Long-wavelength Absorption Band Research Articles

Spectral and luminescent properties of chlorine-substituted derivatives of aniline

The effect of the position and number of chlorine atoms on spectral, geometrical, and photophysical characteristics of individual molecules of chlorine-substituted derivatives of aniline is studied by electronic spectroscopy and quantum-chemical calculations. It is shown that the long-wavelength absorption band of these compounds is formed by several electronic transitions of different nature and intensity. It is ascertained that electronically excited π σ* states involve C-Cl bonds, which weaken upon excitation. According to quantumchemical calculations, the quenching of fluorescence of chlorine-substituted derivatives of aniline is determined by the fact that intersystem crossing is more effective than radiative decay of the S 1 state.

Structure and properties of some cresolphthalein derivatives

The following cresolphthalein (CP) derivatives are prepared from 5,5′-dibromocresolphthalein ( 3): 5,5′-dicyano ( 7), 5,5′-dicarbomethoxy ( 9); 5-bromo-5′-cyano ( 8), 5-bromo-5′-carbomethoxy ( 10), monomethyl ether ( 11) and 5,5′-dibromo monomethyl ether ( 13). The lactone ring remains intact in the monobasic forms of 11 and 12. The dibasic forms of the CP derivatives 3 and 8– 10 show long-wavelength absorption bands, but exist mainly as the closed lactone structure. The dibasic form of 7 does not show long-wavelength absorption and exists only as the closed lactone structure. The proportion of ring-closed dianion goes as the electron-withdrawing ability of the substituents, and it increases in the order H<Br<CO 2CH 3<CN. Acidity constants are reported. They correlate with acidity of the related phenols, but not with the electronic characteristics of the substitutents. The first and second ionization constants are well-resolved in the unsymmetrical derivatives 8 and 10. AM1 calculations predicting optimal structures, uv/vis transitions and various equilibria are reported.

Replacement of 2,2'-bipyridine by 1,4-diazabutadiene acceptor ligands: why the bathochromic shift for [(N empty set N)IrCl(C5Me5)]+ complexes but the hypsochromic shift for (N empty set N)Ir(C5Me5)?

Replacement of 2,2'-bipyridine (bpy) by substituted 1,4-diazabutadiene (R-DAB) alpha-diimine ligands N empty set N leads to a substantial hypsochromic shift of about 0.8 eV for the long-wavelength absorption band in compounds (N empty set N)Ir(C(5)Me(5)) but to a bathochromic absorption shift of about 0.4 eV for the complex ions [(N empty set N)IrCl(C(5)Me(5))](+). DFT calculations on model complexes based on experimental (R-DAB compounds) and geometry-optimized structures (bpy systems) reveal that the low-energy transitions of the cationic chloro complexes are largely of ligand-to-ligand charge-transfer character L'LCT (L = alpha-diimine, L' = Cl) whereas the neutral compounds exhibit pi --> pi transitions between the considerably mixed metal d(pi) and alpha-diimine pi orbitals. The much more pronounced metal-ligand orbital interaction for the R-DAB complexes causes the qualitatively different shifts on replacing the stronger basic bpy by the better pi-acceptors R-DAB. Only the LUMO of the neutral compounds is destabilized on replacement of bpy by R-DAB whereas the LUMO of [(N empty set N)IrCl(C(5)R'(5))](+) and both HOMOs are stabilized through this change.

Evidence of Ground-State Proton-Transfer Reaction of 3-Hydroxyflavone in Neutral Alcoholic Solvents

Photophysical behavior of 3-hydroxyflavone (HF), a molecule that exhibits excited-state intramolecular proton-transfer reaction and has been studied extensively in the past, has been reinvestigated in hydrogen bond donating solvents. In neat alcohols or in an acetonitrile−methanol mixture, a long wavelength absorption band (λmax ≈ 410 nm) has been observed for the first time. It is found that selective excitation of this band does not produce the characteristic fluorescence of the “normal” form or the tautomer; instead, an emission characterized by a featureless band (λmax ≈ 480 nm) is observed. The absorption and emission bands have been found to disappear completely in the presence of water. The influence of temperature suggests that the species responsible for the 410 nm absorption band is produced in higher concentrations at higher temperatures. On the basis of the present results and the available literature, the long-wavelength absorption band of HF has been attributed to its anionic form, generated in the alcoholic media by solvent mediated deprotonation of the 3-hydroxy group of the molecule in the ground state.

