Theoretical Insights into the Structural and Optical Properties of D-π-A-based Cyanostilbene Systems of α and β Variants

The π-conjugated organic molecules containing cyanostilbene motifs have been extensively investigated due to their great potential applications in several optoelectronic and biological fields. Developing efficient molecules in this respect requires strategic structural engineering and a deep understanding of the structure-property relationship at the molecular level. In this context, understanding the impact of positional isomerism in cyanostilbene systems is a fundamental aspect of designing desired materials with improved photophysical properties. Herein, we designed ten donor-π-acceptor (D-π-A) type cyanostilbene derivatives (P1 - P10) with different π linkers and compared their structural and optoelectronic properties arising from the positional variations of the -CN group (α and β- variations) through the utilization of density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The topological analyses of the electron density are used to explain the relatively high stability of α isomer compared to that of β. Frontier molecular orbital analysis reveals that 17 molecules tend to show a reduced highest occupied molecular orbital-lowest unoccupied molecular orbital gap, and most of them showed a greater nonlinear optical (NLO) character compared to the parent molecule. TDDFT calculations indicate that β isomers show higher absorption maxima compared to their α counterparts. Among all the scrutinized molecules, the absorption maximum extended up to 602 nm for P9 and it possesses the highest first-order hyperpolarizability. This study sheds light on positional isomers and their reactivity, absorption spectra, and NLO properties of D-π-A type architecture that can be suitably tuned by appropriating the π-bridge for practical applications.

A theoretical study on the second-order nonlinear optical properties of Pt(II) bis-acetylide complexes: substituent and redox effects

Density functional theory studies on the geometric and electronic structures, UV-vis absorption spectra, and second-order nonlinear optical (NLO) properties of four-coordinate Pt(II) bis-acetylide complexes, cis-[Pt(CNtBu)(ADC)(CCR)2] , have been employed. The effects of ligand variation and the single electron redox process on the structures and NLO response of complexes have also been investigated. It shows that the variations of the ligand and electron have little effect on the geometries of the complexes, but there is a significant effect on their electronic structures and NLO responses. The introduction of a single -NO2 group in acetylide ligands increases the first hyperpolarizability of complex 12 times, while one electron lost in five complexes enhances the first hyperpolarizability 496 times at the most. Both methods are considered effective ways for improving the NLO response of Pt(II) bis-acetylide complexes. Based on the analysis of the electronic and optical properties of fifteen studied complexes, the increase of NLO response is mainly ascribed to strong oscillator strengths, lower electron transition energy, and well-directed effective charge transfer. This work reveals some underlying relationships between the NLO responses and electronic structures of complexes, which is helpful for the design and synthesis of high-performance NLO materials of Pt(II) bis-acetylide complexes.

Substituent and redox effects on the second-order NLO response of Ru(ii) complexes with polypyridine ligands: a theoretical study

Modifying ligands in four studied Ru(II) complexes have slight influence on their static first hyperpolarizabilities (βtot) values. However, the βtot values of complexes 1+/−–4+/− are all enhanced and the largest βtot value is observed for complex 4−.