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J. Phys. Chem. B 2000, 104, 7462-7467
Photochemical Reactions of Phenazine and Acridine Adsorbed on Silver Colloid Surfaces Dae Hong Jeong† and Jung Sang Suh* School of Chemistry and Molecular Engineering, Seoul National UniVersity, Seoul Korea 151-747
Martin Moskovits* Department of Chemistry, UniVersity of Toronto, Toronto, Canada M5S 1A1 ReceiVed: May 8, 2000
The photochemical reactions of phenazine and acridine adsorbed on silver colloid surfaces were studied. Both molecules, when adsorbed on the surface of colloidal silver, were found to photodecompose with the cleavage of a N-C bond in a one-photon process. Phenazine was found to photodecompose further to graphitic carbon under our experimental conditions, particularly when irradiated with short-wavelength laser light. The reaction rate of phenazine, which we propose to adsorb flat on the surface, was faster than that of acridine, which we suggest adsorbs standing up, implying that the reaction rate might depend on the orientation of the molecule with respect to the local metal surface. The reaction rate and degree of photodecomposition of these compounds depend on the irradiation wavelength in a complicated manner. For acridine, the photoreaction rate and the Raman enhancement increased toward the blue, whereas the degree of photodecomposition, i.e., the quantity of photoproduct produced, is a maximum at ∼500 nm.
Introduction Enhanced surface photochemistry of molecules adsorbed on surfaces capable of producing surface-enhanced Raman is now a well-established phenomenon.1-5 Surface-enhanced Raman scattering (SERS) spectroscopy6 is a powerful technique for studying such surface chemical reactions, as it is sensitive to relatively low concentrations of the reagents and products involved. Previously, we described a simple method7,8 for following the kinetics of moderately fast surface-enhanced photochemical reactions using a flow cell. This approach was successfully used to study of the photodecomposition,8,9 photoisomerization,10 and photodesorption7,11 of molecules adsorbed on colloidal silver surfaces. The SERS spectra of phenazine12 and acridine13,14 have been previously reported. Evidence for the photoreaction of adsorbed acridine on colloidal silver was reported but not studied in detail.13 Phenazine and acridine are heterocyclic homologues of anthracene belonging to the point groups C2V and D2h, respectively. In acridine, the 10th carbon atom, and in phenazine the 9th and 10th carbon atoms, of anthracene are replaced by nitrogen. The molecules can adsorb on the surface either as π ligands bonding to the surface through the aromatic rings or as σ ligands bonding through the lone pairs of the nitrogens. The differing bonding options available to the two molecules might result in their adopting different bonding orientation strategies with respect to the local silver surface, thereby presenting us with an opportunity of following the surface photochemical kinetics of two electronically very similar adsorbates that differ primarily in their mode of bonding to the surface. Also, we previously showed that, in heterobicyclic aromatics adsorbed on silver, only the heterocycle was photocleaved. These tricyclic * Corresponding authors. E-mails:
[email protected], mmoskovi@ chem.utoronto.ca. † Present address: Korea Research Institute of Standards and Science, Taejon, Korea.
