J. Phys. Chem. 1981, 85, 2227-2232
2227
Photolysis and Laser-Excited Fluorescence and Phosphorescence Emission of trans-Glyoxal in an Argon Matrix at 13 K Michael Diem, Bruce G. MacDonald, and Edward K. C. Lee* Department of Chemistry, University of California, Imine, California 9271 7 (Received: December 3 1, 1980; In Final Form: March 3 1, 1981)
Matrix-isolated monomers and “cage” dimers of trans-glyoxal in an argon matrix at 13 K photodecompose to give the same photoproducts, HzCO and CO, when they are excited in the Sz So absorption region. The dimers and the monomers in minor sites are photolyzed more easily than the glyoxal monomer in major sites. No photoproducts are formed when they are excited in the SI Soabsorption region, but both fluor_escence and phosphorescence emissions are observed with the latter intensity being predominant. Glyoxal A lA, is efficiently quenched to 5 3A, in an argon matrix, with the matrix playing a role similar to that of collision partners in the gas phase.
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Introduction The study of the spectroscopy and photoreactions of monomeric and dimeric forms of small molecules furnishes an important means of understanding the nature of the molecular excimers and the physical and chemical interactions between the pair members involved in photochemical processes. Therefore, several photochemical studies have been conducted on monomers and “cage” dimers of formaldehyde1p2and sulfur dioxide3 in low temperature matrices. It has been found in these systems that “cage” dimers are more readily photolyzed than matrixisolated monomers, and that new reaction channels open up for “cage” dimers. For example, the dimer of HzCO is photolyzed to give CH30H and CO, whereas the photolysis of HzCO in the gas phase gives Hz + CO or H + HCO. Furthermore, the infrared absorption frequency shifts due to neighbor interactions provide a useful handle in gaining insight into the kinetics and mechanisms of photoreactions involved in the “cage” environment. Recently, the photolysis of trans-glyoxal (HCO-HCO) in Ar matrices at 13 K has been examined. Glyoxal is of interest because of the possibility of its photoisomerization to hydroxyketene, HOCH=C=O. It is the simplest dicarbonyl, a product of the recombination of HCO radicals, and can serve as a prototype for more complex dicarbonyl molecules. In addition, it is interesting to compare the photophysics and photochemistry of glyoxal with those of the formaldehyde systems mentioned above. Numerous studies have been done on the gas-phase photochemistry and photophysics of g l y ~ x a l . ~The , ~ first excited singlet (S,) and triplet states (T,) lie at 21 973 cm-l (454.9 nm) and 19196 cm-l (520.8 nm), respectively. Gas-phase studies by ParmenteP indicate that glyoxal decomposes to the following products: CzH2Oz + hv Hz + 2CO (1) CzHzOz
+ hv
-
-
HzCO
+ CO
(2)
although H2C0 was not directly observed. Yardley7 pro(1) J. R. Sodeau and E. K. C. Lee, Chem. Phys. Lett., 57, 7 1 (1978). (2) M. Diem and E. K. C. Lee, Chem. Phys., 41, 373 (1979). (3) J. R. Sodeau and E. K. C. Lee, J . Phys. Chem., 84, 3358 (1980). (4) (a) P. Avouris, W. M. Gelbert, and M. A. El-Sayed, Chem. Reu., 77, 793 (1977); (b) E. K. C. Lee and G. L. Loper, “Radiationless Transitions”, S. H. Lin, Ed., Academic Press, New York, 1980, p 1. (5) R. S. Lewis and E. K. C. Lee, Adu. Photochem., 10, 1 (1980). (6) C. S. Parmenter, J . Chem. Phys., 41, 658 (1964). (7) J. T. Yardley, J . Chem. Phys., 56, 6192 (1972).
