Bichromophoric Molecules Research Articles

Electronic energy migration and rotation within bichromophoric molecules

Abstract

Energy migration and rotational motion within bichromophoric molecules. II. A derivation of the fluorescence anisotropy

A generalized Förster theory is presented which includes reorientation of the interacting molecules. The stochastic master equation is, for the first time, derived from the stochastic Liouville equation, so that it accounts for the molecular origin to the stochastic transitions rates. A formal solution to the stochastic master equation is given. This equation is compared with its truncated cumulant expansion. The second-order cumulant contains the correlation function 〈κ2(0)κ2(t)〉, where κ denotes the orientational dependence on dipole–dipole coupling. The solution of the master equation is used to formulate the time-dependent fluorescence anisotropy, which is the relevant observable of energy transfer within donor–donor (dd) pairs, or bichromophoric molecules. Depending on symmetry of the local orientational distributions of the donor molecules, and their rates of reorientation, the fluorescence anisotropy decay becomes more or less complicated. Different simplifying conditions are given. The orientational distribution of the bichromophoric molecules is assumed to be isotropic and their rotational motion is taken to be negligible. The effect of using the cumulant expansion on the excitation probability and the fluorescence anisotropy was numerically examined. Brownian dynamics simulations are used to describe isotropic rotation of the d molecules. In the fast case (or dynamic limit), where the rates of transfer are much slower than those of reorientation, the cumulant expansion is always valid. In the intermediate case the approximation becomes questionable, while for the static limit, a numerical evaluation of the formal solution must be performed. The theory presented here is easily modified to account for the influence of reorientation in studies of donor–acceptor transfer.

Electronic energy transfer in supersonic jet expanded naphthalene-(CH 2) n-anthracene bichromophoric molecules

A study of intramolecular electronic energy transfer (Intra-EET) has provided evidence, for the first time, of a dramatic difference in Intra-EET efficiency results obtained under jet cooled and room temperature solution conditions. This was observed with anthracene-(CH 2) n -naphthalene bichromophoric molecules, A1N ( n = 1) and A3N ( n = 3). The rich fluorescence excitation spectrum of the naphthalene moiety in A1N indicates an inefficient EET process whose rate constant is substantially slower than that of the naphthalene moiety fluorescence. It was shown that the EET rate depends on a specific vibronic excitation that affects the molecular conformation, and was found to be at least two orders of magnitude slower in A1N molecule compared to A3N.

Laser induced fluorescence of jet-cooled non-conjugated bichromophores: bis-phenoxymethane and bis-2,6-dimethylphenoxymethane

Abstract The electronic spectroscopy of bichromophoric molecules linked by a OCH 2 O chain, such as bis-phenoxymethane (1), and bis-2,6-dimethylphenoxymethane (3), has been studied in a supersonic free jet by laser induced fluorescence. The experimental results have been compared to calculations resting on the perturbative CIPSI method, in which the di- and tri-excited configurations involving the π as well the v orbitals have been taken into account. The bis-phenoxymethane (1) molecule shows a single 0—0 transition which is blue-shifted relative to anisole by more than 400 cm −1 . This blue-shift has been theoretically related to the conformation of the bichromophore which displays an out-of-plane distorsion of the ether chain relative to anisole. The calculations clearly show that the observed blue-shift of the transition is related to this distorsion, and not to any electronic coupling between both cycles which is very weak. The single transition experimentally observed corresponds to the most stable structure of (1) which is in a gauche-gauche conformation relative to the CO bonds of the chain. In this structure the cycles are equivalent. The study of van der Waals complexes of (1) with usual solvents confirms this interpretation and shows that the equivalence of the cycles is removed by complexation. This contrasts with the bichromophore (3) whose 0—0 transition is blue-shifted by only 25 cm −1 relative to the 2,6-dimethylanisole subunit. Calculations have shown that in this case, the monomeric subunit as well as the bichromophore are in an out-of-plane conformation due to the steric hindrance introduced by the methyl groups. Moreover the excited state of (3) behaves as a weakly fluorescent exciplex whereas the emission resulting from the excitation of (1) is resonant. For the sake of comparison, the fluorescence excitation spectrum of the van der Waals dimer of anisole has also been studied and exhibits a red-shift with respect to bare anisole. The equilibrium geometry and the exciton coupling have also been calculated for the anisole van der Waals dimer, by means of the exchange perturbation theory.

