Luminescent materials are commonly polycyclic aromatic molecules with extended
π-electron conjugation. But a great variety of luminescent molecules that are
electronically nonconjugated are reasonably easy to synthesize.

N,N’-Bis(salicylidene)ethylenediamine (salen, 9 in Figure 3) is such a molecule; its
electronic conjugation is broken by the central ethane bridge.

Haifeng Xiang, Jin Liu, and co-workers at Sichuan University (Chengdu, China)
designed and synthesized a large number of salen derivatives (10) with different
numbers (n) of ethane bridges and combinations of substituents (R) on the phenyl
rings. They then investigated the compounds’ photoluminescence behavior.

Salen derivatives of structure 10 are weakly fluorescent when they are dissolved in
organic solvents because their excited states are nonradiatively annihilated by the
intramolecular movement of the bonds that can freely rotate.

Although the molecules are poorly conjugated, they unexpectedly emit intense blue,
green, or red light in the solid state, with Stokes shifts as high as 162 nm and Φ
values up to 75%. The emission colors can be tuned by changing n and R in the
luminogens. The blockage of intramolecular rotation and the existence of multiple
intermolecular interactions in the solid state are believed to be the cause of the
strong light emission. (J. Mater. Chem. C, DOI: 10.1039/c5tc02555a)