1. Electronic Transitions
Spectroscopy 2:
Electronic Transitions
CHAPTER 14

2. Electronic spectra of polyatomic molecules
Absorption can be traced to specific types of electrons
Groups called chromophores
Transitions involving
d-d transitions
charge-transfer transitions
π* ← π and π* ← n transitions

3. Absorption Involving d − d Transitions
Most transition metal ions are colored (absorb in UV-vis) due to d → d electronic transitions

4. Fig 14.12 Classification of d orbitals in an octahedral field

5. Effect of octahedral field on d-orbital energies in [Ti(H2O)6]3+
Δo ≡ Ligand-field
splitting parameter
(λmax in visible)

6. Charge-transfer transitions
Upon absorption of radiation, an electron is transferred from ligands into d-orbital of metal (LMCT) or vice-versa (MLCT)
Sometimes called “internal redox”
Generally very high molar absorptivities
ε > 10,000
Unusually intense absorption and colors
e.g.,
MnO4–
Cr2O72–
FeSCN–

7. Absorption
spectra of aqueous
charge-transfer
complexes

8. π* ← π and π* ← n transitions
Upon absorption of radiation, a π or n electron is promoted
to a π* orbital

9. Chromophore activity due to
Fig A C=C double bond as a chromophore
Chromophore activity due to
π* ← π

10. Chromophore activity due to
Fig A C=O double bond as a chromophore
Chromophore activity due to
π* ← n

11. π* ← π and π* ← n transitions
Upon absorption of radiation, a π or n electron is promoted
to a π* orbital
In a conjugated chain,
the longer the chain
the more closely spaced the Mos
the longer the λmax in absorption.
e.g., the photochemistry of vision (pg )

12. Scatters/absorbs ~ 43% of photons
From Bill W. Tillery, Physical Science, 6e. McGraw-Hill (2005) )
Scatters/absorbs ~ 43% of photons

13. Pg 491 The photochemistry of vision
π* ← π
In the macular pigment:
reduces chromatic aberration
reduces flux of photons

14. Pg 491 The photochemistry of vision
Molecule in the dark
Attached to opsin protein
(in rods and cones)

15. Pg 491 The photochemistry of vision
Molecule after undergoing photoisomerization
π* ← π
Duration: ~ 200 fs
Efficiency: ~ 67%
Molecule returns to ground state but is trapped in trans-
configuration and presses against opsin molecule
converting conformational energy to a pulse of potential energy

16. Fates of electronically excited states
Two competing processes:
Radiative decay
Fluorescence and phosphorescence
Nonradiative decay
Intra- and intermolecular energy transfer

17. Fig 14.20 Empirical distinction between fluorescence
and phosphorescence
(slow)
(fast)

18. Fig 14.21 Sequence of steps leading to fluorescence

19. Fig 14.22 Relationship between an absorption spectrum
and its fluorescence spectrum

20. Fig 14.23 Solvent shift in absorption and fluorescence

21. Fig 14.24 Sequence of steps leading to phosphorescence

22. Fig 14.25 Jablonski diagram for napthalene

23. Fig 14.26 Absorption to unbound states results in
dissociation and a continuum absorption

24. Fig 14.27 Absorption to a bound state crossed by an
unbound state results in predissociation