Control of emission by intermolecular fluorescence resonance energy transfer and intermolecular charge transfer

Control of emission by intermolecular fluorescence resonant energy transfer (IFRET) and intermolecular charge transfer (ICT) is investigated with the quantum-chemistry method using two-dimensional (2D) and three-dimensional (3D) real space analysis methods. The work is based on the experiment of tunable emission from doped 1,3,5-triphenyl-2-pyrazoline (TPP) organic nanoparticles (Peng, A. D.; et al. Adv. Mater. 2005, 17, 2070). First, the excited-state properties of the molecules, which are studied (TPP and DCM) in that experiment, are investigated theoretically. The results of the 2D site representation reveal the electron-hole coherence and delocalization size on the excitation. The results of 3D cube representation analysis reveal the orientation and strength of the transition dipole moments and intramolecular or intermolecular charge transfer. Second, the photochemical quenching mechanism via IFRET is studied (here "resonance" means that the absorption spectrum of TPP overlaps with the fluorescence emission spectrum of DCM in the doping system) by comparing the orbital energies of the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of DCM and TPP in absorption and fluorescence. Third, for the DCM-TPP complex, the nonphotochemical quenching mechanism via ICT is investigated. The theoretical results show that the energetically lowest ICT state corresponds to a pure HOMO-LUMO transition, where the densities of the HOMO and LUMO are strictly located on the DCM and TPP moieties, respectively. Thus, the lowest ICT state corresponds to an excitation of an electron from the HOMO of DCM to the LUMO of TPP.     

Electronic excited state energy transfer,trapping by dimers and fluorescence quenching in concentrated dye solutions: Picosecond transient grating exp

Picosecond transient grating experiments are used to examine electronic excited state dynamics in concentrated dye solutions. A model based on radiationless excited state transport and trapping by dimers describes the phenomena responsible for fluorescence quenching. The trapping rate constant is found to have a cubic concentration dependence. Rhodamine 6G dimer lifetimes in glycerol and ethanol are 830 ps and <50 ps respectively. The difference arises due to the viscosity dependence of the dimer radiationless relaxation rate.     

Arylketones as triplet donors and quenchers: evidence for chemical quenching and reversible energy transfer

Quenching measurements of phosphorescence intensity and lifetime of benzophenone and its derivatives with electron-attracting substituents by arylketones with electron-releasing substituents, carried out in acetic acid, show that the quenching rate parameter k_q depends on the structure of the two interacting partners. The k_q value increases with the electron-donating power of the substituents in the quencher, approaching the diffusional value with the strongest electron-donating groups, whilst it decreases when electron-withdrawing substituents are present in the quenched species. The first effect is explained by an interaction mechanism where a charge transfer occurs from the quencher to the excited donor. The effect of donor structure on the k_q value is ascribed to the contribution of a partial reversible energy transfer. A study of the dependence of the experimental quenching parameter on the donor concentration gives evidence for the reversible transfer process.     

吖啶橙-罗丹明B能量转移荧光猝灭法测定叶酸

研究了吖啶橙(AO)与罗丹明B(RB)间发生能量转移的最佳条件,在pH=6.80的Britton-Robinson(B-R)缓冲溶液,十二烷基苯磺酸钠的介质中,AO-RB间发生有效能量转移,使RB荧光大大增强,叶酸(FA)的加入使能量转移体系的RB的荧光强度降低,即发生猝灭.以此建立了利用AO-RB能量转移荧光猝灭法测...     

Hg sensing in aqueous solutions: an intramolecular charge transfer emission quenching fluorescent chemosensors

Compounds 4a and 4b, comprising an anthracene moiety as the fluorophore and a pair of dithiocarbamate functionalities as ligating groups, were designed as fluorescent chemosensors for Hg(II). In aqueous solvent systems, upon excitation, in addition to the normal emission bands of locally excited (LE) state of anthracene, both compounds show a prominent pH-independent intramolecular charge transfer (ICT) emissive band, which can be modulated by Hg²⁺ binding. The systems can be exploited to develop a fluorescent sensitive probe for Hg²⁺.     

Stereoselective fluorescence quenching by photoinduced electron transfer in naphthalene-amine dyads

Intramolecular chiral recognition in electron-transfer-induced fluorescence quenching has been observed for diastereomeric dyads composed of a naphthalene chromophore and an amine.     

吖啶橙-罗丹明6G共振能量转移荧光猝灭法测定尿中1-羟基芘

建立了吖啶橙(AO)-罗丹明6G(R6G)共振能量转移荧光猝灭法测定尿中1-羟基芘的新方法.在λex/λem=470/556nm,十二烷基苯磺酸钠(SDBS)存在下,AO-R6G能够发生有效的能量转移,使R6G的荧光大大增强;1-羟基芘(1-OHP)的加入使R6G的荧光猝灭.方法的线性范围是21.3～982 μg/L;检出限为6.4 μg/L;平行7次测定相对标准偏差为0.98%～2.0%;回收率为96.0%～104.4%.该方法用于锅炉工尿样中1-羟基芘的测定,结果与常规的高效液相色谱法一致.     

