Femtosecond-Picosecond Laser Photolysis Studies on Photoinduced Electron Transfer Phenomena in Solutions

Results of our femtosecond-picosecond laser photolysis studies on photoinduced electron transfer phenomena in solutions including exciplex dynamics and its solvent dependences, energy gap dependences of photoinduced charge separation and charge recombination of various geminate ion pairs, mechanisms of chemical reactions via exciplexes and ion pairs, dynamics of photoinduced election transfer in hydrogen bonding complexes, dynamics and mechanisms of photoinduced electron transfer in fixed distance donor acceptor dyads and photosynthetic reaction center models, and mechanisms of electron ejection from solute fluorescent state in polar solutions are summarized and discussed.     

Laser photolysis study of chloranil triplet exciplexes with stilbene

Absorption spectra and decay kinetics of the polar triplet exciplexes (contact radical-ion pairs) formed during quenching of the chloranil triplet state by trans- or cis-stilbenes in benzene with added acetonitrile and methanol, have been studied by laser flash photolysis. The exciplexes include cation-radicals of stilbene dimers, which are deactivated by reverse electron transfer within 10–50 nsec. The dynamics of the intercombination electron transfer and the exciplex dissociation into ion-radicals were determined. The isomerization of stilbene via triplet exciplex formation was not observed.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow 117977. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 572–576, March, 1992.     

Laser photolysis study of the spectral-kinetic characteristics of short-lived triplet exciplexes and the mechanism of atomic hydrogen transfer in the course of 2,6-diphenyl-1,4-benzoquinone triplet quenching with aromatic amines

The spectral-kinetic characteristics of short-lived triplet exciplexes arising in the quenching of 2,6-diphenyl-1,4-benzoquinone triplet with aromatic amines: N,N,N′,N′-tetramethyl-p-phenylenediamine, triphenylamine and diphenylamine have been studied by means of the nanosecond laser photolysis technique. The absorption spectra of triplet exciplexes exhibit distinct maxima characteristic of the absorption spectra of corresponding amine radical cations. The state with complete charge transfer gives the basic contribution to the exciplex structures. A detailed analysis is presented of the kinetic and thermodynamic deactivation characteristics of triplet exciplexes in low-polar solvents.     

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.     

Donor-acceptor complexes of excited states on silica surface and in hexane solution at liquid nitrogen temperature

It is shown that an exciplex between pyrene (Py) and N,N-dimethylaniline (DMA) forms at low temperature (77 K) on the nonporous silica surface and in a solid hexane solution. The fluorescence lifetime of pyrene in the adsorption state increases when the quencher concentration grows at 77 K. The exciplex fluorescence spectra in the adsorption state shifts to a blue wavelength range when the temperature decreases as a result of the dipole-dipole interaction of the exciplex with the hydroxy groups of the silica surface. On the basis of the model suggested the hydroxyl group concentration on the dehydrated at 770 K silica surface has been estimated.     

Effect of Solvent Polarity on the Electronic Structure and Emission Frequencies of Exciplexes of Aromatic Hydrocarbons with Methoxybenzenes and Methylnaphthalenes

The dependence of exciplex emission spectra on the solvent polarity was studied for exciplexes with the Gibbs free energy of excited-state electron transfer, G ^* _et , exceeding –0.1 eV (for pyrene, fluoranthene, 1,12-benzoperylene, and 9-cyanoanthracene with methoxybenzenes or dimethylnaphthalene). These exciplexes showed stronger changes in the spectral shift of exciplex emission and the extent of charge transfer with increasing solvent polarity than the exciplexes having more negative G ^* _et values. The parameters (difference in energy of charge transfer (CT) and locally excited (LE) states in a vacuum, (H ⁰ ₂₂– H ⁰ ₁₁), and the matrix element for electronic coupling of CT and LE states H ₁₂and mrelated to the dipole moment of the CT state and the size of the exciplex) determining the extent of charge transfer, the spectral shift, and other properties of exciplexes were evaluated. The parameters H ₁₂and mfor the exciplexes examined fall in the interval 0.1–0.5 and 1.0–1.7 eV, respectively, and the difference (H ⁰ ₂₂– H ⁰ ₁₁) is proportional to G ^* _et .     