molecules offer us an opportunity to further probe the surface photochemical pathways taken by aromatics. In the present paper, we report the surface-enhanced photochemical kinetics of phenazine and acridine adsorbed on colloidal silver using the aforementioned flow cell method. Experimental Section Silver sols were prepared as described previously.15 Briefly, 60 mL of a 2 × 10-3 M sodium borohydride solution were mixed with 22 ( 2 mL of a 1 × 10-3 M silver nitrate solution. The phenazine was adsorbed on the colloid by adding the adsorbate in a 3:1 water in methanol solution dropwise to the aqueous colloid. The overall concentration was 2.3 × 10-5 M. Acridine was adsorbed by dropwise addition of the adsorbate in aqueous solution to the colloid to an overall acridine concentration of 2.2 × 10-4 M. Poly(vinylpyrrolidone) (PVP, MW 40 000) was added to all of the solutions as a stabilizer, preventing further aggregation and eventual flocculation of the colloid. The final concentration of PVP in solution was approximately 0.007 wt %. Experiments were also carried out in the absence of PVP in order to ascertain that the polymer did not noticeably affect the spectroscopy or the kinetics of interest. The flow cell7,8 used consists of a reservoir made from a graduated cylinder from which the bottom was cut off and to which a stopcock was attached. A standard 1.8-mm Pyrex capillary attached with a plastic tube to the reservoir was used as the Raman cell. The colloid/adsorbate solution was allowed to flow from the reservoir, through the capillary, and into a large basin filled with water to the level of an overflow spout in order to maintain a constant final level. To minimize turbulence, the outlet tube emptied the colloid under the surface of the water in the basin. Colloid samples were used only once to avoid contamination by the photoproduct. The spectrometer was described elsewhere.8 SERS spectra were excited by focusing the Ar ion (Lexel model 3000) or Kr
10.1021/jp001730w CCC: $19.00 © 2000 American Chemical Society Published on Web 07/13/2000
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ion (Lexel model 3500) laser beam onto the center of the glass capillary. Spectra were recorded using a Spex 1887C Triplemate spectrometer equipped with multichannel CCD detection and interfaced to an IBM personal computer. The photodecomposition kinetics were determined as a function of laser power (20-50 mW) at 514.5 and 476.5 nm and as a function of laser wavelength for the six visible (457.9-568.2 nm) laser lines available with 20-mW laser power, as measured at the sample. Two hundred spectra were collected in sequence for each of the experiments using flow, whereas 50 spectra were collected when the experiments were performed without flow. The accumulation times for each spectrum in the two sets of experiments were 5s and 1s, respectively. All of the Raman intensities measured by excitation of the different laser lines were calibrated relative to the Raman lines of CCl4. To check the extent of colloid aggregation, UV-visible spectra were recorded using a Varian Cary 3 UV-visible spectrophotometer. Flow Kinetics. Details of the use of the flow cell for photochemical kinetics measurement were discussed in ref 8. In determining the kinetics of phenazine adsorbed on silver colloid, we made use of the fact that the flow rate through a capillary fed from a continuously emptying reservoir decreases exponentially with time.16 Accordingly, the exposure time of the adsorbate/colloid sample to the laser increases as the flow progresses. The change in the flow rate with time, and hence the laser-exposure time, was calibrated for the system used. To simplify the analysis, we assumed an idealized situation in which the glass capillary has a square cross section illuminated with a laser beam of uniform intensity with a square cross section of side L. We also assumed that the intensity attenuation of the laser beam through the photoreagent solution is negligible and that the reaction is a simple photodecomposition of reagent, A, by a first-order, irreversible reaction initiated by the absorption of n photons. The reagent concentration, A, after exposure to the laser for a time tex is given by
