EXCIPLEX STUDIES–V. ELECTRON EJECTION FROM INDOLE AND METHYL-INDOLE DERIVATIVES*

Abstract— Electron-scavenging experiments with N₂O as scavenger demonstrate at least two electron-producing reactions of the excited singlet states of the exciplex species formed by indole or 1 -methyl-indole with water. Most electrons reacting with N₂O result from collision of the scavenger with a metastable state formed from the initial exciplex state but finite electron yields from indole and 1-methyl-indole at limiting scavenger concentrations suggest that the intermediate states also eject electrons directly into the solvent. The formation of the first metastable state from the fluorescent exciplex state has an activation energy, E_M, estimated to be about 13 kcal/mole for both indole and 1 -methyl-indole water exciplexes. The E_M values for 1-methyl-indole from fluorescence and electron yields are the same, Indicating that at neutral and alkaline pH fluorescence quenching and electron extraction are both being controlled by the formation of the first metastable intermediate. Observed electron yields from indole-water and indole-methanol exciplexes are less than predicted using fluorescence data, although E_M values of 1 kcal/mole are obtained for the indole-methanol exciplex by both methods. At pH 12·0 and 28°C the total electron yields for indole-water and 1 -methyl-indole-water exciplexes are 0·30 and 0·25, respectively. The residual yields attributed to outright formation of hydrated electrons from the initial exciplex excited stateare 0·11 and 0·05, respectively. Electron yields from the indole-water exciplex are strongly pH dependent only near pH 1 where the fluorescence yields as well as the electron yields decrease rapidly with increasing acidity. The 1-methyl-indole-water exciplex shows an additional pH dependence which is first-order in hydrogen-ion activity and has an effective pK_a of about 11·5. Comparable yields for indole and 1-methyl-indole are found only above pH 12. High electron yields are found with indole in the exciplex-forming solvent dioxane and in the non-exciplex forming solvent cyclohexane. For the latter system electrons are probably derived only from the lowest excited state of indole on collision with N₂O.     

Fluorescence quenching of anthracene by indole derivatives in phospholipid bilayers

The quenching of anthracene fluorescence by indole, 1,2-dimethylindole (DMI), tryptophan (Trp) and indole 3-acetic acid (IAA) in palmitoyloleoylphosphatidylcholine (POPC) lipid bilayers was investigated. A very efficient quenching of the anthracene fluorescence in the lipid membrane is observed. Stern-Volmer plots are linear for DMI but present a downward curvature for the other quenchers. This was interpreted as an indication of the presence of an inaccessible fraction of anthracene molecules. By a modified Stern-Volmer analysis the fraction accessible to the quenchers and the quenching constant were determined. The changes in the fluorescence emission spectrum of indole and DMI have been used to calculate the partition constants of these probes into the membranes, and bimolecular quenching rate constants were determined in terms of the local concentration of quencher in the lipid bilayer. The rate constants are lower than those in homogeneous solvents, which may be ascribed to a higher viscosity of the bilayer. No changes in the emission spectra of Trp and IAA are observed in the presence of vesicles, indicating that these probes locate preferentially in the aqueous phase, or in close proximity to the vesicular external interface in a medium resembling pure water. In these cases quenching rate constants were determined in terms of the analytical concentration. In the quenching by DMI a new, red shifted, emission band appears; it is similar to that observed in non-polar solvents and it is ascribable to an exciplex emission. The exciplex band is absent in the quenching by IAA and Trp and only very weakly present when the quencher is indole. From the position of the maximum of the exciplex emission, a relatively high local polarity could be estimated for the region of the bilayer where the quenching reaction takes place.     

