| Abstract
| - Symmetric croconate (CR) and squarylium dyes (SQ) are well-known near-infrared (NIR) dyes and, in general,are considered to be donor−acceptor−donor type molecules. It is established in the literature that CR dyesabsorb in a longer wavelength region than the corresponding SQ dyes. This has been attributed to the CRring being a better acceptor than the SQ ring. Thus increasing the donor capacity should lead to a bathochromicshift in both SQ and CR. On the other hand, some experiments reported in the literature have revealed thatincreasing the conjugation in the donor part of the SQ molecule leads first to red shift, which upon a furtherincrease of the conjugation changes to a blue shift. Hence, to understand the role of the central ring and thesubstitutions in the absorption of these dyes, we carried out high-level symmetry-adapted cluster-configurationinteraction (SAC−CI) calculations of some substituted SQ and CR dyes and compare the absorption energywith the existing experimental data. We found that there is very good agreement. We also carried out SAC−CI calculations of some smaller model molecules, which contain the main oxyallyl substructure. We variedthe geometry (angle) of the oxyallyl subgroup and the substitution in these model molecules to establish acorrelation with the bathochromic shift. We found that the charge transfer is very small and does not play thekey role in the red shift, but on the other hand, the perturbation of the HOMO−LUMO gap (HLG) from boththe geometry and substitution seems to be responsible for this shift. We suggest as a design principle thatincreasing the donor capacity of the groups may not help in the red shift, but introducing groups whichperturb the HLG and decrease it without changing the MO character should lead to a larger bathochromicshift.
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