Alexei I. Tolmachev

Syntheses and photophysical properties of a series of cation-sensitive polymethine and styryl dyes

The syntheses and photophysical properties of 20 cation-sensitive fluoroionophores carrying the tetraoxa monoaza 15-crown-5 receptor are described and discussed. Whereas complexation induces only weak effects for the positively charged hemicyanine probes, the closely related styryl dyes show stronger changes in their photophysical properties upon cation binding in the analytically advantageous near-infrared (NIR) region. The strongest effects in both cation-induced spectral effects and complex stability constants are observed for the uncharged probes of styryl base-type, but these probes usually absorb and emit at shorter wavelengths in the UV/VIS region. For both styryl dyes and styryl bases, in some cases cation-induced fluorescence enhancement or quenching is found.

Unsymmetric cyanines: chemical rigidization and photophysical properties

Cyanines with unsymmetric donor groups are investigated by steady state and time-resolved fluorescence measurements. Several chemically ridigized derivatives are compared, and their synthesis is described. In several cases, rigidization leads to an increase of the nonradiative decay rate constants. This unusual behaviour is explained within a model of multiple emissive states.

Ultrafast Excited-State Dynamics of Donor−Acceptor Biaryls: Comparison between Pyridinium and Pyrylium Phenolates

The excited-state dynamics of two donor-acceptor biaryls that differ by the strength of the acceptor, a pyridinium or a pyrylium moiety, have been investigated using a combination of steady-state solvatochromic absorption, ultrafast fluorescence, as well as visible and infrared transient absorption spectroscopies. The negative solvatochromic behavior of pyridinium phenolate indicates that the permanent electric dipole moment experiences a decrease upon S1 ← S0 excitation, implying that the ground state possesses more zwitterionic character than the excited state. In contrast, pyrylium phenolate exhibits a weakly positive solvatochromic behavior corresponding to a small increase in the dipole moment upon excitation, implying more zwitterionic character in the excited than the ground state. Both compounds are therefore situated at different sides of the cyanine-limit structure, which has equally polar ground and excited states. Despite these differences, both molecules exhibit qualitatively similar excited-state properties. They are characterized by a very short fluorescence lifetime, increasing from about 1 to 20 ps, when varying solvent viscosity from 0.4 to 11 cP. There are, however, characteristic differences between the two compounds: The excited-state lifetimes of the pyrylium dye are shorter and also depend somewhat on polarity. The ensemble of spectroscopic data can be explained with a model where the emitting Franck-Condon excited state relaxes upon twisting around the single bond between the aryl units to a point where the excited- and ground-state surfaces are very close or intersect. After internal conversion to the ground state, the distorted molecule relaxes back to its equilibrium planar configuration, again largely dependent upon solvent viscosity. However, in this case, the kinetics for the pyrylium dye are slower than for the pyridinium dye and the polar solvent-induced acceleration is significantly stronger than in the excited state. This difference of kinetic behavior between the two compounds is a direct consequence of the change of the electronic structure from a normal to an overcritical merocyanine evidenced by steady-state spectroscopy.

The role of internal twisting in the photophysics of stilbazolium dyes

The synthesis of selectively bridged stilbazolium dyes related to DASPMI is described. The comparison of steady-state and time-resolved fluorescence studies as a function of temperature allows one to develop a kinetic model on the basis of which the high photostability and absence of photoisomerization for these dyes can be understood. There are two possible photochemical deactivation channels: non-radiative decay through single-bond twisting which does not lead to a distinguishable photoisomer and through double bond twisting leading to trans–cis photoisomerization. The latter process is more than one order of magnitude slower than single-bond twisting for these compounds and is only observed in the compound where both single bonds are bridged. Lifetime maxima observed at low temperature indicate a further early structural relaxation not connected with bond twisting but with bond length changes.

Femtosecond dynamics of the S2 and S1 fluorescence of ionic styryl dyes in polar solvents

Femtosecond fluorescence upconversion and picosecond time-correlated single-photon counting fluorescence experiments for bridged and unbridged ionic styryl dye compounds in polar solvents are reported. The measured fluorescence transients reveal S2 → S1 internal conversion (IC) with a typical time of 300 fs, independent of bridged or unbridged structure. The lifetime of the relaxed emissive S1 state differs considerably for the bridged and unbridged structures: when the styryl-group single bonds are unbridged, the S1-state lifetime is only about 20 ps and the non-radiative decay to the ground state is very effective. When both single bonds of the styryl dye are chemically bridged and only the double bond is free to rotate, the fast decay is suppressed and the lifetime becomes about 2 ns. In addition, the fluorescence of the ionic styryl dyes shows picosecond transient behavior that is attributed to vibrational cooling in the excited S1 state. Finally, a qualitative discussion is given of the influence of the donor–acceptor strength of the styryl dye compounds on the polymethine and stilbenoid character of the S1 state and how this affects the effectiveness of the single- and double-bond twisting relaxation pathways for this state.

Two heterocyclic merocyanine classes and their optical properties in relation to the donor-acceptor strength of end substituents

The relationship between the first electronic transition energy of heterocyclic merocyanines and donor-acceptor properties of their end substituents has been quailtatively treated and analytically substantiated in terms of the classical valence bond model and the biradicaloid theory. A simple graphical technique has been suggested to classify donor-acceptor-substituted compounds in relation to their electronic structure and to predict the effect of donor-acceptor strength on their optical behavior. In this context, merocyanines can be divided into two classes which differ in charges and electron populations of their donor and acceptor moieties and exhibit mirror-like responses to the variation of donor-acceptor strength. Two families of new heterocyclic merocyanines have been synthesized and their experimentally observed spectroscopic properties have been rationalized using the approach developed.

Spectral and photophysical characteristics of unsymmetric polymethine dyes as model compounds for the colour shift of visual pigments

Selectively bridged unsymmetric cyanines (dimethylamino-vinyl-benzothiazolium dyes) have been studied with respect to their photophysical properties using steady-state and time-resolved fluorescence. The nonradiative deactivation channels strongly depend on the bridging pattern and can be enhanced or slowed down, with the changes mainly connected to the Arrhenius pre-exponential factor, reflecting the photochemical pathways. The spectral shifts, on the other hand, are independent of the bridging pattern and are correlated with the compounds position on a donor acceptor axis including ideal polymethines. Using this model, the colour shift of visual pigments can be explained as an effect of the internal (protein-induced) electric field on the retinal Schiff base, likewise an unsymmetric polymethine dye system.