2428
J. Phys. Chem. 1982, 86,2428-2431
Second Singlet Excited State of Biacetyl D. Ben-Amotzt and I. Y. Chan" Department of Chemistry, Brandeis Universiv, Waltham, Massachusetts 02254 (Received: December 22, 198 I; I n Final Form: February 23, 1982)
The nature of the second excited state of biacetyl was investigated by two-photon excitation spectroscopy. A sharp onset of two-photon absorption was observed at 557 nm. The anticipated B, second singlet is therefore placed at 4.45 eV. At the origin of conventionalnear-UV absorption, the two-photon cross sectio'n is extremely small. This long-wavelength absorption, below the lB, origin, is tentatively ascribed to a small amount of enolic biacetyl in equilibrium with the trans form.
Introduction Because of the photochemical and photophysical interest in biacetyl, there have been numerous spectroscopic' and theoretical2investigations of its electronic states. A major advance toward understanding its electronic structure was achieved in the mid-19709. Through photoelectron studies3 it was recognized that the interaction between the two "nonbonding" orbitals of its two oxygen atoms is large, leading to an energy separation of the n, orbitals of about 2 V. From this perspective, attempts have been made to reexamine the nature of the lower excited states of biacetyl within the one-electron framework of n(b,), n+(a.J, r+*(au), and r-*(bg) orbitals. The lowest singlet state at -437 nm is still considered to be an A, state, arising from a n+ r+* promotion. On the other hand, the second singlet state, first reported by Sidman and McClure (SM)4to have an origin at 318 nm and assigned to an A,(n r-*)state, is now suggested to be a B, state out of a n, r-* promotion. Transition between the ground state and the second excited state will therefore be dipole forbidden; the near-UV band of biacetyl will be vibronic in nature. Under this highly plausible new assignment, many disquieting features concerning the second excited singlet state of biacetyl remain. Firstly, the near-UV absorption spectrum of biacetyl in nonpolar solvents starts at 320 nm, gaining intensity slowly and reaching a Franck-Condon maximum at -273 nm (see Figure 3). Under a pure vibronic interpretation of this band, the molecule would have suffered a large change in C-O bond length. Secondly, the primary photochemical yield of biacetyl is known to be strongly dependent on the excitation wavelength in this region. It changes from 0.27 at 280.4-nm excitation to 0.074 at 313 nma6 And finally, photoacoustic investigation of biacetyl indicated that S2excitation does not relax to the ground state via SI or Taken individually none of these observations constitutes a serious objection. Together, they suggest a need for more refined examination of the second singlet. In this article we report one such investigation using two-photon spectroscopy on crystalline biacetyl. Since the molecule is trans-planar, and neat biacetyl possesses inversion site symmetry,' the g-g selection rule of two-photon transitions will be true. Our original intention was to test the B, nature of the SM origin at 318 nm. As will be seen, this is proven to be not the case. A two-photon allowed transition is observed at higher energy, ascribed to the theoretically suggested B, state. A provocative tentative assignment on the nature of the 318-nm band of SM is also suggested.
