Excited State Mixed Valence Research Articles

Cover Feature: Photoinduced Intervalence Charge Transfers: Spectroscopic Tools to Study Fundamental Phenomena and Applications (ChemPhysChem 20/2022)

The Cover Feature shows a molecule that forms, upon photon absorption, an excited state mixed valence system, which is characterized by a double-well potential energy surface. Inspection of the excited state with light reveals photoinduced intervalence charge transfers. More information can be found in the Concept by Alejandro Cadranel and co-workers.

Synthesis, Structure, and Photophysical Properties of Mo2(NN)4 and Mo2(NN)2(T(i)PB)2, Where NN = N,N'-Diphenylphenylpropiolamidinate and T(i)PB = 2,4,6-Triisopropylbenzoate.

Two dimolybdenum compounds featuring amidinate ligands with a C≡C bond, Mo2(NN)4 (I), where NN = N,N'-diphenylphenylpropiolamidinate, and trans-Mo2(NN)2(T(i)PB)2 (II), where T(i)PB = 2,4,6-triisopropylbenzoate, have been prepared and structurally characterized by single-crystal X-ray crystallography. Together with Mo2(DAniF)4 (III), where DAniF = N,N'-bis(p-anisyl)formamidinate, all three compounds have been studied with steady-state UV-vis, IR, and time-resolved spectroscopy methods. I and II display intense metal to ligand charge transfer (MLCT). Singlet state (S1) lifetimes of I-III are determined to be 0.7, 19.1, and 2.0 ps, respectively. All three compounds have long-lived triplet state (T1) lifetimes around 100 μs. In femtosecond time-resolved infrared (fs-TRIR) experiments, one ν(C≡C) band is observed at the S1 state for I but two for II, which indicate different patterns of charge distribution. The electron would have to be localized on one NN ligand in I and partially delocalized over two NN ligands in II to account for the observations. The result is a standard showcase of excited-state mixed valence in coordination compounds.

Coupled states in dinitrofluorene: relationships between ground state and excited state mixed valence.

The electronic absorption spectrum of 9,9-dimethyl-2,7-dinitrofluorene radical anion in HMPA displays both a NIR intervalence charge transfer and a visible excited state mixed valence transition. These transitions contain a similar vibronic progression resulting from molecular orbitals that are common to both transitions. Vibrational frequency and intensity data are acquired from the resonance Raman spectrum and used to calculate a best fit for the absorption spectrum. The normal coordinate distortions are analyzed in terms of the electronic changes for both transitions to explain their similarity. The Raman scattering intensity decreases at lower excitation wavelength as a result of Raman de-enhancement caused by interference between neighboring excited states.

Dual emission and excited-state mixed-valence in a quasi-symmetric dinuclear Ru-Ru complex.

The synthesis and characterization of the new dinuclear dipeptide [(EtOOC-tpy)Ru(tpy-NHCO-tpy)Ru(tpy-NHCOCH3)](4+) 3(4+) of the bis(terpyridine)ruthenium amino acid [(HOOC-tpy)Ru(tpy-NH2)](2+) 1(2+) are described, and the properties of the dipeptide are compared to those of the mononuclear complex [(EtOOC-tpy)Ru(tpy-NHCOCH3)](2+) 4(2+) carrying the same functional groups. 3(4+) is designed to serve a high electronic similarity of the two ruthenium sites despite the intrinsic asymmetry arising from the amide bridge. This is confirmed via UV-vis absorption and NMR spectroscopy as well as cyclic voltammetry. 4(2+) and 3(4+) are emissive at room temperature, as expected. Moreover, 3(4+) exhibits dual emission from two different triplet states with different energies and lifetimes at room temperature. This is ascribed to the presence of a unique thermal equilibrium between coexisting [Ru(II)(tpy-NHCO-tpy(·-))Ru(III)] and [Ru(III)(tpy-NHCO-tpy(·-))Ru(II)] states leading to an unprecedented excited-state Ru(II)Ru(III) mixed-valent system via the radical anion bridge tpy-NHCO-tpy(·-). The mixed-valent cation 3(5+), on the other hand, shows no measurable interaction of the Ru(II)Ru(III) centers via the neutral bridge tpy-NHCO-tpy (Robin-Day class I). Reduction of 3(4+) to the radical cation 3(3+) by decamethylcobaltocene is bridge-centered as evidenced by rapid-freeze electron paramagnetic resonance spectroscopy. Interestingly, all attempts to observe 3(3+) via NMR and UV-vis absorption spectroscopy only led to the detection of the diamagnetic complex 3-H(3+) in which the bridging amide is deprotonated. Hence 3-H(3+) (and 4-H(+)) appear to reduce protons to dihydrogen. The ease of single and double deprotonation of 4(2+) and 3(4+) to 4-H(+), 3-H(3+), and 3-2H(2+) was demonstrated using a strong base and was studied using NMR and UV-vis absorption spectroscopies. The equilibrating excited triplet states of 3(4+) are reductively quenched by N,N-dimethylaniline assisted by hydrogen bonding to the bridging amide.

