Squaraines as Reporter Units: Insights into their Photophysics, Protonation, and Metal‐Ion Coordination Behaviour

Abstract

The synthesis, photophysical properties, protonation, and metal-ion coordination features of a family of nine aniline-based symmetrical squaraine derivatives are reported. The squaraine scaffold displays very attractive photophysical properties for a signalling unit. These dyes show absorption and weakly Stokes-shifted, mirror-image-shaped emission bands in the visible spectral range and there are no hints of multiple emission bands. The mono-exponential fluorescence decay kinetics observed for all the derivatives indicate that only one excited state is involved in the emission. These data stress the interpretation that squaraines can be regarded as polymethine-type dyes. From a coordination chemistry point of view, the squaraines possess four potential binding sites; that is, two nitrogen atoms from the anilino groups and two oxygen atoms from the central C(4)O(2) four-membered ring. These coordination sites are part of a cross-conjugated pi-system and coordination events with protons or certain metal ions affect the electronic properties of the delocalised pi-system dramatically, resulting in a rich modulation of the colour of the squaraines. The absorption band at around 640 nm is blue-shifted when coordination at the anilino nitrogen atoms occurs, whereas coordination to the C(2)O(4) oxygen atoms results in the development of red-shifted bands. Addition of more than one equivalent of protons or metal cations could additionally entail mixed N,O- or N,N-coordinated complexes, manifested in the development of a broad band at 480 nm or complete bleaching in the visible range, respectively. Analysis of the spectrophotometric titration data with HYPERQUAD yielded the macroscopic and microscopic stability constants of the complexes. Theoretical modelling of the various protonated species by molecular mechanics methods and consideration of some of the title dyes within the framework of molecular chemosensing and molecular-scale "logic gates" complement this contribution.

Energy Transfer Between Squaraine Polymer Sections: From Helix to Zigzag and All the Way Back

“…38-43 These properties allow many applications from dye-sensitized solar cells and organic photovoltaic applications 32, to ion sensors [67][68][69][70][71] and biolabelling. [72][73][74][75][76][77][78][79][80] Unlike conjugated polymers that are based on very small monomers such as styrene (e.g., MEH-PPV) or thiophene (e.g., P3HT) and whose polymer properties are totally different from those of the monomers, 81 squaraine homo-and copolymers are based on squaraine dyes which already show a strong absorption in the red region of the visible spectrum.…”

Outstanding Short‐Circuit Currents in BHJ Solar Cells Based on NIR‐Absorbing Acceptor‐Substituted Squaraines

“…[7] Thus, squaraines are a widely used class of functional dyes for a huge variety of applications. [8] Squaraines have been used in single-layer OPVs and, more recently, in vacuum-deposited and dye-sensitized solar cells. [9] Lately, Silvestri et al reported for the first time on the use of squaraines in solution-processable BHJ solar cells with PCEs of up to 1.2 %.…”

Highly photostable wide-dynamic-range pH sensitive semiconducting polymer dots enabled by dendronizing the near-IR emitters

“…However, based on the work of Ros-Lis et al , protonation on the oxygen atom would result in a slightly red shifted and/or greatly blue-shifted absorption band accompanied by a reduction in the intensity of the normal absorption band. 71 We checked the absorption spectra of our G0Sq monomer in acidic solutions, and found only the main peak at around 640 nm decreased in intensity, and no new absorption band was found (Fig. S3†), thus indicating that the protonation was on the nitrogen atom.…”