Electronic spectra and (hyper)polarizabilities of non-centrosymmetric D–A–D chromophores. An experimentally based three-state model and a theoretical TDDFT study of ketocyanines

Abstract

The electronic structure, spectra and linear and second-order polarizabilities of two symmetric ketocyanines, which are prototypic examples of D-A-D chromophores, have been investigated with two different toolsets: (i) the so-called 'essential-state model', here comprising three states, the ground and two lowest excited (1)ππ* states, has been adapted for these non-centrosymmetric, yet symmetric compounds to determine their permanent electric dipole moments, polarizabilities and first hyperpolarizabilities making use of experimental transition energies and moments; (ii) extensive TDDFT calculations have provided ground-state conformational results consistent with NMR-derived structural information, energies and dipole moments of up to 20 lowest-lying electronic states as well as, within the sum-over-states (SOS) scheme, the most relevant components of the polarizabilities and first hyperpolarizabilities. The two levels of description form consistent pictures of the ketocyanine excited states that provide the most relevant contributions to hyperpolarizabilities: extension of the SOS set beyond the three states of the basic model left unchanged (within ∼10%) the calculated vector component of the second-order polarizability tensor along the direction of the ground-state dipole moment (β(y)). Both approaches indicate that these D-A-D compounds, in spite of their quasi-linear structure, reminiscent of that of centrosymmetric quadrupolar chromophores, feature significant second-order molecular polarizabilities. These rapidly increase with the length of the polyenic bridges in the chromophores. About half of the total value of β(y) is predicted to come from the three-level-term part, β(y,3), most of which derives from the contribution involving the three electronic states of the essential-state model.

Electronic spectra and (hyper)polarizabilities of ketocyanine dye complexes with metal ions

“…Such results is unexpected in view of the fact that symmetric ketocyanines are quasi linear and would have an affinity with centrosymmetric D-A-D chromophore that have vanishing first hyperpolarizabilities. Earlier work on other symmetric ketocyanines has shown similar observation as the calculated first hyperpolarizabilties are quite significant in agreement with our calculations [29].…”

“…The polyenic bridge in KD1 (with D-A-D configuration) retain essentially the same bond length alternation as in the parent merocyanine with D-A configuration. Similar observation has also been made by other workers [29]. In dye-metal ion complexes, however, the bond length alternation is significantly modified as compared to pure dye.…”

“…In recent times, a considerable amount of research attention has been focused on the design of molecule-based materials with large nonlinear optical (NLO) responses [26][27][28]. Very recently, linear and first hyperpolarizabilities of two non-centrosymmetric yet symmetric ketocyanines have been calculated by Ponterini et al on the basis of essential three state model showing substantial vectorial components of first hyperpolarizability which are quite comparable with their parent merocyanines and thus proved to possess significant nonlinear optical properties [29]. In this context their very recent linear and first hyperpolarizability calculation based on three-state-model and TDDFT-SOS analysis of a penta-heptamethine ketocyanine is quite important [30].…”

Electronic spectra and hyperpolarizabilities of structurally similar donor–acceptor dyes. A density functional theory analysis

“…The observed change in the bond length indicates that the percent contribution of the 'VB-CT' form increases due to protonation at the carbonyl O centre of the dyes. Similar observations have been reported for similar dyes undergoing complexation with metal ions at the carbonyl O centre [24,26]. Table 1.…”

Chemical asymmetry and α and β polarizabilities of D-A-D′ chromophores: a three-state-model and TDDFT-SOS analysis of a penta-heptamethine ketocyanine

“…Then, we have set up a three-state model for KC2,3 by a simple numerical procedure using the parameters previously derived from the experimental data for the symmetric (C 2v ) analogues, KC2 and KC3. 8 The model, consistent with the electronic spectrum and the structure of KC2,3, has been used to obtain the whole set of g, c and e state properties, from which hai and b vec have been evaluated according to the SOS approach. In order to verify the capability of the three-state model to correctly account for the effects of the chemical asymmetry, we have performed an accurate analysis of the (hyper)polarizabilities of KC2,3 by TDDFT SOS calculations considering up to twenty excited states.…”