7228
J. Phys. Chem. 1993,97, 1228-1233
Primary Photoprocesses of Cr(V1) in Real-Time Holographic Recording Material: Dichromated Poly(viny1 alcohol) Curusamy Manivannan,. Rupak Changkakoti, and Roger A. Lessard' Centre d'optique, photonique et laser (COPL), Uniuersitt Laual, Pauillon Vachon, Citb Universitaire, Qubbec, Canada G l K 7P4 Gilles Mailhot and Michele Bolte Laboratoire de Photochimie, Ecole Nationale Supbrieure de Chimie, Uniuersitb Blaise Pascal (Clermont-FD II), 631 77 Aubiere Cedex. France Received: February 16, 1993; In Final Form: March 24, 1993
The primary photoprocesses of Cr(V1) in poly(viny1 alcohol) have been studied under various experimental conditions. Aqueous solution and polymer films of dichromated poly(viny1 alcohol) (DCPVA) have been irradiated, and the quantum yield values were determined for the photoreduction of Cr(V1) to Cr(II1) at different pH values. Thermal and photochemical evolution of Cr(V), Cr(III), and polymer radical in DCPVA films was monitored at room temperature using real-time ESR spectrometry. Spin-trapping experiments with N-tert-butyl-a-phenylnitrone(PBN) gave evidence for the formation of radical adduct from the polymer matrix. Effect of p H of the coating solution and concentration of dichromate on the photochemical evolution of intermediates were observed. Possible involvement of all intermediate species from Cr(V1) has been discussed. On the basis of our experimental findings, a suitable mechanism has been suggested, involving the electron transfer from the polymer matrix to Cr(V1) leading to the formation of Cr(V) and polymer radical, which further undergoes reaction to produce Cr(II1) and a cross-linked polymer matrix.
Introduction Oxidation by chromates is a versatile chemical reaction and has been employed for both preparative and analytical purpose^^-^ for more than a century. Dichromated polymeric materials such as dichromated poly(viny1 alcohol) (DCPVA) find many important industrial applications: stencils: printing plate^,^ lithographic plate making,6 and phosphor and black-matrix dot or line patterns in color television picture They also serve as one of the new optical recording materials for holography for the fabrication of holographic optical elements (HOEs).9-11 Our interest in exploiting the photosensitive nature of DCPVA films that can be used to record real-time developingtransmission holograms*la4 using argon ion laser at 488 nm led us to the necessity of investigating the primary photoprocesses of Cr(V1) in the polymer films at normal room temperature. To better understand the mechanism of hologram formation, real-time electron spin resonance (ESR) technique has been employed for studying the PVA-dichromate photoreaction in polymer films. Performing the reactions in polymer films offers some distinct advantages: (i) monitoring the photochemical processes under conditions analogous to those used in the formation of hologram, (ii) identifyingthe intermediates (since in the rigid polymer matrix the life time of the intermediate species generated upon excitation may be long enough to enable their detection by ESR spectroscopy), and (iii) performing the reactions at normal room temperature (thus avoiding work at very low temperature). It is worthwhile to briefly review the earlier work on the oxidation of primary and secondary alcohols by chromium, consideringthe fact that PVA is a polymer (of our interest) derived from a secondary alcohol, namely, vinyl alcohol. The photochemical oxidation of alcohols by potassium dichromate, first observed by Plotnikowlz and Morton13 in aqueous solution, has been investigated in detail by many authors.1628 Thermal and photochemical of isopropyl alcohol by chromate has been extensively studied, and chromatealcohol ester was suggested as the initial active species in the oxidation of a monomeric secondary alcohol. 0022-365419312091-1228$04.00/0
Viewing the earlier work performed on dichromated polymers, Smethurst proposed30 that the photochemical reaction between dichromate and a colloid involved no change in the oxidation state of chromium, whereas Stiehler explained3' that the photochemical reaction involved a change in pH of the solution together with the formation of Cr(II1) species. Later, photoreaction of dichromate in PVA films was studied by Duncalf and D ~ n nand , ~they ~ suggested that the insolubilizationof PVA was caused by the complex formation between PVA and Cr(III), even though no clear evidence was shown for the involvement of Cr(II1). Several authors described the photochemical reaction, based on the absorption of radiation by chromium(V1) ~pecies.3P~~ Recently, detailed studies on the photochemistry of Cr(V1) and Cr(II1) in the polymerization of acrylamide and acrylic acid and in the oxidation of amino acids have been carried 0~t3~a-i by Bolte and co-workers. They suggested a mechanism favoring the formation of Cr(V) as the primary photointermediate. The photochemical reaction between PVA and chromate in aqueous solution has also been studied by UV and EPR spectroscopy, and the proposed mechanism suggeststhat the photoreactionproceeds through the formation of PVA-chromate ester.37
