Effects of metalation state (free base, Mg, Zn, Cd) on excited-state energy transfer in diarylethyne-linked porphyrin dimers

The ground- and excited-state properties of two new porphyrin dimers have been examined using static and time-resolved optical techniques. One dimer consists of a zinc porphyrin and a magnesium porphyrin (ZnMgU), and the other dimer consists of a cadmium porphyrin and a free base (Fb) porphyrin (CdFbU). In both arrays, the porphyrins are joined by a diarylethyne linker at one meso position with mesityl groups at the nonlinking meso positions. The rates of photoinduced energy transfer are faster for ZnMgU ((9 ps)^-1) and CdFbU ((15 ps)^-1) than found previously for ZnFbU ((24 ps)^-1) and MgFbU ((31 ps)^-1). Only for CdFbU does the yield of excited-state energy transfer (87%) drop below the near-quantitative (≥99%) level, and this effect derives solely from competition with a very short inherent lifetime (∼100 ps) of the photoexcited Cd porphyrin. The results further illustrate (1) the efficacy of this dimeric architecture for ultrafast excited-state energy transfer, (2) how molecular/electronic properties can be manipulated to tune photoinduced energy flow in multiporphyrin arrays, and (3) key factors impacting effective inter-porphyrin electronic communication, including porphyrin orbital tuning.