Finding the Most Polar Molecule for LCD Devices

In summary, the conversation discussed the selection of molecules for use in an LCD device. The speaker explained that the most suitable molecule should be polar and have a very electronegative group attached. The conversation also touched on the role of electronegativity in determining polarity, and the importance of a rigid backbone and certain aspect ratio in LCD molecules. The speaker also mentioned the use of surfactants and the molecule "cholesterol" as examples. However, the conversation concluded with the acknowledgement that the topic is more complex than initially thought and requires further research.
  • #1
a.a
127
0
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.
 

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  • #2
The most polar molecule will have a very electronegative group attached. Can you see any?
 
  • #3
C and D?
 
  • #4
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)?
 
  • #5
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?
 
  • #6
a.a said:
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.
 
  • #7
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?)
 
  • #8
Wow, so this is much more complicated then I was led to believe; I'll leave you to it.
 
  • #9
chemisttree said:
Would you say that a monofluoro methyl group attached to an aromatic group is reactive??)
Yes

chemisttree said:
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

chemisttree said:
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.

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

chemisttree said:
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.

chemisttree said:
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...?

chemisttree said:
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?
 
  • #10
Gannon said:
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.
 
  • #11
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.
 

Related to Finding the Most Polar Molecule for LCD Devices

1. What is a polar molecule?

A polar molecule is a molecule that has a separation of electric charge, resulting in a positive and negative end. This occurs when there is an unequal distribution of electrons within the molecule, causing one end to have a slight positive charge and the other end to have a slight negative charge.

2. Why is it important to find the most polar molecule for LCD devices?

LCD devices, such as televisions and computer screens, use liquid crystals that react to electric fields to create images. Polar molecules are necessary for these electric fields to work effectively, so finding the most polar molecule is crucial for the proper functioning of these devices.

3. How do scientists determine the polarity of a molecule?

Scientists use a variety of methods to determine the polarity of a molecule, such as measuring the dipole moment, analyzing the molecular geometry, and using spectroscopic techniques. These methods allow scientists to determine the distribution of charge within a molecule and classify it as polar or nonpolar.

4. What are some examples of polar molecules that are commonly used in LCD devices?

Some examples of polar molecules that are commonly used in LCD devices include nematic liquid crystals, such as 4-cyano-4'-pentylbiphenyl (5CB), which has a dipole moment of 4.8 Debyes, and smectic liquid crystals, such as 4-pentyl-4'-cyanobiphenyl (5CB), which has a dipole moment of 6.2 Debyes.

5. Are there any drawbacks to using highly polar molecules in LCD devices?

While polar molecules are necessary for LCD devices to function, there can be some drawbacks to using highly polar molecules. These molecules can be more difficult to synthesize and may have higher costs associated with them. Additionally, highly polar molecules may also have a shorter lifespan and be more sensitive to environmental factors, which can affect the overall performance of the LCD device.

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