A ) A0 exp(-ktex)
(1)
where A0 is the initial concentration of reagent A. The rate constant k will, in general, depend on the laser intensity, I, as k ) aIn, where n is the apparent number of laser photons required to initiate the photoreaction and a is a wavelengthdependent quantity proportional to the absorption cross section. The concentration of the photoproduct (B) can be determined from the relation B ) A0 - A. The Raman intensity associated with a photoproduct band will have the form
IRaman ) IσBB + b
(2)
where σB is the (band-specific) Raman cross section, I is the laser intensity, and b is a baseline function. This equation was used to express the photoreaction kinetics of acridine in the absence of flow. The flow rate can be written as16
F ) -Agc
dh ) ch dt
(3)
where Agc is the cell cross-sectional area of the cylindrical reagent reservoir, h the instantaneous height of reagent solution in the reservoir, and c a constant. The laser-exposure time of a reagent molecule as a function of the experimental time, t, measured from the instant flow is initiated, is calculated using the expression9,17
tex ) τ0 ln
[
() ]
Acχ t exp +1 Agch0 τ0
(4)
where Ac is the cross-sectional area of the capillary tube used as a Raman cell, h0 is the initial height, τ0 ) Agc/c, and χ is the distance of a point in the illuminating beam measured from the top of laser spot in the direction of flow. Photochemistry takes place from the moment the photoreagent first crosses into the laser spot until it is swept out of the illuminated portion of the capillary. Hence, the measured Raman intensities of product or reagent bands are averages of evolving intensities (and hence concentrations) over a period of time, tex, i.e., by
A h ) (1/L) ) (1/L)
∫0LA dχ
∫0LA0e-kt
ex
dχ
(5) (5′)
This results in the expression
A h)
A0
[(RL + 1)-kτ0+1 - 1]
L(1 - kτ0)R
(6)
and
h B h ) A0 - A where R )
Ac t/τ0 e Agch0
(6′) (7)
The Raman intensity associated with a photoproduct band will have the form
h +b IRaman ) IσBB
(8)
The time evolution of the phenazine Raman bands was fit to this expression. Results and Discussion A representative series of SERS spectra of phenazine recorded during flow are shown in Figures 1 and 2. For most runs, 200 spectra were collected sequentially every 5 s. The spectra reported in Figures 1 and 2 are (bottom to top) the 2nd, 50th, 100th, 150th, and 200th spectra corresponding to the mean times 7.5, 247.5, 447.5, 747.5, and 997.5 s, respectively, after the flow was initiated. A representative series of SERS spectra of acridine recorded sequentially every 1 s without flow, where the colloid sample solution contained in a capillary was irradiated continuously by the laser beam, are shown in Figures 3 and 4. These are (bottom to top) the 1st, 5th, 10th, 25th, and 50th spectra, corresponding to mean exposure times 0.5, 4.5, 9.5, 24.5, and 49.5 s, respectively. The baseline is displaced in equal intervals in the figures, for clarity; in fact, the background signals did not decrease substantially with time. Measurements were also made with 457.9, 476.5, 496.5, 514.5, 530.9, and 568.2 nm excitation using Ar or Kr ion laser lines. Phenazine. The bottom spectrum in Figures 1 and 2, corresponding to the second spectrum of the series of 200 spectra, is rather simple and essentially independent of the excitation wavelength, apart from some minor changes in the relative intensities of some bands. The exposure time while the second spectrum is collected is very short (approximately 5 × 10-5 s). Hence, little photochemistry will have taken place
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Figure 2. Same as Figure 2, but with 20 mW of 457.9-nm Ar+ laser excitation. Figure 1. Five representative spectra out of a series of 200 spectra (bottom to top) corresponding to progressively longer laser-irradiation times for the system phenazine adsorbed on silver colloid. The mean irradiation times to which a molecule was exposed while the spectra presented were recorded are 6.6 × 10-4, 0.061, 6.4, 145, and 391 s, respectively. (The spectra were recorded using multichannel detection and sample flow, and excited with a 568.2-nm Kr+ laser line of 20mW laser power.)
