Salt-induced Charge Separation in Photoinduced Electron Transfer Reactions. The Effect of Ion Size*

The effect of added salts on the efficiency for photoinduced charge separation in two typical electron acceptor (A)/electron donor (D) systems was studied by the technique of laser flash photolysis. We investigate the exciplex-forming pyrene/p-dicyanobenzene (Py/DCB) and pyrene/N,N-dimethylaniline (Py/DMA) systems in ethyl acetate. The salts selected for this study are tetrabutylammonium chloride, tetrahexylammonium chloride, lithium perchlorate, sodium perchlorate, tetrabutylammonium perchlorate, sodium tetraphenylborate and tetrabutylammonium tetraphenyl-borate. In most cases, the salts quench the emission of the exciplexes with rate constants near the diffusional rate limit in ethyl acetate. An apparent red shift of the fluorescence maximum of the exciplexes with increasing salt concentration is also generally observed. Laser flash photolysis experiments showed that in the absence of the salts both A/D systems yield exclusively the triplet excited state of the polyaromatic. However, in the presence of many of the electrolytes studied, induced free radical ion formation is observed. The experimental efficiencies for induced charge separation (n) depend on the A/D system and on the nature of the salt. The measured n values vary between 0 and 0.5. The most striking variation corresponds to the lithium and sodium perchlorates. These salts are almost totally inefficient in quenching the Py/DCB exciplex, while they quench and induce charge separation from the Py/DMA exciplex with a high yield. The effect of the different salts on both exciplexes may be rationalized by using the concept of the soft/hard character of the interacting ions.     

Detection of nucleic acids in situ: novel oligonucleotide analogues for target-assembled DNA-mounted exciplexes

This research describes the effects of structural variation and medium effects for the novel split-oligonucleotide (tandem) probe systems for exciplex-based fluorescence detection of DNA. In this approach the detection system is split at a molecular level into signal-silent components, which must be assembled correctly into a specific 3-dimensional structure to ensure close proximity of the exciplex partners and the consequent exciplex fluorescence emission on excitation. The model system consists of two 8-mer oligonucleotides, complementary to adjacent sites of a 16-mer DNA target. Each probe oligonucleotide is equipped with functions able to form an exciplex on correct, contiguous hybridization. This study investigates the influence of a number of structural aspects (i.e. chemical structure and composition of exciplex partners, length and structure of linker groups, locations of exciplex partner attachment, as well as effects of media) on the performance of DNA-mounted exciplex systems. The extremely rigorous structural demands for exciplex formation and emission required careful structural design of linkers and partners for exciplex formation, which are here described. Certain organic solvents (especially trifluoroethanol) specifically favour emission of the DNA-mounted exciplexes, probably the net result of the particular duplex structure and specific solvation of the exciplex partners. The exciplexes formed emitted at approximately 480 nm with large Stokes shifts ( approximately 130-140 nm). Comparative studies with pyrene excimer systems were also carried out.     

Magnetic field effect on indole exciplexes: a comparative study

A comparative magnetic field effect (MFE) study was done on indole exciplexes with various acceptors, anthracene, pyrene, all-s-trans-1,4-diphenylbuta-1,3-diene and 9-cyanophenanthrene. A surprisingly low magnetic field effect was detected for the 9-cyanophenanthrene exciplexes and was correlated with exciplex geometry. The wavelength dependence of magnetic field effect confirms the presence of single charge-transfer complex for all the exciplexes with 1,2-dimethylindole.     