Experimental Section Materials. Poly(viny1 alcohol) (Aldrich, fiwof11-31000,98% hydrolyzed), (NH4)2Cr207 (Fluka), and N-tert-butyl-a-phenylnitrone (PBN) (Aldrich) were of the best available grade and were used without any further purification. All the solutions were prepared in doubly distilled water. The desired pH was achieved by the suitable addition of HC104 or (NH4)OH and was controlled to h0.02 pH unit. Apparatus and Procedures. Fabrication of Dichromated Polymer Films. Dichromated poly(viny1alcohol) (DCPVA) films have been prepared by the gravity settling method. After the required amount of poly(viny1 alcohol) (PVA) aqueous solution was prepared by dissolving 7 g of PVA in 100 mL of preheated water (60 "C), the weighed amount of (NH4)2Cr207 solid was added and stirred to get a uniform mixture. Then a constant 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7229
Primary Photoprocesses of Cr(V1) volume (4.0 mL) of the above mixture was poured onto a leveled, cleaned glass substrate (size 5 X 7.5 cm2). It was allowed to dry in a dark chamber under normal laboratory conditions (20 OC and relative humidity = 3545%) for 24 h, and orange/yellow, clear transparent films with a thickness of 57-60 pm (measured with Sloan's Dektak IIA apparatus) were obtained. The films were then peeled from the glass plates for the irradiation and ESR spectroscopic measurements. Irradiation Setup. Monochromatic irradiation of dichromated PVA solutions at 365 nm was performed using a high-pressure Hg lamp (Osram HBO 200W) with a Bausch and Lomb grating monochromator. The irradiation beam was parallel, and the reactors were cylindrical quartz cells of 1- or 2-cm path length. The light intensity was measured by ferrioxalate actinometry (Io N 1.3 X 1015 photons s-l cm-2). Similarly, a second experimental setup, consisting of a cylinder made of stainless steel built upon an elliptical base, was used for the irradiation of polymer films. DCPVA films mounted on quartz plates have been irradiated in a second setup delivering more photons (I, 4.0 X 10" photons s-1 cm-2): a high-pressure Hg lamp (Phillips HPW type 125W) whose emission at 365 nm was selected by an inner filter, was located at the focal axis of an elliptical stainless steel cylinder, and the water-jacketed Pyrex reactor tube (diameter = 2.8 cm) was centered at the other focal axis. ESR and UV-Vis Spectroscopic Measurements. The realtime ESR profiles have been obtained by irradiating the dichromated polymer films on a Brucker FR 200D spectrometer at 9.30 GHz with a modulation field of 100 Hz. A Xe-Hg Hanovia lamp was employed for irradiation of samples in the ESR cavity. Diphenylpicrylhydrazyl (DPPH, g = 2.0036) was used as an internal standard. The evolution of intermediates has been monitored during the course of reaction by the change in the peak-to-peakderivative height of ESR signals. Spin-trapping measurements have been performed on dichromated polymer films incorporated with the spin trap, PBN. The UV-vis spectra were recorded on a double-beam Cary 118C spectrophotometer.
Results The photochemical interaction of Cr(V1) with poly(viny1 alcohol) has been studied by (i) observing the changes in the UV-vis spectra of aqueous solution with respect to pH (to understand the nature of the active species), (ii) irradiating the aqueous solution and polymer films having different pHs to determine the quantum yield and to identify the photoproduct, (iii) monitoring the real-time evolution of chromium intermediates in the polymer matrix (fabricated from the starting coating solution with varying pHs) upon polychromatic irradiation, (iv) using spin-trapping measurements to observe the formation of radical adduct, and (v) observing the influence of dichromate concentration on the formation of intermediates. Irradiation Measurementsof DichromatedPoly(viny1alcohol) AqueousSolution and Films. In aqueous solution chromium(V1) exists in different forms, and three species were present in our experimental conditions: HCr04- and Cr2072- in the acidic medium and CrOd2- in the basic medium? H C r O L * Cr0;2HCrO; Cr,O,'-
Cr20,2-
+ H+ + H,O
+ H+ + HCr20,-
pK = 6.49
(1)
kd = 48 mol-' L (2) pK = 0.07
(3)
The UV-vis spectrum varies significantly depending on the pH of the medium. The UV-vis spectrum of HCr04- showed two bands at 260 and 350 nm, with a plateau at 440 nm, which was characteristic of the acidic form, and the Cr042- spectrum showed two bands at 273 and 373 nm (Figure 1). The UV-vis spectrum
4.0
-5.0
9 0
v
W
22.0 0
f]: 0 Lo
2 1.0 460
O.O 200
Wavelength (nm)
5a0
Figure 1. UV-vis spectra of chromium(V1) (1.0 X different pH values. (PVA = 0.07 wt %).
