Electronic energy levels in a homologous series of unsubstituted linear polyenes
| Autor: | Christopher Manos, Ronald L. Christensen, Kevin L. D'Amico |
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| Rok vydání: | 1980 |
| Předmět: | |
| Zdroj: | Journal of the American Chemical Society. 102:1777-1782 |
| ISSN: | 1520-5126 0002-7863 |
| DOI: | 10.1021/ja00526a003 |
| Popis: | Absorption, emission, and excitation spectra of 1,3,5,7-octatetraene, 1,3,5,7,9-decapentaene, and 1,3,5,7,9,1 l-dode- cahexaene have been obtained in room temperature solutions and 77 K glasses. All spectra exhibit the characteristic gap be- tween the origin of the strongly allowed absorption ('Ag - 'B,) and the origin of fluorescence ('Ag* - 'A!): Comparison with results previously obtained for methyl-substituted polyenes shows that the 'B,-'Ag* energy gap is a sensitive function of the degree of substitution. Solvent-effect studies have been used to extrapolate transition energies of the unsubstituted polyenes to gas-phase conditions. For the tetraene, pentaene, and hexaene the 'Bu-IAg* energy differences are 6380, 7050, and 7420 cm-I, respectively. These results are discussed in terms of current theoretical descriptions of polyene electronic states. 1. Introduction Linearly conjugated T electron systems have been subject to renewed experimental and theoretical interest in recent years. This interest in large part has been stimulated by the important roles played by polyene electronic states in several important photobiological processes. For long polyenes (those with more than three double bonds) in condensed phases there now is a great deal of evidence which indicates the existence of low-lying, 'Ag* excited states below the 'B, states respon- sible for the strong absorptions in these molecules. The posi- tions of the 'A,* states are not accounted for in simple mo- lecular orbital treatments. lt thus is important to obtain more detailed experimental data which can guide the development of more appropriate theoretical models. Experiments pointing to the existence in long polyenes of the low-lying 'A,* state can be summarized as follows: (1) observations of large differences between the origins of the strongly allowed absorption ('A |
| Databáze: | OpenAIRE |
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