0022-3654/81/2085-2227$01.25/0
+
posed that glyoxal photoproducts in the gas phase arise from triplet glyoxal decomposition through an intermediate [I] as shown in eq 3 and has suggested for the inCzHzOz(T,) [I] products (3)
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termediate a low-lying lB, or 3B, state, a rearrangement product, or an addition product. Anderson et ala8have proposed that the intermediate may be vibrationally hot ground-state glyoxal (Sot) with 22 000 cm-l of excess energy. It was hoped that examination of the electronic and vibrational spectra and photoproducts of glyoxal in a low-temperature inert gas matrix would provide information helpful in the elucidation of the gas-phase results and in understanding the interactions between molecules in glyoxal dimers, i.e., the photochemistry of glyoxal excimers,
Experimental Section Glyoxal monomer was prepared by heating the solid trimer (Matheson Coleman and Bell) topped with phosphorus pentoxide under vacuum to 120 “C and collecting the gaseous monomer in a trap at 77 K. The argon used was supplied by Liquid Carbonic and had a stated purity of 99.9995%. Gas mixtures of glyoxal and Ar were prepared in a 2-L flask by using standard manometric techniques on a vacuum line free of grease. The mixtures were pulse deposited onto a CsI window which was cooled to 13 K in an Air Products Displex Model 202B cryostat, and 2-20 gmol of glyoxal were deposited. For some experiments, a KBr window was used as a sample support. Infrared spectra were taken with a Nicolet 7199 Fourier transform infrared spectrophotometer a t a resolution of 0.3 cm-l. Photolyses were carried out with light from a 1000-W compact arc, high-pressure mercury lamp (USWIO 1005D). A 3-in. long water filter was used to filter out infrared radiation from the mercury lamp in order to minimize heating the cryogenic sample. No UV filters were used for any of the photolyses with mercury arc. A second light source for photolysis was a Nd:YAG laser-pumped dye laser (Quanta Ray DCR lA/PDL-l) or for some experiments the frequency-quadrupled output a t 266 nm from the Nd:YAG laser itself. The pulse width was approximately 7 ns, and an energy range of 0.5-3 mJ per pulse was used at a repetition rate of 10 Hz. The laser line width was approximately 0.3 cm-l. (8) L. G. Anderson, C. S. Parmenter, and H. M. Poland, Chem. Phys., 1, 401 (1973).
0 1981 American Chemical Society
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The Journal of Physical Chemistty, Vol. 85, No. 15, 1987
Diem et al.
OgO---l OgOr--l
0 00 1740
v
i
0.400
'
i
I
u
-
- " I
L I710
I740
(ern-')
1710
b(crn-l) (b)
(a)
Figure 1. C=O stretch region of glyoxal at 1724 cm-' (MR = 3000 in Ar; Pw = 200 torr): (a) before hotolysis and (b) after 5 4 photolysis with an Hg lamp. The 1740-cm- peak of the H,CO/CO "cage" pair
P
2145
2135
v"
(crn-l)
Flgure 2. CO absorption region after 5-h Hg lamp photolysis of glyoxal = 200 torr). The main peak at 2139 cm-' (MR = 3000 in Ar; PtOtal is due to the CO/H2C0 "cage" pair formed in photolysis.
appears after photolysis. Small spikes at 1734, 1733, and 17 17 cm-' are due to H,O vapor in the spectrometer.
The relative amounts of monomer and dimer formed upon deposition were varied by changing the ratio of Ar to glyoxal (MR) as well as by changing the pressure of each gas mixture. Matrices with MR values ranging from 500 to 10OOO were photolyzed, and the extent of photolysis was monitored by observing the change in absorption intensity of the C=O stretch frequency of glyoxal monomer in Ar a t 1724 cm-l. Laser-induced emission spectra were taken with the Nd:YAG laser-pumped dye laser. Emission was passed though a 3/4-m monochromator (Spex Model 1702) and monitored either by an EM1 9558 photomultiplier tube (S-20) or by an RCA 8575 photomultiplier. Both electronic emission and excitation spectra were taken. Signals were processed by a gated integrator and averaged on an Intel 8085 microprocessor. The luminescence decay time was measured with a boxcar averager (Princeton Applied Research Models 162 and 164). The electronic absorption spectrum was taken with a Cary 219 UV-visible spectrophotometer with a resolution of -0.2 nm.
