Organic long persistent luminescence wood-based materials

In recent years, the development of green materials with functional properties using natural and renewable resources has made huge contributions toward sustainable development [1]. Wood is a traditional natural green structural material with unique characteristics, such as low density, high modulus, high strength, and low thermal conductivity [2], [3], [4], [5], [6]. Due to its porous characteristics, it has been ubiquitous in applications ranging from construction, transportation, and various material fabrications for thousands of years [7], [8], [9]. Wood is mainly composed of cellulose, hemicellulose, and lignin, these bio-based macromolecules contain a series of different functional groups, and the functionalization of wood has been achieved through various physical, chemical, and processing treatments [10], [11], [12], [13]. Among them, transparent wood (TW) prepared from wood exhibits excellent optical properties and has been widely used in the preparation of optoelectronic devices and building materials [14], [15]. For example, Hu and coworkers [16] prepared a novel type of stretchable aesthetic transparent wood (ATW) with good optical and mechanical properties, making ATW a promising candidate for green building materials with additional aesthetic functions. Fu et al. infiltrated quantum dots (CdSe/ZnS) into wood to generate samples with isotropic light scattering and luminescence, which could be used in the design of indoor lamps [17]. By changing the types of quantum dots in the fabrication process, more colors of light can be obtained, which is a potential lighting application of TW [18], [19]. Therefore, TW materials have been widely used due to their abundance, renewability, stability, and sustainability [20], [21], [22], [23]. Especially in optoelectronic applications, TW retains the unique hierarchical structure of wood, providing a rigid environment, dense hydrogen-bonding sites, and an excellent oxygen barrier to suppress the non-radiative deactivation of triplet excitons [24], [25]. However, although there are some reports on room-temperature phosphorescent wood and luminescent TW exhibiting promising applications in anti-counterfeiting, data encryption, sensing and lighting [13], [20], [23], [25], afterglow duration and color tunability are still very limited, and there are no reports on the preparation of organic long persistent luminescent TW.

Moreover, apart from afterglow transparent wood, there is a significant lack of wooden materials that combine natural aesthetics with unique luminescent functionalities in other wood-based systems [26], [27]. For instance, long afterglow wood coatings and adhesives not only possess their inherent coating and adhesive properties but also exhibit excellent visual effects and practical value [28], [29]. These materials can be widely applied in architectural decoration, furniture, and toys, enhancing both functionality and aesthetic appeal while improving visibility and safety in nighttime or low-light conditions [30], [31]. Also, long-afterglow wood-plastic composites can emit a soft glow in the dark, particularly useful as markers for buildings, landscapes, and emergency pathways in parks, adding a special visual beauty [32]. However, the current methods for preparing room-temperature phosphorescent wood-based materials typically involve introducing small amounts of afterglow components into existing materials. This approach has several drawbacks, including low efficiency, short afterglow duration, uneven distribution of afterglow components, and complex preparation processes [29], [30], [31]. Therefore, the development of uniformly stable long afterglow wood coatings, adhesives, and wood-plastic composites holds great promise for various practical applications. To date, there have been no reports of organic long persistent luminescent wood-based products.

Room-temperature phosphorescence (RTP) and organic long persistent luminescence (OLPL) materials are a class of unique luminescent materials with much longer emission lifetimes than fluorescent materials [33], [34], [35]. Recent studies have shown that RTP and organic long persistent luminescent materials have wide applications in oxygen sensing, mapping, anti-counterfeiting, data encryption, background-free bioimaging, and time-gated optical sensing. [36], [37], [38], [39], [40], [41], [42], [43] The long-lifetime properties of RTP and afterglow materials originate from the triplet excited states, which are not easily formed and are prone to oxygen quenching or non-radiative deactivation due to spin-forbidden transition and weak spin–orbit coupling [44], [45], [46], [47]. Pioneering studies in this field have demonstrated that the afterglow quantum efficiency in organic systems can be greatly improved by rational molecular design, aggregation control of molecules, and supramolecular assembly strategies [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62]. For example, in some reported studies, heavy-atom effects and n–π* transitions are often used to enhance the intersystem crossing and phosphorescence decay, thus improving the organic afterglow efficiency [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]. However, the heavy-atom effect usually largely enhances the phosphorescence decay, thus shortening the phosphorescence lifetime [50]. The involvement of n–π* transitions in the organic T₁ state (where T₁ represents the lowest triplet excited state) also accelerates the phosphorescence decay, reducing the phosphorescence lifetime [39], [51], [52], [53]. In donor–acceptor systems, the appropriate combination of organic molecules can generate OLPL lasting for several hours through photo-induced charge separation (CS) and subsequent delayed charge recombination [55], [63]. Recent studies have shown that such donor–acceptor systems can maintain their OLPL properties even when exposed to ambient conditions or dispersed in aqueous media [38], [39], [40], [41], [42], [43], [64], [65], [66], [67], [68]. This binary design strategy allows for the flexible selection of luminescent dopants and organic matrices to construct afterglow materials with various structures and compositions [69], [70], [71], [72], [73], [74].

Herein, we report the fabrication of high-performance organic afterglow systems by pyrylium salt (Pyr) induced photopolymerization systems of methyl methacrylate (MMA) and 9-vinylcarbazole (NVK) in a wood frame, exhibiting unprecedented OLPL transparent wood for the first time (Fig. 1). In-depth studies reveal that intermolecular charge transfer (CT) is responsible for the formation of the charge-separated state in the Pyr-PVK donor–acceptor pairs. Equally important is the role of PMMA-wood as a matrix in suppressing non-radiative decay and oxygen quenching, which is crucial for achieving high-performance OLPL. Furthermore, by coating the pyrylium salt-polymer system onto wood, we have developed a novel functional wood coating with long-lasting afterglow properties. We have also explored its use as a wood adhesive to replace formaldehyde-based adhesives, demonstrating promising application potential. Finally, by incorporating 10 % wood flour into the pyrylium salt-polymer system and curing it, we have produced a unique wood-plastic composite material that combines the fundamental characteristics of wood with the distinctive attributes of the luminescent material.