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.