Studies of solvation in homogeneous and heterogeneous media by electronic spectroscopic method

Solvation characteristics in homogenous (pure and mixed binary solvents) and heterogeneous media (aqueous micelles, β- and γ-cyclodextrine solutions) have been studied by monitoring the emission characteristics of a newly synthesised dye. The longest wavelength absorption and emission band of the dye arise due to transition between S 0 and S 1 state. The maximum energy of electronic transition involving intramolecular charge transfer is found to be dependent on both the hydrogen-bond donating ability and the polarity–polarisability in pure solvent. The dipole moment in the S 1 state, as determined by solvatochromic procedure, agrees well with the value obtained by theoretical calculation at the AM1 level. Preferential solvation of the dye by alcohols has been found to occur in ethanol+water, propan-1-ol+water, propan-2-ol+water binary mixtures. In aqueous micellar media the dye molecule is located at the water–micelle interface. The binding constant for the dye–micelle interaction has also been determined. The results have been compared with those for a structurally related symmetrical ketocyanine dye.

Monolithic Photorefractive Molecular Glass with Electron-Transporting Fluorene Unit

Multifunctional succinates have been prepared as monolithic molecular photorefractive materials which contain an electron-transporting fluorene moiety and an electrooptically active (EO) chromophore. The synthesized succinates form amorphous glass at room temperature with the aid of a flexible diester spacer and butoxycarbonyl group attached to the fluorene unit. Long-wavelength absorption bands were observed in addition to the peaks from fluorene and the EO chromophore, which are due to the charge transfer interaction between electron-accepting fluorene and electron-donating aniline units. Compounds showed photorefractivity without the addition of a photosensitizer, a polymeric binder or a plasticizer, indicating that they are completely monolithic photorefractive materials. Succinate with a 4-dicyanovinylaniline chromophore showed excellent properties, i.e., 32% diffraction efficiency at 55 V/µm and 125 cm-1 coupling gain at 65 V/µm. The electron-transporting nature of space-charge field buildup was confirmed from the coupling direction.

Influence of a zwitterionic surfactant on the chromogenic behavior of a dye‐containing aqueous PVA–polyether gel network

AbstractThe effect of the addition of a zwitterionic sulfobetaine to a dye‐containing polymer gel composed of two polymers – poly(vinyl alcohol) (PVA) and polyether – was investigated. With increasing sulfobetaine content a remarkable reduction of the UV‐vis‐absorption intensity of the longest wavelength absorption band of Phenol Red in the aqueous polymer network was observed, even for sulfobetaine concentrations below its critical micelle concentration (cmc). It can be assumed that this effect is based on the formation of ionic complexes between dye molecules and either single sulfobetaine molecules or aggregates of sulfobetaine. Furthermore, thermotropic behavior occurs in the investigated polymer gel system even at a polyether concentration as low as 0.8 wt%. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Tri(allyl)- and tri(methylallyl)arsine complexes of palladium(ii) and platinum(ii): synthesis, spectroscopy, photochemistry and structures

Tri(allyl)- and tri(methylallyl)arsine complexes of palladium(II) and platinum(II) with the formulae [MX2L2] (M=Pd, Pt and X=Cl, Br, I): [Pd2Cl2(μ-Cl)2L2], [PdCl(S2CNEt2)L] and [Pd2Cl2(μ-dmpz)2L2] [L=As(CH2CHCH2)3 (L′), As(CH2CMeCH2)3 (L″), dmpz=3,5-dimethylpyrazolate] have been prepared. All the complexes have been characterized by elemental analyses and by IR and NMR (1H, 13C, 195Pt) spectroscopy. The stereochemistry of the complexes has been deduced from the spectroscopic data. The molecular structures of complexes [MX2{As(CH2CHCH2)3}2] (M=Pd, X=Cl or Br), [MX2{As(CH2CMeCH2)3}2] (M=Pd, X=Cl or Br; M=Pt, X=Cl) and [Pd2Cl2(μ-Cl)2{As(CH2CMeCH2)3}2] have been established by single crystal X-ray diffraction analyses. The mononuclear complexes exclusively adopt the trans configuration with the exception of [PdCl2L″2], which could be isolated as cis and trans isomers. In the binuclear derivative the arsine ligands are attached to an envelope-shaped Pd–(μ-Cl)2–Pd rectangle with a trans (anti) orientation towards each other. The mononuclear complexes are slightly photoreactive upon irradiation in their long-wavelength absorption band.

