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In the past, the design of light-driven single molecule rotary motors has been mainly guided by the modification of their ground-state conformational properties. Further progress in this field is thus likely to be achieved through a detailed understanding of light-induced dynamics of the system and the ways of modulating it by introducing chemical modifications. In the present theoretical work, the analysis of model organic chromophores and synthesized rotary motors is used for rationalizing the effect of electron-withdrawing heteroatoms (such as a cationic nitrogen) on the topography and branching plane of mechanistically relevant conical intersections. Such an analysis reveals how the character of rotary motion could be changed from a precessional motion to an axial rotational motion. These concepts are then used to design and build quantum chemical models of three distinct types of Schiff base rotary motors. One of these models, featuring the synthetically viable indanylidenepyrroline framework, has conical intersection structures consistent with an axial rotation not hindered by ground-state conformational barriers. It is expected that this type of motor should be capable of funneling the photon energy into specific rotary modes, thus achieving photoisomerization quantum efficiencies comparable to those seen in visual pigments. [ABSTRACT FROM AUTHOR]

1,3-Benzenediamidoethanethiolatemercury [BDT-Hg or BD(S)-Hg] and its derivatives are investigated utilizing the Dirac exact relativistic normalized elimination of the small component method in connection with B3LYP, CCSD(T), and polarizable continuum calculations. It is shown that die chelating energy of BDT-Hg can be significantly increased by replacing sulfur with selenium or tellurium, thus leading to BD(Se)-Hg or BD(Te)-Hg. In this particular case, the chalcogenophilicity of mercury increases from S to Te because increasing the E--Hg bond lengths leads to a reduction of ring strain. Various possibilities of increasing die metal (M) chelating strength in BDT-M complexes are investigated, and suggestions for new chelating agents based on the BDT-M template are made. [ABSTRACT FROM AUTHOR]

A quantum theory of atoms in molecules (QTAIM) and stress tensor analysis was applied to analyze intramolecular interactions influencing the photoisomerization dynamics of a light-driven rotary molecular motor. For selected nonadiabatic molecular dynamics trajectories characterized by markedly different S1 state lifetimes, the electron densities were obtained using the ensemble density functional theory method. The analysis revealed that torsional motion of the molecular motor blades from the Franck-Condon point to the S1 energy minimum and the S1/S0 conical intersection is controlled by two factors: greater numbers of intramolecular bonds before the hop-time and unusually strongly coupled bonds between the atoms of the rotor and the stator blades. This results in the effective stalling of the progress along the torsional path for an extended period of time. This finding suggests a possibility of chemical tuning of the speed of photoisomerization of molecular motors and related molecular switches by reshaping their molecular backbones to decrease or increase the degree of coupling and numbers of intramolecular bond critical points as revealed by the QTAIM/stress tensor analysis of the electron density. Additionally, the stress tensor scalar and vector analysis was found to provide new methods to follow the trajectories, and from this, new insight was gained into the behavior of the S1 state in the vicinity of the conical intersection. [ABSTRACT FROM AUTHOR]

Various types of interactions between halogen (X) and π moiety (X−π interaction) including halogen bonding play important roles in forming the structures of biological, supramolecular, and nanomaterial systems containing halogens and aromatic rings. Furthermore, halogen molecules such as X2 and CX4 (X = Cl/Br) can be intercalated in graphite and bilayer graphene for doping and graphene functionalization/modification. Due to the X−π interactions, though recently highly studied, their structures are still hardly predictable. Here, using the coupled-cluster with single, double, and noniterative triple excitations (CCSD(T)), the Møller-Plesset second-order perturbation theory (MP2), and various flavors of density functional theory (DFT) methods, we study complexes of benzene (Bz) with halogen-containing molecules X2 and CX4 (X = Cl/Br) and analyze various components of the interaction energy using symmetry adapted perturbation theory (SAPT). As for the lowest energy conformers (S1), X2-Bz is found to have the T-shaped structure where the electropositive X atom-end of X2 is pointing to the electronegative midpoint of CC bond of the Bz ring, and CX4-Bz has the stacked structure. In addition to this CX4-Bz (S1), other low energy conformers of X2-Bz (S2/S3) and CX4-Bz (S2) are stabilized primarily by the dispersion interaction, whereas the electrostatic interaction is substantial. Most of the density functionals show noticeable deviations from the CCSD(T) complete basis set (CBS) limit binding energies, especially in the case of strongly halogen-bonded conformers of X2-Bz (S1), whereas the deviations are relatively small for CX4-Bz where the dispersion is more important. The halogen bond shows highly anisotropic electron density around halogen atoms and the DFT results are very sensitive to basis set. The unsatisfactory performance of many density functionals could be mainly due to less accurate exchange. This is evidenced from the good performance by the dispersion corrected hybrid and double hybrid functionals. B2GP-PLYP-D3 and PBE0-TS(Tkatchenko-Scheffler)/D3 are well suited to describe the X−π interactions adequately, close to the CCSD(T)/CBS binding energies (within ∼1 kJ/mol). This understanding would be useful to study diverse X−π interaction driven structures such as halogen containing compounds intercalated between 2-dimensional layers. [ABSTRACT FROM AUTHOR]

While rotary molecular switches based on neutral and cationic organic -systems have been reported, structurally homologous anionic switches providing complementary properties have not been prepared so far. Flere we report the design and preparation of a molecular switch mimicking the anionic p-HBDI chromophore of the green fluorescent protein. The investigation of the mechanism and dynamics of the E/Z switching function is carried out both computationally and experimentally. The data consistently support axial rotary motion occurring on a sub-picosecond time scale. Transient spectroscopy and trajectory simulations show that the nonadiabatic decay process occurs in the vicinity of a conical intersection (CInt) between a charge transfer state and a covalent/diradical state. Comparison of our anionic p-HBDI-like switch with the previously reported cationic N-alkyl indanylidene pyrrolinium switch mimicking visual pigments reveals that these similar systems translocate, upon vertical excitation, a similar net charge in the same axial direction. [ABSTRACT FROM AUTHOR]