A study of the dynamics of electronic energy transfer (EET) in arrays, containing three, four, and six tetraphenylporphine units connected with phenylethynyl spacers is reported. For arrays containing the same chromophores, the EET rate constant was determined from the reorientational dynamics of the transition dipole using the crossed grating technique. EET time constants ranging from 150 ps up to 33 ns were measured, depending on the distance between the chromophores and on the metal ion complexed in the porphyrins. For the trimeric planar arrays, the interchromophoric distance varies by a factor of 2, while the ratio of the through space to through bond distances is constant. By comparing the measured EET rate constants with those calculated using Forster theory, the contributions of the Coulombic, through space, mechanism and of the exchange, through bond, mechanism could be estimated. For the arrays with the shortest spacer (through space distance of 23 A), EET occurs through both exchange and Coulombic interactions with a ratio of about 3:1. This ratio increases up to about 10 as the distance is increased to 34.5 A. At 46.5 A, the ratio decreases and it appears that the Coulombic interaction becomes the dominant mechanism at longer distances. In the tetrahedral compound, the presence of a central saturated carbon strongly alters the electronic conducting properties of the spacer and makes the exchange mechanism inoperative.
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