Aiming at higher stability of deeply coloured dyes, synthetic routes were developed to introduce bridge groups into the dye chromophore. The new methods were employed to synthesize stable dyes intensely absorbing light at wavelengths longer than 1200 nm [4], which was hitherto believed impossible. Also, the most long-wavelength (1620 nm) ππ* absorption of known organics was obtained [4].

Among the most important discoveries were the spectral deviation of unsymmetrical polymethines and the chromophore interaction in bis-cyanines [1]. The pioneering studies addressed the spatial structure of cyanines and the effect of the nature of their substituents as well as solvatochromism [1]. The novel classes of ketocyanines and polyones were investigated [18,19]. The dyes noted for their record large electronic asymmetry (with spectral maximum deviations over 500 nm) and Stokes shifts (Fig. 1) were synthesized [31]. The first organic dyes with conjugated cationic and anionic chromophores (Fig. 2) were prepared [32].

Dyes of unusual structure derived from boron-dipyrromethene and difluorodioxaborine [26−30] were found to have large two-photon absorption cross-sections and high fluorescence quantum yields at 700–900 nm [33,34]. A novel class of polymethine dyes containing carbocyclic instead of commonly used heterocyclic end-groups was obtained and studied spectroscopically [12,13,23,24,35]. The first studies were conducted on the luminescence of polymethines [3,36] and the photonics of their ion pairs [37,38]. The electronic spectral band shapes of polymethine dyes were first analysed [39,40] thus providing an insight into dye absorption selectivity and its relationship to the molecular structure [3,34,40]. A theoretical basis was developed to treat intermolecular interactions of cyanines in liquid solutions and polymer matrices [41−45]. The regularities which govern dye association including J-aggregation were established [3,46−52]. The π-electron distribution in merocyanines was theoretically and experimentally investigated so as to vary the dye electronic structure within the whole range of the three limiting ideal states [neutral polyene – polymethine – dipolar polyene] and also to govern the sign of solvatochromism (positive, negative, or inverse) by varying the donor-acceptor properties of the end-groups, the nature of solvent, and temperature [16,53−59] (Fig. 3).

Juxtaposition of quantum chemical data and a large body of experimental results afforded a qualitatively new understanding of the molecular electronic structure in the ground and excited states and enabled establishment of fundamental relationships between colour and fluorescence of polymethines, on the one hand, and their structure and the nature of the medium (e.g., solvent or polymer matrix), on the other hand. As a result, it became possible to design and obtain dyes with desired properties for diverse light-energy conversion applications.

