Collective photothermal bending of flexible organic crystals modified with MXene-polymer multilayers as optical waveguide arrays

We present a straightforward approach to obtain flexible hybrid organic crystalline materials that are mechanically responsive to infrared light.

The performance of any engineering material is naturally constrained by its structure. While each material may have one or more drawbacks to consider for specific applications, these limitations can be circumvented through hybridization with other materials. By combining organic crystals with MXene as thermal absorbers and positively charged polymers as adhesive counterions, we propose a straightforward approach to obtaining flexible hybrid organic crystal materials that possess mechanical responsiveness to infrared light. The ability to shape control using infrared light expands the range of potential applications for organic crystals, with one direct application being their use as thermally controllable flexible optoelectronic waveguides for signal transmission in flexible organic electronics.

Figure 1. Preparation and thermal properties of the MXene-polymer-crystal hybrids. a Chemical structure of 1. b Scanning electron microscopy (SEM) images of 1@(PDDA/PSS)5 and the 1@(PDDA/MXene)5 surfaces. c Atomic force microscopy (AFM) images of 1@(PDDA/PSS)5 and the 1@(PDDA/MXene)5 surfaces. d (PDDA/MXene)5 coated crystal temperature increase with increasing infrared optical power (ΔT, atmospheric temperature of 25 °C). e A time-dependent temperature increase of the 1@(PDDA/PSS)5 and the 1@(PDDA/MXene)5 surface under the illumination with infrared light (744 mW) at room temperature of 25 °C.

The multilayers of MXene are manufactured using the layer-by-layer (LbL) assembly technique, where poly(diallyldimethylammonium chloride) (PDDA) and MXene nanosheets are alternately deposited onto a solid substrate, resulting in a multilayer structure denoted as (PDDA/MXene)n. In a typical case, the organic compound 1 (Fig. 1a) is coated with five bilayers, 1@(PDDA/MXene)5, with a thickness of approximately 200 nanometers and low roughness (Fig. 1b, c). As illustrated in Fig. 1d, when exposed to infrared light, the surface temperature (ΔT) of 1@(PDDA/MXene)5 increases with the increasing power of the infrared radiation. Under 744 mW of infrared radiation, ΔT reaches 61.2 °C and reaches a steady value within 10 seconds (Fig. 1g), demonstrating an excellent infrared photothermal effect.

Figure 2. Preparation of hybrid organic crystal arrays. a The chemical structure of crystals 24 and photos show their mechanically induced bending. b Process for the preparation of hybrid organic crystal arrays (kept at relative humidity RH = 62%, 64%) and their collective bending induced by illumination with infrared light (250 W).

Some future applications of flexible crystals are based on two-dimensional sensing, which requires an ordered array composed of individual bendable crystals. To explore this potential application direction, we have prepared a two-dimensional array of mixed organic crystals capable of collective bending under infrared light. Elongated elastic crystals of three organic compounds, 2, 3, and 4, measuring centimeters in length (Fig. 2a), were selected. As depicted in Fig. 2b, the newly grown (crystalline) crystals of 2-4 were initially coated with a mixture of PDDA and PSS. Subsequently, a mixture of PDDA and MXene was applied to the surface of the resulting hybrid crystals, 2-4@PDDA/PSS (referred to as 2-4@P), which was described as 2-4@PDDA/MXene@PDDA/PSS for convenience (referred to as 2-4@P2). Finally, a uniform and rapid deposition of polyvinyl alcohol (PVA) along the flexible surface of the hybrid crystals, in combination with PSS, namely PVA/PSS, using a needle tip followed by drying, resulted in the hybrid structure described as 2-4@PVA/PSS@PDDA/MXene@PDDA/PSS (referred to as 2-4@P3). The crystals in the final array undergo multidirectional mechanical motion, further expanding their potential applications in two-dimensional optical transmission and detection.

Figure 3. Assessment of the performance of the hybrid organic crystals. a Testing of reproducibility of deformation of 3,4@P3 in cycling mode. b Dependence of the bending angle of 3,4@P3 as a function of time.

The mechanical robustness, long-term recyclability, and responsive sensitivity of dynamic crystals are essential prerequisites for their application in flexible electronic devices and similar systems. The photodetectors of 3,4@P3 were exposed to both infrared radiation and sunlight. After 100 cycles, the maximum curvature in the bent state remained nearly unchanged (Fig. 3a). The reaction rates of the hybrid crystals were estimated for the first cycle and the 100th cycle (Fig. 3b). The bending and recovery times of 3,4@P3 remained nearly constant. Overall, we conclude that the performance of the photodetectors is sufficiently stable, which is favorable for this material to be considered as a viable candidate for practical applications, such as optical signal transmission.

Figure 4. Optical waveguiding properties of the hybrid organic crystals. a Diagram of infrared light-driven hybrid organic crystal array for optical signal transmission. The top left image is a zoomed-in representation of the crystal tip. b Photographs of a hybrid organic crystal array for optical signal transmission. c A schematic showing the dependence of the optical output point of a hybrid organic crystal on the excitation position. d Photographs showing the change in output of the optical signal of 3@P3 with the position of the infrared light. The insets show 10-fold magnified images of the crystal tip. The broken line circles indicate the position of the optical signal output.

As mentioned above, organic crystals offer advantages such as relatively low optical loss and long-range ordered structures. One of the notable characteristics being actively explored in elongated organic crystals is their ability to serve as optical waveguides. Figure 4a showcases an array of mixed organic crystals driven by infrared light for optical signal transmission. In Figure 4b, one end of the mixed organic crystal is affixed and excited by a 365-nanometer laser, while the optical output at the other end is controlled by infrared radiation. This control is achieved by varying the degree of bending through changes in the excitation point. As depicted in Figures 4c and 4d, 3@P3 is excited by a 365-nanometer laser at its fixed end, and the optical output at the other end varies with the position of 808-nanometer light (184 milliwatts) illumination. These results clearly confirmed that the light transmission through the hybrid organic crystal can be controlled by infrared light.

In summary, we have presented a series of mixed organic crystal materials that undergo deformations driven by infrared light and can be precisely controlled. These mixed materials exhibit favorable characteristics, deriving mechanical flexibility from the mechanical compliance of organic crystals and thermal sensitivity controlled by the MXene layer on their surface. This deformation can be induced not only in individual crystals but also in an array of mixed crystals arranged in a regular two-dimensional pattern. As a proof of concept for the tremendous opportunities this approach brings to dynamic materials, we have demonstrated that infrared light-driven flexible organic crystal waveguides can be constructed using these materials, including ordered arrays of such waveguides. By expanding the palette of available excitation stimuli (such as light, humidity, and magnetic fields) to include infrared light, the prospects for constructing flexible optical and electronic devices based on organic crystals are greatly enhanced.

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