0
2
' I
600
mol L-I) at
3
PH
Figure 2. pH dependence of reduction in quantum yield for Cr(V1) mol L-I; PVA = 0.07 wt %. Cr(II1). [Cr(VI)] = 1.0 X
4
-
of a solution of 1.OX mol L-1 chromium(V1) was not modified by the addition of PVA solution (0.07 wt % solution) under our experimental conditions for the irradiation measurements. PhotoredoxPhenomenonin Chromium(M)-Poly (vinylalcohol) System. The photoconversionofCr(V1) to Cr(II1) wasmonitored by UV-vis spectroscopy from the decrease in absorbance at 350 nm, the Cr(II1) absorbance being negligible at this wavelength. Upon irradiation at 365 nm (employing the first irradiation experimental setup), the absorbance at 350 and 440 nm of the solution ([Cr(VI)] = 1.0 X lo-' mol L-I; PVA = 0.07 wt %) was found to decrease, and the initial quantum yields of Cr(V1) reduction were calculated from the slope of the decrease in absorbance at 350 nm versus irradiation time. They were measured at various pHs. No buffer has been used in order to avoid the eventual complexation with Cr(V1) or Cr(II1). The quantum yields were determined with a 5% conversion, so the change of pH was negligible during the experiment. Duplicate runs yielded values of quantum yield within AS%. The quantum yield of Cr(V1) reduction decreased when moving from pH 1.4 to 2.0, and then it reached a plateau. The quantum yieldisverylowatpH> 4.0(Figure2). TheCr(II1) accumulation was not identifiedin the irradiation of aqueous solution. Polymer films with higher concentrations of Cr(V1) and PVA([(NH4)2Cr20,] = 0.048 mol L-l, PVA = 7.0 wt 5' %)of natural pH have been irradiated using the second experimental setup to detect the formation of Cr(II1). Irradiation for a longer duration was necessary t o build up sufficient concentration of Cr(II1) for identification. Figure 3 showsthe evolutionof Cr(II1) in DCPVA films, showing a maximum at 580 nm. ESR Spectroscopy. ESR spectroscopy offers a convenient technique for the study of photoreactions of dichromated polymer films. Considering the reaction products and intermediates, it is possible to identify and monitor their formation and evolution by ESR spectroscopy upon irradiation. As a matter of fact,
1230 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 0.05
1
I!
:I
Manivannan et ai.
I
""
I
No Irradiation 12 Hr. Irradiation 24 Hr. Irradiation 44 Hr. Irradiation 52 Hr. Irradiation
0.01
PO
60
30
120
150
Irradiation time, in min. 550
600
700
Wavelength (nm) Figure3. UV-vis spectra of chromium(II1) formed in dichromated poly(vinyl alcohol) fiim (of natural pH 4.90); Xi, = 365 nm. [(NH&Cr207] = 0.048 mol L-I; PVA = 7.0 wt 96.
H = Constant
YN
bh
5
YN
10
15
YN
YN
20
25
Figure 5.' (a, top) Real-time ESR profile for the photochemical evolution
of Cr(V) in dichromated poly(viny1 alcohol) film (of natural pH) under polychromatic irradiation. [(N&)2Cr207] = 0.048 mol L-l; PVA = 7.0 wt 96. (b, bottom) Photochemical and thermal decay of Cr(V) signal. h = the height of the Cr(V) signal in au.
Figure 4. ESR spectra of dichromated poly(viny1 alcohol) film at room temperature (T= 20 "C) showing the photochemical evolution of the Cr(V) signal upon polychromatic irradiation. [(NH&Cr207] = 0.048 mol L-1; PVA = 7.0 wt 96;g = 1.9789. Irradiation time (min): (-) 0, (...)2, (---)4, (---) 10, (---) 25. chromium(V1) do is diamagnetic, chromium(V) d1 is paramagnetic with a strong and narrow signal, chromium(1V) d* is paramagnetic only at very low temperature, and chromium(II1) d3 presents a weak and broad signal. In addition, the presence of an organic radicalcan be detected by spin-trapping experiments. ESR experiments have been performed with DCPVA films fabricated with varying pHs of the starting, coating solution. Intermediates Observed in Wchromated Poly(vinyl alcohol) (DCPVA) Films under Polychromatic Irradiation. DCPVA films of natural pH (pH = 4.90) were prepared at normal room temperature (20"C) to observe the intermediates formed in the primary photoprocess. No buffer was added. A narrow signal, AH = 2 G,g = 1.9789, of thermal origin was observed for these films. Upon continuous polychromatic irradiation (X > 320 nm), the intensityof the signal increased without any inductionperiod, showing the building upof the intermediatespecies. Theintensity went through a maximum and then decreased. This strong and narrow signal has been assigned to chromium(V), since among the various chromium valences only Cr(V) (dl) can present such a signaL3M Figure 4 shows the photochemical evolution of the Cr(V) signal. The evolution of the intensity maximum of the signal is depicted in Figure Sa. An on-off experiment was performed in order to compare the thermal and photochemical stability of chromium(V) (Figure 5b). Regarding the identification and evolution of the photoproduct, Cr(III), we obtained the evidence of its formation in polymer
Figure 6. ESR spectra of dichromated poly(viny1 alcohol) film at room temperature (T= 20 "C) showing the photochemical evolution of the Cr(II1) signal upon polychromatic irradiation. Irradiation time (min): A, 0; B, 3; C, 7; D, 12; E, 18; F, 27; G, 40.
