Changfeng Fang

How method-dependent are calculated differences between vertical, adiabatic, and 0-0 excitation energies?

Through a large number of benchmark studies, the performance of different quantum chemical methods in calculating vertical excitation energies is today quite well established. Furthermore, these efforts have in recent years been complemented by a few benchmarks focusing instead on adiabatic excitation energies. However, it is much less well established how calculated differences between vertical, adiabatic and 0-0 excitation energies vary between methods, which may be due to the cost of evaluating zero-point vibrational energy corrections for excited states. To fill this gap, we have calculated vertical, adiabatic, and 0-0 excitation energies for a benchmark set of molecules covering both organic and inorganic systems. Considering in total 96 excited states and using both TD-DFT with a variety of exchange-correlation functionals and the ab initio CIS and CC2 methods, it is found that while the vertical excitation energies obtained with the various methods show an average (over the 96 states) standard deviation of 0.39 eV, the corresponding standard deviations for the differences between vertical, adiabatic, and 0-0 excitation energies are much smaller: 0.10 (difference between adiabatic and vertical) and 0.02 eV (difference between 0-0 and adiabatic). These results provide a quantitative measure showing that the calculation of such quantities in photochemical modeling is well amenable to low-level methods. In addition, we also report on how these energy differences vary between chemical systems and assess the performance of TD-DFT, CIS, and CC2 in reproducing experimental 0-0 excitation energies.

Computational design of faster rotating second-generation light-driven molecular motors by control of steric effects

We report a systematic computational investigation of the possibility to accelerate the rate-limiting thermal isomerizations of the rotary cycles of synthetic light-driven overcrowded alkene-based molecular motors through modulation of steric interactions. Choosing as a reference system a second-generation motor known to accomplish rotary motion in the MHz regime and using density functional theory methods, we propose a three-step mechanism for the thermal isomerizations of this motor and show that variation of the steric bulkiness of the substituent at the stereocenter can reduce the (already small) free-energy barrier of the rate-determining step by a further 15-17 kJ mol(-1). This finding holds promise for future motors of this kind to reach beyond the MHz regime. Furthermore, we demonstrate and explain why one particular step is kinetically favored by decreasing and another step is kinetically favored by increasing the steric bulkiness of this substituent, and identify a possible back reaction capable of impeding the rotary rate.

Computational study of the working mechanism and rate acceleration of overcrowded alkene-based light-driven rotary molecular motors

In recent years, much progress has been made in the design, synthesis and operation of light-driven rotary molecular motors based on chiral overcrowded alkenes. Through consecutive cis–trans photoisomerization and thermal helix inversion steps, where the latter dictate the overall rate of rotation, these motors achieve a full 360° unidirectional rotation around the carbon–carbon double bond connecting the two (rotator and stator) alkene halves. In this work, we report quantum chemical calculations indicating that a particularly fast-rotating overcrowded alkene-based motor capable of reaching the MHz regime, can be made to rotate even faster by the substitution of a rotator methyl group with a methoxy group. Specifically, using density functional theory methods that reproduce the rate-limiting ∼35 kJ mol−1 thermal free-energy barriers shown by the methyl-bearing motor with errors of ∼5 kJ mol−1 only, it is predicted that this substitution reduces these barriers by a significant 15–20 kJ mol−1. This prediction is preceded by a series of benchmark calculations for assessing how well density functional theory methods account for available experimental data (crystallographic, UV-vis absorption, thermodynamic) on the rotary cycles of overcrowded alkenes, and a detailed examination of the thermal and photochemical reaction mechanisms of the original motor of this type.

Assessment of a composite CC2/DFT procedure for calculating 0-0 excitation energies of organic molecules

ABSTRACT The task to assess the performance of quantum chemical methods in describing electronically excited states has in recent years started to shift from calculation of vertical (ΔEve) to calculation of 0–0 excitation energies (ΔE00). Here, based on a set of 66 excited states of organic molecules for which high-resolution experimental ΔE00 energies are available and for which the approximate coupled-cluster singles and doubles (CC2) method performs particularly well, we explore the possibility to simplify the calculation of CC2-quality ΔE00 energies using composite procedures that partly replace CC2 with more economical methods. Specifically, we consider procedures that employ CC2 only for the ΔEve part and density functional theory methods for the cumbersome excited-state geometry optimisations and frequency calculations required to obtain ΔE00 energies from ΔEve ones. The results demonstrate that it is indeed possible to both closely (to within 0.06–0.08 eV) and consistently approximate ‘true’ CC2 ΔE00 energies in this way, especially when CC2 is combined with hybrid density functionals. Overall, the study highlights the unexploited potential of composite procedures, which hitherto have found widespread use mostly in ground-state chemistry, to also play an important role in facilitating accurate studies of excited states.

