Tunable Fluorescence and Afterglow in Organic Crystals for Temperature Sensing.

The modulation of the properties of emission from multiple emission states in a single-component organic luminescent material is highly desirable in data anticounterfeiting, information storage, and bioapplications. Here, a single-component luminescent organic crystal of difluoroboron diphenyl β-diketonate with controllable multiple emission colors is successfully reported. The temperature-dependent luminescence experiments supported by high-level theoretical calculations demonstrate that the ratio of the fluorescence between the monomer and excimer and the phosphorescence maxima of the excimer can be effectively regulated. In addition, the temperature-dependent fluorescence and afterglow dual-emission color changes provide a new strategy for the design of highly accurate double-checked temperature sensors.

Solid organic luminescent materials with multiple emission colors have attracted a great deal of attention due to their fascinating molecular compositions, rich photophysical properties, and enormous potentials in optoelectronic and biomedical applications.[1−14] Multicolor emission systems prepared from composite materials have been widely investigated. However, their preparation is very complex, and the different materials used in preparing them are poorly compatible with each other, which seriously affects their performance in practical applications.[15−22] As a result, there is an urgent need to develop single-component metal-free materials with multicolor emissions, excellent durability, and stability. In reported single-component materials, multicolor emissions are usually achieved via manipulating intermolecular packing modes during the synthesis of the materials and growth of crystals.[16,18,23,24] In contrast, it is still difficult to obtain stimulus-responsive single-component materials with multicolor emissions especially those with more than two emission colors.[25−28]

The luminescence mechanism of organic materials in solid states,[24,29−37] especially single-component organic crystals with multiple emissions, is very complicated.[23,27,28,38] Small variations in molecular conformation, intermolecular packing mode, sample morphology, or even surroundings of materials could affect their luminescence properties.[13,39] Thus, it is very important to explore the luminescence mechanism of multiple-emission materials for their rapid development. The excimer that is formed between a ground-state molecule and an excited-state one is a common luminescence phenomenon found in many chromophores.[40] As the excimer-based sensors precisely probe analytes using the luminescence changes during the monomer–dimer conversion,[41−43] the dynamic formation and dissociation of the excimer may be used as one of the ingenious mechanisms for designing single-component multicolor emitters. Difluoroboron β-diketonate derivatives are some of the potential materials for studying stimulus-responsive luminescence systems with multiple emissions due to their modifiable molecular conformation and intermolecular interaction.[20,26,44−50] In addition, the n−π electronic transition of difluoroboron β-diketonate enables room-temperature phosphorescence and afterglow emission through appropriate regulation of the π system according to El-Sayed’s rule.[17,51−53] These characteristics fully provide the prerequisite for the establishment of a multiple-emission system in a single-component luminescent material.

Here, we present a single-component organic molecular crystal, difluoroboron diphenyl β-diketonate (M1), with temperature-modulated fluorescence and afterglow emissions. The experimental and theoretical calculation results demonstrate that a temperature-dependent fluorescence color change from yellow-green (540 nm, 300 K) to blue (470 nm, 77 K) and an afterglow color change from yellow (560 nm, 300 K) to red (650 nm, 77 K) are due to temperature-modulated intermolecular interactions. At 300 K, the emission of the excimer dominates the whole spectra, but at low temperatures, the formation of the excimer is inhibited and monomer fluorescence increases gradually. Moreover, enhanced intermolecular interaction shifts excimer fluorescence from 540 to 560 nm and phosphorescence from 560 to 650 nm. Therefore, yellow-green excimer fluorescence and yellow excimer afterglow are observed at 300 K, whereas blue monomer fluorescence and red excimer afterglow are observed at 77 K. The ratio of fluorescence between the monomer and excimer correlates linearly with temperature, while phosphorescence maxima have a good linear relationship with temperature. The synergistic temperature response of fluorescence and afterglow of the M1 crystal makes it highly sensitive and accurate in sensing temperature.

