What property is crucial for spectrophotometric determination (visible region) and is exhibited by pNP?

Answer 1

It is crucial to have a molecule that can absorb wavelengths in the visible region...

I guess you are referring to para-nitrophenol. p-Nitrophenol is actually colourless. However its anion is yellow. At alkaline enough conditions you have almost all of the molecules in the dissociated (and yellow) form and then you can do the spectrophotometric determination.
Usually it's modified forms are used as a substrate for monitoring enzymatic activity e.g. p-Nitrophenyl-phosphate is used as a substrate for phopshatases; it is colourless, but the phosphatases cleave of the phosphate, forming yellow p-Nitrophenoxy anion at alkaline reaction conditions or colourless p-Nitrophenol at acidic which is then converted to yellow p-Nitrophenoxy anion by adding base in the reaction mixture.

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Answer 2

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