Medium Effects on Charge Transfer in Metal Complexes

Synthesis, photophysical characterisation, quantum-chemical study and in vitro antiproliferative activity of cyclometalated Ir(III) complexes based on 3,5-dimethyl-1-phenyl-1H-pyrazole and N,N-donor ligands

In this paper, we present the synthesis of four new complexes: the dimeric precursor [Ir(dmppz)2(μ-Cl)]2 (1) (Hdmppz - 3,5-dimethyl-1-phenyl-1H-pyrazole) and heteroleptic bis-cyclometalated complexes: [Ir(dmppz)2(Py2CO)]PF6·½CH2Cl2 (2), [Ir(dmppz)2(H2biim)]PF6·H2O (3), and [Ir(dmppz)2(PyBIm)]PF6 (4), with auxiliary N,N-donor ligands: 2-di(pyridyl)ketone (Py2CO), 2,2'-biimidazole (H2biim) and 2-(2'-pyridyl)benzimidazole (PyBIm). In the obtained complexes, SC-X-ray analysis revealed that Ir(III) has an octahedral coordination sphere with chromophores of the type {IrN2C2Cl2} (1) or {IrN4C2} (2-4). The complexes obtained, which have been fully characterised by physicochemical methods (CHN, TG, FTIR, UV-Vis, PL and 1H, 13C, 15N NMR), were used to continue our studies on the factors influencing the cytotoxic properties of potential chemotherapeutic agents (in vitro). To this end, the following studies are presented: (i) comparative analysis of the effects on the biological properties of N,N-donor ligands and C,N-donor ligands in the studied complexes, (ii) studies of the interactions of the compounds with the selected molecular target: DNA and BSA (UV-Vis, CD and PL methods), (iii) and the reactivity towards redox molecules: GSH, NADH (UV-Vis and/or ESI-MS methods), (iv) cytotoxic activity (IC50) of potential chemotherapeutics against MCF-7, K-562 and CCRF-CEM cell lines.

Poly(dehydroalanine)-Based Hydrogels as Efficient Soft Matter Matrices for Light-Driven Catalysis

Soft matter integration of photosensitizers and catalysts provides promising solutions to developing sustainable materials for energy conversion. Particularly, hydrogels bring unique benefits, such as spatial control and 3D-accessibility of molecular units, as well as recyclability. Herein, the preparation of polyampholyte hydrogels based on poly(dehydroalanine) (PDha) is reported. Chemically crosslinked PDha with bis-epoxy poly(ethylene glycol) leads to a transparent, self-supporting hydrogel. Due to the ionizable groups on PDha, this 3D polymeric matrix can be anionic, cationic, or zwitterionic depending on the pH value, and its high density of dynamic charges has a potential for electrostatic attachment of charged molecules. The integration of the cationic molecular photosensitizer [Ru(bpy)3 ]2+ (bpy = 2,2'-bipyridine) is realized, which is a reversible process controlled by pH, leading to light harvesting hydrogels. They are further combined with either a thiomolybdate catalyst ([Mo3 S13 ]2- ) for hydrogen evolution reaction (HER) or a cobalt polyoxometalate catalyst (Co4 POM = [Co4 (H2 O)2 (PW9 O34 )2 ]10- ) for oxygen evolution reaction (OER). Under the optimized condition, the resulting hydrogels show catalytic activity in both cases upon visible light irradiation. In the case of OER, higher photosensitizer stability is observed compared to homogeneous systems, as the polymer environment seems to influence decomposition pathways.

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.

