Order of magnitude effects of thiazole regioisomerism on the near-IR two-photon cross-sections of dipolar chromophores

We have investigated computationally the two-photon absorption (2PA) properties of donor-acceptor dipolar chromophores the conjugated backbones of which contain two five-membered heterocyclic groups which may be electron-rich (thiophene-2,5-diyl) and/or electron-deficient (thiazole-2,5-diyl). Quantum-chemica, calculations (INDO/MRDCI/S-tensor and Sum-Over-State calculations based on DFT-optimized geometries) indicate that the two-photon cross-sections into the lowest two excited states S₁ and S ₂ can be turned by more than an order of magnitude by varying the nature, order, and, in the case of thiazole, orientation of the heterocycles. Going from one thiazole regioisomer to the other has the strongest impact on the 2PA spectra and can even invert the ratio between the 2PA cross-section of S₁ and S₂. An essential-state analysis reveals that different channels dominate 2PA into S₁ and S₂. The sensitivity of 2PA into S₁ towards the orientation of the thiazole ring stems from a local modulation on the thiazole ring of the change in state dipole moment upon excitation to S₁, Δ_μ01, whereas the dominant essential parameter through which the thiazole orientation affects 2PA into S₂ is the transition dipóle moment between S₁ and S₂, μ₁₂.