Solvatochromatic absorption

Square-planar metallo(diimine)(dithiolene) complexes generally display intense, solvatochromatic absorptions in the visible region of the spectrum that are not found in the corresponding metallo-bis(dithiolene) or metallo-bis (diimine) complexes. Futhermore, the LLCT transition energy does not vary appreciably as a function of the metal ion. Extended Hiickel calculations on Ni, Pt, and Zn metallo(diimine)(dithiolene) complexes indicate that the HOMO is comprised almost entirely of dithiolene orbital character (Figure 2), while the LUMO was found to possess essentially all diimine n orbital character (112, 252, 268). In stark contrast to the spectra of square-planar Ni and Pt metallo (diimine)(dithiolene) complexes, the psuedo-tetrahedral complexes of Zn possess extremely weak LLCT transitions. Now, it is of interest to discuss the differences in LLCT intensity as a function of geometry from a MO point of view. This discussion should help to explain important orientation-dependent differences in photoinduced electron delocalization and charge separation. 

One of the preliminary indications which reveals the potential capacity of a material to generate second harmonics is its solvatochromatic behaviour, i.e. the ability of a material to give rise to considerable differences in its UV-visible spectrum on changing solvent. For example, the electronic absorption spectrum of cw-ferrocenyl-2-(4-nitrophenyl) ethylene in dimethylformamide shows two bands at 340 and 492 nm, respectively, whereas in heptane it displays three bands at 320, 406 and 462 nm, respectively. 

In our study, the properties of the microenvironment of synthetic polymers were investigated by means of various solvatochromic reporters l-methacryloyloxyethyl-4-carboalkoxypyridinium salts (SCHEME I), 1-alkyl-4-/4-hydroxystyryl/pyridinium-betaine (SCHEME II) and spiropyran-merocyanine type (SCHEME ID) all have solvatochromatic bands in the visible absorption l-dimethylaminonaphthalene-5-sulphonamido chro-mophore (SCHEME IV) has a solvatochromic band in the emission spectrum. 

All chromophores are affected by the polarity of the bulk environment to some extent. Some indicators exhibit more pronounced solvatochromatic behaviour, however, showing a pronounced shift in the position and sometimes intensity of an electronic absorption or emission band when the polarity of the medium is changed. The classic example is Reichardt s dye, which shows such an exceptionally large solvatochromatic shift (Amax shifts from 810 nm in diphenyl ether to 453 nm in water) that it was used to establish the commonly used Et(30) spectroscopic scale of solvent polarity [33]. 

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