NOVELTY - New photochromic compositions (PCC's) comprising at least one 1,2-bis-((hetero)aryl)-cycloalkene derivative (I)-(VI) or bis-(cyclo(hetero)alkeno)-cyclohexa-2,6-diene derivative (Ia)-(VIa), optionally together with other polymerized or polymerizable bifunctional monomer component(s), polymerization initiator(s) and/or catalyst(s), is new.    USE - HPCP (optionally in shaped article form) is used as light protectant or colorant in industrial, medicinal or cosmetic products; as a component of analog or digital data memories; in coating compositions; or in micro- or nano-technology (claimed). Typical applications are in 3-D holography, as reversible or irreversible UV-A or UV-B indicators, in UV-A or UV-B protective coatings, as light guides in microelectronics, in radiation-sensitive colored lacquers, in medicinal dosimeters, indicators and displays, as photosensitive printing inks, in colored liquid crystals displays or in molecular optical or electro-optical nano-components (e.g. switches or polarization-sensitive holographic elements).    ADVANTAGE - The polymers, having a photosensitive main chain rather than photosensitive side-chains as in prior art photochromic polymers, show excellent chemical, thermal and photochemical stability, rapid and effective photochromic conversion between the different colored isomers and good processing properties (e.g. film- or fiber forming and adhesion properties).    DETAILED DESCRIPTION - New photochromic compositions (PCC's) comprising (by weight):    (a) 0.01-100 (preferably 30-100) parts of at least one 1,2-bis-((hetero)aryl)-cycloalkene derivative of formula (I)-(VI) or bis-(cyclo(hetero)alkeno)-cyclohexa-2,6-diene derivative of formula (Ia)-(VIa);    (b) 0-99.99 (preferably 10-65) parts other polymerized or polymerizable bifunctional monomer component(s); and    (c) 0-20 (preferably 0.5-5) parts polymerization initiator(s) and/or catalyst(s), 0.1-20 (preferably 1-5) parts radical former being used in the case of radical polymerization for a crosslinked polymer.    R1, R1a, R1b, R2, R2a, R2b, R3, R3a, R3b, R4, R5, T=1-20C alkyl (optionally substituted (os) by halo), 3-20C cycloalkenyl, 3-20C cycloalkyl (os by halo), 2-20C alkynyl, 3-20C cycloalkynyl, 4-50C polyene, 1-20C ether, 1-20C carbonyl group (preferably carboxylic acid, ester, anhydride, amide, urethane, nitrile, aldehyde, ketone, thiocyanate or isocyanate), 1-30C aliphatic or 7-30C aromatic carbonyl compound, maleic acid derivative (preferably carboxylic acid, ester, anhydride or nitrile), 4-10C aryl, 6-50C fused aryl (especially benzo-aryl), 4-30C aromatic compound or derivative (especially (thio)alcohol, amine, carboxylic acid, acid halide, ester, anhydride, amide, urethane, nitrile, aldehyde, ketone, thiocyanate or isocyanate), 2-10C heteroaryl with 1-6 heteroatoms (HA's) (os by halo), 6-50C fused heteroaryl with 1-6C HA's, 4-30C heteroaromatic compound with 1-6 HA's or derivative (especially (thio)alcohol, amine, carboxylic acid, acid halide, ester, anhydride, amide, urethane, nitrile, aldehyde, ketone, thiocyanate or isocyanate), halo, 1-20C amine, 1-20C (thio)alcohol, 1-60C carbohydrate with 1-20 O, 1-60C organophosphorus or organosulfur compound with 1-20 P or S respectively, 1-60C organometallic compound, 1-50C organic complex compound with 1-2 HA's, fullerene or porphyrin or derivative (especially (thio)alcohol, amine, carboxylic acid, acid