Showing papers on "Excimer published in 1975"

Emission properties of vinylcarbazole polymers

Abstract: The emission properties of poly(N‐vinylcarbazole) (PVK), poly(N‐ethyl‐2‐vinylcarbazole), and poly(N‐ethyl‐3‐vinylcarbazole) have been investigated. Dilute fluid solutions of all three vinylcarbazole polymers are observed to exhibit a structureless emission band at energies approximately 5400 cm−1 to the red of the (0,0) band of the normal molecular fluorescence of isolated monomer compounds. On the basis of earlier work appropriate model compounds, this emission is assigned as that from an intrachain excimer state in which two interacting pendant groups, one of which is electronically excited, achieve an overlapping, eclipsed, sandwichlike configuration. The emission spectrum of PVK is characterized by the appearance of a second structureless emission band which appears at higher energy than the intrachain excimer fluorescence common to all three vinylcarbazole polymers. This emission band is unique to PVK and under certain conditions to its model compound, 1,3‐bis(N‐carbazolyl)propane. This emission is assigned as that from a second intrachain excimer. Contrary to the lower energy intrachain excimer, the polymer conformation appropriate for formation of this state exists prior to the initial excitation step.

Electrogenerated chemiluminescence. XXII. Generation of exciplexes in the radical ion reaction

Abstract: Exciplex emission from systems containing tri-p-tolylamine (TPTA) as a donor molecule and various acceptor molecules was observed in both acetonitrile (ACN) and tetrahydrofuran (THF) solutions during electrogenerated chemiluminescence (ecl) studies. Systems containing naphthalene as an acceptor and five donor molecules also showed exciplex emission in A C N solutions. Triplet quenching experiments, magnetic field effect measurements, and temperature studies demonstrated that the exciplex is formed directly in the radical ion reaction. The energy of the exciplex emission showed a linear correlation with the cyclic voltammetric peak potentials for reduction of the acceptor molecules and for oxidation of the donor molecules. Studies of the ecl of the dibenzoylmethane (DBM)-TPTA system in six different electrochemical solvents, where only exciplex emission is observed, demonstrated the importance of solvent dielectric constant on exciplex emission intensity. The role of exciplexes in the general ecl scheme is also discussed. Exciplexes are excited molecular complexes which are dissociated in the ground state.’ They are usually produced by reaction of a photoexcited singlet species (‘A*) with a ground state molecule, D; exciplex emission is usually structureless and red shifted from that of IA* by about 6000 ( 1) ( 2) Previous studies of radical-ion annihilation chemiluminescence reactions and electrogenerated chemiluminescence ( e ~ l ) ~ , ~ in low dielectric constant solvents, such as tetrahydrofuran (THF), have demonstrated the formation of exciplexes by the direct reactions of radical ions ( 3) and it has been suggested that an exciplex may be an intermediate in other cl or ecl reactions, even when exciplex cm-1.1,2 lA* + D -+ ‘(&D+)* l(A-D+)* A + D + hu As+ D * a l(A‘D+)* emission is not observed. It has generally been felt, however, that emission from exciplexes will not be observed in polar solvents, such as acetonitrile (ACN), where dissociation of the exciplex into the radical ions (the reverse of reaction 3) becomes more important.’ We describe here ecl experiments with a number of acceptor and donor molecules in both T H F and ACN which demonstrate the formation of exciplexes and show that the energy of the exciplex emission correlates with the redox potentials of the reactant molecules. Preliminary experiments on the effect of magnetic field, solvent, and temperature variations on the ecl behavior in these systems are also discussed. Experimental Section Chemicals. Tri-p-tolylamine (TPTA) and tri-p-anisylamine (TPAA), kindly provided by Professor R. N. Adams (University of Journal of the American Chemical Society J 97: l l J May 28, 1975

Excimer fluorescence of polymers with aromatic substituents, 1. Dependence of excimer formation on polymer structure

