Intramolecular Exciplex Research Articles

Solution and supersonic jet studies of the intramolecular exciplex of dinaphthyl propanes

The newly synthesized dinaphthyl propanes: 1,3-di-(2-naphthyl)-propane and 1-(1-naphthyl), 3-(2-naphthyl)-propane were studied spectroscopically under supersonic jet conditions and in solution. Under supersonic jet conditions, both molecules exhibit a broad, structureless fluorescence excitation spectrum and relatively short non-exponential decay of the highly red shifted fluorescence, consistent with strong interchromophore coupling in the excited state, resulting in, at least, partial excitation directly into an exciplex-like state. These observations are in contrast to the observed spectra and fluorescence decay of all other dinaphthalene bichromophoric molecules. Solution studies support these findings. The optimal flexible sandwich type geometry of both molecules is probably responsible for the observed exciplex state stabilization.

Competitive occurrence of homolytic N–O and heterolytic C–O bond cleavage in excited-state 1-(arylmethyloxy)-2-pyridones

The irradiation at 340 nm of the title compounds having 9-anthryl and pyren-1-yl groups in methanol was found to give the heterolytic C–O bond cleavage products: 1-hydroxy-2-pyridone and aryl-substituted dimethyl ether (which predominate for the reaction of the former title compound) in addition to 2-pyridone, aryl-substituted methanol and aryl-substituted formaldehyde derived from the homolysis of the N–O bond (which mainly occurs in the photolysis of the latter title compound). It was also found that substitution of the methyl group for hydrogen at the 6-position of the pyridone skeleton in 1-(9-anthrylmethyloxy)-2-pyridone decreases the relative composition of the aryl-substituted dimethyl ether to some extent. These substituent effects on the product compositions were explained in terms of stereoelectronic effects on a charge transfer-type interaction between the aromatic and pyridone rings in the singlet excited state. Analyses of the ground-state conformation for the title compounds by MM2 calculations and 1H NMR spectroscopy, as well as of their singlet excited-state behaviour, substantiated the existence of a non-emissive intramolecular exciplex intermediate which plays a key role in inducing the C–O bond heterolysis.

Ultrafast charge transfer in phytochlorin–[60]fullerene dyads: influence of the attachment position

A femtochemistry study has been performed on two phytochlorin–[60]fullerene donor–acceptor dyads, in which the fullerene moiety is linked via a pyrrolidine bridge either to the position 3 or 7 of the phytochlorin ring. Both dyads form an emissive intramolecular exciplex in less than 1 ps, but only the former dyad relaxes to the ground state via a charge transfer (CT) state.

Charge Transfer Kinetics and Solvatochromism of 1-(9-Anthryl)-3-(4-dimethylaniline) Propane in 1,4-Dioxane: Nonideal Quadrupolar Charge Distribution and the Origin of the Dioxane Anomaly

The solvent 1,4-dioxane behaves like a polar solvent even though its dielectric constant is small. This excess polarity is referred to as the “dioxane anomaly”. Several explanations of this anomaly have been offered in the literature including hydrogen bonding, conformational polarity, and quadrupolar interactions. We have investigated the origins of this anomaly by employing the unique photochemical properties of the ADMA (1-(9-anthryl)-3-(4-dimethylaniline) propane) molecule. ADMA forms an intramolecular exciplex in the excited state. Solvent polarity governs the mechanism of exciplex formation. In nonpolar solvents the decay of the locally excited-state population is monoexponential, while in polar solvents (e > 5.4) the decay of the locally excited-state population is biexponential. Although dioxane has a dielectric constant of 2.2, ADMA dissolved in dioxane exhibits biexponential kinetics. From this result and from the solvatochromic shift of the ADMA exciplex peak in dioxane, we have concluded that ...

