Highly sensitive switching of solid-state luminescence by controlling intersystem crossing

The development of intelligent materials, in particular those showing the highly sensitive mechanoresponsive luminescence (MRL), is desirable but challenging. Here we report a design strategy for constructing high performance On–Off MRL materials by introducing nitrophenyl groups to molecules with aggregation-induced emission (AIE) characteristic. The on–off methodology employed is based on the control of the intersystem crossing (ISC) process. Experimental and theoretical investigations reveal that the nitrophenyl group effectively opens the nonradiative ISC channel to impart the high sensitivity and contrast On–Off behavior. On the other hand, the twisted AIE luminogen core endows enhanced reversibility and reduces the pressure required for the luminescence switching. Thin films can be readily fabricated from the designed materials to allow versatile applications in optical information recording and haptic sensing. The proposed design strategy thus provides a big step to expand the scope of the unique On–Off MRL family.

been found in optical recording and haptic sensing. The design strategy is creative and rational. The experimental data, explanation and results are substantial. I recommend its publication in Nature Communications after some revisions. 1) In the introduction Section, when the authors mentioned the "On the other hand, the morphology of organic molecules with twisted conformations can be easily modulated by mechanical stimulus", some vital references should be provided.
3) Page 5, "They exhibit bright green or yellow fluorescence as well as in thin-films and in aggregates, suggesting the ISC is blocked due to morphology change. (Supplementary Figure 1, Table 1-2)" Table 1-2 are not provided in the main text.

Responses to the Comments and Suggestions of Reviewer 1
Dear Reviewer 1: The reviewer commented that "This is an elaborate piece of work. It puts forward a new train of thought on constructing ON/OFF mechanofluorochromic system, which is general, universal and applicable to other molecules. Thus, this work is recommended to be published on Nature Communications after some minor revisions". He/she also pointed out several minor issues of the manuscript.
We thank the reviewer for his/her recognition of this work and the nice advices he/she made, and we revised the manuscript accordingly. Below are our point-to-point responses to the reviewer's comments.
Our Reply: Thanks for your careful revision. We follow the reviewer's consideration, and revised our manuscript as follows: "intramolecular charge transfer 33,34 and strong π-π interactions modulation 35,36 to control the solid-state luminescence." The two important references have been added to our revised manuscript. Our Reply: Thanks for your kind suggestion. The solution of amorphous aggregates has been changed to the same concentration as the solution of nanocrystalline aggregates. Then, we did the similar experiment of singlet oxygen detection and the same results were got which are shown in the revised Supplementary Information part (Supplementary Fig. 2).

Responses to Reviewers' Comments:
Comments 3: In the application demonstration part, handed heat gun was used to convert amorphous solid to crystalline one, how to ensure the adhesion of powders with the substrates instead of being blown away? Some typos should be corrected, such as "gringding".
Our Reply: Thanks for your careful revision. After thermal treatment by heat gun, the uniform crystalline film can adhere tightly on the surface of different substrates and the crystalline film cannot been blown away easily. There are may be two reasons: First, TPE-4N exhibits good film-forming and easy crystallization abilities (Figure 4), therefore the uniform spherulitic crystalline film can form quickly on the surface of substrates instead of being blown away; Second, the amorphous and crystalline film can adhere tightly on the surface because of the strong Van der Waals force between the molecule's nitro groups and substrates, On other hand, the switching experiments can be repeated for many times without any change and the applications are rewritable or reusable also exclude the possibility of blowing away the powders.
Some typos have been corrected in the revised manuscript.

Responses to the Comments and Suggestions of Reviewer 2 Dear Reviewer 2:
The reviewer commented that "The authors has developed a good strategy to realized high-contrast and highly sensitive ON-OFF MRL materials by manipulating ISC process in AIE compounds, via introducing nitro groups to promote the ISC decay and compete with AIE characteristic. Good applications have been found in optical recording and haptic sensing. The design strategy is creative and rational. The experimental data, explanation and results are substantial. I recommend its publication in Nature Communications after some revisions." He/she also pointed out several minor issues of the manuscript.
We thank the reviewer for his/her recognition of this work and the nice advices he/she made, and we revised the manuscript accordingly. Below are our point-to-point responses to the reviewer's comments.
Comments 1: In the introduction Section, when the authors mentioned the "On the other hand, the morphology of organic molecules with twisted conformations can be easily modulated by mechanical stimulus", some vital references should be provided.
Our Reply: Thanks for your kind suggestion. Four important references have been added to the revised manuscript.
Our Reply: Thanks for your kind suggestion. We follow the reviewer's consideration, and revised our manuscript as follows: "AIEgens often have twisted molecular conformation, such as twisted intramolecular charge transfer conformation, cis-trans isomerized configuration and herringbone conformation etc. Among them, triphenylamine (TPA) and tetraphenyethylene (TPE) are widely used as AIE cores with propeller-like conformation (Fig. 1c)." And three important references have also been added to the revised manuscript.