Two-Photon Absorption Properties and Structure-Property Relationships of Natural 9,10-Anthraquinones: A Curated...

This work provides the first systematic survey of the two-photon properties of 97 natural 9,10-anthraquinones from plants and fungi. A comprehensive computational dataset of two-photon absorption properties calculated using RI-CC2/aug-cc-pVDZ is presented. Single degenerate photon energies required for two-photon excitation span 491.6-1007.9 nm across the five lowest singlet states, with all S₀→S₁ transitions falling within the biological therapeutic window. Remarkably, S₃ state exhibits systematically enhanced TPA efficiency, with 60% of compounds surpassing 1 GM and achieving a mean cross-section of 29.9 GM-substantially higher than S₁ (mean: 7.5 GM). Three compounds demonstrate exceptional performance: cynodontin (73.6 GM, S₂), dermocybin (68.7 GM, S₄), and morindone (50.7 GM, S₃). Orbital analysis reveals that these excitations possess high configurational purity and diagnostics validating the single-reference treatment. The observed spatial separation between hole and particle NTOs, combined with extreme transition dipole anisotropy along the molecular long axis, indicates dipolar charge-transfer enhancement. Comprehensive structure-property analysis establishes that strategic modification may maximise TPA cross-sections. Comparison with aqueous-phase calculations for three compounds reveals non-systematic solvent-induced redistributions of TPA activity across excited states, indicating that gas-phase outcomes serve primarily as internal benchmarks and intrinsic descriptors of structure-property relationships rather than quantitative predictors of photoactivity.