Experimental Section Biacetyl (MCB) was vacuum distilled at 50 "C by using a Vigreux column in an all-glass apparatus. Previous use of this technique in our laboratory has resulted in biacetyl free of any detectable impurity on a Carbowax 20 GC column. The distilled biacetyl was stored in sealed tubes, in the dark, at --16 "C. Biacetyl crystals (58mm3) with well-developed faces were grown from vapor in these sealed tubes. The crystals were handled at low light levels and under inert atmosphere at all times. Naphthalene used in the control experiment was zone refined and dissolved in spectral-grade cyclohexane. The laser system used was a Molectron UV 22 nitrogen laser and a Molectron DL 200 dye laser. The dyes used in the biacetyl experiment included Molectron C495 and the common rhodamines R6G, RB, and R101. For the naphthalene control experiment, 7D4MC and R6G were also used. The photomultipliers (PMs) used were EM1 9558A (primary detector) and EM1 9601B (for monitoring laser intensity). Both of the PMs were shown to respond linearly to light pulse intensity under the conditions of the experiment. The emission was detected at a right angle and was spatially filtered to avoid surface scattering. Phosphorescence from biacetyl was obtained with three Ditric Optics 510-nm short pass interference filters on collimated optics and subsequent counting of the photon pulses in the output of the detector using a Pacific Photometric AD6 pulse discriminator and amplifier and an Ortec 772 gated photon counter. Biacetyl fluorescence was recorded with three additional 500-nm short pass filters on the primary detector, with a PAR 16214 boxcar signal averager and a chart recorder in place of the pulse counting system. With this filter system the scattered laser light is reduced to a few photons per laser shot. The dependence of biacetyl emission intensity on laser intensity (Figure 2) was obtained by comparing the emission signal with the peak anode current output from a PM set up so as to detect scattered dye laser light. Fluorescence from naphthalene was recorded in the same way as that of biacetyl but with three Corning 7-54 filters.
'Department of Chemistry, University of California, Berkeley, CA.
60, 4231.
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(1) Verhaart, G. J.; Brongersma, H. H. Chem. Phys. Lett. 1980,73,176 and earlier references cited therein. (2) Ha, T. K. Chem. Phys. Lett. 1978, 57, 64 and earlier references cited therein. (3) Amett, J. F.; Newkome, G.; Mattice, W. L.; McGlynn, S. P. J. Am. Chem. SOC. 1974,96,4385. (4) Sidman, J. W.; McClure, D. S. J. Am. Chem. SOC. 1955, 77,6461. (5) Bell, W. E.; Blacet, F. E. J. Am. Chem. SOC. 1954, 76, 5332. (6) Kaya, K.; Harshbarger, W. R.; Robin, M. B. J. Chem. Phys. 1974,
(7) Chan, I. Y.; Hsi,
S.Mol. Phys. 1977, 34, 85.
0 1982 Amerlcan Chemical Society
The Journal of Physical Chemistry, Voi. 86, No. 13, 1982 2429
Second Singlet Excited State of Blacetyl
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Figure 1. (A) Fluorescence excitation spectrum of crystalline biacetyl obtained In a two-photon experlment at - 2 K. The orlgln and some of the vibrational features are marked under a two-photon spectroscopic assignment. (b) The C495 dye curve.
Results As a calibration of our experimental procedure, we reproduced portions of the two-photon absorption spectrum of naphthalene in cyclohexane solution (-2 X lob mol/L) as reported by Mikami and Ito.8 Using fluorescence detection, we observed structural features in the 450-470and 570-590-nm regions which were consistent in both position and relative magnitude with those of Mikami and Ito. In addition, a plot of log laser intensity vs. log fluorescent intensity yielded an order of nonlinearity at 460 nm of 2.01 with a correlation coefficient of 0.994 (see Figure 2). We will use the amplitude of the fluorescent signal from the -2 X 10" mol/L naphthalene solution at -460 nm as a rough calibration of biacetyl fluorescent intensity, which was measured by using an essentially identical scheme. Two-photon excitation experiments were carried out over the laser wavelength range 520-650 nm. A t appropriate laser wavelengths, a short-lived emission and a stronger long-lived emission were observed. The long-lived emission at both liquid-helium and liquid-nitrogen temperatures had a lifetime of about 1.3 ms and contained at least 100 times as many photons as the short-lived emission. The lifetime of the short-lived emission was below our 15-nsresolution. By inserting various filters or using a monochrometer in the signal collection optics, we were able to show that the short-lived emission was shorter than -500 nm while the long-lived emission was strongest in a band around 510 nm, with some intensities in the 530560-nm region. As we shall see later we have no reasons to believe these emissions to be anything but the fluorescence and phosphorescence of biacetyl, respectively. In the following presentation we will refer to them as such. A (two-photon) fluorescence excitation spectrum obtained from crystalline biacetyl(m-2 K) is shown in Figure 1. For wavelengths longer than 560 nm, the fluorescence had negligible intensity. The precipitous drop below 525 nm reflected the dye curve. With a bluer dye, the fluorescence output continued to rise slowly, until the laser energy reached the T1 So absorption threshold of the deep trap a t 19806 ~ m - l . ~ ?The ' onset of this masking one-photon transition serves as a natural bound for our experiment. The amplitude of this spectrum at -532 nm is -2000 times weaker than that of the naphthalene solution at 460 nm. No corrections for difference in dye intensity (which could be up to a factor of -2), filter
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(8)Mikami, N.; Ito, M. Chem. Phys. Lett. 1975, 31, 472.
