The change of intermolecular spin exchange interaction within a half-filled band of two-dimensional Langmuir films formed by photoresponsive radicals has been investigated theoretically. The photoswitching of magnetic properties arises as a result of a reversible photochemical reaction within the building units of the Langmuir films. These units consist of a photochromic moiety connected with a polymethine radical. The various contributions to the Heisenberg effective exchange integral, the direct (Coulomb), kinetic, and indirect spin exchange, have been calculated using the band theory of organic open-shell pi-electron systems.