4. Luminescent indicators for optical temperature sensing

Although traditional methods of temperature measurement are very common, optical temperature measurement represents an interesting alternative. Such measurements are free of electromagnetic interferences, can be performed through a transparent glass window and even combined with measurements of other parameters in the same spot (multi-parameter sensors). For some applications (pressure-sensitive paints) contactless sensing of temperature is essential. Molecular thermometers along with thermographic phosphors belong to the most common materials. Although the former show inferior photochemical stability, they are typically much brighter due to high molar absorption coefficients. Additionally, the can be, imbedded e.g. in polymeric nanoparticles to enable optical temperature measurement in very small objects such as cells. Eu(III) complexes benefit from very narrow red emission and long luminescence decay times which are only minor affected by oxygen. However, most of these dyes show excitation in the UV part of the spectrum, which is undesirable for practical applications. We demonstrated that visible-light excitable Eu(III) complexes (Fig. 4.1) are promising molecular thermometers.[27,28] The new dyes are excitable with violet light and emit very strong red luminescence (λmax 617 nm, QY 65- 75 % at 1 °C). The luminescence of the new dyes was highly temperature-sensitive: both luminescence intensity and decay time reduced significantly at higher temperature. Importantly, the optimal response was achieved at ambient temperatures. In an alternative approach we modified polystyrene with covalently bound acridone, which acted as a blue light –excitable antenna for the Eu(III) complex which was embedded into the polymer.[29] Indeed, bright luminescence from the Eu(III) complex was observed under excitation with a blue light. The composite material showed appreciable sensitivity to temperature, but also, unfortunately, some cross-talk to oxygen.
Figure 4.1. Chemical structures of molecular thermometers based on luminescent Eu(III) complexes reprodcued from [27,28].
We also reported a new class of optical temperature probes which utilize thermally-activated delayed fluorescence (TADF). [30] Two different dye groups (carbazole-substituted dicyanobenzenes and anthraquinone-based intramolecular charge-transfer dyes, Fig. 4.2), showed very high temperature dependency of the TADF decay times (1.4 – 3.7 %/K change of the lifetime at 298 K), being particularly advantageous for optical thermometry. Contrary to many state-of-the-art optical thermometers, the dyes show only moderate decrease of TADF intensity at increased temperatures
Figure 4.2. Chemical structures of molecular thermometers based on TADF organic dyes reprodcued from [30].

References:

[27] Borisov, S. M.; Wolfbeis, O. S. Temperature-Sensitive Europium(III) Probes and Their Use for Simultaneous Luminescent Sensing of Temperature and OxygenAnalytical Chemistry 200678 (14), 5094–5101. [28] Borisov, S.; Klimant, I. Blue LED Excitable Temperature Sensors Based on a New Europium(III) ChelateJOURNAL OF FLUORESCENCE 200818 (2), 581–589. [29] Borisov, S. M.; Klimant, I. New Luminescent Oxygen-Sensing and Temperature-Sensing Materials Based on Gadolinium(III) and Europium(III) Complexes Embedded in an Acridone–polystyrene ConjugateAnal Bioanal Chem 2012404 (10), 2797–2806. [30] Steinegger, A.; Klimant, I.; Borisov, S. M. Purely Organic Dyes with Thermally Activated Delayed Fluorescence—A Versatile Class of Indicators for Optical Temperature SensingAdvanced Optical Materials n/a-n/a.