Single-component color-tunable circularly polarized organic afterglow through chiral clusterization

Circularly polarized organic afterglow (CPOA) with both long-lived room-temperature phosphorescence (RTP) and circularly polarized luminescence (CPL) is currently attracting great interest, but the development of multicolor-tunable CPOA in a single-component material remains a formidable challenge. Here, we report an efficient strategy to achieve multicolor CPOA molecules through chiral clusterization by implanting chirality center into non-conjugated organic cluster. Owing to excitation-dependent emission of clusters, highly efficient and significantly tuned CPOA emissions from blue to yellowish-green with dissymmetry factor over 2.3 × 10−3 and lifetime up to 587 ms are observed under different excitation wavelengths. With the distinguished color-tunable CPOA, the multicolor CPL displays and visual RTP detection of ultraviolent light wavelength are successfully constructed. These results not only provide a new paradigm for realization of multicolor-tunable CPOA materials in single-component molecular systems, but also offer new opportunities for expanding the applicability of CPL and RTP materials for diversified applications.


REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): Comments on NCOMMS-21-31539: In this MS, circularly polarized organic afterglow from single component DAACH was reported, through the integration of CTE and chiral moieties. Generally, I think the topic and strategy of this work is simple yet interesting. Also, the spectroscopic characterizations are deserved to be praised. I have the following concerns: 1. The design rationale for single component CPOA: here the authors conjugated the chiral center (trans-1,2-diamidocyclohexane) with the TSC center. I wonder whether such design rationale can be extended with other chiral center or other emission centers? If yes, the authors should provide several other successful examples. 2. For theoretically probing the luminescence mechanism, the authors presented the data of monomer, dimmer, and tetramer. Also, the authors confirmed that the molecular packing and intermolecular electronic communications is of vital importance. Therefore, I think the theoretical calculation of cluster should be more representative. 3. For CPL, |glum| is an important parameter. How about the |glum| of this work when compared with the exiting ones? What's the decisive parameter that controls |glum|? 4. As shown in Figure 2f, DAACH exhibited photo irradiation-enhanced RTP, but no experimental or theoretical explanations were given. 5. The explanations about the color-tunable emission seems plausible. How about the Ex-dependent lifetime? 6. For the application of the color-tunable CPOA: I am not sure what's the role of circularly polarized emission. In fact, only the afterglow emission was explored.
Reviewer #2 (Remarks to the Author): The manuscript by Li and co-authors reported an interesting strategy to realize color-tunable circularly polarized organic afterglow in single component by integrating clusteroluminescence into chiral crystal system. By virtue of the intrinsic excitation-dependent emission of clusters, tunable organic afterglow emissions from blue to yellowish green with circularly polarized property were observed at room temperature. The experimental and theoretical dates are solid. However, I have some concerns about the CPL data shown in the present manuscript. CPL spectrum measured at CPL200 instrument always offers a blend of fluorescence and phosphorescence components. It is impossible to selectively collect the phosphorescence component of the CPL emission by delay setting as traditional photoluminescence instrument. Thus, the CPL data shown in Figure 2b should consist of both fluorescence and afterglow components. Based on such consideration, the DC panel in Figure 2b should be consistent with the steady-state photoluminescence spectra shown in Figure S17 rather than the afterglow spectra shown in Figure S9. However, I found the DC panel in Figure 2b consist of only afterglow component. How did authors realize this? It seems strange. The authors should give a clear description of CPL instrument, parameter setting and measurement procedure. I strongly believe the CPL spectra in Figure 2b consist of dominant circularly polarized fluorescence and tiny amount of circularly polarized afterglow. These concerns needed to be carefully addressed.
Other comments: 1. Crystals always have large birefringence and line dichroism effect, which could potentially result in artificial CPL signal. Is the powder used for the CPL measurement crystal? What is the size of the crystal powder? I have a lot of CPL measurement experiences. I recommend the authors to measure the CPL of powder in KBr pellet to get more reliable CPL signal. Actually, the CPL signal excited by 240 nm shown in Figure 2b is so weak that it is difficult to tell whether it is a real signal or a noise. 2. What is the monitored wavelength for the time-resolved decay profiles in Figure S11 and S23?    (2) For theoretically probing the luminescence mechanism, the authors presented the data of monomer, dimmer, and tetramer. Also, the authors confirmed that the molecular packing and intermolecular electronic communications is of vital importance. Therefore, I think the theoretical calculation of cluster should be more representative.

(3) For CPL, |glum| is an important parameter. How about the |glum| of this work when compared with the exiting ones? What's the decisive parameter that controls |glum|?
Author reply: We appreciate the professional question raised by the referee. The |glum|s of our work are moderate when compared with the recently published dissymmetry factor of the afterglow materials showing |glum|s ranging from 1.4610 -3 to 2.0×10 -2 (Supplementary Fig. 14).
Theoretically, glum can be calculated from the following equation：  (Fig. 2a-b and Supplementary Fig. 14).
Page S12 of the revised Supplementary Information： Experimentally, the dissymmetry factor (glum) can be calculated from following equation: Theoretically, glum is defined as: where |m| and |μ| are the magnitudes of magnetic and electric transition dipole moments vectors, respectively, and the  is the angle between these two dipole moments. Therefore, large |μ| means small glum, while large |m| will result in a high glum value.   (4) As shown in Figure 2f, DAACH exhibited photo irradiation-enhanced RTP, but no experimental or theoretical explanations were given.
Author reply: we appreciate the point raised by the referee and many thanks for the careful review. Actually, according to the previous reports and our understanding, the slightly enhanced RTP intensity upon the increased photo irradiation time is not a photo irradiationenhanced RTP and this is a common phenomenon for organic afterglow materials (Nat. Mater. 2015, 14, 685-690; Nat. Photonics, 2019, 13, 406-411). This phenomenon is due to the fact that organic afterglow emission needs more time to achieve the stable state of triplet excitons for emission owing to the relatively slow and weak intersystem crossing rates to populate the triplet excited state. We have involved the possible explanation in the revised manuscript.
Thanks a lot.

