Aromatic aldols and 1,5-diketones as optimized fragrance photocages.

Aromatic aldols and 1,5-diketones with abstractable γ-hydrogen atoms are highly photoactive cage molecules for the release of fragrance carbonyl compounds (aldehydes and Michael ketones, respectively). Aldols 3a-d are easily accessible by Mukaiyama addition and are cleaved to form the substrates with high quantum yields under solar radiation. By tuning the properties of the chromophores, a series of δ-damascone cages 5 were developed that can be used for selective and fast (5a,e) or slow (5b,d) release of fragrances under air and solar irradiation. The intermediates of the Norrish II process were observed by laser transient absorption spectroscopy.

Photoprocesses of chlorin e6 glucose derivatives.

The ground and excited state processes of chlorin e6, the monomethyl ester C1, the glucose derivative C2 and the 3-heptylchlorin-glucose C3, were studied in solvents of lower and higher polarity. The excited singlet and lowest triplet states of C1-C3 were characterized by spectroscopic methods for several conditions. The quantum yields of formation of singlet molecular oxygen and the other triplet properties of the three chlorins and chlorin e6 are similar, whereas the fluorescence quantum yield decreases on going from C1 to C3. Time-resolved optoacoustic experiments revealed a ca. 30 kJ mol(-1) higher triplet level for C3 with respect to C1/2.

Photooxidation of alkaloids: considerable quantum yield enhancement by rose bengal-sensitized singlet molecular oxygen generation.

The photooxidation of sanguinarine, coralyne and berberine was studied in oxygenated alkaline methanol solutions. Rose bengal as photosensitizer significantly accelerates the process, indicating the importance of singlet molecular oxygen in the reaction mechanism. The quantum yield of sensitized oxidation was found to increase significantly with pH and reaches 0.4 for berberine at pH 13.8. The direct oxidation of alkaloids is less efficient, the quantum yield does not exceed 0.01 even in oxygen-saturated solutions. The photoinduced electron ejection does not play a role in the oxidation. The uncharged pseudobase forms, which are present in alkaline medium, are oxidized much more easily than the alkaloid cations.

Peptide release upon photoconversion of 2-nitrobenzyl compounds into nitroso derivatives.

The photoinduced conversion via the aci-nitro into the nitroso form was studied for 4,5-dimethoxy-2-nitrobenzyl alcohols attached to various leaving groups: amino acids histidine (NHis) and aspartate (NAsp) as well as their fluorenylmethoxycarbonyl derivatives (FHis) and (FAsp). In addition, two peptides containing either of the two amino acids were studied, carrying the photoreactive group attached to a histidine (PHis), or to an aspartate (PAsp). The aci-nitro forms with maximum at λ(aci) = 420 nm were observed for FHis and FAsp after the decay of a triplet-triplet absorption, analogous to those of other 2-nitrobenzyl type compounds. For both FHis and FAsp the quantum yield of photoconversion Φ(p) is 0.03 and for the peptides PHis and PAsp ca 0.01 and 0.005, respectively.

Reduction of isoviolanthrone by reaction with photogenerated radicals.

The properties of isoviolanthrone (V(1)) and its corresponding dihydro form (V(1)-H(2)) in argon-saturated 1-phenylethanol in the presence of acetophenone were examined by photophysical and photochemical methods. Upon photolysis of the sensitizer, a radical-induced reduction of V(1) to V(1)-H(2) occurs. The quantum yield of this reduction is close to unity. The fluorescence of V(1) has a quantum yield of 0.08, for V(1)-H(2)Φ(f) is strongly enhanced and the emission maximum is blue-shifted. The ketyl radical, the semiquinone radical and V(1)-H(2) were observed in this sequence by flash photolysis. V(1)-H(2) is unstable and undergoes oxidation to V(1) on admission of oxygen.

Direct photoreduction and ketone-sensitized reduction of nitrospirobenzopyranindolines by aliphatic amines.

The photoreduction of 6-nitrospiro[2H-1-benzopyran-2,2'-indoline] (N1) and two derivatives (N2 and N3) by diethylamine or triethylamine (TEA) in solution was studied by pulsed and steady-state photolysis. The quantum yield of coloration of the ring-closed Sp form, due to photoinduced ring opening, decreases in acetonitrile with increasing the TEA concentration. The main reason is reaction of TEA with the triplet-excited open merocyanine form. Quenching of this triplet state by amines is rather inefficient for N1-N3; the rate constant for triplet quenching by TEA is k(6) = (2-3) × 10(6) M(-1) s(-1). The secondary transient with an absorption maximum at 420 nm is ascribed to the radical anion. This and the corresponding α-aminoethyl radical subsequently undergo slow termination reactions, yielding a relatively stable product with a maximum at 420-450 nm, which is attributed to a ring-opened dihydromerocyanine (MH(-)). The mechanisms of the two subsequent reduction reactions are discussed. Using acetone as sensitizer the same dihydroproduct was obtained with the Sp form as acceptor, indicating a reaction sequence from photogenerated radicals via a ring-opened radical to MH(-)/MH(2). The effect of TEA concentration on the direct and ketone-sensitized reduction mechanisms was analyzed. Photoreduction by amines, due to competing triplet quenching, is strongly decreased on admission of oxygen.

