Spin-orbit effects on the photophysical properties of Ru(bpy)32+

The Role of Spin-Orbit Coupling in the Linear Absorption Spectrum and Intersystem Crossing Rate Coefficients of Ruthenium Polypyridyl Dyes

The successful use of molecular dyes for solar energy conversion requires efficient charge injection, which in turn requires the formation of states with sufficiently long lifetimes (e.g., triplets). The molecular structure elements that confer this property can be found empirically, however computational predictions using ab initio electronic structure methods are invaluable to identify structure-property relations for dye sensitizers. The primary challenge for simulations to elucidate the electronic and nuclear origins of these properties is a spin-orbit interaction which drives transitions between electronic states. In this work, we present a computational analysis of the spin-orbit corrected linear absorption cross sections and intersystem crossing rate coefficients for a derivative set of phosphonated tris(2,2'-bipyridine)ruthenium(2+) dye molecules. After sampling the ground state vibrational distributions, the predicted linear absorption cross sections indicate that the mixture between singlet and triplet states plays a crucial role in defining the line shape of the metal-to-ligand charge transfer bands in these derivatives. Additionally, an analysis of the intersystem crossing rate coefficients suggests that transitions from the singlet into the triplet manifolds are ultrafast with rate coefficients on the order of 1013 s-1 for each dye molecule.

Ab initio multireference calculation of electronic spectra of the osmium complexes, [Os(bpy) 3 ] 2 + and [Os(phen) 3 ] 2 +

The spin-orbit coupling corrected absorption spectra of osmium complexes, [Os(bpy)   3 ]   2 + and [Os(phen)   3 ]   2 + , were calculated by using ab initio multireference perturbation method (NEVPT2) with relativistic effects taken into account throughout ZORA approximation and corresponding all-electron basis sets. For the same purpose, the time-dependent DFT techniques were used. A very good agreement between NEVPT2 and experimental spectra should be highlighted, especially for the MLCT transitions that occur in visible and near-UV regions ( 16 , 000 - 33 , 000 cm   - 1 ). Moreover, the present study offers description of excited states of titled osmium complexes and their spectra interpretation using molecular orbitals.

Nature of ultrafast dynamics in the lowest-lying singlet excited state of [Ru(bpy)3]2

This work presents mechanisms to rationalize the nature of ultrafast photochemical and photophysical processes on the first singlet metal-ligand charge transfer state (1MLCT1) of the [Ru(bpy)3]2+ complex. The 1MLCT1 state is the lowest-lying singlet excited state and the most important intermediate in the early evolution of photoexcited [Ru(bpy)3]2+*. The results obtained from simple but interpretable theoretical models show that the 1MLCT1 state can be very quickly formed via both direct photo-excitation and internal conversions and then can efficiently relax to its equilibrium geometry in ca. 5 fs. The interligand electron transfer (ILET) on the potential energy surface of the 1MLCT1 state is also extremely fast, with a rate constant of ca. 1.38 × 1013 s-1. The ultrafast ILET implies that the excited electron can dynamically delocalize over the three bpy ligands, despite the fact that the excited electron may be localized on either one of the three ligands at the equilibrium geometries of the three symmetric equivalent minima. Since rapid ILET essentially suggests delocalization, the long-standing controversy in inorganic photophysics-whether the excited electron is localized or delocalized-may therefore be calmed down to some extent.

High Intrinsic Phosphorescence Efficiency and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with π-Aromatic-Rich Cyclometalated Ligands: Attributions of Spin-Orbit Coupling Perturbation and Efficient Configurational Mixing of Singlet Excited States