Photophysical properties of β-(1-pyrenyl)ethyl p-cyanobenzoate in binary solvents of isooctane-ethyl acetate and ethyl acetate-acetonitrile

The effects of solvent polarity on the fluorescence spectra and fluorescence decays of β-(1-pyrenyl)ethyl p-cyanobenzoate (P2CN) were investigated in detail using binary solvents consisting of various mixing ratios of isooctane-ethyl acetate or ethyl acetate-acetonitrile (dielectric constants (ε)=1.94–36.2). Whereas both the intensity and wavelength maxima of an intramolecular exciplex emission (EX) are dependent on the solvent polarity, only the intensity of an emission from the locally excited pyrene (LE) is dependent on the solvents used. When monitored at 377nm, the picosecond SPC (single photon counting) measurements reveal a slow decay (>150ns) in addition to a fast decay (<1ns) of the locally excited P2CN. There are also two decays for the EX which vary the intensity ratios by the monitored wavelength. The decay rate constants, kEX1 and kEX2, have a good linear correlation with the dielectric constants of the solvents, indicating that there exist two kinds of exciplexes. It is suggested that the decays of the locally excited-state of P2CN are so fast due to result of the efficient electron transfer that the two kinds of intramolecular exciplexes are formed from the two discrete conformers in the ground state.

The photochemistry of 2-(1-naphthyl)ethyl benzoates: cycloaddition and intramolecular exciplex formation.

The photochemistry of the 2-(1-naphthyl)ethyl benzoates 6 and 7 was examined in order to compare them to previously studied 2-arylethyl 4-cyanobenzoates that underwent a Norrish Type II fragmentation. The 1-naphthyl group was incorporated to provide a fluorescent chromophore for probing the intramolecular electron transfer proposed previously for the mechanism. The naphthalene fluorescence was quenched for both 6 and 7 although at very different rates. For 6, with the higher thermodynamic driving force (-68.9 kJ/mol), intramolecular electron transfer was fast in all solvents, independent of their polarity (cyclohexane to methanol). For 7, with the lower driving force (-26.5 kJ/mol) the process was fast only in polar solvents. Exciplex emission, observed for 6 (but not for 7), exhibited a large solvatochromic effect possibly indicating a high dipole moment (28 D) in polar solvents (stretched conformation) but a lower one (17 D) in nonpolar solvents (folded conformation). Finally, the 4-cyanobenzoate 6 was very unreactive photochemically. In contrast, benzoate 7 underwent a 2 + 2 cycloaddition of the ester carbonyl to the naphthalene ring to give products 8 and 9, a process for which we have found no precedent.

Intramolecular exciplex and cation-mediated charge-transfer fluorescence in a podand carrying terminal electron donor and acceptor groups

A podand carrying terminal electron donor and acceptor groups (DPA) has been synthesized. There is no intramolecular interaction between the two chromophores in the ground state. Selective excitation of the acceptor moiety, and a very weak and structureless exciplex emission was observed in benzene. In acetonitrile, only quenching of the monomer fluorescence occurred. In the presence of calcium ions conformational modification through complexation with the non-cyclic poly(oxyethylene) chain resulted in the formation of a charge-transfer (CT) complex in the ground state. Selective excitation of the acceptor moiety (380 nm) or at the CT band (340 nm) gave rise to intramolecular excited CT complex emission. The similarity in the fluorescence maxima exhibited by the exciplex and the excited CT complex implies that the electronic structures of their fluorescent states are very similar. The slight difference in the fluorescence quantum yields and decay rate constants between the exciplex and excited CT complex suggests that they have different geometries. Exciplex fluorescence was also observed in γ-CD, depending on the γ-CD concentration.

Time-resolved emission studies of tris-acetylpyrene derivative of a cryptand exhibiting localised monomer, intramolecular exciplex and intramolecular excimer emissions in different environments

The tris-acetylpyrene derivative of a heteroditopic cryptand is found to be a rare supramolecular system exhibiting three emissions due to localised monomer, intramolecular exciplex and excimer emissions in different environments. The lifetime for the monomer emission is much shorter compared to that of free pyrene indicating PET from cryptand moiety to pyrene units. In presence of some transition metal ions, PET is blocked resulting in increase of the lifetime for the metal complex. The major component of the lifetime of the intramolecular exciplex is comparable with that of other polycyclic aromatic tertiary amine systems. Presence of protons in the environment facilitate intramolecular excimer formation whose lifetime is almost twice as that of the intramolecular exciplex emission of the system.

Synthesis and Electronic Properties of Tetrakis[4-(pyrimidyl)phenyl]methanes − A Novel Class of Electronically Active Nanometer-Sized Scaffolds

Tetrakis[4-(arylpyrimidyl)phenyl]methanes 4 can be readily synthesized by using a highly convergent vinamidinium salt cyclocondensation strategy. The conjugated side-chains of these novel nanostructures display interesting cooperative electronic features such as intramolecular exciplex coupling and ECEC redox processes in the cathodic region.

