Showing papers on "Excimer published in 1968"
TL;DR: In this paper, a broad structureless emission band about 5000 cm1 was observed to increase with increasing electron donor concentration at the expense of the fluorescence intensity of the hydrocarbon, thereby following the same Stern-Volmer-type relation as does the well known excimer fluorescence.
Abstract: FLUORESCENCEt Some years ago, while investigating fluorescence quenching of aromatic hydrocarbons (A) by typical electron donors (D), like anilines, we observed1 a broad structureless emission band about 5000 cm1 to the red of the fluorescence of the aromatic hydrocarbon of normal structure. This anomalous fluorescence, as shown in Figure 1, increases in intensity with increasing donor concentration at the expense of the fluorescence intensity of the hydrocarbon, thereby following the same Stern—Volmer-type relation as does the well known excimer fluorescence, e.g. in the case of pyrene2. Extrapolation to infinite donor concentration gives the dashed spectrum (cf. Figure 1) which
282 citations
TL;DR: Chandross, Longworth, and Viscoz as discussed by the authors showed that the emission spectrum comprises two or more components, the spectral distribution of the component shortest in wavelength is similar to that of anthracene fluorescence, but the others are broad, structureless, and located toward the red with respect to the first component.
Abstract: The electrogenerated chemiluminescence (ECL) of anthracene is characterized by emission at the frequency of anthracene fluorescence and also at longer wavelengths. One longer wavelength component is shown to be caused by emission from anthranol produced by decomposition of the cation radical of anthracene and prbbably excited by energy transfer from excited anthracene. Another component, arising from ECL of anthranol itself, is also observed. revious reports of electrogenerated chemiluminescence (ECL) from anthracene solutions in N,Ndimethylformamide (DMF) have noted that the emission spectrum comprises two or more components. The spectral distribution of the component shortest in wavelength is similar to that of anthracene fluorescence, but the others are broad, structureless, and located toward the red with respect to the first component.2s3 This general behavior is common among several polycyclic hydrocarbons and their derivatives. Presently four alternatives are available to explain the longwavelength emission from these systems. Chandross, Longworth, and Viscoz have proposed the formation of an anthracene excited state dimer (excimer), which radiates to produce the low energy emission. A similar explanation is the formation of an anthracene excited state complex with some other species (exciplex). Both the excimer and the exciplex dissociate into component ground state molecules upon deactivation. Zweig, Maricle, Brinen, and Maurer have suggested that solution phosphorescence may be responsible for the longwavelength emission in some of these systems4 Finally, wes have previously pointed out the possibility of emission from an excited state of a product formed during the reaction of the electrogenerated radical ions with their environment. Anthracene has been chosen for study because it is representative of the class of hydrocarbons exhibiting this behavior and because it is available and easily purified. We have performed a number of experiments designed to aid in identifying the emitting species in the anthracene-DMF system, and to help illuminate the means of exciting the species in so lu t ion which do emit. Experimental Section The anthracene used in all experiments was produced by Matheson Coleman and Bell (mp 215-217”). It was purified by triple recrystallization from Baker Spectroquality benzene and Baker Reagent Grade methanol according to a modification of a procedure available in the l i terat~re .~ A portion of the triply recrystallized (1) (a) National Science Fcundation Predoctoral Fellow; (b) to whom correspondence and requests for reprints should be directed. (2) E. A. Chandross, J. W. Longworth, and R. E. Visco, J . A m . Chem. Soc., 87, 3259 (1965). (3 ) A . J. Bard, I<. S . V. Santhanam, S . A. Cruser, and L. R. Faulkner in “Fluorescence,” G. G. Guilbault, Ed., Marcel Dekker, Inc., New York, N. Y . , 1967, Chapter 14. (4) A. Zweig, D. L. Maricle, J. S . Brinen, and A. H. Maurer, J . Am. Chem. Soc., 89, 473 (1967). material was also resublimed twice in DCICUO. No differences in behavior .were found between the material which had been doubly resublimed after recrystallization and that which had merely been recrystallized thrice. For this reason, most subsequent experiments used only the triply recrystallized anthracene. Fluorescence analysis of cyclohexane solutions of the purified anthracene showed no