Breaking the Fixed Output: Harnessing Photonic Reabsorption and Photothermal Effects for Tunable NIR Waveguiding in a Flexible Organic Crystal.

Flexible organic crystals have emerged as promising materials for integrated photonic devices and optical communication systems. However, the output wavelength and intensity of optical signals are typically fixed by the intrinsic molecular structures and crystal packing modes, rendering continuous and reversible modulation of a sole crystal highly challenging. Herein, we design and synthesize a near-infrared flexible organic crystal, in which dual-mode waveguide output modulation of wavelength and intensity is achieved by synergistically exploiting photonic re-absorption and photothermal-driven fluorescence deactivation. Owing to the pronounced overlap between the absorption and emission spectra, the waveguide output wavelength of the crystal can be continuously tuned from 730 to 824 nm based on the light propagation distance. Meanwhile, the crystal exhibits rapid, stable, and highly reversible photothermal conversion under 660 nm laser irradiation. Coupled with temperature-dependent fluorescence deactivation, this photothermal effect enables remote modulation of waveguide output intensity by simply adjusting the irradiation power of the input laser. This work establishes a non-contact, structure-preserving strategy for overall modulation of optical waveguides, providing new opportunities for the development of flexible near-infrared photonic devices and reconfigurable optical systems.