Cyanine-based J-aggregates as a chirality-sensing supramolecular system.

J-aggregates are formed for 3,3'-disulfopropyl-5,5'-dichlorothiacyanine (Tc) and 3,3'-disulfobutyl-5,5'-diphenyl-9-ethyloxacarbocyanine (Oc) in aqueous solution in the presence of NaCl, Mg(NO(3))(2), d/l-tartaric acids, asparagine, proline, DNA, and proteins, such as lysozyme, trypsin, RNase, and gelatin. J-aggregates, which are formed in the presence of chiral additives, are optically active and characterized by sigmoidal kinetics with half-times of 10-1000 s, resonance fluorescence, and large CD amplitudes being up to 2° for Tc. Generally, the induced CD signals of the J-aggregates of both dyes are bisignate and the sign corresponds to that of the additive. The transfer of chirality information occurs in the course of the J-aggregation.

Effect of electrolytes, nucleotides and DNA on the fluorescence of flavopereirine natural alkaloid.

The absorption and fluorescence characteristics of flavopereirine, a pharmaceutically important natural alkaloid, were studied to reveal how the complex formation and the change of the microenvironment affect the deactivation kinetics from the singlet-excited state. The fluorescence lifetime was not influenced by the ionic strength, but a significant deuterium effect was observed showing that hydrogen bonding in the singlet-excited state promoted energy dissipation. Nucleotides caused both static and dynamic quenching. The rate constant of the latter process increased when the nucleobase was capable of donating electron to the excited flavopereirine. The spectrophotometric measurements provided evidence for non-cooperative binding to double-stranded DNA with an equilibrium constant of 4.6 × 10(5) M(-1). Time-resolved fluorescence signals showed that three kinds of complexes are formed with distinct fluorescence lifetimes. Flavopereirine binding to chondroitin sulfate was also found, which led to different fluorescence characteristics at pH 2 and 6.

Direct photooxidation and xanthene-sensitized oxidation of naphthols: quantum yields and mechanism.

The photoinduced oxidation of 1-naphthol to 1,4-naphthoquinone and of 5-hydroxy-1-naphthol to 5-hydroxy-1,4-naphthoquinone was studied by steady-state and time-resolved techniques. The direct photooxidation of naphthols in methanol or water takes place by reaction of the naphoxyl radical ((•)ONaph) with the superoxide ion radical (O(2)(•-)), the latter of which results from the reaction of the solvated electron with oxygen after photoionization. The sensitized oxidation takes place by energy transfer from the xanthene triplet state to oxygen. From the two oxygen atoms, which are consumed, one is incorporated into the naphthol molecule giving naphthoquinone and the second gives rise to water. The effects of eosin, erythrosin, and rose bengal in aqueous solution, pH, and the oxygen and naphthol concentrations were studied. The quantum yield of the photosensitized transformation was determined, which increases with the naphthol concentration and is largest at pH > 10. The quantum yield of oxygen uptake is similar. The pathway involving singlet molecular oxygen is suggested to operate for the three sensitizers. The alternative pathway via electron transfer from the naphthol to the xanthene triplet state and subsequent reaction of (•)ONaph with O(2)(•-), the latter of which is formed by scavenging of the xanthene radical anion by oxygen, does also contribute.

Photoreduction and ketone-sensitized reduction of alkaloids.

The photoprocesses of berberine, palmatine, coralyne, sanguinarine, flavopereirine and ellipticine were studied in several solvents. The quantum yields Φ(Δ) of singlet molecular oxygen formation of berberine, palmatine and sanguinarine are moderate in dichloromethane (0.2-0.6) and much smaller in acetonitrile or trifluoroethanol. For the other alkaloids examined, Φ(Δ) is rather independent of solvent polarity. The direct and ketone-sensitized photolysis, using steady-state irradiation at 313 nm or 248/308 nm laser pulses, was studied by absorption and fluorescence spectroscopy. Thereby, radicals were observed yielding eventually dihydro derivatives as major products, which are thermally back-converted on admission of oxygen. The quantum yield of conversion of alkaloids to dihydroalkaloids is enhanced in the presence of triethylamine. The reaction in the presence of ketones and electron or H-atom donors has a quantum yield of close to unity.

