Accelerated singlet energy transfer in bis(phenylethynyl)phenylene-bridged 5,10,15,20-tetraaryl zinc–free base hybrid diporphyrins

Abstract

A set of three bis(phenylethynyl)phenylene-bridged 5,10,15,20-tetraaryl zinc–free base hybrid diporphyrins 1–3(ZH) has been prepared by the Pd-catalyzed coupling reaction of ethynyl-substituted zinc–porphyrin 7 with diiodobenzene derivatives followed by partial zinc-insertion and separation by chromatography over a silica gel flash column. A similar reaction of 7 with 1,2,4,5-tetraiodobenzene effectively provided porphyrin tetramer 10 in 16% yield in a single step. Intramolecular singlet energy transfer in zinc–free base hybrid diporphyrins 1–3(ZH) has been studied by picosecond time-resolved fluorescence spectroscopy. Determined intramolecular energy transfer rates (k_EN) are 1.5 × 10¹⁰ s^–1, 4.6 × 10⁹ s^–1, and 6.4 × 10⁹ s^–1, respectively, being larger than those of the corresponding 10,20-diaryl-2,3,7,8,12,13,17,18-octaalkyl-substituted diporphyrins. The k_EN rate enhancement has been found to depend on the geometry of the two porphyrins; 3.4-, 8.1-, and 11-fold for 1,2-, 1,3-, and 1,4- isomers, respectively, reflecting the magnitude of the through-bond electronic interactions.