This work describes the influence of gaseous chlorine on thin porphyri
n films. LB films of metal and free base porphyrins based on meso-tetr
a (4-amino sulfonyl) phenylporphyrin [1] exhibit intense optical absor
ption bands in the region 400-700 nm. The change in the molecular envi
ronment of the porphyrin which arises as a result of exposure to the s
ensor gas is reflected in substantial changes to the optical absorptio
n spectrum. This effect is reversible in the absence of the sensor gas
. In-situ optical measurements have been performed to quantify the res
ponse of the films ina stream of gas of specified concentration and th
eir subsequent recovery in air. Factors affecting the response such as
choice of porphyrin, orientation of porphyrin molecules within the LB
film, concentration of sensor gas and temperature have been studied.