Copper curcuminoids containing anthracene groups: fluorescent molecules with cytotoxic activity.

The coordination chemistry of the new curcuminoid ligand, 1,7-(di-9-anthracene-1,6-heptadiene-3,5-dione), abbreviated 9Accm has been studied, resulting in two new copper-9Accm compounds. Compound 1, [Cu(phen)Cl(9Accm)], was synthesized by reacting 9Accm with [Cu(phen)Cl(2)] in a 1:1 ratio (M:L) and compound 2, [Cu(9Accm)(2)], was prepared from Cu(OAc)(2) and 9Accm (1:2). UV-vis, electron paramagnetic resonance (EPR), and superconducting quantum interference device (SQUID) measurements were some of the techniques employed to portray these species; studies on single crystals of free 9Accm, [Cu(phen)Cl(9Accm)] and [Cu(9Accm)(2)(py)] provided detailed structural information about compounds 1 and 2·py, being the first two copper-curcuminoids crystallographically described. In addition the antitumor activity of the new compounds was studied and compared with free 9Accm for a number of human tumor cells. To provide more insight on the mode of action of these compounds under biological conditions, additional experiments were accomplished, including studies on the nature of their interactions with calf thymus DNA by UV-vis titration and Circular Dichroism. These experiments together with DNA-binding studies indicate electrostatic interactions between some of these species and the double helix, pointing out the weak nature of the interaction of the compounds with CT-DNA. The intrinsic fluorescence of the free ligand and both copper compounds provided valuable information over the cellular process and therefore, fluorescence microscopy studies were performed using a human osteosarcoma cell line. Studies in vitro using this technique suggest that the action of these molecules seems to occur outside the nuclei.

Reduction of 4,4'-stilbenequinone and 4,4'-diphenoquinone upon reaction with photogenerated radicals.

The properties of 3,3',5,5'-tetra-tert-butyl-4,4'-stilbenequinone (StQ) were studied by photochemical means. Acetone, acetophenone or benzophenone was photolyzed in the presence of both StQ and a donor, such as alcohols or triethylamine. This initiated reaction of a ketyl radical with StQ to form a semiquinone radical and eventually induce a permanent bleaching due to conversion of StQ to 4,4'-dihydroxystilbene (StQH(2)). The quantum yield of conversion of StQ to StQH(2) increases with the donor concentration. Similar effects were found for the ketone-sensitized radical-induced conversions of the analogous diphenoquinone to the reduction product, diphenol.

Kinetics of spontaneous formation of chiral J-aggregate of N-sulfobutyl oxacarbocyanine.

An oxacarbocyanine with N-sulfobutyl substituents, dye 1, exhibits an ability to form optically active J-aggregates in aqueous solution in the absence of chiral auxiliaries or templates. Optically active J-aggregates are formed in the absence (self-association) and presence of mono- and divalent metal ions. The time course of formation of J-aggregates is described by a sigmoidal time dependence and further characterized by an induction period. The chirality arises either at the step of nucleation or because of conversion of an achiral aggregate (J1) into a chiral one (J2). A monomeric ion pair formed between the anionic dye and the metal cation is the precursor of both aggregates. A red-shifted absorption band, resonance fluorescence, and mono- or bisignate circular dichroism (CD) signals are characteristics of J2. The shape and sign of the CD signal as well as the number of molecules coherently coupled in J2 depend upon the type of metal ion. The presence of N-sulfobutyl groups in 1 results in a relatively high hydrophobicity of the dye molecule, determining the formation of the J2-aggregate.

Properties of 16,17-disubstituted dihydroviolanthrones formed by reaction of violanthrones with photogenerated radicals.

The photophysical and photochemical properties of violanthrone (V1) and two 16,17-R derivatives with R = OC(8)H(17) and OCOC(17)H(35) (V2 and V3, respectively) in solution were examined. The fluorescence quantum yield of V3 is Phi(f) = 0.2, while that of V1 is low (0.01) and V2 even lower. A radical-induced reduction takes place in the presence of a photosensitizer, such as acetone, acetophenone or benzophenone, and an alcohol as donor. The reduction quantum yield in an argon-saturated solution, e.g. in 1-phenylethanol and glycerol triacetate, is close to unity. The products are the corresponding dihydroquinones (VH(2)s), which fluoresce strongly at 560-600 nm, Phi(f) = 0.6-1, and undergo back-oxidation on the admission of oxygen. The flash photolysis of violanthrones in the presence of both sensitizer and donor reveals ketyl radicals after the pulse, the intermediacy of the semiquinone radicals (VH*) at 650-750 nm and the formation of VH(2)s in the 0-1 ms range.

