Yevgen M. Poronik

Octupolar merocyanine dyes: a new class of nonlinear optical chromophores.

A set of new octupolar merocyanine chromophores was designed and synthesized. These compounds were prepared from the reaction of 1,3,5-triformyl-2,4,6-trihydroxybenzene with heterocyclic nucleophiles. Octupolar dyes were formed exclusively in their open-dye form. The one- and two-photon-absorption spectra of the dyes consist of two bands: The long-wavelength band in the two-photon absorption spectrum (a few hundreds GM above 1000 nm) matches well with the intense, long-wavelength-absorption band that is located in the visible region in the linear spectrum. Interestingly, an additional, much-more-intense TPA band in the NIR region is observed at higher energy, which corresponds to a weakly allowed one-photon electronic transition. Changing the peripheral heterocyclic moieties allows tuning of the optical properties to approach the cyanine limit (i.e., polymethine state), thus resulting in a red-shift of the low-energy one-photon-absorption band as well as to the rise of an intense two-photon-absorption band in the NIR region. To the best of our knowledge, this is the first synthesis and TPA characterization of octupolar merocyanine chromophores with typical low-bond-length alternation.

Pyrrolo[3,2-b]pyrroles—From Unprecedented Solvatofluorochromism to Two-Photon Absorption

A combined experimental and theoretical study of the two-photon absorption (2PA) properties of a series of quadrupolar molecules possessing a highly electron-rich heterocyclic core, pyrrolo[3,2-b]pyrrole, is presented. In agreement with quantum-chemical calculations, large 2PA cross-section values, σ2PA ≈10(2) -10(3)  GM (1 GM=10(50)  cm(4)  s photon(-1) ), are observed at wavelengths of 650-700 nm, which correspond to the two-photon allowed but one-photon forbidden transitions. The calculations also predict that increased planarity of this molecule through removal of two N-substituents leads to further increase in the σ2PA values. Surprisingly, the most quadrupolar pyrrolo[3,2-b]pyrrole derivative, containing two 4-nitrophenyl substituents at positions 2 and 5, demonstrates a very strong solvatofluorochromic effect, with a fluorescence quantum yield as high as 0.96 in cyclohexane, whereas the fluorescence vanishes in DMSO.

V-Shaped Bis-Coumarins: Synthesis and Optical Properties

A highly efficient procedure for the synthesis of bis-coumarins fused at the pyranone ring has been developed. The electron-rich phenols reacted with esters of coumarin-3-carboxylic acids, leading to substituted chromeno[3,4-c]chromene-6,7-diones. The reaction is catalyzed by both Lewis acids and 4-dimethylaminopyridine. The most probable mechanistic pathway involves Lewis acid catalyzed or DMAP catalyzed transesterification, followed by intramolecular conjugate addition of α,β-unsaturated esters to phenols and subsequent oxidation of the initially formed intermediate. The reaction is compatible with various functionalities such as NO2, Br, and OMe. Not only benzene derivatives but also dihydroxynaphthalenes are reactive in this reaction, and the structure of the product can be controlled by adjusting the reaction conditions. Furthermore, a double addition is possible, leading to a horseshoe-shaped system comprised of seven conjugated rings. Compounds with four structurally unique skeletons have been obtained and have been shown to strongly absorb in the violet, blue, and/or green regions of the visible spectrum. Most of them display strong greenish yellow fluorescence, which can be modulated by both structural changes and the character of the solvents. Again, introduction of an electron-donating group in the chromeno[3,4-c]chromene-6,7-diones caused a significant red shift in both the absorption and emission maxima, and the effect became especially noteworthy in the case of amino substituents.

Nonlinear Optical Chemosensor for Sodium Ion Based on Rhodol Chromophore

As part of a strategy to identify good fluorescent probes based on two-photon excited fluorescence (TPEF), the sensor for sodium cation has been designed bearing a rhodol chromophore linked with an aza-crown ether. An efficient synthetic route to rhodol derivatives possessing five-membered heterocycles at position 9 and their precursors that contain xanthylium salt has been developed. The synthesis involves condensation of xanthylium salts bearing vinamidinium moiety at position 9, with phenylhydrazine derivatives as the key step. To accomplish the synthesis of derivatives bearing 1-aza-15-crown-5 and 1,10-diaza-18-crown-6, the Buchwald-Hartwig reaction has been employed in the final stage. Electronic spectra of all prepared rhodols display strong absorption in the range of 450-550 nm with well-resolved vibronic bands, which maintains its fine structure in a wide range of solvents. The most intensive two-photon absorption (2PA) band in the rhodol spectrum (165 GM), located at shorter wavelengths, matches well with the short-wavelength absorption band in the linear electronic spectrum and is most probably related to the two-photon allowed electronic transition S0→S2. The influence of cation binding on one- and two-photon spectroscopic properties of rhodol linked with 1-aza-15-crown-5 via the phenylpyrazole bridge has been investigated. This probe exhibits high sensitivity and good selectivity for Na(+) in CH3CN. The mechanism involves the complexation of the Na(+) by 1-aza-15-crown-5 in the probe, which induces prominent fluorescence enhancement via quenching of electron-transfer. Interestingly, the complexation with Na(+) led to a significant increase of the 2PA band in the 750-800 nm region (corresponding to a two-photon allowed, one-photon forbidden transition) for rhodol bearing 1-aza-15-crown-5, which led to the overall enhancement of the TPEF signal (approximately an order of magnitude). Thus, a turn-on fluorescent probe for sodium ion, which does not respond to many other metal species, has been constructed.

