Thia-and/or selenafulvalenyl group-containing compound

A thia- and/or selenafulvalenyl group-containing compound of the formula (I), ##STR31## wherein:

• each of X.sub.1, X.sub.2, X.sub.3, X.sub.4, X'.sub.1, X'.sub.2, X'.sub.3 and X'.sub.4 is independently S or Se,
• Y is an electron donating group selected from the group consisting of a lower alkyl group having 1 to 5 carbon atoms, a lower alkoxy group having 1 to 5 carbon atoms, a thio lower alkoxy group having 1 to 5 carbon atoms, an amino group and a hydroxyl group or an electron accepting group selected from the group consisting of a halogen atom, a cyano group and a nitro group, said Y having a size which is not so large as to prevent molecular overlapping,
• m is an integer of 0 to 4,
• each of Z.sub.1, Z.sub.2, Z'.sub.1 and Z'.sub.2 is independently a hydrogen atom, C.sub.n H.sub.2n+1 in which n is an integer of 1 to 5, or XC.sub.n H.sub.2n+1 in which X is S or Se and n is an integer of 1 to 5, or alternatively, a combination of Z.sub.1 with Z.sub.2 and that of Z'.sub.1 with Z'.sub.2 are C.sub.n H.sub.2n in which n is an integer of 1 to 5, or X(C.sub.n H.sub.2n).sub.n' X in which X is S or Se and n' is an integer of 1 to 3, and
• each of R.sub.1, R.sub.2, R.sub.3, R'.sub.1, R'.sub.2 and R'.sub.3 is independently a hydrogen atom or C.sub.n H.sub.2n+1 in which n is an integer of 1 to 5.