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-chemical calculations (INDO/MRDCI/S-tensor and Sum-Over-States calculations based on DFT-optimized geometries) indicate that the two-photon cross-sections into the lowest two excited states S1 and S2 can be tuned 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-sections 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, Δµ01, whereas the dominant essential parameter through which the thiazole orientation affects 2PA into S2 is the transition dipole moment between S1 and S2, µ12.
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