during the time it was recorded. We, therefore, believe that this spectrum corresponds to the SERS spectrum of phenazine. (The bands at 1014, 2847, and 2950 cm-1 are due to methanol, which was used in the preparation of the phenazine mother solutions.) Only a weak CH stretching band at 3050 cm-1 is observed in this spectrum, aside from the two bands at 2847 and 2950 cm-1 that are due to methanol. It is known18 that aromatic CH stretching vibrations only appear distinctly in SERS spectra when the molecule is adsorbed such that the aromatic ring is oriented perpendicular, or nearly so, with respect to the local surface. We, therefore, conclude that phenazine is adsorbed with its ring slightly tilted with respect to the surface. Upon laser irradiation, the intensity of this SERS band decreases but no new bands appear in the CH stretching region. Elsewhere in the SERS spectrum, a number of new bands were observed to grow in with increasing laser-exposure time. These bands are clearly due to photoproducts resulting from the photoreaction of phenazine. The most intense among the new bands correspond to benzene ring vibrational modes. For example, the band at 1557 cm-1 can be assigned to ν8a, while the bands at 1501, 1470, 1391, 1272, 1167, 618, and 548 cm-1 can be assigned to ν19b, ν19a, ν3, ν1, ν4, ν16a, and ν6a, respectively. The band at 1352 cm-1 is most likely the νCN vibration, as it lies in the appropriate range19 (1320-1405 cm-1). The appearance of a νCN band suggests that one of the NC bonds of
phenazine is broken in the photoprocess. By the breakage of one of its NC bonds, phenazine is converted into a species in which two ortho-substituted benzene rings are connected by a nitrogen atom. A low-frequency band near 500 cm-1, which is a marker band (bending mode) of the substituted -CdN-Ag group, such as was observed in the SERS spectra of phthalazine and its isomers when their NdN or NdC bonds were broken, is not observed in the SERS spectrum of the photoproduct of phenazine. This is consistent with the structure alluded to above, which would not have a substituted -CdN-Ag group; rather, the -N-Ag group is bonded directly to the benzene ring. Some portion of the initial photoproducts formed undergo further photoreactions, some right down to carbon, as the broad graphitic carbon band near 1400 cm-1 is observed to grow in as a broad background. Acridine. Acridine seems to be adsorbed with its plane perpendicular to, or greatly tilted with respect to, the local silver surface, as a strong CH stretching band is observed at 3053 cm-1 in the “early” spectra of this molecule. The molecule likely adsorbs with the short molecular axis of the anthracene ring normal (or almost normal) to the surface by forming a surface bond through the nonbonding electron pair of the nitrogen atom. As the exposure time increases, this CH band does not change appreciably, but a new band at 2940 cm-1 appears. Numerous bands are found to grow in with increasing laser-irradiation time in the spectral range corresponding to C-C vibrational modes (Figures 3 and 4). Here, again, we ascribe the new bands to products resulting from the surface photochemistry of acridine. The photoreaction rate of acridine adsorbed on colloidal silver was much slower than that of phenazine. Hence, the photoreaction of adsorbed acridine could be studied without flow. The bottom spectrum in Figures 3 and 4 is rather simple. The bands correspond to the vibrational modes of acridine.20
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J. Phys. Chem. B, Vol. 104, No. 31, 2000 7465
Figure 4. Same as Figure 3, but with 20 mW of 457.9-nm Ar+ laser excitation. Figure 3. Five representative spectra out of a series of 50 spectra (bottom to top) corresponding to progressively longer laser-irradiation times for the system acridine adsorbed on silver colloid. The mean irradiation times to which a molecule was exposed while the spectra were recorded are 0.5, 4.5, 9.5, 24.5, and 49.5 s, respectively. (The spectra were recorded using multichannel detection and sample without flow, and excited with a 514.5-nm Ar+ laser line of 20-mW laser power.)
The most prominent of the new bands resulting from the surface photochemistry of acridine, once again, correspond to benzene ring modes. For example, the band at 1598 cm-1 can be assigned to ν8b, while, the bands at 1545, 1451, 1387, 1265, 1159, 619, and 543 cm-1 can be assigned to ν19b, ν19a, ν3, ν1, ν4, ν16a, and ν6a, respectively. The two new bands at 543 and 619 cm-1 are very similar to those observed for quinazoline adsorbed on colloidal silver.21 A strong νCN band is observed at 1351 cm-1.19 On the basis of these observations, we believe that adsorbed acridine photodecomposes through the cleavage of the CN bond, resulting in a photoproduct in which two benzene rings are connected by a methylene group. The band at 2940 cm-1 can be assigned to the CH vibration of a methylene group. For both phenazine and acridine, the aggregation of colloid resulting from the addition of the adsorbate solution was very slight. A weak shoulder to the red of the 390-nm UV band corresponding to the absorption of isolated colloid particles is the only indication of a small measure of colloidal aggregation. If the underlying absorption at 390 nm is subtracted, the weak shoulder maximizes at approximately 450 nm. The relative SERS enhancement is found to follow approximately this profile. This also appears to be the case for the measured relative enhancements of the SERS spectra of phenazine and acridine, for which the greatest enhancement was observed with 457.9nm laser excitation, decreasing toward the red.
