Mr. Medhat,

Without quantum chemical computations in solution there is unable an accurate assignment of the bands due to charge redistribution; tautomerism; pi-stacking effects depending on the concentration, etc. 

This is may be due to the  dipole-dipole effects between molecule and polar solvent where water is more polar than ethanol.

Before thinking too much about effects of the solvent on the excitations itself, one should first consider possible effects on the molecular structure, since water and ethanol are both protic solvents. As has been already pointed out, there exist several possible structures due to tautomerism, in particular on the quinolinone side. For example,a proton from the solvent could attach to the ketone side and form a hydroxyl group, which then would lead to a rearrangement in the pi-system and a release of a proton of the former hydroxyl group, which then becomes the new ketone side. Also, the orientation of the carboxyl group is not clear from the scratch.

Nevertheless, I suppose, that the ketone part of the carboxyl group is at least on the same side as the hydroxyl group to gain stabilization by an intramolecular hydrogen bridge. Whether it is on the side shown in the picture or on the other could be cleared with quantum-chemical calculations. As side effect of the intramolecular hydrogen bridge, the proton of the hydroxyl group can easily migrate to the carboxyl group (note, even without a proton of the solvent), giving some kind of zwitterionic structure. As far as I know (don't aks for references), the barrier for the proton migration should be relatively low.

In fact, ethanol and water should have different effects concerning the kinetics and the thermodynamics of this migration reaction due to their different dipole moments, pKa values, etc.. Hence, the ratio of the different structures, for which you have measured your spectra, might depend on your solvent. Since different structures in fact give peaks at different positions in the spectrum, the double peak might be a hint for such kind of solvent dependence. In this context, band shoulders in the spectra might hide some of these effects, so one should be careful with the Gaussian decomposition of the spectra. For me, it seems that there are a lot of shoulders in your spectra. Accordingly, a band, which you see in the ethanol spectrum, might vanish within a shoulder in the water spectrum. Instead, you might see a band arising from another structure, which possibly explains your "inverse" behavior with respect to the solvent.

Nonetheless, it would be a good idea to ask your quantum chemist of trust to do some (TD)DFT calculations. The molecule is not that big, so that calculations are not too difficult (perhaps except the water in the TDDFT part, but one could model some water molecules explicitly, to partially circumvent some problems).

Best regards

If you think ionization is a possibility then a pH variation study might help here. You can even add a pinch of solid NaOH in ethanol to check the effect of acid-base reaction in your absorption spectra. Do you know the origin of the large 350 nm shoulder/plateau in ethanol? I see that the spectrum in ethanol is more structured (vibronic band?) than in water. Proper peak position assignment would be problematic here. Please measure the absorption spectrum in a non-polar solvent to check the possibility of vibronic signal.

Mr. Medhat,

In the context of my former posting you could pay attention to an attachment showing tautomeric forms due to solute-solvent interactions.

They should be taken into consideration within the framework of a theoretical computational quantum chemical treatment in order to obtain an accurate assignment of your experimental data.

The theoretical design could include additional forms depending on the substitutions if there are any other ones in your set of molecules.

In addition, your experimental design involves only polar protic solvents, but these should be taken into consideration the proton(accepting/donating) ability of the solvent and its polarity. So an accurate assignment requires measurements in solvents with a larger polarity-scale and accepting/donating numbers. Moreover, there are needed measurements at different pH, for instance, pH = 1-2, 7 and 10-13. The latter mesurements should be at relatively low concentrations in order to exclude from the effect of the ionic strength. Otherwise you should take ionic strength as an additional experimental parameter computing the theoretical UV-VIS spectra.

The next important point is that there should be take into consideration the effect of the concentration as well. So, your experimental design should include measurements within the framework of a representative set of concentrations in order to exclude from/account for aggregates.

In spite of these comments, the current UV-patterns are more complex. On this view there is needed a mathematical data-processign and a chemometric analysis in order to obtain the accurate experimental parameters. The most obvious are the bands within 300-375 nm. The band at about 315 nm in H2O could pertains to transitions within the framework of form 2 (L21). The theoretical spectrum in H2O reveals a band at 311 nm (attachment). So, a thorough and accurate analyses of your (even current) patterns need computations of a series of forms (meaning different initial electronic structures) of L1 under different experimental conditions in order to obtain a meaningful and reliable assignment.

Because of even the shown only experimental parameters (band-positions) in your figure do not reflect the correct values.

According to the infos belonging to your project:

1) Preparation and Characterization (using IR spectroscopy and 1H NMR) of three new organic ligands:

a. 7-Hydroy-Coumarin-4-Acetic acid (C1).

b. Ethyl-4-hydroxy-1-(4-methoxyphenyl)-2-quinolinone-3-carboxylate (L1).

c. 3-Allyl-Salicohydrazide (B1)

and the valuable answers of the answerers, you have a hard and long way  to become a good spectroscopist  and find the right answer to your question!!  WHY??

- compound a is a commercial product and NOT NEW as ligand !. See

https://pubchem.ncbi.nlm.nih.gov/compound/7-Hydroxycoumarin-4-acetic_acid.

Have you searched the a,b,c compounds on the web??

UV-Vis spectrocopy:

- please note that the UV-Vis spectroscopy is very sensitive to impurities.

- The characterization  with IR or FT-IR and NMR is not enough to show the purity of the compound B (see above) for a project or a paper.

After the synthesis and isolation of B , a thin layer chromatography  must be done with several eluents or a mixture of them (see TLC books).

If you have several spots, a column chromatography will be needed  to isolate them.

After iolation and drying (lab techniques) , infos about the structure can be obtained with NMR and MS.

If you have  B in pure form, you need, after drying) an elemental analysis. If the values of C,H, N, are OK, you can use B in rder to take a UV-Vis spectrum with a known concentration.

How to take a spectrum of B:

Your choice as solvent: water and ethanol

- please note that  the water must be pure  and fresh distill. and taken under N2.

THE CUTOFF of pure water is  180 nm

The CUTOFF of  ethanol is 205 nm.

What a kind of ethanol are using ?  Pure and dry e.g. 99,99 %? Or Uvasol or spectranal  e.g. 95 % ? Company name?

What a kind of spectrometer are you using? single beam, dual beam or diode array detector (simultaneous spectrometer)?

Thank you in advane for your answer.

An answer to your question is obsolete without infos  about right structure and purity!

JRG