D. A. Ivanov

The 1:1 host-guest complexation between cucurbit[7]uril and styryl dye.

The photophysical properties of aqueous solution of styryl dye, 4-[(E)-2-(3,4-dimethoxyphenyl)ethenyl]-1-ethylpyridinium perchlorate (dye 1), in the presence of cucurbit[7]uril (CB[7]) was studied by means of fluorescence spectroscopy methods. The production of 1:1 host-guest complexes in the range of CB[7] concentrations up to 16 μM with K = 1.0 × 10(6) M(-1) has been observed, which corresponds to appearance of the isosbestic point at 396 nm in the absorption spectra and a 5-fold increase in fluorescence intensity. The decay of fluorescence was found to fit to double-exponential functions in all cases; the calculated average fluorescence lifetime increases from 145 to 352 ps upon the addition of CB[7]. Rotational relaxation times of dye 1 solutions 119 ± 14 ps without CB[7] and 277 ± 35 ps in the presence of CB[7] have been determined by anisotropy fluorescence method. The comparison of the results of quantum-chemical calculations and experimental data confirms that in the host cavity dye 1 rotates as a whole with CB[7].

High volume confinement in two-photon total-internal- reflection fluorescence correlation spectroscopy

We report results on two-photon total-internal-reflection fluorescence correlation spectroscopy with radially polarized light. The combination of liquid crystal spatial light modulator, providing radial polarization, with ultrafast laser (picosecond Nd:GdVO4 laser) allowed us to take the advantage of nonlinear optical contrast mechanisms to suppress the side-lobe energy specific for radial polarization and reduce the effective excited volume twice compared to one-photon evanescent wave excitation in fluorescence correlation spectroscopy.

Supramolecular assembler based on cucurbit[8]uril: Photodimerization of a styryl dye in water

Photolysis of aqueous solutions of styryl dye 1 in the presence of cucurbit[8]uril (CB[8]) has been studied by optical spectroscopic methods for the molar ratios n = cCB[8]/c1 in the range of 0 ≤ n ≤ 6. It has been found that the inclusion complexes (1)2@CB[8] dominate in the solution at n ≤ 0.5, whereas the complexes 1@CB[8] dominate at n ≥ 1. The stability constants have been determined for the 1: 1 (log K1 = 6.2 (L mol−1)) and 2: 1 (log β = 11.9 (L2 mol−2)) complexes. The fluorescence decay kinetics of dye 1 in the presence of CB[8] is two-exponential, with the average lifetime increasing substantially at n ≥ 1. It has been shown that the system can operate in the cyclic mode as an assembler (or supramolecular catalyst) in the photodimerization reaction of dye 1 to form cyclobutane derivative 2. The stability constant of the complex 2@CB[8] (log K3 = 5.9 (L mol−1)) and the quantum yield of cycloaddition (⃜ ≈ 0.07 at n ≈ 0.5) have been determined.

Ultrafast photonic-crystal fiber light flash for streak-camera fluorescence measurements

Photonic-crystal fibers (PCFs) provide a high efficiency of frequency upconversion of femtosecond Cr: forsterite laser pulses through the emission of dispersive waves by solitons and third-harmonic generation. Dispersion management allows the central wavelength of the frequency-upconverted signal in PCF output to be tuned within the range of wavelengths from 400 to 900 nm. PCF frequency shifters are employed as excitation sources for time-resolved fluorescence streak-camera measurements on fluorescein solution.

Photophysical properties of aqueous solutions of a styryl dye in the presence of cucurbit[n]uril (n = 5, 6, 8)

Photophysical properties of aqueous solutions of the styryl dye 4-[(E)-2-(3,4-dimethoxyphenyl)-1-ethylpyridinium] perchlorate (1) in the presence of cucurbit[n]urils (CB[n]; n = 5, 6, 8) have been studied by fluorescent spectroscopy methods. The fluorescence intensity of a 10–6 mol L–1 solution of 1 increases by a factor of 12.6 upon the formation of 1 : 1 inclusion complexes with CB[6] or 1.3 in complexes with CB[8]. Upon the formation of inclusion complexes, the average lifetime of the electronically excited state of 1 increases to about 1 ns for both CB[6] and CB[8]. On the basis of fluorescence anisotropy measurements, the rotational relaxation times were estimated to be 408, 314, and 183 ps for the complexes with CB[6], CB[8], and for unbound 1, respectively. Using the fluorescence titration method developed for the case of poorly soluble cavitands, the binding constant of 1 with CB[6] was determined to be 1.1 × 105 L mol–1. The addition of CB[5] does not lead to changes in the photophysical properties of a solution of 1, indicating the absence of complexes between CB[5] and 1. It has been found on the basis of the experimental data that the fluorescence rate constant of 1 decreases about twice in the complex with CB[8], but doubles in the complex with CB[6].

