Stimuli-triggered modulation of solid-state excimer emission in push-pull cyanovinylene dyes

Solid-state light-emitting organic materials that can reversibly alter their optical properties in response to external stimuli have immense potential for applications in information storage, displays, and sensors. However, achieving large, readily perceptible spectral shifts with high reversibility and stability upon exposure to stimuli remains challenging. This study presents cyanovinylene-based dyes (CVn) exhibiting remarkable thermochromism and mechanochromism arising from the controlled formation of emissive excimers in the solid state. By tuning the intermolecular interactions, particularly pi-pi stacking and dipole-dipole interactions, through varying the length of alkoxy side chains (n = 4, 6, 8, 10, 16), a significant modulation of the solid-state photophysical properties was achieved. The emission colors of CVn were engineered to span the visible range from yellow (CV8, 576 nm) through orange (CV6, 594 nm; CV10, 589 nm) to red (CV4, 632 nm; CV16, 671 nm). X-Ray diffraction and excited-state lifetime analyses revealed that CV4, CV6 and CV16 favour the formation of antiparallel dimers that promote excimer-like emission, whereas CV8 can reversibly convert from a metastable polymorph that promotes excimer emission to a second polymorph that lacks the face-to-face stacking interactions responsible for excimer emission. Consequently, CV8 displayed the most dramatic stimuli-responsive behavior, switching between a yellow-emitting monomer-like state (576 nm, CV8-Y) and a red-emitting excimeric state (ca. 660 nm, CV8-R) upon heating or grinding, with the process being reversible upon exposure to CH2Cl2 vapor. This reversible, large spectral shift between two bright emissive states enabled "writing" and "erasing" of fluorescent patterns on surfaces. A series of cyanovinylene dyes (CVn) with varying alkoxy chain lengths exhibits distinct solid-state emission properties. Notably, CV8 achieves stimuli-triggered switching between two emissive states via the controlled formation of emissive excimers.