Observation of ¹MLCT and ³MLCT Excited States in Quadruply Bonded Mo₂ and W₂ Complexes

Abstract

The photophysical properties of the series of quadruply bonded M(2)(O(2)C-Ar)(4) [M = Mo, Ar = phenyl (ph), 1-naphthalene (1-nap), 2-naphthalene (2-nap), 9-anthracene (9-an), 1-pyrene (1-py), and 2-pyrene (2-py); M = W, Ar = ph, 2-nap] complexes were investigated. The lowest energy absorption of the complexes is attributed to a metal-to-ligand charge transfer (1)MLCT transition from the metal-based delta HOMO to the pi* O(2)C-Ar LUMO. The Mo(2)(O(2)C-Ar)(4) complexes exhibit weak short-lived emission (<10 ns) and a nonemissive, long-lived (40-76 mus) excited state detected by transient absorption spectroscopy. The short- and long-lived species are attributed to the (1)MLCT and (3)MLCT excited states, respectively, based on the large Stokes shift, vibronic progression in the low-temperature emission spectrum, and solvent dependence. Comparisons are made to the W(2)(O(2)C-Ar)(4) complexes, which are easier to oxidize and exhibit greater spin-orbit coupling than the Mo(2) systems. From the excited-state energy of the emissive (1)MLCT state and the electrochemical properties of the complexes, it is predicted that this excited state should be a powerful reducing agent. The crystal and molecular structure of Mo(2)(O(2)C-9-an)(4) is also reported together with electronic structure calculations employing density functional theory. To our knowledge, this is the first observation of MLCT excited states in quadruply bonded complexes. In addition, the photophysical properties of the present systems parallel those of organic aromatic molecules and may be viewed as metal-mediated organics. The introduction of the M(2) delta orbital in the complexes in conjugation with the organic pi-system of the ligands affords the opportunity to tune the excited-state energies and redox potentials.

Electron delocalization in the S 1 and T 1 metal-to-ligand charge transfer states of trans -substituted metal quadruply bonded complexes

“…Though allowed by symmetry, the 1 MM δδ* transition is weak due to the relatively poor overlap of the metal d orbitals. In each case, the lowest energy transition is characterized as MLCT (12,13). Although the planar D 2h structure of these molecules is the ground state structure, thermal energy leads to a Boltzmann distribution of conformers/rotamers in which the aryl groups of the ligand deviate from coplanarity with the carboxylate CO 2 or amidinate NCN moieties.…”

Structural Control of 1A2u-to-3A2u Intersystem Crossing in Diplatinum(II,II) Complexes

“…52 And M 2 (O 2 CR) 4 complexes with electronaccepting substituents R show weakly emissive δ→π*(O 2 CR) 1 MLCT states that decay to the corresponding nonemissive triplets in 4 to 20 ps. 53,54 Among mononuclear complexes, picosecond ISC times have been reported for 1 MLCT states of flattened-tetrahedral complexes of Cu I (13−16 ps) 37 and Pt 0 (3.2 ps). 55 Symmetry-forbidden first-order SO coupling between optically excited singlet and lowest triplet states is a common feature for all systems with ps-ns singlet lifetimes.…”

Metal to metal multiple bonds in ordered assemblies

“…Also the photophysical properties of the M 2 -containing complexes are very different from those of Ru(bpy) 2 2ϩ and related polypyridyl complexes that only show emission from the 3 MLCT state (34). The lifetimes of the 1 MLCT states for the M 2 containing species described are sufficiently long that they can be observed directly by transient absorption spectroscopy (35). The lifetime for a series of M 2 -O 2 C(Th) n containing compounds increases with n ϭ 1 3 3, which suggests that the influence of spin orbit coupling that facilitates inter system crossing is decreasing as the charge on the thienyl ligand is removed further from the metal center.…”