NOVELTY - Substituted fused heterocyclic compounds (I) are new.    USE - (I) are useful in: an electronic device, electron transport layer, hole/exciton blocking layer or emitting layer of an apparatus comprising stationary visual display units (visual display units of computers, TV, visual display units in printers, kitchen appliances and advertising panels), illuminations, information panels, mobile visual display units (visual display units in cell phones, laptops, digital cameras, MP3 players, vehicles and destination displays on buses and trains), keyboards, items of clothing, furniture and wallpaper; and electrophotographic photo receptors, photoelectric converters, organic solar cells (organic photovoltaics), switching elements, organic light emitting field effect transistors, image sensors, dye lasers and electroluminescent devices (all claimed).    ADVANTAGE - (I): exhibit improved efficiency, stability, manufacturability, or spectral characteristics of electroluminescent devices; provide organic LED's that are operated at low voltages; and are utilized in blue light emitting phosphorescent emitters that exhibit good efficiency and durability.    DETAILED DESCRIPTION - Substituted fused heterocyclic compounds of formula (I) are new.    E = -OR69, -SR69, -NR65R66, -COR68, -COOR67, -CONR65R66, -CN, or halo;    G = E, 1-18C alkyl, or 6-24C aryl or 2-30C heteroaryl (both optionally substituted by 1-18C alkyl); either    R65, R66 = 6-18C aryl (optionally substituted by 1-18C alkyl or 1-18C alkoxy) or 1-18C alkyl (optionally interrupted by -O-); or    R65R66 = 5 or 6 membered ring;    R67, R69 = 6-18C aryl (optionally substituted by 1-18C alkyl, or 1-18C alkoxy), 1-18C alkyl (optionally interrupted by -O-);    R68 = 6-18C aryl (optionally substituted by 1-18C alkyl, or 1-18C alkoxy), 1-18C alkyl (optionally interrupted by -O-) or H;    X6 = (-N=); and    X7 = (-NR6-); or    X7 = (=N-); and    X6 = (-NR6);    R6 = substituted heterocyclyl moiety of formula (r1) or (r2);    A1 = branching unit having three linkage sites;    A2, A3 = 6-24C arylene or 2-30C heteroarylene (both optionally substituted by G), where A1-A3 are optionally interrupted by at least one -(SiR7R8)-;    R7, R8 = 1-25C alkyl or 6-24C aryl (both optionally substituted by G);    R1, R1a = 1-25C alkyl (optionally substituted by E and/or interrupted by D1), 6-24C aryl or 2-30C heteroaryl (both optionally substituted by G) or H;    L = single bond or a linking group;    X1 = N, or CR9;    X2 = N, or CR10;    X3 = N, or CR48;    X4 = N, or CR49;    X5 = N or CR50; either    R48-R50, R9, R10, R9a, R10a = 1-25C alkyl (optionally substituted by E and/or interrupted by D1), 6-24C aryl or 2-30C heteroaryl (both optionally substituted by G) or H; or    R9R10, R9aR10a = ring (optionally substituted);    p, q = 0-2;    D1 = -CO-, -COO-, -S-, -SO-, -SO2-, -O-, -NR65-, -SiR70R71-, -POR72-, -CR63=CR64- or -CC-;    R63, R64 = 6-18C aryl (optionally substituted by 1-18C alkyl or 1-18C alkoxy), 1-18C alkyl (optionally interrupted by -O-) or H; and    R70-R72 = 1-18C alkyl or 6-18C aryl (optionally substituted by 1-18C alkyl).    Provided that at least one of X3, X4 and X5 is N and 9-(4,6-diphenyl-(1,3,5)triazin-2-yl)-9H-4b,9,10-triaza-indeno(1,2-a)indene is excluded. An INDEPENDENT CLAIM is also included for an electron transport layer, hole/exciton blocking layer or emitting layer comprising (I).