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

Abstract

We report a truxene-based dyad built upon one donor (tri-meso-phenylzinc(II)porphyrin) and two acceptors (octa-β-alkylporphyrin free base) in which the donor exhibits free rotation around a Ctruxene-Cmeso 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 β-methyl groups. This case is interesting because all the structural and spectroscopic parameters affecting the rate for singlet energy transfer according to a Förster 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 (Föster and Dexter) in truxene-based dyads (or polyads) in the S1 excited states.

Excited State N−H Tautomer Selectivity in the Singlet Energy Transfer of a Zinc(II)‐Porphyrin–Truxene–Corrole Assembly

“…Given that the ET process will occur for either the T1 or the T2 tautomer, the true J ‐integral value would need to be calculated using the absorption spectrum of the pure tautomers, which for obvious reasons cannot be obtained. Our previous work on similar systems bridged by truxene has shown that the presence of a conjugated bridge between the donor and acceptor strongly promotes the Dexter mechanism . The experimental ET rate found for [Zn‐COR 2 ] is on the same order of magnitude as those for previously reported compounds; therefore, the assumption that the Dexter process plays an important role is reasonable.…”

“…The k Förster value was determined by calculating κ 2[89, 90] (details in the Supporting Information) using the optimized geometry (DFT computations; Figure ), and the J ‐integral was evaluated using the experimental spectra (Supporting Information, Figure S31). Equation (1) yielded k Förster =1.09×10 13 s −1 , which is three orders of magnitude faster than those for other truxene‐containing polyads from our laboratory (Figure ) …”

“… Structures and singlet energy transfer rates (in s −1 ) for previously reported donor–acceptor dyads built upon a truxene spacer …”

“…We report the synthesis and analysis of a donor–acceptor system, [Zn‐COR 2 ] , based on Zn‐porphyrin, [Zn] , and free‐base corrole, [COR] , bridged by a central ( C 3 v point group) truxene scaffold, an aromatic species known for promoting fast singlet energy transfer . This efficient [Zn] *→ [COR] process is needed to adequately probe the selectivity of one excited‐state tautomer over the other.…”

“…We also measured the rate of excited‐state tautomerization in the donor–acceptor dyad [Zn‐COR 2 ] as well as for two model compounds [COR] and [COR 2 ] (Scheme ). The preparation of the investigated compounds was performed according to a previously published method and is summarized in Scheme . The energy transfer processes is examined from the point of view of both the FRET and Dexter theories, and a strong Dexter dependence is assumed for these systems in relation to the results that were obtained for similar compounds …”