The dynamics of electronic excitations in hybrid associates of Ag2S quantum dots with methylene blue (MB) molecules are studied by femtosecond transient absorption spectroscopy and nanosecond time-resolved spectroscopy. The situation is considered where the relative positions of the energy levels of the associate components allow photoinduced charge transfer. The luminescence quenching for MB+ cations and Ag2S quantum dots in the hybrid associates provides evidence for charge transfer, which is accompanied by a quick return of MB+ to the ground state (absorption is recovered). We propose that there is electron transfer from the MB LUMO level to the energy level of the luminescence center in the quantum dot. In this case, the bleaching band with a peak at 590 nm corresponding to the MBOH center dot form of the dye is formed at a subpicosecond time scale. The corresponding decay time is about 100 ps.

In this paper, we studied the dynamics of transient absorption spectra of a hybrid nanostructure (Ag–CuPc)6 Ag based on island silver films and copper phthalocyanine thin films, as well as individual structural units of this system, by femtosecond transient absorption spectroscopy. It is found that the effects observed for a hybrid nanostructure reflect the existing mutual influence of the plasmonic and organic subsystems on the spectral-kinetic characteristics of each other. The characteristic time of the main component of induced optical density kinetics at a wavelength λ = 525 nm (τ ~ 15 ps) that is caused by singlet-triplet relaxation of the excited electronic states of the organic subsystem of the hybrid structure is noticeably shorter than the relaxation time (τ ~ 50 ps) at the same wavelength for a pure copper phthalocyanine film of 40 nm thickness, just the same as the total thickness of the organic subsystem in the hybrid structure. It is assumed that the presence of silver nanoparticles in the hybrid structure (Ag–CuPc)6 Ag, affects the probability of intersystem conversion in the organic subsystem, accelerating the transition of CuPc molecules to a long-lived triplet state due to a strong local field near the surface of the plasmon particle. The triplet-triplet absorption spectrum of copper phthalocyanine in the spectral range 470–750 nm was obtained.

In this paper, we studied the dynamics of transient absorption spectra of a hybrid nanostructure (Ag–CuPc)6 Ag based on island silver films and copper phthalocyanine thin films, as well as individual structural units of this system, by femtosecond transient absorption spectroscopy. It is found that the effects observed for a hybrid nanostructure reflect the existing mutual influence of the plasmonic and organic subsystems on the spectral-kinetic characteristics of each other. The characteristic time of the main component of induced optical density kinetics at a wavelength λ = 525 nm (τ ~ 15 ps) that is caused by singlet-triplet relaxation of the excited electronic states of the organic subsystem of the hybrid structure is noticeably shorter than the relaxation time (τ ~ 50 ps) at the same wavelength for a pure copper phthalocyanine film of 40 nm thickness, just the same as the total thickness of the organic subsystem in the hybrid structure. It is assumed that the presence of silver nanoparticles in the hybrid structure (Ag–CuPc)6 Ag, affects the probability of intersystem conversion in the organic subsystem, accelerating the transition of CuPc molecules to a long-lived triplet state due to a strong local field near the surface of the plasmon particle. The triplet-triplet absorption spectrum of copper phthalocyanine in the spectral range 470–750 nm was obtained.

The dynamics of the transient absorption spectra of H*- and J-aggregates of an indotricarbocyanine dye was studied using femtosecond pump-probe spectroscopy. Relaxation of the induced transmission of the H*-aggregates occurs with time constants of ~3 and ~30 ps with radiation pumping at λmax = 400 nm and ~30 ps with pumping at λmax = 800 nm. For J aggregates biexponential relaxation with time constants of ~1 and ~20 ps is observed in both cases. The fast component of the kinetics of transmission of the band of the H* aggregates with pumping at λmax =400 nm is due to the possibility of direct transition from the upper edge of the exciton band of the aggregate to the ground state, while the slow component is present as a result of concurrent fast transition from the upper edge of the exciton band to the lower edge and subsequent relaxation from this to the ground state. The fast component of the kinetics of transmission of the band of the J aggregates is determined by the possibility of transition from the unthermalized exciton state to the ground state, while the slow component is due to transition to the ground state after relaxation within the limits of the exciton band. The greater length of delocalization of the excitons in the J aggregates (nine molecules) compared with the H* aggregates (four molecules) appears not only as decrease of the half-width of the stationary absorption band but also as more rapid quenching of the excited states. It was shown that the H* aggregates have a narrow stationary absorption band at 514 nm and weak absorption with a maximum at 756 nm. With pumping at λmax = 800 nm there is a bathochromic shift of the maxima of the transmission band of the system as a result of the appearance of nonuniform broadening.

The dynamics of electronic excitation in hybrid associates of colloidal CdS quantum dots passivated by thioglycolic acid (CdS/TGA) with methylene blue cationic dye molecules (MB +) was studied by femtosecond transient absorption spectroscopy and nanosecond time-resolved spectroscopy. From the luminescence decays it was found that non-radiative resonance energy transfer (FRET) from the recombination luminescence centers accompanied by acceptor fluorescence is observed in the CdS/TGA-MB+ systems with MB+ monomers. It was found that in the exciton decay dynamics of the hybrid systems based on CdS/TGA QDs and methylene blue, there is substantial competition between the resonance energy transfer in MB+ and the energy transfer to MB• and MBOH• forms.