Results It was found that matrices with MR = 10 000,3000, and 2000 appeared the same in IR spectra and gave the same results upon photolysis. In the same way, matrices with MR = 1000 and 500 showed no difference in their photochemical behavior. Thus, for purposes of illustration, spectra are presented from matrices with MR = 3000 for the dilute case and with MR = 500 for the concentrated case. Photolysis with M R = 10000, 3000, and 2000. The C=O stretch region of the glyoxal monomer in Ar (MR = 3000) a t 1724 cm-' is shown in Figure 1: (a) before photolysis and (b) after 5 h of irradiation with the Hg lamp. Accompanying this peak is a broad shoulder on the high-frequency side. This shoulder may be due to aggregates of glyoxal but, at the prevailing dilution cluster formation should be minimal. Thus it is also possible that the shoulder is due to glyoxal monomers in different matrix sites. After photolysis, the monomer has been reduced and the shoulder is essentially gone, as seen in part b of Figure 1. The baseline has risen by 0.2 absorbance units due to increased light scattering by the matrix as the result of irradiation. Also apparent are small peaks at 1734,1733, and 1717 cm-'. These are due to water vapor which was not sufficiently purged from the FT IR spectrometer. Because the absorbances of the dilute samples were often small, the
0 00 I710
1740-
v(crn-') (a)
1740-
1710
v(crn-') (b)
Flgure 3. C=O stretch region of glyoxal at 1724 cm-' (MR = 500 in Ar; f w = 200 torr): (a) before photolysis and (b) after 547 photwsis with an Hg lamp. Two satellite bands at 1722 and 1719 cm-' and the broad shoulder at 1726 cm-' are due to glyoxal dimers and minor monomer des, respectively. These peaks are preferentlally destroyed upon photolysis. The 1740-cm-' peak of the H,CO/CO "cage" pair appears after photolysis.
scale expansion brought out even very weak water peaks. Accompanying the decrease in the intensity of the glyoxal C=O peak at 1724 cm-l is the appearance of a new peak at 1740 cm-l and a fairly strong peak at 2139 cm-' with two weak side bands at 2140 and 2138 cm-l (see Figure 2). Photolysis with MR = 1000 and 500. The C=O stretch region for a concentrated matrix (MR = 500) is shown in Figure 3: (a) before photolysis and (b) after 5-h photolysis with the Hg lamp. In this more concentrated matrix, the monomer peak at 1724 cm-l is accompanied by two satellite peaks at 1722 and 1719 cm-l. Also present is the highfrequency shoulder seen in the dilute matrix, but in this case more pronounced. After a series of irradiation periods totalling 11 h the peak at 1722 cm-l is drastically reduced and the peak at 1719 cm-l disappears. The shoulder at 1726 cm-l also decreases, and the peak at 1724 cm-l appears to increase. A new product peak at 2139 cm-l (due to CO) appears with shoulders at 2140,2138, and 2136 cm-l as shown in Figure 4. As will be discussed, we believe that the satellite peaks and shoulders in the 1724-cm-l region are due to the glyoxal dimers and minor-site glyoxal monomers and that they are photolyzed, forming a set of products consisting of glyoxal monomer, HzCO and CO. Laser-Excited Emission and Excitation Spectra. Laser-excited luminescence excitation spectra show broad and relatively weak zero-phonon lines with very broad phonon wings as seen in Figure 5. Excitation spectra locked onto fluorescence were taken with a gatewidth of -1 ps, and
The Journal of Physical Chemistry, Vol. 85, No. 15, 1981 2229
Photoproducts of trans-Glyoxal in an Argon Matrix
I 1
1
I
I
I
I
580 0
5200
460 0
A (nm) 0 00
I
I
1
2150
,
2130
I
u (cm-') Flgure 4. CO absorption region after 5-h Hg lamp photolysis of glyoxal (MR = 500 in Ar;, ,P = 200 torr). The main peak at 2139 cm-' is due to the CO/H,CO "cage" pair formed in photolysis.
Figure 6. Emission spectrum (uncorrected for spectral sensitivlty) of glyoxal (MR = 500 in Ar; Ptotal 35 torr), excited at 444.4 nm. It shows fluorescence bands from A 'A, (0;= 467.5 nm; 8: = 491 nm) and B 3A, (0; = 537.0 nm; 4: = 568.5 nm; 2; = 591.5 nm).
I
I
I
I
4400
4200
1
I
1
4600
X(nrn)
Figure 7. Electronic absorption spectrum of glyoxal (MR = 3000 in Ar; Ptotal= 200 torr).
