Balancing the phosphorescence and fluorescence of a double-ring porphyrin using different lanthanides for ratiometric oxygen sensing

Abstract

Precise determination of oxygen concentrations is vital in a wide range of applications. Molecules with balanced phosphorescence and fluorescence can act as ratiometric oxygen sensors, and these single-molecule sensors represent the most accurate approach for oxygen sensing. Harnessing the different degrees of singlet–triplet state mixing induced by different lanthanides, a double-ring porphyrin (sinoporphyrin sodium, DVDMS) was simultaneously coordinated to gadolinium(III) and praseodymium(III) to balance the excited-state populations. The fluorescence and phosphorescence quantum yields for this lanthanide complex (PrGd-DVDMS) were 0.17% and 0.66%, respectively. Thus, the phosphorescence-to-fluorescence intensity ratio of PrGd-DVDMS was only 3 : 1; this compares very favorably with that for DVDMS coordinated to two gadolinium(III) ions (Gd₂-DVDMS), which was 30 : 1. Furthermore, PrGd-DVDMS displayed excellent ratiometric oxygen sensing characteristics, and the uncertainty in its phosphorescence-to-fluorescence intensity ratio was considerably lower than that of Gd₂-DVDMS. This new mixed-lanthanide porphyrin therefore has excellent potential as an oxygen indicator for applications such as environmental monitoring that require high reliability.