Probing the binding of the flavonoid, quercetin to human serum albumin by circular dichroism, electronic absorption spectroscopy and molecular modelling methods

The plant derived flavonoid compound quercetin, possesses wide range of biological activities in the human body by interacting with nucleic acids, enzymes and other proteins. As has recently been shown this molecule of polyphenolic type extensively binds to human serum albumin (HSA), the most abundant carrier protein in the blood. Electronic absorption, circular dichroism (CD) spectroscopy and molecular modelling methods were used to characterize optical properties of the quercetin–HSA complex, and to gain information on the binding mechanism at molecular level. The red shift and hypochromism of the longest-wavelength absorption band of quercetin relative to the spectral properties in ethanol suggests that one or more phenolic OH groups of the bound ligand is ionized and that the exocyclic phenyl ring is not coplanar with the benzopyrone moiety. It was found that quercetin shows extrinsic optical activity on interaction with HSA. The induced CD spectra were utilized to calculate the association constant at 37° (1.46±0.21×10 4 M −1) and to probe the ligand binding site. Results of the CD displacement experiments performed with palmitic acid and salicylate were interpreted together with the findings of molecular modelling calculation performed on the quercetin–HSA complex. Computational mapping of possible binding sites of quercetin revealed the molecule to be bound in the large hydrophobic cavity of subdomain IIA. The protein microenvironment of this site was found to be rich in polar (basic) amino acid residues which are able to help to stabilize the negatively charged ligand bound in non-planar conformation. Additionally, the position of quercetin within the binding pocket allows simultaneous binding of other ligands such as warfarin, or sodium salycilate.

Molecular characterization of NikD, a new flavoenzyme important in the biosynthesis of nikkomycin antibiotics.

Nikkomycin antibiotics are potent inhibitors of chitin synthase, effective as therapeutic antifungal agents in humans and easily degradable insecticides in agriculture. NikD is a novel flavoprotein that catalyzes the oxidation of Delta(1)- or Delta(2)-piperideine-2-carboxylate, a key step in the biosynthesis of nikkomycin antibiotics. The resulting dihydropicolinate product may be further oxidized by nikD or converted to picolinate in a nonenzymic reaction. Saturated nitrogen heterocycles (L-pipecolate, L-proline) and 3,4-dehydro-L-proline act as alternate substrates. The ability of nikD to oxidize 3,4-dehydro-L-proline, but not 1-cyclohexenoate, suggests that the enzyme is specific for the oxidation of a carbon-nitrogen bond. An equivalent reaction is possible with the enamine (Delta(2)), but not the imine (Delta(1)), form of the natural piperideine-2-carboxylate substrate. Apparent steady-state kinetic parameters for the reaction of nikD with Delta(1)- or Delta(2)-piperideine-2-carboxylate (k(cat) = 64 min(-1); K(m) = 5.2 microM) or 3,4-dehydro-L-proline (k(cat) = 18 min(-1); K(m) = 13 mM) were determined in air-saturated buffer by measuring hydrogen peroxide formation in a coupled assay. NikD appears to be a new member of the monomeric sarcosine oxidase (MSOX) family of amine oxidizing enzymes. The enzyme contains 1 mol of flavin adenine dinucleotide (FAD) covalently linked to Cys321. The covalent flavin attachment site and two residues that bind substrate carboxylate in MSOX are conserved in nikD. NikD, however, exhibits an unusual long-wavelength absorption band, attributed to charge-transfer interaction between FAD and an ionizable (pK(a) = 7.3) active-site residue. Similar long-wavelength absorption bands have been observed for flavoproteins containing an active site cysteine or cysteine sulfenic acid. Interestingly, Cys273 in nikD aligns with an active-site histidine in MSOX (His269) that is, otherwise, a highly conserved residue within the MSOX family.

