Tetramethylalloxazines as efficient singlet oxygen photosensitizers and potential redox-sensitive agents.

Tetramethylalloxazines (TMeAll) have been found to have a high quantum yield of singlet oxygen generation when used as photosensitizers. Their electronic structure and transition energies (S0 → Si, S0 → Ti, T1 → Ti) were calculated using DFT and TD-DFT methods and compared to experimental absorption spectra. Generally, TMeAll display an energy diagram similar to other derivatives belonging to the alloxazine class of compounds, namely π,π* transitions are accompanied by closely located n,π* transitions. Photophysical data such as quantum yields of fluorescence, fluorescence lifetimes, and nonradiative rate constants were also studied in methanol (MeOH), acetonitrile (ACN), and 1,2-dichloroethane (DCE). The transient absorption spectra were also analyzed. To assess cytotoxicity of new compounds, a hemolytic assay was performed using human red blood cells (RBC) in vitro. Subsequently, fluorescence lifetime imaging experiments (FLIM) were performed on RBC under physiological and oxidative stress conditions alone or in the presence of TMeAll allowing for pinpointing changes caused by those compounds on the intracellular environment of these cells.

5-Deazaalloxazine as photosensitizer of singlet oxygen and potential redox-sensitive agent.

Flavins are a unique class of compounds that combine the features of singlet oxygen generators and redox-dependent fluorophores. From a broad family of flavin derivatives, deazaalloxazines are significantly underdeveloped from the point of view of photophysical properties. Herein, we report photophysics of 5-deazaalloxazine (1a) in water, acetonitrile, and some other solvents. In particular, triplet excited states of 1a in water and in acetonitrile were investigated using ultraviolet-visible (UV-Vis) transient absorption spectroscopy. The measured triplet lifetimes for 1a were all on the microsecond time scale (≈ 60 µs) in deoxygenated solutions. The quantum yield of S1 → T1 intersystem crossing for 1a in water was 0.43 based on T1 energy transfer from 1a to indicaxanthin (5) acting as acceptor and on comparative actinometric measurements using benzophenone (6). 1a was an efficient photosensitizer for singlet oxygen in aerated solutions, with quantum yields of singlet oxygen in methanol of about 0.76, compared to acetonitrile ~ 0.74, dichloromethane ~ 0.64 and 1,2-dichloroethane ~ 0.54. Significantly lower singlet oxygen quantum yields were obtained in water and deuterated water (Ð¤Δ ~ 0.42 and 0.44, respectively). Human red blood cells (RBC) were used as a cell model to study the antioxidant capacity in vitro and cytotoxic activity of 1a. Fluorescence-lifetime imaging microscopy (FLIM) data were analyzed by fluorescence lifetime parameters and distribution for different parts of the emission spectrum. Comparison of multidimensional fluorescent properties of RBC under physiological-like and oxidative-stress conditions in the presence and absence of 1a suggests its dual activity as probe and singlet-oxygen generator and opens up a pathway for using FLIM to analyze complex intracellular behavior of flavin-like compounds. These new data on structure-property relationship contribute to the body of information required for a rational design of flavin-based tools for future biological and biochemical applications.

Co-Sensitization Effects of Indoline and Carbazole Dyes in Solar Cells and Their Neutral-Anion Equilibrium in Solution.

Co-sensitization of two or more light-absorbing compounds on a TiO2 surface has recently become one of the most successful strategies in the development of dye-sensitized solar cells (DSSCs). The specific structure of the dyes for DSSCs implies that they can partly exist in anionic forms in popular solvents used for sensitization. Our study concerns the above two issues being analyzed in detail using the example of the popular carbazole (MK2) and indoline (D205) dyes, studied by stationary absorption and emission, femtosecond transient absorption (in complete cells and in the solutions), current-voltage measurements, DFT and TD-DFT theoretical calculations. After the addition of D205 to DSSC with MK2, the fill factor of the cells was improved, and the electron recombination between TiO2 and the dyes was blocked (observed on sub-nanosecond time scales). Thus, the active co-adsorbent can take the role of the typically used passive additive, like chenodeoxycholic acid. Evidence of the concentration-dependent equilibrium between neutral and anionic forms of dyes with different lifetimes was found in acetonitrile solutions (the best for sensitization), while in ethanol solution the dominant form was the anion (worse for sensitization). Our findings should help in better understanding the operation and optimization of DSSC.

