High-level computational studies of Rhodizonate derivatives: Molecules absorbing in near infrared region due to larger C-C-C angle of the oxyallyl ring

Oxyallyl derivatives Croconate dyes (CR) and Squarylium dyes (SQ) are well-known near infrared (NIR) dyes and in general have been considered to be Donor-Acceptor-Donor type molecules with CR dyes absorbing in the longer wavelength region when compared to the corresponding SQ dyes. Recent high-level calculations have shown conclusively that the NIR absorption in these molecules is not due to the increased donation and acceptance but is due to the diradicaloid nature of the central oxyallyl ring. In continuation of our work on oxyallyl derivatives, we extend the studies to a larger central ring; Rhodizonate (RH) with different substitutions and carry out high-level symmetry-adapted cluster-configuration interaction (SAC-CI) calculations. From the calculations it is predicted that derivatives of RH absorb in the longer wavelength region than the corresponding SQ and CR. We show that the charge transfer is very small and does not play the key role in the red shift but on the other hand it is the perturbation of the HOMO-LUMO gap (HLG) and the diradicaloid nature both due to geometry and substitution, which seem to be responsible for this shift. We reiterate the design principle that increasing the donor capacity of the groups may not help in the red shift, but introducing groups which perturb the HLG and decrease it without changing the MO character should lead to a larger bathochromic shift. We conclude that these RH dyes with absorption greater than 1000 nm will find applications as nonlinear optical (NLO) materials and functional dyes.