Dihydro-1H-Pyrazoles as Donor Blocks in Donor–Acceptor Chromophores for Electro-Optics: A DFT Study of Hyperpolaizability and Electronic Excitations

A diverse set of promising donors for donor–acceptor chromophores based on dihydro-1H-pyrazole (pyrazoline) for use in electro-optics was investigated using DFT at M06-2X/aug-cc-pVDZ level of theory. These calculations showed that it is possible to achieve a molecular hyperpolarizability of up to 1700*10⁻³⁰ esu (up to three times higher compared to conventional diethylaniline donors) for a simple tricyanofuran-based acceptor by carefully tuning the donor structure. It was shown that the molecular hyperpolarizability is mainly affected by the substituents in the aryl rings in positions 3 and 1 of the pyrazoline cycle, while the substituents of the aryl ring in position 5 and the pyrazoline ring itself can be varied without significant effects on the hyperpolarizability. For one of the compounds, a detailed study of the lowest energy electronic excitation was performed using the TD-DFT, confirming the role of the pyrazoline ring as a secondary donor.