Exciplex fluorescence as a diagnostic probe of structure in coordination polymers of Zn2+ and 4,4'-bipyridine containing intercalated pyrene and enclathrated aromatic solvent guests

Coordination polymers based on Zn(II)-4,4'-bipyridine (Zn-bipy) frameworks containing pyrene intercalated between adjacent layers and aromatic solvent molecules enclathrated within the framework cavities have been prepared and characterized for the first time. These compounds are highly fluorescent, and show broad, featureless emission spectra significantly red-shifted relative to pyrene monomer fluorescence; this has been assigned to pyrene-bipy exciplex emission. Single-crystal X-ray structural analysis shows that the presence of the aromatic solvent molecule within the cavities has a profound effect on the architecture of these frameworks: in the case of benzene, toluene, p-xylene, and chlorobenzene, the Zn-bipy framework consists of 1-D ladders, whereas in the case of o-dichlorobenzene (the largest solvent guest), the framework was based on a 2-D square grid. This difference in stoichiometry and architecture was also reflected in significant differences in the fluorescence of these coordination polymers, with three of the four compounds with 1-D ladder geometries having similar fluorescence maxima (ca. 520 nm) and lifetimes (ca. 70 ns), whereas the compound with square grid topology had a significantly blue-shifted maximum (ca. 460 nm) and shorter lifetime (ca. 42 ns). It is proposed that exciplexes form upon excitation of ground-state complexes, involving face-to-face bipy/pyrene complexes (pi-pi stacking interactions) in the case of the 1-D ladder structures, but edge-to-face bipy/pyrene and pyrene/o-dichlorobenzene complexes (C-H...pi interactions) in the case of the 2-D square grid structure.     

Utilization of heavy-atom effect quenching of pyrene fluorescence to determine the intramembrane distribution of halothane

The interaction of halothane (CF₃CHBrCl) with dipalmitoylphosphatidylcholine (DPPC) membranes containing varying amounts of dipalmitoylphosphatidylglycerol (DPPG) was examined via heavy atom effect quenching of pyrene fluorescence by halothane. The effect of halothane on pyrene fluorescence is consistent with a kinetic model based upon the assumption of the existence of two populations of pyrene in the membrane: one accessible to interactions with halothane; the second inaccessible to halothane on the time scale of the pyrene fluorescence excited state. Both populations of pyrene are affected by the presence of halothane in the membrane. The rate of halothane quenching of pyrene fluorescence is increased significantly for all DPPG/DPPC membranes compared to pure DPPC membranes indicating that any DPPG in the membrane facilitates interaction between halothane and pyrene even though the measured partition coefficients indicate that little change in total halothane concentration in the membrane as a whole occurs as a function of percent DPPG in DPPG/DPPC mixtures.
It is speculated that phase boundaries play an important role in determining the behavior of this model system by determining the location of the pyrene probe. The heavy atom effect quenching of pyrene by halothane provides a useful probe of phase boundaries in membranes.     

Nitroanilines as Quenchers of Pyrene Fluorescence

The quenching of pyrene and 1‐methylpyrene fluorescence by nitroanilines (NAs), such as 2‐, 3‐, and 4‐nitroaniline (2‐NA, 3‐NA, and 4‐NA, respectively), 4‐methyl‐3‐nitroaniline (4‐M ‐3‐NA), 2‐methyl‐4‐nitroaniline (2‐M‐4‐NA), and 4‐methyl‐3,5‐dinitroaniline (4‐M‐3,5‐DNA), are studied in toluene and 1,4‐dioxane. Steady‐state fluorescence data show the higher efficiency of the 4‐NAs as quenchers and fit with a sphere‐of‐action model. This suggests a 4‐NA tendency of being in close proximity to the fluorophore, which could be connected with their high polarity/hyperpolarizability. In addition, emission and excitation spectra evidence the formation of emissive pyrene—NA ground‐state complexes in the case of the 4‐NAs and, in a minor degree, in the 2‐NA. Moreover, time‐resolved fluorescence experiments show that increasing amounts of NA decrease the pyrene fluorescence lifetime to a degree that depends on the NA nature and is larger in the less viscous solvent (toluene). Although the NA absorption and the pyrene (Py) emission overlap, we found no evidence of dipole–dipole energy transfer from the pyrene singlet excited state (¹Py) to the NAs; this could be due to the low NA concentration used in these experiments. Transient absorption spectra show that the formation of the pyrene triplet excited state (³Py) is barely affected by the presence of the NAs in spite of their efficiency in ¹Py quenching, suggesting the involvement of ¹Py—NA exciplexes which—after intersystem crossing—decay efficiently into ³Py.     

