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Complete publications list

Selected recent publications:

Buchanan, E. A.; Johnson, J. C.; Tan, M.; Kaleta, J.; Shtukenberg, A. G.; Bateman, G.; Benedict, J. B.; Kobayashi, S.; Wen, J.; Kahr, B.; Císařová, I.; Michl, J. “Competing Singlet Fission and Excimer Formation in Solid Fluorinated 1,3-Diphenylisobenzofurans”, J. Phys. Chem. C2021, 125, 27058.

Abstract: Singlet fission (k_SF) and excimer formation (k_EXC) rate constants along with other photophysical properties of thin solid layers of 1,3-diphenylisobenzofuran and eleven of its fluorinated derivatives have been determined.  The molecular properties of these compounds are similar but their crystal packing varies widely.  Most of them undergo singlet fission whereas excitation in others is trapped in excimers.  The trend in rate constants k_SF agrees qualitatively with results of calculations by a simplified version of the frontier orbital model for a molecular pair and its main shortcoming is discussed.

Jovanovic, M.; Michl, J. “Alkanes versus Oligosilanes: Conformational Effects on σ-Electron Delocalization”, J. Am. Chem. Soc.2022, 144, 463.

Abstract: Observations and computations both suggest that the extent and the conformational dependence of σ-electron delocalization in frontier molecular orbitals are quite different in alkanes C_nH_2n+2 and oligosilanes Si_nH_2n+2, the isosteric and isoelectronic saturated chains built from carbon or silicon atoms, respectively.  We find that the different conformational effects can be understood in simple intuitive terms.  There are two modes of σ-electron delocalization, strongly conformation-sensitive skeletal delocalization through backbone X-X bonds (σ-conjugation and σ-hyperconjugation) and only weakly conformation-sensitive lateral delocalization through lateral X-H bonds (σ-hyperconjugation and σ-homoconjugation). In alkanes, both modes are active and  complement each other, leading to delocalization in all conformations.  In oligosilanes, only skeletal delocalization of holes is important in frontier orbitals, and the even simpler Ladder C model provides an adequate intuitive description of the strong conformational dependence of σ-electron delocalization. Ultimately, the difference is due primarily to the similar electronegativity of carbon and hydrogen as opposed to the lower electronegativity of silicon, which causes a polarization of Si-H bonds.  This understanding has been derived from an analysis of approximate  algebraic solutions of a simple Hückel-level extended Ladder H model for an infinite regular helical chain, using the effective mass of a hole as a measure of delocalization.  This model is derived from the classical Sandorfy H model, and is parameterized by fitting to results of density functional or Hartree-Fock theory.

Rais, D.; Toman, P.; Pfleger, J.; Acharya, U.; Panthi, Y. R.; Menšík, M.; Zhigunov, A.; Thottappali, M. A.; Vala, M.; Marková, A.; Stříteský, S.; Weiter, M.; Cigánek, M; Krajčovič, J.; Pauk, K.; Imramovský, A.; Zaykov, A.; Michl, J. “Singlet Fission in Thin Solid Films of Bis(thienyl)diketopyrrolopyrroles”, ChemPlusChem2020, 85, 2689.

Abstract: Oxidative coupling of a Pt porphyrin Langmuir-Blodgett monolayer with K₂IrCl₆ contained in the aqueous subphase, with simultaneous leaching of the Pt⁺² cations, yields a single layer of porphene (C₂₀N₄H₂)_∞, a periodic fully conjugated analog of graphene that is composed of fused porphyrin rings.  Its structure has been determined by in situ grazing incidence X-ray diffraction (GIXD), X-ray reflection (XR), and Langmuir isotherm mean molecular area.  After transfer to various solid substrates, porphene bilayer forms and has been examined by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning tunneling microscopy (STM), and transmission electron microscopy (TEM).

Rončević, I.; Bastien, G.; Cvačka, J.; Kaleta, J.; Michl, J. “CB₁₁H₁₀⁻ and Related Carborenes,” Inorg. Chem.2020, 59, 12453.

Abstract: Starting with HOOC-CB₁₁Me₁₁⁻ or CB₁₁Me₁₂⁻, collision-induced dissociation has produced a series of methylated analogs of didehydro-closo-carbadodecaborane anions by sequential losses of up to five equivalents of  ethylene.  These reactive intermediates are carborane analogs of doubly pyramidalized alkenes and, more distantly, arynes.  Density functional theory calculations have been used to develop a proposal for the mechanism of the unusual formation of ethylene from the carborane methyl substituents.

Buchanan, E. A.; Michl, J. “Mechanical vs. Electronic Strain: Calculated Configurations of Alkynyl-Pt(II)-Phosphine Macrocycles”, Organometallics2020, 39, 1195.

Abstract: A survey is provided of the current status of singlet fission as a tool for bypassing the Shockley-Queisser limit on the efficiency of single-junction solar cells.