Synthesis of perylene-porphyrin dyads for light-harvesting studies

The spectral coverage of porphyrin-based light-harvesting arrays can be enhanced through the use of suitable accessory pigments. Perylene-monoimide dyes can serve as valuable accessory pigments with porphyrins. To investigate the choice of perylene-monoimide and the effects of molecular architecture on light-harvesting efficacy, five perylene-porphyrin dyads were prepared. Each dyad employs a diphenylethyne linker that bridges the perylene N-imide site and the porphyrin meso-position. Three dyads incorporate a mono-phenoxy perylene at the o-, m-, or p-position of the meso-aryl group on the porphyrin. The two remaining dyads incorporate a perylene-monoimide (bearing zero or three phenoxy substituents) at the p-position of the meso-aryl group on the porphyrin. The introduction of phenoxy groups on the perylenes increases the solubility, a key requirement for use in light-harvesting arrays. The long-wavelength absorption band of the perylene shifts from 506 nm to 532 or 533 nm upon substitution with one or three phenoxy groups, respectively. The synthesis of the dyads entails Pd -mediated coupling of a bromo-perylene and an ethynyl porphyrin, or the mixed-aldehyde condensation with a perylene-aldehyde, mesitaldehyde, and pyrrole. Five perylene-monoimide dyes bearing an ethyne or bromo substituent at the p-position of the N-aryl unit were developed for this modular chemistry. Each perylene-porphyrin dyad exhibits efficient energy transfer from the excited perylene to the ground-state porphyrin.

Study of nonlinear optical characteristics of various media by the methods of z-scan and third harmonic generation of laser radiation

The nonlinear optical parameters of colloidal solutions of metals, semiconductor chalcogenide films, vapours and solutions of organic dyes, as well as of fullerene-doped polyimide films are studied by the methods of z-scan and third harmonic generation (THG). The nonlinear refractive indices, nonlinear susceptibilities, and nonlinear absorption coefficients of these materials are measured at the fundamental 1064-nm emission wavelength of a 35-ps pulsed Nd:YAG laser and its second harmonic at 532 nm. The study of the optical limitation of picosecond and nanosecond light pulses in these media showed that the appearance of the long-wavelength absorption band at the final stage of aggregation of colloidal metals resulted in nonlinear absorption of picosecond pulses. The nonlinear susceptibilities χ(3)(—3ω,ω,ω,ω) of colloidal metals, fullerenes, and dye vapours are measured by the picosecond THG method, and the effect of self-action on THG in these media is analysed.

Substituent Effect on Fluorescence and Photoisomerization of 1-(9-Anthryl)-2-(4-Pyridyl)ethenes

The fluorescence and photoisomerization quantum yields of trans-1-(9-anthryl)-2-(4-pyridyl)ethene (t-4-APyE), 1-(10-methyl-9-anthryl)-2-(4-pyridyl)ethene (t-4-MeAPyE), and 1-(10-chloro-9-anthryl)-2-(4- pyridyl)ethene (t-4-ClAPyE) were measured in cyclohexane, acetonitrile, and methanol at room temperature.Polar solvents result in the drastic reduction of fluorescence quantum yield and increase of photoisomerization quantum yield for all three compounds. These results are probably due to the stabilization of intramolecular charge transfer (ICT) excited state in polar solvent. The higher contribution of ICT in the presence of more electron-donating methyl substituent, manifested by largest positive fluorescence solvatochromism, indicates that the pyridine ring acts as an electron acceptor. Protonation or methylation makes pyridine ring stronger electron acceptor and causes long-wavelength ground state charge transfer absorption band and complete quenching of fluorescence. The fluorescence from t-4-APyE derivatives can be switched off responding external stimuli viz. medium polarity, protonation, or methylation.

Tetraaminoperylenes: their efficient synthesis and physical properties.