Corroles bearing diverse coumarin units — synthesis and optical properties

Two strategies were shown to be efficient in the construction of corroles with appended coumarin units. Direct condensation of formyl-coumarins with dipyrromethanes leads to a diverse range of trans-A2B-corroles in moderate yields. We showed that the strategy consisting of synthesis of various hydroxycoumarins followed by nucleophilic aromatic substitution with pentafluorobenzaldehyde and subsequent condensation of the resulting coumarin-aldehyde with dipyrromethanes is the most general methodology for the preparation of such dyads. The second, more demanding but also more efficient approach is based on Sonogashira coupling of ethynylphenylcorroles with suitably functionalized bromocoumarins. A broad range of structurally diverse coumarins were employed with absorption ranging from 300 to 460 nm. Spectroscopic properties of all eight dyads studied suggest that the linker components are weakly electronically coupled.

Experimental and Theoretical Investigation of the First-Order Hyperpolarizability of Octupolar Merocyanine Dyes

Hyper-Rayleigh scattering experiments and quantum chemical calculations are combined to investigate the second-order nonlinear optical responses of a series of three-arm merocyanine derivatives. They exhibit an octupolar hyperpolarizability response with lower amplitude than crystal violet due to a lower extent of the photoinduced charge transfer and reduced bond length alternation. Strong effects on the second-order optical response measured close to the two-photon absorption level are clearly evidenced; for example, the effective measured polarization ratio deviates below the ideal octupolar value of 3/2 even at very low excitation power. These effects are attributed to two-photon absorption resonance, which we believe modifies dynamically the population of the ground state versus that of the excited state.

Dipolar Dyes with a Pyrrolo[2,3‐b]quinoxaline Skeleton Containing a Cyano Group and a Bridged Tertiary Amino Group: Synthesis, Solvatofluorochromism, and Bioimaging

Two strongly polarized dipolar chromophores possessing a cyclic tertiary amino group at one terminus of the molecule and a CN group at the opposite terminus were designed and synthesized. Their rigid skeleton contains the rarely studied pyrrolo[2,3-b]quinoxaline ring system. The photophysical properties of these regioisomeric dyes were different owing to differing π conjugation between the CN group and the electron-donor moiety. These dipolar molecules showed very intense emission, strong solvatofluorochromism, and sufficient two-photon brightness for bioimaging. One of these regioisomeric dyes, namely, 11-carbonitrile-2,3,4,5,6,7-hexahydro-1H-3a,8,13,13b-tetraazabenzo[b]cyclohepta[1,2,3-jk]fluorene, was successfully utilized in two-photon imaging of mouse organ tissues and showed distinct tissue morphology with high resolution.

2,5-Bis(azulenyl)pyrrolo[3,2-b]pyrroles – the key influence of the linkage position on the linear and nonlinear optical properties

The first route towards pyrrolo[3,2-b]pyrroles containing two azulene moieties at positions 2 and 5 was developed. The key step of this approach is the three-step transformation of pyridine scaffolds into azulene via sequential N-arylation followed by ring-opening and a reaction with cyclopentadiene. The resulting quadrupolar acceptor–donor–acceptor compounds possess interesting optical properties such as bathochromically shifted absorption with the magnitude of the red-shift strongly dependent on the linkage position. Two-photon absorption of these functional dyes is markedly different from that of previously described pyrrolo[3,2-b]pyrroles. The experimental optical spectra were rationalized using time-dependent density functional theory calculations of both the linear and nonlinear optical properties.

How a Small Structural Difference Can Turn Optical Properties of π-Extended Coumarins Upside Down: The Role of Non-Innocent Saturated Rings

The fluorescence properties of two new families of heterocycles possessing either a seven- or five-membered ring attached at the core molecule are entirely different in solution and in the solid state. Crystallization has the effect of inhibiting non-radiative excited-state deactivation pathways, operative in solution for the seven-membered ring compounds, thus leading to significant fluorescence efficiency in the solid state, with quantum yields ranging from 0.10 to 0.36. Conversely, the five-membered ring derivatives, which display notable emission properties in solution, are almost non-emissive in the crystalline state, characterized by a long-range π-stacked arrangement. When embedded in polymeric films, both series show fluorescence features similar to the solution case, with remarkable fluorescence quantum yields ranging from 0.09 to 0.41. According to quantum chemical calculations, 3H-chromeno[3,4-c]pyridine-4,5-diones show the specific mechanism of fluorescence quenching. The derivatives bearing the seven-membered ring undergo, in solution, a significant structural deformation in the excited state, resulting in a large decrease of the energy gap between S1 and S0 and hence to a substantial contribution of the internal conversion in the relaxation process. The fluorescence quenching of the five-membered ring derivatives is in turn related to the intermolecular interaction between adjacent molecules prevailing to a greater extent in the crystal lattice.

The Niementowski reaction of anthranilic acid with ethyl acetoacetate revisited: a new access to pyrano[3,2-c]quinoline-2,5-dione

The reaction of anthranilic acid with ethyl acetoacetate gives rise directly to 4-methylpyrano[3,2-c]quinoline2,5-dione. The mechanism is assumed to involve condensation of the initial product with a second molecule of the β-ketoester. None of the expected compound (2-methyl-4-oxo-1,4-dihydroquinoline-3-carboxylic acid) is formed in the reaction.