Thermally activated delayed fluorescence (TADF) materials have attracted much attention in the last decades as alternative emitters for noble metal-based phosphorescent complexes in OLEDs, due to their comparable ability to harvest both singlet and triplet excitons to produce light.1, 2 Most of the TADF, and indeed most emissive, compounds only emit from a single, low-lying excited state, adhering to Kasha’s rule.3
We report two TADF dendrimers that contain different rigid and planar N-doped polycyclic aromatic hydrocarbons as acceptors (Fig.1). By modulating the molecular geometry, compound 2GCzBPN that possesses a strongly twisted geometry exhibits TADF, while 2GCzBPPZ, possessing a less twisted geometry, shows a complex concentration- and temperature-dependent dual emission associated with emission from both monomer and aggregate, both of which are TADF.
Fig.1Â Chemical structures of 2GCzBPPZ and 2GCzBPN. Comparison of the characteristic photophysical properties of the emitters.
To explore and modulate the emergence of emission from the 2GCzBPPZ aggregates, we measured the PL spectra of 2GCzBPPZ at different concentrations in n-hexane solution. As shown in Fig. 2a, there are two emission bands observed for 2GCzBPPZ where the high-energy emission band at 475 nm converts to the low-energy emission band at 575 nm as a function of increasing concentration of the emitter. The corresponding emission color of 2GCzBPPZ evolves from sky-blue to white and then to yellow with increasing the concentration (Fig. 2b). The integrated emission intensity of the PL spectra increases with increasing concentration due to the combined contribution of emission from monomers and aggregates, and then the intensity decreases as the concentration continues to rise, resulting from aggregation-caused quenching (Fig. 2c). As expected, the emission band at 575 nm gradually intensifies as the concentration increases, owing to the formation of aggregates. At the same time, the absorption maximum associated with the ICT band of 2GCzBPPZ monomer at ~432 nm gradually decreases along with the emergence of a red-shifted absorption tail from 452 to 550 nm (Fig. 2d), indicating the formation of aggregates through strong intermolecular interactions in the ground state. Notably, white emission was observed for the 2 × 10-5 M solutions of 2GCzBPPZ in n-hexane as a result of balanced contributions from two distinct emission bands at λPL of 475 and 575 nm (Fig. 2b). Time-resolved PL studies in n-hexane under degassed conditions demonstrate that the emission at 475 nm decays monoexponentially and there is no long-lived emission, with τp of 7.3 ns, while the emission band at 575 nm decays with biexponential kinetics, with τp of 57.7 ns and τd of 4.1 μs, originated from aggregate-induced TADF (Fig. 2e).
Fig. 2. Aggregation-modulated photophysical behaviour. a Concentration-dependent fluorescence spectra for 2GCzBPPZ in n-hexane solution (λexc = 340 nm). b Corresponding photos under ambient light and using a UV torch (λexc = 360 nm). c Concentration-dependent emission mapping. d Concentration-dependent absorption spectra for 2GCzBPPZ in n-hexane. e PL decay profiles of the emission λem= 475 nm (top) and λem = 575 nm (bottom) under degassed and aerated n-hexane (λexc = 375 nm).
Intrigued by the unusual dual-emissive nature of 2GCzBPPZ, we sought to explore in greater detail the photophysical properties and studied the temperature dependence of the emission in n-hexane. At room temperature, the 1.6× 10-5 M solution of 2GCzBPPZ is dual-emissive and the sample appears to emit white light (Fig. 3a-c), where there are approximately equal contributions from the emission from the monomer (475 nm) and aggregates (575 nm). Upon decreasing the temperature towards the solvent freezing point, the low-energy emission band of 2GCzBPPZ increases in intensity dramatically while the high-energy emission band is completely quenched. The corresponding ratio of the intensity of the emission at 575 to 475 nm (I575/I475) exponentially decreases with increasing temperature from -70 °C to room temperature (r2 = 0.998) (Fig. 3d). On the other hand, increasing the temperature beyond room temperature reveals a complementary effect where the high-energy emission band becomes more intense and the low-energy emission band all but disappears. In this temperature regime, there is a linear relationship between I475/I575 versus temperature (r2 = 0.988 over a temperature region of 25 to 70 °C), corresponding to a ratiometric increase of 6.6% ± 0.2% K−1 (Fig. 3e). Overall, 2GCzBPPZ features excellent temperature sensitivity across a broad range of -70 °C to 70 °C, manifested in distinct colorimetric readout from yellow at -70 °C to white at room temperature and finally to sky blue at 70 °C, corresponding to CIE coordinates of (0.50, 0.49) at -70 °C that shift to (0.23, 0.32) associated with blue emission (Fig. 3c and Movie 1).
The broader range of temperature detection coupled with more significant color change exhibited by 2GCzBPPZ makes it a promising temperature sensor compared to previously reported organic fluorescent temperature sensors. The temperature sensing mechanism originates from variation in the degree of aggregation (specifically variation in distance between monomers) as a function of the temperature, as schematically shown in Fig. 3f, where upon increasing the temperature, the π-stacking interactions necessary for aggregate formation are disrupted and the monomer population increases, reflected in the emergence of blue emission. Recognizing the potential of this compound to act as a temperature sensor, we translated its properties into the solid state by embedding the compound into Paraffin. As shown in Fig. 3g and Movie 2, when photoexcited at 360 nm, the solid Paraffin emits in the yellow at room temperature. As the temperature increases from 20 to 80 °C, the emission gradually blue-shifts from yellow to green. When the temperature increases beyond 160 °C, the liquid paraffin emits in blue, emulating the emission observed in n-hexane beyond 60 °C. This distinctive performance in paraffin makes it ideal as a spatio-temperature probe. 2GCzBPPZ thus shows unrivaled temperature sensitivity and with a broad temperature-dependent spectral response compared to previously reported organic temperature sensors.
Fig. 3. Colorimetric temperature sensing and proposed mechanism. a Temperature-dependent emission spectra of 2GCzBPPZ in n-hexane at a concentration of 1.6´10-5 M (λexc = 340 nm). b Photos of 2GCzBPPZ at various temperatures (UV torch λexc = 360 nm). c CIE plot of the corresponding emission spectra. d Ratiometric plot of I575/I475 vs temperature upon decreasing the temperature below room temperature. e Ratiometric plot of I475/I575 vs temperature upon increasing the temperature from room temperature (right). f Schematic representation of the thermal response and two-state equilibration model describing the observed abnormal temperature-responsive dual emission phenomenon of 2GCzBPPZ. g Spatio-temperature sensor application in paraffin wax (paraffin embedded with 2GCzBPPZ in a test tube (length:160 mm, diameter: 16 mm) excited with a UV torch, λexc = 360 nm).
References
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