Photophysical Properties of MM Quadruply Bonded Complexes Supported by Carboxylate Ligands, MM = Mo2, MoW, or W2

While chemists have extensively studied the photophysical properties of d(6), d(8), and d(10) transition metal complexes, their early transition metal counterparts have received less attention. Quadruply bonded complexes of molybdenum and tungsten supported by carboxylate ligands have intense metal-to-ligand charge transfer (MLCT) absorptions that arise from the electronic coupling of the metal-metal (MM) δ orbital with the CO(2) π-system. This coupling may in turn be linked to an extended π-conjugated organic functional group. The major interaction is akin to the so-called back-bonding in metal carbonyl complexes. By the appropriate selection of MM, its attendant ligands, and the organic group, this absorption can be tuned to span the visible and near IR range, from 400 to 1000 nm. Consequently, these complexes offer potential as photon harvesters for photovoltaic devices and photocatalysis. In this Account, we describe recent studies of dinuclear M(II) containing complexes, where M = Mo or W, and show that there are both parallels and disparities to the monomeric transition metal complexes. These early transition metal complexes have relatively long lived excited state singlets when compared to other transition metal complexes. They also often show unusual dual emission (fluorescence and phosphorescence), with singlet (S(1)) lifetimes that range from 1 to 20 ps, and triplet (T(1)) lifetimes from 3 ns to 200 μs. The fluorescent S(1) states are typically (1)MLCT for both M = Mo and W. These extended singlet lifetimes are uncommon for mononuclear transition metal complexes, which typically have very short lived (1)MLCT states due to rapid femto-second intersystem crossing rates. However, the T(1) states differ. This phosphorescence is MLCT in nature when M = W, while this emission comes from the δδ* state for M = Mo. Through time-resolved femtosecond infrared spectroscopy, we can detect the asymmetric stretch of the CO(2) ligand in both the singlet and triplet δδ* states. Through these analytical methods, we can study how the charge distribution in the singlet and triplet excited states changes over time. In addition, we can detect delocalized or localized examples of MLCT states, which represent class III and I excited state mixed valence in the Robin and Day scheme.

Excited state mixed valence in a dual‐bridged three‐chromophore system

The optical and resonance Raman spectra of the 2,2′: 6′,2″:6″,6‐trioxytriphenyl‐amine cation are measured and interpreted. This molecule contains two simultaneous types of coupling between three chromophores and two types of bridging atoms. The first and conventional coupling involves a single nitrogen bridge that couples all three aryl groups. The second is provided by the three oxygen atoms, each of which bridges two adjacent aryl groups. There are two bands in the visible region of the optical absorption spectrum; their assignment and the interpretation of the contributing orbitals and electronic states are described in terms of the neighboring orbital model that explains the effects of the two types of coupling. The bonding changes that take place in the excited electronic states are probed by resonance Raman spectroscopy intensities and analyzed using the time‐dependent theory of resonance Raman spectroscopy. The optical absorption spectrum was fit using the measured vibrational frequencies and excited state distortions. The distortions correlate well with the bonding changes predicted by the neighboring orbital model. The resonance Raman data and neighboring orbital model analysis reveal that the two optical absorption bands correspond to charge transfers from aryl groups with different nodal structures in their pi orbitals. Copyright © 2012 John Wiley & Sons, Ltd.

