NOVELTY - A photoluminescent composition comprises an acene compound and a benzothiadiazole compound.    USE - As a photoluminescent composition used in a spectrum converter which is used in a solar device (claimed).    ADVANTAGE - The photoluminescent composition for spectrum converters is synergistic and shows enhanced efficiency i.e. the use of acene compound and a benzothiadiazole compound allows a wide portion of the radiations of the solar spectrum to be intercepted (radiation absorbed), re-emitting them at lower frequencies (radiation emitted), within a narrower spectral range, much more effectively than spectrum converters of prior art. The photoluminescent compositions, have high efficiency because of high absorption coefficients and luminescence quantum efficiencies of acene compound and a benzothiadiazole compound, effective superimposition of the emission bands of the acene compounds with the absorption bands of the benzothiadiazole compounds; and distinct separation of the absorption and emission bands of the benzothiadiazole compounds. The distinct separation of the emission and absorption bands which distinguish the benzothiadiazole compounds prevents, or at least considerably attenuates, the luminescence autoquenching phenomenon, which is one of the main reasons for the limited effectiveness of the spectrum converters.    DETAILED DESCRIPTION - A photoluminescent composition comprises an acene compound of formula (I) or (Ia), and a benzothiadiazole compound of formula (II).    n=1-3 (preferably 1);    m=0-2 (preferably 0);    p=1-3 (preferably 1);    R', R'1, R'a, R'1a= optionally substituted aromatic or heteroaromatic group;    X', X'1, X'a, X'1a=covalent bond or a divalent -C?=C- or -CH=CH- group;or    (R'a-X'a-) or (R'1a-X'1a-)=H or 1-5C alkyl;    R1=alkinyl group R3C?=C-, or an aromatic or heteroaromatic group, optionally substituted, conjugated with the benzene ring of the benzothiadiazole;    R3=H, 1-8C alkyl (optionally fluorinated) or optionally substituted 4-10C aromatic or heteroaromatic group;    R2=H or 1-10C alkyl (preferably H).    R' and R'1 are bonded to X' and X'1 respectively by an atom of the aromatic ring, preferably selected from phenyl, pyrroyl, pyridyl, carbazoyl and their substituted derivatives. Provided that at least one of each R'a, R'1a, X'a and X'1a group has the same meaning as the corresponding R', R'1, X' or X'1 groups.