A series of π-aromatic-rich cyclometalated ruthenium(II)-(2,2'-bipyridine) complexes ([Ru(bpy)2(πAr-CM)]+) in which πAr-CM is diphenylpyrazine or 1-phenylisoquinoline were prepared. The [Ru(bpy)2(πAr-CM)]+ complexes had remarkably high phosphorescence rate constants, k RAD(p), and the intrinsic phosphorescence efficiencies (ιem(p) = k RAD(p)/(νem(p))3) of these complexes were found to be twice the magnitudes of simply constructed cyclometalated ruthenium(II) complexes ([Ru(bpy)2(sc-CM)]+), where νem(p) is the phosphorescence frequency and sc-CM is 2-phenylpyridine, benzo[h]quinoline, or 2-phenylpyrimidine. Density functional theory (DFT) modeling of the [Ru(bpy)2(CM)]+ complexes indicated numerous singlet metal-to-ligand charge transfers for 1MLCT-(Ru-bpy) and 1MLCT-(Ru-CM), excited states in the low-energy absorption band and 1ππ*-(aromatic ligand) (1ππ*-LAr) excited states in the high-energy band. DFT modeling of these complexes also indicated phosphorescence-emitting state (Te) configurations with primary MLCT-(Ru-bpy) characteristics. The variation in ιem(p) for the spin-forbidden Te (3MLCT-(Ru-bpy)) excited state of the complex system that was examined in this study can be understood through the spin-orbit coupling (SOC)-mediated sum of intensity stealing (∑SOCM-IS) contribution from the primary intensity of the low-energy 1MLCT states and second-order intensity perturbation from the significant configuration between the low-energy 1MLCT and high-energy intense 1ππ*-LAr states. In addition, the observation of unusually high ιem(p) magnitudes for these [Ru(bpy)2(πAr-CM)]+ complexes can be attributed to the values for both intensity factors in the ∑SOCM-IS formalism being individually greater than those for [Ru(bpy)2(sc-CM)]+ ions.

Electronic States of Tris(bipyridine) Ruthenium(II) Complexes in Neat Solid Films Investigated by Electroabsorption Spectroscopy

We present the electric field-induced absorption (electroabsorption, EA) spectra of the solid neat films of tris(bipyridine) Ru(II) complexes, which were recently functionalized in our group as photosensitizers in dye-sensitized solar cells, and we compare them with the results obtained for an archetypal [Ru(bpy)3]2+ ion (RBY). We argue that it is difficult to establish a unique set of molecular parameter values by discrete parametrization of the EA spectra under the Liptay formalism for non-degenerate excited states. Therefore, the experimental EA spectra are compared with the spectra computed by the TDDFT (time-dependent density-functional theory) method, which for the first time explains the mechanism of electroabsorption in tris(bipyridine) Ru complexes without any additional assumptions about the spectral lineshape of the EA signal. We have shown that the main EA feature, in a form close to the absorption second derivative observed in the spectral range of the first MLCT (metal-to-ligand charge transfer) absorption band in Ru(bpy)3(PF6)2, can be attributed to a delocalized and orbitally degenerate excited state. This result may have key implications for the EA mechanism in RBY-based systems that exhibit similar EA spectra due to the robust nature of MLCT electronic states in such systems.

String-Attached Oligothiophene Substituents Determine the Fate of Excited States in Ruthenium Complexes for Photodynamic Therapy

We explore the photophysical properties of a family of Ru(II) complexes, Ru-ip-nT, designed as photosensitizers (PSs) for photodynamic therapy (PDT). The complexes incorporate a 1H-imidazo[4,5-f][1,10]-phenanthroline (ip) ligand appended to one or more thiophene rings. One of the complexes studied herein, Ru-ip-3T (known as TLD1433), is currently in phase II human clinical trials for treating bladder cancer by PDT. The potent photocytotoxicity of Ru-ip-3T is attributed to a long-lived intraligand charge-transfer triplet state. The accessibility of this state changes upon varying the length (n) of the oligothiophene substituent. In this paper, we highlight the impact of n on the ultrafast photoinduced dynamics in Ru-ip-nT, leading to the formation of the function-determining long-lived state. Femtosecond time-resolved transient absorption combined with resonance Raman data was used to map the excited-state relaxation processes from the Franck-Condon point of absorption to the formation of the lowest-energy triplet excited state, which is a triplet metal-to-ligand charge-transfer excited state for Ru-ip-0T-1T and an oligothienyl-localized triplet intraligand charge-transfer excited state for Ru-ip-2T-4T. We establish the structure-activity relationships with regard to changes in the excited-state dynamics as a function of thiophene chain length, which alters the photophysics of the complexes and presumably impacts the photocytotoxicity of these PSs.