The Effect of Chloride Anions on Charge Transfer in Dye-Sensitized Photoanodes for Water Splitting.

The photoelectrochemical behavior of dye-sensitized photoelectrochemical cells based on a TiO2 layer sensitized with ruthenium components, including an absorber, ruthenium(II)bis(2,2'-bipyridine)([2,2'-bipyridine]-4,4'-diylbis(phosphonic acid)) dibromide (RuP), and a catalyst, ruthenium(II) tris(4-methylpyridine)(4-(4-(2,6-bis((l1-oxidanyl)carbonyl)pyridin-4-yl)phenyl) pyridine-2,6-dicarboxylic acid) (RuOEC), was investigated in the following water-based electrolyte configurations: KCl (pH ≈ 5), HCl (pH ≈ 3), ethylphoshonic acid (pH ≈ 3) with a different KCl concentration, and a standard phosphate buffer (pH ≈ 7). The rate of charge transfer on the photoanode's surface was found to increase in line with the increase in the concentration of chloride anions (Cl-) in the low pH electrolyte. This effect is discussed in the context of pH influence, ionic strength, and specific interaction, studied by cyclic voltammetry (CV) in dark conditions and upon illumination of the photoanodes. The correlations between photocurrent decay traces and CV studies were also observed.

Photodynamic inactivation of Enterococcus faecalis by non-peripherally substituted magnesium phthalocyanines entrapped in lipid vesicles.

Photophysical properties and photodynamic antibacterial potential of magnesium phthalocyanines bearing 2-propoxy, benzyloxy, 3,5-bis(benzyloxy)benzyloxy substituents at non-peripheral positions were studied. The UV-Vis absorption spectra of researched phthalocyanine derivatives were found typical. Extension of peripheral substituent size from 2-propoxy to benzyloxy and finally 3,5-bis(benzyloxy)benzyloxy was accompanied by the rise of quantum yield of fluorescence up to 0.17 and 0.04 in DMF and DMSO, respectively. Similarly, the expansion of the phthalocyanine periphery from the 2-propoxy to benzyloxy and 3,5-bis(benzyloxy)benzyloxy groups resulted in a detectable increase of the singlet oxygen quantum yield values to 0.04, 0.12, 0.14 respectively, which was assessed following direct method of singlet oxygen phosphorescence measurement at 1270 nm. Studied phthalocyanines undergo photobleaching process with the quantum yields at the level of 10-6 in DMSO and 10-5 in DMF. The size of phthalocyanine impacted the process of liposomal formulation. Small liposome vesicles containing non-peripherally substituted phthalocyanines with 2-propoxy and benzyloxy substituents were obtained following extrusion method. The unification process of the liposomes loaded with 3,5-bis(benzyloxy)benzyloxy non-peripherally substituted phthalocyanines was not possible. In in vitro antimicrobial photodynamic inactivation study, log reduction values of bacterial (Enterococcus faecalis) growth at 3.61 and 2.99 were achieved for liposomal formulations containing phthalocyanines with 2-propoxy and benzyloxy substituents respectively, whereas phthalocyanine with 3,5-bis(benzyloxy)benzyloxy substituents was inactive. Phthalocyanine with 2-propoxy substituents exhibited relatively low toxicity in Vibrio fischeri bioluminescence test, whereas phthalocyanine with benzyloxy substituents revealed intense bioluminescence, which could be associated with hormesis phenomenon.

Comparison of charge transfer dynamics in polypyridyl ruthenium sensitizers for solar cells and water splitting systems.

Standard ruthenium components of dye-sensitized solar cells (sensitizer N719) and dye-sensitized photoelectrochemical cells (sensitizer RuP and water oxidation catalyst RuOEC) are investigated in the same solar cell configuration to compare their photodynamics and charge separation efficiency. The samples are studied on time scales from femtoseconds to seconds by means of transient absorption, time-resolved emission and electrochemical impedance measurements. RuP shows significantly slower electron injection into a mesoporous titania electrode and enhanced fast (sub-ns) electron recombination with respect to those of N719. Moreover, RuOEC is found to be responsible for partial light absorption and electron injection with low efficiency. The obtained results reveal new insights into the reasons for the lower charge separation efficiency in water splitting systems with respect to that in solar cells. The important role of the initial processes occurring at the dye-titania interface within the first nanoseconds in this efficiency is emphasized.

