Multifunctional fluorescent diarylethene: time-resolved study of photochemistry.

A study of luminescence and photochromic properties of (E)-2,3-bis(2,5-dimethylthiophen-3-yl)-5-(4-(pyrrolidin-1-yl)benzylidene)cyclopent-2-en-1-one, which is a diarylethene with a push-pull system between carbonyl and dimethylamino groups, was performed using time-resolved methods. The intramolecular charge transfer (ICT) process as well as 6π-electrocyclization and E-/Z-isomerization contribute to the complex light-induced properties of this molecule. Formation of unexpected short-lived intermediates was detected in the time range from 100 fs to 100 µs. A model based on two processes (additional photocyclization and interconversion between conformers) was proposed to rationalize this result. The key intermediates existing in the picosecond time domain are so-called precursors, which are proposed for both parallel (p) and anti-parallel (ap) isomers of the open form. In general, fast light-induced processes for the fluorescent diarylcyclopentenones are much more complicated than for the parent cyclopentenone-based DAE.

Photoinduced Processes in Lysine-Tryptophan-Lysine Tripeptide with L and D Tryptophan.

Optical isomers of short peptide Lysine-Tryptophan-Lysine (Lys-{L/D-Trp}-Lys) and Lys-Trp-Lys with an acetate counter-ion were used to study photoinduced intramolecular and intermolecular processes of interest in photobiology. A comparison of L- and D-amino acid reactivity is also the focus of scientists' attention in various specialties because today, the presence of amyloid proteins with D-amino acids in the human brain is considered one of the leading causes of Alzheimer's disease. Since aggregated amyloids, mainly Aß42, are highly disordered peptides that cannot be studied with traditional NMR and X-ray techniques, it is trending to explore the reasons for differences between L- and D-amino acids using short peptides, as in our article. Using NMR, chemically induced dynamic nuclear polarization (CIDNP) and fluorescence techniques allowed us to detect the influence of tryptophan (Trp) optical configuration on the peptides fluorescence quantum yields, bimolecular quenching rates of Trp excited state, and the photocleavage products formation. Thus, compared with the D-analog, the L-isomer shows a greater Trp excited state quenching efficiency with the electron transfer (ET) mechanism. There are experimental confirmations of the hypothesis about photoinduced ET between Trp and the CONH peptide bond, as well as between Trp and another amide group.

Role of Chiral Configuration in the Photoinduced Interaction of D- and L-Tryptophan with Optical Isomers of Ketoprofen in Linked Systems.

The study of the L- and D-amino acid properties in proteins and peptides has attracted considerable attention in recent years, as the replacement of even one L-amino acid by its D-analogue due to aging of the body is resulted in a number of pathological conditions, including Alzheimer's and Parkinson's diseases. A recent trend is using short model systems to study the peculiarities of proteins with D-amino acids. In this report, the comparison of the excited states quenching of L- and D-tryptophan (Trp) in a model donor-acceptor dyad with (R)- and (S)-ketoprofen (KP-Trp) was carried out by photochemically induced dynamic nuclear polarization (CIDNP) and fluorescence spectroscopy. Quenching of the Trp excited states, which occurs via two mechanisms: prevailing resonance energy transfer (RET) and electron transfer (ET), indeed demonstrates some peculiarities for all three studied configurations of the dyad: (R,S)-, (S,R)-, and (S,S)-. Thus, the ET efficiency is identical for (S,R)- and (R,S)-enantiomers, while RET differs by 1.6 times. For (S,S)-, the CIDNP coefficient is almost an order of magnitude greater than for (R,S)- and (S,R)-. To understand the source of this difference, hyperpolarization of (S,S)-and (R,S)- has been calculated using theory involving the electron dipole-dipole interaction in the secular equation.

Photochemistry of hexachloroosmate(IV) in ethanol.

The photochemistry of the OsIVCl62- complex in ethanol was studied by means of stationary photolysis, nanosecond laser flash photolysis, ultrafast pump-probe spectroscopy and quantum chemistry. The direction of the photochemical process was found to be wavelength-dependent. Irradiation in the region of the d-d and LMCT bands results in the photosolvation (with the wavelength-dependent quantum yield) and photoreduction of Os(iv) to Os(iii), correspondingly. The characteristic time of photosolvation is ca. 40 ps. Photoreduction occurs in the micro- and millisecond time domains via several Os(iii) intermediates. The nature of intermediates and the possible mechanisms of photoreduction are discussed. We believe that the lability of the photochemically produced Os(iv) and Os(iii) intermediates determines the synthetic potential of OsIVCl62- photochemistry.

