Polishchuk V. Dyes with a dioxaborine cycle in the polymethine chain

The dissertation is devoted to the synthesis and study of the spectral properties of dyes with a dioxaborine cycle in the polymethine chain. A synthetic method of obtaining 2,2-difluoro-4,6-dimethyl-5-cyano-(2H)-1,3,2-dioxaborine was developed, which is based on the interaction of 3-cyano-subsituted acetylacetone with boron trifluoride etherate. Cyanosubstitution is an effective way of increasing the reactivity of methyl groups of the dioxaborine complex in cyanine condensations. This paved the path to a simple synthesis of various series of polymethine dyes constituting dioxaborine cycle as an integral part of the polymethine chain. In particular, dyes of the D–π–A–π–D type with various heterocyclic end-groups (D) and different length of the polymethine chain were obtained through the interaction of hemicyanines with cyano-substituted dioxaborine. Depending on the nature of the end-group and the length of the polymethine chain, the synthesized compounds absorb and emit light in the far-red and near-infrared regions of the spectrum (660-830 nm). It was demonstrated that the increase in the donor strength of the end-groups induce a bathochromic shift of the long-wavelength absorption maximum along with increasing of the absorption intensity. However, an enhancement of the donor strength of end-groups also significantly decreases stability of the dyes. A synthetic method of obtaining the polymethine dyes of an unusual D–π–A–π–A′ type constituting central dioxaborine fragment (A), a donor indolenine group (D), and terminal groups of different electron acceptors (A′) was developed. The method includes the reaction of monosubstituted merocyanine (D–π–A) with ethoxyylidene or anilinovinyl derivatives of CH-acids. The obtained dyes possess a unique "hybrid" anionic-merocyanine structure, since both merocyanine (D–π–A) and anionic (A–π–A′) fragments are converged in a single π-conjugated chromophore system. Synthetic approaches to the lengthening of the π-chain in anionic (A–π–A′) as well as in merocyanine (D–π–A) parts of the chromophore system were developed. The extension of the anionic part is a two-stage process, which includes a reaction of merocyanine with dianyl malondialdehyde hydrochloride and subsequent condensa-tion of the resulting hemicyanine with CH-acids. Lengthening of the π-chain in the merocyanine part is achieved through the condensation of dioxaborine with longer-chained cationic hemicyanine and subsequent reaction of the resulting merocyanine with ethoxyylidene derivatives of CH-acids. Correlations of an impact of both the structure of an acceptor end-group (A′) and the length of the polymethine chain on the spectral properties of mero-anionic dyes were established. It was found that the molar extinction coefficient decreases with the lengthening of the polymethine chain in the merocyanine part (D–π–A) of the dye. By contrast, lengthening of the anionic part (A–π–A′) leads to an increase of the absorption intensity of the mero-anionic chromophore. It was found that mero-anionic dyes possess high electronic symmetry of the π-conjugated system both in the ground state S0 and in the first excited state S1. As a result, mero-anionic dyes fluoresce intensely in the visible and near-IR regions of the spectrum. An effective approach to the synthesis of an understudied type of polymethine dyes possessing dianionic nature of the chromophore was developed. Synthetic protocol includes the condensation of dioxaborine-containing bis-hemicyanine with CH-acids yielding a number of polymethines of A′–π–A–π–A′ type containing end-groups (A′) of various structure. It was found that the dioxaborine-containing dianionic dyes are very effective fluorophores with the fluorescence quantum yields reaching 0.90. In addition, the brightness (ε•Φf) of the dye constituting thiobarbituric end-groups reaches a value of 255000 M•cm, which is one of the largest values among known chromophoric systems. An electronic nature of the dianionic dioxaborine-containing chromophore was investigated by physicochemical and quantum chemical methods. It was found that the dianionic π-conjugated chromophore is characterized by a high degree of delocalization of negative charges in the π-conjugated system. At the same time, an enhancing of the acceptor strength of the end-groups leads to an increase in contribution of the resonance structure with negative charges located on the end-groups. The influence of the solvents’ nature on the spectral properties of the dianionic dyes was investigated. The photostability of the obtained dyes was evaluated in comparison with classic cationic and anionic polymethines. During the investigation of the chemical properties of monosubstituted dioxaborine-containing hemicyanines possessing free methyl group it was found, that these compounds undergo oligomerization under strong basic conditions yielding polymethines with polyanionic electronic nature. A detailed study of the chemical behavior