Fluorescent Probes for Monitoring Temperature in Organic Solvents

Abstract

We report two fluorophores, N-(1-pyrenylmethyl)-1-pyrenebutanamide and N-(1-pyrenylmethyl)-1-pyreneacetamide, that exhibit temperature-dependent emission spectra over the temperature range of 20-100°C. The fluorophores are readily synthesized via a one-pot process and can be used in dilute form (<1 µM) for measuring the temperature of various organic solvents nonintrusively. The fluorophores allow measurement of temperatures within a precision of (1°C and provide an internal reference signal that allows factors such as fluorophore concentration, optical path, and excitation light intensity to be ignored.Fluorescence-based temperature sensing is receiving increasing interest as it finds unique applications in monitoring the temperature within micro-sized domains (e.g., a biological cell) or hostile environments (e.g., a microwave-irradiated system). 1-3 The measurement can be made externally to the region of interest and provides useful flexibility in defining the location of temperature measurement and in determining spatial variations in temperature within a region. This method has the advantage over physical probes of temperature (thermometers, thermocouples, etc.) for fluid systems and systems under electromagnetic irradiation in that flow patterns and field lines are not disturbed by the presence of these molecular probes.A number of fluorescence-based optical thermometers have relied on changes in the intensity or wavelength of the fluorescence maximum. Limitations of these systems include the inherent fluctuations in the intensity of the excitation source and observed shifts in wavelength on the order of <0.1 nm/°C. 3,4 Strategies incorporating two fluorescent moieties within one molecule have provided an internal reference source that obviates factors that would affect measurements based on the intensity of a single emission process. Among this class of molecular probes, those containing two pyrenyl units offer high quantum yield and good sensitivity. Limitations for these fluorophores include difficulties associated with their multistep syntheses 5 and their restricted temperature range of operation. Presently, fluorophores of this type have been reported only for measuring temperatures in organic solvents above ∼110°C and below ∼20°C. 2,6,7 Here we describe the synthesis and characterization of a series of easily prepared bipyrenyl fluorophores that are suitable for fluorescence thermometry over the temperature range of 20-100°C, i.e., from room temperature to the boiling point of most common organic solvents.

Luminescent Molecular Thermometers

“…Here we describe situations in which two different fluorescent states equilibrate. Several of these function by forming intramolecular excimers (19,20). Excimer (excited-state dimer) means a complex of two identical fluorophores in the excited state (21).…”

“…Excimer (excited-state dimer) means a complex of two identical fluorophores in the excited state (21). N-(1-Pyrenylmethyl)-1-pyrenebutanamide, 7, is a nice representative (20). When one pyrene fluorophore is excited, then the following equilibrium (eq 1) holds,…”

Excimer‐Based On‐Off Bis(pyreneamide) Macrocyclic Chemosensors

“…This sensitivity of the pyrene emission to the environment has been perceived as an opportunity for the construction of cation sensors . [ ] Systems were designed with (i) podand, crown ether, or calixarene‐based fragments for cation binding, (ii) covalently linked pyrenes acting as fluorophores and, upon the presence or absence of metal cations, (iii) sufficiently different conformations to modulate the fluorescence . [ ] For instance, Nakamura et al .…”

Thermal Characterization of Microheated Microchannels With Spatially Resolved Two-Color Fluorescence Thermometry

“…The fluid temperatures in the heated microchannel sections were measured using temperature sensitive fluorescence dyes [31]. In an optically thin medium, the fluorescent intensity is expressed as,…”