Unraveling the Two-Photon and Excited-State Absorptionsof Aza-BODIPY Dyes for Optical Power Limiting in the SWIR Band

We describe the linear and nonlinear optical properties of substituted aza-boron-dipyrromethene (aza-BODIPY) dyes with a focus on the impact of the spatial position and electronic nature of the substituents. The experimental findings are rationalized through time-dependent density functional theory calculations, pointing out the crucial importance of intramolecular charge transfer to achieve strong two-photon absorption in the near-infrared, with measured cross-sections up to 3300 GM at ca. 1200 nm and 1110 GM at telecommunication wavelengths (1500 nm). The evaluation of the optical power limiting behavior of the series highlights that a [2+1] process, i.e. two-photon absorption followed by excited state absorption, was necessary for high optical power limiting efficiencies. Tetra-functionalized aza-BODIPY, featuring an optimized overlap between two-photon absorption and excited state absorption in the 1200-1600 nm range stands as an ideal platform to obtain efficient broad range optical limiter.