Tuning Two‐Photon Absorption Property of A‐π‐A Type Cyanine Dyes in NIR Region via π‐Electron Structure

This study systematically investigates four A‐π‐A cyanine dyes using density functional theory (DFT) and the sum‐over‐states (SOS) model to reveal how increasing π‐delocalization affects near‐infrared (NIR) two‐photon absorption (TPA) performance. Theoretical results show that extending the π‐plane broadens the conjugated system, enhances π‐electron delocalization, and reduces the extent of orbital overlap. The designed dye 3T‐2 shows prominent localized excitation in the two‐photon transition process. It possesses a specific and concentrated transition pathway, which effectively elevates the transition dipole moments. Consequently, 3T‐2 exhibits excellent TPA properties in the NIR region, with a TPA cross section of 129779.9 GM at 784.4 nm and a third‐order nonlinear coefficient of 44462.8 × 10 −36 esu. It is anticipated that this study offers robust theoretical support for the modulation of nonlinear optical performances in A‐π‐A type NIR dyes, which further facilitates the practical implementation in multiphoton bioimaging and other related fields.