In the early SERS spectra, whose peaks correspond to reagent not yet contaminated by photoproduct, one can find a band whose intensity changes dramatically with excitation wavelength. In the SERS spectra of phenazine, it is the band at 629 cm-1. This band is very strong in the spectrum observed with 457.9-nm excitation; its intensity decreases for longer excitation wavelengths. Contrariwise, in the SERS spectra of acridine, the intensity of the band at 763 cm-1 increases as the excitation wavelength is increased. Changes in intensity of SERS bands with excitation wavelength have been observed in cases in which the distribution of colloidal aggregates is narrow9 and have been interpreted in terms of the so-called surface selection rules for SERS.22 The intensity changes with excitation wavelength of the two bands show opposite behavior, but the enhancements of these two samples show the same behavior, increasing toward the blue. Therefore, one cannot explain the opposite behavior of the two bands simultaneously as being due to surface selection rules. In fact, their differing behavior remains unclear. As a measure of the extent of reaction, we calculated the ratio of a photoproduct peak intensity in the last spectrum of a series of spectra recorded under laser irradiation to the intensity of a reagent band in the second spectrum of such a series. (See Figure 5.) For phenazine, the 548 cm-1 product band and the 1403 cm-1 reagent band were used to construct the ratio. For acridine, the 543 cm-1 product and 1403 cm-1 reagent bands were used. For phenazine, the photochemical efficiency increases toward the red, maximizing at 568.2 nm. With acridine, the maximum photochemical efficiency occurs at 530.9 nm and decreases with decreasing laser wavelength. With phenazine, the broad spectral background observed near 1400 cm-1 due to carbonization increases in intensity as the irradiating wavelength is reduced. This implies that the photoproduct resulting from the surface photochemistry of phenazine decomposes further,
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Figure 5. Measured wavelength dependence of the intensity ratios of the photoproduct peak in the last spectrum to the reagent peak in the second spectrum. Circles represent the ratio of phenazine, and squares acridine.
Figure 7. Time evolution of the SERS intensity of the 1352 cm-1 band of phenazine excited with several Ar and Kr laser wavelength. The laser power was 20 mW in all cases. Intensities were calibrated against Raman spectrum of CCl4, and the baselines were removed.
Figure 6. Time evolution of the SERS intensity of the 1352 cm-1 band of phenazine excited with 496.5-nm Ar+ laser light. Jagged lines are experimental data. The laser powers used were (from bottom to top) 5, 10, 15, and 20 mW.
Figure 8. Time evolution of the SERS intensity of the 543 cm-1 band of acridine excited with 514.5-nm Ar+ laser light. Jagged lines are experimental data. The laser powers used were (from bottom to top) 5, 10, 15, and 20 mW.
