[AoisPokemaster]

Additionally, following what you said just now about how calculating the stokes shift alone was not enough to show solvatochromism, I have measured the maximum absorption values of the curcumin in different compounds here. I am sorry for posting this so late, but I hope to be able receive some more of your help, as I found it really useful. Thank you so much once again. Also, is there a way to explain why propanol and isopropanol does not follow the trend line? like are there characteristics that differentiate these two solvents from the others? The only thing I can think of is the molar mass and the carbon chain length that has caused it. I still need to read more upon it to understand more.

[Corribus]

I don't mean to be preachy, but if you don't know how certain values are calculated or what their units are, how do you expect to interpret data plotted against them as an independent variable? Based on the website you linked to, the "relative polarity" values seem to be determined solvatochromically in some way, possibly by a relative degree of spectral shift in an unknown chromophore reference. We can make some inferences about meaning, but as a numerical scale it's really useless.* My recommendation would be to order your solvents in terms of this value if you want, but dispense with the numerical scale on your plot unless you know exactly what the numerical scale means. Instead, just list the solvent names as your x-axis. Particularly since you are asking us to make some guesses as to things like "isopropanol vs 1-propanol", this is impossible to do without knowing which points on your plot correspond to which of these solvents. Just so you are clear on it, you're running up against something that continues to be a problem in chemistry: how to objectively and quantitatively measure a vague concept like polarity. There's really no universally accepted way to do it, which is why in my opinion they best way to approach it is qualitatively.

Scientific studies of photoluminescence behavior in organic compounds will rarely only present Stokes shifts because they are difference values without an absolute frame of reference. Almost always you will see peak maxima provided as well. Therefore I recommend, in addition to your plots about Stokes shifts, you provide, at least in tabulated form, the Abs and PL maxima of your fluorophore in the different solvents. These values can provide important context to decoding what's going on that you can't get from Stokes shifts alone - particularly when your trends are complicated. If you have an anomalous SS in isopropanol, say, knowing how the solvent affects the positions of the absorption and/or fluorescence bands can tell you something mechanistically that is impossible to know with the SS alone.

1- and 2-propanol differ in the position of the hydroxyl group. This could certainly influence how molecules reorient themselves and stabilize around the polarized excited state. Saying more would be difficult certainly without seeing which points they are on your plot, the specific absorption and fluorescence spectral data, and possibly more sophisticated measurements of, say, the excited state lifetime, polarization, etc. Again, I'll stress that an in-the-weeds approach to explaining every nuanced comparison in your data may lead to disappointment. At best you might be able to do some vague handwavy explanation. I'd focus on the bigger picture and the broad trends you observe.

*It's also kind of redundant. Given that you're plotting a dependent variable against an independent variable whose reference scale was determined against basically the same dependent variable that you're using but a different frame of reference. It'd be like if your friend defined a numerical sweetness scale for different candies based on how sweet they tasted to him, and then you plotted how sweet they tasted to against this numerical scale. Ultimately your experiment isn't saying anything fundamental about sweetness - all you can really make any conclusions about is how your sense of sweetness differs from his.

[wildfyr]

The polarity is "The values for relative polarity are normalized from measurements of solvent shifts of absorption spectra and were
      extracted from Christian Reichardt, Solvents and Solvent Effects in Organic Chemistry, Wiley-VCH Publishers, 3rd ed., 2003."

All relative to water.

I think you have to be careful with this. Because its a bit circular, the polarity was calculated due to solvatochromatic shift of some dye.... which what you are doing. The way each dye interacts with a given solvent will be different.

I sort of like using something totally independent like the diaelectric constant. None are perfect options.