Richard WilsonThe luminescent porous coordination polymers have received considerable interest due to their extensive applications in light-emitting display chemical sensing, optical devices, but also become extremely owing to their multiple nonlinear optical properties. However, if one looks at the field from the structural point of view it turns out that the majority of the research on luminescent coordination polymers concentrated on transition metals (d-block) and well known group of lanthanides (f-block). The incorporation of ions such as europium or terbium ions into a specially tailored organic scaffold has been and remains the most effective method of creating luminescent lanthanide-based coordination polymers. By contrast, the majority of luminescent transition metal based CPs focus on elements that have “closed-shell” d10 configurations.
It is important to note that the trivalent lanthanide ions are typically used in luminescent materials as well as luminescent white light emissions. Regarding luminescent properties, the lanthanides have very specific properties in terms of spectroscopy that result from electron repulsion and the powerful spin-orbit coupling. To a lesser extent, the effect to the crystal field that is that is treated as a slight perturbation of the spectroscopy terms of energies and splitting. This leads to electronic energy level configuration as well as the well-known photophysical characteristics like long lifetimes of the emissions as well as narrow emission bands, not to mention about huge pseudo-Stokes shifts.
A possible use for luminescent lanthanide coordination polymers is in the optical thermometry area. Optical thermometry utilizes the variation in the emission state and the quenching of an emission, and/or the ratio between intensity of emission bands is measured as a function of temperature. These thermometers are luminescent and have the advantages of contactless measurements.
How does the intensity of lanthanide emission may be affected by temperature? All it boils down to energy transfer and processes known as temperature quenching. The process of quenching the temperature of an ion emitting lanthanide level is possible through two fundamental pathways: multiphonon relaxation transfer of energy to ligand localized electronic states, and an energy transfer from 4f ions to the charge-transfer state. These three processes can all be utilized to create ratiometric, linear and nonlinear optical thermometry. This technique uses the ratio of at least two temperature-dependent optical parameters, lifetime, intensity, or spectral position. The most widely used method is to employ the light emission intensities. Additionally self-calibrated optical thermometers built on the light emission intensity method are the most promising method due to its independency of the excitation energy intensity.
In addition, the emission lifetime-based measurements have advantages of avoiding certain problems associated with the use of single emission-band method. One of these is the independency of the source stability of excitation. Also, the sensitivity of detectors is not a problem, and the inhomogeneity of the material is not an issue as well. Thus, the mixed europium/terbium coordination polymers that make use of the lifetime parameter are ideal for thermometry purposes. While the measurement based on intensity has a slightly higher values of the maximum relative sensitivities, the unique benefits of a measurement based on lifetime makes this method appealing.
It is important to note that the number of publications and citations to optical thermometry research papers has been growing dramatically over the last 10 years. Over the course of this decade there was a significant amount of original research papers as well as review articles were released. They thoroughly covered the background, methods available of thermometers’ design, different materials, and finally – all state-of-the-art applications of optical thermometry.
It is worthwhile to point out that tremendous majority of the available optical thermometers are excited by using ultraviolet light. However, a completely new area in luminescence thermometry emerged very recently – that is nonlinear optical thermometry. The very first example of this kind of thermometry utilizes exotic nonlinear optical property (three-photon absorption) of lanthanide coordination polymer within the near-infrared spectrum. In this new technique, referred also to as nonlinear optical temperature sensing, compounds that are normally excited by ultraviolet light can be excited by femtosecond laser pulses at long wavelengths. In numbers, according to this strategy one can use, for example, laser radiation of 800 nm wavelength to get very similar temperature-dependent luminescent response to the one obtained upon excitation with 337 nm wavelength (ultraviolet). This is extremely important, as the near-infrared excitation penetrates much deeper into matter (also biological tissues) than ultraviolet excitation.
This recent report demonstrates how the optical thermometry has moved from purely ‘linear’ domain to that employing nonlinear optical properties of crystalline matter. Given new possibilities that this approach offers, new instances of this kind of luminescence thermometry are likely to be published soon. All in all, thermometry, with every its variation available now, in a stage of rapid growth and significant breakthroughs are expected to be made in this field. This technology has garnered a lot of interest due to the fact that it can solve a variety of current technological problems in the rapidly growing fields of tissue thermometry and large area temperature measurements.
James Robert
