Refractive index dependence of solvatochromism

Design and Photonics of Merocyanine Dyes

Merocyanines, thanks to their easily adjustable electronic structure, appear to be the most versatile and promising functional dyes. Their D-π-A framework offers ample opportunities for custom design through variations in both donor/acceptor end-groups and the π-conjugated polymethine chain, and leads to a broad range of practical properties, including noticeable solvatochromism, high polarizability/hyperpolarizabilities, and the ability to sensitize various physicochemical processes. Accordingly, merocyanines are applied and extensively studied in various fields, such as light-converting materials for optoelectronics, nonlinear optics, optical storage, solar cells, fluorescent probes, and antitumor agents in photodynamic therapy. This review encompasses both classical and novel more important publications on the structure-property relationships in merocyanines, with particular emphasis on the results by A.  I. Kiprianov and his followers in Institute of Organic Chemistry in Kyiv, Ukraine.

Polymerization of Physisorbed Molecular Monolayers via Overhanging Alkynyl Chains: Characterization of Polymerization Kinetics and Monolayer Durability

Monolayers self-assembled by triphenyleneethynylene (TPE) compounds bearing two terminal alkynyl chains were polymerized by Glaser-Hay (G-H) alkyne coupling at the acetonitrile-HOPG interface. The alkynyl chains extend into the solution due to the monolayer's dense-packed morphology. Reacting substructures that have no morphology-determining roles is a potential strategy for preserving monolayer morphology throughout polymerization. Monolayer G-H reaction kinetics and polymerized monolayer durability were characterized by using mass spectrometry and fluorescence. Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (MS) and time-of-flight (TOF) MS were used to identify TPE-oligomers in the monolayer and to track the monolayer populations of TPE-monomer, -dimer, and -trimer as a function of G-H reaction duration. Comparison of the observed kinetics to a Monte Carlo simulation provided evidence of step-growth polymerization. The durability of polymerized monolayers depended strongly on the length of the alkynyl chains linked by G-H reaction. Polymerized T6y monolayers (O(CH2)3C≡CH alkynyl chains) desorbed minimally during 16-h immersion in 90 °C o-dichlorobenzene (oDCB), whereas polymerized T8y (O(CH2)5C≡CH alkynyl chains) and polymerized T11y (O(CH2)8C≡CH alkynyl chains), desorbed 33 and 60%, respectively, of their TPE units after 4 h in 90 °C oDCB. All the polymerized monolayers are much more durable than unpolymerized monolayers, which desorb quantitatively from HOPG when rinsed with 25 μL of oDCB. Polymerized T6y monolayer is a highly durable anchor that may be adapted to build multilayer structures "permanently" attached to the HOPG surface. The alkynyl chain length dependence may be useful for tuning polymerized TPE monolayer durability for specific applications.

‘Hybrid’ mero-anionic polymethines with a 1,3,2-dioxaborine core

The synthesis and characterization of new mero-anionic polymethines of the D–π–A–π–A′ type are described.

Highly Fluorescent Dianionic Polymethines with a 1,3,2-Dioxaborine Core

The difluoroboron β-diketonate ring is ever more used for creating bright polymethine-type fluorophores for the visible and NIR range. Here, we report the synthesis and spectral properties of a series of dianionic cyanine dyes of the rare A¹-π-A-π-A¹ type, with the central dioxaborine ring (A) embedded into the polymethine chain and various electron-acceptor terminal groups A¹. Depending on the nature of the end group, the maxima of their intensive (with molar extinctions up to 380 000 M^-1 cm^-1) and narrow long-wavelength absorption band lie in the range of 530-770 nm. Their absorption and fluorescence bands are nearly mirror-like and characterized by weak solvatochromism; the marked hypsochromic shifts are observed only when going from polar aprotic solvents to methanol. The designed dianionic dyes have fluorescence quantum yields up to 92 % in the visible range, and even for the NIR dyes, the values of 18-37 % are observed in DMF.

The physical significance of the Kamlet–Taft π* parameter of ionic liquids

The Kamlet–Taft dipolarity/polarizability parameters π* for various ionic liquids were determined using 4-tert-butyl-2-((dicyanomethylene)-5-[4-N,N-diethylamino)-benzylidene]-Δ3-thiazoline and 5-(N,N-dimethylamino)-5′-nitro-2,2′-bithiophene as solvatochromic probes.

Probing of local polarity in poly(methyl methacrylate) with the charge transfer transition in Nile red

The permittivity of polymers and its spatial distribution play a crucial role in the behavior of thin films, such as those used, e.g., as sensor coatings. In an attempt to develop a conclusive approach to determine these quantities, the polarity of the model polymer poly(methyl methacrylate) (PMMA) in 600 nm thin films on a glass support was probed by the energy of the charge transfer transition in the oxazine dye Nile red (NR) at 25 °C. The absorption and fluorescence spectra of NR were observed to shift to the red with increasing solvent polarity, because of the intramolecular charge transfer character of the optical transition. New types of solvatochromic plots of emission frequency against absorption frequency and vice versa afforded the Onsager radius-free estimation of the ground and excited states dipole moment ratio. With this approach the values of these dipole moments of 11.97 D and 18.30–19.16 D, respectively, were obtained for NR. An effective local dielectric constant of 5.9–8.3 for PMMA thin films was calculated from the solvatochromic plot and the fluorescence maximum of NR observed in the PMMA films. The fluorescence band of NR in the rigid PMMA films shifted to the red by 130 cm⁻¹ with increasing excitation wavelength from 470 to 540 nm, while in a series of liquids the position of the emission maximum of NR remained constant within same range of the excitation wavelength. It is concluded that the fluorescence spectrum of NR in PMMA undergoes inhomogeneous broadening due to different surroundings of NR molecules in the ground state and slow sub-glass transition (T_g) relaxations in PMMA.