Lecture 28: Photochemistry and Electron Spectroscopy

Read section 20.6 from your textbook.

Electronic Spectra

Electromagnetic radiation interacts with atoms or molecules within a sample. When radiation is absorbed, there is a transition from a low energy state to a higher energy state. Some of this radiation is absorbed and the remainder passes through. A detector measures the radiation that passes through the sample. In a split-beam spectrometer, half of the radiation goes through the sample and the other half goes directly to the detector. The detector compares the intensity of the two signals.
    This is a block diagram of a generalized absorption spectrometer. Light passes alternately through the sample and through a blank. The difference between the two beams is measured, then the output is sent to a printer or other device.

Electromagnetic Energy Spectrum

 The wavelength of the radiation is inversely proportional to its energy. The high energy of X-rays can promote core electrons from low energy to high energy states. Visible light doesn't have enough energy to cause transitions of core electrons but it does have sufficient energy to excite some valence electronic transitions.

Energy Change
(Kcal/mol) 		Wavelength Radiation Transition
>300 10-50 nm X-rays core electron
300 - 40 50-1000 nm UV-visible valence electron
35 - 2 1000-20,000 nm near infrared vibrations
2 - 0.1 20,000-100,000 nm far infrared rotations
0.0001 1-100 nm microwaves rotations
0.000001 100-1000 nm radio waves nuclear spin


When electromagnetic radiation has the same energy as an electronic transition strikes a molecule it is absorbed. This causes an electron to move to a higher energy molecular orbital.

We can obtain a UV-visible spectrum of a compound by passing electronmagnetic radiation in the UV-visible range through a sample of the compound and monitoring the energy absorbed for each wavelength. The lowest energy absorption usually corresponds to the energy required to promote an electron to the lowest occupied molecular orbital (LUMO) from the highest occupied molecular orbital (HOMO).

For transition elements, energy required to promote electrons between d orbitals (or molecular orbitals with d character) is in the visible range, so transition metal compounds are usually colored.

The intensity of d-to-d transitions is much lower than charge transfer transitions.
  • Transfer of an electron from a metal-centered molecular orbital to a higher energy ligand centered molecular orbital (non-bonding, for example) is a metal-to-ligand charge transfer band or MLCT.
  • Transfer of an electron from a ligand-centered molecular orbital to a higher energy metal centered molecular orbital (d orbital) is a ligand-to-metal charge transfer band or LMCT.



Selection Rules

Selection rules tell us which transitions between electronic energy levels are "allowed". Forbidden transitions occur but they are slower and have a lower intensity than the allowed transitions.
  1. Spin selection rule
      The spin multiplicity (the number of unpaired electrons) can't change during the electronic transition.

  2. Laporte selection rule
      The electron can only go to an orbital that is different by 1 in angular momentum quantum number. So s p and p d transitions are allowed but s d and d d are forbidden.

      Because of this, the d d bands in the visible region of the spectrum are weak compared to the charge transfer bands.

Photochemistry

When a molecule A absorbs light, an electron is promoted to a higher energy level and it becomes an excited state molecule (A*). A* is a reducing agent because of the electron in a high energy orbital (easy to transfer this to an oxidizing agent) and an oxidizing agent because of the hole in the low lying orbital (easy to accept an electron from a reducing agent). Several things can happen to this excited molecule:



The rate of a photochemical reaction depends on the the amount of light (with the correct wavelength) that is absorbed by the molecule. We define a quantity called the quantum yield as the rate of the absorption step, kl[A*], divided by the total illumination of the sample.



In a typical photoinitiated reaction of A B with light, there are at least three steps. Each of these has a rate. If we consider that the concentration of the excited state species, A*, is small and constant, we can simplify the rate expression.



Substitution Reactions

Photolysis of a transition metal compound can result in substitution reactions. First, the excited state complex is higher in energy and is more reactive. Second, if the electron promoted to a higher energy level was in a bonding molecular orbital, the bond will be weaker. Photolysis can also promote bond cleavage when an electron is promoted to an antibonding level.

Photolysis is most important for substitution of pi-acceptor ligands, such as CO. These ligands bond strongly to metal and are very high on the spectrochemical series. Under irradiation, the pi bonds are weakened and these ligands can more readily dissociate.



Example: [Ru(bpy)3]2+

Ruthenium (II)-diimine complexes have been used extensively as photosensitizers in solar energy conversion systems. Upon photoexcitation, these Ru(II) complexes can reduce many substances. In principle (though not in current practice), the power "stored" in such a photo-generated redox pair can be used to split water into H2 and O2, two energy rich molecules.