Effects of n → π* Orbital Interactions on Molecular Rotors: The Control and Switching of Rotational Pathway and Speed

Abstract

The role of n → π* orbital interactions in the rotational pathway and barrier of biaryl-based molecular rotors was elucidated through a combined experimental and computational study. The n → π* interaction in the transition state can lead to the acceleration of rotors. The competition between the n → π* interaction and hydrogen bonding further enabled the reversal of the pathway and greasing/braking the rotor in response to acid/base stimuli, thereby creating a switchable molecular rotor.

Analysis of the Orbital and Electrostatic Contributions to the Lone Pair–Aromatic Interaction Using Molecular Rotors

“…This study was enabled by recent examples that successfully employed molecular rotors to measure the TS stabilizing effects of noncovalent interactions such as hydrogen bonding, 14 carbonyl−carbonyl, 12 and ether−carbonyl interactions. 15 Thus, N-phenylimide molecular rotor 1 was designed (Figure 2), which formed a CO•••Ar interaction in the planar bond rotation TS. To assess the electrostatic and orbital delocalization components of the interaction, rotors 1(Ph), 1(Ph-F), 1(Ph-F 3 ), 1(Ph-F 5 ), 1(Ph-NO 2 ), and 1(Ph-(NO 2 )) 2 were prepared 16 with varying numbers of electronwithdrawing fluorine and nitro groups on the phenyl units.…”

“…This study was enabled by recent examples that successfully employed molecular rotors to measure the TS stabilizing effects of noncovalent interactions such as hydrogen bonding, carbonyl–carbonyl, and ether–carbonyl interactions . Thus, N -phenylimide molecular rotor 1 was designed (Figure ), which formed a CO···Ar interaction in the planar bond rotation TS.…”

Chemically Driven Rotatory Molecular Machines

“…[78] This dynamic behavior is attributed to the pyramidalization of the nitrogen center involved in the restricted bond rotation. Upon protonation, the conversion from sp 2 to sp 3 decreases the steric demands around the [76] Scheme 17. An acid-accelerated molecular rotor.…”

Electrostatically-gated molecular rotors

“…More rare were stimuli that increased or accelerated the rate of rotations. 15,19,20,[27][28][29] In this work, we demonstrate the use of attractive electrostatic interactions to stabilize transition states, lower barriers, and increase rates of molecular scale motion. The control of molecular-scale motion using electrostatic interactions has the potential of being integrated with existing micro-and nanoscale devices such as memory and transistors that are electrically addressed and controlled.…”

“…For example, stimuli used to control the rates of rotation of molecular rotors have included: light, [6][7][8][9][10][11] metal ions, [12][13][14] hydrogen bonds, [15][16][17] redox, [18][19][20] anions, 21 guests, 16,22,23 and protons. [24][25][26][27][28][29] The majority of these systems were molecular brakes where the stimuli slowed the rate of rotation. More rare were stimuli that increased or accelerated the rate of rotations.…”