Optimizing nonlinear absorption via modulating π-electron delocalization in multi-ring conjugated cores: from TPA at 515/650 nm to broad temporal response.

Nonlinear absorption across distinct temporal regimes is central to laser-relevant optical attenuation in organic media. Here we engineer branched anthracene chromophores by inverting the direction of intramolecular charge transfer (ICT) between peripheral branches and a polycyclic conjugated core (branch→core versus core→branch), enabling mechanistic comparison within a unified molecular platform. Quantum-chemical calculations (DFT/TD-DFT) together with femtosecond transient absorption spectroscopy corroborate the directional ICT characteristics and reveal the formation of a charge-transfer state (CTS) in both architectures. Open-aperture Z-scan measurements demonstrate two-photon absorption (TPA) at 515 nm and 650 nm under 190 fs excitation, with effective NLA coefficients of (1.2 ± 0.1) × 10-2 cm/GW and (6.3 ± 0.4) × 10-3 cm/GW for Molecule 2, respectively (negligible in the reference analogue). In addition, pronounced nanosecond excited-state absorption (ESA) at 532 nm under 4 ns excitation is observed for Molecules 2-4, whereas the reference analogue (Molecule 1) shows negligible response. Mechanistically, CTS formation enhances nanosecond ESA, whereas core→branch ICT markedly strengthens the femtosecond TPA by expanding π-electron delocalization, as supported by computed transition densities. These results establish a practical structure-property rule linking ICT direction, π-delocalization, and CTS-mediated pathways for tailoring broadband ESA and multi-timescale nonlinear absorption in solution-processable organic systems.