Origin of the temperature dependence of the rate of singlet energy transfer in a three-component truxene-bridged dyads

We report a truxene-based dyad built upon one donor (tri-meso-phenylzinc(II) porphyrin) and two acceptors (octa-beta-alkylporphyrin free base) in which the donor exhibits free rotation around a C-truxene-C-meso single bond at 298 K in fluid solution but not at 77 K in a glass matrix, whereas the acceptors have very limited motion as they are blocked by beta-methyl groups. This case is interesting because all the structural and spectroscopic parameters affecting the rate for singlet energy transfer according to a Forster Resonance Energy Transfer are only weakly temperature dependent, leaving only the Dexter mechanism explaining the larger variation in rate of energy transfers with the temperature hence providing a circumstantial evidence for a dual mechanism (Foster and Dexter) in truxene-based dyads (or polyads) in the S-1 excited states.