Bichromophoric Compounds Research Articles

N-(1-naphthylacetyl)glycine phenacyl ester and phenacyl (1-naphthylacetoxy)acetate.

The structures of the bichromophoric compounds N-(1-naphthylacetyl)glycine phenacyl ester, C(22)H(19)NO(4), (I), and its oxygen analogue, phenacyl (1-naphthylacetoxy)acetate, C(22)H(18)O(5), (II), have been determined. The molecules of (I) are held together by intermolecular N-H...O hydrogen bonds between the carbonyl and N-H groups, while compound (II) does not show any hydrogen bonding in the crystal.

First Observation of Monomer and Dimer Radical Cation upon Photoreduction of Cyanopyridinium Derivatives in Solution by Electron Spin Resonance and Absorption Spectroscopy

The monomer radical and the dimer radical cation of cyanopyridinium derivatives have been studied by absorption and electron spin resonance (ESR) spectroscopies at room temperature in acetonitrile upon steady photolysis. The stable cyanopyridinyl radical of 1,3-bis(4-cyanopyridiniumyl)propane (acceptor) formed by one-electron transfer from tetraphenylborate anion (donor) interacted intramolecularly with a parent cation, forming a dimer radical cation. A charge resonance band was observed at 1360 nm and assigned to the formation of intramolecular dimer radical cations. The ESR spectra revealed that one unpaired electron in the intramolecular dimer radical cation was completely shared between two chromophores of photoreduced 1,3-bis(4-cyanopyridiniumyl)propane (1). On the other hand, the intermolecular dimer radical cation of 1-(4-cyanopyrdiniumyl)hexadecane (2) as a monochromophoric compound was not formed because no charge resonance band was observed in the near-infrared region. Clearly resolved ESR spectra of both the dimer radical cation and the monomer radical were observed, for the first time, upon photoreduction of these cyanopyridinium derivatives. The effect of the chromophore size on the charge resonance band in the near-infrared region was also demonstrated by homo- and hetero-type bichromophoric compounds containing 4-cyano- and 4-nitrostyryl-pyridinium.

Intramolecular electronic energy transfer in bichromophoric macrocyclic complexes.

Efficient intramolecular electronic energy transfer (EET) has been demonstrated for three novel bichromophoric compounds utilizing a macrocyclic spacer as the bridge between the electronic energy donor and acceptor fragments. As their free base forms, emission from the electronically excited donor is absent and the acceptor emission is reductively quenched via photoinduced oxidation of proximate amine lone pairs. As their Zn(II) complexes, excitation of the donor results in sensitization of the electronic acceptor emission.

Intramolecular Excitation Energy Transfer in Bichromophoric Compounds in Stretched Polymer Films

Intramolecular excitation energy transfer has been investigated with bichromophoric compounds in which naphthalene and anthracene as donor (D) and acceptor (A), respectively, are linked through a methylene chain, i.e., 9-anthryl-(CH2)n-1-naphthyl (AnN) with n = 1, 3, and 6 methylene units as spacer. The respective compounds were dispersed in stretched PVA films with the expectation that the methylene chain takes preferred conformations elongated in the direction of stretching polymer films so that the D−A distances are determined by the methylene chain length. From analyses of picosecond time-resolved fluorescence spectra and the fluorescence decay kinetics, it has been found that the energy transfer rates show a systematic change depending on the number of the methylene chain units: 47.6 × 1010 s-1 in A1N, 1.96 × 1010 s-1 in A3N, and 0.15 × 1010 s-1 in A6N at 10 K. These changes in the energy transfer rate are basically consistent with the theoretical estimation based on the Forster's mechanism, with so...

