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Bacteriochlorophyll (BChl) are pigments that occur in the photosynthetic mechanisms of bacteria. I am studying some papers on the excitonic properties of BChl's, and the term $Q_y$ transition comes up a lot, but I haven not found any explanation of its meaning.

Can someone explain to me the meaning of this term?

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The $Q_x$ and $Q_y$ transitions are electronic excitations in the conjugated $\pi$ orbitals of the Bchl a molecule. They involve two different sets of conjugated bonds. The $Q_x$ involves a shorter chain of conjugated bonds so it occurs at a higher energy/frequency. I couldn't find a really good diagram to show which bonds are involved in the in the $Q_x$ and which in the $Q_y$ excitations, but figure 1 in this paper has a reasonable illustration.

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I am sorry but I don't understand. I should mention that I am a student just starting to learn about these things. What are the conjugated $\pi$ orbitals? What is a conjugated bond? ... I am sure your answer is very good but I understand like 10% of it. –  becko Mar 4 '13 at 4:13
    
If you could reference me to some introductory material somewhere that would also be great. –  becko Mar 4 '13 at 4:14
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Gosh, you're asking for an explanation of a huge area of chemistry :-) Have a look at en.wikipedia.org/wiki/Pi_bond for an explanation of what a $\pi$ orbital is, then en.wikipedia.org/wiki/Conjugated_system for an explanation of how $\pi$ bonds can conjugate. –  John Rennie Mar 4 '13 at 7:10
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