We have made a selection of the leading structures among cationic polymethine dyes with different structures of terminal heterocyclic groups and polymethine chains for passive Q-switch of a diodepumped solid-state neodymium laser. Such dyes have a maximum absorption in the region of 1060 nm and a minimum absorption in the region of 810 nm. It was found that the 4-((E)-2-((E)-2-chl oro-3-(2-(2,6-diphenyl-4H-thiopyran-4-ylidene)ethylidene)cyclohex-1-en-1-yl)vinyl)-2,6-diphenylthio pyrylium tetrafluoroborate dye most closely matches these characteristics. The nature of its longwavelength and higher electronic transitions was interpreted based on quantum-chemical calculations by the DFT/B3LYP/6-31G (d, p) method and TDDFT taking into account the polarity of the medium by the PCM method. The use of this dye as a laser Q-switch made it possible for the first time to obtain passive mode locking in diode-pumped neodymium lasers. Copyright (c) 2022 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the EastWest Chemistry Conference 2021.

The spectral and luminescent properties of trans-2-(4’-dimethylaminebenzylideneacetyl)-5,5-dimethylcyclohexane-1,3-dione and 10-hydroxy-3,3,6,6,9-pentamethyl-1,2,3,4,5,6,7,8,9,10-decahydroacridine-1,8-dione have been studied depending on the acid-base properties of the medium. It has been shown that the first compound can exist in the form of three different structural forms, while the second compound exhibits dual fluorescence in solvents with a high basicity parameter. These properties of chromophores can be used in chemical analysis to indicate the acid-base equilibrium of the medium.

Stationary luminescence and pulse fluorometry methods were used to study the spectral-kinetic and photophysical properties of a 9-methyl-10-hydroxyacridine(1,8)dione derivative dye (9,10-ACD) in polar protic and aprotic solvents at 293 and 77 K. We are the first to report a new long-wavelength fluorescence band (LF, 670–710 nm) in both polar protic (methanol and ethanol) and aprotic solvents (N,N-dimethylformamide and dimethyl sulfoxide) at room temperature. This band is attributed to the high-lying singlet excited state (in the range 460–490 nm) responsible for the short-wavelength fluorescence band (SF). The LF is not found in low-polar (methylene chloride, polar aprotic (acetonitrile) and highly viscous protic solvents (glycerol) at 293 K as well as at 77 K in ethanol. This band arises as a result of deprotonation of a molecule in an excited state and emission of the resultant anion in the long-wavelength region. Analysis of the spectral and kinetic data for SF decay and the LF rise indicates a mechanism, in which the dye anion formation depends on the basicity of the polar solvent. Possible practical applications of this spectral effect in biology, medicine, and analytical chemistry are proposed.

Polymethine dye-based optical compounds characterized by low absorption at the pump radiation wavelength 808 nm and at the same time by high absorption at the lasing wavelength 1064 nm have been developed for the diode pumped Nd:YAG laser systems. It has been theoretically shown that the thiopyrylotricarbocyanines with saturated bridging groups in the polymethine chains are the most effective medium for such applications. The passive Q-switch unit as well as the saturable absorber mirror fabricated using the developed polymethine dye-compounds were examined experimentally. It has been shown that these dye-based optical components can provide the efficient lasing in the modes of the Q-switching and formation of the picosecond pulse train.

It was found that the long-wavelength absorption band of the laser dye IR 1061 and its analogue with an unsubstituted polymethine chain is strongly broadened and decreases in intensity in polar solvents, while the fluorescence band remains narrow and practically does not change in a wide range of solvent polarities. Based on the quantum-chemical calculations of these dyes by the ab initio DFT/B3LYP/6-31G (d,p) and TDDFT methods, taking into account the polarity of the medium by the PCM method, it is shown that the reason for this difference is the weakening of solvation in the fluorescent state as compared to the ground state due to the greater equalization of the charge in the first than in the latter. An increase in the alternation of bond orders in the polymethine chain in the fluorescent state was found, which causes an increase of vibronic interactions in the radiative transition as compared to the absorptive one. Spectral effects caused by a change in the angle of rotation of phenyl groups in the thiopyrylium cycle upon excitation have been analyzed.