Photokinetic behaviour of biphotochromic supramolecular systems: Part 2. A bis-benzo-[2H]-chromene and a spirooxazine–chromene with a ( Z-)ethenic bridge between each moiety

The photochromic behaviour of two new bichromophoric molecules (a symmetric one, formed by two 3,3-diphenyl-[3H]-naphtho[2,1-b]pyran moieties, and an asymmetric one, formed by a naphthopyran and a spiro[indoline-naphthoxazine] moiety) linked through an ethenic double bond has been studied. A model compound, where the chromene moiety is linked to a naphthalene group, was also investigated for comparison purpose. The photochemical and thermal evolution of the bichromophoric molecules were followed spectrophotometrically and by high performance liquid chromatography (HPLC). The photochromic reaction was found to be only partially photoreversible. Massive irradiation with visible light led to degradation. The reaction mechanism for these molecules containing a chromene moiety is characterised by an intermediate, which thermally evolves to different products. The quantum yield of formation and the first-order kinetic constant of the thermal evolution of the intermediate were determined. The identification of the photoproducts was helped by information from a parallel NMR study. Comparison of these results with those previously obtained on related monochromophoric and bichromophoric compounds allowed reasonable photoreaction mechanisms to be proposed.

Studies of Charge Resonance Bands Formed in .ALPHA.- and .GAMMA.-Styrylpyridinium Tetraphenylborate Derivatives.

The charge resonance (CR) interactions in α-substituted-(4-nitrostyryl)pyridinium derivatives have been investigated. The CR bands of dimer radical cation in α-substituted bichromophoric compounds were observed with a peak around 1010nm upon irradiation at 405nm in acetonitrile (ACN) at room temperature. These bands were weak because of their restricted geometrical conformation as compared with γ-substituted one. The CR band of dimer radical cation in α-substituted monochromophoric compound was observed at 993nm in 1, 2-dimethoxyethane. No CR band for this compound in ACN as a polar solvent strongly suggested that radical is difficult to be generated because of increased distance between the tetraphenylborate anion as a donor to the styrylpyridinium chromophore as an acceptor.

Excited state interactions in phenol/olefin bichromophoric compounds: direct detection of an intramolecular exciplex

The emission from an intramolecular exciplex detected in trans-4-methoxy-2-(3-phenyl-2-propenyl)phenol (1) is the first direct evidence for the excited state interchromophoric interaction in phenol–styrene systems.

Enantioselective Discrimination in the Intramolecular Quenching of an Excited Aromatic Ketone by a Ground-State Phenol

Chiral recognition is a subject of increasing interest in photochemistry.1 In principle, this can be achieved with optically active photosensitizers if an enantiodifferentiating interaction occurs in the relevant excited state. Photosensitized electron transfer processes appear specially suitable for such purposes, as the donor and the acceptor must form a highly coupled system for some time.2 The use of chiral compounds as both excited sensitizers and quenchers could enable examination of pure chiral discrimination through comparison of the quenching rates, since enantiomers have otherwise the same electronic nature and the same size. Leigh et al.3 have suggested that hydrogen abstraction from phenols by carbonyl π,π* triplets involves electron transfer within a hydrogen-bonded triplet exciplex, followed by proton transfer. In this context, we have prepared bichromophoric compounds containing enantiomerically pure aryl ketones and phenols, to check the stereochemical aspects of intramolecular electron transfer by means of laser flash photolysis studies. Benzoylthiophene is a heterocyclic diaryl ketone with a lowest lying π,π* triplet.4 This excited state involves predominantly the thenoyl chromophore; it has an energy4 of ca. 63 kcal mol-1 and is able to react with phenols, such as p-cresol and tyrosine methyl ester.5 Both substructures are present in the four enantiomerically pure bichromophoric compounds prepared in this work: TPATyr [(R,S)-1, (S,S)-1] and SUP-Tyr [(R,S)-2, (S,S)-2] (Scheme 1). We have studied the intramolecular electron-transfer process by laser flash photolysis through phenol quenching of the ketone triplet state and found a high degree of stereoselectivity, which agrees with a geometry where interaction between the π-systems of the phenol and the aromatic ketone is maximized. Compounds 1 and 2 differ in the relative aromatic ketone versus phenol orientation. They were obtained by condensation of the corresponding racemic acid, tiaprofenic acid (TPA) or suprofen (SUP), with the methyl ester of the natural occurring amino acid (S)-tyrosine, in the presence of a carbodiimide {(1-ethyl-3-[3dimethylamino]propylcarbodiimide), EDC}. In both cases, after purification of the resulting amides, one of the diastereoisomers crystallized. The other stereoisomers were further purified by HPLC chromatography [potassium dihydrogen phosphate (0.06 M):acetonitrile:triethylamine (317:183:0.1 by volume)]. All the bichromophoric compounds were fully characterized by 1H NMR, 13C NMR, and elemental analysis.

