Symmetry-lowering distortion of near-infrared polymethine dyes—a study by first-principles methods

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The shape of the longest-wavelength absorption band of near-infrared (NIR) polymethine dyes in polar solvents is not welli-understood. Among different tentative explanations a lowering of the molecular symmetry has been previously discussed. Polymethines of the symmetry point group C 2 v may undergo Peierl's distortion resulting in a structure of lower symmetry. A symmetry collapse was reported based on semiempirical AM1 calculations. The potential change of the molecular symmetry of polymethines was reexamined by first-principle methods. As in the case of AM1 calculations spin-restricted ab initio Hartree–Fock (RHF) calculations of long-chain strepto[ N]methines ( 1) and pyrido[ N]methines ( 2) resulted in symmetry-lowered ground state structures. This occurs with N=19 for 1 and of N=11 with 2 at the RHF/6-31G* level. However, if electron correlation is taken into account by Møller–Plesset perturbution theory of second-order (MP2) or by density functional theory (DFT) the classical symmetric structures are favored. In contrast to spin-restricted DFT calculations with Becke's three-parameter exchange functional (B3LYP), symmetry-breaking occurs at the DFT BHLYP level but at much longer-chain lengths than in the HF approximation. According to polarized continuum model (PCM) calculations, low-symmetry structures are favored in polar solvents. The interpretation of the absorption spectra of the polymethine dyes dissolved in polar solvents is more difficult as earlier assumed and additional experimental studies are necessary to rationalize the nature of the NIR absorptions.