Deep‐red Emitting Copper(I) Indenediyltrisphosphine Complexes with Minimized Skeletal Vibrations and Configurational Disorder.

The full width at half maximum (FWHM) of emission spectra, which plays an important role in determining color purity, may not always receive sufficient attention in the design of emissive materials. Particularly for the red emitter, the traditional focus has been on emission maxima, yet broad FWHM values can significantly change the perceived color. For example, the red color (λem=616–677 nm) emitted from reported Cu(I) complexes is perceived as orange to yellow if FWHM is large. To reduce FWHM, we incorporated a strained and rigid metalaphosphadicyclopenta[<italic>a,f</italic>]pentalene motif into Cu(I) complexes using trisphosphine ligands featuring a 1<italic>H</italic>‐indene‐2,3‐diyl backbone (<bold>ITP</bold>). Herein, we present the synthesis, structure, and emission properties of <bold>ITP‐MX</bold> and the congeners, showcasing genuinely deep‐red emission with narrow FWHM values of 56 nm. These materials exhibit color coordinates close to pure red on the CIE diagram, unlike reported broader‐emitting counterparts. Measurement of the entropy of disorder of the emissive crystal by a recently reported statistical mechanical method revealed a quantitative correlation between FWHM and the increase in the number of microstates (the degree of freedom) of the crystals and suggested that mechanical stress can increase the entropy of the crystal, which results in emission broadening. [ABSTRACT FROM AUTHOR]