Hydrogen-bonding and Proton Transfer Interactions between Harmane and Trifluoroethanol in the Ground and Excited Singlet States

A study of the hydrogen-bonding and proton transfer reactions of the ground and excited states of harmane (1-methyl-9H-pyrido/3,4-b/indole) and its N ₉-methyl derivative with 2,2,2-trifluoroethanol in cyclohexane is reported. Spectral measurements (UV–visible, Fourier trans-form IR, steady-state and time-resolved fluorescence) show the formation of fluorescent ground-state hydrogen-bonded complexes. The results have been interpreted assuming a tautomeric equilibrium between a 1:1 hydrogen-bonded complex and its 1:2 proton transfer tautomer (hydrogen-bonding ion pair). Upon excitation to its singlet excited state, the proton transfer tautomer of harmane reacts with an additional 2,2,2-trifluoroethanol molecule to give a zwitterionic exciplex, which fluoresces at longer wavelength.     

Dynamic study of excited state hydrogen-bonded complexes of harmane in cyclohexane-toluene mixtures

Photoinduced proton transfer reactions of harmane or 1-methyl-9H-pyrido[3,4-b]indole (HN) in the presence of the proton donor hexafluoroisopropanol (HFIP) in cyclohexane-toluene mixtures (CY-TL; 10% vol/vol of TL) have been studied. Three excited state species have been identified: a 1:2 hydrogen-bonded proton transfer complex (PTC), between the pyridinic nitrogen of the substrate and the proton donor, a hydrogen-bonded cation-like exciplex (CL*) with a stoichiometry of at least 1:3 and a zwitterionic exciplex (Z*). Time-resolved fluorescence measurements evidence that upon excitation of ground state PTC, an excited state equilibrium is established between PTC* and the cationlike exciplex, CL*, lambdaem approximately/= 390 nm. This excited state reaction is assisted by another proton donor molecule. Further reaction of CL* with an additional HFIP molecule produces the zwitterionic species, Z*, lambda(em) approximately/= 500 nm. From the analysis of the multiexponential decays, measured at different emission wavelengths and as a function of HFIP concentration, the mechanism of these excited state reactions has been established. Thus, three rate constants and three reciprocal lifetimes have been determined. The simultaneous study of 1,9-dimethyl-9H-pyrido[3,4-b]indole (MHN) under the same experimental conditions has helped to understand the excited state kinetics of these processes.     

A fluorescent chemosensor for Zn(II). Exciplex formation in solution and the solid state

The macrocyclic phenanthrolinophane 2,9-[2,5,8-triaza-5-(N-anthracene-9-methylamino)ethyl]-[9]-1,10-phenanthrolinophane (L) bearing a pendant arm containing a coordinating amine and an anthracene group forms stable complexes with Zn(II), Cd(II) and Hg(II) in solution. Stability constants of these complexes were determined in 0.10 mol dm(-3) NMe(4)Cl H(2)O-MeCN (1:1, v/v) solution at 298.1 +/- 0.1 K by means of potentiometric (pH metric) titration. The fluorescence emission properties of these complexes were studied in this solvent. For the Zn(II) complex, steady-state and time-resolved fluorescence studies were performed in ethanol solution and in the solid state. In solution, intramolecular pi-stacking interaction between phenanthroline and anthracene in the ground state and exciplex emission in the excited state were observed. From the temperature dependence of the photostationary ratio (I(Exc)/I(M)), the activation energy for the exciplex formation (E(a)) and the binding energy of the exciplex (-DeltaH) were determined. The crystal structure of the [ZnLBr](ClO(4)).H(2)O compound was resolved, showing that in the solid state both intra- and inter-molecular pi-stacking interactions are present. Such interactions were also evidenced by UV-vis absorption and emission spectra in the solid state. The absorption spectrum of a thin film of the solid complex is red-shifted compared with the solution spectra, whereas its emission spectrum reveals the unique featureless exciplex band, blue shifted compared with the solution. In conjunction with X-ray data the solid-state data was interpreted as being due to a new exciplex where no pi-stacking (full overlap of the pi-electron cloud of the two chromophores - anthracene and phenanthroline) is observed. L is a fluorescent chemosensor able to signal Zn(II) in presence of Cd(II) and Hg(II), since the last two metal ions do not give rise either to the formation of pi-stacking complexes or to exciplex emission in solution.     

