Ligand-centered State Research Articles

A New Blue Photoluminescent Salen-like Zinc Complex with Excellent Emission Quantum Yield

Abstract A new salen-like compound, H2L, bearing a 12-carbon atom length aliphatic chain bridging iminic nitrogen atoms, was successfully synthesized and characterized. Interestingly, its zinc complex, ZnL, showed an extraordinary high fluorescent quantum yield value of 75%, originating from a metal-perturbed ligand-centered state.

Rapid intersystem crossing in highly phosphorescent iridium complexes

Femtosecond time-resolved absorption measurements on Ir complexes are performed. On excitation at wavelength 400 nm, singlet metal-to-ligand charge-transfer (MLCT) states are excited. On probing at 580 nm, a transient absorption appears with time constant 70–100 fs. This transient state rising rapidly but decaying very slowly is assigned to be the lowest triplet MLCT state. Accordingly intersystem crossing in these Ir complexes is rapid. Measured with a nanosecond laser, emission from the lowest triplet MLCT state decays with lifetimes 1.55, 1.65 and 1.7 μs for Ir(ppy) 3, Ir(DBQ) 2(acac) and Ir(MDQ) 2(acac), respectively. On 266-nm excitation with a femtosecond laser, high-energy states with mostly π–π * character (ligand-centered states) are accessed; they relax to the lowest triplet MLCT state with time constant 100–350 fs. The efficient flow of energy from high-energy electronic states to 3MLCT results in high quantum yield in electroluminescent phosphorescence in organic light-emitting diodes.

Influence of electron donor and acceptor substituents on the excited state properties of rhenium(I) tetracarbonyl chelate complexes

The new complexes [Re(CO)4(meppy)], [Re(CO)4(dmdcb)]PF6 and [Re(CO)4(dmeob)][CF3SO3] [Hmeppy = 3-methyl-2-phenylpyridine, dmdcb = 4,4′-di(methoxycarbonyl)-2,2′-bipyridine and dmeob = 4,4′-dimethoxy-2,2′-bipyridine] have been synthesized. The crystal and molecular structure of [Re(CO)4(meppy)] was determined by X-ray diffraction. High-resolution optical absorption, luminescence and Raman spectroscopy on single crystals and glasses at cryogenic temperatures revealed the lowest excited state for all complexes as a nominally 3π–π* ligand centered state with some 1MLCT character mixed in by spin–orbit coupling. This leads to the occurrence of metal–ligand vibrational sidebands in the absorption and luminescence spectra. The lowest excited state has a charge-transfer character of 1.7, 1.3 and 1.0% for [Re(CO)4(meppy)], [Re(CO)4(dmdcb)]PF6 and [Re(CO)4(dmeob)][CF3SO3], respectively. These results are compared with those for the unsubstituted complexes [Re(CO)4(ppy)] and [Re(CO)4(bpy)]PF6 (Hppy = 2-phenylpyridine and bpy = 2,2′-bipyridine). The influence of the σ-donor group on the energy of the excited states and on the mixing of 1MLCT in the 3LC state is negligibly small. π-Electron donor substituents increase the energy of the excited states and increase the singlet–triplet splittings; π-electron withdrawing groups have the opposite effect and increase the 1MLCT character in the first excited state. The luminescence decay of [Re(CO)4(meppy)] is not single exponential due to a large inhomogeneous distribution of chromophores in the glass. This large distribution is due to the steric interaction of the methyl group with the phenyl part of the ligand.

Synthesis, Crystal Structure, High-Resolution Optical Spectroscopy, and Extended Hückel Calculations on Cyclometalated [Re(CO)4(ppy)] (ppy = 2-Phenylpyridine)

The cyclometalated complex [Re(CO)4(ppy)], where ppy- = 2-phenylpyridine, was synthesized. Its crystal and molecular structure was determined by X-ray diffraction. The lowest energy electronic excitations were studied by high-resolution optical spectroscopy at cryogenic temperature. The first excited state in [Re(CO)4(ppy)] is nominally a triplet ligand-centered state, with 1.8% metal-to-ligand charge transfer character mixed in through spin−orbit coupling. This induces a shortened lifetime (89 μs at 10 K) and the occurrence of metal−ligand vibrations in both absorption and luminescence spectra. The transition moment of the first electronic excitation lies in the molecular plane with a tilt toward the pyridine part of the ppy- ligand. This is derived from the polarized absorption measurements and confirmed by an extended Hückel calculation. Splittings in the IR absorption bands are resulting from the presence of two crystallographically equivalent complexes in the unit cell which are tilted with respect to each other.

Multiphoton-induced picosecond photophysics of chromium(III)-polypyridyl complexes

The photophysics of Cr(III)-polypyridyl complexes, Cr(NN)3 3+, have been the object of many studies in attempts at understanding the behaviour of these systems on absorption of light (Jamieson 1981; Serpone 1983). The usual photochemically relevant states of Cr(NN)3 3+ (Fig. 1) are: the ground state, \({\!^4{\text{A}}_2}\); the lowest energy spin-allowed quartet state, \({\!^4{\text{T}}_2}\); and the lowest energy spin-forbidden doublet states, \({\!^2{\text{T}}_1}/ {\!^2{\text{E}}}\). Also, shown are the other higher energy quartet and doublet states, and the ligand-centred states (Ohno 1983 1986) depicted as dashed curves? not shown are some 12 additional doublet states. Several studies have been Open image in new window Fig. 1 Qualitative energy level diagram as a function of a distort- ion coordinate. Solid lines denote metal-centred state surfaces; dashed lines show ligand-centred surfaces. From (Serpone 1987). directed toward the long-lived (τ ~ 0.1 - 1 ms) luminescent \({\!^2{\text{T}}_1}/ {\!^2{\text{E}}}\) states; prompt fluorescence from \({\!^4{\text{T}}_2}\) has not been observed in fluid media at ambient temperatures, implying either that the lifetime of \({\!^4{\text{T}}_2}\) is too short or that this state is not populated. Recent pulsed laser transient absorption spectroscopy experiments have indicated that τ of \({\!^4{\text{T}}_2}\) of Cr(acac)3, Cr(NCS)6 3-, t-Cr(NH3)2(NCS)4 -, t-Cr(en)2(NCS)2 +, Cr(en)3 3+, and Cr(bpy)3 3+ are a few ps or less (Kirk 1976; Rojas 1986 1987; Pyke 1978; I.eSage 1983 ; Nicollin 1980).