Temperature effect on the vapor-phase exciplexes of cyano-substituted anthracenes

The emission spectra and lifetimes of the vapor-phase exciplexes of four cyano-substituted anthracenes with N,N-dimethylaniline (DMA) as a donor are examined over a wide temperature range. The activation energies associated with the exciplex dissociation are calculated to be of the order of 10 kcal/mol. The entropy change in forming the exciplexes is discussed in relation to the efficiency of the exciplex formation. For various donors other than DMA, preliminary results on how they interact with excited 9,10-dicyanoanthracene are also given.     

Exciplex fluorescence emission from simple organic intramolecular constructs in non-polar and highly polar media as model systems for DNA-assembled exciplex detectors

Organic intramolecular exciplexes, N-(4-dimethylaminobenzyl)-N-(1-pyrenemethyl)amine (1) and N'-4-dimethylaminonaphthyl-N-(1-pyrenemethyl)amine (2), were used as model systems to reveal major factors affecting their exciplex fluorescence, and thus lay the basis for developing emissive target-assembled exciplexes for DNA-mounted systems in solution. These models with an aromatic pyrenyl hydrocarbon moiety as an electron acceptor appropriately connected to an aromatic dimethylamino electron donor component (N,N-dimethylaminophenyl or N,N-dimethylaminonaphthyl) showed strong intramolecular exciplex emission in both non-polar and highly polar solvents. The effect of dielectric constant on the maximum wavelength for exciplex emission was studied, and emission was observed for 1 and 2 over the full range of solvent from non-polar hydrocarbons up to N-methylformamide with a dielectric constant of 182. Quantum yields were determined for these intramolecular exciplexes in a range of solvents relative to that for Hoechst 33,258. Conformational analysis of 1 was performed both computationally and via qualitative 2D NMR using (1)H-NOESY experiments. The results obtained indicated the contribution of pre-folded conformation(s) to the ground state of 1 conducive to exciplex emission. This research provides the initial background for design of self-assembled, DNA-mounted exciplexes and underpins further development of exciplex-based hybridisation bioassays.     

Experimental study of singlet-triplet energy transfer in liquid solutions

It has been found that fluorescence quenching of 9,10-dibromo-,9,10-dichloro-and 9,10-dicyano-anthracene by aromatic hydrocarbons is due not to singlet-triplet energy transfer but to donor-acceptor interactions leading to the formation of short-lived exciplexes. The 9,10-dicyanoanthracene exciplexes fluoresce and their radiationless deactivation occurs via population of the dicyanoanthracene triplet state. The decay of exciplexes of halo-anthracenes with arenes due to the heavy-atom effect occurs exclusively via intersystem crossing to the halo-anthracene triplet state.     

INTRAMOLECULAR EXCIMER FLUORESCENCE: A NEW PROBE OF PHASE TRANSITIONS IN SYNTHETIC PHOSPHOLIPID MEMBRANES

Abstract— The molecule (1,1'-dipyrenyl)-methyl ether [dipyme] shows intramolecular excimer fluorescence in competition with fluorescence from the locally excited pyrene chromophore. This intensity ratio I_c/I_m is sensitive to solvent viscosity. The molecule is soluble in synthetic phospholipid membranes. ¹H NMR studies suggest that the molecule is localized in the hydrocarbon region of the membrane. Fluorescence measurements at various temperatures of dipyme dissolved in these membranes show that I_c/I_m is exceedingly sensitive to fluidity changes accompanying both the pretransitions and the gel to liquid crystalline transitions of the membrane. These studies can be carried out at a mole ratio of probe to lipid 10²-10³ smaller than that necessary to observe bimolecular pyrene excimer formation.     