luminescence bands other than those directly attributable to anthracene.6 Maxima in fluorescence intensity were found at 378, 397, 420, 447, and ca. 475 mp. In particular tetracene was shown by absorption spectroscopy and by fluorescence measurements to be present in amounts less than 0.1 z, since none was detectable by these methods. The solvent used in every case was N,N-dimethylformamide which was also supplied by Matheson Coleman and Bell (bp 152154”). The solvent was further purified,by two methods. Method A involved storing the solvent over anhydrous cupric sulfate for several days to complex water and dimethylamine. The solvent was then decanted and distilled at a reflux ratio of 5 from a glass bead packed column 100 cm high under a nitrogen pressure of 20 mm. The middle fraction was retained for use. Method B also involved storage over anhydrous cupric sulfate For a period of several days. The distillation which followed was under the same conditions as above except that the reflux ratio was unity. Following this distillation, the solvent was stored over Linde Type 4A Molecular Sieves for a period of 48 hr. Then the material was decanted and redistilled using a reflux ratio of .l. Once again, only the middle fraction rvas taken. The solvent was stored under an inert helium atmosphere. Neither solvent batch showed fluorescence bands, even under the most sensitive conditiuris. The supporting electrolyte used in all experiments was tetra-rzbutylammonium perchlorate (TBAP), Polarographic grade, supplied by Southwestern Analytical Chemicals, Austin, Texas. The TBAP was used without further purification, but was dried in a vacuum oven for 48 hr at a temperature of 100” and then stored in a desiccator over magnesium perchlorate. The TBAP contained no fluorescent impurities. The electrolysis cell used for ECL emission measurements consisted of two platinum helices inserted through graded seals into the Pyrex wall of a 14/35 standard taper joint, as shown in Figure 1. An adapter was provided so that the cell could be evacuated easily. The electrodes were 2-5 mm apart. It was generally found that greatest emission intensities were incident upon the monochromator entrance slits when the slits and the two electrodes were arranged colinearly. This arrangement was used uniformly in the experiments. Immediately after loading the cell, it was degassed on a vacuum line similar to that described previously7 using two freeze-pumpthaw cycles. Minimum pressure over the frozen solution on the second cycle was at most 10-4 torr in every case. The voltage applied to the cell was simply the 60-cycle sinusoidally alternating line voltage which was reduced from 110 V root mean square to any (5) T. Takeuchi and M . Furusawa, Kogpo Kagaku Zasshi, 68, 474 ( 6 ) I. Berlman, “Handbook of Fluorescence Spectra of Organic (7) K. S. V. Santhanam and A. J. Bard, J . A m . Chem. Soc., 88, 2669 (1965); Chem. Abstr., 63, 4060e (1965). Molecules,” Academic Press, New York, N . Y., 1965.
88 citations
23 Apr 1968
TL;DR: In this article, an exciton resonance state originating from the Ba state of the pyrene molecule was determined as a function of the intermolecular separationr (A) function.
Abstract: are determined as functions of the intermolecular separationr (A). V' is shown to be consistent with an exciton resonance state originating from the 'Ba state of the pyrene molecule.
69 citations
TL;DR: In this article, the authors investigated the effect of high pressure on the fluorescence spectra of ultrapure crystals of naphthalene, anthracene, and phenanthrene.
Abstract: Fluorescence spectra of ultrapure crystals of naphthalene, anthracene, and phenanthrene have been studied at pressures as great as 50 kbar. These spectra exhibit complex irreversible effects superimposed upon the expected reversible shifts in the energies of the normal fluorescence spectra. The irreversible effects include a loss of the intensity of the normal fluorescence with increasing pressure and the almost simultaneous appearance of a broad, featureless emission at energies about 3000–6000 cm−1 lower than that of the normal fluorescence. This emission is assigned as the fluorescence of excimers formed upon optical excitation of the crystals under high pressure. The crystals continue to exhibit this excimer fluorescence after the pressure is reduced to atmospheric pressure, but the irreversible effects can be removed by thermal annealing of the crystal at atmospheric pressure. The irreversibility is attributed to trapping of pairs of molecules in an excimer‐like orientation as a crystal defect after they lose the excitation energy of the excimer. These defects then act as traps for excitation energy in the crystal. Other possible interpretations of the irreversible effects are discussed.