A fragment of iconostasis of the 18th century (Tallinn): the image on the left was obtained on the KH-2 nano-chromatic film and the image on the right on the I-1060B infra-chromatic film Fig. 4. A fragment of iconostasis of the 18th century (Tallinn): the image on the left was obtained on the KH-2 nano-chromatic film and the image on the right on the I-1060B infra-chromatic film
• Dye-based infra-chromatic photographic films [60] having much higher resolution and sensitivity than their analogues (including Kodak Aerographic Film 3432). They enable detection of hidden details behind the haze or under a coating and are employed to explore the earth surface and sea shelves, to assess the quality of cultivated areas and woodlands, and also to produce general- and special-purpose maps. The application of the I-1060B film is illustrated in Fig. 4.
• Novel classes of active laser media generating light at 600−900 nm which are based on unsymmetrical dyes with record-broad absorption bands and record-large Stokes shifts [31,61,62] (Fig. 1). These features allow the same dye to be used in various common laser pump sources (λ = 308, 510, 532, 578, and 694 nm).
• A new polymer active laser medium, which first provided tuneable dye-laser generation with pumping at 1060 nm [62]. The conversion efficiency at wavelengths over 1100 nm amounts to 43% thus comparing well with the best active media for visible dye lasers.
Passive mode locking of an erbium laser. The absorption spectra of the NIR dye in acetonitrile (blue line) and o-dichlorobenzene (red line) are shown Fig. 5. Passive mode locking of an erbium laser. The absorption spectra of the NIR dye in acetonitrile (blue line) and o-dichlorobenzene (red line) are shown
• A passive Q-switch which first enabled passive mode locking of an erbium laser (λem = 1540 nm), a primarily important device in fiber optics, optical ranging, and ophthalmology [63] (Fig. 5).
• High-efficiency polymer passive Q-switches for nano- and picosecond solid-state lasers (λem = 694 [64], 1060 [65−67], and 1300 nm [68]), which have a long service life in the technology of “single pulse at the same point interval more times”.
• Dyed-polymer based absorption light filters protecting against laser radiation of 1060 nm wavelength [69]. They attenuate radiation by 4–6 orders of magnitude (as required), with 40−90% visual transmission, and can operate at any radiation incidence angle, in contrast to interference light filters.
• Dyes exhibiting a dramatic increase/decrease in dipole moment upon electronic excitation. A number of such compounds have been found to have pronounced nonlinear optical properties [33,34,70−73].
• New holographic recording media based on a dye-doped photothermoplastic polymer which are applied for nondestructive metal quality control [74,75]. Unlike many analogous items, they need no photoprotection during latent image formation and can therefore be used under field conditions.
• Photovoltaic systems with the photosensitivity around 700 nm (where the maximum solar photon flux occurs), employed in the fabrication of solar cells [76−78].
• Dyes for the compositions photopolymerized with a semiconductor laser (λem = 405, 650, 780, 830, 860, and 980 nm). A hitherto unknown effect of organic dyes has been revealed: polymethine dyes initiate/inhibit methyl methacrylate thermal polymerization in the absence of a common initiator [79].
High-contrast fluorescent probes for medical and biological applications
• High-contrast fluorescent probes for medical and biological applications [80–82]
• Thiasquaraines exhibiting nearly 100% triplet quantum yield and hence efficiently sensitizing singlet oxygen formation [83].
• Chemosensors for food quality control.
• New selective chemosensors and labels for cations [84] and anions [85] derived from pyridine-2,6-dicarboxamides. Starting from them, a new receptor-ionophore system has been developed which contains multiple coordination centres in the molecule. Co2+, Zn2+ and Ni2+ions complex with the pyridine moiety to cause a bathochromic absorption shift, whereas Pb2+, Hg2+, Cd2+, Mg2+ і Ca2+ ions form an aza-crown complex accompanied by a hypsochromic shift (Fig. 7).

Dioxaborine dyes showing much promise as efficient luminescent indicators of primary and secondary aliphatic amines (Fig. 8), with a limit of detection down to 1 µМ (1 ppm) [86]. As compared to Indophenol Blue, a well-known ammonia indicator, the dyes concerned are close in sensitivity being much more time-saving, having a wider action range, and needing no special chemicals. They can be employed for ecological control in the amine industry.

The regularities of polyene–polymethine structural variations of merocyanine dyes in the crystal state have been explored [87,88]. It is found that for positively solvatochromic compounds they are similar to the regularities in solutions, that is, their dipolarity decreases with the polymethine chain lengthening. The opposite tendency is observed for negatively solvatochromic merocyanines, i.e. an increase of the contribution of the dipolarity for higher vinylogues. It has been shown that for merocyanines in the crystal the ideal polymethine state can be achieved via a judicious choice of the donor-acceptor properties of their end-groups (Fig. 9).

Water-soluble squarylium dyes with the anchoring sulfo-groups were synthesized to enhance the sorption ability of squaraines towards TiO2 nanoparticles [89]. This results in an improvement of the photovoltaic characteristics of the Grätzel solar cells on their basis.

Novel neutral D-π-A-π-D and dianionic A1-π-A-π-A1 polymethine dyes have been synthesized, in which the 1,3,2-dioxaborine ring is the central part of the polymethine chain [90,91]. The obtained compounds are found to be intense fluorophores for the red and NIR spectrum region.

