Enhanced Two-Photon Absorption and Promising Broad Energy Range Optical Power Limiting Properties of Transoid and Cisoid Benzodipyrrolenine-Fused Squaraine Dimers

Molecular symmetry is a decisive factor for the two-photon absorption (2PA) properties of an organic chromophore. In order to explore its impact on the 2PA cross section of squaraine dyes, we synthesized two isomers in which two indolenine squaraines are fused by two different benzodipyrrolenine bridges leading to either a chromophore with C-2h symmetry or with C-2v symmetry. The excited states of these two chromophores were characterized by steady-state one-photon absorption spectroscopy, one-photon fluorescence excitation anisotropy, and 2PA-induced fluorescence spectroscopy in comparison with the time-dependent and quadratic response density functional theory. Both chromophores show strong absorption and emission in the near infrared spectral region. While the C-2v symmetric chromophore displays significant 2PA cross sections over the entire manifold of excited states, the C-2h symmetric chromophore shows the expected symmetry selection for totally symmetric and asymmetric excited states. In this chromophore, the 2PA cross section reaches extremely high values of ca. 14,000 GM at 16,000 cm(-1) and 90,000 GM at 22,000 cm(-1), which are among the highest reported for organic chromophores to date. The C-2h symmetric chromophore could accordingly be used as an efficient optical power limiter both in solution and as a film in the polystyrene matrix throughout the excited-state manifold.