A series of six open-chain tetrapyrroles has been synthesized and used as chromophores for the plant photoreceptor protein phytochrome. The novel chromophores vary in the size of substituents 17 and 18 at ring D. This ring undergoes maximal conformational change upon light excitation (Z -> E photoisomerization of the 15,16-double bond). Instead of methyl and vinyl substituents (positions 17, 18) as present in the native chromophore phytochromobilin, dimethyl, methyl and isopropyl, methyl and tert-butyl, ethyl and methyl, vinyl and methyl, and isopropyl and methyl substituents have been generated. All novel chromophores assemble with the apoprotein. The obtained chromoproteins show hypsochromic shifts of the absorbance maxima by 10 nm maximally, compared to the native pigment, except for the 17-isopropyl-18-methyl-substituted compound which showed a 100 nm hypsochromic shift of selectively the P-r form. The assembly kinetics were slowed down in correlation to the increasing size of the substituents, with stronger effects for modified substituents at position 17. The thermal stability,of the photoinduced P-fr form for the 18-isopropyl and the 18-tert butyl substituents was even greater than that of the native pigments. Those chromophores carrying substituents at position 17 larger than the methyl group (ethyl and isopropyl) showed a very low stability of the respective P-fr forms. Time-resolved detection of the P-r to P-fr conversion (laser-induced flash photolysis) revealed a slower formation of the P-fr form for those chromophores carrying larger substituents at position 18, whereas the rise and decay kinetics of the early intermediates are only moderately changed. Introduction of larger substituents at position 17 (ethyl, vinyl, and isopropyl) causes drastic changes in the kinetics; in particular the formation of the first thermally stable intermediate, I-700, is significantly slowed, making a detection of its rise possible.