Hello everyone,
i need to know, why there is an red shift in linear absorbance spectra as it measured from different solvents (Methanol,acetone and DMF). is is due to polarity of solvents? or any other kind of reasons?
Hey Vinay,
yes, polarity is the main reason effecting the general shape and band position in absorbance. If you observe significant bathochromic shifts with increasing solvent polarity you might have a charge transfer absorbance. You will always observe it in cases were the excited state shows a higher degree of charge delocalization than the ground state.
To be sure about the charge transfer character, it is advisable to check for emission spectroscopy as well - there, a similar bathochromic shift should be observed (in general with decreasing emission intensity for higher polarity).
Also, if soluble, I would recommend to measure again in low polarity solvents - like toluene or slightly higher in polarity anisole or chloro-benzene.
Is it really polarity (or just polarity)? A simple increase of the dielectric constant of the surrounding medium always leads to a redshift irrespective if the medium/solvent is polar or not...
Methanol: eps=1.77, Acetone: eps = 1.85, DMF: eps=2.04
I should add that the values I provided are the squared indices of refraction and not those at zero frequencies...
And here is a concrete example in the infrared spectral range: Article Deviations from Beer's law on the microscale - Nonadditivity...
Professor Thomas Mayerhöfer , your point is fine but the absorption spectra depend on the induced dipole moment. Definitely polarity controls it.
Professor Agnishwar Girigoswami , I do not doubt this! But how is experimentally one effect separated from the other? It would be very kind if you could provide corresponding paper which show how this separation is achieved!
You should go back to the original work on Solvatochromism in the 1950s, some of which is by:
McRae E.G. Jour. Phys Chem, 61, 562 (1957.)
Lippert E; Z. Elektrochem; 61; 962; (1957)
which draws on the work on solvation by, for example
Onsager L; Jour. Am. Chem. Soc; 58; 1486-1493; (1936)
But a more recent text in the area is probably the best place to start : e.g.
Suppan P. and Ghoneim N;
Solvatochromism; Royal Society of Chemistry Information Services (1997)
It seems that in the original literature, like in McRae E.G. Jour. Phys Chem, 61, 562 (1957), people were well aware of the fact that both effects, polarity and change of the index of refraction/dielectric function, contribute to band shifts in absorbance, but it seems that in newer literature like in Article What is Solvatochromism?
this knowledge has been lost.
Thomas Mayerhöfer Strange - as far as I can remember, it should be implicit in the Debye and Clausius-Mosotti equations. In the past I have used the solvatochromatic parameters: f(D) adn f(n^2). Correlation of Absorption (or fluorescence) maxima with f (D) is most generally applied for polar solute molecules in solvent of varying dielectric constants. For non-polar molecules this is replaced by a f (n2) correlation.
West W; Geddes A.L; Jour. Phys. Chem; 68; 837; (1964).
Hugh J Byrne I think the problem is that local fields do not change the dielectric function/the energy difference between ground state and excited state, while polarity does. Accordingly, if you just view the absorbance peak shift as indication that the difference between ground and excited state changse or see it from a mere quantum dynamical point and model the influence of the solvent just by a handful solvent molecules around the molecule of interest, then you certainly forget about the influence of local fields. Best indication for this is that the authors of the paper I mentioned above noticed (different) peak shifts in apolar solvents (having different indices of refraction) and could not explain them...
Thomas Mayerhöfer - I haven't dealt with this for years (almost decades!) Given that there has to be a change in parity/symmetry between the ground and excited states, as far as I can remember the impact on the ground and excited states is different, and different for polar/nonpolar solute/solvent conbinations, and can be probed by comparing absorption/fluorescence shifts. But yes of course, any of these treatments include approximations which means they are best applied in examples of extreme examples of polarity.
Hugh J Byrne From the start of the discussion it seemed to me that the knowledge about local field effects has somehow vanished. I would not go as far as one of my friends, who says that every ten years you can sell something old as something new, because people have forgotten about it, but I have to say at times he is right.
Banchhanidhi Prusti I think the effect Vinay Parol ask about is something different, because in your paper you describe a strong redshift while the index of refraction of your solvents (hexane -> acetonitrile) is nearly the same (1.375 vs. 1.344), and if, then it should actually cause a blueshift because the index of refraction decreases.
We have recently investigated the reason for the increasing redshift due to an increase of the index of refraction of the solvent, more or less by accident. The first time this was observed/published was already in 1878 and it is known under "Kundt's rule". It holds for about half of the solvents. As we could show, the reason for it is the "local field of Lorentz" which leads to a coupling of all oscillators of solute and solvent, regardless of spectral position (i.e. from Terahertz to UV). The other half interacts chemically with the solute which leads to aggregation, dissociation etc. and changes its polarizability, or, in other words, the energy differences between ground and excited states...
Article Beyond Beer's Law: Revisiting the Lorentz-Lorenz Equation
It is really amazing that the redshifts can be as large as nearly 10 000 cm-1. Not only that, but while shifting to the red, the absorbance coefficients can increase tremendously by 600 % and more. The local field effect is certainly not the only effect at play, but it can explain such shifts and increases as we found recently: Article The Negative Solvatochromism of Reichardt's Dye B30 - A Comp...
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