Two-Photon Absorption and Dynamics of Excited States in Bromochalcone Derivatives.

Two-photon absorption (2PA) in organic compounds has gained significant interest due to its applications in nonlinear optics, including two-photon fluorescence microscopy, photodynamic therapy, and laser microfabrication. This study investigates the two-photon absorption cross section (2PACS) and the excited-state dynamics of 4'-bromochalcone derivatives using a combination of experimental and computational approaches. Quantum chemical calculations employing density functional theory (DFT) and time-dependent DFT (TD-DFT) were performed to analyze the electronic properties of the molecules. Experimental characterization involved linear optical measurements (UV-vis absorption and fluorescence spectroscopy), femtosecond transient absorption spectroscopy (TAS), and open-aperture (AO) Z-scan measurements to determine the degenerate 2PACS (D-2PACS). Our results reveal that resonant donor substituents, such as the dimethylamino group, enhance nonlinear absorption, especially in the lower-energy band. Furthermore, TAS revealed an intriguing dynamic associated with the formation of a twisted intramolecular charge transfer (TICT) state, which was corroborated by anisotropy and solvatochromism measurements as well as by computational simulations. These findings provide insights into the relationship between the molecular structure and nonlinear optical properties, contributing to the development of optimized materials for photonic applications.