I
1
I
450 0
I
440 0
1
I
1
4300
A (nm)
Flgure 5. Luminescence excitation spectrum (unnormalized)of glyoxal (MR = 2000 in Ar; Ptov = 200 torr). Observed peaks at 5; = 449.5 nrn; 8; = 444.6 nm; 4,(?) = 440.5 nm; 2; = 432.0 nm.
those locked onto phosphorescence were taken with a gatewidth of -1 ms on the gated integrator. No differences in the excitation intensities were observed. Emission spectra were taken at laser excitation wavelengths of 448.7, 444.4,443.5,439.0,433.5,430.6, and 425.7 nm. The spectra showed no changes in relative band intensities as a function of excitation wavelengths. Emission spectra showed predominantly triplet-sta..te emission. The intensity of the 0 4 band of the B 3A, X lA, transition located near 530 nm is an order of magnitude greater than that of the G O band X lA, transition located near 460 nm. A of the A 'A, typical emission spectrum (MR = 500) obtained by using an excitation wavelength of 444.4 nm is shown in Figure 6. The phosphorescence decay time in an Ar matrix was -3 ms when measured with a boxcar averager. This is comparable to the 3.29 f 0.1 ms lifetime of the B 3A1 state reported in the gas phase.7 Electronic Absorption Spectrum. The electronic absorption spectrum of a dilute matrix (MR = 3000) is given in Figure 7. In this case, the matrix was deposited to give monomeric glyoxal. The absorption spectrum was taken with a Cary 219 spectrometer, and the sample was then heated to 25 K to allow diffusion for 10 min. In this way, the IR spectrum taken after diffusion resembled that of a concentrated (MR = 500) matrix. The electronic absorption spectrum after diffusion contains no features which are not observable in the spectrum taken before diffusion. This lack of difference in the visible absorption spectra between isolated and aggregated glyoxal is also
-
-
apparent in the cases of the excitation and emission spectra. Pulsed Laser Photolysis. Photolysis with the Nd:YAG laser-pumped dye laser in the Sz So absorption region at 288.0 nm resulted in formation of HzCO and CO, but photolysis in the SI So absorption region at wavelengths of 445.0 to 440.0 nm gave no observable products. In one irradiation experiment, the direct, frequency-quadrupled output (at 266 nm) from the Nd:YAG laser was used for photolysis. At an energy input level of 1mJ per pulse, the glyoxal sample was rapidly photolyzed. However, at an energy input level of 3 mJ per pulse, the matrix was destroyed through heating and evaporation.
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Discussion The frequencies and assignments of a number of IR peaks observed in the argon-glyoxal matrices are listed in Table I. The frequencies assigned to HzCO/CO, CO/CO, CO, and CzHzOZ/HzCOwere obtained from authentic samples of these substances in Ar at 13 K. Because the peaks at 1722 and 1719 cm-l are concentration dependent, and because they are nonexistent in dilute Ar matrices we believe that these peaks arise from glyoxal dimers, and possibly higher aggregates. The broad shoulder at 1726 cm-' is also concentration dependent, but is present even in the most dilute (MR = 10000) matrices, when the peaks at 1722 and 1719 cm-' are gone. For this reason, and the fact that the shoulder lies to the blue of the monomer peak, while the assumed dimer peaks lie to red, it is believed that the shoulder is due to monomeric glyoxal in several different matrix sites. The monomers in these sites are more photoreactive, and thus decrease more quickly than the "normal" monomer (1724 cm-l) after photolysis. Photolysis. It can be seen from Figures l b and 3b that the concentrated matrix was photolyzed more rapidly than the dilute matrix, giving a greater amount of HzCO a t the expense of glyoxal dimers and minor-site monomers. Thus,
2230
Diem et al.
The Journal of Physical Chemistry, Vol. 85,No. 15, 1981
TABLE I: Infrared Frequencies ( u ) and Assignments of Some Characteristic Absorption Peaks in Ar/Glyoxal, Ar/H,CO, Ar/CO, and N,/Glyoxal Matrices (13 K ) u,
species
mode
H,CO H,CO/H,CO H,CO/CO
c=os t ( u , )
C,H,O, (in Ar)
C=O s t C = O s t (H,CO) (CO) C = O st ( u l o ) CH s t
this work 1742.2 1738.6 1740.0 2140.0 1724.3 2855.8
I40
cm-’ other works
1742a 173ga
z! 0 a
7c
1 7 3 2 b (gas) 2835.076
CH st (VI,
CH rock ( u l l ) C,H,O, (in N2) C = O s t ( u , ” ) CH s t ( u o ) CH rock’(u,,) C,H,O,/H,CO C = O s t (C,H,O,) C,H,O,/C,H,O, C=O s t (in Ar) C,H,O,/C,H,O, (in N,)
C=O st
co co/co
4 co 1 V(C0)
Pas)
1 3 1 2 d 1312.46 (gas) 1731.2 2857.8 1322d 1731.3 1722.4 1719.2 1728.0 1726.4 2138.4 2138‘ 2136.5 2140‘
Reference 1 0 a Reference 9 (Koshkoo and Nixon). Reference (Cole and Osborne, gas-phase frequencies). 11 (Dubost). Broad.