Insights into the limitations of solar cells sensitized with ruthenium dyes revealed in time-resolved spectroscopy studies.

The substitution of iodide electrolytes with cobalt ones has led to the current champion laboratory efficiencies for dye-sensitized solar cells (DSSCs). However, unlike with organic dyes, this strategy does not work with classical ruthenium dyes. Therefore, we compare DSSCs sensitized with a popular Ru dye (N719) using both types of electrolytes by exploring the electron dynamics occurring from sub-ps to seconds. An important limitation in the photocurrent of cobalt-based cells is revealed to be due to electron recombination between titania and oxidized Ru dyes, which is much higher than that in iodide-based cells and occurs on the time scale of tens and hundreds of ps. Electron recombination between titania and the electrolyte, taking place on the millisecond time scale, is responsible for further lowering of the photovoltage and fill factor of cobalt-based cells. Ruthenium dyes also exhibit lower absorption coefficients with respect to their organic counterparts. For this reason, we also investigate the effect of the changes in the titania layer thickness, addition of scattering nanoparticles and modifications in the TiCl4 treatment on DSSC performance.

Effects of Post-Assembly Molecular and Atomic Passivation of Sensitized Titania Surface: Dynamics of Electron Transfer Measured from Femtoseconds to Seconds.

The dynamics of electron transfer at the dye-titania and titania-electrolyte interfaces is investigated in two post-sensitization processes: (i) atomic layer deposition of blocking alumina coating and (ii) hierarchical molecular multicapping. To measure the electron transfer dynamics, time-resolved spectroscopic methods (femtosecond transient absorption on the time scale from femtoseconds to nanoseconds and electrochemical impedance spectroscopy on the time scale from milliseconds to seconds) are applied to the complete dye-sensitized solar cells with cobalt-based electrolyte and champion ADEKA-1 dye (with silyl-anchor unit) or its popular carboxyl-anchor analogue, MK-2 dye. Both molecular capping and alumina blocking layers slow down the electron injection process (the average rate constant decreases from 1.1 ps-1 to 0.4 ps-1) and partial sub-nanosecond back electron transfer from titania to the dye (from ca. 10 ns-1 to 5 ns-1). Very small alumina layers (of 0.1 nm thickness) have the highest impact on reducing the rate constants of these electron transfer processes, and for the thicknesses greater than 0.3 nm the rate constants hardly change. In contrast, the electron recombination between titania and electrolyte, occurring on the millisecond time scale, starts to be significantly suppressed for the blocking layers of 0.3 nm or more in thickness (up to ca. 20 times for 0.5 nm thickness with respect to that for untreated sample), improving open circuit voltage and fill factor of the cells. The amplitude of the relative photocurrent (short circuit current per number of absorbed photons) is found to depend almost exclusively on the ultrafast and fast processes taking place in the first nanoseconds after dye excitation. The positive impact of coadsorbents on the solar cells performance for both ADEKA-1 and MK-2 is also studied.

Chemical quenching of singlet oxygen by betanin.

Betanin is the best known natural dye belonging to the betacyanin family. In this work, efficient singlet oxygen quenching by betanin in deuterated water with the rate constant 1.20 ± 0.15 × 10(8) M(-1) s(-1) is reported, deduced from the (1)O2 phosphorescence decays measured as a function of betanin concentration. The quenching occurs by a chemical mechanism, as confirmed by the analysis of the transient absorption kinetics at the probe λ ∼ 535 nm, by comparison of the initial triplet signal amplitude of perinaphthenone acting as the (1)O2 photosensitizer with the final bleaching signal of betanin. The main betanin oxidation product is 2-decarboxy-2,3-dehydrobetanin, with its formation observed as the transient absorption signal at λ ∼ 445 nm. LC-MS/MS analysis of the photolyzed solutions supports the product identification as 2-decarboxy-2,3-dehydrobetanin, based on the molecular ion [M](+) observed at m/z 505. Isobetanin also undergoes a similar photooxidation reaction.