Magnetic Field Effect on the Intramolecular Exciplex Fluorescence of Chain-Linked Pyrene/N,N-Dimethylaniline Systems

The magnetic field effect (MFE) (≤ 13 T) on the intramolecular exciplex fluorescence of chain-linked pyrene/N,N-dimethylaniline systems is studied. In the case of pyrenyl-(CH2)2-O-(CH2)10-N(CH3)-phenyl, the exciplex fluorescence lifetimes are 34.2 (0 T), 50.6 (1 T), and 36.1 ns (13 T) in tetrahydrofuran/N,N-dimethylformamide (1 : 1) mixed solvent at room temperature. The lifetime decrease above 1 T is explained in terms of a Δg mechanism. Solvent dependence of the MFE maximum is discussed in terms of stabilization energy of the intramolecular radical ion pair, which is in fast dynamic equilibrium with the singlet exciplex. Influences of chain length and temperature are also discussed.

A novel fluorescence probe for micellization of amphiphilic molecules in water based on intramolecular exciplex formation

A novel friction-sensitive fluoresence probe based on intramolecular exciplex formation, (9-anthrylmethyloxymethyl)pyrene (AMOP), was used to study the micellization of amphiphilic molecules such as sodium dodecyl sulfate (SDS) and hydrocarbon-modified polyethylene glyco (HM-PEG) in water. Results demonstrate that AMOP can serve as a sensitive probe for micellization of both SDS and HM-PEG. In addition, AMOP reveals information about the microviscosity of SDS and HM-PEG micelles, wich is in good agreement with the results obtained from other fluorescence probes based on intramolecular excimer such as dipyrenylpropane (Py(CH 2 ) 3 Py) and bis(1-pyrenylmethyl) ether (dipyme, PyCH 2 OCH 2 Py).

Photophysical properties of tris-acetylpyrene derivative of a cryptand in different environments

The three secondary amino nitrogens of a heteroditopic cryptand have been derivatized with acetylpyrene to have a fluorophore–spacer–receptor configuration. The localized monomer fluorescence intensity is reduced in this system due to quenching via photoinduced intramolecular electron transfer from nitrogen atoms in the receptor cryptand moiety to the fluorophores. The system also exhibits a red-shifted broad structureless emission in different solvents which is assigned to intramolecular exciplex formation. Among the different metal ions, which bind in the cryptand cavity, the local monomer emission can be recovered to different extents in the presence of Zn(II), Co(II) and Mn(II) ions while binding of Ni(II) or Cu(II) did not result in any fluorescence enhancement. Presence of protons, however, facilitates intramolecular excimer formation in the free ligand. The intensity ratio of monomer and excimer emission is found to be a function of proton concentration. The system is thus a rare one exhibiting monomeric, excimeric and exciplex emissions depending on the environment.

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.

Effect of high magnetic fields on intramolecular exciplex fluorescence of pyrene and dimethylaniline systems

The effect of high magnetic fields (〈13 T) on the intramolecular exciplex fluorescence generated from chain-linked pyrene and dimethylaniline systems is studied in acetonitrile. On increasing the magnetic field from 0 to ca. 1 T, the exciplex fluorescence intensity increases and then decreases gradually for fields up to ca. 8 T. The exciplex fluorescence lifetime exhibits a magnetic field dependence similar to that for the intensity (〈13 T). The reversal of the effect in the high magnetic field region is interpreted in terms of a Δ g mechanism.

Intermolecular interactions of 4-pyrrolidino pyridine: a simulation study

Simulations of intermolecular interaction by the AM1 method have been performed. The hydrogen bonding complex between and 4-pyrrolidino pyridine (PP) and water molecules with 1:n complexes have been considered to investigate possible stable complex configurations and to calculate the stable interaction energy. These calculations confirm the influence of water molecules on twisting of the pyrrolidino group in ground state. The excited state simulation predicts that the energy minimized geometry of the PP molecule takes an almost sandwich like structure, confirming the formation of an intramolecular exciplex in the gas phase as well as in nonpolar or in weakly polar solvents in line with the experimental findings.