New conducting pyrrole–thiophene co-polymer from an oligomer precursor: electrochemical characterisation

A new oligomer of N,N′-bis(2-pyrrolyl methylene)-3,4-dicyano-2,5-diaminothiophene possessing cyano-substituted thiophene and pyrrole residues linked together by azomethine groups was used for the electrochemical polymerisation of conducting films. The approach used for the oligomer design favours inter-chain interactions through hydrogen bonding and negative charge stability through the cyano substitute thiophene. The oligomer was successfully electropolymerised at 0.67 V vs Ag⁺/Ag from 0.1M tetrabutylammonium tetrafluoroborate (TBABF₄)/acetonitrile as a dark blue film on the surface of platinum electrodes. Cyclic voltammetry has been used to investigate the redox behaviour of the films. The electrically conducting polymer showed p-doping/neutralisation behaviour. The effect of different electrolytes such as TBABF₄, tetrabutylammonium perchlorate (TBAClO₄), lithium perchlorate (LiClO₄) and sodium perchlorate (NaClO₄) on the redox switching and the stability of the polymer films was investigated. Infrared and UV-vis spectra of oligomer and polymers are presented. The evolution of the film growing process is shown by UV-vis spectroscopy.     

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.     

Mechanism of Back Electron Transfer in an Intermolecular Photoinduced Electron Transfer Reaction: Solvent as a Charge Mediator

Back electron transfer (BET) is one of the important processes that govern the decay of generated ion pairs in intermolecular photoinduced electron transfer reactions. Unfortunately, a detailed mechanism of BET reactions remains largely unknown in spite of their importance for the development of molecular photovoltaic structures. Here, we examine the BET reaction of pyrene (Py) and 1,4‐dicyanobenzene (DCB) in acetonitrile (ACN) by using time‐resolved near‐ and mid‐IR spectroscopy. The Py dimer radical cation (Py₂^.+) and DCB radical anion (DCB^.?) generated after photoexcitation of Py show asynchronous decay kinetics. To account for this observation, we propose a reaction mechanism that involves electron transfer from DCB^.? to the solvent and charge recombination between the resulting ACN dimer anion and Py₂^.+. The unique role of ACN as a charge mediator revealed herein could have implications for strategies that retard charge recombination in dye‐sensitized solar cells.     

Leitfähigkeitsuntersuchungen in Hexamethylphosphorsäuretriamid und Propandiol-1,2-carbonat

Limiting conductances, dissociation constants and ion conductances have been determined for a number of tetrabutylammonium and lithium salts in the solvents hexamethylphosphoric triamide (HMPT) and propanediol-1.2-carbonate (PDC). LiCl and LiBr are completely dissociated inHMPT, but are associated inPDC. The reverse behaviour is found for the corresponding tetrabutylammonium salts. Conductance measurements show thatHMPT is a strongly differentiating solvent for the donor strength of halide and perchlorate ions.     

Thermodynamic investigation of salting effect of ovalbumin in various salts solutions

Calorimetric and volumetric investigations of ovalbumin salting by sodium chloride, lithium chloride, potassium chloride, lithium sulfate and ammonium sulfate buffered solutions in the range of concentrations of about 0.2–1.2 M of salts are reported. The evolution of enthalpies and apparent molar volumes vs. salts concentration was investigated using ITC MicroCal titration microcalorimeter and Anton Paar DMA 60/602 digital densimeter. It was found that the changes in enthalpies of salting and apparent molar volumes follow a similar trend. Conclusions about the ability of salts: to precipitate ovalbumin solution are presented.     