Observation of parallel intersystem crossing and charge transfer-state dynamics in [Fe(bpy)3]2+ from ultrafast 2D electronic spectroscopy

Transition metal-based charge-transfer complexes represent a broad class of inorganic compounds with diverse photochemical applications. Charge-transfer complexes based on earth-abundant elements have been of increasing interest, particularly the canonical [Fe(bpy)3]2+. Photoexcitation into the singlet metal-ligand charge transfer (1MLCT) state is followed by relaxation first to the ligand-field manifold and then to the ground state. While these dynamics have been well-studied, processes within the MLCT manifold that facilitate and/or compete with relaxation have been more elusive. We applied ultrafast two-dimensional electronic spectroscopy (2DES) to disentangle the dynamics immediately following MLCT excitation of this compound. First, dynamics ascribed to relaxation out of the initially formed 1MLCT state was found to correlate with the inertial response time of the solvent. Second, the additional dimension of the 2D spectra revealed a peak consistent with a ∼20 fs 1MLCT → 3MLCT intersystem crossing process. These two observations indicate that the complex simultaneously undergoes intersystem crossing and direct conversion to ligand-field state(s). Resolution of these parallel pathways in this prototypical earth-abundant complex highlights the ability of 2DES to deconvolve the otherwise obscured excited-state dynamics of charge-transfer complexes.

Intramolecular Through-Space Double-Electron Transfer Between A Pair of Redox-Active Guanidine Units Aligned by Dithiolate Bridges

Using unconventional synthesis protocols, two redox-active triguanidine units are connected by a dithiolate bridge, aligning the two redox-active units in close proximity. The reduced, neutral and the tetracationic redox states with two dicationic triguanidine units are isolated and fully characterized. Then, the dicationic redox states are prepared by mixing the neutral and tetracationic molecules. At low temperatures, the dications are diamagnetic (singlet ground state) with two different triguanidine units (neutral and dicationic). At room temperature, the triplet state with two radical monocationic triguanidine units is populated. At low temperature (210 K), chemical exchange by intramolecular through-space electron-transfer between the two triguanidine units is evidenced by EXSY NMR spectroscopy. Intramolecular through-space transfer of two electrons from the neutral to the dicationic triguanidine unit is accompanied by migration of the counterions in opposite direction. The rate of double-electron transfer critically depends on the bridge. No electron-transfer is measured in the absence of a bridge (in a mixture of one dicationic and one neutral triguanidine), and relatively slow electron transfer if the bridge does not allow the two triguanidine units to approach each other close enough. The results give detailed, quantitative insight into the factors that influence intramolecular through-space double-electron-transfer processes.

Design Strategies for Luminescent Titanocenes: Improving the Photoluminescence and Photostability of Arylethynyltitanocenes

Complexes that undergo ligand-to-metal charge transfer (LMCT) to d0 metals are of interest as possible photocatalysts. Cp2Ti(C2Ph)2 (where C2Ph = phenylethynyl) was reported to be weakly emissive in room-temperature (RT) fluid solution from its phenylethynyl-to-Ti 3LMCT state but readily photodecomposes. Coordination of CuX between the alkyne ligands to give Cp2Ti(C2Ph)2CuX (X = Cl, Br) has been shown to significantly increase the photostability, but such complexes are not emissive in RT solution. Herein, we investigate whether inhibition of alkyne-Ti-alkyne bond compression might be responsible for the increased photostability of the CuX complexes by investigating the decomposition of a structurally constrained analogue, Cp2Ti(OBET) (OBET = o-bis(ethynyl)tolane). To investigate the mechanism of nonradiative decay from the 3LMCT states in Cp2Ti(C2Ph)2CuX, the photophysical properties were investigated both upon deuteration and upon rigidifying in a poly(methyl methacrylate) film. These investigations suggested that inhibition of structural rearrangement may play a dominant role in increasing emission lifetimes and quantum yields. The bulkier Cp*2Ti(C2Ph)2CuBr was prepared and is emissive at 693 nm in RT THF solution with a photoluminescent quantum yield of 1.3 × 10-3 (τ = 0.18 μs). Time-dependent density functional theory (TDDFT) calculations suggest that emission occurs from a 3LMCT state dominated by Cp*-to-Ti charge transfer.

