Molecular thermometer for cryogenic range based on Pd-porphin phosphorescence

Abstract

In this paper we report on the possibility to design the luminescent molecular thermometer for the cryogenic temperatures. The approach is based on the high temperature sensitivity of the Pd-porphin phosphorescence in <i>n-</i>alkane matrices. The dependence of radiative deactivation of Pd-porphin triplet states on temperature has been studied in Shpol'skii matrices in the temperature interval of 1.2-210&Kappa; . The dramatic changes in the phosphorescence spectra were observed upon temperature rise, which were due to inclusion of the thermally activated states of Pd-porphin in the processes of radiative deactivation. At liquid helium temperature <i>T</i>₁&rarr;<i>S</i>₀ emission is observed only, and an increase in temperature leads to the activation of <i>T</i>_i&rarr;<i>S</i>₀ radiative transitions, so new additional bands appear. Activation energy <i>E</i>_a of these states has been measured in <i>n-</i>octane and <i>n-</i>nonane matrices. The splitting value of the lowest quasidegenerate triplet state &Delta;<i>E</i>(<i>T</i>₂-<i>T</i>₁) has been determined for both planar and distorted conformations of Pd-porphin macrocycle in noctane matrix, amounting to 40 and 57 cm^-1, respectively. The workability of dependence of Pd-porphin phosphorescent properties on temperature in the design of the molecular thermometers for the temperature range from few K up to temperatures close to the melting point of <i>n</i>-alkane matrix (150-200 K) was shown.