films upon polychromatic irradiation through UV-vis spectroscopy (Figure 3). Further, we could also follow the formationof Cr(II1) through ESR spectroscopy. On continuous irradiation, a very broad signal was observed in addition to the signal observed for Cr(V), which hasbeenassigned t o ~ h r o m i u m ( I I I ) .Thekinetics ~~ of photochemical evolution of Cr(II1) has been followed, and Figure 6 shows the nature of the observed signal and its irradiation time dependence. Spin-TrappingExperiments. To answer the question of whether or not the polymer matrix is involved in the primary photoprocess and in turn to confirm the involvement of an electron-transfer process, we performed spin-trapping measurements on DCPVA films. No report has been made in the earlier studies regarding the formation of intermediates from the polymer. N-tert-Butyl-a-phenylnitrone(PBN) has been chosen as a spin trap, since it is water soluble (easy to incorporate into DCPVA films), does not absorb at wavelengths higher than 340 nm, and is known to form readily observable adducts with alkyl, aryl,
Primary Photoprocesses of Cr(V1)
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7231 l5
0 8 0
1OG
Figure 7. ESR spectra of the radical adduct of N-tert-butyl-a-
phenylnitrone (PBN)in polymer film at room temperature (Xi, > 345 nm). Irradiation time (min): A, 0; B, 30 s; C, 1; D, 2; E,4; F, 10; G,
10
5
H
15
20
Irradiation time, in min. -1
15.
alkoxyl, and acyloxyl radi~als.3~DCPVA-PBN films were prepared ([(NH&Cr207] = 0.048 mol L-l; PVA = 7.0 wt 5%; [PBN] = 0.01 mol L-l) and exposed with filtered light (Xim > 345 nm, to avoid any absorption of the light by PBN). Upon irradiation, two types of signals appeared: the narrow signal at g = 1.9789 already observed without PBN (for Cr(V)) and a triplet signal ( g = 2.0059, aN = 14.7 G ) . The intensity of the triplet signal shows a slow continuous increase even for longer irradiations, with a faster increase during the initial stage and a slow rate of increase at the later stage (Figure 7). As observed for the evolution of Cr(V) signal, there was no induction period. Irradiation of plain PVA films (of same concentration) under similar conditions showed no evidence of formation of any paramagnetic species. Effect of pH on the Intermediates of Chromiumand Poly(vinyl alcohol). On employing this material (DCPVA films) for holographic recording, it has been observed that the pH of the starting solution (from which the polymer films have been casted) plays a significant role in the holographic efficiency (measured as diffractionefficiency).lla+ On moving from theacidic to basic pH, the real-timediffractionefficiency has been found to increase from 40% to 60%. Similar ESR signals for Cr(V), Cr(III), and polymer radical have been observed, when irradiating different DCPVA films made from solutions of various pHs. The kinetics of Cr(V) and polymer radical signal as a function of irradiation time are reported in Figure 8a,b. Effect of Dichromate on the Cr(V) Evolutionin DCPVA Films of Natural pH. The amount of chromium doped in the polymer films should influence the extent of formation of intermediates upon irradiation. To get an idea of the optimum concentration of Cr(V1) for achieving themaximumCr(V) concentration,which in turn will give important information regarding the exact amount of needed chromium (so that loading of excess dichromate in polymer films could be avoided while holographic recording), polymer films of natural pH have been cast with varying concentration of (NH4)2Cr207in the range 7.93 X to 5.55 X 10-2mol L-1. They have been irradiated at room temperature, and the intensity of the Cr(V) ESR signal has been compared for an irradiation time of 15 min (Figure 9). The intensity of the Cr(V) signal increased and attained a maximum at 3.17 X 10-2 mol L-1 and then decreased when the chromium(V1) concentration in the polymer film continuously increased.
Discussion On the basis of the results observed from the irradiation experiments on solution and polymer films, ESR spectroscopic measurements, and the spin-trapping experiments, the primary photoprocesses of Cr(V1) in poly(viny1 alcohol) films can be explained.
