Spectroscopy notes for uni: UV-vis

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A tray of cuvettes containing indigo carmine, a dye which emits blue light

I am only publishing these as convenient access points for me. Some of this can be used at A Level, I think, but I'd hesitate before relying on them. 

UV-vis spectroscopy

  • Used with electronic transitions; when you shine light on a sample, it will excite an electron to a higher energy level, which corresponds to a change in energy. These electronic transitions are very fast, and the period of time that excitation takes place is short. UV-vis spectroscopy effectively measures what type of light has been absorbed by the sample. 
  • Light obviously being along the visible and UV parts of the EM spectrum, with red light having wavelengths of about 700nm and violet light having wavelengths closer to 400nm. 
  • When you have a cuvette containing a given sample of solution, and you shine light through the cuvette, some of the light will be absorbed by the solution, whilst some will pass through. The ratio between the light that's passed through (I) and the initial light (I0) is known as the transmittance. 
  • Absorbance is similar, but it instead measures how much light has been absorbed by the sample. The relationship between absorbance and transmittance is:
  • A = 10-T, where A is absorbance and T is transmittance. Both A and T are unitless.
  • Beer-Lambert law: here, A = εcl, where ε is the molar absorption/extinction coefficient, c is the concentration of the sample, and l is the path length of the cuvette where you are measuring the absorption of light. This law is only valid for dilute solutions.
  • ε is a measure of how likely a transition is to occur - the greater ε, the higher the probability of transition.  
  • Organic compounds which can absorb UV-vis light are known as chromophores. Examples include alkenes (C=C), carbonates (C=O), and nitromethane (N=O), and all of them have π bonds present. Chromophores must have π bonds:
    • Electronic transitions occur from a bonding orbital to an anti-bonding orbital, as per the movement of electrons during excitation. 
    • These transitions all correspond to a wavelength, as per E = hc/λ. However, the only transitions which have wavelengths that match the UV-vis part of the EM spectra are those from non-bonding or π orbitals to a π* anti-bonding orbital (the * means anti-bonding).
  • Conjugation effects: 
    • The greater the effects of conjugation - that is, having alternating π bonds in a molecule - the lower the energy of π to π* transitions.
    • Therefore, these transitions will have larger wavelengths.
    • This is known as either red shift or bathochromic shift (notice red shift is used similarly to how it is used in astronomy).
    • This also applies in aromatic systems - the greater the degree of aromaticity, the greater the red shift. (So naphthalene - which is two benzene rings fused together - will have lower energy transitions than benzene. You'd therefore expect naphthalene to have higher wavelength transitions.)
  • Colour
    • Chromophores will absorb certain wavelengths of light very strongly.
    • The colour that is emitted will be the complementary colour to the light absorbed.
    • For example, a chromophore that absorbs light around 400nm long will mainly absorb violet light. So the colour the molecule emits will be the complementary colour to violet, which happens to be red.
  •  Isobestic points
    • During an equilibrium, you might see a colour change. 
    • In this reaction, if the reactant and product have the same value of ε for a given wavelength, the rate of absorption will be constant, independent of the concentration of the reactant.
    • The wavelength at which this occurs is known as the isobestic point. This also happens to indicate when there is an interconversion between the reactants without any side reaction going on. This is a clean equilibrium.
  • Transition metal complexes
    • As mentioned in a previous post, transition metal complexes can have various colours. This can be due to two different reasons: 
      • Transitions between d-orbitals
        • d-orbitals are at different energy levels to each other (they are degenerate to each other) in a metal complex, so you can have electronic transitions between them.
        • They are often very weak, but they do exist.
      •  Charge transfer between ligands and metals
        • You'll remember that ligands transfer a lone pair of electrons to a metal ion to form a metal complex. The reverse can also happen!
        • These transfers in charge result in stronger absorptions, and bolder colours. 
    • Typically, expect the colours in transition metal complexes to be less bold than those in organic dyes which contain chromophores with high degrees of conjugation. 

These are all metal complexes dissolved in water, with the water molecules acting as ligands. The colours aren't particularly bold, so I'm assuming the colours are derived from electronic transitions between d orbitals.

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