Materials exhibiting reversible changes in optical properties upon light irradiation have shown great potential in diverse optoelectronic areas. In particular, the modulation of photochromic behavior on demand for such materials is of fundamental importance, but it remains a formidable challenge. Here, we report a facile and effective strategy to engineer controllable photochromic properties by varying the counterions in a series of zinc complexes consisting of a spirolactam-based photochromic ligand. Colorability and coloration rate can be finely tuned by conveniently changing their counterions. Through utilization of the reversible feature of the metal-ligand coordination bond between Zn2+ and the spirolactam-based ligand, dynamic manipulation of photochromic behavior was achieved. Furthermore, we demonstrated the practical applications of the tunable photochromic properties for these complexes by creating photochromic films and developing multilevel security printing. These findings show opportunities for the development of smart materials with dynamically controllable responsive behavior in advanced optoelectronic applications.

Pure organic materials with tunable room temperature phosphorescence (RTP) have attracted considerable interest because they are promising candidates for a wide range of optoelectronic applications. Herein, a series of organic compounds of (4‐(9H‐carbazol‐9‐yl)butyl) triphenylphosphonium (CBTP) with different halide anions (CBTP‐Cl, CBTP‐Br, and CBTP‐I) are synthesized. They show emission color changes from blue to orange‐red in the solid state. Single‐crystal X‐ray diffraction analysis and theoretical calculations demonstrate that the RTP is primarily caused by the external heavy‐atom effect (EHE), which enhances the spin–orbit coupling between the singlet and triplet excited states to facilitate the intersystem crossing rate. Distinct white light emission can be achieved using the controllable RTP by doping a certain ratio of potassium iodide (KI) into a polymer matrix containing CBTP‐Cl. Moreover, luminescent information can be recorded on a paper substrate made from a polymer film containing CBTP‐Cl with KI aqueous solution as the ink. The results suggest that rational control of the EHE of these pure organic materials is promising for different optoelectronic applications, including solid‐state lighting, data recording, and security protection.

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Information recording on paper has always been the most important approach to keep records of human activity and to spread civilization. With the progress of science and technology, paper with different functions should be exploited to conform to the increasing demands in various scenarios. In one aspect, traditional paper can only be used once, and using large amounts of paper causes deforestation, additional solid waste treatment, environmental pollution, and high energy consumption. Consequently, the development of rewritable paper that is environment-friendly, low cost, and can save resources is significant for green printing. In the other aspect, information leakage brings security issues, which may lead to severe consequences, such as war outbreak, economic loss, social problems, and so on. Therefore, the development of security printing has also attracted wide interests. Stimuli-responsive photofunctional materials that have reversible variations in absorption or emission in response to changes in the external environmental have a great potential for the achievement of green and security printing. To date, much progress has been made in these research areas. This paper lists different smart materials that respond to various external stimuli, such as light, water, pH, heat, and metal ions, and summarizes the recent advances towards green and security printing. Also, we discuss the current challenges and future directions in this rapidly growing research field. It is expected that this review article will stimulate and guide future studies for the advanced green and security printing.

Luminescent materials with tunable emission lifetimes have gained broad research interest, because they are ideal candidates for optical multiplexing applications, such as biolabeling, security printing, and data storage. To date, a few studies have reported that the efficient regulation of emission decay times can be achieved for lanthanide nanomaterials. However, it remains challenging to engineer the luminescence lifetimes of small molecules due to the lack of effective methods. Herein, a new strategy is put forward to control the emission lifetimes of ionic Cu(I) complexes by changing the electrostatic interactions between cationic complexes and counter anions. The Cu(I) complexes prepared with different counterions (ClO4−, PF6−, NO3−, and BF4−) exhibit distinct luminescence lifetimes from 12.9 to 22.3 µs in the solid state. Importantly, by simply doping different contents of CuBF4 into polymethyl methacrylate, the emission lifetimes could be varied in a linear manner in the range from 11.4 ± 0.3 to 20.7 ± 0.2 µs. Based on the tunable emission lifetimes, optical multiplexing is achieved by using time‐resolved luminescence imaging technique.

materials, which can be broadly applied as bioprobes, [2] environmental monitors, [3] optical data storage medium, [4] etc. Generally, the stimuli-responsive luminescent materials are designed based on an inner sphere mechanism for electron transfer, and there are very few examples of such materials based on the outer sphere electron transfer mechanism. In fact, the design and preparation of stimuli-responsive materials based on inner sphere electron transfer mechanism is more difficult because tedious synthetic procedures or complex covalent modifications are often involved (Figure 1a). Supramolecular interactions constructed materials can be considered as ideal candidates to overcome this limit. Ion pairs, particularly those in which one or both of the components are transition metal complexes, have attracted increasing attention recently. [5] The ion-paired materials are of practical importance in the development of organic light-emitting diode, [6] dye-sensitized solar cells, [7] catalysts, [8] chemical sensors, [9] as well as smart electronic devices. [10] Importantly, ionic donor− acceptor interactions may provide an outer sphere mechanism for electron transfer that would enable the construction of stimuli-responsive materials.Ionic phosphorescent iridium(III) complexes have received significant attention due to their appealing photophysical properties, such as high photoluminescence (PL) quantum yields, excellent photostability, significant Stokes shifts, and high sensitivity toward surrounding environments. [11] Among the various ionic iridium(III) complexes, bis-terpyridine iridium(III) complexes are one of the most attractive candidates for the formation of stimuli-responsive ion pairs due to the following features: i) rich photochemistry, complicated excited states, and remarkable photophysical properties are ideal for stimuli-responsive character, [12] ii) high cationic character (charge +3) enables strong electrostatic interactions between donor and acceptor, and iii) very high energy content in the excited states makes them powerful photo-oxidants, [13] ensuring efficient PET processes. Based on these facts, it was hypothesized that the emission intensity of a bis-terpyridine iridium(III) complex would be quenched by the addition of a strong electron donor via a PET process. Subsequently, the consumption Stimuli-responsive luminescent materials are of scientific and technological interest due to their wide range of optoelectronic applications. The utilization of photoinduced electron transfer (PET) reactions is an effective strategy to design various stimuli-responsive luminescent materials. To date, most of these materials are based on the inner sphere electron transfer mechanism, which refers to a redox chemical reaction that proceeds via a covalent bridge. In contrast, the method of using outer sphere electron transfer mechanism is superior to the covalent approach as it does not require tedious synthesis. Ion pairs, which are composed of two oppositely charged components, are ideal for t...

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