Metal-Metal Charge Transfer Properties of a Series of Trinuclear Fe2 Ru and Corresponding Pentanuclear Fe2 Ru2 Ag Cyanido-Bridged Complexes

A series of trimetallic cyanidometal-bridged compounds [Men Cp(dppe)FeII -(μ-NC)-RuII (MeOpy)4 -(μ-CN)-FeII (dppe)CpMen ] - [PF6 ]2 (N[PF6 ]2 , n=0, N =1; n=1, N=2; n=3, N=3; Cp=cyclopentadiene, dppe=1,2-bis(diphenylphosphino)ethane, MeOpy=4-methoxypyridine) and their one- and two-electron oxidized compounds N3+ and N4+ were synthesized and characterized. Meanwhile, a series of corresponding linear cyanido-bridged pentanuclear compounds [Men Cp(dppe)FeIII -(μ-NC)-RuII (MeOpy)4 -(μ-NC)-AgI -(μ-CN)-RuII (MeOpy)4 -(μ-CN)-FeIII (dppe)CpMen ][BF4 ]5 (M[BF4 ]5 , n=0, M=4; n=1, M=5; n=3, M=6) were also obtained and well characterized. The investigations suggest that in the trinuclear system there exists remote interaction between the two Fe centers, but no significant interactions exist across the central silver unit between the metals on the two sides of the silver center in the pentanuclear system. In both the trinuclear N4+ and the pentanuclear M5+ complexes, there exists the neighboring RuII →FeIII MM'CT transitions, and the MM'CT energy in the corresponding trinuclear system is higher than those in the pentanuclear system in which no remote metal-metal interaction occurs. Meanwhile, as the substituted methyl groups on the cyclopentadiene increases, the redox potential of the ruthenium in the trinuclear N4+ series increases, but that in the pentanuclear M5+ complexes decreases.

Mechanism of Oxygen Quenching of the Excited States of Heteroleptic Chromium(III) Phenanthroline Derivatives

In this study, we report the synthesis and characterization of some heteroleptic Cr(III) complexes of the form [Cr(Phen)2L](OTf)3, where Phen = 1,10-phenanthroline and L is either 2,2'-bipyridine (bpy) or its derivatives, such as 4,4'-dimethyl-2,2'-bipyridine (4,4'-DMB), 4,4'-dimethoxy-2,2'-bipyridine (4,4'-DMOB), 4,4'-ditert-butyl-2,2'-bipyridine (4,4'-dtbpy), 5,5'-dimethyl-2,2'-bipyridine (5,5'-DMB), 4,4'-dimethoxycarbonyl-2,2'-bipyridine (4,4'-dmcbpy) or 1,10-phenanthroline derivatives, such as 5-methyl-1,10-phenanthroline (5-Me-Phen) and 4,7-dimethyl-1,10-phenanthroline (4,7-DMP). Heteroleptic complexes were prepared in two stages via the intermediate [Cr(Phen)2(CF3SO3)2](CF3SO3) and five examples have been crystallographically characterized. Steady-state absorption and luminescence emission characteristics of these complexes were measured in 1 M HCl solutions. The luminescence quantum yield of these complexes was found to be the lowest for [Cr(Phen)2(4,4'-dmcbpy)](OTf)3 and the highest for [Cr(Phen)2(4,4'-DMB)](OTf)3 with values of 0.31 × 10-2 and 1.48 × 10-2, respectively. The calculated excited state energy, E0-0, was found to vary within the narrow range of 163.1-165.0 kJ mol-1 across the series. Transient absorption spectra in degassed, air-equilibrated, and oxygen-saturated 1 M HCl aqueous solutions were also measured at different time decays and demonstrated no significant differences, indicating the absence of any ion-separated species in the excited state. Excited-state decay traces at the wavelength of maximum absorption were used to calculate oxygen quenching rate constants, kq, which were found to be in the range 3.26-5.27 × 107 M-1 s-1. Singlet oxygen luminescence photosensitized by these complexes was observed in D2O, and its luminescence intensity at 1270 nm was used for the determination of singlet oxygen quantum yields for these complexes, which were in the range of 0.20-0.44, while the fraction of the excited 2E state quenched by oxygen was in the range of 0.22-0.68, and the efficiency of singlet oxygen production was in the range of 0.44-0.90. The mechanism by which the excited 2E state is quenched by oxygen is explained by a spin statistical model that predicts the balance between charge transfer and noncharge transfer deactivation pathways, which was represented by the parameter pCT that was found to vary from 0.35 to 0.68 for this series of Cr(III) complexes.

