Enhanced Diradical Nature in Oxyallyl Derivatives Leads to Near Infra Red Absorption: A Comparative Study of the Squaraine and Croconate Dyes Using Computational Techniques
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- 作者：Kola Srinivas ; Ch. Prabhakar ; C. Lavanya Devi ; K. Yesudas ; K. Bhanuprakash ; V. Jayathirtha Rao
- 刊名：Journal of Physical Chemistry A
- 出版年：2007
- 出版时间：May 3, 2007
- 年：2007
- 卷：111
- 期：17
- 页码：3378 - 3386
- 全文大小：157K
- 年卷期：v.111,no.17(May 3, 2007)
- ISSN：1520-5215
文摘
We apply many criteria to estimate the diradical character of the ground state singlets of several oxyallylderivatives. This is carried out as the oxyallyl derivatives like squaraine and croconate dyes can be representedby both mesoionic and diradical formulas, the domination of which would characterize its lowest energytransition. One criterion applied is the singlet-triplet gap, which is known to be inversely proportional to thediradical character. Another criterion is the occupation number; this is determined for the symmetry brokenstate of the molecules in the unrestricted formalism, and the difference of occupation in the HOMO andLUMO is related to the diradical character. The diradical character of all of the croconates and few squarainesis 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 dominanceof the diradical character. To understand the reliability of the DFT methods for the absorption propertypredictions of these molecules, TD-DFT studies to calculate the vertical excitation energies have been carriedout, using the B3LYP/ BLYP exchange correlation functionals and the LB94 asymptotic functional with andwithout 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 symmetricsquaraines yields an R value of 0.92 and, for the unsymmetric squaraines, an R value of 0.936. With inclusionof 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 croconatedyes, however, have larger deviation from the experimentally observed values in all of the functionals studiedeven after inclusion of the solvent effects. The deviations are also not systematic. The deviation with respectto the experiment in this case is attributed to the average larger diradical character in this series.