I-
or0
DH-6.6
I
‘
4
12
8
20
16
Irradiation time, in min. Figure 8. (a, top) Irradiation time dependence of the Cr(V) ESR signal intensity for dichromated poly(viny1 alcohol) films of varying pH upon polychromatic irradiation. (b, bottom) Irradiation time dependence of polymer radical’s ESR signal intensity for dichromat4 poly(viny1alcohol) films of varying pH upon polychromatic irradiation.
I
4
0 0
1
2
3
4
5
6
[(NH&,Cr,q] x l o 2 , moll-’
Figure 9. Cr(V) signal dependence on the (NH&Cr20, concentration in dichromated poly(viny1 alcohol) films of natural pH (PVA = 7.0 wt 9%) upon polychromatic irradiation a t room temperature (T = 20 OC). Irradiation time = 15 min.
Active Specia of Chromium. Considering the active species of chromium, it exists either as HCr04- and/or Cr2O7” in the acidic medium or as Cr042- in the basic medium38 (eq 1-3). The equilibrium involving monomer HCr04- and dimer C r ~ 0 7 does ~not depend on the pH of the medium but on the total concentration of Cr(V1). In the pH range 2.0-4.5, it is possible to evaluate the concentration of each species from the equation3*
kd = [Cr2072-]/[HCrO;]2 = 48 mol-’ L and the calculated percentage of C r ~ 0 7 is ~ -presented in Table I. In experiments involving the polymer films, with a total Cr-
Manivannan et al.
7232 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993
TABLE I: Concentration of Cr(VI) as CrZO+
account for the formation of polymer radical.
Cr(V1) as Cr20+, 8
[Cr(VI)], mol L-1
[ ~ r " ]*
46.2 8.4 4.6 0.95 0.10
10-2 10-3
5XlO-l 1O-l 10-5
(VI) concentration of -0.10 mol L-l, it appears from the values listed in Table I that the monomer and dimer exist in a 1:I.S ratio. On the basis of this evaluation, the Cr(V1) will be marked as such in the reaction mechanism without specifically indicating the species. On the contrary, under the experimental conditions used for the determination of the quantum yields, HCr04- (the monomeric form) was the major absorbing species. No detectable spectral modifications due to the presence of chromate ester were observed both in aqueous solution and in polymer film. Involvement of Polymer Matrix. The involvement of polymer matrix in the photoprocess with Cr(V1) to generate Cr(II1) is evidenced through the spin-trapping measurements with PBN, according to the reaction 'h
-0' (4)
The ESR spectrum for radical adduct only reveals a triplet and not three doublets, as usually observed in solution. It can be due to the fact that, in solid matrix, the band width and a H were of the same order of magnitude. It did not allow the occurrence of a coupling with H to be seen, and as a result the envelope of the H coupling signal has been observed, as in the case of acry10nitrile.~~ Hence, the observation of polymer radical adduct with PBN strongly supports the involvement of an electrontransfer mechanism with the polymer matrix. Formation of Cr(V) in the Primary Photoprocess. Reviewing the results obtained through the ESR spectroscopic measurements, the presence of Cr(V) of thermal origin has been observed in all polymer films. Photochemical evolution of Cr(V) was monitored for different DCPVA films fabricated under various experimental conditions, which points out that Cr(V) is the primary species responsible for the photoreaction in the investigated pH range 2.5-10.2. Onexcitation by light, the Cr(V1) in the polymer matrix undergoes an electron-transfer reaction with the polymer matrix to produce Cr(V) and a polymer radical. The Cr(V) signal shows a rapid increase to reach a maximum and then decreases slowly. Henning et al. already described40 the intervention of Cr(V) in the photooxidation of alcohol by HCr04-. Rahman and R&k ruled outz3 the possibility of Cr(V) formation as a secondary reaction, Cr(V1) + Cr(1V) 2Cr(V), from the oxidation of isopropyl alcohol by chromic acid with the estimated equilibrium Moreover, a similar observation by Lee constant of 4 X Van Nice and Farlee also supported the formation of Cr(V) as the primary ph~tointermediate.