Bacteriochlorophyll is the main structural unit of photosynthetic components of bacteria, in which photosynthesis is conducted without production of oxygen. They include purple and green sulphur bacteria. Various bacteria contain different types of bacteriochlorophylls (a, b, c, d, e, g). They form the basis of light-harvesting antennae and make part of the photosynthetic reaction centre.

Bacteriochlorophyll in bacteria performs the same functions as chlorophyll in plants. In general, bacteriochlorophylls absorb light of a longer wavelength than chlorophylls. The position of the absorption maximum is observed in the red or infra-red region and depends on the type of bacteriochlorophyll and its protein environment. Bacteriochlorophylls are denoted by the numbers, with the letter B denoting the position of the long-wavelength absorption peak. Thus, B850 refers to bacteriochlorophyll, whose maximum absorption coefficient is reached at 850 nm. Bacteria, in contrast to plants, are capable of photosynthesis not only in the visible spectrum, but also outside of it, in the infra-red region (800-1035 nm). This unique feature of such bacteria among all known phototrophic organisms explains the possibility of their functioning in the dark and in places that are unsuitable for other photosynthetic systems (deep in the soil or in the sea, or in the internal organs of living organisms). These properties offer attractive prospects for use of bacteriochlorophylls, especially as elements of nanostructures in photodynamic therapy and nanomedicine.