Added text:
Page 3 of the revised manuscript, right column, lines 2-7: Possibly due to the relatively slow and weak intersystem crossing rates, it needs considerable time to achieve the stable state of triplet excitons for afterglow emission, resulting in the slightly enhanced RTP with the increase of photo irradiation time ( Fig. 2f and Supplementary   Fig. 23).    Author reply: We appreciate the concern raised by the referee. As to the role of circularly polarized emission in the application, we think that the chirality features of (S, S)-DAACH and (R, R)-DAACH render the multicolor afterglow displays with different circularly polarized emission features. As shown in Fig. 4c, although quite similar afterglow emission can be found in (S, S)-DAACH and (R, R)-DAACH powders, the inherent difference of chirality renders this pair of enantiomers with different circularly polarized emission, showing a negative CPL signal for "C", and a positive CPL signal of "OO". We indeed know that this is a very primary attempt to develop multicolor CPOA displays, but we believe that this multicolor CPOA material system may have great application potential in chiral encryption and 3D displays. To make this CPL properties of displayed patterns more intuitive to the reader, we have updated Fig. 4c in the revised manuscript. We hope these descriptions can clarify the concern raised by the referee.
Many thanks.  Specifically, because of the very small stokes-shift between the fluorescence and phosphorescence spectra excited by 240 nm, we can not use the filter to eliminate the influence of fluorescence signal of CPL emission on phosphorescence signal of CPL emission. Therefore, the CPL spectra of SSPL in Fig. 2b excited by 240 nm UV light indeed consist of dominant circularly polarized fluorescence and tiny amount of circularly polarized afterglow; thus, the curve (black, DC panel in Fig. 2b) excited by 240 nm is similar to that of the SSPL emission excited by 240 nm with main emission peak at ~434 nm (Supplementary Fig. 26a), rather than the afterglow emission excited by 240 nm (Supplementary Fig. 18). Since the fluorescence component can be effectively eliminated by means of long-pass filter with a cut-on wavelength of 495 nm, for 360 nm excited CPL measurement, the curve (red, DC panel in Fig. 2b) is close to that of the afterglow emission excited by 360 nm (Supplementary Fig. 18). To further demonstrate the excitation-dependent CPOA properties, CPL properties of the organic afterglow of (R, R)-DAACH and (S, S)-DAACH powders excited by 340 nm were further performed. As shown in updated Fig. 2b, the curve (blue, DC panel in Fig. 2b) excited by 340 nm is also identical to that of the afterglow emission excited by 340 nm (Supplementary Fig. 18) and the clearly red-shifted CPOA properties were obviously observed in DC panel when changed

Added text and updated figure:
Page S12 of the revised Supplementary information: To individually achieve the fluorescence and phosphorescence signal of CPL emission, the CPL spectra can be split into fluorescence and phosphorescence parts through using the short-pass and long-pass filters owing to the large stokes-shift of the fluorescence and phosphorescence in organic afterglow materials. Experimentally, when we measured the fluorescence  understand the concern that concentrated on the CPL signal excited by 240 nm, owing to its relatively large fluctuations in the CPL profiles. To eliminate the influence of noise on the true CPL signal, the superimposed measurements, which have been recognized as an effective method to increase the ratio of signal to noise of CPL signal, were adopted with total numbers of 10~30 cycles when we performed the CPL measurement. Moreover, the CPL curves of (R, R)-DAACH and (S, S)-DAACH powders excited by 240 nm demonstrate good configuration symmetry. Therefore, we believe that the CPL signal excited by 240 nm in Fig. 2b in our manuscript is true and reliable. We hope these discussions would clarify the referee's concern.
Many thanks.

Added text and figures:
Page 2 of the revised manuscript, right column, lines 8-11 and Supplementary Fig. 15: The chiral characteristics were also observed, when (R, R)-DAACH and (S, S)-DAACH powders were dispersed (50 wt%) in potassium bromide slice (Supplementary Fig. 15). (2) What is the monitored wavelength for the time-resolved decay profiles in Figure S11 and S23?
Author reply: We thank the question raised by the referee. We are very sorry that we forgot to add the monitored wavelength for the time-resolved decay profiles in Supplementary Figs. (6) When people measure and discuss CPL spectra, CD spectra are always simultaneously provided to give more fruitful information. Since (R,R)-DAACH in ethanol solutions showed observable absorption spectra, CD spectra should be also provided.
Author reply: Thank the referee for the professional and constructive suggestion. The CD spectra have been measured and the corresponding discussion and spectra had been provided in the revised manuscript and Supplementary Information. Thanks again.

Added text and figure:
Page 2 of the revised manuscript, right column, lines 11-15 and Supplementary Fig. 16: The almost mirror circular dichroism signals of (S, S)-DAACH and (R, R)-DAACH ethanol solutions with strong signals at ~205 nm further confirm the successful introduction of chirality in these materials (Supplementary Fig. 16).

A list of changes made:
(1) The list of reported chiral organic afterglow materials with corresponding lifetimes, efficiencies and glum has been provided in the revised Supplementary Fig. 14. (2) The numbers assigned to references and Supplementary figures have been updated.