A compound of the general formula where R 1 and R 2 represent fluorine and R 3 and R 4 represent hydrogen, or R 1 and R 3 represent fluorine and R 2 and R 4 represent hydrogen, or R 1 and R 4 represent fluorine and R 2 and R 3 represent hydrogen, Z 1, Z 2, Z 3 and Z 4 represent 0, R a and R b independently of one another represent hydrogen or unsubstituted alkyl, alkenyl, alkadienyl, alkynyl, cycloalkyl, bicycloalkyl, cycloalkenyl, heterocycloalkyl, aryl or heteroaryl, or one of the radicals Z 1 or Z 2 represents NR c and R a and R c together represent a bridging group comprising 2 to 5 atoms between the flanking bonds and/or one of the radicals Z 3 or Z 4 represents NR d and R b and R b together represent a bridging group comprising 2 to 5 atoms between the flanking bonds, wherein the term alkenyl encompasses straight-chain or branched alkenyl groups which, depending on the chain length, may bear one or more double bonds, the term alkynyl encompasses alkynyl groups comprising one or more nonadjacent triple bonds, the term cycloalkenyl encompasses monocyclic monounsaturated hydrocarbon groups comprising 3 to 8 carbon ring members, the term bicycloalkyl encompasses bicyclic hydrocarbon radicals comprising 5 to 10 C atoms, the term aryl encompasses mono- or polycyclic aromatic hydrocarbon radicals, the term heterocycloalkyl encompasses nonaromatic, unsaturated or fully saturated cycloaliphatic groups comprising 5 to 8 ring atoms in which 1, 2 or 3 of the ring carbon atoms have been replaced by heteroatoms selected from oxygen, nitrogen, sulfur and an -NR e - group, the term heteroaryl encompasses heteroaromatic mono- or polycyclic groups. R 1 and R 2 represent fluorine and R 3 and R 4 represent hydrogen, or R 1 and R 3 represent fluorine and R 2 and R 4 represent hydrogen, or R 1 and R 4 represent fluorine and R 2 and R 3 represent hydrogen, Z 1, Z 2, Z 3 and Z 4 represent 0, R a and R b independently of one another represent hydrogen or unsubstituted alkyl, alkenyl, alkadienyl, alkynyl, cycloalkyl, bicycloalkyl, cycloalkenyl, heterocycloalkyl, aryl or heteroaryl, or one of the radicals Z 1 or Z 2 represents NR c and R a and R c together represent a bridging group comprising 2 to 5 atoms between the flanking bonds and/or one of the radicals Z 3 or Z 4 represents NR d and R b and R b together represent a bridging group comprising 2 to 5 atoms between the flanking bonds, wherein the term alkenyl encompasses straight-chain or branched alkenyl groups which, depending on the chain length, may bear one or more double bonds, the term alkynyl encompasses alkynyl groups comprising one or more nonadjacent triple bonds, the term cycloalkenyl encompasses monocyclic monounsaturated hydrocarbon groups comprising 3 to 8 carbon ring members, the term bicycloalkyl encompasses bicyclic hydrocarbon radicals comprising 5 to 10 C atoms, the term aryl encompasses mono- or polycyclic aromatic hydrocarbon radicals, the term heterocycloalkyl encompasses nonaromatic, unsaturated or fully saturated cycloaliphatic groups comprising 5 to 8 ring atoms in which 1, 2 or 3 of the ring carbon atoms have been replaced by heteroatoms selected from oxygen, nitrogen, sulfur and an -NR e - group, the term heteroaryl encompasses heteroaromatic mono- or polycyclic groups.