~~ Here, it is important to consider the formation of Cr0Z2+during the oxidation of (CH3)zCHOH by HCr04-, studied through a stopped-flow technique in aqueous HC104 solution.41 The proposed mechanism (Scheme I11in ref 43) involvesthe formation of Cr02+ as the primary photointermediate, which reacts with the alcohol to generate Cr(II), and Cr(V) is generated in the subsequent reactions without involving any radical formation. But, in our experiments involving polymer films, formation of a polymer radical clearly confirms the involvement of a one-electrontransfer process and not a two-electron oxidation. Moreover, there was no induction period observed for the photochemical generation of Cr(V) and polymer radical. Another possible set of reactions involving the formation of Cr(V) as tertiary photointermediate can also be put forward to
-
+ polymer
-
2e
[Cr"]
+ polymer
[Cr"]
+ polymer radical
Cr"'
Cr"
+ polymer
+ polymer radical
(6)
+ polymer
(7)
-
Cr"
If this sequence of reactions is taking place, then an increase in the intensity of the Cr(V) signal implies the decrease in polymer radical intensity. On the contrary, the thermal and photochemical evolution of Cr(V) and that of polymer radical are similar. The absence of an induction period either for the evolution of Cr(V) or for the polymer radical and the similar trend of evolution of Cr(V) and polymer radical strongly suggest that Cr(V) and polymer radical are forming in the primary photoprocess as
[Cr"]
+ polymer
hv
CrV+ polymer radical
(8) with the subsequent photochemical reactions to form Cr(II1) and cross-linked polymer matrix. Even though the observation of Cr(V) signals in the absence of light gives evidence for some thermal reaction (during the preparation and storage of films), light on-off cycle experiments (Figure 5b) indicate that the thermal reaction pathway is a minor one compared to the photochemical pathway. Concerning the thermal pathway, participation of reactions similar to eq 5-7 can be admitted, since involvement of these reactions can explain the formation of Cr(V) through the thermal reaction. Photoproduct Cr(III). Formation of the photoproduct, Cr(111), has been evidenced from the irradiation of polymer films also (Figure 3). Due to the width and the very weak intensity of the chromium(II1) signal it was impossible to assess whether chromium(II1) was complexed or not with the polymer matrix. Experiments also were carried out to identify the possible generation of Cr(II1) due to thermal reaction. It has been found that evolution of Cr(II1) of thermal origin is quite insignificant both in polymer films and in aqueous solution monitored by ESR and UV-vis spectroscopy, respectively. AnotherPossiblehtermediate:Cr(IV). The low concentration of Cr(1V) available by conventionalmethods, itslackofsignificant absorbance in the visible region, and its relatively short life time preclude direct characterization of Cr(IV).41 This explains why this species has not been identified despite numerous studies of chromium redox reactions (reviewed earlier). However, due to the presence of two unpaired electrons, all known Cr(1V) compounds with oxygen ligands produce ESR signals a t very low temperatures.4"~~ Similar to Cr(V), Cr(1V) also has been shown2~23J5to be an important species in the chromium(V1) oxidation of secondary alcohols. By analogy, we could postulate that the oxidation of PVA would also be affected by Cr(1V) after the Cr(V) disproportionationz7 (2Cr(V) = Cr(1V) Cr(VI), equilibrium constant 4 X 1014) in the PVA-chromate photoreaction. This undoubtedly would occur rapidly, and therefore the concentration of Cr(IV) would be below the sensitivity limit of ESR. This step of the reaction would also yield oxidized PVA and Cr(II1). Even though a two-electron-transfer reactionleading to the formation of Cr(1V) as the primary reaction product is possible in the thermal pathway, the photochemical pathway mainly involves the generation of Cr(V) and polymer radical as primary photointermediates. Role of pH of the Medium. The pH of the medium plays an important role in controlling the photoredox reaction. In aqueous dilute solution, consumption of H+ions during the reduction of Cr(V1) to Cr(II1) has been observed,36dmaking the solution more and more basic. When Cr042-(the deprotonated form) present in the aqueous basic medium becomes the major absorbingspecies, the equilibrium reaction (eq 1) stops. Unlike what is observed36d