66 citations
TL;DR: In this paper, the authors measured the phosphoresence spectra of phenanthrene and naphthalene in liquid ethanol solutions and ascribed the structured component to the triplet monomer, the structureless one to triplet excimer phosphorescence emission. Decay constants for the two emissions have been measured as a function of the temperature.
48 citations
TL;DR: In this paper, the role of excited molecules and excimers in transfer to the solute molecules is considered in terms of the Voltz relations, which include the Forster critical transfer distance, the molecular diffusion coefficients, and the solvent excitation migration coefficient.
Abstract: The rate parameters of solvent-solute energy transfer and of oxygen-solvent quenching have been determined for solutions of 2, 5-diphenyloxazole in benzene, toluene, p -xylene and mesitylene. The role of excited molecules and excimers in transfer to the solute molecules is considered in terms of the Voltz relations, which include the Forster critical transfer distance, the molecular diffusion coefficients, and the solvent excitation migration coefficient. It is proposed that the migration is due to excimer formation and dissociation, and that the energy transfer occurs by a diffusion/migration-controlled collisional process. Dilution of the solvent decreases the migration, but increases the transfer distance, so that the transfer efficiency remains practically constant. The excimer formation and dissociation rate parameters in the pure alkyl benzenes are evaluated.
48 citations
TL;DR: The semi-empirical theory of excimer luminescence in the Pariser-Parr-Pople scheme is shown to be formally equivalent to, but much simpler than, the theories based on the molecular exciton-charge resonance concept as discussed by the authors.
Abstract: The semiempirical theory of excimer luminescence in the Pariser–Parr–Pople scheme [Paper I, J. Chem. Phys. 48, 2589 (1968)] is shown to be formally equivalent to, but much simpler than, the theories based on the molecular exciton–charge resonance concept. On the basis of an assumption that excimers possess D2h equilibrium conformation, the question of the stability of the excimer triplet state and the thermal distortion of the excimer as the possible source of dipole‐allowed character of excimer fluorescence is examined. It is shown that excimer triplet states of many polycyclic hydrocarbons are very likely unstable with respect to dissociation into monomer triplet and monomer ground state, and that thermal excitation of torsional distortion can account for the observed radiative lifetime of excimer fluorescence in naphthalene.
47 citations
TL;DR: In this article, a semi-empirical theory based on limited configuration interaction has been developed for the excimer luminescence of alternant hydrocarbons, which adequately accounts for the energy of excimer fluinescence (fluorescence and phosphorescence).
Abstract: A semiempirical theory based on limited configuration interaction has been developed for the excimer luminescence of alternant hydrocarbons. The theory adequately accounts for (1) the energy of excimer luminescence (fluorescence and phosphorescence), (2) the linear relationship between the energies of the excimer fluorescence and the p band of the monomer, and (3) the near constancy of the energy difference between the monomer and excimer fluorescence.
46 citations
TL;DR: In this article, the electronic absorption spectra of three crystalline modifications of anthracene derivatives have been measured; they are the α and β form of 9, 10-dichloroanthracene and 9-10-dibromo-anthracene.
Abstract: The electronic absorption spectra of three crystalline modifications of anthracene derivatives have been measured; they are the α and β form of 9, 10-dichloroanthracene and 9, 10-dibromo-anthracene. The assignment has been presented for each absorption band. The charge transfer absorption which is regarded to the transition between two stacked molecules has been observed in the α-form of 9, 10-dichloroanthracene; also a Rydberg type transition which has an out of plane polarization is found in this crystal starting at 290 mμ. It is unusually broad and is regarded as a band to band transition. The emission band has been measured at different temperature; it is found that excimer type emission is most prominent in the β-form of 9, 10-dichloroanthracene, in which intermolecular approach in the excited state will occur with less difficulty. A criterion for strong, medium, and weak interactions in the excited state is presented by comparing the shift, and the shape of the emission band with the intermolecular ...
44 citations
TL;DR: In this article, the theory of intermolecular interaction in excimers based on configuration interaction between molecular exciton and charge resonance states was tested with oxazole and oxadiazole derivatives and a clear correlation between the relative location of the interacting states and the ability to form an excimer was found.