Mechanistic study of the trans,cis,cis-[RuCl2(DMSO)2(H2O)2] complex photochemistry in aqueous solutions.

It is known that trans,cis,cis-[RuCl2(DMSO)2(H2O)2] (1a) complexes, which are formed upon dissolution of trans-[RuCl2(DMSO)4] in water, demonstrate light-induced cytotoxicity. The mechanistic study of 1a photochemistry has been performed using ultrafast pump-probe spectroscopy, laser flash photolysis and stationary photolysis. The first stage of 1a photochemistry is the photoexchange of a DMSO ligand to a water molecule; its quantum yield is wavelength-dependent (estimating by values 0.3 and 0.04 upon irradiation at 308 and 430 nm, respectively). The mechanism of photoexchange is complicated involving at least four Ru(ii) intermediates. Two tentative mechanisms of the process are proposed.

Photochemistry of tris(2,3,5,6-tetrathiaaryl)methyl radicals in various solutions.

During the last decades, persistent tris(2,3,5,6-tetrathiaaryl)methyl radicals (TAMs) have attracted much attention due to their applications in oximetry, EPR tomography, and as spin labels in pulsed dipolar EPR spectroscopy. Recently, researchers proposed to use TAM radicals as spin labels and/or a partner for photoinduced spin labels. Thus, the questions of their photochemical stability and mechanism of degradation under UV irradiation have become relevant and important. In this study, steady-state photolysis and flash photolysis of TAM radicals were investigated. A detailed mechanism of TAM phototransformations was proposed and confirmed by NMR, gel permeation chromatography, and mass-spectrometric analyses of the products.

Mechanism of photochromic transformations and photodegradation of an asymmetrical 2,3-diarylcyclopentenone.

A mechanistic study of the photochromic properties and photodegradation processes of an asymmetrical diarylcyclopentenone bearing thiophene and benzothiophene units using stationary photolysis, nanosecond laser flash photolysis and time-resolved luminescence was performed. It was found that the light-induced reversible isomerization of (3-(2,5-dimethyltiophen-3-il)-2-(2-methyl-1-benzylthiophen-3-il)cyclopent-2-en-1-one, compound 1) from open to closed form is a common photochromic transformation inherent to diarylethenes, while the photodegradation process proceeds in two ways. The first is a formal 1,2-dyotropic rearrangement, proceeding without the participation of oxygen. The second is the oxygen-dependent mechanism involving the excitation of the open form 1A into the triplet state, quenching of the latter by dissolved oxygen, and oxidation of the initial compound by singlet oxygen.

Stereoselectivity of Electron and Energy Transfer in the Quenching of (S/R)-Ketoprofen-(S)-Tryptophan Dyad Excited State.

Photoinduced elementary processes in chiral linked systems, consisting of drugs and tryptophan (Trp) residues, attract considerable attention due to several aspects. First of all, these are models that allow one to trace the full and partial charge transfer underlying the binding of drugs to enzymes and receptors. On the other hand, Trp fluorescence is widely used to establish the structure and conformational mobility of proteins due to its high sensitivity to the microenvironment. Therefore, the study of mechanisms of Trp fluorescence quenching in various systems has both fundamental and practical interest. An analysis of the photo-chemically induced dynamic nuclear polarization (CIDNP) and Trp fluorescence quenching in (R/S)-ketoprofen-(S)-tryptophan ((S/R)-KP-(S)-Trp) dyad carried out in this work allowed us to trace the intramolecular reversible electron transfer (ET) and obtain evidence in favor of the resonance energy transfer (RET). The fraction of dyad's singlet excited state, quenched via ET, was shown to be 7.5 times greater for the (S,S)-diastereomer than for the (R,S) analog. At the same time, the ratio of the fluorescence quantum yields shows that quenching effectiveness of (S,S)-diastereomer to be 5.4 times lower than for the (R,S) analog. It means that the main mechanism of Trp fluorescence quenching in (S/R)-KP-(S)-Trp dyad is RET.

Excitation-Wavelength-Dependent Emission and Delayed Fluorescence in a Proton-Transfer System.

Manipulating the relaxation pathways of excited states and understanding mechanisms of photochemical reactions present important challenges in chemistry. Here we report a unique zinc(II) complex exhibiting unprecedented interplay between the excitation-wavelength-dependent emission, thermally activated delayed fluorescence (TADF) and excited state intramolecular proton transfer (ESIPT). The ESIPT process in the complex is favoured by a short intramolecular OH⋅⋅⋅N hydrogen bond. Synergy between the excitation-wavelength-dependent emission and ESIPT arises due to heavy zinc atom favouring intersystem crossing (isc). Reverse intersystem crossing (risc) and TADF are favoured by a narrow singlet-triplet gap, ΔEST ≈10 kJ mol-1 . These results provide the first insight into how a proton-transfer system can be modified to show a synergy between the excitation-wavelength-dependent emission, ESIPT and TADF. This strategy offers new perspectives for designing ESIPT and TADF emitters exhibiting tunable excitation-wavelength-dependent luminescence.

