Dear Bijoya Das,

unfortunately your question is so short and non-specific, that it is impossible to give a meaningful answer. What kind of molecules do you work with? Your solvent is water, I suppose? How do you know that clustering occurs? Are you interested in change caused by pH or by a specific acid? After information on all these conditions it may be possible to answer your question.

Kind regards

Max Lippitsch 

Mrs/Miss Das,

The solven effect depending on the properties of the solvent, as direction and magnitude of pi-pi and n-pi transitions in organics can be found in any textbooks with topic instrumental analysis; analytical instrumentation and more.