Ligand Based Dual Fluorescence and Phosphorescence Emission from BODIPY Platinum Complexes and Its Application to Ratiometric Singlet Oxygen Detection

Four new 4,4-difluoro-4-bora-3a,4a-diaza-s-indacen-8-yl
(BODIPY) platinum­(II) complexes of the type cis-/trans-Pt­(BODIPY)­Br­(PR3)2 (R = Et or
Ph) were synthesized and characterized by NMR, electronic absorption,
and luminescence spectroscopy. Three of the complexes were also studied
by single crystal X-ray diffraction. The absorption profiles of the
four complexes feature intense HOMO → LUMO π →
π* transitions with molar extinction coefficients ε of
ca. 50 000 M–1cm–1 at around
475 nm and vibrational progressions that are characteristic of BODIPYs.
Most remarkably, most complexes exhibit dual emissions through fluorescence
at ca. 490 nm and phosphorescence at ca. 650 nm that originate from
Pt-perturbed BODIPY-centered 1ππ* or 3ππ* states, respectively. Electronic absorption and luminescence
spectroscopy data are in good agreement with our TD-DFT calculations.
While the emission of the cis-complexes is dominated
by fluorescence, their trans-isomers emit predominantly
through phosphorescence with a phosphorescence quantum yield for trans-Pt­(BODIPY)­Br­(PEt3)2 (trans-1) of 31.2%. trans-1 allows for ratiometric one-component
oxygen sensing in fluid solution up to atmospheric concentration levels
and exhibits a remarkably high Stern–Volmer constant for the
quenching of the excited triplet state by oxygen of ca. 350 bar–1 as determined by changes in phosphorescence intensity
and lifetime.