Energy-band structure for triplet excitons in space group c 52h

The triplet exciton energ-band structure for organic molecular solids belonging to space group C_2h⁵ containing in general four molecules at arbitrary positions in the unit cell is derived in the restricted Frenkel limit for the principal symmetry directions in the zone. The dispersion relations can be obtained without a priori assumptions on the relative strengths of the transfer matrix elements between the different sublattices. The band structure of crystals of current experimental interest like pyrene and 1,4-dibromonaphthalene is discussed.