The Energy Structure of Spin States in Reducing the Nonradiative Voltage Loss in Organic Solar Cells

In organic solar cells (OSCs), the nonradiative voltage loss (ΔVnr) has been identified as a critical factor for the relatively lower open-circuit voltage. Under open-circuit conditions, most of the charge recombination processes occur via the triplet exciton state, underscoring the importance of the energy structures concerning the local exciton (LE) and charge transfer (CT) spin states. In this Letter, we propose a five-state model to explore the spin state energy structures to reduce ΔVnr. Our calculations reveal that, to minimize ΔVnr, the spin singlet state for LE should possess a lower energy than the triplet state, ES1 < ET1. In contrast, the energies of the CT spin states have a negligible effect on ΔVnr. We identify the best energy structure as ES1 < ET1 ∼ ECT1/CT3. Moreover, our calculations demonstrate that strong couplings between these spin states, particularly involving spin flip, can effectively mitigate ΔVnr. These findings present novel insights for the advancement of OSCs.

Controlling Intramolecular and Intermolecular Electronic Coupling of Radical Ligands in a Series of Cobaltoviologen Complexes

Controlling electronic coupling between multiple redox sites is of interest for tuning the electronic properties of molecules and materials. While classic mixed-valence (MV) systems are highly tunable, e.g., via the organic bridges connecting the redox sites, metal-bridged MV systems are difficult to control because the electronics of the metal cannot usually be altered independently of redox-active moieties embedded in its ligands. Herein, this limitation was overcome by varying the donor strengths of ancillary ligands in a series of cobalt complexes without directly perturbing the electronics of viologen-like redox sites bridged by the cobalt ions. The cobaltoviologens [1_X-Co]ⁿ⁺ feature four 4-X-pyridyl donor groups (X = CO₂Me, Cl, H, Me, OMe, NMe₂) that provide gradual electronic tuning of the bridging Co^II centers, while a related complex [2-Co]ⁿ⁺ with NHC donors supports exclusively Co^III states even upon reduction of the viologen units. Electrochemistry and IVCT band analysis indicate that the MV states of these complexes have electronic structures ranging from fully localized ([2-Co]⁴⁺; Robin-Day Class I) to fully delocalized ([1_CO2Me-Co]³⁺; Class III) descriptions, demonstrating unprecedented control over electronic coupling without changing the identity of the redox sites or bridging metal. Additionally, single-crystal XRD characterization of the homovalent complexes [1_H-Co]²⁺ and [1_H-Zn]²⁺ revealed radical-pairing interactions between the viologen ligands of adjacent complexes, representing a type of through-space electronic coupling commonly observed for organic viologen radicals but never before seen in metalloviologens. The extended solid-state packing of these complexes produces 3D networks of radical π-stacking interactions that impart unexpected mechanical flexibility to these crystals.

Cyclometalated platinum(II) complexes with acyclic diaminocarbene ligands for OLED application

A novel series of cyclometalated platinum(II) complexes bearing acyclic diaminocarbene (ADC) ancillary ligands were designed and prepared. Their photophysical properties were systematically studied through experimental and theoretical investigations. All complexes exhibit green phosphorescence with a quantum efficiency of up to 45% in 2 wt% doped PMMA film at room temperature. The complexes are used as light-emitting dopants for organic light-emitting diode (OLED) fabrication. The devices displayed a green emission with a maximum current efficiency of 2.9 cd A^-1 and a luminance of 2700 cd m^-2. These results show that these cyclometalated platinum(II) complexes can be used as efficient green emitting components of OLED devices.