Abstract: Experimental results, concerning excimer formation with oxazole and oxadiazole derivatives, are used to test the theory of intermolecular interaction in excimers based on configuration interaction between molecular exciton and charge resonance states. SCF molecular orbitals are computed and used to obtain the approximate energies of the molecular exciton and charge resonance states. The polar nature of the considered molecules is taken into account. A clear correlation between the relative location of the interacting states and the ability to form an excimer is found, which strongly supports the above‐mentioned theory.
31 citations
TL;DR: In this article, the lifetime of excimer fluorescence can be satisfactorily explained in two ways: (1) the transition is made vibronically allowed in much the same way as the 1B2u→1A1gtransition of benzene; or (2) the excimer possesses C1 or C2υ symmetry, in which case a very small relative tilting of the benzene planes can induce transition moments of proper magnitude without invoking vibronic effects at all.
Abstract: Semiempirical charge self‐consistent MO calculations have been carried out for a large number of benzene dimers of different geometric conformations (e.g., D6h, C2h, S2, C6, S12, C2υ, etc.). It has been found that the most stable excimer conformation is not the most symmetric (i.e., D6h), but rather one in which the rings are rotated relatively about the D6h axis and/or tilted, one with respect to the other. The lifetime of excimer fluorescence can be satisfactorily explained in two ways: (1) The transition is made vibronically allowed in much the same way as the 1B2u→1A1gtransition of benzene; or (2) the excimer possesses C1 or C2υ symmetry, in which case a very small relative tilting of the benzene planes can induce transition moments of proper magnitude without invoking vibronic effects at all. A number of suggestions are made concerning solvent effects, and the presence of underlying continua in discrete absorption spectra, etc. In particular, a concept of contact excimer absorption is broached and ra...
TL;DR: In this paper, the conditions necessary for the existence of an isosbestic point in the fluorescence spectra of a compound which emits from the monomer as well as the excimer state were discussed.
TL;DR: In this article, an experimental evidence is presented on an excimer-excimer interaction in pyrene crystals and solutions containing a high concentration of pyrene, which is used to explain in part why scintillation pulse shapes when excited by α-particles contain less of a excimer component than when excited with β-particle.
TL;DR: In this article, the relative fluorescence yield on excitation wavelength and concentration is analyzed in terms of a model kinetic scheme involving the formation of higher excimer states D from the higher excited molecular states X**.
Abstract: The fluorescence excitation spectra of liquid benzene, toluene, p-xylene, mesitylene, 2-methylnaphthalene, 1,6-dimethylnaphthalene, naphthalene, fluorobenzene and fluoronaphthalene, and of cyclohexane solutions of the first six compounds, were observed in deoxygenated systems at wavelengths not less than 195 nm. The observed dependence of the relative fluorescence yield on excitation wavelength and concentration is analysed in terms of a model kinetic scheme involving the formation of higher excimer states D** from the higher excited molecular states X**. Data are obtained on the relative efficiencies of internal conversion from X** and D** to the corresponding fluorescent states. The nature of the competing processes is discussed.
TL;DR: The most significant effects of pressure on the spectra of the host crystals are a loss of intensity of the normal fluorescence with increasing pressure and the almost simultaneous appearance of a broad, featureless emission at energies about 3000-6000 cm−1 lower than that of the standard fluorescence.
Abstract: Fluorescence spectra of crystals of anthracene, naphthalene, and phenanthrene doped with other polycyclic aromatic hydrocarbons have been studied at pressures as great as 50 kbar. The most significant effects of pressure on the spectra of the host crystals are a loss of intensity of the normal fluorescence with increasing pressure and the almost simultaneous appearance of a broad, featureless emission at energies about 3000–6000 cm−1 lower than that of the normal fluorescence. This emission is assigned as the fluorescence of excimers formed upon optical excitation of the crystals under high pressure. The growth of the intensity of the excimer emission with increasing pressure, relative to the intensity of the normal fluorescence, is the same in doped crystals as in pure crystals. The intensities of the guest molecule fluorescence relative to the intensity of the normal host fluorescence in the various doped crystals increase with pressure at about the same rate as the intensity of the excimer emission. These observations are interpreted as indicating that excimers are the lowest‐energy excited singlet species of aromatic molecular crystals at these pressures and that when the excimer concentration is appreciable, the path of energy transfer is from the host via excimers to the guest.
