Chemistry Finding the Most Polar Molecule for LCD Devices

[a.a]

We were given a set of molecules and asked determine the most apporiate choice to be used in an LCD divice, I know that the molecules have to be polar but I don't understand how to find the most polar molecule.

 

[Gannon]

The most polar molecule will have a very electronegative group attached. Can you see any?

 

[a.a]

C and D?

 

[Gannon]

Electronegativity is a trend on the periodic table: electronegativity increases as the table moves from LEFT to RIGHT and from BOTTOM to TOP. C and H have similar electronegativities, which make their association nonpolar. The cyano group (CN) is a pretty good one, as nitrogen (N) is far to the TOP and RIGHT. What about Fluorine (F)?

 

[a.a]

Fluorine is very elecronegative, even compared to N.
So I would then be A and D.

Just on the side, takeing a second look at C, it wouldn't be it because CH3 is on both sides, do does that mean its non-polar?

 

[Gannon]

Fluorine is very elecronegative, even compared to N.
So I would then be A and D.

Just on the side, takeing a second look at C, it wouldn't be it because CH3 is on both sides, do does that mean its non-polar?

Exactly, so assuming that an LCD-appropriate molecule must be polar, I would say A.

 

[chemisttree]

Would you say that a monofluoro methyl group attached to an aromatic group is reactive? Are LCD's supposed to be reactive? LCD's are supposed to crystallize loosely to remain liquid crystalline over a certain temperature range. How might the length of the aliphatic tail of the molecule affect melting point? Is an 11 carbon aliphatic group better or worse than a 5 carbon chain in this regard? How does a polar group affect a molecule's suitability as an LCD? Surfactants have polar substituents and non-polar tails. Are surfactants good candidates for LCD's? Are molecules that have rigid backbones and will stack efficiently good candidates for LCD's? (hint: the prototype for the 'cholesteric' liquid crystal phase is the molecule "cholesterol". How polar is cholesterol?)

 

[Gannon]

Wow, so this is much more complicated then I was led to believe; I'll leave you to it.

 

[a.a]

Would you say that a monofluoro methyl group attached to an aromatic group is reactive??)

Yes

Are LCD's supposed to be reactive?

I think No? If it would then it wouldn't be a very effective device, unless we say that there are no other solutions in the device that it can react with

How might the length of the aliphatic tail of the molecule affect melting point?

Not sure on this, please correct me, but the longer the length the lower the melting point.

Is an 11 carbon aliphatic group better or worse than a 5 carbon chain in this regard?

Better?

How does a polar group affect a molecule's suitability as an LCD?

The molecule has to be polar because LCD devices use a magnet to charge to seprate the polar and non-polar portions so that the crystials allign in a certian way. This or something of this sort was the explanation that our professor gave us during the lecture on this topic, but she didnt spend much time on it.

Surfactants have polar substituents and non-polar tails. Are surfactants good candidates for LCD's?

I think that it woud be best if the tails are polar...?

Are molecules that have rigid backbones and will stack efficiently good candidates for LCD's? (hint: the prototype for the 'cholesteric' liquid crystal phase is the molecule "cholesterol". How polar is cholesterol?)

Cholesterol, if I am not mistaken is non-polar, which gets me confused. How would stacking affect the efficiency of liquid crystals in a LCD device?

 

[a.a]

Wow, so this is much more complicated then I was led to believe; I'll leave you to it.

Sigh... agreed, and Chemistry is my weakest link.

 

[chemisttree]

You were on the right track but you oversimplified the rationale for choosing an LCD by looking only at the most polar molecule. LCD's need to be rigid, have a certain aspect ratio, preferably have a polarizable functionality, be liquid at about room temperature, perhaps be colored (if used in color displays), and so forth. You need to choose the best candidate for all of these reasons not just because one molecule is more polar than another.