylene there exist four possible quinone/hydroquinone derivatives: 3,10-, 3,9-, and 1,12-perylenequinones and 3,4,9,10-tetraone.28 Since we can exclude the possibility of formation of perylenetetraone due to substitution with the dodecanoic acid chain at the 3-position, the number of possible quinone forms of perylene (three) is equal to the number of stable redox signals at cyclic voltammograms. We believe the three redox signals are directly related to the three isomeric forms of perylenedione/diol. To assign the signals at 0.34 and 0.19 V to the specified isomeric forms of perylenequinone, we performed a number of experiments with PTA, a perylene molecule substituted with four carboxylic acid groups. We believe that understanding the redox reactions of this compound will allow more precise determination of the mechanism of transformation of nonsubstituted perylene. Transformations of Surface-Attached Perylenetetracarboxylic Acid. The covalent immobilization of perylenetetracarboxylic acid on an oxide surface was performed in the same way as for PDA. The excitation and emission spectra of the compound attached to quartz are shown in Figure 6. The excitation spectrum reveals three bands at 420, 440, and 470 nm, and the emission bands are centered at 495, 520, and 570 nm. These spectra are similar to those of surface-bound PDA but are redshifted ca. 30 nm. The well-resolved vibronic structure of the excitation and emission bands should be noted. We believe that because no heat was applied to this surface reaction, the interaction between the surface amines and PTA does not form imide linkages. If imides had formed, the excitation and emission bands would likely be broad and featureless, and shifted significantly to longer wavelengths (emission signals near 600 nm).14 The spectral response of PTA is, in fact, somewhat more complicated than that of PDA, and we discuss this issue next. The excitation and emission spectra of PTA depend strongly on the pH of the solution (the spectra of PDA do (28) Boldt, P. Extended quinones. In Chemistry of the quinoid compounds; Wiley & Sons: New York, 1988; Vol. 2, Part 2, p 1437.
Oxidative Transformations of Perylene
not) in which the sample with immobilized fluorophore is immersed, due to the presence of carboxylic acid groups conjugated to the chromophore. In the inset to Figure 6 we show the emission spectra of PTA attached to quartz, immersed in water and aqueous 0.5 M H2SO4. The spectrum recorded in water at pH 7 is similar to that recorded for surface-bound PTA in air. The spectrum is altered significantly when the sample is placed in an acidic environment. The fluorescence intensity decreases, and only a weak signal centered at ca. 540 nm is seen (a relatively intense Raman band of water is seen at 515 nm, and this feature is also present but less prominent in the pH 7 spectrum). These spectral changes are consistent with protonation of carboxylic groups, which will serve to alter the extent of conjugation in the chromophore. When PTA is in pH 7 water, it is ionized to some extent, and in acidic solution, PTA exists in its fully protonated form. We consider next the electrochemical behavior of surface-bound PTA (attached to ITO). Two consecutive cyclic voltammograms recorded in 0.5 M H2SO4 are presented in Figure 7a. In the first cycle an irreversible oxidation signal is seen with a maximum at ca. 1.25 V. In the second (and subsequent) scans this signal diminishes, indicating an irreversible transformation of PTA into a new species. This result is qualitatively similar to that for PDA, but the oxidation peak occurs at significantly higher potential for PTA. Following irreversible oxidation, several new features appear in the voltammograms: a pair of peaks at 0.45 V and a pair at ca. 0.55 V, with the reduction peak of this signal being poorly resolved. The intensity of the signals is relatively small, suggesting that some other oxidation products may exist, but they appear to be electroinactive and we have no information about them. When PTA bound to ITO is studied in neutral solution (aqueous 0.5 M LiClO4), its oxidation behavior is significantly different (Figure 7b). The oxidation starts at a much lower potential (above 0.8 V), and the currents are significantly higher. We believe the oxidation current must be associated with some additional Faradaic process. The oxidation products give voltammetric signals at ca. 0.05 and 0.2 V. However, due to the different pH (neutral), their formal potentials cannot be compared directly to the potentials of PDA oxidation products in acidic solution. The sample was transferred to 0.5 M H2SO4 after oxidation in aqueous LiClO4, and the cyclic voltammograms in 0.5 M H2SO4 are shown in Figure 7c. These data reveal the same pattern as observed previously for the oxidation products of PDA. There are three overlapping redox peak pairs at 0.15, 0.25, and 0.42 V, which are only slightly anodically shifted compared to PDA products. These electrochemical results allow us to draw some more general conclusions. When surface-bound PTA is oxidized in acidic solution, a new peak pair at 0.45 V appears (we will consider the origin of the second pair at 0.55 V below). As the positions 3, 4, 9, and 10 are blocked by carboxylic acid or amide groups, it can be assumed that the substitution of the perylene ring system with hydroxyl/dione functionalities is not feasible at these positions. Thus, the only substitution leading to the quinone/hydroquinone redox couple would be the 1,12isomer. In this picture, PTA is oxidized to the 1,12perylenedione/perylenediol system as the only electroactive product. When the same experiment is performed in neutral solution, the oxidation leads to formation of three redox couples (Figure 7c). The similarity of these signals to the products of PDA oxidation suggests that three isomeric quinone forms are produced: 3,9; 3,10; and 1,12. If this is the case, the carboxylic acid groups of PTA
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Figure 7. Consecutive cyclic voltammograms for PTA covalently bound to ITO; sweep rate, 0.1 V/s: (a) recorded in aqueous 0.5 M H2SO4; (b) recorded in aqueous 0.5 M LiClO4; (c) recorded in aqueous 0.5 M H2SO4 after preceding oxidation in aqueous LiClO4.