Understanding of [RuL(ONO)]n+ acting as nitric oxide precursor, a theoretical study of ruthenium complexes of 1,4,8,11-tetraazacyclo- tetradecane having different substituents: How spin multiplicity influences bond angle and bond lengths (Ru-O-NO) in releasing of NO

Generation of nitric oxide has been a great interest in cell biology as it involves a wide range of physiological functions including the blood pressure control; thus the exploitation of ruthenium chemistry has been motivated in biochemical and clinical points of view. Herein, the structural and electronic properties of ruthenium(II) complexes of 1,4,8,11-tetraazacyclotetradecane containing pyridyl, imidazole and benzimidazole (L¹, L², L³) were analyzed theoretically in the context of how spin multiplicity plays a crucial role influencing the NO release from the LRu-ONO moiety. The results show that β-cleavage of nitrito in the complex motivates the release of NO as it depends highly on total spin multiplicity of metal ion altering significantly the geometrical parameters; particularly, a decrease of bond length of Ru-ONO is highly associated with an increase of RuO-NO bond distance that correlates with the decrease of the Ru-O-NO bond angle ultimately leading to the release of NO; apparently, the bending nature of Ru-O-NO defines its release from the complex. This is consistent with orbital energy (d_x²-_y²) where the stabilization of axial Ru-O bond in the complex was observed, and proved by molecular orbital studies. In the excitation of the complex (singlet to triplet or singlet to quintet), the NO release has been facilitated, agreeing with the Gibbs free energy data where a lower energy for NO release was obtained compared to other types of excitations. In the calculated electronic spectra, a visible broad band with relatively high intensity for [RuL¹ONO]⁺ was observed, agreeing approximately with reported experimental results.

Quantitative calculations of the non-radiative rate of phosphorescent Ir(III) complexes

It has recently been proposed that the dominant non-radiative decay mechanism in blue Ir(iii) phosphors at room temperature is due to the low-lying non-radiative metal-centred triplet states. These are populated thermally via an activated transition from the highly radiative metal-to-ligand-charge-transfer states that are initially populated due to intersystem crossing following the radiative or electronic excitation of the phosphor. We apply transition state theory to quantitatively calculate the non-radiative decay rate of a family of Ir(iii) complexes containing N-heterocyclic carbene (NHC) ligands. We compare the, computationally inexpensive, one-dimensional theory with the, more accurate, multi-dimensional theory. Both methods find a non-radiative rate with an Arrhenius form (knr = kae-ΔE/kBT). The pre-exponential factors, ka, and activation energies, ΔE, are evaluated via density functional theory (DFT). The multi-dimensional theory shows that there is an order of magnitude variation in ka within this family of materials (between 3 × 1011 s-1 and 3 × 1012 s-1). This is not captured by the one-dimensional theory, which predicts very uniform rate constants in the middle of this range (∼1012 s-1). Nevertheless, the activated process involved, and the linear relationship between ka and knr, mean that ka plays a subtle role in determining knr. Consistent with this we find that both methods capture the trend observed experimentally in the non-radiative rates. Furthermore, the magnitude of the calculated knr is similar in both methods and in good agreement with experimental values [except for one complex with a very shallow activation barrier (<0.1 eV)]. It has previously been demonstrated that radiative decay rates can be accurately calculated from DFT. Combined with our results for the non-radiative rates, this implies that DFT methods can accurately predict the emission efficiency in Ir(iii) phosphors. Therefore, DFT calculations are both fast and accurate enough to play a significant role in the design of new deep blue Ir(iii) phosphors with high emission efficiency. Even the one-dimensional theory provides reasonable agreement with experiment. This suggests that a funneling approach - where only the best performing molecules, according to the one-dimensional theory, are studied in the more laborious multi-dimensional framework - could be a powerful strategy for designing active materials for phosphorescent organic light-emitting diodes (PHOLEDs) from first principles.