Enhancement of Intramolecular Photocycloaddition of Bichromophoric Compounds via Inclusion in Low-Density Polyethylene Films

The photocycloaddition of tetra- and penta(ethylene glycol) labeled at the chain terminals with 2-naphthoyl groups (N-Pn-N; n = 4 or 5) and tetra(ethylene glycol) terminated by 7-(4-methylcoumarinyl) groups (C-P4-C) has been investigated in hexane solutions and in low-density polyethylene (LDPE) films. At loading levels less than 1 × 10-2 mol/g-film, irradiation of the compounds in LDPE leads to formation of intramolecular photocyclomers to the exclusion of intermolecular products. Irradiations in hexane containing 1 × 10-3 M of a bichromophoric compound produce large amounts of oligomeric material. The difference between the results in solution and the film is explained in terms of the compartmentalization of the guest molecules in the LDPE amorphous regions. Cold-stretching the LDPE films significantly increases the relative efficiency of the intramolecular photocycloadditions due to reduction of the free volume at the reaction sites. This work reports a new approach to synthesize large-ring compounds in high chemical yields at relatively high substrate concentrations.

Intramolecular excimer formation in short- and long-chainlength di(9-anthryl) bichromophoric compounds and relation to ground state properties

The ground state conformational, UV absorption and fluorescence spectral as well as decay time data for a series of polymethylene- and one polyoxyethylene-linked di(9-anthracenes) in solution are compared, for chainlengths varying from n=0 (directly linked system A0A, 9,9′-bianthryl) to n=∞ (A∞, reference system 9-decylanthracene). 1H NMR spectroscopy of A2A to A11A shows that there is no clearly preferred ground state conformation of the anthracene moieties. The crystal structure of A2A was solved and the anti conformation of the anthracene units was established. The UV spectra show a redshift interpreted within the Forster exciton model (predominance of extended conformations). Fluorescence spectra for (CH2)n-bridged systems show no clear excimer band for short-chain systems but redshifted excimer emission for A4A to A9A. Loss of structure and fluorescence band shape changes as well as biexponential decay times allow the conclusion that excimer formation occurs in all chained systems. For a related polyoxyethylene-chained system, the excimer band is substantial, probably a consequence of the increased flexibility of the chain and moderate photoreactivity. Solvent polarity changes are strong for A0A [formation of a twisted intramolecular charge transfer (TICT) state] but also significant in the systems A1A and A2A. This is rationalized by the possiblity for competitive formation of an excimer-type (parallel but tilted chromophore arrangement) and a TICT-type (near perpendicular) conformation. For AnA (n>2) the solvent polarity was found to have virtually no influence.

Photobinding of Tiaprofenic Acid and Suprofen to Proteins and Cells: A Combined Study Using Radiolabeling, Antibodies and Laser Flash Photolysis of Model Bichromophores

Drug photoallergy is a matter of current concern. It involves the formation of drug-protein photoadducts (photoantigens) that may ultimately trigger an immunological response. Tyrosine residues appear to be key binding sites in proteins. The present work has investigated the photobinding of tiaprofenic and (TPA) and the closely related isomer suprofen (SUP) to proteins and cells by means of radioactive labelling and drug-directed antibodies. To ascertain whether preassociation with the protein may play a role in photoreactivity, two model bichromophoric compounds (TPA-Tyr and SUP-Tyr) have been prepared and studied by laser flash photolysis. The results of this work show that (a) TPA and SUP photobind to proteins with similar efficiencies, (b) both drugs form photoadducts that share a basic common structure, as they are recognized by the same antibody and (c) drug-protein preassociation must play a key role in photoreactivity, as indicated by the dramatic decrease in the triplet state lifetimes of the model bichromophores compared to the parent drugs.