Intramolecular Exciplex Formation and Complexing Behavior of Naphthalene Derivatives as Fluorescent Chemosensors for Calcium and Barium Ions

Intramolecular Exciplex Formation and Complexing Behavior of Naphthalene Derivatives as Fluorescent Chemosensors for Calcium and Barium Ions

Formation and behavior of intramolecular N-(styrylalkyl)aniline exciplexes

The photophysical and photochemical behavior of several aniline spacer - styrene molecules in which the spacer is a short polymethylene chain has been investigated. Long wavelength excitation selectively excites the aniline chromophore which undergoes intramolecular quenching by the styrene to yield an intramolecular exciplex. Exciplex fluorescence is observed for both of the tertiary anilines investigated and for one of the three secondary anilines. More rapid exciplex formation in nonpolar vs. polar solvents is attributed to the necessity of desolvating the polar aniline singlet in polar solvents. Intramolecular adduct formation is the major photochemical process for the secondary anilines, but is highly inefficient for the tertiary anilines. The behavior of the intramolecular aniline-styrene exciplexes is compared to that of the styrene-amine exciplexes previously investigated in this laboratory.Key words: intramolecular exciplex, fluorescence, aniline, styrene.

Solvent parameter based on the exciplex emission of (2-naphthoxy)polyoxalkyl p-cyanobenzoates

The fluorescence spectra of (2-naphthoxy)polyoxalkyl p-cyanobenzoates ( 1–3) were analyzed in terms of emission maxima, lifetimes, and quantum yields of the emissions from the locally excited state and the exciplexes (EX) state in 15 kinds of solvents from cyclohexane to acetonitrile. The emission in benzene, toluene, and 1,4-dioxane were deviated from usual Mataga–Lippert plots of EX-emission maxima of 1–3 versus solvent parameter, Δ f. In order to modify Δ f parameter, we develop new empirical solvent parameter ( E M(2)) based on the intramolecular EX emission of 5-(2-naphthoxy)-3-oxa-1-pentyl p-cyanobenzoate ( 2).

Kinetics and thermodynamics of free flavins and the flavin-based redox active site within glucose oxidase dissolved in solution or sequestered within a sol-gel-derived glass.

We report on the steady-state and time-resolved fluorescence from the redox active site flavine adenine dinucleotides (FADs) that are bound to glucose oxidase (GOx) when this enzyme is dissolved in aqueous solution or sequestered within a sol-gel-derived glass. To the best of our knowledge, this represents the first report on the actual dynamics of an enzyme active site when the enzyme is part of a sol-gel-derived glass. The results from these experiments show that the "free" FAD intramolecular folding/unfolding kinetics are slowed 3-10-fold within the glass vs solution. The intramolecular exciplex formation event (i.e., excited-state FAD residue folding/unfolding) is completely arrested for the GOx-bound FAD if the enzyme is sequestered within a glass in the absence of glucose. This is significantly different from the behavior of GOx dissolved in solution. However, despite this difference in behavior, the GOx molecules that are sequestered within the glasses continue to function somewhat like GOx dissolved in aqueous solution if they are challenged with glucose. We also found that the GOx molecules do not leach from the glass and they exhibit rotational mobility that is only 2-fold less than GOx dissolved in aqueous solution at 20 degrees C. In aqueous solution or within these glasses, the enzyme pocket that hosts the FAD redox sites opens up by 25-30% when GOx is challenged with glucose. Finally, we present preliminary analytical results for film-based sol-gel-derived biosensors that contain GOx, L-amino acid oxidase or cholesterol oxidase wherein the intrinsic FAD fluorescence produces the analytical signal.

Formation and behavior of intramolecular &lt;i&gt;N-&lt;/i&gt;(styrylalkyl)aniline exciplexes

The photophysical and photochemical behavior of several aniline spacer - styrene molecules in which the spacer is a short polymethylene chain has been investigated. Long wavelength excitation selectively excites the aniline chromophore which undergoes intramolecular quenching by the styrene to yield an intramolecular exciplex. Exciplex fluorescence is observed for both of the tertiary anilines investigated and for one of the three secondary anilines. More rapid exciplex formation in nonpolar vs. polar solvents is attributed to the necessity of desolvating the polar aniline singlet in polar solvents. Intramolecular adduct formation is the major photochemical process for the secondary anilines, but is highly inefficient for the tertiary anilines. The behavior of the intramolecular aniline-styrene exciplexes is compared to that of the styrene-amine exciplexes previously investigated in this laboratory.