NOVELTY - Fused heterocyclic compounds (I), are new.    USE - (I) or the oligomers, polymers or dendrimers are useful: in an electronic device, where the electronic devices are organic electroluminescent devices, organic integrated circuits, organic field-effect transistors, organic thin film transistors, organic light emitting transistors, organic solar cells, organic optical detectors, organic photoreceptors, organic field-quench devices, light-emitting electrochemical cells and organic laser diodes; and as a host material, preferably for fluorescent dopants, or as fluorescent dopants, a hole transport material, hole injection material or as electron transport material (all claimed).    ADVANTAGE - (I): provide organic light emitting diodes with a long operational life, good efficiency and low operating voltage; and exhibit improved thermal stability.    DETAILED DESCRIPTION - Fused heterocyclic compounds of formula (I), are new.    Ar1 = 15-60 membered (hetero)aryl (optionally substituted with one or more R);    Ar2 = 6-10 membered (hetero)aryl (optionally substituted with one or more R1);    Y1 = CR2 or N, with the proviso that no more than 2 adjacent Y1 represent N;    X = CR3 or N, with the proviso that no more than 2 adjacent X represent N;    n = 1-4, preferably 1; either    R, R1, R2 = H, D, F, Cl, Br, I, CHO, N(R4)2, C(=O)R4, P(=O)(R4)2, S(=O)R4, S(=O)2R4, CR4=C(R4)2, CN, NO2, Si(R4)3, B(OR4)2, OSO2R4, OH, or linear 1-40C-(thio)alkyl or -alkoxy, branched or cyclic 3-40C-(thio)alkyl or -alkoxy, 2-40C-alkenyl or -alkynyl (all optionally substituted by one or more R4, and where one or more non-adjacent CH2-groups are optionally replaced by R4C=CR4, C?=C, Si(R4)2, Ge(R4)2, Sn(R4)2, C=O, C=S, C=Se, C=NR4, P(=O)(R4), SO, SO2, NR4, O, S or CONR4, and one or more H-atoms are replaced by D, F, Cl, Br, I, CN or NO2), or 5-60 membered mono- or polycyclic (hetero)aromatic ring system or 5-60 membered (hetero)aryloxy (both optionally substituted by one or more non-aromatic residue of R4); or    two or more R, R1, R2 bonded together = mono or polycyclic, aliphatic or aromatic ring system; either    R3 = H, D, F, Cl, Br, I, CHO, N(R4)2, C(=O)R4, P(=O)(R4)2, S(=O)R4, S(=O)2R4, CR4=C(R4)2, CN, NO2, Si(R4)3, B(OR4)2, OSO2R4, OH, or linear 1-40C-(thio)alkyl or -alkoxy, branched or cyclic 3-40C-(thio)alkyl or -alkoxy, 2-40C-alkenyl or -alkynyl (all optionally substituted by one or more R4, and where one or more non-adjacent CH2-groups are optionally replaced by R4C=CR4, C?=C, Si(R4)2, Ge(R4)2, Sn(R4)2, C=O, C=S, C=Se, C=NR4, P(=O)(R4), SO, SO2, NR4, O, S or CONR4, and one or more H-atoms are replaced by D, F, Cl, Br, I, CN or NO2); or    two or more R3 bonded together = mono or polycyclic, aliphatic or aromatic ring system; and either    R4 = H, D, F or 1-20C-aliphatic, (hetero)aromatic organic residue, in which one or more H-atoms are optionally replaced by F; or    two or more R4 bonded together = mono- or polycyclic, aliphatic or aromatic ring system, where in the case that Ar1 represents a benzo(a)anthracene, which is bonded in positions 1, 2, 3, 4, 5, 6, 8, 9, 10, 11 or 12 on the group Ar2.    INDEPENDENT CLAIMS are included for:    (1) the preparation of (I);    (2) oligomers, polymers or dendrimers containing (I); and    (3) a formulation comprising (I) or the oligomers, polymers or dendrimers, and a solvent.