Unlocking Giant Two‐Photon Absorption Through Iodine Coordination in A‐π‐D‐π‐A Carbazole‐Based Probes

ABSTRACT The strategic modulation of two‐photon absorption (TPA) in A‐π‐D‐π‐A carbazole‐based probes, achieved through N‐methylation of terminal pyridine groups and subsequent iodide ion coordination, is unveiled by density functional theory (DFT)/TD‐DFT calculations. We demonstrate that methylation transforms pyridine into pyridinium nitrogen, markedly enhancing intramolecular charge transfer efficiency. The subsequent introduction of iodide ions uniquely coordinates with the pyridinium nitrogen, triggering a dramatic surge in the long‐wavelength TPA cross‐section. This enhancement is primarily attributed to a resonant one‐step TPA transition mechanism. Our findings establish a potent strategy for the rational design of high‐performance biological imaging probes with tailored response wavelengths.