‘
the glyoxal dimer and the minor-site monomers are more easily photolyzed than the “normal” monomer a t 1724 em-’. This conclusion is also born out in Figure 8 where the major monomer (1724 cm-l) peak is observed to be more intense after 11 h of photolysis than it is before photolysis begins. It seems likely that photolysis of a glyoxal dimer would result in photodissociation of one member, producing a monomer which is more stable toward photolysis than the dimer. However, such results are sometimes difficult to interpret quantitatively with confidence, since the matrix undergoes changes during irradiation. Following photolysis in the concentrated matrix, a peak appears at 1731 cm-l. From spectra of authentic mixtures, this peaks is known to arise from a glyoxal/formaldehyde complex. It is most likely due to the C=O stretch of glyoxal perturbed by H2C0. If it were due to the HzCO C=O stretch, this would mean an 11-em-’ shift to the red from the C=O stretch frequency of isolated H2C0. Since H&O perturbed by HzCO (Le,, dimer H2CO) has its C=O stretch frequency red-shifted by only 4 cm-l, from 1742 to 1738 cm-l, it seems unlikely that perturbation by the less polar glyoxal would be twice as great. To explain the shift to higher frequency of the glyoxal C=O stretch, we visualize the H2C0 in the formaldehyde/glyoxal pair as somehow contorting the glyoxal out of planarity, leading to loss of conjugation and an increase in energy of the C=O bonds. Another explanation may be that in destroying the planarity of the glyoxal, the H2C0 may cause the usually inactive symmetric C=O stretch (vz) to become active. The effect of glyoxal on the C=O stretch of H2C0 is not evident. In authentic samples of formaldehyde/glyoxal, the only peaks observed were attributable to HzCO monomer, H2C0 dimer, glyoxal monomer, glyoxal dimer, and to the peak at 1731 cm-l, assigned to glyoxal perturbed by HzCO. Since no other peak was observed, it is likely that HzCO monomer and H2C0 perturbed by glyoxal have coincident C=O stretch frequencies. This conclusion is
0 00
I
1740
I
I
1
1710
u(cm-‘)
Flgure 8. C=O stretch region of glyoxal at 1724 cm-’ (MR = 500 in Ar; P,,, = 150 torr) after 11-h photolysis to be compared to Figure 3b (5 h). The spikes at 1734, 1733, and 1717 cm-’ are due to H,O vapor in the spectrometer. The dimer and aggregate peaks at 1722 and 17 19 cm-’ have completely disappeared. The 1740-cm-’ peak due to the H,CO/CO “cage” pair is strong.
supported by spectra of authentic samples of CO and glyoxal. In these spectra, there were no new CO peaks observed in the presence of glyoxal. Thus, it appears that glyoxal does not perturb the CO frequency or the C=O stretch frequency of H2C0. The products of photolysis of both the monomer and the dimer are the same: H,CO and CO (and H2, presumably present but undetected in the IR spectra) as indicated in reactions 1 and 2. In this respect glyoxal does not follow the pattern observed in photolysis of H2C0 in Ar,lpZor SOz in Ar and O2mat rice^.^ As stated previously, these systems show that dimer photolysis leads to products different from those of monomer photolysis. Apparently, H2C0 and SOz monomers are less reactive photochemically (for unimolecular reactions) than glyoxal monomer. This is not unreasonable in Ar matrices, since SOz monomer has no feasible reaction pathway to follow upon photolysis at low excitation energies, and HzCO may rearrange to HCOH which could return to HzCO with a high probability in a photolytic environment. The results of glyoxal photolysis in the matrix agree only qualitatively with those of the gas-phase studies cited.&@J2 In the present study, HzCO has been directly observed as a product of glyoxal photolysis by the decomposition process (reaction 1) which has been proposed to be the main decomposition route in the previous gas-phase photolysis studies.6-8*12However, it should be noted in our work that pulsed irradiation with 445.0- and 299.5-nm output from the Nd:YAG pumped dye laser gave no product while the irradiation with 288.0 and 266.0 nm output gave photolysis products readily. The gas-phase photolyses at 435@ and 444.72 nm,12 for example, gave CO as a photodecomposition product. Therefore, the wavelength dependence of photolysis in the matrix is contrary to results of gas-phase studies. The product formation in the matrix photolysis seems to require the photoexcitation of glyoxal to the S2 manifold, whereas in the gas-phase photolysis the excitation to the S1manifold is sufficient to give the same products. (9) H. Koshkoo and E. R. Nixon, Spectrochirn. Acta, Part A , 29,603 (1973). (10) A. R. H. Cole, and G. A. Osborne, Spectrochirn. Acta, Part A , 27, 2461 (1971). (11) H.Dubost, Chem. Phys., 12, 139 (1976).
