Absorption (ABS) and circular dichroism (CD) spectra were recorded for 3 concentrations (2.3, 19 and ca. 75 μM) of 4-isobutyl homologs. Monomer spectra were the same as those for 4-n-propyl-5-ethyl farnesyl bacteriochlorophyll c. Pure polymer spectra were obtained by subtracting the 2.3-μM spectra appropriately scaled from the ca. 75-μM spectra. The polymer showed an ABS peak at 742 nm and a CD trough at ca. 742 nm. These properties are in harmony with the aggregate model proposed by Smith KM, Kehrs LA and Fajer J (1983, J Am Chem Soc 105: 1387-1389). A log-logplot of absorbance at 742 nm vs. monomer concentration could be fitted by a straight line of slope 1.6.