M1 crystals exhibit yellow fluorescence at 540 nm with a small shoulder at 470 nm; they also show a phosphorescence band at 560 nm with a very weak shoulder at 650 nm at room temperature (Figure a and Figure S1). The concentration-dependent fluorescence spectra of M1 in a chloroform solution were recorded at room temperature (Figure b). A new unstructured broadened emission band at ∼540 nm appeared and gradually increased as the concentration of M1 increased from 1 μM to 200 mM. This is in line with the emission characteristics of the excimer. The emission band of the monomer at 400 nm gradually red-shifted to 470 nm, which could probably be caused by the enhanced intermolecular interaction at high concentrations. The peak positions of the monomer and excimer fluorescence are basically consistent with the peak in the crystal. Therefore, the emission at 540 and 470 nm in the crystal is assigned to the fluorescence of the excimer and monomer, respectively.

Figure 1

(a) Fluorescence (green line) and phosphorescence (red line, delayed 1 ms) spectra of M1 crystals. (b) Concentration-dependent emission spectra and corresponding emission mapping of M1 in chloroform solutions. Fluorescence (at room temperature) and phosphorescence (delayed 1 ms, at 77 K) spectra of (c) 1 wt % and (d) 50 wt % M1 doped in PMMA films.

To determine the attribution of these two phosphorescence bands at 560 and 650 nm, the emission spectra of M1 in a poly(methyl methacrylate) (PMMA) film (Figure c,d) and a chloroform solution with different concentrations were recorded (Figures S2 and S3). At room temperature, only the monomer’s fluorescence at 400 nm was observed (1 wt % in PMMA or 1 μM in a chloroform solution). The delayed emission band at around 525 nm could be observed only at 77 K, which is assigned to the phosphorescence of the monomer. Interestingly, a new broad phosphorescence band at 650 nm was detected at 77 K as the concentration of M1 increased to 50 wt % in PMMA (Figure d) or 200 mM (Figure S3) in a chloroform solution, which is assigned to the excimer-correlated phosphorescence.

The most important finding of this work is the temperature-dependent fluorescence and afterglow color changes in M1 crystals. The fluorescence intensity at 470 nm subtly increased, while the fluorescence at 540 nm increased and gradually red-shifted to 560 nm as the temperature decreased from 300 to 200 K, due to the stabilizing effect of cooling on the excimer complex. The further increase in the fluorescence at 470 nm and the decrease in the fluorescence at 560 nm resulted from the suppression of the excimer formation process (Figure a). At the same time, the phosphorescence at 560 nm gradually red-shifts to 650 nm with the intensity increasing first and then decreasing. This is also consistent with the temperature stabilization and suppression effect on the excimer formation process (Figure b).

Figure 2

Temperature-dependent (a) fluorescence and (b) phosphorescence (delayed 1 ms) spectra of M1 crystals and corresponding emission mappings. (c) Selected crystal patterns, volumes, and densities of M1 crystals at different temperatures.

The red-shift of the excimer fluorescence and phosphorescence spectra was further investigated by the crystal structures of M1 at different temperatures (Figure c and Table S1). It is noteworthy that there was no significant overlap between the dioxaborine ring and the adjacent molecules. The antiparallel alignment of neighboring molecules indicated moderate intermolecular interactions at room temperature. At low temperatures, the centroid–centroid distance of the two phenyls between two adjacent molecules gradually decreased from 3.763 to 3.685 Å while the distance between the difluoroboron β-diketonate moiety and the phenyl group from the molecule in the vicinity decreased from 3.751 to 3.627 Å. Macroscopically, the volume of the recorded crystal decreases from 2604.4 to 2495.3 Å3, which corresponded to an increase in the crystal density from 1.388 to 1.448 g/cm3 at low temperatures. Therefore, the enhanced intermolecular interaction was observed at low temperatures, which is the main contribution of the red-shift of the fluorescence and phosphorescence.