Reorganization energy of electron transfer

The theory of electron transfer reactions establishes the conceptual foundation for redox solution chemistry, electrochemistry, and bioenergetics. Electron and proton transfer across the cellular membrane provide all energy of life gained through natural photosynthesis and mitochondrial respiration. Rates of biological charge transfer set kinetic bottlenecks for biological energy storage. The main system-specific parameter determining the activation barrier for a single electron-transfer hop is the reorganization energy of the medium. Both harvesting of light energy in natural and artificial photosynthesis and efficient electron transport in biological energy chains require reduction of the reorganization energy to allow fast transitions. This review article discusses mechanisms by which small values of the reorganization energy are achieved in protein electron transfer and how similar mechanisms can operate in other media, such as nonpolar and ionic liquids. One of the major mechanisms of reorganization energy reduction is through non-Gibbsian (nonergodic) sampling of the medium configurations on the reaction time. A number of alternative mechanisms, such as electrowetting of active sites of proteins, give rise to non-parabolic free energy surfaces of electron transfer. These mechanisms, and nonequilibrium population of donor-acceptor vibrations, lead to a universal phenomenology of separation between the Stokes shift and variance reorganization energies of electron transfer.

The effects of introducing terminal alkenyl substituents into the 2,2'-bipyridine domain in [Cu(N^N)(P^P)]+ coordination compounds

The N^N chelating ligands 6,6'-bis(but-3-en-1-yl)-2,2'-bipyridine (1), 6-(but-3-en-1-yl)-6'-methyl-2,2'-bipyridine (2), 6,6'-bis(pent-4-en-1-yl)-2,2'-bipyridine (3) and 6-(pent-4-en-1-yl)-6'-methyl-2,2'-bipyridine (4) have been prepared, characterized, and incorporated into the heteroleptic [Cu(N^N)(P^P)][PF₆] complexes in which P^P is either POP (bis(2-(diphenylphosphanyl)phenyl)ether) or xantphos (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane). The eight coordination compounds have been fully characterized, including the single crystal structures of [Cu(1)(xantphos)][PF₆], [Cu(1)(POP)][PF₆]·CH₂Cl₂, [Cu(2)(xantphos)][PF₆], [Cu(2)(POP)][PF₆] and [Cu(3)(POP)][PF₆]·0.5Et₂O. The [Cu(N^N)(P^P)]⁺ cations exhibit a partially reversible or irreversible Cu⁺/Cu²⁺ oxidation at more positive potentials than the benchmark [Cu(bpy)(P^P)]⁺ and [Cu(Me₂bpy)(P^P)]⁺ complexes consistent with the increase in steric hindrance of the terminal alkenyl substituents. When excited in the region of the metal-to-ligand charge transfer (MLCT) absorption, solutions of the [Cu(N^N)(P^P)][PF₆] complexes are weak emitters with λmaxem in the range 565-578 nm. However, powdered samples achieve photoluminescence quantum yields in the range of 28.5 to 62.3%, with the highest PLQY found for [Cu(3)(POP)][PF₆] with an excited-state lifetime, τ, of 16.1 μs. For [Cu(3)(POP)][PF₆], the excited state lifetime was measured in MeTHF at 293 and 77 K, and the increase in τ from 1.77 to 59.4 μs upon cooling supports thermally activated delayed fluorescence (TADF) at ambient temperatures.

Role of the Trifluoropropynyl Ligand in Blue-Shifting Charge-Transfer States in Emissive Pt Diimine Complexes and an Investigation into the PMMA-Imposed Rigidoluminescence and Rigidochromism