TL;DR: In this article, the effects of increasing aromatic concentration on the fluorescence spectrum and intensity of benzene and p‐xylene in cyclohexane were investigated and it was shown that an increase in aromatic concentration results in the appearance of a new fluorescence band in benzene but has only a small effect on the spectrum of p•xylene, while the increase in excimer fluorescence could not correlate with the decrease in monomer fluorescence as predicted by a simple monomer-excimer mechanism.
Abstract: Experimental results of the effects of increasing aromatic concentration on the fluorescence spectrum and intensity of benzene and p‐xylene in cyclohexane are presented. Other authors have studied these systems but there is disagreement as to results and interpretations. In agreement with previous results, an increase in aromatic concentration results in the appearance of a new fluorescence band in benzene but has only a small effect on the spectrum of p‐xylene. The fluorescence spectrum of benzene at high concentrations, however, cannot be simply resolved into monomer and excimer spectra. Further, the increase in excimer fluorescence could not be correlated with the decrease in monomer fluorescence as predicted by a simple monomer–excimer mechanism. The luminescence decay time for both p‐xylene and benzene remains constant within 10% in going from very dilute solutions to undiluted aromatic. This constancy in decay time can be correlated with changes in the monomer fluorescence intensity by means of a mo...
TL;DR: In this article, the absorption and fluorescence shifts of perylene at high pressures are compared for the two crystalline forms, and it is observed that −Δνf < 0.
Abstract: The absorption and fluorescence shifts of perylene at high pressures are compared for the two crystalline forms. The red shift − Δν at 20 kbar is greater for β‐perylene than for α‐perylene. Also, it is observed that − Δνf < − Δνa for α‐perylene and − Δνf ∼ − Δνa for β‐perylene. The smaller shift of the excimer fluorescence compared to the pressure shift in absorption of α‐perylene is indicative of large specific interactions in the excited state. The condition − Δνf < − Δνa has been previously observed in charge‐transfer complexes so that charge‐transfer forces could be responsible for the stabilization of the excimeric state although the present data cannot exclude other explanations for the specific interactions. Intensity changes in the crystal absorption band are not easily related to vibronic coupling mechanisms. Fluorescence intensities are reduced for both crystals under compression.
TL;DR: The transient absorption reported in the nanosecond pulse radiolysis of benzene and toluene is assigned to the B 1g → E 1u excimer transition as discussed by the authors.
TL;DR: In this paper, the fluorescence lifetimes and quantum yields of toluene solutions in n-hexane from -85 °c to 50 °c have been analyzed to evaluate the molecular and excimer fluorescence and internal quenching rate parameters, their frequency factors and activation energies and other related parameters.
Abstract: Observations of the fluorescence lifetimes and quantum yields of toluene solutions in n-hexane from -85 °c to 50 °c have been analysed to evaluate the molecular and excimer fluorescence and internal quenching rate parameters, their frequency factors and activation energies and other related parameters. An excimer binding energy of 029 ev is obtained.
TL;DR: In this article, fluorescent emission of solutions of pyrene in liquid paraffin-hexane mixtures and in benzene containing dissolved polystyrene is observed and the difference in the viscosity effect of the two solvents on excimer formation is explained in terms of the erroneous use of the macroscopic viscosities.
01 Jan 1968
TL;DR: In this article, the authors measured the ratio of excimer to molecular fluorescence intensities in both the delayed and total emission spectra of pyrene in ethanol, as a function of solute concentration and temperature from 150-350°K.
Abstract: The ratio of excimer to molecular fluorescence intensities in both the delayed and total emission spectra of pyrene in ethanol has been measured as a function of solute concentration and temperature from 150–350°K. An analysis of the data in terms of triplet-triplet annihilation leading directly to excimer formation (process 1) or production of the lowest excited molecular singlet state (process 2) shows that the relative probabilities k 1/k 2 of these processes increases from zero at low temperatures to a maximum of 1.8 at higher temperatures.
TL;DR: In this paper, a pulsed laser bean at 3371 A was used to demonstrate the effect of high excitation intensity on excimer formation of pyrene in solution, and a saturation effect was predicted and found.