Synthesis and excited-state photodynamics in perylene-porphyrin dyads 2. Effects of porphyrin metalation state on the energy-transfer, charge-transfer, and deactivation channels

The photophysical properties of two perylene-porphyrin dyads have been examined in detail with the aim of expanding the functional utility of these constructs for molecular optoelectronics applications. The dyads consist of a perylene-bis(imide) dye (PDT) connected to either a magnesium porphyrin (Mg) or a free base porphyrin (Fb) via a diphenylethyne (pep) linker. The photophysical behavior of these two dyads show both similarities and differences to one another and to the dyad containing a zinc porphyrin (Zn) that was examined in the previous paper in this series. In the case of both PDI-pep-Fb and PDI-pep-Mg in toluene. the excited perylene unit (PDI*) decays rapidly (Fb = 2.9 ps; Mg = 2.5 ps) by energy transfer to the porphyrin forming PDI-pep-Por* in relatively high yield (Fb similar to 85%; Mg similar to 50%) and hole transfer to the porphyrin forming PDI--pep-Por(+) (Fb similar to 15%; Mg similar to 50%). This behavior parallels that observed for PDI-pep-Zn. for which rapid (2.5 ps) decay of PDI* affords PDI-pep-Zn* and PDI--pep-Por+ with yields of 80% and 20%, respectively. The subsequent behavior of the Fb-containing dyad is distinctly different in two ways from that of the Zn or Mg porphyrin-containing dyads. (1) Charge recombination within PDI--pep-Fb(+) primarily forms PDI-pep-Fb*, thereby complementing the formation of the latter species from PDI*pep-Fb. (2) PDI-pep-Fb* subsequently decays to the ground state via fluorescence emission with a rate and yield that are nearly identical to those of an isolated Fb porphyrin. In contrast. for both PDI-pep-Mg and PDI-pep-Zn, the predominant decay process for PDI-pep-Por* is electron-transfer yielding PDI--pep-Por(+) (Zn similar to 80%; Mg > 99%). The rapid electron-transfer quenching of PDI-pep-Por* and the nonemissive character of PDI--pep-Por(+) leads to negligible fluorescence from the two metalloporphyrin-containing dyads after photoexcitation. The PDI--pep-Por(+) charge- separated product with Por = Mg or Zn is very long-lived (> 10 ns) in toluene but decays much more rapidly (<0.5 ns) in acetonitrile. The differences in the rates of the various charge-transfer and charge-recombination processes of all of the dyads are consistent with a rate versus free-energy-gap profile (based on the relative redox potentials of the porphyrin constituents) that is in qualitative accord with electron-transfer theory. Collectively, the studies reported in this and the previous paper indicate that PDI-pep-Fb has the greatest potential utility in photonics applications wherein light harvesting by an accessory pigment, energy transport to an output chromophore, and emission (or energy transfer to another chromophore) are desired. On the other hand, PDI-pep-Mg (like PDI-pep-Zn) would be most useful as an all-optical gating element in which excited-state energy in an appended chromophore chain can be quenched by the charge- separated state of the perylene-porphyrin dyad, thereby shunting the light output or flow of energy.