A model electronic Hamiltonian to describe low-lying d-d and metal-to-ligand charge-transfer excited states of [Fe(bpy)3 ]2

A simple practical method to compute both d-d and metal-to-ligand charge-transfer (MLCT) excited states of iron(II) polypyridyl complexes is proposed for use in simulation studies. Specifically, a model electronic Hamiltonian developed previously for d-d excited states of [Fe(bpy)₃ ]²⁺ is extended to deal with low-lying MLCT excited states simultaneously by including the MLCT electronic configurations into the basis functions of the model Hamiltonian. As a first attempt, parameters in the model Hamiltonian matrix elements are determined by using density functional theory (DFT) and time-dependent (TD-)DFT calculation results as benchmarks. To examine the performance of the model Hamiltonian, the potential energy curves along the interpolation between the lowest singlet and quintet state structures are compared to those from the (TD-)DFT calculations and to those from CASPT2 calculations in literature. The electronic absorption spectrum computed through molecular dynamics simulation is compared to the experimental spectrum. The spin-orbit couplings at the ground state structure are also compared to those from wavefunction-based ab initio electronic structure calculations. The results indicate that the constructed model Hamiltonian provides reasonable information on both the low-lying d-d and MLCT excited states of [Fe(bpy)₃ ]²⁺ .

On the Possible Coordination on a 3MC State Itself? Mechanistic Investigation Using DFT-Based Methods

Understanding light-induced ligand exchange processes is key to the design of efficient light-releasing prodrugs or photochemically driven functional molecules. Previous mechanistic investigations had highlighted the pivotal role of metal-centered (MC) excited states in the initial ligand loss step. The question remains whether they are equally important in the subsequent ligand capture step. This article reports the mechanistic study of direct acetonitrile coordination onto a 3MC state of [Ru(bpy)3]2+, leading to [Ru(bpy)2(κ1-bpy)(NCMe)]2+ in a 3MLCT (metal-to-ligand charge transfer) state. Coordination of MeCN is indeed accompanied by the decoordination of one pyridine ring of a bpy ligand. As estimated from Nudged Elastic Band calculations, the energy barrier along the minimum energy path is 20 kcal/mol. Interestingly, the orbital analysis conducted along the reaction path has shown that creation of the metallic vacancy can be achieved by reverting the energetic ordering of key dσ* and bpy-based π* orbitals, resulting in the change of electronic configuration from 3MC to 3MLCT. The approach of the NCMe lone pair contributes to destabilizing the dσ* orbital by electrostatic repulsion.

Low-Temperature Spectra and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with Cyclometalated Ancillary Ligands: The Excited State Spin-Orbit Coupling Origin of Variations in Emission Efficiencies

The 77 K emission spectra of cyclometalated ruthenium(II)-2,2'-bipyridine (CM-Ru-bpy) chromophores are very similar to those of related Ru-bpy complexes with am(m)ine or diimmine ancillary ligands, and density functional theory (DFT) modeling confirms that the lowest energy triplet metal to ligand charge transfer (³MLCT) excited states of CM-Ru-bpy and related Ru-bpy complexes have very similar electronic configurations. However, the phosphorescence decay efficiencies of CM-Ru-bpy excited states are about twice those of the conventional Ru-bpy analogues. In contrast to the similar ³MLCT excited state electronic configurations of the two classes of complexes, the CM-Ru-bpy chromophores have much broader visible region MLCT absorptions resulting from several overlapping transitions, even at 87 K. The emitting excited-state emission efficiencies depend on spin-orbit coupling (SOC) mediated intensity stealing from singlet excited states, and this work explores the relationship between the phosphorescence efficiency and visible region absorption spectra of Ru-bpy ³MLCT excited states in the weak SOC limit. The intrinsic ³MLCT emission efficiency, ι_em, depends on mixing with singlet excited states whose Ru^III-dπ-orbital angular momenta differ from that of the emitting state. DFT modeling of the ¹MLCT excited-state electronic configurations that contribute significantly to the lowest energy absorption bands have Ru^III-dπ orbitals that differ from those of their emitting ³MLCT excited states. This leads to a very close relationship between ι_em and the lowest energy MLCT band absorptivities in Ru-bpy chromophores. Thus, the larger number of ¹MLCT transitions that contribute to the lowest energy absorption bands accounts for the enhanced phosphorescence efficiency of Ru-bpy complexes with cyclometalated ancillary ligands.

