Synthesis of a Novel Chiral Squaraine Dye and Its Unique Aggregation Behavior in Solution and in Self‐Assembled Monolayers

Abstract

Squaraine dyes have been studied extensively in the last few decades due to their advantageous optical properties that enable applications including imaging technologies and photoconducting devices, photovoltaics and non-linear optics, and sensing and photodynamic therapy. The isolated squaraine chromophore exhibits an intense and narrow absorption band in the long-wavelength range of the visible spectrum. Furthermore, the dye displays a pronounced tendency to form Hand J-type aggregates in different homogenous and heterogeneous media. In order to harness the beneficial opto-electronic characteristics of this unique class of chromophores for the construction of improved devices, control over their aggregation behavior is absolutely critical since it influences electronand energy-transport processes. Since chirality has been used as a guiding element to direct self-assembly processes, we investigated the influence of introducing a chiral bias to squaraine aggregation. Here, we present our initial results on the synthesis of a novel chiral squaraine dye and the investigation of its unique aggregation behavior in aqueous solution and on a solid substrate. Condensation of electron-rich aromatics with squaric acid in an azeotropic solvent mixture such as toluene/1-butanol provides a simple access to squaraines. Highly activated N,N-dialkylated 3,5-dihydroxyanilines are conveniently generated in situ by condensation of secondary amines with 1,3,5trihydroxybenzene (phloroglucinol), and subsequent reaction with squaric acid in the same pot leads to formation of bis(4-dialkylamino-2,6-dihydroxyphenyl)squaraines, which offer the advantages of much higher intrinsic fluorescence quantum yields, increased relative chemical stability, and stronger aggregation tendency than their non-hydroxylated analogues. Such a two-step, one-pot procedure can also be applied to the synthesis of polysquaraines, useful for cation sensing. Since secondary amines are mandatory to avoid squaramide formation, the natural amino acid L-proline is an obvious choice for a chiral building block. Preliminary experiments indicated that an ester linkage to the side chain was not stable during the squaraine condensation step due to transesterification and therefore, initial reduction to L-prolinol, and attachment of the side chains via ether linkages was pursued. Our route (Scheme 1) involves preparation of the alkylated L-prolinol followed by the two-step, one-pot procedure to access chiral squaraine 1 carrying nonpolar, linear n-alkyl side chains. This synthesis takes advantage of the enhanced nucleophilicity of aromatics possessing cyclic, i.e., pyrrolidino, rather than acyclic secondary amine substituents. Compound 1 showed a characteristic sharp (full width at half maximum, FWHM ∼ 20 nm) and intense (molar absorption e ∼ 300 000 M cm) absorption band centered around 640 nm that is indicative of the presence of mostly monomeric, i.e., non-aggregated, squaraine chromophores. Significant variation of solvent polarity, from cyclohexane to chloroform to acetonitrile, led to negligible bathochromic shifts (637→ 638→ 642 nm). Emission measurements revealed sharp fluorescence spectra with relatively small Stokes shifts (Dkmax ∼ 17 nm) and high fluorescence quantum yields (Uf ∼ 0.85), illustrating the rigidified character of the chromophore due to intramolecular hydrogen bonding. Due to the hydrophobic nature of the squaraine moiety and the unique quality of water to promote aggregation as a consequence of the operating hydrophobic effect, solvent titration experiments involving a systematic variation of the water content in acetonitrile solution were carried out (Fig. 1). The UV-vis spectra (Fig. 1, top) clearly show that aggregation is induced above a critical water concentration of ∼ 10 vol.-%. Above this threshold value, the monomer band initially rapidly decreases, and, at ∼ 18 vol.-% water content, aggregate bands start to emerge. At 30 vol.-% water content, the monomer band is greatly reduced to ∼ 10 % of its initial intensity, and both blueand red-shifted bands at 555 and 761 nm, respectively, were observed. The corresponding C O M M U N IC A IO N S