Unusual temperature-range-tunable fluorescence characteristic of C6-unsubstituted tetrahydro-pyrimidines: Influence factors, sensitivity evaluation and application in different temperature ranges

Abstract

Fluorescence thermometers have attracted wide interesting owing to their advantages of ultra-high sensitivity, fast response speed, high spatial resolution, safe and non-invasive detection signal etc. However, conventional fluorescence thermometers are sensitive only in a certain range. In 2016, we found that one of C6-unsubstituted tetrahydro-pyrimidines (THPs) that synthesized by an efficient five-multicomponent reaction (5CR) shows unusual temperature-range-tunable (TRT) fluorescence characteristic: showing sensitive fluorescence-color/-on-off changes in different temperature ranges by only changing excitation wavelength. Here, the influences of molecular structures, packing modes and chirality on the TRT fluorescence characteristic of THPs were investigated in detail. In addition, the crucial parameter evaluating the TRT characteristics and the selection of the suitable excitation wavelengths in different T-ranges are explored. The obtained experimental results indicate that: (a) the TRT characteristics of THPs mainly depends on molecular structures rather than their R- and S-enantiomer molecular packing modes; (b) although the pure enantiomeric alignments of THPs significantly decrease their fluorescence quantum yields, they favor the TRT characteristics; (c) the response (Sλrex) of red-edge excitation wavelength (λrex) to temperature is a crucial parameter for evaluating TRT characteristic; (d) the suitable excitation wavelengths for fluorescence measurement in different temperature ranges can be simply identified by the good linear relationship between λrex and temperature; (e) Racemic THP 5c is the most sensitive thermometer in different T-ranges. From 10–430 K (−263 °C−157 °C), 5c could show sensitive fluorescence-on-off change in any narrow T-range (<40 K), such as 10–30 K, 30–60 K … 300–330 K, 400–430 K, by selecting suitable λrex values (the thermo-response sensitivities ranges from 0.83 to 4.20% K−1). The work reported here is expected to promote the development and application of different kinds of fluorescence thermometers with TRT characteristics.