Exploring the organization and dynamics of hippocampal membranes utilizing pyrene fluorescence

The organization and dynamics of cellular membranes in the nervous system are crucial for the function of neuronal membrane receptors and signal transduction. Previous work from our laboratory has established hippocampal membranes as a convenient natural source for studying neuronal receptors. In this paper, we have monitored the organization and dynamics of hippocampal membranes and their modulation by cholesterol using pyrene fluorescence. The apparent dielectric constant experienced by pyrene in hippocampal membranes turns out to be approximately 20+/-3, depending on the experimental condition. Our results show that the polarity of the hippocampal membrane is increased upon cholesterol depletion, as monitored by changes in the ratio of pyrene vibronic peak intensities (I1/I3). This is accompanied by an increase in lateral diffusion, measured as an increase in the pyrene excimer/monomer ratio. These results are relevant in understanding the complex organization and dynamics of hippocampal membranes and could have implications in neuronal diseases characterized by defective cholesterol metabolism.     

Biphasic photochemistry: time-resolved spectra of adsorbed hydrocarbons

Time-resolved fluorescence spectra were measured for pyrene and naphthalene adsorbed on a silica gel surface using the time-correlated photon-counting technique Excimer-Iike emission was observed within an excitation lamp duration of a few nanoseconds with both pyrene and naphthalene. It is suggested, from the excitation spectra, that ground state complexes of the adsorbates are responsible for the formation of the excimer-like emission.     

Lifetimes of partial charge transfer exciplexes of 9-cyanophenanthrene and 9-cyanoanthracene

The fluorescence decays of several exciplexes with partial charge transfer have been investigated in solvents of various polarity. The measured lifetimes are found to be in reasonable agreement with the activation enthalpy and entropy of exciplex decay obtained earlier from the temperature dependence of the exciplex emission quantum yields. For exciplexes with 9-cyanophenanthrene substantial contribution of the higher local excited state into the exciplex electronic structure is found and borrowed intensity effect enhances the exciplex emission rate constants.     

Investigation of geminate recombination of radical ion pairs generated by dissociation of exciplexes in moderately polar solvents using the photoconductivity technique

A new approach to determination of the recombination rate of radical ion pairs in moderately polar solvents is presented. It is based on an investigation of transient photocurrents caused by dissociation of exciplexes generated in photoinduced electron transfer reactions. It has been shown that the recombination rate of geminate ion pairs can be found from the photocurrent rise time. We have applied such an approach to transient photocurrents observed by Hirata et al. [Y. Hirata, Y. Kanda, N. Mataga, J. Phys. Chem. 87 (1983) 1659] for the pyrene/dicyanobenzene system in solvents of moderate polarity. The increase of the obtained recombination rate of photogenerated ions with increasing polarity of solvent testifies that ions recombine mainly by the backward electron transfer from the dicyanobenzene anions to solvent-separated cations of pyrene.     

Exciplex luminescence studies of the surface polarity of end-capped bonded phases

The fluorescence intensities and emission maxima of surface-bound exciplexes were studied to determine the relative polarity of microparticulate silica before and after derivatization with trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDS). The results indicate that HMDS yields a bonded phase with greater average polarity relative to TMCS at equivalent surface coverages. This difference is attributed to the formation of basic sites on the surface by chemisorption of ammonia generated during derivatization with HMDS. The observation of exciplex emission on silica gel is novel and strongly supports clustering of surface-bound molecules into high-density regions.     

Computer simulation study on the structural-optical related properties of a pyrene-functionalized fluorescent film

Molecular dynamics simulations were carried out to investigate the structural properties and the structure related optical properties of a pyrene-functionalized film and also the film in the presence of nitrobenzene. It has been shown that at equilibrium (1) the pyrene molecules of the film, of which the fluorophore molecules have been attached to a glass plate surface via relatively long flexible spacers, adopt quasi-coplanar structures; (2) the distance between a pair of pyrene rings populates from 4 A to longer than 10 A, but quite a large number of pyrene molecules populate within distances between 4 and 5 A, indicating that the fluorescence of the film should be characterized by both monomer emission and excimer emission; (3) introduction of nitrobenzene into the system results in a decrease of the population of pyrene molecules within the distances suitable for the formation of excimers, suggesting that excimer emission of the film would be decreased; and (4) the incoming nitrobenzene molecules insert themselves in between the previously formed coplanar structures of pyrene rings and form a complex with one of them. Considering that nitrobenzene is an electron-poor compound and nonfluorescent and that pyrene is an electron-rich one, it is expected that the formation of the complex must result in an excitation transfer from pyrene to nitrobenzene, provided that the distance between them and the orientations of them are reasonable. This indicates that the introduction of nitrobenzene not only decreases the number of excimers of pyrene but also quenches the monomer emission of the fluorophore. All the expectations from the simulation studies are basically consistent with the experimental observations.     

