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This is essentially a chemistry question, not a biology question. The short answer is, accessory pigments are molecules which are somewhat loosely defined as all light absorbing compounds in leaves except chlorophyll a. At the molecular level, the term 'light energy is absorbed' means that light energy raises an electron from a low energy orbital to a high energy orbital. Carotenes consist mainly of chains (usually eight or more) of conjugated double bonds. Conjugated double bonds raise energy of the highest occupied molecular orbital (HOMO) and lower the energy of the lowest unoccupied molecular orbital (LUMO), so it takes comparatively less energy to excite the electrons.
i never saw carotene str and is dis qn is related to chem wait den lemme gogle hope i can get a pic of it :)
The distinctions between biology, molecular biology, biochemistry, organic chemistry, bioorganic chemistry and physical chemistry are open to interpretation and (arguably) arbitrary. Personally, I see it as a spectrum of different theoretic and experimental approaches to the same problems and molecules with some distinctions imposed between the disciplines largely for organizational purposes. Carotenes are definitely organic molecules - that is, they are produced by organisms. As I noted above, the term 'carotene' refers not to one single molecule but to a class of molecules all of them 40 carbon unsaturated hydrocarbons containing extensive systems (8 or more) conjugated double bonds. And they are produced only by all plants (although as carrots contain only one unusual kind of carotene, they are the oft cited but not all that representative example) but not by anything else. But the question is about the physical properties of carotenes that make them better able to absorb light energy than most other molecules. These actual, intrinsic properties of the molecules are quite independent of context they are in - so I read as a cut and dried organic chemistry question, not a biology question.