Interaction between intramolecular charge transfer state of p-N,N-dimethylaminobenzonitrile and amines. Stereo-controlled fluorescence quenching

The first excited singlet state, an intramolecular charge transfer state, of p-N,N-dimethylaminobenzonitrile (DMABN) is quenched by tertiary amines. The quenching rate cannot be correlated with the oxidation potential of the amines, but is controlled by the size of the amine alkyl group. This unusual steric effect indicates a short-range interaction, which we attribute to three-electron bonding. Excited-state three-electron bonding interaction should be a general phenomenon and can lead to the formation of sigma-bonded exciplexes. The implication of this result on the origin of the anomalous dual fluorescence of DMABN is discussed.     

Positronium inhibition and quenching by organic electronic acceptors and charge transfer complexes

Positron lifetime measurements were performed on a series of organic electron acceptors and charge-transfer complexes in solution. The acceptors cause both positronium (Ps) inhibition (with maybe one exception) and quenching, but when an acceptor takes part in a charge-transfer complex the inhibition intensifies and the quenching almost vanishes. The reaction constants between ortho-Ps and the acceptors were determinded to be: 1.5 × 10¹⁰ M⁻¹ s⁻¹ for SO₂ in dioxane 3.7 × 10¹⁰ M⁻¹ s⁻¹ for SO₂ in n-heptane, 3.4 × 10¹⁰ M⁻¹ s⁻¹ for tetracyanoquinodimethane in tetrahydrofurane and 1.6 × 10¹⁰ M⁻¹ s⁻¹ for tetracyanoethylene in dioxane. From the ortho-Ps lifetimes in solutions containing charge-transfer complexes complexity constants were determined that were in reasonable agreement with constants obtained from optical data. The influence of acceptors and charge-transfers complexes on the Ps yield was interpreted in terms of the spur reaction model of Ps formation. Correlation was also made to gas phase reaction between electron acceptors and free electron, as well as to pulse radiolysis data.     

Variation of the ultrafast fluorescence quenching in 2,6-sulfanyl-core-substituted naphthalenediimides by electron transfer

The ultrafast fluorescence quenching of 2,6-sulfanyl-core-substituted naphthalenediimides was investigated by transient spectroscopy. We find a strong dependence of the relaxation on the chemical structure of the substituent. Direct linking of an aryl rest to the sulfur atom leads to a strong red shift of the fluorescence in 1 ps and the disappearance of the emission in 5-7 ps depending on the polarity and viscosity of the solvent. This complex behavior is interpreted with the help of quantum chemical calculations. The calculations suggest that the initial relaxation corresponds to a planarization of the substituents and an associated partial electron transfer. This is followed by a twisting of the phenylsulfanyl substituents out of the molecular plane that allows a complete localization of the electron-donating orbital on the aryl group. Finally the back transfer happens in another 5-7 ps. For an additional methylene spacer group between the sulfur and the aryl, this sequence of relaxation steps is not possible and a simple exponential decay, slower by about 1 order of magnitude, is found.     

A reversible optode membrane for picric acid based on the fluorescence quenching of pyrene

Pyrene immobilized in plasticized poly(vinyl chloride) (PVC) membrane is able to extract selectively picric acid from the sample solution into the organic membrane phase. Since this extraction equilibrium is accompanied by a change in the fluorescence spectrum of pyrene, the chemical recognition process can be directly translated into an optical signal. With the optode membrane described, picric acid in sample solutions from 8.7 x 10(-6) to 4.3 x 10(-3)M can be determined. The calibration curve of the optode membrane for picric acid shows a good correlation with the mathematically derived formalism and thus confirms the theoretically expected behaviour. Besides a high reproducibility of the optical signals, the very short response times less than 35 sec are realized. The optode membrane presented exhibits good selectivity for picric acid over many other usual hydrophobic anions.     

A reversible chemosensor for nitrite based on the fluorescence quenching of a carbazole derivative

The carbazole derivative, with an amino group in 9-position (9-methylacryloylamino carbazole (MAC), has been utilized to prepare a fluorescent sensor and used for the determination of NO₂⁻ based on the reaction between nitrite (NO₂⁻) and excess I⁻ to form I₃⁻, which can quench the fluorescence of carbazole derivative. MAC, as a fluorescent carrier, has a terminal double bond and is covalently immobilized on a quartz glass plate surface by photo-polymerization to prevent the leakage of the dye. The sensor shows sufficient repeatability, selectivity, operational lifetime of 8 weeks, and a fast response of less then 30 s. NO₂⁻ can be determined in the range between 1.0×10⁻⁶ and 1.0×10⁻⁴ mol l⁻¹ with a detection limit of 8.0×10⁻⁷ mol l⁻¹ at pH of 2.0. The quenching mechanism is discussed. Most commonly coexisting ions do not interfer with the NO₂⁻ assay.     

