Irradiation of a ferroelectric S _C * liquid crystal phase induced by the photochromic dopants (R,R)-6,6′-bis(2-octyloxy)-5,5′-dinitrothioindigo and (R,R)-5,5′-dichloro-6,6′-bis(2-octyloxy)thioindigo at λ=514 and 532 nm, respectively, causes an increase in spontaneous polarization (P _S ) by a factor ranging from 1.4 to 4.0. This increase in P _S is achieved without concomitant destabilization of the S _C * phase, and is consistent with an increase in transverse dipole moment of the thioindigo core as a result of trans-cis photoisomerization. The contribution of the thioindigo core towards P _S is achieved through stereo-polar coupling with the chiral side-chains, which is enhanced by the presence of the nitro and chloro substituents.