FLUORESCENCE QUENCHING OF INDOLES BY N,N-DIMETHYLFORMAMIDE

Abstract. The quenching of the excited state of indoles by small polar molecules is believed to occur via the formation of excited state complexes. The rate of excited state complex formation of indoles with dimethylformamide is shown to be a function of the group appended to the indole ring at the 3- or 5-position. Substituents which can strengthen the charge transfer character of the exciplex are observed to enhance the rate of fluorescence quenching. In contrast to other amides, dimethylformamide appears to be a moderately good quencher of indole fluorescence, and may be a useful probe for proteins.     

Exciplex-type behavior and partition of 3-substituted indole derivatives in reverse micelles made with benzylhexadecyldimethylammonium chloride, water and benzene

The fluorescence properties of 3-methylindole (MI), 3-indoleacetic acid (IAA), 3-indoleethyltrimethylammonium bromide (IETA), L-tryptophan (Trp) and tryptamine hydrochloride (TA) were studied in reverse micelles solutions made with the cationic surfactant benzylhexadecyldimethylammonium chloride (BHDC) in benzene as a function of the molar ratio water/surfactant R (= [H2O]/[BHDC]). The fluorescence quenching of the model compound MI by benzene in cyclohexane solutions and by BHDC in benzene solutions were also studied in detail. The fluorescence of MI in benzene is characteristic of a charge-transfer exciplex. The exciplex is quenched by the presence of BHDC, due to the interactions of the surfactant ion pairs with the polar exciplex. In reverse micelle solutions at low R values, all the indoles show exciplex-type fluorescence. As R increases, the fluorescence behavior strongly depends on the nature of the indole derivative. The anionic IAA remains anchored to the cationic interface and its fluorescence is quenched upon water addition due to the increases of interface's micropolarity. For IETA, TA and Trp an initial fluorescence quenching is observed at increasing R, but a fluorescence recovery is observed at R > 5, indicating a probe partition between the micellar interface and the water pool. For the neutral MI, the fluorescence changes with R indicate the partition of the probe between the micellar interface and the bulk benzene pseudophase. A simple two-site model is proposed for the calculation of the partition constants K as a function of R. In all cases, the calculation showed that even at the highest R value, about 90% of the indole molecules remain associated at the micellar interface.     

Photophysics of anthracene-indole systems in unilamellar vesicles of DMPC and POPC: Exciplex formation and temperature effects

The quenching of anthracene fluorescence by indole (IN), 1,2-dimethylindole (DMI), tryptophan (Trp) and indole 3-acetic acid (IAA) in dimiristoylphophatidylcholine (DMPC) and palmitoyloleoylphosphatidylcholine (POPC) lipid bilayers was investigated. The studies were carried out at 25 degrees C in POPC vesicles and below (15 degrees C) and above (35 degrees C) the phase transition temperature (24 degrees C) of DMPC. A very efficient quenching of the anthracene fluorescence by IN and DMI in the lipid membrane is observed in all cases. It is less efficient in the case of Trp and IAA. Stern-Volmer plots are linear for DMI but present a downward curvature for the other quenchers. This was interpreted as an indication of the presence of an inaccessible fraction of anthracene molecules. By a modified Stern-Volmer analysis the fraction accessible to the quenchers and the quenching constant were determined. Partition constants of the quenchers were obtained from the changes in the fluorescence emission of the indole moiety caused by the presence of the phospholipid. Using the partition constants bimolecular quenching rate constants were determined in terms of the local concentration of quencher in the lipid bilayer. These corrected rate constants are lower than those in homogeneous solvents. In the case of DMPC values the gel phase are higher than in the liquid-crystalline phase. In the quenching by IN and DMI a new, red shifted, emission band appears which could be assigned to an exciplex emission. The exciplex band is absent in the quenching by IAA and Trp.     