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The Journal of Physical Chemistry, Vol. 86, No. 13, 1982
I ' -
"
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c
m, X
:I 0
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Figure 3. Absorption of biacetyl In the near-UV region: (A) cyclohexane solution of biacetyl at room temperature; (6)two-photon spectrum of biacetyl as a function of laser half-wavelength at 2 K (this work), not corrected for dye intensity; (C) rough sketch of Sidman and McClure's reported absorption by biacetyl at 20 K.
doubled light). The green phosphorescence of biacetyi was easily visible with the naked eye. With excitation wavelength approaching 320 nm, the emission decreased markedly. At 318 nm (near the peak of the RlOl dye) the phosphorescence was down by about a factor of 500 from the value at 305-nm excitation. We have thus qualitatively reproduced the spectral features of SM. Yet in the same region, two-photon absorption activity was clearly absent.
Discussion In scanning the dye laser through four dyes, representing the 520-650-nm region, the only substantial biacetyl emission that we found was that shown in Figures 1and 3B. This spectrum, which was observed by using both fluorescence and phosphorescence with essentially identical results, appears to originate at -557 nm. The wavelengths and the lifetimes of the emitted photons were consistent with those of biacetyl fluorescence and phosphorescence, respectively. However, the possibility that an impurity or a minor isomeric form of biacetyl was responsible for the initial absorption of radiation must first be discounted. The possibility of a minor impurity serving as a gateway in a consecutive two-photon process can be easily put aside by the following observations: (1)We followed a proven purification routine. (2) Identical spectra were obtained from samples constituting different fractions in trap-totrap distillation. (3) Most nontrivial contaminants of biacetyl, such as acetone, keto alcohols, and ethereal oligomers, do not absorb in the visible. (4) Above all, any impurities which could absorb our green excitation would also be expected to appear in the emission. Neat biacetyl emits mainly from a deep trap at 19 806 cm-1.49799 There are no known traps with lower energy. Given the efficient trap-to-supertrap energy transfer, the existence of such traps should be manifest in previous photophysical studies. Thus, although impurities or minor isomeric forms probably exist (see below), they are extremely unlikely to serve as one-photon gateways in our observed spectra. The possibility of two-photon absorption into carbonyl impurities (as mentioned in observation 3) can likewise be discounted. These impurities should have two-photon cross sections similar to that of biacetyl and therefore suffer from the effect of high dilution. Furthermore, the (9) Brand, J. C. D.; Mao, A. W. H. J.Am. Chem. SOC.1974,96,4380.