Square-planar Pt^II complexes are of interest as dopants for the emissive layer of organic light-emitting diodes. Herein, the photophysics of three Pt bipyridyl complexes with the strongly e^- withdrawing, high-field, 3,3,3-trifluoropropynyl ligand has been investigated. One complex, (phbpy)PtC₂CF₃ (phbpy = 6-phenyl-2,2'-dipyridyl), has also been characterized by single-crystal X-ray diffraction. All complexes reported are emissive in both RT CH₂Cl₂ solution (Φ_PL = 0.007 to 0.027) and PMMA film (Φ_PL = 0.25 to 0.42). The trifluoropropynyl ligand elevates the energy of the MLCT and LL'CT states above that of the IL π-π* state, resulting in IL emission in all cases. The emission energies of the trifluoropropynyl compounds are also blue-shifted relative to the analogous pentafluorophenylethynyl compounds, suggesting that the trifluoropropynyl ligand is one of the most electron-withdrawing alkynyl ligands. Rate constants for radiative and nonradiative deactivation were determined from experimentally determined values of Φ_PL and excited-state lifetimes in both solution and PMMA films. The increase in Φ_PL upon incorporation into PMMA film (rigidoluminescence) results from a decrease in the rate constant for non-radiative relaxation. Experimental activation energies for excited-state decay in combination with TDDFT are consistent with the rigidoluminescence resulting from an increase in the energy of the non-emissive triplet metal-centered state. Two of the complexes investigated, (^Ph2bpy)Pt(C₂CF₃)₂ and (^t-Bu2bpy)Pt(C₂CF₃)₂, where ^t-Bu2bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl and ^Ph2bpy = 4,4'-diphenyl-2,2'-dipyridyl, exhibit concentration-dependent excimer emission (orange) along with monomer emission (blue), enabling fine-tuning of the emission color. However, excimer emission was absent in cured PMMA films up to the solubility limit for solution processing of (^Ph2bpy)Pt(C₂CF₃)₂ in CH₂Cl₂, demonstrating the diffusional nature of excimer formation.

Novel NIR-Phosphorescent Ir(III) Complexes: Synthesis, Characterization and Their Exploration as Lifetime-Based O2 Sensors in Living Cells

A series of [Ir(N^C)2(N^N)]+ NIR-emitting orthometalated complexes (1-7) has been prepared and structurally characterized using elemental analysis, mass-spectrometry, and NMR spectroscopy. The complexes display intense phosphorescence with vibrationally structured emission bands exhibiting the maxima in the range 713-722 nm. The DFT and TD DFT calculations showed that the photophysical characteristics of these complexes are largely determined by the properties of the metalating N^C ligands, with their major contribution into formation of the lowest S1 and T1 excited states responsible for low energy absorption and emission, respectively. Emission lifetimes of 1-7 in degassed methanol solution vary from 1.76 to 5.39 µs and show strong quenching with molecular oxygen to provide an order of magnitude lifetime reduction in aerated solution. The photophysics of two complexes (1 and 7) were studied in model physiological media containing fetal bovine serum (FBS) and Dulbecco's Modified Eagle Medium (DMEM) to give linear Stern-Volmer calibrations with substantially lower oxygen-quenching constants compared to those obtained in methanol solution. These observations were interpreted in terms of the sensors' interaction with albumin, which is an abundant component of FBS and cell media. The studied complexes displayed acceptable cytotoxicity and preferential localization, either in mitochondria (1) or in lysosomes (7) of the CHO-K1 cell line. The results of the phosphorescence lifetime imaging (PLIM) experiments demonstrated considerable variations of the sensors' lifetimes under normoxia and hypoxia conditions and indicated their applicability for semi-quantitative measurements of oxygen concentration in living cells. The complexes' emission in the NIR domain and the excitation spectrum, extending down to ca. 600 nm, also showed that they are promising for use in in vivo studies.

A Single Solvating Benzene Molecule Decouples the Mixed-valence Complex through Intermolecular Orbital Interactions

Characterization of covalency of intermolecular interactions in the van der Waals distance limit remains challenging because the interactions between molecules are weak, dynamic, and not measurable. Herein, we approach this issue in a series of supramolecular mixed-valence (MV) donor(D)-bridge(B)-acceptor(A) systems consisting of two bridged Mo₂ units with a C₆H₆ molecule encapsulated, as characterized by the X-ray crystal structures. Comparative analysis of the intervalence charge transfer spectra in benzene and dichloromethane substantiates the strong electronic decoupling effect of the solvating C₆H₆ molecule that breaks down the dielectric solvation theory. Ab initio and DFT calculations unravel that the intermolecular orbital overlaps between the complex bridge and the C₆H₆ molecule alter the electronic states of the D-B-A molecule through intermolecular nuclear dynamics. This work exemplifies that site-specific intermolecular interaction can be exploited to control the chemical property of supramolecular systems and to elucidate the functionalities of side-chains in biological systems.

