Cation-Controlled Excimer Packing in Langmuir–Blodgett Films of Hemicyanine Amphiphilic Chromoionophores

Abstract

Supramolecular structure of ultrathin films of hemicyanine dye bearing a crown ether group (CrHCR) was tuned by lateral pressure and investigated by means of compression isotherms, UV-vis and fluorescence spectroscopies, and X-ray reflectivity. Two different types of aggregation were revealed, depending on the absence or the presence of metal cations in the water subphase. While CrHCR forms at high surface pressures head-to-tail stacking aggregates on pure water, changing the subphase to a metal-cation-containing one leads to the appearance of well-defined excimers with head-to-head orientation. The structure of monolayers transferred onto solid supports by the Langmuir-Blodgett (LB) technique was examined by use of X-ray reflectivity measurements and molecular modeling. A model of cation-induced excimer formation in hemicyanine Langmuir monolayers is proposed. Finally, fluorescence emission properties of LB films of CrHCR can be managed by appropriate changes in the subphase composition, this last one determining the type of chromophore aggregation.

The Effect of Alkyl Substituent Length on Receptor Properties of Dithiaaza-Сrown-Hemicyanine MonolayersThe Effect of Alkyl Substituent Length on Receptor Properties of Dithiaaza-crown - Hemicyanine Monolayers

“…[7,20,33,42] In this case, recorded monolayer compression isotherms (Figure 4) are significantly different to the ones obtained on pure water subphase in overall form and the position of the bend. In addition, more importantly, the value of mean molecular area, at which surface pressure begins to increase, is shifted towards higher values for all three studied compounds.…”

“…Moreover, the difference between these mean molecular area values for all three molecules on barium-containing subphase is much smaller (around 20 Å 2 ) than in case of pure water subphase (60 Å 2 between ChIP12 and ChIP16, and around 40 Å 2 between ChIP16 and ChIP21). Most probably, barium cations preorganize monolayer via weak interaction with dithiaaza-crown ether moiety of the studied molecules in the same way as it was for analogous azacrown substituted hemicyanine dye [7,20,33,42] and the resulting repulsion between the likely charged chromoionophoric groups provides the main contribution to the monolayer structure, while the length of alkyl chains are not as important (Scheme 3,b).…”

“…[1][2][3][4] From this standpoint, crown ether substituted fluoro-and chromophores capable of significant changes of photophysical properties upon selective binding of cations are of special interest. [5][6][7][8][9][10][11] Modification of the crown ether cycle allows achieving high selectivity and binding constants for biologically and environmentally important metal ions. For example, replacement of one or two oxygen atoms in this cycle by sulfur ones can lead to higher affinity for mercury and copper cations.…”

“…[12][13][14][15][16][17][18] Moreover, simultaneous inclusion of nitrogen atom into the crown-substituent of a coloured molecule as a part of both ionophoric and chromophoric moieties is an easy way to impart the chromoionophoric power to the dye molecule and obtain a spectral response to metal cation binding since their interaction with the nitrogen atom will reduce the overall chromophore conjugation and thus change the photophysical properties of the system. [5][6][7][8][9][10][11][19][20][21][22][23] In this regard, aza-crown substituted hemicyanines represent one of the most promising compounds for sensory applications due to the combination of hemicyanine spectral properties (absorbance and fluorescence in visible range), crown ether receptor ability and ease of chemical modification. [5,12,17,[24][25][26][27][28][29][30][31][32] However, most studies concerning the sensory properties of such crown-substituted hemicyanines were done in solution.…”

“…One of the most efficient methods to achieve this is LangmuirBlodgett technique, which allows formation of ultrathin organized films on almost any type of substrates. [7,20,[33][34][35][36][37][38][39] One of the principal chemical modifications of the hydrophilic hemicyanine molecules needed to use this technique is addition of hydrophobic alkyl chains to attain amphiphility in order to make the molecule surface active. However, the effect of the length of such substituent on the sensory properties of different compounds remains absolutely unstudied.…”