We found that the long-wavelength absorption band of the laser dye IR 1061 and its analogue with an unsubstituted polymethine chain is strongly broadened and decreases in intensity in polar solvents, while the fluorescence band remains narrow and does not change in a wide range of solvent polarities. Based on the quantum-chemical calculations performed for these dyes by the ab initio DFT/B3LYP/6-31G(d,p) method and TDDFT and taking into account the polarity of the medium, we demonstrated by the PCM method that the reason for this difference is the weakening of solvation in the fluorescent state as compared to the ground state due to the greater equalization of the charge in the former dye than in the latter one. An increase in the alternation of bond orders in the polymethine chain in the fluorescent state was found, which causes an increase in vibronic interactions in the radiative transition compared to the absorptive one. Spectral effects caused by changes in the angle of rotation of phenyl groups in the thiopyrylium cycle upon excitation have been analyzed.

Fluorescence quenching of positively solvatochromic merocyanines with a bridge group in the polymethine chain increases linearly with increasing temperature and is enhanced with increased chain length. Analysis of the kinetics of the fluorescence quenching gave the activation energy of this process. The absorption and fluorescence spectra in solvents of different viscosity and polarity at room temperature and 77 K showed that the major channel for fluorescence quenching is photoisomerization of the bonds in the polymethine chain. This conclusion was supported by TDDFT quantum-chemical calculations for the bond lengths of the merocyanines in the ground and excited states.

Octahydroacridino[4,3-c]acridine-1,9(2H,5H)dione derivatives were synthesized via condensation of bisazomethines (N 1 ,N 5 -di-R-benzylidenenaphthalene-1,5-diamines) with dimedone in butanol-1. The structures of the prepared compounds were confirmed using NMR and IR spectroscopy. Studies of the spectral and luminescent properties of the synthesized compounds in EtOH and DMF at 293 and 77 K showed that the absorption spectra were composed of three electronic transitions S n ← S 0 (n = 1–3) with frequencies of 24,400 and 20,800 cm–1 for the 0–0-transitions from the singlet (S 1 ) and triplet (T 1 ) states, respectively. Fluorescence quenching by EtOH was due to the formation of an H-bonded complex, the lifetime of which was <10 –9 s. The naphthalene fragment in the molecule was responsible for phosphorescence in the studied compounds.

Derivatives of octahydroacridino[4,3-c]acridin-1,9(2H,5H)dione are synthesized using the condensation of diazomethines (N 1 ,N 5 -di-R-benzylidenenaphthalen-1,5-diamines) with dimedone in butanol. The structure of the prepared compounds is confirmed by the methods of nuclear magnetic resonance and IRspectroscopy. The spectral and luminescent properties of the synthesized compounds are studied in ethanol and DMF as solvents at 293 and 77 K. It is found that the absorption spectra are formed by three S n ←S 0 (n = 1-3) electronic transitions and the frequencies of 0-0 transitions from singlet (S 1 ) and triplet (T 1 ) states are equal to 24400 and 20800 cm - 1 , respectively. The fluorescence quenching by ethanol is due to the formation of the complex with the hydrogen bonds; the lifetime of this complex is less than 10 - 9 s. Phosphorescence in the studied compounds is caused by the existence of a naphthalenic fragment in the molecule.

Using femtosecond spectroscopy and steady-state luminescence methods, the mechanisms of very fast non-radiative deactivation (knr ~ 5 ? 1011 ?-1) of the electronic excitation energy at room temperature in organic and buffer solutions of well-known natural antioxidants rutin and quercetin have been studied.

