A vinylogous series of highly dipolar merocyanines were designed to study their solvatochromism in a wide range of solvents including low-polarity alkanes. It has been revealed that the lower vinylogues indeed have negative solvatochromism in the full range of solvent polarities starting from n-hexane, while the hexamethinemerocyanine exhibits reversed solvatochromism. With the extreme ranges of solvatochromism, the studied dyes possess 5-7 times weaker solvatofluorochromism, which can be rationalized via their decreased dipolarity in the fluorescent state. They also demonstrate an inverse dependence of their fluorescence quantum yield on solvent polarity and have near-record Stokes shifts in high-polarity media. The experimental data are supplemented by the results of DFT quantum chemical analysis of dye electronic structures in both the ground and excited states with PCM solvent field simulation.
A series of merocyanines derived from 1,3‐indanedione and heterocycles of various electron‐donating properties was studied in detail. Their solvatochromic properties were explored in a wide range of solvent polarities to reveal the dependences of their chromacity and electronic structure on the key structural parameters – the properties of a donor heterocycle and the polymethine chain length. Also the dyes were studied by NMR spectroscopy and by quantum chemical calculations, both with the semiempirical AM1 and the non‐empirical density functional theory/B3LYP method. The solvatochromic properties of the explored dyes are rather close to those of merocyanines derived from malononitrile as acceptor group. Appreciable distinctions were observed only in protic ethanol; obviously, they are connected with the formation of solvent–solute H‐bonds in the case of 1,3‐indanedione derivatives. The electron‐acceptor properties of 1,3‐indanedione were found to be somewhat stronger in comparison with those of malononitrile even in aprotic solvents, contrary to the known literature data. Analysis of the merocyanines' molecular orbitals and simulation of their electronic spectra were carried out both in vacuum and in the solvent matrix, and the absorption electronic transitions were analyzed. Static nonlinear optical properties were calculated for both the new merocyanines and the corresponding cationic and anionic cyanine dyes. Copyright © 2010 John Wiley & Sons, Ltd.
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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