Bright Near‐Infrared Fluorophores Based on Squaraines by Unexpected Halogen Effects

Abstract

Unverhofft kommt oft: Ein zufällig entdeckter Halogeneffekt in Squarainfarbstoffen führte zur Entwicklung neuer Nahinfrarotfluorophore, die im sichtbaren Bereich nahezu vollständig transparent sind und im Nahinfrarot jenseits von 900 nm Licht emittieren (siehe Bild).

Diketopyrrolopyrrole Organic Thin‐Film Transistors: Impact of Alkyl Substituents and Tolerance of Ethylhexyl Stereoisomers

“…Thus, while the parent H‐DPP dye is an outstanding fluorophore with a fluorescence quantum yield ( Φ fl ) of unity, the fluorescence quantum yields of the Cl‐DPP and Br‐DPP derivatives are still very high (0.84 each), while that of the cyano derivative CN‐DPP is slightly smaller with 0.7 (Table ). It is interesting to note that, as in the case of other halogenated dyes, no heavy‐atom effect leading to triplet formation is observed for this series of dyes.…”

Efficient NIR‐Light Emission from Solid‐State Complexes of Boron Difluoride with 2′‐Hydroxychalcone Derivatives

“…Although a large number of fully organic fluorophores have been found to be efficient in the visible spectrum, those that emit deep‐red‐ or near‐infrared (NIR) light with significant efficiency in the condensed phase still remain much less common 2. The most‐popular NIR‐emissive organic dyes belong to the class of polymethines3 but also include oxazine and thiazine derivatives,3 rylenes,4 squaraines,5 borondipyrromethanes (BODIPYs),6 and low‐band‐gap π‐conjugated oligomers 7. Despite numerous studies, only very few of these compounds have been reported to be effective as solid emitters and values of their photoluminescence quantum yields in the condensed phase are very scarce and usually low.…”

Halogen–Arene Interactions Assist in Self‐Assembly of Dyes

“…The absorption spectra of monomers of these squaraines in dichloromethane show the expected characteristic features, namely an intense main band in the long‐wavelength region with a maximum at λ =870 nm for both SQ‐H and SQ‐Me , λ =885 nm for SQ‐Cl , λ =891 nm for SQ‐Br , and λ =900 nm for SQ‐I (see Figures S1–S5 in the Supporting Information) 8. By changing the solvent to the less polar solvent toluene, a bathochromic shift of the main absorption band is observed with concomitant band narrowing and more pronounced vibronic progressions for the whole series of dyes, thus leading to absorption maxima at λ =903 ( SQ‐H , SQ‐Me ), 915 ( SQ‐Cl ), 923 ( SQ‐Br ), and 928 nm ( SQ‐I ).…”

“…To study the aggregation behavior of acceptor‐substituted squaraines in less polar solvents such as toluene, we chose a series of 4‐methylquinolinium‐based acceptor‐substituted squaraines which feature dodecyl chains as lipophilic anchors at the nitrogen atoms of the quinolinium groups (Figure 1). The synthesis and NIR fluorescence properties of these squaraines have been recently published 8. Within this series, the substitution of a hydrogen atom at the donor moiety by different halogen atoms (Cl, Br, I), on one hand, and by a methyl group, on the other hand, allows the elucidation of halogen effects on the aggregation properties of these dyes.…”

“…This model, also known as the equal‐ K model, is based on the assumption that each monomer addition to the growing aggregate is governed and described by one single equilibrium constant, K e . Considering the structural nature of squaraine aggregates ( H ‐type as revealed by strong hypsochromic shift), the assumption of such an isodesmic growth is reasonable since squaraines are, as a result of their rigid molecular structure,8 planar compounds with two equal π faces. Thus, for the formation of the initial dimer in solution neither a major driving force will be required to bring the molecules into a favorable conformation, nor the remaining free π faces would be altered to discriminate a further growth of the aggregate as required for a cooperative or anticooperative mechanism.…”