Computational dye design by changing the conjugation order: Failure of LR-TDDFT to predict relative excitation energies in organic dyes differing by the position of the methine unit

In dye design by changing the conjugation order, computational errors behave differently and are more important than when changing the size of conjugation. A different position of the methine unit in dyes 2-Cyano-3-[5′-(4″-(N,N-dimethylamino) phenyl) thiophen-2′-yl]-acrylic acid and Cyano-[5-(4′-(N,N-dimethylamino) benzylidene)-5H-thiophen-2-ylidene]-acetic acid induces an uncommonly large difference in absorption spectrum. Long-range effects cause a failure of TDDFT using ab initio and hybrid functionals to predict even relative transition energies in spite of a small degree of charge transfer. Orbital energies remain good predictors of the relative energetics and are used to compute the effect of nuclear motions on absorption spectrum.