Keywords: Ab initio calculations / Absorption / Dioxaborine / Fluorescence / Polymethine dyeThe natures of the chromophores in symmetric polymethine dyes derived from 2,2-difluoro-1,3,2-dioxaborine have been investigated. Ab initio quantum chemical calculations demonstrated that the presence of dioxaborine end residues stabilizes the frontier levels of the corresponding polymethine dye and makes electron-density distribution over the oxygen atoms in the chelate ring more even than in the analogous dye structure with boron-free acyclic end groups. A series of novel symmetric polycarbocyanines and a tricarbocyanine series with variously bridged polymethine chromophores have been synthesized from hitherto unknown pyrimidino- IntroductionSince the second half of the last century, it has been known that 2,2-difluoro-1,3,2-dioxaborines bearing a methyl or methylene group at position 4 of the heterocycle can form deeply coloured polymethine dyes (PDs). [1][2][3][4][5][6] A special interest in π-conjugated systems derived from 2,2-difluoro-1,3,2-dioxaborines has stemmed from their peculiar electronic and spectral luminescent properties, such as high hyperpolarizabilities, [7,8] wide ranges (from UV to near IR wavelengths) and high intensities of absorption [1,2,[4][5][6][7][8][9][10][11][12] and fluorescence, [10][11][12][13] photosensitizing activities towards photoconducting materials [3,4] and metallic silver, [13] and large two-photon cross sections.[13] The wide application of 2,2-difluoro-1,3,2-dioxaborines has been reviewed. [14,15] An effective instrument for the design of PDs with specified parameters is offered by structural variation in the polymethine chains (PCs) and heterocyclic end groups.[16] This approach calls for a deep insight into the relationships between the electronic structures and the spectral properties of the symmetric PDs concerned. However, the literature devoted to 2,2-difluoro-1,3,2-dioxaborine PDs is mostly focused on synthetic [1][2][3][4][5][6]8] and some colour [12,17] well as on various physical properties. [7,8,13,15,17] The natures of the chromophore systems and the general colour regularities have so far remained unexplored.This study, based on the nonempirical ab initio method (UHF/6-31G**), addresses the effect of the ring-forming BF 2 moiety on the frontier level positions and the electrondensity distributions in the molecules of symmetric carbocyanines with variously derivatized 2,2-difluoro-1,3,2-dioxaborine end residues. A novel polycarbocyanine series has been synthesized from previously unknown pyrimidinoannelated 2,2-difluoro-1,3,2-dioxaborines, and the spectral properties of the dyes obtained have been investigated. Results and DiscussionThe simplest symmetric 2,2-difluoro-1,3,2-dioxaborine PD 1, with its first absorption maximum found at λ max = 519 nm in CH 3 CN, was synthesized by us starting from the boron complex of acetylacetone: [17] Dyes of this kind are regarded as oxonoles: that is, oxygen analogues of cyanine dyes bearing delocalised negative charges.[6] A cla...

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