M2δ to ligand π-conjugation: testbeds for current theories of mixed valence in ground and photoexcited states of molecular systems

Redox active quadruply bonded units, M(2), can be combined so that they either (i) are bridged by an organic linker or (ii) function as a bridge between two identical organic ligands. When two M(2) units are linked together by an organic group that affords M(2)δ-bridge π-conjugation the electronic structure of each M(2) unit is perturbed by the other in the ground state, the photoexcited states, and the mixed valence oxidized form. Similarly when a M(2) center links two organic π systems represented by L, the two organic units are coupled by Lπ*-M(2)δ-Lπ* interactions in their ground state, their photoexcited states, and the mixed valence reduced state. The photoexcited states of the neutral complexes (both case i and ii) provide examples of excited state mixed valence. In case (i), the positive charge may be localized on one dinuclear center or may be delocalized over both M(2) units. Similarly in (ii), the electron may be localized on one ligand or delocalized over both. In this tutorial review, spectroscopic studies (UV-vis-NIR absorption, steady state emission, EPR, and time resolved infrared) of these mixed valence systems employing carboxylate tethers are described and the data are discussed in terms of contemporary theories of mixed valence ions.

Three-Chromophore Excited-State Mixed Valence

The lowest energy optical electronic absorption band of the three-chromophore system tris(4-bromophenyl)amine radical cation is analyzed. The lowest energy electronic transition corresponds to a p-bromophenyl orbital to nitrogen p orbital transition that places the positive charge on three equivalent p-bromophenyl chromophores. The excited electronic state is an example of excited-state mixed valence (ESMV), and the spectrum is interpreted using two ESMV models. The simplest model invokes the concept of an "effective coupling" between the three identical chromophores with an excited-state energy splitting equal to three times the coupling. A more accurate model, the "neighboring orbital model", utilizes the coupling between the bridge's and charge-bearing unit's orbitals closest in energy. The three-chromophore system provides a striking illustration of the failure of an effective coupling term to account for ESMV splitting. The calculated relative energies of the diabatic and adiabatic states are different, but the calculated absorption spectra of the two models show nearly identical vibrational fine structure. Resonance Raman data and the time-dependent theory of electronic and resonance Raman spectroscopies are used to calculate the spectra.

Resonance Raman De-Enhancement Caused by Excited State Mixed Valence

Resonance Raman and absorption spectra of 9,10-bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)-anthracene (2) are measured and analyzed. The contribution of the individual vibrational normal modes to the reorganization energy is investigated. Excited-state mixed valence in this system is analyzed using density functional theory electronic structure calculations. The resonance Raman excitation profiles exhibit a resonance de-enhancement effect around 20 725 cm-1, but a corresponding feature is not observed in the absorption spectrum. This unusual observation is attributed to the presence of a dipole-forbidden, vibronically allowed component of the split mixed valence excited state. The de-enhancement dip is calculated quantitatively and explained in terms of the real and imaginary components of the polarizabilities of the two overlapping excited states.

Excited-State Mixed-Valence Distortions in a Diisopropyl Diphenyl Hydrazine Cation

Excited-state mixed valence (ESMV) occurs in the 1,2-diphenyl-1,2-diisopropyl hydrazine radical cation, a molecule in which the ground state has a symmetrical charge distribution localized primarily on the hydrazine, but the phenyl to hydrazine charge-transfer excited state has two interchangeably equivalent phenyl groups that have different formal oxidation states. Electronic absorption and resonance Raman spectra are presented. The neighboring orbital model is employed to interpret the absorption spectrum and coupling. Resonance Raman spectroscopy is used to determine the excited-state distortions. The frequencies of the enhanced modes from the resonance Raman spectra are used together with the time-dependent theory of spectroscopy to fit the two observed absorption bands that have resolved vibronic structure. The origins of the vibronic structure and relationships with the neighboring orbital model are discussed.

