Functional materials riassunto

Aromaticity

A cyclic molecule with 4n+2 delocalized electrons describes the concept of aromaticity.

Orto-meta-para: molecules to remember

Toluene is a key molecule to remember in this context.

5 Members heterocycles

• Furan (O)
• Thiophene (S)
• Pyrrole (NH)

In the skeleton, usually, the bond is in the α position (2), while the lateral substituent is bound in the β position (3).

Acenes: fused benzene rings

• 2: Naphthalene
• 3: Anthracene
• 4: Tetracene
• 5: Pentacene

Poly-acetylene

[–CH=CH–] is the simplest polymer that presents conjugation, not soluble and not meltable. It exists in various stereoisomers:

• Trans-transoid (all trans PA)
• Cis-transoid
• Trans-cisoid
• Cis-cisoid (all cis PA): cannot be obtained due to steric hindrance of the chain

Can be synthesized in various methods:

1. Catalytic polymerization of acetylene: Through a proper Ziegler-Natta catalyst, we obtain a chain growth polymerization with high control on regioregularity and stereoregularity, avoiding chain transfer (branching). If the polymerization is performed at low T (−80°C), a cis-polymer is obtained, while at high T (+100°C), a trans-polymer is obtained. Catalysts can be homogeneous (soluble in reaction bath) or heterogeneous (not soluble); in general, heterogeneous are preferred, but they cannot be used if the polymer is not soluble (difficult separation).
2. Non-catalytic polymerization of acetylene:
3. Catalytic polymerization of a monomer different from acetylene: Use metathesis catalyst and cyclooctatetraene as precursor (creates double bonds instead of single ones as Ziegler-Natta catalysts) to cause ring opening metathesis polymerization. Through this method, we obtain cis-PA, except where the double bond is broken and recreated → 75% cis, 25% trans.
4. Indirect method: From the monomer, we synthesize a polymeric precursor (stable and soluble), which will then be converted to PA; in this way, we can easily process and characterize the precursor before it becomes non-soluble PA. Examples:
   • Example 1: PA obtained from PVC by expulsion of hydrochloric acid (dehydrohalogenation); yielding is not perfect, I obtain a non-crystalline non-conjugated PA due to some chloride left.

Durham method: Polymers are prepared by ring-opening metathesis polymerization, and a subsequent heat-induced reaction yields the final polymer, as well as a volatile side product.

Poly-aniline

Different poly-aniline based on the ratio between the reduced form (y) and the oxidised form (1-y): Doping of polyaniline: acidic doping / redox doping

• Protonation requires some N in sp³ hybridization, performed with HCl

• Leucomeraldine: y=1, N Y fully reduced
• Emeraldine: y=0.5 Y Y
• Pernigraniline: y=0, fully Y N oxidised

Examples of mesogens

• Disc-like (discotic) mesogen: R are alkyl-chains (flexible substituents are fundamental for mesogens) and electroactive groups to allow for an external control through electromagnetic fields.
• Rod-like (calamitic) mesogen
• Cholesteric rod-like mesogen

Classification of smart materials

1. Based on output type

• Type 1: Change of a property
• Type 2: Produces energy

2. Considers both type of output and input

• Photovoltaics (input=optical, output=electrical)
• Photochromic (input=optical, output=optical)
• Electrochromic (input=electrical, output=optical)
• Electroluminescent (input=electrical, output=optical)
• Chemical or biochemical sensor (input=chemical, output=optical or electrical)
• Thermochromic (input=thermal, output=optical)

Fundamental features of smart materials

1. Reversibility
2. Direction of change
3. Response rate
4. Transient response or response by dose
5. Sensitivity
6. Selectivity
7. Self-actuation

Structure of smart materials

Smart materials’ structure consists of:

• Skeleton: Responsible for the smart response
• Lateral substituent: May have different purposes:
  • Increase solubility: Wet techniques are cheaper than vacuum ones. The best for this purpose are alkyl chains (methyl, ethyl, propyl...)
  • Electroactive: Tune the smart response
  • Inductive effect: Electronegativity difference creates a dipole moment. The effect becomes weaker going farther from the source of the distortion.

Acceptors I-: Cl, NH₂, NO₂, CN, C₆H₅, F

Donors I+: B, CH₃, Si(CH₃)₃

N.B.: Hybridised molecules electronegativity sp³ < sp² < sp

Mesomeric (resonance) effect: Presence of different resonance structures creates an electroactive effect, usually higher than inductive one (except for F).

In general, the higher is the number of double bonds, the higher will be the stability (and probability) of a resonance structure; non charge-separated structures are also more stable.

Fundamental building block (ex: liquid crystals must have a rigid skeleton and flexible substituent)

Inorganic materials are usually more stable, while organic ones suffer degradation (ambient is fundamental).

Electron delocalization and E_gap reduction

π-conjunction: Mesomeric effect is also present when we have a linear chain with alternating double bonds → if a lot of π-bonds overlap (lowest energy resonance structure), we obtain a quasi-conduction band and a quasi-valence band, with an energy gap that decreases as the number of monomers increases (up to an asymptote). Electron delocalization highly depends on the planarity of the molecule (higher planarity, higher conjugation).

There are other ways to obtain delocalization of electrons, for example, by connecting a double bond and an element of another group with lone electron pairs; in this case, the energy level that creates is placed in the energy gap.

Bond length alternation

The length of bonds gives an idea of the conjugation.

Doping

Reversible introduction of charge carriers in a semiconductor in order to increase its conductivity. It can be chemical, electrochemical, photodoping, acidic doping. Materials are easily p-doped if they have a high HOMO level.