Enhanced rectifying performance by asymmetrical gate voltage for BDC20 molecular devices

By applying the asymmetrical gate voltage on the 1,4-bis (fullero[c]pyrrolidin-1-yl) benzene BDC20 molecule, we investigate theoretically its electronic transport properties using the density functional theory and nonequilibrium Greens function formalism for a unimolecule device with metal electrodes. Interestingly, the rectifying characteristic with very high rectification ratio, 91.7 and 24.0, can be obtained when the gate voltage is asymmetrically applied on the BDC20 molecular device. The rectification direction can be tuned by the different gate voltage applying regions. The rectification behavior is understood in terms of the evolution of the transmission spectrum and projected density of states spectrum with applied bias combined with molecular projected self-consistent Hamiltonian states analyses. Our finding implies that to realize and greatly promote rectifying performance of the BDC20 molecule the variable gate voltage applying position might be a key issue.

Rectifying behaviors of an Au/(C20)2/Au molecular device induced by the different positions of gate voltage

The electronic transport properties of a gated Au/(C20)2/Au molecular device are studied using nonequilibrium Greens function in combination with density functional theory. The results show that different applied positions of the external transverse gate voltage can effectively tune the current–voltage (I–V) characteristic of molecular devices. Rectifying behaviors of the device can be realized when the gate voltage is applied asymmetrically on the left C20 molecule, and the rectification directions can also be modulated by the positive or negative value of the gate voltage. These results provide an important theoretical support to experiments and the design of a molecular rectifier.

Edge hydrogenation-induced spin-filtering and negative differential resistance effects in zigzag silicene nanoribbons with line defects

We investigate the effects of line defects (558-defect and 57-defect) and edge hydrogenation (mono-hydrogenation and di-hydrogenation) on magnetism and spin transport of zigzag silicene nanoribbons (ZSiNRs) by first-principles calculations. The line defects and edge hydrogenation are able to tune the edge and interedge spin polarization in the defective ZSiNRs. The ZSiNRs can be formed into antiferromagnetic (AFM)-metals, ferromagnetic (FM)-metals, AFM and FM semiconductors through modulating the line defects and edge hydrogenation. Moreover, the perfect spin-filtering effect (SFE) with 100% spin polarization and negative differential resistance (NDR) effect can be achieved by di-hydrogenation in our proposed devices. Our findings demonstrate that the ZSiNRs with diverse line defects and edge hydrogenation are promising materials for spintronic applications.

Vacancy-induced spin polarization in graphene and B–N nanoribbon heterojunctions

By using nonequilibrium Greens functions (NEGF) and density functional theory (DFT), we investigate the spin-separated electronic transport properties in heterojunctions constructed by zigzag graphene and boron nitride nanoribbons. The results show that the heterojunctions exhibit a strong spin polarization and ferromagnetic state when there is a vacant position in the boron nitride nanoribbons (BNNRs). The spin-filter effect can be significantly tuned and improved by the species of the nitrogen and boron vacancy and the location of the vacancy in the boron nitride nanoribbons with the spin-filter efficiency (SFE) up to nearly 100%. The spin negative differential resistance (SNDR) properties at low bias can also be found in the proposed molecular spin devices. Mechanisms for the results are suggested, and these findings open up new possibilities for developing nano-spintronic devices.

Design of boron vacancy enhanced spin filtering graphene/BN zigzag nanoribbon heterojunctions

The spin-polarized electronic transport properties of zigzag graphene nanoribbons (ZGNRs) and boron nitride nanoribbons (ZBNNRs) heterojunctions with a boron vacancy are investigated by using non-equilibrium Greens function and density functional theory, especially under an external electric field. The model we used in this paper is chosen from the last essay we researched, the I–V curves in the ferromagnetic states for (ZBNNR)5–(ZGNR)3–Bv devices are investigated, and the results show that with an external electric field, the heterojunctions are promising multifunctional devices in molecular spintronics due to their nearly perfect spin-filter effect, high rectification ratio and spin negative differential resistance properties at low biases. Mechanisms for such phenomena are proposed and these findings suggest a new opportunity for developing molecular spintronic devices.