eventually down to carbon, and that the overall or ratedetermining rate of this subsequent reaction increases toward the blue. With acridine, residual reagent bands are clearly visible even after 50 s of irradiation. With 457.9-nm laser irradiation, the photoprocess produces almost no photoproduct, even after prolonged irradiation. Moreover, carbonization is not observed with this reagent, suggesting that little photochemistry occurs after the first surface photochemical event. In analyzing the photokinetics of phenazine, we should, in principle, use a model that reflects fact that at least two consecutive photoreactions occur, i.e., at least a model of the form A f B f C, the evidence of a second photochemical step being the appearance of surface graphitic carbon. The observed time evolution of the carbon band was found to be so slow, however, (see Figures 6 and 7) that an analysis of the photochemical kinetics in terms of the simpler, approximate scheme, A f B, suffices. Figure 6 shows the time evolution of the SERS intensity of the photoproduct band at 1352 cm-1 obtained by irradiating adsorbed phenazine with 496.5-nm Ar+ laser light. In Figure 6, the jagged lines are the experimental data, and the smooth lines are recalculated curves after the data were fit to eq 8 using eq 4. The best fit was obtained with a ) 0.34 s-1 mW-1 and n ) 0.97, where these constants refer to the rate constant k1 ) aIn. The four curves in Figure 6 were fit simultaneously using common values of the parameters but independent values for the baselines. The value of a was calculated assuming Agc/Ac ) 77.6, h0 ) 4.3 cm, and L ) 84 µm. The value of τ0 ) 76.6 s was returned by the fit. For acridine, the time evolution of
the SERS intensity of the photoproduct band at 543 cm-1 observed when the colloid is irradiated with 514.5-nm Ar+ laser light is shown in Figure 8. The smooth lines are recalculated curves after the data were fit to eq 2. The best fit was obtained with a ) 0.015 s-1 mW-1 and n ) 0.90. For both reactions of phenazine and acridine, n is near unity, implying that both reactions are initiated by a one-photon process. The time evolutions of the SERS intensity of the phenazine photoproduct band at 1352 cm-1 and the acridine photoproduct band at 543 cm-1 are shown in Figures7 and 9, respectively, for six irradiation wavelengths. The rate constants are plotted in Figure 10 as a function of the irradiation wavelength. For acridine, the measured photoreaction rate increases toward the blue. (We could not measure the photochemical rate with 457.9nm laser irradiation because the intensities of the photoproduct bands were too weak.) For phenazine, the maximum photoreaction rate is observed at ∼500 nm. Hence, the photochemistry of the two molecules adsorbed on silver is quite distinct, even though they show very similar SERS enhancement as a function of excitation wavelength. This implies that, although enhancement is needed in order to bring about the facile surface photochemistry observed, other resonance conditions must be satisfied that are specific to the two adsorbates. So far, it is not clear why two such similar molecules should undergo such distinct surface photochemical transformations. The reaction rate of phenazine is much faster than that of acridine (Figure 8). Previously, we suggested that phenazine is adsorbed flat on the surface and acridine stands up. We also showed, with a series of phthalazine isomers, that their surface
Photochemical Reactions of Phenazine and Acridine
J. Phys. Chem. B, Vol. 104, No. 31, 2000 7467 reaction need not be directly related. This is not, in itself, puzzling; it means that one can have situations in which a molecule can be adsorbed at more than one surface site or with more than one bonding geometry and that only the minor site or geometry undergoes photochemistry, but it does so rapidly. This is what we seem to observe with phenazine and acridine. Acknowledgment. J.S.S. acknowledges with thanks the financial support of the Korean Research Foundation made in the 1998 program year and the KOSEF through CMS. M.M. is grateful to NSERC and CIPI for financial support. References and Notes
Figure 9. Time evolution of the SERS intensity of the 543 cm-1 band of acridine excited with several Ar and Kr laser wavelength. The laser power was 20 mW in all cases. Intensities were calibrated against Raman spectrum of CCl4, and the baselines were removed.
Figure 10. Measured wavelength dependence of the reaction rates of phenazine and acridine adsorbed on silver colloid surfaces. Circles represent the ratio of phenazine and squares acridine.
photochemistry is affected critically by their mode of adsorption on the surface. Quinazoline, for example, photoreacted rapidly only when it adsorbed flat. No photoreaction was observed for the molecule adsorbed edgewise on the surface. However, this was not a general rule. For example, with cinnoline, it was the perpendicular mode of adsorption that resulted in the slowest photochemical rate. Nevertheless, it is clear that the adsorption geometry can affect the surface photochemical rate critically. Yet another intriguing result came out of the current study: the fact that the photochemical rate and the extent of photo-
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