Merocyanine dyes electronic structure

Of particular interest are the intramolecularly ionic meropolymethine dyes (especially the merocyanines), whose electronic structure lies somewhere between that of polyenes and that of polymethines depending on the nature of X and X as well as on solvent polarity [20], These are systems in which an electron-donating group, D, is linked by a conjugated system, R, to an electron-accepting group, A. Their intermediate r-electronic structure can be described in terms of two mesomeric structures, D—R—A D —R—A , as, for example, this special vinylogous merocyanine dye ( = 0,1,2,...) ... 

Merocyanine dyes 50-55 (Fig. 5.18) possess an electronic structure at the meso-meric center between neutral and zwitterionic electron distribution and have high polarizabilities and dipole moments and exhibit absorption spectra with sharp bands that give rise to exceptionally brilliant magenta hues [60],... 

Combining the idea of solvent-induced changes in molecular structure with the concept of a solvent continuum around the solvatochromic molecule, a micro-structural model of solvatochromism has been developed by Dahne et al., which reproduces, qualitatively correctly and quantitatively satisfactorily, the solvatochromic behavior of simple merocyanine dyes [95b], The results obtained with this model for 5-(dimethylamino)penta-2,4-dienal are in good agreement with the solvent-dependent experimental data such as transition energies, oscillator strengths, r-electron densities, and r-bond energies [95b] cf. also [326, 327],... 

The absorption, emission, and redox properties of squaraines make them highly suited for applications as photosensitizers. In view of this, the early studies on squaraines were focused on thin photovoltaic and semiconductor photosensitization properties [1,4,5,91-97], Champ and Shattuck [98] first demonstrated that squaraines could photogenerate electron-hole (e-h) pairs in bilayer xerographic devices. Subsequently, extensive work has been carried out on the xerographic properties of squaraines [2,24,34,47,48,99,100], and these properties have been reviewed recently [11]. In an extensive smdy on the correlation s between cell performance and molecular structure in organic photovoltaic cells, squaraines were found to have much better solar energy conversion efficiencies than a variety of other merocyanine dyes [4,5]. 

To test the effectiveness of the new CS INDO-based procedure we searched for compounds exhibiting large solvent effects on their structural and optical properties. From this point of view merocyanine dyes are most suitable in view of the dramatic dependence of their UV/vis spectra on the solvent polarity [25,26]. The current interpretation of this behaviour is based on the fact that merocyanines are formed by an amino group (electron donor) and a carbonyl... 

Solvatochromism of merocyanine dyes is accompanied also with the solvent dependence of their dipole moment [Pa 80], So dipole moment measurements have also contributed to the explanation of solvatochrom behaviour of these systems. In such systems the electronic ground and excited states of a molecule are described by two resonance structures non-polar (quinoid) and polar (benzenoid). 

The proposed calculation procedure will be first tested by analysing in detail the effects of the solvent polarity on the structure and electronic spectra of the simple merocyanine Ml. Afterwards, the selected calculation procedure will be applied to the more complex dyes M2 and M3, characterized by equal length of the conjugated path connecting the donor and acceptor group, but exhibiting opposite solvatochromic effects. To be precise, the acyclic merocyanine M2 shows, like the simpler chromophore Ml, positive solvatochromism [25] (i.e. bathochromic shift of the first absorption band on increasing solvent polarity),... 

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