Question

Which compound in each of the following pairs would you expect to have a greater fluorescence quantum yield? Explain.

Short answer

Part (a) Fluorescein has a higher quantum yield.

Part (b) $o,o\text{'}-dihydroxyazobenzene$has a higher fluorescence quantum yield.

Step-by-step solution

Step 1. Given information

In the following situations, the quantum yield (quantum efficiency) of fluorescence is higher:

(i) Compounds with aromatic systems have a higher fluorescence quantum yield than those with aliphatic or alicyclic carbonyl structures.

(ii) Fluorescence quantum yield is higher in compounds with fused rings.

(iii) Fluorescence quantum yield is higher in compounds with strongly conjugated double bond structures than in compounds with unconjugated double bonds.

(iv) Fluorescence quantum yield is higher in compounds with a more rigid structure.

Part (a) Step 1. Comparing structures 

The structure of phenolphthalein and fluorescein are as follows:

Compared to phenolphthalein, fluorescein has fused rings and a more rigid structure. As a result, fluorescein has a higher quantum yield.

 Part (b) Step 1. Comparing compounds

The structure of $o,o\text{'}-dihydroxyazobenzene$and $bis(o-hydroxyphenyl)hydrazine$are as

The double bond between the two nitrogen groups in compound $o,o\text{'}-dihydroxyazobenzene$is in conjugation between the aromatic rings, but the double bond between the two nitrogen groups in compound bis $(o-hydroxyphenyl)hydrazine$is missing.

As a result, $o,o\text{'}-dihydroxyazobenzene$has a higher fluorescence quantum yield.