A cis,fac-[RuCl2(DMSO)3(H2O)] complex exhibits ultrafast photochemical aquation/rearrangement.

It is known that both cis,fac-[RuCl2(DMSO)3(H2O)] (1a) and trans,cis,cis-[RuCl2(DMSO)2(H2O)2] (2a) complexes, which are formed on the dissolution of trans and cis-isomers of [RuCl2(DMSO)4] in water, demonstrate light-induced anticancer activity. The first stage of 1a photochemistry is its transformation to 2a occurring with a rather high quantum yield, 0.64 ± 0.17. The mechanism of the 1a → 2a phototransformation was studied by means of nanosecond laser flash photolysis and ultrafast pump-probe spectroscopy. The reaction occurs in the picosecond time range via the formation and decay of two successive intermediates interpreted as Ru(ii) complexes with different sets of ligands. A tentative mechanism of phototransformation is proposed.

Mechanistic Aspects of Photoinduced Rearrangement of 2,3-Diarylcyclopentenone Bearing Benzene and Oxazole Moieties.

Photoinduced rearrangement of diarylethenes to naphthalenes or isoelectronic benzoannulated heterocycles is a novel reaction in preparative organic photochemistry. Recently it was shown that unsymmetrical diarylethenes containing benzene and oxazole derivatives efficiently undergo this transformation leading to amide derivatives of naphthalene. Mechanistic study of skeletal rearrangement for a typical representative of these compounds, namely 3-(5-methyl-2-phenyl-1,3-oxazol-4-yl)-2-phenylcyclopent-2-en-1-one, was performed by stationary and laser flash photolysis as well as density functional theory (DFT) calculations. The mechanism of the rearrangement was found to comprise several thermal stages. Both singlet and triplet states of the initial compound can be transformed to the reaction product, which results in the dependence of the quantum yield vs concentration of dissolved oxygen. Three reactive intermediates were registered in the laser flash photolysis experiment; the predicted structures were in accordance with DFT calculations of the electronic absorption spectra. In addition to the previously proposed mechanism of skeletal rearrangement based on a sigmatropic shift of hydrogen, two new parallel pathways based on formation of a carbanion/carbocation were determined.

Primary photophysical and photochemical processes for hexachloroosmate(iv) in aqueous solution.

The photoaquation of the OsIVCl62- complex was studied by means of stationary photolysis, nanosecond laser flash photolysis and ultrafast kinetic spectroscopy. The OsIVCl5(OH)2- complex was found to be the only reaction product. The quantum yield of photoaquation is rather low and wavelength-dependent. No impact of redox processes on photoaquation was revealed. The total characteristic lifetime of the process is about 80 ps. Three intermediates were recorded in the femto- and picosecond time domains and assigned to different Os(iv) species. The nature of intermediates and possible mechanisms of photoaquation are discussed.

Photochemical transformation of an iron(III)-arsenite complex in acidic aqueous solution.

Surface complexation between arsenious acid anions (As(III)) and ferric (hydr)oxides in water is important for the transformation and transfer of inorganic arsenic species. The mechanisms of formation and the photochemistry of dissolved Fe(III)-As(III) complexes in acidic aqueous solution are still unclear. Here, the photooxidation of As(III) in the presence of Fe(III) ions in acidic media has been investigated by laser flash and steady-state photolysis. At low arsenite concentrations (<1 mM), As(III) is oxidized by the ˙OH radical generated by photolysis of the FeOH(2+) complex. At higher arsenite concentrations (>10 mM), photoactive Fe(III)-As(III) complexes are formed (ϕ≈ 0.012). At all arsenite concentrations, a white FeAsO4 colloid is formed during As(III) photolysis in the presence of Fe(III) ions. Solid Fe(III)-As(III) complexes have been prepared and characterized, and the photochemical transformation of As(III) into As(V) in solid Fe(III)-As(III) complexes has been confirmed. These findings are important for a better understanding of the evolution of As(III) species under environmental conditions and should provide guidance for detoxification of As(III)-polluted water systems.

Photochemistry of Dithiocarbamate Cu(S2CNEt2)2 Complex in CHCl3. Transient Species and TD-DFT Calculations.