Quencing of perylene fluorescence by Ag+ ions via exciplex formation

The fluorescence of perylene in alcoholic solution is dynamically quenched by AgClO₄ resulting in a new emission band with its maximum around 480 nm. Analysis of fluorescence decay curves at various emission wavelengths proves the formation of a perylene/Aa⁺ ion exciplex in the quenching reaction.     

Magnetic field effect in fluorescence of excited fluorophore equilibrated with exciplex that reversibly dissociates into radical-ion pair undergoing the spin conversion

The fluorescence of the photoexcited electron acceptor, (1)A?, and the exciplex, (1)[D(+δ)A(-δ)] formed at contact of (1)A? with an electron donor (1)D, is known to be very sensitive to a magnetic field, assisting the spin conversion in the resulting geminate radical ion pair (RIP), (1, 3)[D(+)...A(-)]. The relative increase of the fluorescence in the highest magnetic field compared to the lowest one, known as the magnetic field effect, crucially depends on the dielectric constant of the solvent, ?. This phenomenon first studied experimentally is at first reproduced here theoretically by means of the so called integral encounter theory. It was shown to be very sensitive to the position of the exciplex energy level relative to the levels of exciplex precursors and the charged products of its dissociation. The results obtained strongly depend on the dielectric properties of the solvents as well as on the exciplex and RIP formation rates.     

无色孔雀绿光氧化反应的途径

前一篇文章已说明,无色孔雀绿(LMG)经光氧化生成孔雀绿(MG⁺)染料是一个复杂的光化学过程。萘加入反应溶液猝灭了LMG的激发态,但却使反应速率升高。对这一实验结果我们曾推断为:可能是由于萘与LMG之间发生了电子转移作用。本文将对这一推断作进一步实验论证,并将说明在没有电子受体参加时,LMG的光氧化主要通过其激发三线态途径进行。已得到的实验结果证明,我们的上述推断是正确的,现予以报道。     

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.     

Exciplex Emission from a Boron Dipyrromethene (Bodipy) Dye Equipped with a Dicyanovinyl Appendage

The photophysical properties of a prototypic donor–acceptor dyad, featuring a conventional boron dipyrromethene (Bodipy) dye linked to a dicyanovinyl unit through a meso‐phenylene ring, have been recorded in weakly polar solvents. The absorption spectrum remains unperturbed relative to that of the parent Bodipy dye but the fluorescence is extensively quenched. At room temperature, the emission spectrum comprises roughly equal contributions from the regular π, π* excited‐singlet state and from an exciplex formed by partial charge transfer from Bodipy to the dicyanovinyl residue. This mixture moves progressively in favor of the locally excited π, π* state on cooling and the exciplex is no longer seen in frozen media; the overall emission quantum yield changes dramatically near the freezing point of the solvent. The exciplex, which has a lifetime of approximately 1 ns at room temperature, can also be seen by transient absorption spectroscopy, in which it decays to form the locally excited triplet state. Under applied pressure (P<170 MPa), formation of the exciplex is somewhat hindered by restricted rotation around the semirigid linkage and again the emission profile shifts in favor of the π, π* excited state. At higher pressure (170<P<550 MPa), the molecule undergoes reversible distortion that has a small effect on the yield of π, π* emission but severely quenches exciplex fluorescence. In the limiting case, this high‐pressure effect decreases the molar volume of the solute by approximately 25 cm³ and opens a new channel for nonradiative deactivation of the excited‐state manifold.     