Dependence of the intramolecular electronic energy transfer in bichromophoric molecules on the interchromophore bridge

Results are presented on the intramolecular electronic energy transfer (intra-EET) in bichromophoric molecules of the type benzene-α-diketone, in which the interchromophore bridge contains methyl substituents β to the α-dicarbonyl acceptor chromophore. These results show that singlet-singlet intra-EET is independent of substitution and can be explained by a model assuming Dexter-type short-range exchange interaction. For singlet-triplet and triplet-triplet transfer, there are indications that the phosphorescence yield of the acceptor is larger than that for non-substituted bichromophoric molecules. This can be explained by through-bond inteaction promoting EET via a long-range superexchange mechanism, by variations in the non-radiative decay of the triplet state of the acceptor or by chemical reaction.

Photophysical Properties and Photosubstitution and Photoredox Reactions of Aromatic Nitro Compounds

Abstract Orientation rules for photochemical nucleophilic substitutions are deduced from intramolecular photoreactions of some bichromophoric chain molecules, p-O2NC6H4O(CH2)nNHC6H5. The lower homologues undergo photochemical nucleophilic substitutions which can be classified as a Smiles-type photorearrangement. The photochemistry of this type of bifunctional chain species depends upon the methylene chain length. When the chain length is sufficiently long, the photorearrangement is totally forbidden. Photoredox reactions are the only accessible reaction processes for the higher homologues. The photoredox reaction consisting of cage and escape pathways is modulated by an external magnetic field. The magnetic field effects on the end product distribution are due to changes in the intersystem crossing rate induced by hyperfine interaction within biradical intermediates.

Conformational Analysis of C60 Ball-and-Chain Molecules: a Molecular Orbital Study

A computational study of the conformational analysis of a series of C60 ball-and-chain bichromophoric molecules (1)-(6) has been carried out by means of the AM1 and HF/3-21G//AM1 theoretical methods. It is found that the AM1 method underestimates both the relative conformational energies and the magnitude of the energy barriers folded and extended conformers. In contrast, the HF/3-21G//AM1 method gives reasonable values for these quantities. The HF/3-21G//AM1 calculated energy differences between the folded and extended conformers of systems (2)-(5) are less than 5 kJ/mol and the barriers to their interconversion are c. 30 kJ/mol. The conformational bias in these systems may be modified by the judicious placement of methyl groups in the region of the hydrocarbon chain closest to the C60 cage. It is found that the length of the chain and the nature of the non-C60. chromophore have little effect on the conformational energetics.

Photoinduced through-bond electron transfer and rearrangement in bichromophoric chain molecules

Relative quantum yields for the intramolecular electron transfer in N-[ ω-( p-nitrophenoxy)alkyl]anilines 1 and the analogues are determined. The role of through-bond and through-space pathways in the photorearrangement of 1( n=2−6) is discussed in connection with the quantum yields of the reaction in acetonitrile and benzene.

1,3-di-2,2′-Naphthylpropane as a fluorescent probe for silica surfaces

The bifunctional molecule 1,3-di-2,2′-naphthylpropane, N(3)N, has been investigated as a thermodynamic surface interaction probe of silica surfaces. Adsorption of N(3))N onto a silica surface from cyclohexane follows a Freundlich adsorption isotherm, demonstrating a range of weak interactions between the molecule and surface active sites. A moderate excimer emission ( λ max = 400 nm) observed at room temperature for low probe concentrations (below 5% of a monolayer) is attributed to an intramolecular process. Our results indicate that at low surface coverages more than 70% of the isolated bichromophoric molecules exist in an intramolecular excimer form. Thermodynamics data obtained from fluorescence studies reveal that excimer emission increases concurrently with a decrease in monomer emission as the temperature is raised. From an Arrhenius plot of the data an activation energy of about 2.2 kcal mol −1 is calculated for the intramolecular excimer formation on silica surface. Gas phase molecular mechanics calculations predict an activation energy of 2.3 kcal mol −1 for the interconversion of an anti-gauche conformer (non-excimer emitting) to a gauche-gauche conformer responsile for intramolecular excimer emission. Since the calculation does not take into accountsurface interactions, the anti-gauche conformation may not be the predominant non-excimer emitting species present on silica surface.

