Modification of the Second Harmonic Generation and Fluorescence Efficiency of D289 Dye Based on a Donor–Acceptor Structure

Second harmonic generation (SHG) spectroscopy and microscopy have been widely applied in studying the interfacial processes that occur in chemical, physical, and biological systems. Efficient SHG probes are crucial for these investigations. Herein, we designed two water-soluble dyes with conjugated donor–acceptor structures (TPAP and TPAPM) and studied their second-order nonlinear optical (NLO) properties and fluorescence efficiencies. The modified molecules emitted much less two-photon fluorescence than dye D289 due to nonradiative transitions caused by molecular motions in solution. TPAPM exhibited higher NLO performance than D289 and TPAP because of the strong electron pull–push effect operating along the D−π–A backbone, and it is a promising SHG probe molecule for multimodal imaging in biological applications.