(12) G. H. Atkinson, M. E. McIlwain. and C. G. Venkatesh, J. Chem. Phys., 68, 726 (1978)
Photoproducts of trans-Glyoxal in an Argon Matrix
The Journal of Physical Chemistry, Vol. 85, No. 15, ‘1981 2231
No intermediates were observed in IR spectra after any of the photolyses. There are several possible reasons for the lack of such species: (i) Because most of the photolyses were carried out with unfiltered light from a high-pressure Hg lamp (giving a quasi-continuum of wavelengths) it is possible that secondary photolysis occurred. Thus, any intermediate formed would be directly photolyzed to products. Photolysis with pulsed laser light did not produce observable intermediates. It may be that, like glyoxal, any intermediate would have an electronic absorption continuum in the photolysis region (288-266 nm). The intermediate in reactian 3 is likely to contain a carbonyl group. Most carbonyls larger than H2C0 have little or no structure in their electronic absorption bands,13so it is not unreasonable to assume that light of any wavelength within the glyoxal transition may lead to photolysis of any possible intermediate. Of course, if an intermediate were a vibrationally hot ground-state glyoxal, it would rapidly decay to the ground level in the matrix and could not be seen in an IR spectrum. (ii) An intermediate, once formed, may have its IR peaks coincident with those of glyoxal or one of the products. Since any intermediate would probably be present only in small amounts, its weak absorptions could easily be buried in the larger peaks of the stable molecules present. (iii) Any intermediate formed has such a low steady-state concentration that it cannot be detected in IR spectra. Emission. The fact that the triplet emission intensity i,s much greater than that of singlet emission implies that A ‘A, 5 3A, intersystem crossing is important in the matrix. However, what is puzzling is that the fluorescence is observed a t all. Gas-phase studies by Yardley et al.14 have shown _thatAr quenches the lA, state of glyoxal via B 3A,. The matrix apparently enthe process A lA, haces relaxation to the triplet in a manner similar to that of gas-phase collision partners. The radiative lifetime of the zeroth vibrational level of glyoxal A lA, is 4.1_.~s.’~ Therefore, to some extent the radiative transition A ‘A, X lA, is cqmpeting with the matrix-induced radiationB 3A,, in a microsecond time scale. less process, A lA, Emission spectra appear identical, independent of the excitation wavelength. This implies that emissions occur from the same vibrational level (of S1 as well as TJ, more specifically the zeroth level, due to rapid vibrational relaxation on a microsecond time scale or faster in the matrix, prior to emission. It is interesting to compare the effect of pressure as well as the effect of matrix on the radiationless processes in HzCO (or D2CO) and glyoxal. In formaldehyde, neither collision-induced intersystem crossing? S1 T1, nor any T1procesP has been observed. In matrix-induced S1 glyoxal, both collision-inducedgand matrix-induced S1 T1 processes are observed. Therefore, the effect of gasphase collisions appears similar to the effects of the inert gas matrix on the radiationless processes of these small and intermediate size molecules. The similarity in the band shapes of the electronic absorption spectra and the excitation spectra indicates that the quantum efficiencies for the radiative and nonradiative processes are rather uniform over the entire phonon sideband within the resolution of our experiment.
