Directing Energy Transfer in Panchromatic Platinum Complexes for Dual Vis–Near-IR or Dual Visible Emission from σ‑Bonded BODIPY Dyes

We report on the
platinum complexes trans-Pt­(BODIPY)­(8-ethynyl-BODIPY)­(PEt3)2 (EtBPtB) and trans-Pt­(BODIPY)­(4-ethynyl-1,8-naphthalimide)­(PR3)2 (R = Et, EtNIPtB-1; R = Ph, EtNIPtB-2),
which all contain two different dye ligands that are connected to
the platinum atom by a direct σ bond. The molecular structures
of all complexes were established by X-ray crystallography and show
that the different dye ligands are in either a coplanar or an orthogonal
arrangement. π-stacking and several CH···F and
short CH···π interactions involving protons at
the phosphine substituents lead to interesting packing motifs in the
crystal. The complexes feature several strong absorptions (ε
= 3.2 × 105–5.5 × 105 M–1 cm–1) that cover the regime from
350 to 480 nm (EtNIPtB-1 and EtNIPtB-2)
or from 350 to 580 nm (EtBPtB). Besides the typical absorption
bands of both kinds of attached dyes, they also feature an intense
band near 400–420 nm, which is assigned by time-dependent density
functional theory calculations to a higher-energy transition within
the ethynyl-BODIPY (EtB) ligand or to charge transfer between the
BODIPY (B) and naphthalimide (NI) chromophores. All complexes show
dual fluorescence and phosphorescence emission from either the B (EtNIPtB-1 and EtNIPtB-2) or EtB (EtBPtB) ligand with a maximum phosphorescence quantum yield of 41% for EtNIPtB-1. The latter seems to be the highest reported value
for room temperature phosphorescence from a BODIPY dye. The complete
quenching of the emission from the chromophore absorbing at the higher
energy and the appearance of the corresponding absorption bands in
the fluorescence and phosphorescence excitation spectra indicate complete
and rapid energy transfer to the chromophore with the lower-energy
excited state, i.e., EtNI → B in EtNIPtB-1 and EtNIPtB-2 and B → EtB in EtBPtB. The latter
process was further investigated by transient absorption spectroscopy,
indicating that energy transfer is complete within 0.6 ns. EtNIPtB-1 catalyzes the photooxidation of 1,5-dihydroxynaphthalene with photogenerated 1O2 to Juglone at a much faster rate than methylene
blue but with only modest quantum yields of 37% and with the onset
of photodegradation after 60 min.