Solute-solvent electronic interaction is responsible for initial charge separation in ruthenium complexes [Ru(bpy)3]2+ and [Ru(phen)3]2+

Origin of the initial charge separation in optically-excited Ruthenium(II) tris(bidentate) complexes of intrinsic D3 symmetry has remained a disputed issue for decades. Here we measure the femtosecond two-photon absorption (2PA) cross section spectra of [Ru(2,2′-bipyridine)3]2 and [Ru(1,10-phenanthroline)3]2 in a series of solvents with varying polarity and show that for vertical transitions to the lower-energy 1MLCT excited state, the permanent electric dipole moment change is nearly solvent-independent, Δμ = 5.1–6.3 D and 5.3–5.9 D, respectively. Comparison of experimental results with quantum-chemical calculations of complexes in the gas phase, in a polarizable dielectric continuum and in solute-solvent clusters containing up to 18 explicit solvent molecules indicate that the non-vanishing permanent dipole moment change in the nominally double-degenerate E-symmetry state is caused by the solute-solvent interaction twisting the two constituent dipoles out of their original opposite orientation, with average angles matching the experimental two-photon polarization ratio.

A Key Factor Dominating the Competition between Photolysis and Photoracemization of [Ru(bipy)3]2+ and [Ru(phen)3]2+ Complexes

Photolysis and photoracemization are two important photochemical phenomena of the prototype complexes [Ru(bipy)₃]²⁺ and [Ru(phen)₃]²⁺ (bipy = 2,2'-bipyridine, phen = 1,10-phenanthroline), but little is known about their relations. To solve this issue, the photoinduced chiral inversion Δ⇌Λ of the complexes was analyzed theoretically. The results indicated that the photoracemization reaction proceeds on the lowest triplet potential energy surface in three steps ³CT_Δ↔³MC_Δ, ³MC_Δ↔³MC_Λ, and ³MC_Λ↔³CT_Λ (CT = charge transfer state; MC = metal-centered state). Where the first and third steps are fast processes of picoseconds, the second is the rate-determining step (RDS) of microseconds. Such a slow step for the racemization leads to the excited molecule lingering around the bottom of ³MC state after the first step and, therefore, greatly enhances the possibility of deexcitation and photolysis mostly at the triplet-singlet crossing point. In other words, the photoracemization and photolysis of the complexes have a competition relation, not a slave relation as assumed by the photoracemization model suggested in literature. They are dominated by the RDS. This conclusion is also consistent with the Δ(δ S)⇌Λ(δ S) chiral inversion of the [Ru(bipy)₂(L-ser)]⁺ series complexes, which is reversible with no detectable photolysis, as its second step is a fast one. Note that, although the photoracemization of the prototype complexes is very slow, it passes through the three steps reversibly and ends with a photon emitting, which could be detected with the time-resolved circularly polarized luminescence and related techniques. These findings are helpful to understand and control the photochemical behavior of the complexes in practice.

Theoretical Study and Design of Phosphorescent Cyclometalated (C∧C*)PtII(acac) Complexes: The Substituent Effect Controls the Radiative and Nonradiative Decay Processes

