Charge-transfer band

Charge-transfer bands are a characteristic feature of the optical spectra of many compounds. These bands are typically more intense than d–d transitions. They typically exhibit <a href="/facts/Solvatochromism/EAoYgJrj">solvatochromism</a>, consistent with shifts of electron density that would be sensitive to solvation.
CT absorptions bands are intense and often lie in the ultraviolet or visible portion of the spectrum. For <a href="/facts/Coordination_complex/1tCyeQUm">coordination complexes</a>, charge-transfer bands often exhibit molar absorptivities, ε, of about 50000 L mol−1 cm−1. By contrast ε values for d–d transitions are in the range of 20–200 L mol−1 cm−1. CT transitions are spin-allowed and <a href="/facts/Laporte_rule/lNnV2zoF">Laporte</a>-allowed. The weaker d–d transitions are potentially spin-allowed but always Laporte-forbidden.
Charge-transfer bands of transition metal complexes result from shift of charge density between <a href="/facts/Molecular_orbitals/0dP30jaa">molecular orbitals</a> (MO) that are predominantly metal in character and those that are predominantly ligand in character. If the transfer occurs from the MO with ligand-like character to the metal-like one, the transition is called a ligand-to-metal charge-transfer (LMCT). If the electronic charge shifts from the MO with metal-like character to the ligand-like one, the band is called a metal-to-ligand charge-transfer (MLCT). Thus, a MLCT results in oxidation of the metal center, whereas a LMCT results in the reduction of the metal center.

Charge-transfer band open-in-new

Charge-transfer band