Glaser-mediated synthesis and photophysical characterization of diphenylbutadiyne-linked porphyrin dyads

The Pd-mediated Glaser coupling of a zinc monoethynyl porphyrin and a magnesium monoethynyl porphyrin affords a mixture of three 4,4′-diphenylbutadiyne-linked dyads comprised of two zinc porphyrins (Zn-pbp-Zn), two magnesium porphyrins (Mg-pbp-Mg), and one metalloporphyrin of each type (Zn-pbp-Mg). The latter is easily isolated due to the greater polarity of the magnesium versus the zinc chelate. Exposure of Zn-pbp-Mg to silica gel results in selective demetalation, affording Zn-pbp-Fb where Fb = free base porphyrin. This synthesis route employs the magnesium porphyrin as a latent form of the Fb porphyrin, thereby avoiding copper insertion during the Glaser reaction, and as a polar entity facilitating separation. The absorption spectrum of Zn-pbp-Mg or Zn-pbp-Fb is the sum of the spectra of the component parts, while in each case the fluorescence spectrum upon illumination of the Zn porphyrin is dominated by emission from the Mg or Fb porphyrin. Time-resolved absorption spectroscopy shows that the energy-transfer rate constants are (11 ps)^-1 and (37 ps)^-1 for Zn-pbp-Mg and Zn-pbp-Fb, respectively, corresponding to energy-transfer quantum yields of 0.995 and 0.983, respectively. The calculated Förster through-space rates are (1900 ps)^-1 and (1100 ps)^-1 for Zn-pbp-Mg and Zn-pbp-Fb, respectively. Accordingly, the through-bond process dominates for both dyads with a through-bond:through-space energy-transfer ratio of ≥ 97:1. Collectively, the studies show that the 4,4′-diphenylbutadiynyl linker supports fast and efficient energy transfer between Zn and Mg or Fb porphyrins.