--
--
-
--
--
--
--
(13) J. G. Calvert and J. M. Pitts, Jr., “Photochemistry”, Wiley, New York, 1966, pp 368-9. (14) J. T. Yardley, G. W. Hollerman, and J. I. McIlwain, Chem. Phys. Lett., 10, 266 (1971). (15) B. G. MacDonald and E. K. C. Lee, J. Chem. Phys., 71, 5049 (1979). (16) (a) L. T. Molina, K. Y. Tang, J. R. Sodeau, and E. K. C. Lee, J . Phys. Chem., 82, 2575 (1978); (b) J. Goodman and L. E. Brus, Chem. Phys. Lett., 58, 399 (1978).
1401-1 - 000
2150
-
2120
u (ern-')
Figure 9. CO absorption region after 11-h Hg lamp photolysis of glyoxal (MR = 500 In Ar; Ptolal= 150 torr) to be compared to Figure 4 (5 h). The main peak at 2139 cm-’ is due to the CO/H2C0 “cage” pair.
Molecular Interaction. The IR spectra of glyoxal in Ar showed two CH stretch peaks, at 2861 and 2856 cm-l. By its Ca symmetry, glyoxal should exhibit only one IR-active CH stretch. A spectrum of glyoxal in a N2matrix did show only one CH stretch, at 2858 cm-l. The relative intensities of the two peaks were the same in dilute and concentrated matrices, therefore aggregation is not involved. Apparently the doublet is due to a matrix effect present in Ar and not present in N2. It may be that the CH bonds see an aymrnetric matrix environment in Ar, causing the symmetric CH stretch to become IR active. However, the local matrix environment around the C=O bonds appears to be symmetric, since there is no observed splitting of the C=O stretching frequency. This site effect in Ar may also be involved in the reactivities of glyoxal monomers and dimers upon photolysis. In N2,the C-H stretching frequency is not split, thus implying a symmetric matrix environment, unlike the case with Ar. The size of the substitutional site in N2 at 4 K is ca. 4.52 X 3.42 X 3.42 A,17greater than that in Ar a t 4 K (3.755 A diameter),18and it may be this difference in cage sizes which accounts for the varying shifts in infrared frequencies. After 11h of photolysis with the Hg lamp the following set of peaks appeared in the CO absorption region, 2149 (weak, broad), 2139 (medium strong), 2138 (medium, shoulder), and 2136 (weak, shoulder) as shown in Figure 9. Other matrix spectra have shown that these are due to CO/H20, CO/H2C0, CO monomer, and CO/CO, respectively. Glyoxal photolysis is expected to give all of these products, except CO/H20 and the monomeric CO, which are, therefore, anomalous. The reactions of glyoxal to give either HzCO and CO, or CO/CO and H2, are both slightly end other mi^.'^ However, the excess energy from the incident radiation may cause local heating of the matrix sufficient to allow the light CO molecule to diffuse away from the reaction site. In this way, isolated CO, not directly produced from photolysis, could ultimately be formed. Another process leading to monomeric CO could be the absorption of light by HzCO/CO (product from (17) W. F. Giauque and J. 0. Clayton, J. Am. Chem. SOC.,55,4875 (1933). ‘ (18) 0. G. Peterson, D. N. Batchelder, and R. 0. Simmons, Phys. Rev., 150, 703 (1966). (19) “Selected Values of Chemical and Thermodynamic Properties”, Natl. Bur. Stand. U S . Circ., No.500 (1952).
