Abnormal thermally-stimulated dynamic organic phosphorescence

Abstract

Dynamic luminescence behavior by external stimuli, such as light, thermal field, electricity, mechanical force, etc., endows the materials with great promise in optoelectronic applications. Upon thermal stimulus, the emission is inevitably quenched due to intensive non-radiative transition, especially for phosphorescence at high temperature. Herein, we report an abnormal thermally-stimulated phosphorescence behavior in a series of organic phosphors. As temperature changes from 198 to 343 K, the phosphorescence at around 479 nm gradually enhances for the model phosphor, of which the phosphorescent colors are tuned from yellow to cyan-blue. Furthermore, we demonstrate the potential applications of such dynamic emission for smart dyes and colorful afterglow displays. Our results would initiate the exploration of dynamic high-temperature phosphorescence for applications in smart optoelectronics. This finding not only contributes to an in-depth understanding of the thermally-stimulated phosphorescence, but also paves the way toward the development of smart materials for applications in optoelectronics.

Fig. 1. A schematic representation of thermally-stimulated dynamic organic phosphorescence.

a Energy transfer processes for phosphorescence in organic phosphors. Promoting the intersystem crossing (ISC) process (step 1) and reducing the rate of nonradiative transition (step 2) are key points for boosting phosphorescence. b Organic molecular motion model under thermal stimulus. c Presentation for influence of molecular stacking on phosphorescent behavior under thermal stimulus in previous and this works. Notably, intermolecular stacking distance and interactions along the direction of π-π stacking are usually even. The phosphorescence fades away as temperature increasing in previous work. On the contrary, there exists uneven intermolecular stacking in this work. Under thermal stimulus, the excited molecular conformations undergo deformation, thus affording the abnormally dynamic phosphorescence.

Fig. 2. Investigation of the photoluminescence for crystalline FPO powder.

a Temperature dependent phosphorescence spectra excited by 400 nm. Note that the phosphorescence was collected with a delay time of 8 ms. b A CIE chromaticity coordinate diagram of the phosphorescence at different temperature. It is recorded by regulating temperature from 193 to 358 K. The inset images show the long-lived luminescence for FPO phosphors at 298, 308, 318, 338 and 358 K, respectively. c A plot of phosphorescence intensity monitoring the emission bands at 479 and 579 nm as a function of temperature. d Time-resolved emission spectra excited by 400 nm at room temperature. e Excitation–phosphorescence emission mapping at 298 and 348 K.

Fig. 3. A plausible mechanism for abnormal thermally-stimulated dynamic phosphorescence.

a Normalized phosphorescence spectra of FPO molecule in m-THF solution and PMMA film at 77 K. b Molecular arrangement of FPO molecules viewed along a axis (left) and π-π stacking (right) in crystal. c Thermal expansion coefficient of FPO crystal along the principal axes (a–c) at different temperature from 90 to 360 K. d Natural transition orbitals (NTOs) contributing to the lowest-energy triplet transitions of FPO trimer and dimer models in crystal. The inset structures show the molecular conformation in excited state of trimer (left) and dimer (right) models. e Proposed energy transfer processes for thermally-stimulated dynamic phosphorescence. Note that molecules at ground state (S₀) reaches to S₁ (step 1) after absorption of photons, and then transform to T₁ through ISC (step 2). Under lower temperature, the excited molecules at T₁ are further stabilized by the triplet excited state (¹T₁^*) of trimer with a lower energy level (step 3), enabling long-lived yellow phosphorescence (step 4). As temperature increases, the excited molecule experiences large conformation adjustment in an environment of uneven intermolecular interactions, which was further stabilized for ²T₁^* (step 5) of dimer. Finally, a bright cyan-blue phosphorescence was observed via radiative transition from ²T₁^* (step 6).

Fig. 4. Phosphorescence properties and crystal structures of the control phosphors.

a Temperature dependent phosphorescence spectra of the CPO, FEO CEO, and FMO phosphors excited by 400 nm. b Afterglow photographs of the crystalline CPO, FEO CEO, and FMO materials taken by varying temperature from 303 to 343 K. c Intermolecular π-π stacking in CPO and FMO crystals at room temperature. d Thermal expansion coefficient of the CPO crystal along the principal axes (a–c) at the temperature from 273 to 353 K. e Conformational energies estimated for the optimized structures of the CPO (left) and FMO (right) molecules in dimer and trimer state.

Fig. 5. Demonstration of thermal-responsive phosphors for smart pigments and colorful afterglow display.

a Schematic diagram for screen printing pattern of landscape. b Afterglow photographs of landscape pattern taken as temperature change from 323 K to room temperature. Note that the photographs from I to IV represents the seasonal change from summer to autumn. c A schematic of colorful afterglow display device (left) and afterglow photographs with specific color values through heating for different times (right). d–k Colorful afterglow patterns with integrated circuit by programmable regulation of the UV lamps and heating plate on-off, respectively.