Conductance of Some 1:1 Electrolytes in N,N-Dimethylacetamide at 25°C

Precise conductance measurements of solutions of lithium chloride, lithium bromide, lithium iodide, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, tetrabutylammonium bromide, and tetrabutylammonium tetraphenylborate in N,N-dimethylacetamide are reported at 25°C in the concentration range 0.005–0.015 mol-dm^–3. The conductance data have been analyzed by the 1978 Fuoss conductance equation in terms of the limiting molar conductance (⁰), the association constant (K _a), and the association diameter (R). The limiting ionic conductances have been estimated from an appropriate division of the limiting molar conductivity of the reference electrolyte Bu₄NBPh₄. Slight ionic association was found for all these salts in this solvent medium. The results further indicate significant solvation of Li⁺ion, while the other ions are found to be unsolvated in N,N-dimethylacetamide.     

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.     

Disorder-Enhanced Charge-Transfer-Mediated Room-Temperature Phosphorescence in Polymer Media

Room-temperature phosphorescence (RTP) polymers have important applications for biological imaging, oxygen sensing, data encryption, and photodynamic therapy. Despite the many advantages polymeric materials offer such as great control over gas permeability and processing flexibility, disorder is traditionally considered as an intrinsic negative impact on the efficiency for embedded RTP luminophores, as various allowed thermal motions could quench the emitting states. However, we propose that such disorder-enabled freedoms of microscopic motions can be beneficial for charge-transfer-mediated RTP, which is facilitated by molecular conformational changes among different electronic transition states. Using the “classic” pyrene-aniline exciplex as an example, we demonstrate the mutual enhancement of red/near-infrared and green RTP emissions from the pyrene and aniline moieties, respectively, upon doping the aniline polymer with trace pyrene derivatives. In comparison, a pyrene-doped crystal formed with the same aniline structure exhibits only charge-transfer fluorescence with no red or green RTP observed, suggesting that order suppresses the RTP channels. The proposed polymerization strategy may be used as a unified method to generate multi-emissive polymeric RTP materials from a vast pool of known and unknown exciplexes and charge-transfer complexes.     

Exciplex formation in aromatic copolymers and photoinduced electron transfer from exciplex to acceptor

The exciplex formation in 9-vinylphenanthrene-p-N,N-dimethylaminostyrene copolymers, its characteristics, and the electron transfer process in polar solvents were studied. The copolymer exhibited a more intense intramolecular exciplex fluorescence than the low-molecular-weight model system, phenanthrene-N,N-dimethylaniline, in which the intermolecular exciplex formation occurred. Intensities of the exciplex fluorescence, which were unchanged regardless of the copolymer composition, led us to speculate that the efficient energy migration takes place from an excited phenanthrene unit to an exciplex forming site on the polymer chain. The electron transfer in the copolymer-p-dicyanobenzene system was studied in polar media. The formation of p-dicyanobenzene anion radical was measured by flash photolysis and electron spin resonance (ESR). p-Dicyanobenzene anion radical was generated by the electron transfer process via exciplex and the direct electron transfer process from the excited phenanthrene unit in the copolymer.     

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 glycol (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, which is in good agreement with the results obtained from other fluorescence probes based on intramolecular excimer such as dipyrenylpropane (Py(CH₂)₃Py) and bis(1‐pyrenylmethyl) ether (dipyme, PyCH₂OCH₂Py).     

Investigation of perchlorate, phosphate and ion-pairing eluent modifiers for the separation of cephalosporin epimers

The retention behavior of several pairs of 7 alpha- and 7 beta-cephalosporin epimers was investigated using perchlorate, phosphate and ion-pairing eluent modifiers. At pH 2.5, sodium perchlorate, sodium phosphate and sodium pentanesulfonate all provided separation of epimers with free 7-amino groups. When the 7-amino group was blocked, as in cephalexin and cefaclor, sodium perchlorate gave the best separation at pH 2.5. A tetrabutylammonium ion-pairing system at pH 7.0 provided separation of all epimer pairs containing a free carboxylic acid at the 3 position. Hydrophobic, residual silanol and ionic interactions were factors in the retention mechanism of the cephalosporins under the conditions investigated. An ionic interaction of perchlorate with the protonated amine of the cephalosporin was postulated as an explanation of the retention and selectivity effects observed with perchlorate as an eluent modifier.     

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.     

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.