The temperature-dependent fluorescence and afterglow emissions are rationalized by theoretical calculations. The S0 structures are optimized with the B3LYP method, while the S1 and T1 structures are determined with the TD-B3LYP method (see the Supporting Information for details).[54−58] The polarizable continuum medium (PCM) model is also used to consider solvent effects.[59] The energies of all of the optimized structures are further refined at the CASPT2 and CASPT2/PCM levels.[60,61] The vertical fluorescence and phosphorescence emission energies in chloroform solutions at 408 and 543 nm are consistent with the experimental data at 400 and 525 nm, respectively. These results show that the fluorescence and phosphorescence stem from the S1 and T1 states (Table S4).

The optimized S0, S1, and T1 structures of the M1 monomer and dimer in the crystal are also determined at the QM(CASPT2)/MM level (Figure ).[62,63] The vertical fluorescence emission energy of the monomer at 443 nm matches well with that at 470 nm measured by the experiment. The fluorescence and phosphorescence emissions of the dimer at 536 and 548 nm are in accordance with the experimental emissions at 540 and 560 nm, respectively, in the crystal. The red-shift of the fluorescence and phosphorescence of the M1 dimer at low temperatures is also confirmed by the calculation results. As the centroid–centroid intermolecular distance within one dimer is decreased, the calculated fluorescence and phosphorescence of the dimer are red-shifted to 552 and 627 nm. The red-shifts of the fluorescence and phosphorescence spectra at shorter intermolecular distance from the theoretical simulation are in good agreement with the results of temperature-dependent spectroscopy experiments. This further confirms that the dual temperature-dependent characteristics of fluorescence and afterglow emission of M1 crystals are caused by the change in intermolecular interactions at different temperatures.

Figure 3

Frontier molecular orbitals at the S0 minima of (a) monomer and (b) dimer models of M1 crystals. Proposed luminescence models of the (c) monomer and (d) dimer. The S0, S1, and T1 energies (kilocalories per mole) are calculated at the QM(CASPT2)/MM level for optimized T1 and S1 structures. The vertical emission energies are the energy differences between S1 and S0 (EF) or T1 and S0 (EP) calculated above.

To date, most of the traditional temperature-responsive luminescent materials use the single emission intensity or lifetime changes to monitor temperature fluctuation, which is also influenced by the surroundings. To improve the accuracy and sensitivity of thermometers, ratiometric temperature sensors are a good alternative, which can accurately measure temperature by the change in the ratio of the intensity or lifetime of two or more emissions. Therefore, the temperature-dependent fluorescence and afterglow emission color changes of M1 crystals provide a new strategy for designing double-checked temperature sensors with significantly improved sensitivity and accuracy. The fluorescence ratio between the monomer and excimer linearly correlates with temperature (Figure a), while the phosphorescence maximum has a good linear relationship with temperature (Figure b). The synergistic temperature response of the fluorescence and afterglow luminescence of the M1 crystal endow it with high sensitivity and accuracy in sensing temperature. In addition, the temperature-dependent fluorescence and afterglow color change can be clearly and directly observed by the naked eye or with a camera or by comparison with the temperature-dependent CIE chromaticity diagram (Figure c,d).

Figure 4

(a) Linear relationship between IFLM/FLE and temperature (IFLM/FLE is the ratio of the fluorescence intensities between the M1 monomer and excimer). (b) Linear relationship between the phosphorescence maxima of M1 crystals and temperature. (c and d) CIE coordinates transformed from panels a and b, respectively, of Figure .

In summary, a single-component organic crystal difluoroboron 1,3-diphenyl β-diketonate (M1) with stimulus-responsive multiple emissions was successfully constructed and investigated. The temperature-dependent spectroscopy experiments supported by high-level theoretical calculations demonstrate that the temperature-dependent fluorescence color of M1 crystals changes from yellow-green (540 nm, 300 K) to blue (470 nm, 77 K) and the afterglow color changes from yellow (560 nm, 300 K) to red (650 nm, 77 K) due to the temperature-modulated intermolecular interactions. The ratio of the fluorescence between the monomer and excimer correlates linearly with temperature, while the phosphorescence maxima have a good linear relationship with temperature. The synergistic temperature response of the fluorescence and afterglow emissions of M1 crystals make it sensitive and accurate in sensing temperature.