Site-selective and stereoselective intramolecular (2pi + 2pi) photocycloaddition of arylalkenes to pyrene and its photocycloreversion

Irradiation of a benzene solution containing methyl p-(1-pyrenylmethoxymethyl)cinnamate (1a) with a high-pressure Hg lamp through Pyrex filter stereoselectively gave an intramolecular (2pi + 2pi) photocycloadduct (2a) in an 83% yield in a site-selective manner at the 4,5-position of the pyrene ring. Similar irradiation of an ortho-substituted derivative (3) afforded the corresponding (2pi + 2pi) cycloadduct (4) as a sole product at the 9,10-position of pyrene. The site-selective photocycloaddition can be reasonably explained by the intramolecular sandwich-type singlet exciplexes between the pyrene and phenyl rings.     

Unusual fluorescence spectral response of 1-(4-N,N-dimethylaminophenylethynyl)pyrene towards the thermotropic phase change in lipid bilayer membranes

The applicability of 1-(4-N,N-dimethylaminophenylethynyl)pyrene (DMAPEPy), a pyrene derivative showing intramolecular charge transfer, as a prospective probe for lipid bilayer membranes has been evaluated. High sensitivity of DMAPEPy to solvent polarity and viscosity makes it to act both as a polarity-sensitive probe and as a fluorescence anisotropy probe. The molecule shows high partition efficiency towards bilayer membranes in both solid gel as well as in the liquid crystalline phases. The emission spectrum, quenching experiment and lifetime data suggest bimodal distribution of DMAPEPy in the bilayer. Using the solvent polarity scales the polarity parameters of the two locations in lipid bilayer have been estimated. In the bilayer environment it exhibits remarkable spectral changes with temperature. The thermotropic phase change of the bilayer is sensitively monitored by fluorescence intensity as well as fluorescence anisotropy parameters. DMAPEPy is also capable of sensing the changes induced by membrane modifiers like cholesterol.     

Bonded exciplex formation: electronic and stereoelectronic effects

As recently proposed, the singlet-excited states of several cyanoaromatics react with pyridine via bonded-exciplex formation, a novel concept in photochemical charge transfer reactions. Presented here are electronic and steric effects on the quenching rate constants, which provide valuable support for the model. Additionally, excited-state quenching in poly(vinylpyridine) is strongly inhibited both relative to that in neat pyridine and also to conventional exciplex formation in polymers, consistent with a restrictive orientational requirement for the formation of bonded exciplexes. Examples of competing reactions to form both conventional and bonded exciplexes are presented, which illustrate the delicate balance between these two processes when their reaction energetics are similar. Experimental and computational evidence is provided for the formation of a bonded exciplex in the reaction of the singlet excited state of 2,6,9,10-tetracyanoanthracene (TCA) with an oxygen-substituted donor, dioxane, thus expanding the scope of bonded exciplexes.     

Emission from aromatic hydrocarbon-diene and -olefin exciplexes

Luminescence from aromatic hydrocarbon-olefin and -diene exciplexes, providing strong evidence for their intermediary in singlet quenching processes, is reported. Solvent dependence of the emission maximum gives a value of 10.8D for the dipole moment of the 1-cyanonaphthalene-1,2-dimethylcyclopentene exciplex while the temperature dependence affords a value of 6.7 kcal/M for the heat of formation. Linear-free-energy correlations of rate constants for 1-cyanonaphthalene and naphthalene fluorescence quenching by dienes and olefins and strained hydrocarbons with the adiabatic ionization potentials of the quenchers are consistent with major contributions from charge-resonance in the exciplex formation process.