The possibilities of changing the direction of electronic excitations transfer in hybrid associates are analyzed. Studies of hybrid associates of colloidal Ag2S quantum dots (QDs), stabilized with thioglycolic acid (TGA) with thionine (TH+) molecules, were realized by transmission electron microscopy, absorption and luminescence spectroscopy, and photoluminescence decay studies (time correlated single photon counting). The increase in the average size of QDs from 1.8 to 5.5nm, as well as shift of QDs luminescence peak from 630 to 950nm were observed, when TGA molecules together with sodium sulfide was used as a sulfur precursor. The hybrid association of QDs (1.8nm) with TH+ molecules leads to quenching of QDs luminescence in the luminescence band with peak at 630nm and simultaneously decreases in luminescence lifetime from 13.7 to 6.5ns. In the case of QDs association with a luminescence peak at 950nm with TH+ molecules, it leads to quenching of TH+ luminescence and decrease in its luminescence lifetime from 0.43 to 0.3ns. It is concluded that the decreasing in the luminescence lifetime is caused by nonradiative resonance energy transfer between components of associates. An increase in the average size of QDs leads to a change in the direction of energy transfer between associate components.

Nitro derivatives of xanthione, 2,7-dinitro-9 H-xanthene-9-thione and 2,4,7-trinitro-9 H-xanthene-9-thione, have been first synthesized and their stationary and transient spectra have been measured. The stationary spectra show that the attachment of the nitro groups to the xanthione scaffold leads to strong quenching of S2 → S0 fluorescence and the decrease of the oscillator strength of the S2 ← S0 electronic transition. Analysis of the transient absorption spectra uncovers the ultrafast stimulated emission quenching from the second excited state, S2, in the both derivatives. A kinetic scheme has been suggested to rationalize the complex spectral dynamics of the transient absorption signal. The kinetic scheme is deduced from the analysis of the transient spectra and supported by the quantum-chemical calculations, which predict the existence of a dark state and S2 state splitting into two close levels. The ultrafast transitions between S2 state sublevels and the transition into the dark state play a crucial role in spectral dynamics. These new features discovered in the nitro derivatives of xanthione distinguish essentially their spectral dynamics from that observed in xanthione.

New fluorescent molecular rotors (FMRs) were developed by modification of the Thioflavin T (ThT) structure via introduction of methyl and methoxy groups. Effects of the substituents on fluorescence properties and twisted intramolecular charge transfer (TICT) rate in the excited state of the molecules were studied using steady-state fluorescence and time-resolved absorption spectroscopy. Quantum chemical calculations of the molecules in the ground and excited states were carried out to aid interpretation of the experimental results. Only cationic forms of ThT derivatives have FMR properties and exhibit viscosity-dependent fluorescence. The TICT rate was found to be affected by the size of the molecular fragments, which experience mutual rotation, dihedral angle [Formula: see text] between the fragments in the ground state as well as their donor/acceptor properties.

Fluorescent molecular rotors (FMR) have wide range of applications due to high sensitivity of their emission intensity to microenvironment viscosity. Thioflavin T (ThT), which exhibits FMR properties, is widely used as fluorescent probe for in vitro detection of amyloid fibrils (AF) due to its high affinity and "light-up" feature (fluorescence quantum yield of ThT changes by similar to 3 orders of magnitude upon binding to AF). At physiological pH, ThT is positively charged and, therefore, it does not cross blood-brain barrier (BBB). It has been proposed that neutral derivatives of ThT are more likely to cross BBB, which should make them suitable for in vivo applications. However, for in vivo applications as fluorescent imaging agents, the neutral ThT derivatives must retain the FMR properties of ThT. In this paper, we examined whether neutral ThT derivatives exhibit FMR properties by studying the effect of solvent viscosity on their fluorescence intensity and decay lifetime. We observed that while the cationic ThT derivatives possess FMR properties, the neutral forms behave as regular highly-emitting fluorophores. Further, quantum chemical calculations in gas phase showed significant differences in the shape of excited state potential energy surfaces for the neutral and cationic derivatives of ThT. While, for charged ThT derivatives, the E(S-1(*)) energy is minimal for the twisted conformation with dihedral angle phi = 90 degrees between molecular fragments, the coplanar conformation with phi = 0 degrees (or 180 degrees) is more favorable for the neutral derivatives. From our experimental and theoretical studies we conclude that the neutral ThT derivatives lack FMR properties as their photoexcitation does not induce twisting motion coupled with internal charge transfer and, therefore, their specificity as fluorescent imaging agents for AF detection is lower than that of parent ThT. (C) 2018 Elsevier B.V. All rights reserved.