The spectroscopic and photophysical properties of the biologically important plant antioxidant quercetin in organic solvents, polymer films of polyvinyl alcohol, and a buffer solution at pH 7.0 are studied by stationary luminescence and femtosecond laser spectroscopy at room temperature and 77 K. The large magnitude of the dipole moment of the quercetin molecule in the excited Franck–Condon state μ e FC = 52.8 C m indicates the dipolar nature of quercetin in this excited state. The transient induced absorption spectra S 1 → S n in all solvents are characterized by a short-wave band at λ abs max = 460 nm with exponential decay times in the range of 10.0–20.0 ps. In the entire spectral range at times of >100 ps, no residual induced absorption was observed that could be attributed to the triplet–triplet transitions Т 1 → Т k in quercetin. In polar solvents, two-band fluorescence was also recorded at room temperature, which is due to the luminescence of the initial enol form of quercetin (~415 nm) and its keto form with a transferred proton (550 nm). The short-wave band is absent in nonpolar 2-methyltetrahydrofuran (2-MTHF). The spectra of fluorescence and fluorescence excitation exhibit a low dependence on the wavelength of excitation and detection, which may be related to the solvation and conformational changes in the quercetin molecule. Decreasing the temperature of a glassy-like freezing quercetin solution in ethanol and 2-MTHF to 77 K leads to a strong increase in the intensity (by a factor of ~100) of both bands. The energy circuits for the proton transfer process are proposed depending on the polarity of the medium. The main channel for the exchange of electronic excitation energy in the quercetin molecule at room temperature is the internal conversion S 1 ⇝ S 0 , induced by the state with a proton transfer.

Steady-state and pulsed spectroscopic methods are used to study the spectroscopic and photophysical properties of the biologically important plant pigment rutin at room temperature and 77 K in organic solvents and a buffer solution at pH 7.0. The large dipole moment μ e = 13.3 D of the rutin molecule in a Franck–Condon excited state indicates that rutin is dipolar in this excited state. The nonstationary S 1 → Sn induced absorption spectra are characterized by a short-wavelength band at λ abs max = 460 nm and low-intensity absorption in the 500–750 range which clearly belongs to associates of rutin. No residual induced absorption which might be related to triplet-triplet T 1 →T k transitions in rutin was observed over the entire spectral range for times >50 ns. S 1 → S 0 fluorescence with a quantum yield Φ fl ~ 10 –4 was also observed at room temperature. The fluorescence and fluorescence excitation spectra manifest a weak dependence on the excitation and detection wavelengths, which may be related to the presence of conformers in the solution owing to rotation of the phenol B ring around a single 1′–2 bond. Lowering the temperature of a glassy frozen solution of rutin in ethanol to 77 K raises Φ fl by a factor of 750. A rate constant k ic = 3.7·1011 s –1 for internal conversion from the S 1 state at room temperature is calculated from the spectral-luminescence data. It is found that the main channel for exchange of electronic excitation energy in the rutin molecule at room temperature is S 1 (π,π * ) ~~> S 0 -internal conversion induced by the charge-transfer state.

The spectral and polarization characteristics of optically anisotropic polyvinyl alcohol (PVA) films containing 4,4'-bis[4-(phenylamino)-6-(methoxy-1,3,5-triazin-2-yl)amino]stilbene-2,2'-disulfonic acid as dichroic dye, which has intense blue fluorescence, were investigated by polarized luminescence and absorptionspectroscopy in the IR and UV regions. With fourfold uniaxial stretching of the film the orientation parameter of the dye amounts to 0.82–0.86, the maximum polarizing ability of the film is 96% (at the maximum of the absorption band at 375 nm), and the degree of polarization and quantum yield of fluorescence at the optimum concentration of the dye amount to 0.90 and 0.91. The degree of orientation of the dye molecules depends weakly on the concentration (0.01–0.50 wt.%) and increases with increase of the uniaxial stretching of the film. The insertion of the dye molecules between the PVA chains leads to a reduction of the crystallinity of the polymeric matrix.

