Structural properties of a series of photochromic fluorinated indolylfulgides

Fluorinated indolyl­fulgides are a class of photochromic organic compounds that meet many of the requirements for use as optical memory media and optical switches. The X‐ray crystal structures of a series of five photochromic fluorinated indolyl­fulgides have been determined, namely (3Z)‐3‐1‐(1,2‐di­methyl‐1H‐indol‐3‐yl)‐2,2,2‐tri­fluoro­ethyl­idene]‐4‐(1‐methyl­ethyl­idene)­dihydrofuran‐2,5‐dione (tri­fluoro­methyl­iso­propyl­idene­indolyl­fulgide), C₁₉H₁₆F₃NO₃, (I), (3Z)‐3‐1‐(1,2‐dimethyl‐1H‐indol‐3‐yl)‐2,2,3,3,3‐penta­fluoro­propyl­idene]‐4‐(1‐methyl­ethyl­idene)­dihydrofuran‐2,5‐dione (penta­fluoro­ethyl­iso­propyl­idene­indolyl­fulgide), C₂₀H₁₆F₅NO₃, (II), (3Z)‐3‐1‐(1,2‐di­methyl‐1H‐indol‐3‐yl)‐2,2,3,3,4,4,4‐hepta­fluoro­butyl­idene]‐4‐(1‐methyl­ethyl­idene)­dihydrofuran‐2,5‐dione (hepta­fluoro­propyl­iso­propyl­idene­indolyl­fulgide), C₂₁H₁₆F₇NO₃, (III), (3Z)‐3‐1‐(1,2‐di­methyl‐1H‐indol‐3‐yl)‐2,2,2‐tri­fluoro­ethyl­idene]‐4‐(tri­cyclo­3.3.1.1^3,7]­decyl­idene­)dihydrofuran‐2,5‐dione (tri­fluoro­methyl­adamantyl­idene­indolyl­fulgide), C₂₆H₂₄F₃NO₃, (IV), and (3Z)‐3‐1‐(1,2‐di­methyl‐1H‐indol‐3‐yl)‐2,2,3,3,4,4,4‐hepta­fluoro­butyl­idene]‐4‐(tri­cyclo­3.3.1.1^3,7]­decyl­idene­)dihydrofuran‐2,5‐dione (hepta­fluoro­propyl­adamantylidenein­dolyl­fulgide), C₂₈H₂₄F₇NO₃, (V). The photochromic property of fulgides is based on the photochemically allowed electrocyclic ring closure of a hexatriene system to form a cyclo­hexa­diene. For each fulgide examined, the bond lengths within the hexatriene system alternate between short and long, as expected. Comparing the structures of the five fulgides with each other demonstrates no significant difference in bond lengths, bond angles or dihedral angles within the hexatriene systems. The distance between the bond‐forming C atoms at each end of the hexatriene system does vary. Correlations of structural properties with optical properties are addressed.