Organic Energy Materials

Question 1

Limitations of current energy storage solutions?

Answer 1

Use inorganic TM which are expensive, relatively heavy, brittle, limited supplies

Question 2

Limitations of renewable energy (RE) sources?

Answer 2

Intermittent and sensitive to external factors - which limits how much we can benefit from RE

Question 3

What electronic properties do conjugated 𝜋 - systems have influence on?

Answer 3

Conductivity
Light harvesting
Light emission
Electrochemical reversibility

Question 4

What 2 ways can 𝜋 conjugation occur?

Answer 4

Through bond
Through space

Question 5

Huckels rule for aromaticity

Answer 5

4n + 2 𝜋 electrons in a cyclic planar array

Question 6

How does 𝜋 - conjugation effect thermal and chemical stability?

Answer 6

Compared to non-conjugated dienes heat of hydrogenation of a conjugated diene is significantly lower. Suggesting 𝜋 - conjugation has a stabilising effect

Question 7

What is the band gap, Eg ?

Answer 7

Energy gap between the 𝜋 orbital (valence band) and 𝜋* orbital (conduction band) of a 𝜋 - conjugated system

Question 8

What effect does extended 𝜋 - conjugation have on the band gap?

Answer 8

Results in a smaller band gap

Question 9

How to promote conductivity in conjugated system

Answer 9

In ground state e are not able to move freely/ conduct so must promote e into conduction band (hv, heat)

Question 10

What is an exciton

Answer 10

electron-hole pair

Question 11

Exciton binding energy, Eb?

Answer 11

Eb = Eg - Ex
Eg = Energy of band gap
Ex = Energy of phonon emitted upon charge recombination

Question 12

Issue with excitons in organic semiconductors? and how to fix

Answer 12

Excitons are tightly bound ( electron tightly bound to hole)
So donor-acceptor strategies are typically employed to achieve greater and longer lived charge separation

Question 13

What are the different donor-acceptor interfaces of photovoltaic devices?

Answer 13

Monolayer
Bilayer
Interdigitated
Bulk heterojunction

Question 14

UV visible electronic transitions

Answer 14

of possible allowed transitions only 𝜋 -> 𝜋* and n -> 𝜋* transitions are accessible in UV vis absorption range
Therefore need to maximise solar harvesting efficiencies by designing conj organic molecules that can absorb more of solar spectrum

Question 15

What does a smaller band gap mean in terms of wavelengths of light?

Answer 15

A smaller band gap means longer wavelengths of light can promote the e into conduction band, taking advantage of all wavelengths of light

Question 16

What is ionisation potential, IP?

Answer 16

Ability to lose electrons (become oxidised)

Question 17

What is electron affinity, EA?

Answer 17

Ability to gain electrons (become reduced)

Question 18

How to determine the band gap?

Answer 18

Absorption spectroscopy
The optical energy band gap is the energy required to photoexcite an electron from the HOMO to the LUMO (units eV)
obtained experimentally from 𝛌onset

Question 19

Equation for Eg,opt

Answer 19

Eg,opt = hc/𝛌onset

Question 20

What effect does increasing conjugation have on the absorption profile of a molecule?

Answer 20

This shifts the absorption profile to a higher 𝛌onset (red shift) (bathochromic shift) and decreases the band gap

Question 21

What effect does heteroatom have on absorption profile?

Answer 21

Inclusion of heteroatom joining benzene rings means the molecule is more planar and enhances conjugation resulting in bathochromic shift with higher 𝛌onset and therefore a smaller Eg

Question 22

What effect does a regioregular backbone have on absorption profile?

Answer 22

Will polarise conjugated systems leading to bathochromically shifted, higher 𝛌onset and smaller Eg

Question 23

What effect do substituents have on absorption profile?

Answer 23

+M groups increase conjugation efficiency across the chromophore leading to bathochromic shift with smaller Eg opt
-M groups limit conjugation efficiency across molecule leading to hypochromic shift with a larger Eg opt

Question 24

Add OH to benzene ring

Answer 24

1) SO3/H2SO4
2) NaOH
3)H+

Question 25

In energy storage applications how can we make it more energetically favourable to store charge

Answer 25

By stabilising the LUMO of the electron acceptors to enhance their electron affinity

Question 26

What is linear voltammetry?

Answer 26

Sweep applied voltage in single direction between 2 potentials
Useful for determining half potential

Question 27

What is cyclic voltammetry?

Answer 27

Sweep back and forth between 2 potentials, typically same start and end point
More useful technique for determining reduction and oxidation potential as well as half potentials
Also useful for determining if electron transfer process is electrochemically reversible

Question 28

Equation to determine if an electrochemical redox reaction is reversible

Answer 28

ΔE = |Ered - Eox| ~ 59/n mV
where n = number of e- transfered

Question 29

What are the effects of electron donor with destabilised HOMO?

Answer 29

Higher HOMO level easy to oxidise because of lower ionisation potential

Question 30

What are the effects of electron acceptors with stabilised LUMO

Answer 30

Lower level LUMO easier to reduce as it has a higher/greater electron affinity

Question 31

What is a battery?

Answer 31

Electrochemical cells that convert chemical energy into electrical energy

Question 32

During battery discharge electrons flow towards…

Answer 32

the cathode

Question 33

in a battery oxidation occurs at…

Answer 33

The anode

Question 34

In a battery reduction occurs at…

Answer 34

The cathode

Question 35

Primary batteries

Answer 35

Irreversible disposable

Question 36

Secondary batteries

Answer 36

Rechargeable, electrolytic cells

Question 37

1V =

Answer 37

1J / 1C

Question 38

The voltage of multiple batteries in series is…

Answer 38

sum of individual cell voltages

Question 39

The current of