Photoinitiated domino reactions: N-(adamantyl)phthalimides and N-(adamantylalkyl)phthalimides.

Phthalimides 1-6 undergo photochemical reactions upon direct irradiation as well as triplet sensitization and give rise to new products. Besides formation of the primary photoproducts, the first photochemical step initiates a subsequent thermal domino reaction or a domino sequence of a thermal and a photochemical reaction. The latter, involving two photochemical intramolecular gamma-H abstractions, was observed with phthalimides 1, 3, and 6 and delivered stereospecifically the hexacyclic benzazepine products 12, 19, and 27, respectively. The lowest triplet states of 1-6 were characterized in several solvents upon direct and acetone-sensitized excitation. The intermolecular electron transfer from triethylamine and DABCO was studied, and the radical anions were observed. Electrochemical measurements showed that intramolecular electron transfer from the adamantyl group of 1-6 to the lowest triplet state of the phthalimides is not feasible. The formation of products can be explained by intramolecular H-abstraction from the (alkyl)adamantane to the excited phthalimide, either from the excited singlet state or a hidden upper excited triplet state.

Reduction of the polynuclear quinonoid dyes 16,17-dihydroxy- and dimethoxyviolanthrone with photogenerated radicals.

The radical-induced reduction of 16,17-dihydroxy- (M1O2) and 16,17-dimethoxyviolanthrone (M2O2) in argon-saturated 1-phenylethanol or mixtures of dimethyl sulfoxide and acetonitrile or glycerol triacetate was studied by photochemical methods. The photoproduct of MiO2 (i=1, 2) in the presence of a sensitizer, such as acetone, acetophenone or benzophenone, and a donor, e.g. an alcohol or triethylamine, is the fluorescing dihydroquinone, Mi(OH)2 which undergoes back-oxidation. Flash photolysis reveals ketyl radicals after the pulse and indicates radical formation in the 1 ms range en route to Mi(OH)2, while the putative donor and semiquinone radicals are undetectable intermediates. The concentrations of quinonoid dyes and donors were varied to explore the phenomena and to provide data for a mechanistic interpretation.

Dimer-promoted fluorescence quenching of coralyne by binding to anionic polysaccharides.

The effects of anionic chondroitin or dextran sulfates on the absorption and fluorescence properties of coralyne, a cationic benzo[c]phenanthridine type alkaloid, were studied in aqueous solution. The remarkably strong binding to both polysaccharides promotes the coralyne dimer formation, which was evidenced by the changes in the absorption and fluorescence spectra and the fluorescence decay. The extent of dimerization, induced by chondroitin, shows a significant pH dependence because the competitive protonation of the carboxylate moieties of the polymer chain decreases the number of binding sites. A larger molecular weight of dextran sulfate stabilizes the coralyne dimer more efficiently.

Flavin-sensitized photo-oxidation of lysozyme and serum albumin.

The excited state processes of riboflavin, flavin mononucleotide and flavin adenine dinucleotide in argon-saturated aqueous solution were studied in the presence of lysozyme or bovine serum albumin (BSA). UV-Vis absorption and fluorescence spectroscopy indicates that the noncovalent flavin-protein binding is relatively weak. Quenching of the flavin triplet state by BSA, observed by time-resolved photolysis, is less efficient than by lysozyme. Light-induced oxidation of the two proteins and reduction of the three flavins were studied. The quantum yields of the former and latter in the absence of oxygen are up to 0.1 and 0.04, respectively. The effects of pH and sensitizer and protein concentrations were examined in greater detail. The proposed reaction is electron transfer from the tryptophan moiety to the flavin triplet rather than excited singlet state.

Photoprocesses of xanthene dyes bound to lysozyme or serum albumin.

The ground and excited state processes of eosin, erythrosin and rose bengal in aqueous solution were studied in the presence of lysozyme or bovine serum albumin (BSA). Noncovalent protein-dye binding was analyzed by circular dichroism (CD), fluorescence and UV-Vis absorption spectroscopy. The effects of protein concentrations and pH were studied. Fluorescence quenching of the dye takes place due to binding to lysozyme and fluorescence enhancement due to low loading to BSA. The effects of proteins on the xanthene triplet state and its precursor were observed by time-resolved 530 nm photolysis. The triplet lifetime is quenched by lysozyme and prolonged by loading to BSA. Light-induced damages on both the dyes and proteins were observed under exclusion of oxygen. Photo-oxidation is efficient for lysozyme and lower for BSA. The CD signal of the eosin/BSA system is maximum at pH 4, where the photo-oxidation is minor.