Excimer

About: Excimer is a(n) research topic. Over the lifetime, 3725 publication(s) have been published within this topic receiving 75104 citation(s).

Calculation of ground and excited state potential surfaces of conjugated molecules. I. Formulation and parametrization

Abstract: A formulation is developed for the consistent calculation of ground and excited state potential surfaces of conjugated molecules. The method is based on the formal separation of u and 7r electrons, the former being represented by an empirical potential function and the latter by a semiempirical model of the Pariser-Parr-Pople type corrected for nearest-neighbor orbital overlap. A single parameter set is used to represent all of the molecular properties considered; these include atomization energies, electronic excitation energies, ionization potentials, and the equilibrium geometries and vibrational frequencies of the ground and excited electronic states, and take account of all bond length and bond angle variations. To permit rapid determination of the potential surfaces, the u potential function and SCF-MO-CI energy of the r electrons are expressed as analytic functions of the molecular coordinates from which the first and second derivatives can be obtained. Illustrative applications to 1,3butadiene, 1,3,5-hexatriene, a,w-diphenyloctatetraene, and 1,3-cyclohexadiene are given. detailed interpretation of electronic transitions and A concomitant photochemical processes in conjugated molecules requires a knowledge of the ground and excited state potential surfaces. The determination of such surfaces has long been a goal of theoretical chemistry. Difficulties in a reliable a priori approach to the problem for a system as simple as ethylene2 are such that calculations for more complicated molecules are prohibitive at present. Consequently, a variety of methods that utilize experimental data have been introduced. Completely empirical treatments, in which the energy surface is expressed as a function of potential parameters fitted to the available information (1) Supported in part by Grant EY00062 from the National Institute of Health. (2) U. Kaldor and I. Shavitt, J . Chem. Phys., 48, 191 (1968); R. J. Buenker, S. D. Peyerimhoff, and W. E. Kammer, ibid., 55, 814 (1971). (equilibrium geometry, vibrational frequencies, etc.), have had considerable success in applications to molecules for which a localized electron description is app l i~ab le .~ The great advantage of this type of approach, which leaves open questions of reliability when extended from one class of molecules to another, is the ease and speed of the calculations; this had made possible applications to systems as large as certain nucleic acids and globular proteins. For conjugated molecules, however, the importance of delocalization introduces difficulties into such an empirical treatmenL5 (3) (a) See, for example, J. E. Williams, P. J . Stand, and P. v. R. Schleyer, Annu. Reu. Phys. Chem., 19, 531 (1969); (b) S. Lifson and A. Warshel, J . Chem. Phys., 49, 5116 (1968); A. Warshel and S . Lifson, ibid., 53, 8582 (1970). (4) M. Levitt and S. Lifson, J. Mol. B i d , 46, 269 (1969); M. Levitt, Nature (London), 224, 759 (1969). ( 5 ) C. Tric, J . Chem. Phys., 5 1 , 4778 (1969). Journal of the American Chemical Society 1 94:16 1 August 9, 1972

Acid-Base Properties of Electronically Excited States of Organic Molecules

Abstract: Publisher Summary This chapter discusses acid–base properties of electronically excited states of organic molecules. Excited state pK-values are most easily accessible through the use of the Forster cycle. To perform this calculation for a particular molecule, it is necessary to know the ground state equilibrium constant for the reaction in question and to have some measure of the energy difference between the lowest vibrational level of the ground and the excited state in both the B and BH + forms. The effects of solvation on 0–0 energies are discussed. The changes in molecular fluorescence with acidity give information about the protolytic behavior of the excited singlet state of a compound. Two techniques, phase and pulse fluorometry, are used for the direct measurement of fluorescence decay rates. The excited state acid-base behavior of molecules has direct implications in the field of analytical fluorimetry and phosphorimetry.

