  This study delves into the enhancement of two-photon absorption (2PA)
properties in diazaacene-bithiophene derivatives through a synergistic approach
combining theoretical analysis and experimental validation. By investigating
the structural modifications and their impact on 2PA cross sections, we
identify key factors that significantly influence the 2PA efficiency. For all
molecular systems studied, our state-of-the-art quantum chemical calculations
show a very high involvement of the first excited singlet state (S1) in the 2PA
processes into higher excited states, even if this state itself has only a
small 2PA cross section for symmetry reasons. Consequently, both the oscillator
strength of S1 and the transition dipole moments between S1 and other excited
states are of importance, underscoring the role of electronic polarizability in
facilitating effective two-photon interactions. The investigated compounds
exhibit large 2PA cross sections over a wide near-infrared spectral range
reaching giant values of 42000 GM. The introduction of diazine and diazaacene
moieties into bithiophene derivatives not only induces charge transfer but also
opens up pathways for the creation of materials with tailored nonlinear optical
responses, suggesting potential applications in nonlinear optics.
