| Abstract
| - It is well-known from experimental studies that the oxyallyl-substructure-based squarylium and croconiumdyes absorb in the NIR region of the spectrum. Recently, another dye has been reported (J. Am. Chem. Soc.2003, 125, 348) which contains the same basic chromophore, but the absorption is red-shifted by at least 300nm compared to the former dyes and is observed near 1100 nm. To analyze the reasons behind the large redshift, in this work we have carried out symmetry-adapted cluster−configuration interaction (SAC−CI) studieson some of these NIR dyes which contain the oxyallyl substructure. From this study, contrary to the earlierreports, it is seen that the donor groups do not seem to play a major role in the red-shift of the absorption.On the other hand, on the basis of the results of the high-level calculations carried out here and using qualitativemolecular orbital theory, it is observed that the orbital interactions play a key role in the red shift. Finally,design principles for the oxyallyl-substructure-based NIR dyes are suggested.
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