Temperature Insensitive Fluorescence Intensity In A Coumarin Monomer -Aggregate Coupled System

Temperature is a fundamental parameter affecting various physical and chemical processes; Fluorescent sensing of temperature has recently attracted considerable research interests across different fields, such as tracking cellular events in biological systems and exploring the flow mixing in fluid dynamics, with high spatial and temporal resolutions. In most fluorescent systems, fluorescence intensities decrease with a rise in temperature owing to the thermal activation of nonradiative de-excitation pathways. The temperature dependence of fluorescence can be used to construct thermometers. In such applications, ratiometric measurements are preferred because it provides a built-in correction to permit signal ratio ing and allow accurate and quantitative measurements. Ratiometric fluorescent thermometers usually contain one temperature-sensitive fluorophore as a sensor and one relatively temperature-insensitive fluorophore as a reference; The emission intensity ratios of the sensor and the reference afford temperature information.Following the recent exploration of molecular aggregates and their functionalities, we propose and demonstrate a new design concept to achieve ultra-low temperature dependence of emission intensity using a monomer-aggregate coupled system. In the most simplified version of this design, the QE of a dye drops as temperature increases; while at the same time, the rising temperature leads to the dissolution of non-emissive molecular aggregates, boosting the quantity of emissive monomers. By achieving a proper balance between these two contrasting temperature dependent emission characteristics, the overall emission intensity of the fluorescent system could be maintained at a constant level.Our model system is based on 7-(dimethylamino)-coumarin-3-carbaldehyde (1) in chloroform and coumarin 545(2) in methanol. Aggregation is proved through absorption spectra?emission spectra?1H-NMR?fluorescent lifetime measurements. The emission intensities of a fluorescent monomer-aggregate coupled system, based on 7-(dimethylamino)-coumarin-3-carbaldehyde, exhibit ultra-low temperature dependence and afford a temperature coefficient, of only 0.05% per?, by judicious selection of the excitation wavelength. The temperature coefficient of coumarin 545 (2) in methanol is as low as 0.025% per?.This fluorescence system can thus be used as a temperature-insensitive reference. Moreover, based on the unique temperature dependent emission, ratiometric temperature measurements can be performed, either in conjunction with another highly temperature-sensitive dye, or by exploring different temperature sensitivities of the monomer and H-aggregate emissions. This study has significant implications to the development of fluorescence based thermometer.