We have investigated spectral and spectral-kinetic properties of hybrid (organic semiconductor-plasmonic metal) nanocomposites on the base of silver nanoparticle monolayers and thin nickel phthalocyanine (NiPc) films. The spectroscopic results obtained by the femtosecond pump-probe technique demonstrate an enhancement of the fast optical response in the range of long-wavelength NiPc electronic absorption bands when the NiPc thin films contact with silver nanoparticles. This could be assigned to the plasmon-related modification and the enhancement of hybrid structures absorption at its steady-state absorption spectra.

We have investigated spectral and spectral-kinetic properties of hybrid plasmonic nanocomposites on silver nanoparticle monolayers and thin nickel phthalocyanine films. The spectroscopic results obtained by the femtosecond pump-probe technique demonstrate that a fast optical response in the range of long-wavelength nickel phthalocyanine electronic absorption bands becomes observable and registered when the nickel phthalocyanine thin films contact with silver nanoparticles. This could be assigned to the plasmon-related modification and the enhancement of organic subsistent absorption at steady-state absorption spectra of hybrid nanostructures.

Using methods of stationary and time-resolved femtosecond laser spectroscopy, as well as time-dependent density functional theory, the spectral-kinetic properties of excited electronic states of synthesized 2,7-diaminoxanthone (DAX) have been studied in polar protic and aprotic solvents. The intramolecular charge transfer from amino-groups to the central oxygen-containing moiety manifests itself in appearance of a new intensive long-wavelength absorption band in DAX. Considerable bathochromic Stokes shift observed in DAX fluorescence spectra with increasing aprotic solvents polarity indicates growth of charge transfer degree and, respectively, of transition dipole moment between the ground to the first singlet-excited states. In alcohols, hydrogen bond formation between DAX and solvent results in an increase of Stokes shift and fluorescence quenching. The observed spectral dynamics reflects the solvent relaxations and is well reproduced with the dynamic parameters of the solvents measured and reported elsewhere.

Non-linear absorption spectroscopy in pump and probe configuration has been used to test the population of non-equilibrium carriers in Ce-doped Y3Al5O12 (YAG), Lu3Al5O12 (LuAG), and Gd3AlxGa(5-x)O12 (GAGG) crystals with and without codoping by Mg2+ ions. A faster rise time of the induced optical density has been observed in all crystals codoped with Mg with respect to that in Mg-free samples. A significant difference in the time evolution of the differential optical density in GAGG with respect to YAG and LuAG crystals has also been measured. In both GAGG:Ce and GAGG:Ce,Mg an absorption band with maximum in the blue-green range and a decay time of 1.4 ps is present. This band is due to the absorption by free electrons before they are trapped or re-captured by Ce3+ ions. A broad absorption band in the yellow-red region with very short rise time and a decay time longer than 150 ps has been observed in all the Ce-doped garnets under study and can be attributed to the absorption from the Ce3+ excited states.

Investigations of dynamics of exciton relaxation in colloidal thioglycolic acid (TGA)-capped CdS/ZnS core/shell systems with diameter of 3.6 nm by means of femtosecond transient absorption spectroscopy, thermostimulated luminescence (TSL), and decay of luminescence are presented in this paper. It was found that the intensity of trap-state luminescence increases when one and two ZnS monolayers are formed. Also, the lifetime of trap-state luminescence increases. Two types of trap states with different depths were found, using thermostimulated luminescence technique. Localized states of the first type with depth of 0.085 eV do not change their concentration during sell formation. In contrast, trap state of the second type with depth of 0.125 eV are almost completely removed. It was found that the electron lifetime, investigated femtosecond transient absorption is not changed during formation of ZnS shell. It was concluded that localized states are channels of non-radiative recombination, direct quenching the center of trap-state luminescence. The absence of exciton luminescence is caused by rapid localization of holes at luminescence center.

In this study, we investigate transient phenomena in synthetic diamonds obtained by High Pressure High Temperature and Chemical Vapor Deposition methods. Study was aimed at searching for inorganic crystalline media combining ionizing radiation detecting properties and non-linear absorption of ultra-short laser pulses. The nonlinear pump-and-probe optical absorption technique with of 140 fs laser pulses was used to study the effects.

A new compound, 1,3-dimethoxy xanthione (DXT), has been synthesized and its absorption (stationary and transient) and luminescence spectra have been measured in n-hexane and compared with xanthione (XT) spectra. The pronounced broadening of xanthione vibronic absorption band related to the electronic transition to the second singlet excited state has been observed. Distinctions between the spectra of xanthione and its methoxy derivatives are discussed. Quantum chemical calculations of these compounds in the ground and excited electronic states have been accomplished to clarify the nature of electronic spectra changes due to modification of xanthione by methoxy groups. Appearance of a new absorption band of DXT caused by symmetry changes has been discussed. Calculations of the second excited state structure of xanthione and its methoxy derivatives confirm noticeable charge transfer (about 0.1 of the charge of an electron) from the methoxy group to thiocarbonyl group. Fitting of the transient spectra of XT and DXT has been fulfilled and the time constants of internal conversion S2→S1 and intersystem crossing S1→T1 have been determined. A considerable difference between the time constants of internal conversion S2→S1 in XT and DXT is uncovered.

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.