Gas-phase absorption spectra of a vinylogous series of merocyanines have been studied for the first time [92]. It is found that in vapours their electronic structure is considerably less dipolar than even in nonpolar n-hexane, what is manifested both in the hypsochromic shift of the long-wavelength absorption band and its broadening, symmetrization, smaller vinylene shift. Juxtaposition of the TDDFT calculations with the experimental data shows their better correlation in the gas phase than in solutions. Thus, the gas-phase spectral data provide the useful reference for assessment of various quantum chemical approaches [92,93].

Thermochromism of regular series of merocyanines has been studied in ethanol in going from 293 to 77 K [53,94,95]. It has been found from the spectral data that at low temperature the electronic structure of the more dipolar dyes becomes highly zwitterionic in both the ground and fluorescent states. In the extreme cases this results in unusual effects: a decrease of the fluorescence quantum yields and an increase of the Stokes shift in the frozen matrix [95].

Stilbazolium merocyanine dyes with the 2,6-di-tert-butyl substituted donor pyridine moiety were synthesized for the first time [96]. Their absorption and fluorescence spectra in a series of solvents of various polarities were compared with those of Brooker's merocyanine and its derivative with the bulky substituents in the acceptor end-group. Hereby, the effects of nucleophilic and electrophilic solvation on their solvatochromism have been discriminated, revealing that the former, although it has lesser impact in this case, is an essential factor in such solvents as pyridine and DMF.

The study of series of merocyanines with the 1,2-diphenyl-3,5-pyrazolidinedione moiety as the acceptor end-group has revealed that its acceptor strength—contrary to the literature data—is inferior to that of the thiobarbituric end-group [97]. The weak fluorescence of some of the studied dyes has been explained, based on the TD-DFT and CC2 calculations, by the fast competitive non-radiative decay associated with the non-typical low-lying non-fluorescent ππ*-excited state.

The first systematic cyclic voltammetry study of the electrochemical properties cationic, anionic and merocyanine polymethine dyes has allowed to analyse the effects of structural parameters, viz. the chromophore charge, the donor-acceptor properties of the terminal groups, the polymethine chain length, and the molecular symmetry on the redox-potentials of polymethines [98]. The statistical analysis of the obtained voltammetry data and the DFT calculations has allowed to suggest linear correlation equations that may be employed for credible prediction of the HOMO and LUMO energies in the search and design of molecules with required redox properties.

For a practically important NIR spectrum range, an ideology and strategy of design of merocyanines with a higher absorption intensity and fluorescence capacity than the corresponding ionic polymethines have been developed [99]. By tailored choice of end-groups, this effect can be achieved in both high- and low-polarity media, including polymer matrices. The developed merocyanine have significant advantages over ionic dyes when used in solar cells and holographic recording media, since, due to their nonionic nature, merocyanines, in contrast to ionic dyes, do not cause harmful dark current and practically do not aggregate.

Based on the quantum chemical analysis, the features of the benzo[cd]indole moiety have been identified, which are responsible for short lifetimes and low fluorescence quantum yields of the respective dyes [100]. It is found that the benzo[cd]indole moiety, in contrast to indole or benzimidazole, makes a significant contribution to the boundary molecular orbitals of the polymethine dyes, resulting in an increase of vibronic interactions in the long-wavelength transition and thus in an increase of internal conversion.

On the basis of the quantum chemical analysis of the electronic structure of organic molecules and the light energy degradation in them, the novel efficient radioluminophores derived from pyrazoline have been designed and synthesized. They have both substantial Stokes shifts (80–100 nm) and high fluorescence quantum yields (75–98%). Synthesized luminophores have high fluorescence brightness under α-, β- and γ-irradiation in the sensitivity range of common photoelectron radiation detectors. In terms of fluorescence intensity, they are 5 times as good as the best of known analogues. This allowed together with scientists from Clemson University (USA) to develop and patent polymer scintillators and dosimeters based on them, capable of responding not only to the dose, but also to the type of radiation [101].