Abstract: Poly(α-methylstyrene) exhibits the same excimer fluorescence as polystyrene although it would be expected that repulsions between the α-methyl groups would hinder excimer formation. An alternating copolymer of stilbene and maleic anhydride exhibits excimer fluorescence, although this is absent in styrene/maleic anhydride copolymer, in 1,2-diphenylethane, and in 1,4-diphenylbutane. With polyacenaphthylene, excimer fluorescence is strong and independent of chainlength if the sample was obtained by free radical polymerization. In polyacenaphthylene obtained by cationic initiation, excimer emission increases in intensity with a decreasing polymerization temperature. Alternating acenaphthylene copolymers exhibit strong excimer emission if the comonomer is maleic anhydride, methyl methacrylate or methacrylonitrile, while methyl acrylate and acrylonitrile copolymers form very little excimer. These results suggest that excimer formation is very sensitive to conformational changes due to a change of tacticity or the nature of the comonomer and this is also reflected in NMR spectra. The use of paracyclophanes as model compounds indicates that excimer formation is compatible with a considerable deviation from a parallel alignment of the interacting aromatic residues. Poly(α-methylstyrol) zeigt die gleiche Excimeren-Fluoreszenz wie Polystyrol, obwohl man annehmen konnte, das die Abstosungskrafte zwischen den α-Methylgruppen die Bildung von Excimeren verhindern wurden. Obwohl das Copolymer aus Styrol und Maleinsaureanhydrid sowie 1,2-Diphenylathan und 1,4-Diphenylbutan keine Excimeren-Fluoreszenzaufweisen, zeigt ein alternierendes Copolymer aus Stilben und Maleinsareanhydrid eine solche. Polyacenaphthylen zeigt eine starke von der Kettenlange unabhangige Excimeren-Fluoreszen, wenn die Probe durch radikalische Polymerisation hergestellt wurde. Polyacenaphthylen, das durch kationische Polymerisationsauslosung erhalten wurde. zeigt eine Excimeren-Emission, deren Intensitat mit a bnehmender Polymerisationstemperatur zunirnmt. Alternierende Acenaphthylen-Copolymere weisen starke Excimeren-Emission auf, wenn das Comonomer Maleinsaureanhydrid, Methylmethacrylat oder Methacrylnitril ist, wahrend Methylacrylat- und Acrylnitril-Copolymere sehr wenig Excimere bilden. Diese Ergebnisse lassen die Vermutung zu, das die Bildung von Excimeren sehr empfindlich auf Anderungen der Konformation, hervorgerufen durch eine Anderung der Taktizitat oder der Natur des Comonomers, ist, und dies kommt auch in den NMR-Spektren zum Ausdruck. Die Verwendung von Paracyclophanen als Modellverbindungen zeigt, das sich die Bildung von Excimeren mit einer betrachtlichen Abweichung von einer parallelen Ausrichtung der miteinander in Wechselwirkung tretenden aromatischen Reste vereinbaren last.

Studies of E-Beam Pumped Molecular Lasers

Abstract: : This report contains a brief review of the results obtained in this program which was designed to examine prospective high power, high efficiency laser media These studies included measurements of the optical emissions and temporal behavior of excited state in dense vapors and gas mixtures excited by intense, pulsed electron beams In addition, detailed models of the excitation, relaxation, and decay kinetics of excited states have been realized for the case of AR-N2, Xe-O2, and Xe-Hg mixtures We also examine in detail the spectral and temporal properties of emissions coming from mixtures of Xe and Hg, a prototype of rare gas-metal atom excimer systems, and determine some relevant rate constants Significant fluorescent yields from weakly bound Xe-Hg excimer radiations suggest the possibility of a good UV pump source (near 270 nm) which could be used for photolytic production of I(2P(1/2)) A model of the energy pathways has been constructed, and it is concluded that laser action may be possible on the bands arising from the Hg 7 3S levels Keywords: Electron beam; Energy transfer; Molecular laser; Rare gas dimers; Xenon; Mercury; Ultraviolet lamp; and Ultraviolet spectroscopy

Calculated and experimental electronic structure of gaseous MnO3F and MnO3Cl

Abstract: The gas phase electronic absorption spectra of MnO3F and MnO3Cl have been measured in the visible and near ultraviolet regions. Interpretation has been made by comparison with a molecular calculation for each molecule by the SCF–Xα–SW method in terms of C3v symmetry. The calculated ground and excited states and observed electronic transitions correlate directly with those for the parent MnO−4 cluster. The lowest excited state, 1Ea, which correlated with the first 1T1 state of MnO−4, retains the characteristics of the parent state. The charge transfer originating from the halide atom is first observed in the 1Ec excited state in each molecule correlating with the second lowest 1T2 state in MnO−4.

Weak Fluorescence from [2.2]Metacyclophane

Abstract: Weak fluorescences from [2.2]metacyclophane (MCP) were observed: the normal and intramolecular excimer fluorescences were at 305 and 361 nm respectively. It has been shown that a rapid radiationless deactivation due to the interaction between the two benzene rings in MGP occurs preferentially and the excimer formation responsible for the photocyclization takes place to a small extent.