Free energy correlation of rate constants for electron transfer between organic systems in aqueous solutions

Recent experimental data concerning the rate constants for electron transfer reactions of organic systems in aqueous solutions and their equilibrium constants is examined for possible correlation. The data is correlated quite well by the Marcus theory, if a reorganization parameter, λ, of 18 kcal/mole is used. Assuming that the only contribution to λ is the free energy of rearrangement of the water molecules, an effective radius of 5A?for the reacting entities is estimated. For the zero free energy change reaction, i.e., electron exchange between a radical ion and its parent molecule, a rate constant of about 5 × 10⁷ M^?1 s^?1 is predicted.     

Migration of excitation energy and fluorescence quenching in regular lattices

Intermolecular transfer of excitation energy is studied in model systems containing luminescent donor molecules (D) and acceptor molecules (A) which constitute deep energy traps. It has been assumed that the donor and acceptor molecules form a regular lattice in which the energy transfer takes place in a hopping manner. Within the limits of the model, it has further been assumed that elementary processes are responsible for the deactivation of excited donor molecules (D^*). These processes are: fluorescence, internal conversion and non-radiative energy transfer D^* → D and D^* → A. The general considerations concern the partial and total fluorescence quantum yield of the system and the number of energy transfers before its deactivation. A more detailed analysis and calculations, leading to analytical expressions describing these quantities, have been made for linear systems. It is shown, that has approach, under the conditions where in the system only the migration of energy is observed and no other means of energy deactivation exist leads to the same value of the average number of energy transfers as was obtained earlier by Montroll and a mean relaxation time as given by Movaghar et al.     

Application of the energy gap law to the decay of charge transfer excited states,solvent effects

Values of non-radiative decay rate constants (k_nr) and emission energies (E_cm) have been obtained for Os(Phen₃)²⁺ in a series of solvents and the results are consistent with the energy gap law. For hydroxylic solvents like water or methanol related studies suggest the existence of strong, specific contributions to the vibrational trapping energy of the solvent.     

A photochromic acceptor as a reversible light-driven switch in fluorescence resonance energy transfer (FRET)

A method has been developed for the quantitative determination of fluorescence resonance energy transfer (FRET) based on the modulation of donor fluorescence upon the reversible photoconversion of a photochromic acceptor. A model system was devised, consisting of Lucifer Yellow cadaverine (LYC, donor) conjugated to the photochromic molecule, 6-nitroBIPS (1′,3′-dihydro-1′-(2-carboxyethyl)-3′,3′-dimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indoline]). Near-ultraviolet irradiation catalyzes the conversion of the colorless spiropyran (SP) to the colored merocyanine (MC) form of 6-nitroBIPS. Only the MC form absorbs at the emission wavelengths of the donor, thereby potentiating FRET, as demonstrated by quenching of the donor. Subsequent irradiation in the visible MC absorption band reverts 6-nitroBIPS to the SP form and FRET is inactivated. The acceptor exhibited high photostability under repeated cycles of alternating UV–Vis irradiation. In this model system, the intramolecular FRET efficiency was close to 100%. The observed maximal donor quenching of 34±3% was indicative of an equilibrium determined by the high quantum efficiency of forward conversion (SP→MC) induced by near-UV irradiation and a low but finite quantum efficiency of the back reaction resulting from excitation of the MC form directly as well as indirectly (by FRET via the donor). A quantitative formalism for the photokinetic scheme was developed. Photochromic FRET (pcFRET) permits repeated, quantitative, and non-destructive FRET determinations for arbitrary relative concentrations of donor and acceptor and thus offers great potential for monitoring dynamic molecular interactions in living cells over extended observation times by fluorescence microscopy.     

A schematic model for energy and charge transfer in the chlorophyll complex

A theory for simultaneous charge and energy transfer in the carotenoid-chlorophyll-a complex is presented here and discussed. The observed charge transfer process in these chloroplast complexes is reasonably explained in terms of this theory. In addition, the process leads to a mechanism to drive an electron in a lower to a higher-energy state, thus providing a mechanism for the ejection of the electron to a nearby molecule (chlorophyll) or into the environment. The observed lifetimes of the electronically excited states are in accord/agreement with the investigations of Sundstr?m et al. and are in the range of pico-seconds and less. The change in electronic charge distribution in internuclear space as the system undergoes an electronic transition to a higher-energy state could, under appropriate physical conditions, lead to oscillating dipoles capable of transmitting energy from the carotenoid-chlorophylls chromophore to the reaction center by sending an electromagnetic wave (a photon) which provides a novel new mechanism for energy production. In the simplest version of the F?rster?CDexter theory, the excitation energy of a donor is transferred to an acceptor and then de-excited to the ground state by fluorescence with no electron being transferred. In the process proposed herein, charge and energy both are transferred from donor to acceptor which can further de-excite by fluorescence. The charge transfer time scale involving an actual transfer of electron is in the pico-second range.