Characterization of PTCs with and without THF at ambient conditions. The PTCs were prepared by simply dissolving equimolar quantities of perylene and TCNB in THF solvent, yielding red PTCs-THF. The PTCs-THF crystals turned greyish-green within two minutes and transformed into PTCs due to the escape of THF from PTCs-THF. Figure 1a shows the X-ray diffraction (XRD) patterns of the PTCs and PTCs-THF sealed in two separated glass capillary tubes. Both are in good agreement with the corresponding XRD patterns of our simulated structures. Both the as-prepared PTCs and the PTCs-THF crystallize in a monoclinic structure but with different cell parameters, as summarized in Supplementary Table S1. Their molecular packings in the corresponding crystals are presented in Fig. 1b and Supplementary Figs. S1-S2. In the cocrystals, perylene is a typical polycyclic aromatic hydrocarbon chromophore as the donor (D) component, while TCNB is the acceptor (A). The donor and acceptor molecules are arranged alternately in a similar molecular column (-DADA-), while neighboring molecular columns are connected to each other, forming a tightly packed stack11. For the PTCs, it can be seen that the molecular centers of TCNB and perylene are not above each other, but show a 35% deviation from the accumulation axis29; for the PTCs-THF, their center goes back to the same vertical plane. Note that the intercalation of THF molecules would force TCNB to stack toward the edge of perylene. The π-π overlap between one TCNB molecule and the adjacent perylene molecules is about 50% of a perylene plane30.
Piezochromic luminescent properties of PTCs and PTCs-THF. PL spectra of PTCs upon compression up to 5.68 GPa are shown in Fig. 2a. As we can see from the figure, the PL emission of PTCs shows a normal red shift from 720 to 776 nm as pressure increase, accompanied with an obvious decrease in PL intensity. Such pressure-induced emission changes and fluorescence quenching at certain pressures have also been observed in compression experiments on other cocrystals15,26.
To tune the intermolecular interactions between donor and acceptor, THF has been inserted into the lattice of the PTCs to study its effect on the PL emission of the cocrystals. For this, PTCs were soaked in liquid THF in a diamond anvil cell where the greyish-green cocrystals quickly turned red, suggesting that THF molecules penetrated into PTCs, forming PTCs-THF. In this case, THF can be stabilized in the cocrystal by applying pressure and also acts as pressure transmission medium surrounding the sample. Remarkably, PTCs-THF exhibits an abnormal PL behavior as pressure increases. As shown in Fig. 2b, the PL emission bands show a clear blue shift from 655 to 619 nm as pressure increases up to 3.12 GPa, accompanied with a significant enhancement of PL intensity. At higher pressure above 3.12 GPa, the PL intensity starts to decrease gradually, but the PL emission bands still keep blue-shifting as pressure increases. Supplementary Fig. S3 demonstrates the piezochromic PL spectra of these two cocrystals up to 20 GPa. To study how the band gap of PTCs-THF changes under pressure, in situ UV-Vis absorption spectra of the material were measured during compression (Fig. 2c). The absorption edge of PTCs-THF is located at 616.6 nm at 0.08 GPa (the corresponding band gap is 2.01 eV), and exhibits an obvious blue shift during compression. As pressure increases up to 5.45 GPa, the absorption edge of the cocrystal moves to 603.9 nm (corresponding to band gap 2.06 eV) (insert, Fig. 2c). The band gap of the cocrystal thus increases due to the THF insertion, causing the anomalous emission blue-shift as pressure increases. Note that the absorption edge of the cocrystal becomes less sharp and gradually broadens as pressure increases at above 5.45 GPa (Supplementary Fig. S3c), which makes it challenging to derive an accurate band gap of the compressed cocrystal by curve fitting. This also indicates that interactions in the cocrystal become stronger31.
Structural evolution of PTCs with THF insertion upon compression. To give a further understanding of the effect of THF insertion on the novel PL emission of our cocrystals upon compression, a high-pressure XRD experiment on PTCs-THF has been performed and the recorded XRD patterns are shown in Fig. 3a. All the diffraction peaks shifted to lower d-values, indicating compression of the lattice. We also present the variation of the unit cell volume with pressure in Fig. 3b. The results suggest that no structural transition happened to PTCs-THF during compression. Instead, the a-, b- and c-axes exhibit different pressure evolutions upon compression, indicating an anisotropic compression of the lattice (insert, Fig. 3b). Note that above 3 GPa, the c-axis was more compressible than the a-axis and b-axis, indicating that the molecules become more parallel and more closely packed in the ab plane (Fig. 3c), which could increase the π-π interactions. Therefore, beyond 3.12 GPa, the reduced distance between D and A in the cocrystal promotes effective π-π stacking interactions that should be responsible for the emission quenching32.
