Charge-transfer octupolar molecules can form clusters in solution through intermolecular hydrogen bonds. In the present work we explore the role of such clustering on two-photon absorption (TPA) spectra assuming 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) as a model system. Using density functional quadratic response theory we examine different cluster structures of TATB dimers, trimers, and tetramers taken from snapshots of molecular dynamics simulations. In comparison with the TPA spectrum of a monomer, significant red shifts of charge-transfer states are predicted for all chosen clusters, which mainly is the result of the distortion of the structures induced by the aggregation. The TPA spectra for dimers and trimers show strong conformation dependence, whereas they turn out to be more stable for tetramers. Enhancements of TPA absorption have also been found for clusters containing less distorted molecules connected by hydrogen bonds.
