Bond-length alternation in rigidized merocyanines

The two merocyanines 7-(4-dimethylaminophenyl)-2,3, 4,4a,5,6, 10, 10a-octah ydroanthracen-2-onc [C22H25NO, (2)] and [2,3,4,4a,5,6,10, 10a-octahydro-7-( 5-piperidino-2-thienyl)- 2-anthrylidene]methanedicarbonitrile [C26H27N3S, ( 4)] show somewhat shortened single bonds [1.434(4)-1.452 (4) Angstrom in (2 ) and 1.421(4)-1,425(4)Angstrom in (4)] and elongated double bonds [1.345(4 )-1.358 (4) Angstrom in (2) and 1.355(4)-1.365(4) Angstrom in (4)] compared to standard bond lengths in polyenes, A comparison of the merocyanines rev eals that (4) exhibits a slightly smaller bond length alternation within th e rigidized hexatriene moiety than (2), probably because of an increased gr ound-state polarization.