Assessing the Structure of Octastate Molecular Switches Using ¹H NMR Density Functional Theory Calculations

Abstract

Density functional theory calculations are used to reveal the relationships between the structures, energies, and NMR signatures of an octastate molecular switch composed of a dithienylethene (DTE) unit covalently linked to an indolino­[2,1-b]­oxazolidine (BOX) moiety through an ethylenic junction. Both the DTE and BOX moieties can adopt open or closed forms. The ethylenic junction can be Z or E, but the latter has been confirmed to be, by far, more stable than the former for all BOX/DTE combinations. In addition, when the DTE is open, the two thienyl units can fold to form parallel conformers, by opposition to the antiparallel or unfolded conformers. Usually parallel conformers present a higher energy than the antiparallel ones, but in the case of compound 2 having a bulky substituent (R = pPh-SMe) on the terminal thienyl group, the enthalpy of one conformer is very close (1–2 kJ mol–1) to that of the most stable antiparallel one, making photocyclization less efficient. These conformational differences and the presence of parallel DTE forms have been substantiated by analyzing experimental 1H NMR chemical shifts in light of their calculated values. These 1H NMR chemical shift calculations led to the following statements: (i) Going from state I (DTE open, BOX closed) to state II (both DTE and BOX are open) the H8 proton of compound 1 (R = Me) is deshielded by ∼0.15 ppm. (ii) The deshielding of H8 proton of compound 2 is larger and attains 0.41 ppm whereas H7 is more shielded by 0.11 ppm. (iii) Then, going from compound 1 to compound 2 leads to deshielding of both H7 and H8 protons. As a consequence, the difference of photochromism gating efficiency among compounds 1, 2, and 3 (R = pPh-OMe) can be attributed to the stabilization of parallel conformer due to an establishment of an intramolecular interaction with BOX opening.

Binding structure, breaking forces and conductance of Au-Octanedithiol-Au molecular junction under stretching processes: a DFT-NEGF study

“…The molecular junction with one or several molecules adsorbed between two electrodes has been widely studied both experimentally and theoretically in the past several decades [1][2][3][4][5][6]. Different electric functions such as rectification effect [7], switching effect [8,9] and negative differential resistance effect [9] are closely associated with electrode materials, chemical properties of molecular wires, and the binding interaction between electrodes and molecular anchors. Experimentally, the most popular characterization techniques, such as scanning tunneling microscopy (STM), atomic force microscopy (AFM), and mechanically controllable break junctions, are widely used [10][11][12][13] were thought to provide the high spatial resolution, it is not easy to get atomic precise resolution when the molecular wire is measured in dynamic states.…”