[darkdevil]

Hi all,

I synthesized a compound and it is fluorescence at very diluted form. And I dropped some of the diluted compound on a filter paper ,dried it with a dryer to remove the solvent, and fluorescence is still present.

When I obtain the powder form of the compound, the material turned in to a brown powder with no fluorescent properties under UV light (also the concentrated solution form)

The TLC spots are also different for both the diluted and concentrated solutions. I tried to dilute the conc. solution and get its TLC, the spot turned into the diluted one. Is it possible for a same compound to have 2 different spot (very near to each other) on a same TLC plate? and to have such defferent luminescence properties in bulk powder and "thin film" forms?

Thank you!

[discodermolide]

Looks like you have two different compounds.
What solvent did you dissolve it in?

[Corribus]

Fluorescence is one property that is not very good to measure "by eye".  Hand-held UV lamp irradiation can tell you a little, but nothing quantitative.  You are better off using a fluorimeter if you really want to get an idea of what is fluorescent and what isn't. 

More to your question, there are lots of examples of compounds which have quenched fluorescence (or altered fluorescence) when concentrated.  A classic example is pyrene, which is certainly fluorescent when dilute but forms a very highly fluorescent (and different color fluorescence) when concentrated due to excimer formation.  Fluorophores tend to be flat, rigid molecules and thus they also tend to have good intermolecular interactions when concentrated.  These interactions do all kinds of strange things to the fluorescence in the solid state.  Usually these interactions are deleterious (meaning fluorescence is quenched, red-shifted, or both) because interactions of adjacent pi-systems introduces new efficient pathways for nonradiative deactivation of the excited (fluorescent) electronic state.  I'm not sure about thin film vs powder off the top of my head without knowing more about your molecular structure, but I can tell you that not all solids are created equal.  Intermolecular interactions in a thin film or dropped from a dilute solution can be quite different than in an amorphous powder.  This alone could explain the differences you are seeing.

Then again, it might just be the insensitivity of your eye, so I wouldn't make any real conclusion until you do a more analytical fluorescence experiment with a fluorimeter.

[BobfromNC]

What you are seeing is classic in certain fluorescent dyes, such as fluoroscein, which can exist in at least two forms, an open, carboxy form, and a closed lactone type form, and the two have very different properties.   That is very similar to the pH dyes that change color at different pH.  The form can also depend on concentration, solvent, and solvation.   So the dye may be the same material, per say, but in two interchangable structures, like an enol verses a ketone. 

If you look here: http://en.wikipedia.org/wiki/Fluorescein

the top structure is the open form, with the carboxy free to deprotonate, found in higher pH, the structure 2 6-FAM, shows a similar molecule in the closed form, more prevalent at lower pH, and non-aqueous environments.    These forms have very different colors and fluorescence characters.   That is a common characteristic of almost all fluoroscein and rhodamine type dyes and many pH indicators. 

[darkdevil]

What you are seeing is classic in certain fluorescent dyes, such as fluoroscein, which can exist in at least two forms, an open, carboxy form, and a closed lactone type form, and the two have very different properties.   That is very similar to the pH dyes that change color at different pH.  The form can also depend on concentration, solvent, and solvation.   So the dye may be the same material, per say, but in two interchangable structures, like an enol verses a ketone. 

If you look here: http://en.wikipedia.org/wiki/Fluorescein

the top structure is the open form, with the carboxy free to deprotonate, found in higher pH, the structure 2 6-FAM, shows a similar molecule in the closed form, more prevalent at lower pH, and non-aqueous environments.    These forms have very different colors and fluorescence characters.   That is a common characteristic of almost all fluoroscein and rhodamine type dyes and many pH indicators.

Thank you so much for this information. What I am trying to synthesize is an indigo derivative. I am trying to acylate the two/one amine on the indigo molecule. (structure is here--> http://en.wikipedia.org/wiki/Indigo_dye)
But I cannot characteize it using NMR... I tried to dissolve soem powder n acetone, but they are not likely to dissolve. But the compound is soluble in acetone when in small amount, and exhibits fluorescent property under UV.