[2+2] Photocycloaddition in a bichromophoric dyad: photochemical concerted forward reaction following Woodward-Hoffmann rules and photoinduced stepwise reverse reaction of the ring opening via predissociation.

A novel biphotochromic dyad with styrylbenzo[f]quinoline photochromes was designed and synthesized to study the [2+2] photocycloaddition (PCA) reaction leading to cyclobutane with two benzo[f]quinoline (BQ) substituents and the reverse four-membered ring opening reaction. In the dyad, PCA occurs in a concerted manner according to the Woodward-Hoffmann rules in the ππ* excited state after excitation of the whole conjugated π-system comprising the ethylene group. Nanosecond time-resolved emission spectroscopy indicated formation of an excimer as a possible intermediate of the PCA reaction. The reverse reaction of photoinduced cyclobutane ring opening is assumed to proceed stepwise according to the predissociation mechanism: after excitation of the BQ substituent, energy transfer (ET) occurs from the bonding ππ* term localized on the BQ substituent to the dissociative πσ* term localized on cyclobutane; the efficiency of such a process was measured for the first time. For the first time, it is suggested that the predissociation mechanism is common to the ring-opening reaction of any cyclobutane with an unsaturated substituent where the π-system of the substituent rather than the σ-system of cyclobutane is excited under light irradiation.

Oxidative C-H/C-H Coupling of Dipyrromethanes with Azines by TiO2-Based Photocatalytic System. Synthesis of New BODIPY Dyes and Their Photophysical and Electrochemical Properties.

Oxidative C-H/C-H coupling reactions of dipyrromethanes with azines in the presence of a heterophase oxidative photocatalytic system (O2/TiO2/visible light irradiation) were carried out. As a result of cyclization of obtained compounds with boron trifluoride etherate, new hetaryl-containing derivatives of 4,4-difluoro-4-boron-3a,4a-diaza-s-indacene were synthesized. For the obtained compounds, absorption and luminescence spectra, quantum yields of luminescence as well as cyclic volt-amperograms were measured.

Studying the size-selective precipitation of colloidal quantum dots by decomposing the excitation-emission matrix.

The exciton peak in the excitation-emission matrix (EEM) of colloidal quantum dots implicitly contains information about inhomogeneous broadening and the photoluminescence (PL) and photoluminescence excitation (PLE) spectra of individual particles in the vicinity of the absorption onset. A numerical procedure for extracting this information has been developed and applied to the EEMs of polydisperse InP/ZnS core/shell colloidal quantum dots and their supernatant solutions obtained by partial precipitation with a non-solvent. The inhomogeneous broadenings obtained in this way have been converted by the sizing curve into particle-size distributions. These distributions have been found to be in agreement with the size-selective precipitation theory proposed recently. The homogeneous PL and PLE line shapes obtained by analyzing the EEMs were found to satisfy the Kennard-Stepanov relation, which is the first more or less direct evidence of its validity for colloidal quantum dots.

Styrylbenzoquinoline dyads as a new type of fluorescing photochromes operating via [2 + 2] photocycloaddition mechanism: Optimization of the structure.

We synthesized and studied a novel bichromophoric dyad in which bridging methylene groups link two styrylbenzo[f]quinoline (SBQ) photochromes to a salicylic acid residue. The dyad was designed for use as a fluorescent P-type photochrome acting via a [2 + 2] photocycloaddition (PCA) reaction. Compared to previously studied dyads, a change in the attachment handle and shortening of the bridging groups resulted in simultaneous rise of the quantum yields of both fluorescence and PCA. Under light irradiation, two competitive reversible reactions occurred in the dyad. The first is photoisomerization between the trans- and cis- isomers of the SBQ moieties. The second is PCA. The latter process was predominant and resulted in the formation of the cyclobutane ring bearing two benzo[f]quinoline (BQ) groups. In the ground S0 state, NMR data and DFT calculations indicated the formation of folded dyad conformers whose structure is pre-organized for PCA due to π-stacking interactions of two SBQ moieties. In the excited dyad, steady-state and time-resolved nanosecond fluorescence spectroscopy revealed the formation of an excimer, which was assumed to be a precursor of cyclobutane. Due to the fluorescence properties of SBQ and BQ, both dyad and cyclobutane fluoresce and can serve as a color-correlated multicolor fluorescence photoswitch. A simple approach is proposed for predicting the relationship between the spectral properties of the dyad and cyclobutane, which are the open and closed isomers of a new type of photochromes. The approach uses the dependence of the position of the maximum of the absorption band of an aromatic compound on the size of the π-system, as well as the fact that the sizes of the π-systems of the dyad and cyclobutane are related by a simple relation.

Influence of surfactant polydispersity on the structure of polyoxyethylene (5) nonylphenyl ether/cyclohexane/water reverse microemulsions.

The distribution of the number of oxyethylene groups in molecules of polyoxyethylene (5) nonylphenyl ether (NP5) is shown to significantly affect the structure of NP5/cyclohexane/water reverse microemulsions. The effect is caused by the difference in solubility of NP5 components. This difference leads to fractionation: surfactant molecules dissolved in the intermicellar nonpolar medium have, on average, shorter polyoxyethylene chains than surfactant molecules forming reverse micelles. A model is proposed which takes into account the influence of this fractionation on reverse micelle size and free surfactant concentration. The validity of this model for NP5/cyclohexane/water reverse microemulsions is experimentally confirmed by dynamic light scattering. It is found that changing the degree of surfactant fractionation in the above mentioned microemulsions can result in the formation of non-spherical reverse micelles.

On the composition fluctuations of reverse micelles.

The polydispersity of the reverse micelles is determined mainly by the fluctuations of their composition. The composition of the reverse micelle is a two-dimensional random variable whose components are the numbers of water (i) and surfactant (j) molecules. In this study the fluctuations of the composition of the reverse micelles are considered in the Gaussian approximation. It is shown that the standard deviation of the quantity w=i/j may be calculated from the dependence of the water vapor pressure above the microemulsion on the molar ratio W=[water]/[surfactant]. The estimation based on the literature data for microemulsion system sodium bis(2-ethylhexyl)sulfosuccinate/water/isooctane at 37°C in the range W=0-18 has shown that the relative standard deviation of the quantity w is about 10%. It is shown that the value of the composition fluctuations is related to the dependence of average composition on the concentration of reverse micelles at constant parameter W.