Through-space charge transfer enabled design of nonlinear optical chromophores with balanced absorption and...

The design of high-performance nonlinear optical (NLO) materials often faces a fundamental trade-off between enhancing the charge transfer and controlling the red shift of the optical absorption. In this study, we employed density functional theory (DFT) and time-dependent DFT (TD-DFT) to investigate a molecular design strategy that introduces a through-space charge transfer (TSCT) pathway in addition to conventional through-bond charge transfer (TBCT). Using a π-conjugated donor-π-acceptor (D-π-A) framework, we demonstrate that the incorporation of TSCT not only intensifies the intramolecular charge transfer (ICT), which is crucial for boosting the NLO response, but also suppresses further red-shifting of the absorption edge, which is typically accompanied by extended conjugation in high-performance NLO chromophores. Our findings reveal that the cofacial arrangement of the donor and acceptor units via a π-bridge plays a crucial role in modulating the excitation characteristics without compromising the NLO efficiency. This approach offers a promising computational design strategy for next-generation NLO materials, where the simultaneous optimisation of spectral transparency and NLO efficiency is required.