New red-emitting Schiff base chelates: promising dyes for sensing and imaging of temperature and oxygen <i>via</i> phosphorescence decay time

Abstract

New complexes of Zn(ii), Pd(ii) and Pt(ii) with Schiff bases are prepared in a one-step condensation of 4-(dibutylamino)-2-hydroxybenzaldehyde and 4,5-diaminophthalonitrile in the presence of a metal salt. The complexes possess efficient absorption in the blue-green part of the spectrum with molar absorption coefficients up to 98 000 M^-1 cm^-1. The Pt(ii) complex shows very strong red phosphorescence in anoxic solutions at room temperature with a quantum yield of 65% in toluene which places it among the brightest emitters available for this spectral range. The phosphorescence of the Pd(ii) complex under the same conditions is very weak (<i>Φ</i> < 1%) but is enhanced to <i>Φ</i> > 10% upon immobilization into polymers. Optical thermometers based on self-referenced lifetime read-out are prepared upon immobilization of the dyes into gas-blocking poly(vinylidene chloride-<i>co</i>-acrylonitrile). At 25 °C, the materials based on Pd(ii) and Pt(ii) complexes show sensitivities of -2.1 and -0.52%<i>τ</i>/K, respectively. Application of the sensors for imaging of temperature on surfaces (planar optode) and for monitoring of fast temperature fluctuations (fiber-optic microsensor) is demonstrated. Immobilized into a gas-permeable matrix, the Pt(ii) complex also performs as a promising oxygen-sensing material. The new systems are also attractive for imaging of oxygen or temperature with the help of multi-photon microscopy, due to a good match with the biological optical window and much better brightness under two photon excitation compared to that of the conventional Pt(ii) <i>meso</i>-tetra-(pentafluorophenyl)porphyrin.