Excimer

About: Excimer is a research topic. Over the lifetime, 3725 publications have been published within this topic receiving 75104 citations.

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.