Enhanced Diradical Nature in Oxyallyl Derivatives Leads to Near Infra Red Absorption:  A Comparative Study of the Squaraine and Croconate Dyes Using Computational Techniques^†

Abstract

We apply many criteria to estimate the diradical character of the ground state singlets of several oxyallyl derivatives. This is carried out as the oxyallyl derivatives like squaraine and croconate dyes can be represented by both mesoionic and diradical formulas, the domination of which would characterize its lowest energy transition. One criterion applied is the singlet-triplet gap, which is known to be inversely proportional to the diradical character. Another criterion is the occupation number; this is determined for the symmetry broken state of the molecules in the unrestricted formalism, and the difference of occupation in the HOMO and LUMO is related to the diradical character. The diradical character of all of the croconates and few squaraines is estimated to be large. All of these have absorption above 750 nm and can be classified as near infrared (NIR) dyes, leading to the inference that NIR absorptions in these molecules are largely due to the dominance of the diradical character. To understand the reliability of the DFT methods for the absorption property predictions of these molecules, TD-DFT studies to calculate the vertical excitation energies have been carried out, using the B3LYP/ BLYP exchange correlation functionals and the LB94 asymptotic functional with and without the inclusion of solvent. The deviations, in both the squaraine series (average lower diradical character), are found to be systematic, and with the inclusion of the solvent in the calculation, the deviations decrease. The best least-squares fit with the experimentally observed values using B3LYP /6-311G(d, p) for the symmetric squaraines yields an R value of 0.92 and, for the unsymmetric squaraines, an R value of 0.936. With inclusion of the solvent, the R value is 0.96 for the symmetric squaraines and 0.961 for the unsymmetric squaraines, indicating that these DFT functionals with linear scaling may be used to study these systems. The croconate dyes, however, have larger deviation from the experimentally observed values in all of the functionals studied even after inclusion of the solvent effects. The deviations are also not systematic. The deviation with respect to the experiment in this case is attributed to the average larger diradical character in this series.

Ni Dithiolenes – A Theoretical Study on Structure–Property Relationships

“…The diradical character was also determined by using the expression: [48] E 1 = E RDFT (singlet) -E UDFT (singlet)…”

Comparative Study of the Semiconducting Properties of Benzothiadiazole and Benzobis(thiadiazole) Derivatives Using Computational Techniques

“…[65] There is a small lifting of degeneracy (right side in Figure 5), which is stabilized when a geometrical or electronegative perturbation occurs, but the BRC is retained. [19,66] The VB pictures shown in the Supporting Information ( Figure S4 and Table S2) of these molecules confirm the biradical nature of the molecules. [67,68] Thus, one can consider TDI, BBI and BBT to contain the substructure TMB.…”

“…[50][51][52][53][54] The reorganization energy consists of both intramolecular and intermolecular contributions. [66] The intramolecular contribution is correlated to the geometry changes of the molecule and the intermolecular contributions are correlated to the polarization of the surrounding molecules upon oxidation/reduction. [55] The intermolecular reorganization energies, that is polarization contribution in this case, are expected to be small and hence neglected.…”

Croconaine Dyes – the Lesser Known Siblings of Squaraines

“…The years 2007 and 2008 were particularly abundant; on three papers on croconaine dyes were published in 2007 [58][59][60] and six in 2008. [61][62][63][64][65][66] Of the three papers in 2007, one was a theoretical paper covering type 2, 4, 5, 13, 16, 23 and 24 croconaines, [58] while the other two were experimental papers, both covering type 4 [59,60] and 4b [59] croconaines, prepared by variants of previously reported methods.…”