Red shift is caused when excited state is more polar as compared to ground state so the polar solvents stabilize excited state more than ground state. So overall there is decrease in the energy gap between excited and ground state resulting in red shift. When ground state is more polar than excited state, the polar solvents stabilize  ground state more than  excited state. So overall there is increase in the energy gap between excited and ground state resulting in blue shift.

Red shift is caused when excited state is more polar as compared to ground state so the polar solvents stabilize excited state more than ground state. So overall there is decrease in the energy gap between excited and ground state resulting in red shift. When ground state is more polar than excited state, the polar solvents stabilize  ground state more than  excited state. So overall there is increase in the energy gap between excited and ground state resulting in blue shift.

I agree with Peter and Gopal 's answers.

If you need more infos, please launch Google Scholar and NOT Google  and you 'll a lot of hits (here the link)

https://scholar.google.de/scholar?hl=de&q=How+red+shift+and+blue+shift+depends+on+the+polarity+of+solvent%3F+&btnG=&lr=

http://pubs.acs.org/doi/abs/10.1021/cr00032a005?journalCode=chreay

Good luck

JRG

Thank you all of you Sir. Thank you very much.

Gopal Singh gave you the correct answer. Details can be found in the minireview:

http://onlinelibrary.wiley.com/doi/10.1002/cphc.201200426/abstract

Thanks Mr. Gopal. Very nice and easy answer.

But what decides the amount of shift of the ground and excited states? Why the shift intensities are different?

And can you give me some example where ground state is more polar than excited state? what are the reasons behind it?

Hi,

I want to know ,what is the reason when there is red shift during quenching by a quencher ? Please help.

With Best Regards,

Bikash

It's the dipole moment of the compound which decides it's interactions with solvent dipole. More polar the excited state, more it will interact with the Polar solvents and more will be the shift. Intensity is governed by the type and extent of change in conformation of molecules in excited state. Common example given for more polar ground state is carbonyl compounds. You can find it in any book on spectroscopy.

A quencher can interact with fluorophore in variety of ways. Interaction depends upon the structure and type of quencher as well as the fluorophore. Red shift can be achieved by increasing conjugation through some conformational changes in fluorophore by the quencher, changing excited state energy levels of fluorophore by direct dipole-dipole interactions with the quencher etc. For better insight into the phenomenon please read principles of fluorescence spectroscopy by Lacowicz and go through as much literature as possible.

If the excited state molecule is more polar than ground state molecule, red shift will occurred in polar solvent. In case, we have non-polar solvent what will happen, is it possible to happening blue shift?

Either there will not be any shift or it will be very small.

Gopal Singh

Hi, I have a question.

What can be the reason of high intensity blue peak for H-aggregation structure/high intensity red peak for J-aggregation?

When I am trying the UV spectrum calculation using DFT, I am getting the high intensity blue peak for H-aggregated solar dyes. In the same time, when I am trying the J-aggregated structure, I am getting the high intensity red shift peak. Why the intensity of the UV absorption shifting from blue shift to red shift because of changing the H-aggregation structure to J-aggregation structure?

thanks in advances.

Mamun

Koushik Majhi Debasish Das Mahanta

For alkenes and arenes, the pi-pi* transition shifts to longer wavelength (red shift or bathochromic shift) if solvent polarity is increased. This is because in ground state they aren't polar hence there is negligible stabilization, however, in excited state (pi*) they have significant polarization especially in push-pull systems which leads to stabilization of pi* in polar solvents and reduced energy gap between pi-pi*.

For carbonyl compounds, the n–pi* transition of the keto group shifts towards shorter wavelength (blue-shift or hypsochromic shift) if solvent polarity is increased. The reason being, the non-bonding electrons on oxygen interact strongly (Hydrogen bonding) with hydrogen of polar solvents like EtOH, MeOH, H2O etc. In the excited state, the electron is promoted to pi* or in other words the electron has shifted towards the less electronegative carbon. Even though stabilization would exist for pi* as well but not to the extent to which n is stabilized. This leads to increase in energy gap between n-pi*.

As an example in alpha-beta unsaturated compounds, n-pi* transition undergo hypsochromic shift while pi-pi* transition undergo bathochromic shift.

Fun Fact - Reichardt's dye is popular solvatochromic dye which changes color in different solvents which is easily observable by the naked eye.