DFT studies of squarylium and core‐substituted squarylium dye derivatives: understanding the causes of the additional shorter wavelength absorption in the latter

Recent reports indicate that the core-substituted squarylium (CSQ) dyes (obtained when one oxygen atom of the SQ moiety is replaced by an electron withdrawing group or sulfur) show bathochromic shift of the absorption maxima and an additional shorter wave length absorption in visible when compared to parent SQ dyes. To investigate this interesting property of these dyes which will be more suitable for applications in DSSC, a comparative study using computational techniques of some selected SQ and CSQ derivatives has been carried out. The effect of this core substitution on geometries is studied. The ground state charge distribution is analyzed by natural population analysis. It is noticed that the biradical character, which is normally large in SQ derivatives, is reduced in CSQ due to the substitution and the zwitterionic character is increased. The absorption maxima for both parent SQ and CSQ dyes obtained with TD-DFT methods using various functional like B3LYP, M06-2X and CAM-B3LYP methods do not match the experimental results. However, results obtained using SAC-CI methodare better. Charge transfer (CT) data based on Mulliken charges of both ground and excited states is obtained from SAC/SAC-CI studies. It is seen that on excitation substantial CT from the side groups to the central core is taking place in parent SQ molecules. In contrast, intense CT occurs from – Xt o side groups through central core in the case of CSQ molecules. This study will be helpful in designing and synthesizing new CSQ dyes which makes them suitable for solar cell applications. Copyright © 2012 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this paper