The compound under study exhibits large stokes shift in polar and nonpolar solvents ranging from 90:170 nm. what are all rationale for this large shift?
e.g.
- excited state intramolecular hydrogen transfer
- aggregates
- exciplex
....... etc
Normally there is a red shift due to vibrational relaxation of the initial excited state to the lowest vibrational state of the first electronic excited state. Emission occurs to a manifold of vibrational states in the ground state, causing a red shifted emission.
It may also have to do with formation of a highly polar excited state which undergoes relaxation of the solvent cage around it, right after the excitation, leading to a red shift in the emission. Usually this will show up as a stokes shift that increases with the solvent polarity. I think this could be what is happening in your case.
However, it is difficult to know exactly what could be the cause without knowing some more details about the compound you are studying. A large stokes shift may indicate a fast relaxation from the initial state to the emissive state, could also be due to intramolecular energy-transfer (part of the molecule acts as a donor, absorbing light and another portion of the molecule acts as an acceptor, which emits light with a significant red shift).
Stokes shift is the difference (in wavelength or frequency units) between positions of the band maxima of the absorption and emission spectra (fluorescence and Raman being two examples) of the same electronic transition.[
To add to what has been said. Large Stokes shift can also occur if emission is from a intramolecular charge transfer state. When a molecule absorbs light it is excited to a locally excited state (LE state) and from the LE state the excitation is transferred to a intramolecular charge transfer (ICT state) and emission occurs from the ICT state. Excitation transfer from LE state to ICT state occurs on a fast time scale. It is another type of excited state reaction as mentioned by Prof. Gert.
what will be the reasons for the origination of Stokes shift in Perovskites?
It should be noted that the maxima in the absorption, excitation and emission spectra may be strongly affected by the envirpnment of the molecule so that the Stokes shift is not an intrinsic property of the molecule under study. It can only be measured experimentally.
@ Gert Van der Zwan! Why usually polar solvents shows large stokes shift than non polar?
Article Quantum-classical mechanics: Luminescence spectra in polymet...
A large stokes shift may indicate a fast relaxation from the initial state to the emissive state, could also be due to intramolecular energy-transfer (part of the molecule acts as a donor, absorbing light and another portion of the molecule acts as an acceptor, which emits light with a significant red shift).
This is a great question and many answers of others are very good too! I have been solving the same one about 6-7 years ago. In case of our compounds (a small library of xanthene dyes), the reason was, that the bond-order of the exocyclic substitutens was changing a lot between the ground and excited states and this also invovles large reorganization of solvent molecules, so the energy difference of HOMO and LUMO in ground and excited states were large. This manifested as a large Stokes shift (up to 10000 cm-1 in polar protic media). We published this particular work in 2015: Article Small-Molecule Fluorophores with Large Stokes Shifts: 9-Imin...
Interestingely, others suggested that in isoelectronic system (coumarine dyes) the same effect was caused by easily allowed or restricted rotation:Article Molecular Design of UV-vis Absorption and Emission Propertie...
which we does not found true for our system.In general I thinks there is no single reason explaining the large Stokes shifts and it depends at least on the molecule of interest and solvent system. So it would help a lot for the dicussion, if you could disclose the structure of your molecule(s)...
Large Stokes shifts (8000-10000 cm-1) are usually indicative of excited state reactions, most often excited state proton transfer. Purely vibrational shifts (due to displacement of the potential energy curve in the excited state with respect to the ground state, suggested by Bin Wu) are in general rather small.
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