Design of Triads for Probing the Direct Through Space Energy Transfers in Closely Spaced Assemblies

Using a selective stepwise Suzuki cross-coupling reaction, two trimers built on three different chromophores were prepared. These trimers exhibit a <b>D^∧A</b>_<b>1</b>-<b>A</b>_<b>2</b> structure where the donor <b>D</b> (octa-β-alkyl zinc­(II)­porphyrin either as diethylhexamethyl, <b>10a</b>, or tetraethyltetramethyl, <b>10b</b>, derivatives) through space transfers the S₁ energy to two different acceptors, di­(4-ethylbenzene) zinc­(II)­porphyrin (<b>A</b>_<b>1</b>; acceptor 1) placed cofacial with <b>D</b>, and the corresponding free base (<b>A</b>_<b>2</b>; acceptor 2), which is <i>meso–meso</i>-linked with <b>A</b>_<b>1</b>. This structure design allows for the possibility of comparing two series of assemblies, <b>9a,b</b> (<b>D^∧A</b>_<b>1</b>) with <b>10a,b</b> (<b>D^∧A</b><b>̂</b>_<b>1</b>-<b>A</b>_<b>2</b>), for the evaluation of the S₁ energy transfer for the global process <b>D</b>*→<b>A</b>_<b>2</b> in the trimers. From the comparison of the decays of the fluorescence of <b>D</b>, the rates for through space energy transfer, <i>k</i>_ET for <b>10a,b</b> (<i>k</i>_ET ≈ 6.4 × 10⁹ (<b>10a</b>), 5.9 × 10⁹ s^–1 (<b>10b</b>)), and those for the corresponding cofacial <b>D</b><b>^∧A</b>_<b>1</b> systems, <b>9a,b</b>, (<i>k</i>_ET ≈ 5.0 × 10⁹ (<b>9a</b>), 4.7 × 10⁹ s^–1 (<b>9b</b>)), provide an estimate for k_ET for the direct through space <b>D</b>*→<b>A</b>_<b>2</b> process (i.e., <i>k</i>_ET(<b>D^∧A</b>_<b>1</b>-<b>A</b>_<b>2</b>) – <i>k</i>_ET(<b>D^∧A</b>_<b>1</b>) = <i>k</i>_ET(<b>D</b>*→<b>A</b>_<b>2</b>) ∼ 1 × 10⁹ s^–1). This channel of relaxation represents ∼15% of <i>k</i>_ET for <b>D</b>*→<b>A</b>_<b>1</b>