Vibronic coupling and energy transfer in bichromophoric molecules: The effect of symmetry

The fluorescence spectra of a series of bichromophoric molecules consisting of covalently linked fluorene units were investigated in a supersonic jet. In three of the systems (spirobifluorene, d8h8-spirobifluorene and 1-methyl spirobifluorene) no electronic coupling and no corresponding exciton splitting were detected in the zero-point level of the S1 state. Only 9,9′-bifluorene exhibited an exciton splitting in the v=0 state. The lack of coupling was attributed to symmetry; in the spirobifluorenes the planes of the fluorene moieties and the S1←S0 transition moments are perpendicular. When low vibrational levels were excited, state mixing, and energy transfer between the chromophores was observed. This behavior is characteristic of the ‘‘small molecule’’ regime of radiationless transition theory. When higher vibrational levels were excited, the systems exhibited typical ‘‘large molecule’’ behavior. In this limit, both electronic energy transfer, as well as intramolecular vibrational relaxation contribute to the decay of the initially excited state. Intramolecular dispersive interactions were also investigated by comparing the bifluorenes with a series of reference compounds.

Emission of a Bichromophoric Molecule in the Presence of an Added Quencher. Study of the Time-Resolved Fluorescence by Global Compartmental Analysis with and without the Use of a Model Compound

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEmission of a Bichromophoric Molecule in the Presence of an Added Quencher. Study of the Time-Resolved Fluorescence by Global Compartmental Analysis with and without the Use of a Model CompoundJan Van Stam, Luc Van Dommelen, Noeel Boens, Klaas Zachariasse, and Frans C. De SchryverCite this: J. Phys. Chem. 1995, 99, 23, 9386–9396Publication Date (Print):June 1, 1995Publication History Published online1 May 2002Published inissue 1 June 1995https://doi.org/10.1021/j100023a014RIGHTS & PERMISSIONSArticle Views53Altmetric-Citations8LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (987 KB) Get e-Alerts

Luminescence studies of biochromophoric 9-acetoxyanthracene derivative

The absorption spectra of the bichromophoric 9-acetoxyanthracene compounds contain three electronic transitions in the visible and UV region. The spectra have similar shapes as the spectrum of the free chromophor 9-acetoxy-10-(2′,4′-acetoxyphenyl)anthracene. The molar extinction coefficients of the respective band differ from those of the free chromophor. For all molecules the long-wavelength absorption band does not display the mirror-symmetry with the fluorescence spectrum. The fluorescence quantum yield and decay time for each bichromophoric molecule are different and they differ from that of the free chromophor. The Φ F and τ F values are about 10% smaller than those of molecule IV. The determined spectroscopic properties of the bichromophoric molecules, with the respective data of the free chromophor, allow us to state that in the molecules studied a non-radiative excitation energy transfer occurs. The transfer efficiency parameter E, and the rate constant of energy transfer k T are equal to 0.10 ± 0.03 and, 1.36 × 10 7 s −1, respectively. For the molecules studied k T is one order of magnitude smaller than the constant k F.

Intramolecular Energy Transfer from Upper Triplet States in Rigidly-Linked Bichromophoric Molecules

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTIntramolecular Energy Transfer from Upper Triplet States in Rigidly-Linked Bichromophoric MoleculesYijin Ren, Zhijun Wang, Hong Zhu, Stephen J. Weininger, and W. Grant McGimpseyCite this: J. Am. Chem. Soc. 1995, 117, 15, 4367–4373Publication Date (Print):April 1, 1995Publication History Published online1 May 2002Published inissue 1 April 1995https://doi.org/10.1021/ja00120a019RIGHTS & PERMISSIONSArticle Views167Altmetric-Citations9LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (746 KB) Get e-Alerts Get e-Alerts

Kinetic and standard thermodynamic parameters of photoinduced charge separation reactions in a bichromophoric anthracene-containing molecule from time-resolved fluorescence measurements in different solvents and solvent mixtures and at different temperatures

The determination of kinetic and standard thermodynamic parameters (reorganization energies, electronic coupling elements, free energy, enthalpy and entropy changes) of intramolecular photoinduced charge separation reactions in ω-(10-Phenyl-9-anthryl)propiophenone from the analysis of biexponential fluorescence decay curves exhibits distinct differences between the electronic factors in aprotic (benzene, acetonitrile, and mixtures thereof) and protic (methanol, ethanol) solvents and an unexpectedly small variation of the standard free energy change with solvent polarity.