Trimethylsilylation of 1,8-diaminonaphthalene gave 1,8-bis(trimethylsilylamino)naphthalene (1 a), which was in turn lithiated with two molar equivalents of n-butyllithium to give the tris(thf)-solvated dilithium diamide [1,8-[(Me(3)SiN)Li(thf)](2)C(10)H(6)](thf) (2 a). Metal exchange of 2 a with TlCl was carried out in two steps, via the previously characterized mixed-metal amide [1-[(Me(3)SiN)Li(thf)(2)]-8-[(Me(3)SiN)Tl]C(10)H(6)], to give the dithallium diamide [1,8-[(Me(3)SiN)Tl](2)C(10)H(6)] (3 a). Thermolysis of 3 a cleanly gave a 1:1 mixture of the 4,9-bis(trimethylsilylamino)perylenequinone-3,10-bis(trimethylsilylimine) (4 a) and 1 a. By this route, a whole series of silylated homologues of 4 a was obtained in good yields, while the same method proved to be inefficient for the synthesis of the alkyl-substituted analogues. Compound 4 a and its tert-butyldimethylsilyl derivative 4 d were reduced with sodium amalgam to give, after protonation, the corresponding 3,4,9,10-tetraaminoperylenes 7 a and 7 d. Cyclic voltammetry showed two reversible, closely spaced reduction waves (E(red 1)=-1.39, E(red 2)=-1.59 V versus SCE) corresponding to this conversion. The perylenes 7 a and 7 d are thought to be the primary products in the reaction cascade leading to the perylene derivatives, involving the thermal demetalation of the thallium amides, possibly via Tl(II)bond;Tl(II) intermediates, first to give 7 a and its analogues. The final oxidation of the tetraaminoperylenes by one molar equivalent of 3 a and analogous thallium amides gave the quinoidal derivatives such as 4 a and 4 d, a step that could be studied by direct reaction of the isolated species. The UV/Vis absorption spectra of the 4,9-bis(silylamino)perylenequinone-3,10-bis(silylimines) are characterized by a long-wavelength absorption band with a pronounced vibrational structure (lambda(max)=639 nm, lg epsilon =4.53) attributed to a pi*<--pi and a pi*<--n absorption band at 454 nm (lg epsilon 4.83), along with intense absorption in the UV region. A weak red emission with a rather low quantum yield (Phi(fl)=0.001, lambda(max)=660 nm) is observed upon irradiation of a sample; the lifetime of the emission is only 66 ps. The low emission quantum yield is attributed to the *pi<--n transition of the amino perylene, which induces strong spin-orbit coupling, leading to a large triplet yield. The triplet state was probed by transient absorption spectroscopy and found to have a lifetime of 200 ns in air, and 1100 ns in argon-flushed solution. Treatment of 4 a with a stoichiometric amount of KF in methanol/water under phase-transfer conditions (with the cryptand [C 222]) gave an almost quantitative yield of the parent compound 4,9-diaminoperylenequinone-3,10-diimine (8). Treatment of 8 with two molar equivalents of the ruthenium complex [Ru(bpy)(2)(acetone)(2)](PF(6))(2), generated in situ, yielded the blue dinuclear ruthenium complex [(bpy)(4)Ru(2)[mu(2)-N,N':N",N"'-[[4,9-(NH(2))(2)-3,10-(NH)(2)]C(20)H(8)]]](PF(6))(4) (9), the redox properties of which were studied by cyclic voltammetry. The difference in the potentials of the two one-electron redox steps (225 mV) indicates strong coupling of the metal centers through the 4,9-diaminoperylenquinone-3,10-dimine bridging ligand and corresponds to a comproportionation constant K(c) of 6.3 x 10(3). The UV/Vis absorption spectrum of the mixed valent form, which is stable in air, has a characteristic intervalence charge-transfer (IVCT) band in the near infrared at 930 nm (lg epsilon =3.95), from which an electronic coupling parameter J of 760 cm(-1) could be estimated, placing compound 9 at the borderline between the class II and class III cases in the Robin-Day classification.