Nanosecond laser flash photolysis was used to study the mechanism of photochemical transformations of the diethyldithiocarbamate Cu(II) complex (Cu(dtc)2, where dtc- ≡ -S2CNEt2 anion) in chloroform solutions. The electron transfer from the excited Cu(dtc)2 complex to a solvent molecule leads to the appearance of the primary intermediate, the [ClCu(dtc)(dtcCHCl2)] complex, where a dtcCHCl2 molecule is coordinated with a copper ion via one sulfur atom. In the fast reaction (k = 2.1 × 109 M-1 s-1) with Cu(dtc)2, this complex forms a long-lived dimer [ClCu(dtc)(dtcCHCl2)Cu(dtc)2]. This intermediate decays during several seconds (k = 5.6 × 10-2 s-1) into the final product, i.e., a diamagnetic dimer [ClCu(dtc)Cu(dtc)2]. To determine the structure of intermediate complexes the quantum chemical calculations were carried out using DFT, TD-DFT, and PCM (Polarizable Continuum Model) methods.

Laser flash photolysis study of photocatalytic properties of pillared interlayered clays and Fe,Al-silica mesoporous catalysts.

Laser flash photolysis was applied to determine the primary photochemical processes over iron-containing clay (montmorillonite KSF), pillared interlayered clays (PILCs) and mesoporous mesophase iron silicate materials (MMMs). For KSF, the homogeneous photochemical reaction of Fe(III) leached from the clay material resulted in the formation of OH radicals, which were monitored by means of their reaction with methyl viologen dication (MV(2+)). For PILCs and MMMs, no leaching of Fe(III) to the solution nor hydroxyl radical formation were observed. Nevertheless, these catalysts were found to exhibit a sufficient effect on phenol photoionization. The increase in quantum yields of PhO radicals is caused by the effect of PILCs and MMMs and is explained by heterogeneous processes on the surface of catalyst particles.

A mechanistic study of the photodegradation of herbicide 2,4,5-trichlorophenoxyacetic acid in aqueous solution.

Photodegradation of herbicide 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in aqueous solution was investigated by stationary (254 nm) and nanosecond laser (266 nm) flash photolysis techniques. It was shown that in the primary photochemical step both photoionization (which generates a hydrated electron-radical cation pair) and heterolytic cleavage of a C-Cl bond takes place. The major products of substitution of one of the chlorine atoms in the 2-, 4- or 5-position by a hydroxyl group as well as the products of hydroxylation of the benzene ring in 3- and 6-positions were identified by HPLC and LC-MS methods. The complexation of 2,4,5-T with ß- and γ-cyclodextrins (ß(γ)CD) was investigated. The influence of such complexation on the quantum yield of herbicide photolysis and on the ratio of photodegradation products was determined.

Photophysics of 1,8-naphthalimide/Ln(III) dyads (Ln = Eu, Gd): naphthalimide → Eu(III) energy-transfer from both singlet and triplet states.

Transient absorption and time resolved luminescence spectroscopy were used to study photophysical processes in the macrocycle-appended 1,8-naphthalimide compound H3L, and its Eu(III) and Gd(III) complexes Eu·L and Gd·L, in particular the naphthalimide-Eu(III) energy-transfer process. In all cases aggregation of the naphthalimide chromophores results in a low-energy emission feature in the 470-500 nm region in addition to the naphthalimide fluorescence; this lower-energy emission has a lifetime longer by an order of magnitude than the monomer naphthalimide fluorescence. Transient absorption spectroscopy was used to measure the decay of the naphthalimide triplet excited state, which occurs in the range 30-50 µs. In Eu·L, partial energy-transfer from the naphthalimide chromophore results in sensitized Eu(III)-based emission in addition to the naphthalimide-based fluorescence features. Time-resolved measurements on the sensitized Eu(III)-based emission reveal both fast (~10(9) s(-1)) and slow (~10(4) s(-1)) energy-transfer processes from the naphthalimide energy-donor, which we ascribe to energy-transfer occurring from the singlet and triplet excited state of naphthalimide respectively. This is an unusual case of observation of sensitization of Eu(III)-based emission from the singlet state of an aromatic chromophore.

Efficient emission of Zn(II) and Cd(II) complexes with nopinane-annelated 4,5-diazafluorene and 4,5-diazafluoren-9-one ligands: how slight structural modification alters fluorescence mechanism.

