Photochemistry of Furyl- and Thienyldiazomethanes: Spectroscopic Characterization of Triplet 3-Thienylcarbene

Photolysis (λ > 543 nm) of 3-thienyldiazomethane (<b>1</b>), matrix isolated in Ar or N₂ at 10 K, yields triplet 3-thienylcarbene (<b>13</b>) and α-thial-methylenecyclopropene (<b>9</b>). Carbene <b>13</b> was characterized by IR, UV/vis, and EPR spectroscopy. The conformational isomers of 3-thienylcarbene (<i>s</i>-<i>E</i> and <i>s</i>-<i>Z</i>) exhibit an unusually large difference in zero-field splitting parameters in the triplet EPR spectrum (|<i>D</i>/<i>hc</i>| = 0.508 cm^–1, |<i>E</i>/<i>hc</i>| = 0.0554 cm^–1; |<i>D</i>/<i>hc</i>| = 0.579 cm^–1, |<i>E</i>/<i>hc</i>| = 0.0315 cm^–1). Natural Bond Orbital (NBO) calculations reveal substantially differing spin densities in the 3-thienyl ring at the positions adjacent to the carbene center, which is one factor contributing to the large difference in <i>D</i> values. NBO calculations also reveal a stabilizing interaction between the sp orbital of the carbene carbon in the <i>s</i>-<i>Z</i> rotamer of <b>13</b> and the antibonding σ orbital between sulfur and the neighboring carbonan interaction that is not observed in the <i>s</i>-<i>E</i> rotamer of <b>13</b>. In contrast to the EPR spectra, the electronic absorption spectra of the rotamers of triplet 3-thienylcarbene (<b>13</b>) are indistinguishable under our experimental conditions. The carbene exhibits a weak electronic absorption in the visible spectrum (λ_max = 467 nm) that is characteristic of triplet arylcarbenes. Although studies of 2-thienyldiazomethane (<b>2</b>), 3-furyldiazomethane (<b>3</b>), or 2-furyldiazomethane (<b>4</b>) provided further insight into the photochemical interconversions among C₅H₄S or C₅H₄O isomers, these studies did not lead to the spectroscopic detection of the corresponding triplet carbenes (2-thienylcarbene (<b>11</b>), 3-furylcarbene (<b>23</b>), or 2-furylcarbene (<b>22</b>), respectively)

Highly Stable Vapor-Deposited Glasses of Four Tris-naphthylbenzene Isomers

Recent reports have shown that physical vapor deposition can prepare organic glasses with remarkably high kinetic stability and low enthalpy in comparison with ordinary glasses formed by cooling the supercooled liquid. We have prepared vapor-deposited glasses of four isomers of tris-naphthylbenzene (TNB) and characterized them using differential scanning calorimetry. We find that highly stable glasses can be prepared from all four isomers of TNB by vapor deposition at 0.85 <i>T</i>_g. Glasses with low enthalpy were prepared for at least three of the four isomers. Because the TNB family of isomers contains both good and poor glass-formers, these results indicate a new dimension to the generality of stable organic glass formation via physical vapor deposition