FRET Sensitization of Tungsten–Alkylidyne Complexes by Zinc Porphyrins in Self-Assembled Dyads

The photophysical properties of self-assembled zinc–porphyrin/tungsten–alkylidyne
dyads have been investigated with the aim of determining whether the
porphyrin S1 excited state sensitizes the tungsten–alkylidyne 3[dπ*] state. The luminescent metalloligand W­(CC6H4CCpy)­(dppe)2Cl (1; dppe
= 1,2-bis­(diphenylphosphino)­ethane) has been synthesized and shown
by electronic and NMR spectroscopy to coordinate axially to ZnTPP
and ZnTPClP (TPClP = tetra­(p-chlorophenyl)­porphyrin) via the terminal pyridyl group. Coordination
of 1 to ZnPor results in partial quenching of porphyrin
S1 fluorescence and a decrease in the 3[dπ*]
excited-state lifetime of 1. Transient-absorption spectroscopy
shows that fluorescence quenching occurs via intramolecular Förster
resonance energy transfer from the porphyrin S1 state to
the 1[dπ*] excited state of 1, which
then undergoes rapid singlet–triplet intersystem crossing to
produce the 3[dπ*] excited state. Sensitization of
the 3[dπ*] state occurs with high overall efficiency
(ϕEnT ≈ 80%), thus strongly enhancing light
harvesting for the tungsten–alkylidyne compound. The mechanism
and rates of the net S1→3[dπ*]
energy transfer are found to differ significantly from those for previously
reported zinc–porphyrin/tungsten–alkylidyne dyads that
are constructed from similar components but connected instead with
covalent bonds at the porphyrin edge. Density functional theory calculations
indicate that these differences are due in part to the degree of orbital
mixing between the porphyrin and metal–alkylidyne subunits.