Interpretation of Unusual Absorption Bandwidths and Resonance Raman Intensities in Excited State Mixed Valence

Excited state mixed valence (ESMV) occurs in molecules in which the ground state has a symmetrical charge distribution but the excited state possesses two or more interchangeably equivalent sites that have different formal oxidation states. Although mixed valence excited states are relatively common in both organic and inorganic molecules, their properties have only recently been explored, primarily because their spectroscopic features are usually overlapped or obscured by other transitions in the molecule. The mixed valence excited state absorption bands of 2,3-di-p-anisyl-2,3-diazabicyclo[2.2.2]octane radical cation are well-separated from others in the absorption spectrum and are particularly well-suited for detailed analysis using the ESMV model. Excited state coupling splits the absorption band into two components. The lower energy component is broader and more intense than the higher energy component. The absorption bandwidths are caused by progressions in totally symmetric modes, and the difference in bandwidths is caused by the coordinate dependence of the excited state coupling. The Raman intensities obtained in resonance with the high and low energy components differ significantly from those expected based on the oscillator strengths of the bands. This unexpected observation is a result of the excited state coupling and is explained by both the averaging of the transition dipole moment orientation over all angles for the two types of spectroscopies and the coordinate-dependent coupling. The absorption spectrum is fit using a coupled two-state model in which both symmetric and asymmetric coordinates are included. The physical meaning of the observed resonance Raman intensity trends is discussed along with the origin of the coordinate-dependent coupling. The well-separated mixed valence excited state spectroscopic components enable detailed electronic and resonance Raman data to be obtained from which the model can be more fully developed and tested.

Spectroscopic consequences of a mixed valence excited state: quantitative treatment of a dihydrazine diradical dication.

A model for the quantitative treatment of molecular systems possessing mixed valence excited states is introduced and used to explain observed spectroscopic consequences. The specific example studied in this paper is 1,4-bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)-2,3,5,6-tetramethylbenzene-1,4-diyl dication. The lowest energy excited state of this molecule arises from a transition from the ground state where one positive charge is associated with each of the hydrazine units, to an excited state where both charges are associated with one of the hydrazine units, that is, a Hy-to-Hy charge transfer. The resulting excited state is a Class II mixed valence molecule. The electronic emission and absorption spectra, and resonance Raman spectra, of this molecule are reported. The lowest energy absorption band is asymmetric with a weak low-energy shoulder and an intense higher energy peak. Emission is observed at low temperature. The details of the absorption and emission spectra are calculated for the coupled surfaces by using the time-dependent theory of spectroscopy. The calculations are carried out in the diabatic basis, but the nuclear kinetic energy is explicitly included and the calculations are exact quantum calculations of the model Hamiltonian. Because the transition involves the transfer of an electron from the hydrazine on one side of the molecule to the hydrazine on the other side and vice versa, the two transitions are antiparallel and the transition dipole moments have opposite signs. Upon transformation to the adiabatic basis, the dipole moment for the transition to the highest energy adiabatic surface is nonzero, but that for the transition to the lowest surface changes sign at the origin. The energy separation between the two components of the absorption spectrum is twice the coupling between the diabatic basis states. The bandwidths of the electronic spectra are caused by progressions in totally symmetric modes as well as progressions in the modes along the coupled coordinate. The totally symmetric modes are modeled as displaced harmonic oscillators; the frequencies and displacements are determined from resonance Raman spectra. The absorption, emission, and Raman spectra are fit simultaneously with one parameter set. The coupling in the excited electronic state H(ab)(ex) is 2000 cm(-1). Excited-state mixed valence is expected to be an important contributor to the electronic spectra of many organic and inorganic compounds. The energy separations and relative intensities enable the excited-state properties to be calculated as shown in this paper, and the spectra provide new information for probing and understanding coupling in mixed valence systems.