halide, ester, anhydride, amide, urethane, nitrile, aldehyde, ketone, thiocyanate or isocyanate), 6-50C aromatic diazo compound with 1-20 HA's, quinone or heteroquinoid derivative with 2-20C and 1-10 HA's, 1-20C aliphatic nitro compound with 1-10 NO2 groups, 6-50C aromatic nitro compound with 1-20 NO2 groups, 3-100C polycyclic hydrocarbyl with 1-30 HA's (sic) or derivative (especially (thio)alcohol, amine, carboxylic acid, acid halide, ester, anhydride, amide, urethane, nitrile, aldehyde, ketone, thiocyanate or isocyanate), 5-60C isoprenoid, 1-10C aminoacid with 1-5 amino groups (or peptide formed from 2-100000 such aminoacids), (poly)nucleotide with 1-100000 nucleotide units, polymer (especially polyalkane, polyalkene, polyalkyne, polyester, polyamide, polyimide, polyurethane or polyarylene ether containing 10-10000000C and optionally HA's or halo) or H;    R1+R1 or R1a+R1b=1-20C alkyl bridge (os by halo, CN, 1-20C hydroxyalkyl, 1-20C alcohol ester residue or O); or    R1a + R1b=os 5-7 membered heterocycle (containing at least one N, O or S and optionally 1-3 double bonds); or a benzo- or anthraquinone structure (os by halo);    R6 - R8, Y1, Y2=1-20C aliphatic (thio)alcohol, (thio)aldehyde, (thio)ketone or (thio)isocyanate with up to 5 S, 6-36C (hetero)aromatic (thio)alcohol, (thio)aldehyde, (thio)ketone, carboxylic acid, acid halide, nitrile or (thio)isocyanate with 0-5 HA's, 1-20C aliphatic carboxylic acid, acid halide or nitrile, 1-50C aliphatic amine with 1-10 N, 6-50C aromatic amine with 1-20 HA's, halo, organometallic compound, 4-50C alkene with 1-25 double bonds (os by halo), 3-20C cycloalkenyl with 1-10 double bonds (os by halo), 2-20C alkynyl with 1-10 double bonds (os by halo), 3-20C cycloalkynyl with 1-10 double bonds (os by halo), 1-20C linear or branched sulfonic, phosphoric or phosphonic acid with 1-5 sulfonic, phosphoric or phosphonic acid groups respectively, linear or branched siloxane with 1-20 Si and up to 40 O, linear or branched siloxane with 1-20 Si, 1-20C and up to 40 O or H;    X1-X4=C, Si, Ge, Sn, N (secondary, tertiary or quaternary), P, As, Sb, O, S, Se or Te;    Z1, Z2=urethane, ester, amide, imine, (thio)ether, sulfone, carbonate, alkene, alkylene, (hetero)aryl ether or (hetero)arylene;    n=1-1000000000 (preferably 1000-1000000, especially 10000-100000);    Heteroatoms are preferably O, S or N.    Provided that R8 is H if R6 and R7 are the same; or R8 may also be a bond to a further urethane, ester, amide, ether, sulfonic, phosphoric or phosphonic acid derivative, siloxane, carbosilane, alkyne, alkene, alkyl, heterocycle, carbocycle, imine, poly(hetero)aryl or polyene.    INDEPENDENT CLAIMS are included for:    (1) the preparation of (I)-(VI) and (Ia)-(VIa);    (2) producing (Ia)-(VIa) by exposing (I)-(VI) (optionally in a solvent) to electromagnetic radiation of wavelength less than 400 (preferably 190-330) nm;    (3) the production of (I)-(VI) by exposing (Ia) - (VIa) (optionally in a solvent) to electromagnetic radiation of wavelength more than 400 (preferably 500-800) nm;    (4) the production of a hardened photochromic polymer (HPCP) by (co)polymerization of PCC in solution, in the gas phase or in solid or liquid bulk form;    (5) shaped articles (specifically in film, fiber and/or granule form) of HPCP; and    (6) a ready-for-use solid or liquid composition (specifically a lacquer) comprising 0.01-100 (preferably 10-60) %, by weight PCC, optionally dispersed or dissolved in a solvent.