Question

4. Pyridine shows significantly lower fluorescence (quantum yield) than the fused ring molecule isoquinoline. Suggest a reason for that difference. pyridine isoquinoline

Answer 1

very first thing to observe here is that in Pyridine and isoquinoline both are conjugated system. pi electrons that is pi bonds are resonating. since in conjugated systems the energy difference between HOMO and LUMO is very small, hence Fluorescence occurs at very low exciting energy to get excited , now these excited electrons produce visible light while returning back to HOMO from LUMO.Also we know increase of the conjugation length of the pi system causes a shift of the emission spectra towards longer wavelength that means towards lower energy.

Hence in Isoquinoline there is extended conjunction in comparison to Pyridine. So isoquinoline has longer pi system that is Fluorescence occur at lower energy and in greater yield.

Conclusion- Isoquinoline is an extented pi system of conjugated electron require lesser energy to get excited and gives more Yield.

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