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

Abstract

The photophysical properties of self-assembled zinc-porphyrin/tungsten-alkylidyne dyads have been investigated with the aim of determining whether the porphyrin S excited state sensitizes the tungsten-alkylidyne (3)[dπ*] state. The luminescent metalloligand W(≡CC(6)H(4)CCpy)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) has been synthesized and shown by electronic and NMR spectroscopy to coordinate axially to ZnTPP and ZnTP(Cl)P (TP(Cl)P = tetra(p-chlorophenyl)porphyrin) via the terminal pyridyl group. Coordination of 1 to ZnPor results in partial quenching of porphyrin S(1) 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 S(1) 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 (φ(EnT) ≈ 80%), thus strongly enhancing light harvesting for the tungsten-alkylidyne compound. The mechanism and rates of the net S(1)→(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.

Photoinduced Charge Separation in Zinc–Porphyrin/Tungsten–Alkylidyne Dyads: Generation of Reactive Porphyrin and Metallo Radical States

“…In this context, we recently described the structures, bonding, and excited-state properties of self-assembled zinc-porphyrin/tungsten-alkylidyne dyads, in which the luminescent tungsten-alkylidyne metalloA 1) is coordinated via its pendant pyridyl moiety to the zinc center of a tetraarylporphyrin (Scheme 1). [12] These dyads, generically denoted hereafter as ZnPor(1), were designed to take advantage of the valuable light-harvesting, photophysical, and redox properties of zinc porphyrins. [13] Density-functional theory calculations on ZnPor(1) showed that the HOMO is the redox-active d xy orbital of 1, and that this and most other frontier orbitals are more than 99 % localized on the parent subunit (Figure 1 a).…”

“…10 11 s À1 ) to produce the 3 A C H T U N G T R E N N U N G [dp*] state (Figure 1 b). [12] Thus, the ZnPor(1) architecture significantly enhances light harvesting for the photochemically useful 3 A C H T U N G T R E N N U N G [dp*] state of 1.…”

“…(b) Jablonski diagram of principal photophysical processes (solid arrows) observed for ZnPor(1) in toluene solution; shaded boxes indicate ranges of excitedstate energies for coordinated 1. [14] Results and Discussion Spectroscopic and redox properties of ZnPor(1) dyads in fluorobenzene: In our earlier report, [12] it was shown that the molecular and electronic structures of ZnTPP(1) and ZnTP Cl P(1) (Scheme 1 and Figure 1) in toluene solution are similar in many respects to those of simple axially substituted ZnPor(L) complexes, such as ZnPor(py), as evidenced by their comparable equilibrium binding constants (ZnTPP(py), K b = 6000 m À1 ; [17] ZnTPP(1), 7700 m À1 ; ZnTP Cl P(1); 9100 m À1 ), identical electronic-absorption band maxima, and the axial geometries predicted by density functional theory calculations. In the present study, the axially ligated ZnPor(1) dyads are observed also to form in fluorobenzene.…”

“…, l max = 403 nm, e = 35 000 m À1 cm À1 ). [12] Consequently, it is possible to selectively excite the porphyrin-centered S 1 states of the dyads.…”

“…intersystem crossing is very fast (k iscÀW ! 10 11 s À1 ) [12] it is unlikely that formation of [ZnPor À ] [1 + ] from the 1 A C H T U N G T R E N N U N G [dp*] state is kinetically feasible, and so this pathway is not considered further (or shown in Figure 3).…”