Intramolecular Excimer Formation. I. Diphenyl and Triphenyl Alkanes

Abstract: Fluorescence spectra have been measured for a variety of diphenyl and triphenyl alkanes in cyclohexane and in p‐dioxane. A class of compounds which are so structured that the phenyl groups along a main alkane chain are separated by exactly three carbon atoms, e.g., 1,3‐diphenylpropane, 1,3,5‐triphenylpentane, has been found to possess unique fluorescence characteristics. These are the appearance of a long‐wavelength band in the region of 330 mμ and a marked decrease in the fluorescence yield. The long‐wavelength band is attributed to an emission from an excimer (a transient dimer) formed intramolecularly by the association of excited and unexcited phenyl groups. Formation of excimers in the specific class of compounds is discussed in relation to molecular configuration. Efficiency of excimer formation, solvent effects, and quenching by dissolved oxygen are some of the topics discussed through kinetic considerations.

Lateral diffusion in the hydrophobic region of membranes: use of pyrene excimers as optical probes.

Abstract: A new (optical) method of diffusion measurement is described which allows the determination of the coefficient of lateral diffusion, Ddiff, of aromatic molecules in the hydrophobic region of lipid bilayers. In the present work pyrene is used as a fluorescence probe. The method is based on the finding that the formation of excited pyrene dimers (excimers) in fluid membranes is a diffusion controlled process. The value of Ddiff is obtained from the second order rate constant of excimer formation which is determined from the ratio of the excimer to the monomer fluorescence quantum yields The method has been applied to dipalmitoyllecithin membranes and mixed dipalmitoyllecithin-cholesterol bilayers. The diffusion coefficient for pyrene in dipalmitoyllecithin membranes at 50 °C (that is above the lipid phase transition) is Ddiff = 1.4 · 10−7 cm2/s. The activation energy for the pyrene diffusion is ΔE = 8.8 kcal/mole. Below the lipid phase transition pyrene aggregates into small clusters embedded in the lipid matrix. For bilayers of unsonicated lipid dispersions the cluster formation is observed at very low pyrene concentration of 0.1 mole %, showing that the lipid matrix forms a rather regular crystal structure. Above the lipid phase transition cholesterol reduces the lateral mobility of pyrene considerably (at 30 mole cholesterol: Ddiff = 0.64 · 10−7 cm2/s). Below the phase transition cholesterol suppresses the cluster formation. The excimer formation of pyrene is also sensitive to the pretransition of the lipid matrix occurring about 10 °C below the main lipid phase transition at Tt = 41 °C.

MLCT excited states of cuprous bis-phenanthroline coordination compounds

Abstract: Cuprous bis-phenanthroline compounds possess metal-to-ligand charge transfer, MLCT, excited states. Phenanthroline ligands coordinated to Cu(I) that are disubstituted in the 2- and 9-positions with alkyl or aryl groups, abbreviated CuI(phen′)2+, have long-lived excited states at room temperature. The parent CuI(phen)2+ compound is non-emissive under the same conditions with a short excited state lifetime, τ<10 ns. Disubstitution in the 2,9-positions stabilizes the Cu(I) state and increases the energy gap between the MLCT and the ground state. The prototypical and most well studied compound is CuI(dmp)2+, where dmp is 2,9-(CH3)2-1,10-phenanthroline. In dichloromethane solution at room temperature, CuI(dmp)2+ displays broad MLCT absorption with λmax=454 nm, a broad unstructured emission with λmax=730 nm, and an excited state lifetime of 85 ns. The emission arises from two closely spaced MLCT excited states, separated in energy by 1800 cm−1, that behave as one state at room temperature. CuI(dmp)2+* excited states are quenched in the presence of Lewis bases and coordinating solvents. A 5-coordinate excited state complex, or exciplex, is proposed to account for temperature dependent quenching data. The substantial inner-sphere reorganizational energy changes that follow light excitation are novel features of these MLCT excited states. This review attempts to cover all the existing data reported on CuI(phen′)2+ excited states and contrast it with well-known MLCT behavior of (dπ)6 transition metal compounds.