Persistent and Transient Open-Shell Species Derived from C60−TTF Cyclohexene-Fused Dyads

A new series of donor−acceptor fused dyads consisting of a C60 doubly tethered to a substituted TTF moiety has been synthesized. Cyclic voltammetry of the new fullerene derivatives in solution shows a modulation in the difference between the first reduction potential of the fullerene moiety and the first oxidation potential of the TTF moiety with the substituents of the TTF addend. Along with the neutral bichromophoric compounds, we also report the generation and characterization by EPR and absorption spectroscopies of the corresponding persistent open-shell species obtained electrochemically, namely their radical cations and radical anions. Spin density distributions of radical cations and radical anions derived from dyads 1a−c are mainly located on the TTF and fullerene moieties, respectively, as ascertained from the g values and 33S hyperfine coupling constants. Interestingly, the EPR of the radical anion derived from the bisadduct 1d exhibits a structured signal (g = 2.0005) arising from the coupling ...

Electronic energy transfer in solution in naphthalene-anthracene, naphthalene-acridine and benzene-DANS bichromophoric compounds

Intramolecular energy transfer (intraEET) in naphthalene-anthracene ( I), naphthalene-acridine ( II) and benzene-DANS ( III) bichromophoric compounds was investigated by stationary absorption and fluorescence measurements in solution. Computer simulations of conformational distributions of compounds I (with n = 1, 3 and 6 methylene units as spacer groups) in the gas phase were performed and predict similar molecular structures for n = 3 and 6, which is consistent with previous observations in a supersonic jet expansion. In compounds I, fast intraEET (rate constant k EET > 1 × 10 10 s −1) was observed upon electronic excitation of the naphthalene moieties, whereas in compound III with 6 methylene units and an amide group as molecular spacer, a significantly slower intraEET was found ( k EET = (1.0 0 5) × 10 8 s −1 ). The result is discussed within the framework of the Fermi Golden Rule expression. Differences in Franck-Condon weighted density of EET acceptor states and of the mutual orientation of the two chromophoric units were found to account for the different intraEET efficiencies. This work provides no evidence for intraEET in compounds III.

Intramolecular Triplet Energy Transfer. 4. A Carbazole−Naphthalene System Having a Flexible Alkyl Spacer Doped in Poly(methyl methacrylate) Matrixes

The rates of intramolecular triplet energy transfer from carbazole (Cz) to naphthalene (Np) chromophore for the bichromophoric compounds with a flexible alkyl spacer, Cz−(CH2)n−Np, n = 4−12, doped in poly(methyl methacrylate) matrixes were measured by phosphorescence spectroscopy and laser flash photolysis. The results were interpreted in terms of a model that allows a distribution of donor−acceptor distances for a given spacer length by using Dexter's equation. This model provided the transfer rate for all the compounds tested, and the comparative study with experimental results indicated that the “through-space” mechanism mainly governs the intramolecular triplet−triplet energy transfer in flexible spacer compounds. The temperature dependence of the rates of triplet energy transfer was also investigated.

Ground and excited-state intramolecular interactions in phenol–olefin bichromophoric compounds

The intramolecular OH· · ·π interactions of the phenolic and olefinic chromophores in o-propenyl and o-butenyl phenols 1a–d in the ground and excited states have been studied by (a) ab initio Hartree–Fock calculations using the 3-21G and 6-31G** basis sets, (b) FTIR spectral measurements in the gas phase, (c) fluorescence emission studies and (d) product studies. Such interactions appear to play a key role in the photochemical behaviour of the four compounds.