Induced chirality upon binding of cis-parinaric acid to bovine β-lactoglobulin: spectroscopic characterization of the complex

Binding of the polyunsaturated cis-parinaric acid to bovine β-lactoglobulin (BLG) was studied by circular dichroism (CD), electronic absorption spectroscopy and mass spectrometry methods. Upon protein binding, the UV absorption band of parinaric acid is red shifted by ca. 5 nm, showing hypochromism and reduced vibrational fine structure, suggesting that the ligand binds as a monomer in non-planar geometry. In the CD spectra measured at pH 7.36 and 8.5 a strong, negative Cotton band appears centered at 310 nm (Δ ϵ=−25 M −1 cm −1) corresponding to the long-wavelength absorption band of cis-parinaric acid. The source of this induced optical activity is the helical distortion of the polyene chromophore caused by the chiral protein environment. From CD spectral data the value of the association constant was calculated to be 4.7×10 5 M −1 at pH 7.36. CD and mass spectrometry measurements showed that parinaric acid binds weakly to BLG in acidic solution, though small peaks at mass 18559 and 18645 can be obtained in the reconstructed electrospray mass spectrum; these correspond to the binding of parinaric acid in 1:1 stoichiometry to both monomer variants of BLG B and A. The hydrophobic interior cavity of BLG was assigned as the primary binding site of cis-parinaric acid.

Magnetic Resonance Studies and Molecular Orbital Calculations on the Doublet and Triplet States of Bacteriopurpurin: a Potential Second-Generation Photosensitizer for Photodynamic Therapy

Bacteriopurpurin is a macrocyclic compound at the saturation level of bacteriochlorins that was specifically synthesized as a potential photosensitizer in photodynamic therapy. It exhibits a long-wavelength Qy absorption band at 843 nm, which is a necessary prerequisite for a deep tissue penetration. Magnetic resonance spectroscopic properties of bacteriopurpurin have been examined by fluorescence-detected magnetic resonance (FDMR) and electron paramagnetic resonance (EPR) in conjunction with molecular orbital calculations. The electronic structure of the radical cation state obtained by oxidation of bacteriopurpurin with iodine is mapped by the determination of isotropic proton hyperfine coupling constants from electron−nuclear double-resonance spectroscopy at 298 K. A tentative assignment of the couplings to the various proton positions in the molecule is achieved by comparison of the experimental values with simulated ones based on semiempirical intermediate neglect of differential overlap calculations...

Characterization of a metalloregulatory bismuth(III) site in Staphylococcus aureus pI258 CadC repressor.

Staphylococcus aureus pI258 CadC is a metal sensor protein that regulates the expression of the cad operon which encodes metal ion resistance proteins involved in the efficient efflux of Cd(II), Pb(II), Zn(II) and, according to one report, Bi(III) ions. In this paper, direct evidence is presented that Bi(III) binds to CadC and negatively regulates cad operator/promoter (O/P) binding. Optical absorption spectroscopy reveals that dimeric CadC binds approximately 0.8 mol equivalents of Bi(III) per CadC monomer to form a coordination complex characterized by three S(-)-->Bi(III) ligand-to-metal charge transfer transitions, with the longest wavelength absorption band centered at 415 nm (epsilon(415)=4000 M(Bi)(-1) cm(-1)). UV-Vis absorption spectra of wild-type and mutant Cys-->Gly (Ser) substitution CadC mutants compared to [Bi(DTT)(2)], [Bi(GSH)(3)] and [Bi(NAC)](3) model complexes reveal that Cys7, Cys11, Cys60 and Cys58 directly coordinate Bi(III) in a tetrathiolate coordination complex. The apparent affinity derived from a Bi(III)-displacement optical titration with Cd(II) is estimated to be K(Bi)< or =10(12) M(-1). Apo-CadC binds with high affinity [ K(a)=1.1(+/-0.3)x10(9) M(-1); 0.40 M NaCl, pH 7.0, 25 degrees C] to a 5'-fluorescein-labeled cad O/P oligonucleotide,while the binding of one molar equivalent of Bi(III) per CadC monomer (Bi(1)-CadC) reduces the affinity by approximately 170-fold. Strikingly, Bi(III)-responsive negative regulation of cad O/P binding is abrogated for Bi(1)-C60G CadC and severely disrupted in Bi(1)-C7G CadC, whose relative affinity is reduced only 10-fold. The mechanism of Bi(III)-responsive metalloregulation is discussed, based on the findings presented here. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00775-001-0336-9.