Chemical doping

RedOx reaction, usually carried out with a salt, aimed at introducing a dopant in the structure of the material. Dopant can be:

• Donor substituent increases both HOMO and LUMO (but less) → n-doping
• Acceptor substituent decreases both LUMO and HOMO (but less) → p-doping

Therefore, E_gap decreases in both cases (if planarity is not damaged!).

Can a material be a p-dopant?

• Intuitive way: We need to have a cation in the highest valence state (it has to be reduced); remember that atoms dislike being reduced to 0 and that also a negative valence state can occur in the polymer.
• Correct way: Compare the oxidation potential of the polymer and cation (polymer’s one should be higher to oxidise it and obtain p-doping)

Doping is usually not homogeneous (surface gets doped more), so it is useful to consider the doping ratio. Just for PA, it is evaluated as Y = [n° dopant mol]/[n° C mol].

Usually, the smaller is the dopant, the higher will be the doping ratio. Typical doping ratios are: 0 < Y ≤ 0.3.

N-type material: Pristine material with relatively low LUMO (easily gets an electron) → material with increased number of electrons (with a donor) → higher HOMO

P-type material: Pristine material with relatively high HOMO (easily loses an electron) → material with increased number of holes (with an acceptor) → lower LUMO. Much easier to obtain since stability is higher (both of material and dopant) and there are not many materials that accept n-doping.

N.B.: The best way to decrease the E_gap is to add a donor on one side and an acceptor on the other (even better than increasing the conjunction).

Electrochemical doping

Material is used as an electrode of an electrochemical cell to deposit the dopant or to inject charge carriers directly.

Photodoping

Material to be doped is irradiated with photons to excite electrons. Not stable at all, when irradiation stops the system recovers immediately.

Acidic doping

Applicable only on a few materials, a strong acid adds positive charge to the material (protonation, see emeraldine). De-doping is performed with a base.

Defects

Soliton – ex: PA (poly-acetylene)

Solitons are neutral defects that can occur when a polymer has 2 energetically equal resonance structures; it consists in the separation of an electron pair of a double bond (when a soliton is present, an equal anti-soliton is present). The system tends to relax, so the soliton is delocalized over a certain length of the chain (usually over around 8 C atoms, 4 from each side).

Solitons are neutral charge carriers, so they should increase conductivity; in reality, conductivity is not changed since:

• Density of solitons is low (around 400 every 10⁶ C atoms)
• Soliton is neutral (voltage difference doesn’t move it)
• It is an intramolecular carrier, while intermolecular transport (hopping) is the most important (solitons cannot hop)

I can dope the material with an acceptor (positive soliton) or with a donor (negative soliton), but the mathematical description will stay unchanged.

Polaron – ex: PPP (poly-para-phenylene: quinoidal form and aromatic form)

When a polymer has two resonance structures at different energy levels, if an electron is withdrawn or added by a dopant, a polaron may form. Defect and anti-defect will this time be of different nature, since polaron is a charge while anti-polaron is a radical. Since the quinoidal form is more energetic, its extension is usually limited to 4 or 5 monomeric units. Polarons are defects with both spin and charge and can hop, they increase conductivity.

Bi-polaron

After the formation of a polaron, if the radical is withdrawn by an acceptor or combines with another radical to form a double bond, a bi-polaron is formed. This creates intermediate energy states (lower E_gap) and can hop: increases conductivity (spin-less conductivity). By doping a material, I form both polarons and bi-polarons, but if I increase the doping ratio, more bi-polarons form.

Electrochromic devices

UV-visible spectroscopy measures the optical energy gap (maximum lambda adsorbed, onset of the peak, not the max) to evaluate which material I am analyzing. Conversion factor from nm to eV: 1240 nm*eV. Two lamps are needed: a deuterium lamp and a tungsten lamp.

If I change the conjugation, the spectrum will change: increase in conjugation leads to adsorption at lower energies (redshift/bathochromic effect; the opposite is called blueshift/ipsocromic effect) and to an increase in absorbance (hyperchromic effect).

ί ε∗b∗c Lambert-Beer law: ε=absorption coeff., b=optical path, c=property of the material

N.B.1: Absorbance of the solvent needs to be considered (often we compare the signal with a reference with solvent alone).

N.B.2: Physical state influences the spectrum: solid is redshifted and with wider peaks.

Optical properties associated with doping

Since in doped materials we observe a redshift in absorbance, different things can happen:

• Uncolored material becomes colored (peak of absorption shifts from UV to visible)
• Colored material changes color (peak of absorption shifts in the visible range)
• Colored material becomes uncolored (peak of absorption shifts from visible to near IR)

Rely on electrochemical doping to make a material change from transparent to colored and vice-versa.

Components of electrochromic devices

• Glass (more stable) or plastic (flexible and cheaper) substrates
• Transparent electrodes (TCO=Transparent Conductive Oxide, deposited by vacuum techniques, the most used is ITO=Indium Tin Oxide)
• Electrochromic layer
• Ion storage layer, needed since no counter redox reaction occurs; can be substituted by a proper additional electrochromic layer (common pairing is WO₃ with NiO).
• Electrolyte: Needs to be electronically insulating but ionically conductive, usually organic molecules dissolved in water

N.B.: Usually electrochromic materials show a response by dose. A side effect is that when colored, absorption in IR becomes more important, so the material heats up (comfort effect).

Electrochromic materials

In general, electrochromic materials can be divided into:

• Type I: Soluble in all forms (viologens)
• Type II: Soluble in the uncolored form but turns into a colored solid on the electrode from the redox reaction
• Type III: Solid in all states (inorganic)