Theoretical study on the two-photon absorption properties of pyrazole aluminum compounds

An extensive series of pyrazole aluminum compounds containing an Al₂N₄ center as a pseudoconjugated system were theoretically investigated for their one-photon absorption and two-photon absorption (OPA and TPA) properties by density functional theory (DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods. The results indicate that pyrazole aluminum compounds are good TPA materials and that the TPA maximal absorption cross-section (δ_max) can reach 2860.1 GM (1 GM=10^-50 cm⁴·s·photon^-1). By incorporating electron-acceptors in the central core, a π-conjugated bridge and terminal groups, the OPA and TPA properties can be modulated. This research provides strategies for the enhancement of molecular TPA in the target region. The origin of the large δ_max of some of the studied molecules was determined using a three-level energy model. We conclude that an increase in the intramolecular charge transfer can enhance δ_max. Moreover, the pyrazole aluminum compounds behave in a similar manner to pyrazabole chromophores in terms of linear optical, and TPA properties and they possess an increased δ_max to some extent.