ChemInform Abstract: Photoinduced Charge Transfer in Bichromophoric Molecules in the Gas Phase

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Photoinduced charge transfer in bichromophoric molecules in the gas phase

We have studied a series of bichromophores of the form A-(CH2)n-D where A is a 9-anthryl group, D is either a N-methylanilino group or a N-methyl-p-methoxyanilino group, and n=1,2,3, or 4. The fluorescence excitation and emission spectra of these molecules have been observed in a supersonic jet and in a variety of solvents. In the gas phase, in the n=1 and n=3 molecules, and in an extended conformer of the n=4 molecules interactions are weak resulting only in a slightly redshifted anthracene spectrum. No redshifted emission was observed in these cases. For the n=2 molecules and a second conformer of the n=4 molecules we observed a redshifted emission in the gas phase. This emission is assigned as coming from a charge transfer (CT) state or exciplex. On the basis of the excitation and emission spectra we observed two different pathways to reach the CT state. For the molecules with donor N-methyl-N-alkylaniline, excitation took place to the locally excited (LE) state followed by relaxation to the CT state. For the molecules with donor N-methyl-N-alkyl-p-methoxyaniline, the CT state was low enough to interact with the ground state forming an intramolecular electron–donor–acceptor (EDA) complex. From this EDA ground state direct excitation to the CT state occurred. In the molecule with n=2 and donor N-methyl-p-methoxyaniline the EDA interaction is very weak allowing excitation to the LE state as well as to the CT state. This set of molecules exhibits the full range of spectroscopic behavior expected in bichromophores.

Mediated Electronic Energy Transfer: Effect of a Second Acceptor State

Intramolecular electronic energy transfer was observed in bichromophoric molecules with the general structure donor-spacer-acceptor where indole is the donor, naphthalene is the acceptor, and cyclohexane is the inert spacer. Measurements were performed in a supersonic jet where both the absorption and emission spectra of the two chromophores were well-resolved and indole could be selectively excited with essentially no excitation on naphthalene. The emission produced on excitation of indole showed strong naphthalene emission and relatively weak indole emission. Moreover, the fluorescence lifetime was similar to the fluorescence lifetime of naphthalene, indicating that the energy transfer occurred on a time scale faster than the fluorescence lifetime of the donor

Molecular engineering of cyclic ?-diketone aromatic ring bichromophoric molecules for studies of intramolecular electronic energy transfer

The design, structures and spectral properties of a number of bichromophoric molecules are presented. These bichromophoric molecules are composed of an aromatic ring connected by two methylene chains to an α-diketone moiety. Both absorption and emission spectra can be attributed to a superposition of the individual spectra of the separate chromophores. The critical transfer radius for electronic energy transfer from the aromatic (donor) chromophore to the α-diketone (acceptor) chromophore was calculated from the spectral overlap between the fluorescence spectrum of the aromatic moiety and the absorption spectrum of the α-diketone moiety. The results show that this series of molecules is well suited for a mechanistic study of short-range intramolecular electronic energy transfer.

Electronic energy transfer: density of states

Intramolecular electronic energy transfer has been observed in one isomer of the bichromophoric molecule D-Sp-A where the donor (D) is naphthalene, the spacer (Sp) is cyclohexane, and the acceptor (A) is dimethylaniline (DMA). Because the emission spectra of both the donor and acceptor overlap, the emission wavelength could not be used as a diagnostic for energy transfer. However, the donor and acceptor have very different fluorescence lifetimes, and large changes in the fluorescence lifetimes of the vibronic states of naphthalene due to the presence of DMA could be used as a measure of the mixing and energy transfer between the two chromophores

Multichromophoric cyclodextrins. 2. Inhomogeneous spectral broadening and directed energy hopping

A [beta]-cyclodextrin bearing seven 2-naphthoyloxy chromophores is a good model for the study of the effect of the excitation wavelength on energy hopping among chromophores in well-defined positions, as in photosynthetic antennae. Absorption spectra, emission spectra, and excitation polarization spectra were recorded in a propylene glycol-dioxane glass at 200 K. Comparison is made with a bis(naphthoate) bichromophoric molecule. The parallelism between the increase of emission spectrum displacement and fluorescence anisotropy observed for the red edge of most vibronic bands, and especially for the 0-0 one, is established for the first time. It can be interpreted in terms of inhomogeneous spectral broadening due to solvation heterogeneity. The decrease of energy transfer that is observed upon red-edge excitation is evidence that energy hopping is not chaotic but directed toward lower-energy chromophores. 43 refs., 7 figs.