do not block the substitution and must be cleaved from the perylene ring structure during the oxidation process, likely releasing carbon dioxide. The large currents observed during oxidation of PTA in LiClO4 solution (Figure 7b) are most likely associated with at least partial decarboxylation, a process which is known for other aromatic carboxylic acids.29-31 This process occurs in neutral solution but it is inhibited in acidic media. With this information, we can assign the voltammetric signals (Figure 7c for PTA, Figure 3 for PDA) to isomeric forms of perylenequinones. The signal at 0.42 V (0.34 V for PDA) in this interpretation is associated with the 1,12 isomer; the signal at 0.25 V (0.23 V for PDA) is the 3,10 isomer, (29) Montilla, F.; Morallon, E. Vazquez, J. L. Langmuir 2003, 19, 10241. (30) Andrieux, C. P.; Gonzalez, F.; Saveant, J. M. J. Electroanal. Chem. 2001, 498, 171. (31) Corrigan, D. S.; Weaver, M. J. Langmuir 1988, 4, 599.
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Scheme 1. Oxidative Transformations of Immobilized Perylene; Structures of Isomeric Forms of Perylenequinones
and the 0.15 V (0.19 V for PDA) feature is thus the 3,9 isomer. The origin of the signal at 0.55 V observed in acidic solutions for both PTA and PDA needs to be determined. The signals decrease in subsequent scans, which is consistent with these species being transformed into final products. We believe the signals at 0.55 V are monohydroxyl derivatives, which after initial oxidation are readily transformed into the dihydroxyperylene/perylenedione redox couple. This finding is similar to the redox chemistry of two other polycyclic aromatic hydrocarbons, pyrene and anthracene, where monohydroxyl derivatives are produced as intermediate species.8,19 Determination of the Surface Concentration of PDA. The results we report here suggest a mechanism of oxidative transformation of perylene that is indicated in Scheme 1. The first step in the reaction sequence is the oxidation of the perylene moiety to a radical cation, which reacts with water to produce monohydroxy derivative(s). We assume that, regardless of the substitution position, all possible monohydroxy derivatives exhibit voltammetric peaks at more or less the same potential of ca. 0.55 V. Monohydroxy derivatives, as intermediate species, are subsequently oxidized further and react with water to produce isomeric redox couples of perylenediones/perylenediols. We assign the voltammetric signal at 0.19 V to 3,9perylenedione/3,9-perylenediol, the signal at 0.23 V to 3,10-
perylenedione/3,10-perylenediol, and the signal at 0.34 V to 1,12-perylenedione/1,12-perylenediol. Based on this information, we can determine the surface loading density of surface-bound perylene moieties. We can determine this quantity in a number of ways, but we believe the most reliable method is to measure the charge exchanged during the quinone/hydroquinone reaction. The total charge under the voltammetric peaks (Q) at 0.19, 0.23, and 0.34 V is equal to 2.7 µC/cm2. The total charge associated with the quinone/hydroquinone redox reaction was calculated numerically. First, we subtracted the baseline below the anodic peaks, using the first scan as the baseline, and then integrated the current in the range 0.1-0.4 V. The potential axis was converted to time using the relationship t ) E/ν, to get the charge. Assuming a two-electron reaction (vide supra), the surface concentration can be determined according to eq 2:
Γ)
Q ) 1.4 × 10-11 mol/cm2 2F
(2)
The surface loading obtained is comparable to the values reported for other fluorophores and different attachment procedures. For instance, the attachment of aminopyrene to ITO using acid chloride chemistry produces the surface coverage of 5.3 × 10-11 mol/cm2.19 The same fluorophore attached “in situ” to mercaptoundecanoic acid on gold gives the surface loading of 7.7 × 10-10 mol/cm2.8 On the other
