The effect of non-covalent conformational locks on intra-molecular charge transport of OPV units

Impact of the chemical insertion of the dimethylamino group on the electronic and optical properties of the 4-(methoxyphenyl acetonitrile) monomer (MPA): a DFT theoretical investigation

CONTEXT

Density functional theory (DFT) calculations on the ground and the first excited state are performed on the modified and unmodified 4-(methoxyphenyl acetonitrile) monomer (referred to as MPA). The modified monomer named MFA is obtained by Knoevenagel condensation of MPA with dimethylformamide dimethyl acetal (DMF-DMA). DFT computations show that the chemical grafting of the dimethylamino group onto the MPA unit induces a great change in the geometric, electronic, and optical properties. Going from MPA to MFA monomer, a great change in the frontier orbitals of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the ground and the first excited state is observed. Consequently, a reduction in the energy gap HOMO-LUMO and an enhancement in the absorption and emission properties are observed under the chemical modification. The observed modifications in the electronics and optical properties are the result of the charge transfer appearing between the cyano (C≡N) acceptor group and the dimethylamino (DMF-DMA)-grafted group donor ring.

METHODS

Quantum chemical calculations were performed in the ground and the first excited state using the density functional theory (DFT), and it extends the time-dependent density functional theory (TD-DFT), implemented in the Gaussian 09 software package. The ground state is obtained by optimization of the studied molecular geometries by employing the DFT/M062X/6-31G(d,p) level of theory. The first excited state is obtained by re-optimization of the ground state geometries using the TD-DFT/M062X/6-31G(d,p) level of theory. The contour plots of the frontier orbitals and the molecular electrostatic potential (MEP) maps are obtained from the ground and the first excited state, optimized geometries, and drawn using Gaussview software.

Effect of S⋯π interactions on the charge transport properties of the DPP framework

In this work, we designed and synthesized two similar π-conjugated molecules, N-alkyl (DPP-R) and N-aryl (DPP-B), to comparatively explore the S⋯π interactions using a scanning tunneling microscopy-based break junction (STM-BJ) technique. The conductance results of the STM-BJ experiments indicated that DPP-R has a 66% greater conductance (G) than DPP-B. Combined with molecular simulations, it was demonstrated that the presence of S⋯π interactions led to a certain degree of orbital overlap of the highest occupied molecular orbital (HOMO), and created a favorable channel for electron transport in the DPP-B junction.