Preparation, spectral characteristics and photolytic stability of anthracene-hindered amine probes: influence of the medium

New probes, esters of 3-(9-anthracene)propionic acid with 2,2,6,6-tetramethyl-4-hydroxypiperidine and its oxidized and protonized form, were prepared and spectrally characterized. Spectral measurements were performed in solution and in polymer matrices as polyethylene, isotactic polypropylene, polystyrene, polymethyl methacrylate and polyvinylchloride. Absorption spectra of these derivatives were shifted about 10 nm bathochromically in comparison with anthracene. The longest wavelength absorption band of the parent amine (probe III) and its derivatives exhibited the fine vibrational structure in solution and in all polymer matrices. Fluorescence spectra of the probes were red-shifted as compared to anthracene. The vibrational structure of this emission is well preserved in all media. Quantum yield of fluorescence of parent amine is slightly higher than anthracene in methanol. The range of values of quantum yield relative to anthracene of parent amine is rather large (0.3–11) in polymer matrices. The lifetime of fluorescence of parent amine is 3.8 ns in methanol and 4.1 ns in cyclohexane but it is larger in polymer matrices. Both quantum yield and lifetime of oxidized form were smaller in comparison with parent amine. The quantities Φ NH/ Φ NO and τ NH/ τ NO expressing the extent of intramolecular quenching (electronic energy transfer) are low for this type of probes in methanol solution. In all polymer matrices these values are higher (2–7.5) but there are some differences between ratio calculated from quantum yields and from lifetimes. The photolytical stability of the novel probes is low in polymer matrices at 366 nm irradiation. 2,2,6,6-Tetramethylpiperidine structural unit as parent amine or as stable nitroxyl radical does not exhibit any stabilization effect.

Deactivation behavior and excited-state properties of (coumarin-4-yl)methyl derivatives. 2. Photocleavage of selected (coumarin-4-yl)methyl-caged adenosine cyclic 3',5'-monophosphates with fluorescence enhancement.

A series of axial and equatorial diastereomers of (coumarin-4-yl)methyl-caged adenosine cyclic 3',5'-monophosphates (cAMPs), 1-6, having methoxy, dialkylamino, or no substituent in the 6- and/or 7-positions, and their corresponding 4-(hydroxymethyl)coumarin photoproducts 7-12 have been synthesized. The photochemical and UV/vis spectroscopical properties (absorption and fluorescence) of 1-6 and 7-12 have been examined in methanol/aqueous HEPES buffer solution. Donor substitution in the 6-position causes a strong bathochromic shift of the long-wavelength absorption band, whereas substitution in the 7-position leads only to a weak red shift. The photochemical cleavage of the caged cAMPs was investigated, and the photoproducts were analyzed. Photochemical quantum yields, fluorescence quantum yields, and lifetimes of the excited singlet states were determined. The highest values of photochemical quantum yields (photo-S(N)1 mechanism) were obtained with caged cAMPs having a donor substituent in the 7-position of the coumarin moiety, caused by electronic stabilization of the intermediately formed coumarinylmethyl cation. With donor substitution in the 6-position, the resulting moderate electronic stabilization of the coumarinylmethyl cation is overcompensated by the strong bathochromic shift, reducing the energy gap between the excited-state S(1) and the corresponding coumarinylmethyl cation. The rate constant for the ester cleavage and liberation of cAMP is about 10(9) s(-1), estimated for the axial isomer of 6 by analysis of the fluorescence increase of the alcohol 12 formed upon laser pulse photolysis.

Photochemistry of Aliphatic Nitroxide Radicals Grafted onto Activated Aerosil Surface

The mechanism of photoinduced transformations of nitroxide radicals ≡SiON(O·)CH3 (I) and ≡SiOCH2N(O·)CH3 (II) obtained on the activated aerosil surface was studied. The nitroxides were photolyzed with 436-nm light at 77 K. It was shown that the action of light in the long-wavelength absorption band corresponding to the n–π* transition resulted in the dissociation of the O–N or C–N bond in radicals I or II, respectively. The quantum yields of these reactions were found to be ≈0.6 and ≈0.002, respectively.