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 b een examined using static and time-resolved optical techniques. One dimer c onsists of a zinc porphyrin and a magnesium porphyrin (ZnMgU), and the othe r dimer consists of a cadmium porphyrin and a free base (Fb) porphyrin (CdF bU). 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)) an d CdFbU ((15 ps)(-1)) than found previously for ZnFbU ((24 ps)(-1)) and MgF bU ((31 ps)(-1)). Only for CdFbU does the yield of excited-state energy tra nsfer (87%) drop below the near-quantitative (greater than or equal to 99%) level, and this effect derives solely from competition with a very short i nherent lifetime (similar to 100 ps) of the photoexcited Cd porphyrin. The results further illustrate (1) the efficacy of this dimeric architecture fo r ultrafast excited-state energy transfer, (2) how molecular/electronic pro perties can be manipulated to tune photoinduced energy flow in multiporphyr in arrays, and (3) key factors impacting effective inter-porphyrin electron ic communication, including porphyrin orbital tuning.