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    A level chemistry question;

    Why does increasing the length of a chromophore in a dye molecule decrease the frequency of light absorbed?

    Help please!!!!
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    As E=hv, the amount of energy needed to promote electrons to a higher energy level must increase, therefore a higher frequency corresponding to this energy must be absorbed.

    Not sure why though, but if it's an exam question this might get you some marks.
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    I thought, as the size of the delocalised electron system increased, the amount of energy to excite an electron decreased? And since E is proportional to V then the frequency decreases, and will transmit in the opposite end of the spectrum.
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    I don't think you'd get asked this on an A level paper since the (correct) answer requires knowledge of basic molecular orbital theory which isn't on the syllabus. But if you really want to know:

    The light observed is due to excitation and subsequent relaxation of an electron between the higest occupied molecular orbital (HOMO), which is a pi orbital of a C=C double bond, and the lowest unnocupied molecular orbital (LUMO). The size of this energy gap determines the exact frequency of light absorbed accroding to E = hv. Adding extra pi orbitals (extra double bonds) increases the energy level of the HOMO, and also lowers the energy level of the LUMO, so the energy gap between them is smaller. According to E=hv, lower energy gap means lower frequency light as you increase chromophore (conjugation) length.

    Maybe there's a simpler, A level friendly explanation, but that's the reason a longer chromophore length means lower frequency absorption. I'd imagine you just have to know what RobG93 said in the post above mine.
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    Thank you everyone! Yeah, I don't think I'll get that question in an exam either. But its in my end of topic test which apparently is always harder. I suppose its better, because you have a greater depth of knowledge (:

    You've all been really helpful though! I agree, I think I will go with RobG93's explanation. Seems A level friendly!

    This was my first post too. How scary :P
 
 
 
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