Squaraine dyes for non-linear optics and organic electronics

This dissertation describes the investigation of the synthesis and characterization of new squaraine-based photonic and electronic materials. In the first part of this thesis, squaraine dyes with large conjugation systems, including extended squaraines consisting of bis(donor)substituted vinylene-heterocycles and bis(indolinylenemethyl)squaraine-based oligomers linking through different pi-bridges were designed, synthesized and characterized to achieve strong two-photon absorption (2PA) for optical-power limiting applications in the near-infrared (NIR). It was found that using strong pi-donating amine donor and alternate vinylene and heterocycle bridges, the extended squaraine exhibited strong 2PA band in the NIR region and in some cases the 2PA bands cover a broad spectrum window. Utilizing some of these dyes, strong optical suppressions for nanosecond and femtosecond laser pulses at selected wavelengths were obtained. It is worth noting that one of the dendronized squaraine forms smooth and high optical quality films with large NIR transparency which makes it promising candidate for practical OPL and other 2PA-induced applications. The oligomers approach achieved materials with large 2PA cross-sections at the wavelength close to one-photon resonance. Use these oligomers as light harvesting donors in organic solar cells revealed enhanced photovoltaic performance with larger oligomers. In the second part of this thesis, a series of squaraine- and phthalocyanine-acetylide ligands were coordinated to platinum and gold, and the photophysical properties of these complexes were studied. Despite of the presence of heavy atom(s), these dyes did not exhibit impressive triplet quantum yields but with surprisingly high fluorescence quantum yields observed in the monomeric squaraine-platinum and gold complexes. The low triplet quantum and unchanged redox potentials in comparison to the ligand of these dyes indicate possible poor electronic coupling between the metal d-orbital and the ligand pi-system. However, using these compounds in organic solar cells showed significant enhancement compared with their metal-free counterparts. In Chapter 5, an effective approach on optimizing bis(indolinylenemethyl)-based squaraine sensitizers with various surface anchor groups and pi-linkers, achieved high power conversion efficiencies (PCEs) of 6.7% in liquid dye-sensitized solar cells (DSSCs) and 2.7% in solid-state DSSCs, which stand out all the previous reported squaraine-based sensitizers, as does the very broad range over wavelengths over which high incident-photon-to-current efficiencies (IPCEs) were obtained.