Study of photophysical properties of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine in dependence of pH using different spectral techniques.

The photophysical properties of 5-deazaalloxazine and 1,3-dimethyl-5-deazalloxazine at different pH values were characterized using absorption spectra, fluorescence emission spectra, fluorescence excitation spectra, synchronous fluorescence spectra and total fluorescence spectra. Their ionised and/or neutral forms were discussed in comparison with those obtained for other derivatives of 5-deazaalloxazine and/or 5-deazaisoalloxazine. Steady-state and time-resolved techniques were used to study the protonation/deprotonation equilibria between cationic and neutral forms of both compounds and between neutral and monoanionic forms of 5-deazalloxazine, as well as between monoanionic forms of this compound and its dianion. We estimated pKa values for these equilibria both in the ground and excited states. Our steady-state and time-resolved measurements indicate that the cation of 5-deazaalloxazine in its isoalloxazinic form exhibits fluorescence that is quenched by protons in a dynamic process. Contrary to that, the cation of 1,3-dimethyl-5-deazaalloxazine has almost no fluorescence. Additionally, we found that the neutral forms of 5-deazalloxazine and 1,3-methyl-5-deazalloxazine are also quenched in acidic conditions by protons. In basic conditions, 5-deazaalloxazine forms two structurally different anions, namely the alloxazinic monoanion and the isoalloxazinic monoanion; both simultaneously dissociate into the isoalloxazinic dianion at even higher pH values. The synchronous fluorescence spectra and total fluorescence spectra demonstrated their suitability to characterize and differentiate different fluorescent forms of 5-deazalloxazine, namely: the cation, the neutral form, two monoanions, and the dianion, in a wide pH range.

Synthesis, spectroscopy, theoretical and biological studies of new gramine-steroids salts and conjugates.

New gramine connections with bile acids (lithocholic, deoxycholic, cholic) and sterols (cholesterol, cholestanol) were synthesized. The structures of products were confirmed by spectral (NMR, FT-IR) analysis, mass spectrometry (ESI-MS) as well as PM5 semiempirical methods. Unexpectedly, the products of the reaction of gramine with cholesterol and cholestanol were symmetrical compounds consisting of two molecules of sterols connected by N(CH3)2 group. All new synthesized compounds interact in vitro with the human erythrocyte membrane and alter discoid erythrocyte shape inducing stomatocytosis or echinocytosis. Increase in the incorporation of the fluorescent dye merocyanine 540 (MC540) into the erythrocyte membrane indicates that new compounds at sublytic concentrations are capable of disturbing membrane phospholipids asymmetry and loosening the molecular packing of phospholipids in the bilayer. Gramine significantly decreases the membrane partitioning properties as well as haemolytic activity of lithocholic acid in its new salt. Moreover, both deoxycholic and cholic acids completely lost their membrane perturbing activities in the gramine salts. On the other hand, the capacity of new gramine-sterols connections to alter the erythrocyte membrane structure and its permeability is much higher in comparison with sterols alone. The dual effect of gramine on the bile acid and sterols cell membrane partitioning activity observed in our study should not be neglected in vivo.

Photophysics, excited-state double-proton transfer and hydrogen-bonding properties of 5-deazaalloxazines.

The photophysical properties of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine were studied in different solvents. These compounds have higher values of fluorescence quantum yields and longer fluorescence lifetimes, compared to those obtained for their alloxazine analogs. Electronic structure and S0 -Si transitions were investigated using the ab initio methods [MP2, CIS(D), EOM-CCSD] with the correlation-consistent basis sets. Also the time-dependent density functional theory (TD-DFT) has been employed. The lowest singlet excited states of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine are predicted to have the π, π* character, whereas similar alloxazines have two close-lying π, π* and n, π* transitions. Experimental steady-state and time-resolved spectral studies indicate formation of an isoalloxazinic excited state via excited-state double-proton transfer (ESDPT) catalyzed by an acetic acid molecule that forms a hydrogen bond complex with the 5-deazaalloxazine molecule. Solvatochromism of both 5-deazaalloxazine and its 1,3-dimethyl substituted derivative was analyzed using the Kamlet-Taft scale and four-parameter Catalán solvent scale. The most significant result of our studies is that the both scales show a strong influence of solvent acidity (hydrogen bond donating ability) on the emission properties of these compounds, indicating the importance of intermolecular solute-solvent hydrogen-bonding interactions in their excited state.

