Abstract
Molecular aggregates of two squaraine dyes of different ability for hydrogen-bond formation are investigated with computational chemistry tools. Oligomers up to hexamers are included into the present analysis. Potential energy surface scan for dimers and small oligomers indicates a preference for regular staircase and zig-zag structures. A decomposition of interaction energy of the dimers with SAPT0 procedure allows to notice a clear predominance of dispersion-bound structures in the case of 2,4-bis(4-dimethylaminophenyl)squaraine and susceptibility of the 2,4-bis(aminophenyl)squaraine for hydrogen-bond formation, what is consistent with predictions based on molecular structure. An influence of aggregation on spectroscopic properties is analyzed. An elongation of the squaraine oligomer leads to a shift on the maximum absorption wavelength and an increase of the intensity of the signal. Moreover, in the case of the 2,4-bis(aminophenyl)squaraine, the new interaction-induced signals appear in the long-wavelength part of the UV–Vis spectrum.
Highlights
Well-ordered supramolecular assemblies are known and applied in various branches of science, technology and engineering. Their presence can be undesirable for some applications such as in lasers or dye-sensitized solar cells
The H-aggregates exhibit blue-shifted signals in the absorption spectrum and large Stokes shift for the weak fluorescence. This contrasting behavior of the two type of aggregates arises from different mutual orientation of transition dipole moments of their composing molecules: H-aggregates are built of the molecules arranged face-to-face with parallel orientation of the dipoles and for the J-aggregates the displacement of the molecules is observed giving a “coplanar inclined” orientation of transition moments [1, 2]
The dimers of the two analyzed squaraine dyes are presented in Figs. 2 and 3
Summary
Well-ordered supramolecular assemblies are known and applied in various branches of science, technology and engineering. An explanation of the shifts of the absorption bands upon aggregation has been given by Kasha et al [1] According to their exciton model, the shift arises due to a strong coupling between the components transition dipoles that cause an exciton splitting of the excited state. The squaraines have been obtained with simultaneous coexistence of both types of aggregates in different domains [11] In this case, the absorption band splitting is observed and the two parts appear on the spectrum: one that is bathochromically and the other-hypsochromically shifted. The current contribution aims to carefully analyze mutual interactions in the small squaraine aggregates and their influence on the spectroscopic properties
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