Photophysical Studies of Metal to Ligand Charge Transfer Involving Quadruply Bonded Complexes of Molybdenum and Tungsten

Abstract

Photoinduced metal-to-ligand charge transfer transitions afford numerous applications in terms of photon energy harvesting. The majority of metal complexes studied to date involve diamagnetic systems of d(6), d(8), and d(10) transition metals. These typically have very short-lived, ∼100 fs, singlet metal to ligand charge transfer ((1)MLCT) states that undergo intersystem crossing to triplet metal to ligand charge transfer ((3)MLCT) states that are longer lived and are responsible for much of the photophysical studies. In contrast, the metal-metal quadruply bonded complexes of molybdenum and tungsten supported by carboxylate, O2CR, and related amidinate ligands (RN)2C(R') have relatively long-lived (1)MLCT states arising from M2δ to Lπ* transitions. These have lifetimes in the range 1-20 ps prior to intersystem crossing to T1 states that may be (3)MLCT or (3)MMδδ* with lifetimes of 1-100 ns and 1-100 μs, respectively. The M2 quadruply bonded complexes take the form M2L4 or M2L4-nL'n where n = 1-3. Thus, in their photoexcited MLCT states, these compounds pose the question of how the charge resides on the ligands. This Account reviews the current knowledge of how charge is positioned with time in S1 and T1 states with the aid of active IR reported groups located on the ligands, for example, C≡X multiple bonds (X = C, N, or O). Several examples of localized and delocalized charge distributions are noted along with kinetic barriers to the interconversion of MLCT and δδ* states. On the 50th anniversary of the recognition of the MM quadruple bond, these complexes are revealing some remarkable features in the study of the photophysical properties of metal-ligand charge transfer states.

Synthesis, Structure, and Photophysical Properties of Mo2(NN)4 and Mo2(NN)2(TiPB)2, Where NN = N,N′-Diphenylphenylpropiolamidinate and TiPB = 2,4,6-Triisopropylbenzoate

“…Considering that the electron is distributed evenly over all available NN ligands in I − and II − , the shift would be around 200−300 cm −1 when the charge is localized on one ligand. The 3 I and 3 II calculations predicted that the ν(CC) band in the 3 MoMoδδ* T 1 state would reside close to the ground state, ∼5 cm −1 higher in energy.…”

“…At longer time delays, we observed a strong ν(N−C−N) bleach at 1456 cm −1 and an associated strong absorption feature grows at 1480 cm −1 . The CN 2 stretch moves to high energy because less electron density would donate to the CN 2 π* orbital from the Mo 2 δ bond in the 3 MoMoδδ* T 1 state in comparison to the ground state. Also at higher energy, we see a weak peak at 2205 cm −1 due to ν(C C) in the T 1 state, shifted ∼5 cm −1 to lower energy from the ground state (rather than to higher energy, as predicted from calculations).…”

“…The transition typically generates a chargeseparated, metal to ligand charge transfer singlet state ( 1 MLCT) that subsequently undergoes intersystem crossing (ISC) to a metal-centered triplet state ( 3 MoMoδδ*). The 1 MLCT S 1 lifetimes are on the order of 1−20 ps and 3 MoMoδδ* T 1 lifetimes are on the order of 10−100 μs. 16 Typically, in the compounds Mo 2 (O 2 CR) 4 and Mo 2 (O 2 CR) 2 (T i PB) 2 , where R = −C 4 H 2 SCCH, −C 6 H 4 -p-CCH, extensive charge delocalization (between ligands that are trans to each other) in the 1 MLCT state was found.…”

“…1−3 One famous example is the charge distribution in the MLCT singlet state ( 1 MLCT) of Ru(bpy) 3 2+ . 1,4−6 Having a C 3 axis means that the electron may be distributed equally or unequally over the three bpy ligands following photon absorption.…”

“…17 As proposed by Zink, a class I (similar to the Robin−Day scheme) MLCT excited state is one where just a single ligand possesses the negative charge, a class III MLCT excited state is one in which the charge is completely delocalized, and a class II MLCT excited state is defined as an intermediate, where the charge is localized but has a low barrier to interconversion. Thus, we assigned the aforementioned localized 3 MLCT T 1 state of Ru(bpy) 2+ as class I while assigning the delocalized 1 MLCT S 1 states of the aforementioned Mo 2 (O 2 CR) 4 and Mo 2 (O 2 CR) 2 (T i PB) 2 compounds as class III. Yet, there was one such amidinate compound that was examined previously, Mo 2 [( i PrN) 2 CCCPh] 2 (O 2 CMe) 2 , in which we discovered that the charge is primarily localized on one ligand but with some charge spillover to the other ligand.…”