Spectroscopy and Photophysics of Monomethyl-Substituted Derivatives of 5-Deazaalloxazine and 10-Ethyl-5-Deaza-Isoalloxazine.

Steady-state and time-resolved spectra were used to describe the singlet and triplet states of 8-methyl-5-deazaalloxazine (8-Me-5-DAll), 9-methyl-5-deazaalloxazine (9-Me-5-DAll) and 10-ethyl-5-deaza-isoalloxazine (10-Et-5-DIAll). Solvatochromic properties were described using different polarity scales, including Δf and the four-parameter scale proposed by Catalán. The results indicate that the Catalán scale shows a strong influence of solvent acidity (hydrogen-bond donating ability) on the emission properties of 8-Me-5-DAll and 9-Me-5-DAll. These results indicate the importance of intermolecular solute-solvent hydrogen-bonding interactions in the excited state of these compounds. Contrary to deazaalloxazines, solvent acidity affects the absorption spectra of 10-Et-5-DIAll. Fluorescence lifetimes and quantum yields and also transient absorption spectra were determined for all of the compounds studied. Electronic structure and S(0)-S(i), S(0)-T(i ), T(1)-T(i) transitions energies and oscillator strengths were calculated using the TD-DFT methods. Theoretical calculations were compared to experimental data.

Acid-base equilibriums of lumichrome and its 1-methyl, 3-methyl, and 1,3-dimethyl derivatives.

Lumichrome photophysical properties at different pH were characterized by UV-vis spectroscopy and steady-state and time-resolved fluorescence techniques, in four forms of protonation/deprotonation: neutral form, two monoanions, and dianion. The excited-state lifetimes of these forms of lumichrome were measured and discussed. The results were compared to those obtained for similar forms of alloxazine and/or isoalloxazine, and also to those of 1-methyl- and 3-methyllumichrome and 1,3-dimethyllumichrome. The absorption, emission, and synchronous spectra of lumichrome, 1-methyl- and 3-methyllumichrome, and 1,3-dimethyllumichrome at different pH were measured and used in discussion of fluorescence of neutral and deprotonated forms of lumichrome. The analysis of steady-state and time-resolved spectra and the DFT calculations both predict that the N(1) monoanion and the N(1,3) dianion of lumichrome have predominantly isoalloxazinic structures. Additionally, we confirmed that neutral lumichrome exists in its alloxazinic form only, in both the ground and the excited state. We also confirmed the existence and the alloxazinic structure of a second N(3) monoanion. The estimated values of pK(a) = 8.2 are for the equilibrium between neutral lumichrome and alloxazinic and isoalloxazinic monoanions, with proton dissociation from N(1)-H and N(3)-H groups proceeding at the almost the same pH, while the second value pK(a) = 11.4 refers to the formation of the isoalloxazinic dianion in the ground state.

Influence of pH on photophysical properties of (E)-1-(4-chlorobenzyl)-4-(4-hydroxystyryl)pyridinium chloride.

The protonation/deprotonation equilibrium was investigated for N-p-chlorobenzyl-substituted (E)-4'-hydroxy stilbazolium halide, namely (E)-1-(4-chlorobenzyl)-4-(4-hydroxystyryl)pyridinium chloride (EPC). Absorption, emission and synchronous scanning spectra were used to explain the observed phenomena. The excited state lifetimes of the protonated and deprotonated forms of EPC were measured and discussed. Absorption spectra were used to determine its pK(a) value in the ground state. We conclude that the protonation/deprotonation equilibrium is not attained in the first excited state of EPC, for kinetic reasons. The quinoid and benzenoid structures of EPC in the ground and excited state are discussed in acidic and basic range of pH. Aqueous solutions of EPC were yellow at pH < 7 and red at pH > 7, and addition of alcohols (methanol or 2-propanol) enhanced this change. Therefore, quaternary stilbazolium salts were investigated for application as acid-base indicators.