Ligand-to-Ligand Charge Transfer in (2,2′-Biquinoline)bis(cyclopentadienyl)zirconium(IV) – Absorption and Emission in the Visible Range

Abstract

The lowest energy electronic transition of [(cyclopentadienyl)2ZrIV(2,2′‐biquinoline)]2+, which appears in absorption at λmax = 360 nm and in emission at λmax = 518 nm, is of the Cp− → biq ligand‐to‐ligand charge‐transfer‐type.

Luminescence and spectroscopic studies of organometallic rhodium and rhenium multichromophore systems carrying polypyridyl acceptor sites and phenylethynyl antenna subunits

“…In this context it is important to note that several other reasons for a more complex photophysical behaviour may exist, which should also be taken into account for an interpretation of the luminescence properties of heteroleptic rhodium (III) diimine complexes such as 2. Besides low-lying intraligand excited states, the deactivation pathways of such systems may also involve metal-centered (MC) triplet states of 3 ded origin [36] or may be complicated by ligand-to-ligand charge transfer (LLCT) excited states at rather low energies, which is sometimes the case in the presence of cyclopentadienyl donor ligands [37]. Frequently, the energetic separation of close-lying electronic levels of different orbital parentage is small, which makes these systems very sensitive to minor chemical modifications and may also lead to a pronounced temperature dependency of the photophysical properties.…”

Ligand to Ligand Charge Transfer in (Hydrotris(pyrazolyl)borato)(triphenylarsine)copper(I)

“…An uncommon type of charge-transfer transition is ligand to ligand (LLCT) or interligand charge transfer. In comparison to the vast literature on MLCT and LMCT, very little has been published on LLCT. − In many cases LLCT bands are difficult to detect in absorption spectra; reasons for this difficulty include the fact that these bands may be hidden under or obscured by absorption bands of different origins, or they may occur at energies very different from those ordinarily studied. The molar absorptivities of the LLCT bands may be low because of the poor overlap between the orbitals involved.…”

Optical properties of ethylenebis(indenyl)zirconium(IV) binolate with binol=1,1′-binaphthyl-2,2′-diol. Binolate intraligand phosphorescence under ambient conditions

“…Since the emitting state is a triplet, zirconium and hafnium as metals of the second and third transition series facilitate the phosphorescence owing to their heavy-atom effect which leads to increased spinorbit coupling. Accordingly, the phosphorescence of dissolved Cp 2 Ti IV X 2 appears only at low temperatures [15] while Cp 2 Zr IV X 2 and Cp 2 Hf IV X 2 also phosphoresce at room temperature [8][9][10][11]. The emission originates from an LMCT state in agreement with the oxidizing nature of M(IV) and the reducing character of Cp À and X À .…”

“…Various complexes of the general formula Cp 2 M IV X 2 with M ¼ Ti [15], Zr and Hf [8][9][10][11][12][13] have been shown to be luminescent. Since the emitting state is a triplet, zirconium and hafnium as metals of the second and third transition series facilitate the phosphorescence owing to their heavy-atom effect which leads to increased spinorbit coupling.…”

“…Since Zr(IV) has a d 0 electron configuration any interference by LF (ligand field) states is avoided and only low-energy LMCT and IL states are accessible. Moreover, various complexes of the type Cp 2 Zr IV X 2 with Cp ¼ C 5 H 5 have been reported to emit [8][9][10][11][12][13]. These emissions come from triplet states and appear at r.t. [8][9][10][11] binaphthyl group provides pp à IL states at relatively low energies [14].…”