A comparative study of semi-squaraine and squaraine dyes using computational techniques: tuning the charge transfer/biradicaloid character by substitution

Abstract

Semi-squaraines (SMSQ) are known as donor-acceptor (D-A) type molecules whereas squaraines (SQ), which differs from SMSQ by an extra donor group, are more or less biradicaloids in nature. The effect of the additional donor group in SQ, which changes the nature of the molecule, on geometrical and electronic structure are studied here and compared with the corresponding SMSQ. It is noticed from the geometrical parameters that, a strong resonance exists in SQ whereas disparity in carbon-carbon bond lengths of central C4 ring is seen in SMSQ dyes. The increasing and decreasing of antibonding interactions between central C4 ring and side donor groups cause destabilization of HOMO and stabilization of LUMO respectively in case of SQ compared to SMSQ molecules. This leads to decreasing the HOMO-LUMO gap and promotes biradicaloid character of SQ. The absorption maxima obtained by using TD-DFT method with BLYP, B3LYP, BHandHLYP, CAM-B3LYP and M06-2X functionals are not in good agreement with experimental results. On the other hand SAC-CI method gives better results for all the molecules. From this work we can evolve a design principle of these molecules which play a role as sensitizers in dye sensitized solar cells.

Molecular aspects of squaraine dyes aggregation and its influence on spectroscopic properties

“…From the computational point of view, squaraine dyes still are challenging and only scarce study is present in the literature mostly for single isolated molecules or simple models of dye-sensitized solar cell [14,[21][22][23][24][25][26][27][28][29][30]. Thus, the current contribution aims to carefully analyze mutual interactions in the small squaraine aggregates and their influence on the spectroscopic properties.…”

Visible to NIR absorbing C–N and C–C bonding squaraines: A computational study

“…The electronic excitations of CSQ molecules are underestimated by TDDFT methods, and the reason may be due to presence of some diradical character (DC) in these molecules . Various methods are available to estimate the DC .…”

“…Further details of this method are available in literature . This method has been used earlier successfully for CSQ molecules (a difference of 0.1 eV with the experimental values was observed) . The singlet geometries of CSQ molecules obtained at B3LYP/6‐311G (d, p) level are subjected to SAC‐CI calculations with 6‐311G (d, p) basis set.…”

“…Lowest 5 singlet excitation energies were calculated using the time‐dependent density functional theory (TDDFT) methodology with 6‐311G (d, p) basis set. The SAC‐CI calculations were carried out with the active space option, and active space was chosen with the window option, depending on the molecule size; some core and some virtual orbitals are not considered in active space (the minimum is not less than 200, and the maximum is 360 orbitals) . Frontier molecular orbitals (FMOs) are responsible for the electronic excitations leading to absorption maxima pictorially visualized from the population analysis carried out at B3LYP/6‐311G (d, p) level.…”

“…Squaraines consist of 2 donor moieties (D), which are based on arylamine or heterocyclic derivatives connected to central ring of 4‐membered (C 4 O 2 ) electron withdrawing group (A). In our earlier reports, we have carried out by symmetry adopted cluster configuration interaction (SAC‐CI) calculations for some NIR absorbing C–C bonding squaraine (CSQ) molecules and shown that there is no large charge transfer from side donor groups to the central acceptor ring . It is also found that the major transition of these molecules is from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO), which is mainly localized at central acceptor ring.…”