Ultrafast kinetics of fluorescence decay of aqueous solutions of styryl dye derivatives and their complexes with cucurbit[7]uril

The ultrafast relaxations of electronically excited states of styryl dyes, viz., 4-[(E)-2-(3,4-dimethoxyphenyl)- 1-ethylpyridinium] perchlorate (1) and 1-(3-ammoniumpropyl)-4-[(E)-2-(3,4-dimethoxyphenyl) vinyl]pyridinium diperchlorate (2), and their inclusion complexes with cucurbit[7]uril (СВ[7]) have been studied in aqueous solutions as a function of the fluorescence wavelength by femtosecond laser-induced fluorescence spectroscopy.For all dyes and their complexes, the fluorescence decay curves are fitted satisfactorily by the sum of two and three exponentials. The relaxation of electronically excited states in the blue wing of fluorescence band contains a component with a short decay time of about 100–200 fs and the red wing displays a rising component with a characteristic time of the same order of magnitude. In addition, the fluorescence decay includes a component with a characteristic time of about 2 ps, which is independent of complexation and fluorescence wavelength. The addition of СВ[7] to aqueous solutions of dyes 1 and 2 results in an increase in the longer relaxation times from ≈50 ps for the free dyes to 100–150 ps for the bound ones. The extra positive charge of dication 2 has almost no effect on the photophysical properties compared to cation 1, which agrees with quantum chemical calculations of the structures of inclusion complexes. A considerable increase in the fluorescence lifetime can be attributed to the increase in the potential energy barrier between the excited state of dye and the conical region on the potential energy curve, which is assumed to be caused by the displacement of the dye cation from the equilibrium state deep into the cavity within the first several picoseconds after laser pulse excitation.

Time-resolved fluorescence anisotropy of styryl dye–cucurbituril complexes

It is known that photophysical properties of a dye molecule change significantly in supramolecular host–guest complexes of organic luminophores with cucurbit[n]urils (CBn) [1]. For example, the f luorescence intensity of a styryl dye–CB6 complex increases more than 12-fold in comparison with the free dye [2]. This phenomenon is relatively poorly studied and is very interesting property from both fundamental and applied points of view. In this communication, we present new experimental results on ultrafast f luorescence depolarization of a styryl dye, 4-[4-(dimethylamino)styryl-1-methylpyridinium] iodide (DSM), in the host-guest complexes with CB6, in ethylene glycol and water solutions. Ethylene glycol (Khimmed, analytical grade), water (Millipor Simplicity), cucurbit [6]uril (Sigma Aldrich), and DSM (Sigma Aldrich) were used without additional purification. The measurements were carried out at ambient temperature, with the DSM concentration being 5 × 10–6 mol/L in steady-state experiments or 2 × 10–4 mol/L in the study on picosecond timescale. The concentration of CB6 aqueous solution was 2.3 × 10–4 mol/L. Steady-state f luorescence spectra and fluorescence anisotropy were measured on a Fluorolog 3 Tau spectrofluorimeter. Fluorescence decay kinetics on nanosecond timescale was studied by correlated single-photon counting on a PicoQuant spectrofluorimeter. Femtosecond fluorescence depolarization was studied by the upconversion (frequency summation) method using a chromium-forsterite laser (pulse duration of 90 fs, first harmonics of 1280 nm). The detailed description of the experimental setup is given elsewhere [3]. The upconversion nonlinear BBO crystal is physically a polarizer (analyzer). The transmission direction of the crystal is determined by the direction of oscillations of the electric vector of a linearly polarized gate pulse. The time-resolved f luorescence anisotropy as an important characteristic of the f luorophore was determined experimentally as

A Fluorimeter on the Basis of a Femtosecond Cr+4: Forsterite Laser

An apparatus for measuring luminescence lifetimes using the femtosecond up-conversion method is presented. A laser system based on a chromium–forsterite crystal with a lasing wavelength of 1250 nm is used to generate ultrashort pulses. Luminescence is excited in a sample by optical harmonics that are generated in nonlinear optical crystals. The width of the instrument function of the system is 200 fs. Methods for optimizing the signal-to-noise ratio in measurements are discussed. The results of measurements of the fluorescence- decay dynamics and the anisotropy of aqueous solutions of an inclusion complex of the 4-DASPI styrene dye in cucurbit[6]uril on short (5 ps) and long (500 ps) time scales are presented.

Effect of Heavy Water on Ultrafast Dynamics of the Fluorescence Stokes Shift for a Styryl Dye and its Complexes with Cucurbiturils

It has been recently found [1] that upon excitation, host–guest complexes of styryl dye cations with cucurbit[n]urils (CBn), which are barrel-shaped hollow macrocyclic molecules consisting of n glycouril units (usually n = 5–8) linked with methylene groups, operate as a molecular machine with a function of switch [2]. An external stimulus (light in this case) causes high-amplitude translational motion of the guest molecule relative to the macrocycle. The physical origin of such molecular motion is Coulomb interaction between the negatively charged portal of cucurbituril and the charge of the dye molecular cation. Photoexcitation results in charge transfer from the dye pyridine moiety to the styryl group, thereby creating Coulomb forces that act to shift the electronically excited cation into the cavitand cavity. After relaxation of the excited state within 102 ps [1], the system returns to the initial equilibrium state.

Adsorption of сucurbit[6]uril and styryl dye complexes on the surface of silver nanoparticles

Fluorescence spectroscopy and electrochemistry methods have been applied to study the adsorption of complexes of styryl dye, trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide (DSMI), with cucurbit[6]uril (CB6) on the surface of silver nanoparticles (AgNPs) (with an average size of 15 nm) prepared by silver nitrate reduction using sodium borohydride. It is established that in the presence of AgNPs the intensity of fluorescence of the DSMI@CB6 complex decreases by approximately 40%; the fluorescence of only free complexes in the bulk solution is observed, and the fluorescence of the complexes, which were adsorbed on the AgNPs surface, is completely quenched. The Frumkin isotherm (adsorption equilibrium constant B = 7.0 × 106 L/mol) is used to describe the adsorption. In addition, the binding constant of the inclusion host–guest DSMI@CB6 complex is adjusted: K = 106 L/mol.