2428 J. zyxwvuts Phys. Chem. 1982, zyxwvut 86, zyxwvu 2428-2431 Second Singlet Excited State of Biacetyl D. Ben-Amotzt and zyxwvutsrq I. Y. Chan" Department of Chemistry, Brandeis Universiv, Waltham, Massachusetts 02254 (Received: December 22, 198 zyxw I; In 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)4 to 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-* pro- motion. 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 dis- quieting 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 vi- bronic 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 S2 excitation does not relax to the ground state via SI or Taken individually none of these observations constitutes a serious objection. To- gether, 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 in- version 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. 'Department of Chemistry, University of California, Berkeley, CA. 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 (58 mm3) 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. Phos- phorescence 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 ob- tained 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. (1) Verhaart, G. J.; Brongersma, H. H. Chem. Phys. Lett. 1980,73,176 (2) Ha, T. K. Chem. Phys. Lett. 1978, 57, 64 and earlier references and earlier references cited therein. cited therein. Chem. SOC. 1974,96,4385. (3) Amett, J. F.; Newkome, G.; Mattice, W. L.; McGlynn, S. P. J. Am. (4) Sidman, J. W.; McClure, D. S. zyxw 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, 60, 4231. (7) Chan, I. Y.; Hsi, S. Mol. Phys. 1977, 34, 85. 0 1982 Amerlcan Chemical Society