+
The Journal of Physical Chemistry, Vol.97, No. 28, 1993 7233
Primary Photoprocesses of Cr(V1)
SCHEME I
PVA matrix Cr" in PVA matrix (at r w m temperature)
a
CrV+ (PVA)'
in aqueous solution, C r 0 2 - appears to be an active species in terms of redox process with the polymer matrix, as evidenced by the presence of ESR signals of Cr(V) and the polymer radical. Our earlier observationslls+ also indicated that the holographic recording efficiency is high under these conditions. A direct comparison of quantum yield measuring the reduction of Cr(V1) into Cr(II1) in aqueous solution and the photochemical phenomenon in polymer films yielding Cr(V) cannot be made. Reaction Mechanism. On the basis of experimental observations and discussions, it can be inferred that light-activated Cr(VI) in the polymer matrix generates Cr(V) and polymer radical involving an electron-transfer process. They undergo further reaction to generate the final products, Cr(II1) and cross-linked PVA matrix. The mechanism of the photoredox phenomenon between Cr(V1) and PVA in films is presented in Scheme I, based on the observed results. Polymer is oxidized by a oneelectron-transfer reaction leading to the formation of polymer radical, which has been clearly confirmed by the spin-trapping studies. Formation of the photoproduct, Cr(III), has also been confirmed by the UV-vis and ESR spectroscopy. The thermal oxidation reaction pathway was found to be a minor one compared to the photochemical pathway. The pH of the starting solution plays an important role in enhancing the generation of intermediates of chromium and polymer. The proposed mechanism will also help to explain the nature of photochemicalreactionsduring the holographic recording, since the photochemical phenomenon is identicalz*'upon excitation at 365,436, or 5 14 nm. In continuation of this work and to increase the spectral sensitivity and holographic efficiency, DCPVA films incorporated with electron donorsllbc and dyes (Eosin Y, Fluorescein, and Rose Bengal) have been studied.llb.d Further studies are in progress involving these systems (DCPVA-dyes) to identify the intermediates and the influence of dyes upon polychromatic/filtered irradiation, which will constitute a forthcoming paper.43
Acknowledgment. We are indebted to the Natural Science and Engineering Research Council of Canada (under Grant NSERC-A0360), Gouvernement du Qutbec (under Grants FCAR 90-AS-2765 and 90-PR-0344), Associationof Universities and Colleges of Canada (under the scheme 'Horizon Le Monde - Europe 1992'), and EDF Photochimie, France, for financial assistance. We thank Prof. J. Lemaire and Prof. G. Mousset of Universit6 Blaise Pascal for providing the necessary instrumental facilities. References and Notes (1) Westheimer, F. H. Chem. Rev. 1949, 45, 419. (2) (a) Eder. J. M. J . Prakt. Chem. 1879, 14, 294. (b) Eder, J. M. J. Prakt. Chem. 1885,6,495. (3) Kosar, J. Light Sensitive Systems; Wiley: New York, 1965; p 46. (4) Ritt, M. P.; Saulnier-Ebert, L. M. U. S. Patent 4561931A.
( 5 ) Bravar, M.; Rek, V.; Kostelac-Biffl, R. J . Polym. Sci., Polym. Symp. 1973, 40, 19. (6) Schlaepfer, K. Adv. Prinf.Sci. Technol. 1971, 6, 1. (7) Morrell, A. M.; Law, H. B.; Ramberg, E. G.; Herold, E. W. Color Television Picture Tubes; Academic Press: New York, 1974. (8) Grimm, L.; Hilke, K.-J.; Scharrer, E. J . Electrochem. Soc. 1983, 130, 1767. (9) Ziping, F.; Juqin, Z.; Dahsiung, H. GuangxueXuebao 1984,41101. (10) Lelibre, S.;Couture, J. J. A. Appl. Opt. 1990, 29, 4384. (1 1) (a) Lessard, R. A.; Changkakoti, R.; Manivannan, G. Photopolymer Device Physics, Chemistry and Applications II 1991, SPIE Vol. 1559, 438.
