Intermolecular Electronic Coupling of Organic Units for Efficient Persistent Room‐Temperature Phosphorescence

Abstract Although persistent room‐temperature phosphorescence (RTP) emission has been observed for a few pure crystalline organic molecules, there is no consistent mechanism and no universal design strategy for organic persistent RTP (pRTP) materials. A new mechanism for pRTP is presented, based on combining the advantages of different excited‐state configurations in coupled intermolecular units, which may be applicable to a wide range of organic molecules. By following this mechanism, we have developed a successful design strategy to obtain bright pRTP by utilizing a heavy halogen atom to further increase the intersystem crossing rate of the coupled units. RTP with a remarkably long lifetime of 0.28 s and a very high quantum efficiency of 5 % was thus obtained under ambient conditions. This strategy represents an important step in the understanding of organic pRTP emission.

High resolution mass spectra (HRMS) were obtained using a Thermo MAT95XP spectrometer. Elemental analysis was conducted using an Elementar Vario EL analyzer. X-ray crystallographic intensity data were collected by using a Bruker Smart 1000 CCD diffractometer equipped with a graphite monochromated Enhance (Mo) X-ray source. The CCDC numbers of the compounds are 1402467-1402470. The UV−vis absorption spectra were obtained using a Hitachi U-3900 spectrophotometer.
The photoluminescence (PL) spectra and time-resolved emission decay data were obtained using a spectrometer (FLSP980) from Edinburgh Instruments. The photoluminescence quantum yields were measured on a calibrated integrating sphere.
The persistent phosphorescence spectra and transient PL decay images were obtained in delay of 25 ms from an Ocean-optical Maya 2000pro spectrometer. The WAXD measurements were obtained using a Rigaku X-ray diffractometer (D/max-2200) with an X-ray source of Cu Kα (λ= 0.15406 nm) at 40 kV and 30 mA, at a scan rate of 2° (2θ) per 1 min. The glass transition temperature of Cz-BP was evaluated via DSC on a NETZSCH DSC 204 F1 instrument in nitrogen at a heating rate of 10 °C/min.   [2]. For a molecule with data in two rows, the upper row is for a nonpolar solvent and the lower row for a polar solvent, respectively. Phos., phosphorescence; ISC, intersystem crossing; fluo., fluorescence. No data is indicated as "−".
As shown in Table S1, the intersystem crossing (ISC) process in organic molecules (containing lone-pair electrons) with an nπ* excited state configuration is through an allowed transition between nπ* state and ππ* states. When isolated in the solid solvents at low temperature, they usually exhibit short phosphorescence lifetimes (<10 ms) and high phosphorescence quantum efficiencies (≥ 50%, except acetone).
On the contrary, the ISC process in typical hydrocarbon organic molecules with a ππ* excited state configuration is through the forbidden transition between two different ππ* states. Hence, such compounds generally exhibit phosphorescence with long lifetimes (>10 ms) and lower quantum efficiencies (< 10%, except benzene). As shown in Figure S1, close intermolecular interactions of the n-π* unit (n unit) and π-π* unit (π unit) are present in the single-crystal structures of these previously-reported organic persistent RTP molecules. This suggests our new explanation for organic persistent RTP may be generally applicable for a wide range of systems. The spectrum of 4-(9H-carbazol-9-yl)benzophone (Cz-BP) reported by Yuan et al. 6 is different from our result, however, similar with that of the first observation of weak persistent RTP in reference [7]. The Cz-BP molecules also exhibit a different stacking in the single-crystal, compared with our result.
Nevertherless, its dimer stacking between two carbazolyl units is similar to that of the planar compounds in reference [7], indicating that their long-lived RTP may come from the same mechanism that is different from other organic persitent RTP compounds. As carbazole has a ground state dipole moment, 8

