Electric-field induced second harmonic generation responses of push-pull polyenic dyes: experimental and theoretical characterizations

Solvent effects on the second harmonic responses of donor-acceptor Stenhouse adducts: from implicit to hybrid solvation models

The effect of conformational dynamics and solvent interactions on the second-order nonlinear optical (NLO) responses of the open and closed forms of a donor-acceptor Stenhouse adduct (DASA) are investigated by a mixed quantum/classical computational approach, which couples molecular dynamics (MD) simulations and time-dependent density functional theory (TD-DFT) calculations. The latter are further combined with various solvation schemes, including polarizable continuum models, hybrid QM/MM approaches using either non polarizable or polarizable electrostatic embedding, and QM/QM' schemes with explicit treatment of a few molecules of the first solvation shell. The performances of the different solvation models are discussed in the context of comparisons with experimental data obtained from hyper-Rayleigh scattering measurements.

Merocyanines: Electronic Structure and Spectroscopy in Solutions, Solid State, and Gas Phase

Merocyanines, owing to their readily tunable electronic structure, are arguably the most versatile functional dyes, with ample opportunities for tailored design via variations of both the donor/acceptor (D/A) end groups and π-conjugated polymethine chain. A plethora of spectral properties, such as strong solvatochromism, high polarizability and hyperpolarizabilities, and sensitizing capacity, motivates extensive studies for their applications in light-converting materials for optoelectronics, nonlinear optics, optical storage, fluorescent probes, etc. Evidently, an understanding of the intrinsic structure-property relationships is a prerequisite for the successful design of functional dyes. For merocyanines, these regularities have been explored for over 70 years, but only in the past three decades have these studies expanded beyond the theory of their color and solvatochromism toward their electronic structure in the ground and excited states. This Review outlines the fundamental principles, essential for comprehension of the variable nature of merocyanines, with the main emphasis on understanding the impact of internal (chemical structure) and external (intermolecular interactions) factors on the electronic symmetry of the D-π-A chromophore. The research on the structure and properties of merocyanines in different media is reviewed in the context of interplay of the three virtual states: nonpolar polyene, ideal polymethine, and zwitterionic polyene.

Calculation of the geometry, absorption spectrum, and first hyperpolarizability of 4,5-dicyanoimidazole derivatives in solution. A multiscale ASEC-FEG study

We investigate the effects of solvents on the geometry, absorption spectrum, and first hyperpolarizability of six push-pull molecules, each containing a 4,5-dicyanoimidazole group as an electron acceptor and a N,N-dimethylamino group as an electron donor, with systematically extended π-conjugated systems. Geometry optimizations in dichloromethane, methanol, water, and formamide under normal thermodynamic conditions were performed using the average solvent electrostatic configuration-free energy gradient method, which employs a discrete solvent model. The conformational structure of molecules is moderately affected by the environment, with the π-conjugated system becoming more planar in protic solvents. Solvent effects on the first hyperpolarizability result in marked increases that are in line with the red shifts of the absorption spectrum. The hyperpolarizability of smaller molecules within the set may be significantly influenced by the effects of geometry relaxation in highly polar protic solvents. The results illustrate the role of hydrogen bonds in the structure and electronic properties of push-pull molecules in protic environments. For smaller molecules, hydrogen bonds significantly contribute to enhancing the hyperpolarizability, but the effect of these specific interactions becomes less significant with the length of the π-conjugated system.

Dynamic effects on the nonlinear optical properties of donor acceptor stenhouse adducts: insights from combined MD + QM simulations

The second-order nonlinear optical (NLO) responses of a donor-acceptor stenhouse adduct (DASA) are investigated by using a computational approach combining molecular dynamics simulations and density functional theory (DFT) calculations. Specific force fields for the open and closed photoswitching forms are first parameterized and validated according to the Joyce protocol, in order to finely reproduce the geometrical features and potential energy surfaces of both isomers in chloroform solution. Then, DFT calculations are performed on structural snapshots extracted at regular time steps of the MD trajectories to address the influence of the thermalized conformational dynamics on the NLO responses related to hyper-Rayleigh scattering (HRS) experiments. We show that accounting for the structural dynamics largely enhances the HRS hyperpolarizability (βHRS) compared to DFT calculations considering solely equilibrium geometries, and greatly improves the agreement with experimental measurements. Furthermore, we show that the NLO responses of the NLO-active open form are correlated with the bond order alternation along the triene bridge connecting the donor and acceptor moieties, which is rationalized using simple essential state models.

High-throughput virtual screening of organic second-order nonlinear optical chromophores within the donor-π-bridge-acceptor framework

In view of the theoretical importance and huge application potential of second-order nonlinear optical (NLO) materials, it is of great significance to conduct high-throughput virtual screening (HTVS) on a compound library to find candidate NLO chromophores. Under the donor-π-bridge-acceptor structural framework, a virtual compound library (size = 27 090) was constructed by enumeration of structural fragments. The kernel property adopted for optimization is the static first hyperpolarizability (β0). By combining machine learning and quantum chemical calculations, we have performed an HTVS procedure to sieve NLO chromophores out, and the response mechanism of the selected optimal NLO chromophores was examined. We have found: (a) The multi-layer perceptron/extended connectivity fingerprint combination with 20% selection ratio gives the highest prediction accuracy for the studied systems. (b) The two optimal donors are bis(4-diphenylaminophenyl)aminyl and bis(4-tert-butylphenyl)aminyl; the optimal π-bridges are composed of two thiophenyl, selenophenyl or furanyl units; and the two optimal acceptors are tri-s-triazinyl and 2,3-dicyanopyrazinyl. (c) The no. 1 candidate molecule can exhibit a calculated β0 equal to 8.55 × 104 a.u. (d) The difference in NLO responses of the optimal 16 molecules comes from the synergistic interaction of ES1, Δμ and f, by employing the two-level model. In addition, the sizable Δμ and f allow the studied optimal molecules to obtain a large NLO response in the meantime keeping a not-too-low excitation energy (retaining good optical transparency in the restricted range of the visible spectrum region). (e) With further modification on the acceptor, the designed DPA-π-TRZ-A' (A' = CN or NO2, π = oligo-thiophenyl or selenophenyl) systems can exhibit a rather large NLO response (maximum β0 = 3.17 × 105 a.u.), hence should have considerable potential as second-order NLO chromophores. With the above observations, we expect to provide some insight for the research community into the HTVS of organic second-order NLO chromophores.