Density functional theory (DFT) and time-dependent DFT calculations were performed to evaluate the influence of substituent effect of (1) R = 4-Me, (2) R = 4-OMe, and (3) R = 2,3-OC₆H₄ on the phenyl ring of (C^∧C*)Pt^II(acac) (C^∧C* = phenylimidazole, acac = acetylacetone), respectively, on absorption and phosphorescent spectra properties, as well as the radiative and nonradiative processes. We found that emissions of complexes 2 and 3 originate from the Kasha-like T₁ state, whereas that of complex 1 originates from non-Kasha T₂ state. Compared with the emission of complex 1, the emission peaks of 2 and 3 are red-shifted, which is attributed to p-π and π-π conjugation effects resulting from the electron-donating groups -OCH₃ and -OC₆H₄ with ligand C^∧C*, respectively. The radiative rate constants (κ_r) of 2 and 3 are larger than that of 1, namely, κ_r(1) < κ_r(2) < κ_r(3), indicating that κ_r can be efficiently increased by enlarging π-conjugation at the main ligand of (C^∧C*)Pt^II(acac), which can cause the increase of spin-orbit coupling (SOC) matrix elements. At the same time, the activation energy barriers for the rate-limiting step can be largely raised accompanied by enlarging the ability of electron-donation of the substituent group at the main ligand of (C^∧C*)Pt^II(acac), which can cause the decrease of the nonradiative rate constant (κ_nr), namely, κ_nr(1) > κ_nr(2) > κ_nr(3). According to Φ_P = κ_r/(κ_r + κ_nr), the quantum yields should have the sequence Φ_P(1) < Φ_P(2) < Φ_P(3), which is in accordance with the experiment. In addition, to guide experimental synthesis of highly efficient (C^∧C*)Pt^II(acac), a new complex 4 through extending the π-conjugation in the C^∧C* ligand of (C^∧C*)Pt^II(acac) was theoretically designed, which has a larger quantum yield than 1-3.

Excited state relaxation processes of H2-evolving Ru-Pd supramolecular photocatalysts containing a linear or non-linear bridge: a DFT and TDDFT study

In this study, the early-time excited state relaxation processes of bimetallic Ru-Pd supramolecular photocatalysts containing a linear 2,2':5',2''-terpyridine or a nonlinear 2,2':6',2''-terpyridine bridging ligand (BL) were investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) approaches. The bridge based metal-to-ligand charge transfer triplet (³MLCT) state of the metal complex containing a linear bridging ligand was calculated to be the lowest energy triplet (T₁) state which is closely related to the photocatalytic H₂ production, while for that having a nonlinear bridging ligand, the T₁ state is a Ru metal-centered (MC) triplet (³MC_Ru) state that is short-lived and rapidly decays to the ground electronic state (S₀). Our simulation provides an alternative explanation for the smaller interligand electron transfer (ILET) rate in the Ru-Pd complex containing a linear bridge compared to the corresponding monometal Ru complex. Based on the calculation, we also suggest that the successive ³MLCT → ³MC_Ru → S₀ conversion is responsible for the inefficiency of the Ru-Pd complex containing nonlinear bridge as a photocatalyst for H₂ production. This study provides theoretical insights into the key steps of the photoinduced processes of the bimetallic H₂-evolving supramolecular photocatalyst.

Solvation structure around ruthenium(II) tris(bipyridine) in lithium halide solutions

The solvation of the ruthenium(II) tris(bipyridine) ion ([Ru(bpy)3](2+)) is investigated with molecular dynamics simulations of lithium halide solutions in polar solvents. The anion distribution around the [Ru(bpy)3](2+) complex exhibits a strong solvent dependence. In aqueous solution, the iodide ion forms a solvent shared complex with [Ru(bpy)3](2+), but not in the other solvents. Between Cl(-) and [Ru(bpy)3](2+), the strong hydration of the chloride ion results in a solvent separated complex where more than one solvent molecule separates the anion from the metal center. Hence, tailored solvation properties in electrolytes is a route to influence ion-ion interactions and related electron transfer processes.

Excited state decay of cyclometalated polypyridine ruthenium complexes: insight from theory and experiment

This perspective article tackles the open question why cyclometalated polypyridine ruthenium(ii) complexes typically only emit very weakly at room temperature and delivers answers beyond the standard schemes involving ³MC and tunneling decay channels.

Excited-State Proton Transfer Controls Irreversibility of Photoisomerization in Mononuclear Ruthenium(II) Monoaquo Complexes: A DFT Study