Ben-Amotz and Chan
first singlet absorption band of acetone, acetoin, and similar compounds lies longer than 300 nm. They cannot account for our observed spectrum. Summing up, our (two-photon) spectrum is most reasonably ascribed to biacetyl itself. In the remaining portion of this paper we will discuss the relationship between our two-photon spectrum and the well-known near-UV absorption of biacetyl which has a moderately strong band appearing to originate at around 320 nm. In the process of these discussions, we will argue both for the assignment of our absorption origin to a B, transition of trans-biacetyl and for a provocative suggestion that the 320-nm origin is due to the -0.1% enol form of biacetyl which is present in equilibrium with trans-biacetyl at room temperature. It has been suggested that the near-UV absorption of biacetyl, with an origin at 31 475 cm-' (318 nm or 3.9 eV) first reported by SM, is a 'B, (n, T+*) transiti~n.~?~ This is a logical consequence of a large n, splitting. The available theoretical calculations for the location of the lB, state are certainly in the right general region (3.53 eV for ref 10 and 5.15 eV for ref 2). Now if the 318-nm origin were due to a Bg (two-photon allowed) transition, we would have expected to see it in our experiments. Since we observed only a sharp onset of absorption at 556.9 nm, we believe that the 318-nm origin is not due to a lB, transition and that the only symmetry-allowedtwo-photon absorption in the 275-320-nm near-UV region originates at 278.5 nm (see Figure 3B). It might be suggested that the SM origin is in fact due to an A, transition of trans-biacetyl and thus our observed onset represented a false origin resulting from parity inverting vibrational modes. However, the energy difference between our onset and the 318-nm origin (AE 4300 cm-l) argues against this possibility; 4000 cm-l is too large for a reasonable b, vibrational frequency of electronically excited biacetyl. We are thus led to the conclusion that our two-photon origin represents the true 0-0 B, absorption of trans-biacetyl. The discrepancy in the theoretical and experimental values is not surprising in view of the fact that even in the ab initio SCF and CI calculations of ref 2 no attempt was made to optimize biacetyl geometry. A further corroboration of the B, origin at 278.5 nm comes from analyzing the vibrational features in the lifetime-broadened spectrum (see Figure 1). The 1098- and 1620-cm-' modes may comfortably be assigned to C-C and C-0 stretching, respectively. However, a question as to the nature of the 318-nm UV absorption origin remains to be resolved. The best available calculations suggest that the next higher singlet absorption above S1 (437-nm origin) has B, sy"etry.2!'0 If, as we claim, this B, transition originates at 278.5 nm, then the lower-lying 318-nm origin becomes rather enigmatic. We would like to suggest the possibility that the UV absorption band of biacetyl originating at around 320 nm is due primarily to the small quantity of enol biacetyl which is present in equilibrium with its normal trans form at room temperature.
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An absorption for the enol form of biacetyl in this region has previously been suggested by Lemaire in his work on the photoenolization of biacety1.l' Lemaire observed an (10) Leclereg, J. M.; Mijoule, C.; Yvan, P.J. Chem. Phys. 1976, 64, 1464. (11) Lemaire, J. J. Phys. Chem. 1967, 71, 2653.
Second Singlet Excited State of Biacetyl
The Journal of Physical Chemistry, Vol. 86, No. 13, 1982
increase in the near-UV absorption of various biacetyl solutions after they were subjected to prolonged UV irradiation. For the n-heptane solution of biacetyl Lemaire's data show that its near-UV band simply increases in intensity without any obvious shape change (the alcoholic and aqueous solutions that he studied were complicated by hemiketal and hydrate absorption in this same region). Lemaire argued for the attribution of this increased absorption to the photochemical formation of enol biacetyl and went on to derive an extinction coefficient for this enol of -10000 L/(mol cm) with a maximum absorption occurring around 275 nm. This strong absorption was attributed to the AT* absorption of enolic biacetyl. Although Lemaire did not go so far as claiming that the entire near-UV absorption before UV irradiation was also due to the enol, he did imply that, if as little as -0.1% enol were present in equilibrium with trans-biacetyl, this would in fact be the case. Early analytical work by Gero12claimed a value of 1.1% equilibrium enol for biacetyl at room temperature. A later study utilizing measurements of the yield of hydrogen from yirradiated aqueous solutions of various organic compounds13produced qualitative results which were consistent with