Organic semiconductor

A monomer of formula VIa or VIb

• R7—(Ar1)a—U—(Ar2)c—R8 VIa
• R7—U—(Ar1)a—U—R8 VIb
• wherein a and c are each independently 0, 1, or 2, U is a unit of formula I, Ar1 and Ar2 are each independently aryl or heteroaryl with 5-30 ring atoms, optionally substituted by Rs, which is on each occurrence identically or differently F, Br, Cl, —CN, —NC, —NCO, —NCS, —OCN, —SCN, —C(O)NR0R00, —C(O)X0, —C(O)R0, —NH2, —NR0R00, —SH, —SR0, —SO3H, —SO2R0, —OH, —NO2, —CF3, —SF5, optionally substituted silyl, carbyl or hydrocarbyl with 1 to 40 C atoms that is optionally substituted and optionally comprises one or more hetero atoms R7 and R8 are selected from the group consisting of Cl, Br, I, O-tosylate, O-triflate, O-mesylate, O-nonaflate, —SiMe2F, —SiMeF2, —O—SO2Z1, —B(OZ2)2, —CZ3═C(Z3)2, —C≡CH, —C≡CSi(Z1)3, —ZnX0 and —Sn(Z4)3, wherein X0 is halogen, Z1-4 are selected from the group consisting of alkyl and aryl, each being optionally substituted, and two groups Z2 may also together form a cyclic group.