Anomalous planar structure observed in (E)-10-(1,2-dibromoprop-1-en-1-yl)-10H-phenothiazine

•Bromine addition to 10-propynylphenothiazine proceeds without stereoselectivity.•Crystal structures of E and Z isomers are discussed based on DFT calculations.•The E isomer shows an anomalous planar structure owing to CBr⋯π halogen bonds.

Both Z and E isomers of 10-(1,2-dibromoprop-1-en-1-yl)-10H-phenothiazine were given by bromine addition to 10-(prop-1-yn-1-yl)-10H-phenothiazine with little stereoselectivity. The crystal structure of the Z isomer showed a usual butterfly structure, while that of the E isomer afforded an anomalous planar structure which was usually observed in positively charged phenothiazines. The reason for the planar structure was concluded to originate in intermolecular charge transfer interaction through the CBr⋯π halogen bonds.