Zinc(II) and cadmium(II) chlorido complexes with an N,N-chelating nopinane-annelated 4,5-diazafluoren-9-one ligand (LO) were synthesized. While the zinc(II) complex is mononuclear and adopts a tetrahedral ZnN2Cl2 coordination geometry, its cadmium(II) analogue features a 1D polymeric structure due to the bridging coordination of chlorido ligands with Cd2+ ions having an octahedral CdN2Cl4 coordination geometry. The photophysical properties of the oxygen-containing LO ligand and its zinc(II) and cadmium(II) complexes were studied in solution and in the solid state and matched against the properties of its oxygen-free 4,5-diazafluorene congener L and its complexes of the same metal ions. Comprehensive experimental and theoretical studies revealed the impact of the oxygen atom in the ligand core on the luminescence of the ligands and the complexes. For the oxygen-free L ligand and L-based complexes, the structural differences between the S0 and S1 geometries are small, which leads to fluorescence with extraordinarily small Stokes shifts. The emission of these compounds is of locally excited character for L and of mixed locally excited + ligand-to-halide charge transfer character for the L-based complexes. The introduction of the oxygen atom in the ligand core results in a drastic red-shift of the emission band due to short-range charge transfer. The differences between the S0 and S1 geometries are much more pronounced for LO and LO-based compounds than those of their oxygen-free analogues, leading to an order of magnitude larger Stokes shifts. On going from solution to the solid state, LO and its complexes exhibit aggregation-induced emission (AIE) behaviour with photoluminescence quantum yields (PLQYs) reaching tens of percent.

Direct UV photodegradation of nalidixic acid in aqueous solutions: A mechanistic study.

Mechanism of direct UV photolysis of nalidixic acid (NA), a model quinolone antibiotic, was revealed using a combination of steady-state photolysis coupled with high resolution LC-MS and DFT quantum-chemical calculations. Both quantum yields of photodegradation and detailed identification of final products were performed for the first time for two main forms of NA: neutral and anionic. The quantum yield of NA photodegradation is 0.024 and 0.0032 for the neutral and anionic forms in the presence of dissolved oxygen and 0.016/0.0032 in deoxygenated solutions, respectively. The main process is photoionization with the formation of a cation radical, which undergoes transformation into three different neutral radicals and further into final photoproducts. It is shown that the triplet state does not play a role in the photolysis of this compound. The main products of photolysis are the products of the loss of carboxyl, methyl and ethyl groups in the NA molecule, as well as the dehydrogenation of the ethyl group. The results obtained may be important for understanding the fate of pyridine herbicides in the processes of disinfection by UV and in natural waters under the action of sunlight.

Luminescence of ESIPT-capable zinc(II) complexes with a 1-hydroxy-1H-imidazole-based ligand: exploring the impact of substitution in the proton-donating moiety.

The rational design of ESIPT-capable metal complexes (ESIPT - Excited State Intramolecular Proton Transfer) requires two sites, namely, an ESIPT site and a metal binding site, to be spatially separated into the ligand core. Ligands featuring such sites are able to bind metal ions without being deprotonated upon their coordination. The use of ESIPT-capable ligands for the synthesis of metal complexes paves the way toward the exploration of ESIPT in the field of coordination chemistry. In this study, we present a new ESIPT-capable ligand on the base of 1-hydroxy-1H-imidazole, 1-hydroxy-5-methyl-4-[(2,2'-bipyridin)-6-yl]-2-(pyridin-2-yl)-1H-imidazole (HLb), and a series of ESIPT-capable zinc(II) halido complexes, [Zn(HLb)X2] (X = Cl, Br, I). Due to the incorporation of a (2,2'-bipyridin)-6-yl group at position 4 of the imidazole cycle, HLb acts as an N,N,N-chelating ligand. In the solid state, HLb and [Zn(HLb)X2] emit in the yellow region of the spectrum with excited state lifetimes in the nanosecond domain. Chelation-induced emission enhancement (CHEF) effect in zinc(II) complexes leads to an increase in the photoluminescence quantum yield (PLQY) for these compounds in comparison with free HLb ligand. The ESIPT process in HLb and [Zn(HLb)X2] is barrierless. The emission of [Zn(HLb)X2] is associated with the S1T → S0 transition in the tautomeric form (T-form). In contrast, due to (i) the dark nature of the S1 state and the bright nature of the S2 state and (ii) the large S1-S2 energy gap, HLb shows weak S2T → S0 fluorescence, in violation of Kasha's rule. Finally, the analysis of atomic charges in a series of ESIPT-capable 1-hydroxy-1H-imidazoles and their zinc(II) complexes allowed us to reveal the influence of expanding π-conjugation in the proton-donating and proton-accepting moieties on the stabilization/destabilization of the T-form and on the position of the emission band.