Reversing the Bond Length Alternation Order in Conjugated Polyenes by Substituent Effects

We have synthesised several push‐pull substituted conjugated polyenes and determined their accurate C−C bond lengths and charge‐density distributions by utilising quantum crystallographic techniques. In a series of alkene, dienes, and triene bearing two (trifluoromethyl)sulfonyl (triflyl) groups on the terminal carbon atom, unique reversal of the bond‐length alternation (BLA) order has been observed. This is a pronounced aberration from the molecular structure predicted by the Lewis‐structure‐based neutral resonance structure. Such reversal of BLA order has not been observed in push‐pull compounds bearing conventional electron‐withdrawing groups such as carbonyl and cyano groups instead of triflyl groups. Bonding behaviour of both normal and reversed bond length alternating systems has been revealed by complementary bonding analysis using several bond descriptors based on the experimentally fitted wavefunctions. Bond lengths are widely utilised as observable structure parameters to estimate the contributions of particular resonance structures. Herein, it is demonstrated that outstandingly strong electron‐withdrawing groups can reverse the textbook bond‐length alternation order in polyene systems. This is a pronounced aberration from the molecular structure predicted by the neutral Lewis resonance form.