Chromophores

Question 1

Explain the subtractive colour model

• White light? Black light? Clear?

Answer 1

Subtractive colour model: light absorbed by an object is subtracted from light transmitted to produce the colour we see

• White = everything reflected
• Clear = everything transmitted
• Black = everything absorbed

Question 2

Explain the process of UV-vis

Answer 2

1. Light is passed through a monochromator which selects a single λ
2. A splitter creates the reference beam and the incident beam
3. The incident beam passes through the sample producing the transmitted beam
4. The transmitted beam (photons) is subtracted from the reference beam to measure absorbance
   
   • Outputs an absorption spectrum (absorbance vs. wavelength)

Question 3

What happens when a molecule absorbs light?

Answer 3

When a molecule absorbs light, an electron in the HOMO is promoted to another orbital (usually LUMO)

• HOMO: highest occupied molecular orbital
• LUMO: lowest unoccupied molecular orbital

Question 4

Describe the singlet stats

Answer 4

• Singlet state (S): derived from 2(sum of ms) + 1 = 1
  
  1. Ground state (S₀): unexcited electrons
  2. S₁ state: electrons excited from HOMO to LUMO
  3. S₂ state+: shorter wavelengths can promote from HOMO to LUMO+1
  • If this happens, the electron quickly relaxes back to LUMO/S1 state

Question 5

What 5 events can occur following light absorpion

Answer 5

• After absorption, the system undergoes fast vibrational cooling and returns to the S1, v0 state (i.e. falls to LUMO, if needed)
• After this, the system must return to ground state (S0, v0)
  
  1. Fluorescence may occur as it returns
  2. Internal conversion may occur
  3. Intersystem crossing (phosphorescence) to the triplet state may occur
  4. Energy transfer to another molecule may occur (ex., pigments, chromophores)
  5. Chemical reactions may occur (ex., the formation of thymine dimers in DNA)

Question 6

As the system returns to ground state, fluorescence can occur. What is fluorescence?

Answer 6

• Fast return
  
  • The larger the fall, the shorter the wavelength
• Emission of radiation (light)
• Usually requires additional cooling to v0

Question 7

How can fluorescence be measured?

Answer 7

• Using a fluorescence spectrometer/fluorimeter

1. Light is passed through a monochromator which selects a single λ
   
   • Usually we select the wavelength of maximum absorption
2. This ‘excitation beam’ passes through the sample
3. The sample creates an emission beam which is monitored perpendicular to the excitation beam
   
   • Prevents any transmitted light from hitting the detector
4. A second monochromator scans all λ the sample is emitting
5. The detector receives the light and interprets
   
   • Outputs as a fluorescence emission spectrum (fluorescence vs. emission λ)

Question 8

As the system returns to ground state, internal conversion can occur. What is interal conversion?

Answer 8

• The electron returns immediately to S0, v0 (no further cooling) and all energy is lost as heat
• Speed varies

Question 9

As the system returns to ground state, intersystem crossing can occur. What is intersystem crossing?

Answer 9

• An excited electron can undergo a spin flip (intersystem crossing) to the triplet state T1
  
  • T1 is lower energy than S1 (makes sense)
  • Vibrational cooling to T1, v0 may occur if needed
• Intersystem crossing (an additional spin flip) occurs to return the electron to the S0 state
  
  • Vibrational cooling to S0, v0 may occur if needed
  • This process can release radiation via phosphorescence (slower than fluorescence) or can release the energy as heat via internal conversion

Question 10

Summarise everything that can happen during the return from S1, vx to S0, v0 using a Jablonski diagram

Answer 10

Question 11

What causes the HOMO-LUMO gap to narrow? (i.e., longer wavelengths can be absorbed?)

Answer 11

1. Extended conjugation
2. Polar groups

Question 12

Describe the effect of conjugation on a chromophore

Answer 12

• Shorter consecutive chains = higher energy, shorter wavelength
• Longer consecutive chains = lower energy, longer wavelength
  
  • Due to an increase in the size of the molecular orbital

Question 13

Describe the effect of polar groups on chromophores

Answer 13

Less conjugation is required for a molecule to exhibit colour (absorb a lower wavelength) when it has polar or charged functional groups

Question 14

Describe the structure of indigo

Answer 14

Question 15

Describe the structure of quinones

Answer 15

They have an auxochrome: a group or substance on a core chromophore that influences the absorption of the chromophore

Question 16

Describe the structure of triarylmethanes

Answer 16

Question 17

Phenolphthalein is a type of triarylmethane. Show how it reacts with a base to go from a colourless compound to a pink one

Answer 17

Question 18

Describe the structure of the porphyrin heme

Answer 18

Heme (hemoglobin) is coordination number 6 (octahedral) that binds Fe

Question 19

Describe the structure of the porphyrin chlorophyll

Answer 19

Chlorophyll is coordination number 4 (square planar) that binds Mg

Question 20

Describe the structure of diaryl azo

Answer 20

Chromophore in synthetic dyes