Intermolecular interactions of PTCs with THF insertion by IR and Raman spectroscopies. As no structural transition occurred in the compressed cocrystal, the novel PL emission of PTCs-THF should be related to the changes of intermolecular interactions in the cocrystals upon compression. These interactions were studied by Infrared (IR) spectroscopy. As shown in Fig. 4a, the IR spectra of PTCs, THF and PTCs-THF were measured. Each of their vibrational modes could be assigned according to our theoretically calculated IR spectra (Supplementary Fig. S4). The spectroscopic features from both perylene and TCNB can be clearly distinguished, suggesting only weak van der Waals interactions between the molecules33. Upon compression, the IR peaks of PTCs were gradually blue-shifted and broadening (Fig. 4b), showing a common pressure evolution as observed in other molecular crystals25,34,35. In contrast, PTCs-THF show obvious differences in the IR spectra (Fig. 4c) compared with those of PTCs under pressure, due to the insertion of THF molecules into the lattice. The insertion of THF clearly results in the formation of H-bonding between THF and TCNB. The C-O-C symmetrical stretching vibration s(C-O-C) from THF, located at 895 cm− 1, exhibited a clear red shift as pressure increased, indicating a strengthening of C-H···O hydrogen bonds. As for TCNB, the formation of C-H···O hydrogen bonds also leads to a much higher blue-shift rate of the C-H stretching vibrations ν(C − H) at 3028 and 3108 cm− 1 in the PTCs-THF than for that in the PTCs upon compression. A similar effect of the H-bond formation on the blue shift of the C-H stretching mode has also been observed in polyglycine II36. In addition, the IR peaks at 914 and 1246 cm− 1 (a new peak appearing at 1.13 GPa), which can be assigned to C-H wagging vibrations ω(C-H) and C-H bending vibrations β(C-H) in TCNB37, exhibit an obvious enhancement in intensity (Supplementary Fig. S5), indicating that the polarity of the C-H bond of TCNB increases. This further supports the formation of a blue-shifted H-bond. Consequently, the H-bonding stabilized the TCNB38–41. On the other hand, the insertion of THF causes a distortion of the perylene molecule. This is evidenced by the gradual asymmetrization and split of the initially asymmetrical deformation vibrations δas(C-Cring) of perylene (peak at 1586 cm− 1) during compression42 (Supplementary Fig. S6). The distortion of perylene should reduce the π-conjugation43,44.
Our Raman measurements give further support for the formation of H-bonding when THF is inserted into the cocrystal upon compression. The recorded Raman spectra at ambient and high pressures are shown in Fig. 4e and Fig. 4f. Each vibrational mode of PTCs and PTCs-THF can be assigned, as shown in Fig. 4d and Supplementary Fig. S7. The dependence of peak positions and intensities on pressure for some selected Raman modes is shown in Supplementary Fig. S8. It is clear that the relative peak intensity of the carbon ring stretching vibration ν(C-Cring) at 1541 cm− 1 becomes stronger as pressure increases from ambient to 3.53 GPa37 (Supplementary Fig. S8a), indicating that the polarizability of the C-H bond of TCNB increases39–41. This should be related to the enhancement of hydrogen bonding. The Raman peak at 1367 cm− 1 from symmetrical deformation vibrations δs(C-Cring) of perylene gradually splits during compression42 (Supplementary Fig. S9), which indicates the deformation of perylene, in agreement with our IR results. In addition, the peak located at 549 cm− 1 from C-C ≡ N out-plane bending vibration β(C-C ≡ N) of TCNB exhibits a split at 1.1 GPa and one of the split peaks (545 cm− 1) downshifts to low frequency up to 3.03 GPa37 (Supplementary Fig. S8b), which indicates that THF restricts the C-C ≡ N out-of-plane bending vibration (Supplementary Fig. S9). This could inhibit non-radiative emission and thus promote PL enhancement.
Besides its capability to form H-bonding with TCNB and affect the molecular vibrations, as well as to distort the perylene conformation, the THF inserted into the cocrystal also acts as spacer to separate and stabilize the TCNB and perylene molecules due to its “inert” properties (the inability to form covalent bonds). Neither Raman nor IR measurements show any obvious weakening or broadening of any IR or Raman peak from TCNB or perylene upon compression, in contrast to the common pressure evolution of weakening and broadening of the corresponding modes in PTCs upon compression.
Calculation of the molecular orbitals (MO) and photoluminescent properties. The MO were further calculated to analyze the change of the HOMO-LUMO energy gap15,19 (Fig. 5a). Upon THF insertion, the energy gap is increased from 1.836 eV in PTCs to 2.093 eV in PTCs-THF at ambient pressure (Supplementary Fig. S10). The HOMO is distributed mainly on perylene, while the LUMO is distributed mainly on TCNB. Note that THF is not involved in the observed frontier orbitals distribution. The distribution of frontier orbitals in the cocrystal does not change obviously, but the energy gap of PTCs-THF increases from 2.093 eV to 2.654 eV upon compression from 0 to 20 GPa. Meanwhile, the vertical energy from our calculation also exhibits a similar pressure evolution as the HOMO-LUMO energy gap and becomes larger as pressure increases (Fig. 5b), which agrees well with the experimentally observed blue-shifted emission. The oscillator strengths of the S1→S0 electronic transition were calculated to analyze the change of the PL intensity45. The oscillator strength of PTCs-THF is increased from 0.025 to 0.0362 when pressure is increased from 0 to 5 GPa, while it decreases as pressure increases at above 5 GPa (Fig. 5c). These changes in the oscillator strength, suggesting an increase in emission intensity as pressure increases up to 5 GPa and a gradual quenching above 5 GPa, are in very good agreement with our experiments.
Analysis of noncovalent interactions (NCI) and molecular configurations. The experimentally observed changes in NCI in the PTCs-THF system have been further studied by the Multiwfn software46. As shown in Fig. 6a, the hydrogen bond strength C-H···O increases gradually (colour change from dark green to cyan) as pressure increases47 (red circles in Fig. 6a). Besides this, the THF insertion also plays a role for isolation and stabilization of TCNB (blue and red circles in Fig. 6a) in the cocrystal upon compression. Moreover, we find that, due to the rigidity of THF molecules, the THF insertion also causes a configurational distortion of the perylene upon compression (see Fig. 6b, the planar perylene turns to a configuration with twisted angle of 7.705 ° at 20 GPa). All our theoretical calculations and experiments thus show that the THF insertion significantly affects the molecular configuration of the donor and the acceptor and their intermolecular interactions, and is thus the main reason for the anomalous pressure-induced blue shift and emission enhancement by affecting the HOMO-LUMO energy gap.