Study of Microscopic Molecular Environment Through which the Natural Radiative Lifetime is Determined

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In order to understand the nature of microscopic molecular environments through which radiative lifetime of organic molecules are determined, the fluorescence lifetimes of a number of anthracene derivatives were measured in a wide variety of specially designed systems. The obtained results are summarized as follows.1. The radiative lifetime is determined for several anthracene derivatives both in supersonic free jets (gas phase) and in solution. The values in the two phases can be related through the refractive index of the solvents used as n^2zeta_f (solution) =zeta_f (gas phase)2. The relation established through a comparison of the radiative lifetimes in the two phases was further examined in solution. The refractive index of the solvent was varied by the application of hydrostatic pressure. For five anthracene derivatives, whose fluorescence quantum yields are practically equal to unity, n^2-relationship was found to be followed excellently over the range of pressure from 0.1 MPa to 400 MPa except for 9, 10-dimethoxyanthracene.3. Microscopic molecular environment created around a central molecule is dependent on the shape of the molecule in the center. This may lead to a n-dependence different from n^2-dependence. To confirm this, the effect of pressure on the fluorescence lifetimes of POPOP, QUI, BBQ and Bis-MSB, all of which are of a long-chain shape. In addition, the fluorescence-lifetime measurements were performed in the rare-gas matrixes or in droplets in order to obtain basic data which are necessary to expand the present study.