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substrates, where the samples are immersed in an aqueous solution containing ammonium persulfate, a strong oxidant. The fluorescence intensity of the perylene chromophore decreases (due to transformation to quinones) relatively slowly, over tens of minutes, while the pyrene signal diminishes over several minutes. This result suggests significant differences in kinetics of oxidative transformation of different PAHs, rather than a simple dependence on absorption wavelength. While the oxidative degradation mechanism for PAHs appears to be general to this family of molecules, the kinetics of oxidative degradation varies significantly between compounds, and this is an issue that bears further investigation. Our data show that for structurally different PAHs (perylene, pyrene, anthracene),8,19 the oxidative transformation mechanism is substantially the same. The molecules can be oxidized in aqueous media to reactive radical cations, and then they are transformed initially to monohydroxy derivatives and subsequently to quinone/ hydroquinone redox couples. The generality of the mechanism seems to be an important finding, and it may be useful in designing methodologies for the efficient degradation of PAH-containing wastes. Figure 8. Consecutive emission spectra of (a) PDA at 0, 6, 11, 33, 44, 55, 66, 77, 88, and 99 min after immersion into aqueous 0.25 M ammonium persulfate solution (excitation at 420 nm) and (b) aminopyrene at 0, 3, and 6 min after immersion into aqueous 0.25 M ammonium persulfate solution (excitation at 320 nm). Both chromophores were attached to quartz. The signals at 482 nm for PDA (overlapped with a vibronic band of perylene at 470 nm) and 360 nm for pyrene are water Raman bands.
hand, the aminopyrene attached to boron-doped diamond using adipoyl chloride as a spacer results in a surface concentration of only 4.25 × 10-12 mol/cm2.32 We find that, compared to other PAHs such as pyrene and anthracene, surface-attached perylene molecules are relatively stable when exposed to an oxygen-rich environment and UV-light. The emission spectrum of PDA bound to quartz or ITO does not change significantly for hours under ambient illumination. However, relatively highintensity light (e.g. laser irradiation) causes a rapid degradation of the fluorescence intensity due to photooxidation. The comparative stability of perylene relative to pyrene and anthracene8 may be the result of the longer wavelength absorption of perylene. The experimental data suggest that, although the oxidation potential of perylene is slightly lower than that for pyrene or anthracene, perylene is significantly less susceptible to chemical oxidation. In Figure 8 we show the time-resolved emission spectra of PDA (a) and pyrene (b) attached to silica (32) Krysinski, P.; Show, Y.; Stotter, J.; Blanchard, G. J. J. Am. Chem. Soc. 2003, 125, 12726.
Conclusions Substituted perylene chromophores can be immobilized on oxide surfaces such as silica and ITO using well established covalent interface chemistry, and these chromophores retain their fluorescent properties following covalent attachment. Perylene can be oxidized irreversibly in aqueous medium in a process that involves several reaction steps. The oxidation of perylene results in the formation of radical cations, which, once formed, react with water to form monohydroxy derivatives. Subsequent oxidation and reaction with water produces several isomeric forms of the perylenequinone/hydroquinone redox couple. It was our purpose in this work to establish the generality of the mechanism of PAH oxidative degradation. Based on our spectroscopic and electrochemical data, we believe the oxidative transformation of perylene is consistent with that seen for other PAHs such as pyrene and anthracene8 and that this mechanism is characteristic of most polycyclic aromatic hydrocarbons, tethered or not. With this understanding of the surface chemistry characteristic of PAHs, we can determine their surface loading density and we can tailor their local environments to minimize exposure to oxygen in applications where the optical properties are utilized. Acknowledgment. We are grateful to the U.S. Department of Energy for support of this work through Grant DEFG0299ER15001. LA0479509
