Theoretical investigation of the absorption spectrum of thioindigo dyes

The low-lying excited states of trans-thioindigo derivatives have been determined by using time-dependent density functional theory. A large panel of atomic basis sets has been tested and it turns out that 6-311G(d,p) provides accurate geometries, whereas a more extended basis, 6-311+G(2d,p), is necessary for describing excited states. Adamo and Barone's fitted-parameter-free hybrid functional (PBE0) gives λmax in very good agreement with experiment once bulk solvent effects are taken care off. Using the PCM-TD-PBE0/6-311+G(2d,p)//PCM-PBE0/6-311G(d,p) level of theory, the effects of various types of substitution are accurately reproduced; with the largest λmax deviation being limited to 10 nm.