Abstract
Electronic coupling magnitudes in donor−bridge−acceptor (DBA) molecules are influenced by the detailed structure of the bridge and its connections to the D and A groups. The influence of different symmetry, initial and final states on the electronic coupling magnitude, and on transfer dynamics has been investigated in single conformation (“rigid”) DBA molecules with mirror plane symmetry elements. There is no uniformity of opinion on the magnitude of the “symmetry effect” in such systems. In this manuscript, the magnitude of the formally, symmetry forbidden electronic coupling between an excited anthracene donor, D*, and a cyclobutenediester acceptor across a three-bond bridge, DBA 1, is determined through an analysis of the fluorescence polarization anisotropy of the charge transfer state to ground state (CT→S0) charge recombination emission. The wavelength dependence of the anthracene's absorption and emission band polarization are determined in a structurally related DBA molecule, 2, that does not exhibit charge transfer absorption or emission bands. The polarization of these transitions agrees with the literature results for related anthracenes. Using the polarization data for the lowest energy anthracene absorption band (S0→S1), the polarization of the charge recombination emission in 1 is determined to be wavelength independent and to lie within the mirror plane symmetry element of the molecule (orthogonal to the anthracene short axis). These results demonstrate that the charge recombination emission derives negligible oscillator strength from the anthracene S1→S0 transition and that the electronic coupling between D* and A (i.e., between the S1 and CT states) is too small to determine accurately by this method. The absorption polarization data from 1 provides evidence of a previously undetected, weak charge transfer absorption band (S0→CT) on the red edge of the S0→S1 transition.
Published Version
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