SELECTED RECENT PUBLICATIONS:
Schrauben, J.; Ryerson, J.; Michl, J.; Johnson, J. "The Mechanism of Singlet Fission in Thin Films of 1,3-Diphenylisobenzofuran", J. Am. Chem. Soc. 2014, 136, 7363.
ABSTRACT: In order to elucidate the mechanism of singlet fission in thin films of 1,3-diphenylisobenzofuran (1), we have performed ultrafast transient absorption spectroscopy as a function of sample temperature and excitation fluence on polycrystalline thin films composed of two polymorphs. Our earlier investigations revealed that films enriched in a particular polymorph of 1 displayed near 200% efficiency for triplet formation at 77 K, while films composed primarily of a second polymorph had a very low triplet quantum yield. Present data confirm the triplet yield disparities in the two polymorphs and demonstrate the distinct fates of the initially prepared singlets in films of different structure. Singlet fission is inhibited in the more stable polymorph due to rapid excimer formation and trapping. The less stable polymorph undergoes highly efficient singlet fission with a dominant time constant of 10-30 ps and without strong thermal activation. Transient absorption measurements with varying excitation fluence indicate that singlet-singlet annihilation is the primary competitor of singlet fission, and that fission from higher-lying and vibrationally hot singlet states can also contribute to the triplet formation process.
Kobr, L.; Zhao, K.; Shen, Y.; Shoemaker, R. K.; Rogers, C. T.; Michl, J. "Tris-o-phenylenedioxycyclophosphazene (TPP) Inclusion Compounds Containing A Dipolar Molecular Rotor", Cryst. Growth Des. 2014, 14, 559.
ABSTRACT: A rod-shaped molecular rotor consisting of a p-terphenyl shaft attached to p-carborane whose antipodal position carries a dipolar 2,3-dichlorophenyl rotator forms an inclusion compound with hexagonal tris-o-phenylenedioxycyclotriphosphazene (TPP). Results of solid-state NMR spectroscopy, X-ray powder diffraction, dielectric loss spectroscopy, and density functional theory calculations lead us to propose that the whole molecule inserts into the TPP channels, with the rotator located in the outermost surface layer. Although the placement and alignment of the dipoles at the surface appear favorable, the sample does not exhibit collective behavior even at 7 K, presumably due to the relatively large barrier to rotation (~8.6 kcal/mol). In incompletely annealed samples of the inclusion compound, some of the rotators protrude outside the surface and have a rotational barrier of ~3.4 kcal/mol. In the inclusion compound of an analog in which the rotator is replaced with a methyl group, some of the methyl substituents are located inside the surface layer of TPP and others protrude above it.
Douvris, C.; Michl, J. “Update 1 of: Chemistry of the Carba-closo-Dodecaborate(-) Anion, CB11H12−”, Chem. Rev. 2013, 113, PR179.
Kaleta, J.; Akdag, A.; Crespo, R.; Piqueras, M.-C.; Michl, J. "Evidence for an Intermediate in the Methylation of CB11H12- with MeOTf. Comparison of Electrophilic Substitution in Cage Boranes and in Arenes", ChemPlusChem 2013, 78, 1174.
ABSTRACT: The trideuteriomethylation of BH vertices in CB11H12- and its derivatives with CD3OTf yields a mixture of B-CD3 and B-CHD2 substitution products, demonstrating the intermediacy of a species with a long enough lifetime for hydrogen scrambling between the boron vertex and the methyl substituent. No such scrambling is observed when CD3OTf is used to methylate toluene. According to density functional theory calculations, the intermediate in BH vertex methylation is a three-center bonded σ adduct of a methyl cation to the BH bond and the proton scrambling occurs via a transition structure containing a distorted square pyramidal methane attached axially to a "naked" boron vertex. The subsequent proton or deuteron loss is presently not understood in detail. A general comparison of electrophilic substitution on closo boranes and arenes is provided and similarities as well as differences are discussed. A recalculation of the optimized geometry of the CB11Me12∙ radical produced a second Jahn-Teller distorted minimum and resulted in a somewhat improved agreement between calculated and measured proton hyperfine coupling constants.