Excitation conversion in excimers and exciplexes of aromatic hydrocarbons

Flurorescence spectra of a number of aromatic hydrocarbons, their excimers and exciplexes with diethylaniline have been measured in n-hexane under aerobic conditions. It has been found that the value of the red shift of the structureless excimer or exciplex band in reference to the monomer fluorescence band is about 4200 or 3000 cm⁻¹, respectively. For indole—ethanol exciplexes the red shift was about 3700 cm⁻¹. It is suggested, that the shift depends on the intramolecular vibrations in the quencher accepting the vibrational energy from the electronically excited donor molecule. It has been proposed that excimer and exciplex fluorescence occurs due to the donor emission of a number of quanta from the virtual levels.     

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.     

N-n-butyl-N,N-di(2-naphthylmethyl)amine: A compound exhibiting intramolecular excimer and exciplex formation

The dinaphthylamine (3) is unique in that it exhibits both intramolecular excimer and exciplex formation. The relative amounts of each is dependent upon the solvent polarity. Highly polar solvents, e.g. methanol, lead to the emission being entirely that of the exciplex but addition of acid to these solutions prevents exciplex formation and only excimer is observed.     

Study on the Photophysical Process in N-Phenyl Phenothiazine and N-(2-Pyridine) Phenothiazine

By the measurements of the solvent and temperature effects of the fluorescence emission spectra and lifetime of N-phenyl phenothiazine (PHZ) and N-(2-pyridine) phenothiazine (PYZ),the existence of strongly twisted intramolecular charge transfer (TICT) state in PYZ emission spectra is proved.The exciplex formed by PHZ, PYZ and dimethyl terephthalate (DMTP) is investigated, and moreover, the transient absorption spectra of positive and negative ions as the results of the dissociation of the exciplex in the polar solvents is observed through the flash photolysis.     

Etudes photochimiques—XIII : Reactions de photoaddition du benzo[b]thiophene avec des amines et avec le pyrrole

The photochemical behavior of benzo[b]thiophene is different from that of thiophene. The former does not undergo photoisomerization and is not converted into an indole when irradiated in the presence of a primary amine; its photochemical behavior resembles that of naphthalene. Thus photoexcited benzo[b]thiophene gives adducts with primary amines (1(H)), secondary amines (2), and pyrrole (3(H)): an exciplex is an intermediate. The existence of the exciplex is supported by the formation of aldehyde 7 when benzo[b]thiophene is irradiated in aqueous propylamine, the finding that photoexcited benzo[b]thiophene does not react with an alcohol and that photoexcited indole benzo[b]furan, 2-methylbenzo[b]thiophene, and 3-methylbenzo[b]thiophene do not react with propylamine or with piperidine. The results are interpreted in terms of spin densities calculated for the anion radicals of the compounds under study.     

Exciplex formation and energy transfer in a self-assembled metal-organic hybrid system

Exciting assemblies: A metal-organic self-assembly of pyrenebutyric acid (PBA), 1,10-phenanthroline (o-phen), and Mg(II) shows solid-state fluorescence originating from a 1:1 PBA-o-phen exciplex. This exciplex fluorescence is sensitized by another residual PBA chromophore through an excited-state energy-transfer process. The solvent polarity modulates the self-assembly and the corresponding exciplex as well as the energy transfer, resulting in tunable emission of the hybrid (see figure).     

外围萘修饰PAMAM树枝形聚合物的合成及其质子化过程研究

合成了外围修饰有萘基团的0～3代聚酰胺-胺树枝形聚合物GnN (n＝0～3), 化合物通过了IR, 1H NMR, 13C NMR和MALDI TOF的表征. 稳态光物理研究表明, 甲醇溶液中GnN外围萘基团与骨架胺之间发生电子转移过程, 形成最大发射峰在450 nm的激基复合物, 萘的荧光被明显猝灭; 当GnN骨架被质子化, 分子内光致电子转移过程和萘与骨架胺基间激基复合物的形成被抑制, 萘单体荧光发射大大增强; 由于质子化后树枝形聚合物骨架趋于伸展构象, 外围萘基团间相互作用增强, 部分形成最大发射峰在400 nm的激基缔合物.