Electronic Transitions Spectroscopy 2: Electronic Transitions CHAPTER 14

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

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

Fig 14.12 Classification of d orbitals in an octahedral field

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

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–

Absorption spectra of aqueous charge-transfer complexes

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

Chromophore activity due to Fig 14.14 A C=C double bond as a chromophore Chromophore activity due to π* ← π

Chromophore activity due to Fig 17.13 A C=O double bond as a chromophore Chromophore activity due to π* ← n

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 490 - 492)

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

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

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

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

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

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

Fig 14.21 Sequence of steps leading to fluorescence

Fig 14.22 Relationship between an absorption spectrum and its fluorescence spectrum

Fig 14.23 Solvent shift in absorption and fluorescence

Fig 14.24 Sequence of steps leading to phosphorescence

Fig 14.25 Jablonski diagram for napthalene

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

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