CHEM2077: Selection Rules and Charge-Transfer Transitions

Vocabulary and key concepts regarding electronic selection rules, charge-transfer mechanisms, and luminescence in transition metal chemistry.

Spin Selection Rule

A rule stating that electronic transitions are only allowed if $\Delta S = 0$, meaning transitions between states of different multiplicities are forbidden.

Spin-orbit coupling

A phenomenon that mixes states of different multiplicities, particularly in 2nd and 3rd-row transition metals, allowing "forbidden" transitions to become observable but weak.

Laporte (Symmetry) Selection Rule

In centrosymmetric molecules like octahedrons, transitions must involve a change in parity ($g \leftrightarrow u$); d-d transitions are forbidden because all d-orbitals are gerade ($g$).

Vibronic Coupling

A relaxation of the Laporte rule where asymmetric vibrations temporarily remove a molecule's center of inversion, allowing weak absorption with $\epsilon \approx 1 - 100\,\text{M}^{-1}\text{cm}^{-1}$.

d-p Mixing

Occurs in non-centrosymmetric geometries like Tetrahedral, where d and p orbitals mix to provide $u$ character, increasing intensity to $\epsilon \approx 100 - 1000\,\text{M}^{-1}\text{cm}^{-1}$.

Charge-Transfer (CT) Transitions

Laporte-allowed and Spin-allowed transitions involving electron movement between metal-centered and ligand-centered orbitals, resulting in intensities of \epsilon > 1000\,\text{M}^{-1}\text{cm}^{-1}.

Ligand-to-Metal Charge Transfer (LMCT)

Transition where an electron moves from a ligand orbital to a metal d-orbital; favored in high oxidation state metals with $\pi$-donor ligands like $O^{2-}$ or $Cl^{-}$.

Metal-to-Ligand Charge Transfer (MLCT)

Transition where an electron moves from a metal d-orbital to a ligand $\pi^*$ orbital; favored in low oxidation state metals with $\pi$-acceptor ligands like bipyridine or CO.

Permanganate ($[MnO_4]^{-}$)

A complex whose deep purple color is derived from LMCT from the $O^{2-}$ ligand to the $Mn(VII)$ center.

$$[Ru(bpy)_3]^{2+}$$

A complex known for its intense orange color caused by Metal-to-Ligand Charge Transfer (MLCT).

Luminescence

The emission of light from an excited state, categorized as either fluorescence (singlet to singlet) or phosphorescence (triplet to singlet).

OLEDs (Organic Light-Emitting Diodes)

Devices that use transition metal complexes like $Ir(ppy)_3$ to harvest both singlet and triplet excitons for light emission.

Heavy Metal Effect in OLEDs

3rd-row metals like Iridium have high spin-orbit coupling that mixes singlet and triplet states, allowing formally forbidden triplet-to-singlet emission to occur efficiently.