| Abstract
| - The vibronic spectroscopy of ortho-, meta-, and para-diethynylbenzene (oDEB, mDEB, and pDEB) wasstudied by two-color resonant two photon ionization (R2PI). The symmetry allowed S0−S1 origins of oDEB,mDEB, and pDEB were located at 33 515, 33 806, and 34 255 cm-1, respectively, with the vibronic structureextending about 2000 cm-1 above the origin in oDEB and mDEB, but more than 3000 cm-1 in pDEB. Majorpeaks in each spectrum were attributed to vibronically induced bands, indicating strong coupling of the S1state to the S2 state. Ground-state infrared spectra in the C−H stretch region (3000−3360 cm-1) were obtainedusing resonant ion-dip infrared spectroscopy (RIDIRS). In all three isomers, the acetylenic C−H stretchfundamental was split by Fermi resonance with a combination band composed of the C⋮C stretch and twoquanta of the C⋮CH bend. This Fermi resonance was detuned in the overtone region of pDEB, whichshowed a single peak at 6556 cm-1. Infrared spectra were also recorded in the excited electronic state usinga UV−IR−UV version of RIDIR spectroscopy. In all three isomers, the acetylenic C−H stretch fundamentalwas unshifted from the ground state, but no Fermi resonance was seen. In addition to the sharp C−H stretchfeatures, the oDEB S1 infrared spectrum showed a broad absorption stretching from 3050 to 3250 cm-1.Selective deuteration of oDEB at the acetylenic hydrogens led to infrared spectra that showed the broadabsorption despite the absence of the acetylenic C−H stretch, indicating that the IR absorption is electronicin nature. Characteristics of this second excited state and its potential relevance as a gateway to a photochemicalBergman cyclization are discussed.
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