The elastic crystals discovered recently breaks the stereotype “crystals equate brittleness”. The mechanism behind the elasticity of crystals has been studied over the years and intermolecular interactions were found to be the vital factor for the elasticity. Organic crystals based on π-conjugated small organic molecules display luminescent properties which can be tuned by changing molecular structures. And in another away, varying intermolecular interactions and the molecular arrangement also affects the emission of crystals. Their chemical versatility, anisotropy in structure and properties, and long-range structural order has led to increased recognition of these compounds as a new materials class for organic optoelectronic components, such as resonators, circuits and lasers. A particularly important discovery of elasticity has also been pronounced in the organic fluorescence crystals recently. This newly realized property overcame the shortcoming that the classical fluorescence crystalline material is fragile in applications. Organic crystals were proved to be used as an excellent optical transmission media, such as optical waveguides for passive transduction of information in both the visible and near-infrared spectral regions. Furthermore, Opportunities for application of the fluorescence of some flexible organic crystals have also been implicated for active signal transmission.
Our group developed some single crystals with different emission wavelength as optical waveguides. They can transmit optical signals to another end in active mode: the excited part of the crystal can be considered as an optical source (input) and the remaining portion of the crystal plays the role of optical transmission medium (Figure 1a). These crystals with different luminescence were based on various molecular structures. Is it possible to design a crystal material that can transmit different light in different conditions? Herein we reported a crystal with green fluorescence in room temperature based on small π-conjugated molecules, the color of emission of crystals changes visibly from green to orange when they are transferred from room temperature to liquid nitrogen (Figure 1b). The color change is due to a red-shift of the emission maximum upon cooling, whereupon the emission intensity gradually increases (Figure 1c). The maximum emission wavelength changes from 540 to 580 nm upon cooling from 277 to 77 K, and the dependence is linear in the temperature range from 77 to 277 K. The change of emission intensity with temperature is also linear within a certain temperature range. Which translates into an opportunity for reproducible optical temperature measurement based on fluorescence.
At the same time, the crystals maintained excellent optical waveguide property at room temperature or low temperature, whatever they are straight or bending. The optical loss coefficient of a straight crystal at room temperature was found to be 0.16 dB mm‒1. A bent crystal had a nearly identical optical loss, 0.17 dB mm‒1. At 77 K, the optical loss factors were found to be 0.17 dB mm‒1 for a straight crystal and 0.20 dB mm‒1 for a bent crystal. We hypothesized that the combination of linear dependence of the emission wavelength on temperature and the favorable optical waveguide capacity of crystals at low temperature could carry some potential for the development of flexible temperature sensors. Along this line of thought, a crystal of about 1.0 cm in length was selected, and liquid nitrogen was dropped continuously on a small area at one end. The cold spot was excited with a laser, and the optical output was collected and analyzed at both ends of the crystal. Consistent with the temperature difference, the cold end of the crystal emitted orange light, while the opposite end that was at higher temperature emitted green light. The position of laser excitation was then changed, and the crystal was excited at the higher temperature end while it was being cooled at the opposite end. Interestingly, the warm end emitted green light and the cold end emitted orange light. These results demonstrate that the crystal can be indeed cooled locally, and it does not equilibrate thermally during the experiment. These experiments proved that, when the crystal is used as a medium for optical transduction, the output signal depends only on the temperature at the point of excitation; the output is not affected even when the intermediate section of the crystal is at lower temperature (Figure 1d).
We performed DFT calculations to investigate the root-cause of the temperature dependence of fluorescence. The calculation results indicate that the S2 and S3 excitation energies are close to the S1 excitation energy, and the S3 population will be increased when the temperature is elevated, which is beneficial to enhance the luminescence from S3 and results in blueshift in the emission. We also obtained the X-ray crystal diffraction data at room temperature and low temperature. It is clear to know that the abundant weak intermolecular interactions like C–H···O interactions and F···F interactions ensured the elasticity of the crystal. Additionally, the π···π distance decreasing upon cooling is the most likely contributor to the change in emission.
In summary, we found a flexible organic crystalline material with temperature-dependent spectral changes which could be used for measurement of temperature by optical means in a wide temperature range, and particularly at low temperatures. The fluorescence of the crystals shows linearity in response to both the maximum and intensity of the emission. The crystal can convert the temperature at the excited position into a more stable optical signal output. The extraordinary elasticity of the crystal provides better durability and resistance to mechanical damage when it is used as an optical waveguide. This work expands the scope of application of elastic organic crystals as optical waveguides at low temperatures, and gives an opportunity to develop fluorescence thermometric devices by using soft and light organic crystalline materials as active sensing medium.