(b) Lessard, R. A,; Changkakoti, R.; Manivannan, G. Optical Memory and Neural Networks 1992, I , 75. (c) Manivannan, G.; Changkakoti, R.; Lesrard, R.A. Opt. Eng. 1993,32,671. (d) Changkakoti,R.;Manivannan,G.;Lessard, R. A.CanadianAssociationofPhysicists1992Congress,UniversityofWmdsor, Windsor, Canada, June 14-17; Physics in Canada 1982, 48, 75. (12) (a) Plotnikow, J. Z . Wiss. Phot. 1919, 19,40. (b) Plotnikow, J. Z. Wiss. Phot. 1926, 32, 13. (13) Morton, D. S.J . Phys. Chem. 1929, 33, 1135. (14) (a) Bowen, E. J.; Bunn,C. W. J . Chem.SOC.1927,2353. (h) Bowen, E. J.; Chatwin, J. E. J . Chem. Soc. 1932, 2081. (15) Leo, A.; Westheimer, F. H. J. Am. Chem. SOC.1952, 74, 4383. (16) Cohen, M.; Westheimer, F. H. J. Am. Chem. SOC.1952.744387. (17) Westheimer, F. H.; Novick, A. J . Chem. Phys. 1943, ZI, 506. (18) Watanabe, W.; Westheimer, F. H. J . Chem. Phys. 1949, 17, 61. (19) (a) Wiberg, K. B.; Schafer, H. J. Am. Chem.Soc. 1967,89,455. (b) Wiberg, K. B.; Schafer, H. J. Am. Chem. SOC.1969, 91, 927. (c) Wiberg, K. B.; Schafer, H. J . Am. Chem. SOC.1969, 91,933. (20) (a) Wiberg, K. B.; Mukherjee, S . K. J. Am. Chem. Soc. 1971, 93, 2543. (b) Wiberg, K. B.; Mukherjee, S . K. J. Am. Chem.Soc. 1974,96,1884. (21) Wiberg, K. B.; Szeimies, G. J . Am. Chem. SOC.1974,96, 1889. (22) (a) Rofek, J.; Radkowsky, A. E. J. Am. Chem. Soc. 1968,90,2986. (b) Rofek, J.; Radkowsky, A. E. J. Am. Chem. SOC.1973,95,7123. (23) Rahman, M.; Rofek, J. J. Am. Chem. Soc. 1971, 93, 5462. (24) Hasan, F.; RoEek, J. J. Am. Chem. Soc. 1972, 94, 8946. (25) Rahman, M.; R&k, J. J . Am. Chem. SOC.1971, 93, 5455. (26) Hasan, F.; R&k, J. J. Am. Chem. SOC.1972, 94, 9073. (27) Srinivasan, V.; R&k, J. J. Am. Chem. SOC.1974, 96, 127. (28) (a) Hasan, F.; Rofek, J. J. Am. Chem. SOC.1972, 94, 3181. (b) Hasan, F.; Rofek, J. J. Am. Chem.SOC.1973,95,5421. (c) Hasan, F.; RoEek, J. J. Am. Chem. SOC.1974,96, 534. (29) (a) Klaning, U. Acta Chem. Scand. 1958,12,807. (b) Klaning, U. Acta Chem. Scand. 1959,13,2152. (c) Klaning, U. Bull. Soc. Chim. Belg. 1962, 71, 819. (30) Smethurst, P. C. Sci. Ind. Phorogr. 1947, 18, 23. (31) Stiehler, H. Intern. Bull. 1956, 12. (32) Duncalf, B.; Dunn, A. S.J . Appl. Polym. Sci. 1964.8, 1763. (33) O'Brien, Jr., B. J . Am. Opt. Soc. 1952,42, 101. (34) Koch, R.; Byers, D. J.; Rossell, R. E. Proc. 4th Ann. Tech. Meeting TAGA 1952, 105. (35) Branin, P. B.; Fonger, W. H. J. Electrochem. SOC.1975, 122, 94. (36) (a) Robert, B.; Bolte. M.; Lemaire. J. J . Chim. Phys. 1985,82,361. (b) Bolte, M.; Robert, B.; Lemaire, J. Can. J . Chem. 1986, 64, 1864. (c) Galcera, T.; Jouan, X.;Bolte, M. J . Photochem. Photobiol.,A 1988,45,249. (d) Fageol, P.; Bolte, M.; Lemaire, J. J. Phys. Chem. 1988,92,239. (e) Bolte, M.; Lemaire, J. J. Photochem. Photobiol., A 1989, 46, 285. (f) Fageol, P.; Bolte, M. Makromol. Chem. 1989, 190, 367. (g) Mailhot, G.; Bolte, M. J. Photochem.Photobiol..A 1991.56.387. (h) Mailhot.G.:Bolte.M. Polvhedron 1991,10,237. (i) Mailhot, G.1 Philippa;t,'J. L.; Boite,'M. Polym. C&". 1991, 32, 229. (37) Lee Van Nice, H.; Farlee, R. Polym. Eng. Sci. 1977, 17, 359. (38) (a) Pourbaix, M. Atlas d'6quilibres Clectrochimiques L 25OC; Gauthier-Villars: Paris, 1963;p 258. (b) HandbookofChemistry andPhysics, 64th ed.;CRC: Cleveland, 1983-1984; p D169. (c) Linge, H. G.; Jones, A. L. Aust. J . Chem. 1968, 21, 2189. (d) Tong, J. Y. P.; King, E. L. J. Am. Chem. SOC.1953, 75,6180. (39) Janzen, E. G. Acc. Chem. Res. 1971, 4, 31. (40) Henning, H.; Scheibler, P.; Wagner, R.; Thomas, P.; Rehorek. D. J. Prakt. Chem. 1982, 324,279. (41) (a) Scott, S.L.; Bakac, A.; Espenson, J. H. J . Am. Chem.Soc. 1991, I 13, 7787. (b) Ibid 1992, 114, 4205. (42) (a) Ward, G. A.; Kruse, W.; Bower, B. K.; Chien, J. C. W. J . Organometal.Chem.1972,42,C43. (b) Mowat, W.;Shortland, A.;Yagupsky, G.; Hill, N. J.; Yagupsky, M.; Wilkinson, G. J . Chem. Soc., Dalton Trans. 1972, 533. (43) Manivannan, G.; Changkakoti, R.; Lessard, R. A.; Mailhot, G.; Bolte, M. Paper accepted for presentation in SPIE's 1993 International Symposium on Holography, Microstructures and Laser Technologies, Aug. 15-21, Qu6boc, Canada; 1993, SPIE Vol. 2042.