IV. Molecular stacking in crystals
Close stacking between carbonyl (or sulfonyl) and carbazolyl units (i.e., n and π units) are found in single crystal X-ray structures of all the compounds. It is interesting that two stacking modes coexist in the single crystal of Cz-BP, including carbonyl stacks with carbazolyl group and two stacked carbazolyl units ( Figure S5).
However, the distance (3.373 Å and 3.561 Å) between the carbonyl and carbazolyl groups is shorter than that of two stacked carbazolyl groups (3.653 Å). The ISC transition channels are found mainly between the carbonyl and carbazolyl group (i.e., n and π units) from the TD-DFT calculated results (Figures 3, S9 and S11). The stacking between carbonyl and carbazolyl units thus plays the major role in the processes of persistent RTP of Cz-BP.    the major ISC channels are mainly determined based on two elements. Firstly, the ratio of the same transition configuration in S1 and Tn should be large in all the transition orbital compositions. Secondly, the energy gap between S1 and the specific Tn state should be small. When the energy of Tn is lower than S1, the first element is considered to be more important. The determination of minor ISC channels is vice versa. In the following schematic representations of Figures S11, S12, S13, and S14,   As is clearly shown in Figure S9 and S10, as expected, the HOMO and HOMO-n orbitals are primarily located at the carbazolyl moieties, while the LUMO and LUMO+n orbitals are found at the carbonyl moieties. Interestingly, for coupled BCz-DPS, the LUMO orbital partially extends to the carbazolyl group of the close coupled molecule, and it becomes more obvious for the LUMO+2 orbital. This indicates that the transitions involving both LUMO and LUMO+2 orbitals contain an intermolecular component. Figure S11. Schematic representations of the TD-DFT calculated energy levels, main orbital configurations and possible ISC channels of (a) coupled Cz-BP@ππ (according to the ππ stacking shown in Figure S5b), (c) coupled Cz-BP@ππn (according to the ππn stacking shown in Figure S5c) and (e) coupled Cz-BP@nππ (according to the nππ stacking shown in Figure S5d) at the singlet (S1) and triplet (Tn) states. The schematic representations in (b) coupled Cz-BP@ππ, (d) coupled Cz-BP@ππn and (f) coupled Cz-BP@nππ are the relative possible intramolecular and intermolecular ISC channels.
As can be seen above, and also in Figure 3c-f, all of the ISC transitions are from the Cz groups to the carbonyl groups. In an isolated Cz-BP molecule (Figure 3e), intramolecular ISC processes are exclusive. However, significant intermolecular ISC channels are found in coupled Cz-BP (Figure 3f, S11d and S11f). For coupled Cz-BP@nπ, coupled Cz-BP@ππn and coupled Cz-BP@nππ there are similar intermolecular ISC channels. In contrast, in coupled Cz-BP@ππ, only the intramolecular ISC channels are observed ( Figure S11b). These results reveal that the stacks between Cz groups (π unit) play minor roles in the intermolecular ISC transitions of the system. These results also indicate, as above, that the coupling of carbonyl and Cz groups (i.e., n and  units) in Cz-BP molecules is the main reason for enhanced ISC transition for triplet excitons, which will lead to persistent RTP.
Additionally, as all the major ISC transition channels in the systems of coupled Cz-BP@ππn and coupled Cz-BP@nππ can be found in the coupled Cz-BP@nπ system, the stacks between Cz groups (π unit) in the two former structures can be ignored. Thus, the result of a coupled Cz-BP@nπ system can be used as the representative one for coupled Cz-BP molecules. As can be seen in Figure S12, all of the ISC transitions are from the Cz groups to the carbonyl groups. In an isolated BCz-BP molecule ( Figure S12c), intramolecular ISC processes are exclusive. However, only intermolecular ISC channels are found in coupled BCz-BP ( Figure S12d). These results indicate that the coupling of carbonyl and carbazolyl groups (i.e., n and  groups) in BCz-BP molecules is the main reason for the enhanced ISC transition for triplet excitons, which will lead to persistent RTP. arrows refer to the ISC channels from S1 and S2, respectively.
As can be seen in Figure S13, all of the ISC transitions are from the Cz groups to the carbonyl groups. In an isolated Cz-DPS molecule ( Figure S13c), intramolecular ISC processes are exclusive. However, intermolecular ISC channels are found in coupled Cz-DPS through S2 to T7 and S2 to T7 ( Figure S13d). These results indicate that the coupling of carbonyl and carbazolyl groups (i.e., n and  groups) in Cz-DPS molecules is the main reason for the enhanced ISC transition for triplet excitons, which will lead to persistent RTP.         ultraviolet irradiation, the ground part (light blue) of security letter 'π' was clearly recognized from the crystal part (yellow). After the excitation is turned off, only the crystal part (orange) of the letter 'π' can be observed.