Photocyclization of 2-cinnamylphenols via excited state proton transfer (ESPT) involving the lowest-lying styrenic singlet

The attachment of substituents to the styrenic ring of trans-2-cinnamylphenol produces a decrease in the singlet energy of the styrenic moiety. As a consequence, the resulting chromophore becomes responsible for the photophysical and photochemical properties of this type of bichromophoric compounds. Both fluorescence emission and photocyclization occur predominantly from the lowest-lying styrenic singlet. The result is a marked regioselectivity towards 6-membered ring products, via an excited state proton transfer.

Intramolecular energy transfer in naphthalene-containing polyesters: Experiment and simulation for model compounds derived from five aliphatic dicarboxylic acids and 2-hydroxynaphthalene

Steady-state fluorescence depolarization measurements and molecular dynamics simulations have been used to study the efficiency of nonradiative intramolecular singlet energy transfer between 2-naphthoxy groups, denoted N, in model compounds for polyesters derived from 2,6-dihydroxy-naphthalene and aliphatic dicarboxylic acids. The five bichromophoric compounds studied are the diesters abbreviated as N-OOC-(CH2)n-COO-N;n = 2–6, which are condensation products obtained from 2-naphthol and aliphatic dicarboxylic acids. The anisotropy of the fluorescence of these compounds dispersed in a solid matrix of glassy poly(methyl methacrylate) indicates that there is nonradiative singlet energy transfer between naphthoxy groups. The efficiency of this transfer depends onn. A theoretical treatment using molecular dynamics simulations for the conformations of the five model compounds has been performed in order to evaluate the parameters related to the efficiency of the transfer. The experimental and theoretical variation of such parameters withn is consistent with the estimated Forster radius of 9–10 A for this system.

Effect of the directionality of the ester group on the formation of hairpins in polyesters containing naphthalene and a flexible spacer

Steady-state fluorescence measurements and molecular dynamics simulations have been used to study the intramolecular formation of excimers in five model compounds for polyesters containing naphthalene groups separated by flexible spacers. The model compounds are derived from 2-hydroxynaphthalene and HOOC--(CH 2 ) n -COOH, n = 2-6. The ratio of the intensity of excimer and monomer emissions, I n / I M , is nearly independent of the viscosity of the medium, n, over the range covered in dilute solution. Although I D /I M is always very small, it shows an odd-even effect for the first four members of the series, with maxima when n is odd. Molecular dynamics simulations provide an explanation for the small values of I D /I M , their weak dependence on n, and the trend of I D /I M with n. The results for the present series of model compounds are compared with previous work, which reported larger values of I D /I M , and a stronger dependence of I D /I M on n, for bichromophoric compounds derived from 2-naphthoic acid and aliphatic glycols, where the direction of the ester groups is reversed. The origin of the difference in the behavior of I D /I M in the two series is identified.

Mechanisms of excitation energy transport in macromolecules

Recent developments in our understanding of the mechanisms of singlet excitation energy transfer (EET) in multichromophoric assemblies are reviewed. Particular emphasis is placed on the role of superexchange interactions involving the molecular framework linking the chromophores in the mediation of EET processes. Experimental measurements of the rates of EET in rigidly linked model bichromophoric compounds demonstrate that, under certain conditions, a through-bond interaction can dominate the energy transport process. High rates of EET in synthetic macromolecules are also observed, and the possible pathways of energy migration and trapping in these systems are outlined.

Molecular design of asymmetric bichromophoric compounds

The goal of the present study is a better understanding of photoinduced electron or energy transfer in a single molecule. The growing interest of this field is caused by its implication in life phenomena such as photosynthesis and in the development of molecular devices. We present molecular design of new compounds consisting of a well-known luminescent organic moiety (porphyrin, carbazole, phthalocyanine) bound to an inorganic one by a bridging ligand. This latter is in the simplest case a triple bond that can be extended easily. In the present work, the inorganic part is a pentaammine—ruthenium which has been extensively studied previously. Our interest is focused on the possible ability of the ruthenium moiety to quench the light emission of the luminescent organic chromophore.