The detailed DFT investigation clears the working mechanism of the irreversible photoisomerization of trans-[Ru(tpy)(pynp)(OH2)](2+) (TA) and cis-[Ru(tpy)(pynp)(OH2)](2+) (CA) complexes. Both TA and CA complexes present two types of low lying triplet states, one resulting from a triplet metal-ligand charge-transfer (TMLCT) and the other from a triplet metal-centered d-d transition (TMC). The vertical excitation of the singlet ground state of the complexes leads to a singlet excited state, which undergoes ultrafast decay to the corresponding TMLCT. For TA, this TMLCT transforms with a low barrier to a TMC state. The dissociative nature of the TMC state leads to easy water removal to produce a five-coordinate intermediate that can isomerize via rotation of a pynp ligand and proceed towards the CA product. For CA, however, during this excitation and intersystem crossing process, an excited-state proton transfer (ESPT) occurs and the resultant TMLCT is very much stabilized with a very strong Ru(II)-OH bond; the high barrier from this TMLCT blocks conversion to a TMC state and thus prevents isomerization from the cis to the trans isomer. This high barrier also prevents the possibility of the isomerization process from TA to CA solely on the adiabatic triplet pathway. Instead, crossing points (XMC-CB, XMC-CA) near the minimum of the triplet metal-centered state of the cis isomer provide nonadiabatic decay channels to the ground-state S0--CA, which completes the photoisomerization pathway from TA to CA.

A new photoactive Ru(ii)tris(2,2′-bipyridine) templated Zn(ii) benzene-1,4-dicarboxylate metal organic framework: structure and photophysical properties

It has now been demonstrated that Ru(ii)tris(2,2′-bipyridine) (RuBpy) can be utilized to template the formation of new metal organic framework (MOF) materials containing crystallographically resolved RuBpy clusters with unique photophysical properties.

A density functional theory and spectroscopic study of intramolecular quenching of metal-to-ligand charge-transfer excited states in some mono-bipyridine ruthenium(II) complexes

The complexes [Ru(NCCH3)4bpy]2+ and [Ru([14]aneS4)bpy]2+ ([14]aneS4 = 1,4,8,11-tetrathiacyclotetradecane, bpy = 2,2′-bipyridine) have similar absorption and emission spectra but the 77 K metal-to-ligand charge-transfer (MLCT) excited state emission lifetime of the latter is less than 0.3% that of the former. Density functional theory modeling of the lowest energy triplet excited states indicates that triplet metal centered (3MC) excited states are about 3500 cm−1 lower in energy than their 3MLCT excited states in both complexes. The differences in excited state lifetimes arise from a much larger coordination sphere distortion for [Ru(NCCH3)4bpy]2+ and the associated larger reorganizational barrier for intramolecular electron transfer. The smaller ruthenium ligand distortions of the [Ru([14]aneS4)bpy]2+ complex are apparently a consequence of stereochemical constraints imposed by the macrocyclic [14]aneS4 ligand, and the 3MC excited state calculated for the unconstrained [Ru(S(CH3)2)4bpy]2+ complex (S(CH3)2 = dimethyl sulfide) is distorted in a manner similar to that of [Ru(NCCH3)4bpy]2+. Despite the lower energy calculated for its 3MC than 3MLCT excited state, [Ru(NCCH3)4bpy]2+ emits strongly in 77 K glasses with an emission quantum yield of 0.47. The emission is biphasic with about a 1 μs lifetime for its dominant (86%) emission component. The 405 nm excitation used in these studies results in a significant amount of photodecomposition in the 77 K glasses. This is a temperature-dependent biphotonic process that most likely involves the bipyridine-radical anionic moiety of the 3MLCT excited state. A smaller than expected value found for the radiative rate constant is consistent with a lower energy 3MC than 3MLCT state.

Unravelling the S → O linkage photoisomerization mechanisms in cis- and trans-[Ru(bpy)2(DMSO)2](2+) using density functional theory

A mechanistic study of the intramolecular S → O linkage photoisomerization in the cis and trans isomers of [Ru(bpy)2(DMSO)2](2+) was performed using density functional theory. This study reveals that for the cis isomer the linkage photoisomerization of the two DMSO ligands occurs sequentially in the lowest triplet excited state and can either be achieved by a one-photon or by a two-photon mechanism. A mechanistic picture of the S → O photoisomerization of the trans isomer is also proposed. This work especially highlights that both adiabatic and nonadiabatic processes are involved in these mechanisms and that their coexistence is responsible for the rich photophysics and photochemical properties observed experimentally for the studied complexes. The different luminescent behavior experimentally observed at low temperature between the cis and trans isomers is rationalized based on the peculiarity of the topology of the triplet excited-state potential energy surfaces.