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J. Phys. Chem. 1981, 85,2232-2238
glyoxal), with no reaction. The photoexcited HzCO* may dispose of its excess energy t o the matrix causing local heating and allowing the diffusion of the CO molecule. The relative intensities of the various peaks in the glyoxal photolysis may be used to gain some insight into the relative importance of reactions 1 and 2. It was found in the photolyses of H2C0 in Ar that, for equimolar amounts of H 2 C 0 and CO, the intensity of the C=O stretch of H2C0 is approximately 1.5 times that of the CO frequency as measured by peak areas. Therefore, if reaction 2 were the only process resulting from photolysis, the CO peak should be two-thirds as strong as the stretch of H2C0. However, in the case of the concentrated matrix, these two peaks are of approximately equal intensity throughout the photolysis of the sample. Therefore, CO is produced from an additional source to reaction 2. It has been shown also that the intensities of the C=O stretch frequencies for equal amounts of glyoxal and H2C0 should have roughly the same value. But the loss of glyoxal in the concentrated matrix is not compensated by the increase in H2C0. Thus secondary photolysis of H2C0 is an important reaction H2CO + hv H2 + CO (4) The H2C0 is photolyzed more quickly than expected from previous work: and this may be due to the fact that glyoxal reacts to give a HzCO/CO pair, with CO enhancing the photoreactivity of H2C0. Even if reaction 4 were not important, then reaction 2 would occur to 1.5 times the extent of reaction 1to account for the products. Since reaction 4 is known to occur,z it represents an added source of CO. Therefore, the quantum yield of reaction 2 is a t least 1.5 times as great as the quantum yield of reaction 1. Thus, under the conditions of our experiment, reaction 2 seems to be the more im+
portant pathway for glyoxal photolysis. Quantum Efficiencies of Photolysis. The dimer photolyzes more readily than the major site monomer in the S2 So absorption region (320-260 nm). We are unable to determine the relative quantum efficiencies for product formation, since we were unable to measure relative absorption coefficients of this transition for the dimer vs. the major site monomer. However, it is plausible that the dimer in Ar matrix could have an enhanced absorption coefficient for this highly forbidden transition relative to the monomer in Ar by lowering the symmetry due t o dimerization.
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Conclusions In the present study it has been found that (1)glyoxal dimers and monomers in different Ar matrix sites are photolyzed more readily than the most common glyoxal monomers; (2) both monomers and dimers give the same photoproducts, H2C0and CO; (3) it is not known whether the higher product yields from photolysis of dimers and perturbed monomers are due to greater quantum efficiency or enhanced absorption of these species; (4) the photochemical threshold for the formation of H2C0 and CO in an Ar matrix appears to lie between 300 and 288 nm on the Sz manifold; and (5) glyoxal A lA, is quenched to 5 3Au in an Ar matrix, with the matrix playing a role similar to that of collision partners in the gas phase. Acknowledgment. This report is based upon research supported by the National Science Foundation under Grant CHE-79-25451 for product studies and the Department of Energy (Office of Basic Energy Sciences) under Contract DE-AT-03-76-ER 70217 for emission studies. The FT IR spectrometer was furnished by the NSF Departmental Research Instrumentation Program.
Electrodic Dye Layers in the Gold-Rhodamine B Photoelectrochemical Cell T. I. Quickenden” and I?.L. Bassett Department of Physical and Inorganic Chemistty, University of Western Australia, Nedlands, W.A., 6009, Australia (Received: January 6, 196 1; In Final Form: March 24, 196 I)
Dye layers have been deposited on a gold electrode from an aqueous solution of rhodamine B (ethanaminium, N-[9-(2-carboxyphenyl)-6-(diethylamino)-3H-xanthen-3-ylidene]-N-ethyl-, chloride), under applied oxidizing potentials in excess of 0.85 V relative to a saturated calomel electrode. Such layers were observed to increase the power conversion efficiency of the gold-rhodamine B photoelectrochemical cell by up to 14 times. The dye layers were shown, by absorption, fluorescence, and fluorescence excitation spectroscopy, to be composed of rhodamine B and its completely deethylated form, rhodamine 110. Photovoltage action spectra were determined for the dye-coated photoelectrodes and were found to resemble the relevant absorption spectra, except for the presence of a photovoltage peak at 400 nm. This peak was not observed in the action spectra of uncoated gold, electrodes immersed in rhodamine B solutions.
Introduction The formation of electrodic dye layers in photoelectrochemical cells containing solutions of fluorescent dyes was first noted briefly by Miller1 in 1962, in his comprehensive study of the iron-thionine cell. However, it is only in the last few years that substantial evidence for the appearance
of dye layers in such systems has become available. In 1978 Yim2 reported that, when cells containing gold electrodes in contact with rhodamine B solutions were irradiated for long periods, a layer of colored material was deposited on the photoelectrode and the photovoltage concurrently increased. Further studies by Quickenden, Yim, and Herring3 on the gold-rhodamine B cell, using a
(1)L. J. Miller, “A Feasibility Study of a Thionine Photogalvanic Power Generation System”, Final Report, Contract No. AF33(616)-7911, Sunstrand Aviation, ASTIA Document No. 282878, 1962.
(2) G. K. Yim, Ph.D. Thesis, University of Western Australia, Nedlands, Australia, 1978.
0022-3654/81/2085-2232$01.25/00 1981 American Chemical Society