9,9-Dimethyl-12-[(5-aryl-2-furyl) (or 5-aryl-2-thienyl, or 5-aryl-1-methyl-1H-pyrrolyl)]-7,8,9,10,11,12-hexahydrobenzo[a]acridin-11-ones were synthesized, and their absorption and luminescence spectrum characteristics in ethanol at room temperature and at 77 K were studied. The spectra suggest the existence of these compounds in a liquid solution as mixtures of conformers, each of which is characterized by its own absorption and fluorescence spectra.

Using methods of steady state luminescence and femtosecond spectroscopy, we have studied the mechanism of intramolecular proton transfer in synthesized 3,7-dihydroxy-2,8-di(4-methoxyphenyl)-4H,6H-pyrano[3,2-g]chromen-4,6-dion in polar and nonpolar solutions, films, and polycrystals at 293 and 77 K. In an excited singlet state, intramolecular proton transfer occurs in two stages. At the first stage, a tautomer with one transferred proton (OTP tautomer) is formed from the Franck-Condon state within τ1 = 0.6 ps. At the second stage, the second proton is transferred within τ2 = 3.1 ps and a tautomer with two transferred protons (TTP tautomer) is formed, which fluoresces in toluene at 293 K with a high quantum yield, Φ f = 0.66, and the fluorescence spectrum of which is characterized by a large Stokes shift, 9900 cm−1. At 293 K, polar solvents (dimethylformamide, dimethyl sulfoxide, ethanol, etc.) solvate the BFV molecule in the ground state, while, in the excited state, an OTP tautomer is mainly formed. In polar ethanol at 77 K, a dual fluorescence spectrum is observed, which is caused by the fluorescence emission of polysolvates with λ max f = 460 nm and TTP phototautomers at λ max f = 610 nm.

By a three-component condensation of 8-aminoquinoline, aromatic aldehydes, and 1,3-diketones accompanied by the Hofmann-Martius rearrangement hydrobenzophenanthroline derivatives were synthesized. Their spectral and luminescence properties in ethanol solution were investigated at 293 and 77 K.

Hydrobenzophenanthrolinone derivatives were synthesized by three-component condensation of 8-aminoquinoline, aromatic aldehydes, and dimedone. The structures of the obtained substances were confirmed by NMR and IR spectroscopy and mass spectrometry. Spectral-luminescent investigations of the synthesized compounds showed that they were characterized by high oscillator strengths for allowed electronic transitions S n ← S 0 (n = 1–3). Low fluorescence quantum yields in EtOH (Φfl ~ 10–4–10–3) and an increase of the Φfl values in toluene (~10–2) at room temperature and with lowering the temperature to 77 K (Φfl ~ 10–1) for a number of the compounds under study were satisfactorily explained within the framework of Marcus theory.

The three-component condensation of 1- or 2-naphthylamines, aromatic aldehydes, and methyl 2-(benzo[1,3]dioxol-5-yl)-4,6-dioxocyclohexane-1-carboxylate led to the formation of methyl 9-( cis,trans )-10-(1,3-benzodioxol-5-yl)-7-aryl-8-oxo-7,8,9,10,11,12-hexahydrobenzo[c]acridine-9-carboxylates or methyl 10-( cis,trans )-9-(1,3-benzodioxol-5-yl)-12-aryl-11-oxo-7,8,9,10,11,12-hexahydrobenzo[a]acridine-10-carboxylates. The spectral luminescence properties of compounds obtained were investigated in ethanol at 293 and 77 K.

A three-component condensation of 2-naphthylamine, aromatic aldehydes, and methyl 2,2-dimethyl-4,6-dioxocyclohexanecarboxylate afforded methyl ( cis,trans )-12-aryl-9,9-dimethyl-11-oxo-7,8,9,10,11,12-hexahydrobenzo[ a ]acridine-10-carboxylates. Spectral luminescence and nonlinear optical properties of compounds obtained were investigated.