Structure, Bonding, and Lowest Energy Transitions in Unsymmetrical Squaraines: A Computational Study

Abstract

Natural resonance theory (NRT) and natural bond orbital (NBO) analysis have been carried out on a simple symmetrical and an unsymmetrical substituted squaraine with a view of understanding the structure of the latter type of squaraines. It is found that there are some fundamental differences in the structure and bonding between these two types of squaraines particularly in the resonance weights and delocalization energies. These differences are expected to reflect in the low energy transitions and charge transfer in these squaraines. To investigate this, the nature of the lowest energy transitions occurring on excitation in unsymmetrical squaraines has been studied using high-level symmetry adapted cluster-configuration interaction method (SAC/SAC-CI) and compared with reported experimental observations. In general the agreement with the experimental data is very good. The transition dipole moment always lies on the pi-backbone and is quite large in magnitude. The ground state dipole moment in some cases does not change in the excited state upon excitation while in some other cases there is a large reduction/enhancement in the magnitude indicative of some charge rearrangement in this direction. Inclusion of the solvent using the IEFPCM model, a slightly better agreement with the experiment is found in some cases. Studies are carried out with a different basis set and it is found that the change in basis set has very little effect on the transition energies. In the case of weak side donor groups attached to the central ring the larger charge transfer to the central acceptor ring in general takes place from the O- atoms of the squarylium moiety while in the case of strong donors the charge transfer from the O- atoms to the central rings drop down. We have not observed any correlation between the charge transfer in the excited state to the central ring from the side donor groups and the lowest energy excitation in the molecules. Reduction of the HOMO-LUMO gap (an indication of increase of the diradicaloid character) always leads to a bathochromic shift.

Influence of a polarizable surrounding on the electronically excited states of aggregated perylene materials

“…To study solvent effects we first employed the integral equation formalism variant of PCM (IEFPCM) in combination with HF for the ground state and with TD‐HF for the description of excited state properties . As solvents we studied cyclohexane ( ε = 2.028, n = 1.424) and water ( ε = 78.39, n = 1.333) to include environments with large dielectric constants ε as well as high refractive indices n .…”

Electronic Structure of Bis(4‐dimethylaminophenyl)squaraine

“…The representation in Figure may be regarded as one of the least preferred resonance forms of the molecule. In fact, assigning −1 charge on oxygen suggests 1,3‐interaction between oxygen‐bearing carbon centers of the squaraine unit while a zero charge representation on oxygen promotes 1,3‐interaction between phenyl ring bearing carbon centers (Figure ) . The well‐known solvatochromic behavior of the dye and its intense absorption in the red and near‐infrared region [12] of electromagnetic spectra are intimately related with the quadrupolar character.…”

Near infrared absorbing benzobis(thiadiazole) derivatives: computational studies point to biradical nature of the ground states

“…have reported isolable stable singlet biradicaloids, the biradical character confirmed by temperature dependent magnetic susceptibility experiments and supported by theoretical studies 32. These molecules resonate between the biradical and zwitterionic forms in the ground state and they have absorption in the NIR region 26–33…”

“…Some promising NIR dyes are the squarylium dyes, croconate dyes, and recently synthesized derivatives of di‐(benzofuranonyl)methanolate (BM dye) all of which contain the meso‐ionic oxyallyl substructure 1–4, 12–14. We have recently carried out high level computational studies of these dyes and attributed the NIR absorption to the singlet biradicaloid character of the oxyallyl substructure 26–31. More recently Koide et al .…”