The Electronic Structure of the Electron-donor-acceptor System in the Excited State. The Exciplex and the Fluorescent State of EDA Complex

Abstract: The semiempirical SCF-MO-CI calculation was applied to the electron donor-acceptor (EDA) interaction in 9,10-dicyanoanthracene and naphthalene system which exhibits the exciplex fluorescence and also the fluorescence due to the EDA complex stable in the ground state. On the basis of energy contour maps of the ground and excited states and of electronic transitions in the EDA system, the relaxation process from the F.G. excited state of the EDA complex to an identical fluorescent state with the exciplex was discussed on the intermolecular interaction, and also on the intramolecular interaction in (9,10-dicyanoanthracene)-(CH2)3- (naphthalene) systems.

Interaction of aromatic amine exciplexes with polar molecules

Abstract: The interaction between polar molecules and the intramolecular exciplex forming molecule 1-hydroxy-1-(2-naphthyl)-3-(N-piperidinyl)propane has been investigated using steady state and nanosecond time resolved spectroscopy. Values for the various excited state rate constants have been obtained which suggest a one polar molecule fluorophore interaction. Evidence has been found for ground-state associations although the extent of this is small compared with the excited state interactions.

Analysis of Optically Excited Mercury Molecules

Abstract: Report presenting new measurement techniques used to obtain potential curves, f-values, and kinetic behavior for Hg2. The Hg2 molecule is representative of a class of molecules which have dissociative ground states and bound excited states. It can therefore be used as a prototype of this class of molecules which are of interest as potential new laser candidates. Because of the non-bound ground state, standard absorption spectroscopic techniques cannot be used to obtain the necessary information about the excited states.

Electronic Spectra and Structure of p-Benzoquinone

Abstract: Ground and excited state electronic structure and spectra of p-benzocuinone have been studied using the variable electronegativity SCF-MO-CI method. The calculated values of ionization potential and excitation energies are in good agreement with experiment. It is found that the electronic structure and geometry of the molecule are appreciably different in the excited singlet state as compared to the ground state.

Luminescence of poly(N-vinylcarbazole) in the solution and solid state

Abstract: Fluorescence spectra of poly(N-vinylcarbazole) have been studied as a function of various environments and temperature. Fluorescence spectra have been excited with UV light from 200 W Xenon lamp or with second-harmonics of a pulsed ruby laser. Polymer films and powders show exclusively excimer fluorescence at room temperature. At high intensities of excitation the observed fluorescence spectra have a weak quasiperiodic structure.

The effect of 9-ethyladenine on the photodimerization of 1-cyclohexylthymine.

Abstract: — The photodimerization of 1-cyclohexylthymine (l-CT).and the effect of adding 9-ethyladenine (9-EA) on this reaction were studied in chloroform and/or acetonitrile solutions. The photodimerization of 1-CT is shown to occur through an excited triplet state. 9-EA works as an inhibitor against this reaction by quenching the excited singlet state of 1-CT. It is suggested that a singlet exciplex made up from the excited state 1-CT and the ground state 9-EA by hydrogen-bonding may play an important role in the quenching process.

Studies of e-beam pumped molecular lasers. Final report, 15 April 1972-30 June 1975

Abstract: This report contains a brief review of the results obtained in this program which was designed to examine prospective high-power, high-efficiency laser media. These studies included measurements of the optical emissions and temporal behavior of excited state in dense vapors and gas mixtures excited by intense, pulsed electron beams. In addition, detailed models of the excitation, relaxation, and decay kinetics of excited states have been realized for the case of Ar-N/sub 2/, Xe-O/sub 2/, and Xe-Hg mixtures. Also examined in detail are the spectral and temporal properties of emissions coming from mixtures of Xe and Hg, a prototype of rare gas-metal atom excimer systems, and determine some relevant rate constants. Significant fluorescent yields from weakly bound Xe-Hg excimer radiations suggest the possibility of a good UV pump source (near 270 nm) which could be used for photolytic production of I(2P(1/2)). A model of the energy pathways has been constructed, and it is concluded that laser action may be possible on the bands arising from the Hg 7 3S levels.

New laser measurement techniques for excited electronic states of diatomic molecules

Abstract: Introduction This paper will briefly outline several new laser measurement techniques which we have developed for the analysis of excited elecronic states of diatomic molecules. For molecules which have bound ground states, a visible laser is used to selectively excite a single vibration-rotation level in the electronic state of interest. We then use DC Stark effect, RF double resonance and various fluorescence techniques, as discussed in Sections I and II, to obtain excited state dipole moments, lifetimes, quenching cross sections, transition moments as a function of internuclear distance, and other molecular structure data. We have also developed new laser excitation techniques and used optical double resonance methods for excimer molecules which have repulsive ground states and are bound only in their excited states. These techniques, discussed in Sections III and IV, have been used to obtain potential energy curves, fvalues, lifetimes and various kinetic rates.