New push–pull ladder-type chromophores with large hyperpolarizability for nonlinear optical application

A novel series of ladder-type chromophores TA0-5 with strong donor and acceptor were designed and theoretically investigated for applications in nonlinear optics. Chromophores TA0-5 possess ladder-type bridges with different fused-ring structures. The influence of varying structures of ladder-type bridges and introduction of strong donor and acceptor on the first hyperpolarizabilities in this innovative push–pull fused system were analyzed by multiple methods, such as (hyper)polarizability density, coupled perturbed Kohn–Sham (CPKS) method, sum-over-states (SOS) model and two states model. Through density of states analysis, TA0–TA5 all show deep HOMO levels and low energy gaps, indicating the excellent air-stable property of them. Through tuning the number of thiophene rings in the bridge and introduction of strong push–pull architecture, the first hyperpolarizabilities of new chromophores are significantly enhanced. Importantly, TA5 exhibits remarkable first hyperpolarizability ( β_tot value is 5882.1 × 10⁻³⁰ esu) in AN. The two-dimensional second-order nonlinear optical (NLO) spectra predicted by the SOS model show the strong NLO responses under external fields. All the designed chromophores exhibit outstanding electro-optical Pockels and optical rectification effects. The results reveal that this new series of push–pull ladder-type chromophores can be high-performance NLO materials and are expected to be prevalent for NLO and optoelectronic application.