an equilibrium enol content for aqueous biacetyl of >0.1%. We have looked into this problem with modern NMR spectroscopy. Our NMR spectrum of deoxygenated neat liquid biacetyl showed a small peak at 4.4 ppm downshifted from Me&, which may reasonably be assigned to enolic protons. The intensity of this small peak was roughly 0.07% of the main biacetyl peak at 2.3 ppm. In addition, after 15-min exposure to an unfocused 100-W Hg lamp (-7 cm away, filtered through 1cm of water), the magnitude of this small peak increased to 0.2% of the main peak. Assuming that the 2.3 ppm peak is due to the six methyl protons of biacetyl and that the 4.4 ppm peak is due to the two vinyl protons of the enol, our results suggest the presence of about 0.2% enol in biacetyl before UV irradiation and about 0.6% enol after the brief exposure to UV light. The increased magnitude of the 4.4 ppm peak upon UV irradiation is consistent with Lemaire's proposed photochemical formation of enol and thus serves to reaffirm our assignment of this peak to enolic biacetyl. Evidence for the existence of a m*absorption with Aat about 275 nm for some enols and biacetyl in particular has been presented elsewhere."J4 Since the A-A* absorption for the enol is a factor of 100-1ooO times stronger than the n-A* visible absorption of bia~etyl,~,"*'~ the presence of about 0.1-1% enol in biacetyl would make the near-UV absorption of the enol as strong as the visible (S,) band of trans-biacetyl. Since the near-UV absorption of biacetyl is in fact comparable in magnitude to the visible
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(12) Gero, A. J. Org. Chem. 1954, 19, 1960. (13) Witter, R. A.;Nata, P. J. Org. Chem. 1973, 38, 484. (14) Hammond, G. S.;Borduin, W. G.: Guter, G. A. J. Am. Chem. SOC. 1959,81,4682. (15) Kelder, J.; Cerfontain, H.; Eweg, J. K.; Rettschinck, R. P. H. Chem. Phys. Lett. 1974, 26, 491.
2431
S1band (solution," gas phase6),it is evident that our value of 0.2% enol could serve to account for essentially the entire near-UV absorption of biacetyl as enol absorption. As a comparison it is instructive to look at the visible and near-UV absorptions of the closely related glyoxal molecule, which is unable to form an enol. The two absorption bands of glyoxal are roughly in the same region as those of biacetyl, but in the case of glyoxal vapor the near-UV absorption is more than a factor of 10 smaller than the visible A,, absorption.15 It is thus reasonable to suggest that biacetyl does have a weak B, vibronic absorption in the near-UV region, which is overwhelmed by the strong absorption of the enol. Finally we wish to mention that this new picture of the second absorption band of biacetyl reconciles with many other observations concerning the second singlet excited state of biacetyl. The three troublesome behaviors of biacetyl mentioned in the Introduction may now be interpreted in a straightforward manner. Especially, although we have not determined a complete Franck-Condon profile, the onset of the two-photon spectrum is apparently steeper than the conventional S2band (see Figure 3). This behavior does not demand an unreasonable change in bond length for its explanation. More intriguingly, because of the finite probability of photoenolization even at low temperature, the conventional way of taking UV absorption spectra, as was employed by SM, might actually enhance the long-wavelength enol absorption. Last but not least, the presence of a significant fraction of enol might be associated with the existence of a deep trap at 19804 cm-', which plays such an overwhelming role in the photophysics of crystalline biacetyl. This very deep x-trap persists despite incessant purification efforts, nor does it depend upon the mode of crystal growing (from melt, solution, or vapor). Brand and Maog have found that its concentration varied with photochemical history. This unusual x-trap might conceivably be caused by enols in equilibrium with the lattice molecules, a situation from which a chemist may find no escape. Conclusion The primary result of our investigation is that the two-photon spectrum of biacetyl contains a sharp origin at 556.9 nm (278.5-nm half-wavelength). We have attributed this origin to a true B, 0-0 absorption of trans-biacetyl. In addition, we have suggested that essentially all of the UV absorption of biacetyl at wavelengths longer than the 278.5-nm B, origin is due to a small amount of enol which is associated with the predominant trans form of biacetyl. Acknowledgment. We thank Barbara Goldenberg for supplying the distilled biacetyl as well as for much helpful advice and assistance to one of us (D.B.) around the laboratory. We thank Professor Philip Keehn for helpful discusions on NMR spectroscopy. This work is supported in part by the National Science Foundation (CHE 8113452).