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Decoupling mixed-valence complexes by an encapsulated benzene molecule

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Demonstrating intermolecular orbital interactions in the van der Waals distances

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Illustrating interplay between intermolecular electronic and nuclear degrees of freedom

Reorganization Energy of Electron Transfer in Ionic Liquids

Bandshape analysis of charge-transfer optical bands in room-temperature ionic liquids (ILs) was performed to extract the reorganization energy of electron transfer. Remarkably, the reorganization energies in ILs are close to those in cyclohexane. This result runs against common wisdom in the field since conducting ILs, which are characterized by an infinite static dielectric constant, and nonpolar cyclohexane fall to the opposite ends of the polarity scale based on their dielectric constants. Theoretical calculations employing structure factors of ILs from molecular dynamics simulations support the low values of the reorganization energy. Standard dielectric arguments do not apply to solvation in ILs, and nonergodic reorganization energies are required for a quantitative analysis.

The ligand-to-metal charge transfer excited state of [Re(dmpe)3]2

The ligand-to-metal charge transfer (LMCT) transitions of [Re(dmpe)₃]²⁺ (dmpe = bis-1,2-(dimethylphosphino)ethane) were interrogated using UV/Vis absorbance spectroscopy, photoluminescence spectroscopy, and time-dependent density functional theory. The solvent dependence of the lowest energy charge transfer transition was quantified; no solvatochromism was observed. TD-DFT calculations reveal the dominant LMCT transition is highly symmetric and delocalized involving all phopshine ligand donors in the charge transfer, providing an understanding for the absence of solvatochromism of [Re(dmpe)₃]²⁺.

Unusual fluorescence behaviour of a heteroleptic Cu(i) complex featuring strong electron donating groups on a diimine ligand

Synthesis of a novel bulky diimine ligand and its corresponding heteroleptic Cu(i). Unusual fluorescence behavior of a novel Cu(i) complex due to a strong electron-donor diimine ligand.

A Class III asymmetric binuclear cyanido-bridged mixed-valence complex

Asymmetric binuclear cyanido-bridged complexes, [Cp*(dppe)RuII–CN–RuII(bpy)2Cl]+ (1+), and their one-electron oxidized complexes 12+ have been synthesized and characterized. The investigations show that 12+ may be a Class III mixed-valence complex.

Phosphorescent NIR emitters for biomedicine: applications, advances and challenges

Application of NIR (near-infrared) emitting transition metal complexes in biomedicine is a rapidly developing area of research. Emission of this class of compounds in the "optical transparency windows" of biological tissues and the intrinsic sensitivity of their phosphorescence to oxygen resulted in the preparation of several commercial oxygen sensors capable of deep (up to whole-body) and quantitative mapping of oxygen gradients suitable for in vivo experimental studies. In addition to this achievement, the last decade has also witnessed the increased growth of successful alternative applications of NIR phosphors that include (i) site-specific in vitro and in vivo visualization of sophisticated biological models ranging from 3D cell cultures to intact animals; (ii) sensing of various biologically relevant analytes, such as pH, reactive oxygen and nitrogen species, RedOx agents, etc.; (iii) and several therapeutic applications such as photodynamic (PDT), photothermal (PTT), and photoactivated cancer (PACT) therapies as well as their combinations with other therapeutic and imaging modalities to yield new variants of combined therapies and theranostics. Nevertheless, emerging applications of these compounds in experimental biomedicine and their implementation as therapeutic agents practically applicable in PDT, PTT, and PACT face challenges related to a critically important improvement of their photophysical and physico-chemical characteristics. This review outlines the current state of the art and achievements of the last decade and stresses the most promising trends, major development prospects, and challenges in the design of NIR phosphors suitable for biomedical applications.

Photochemistry of Rhenium(i) Diimine Tricarbonyl Complexes in Biological Applications

Luminescent rhenium complexes continue to be the focus of growing scientific interest for catalytic, diagnostic and therapeutic applications, with emphasis on the development of their photophysical and photochemical properties. In this short review, we explore such properties with a focus on the biological applications of the molecules. We discuss the importance of the ligand choice to the contribution and their involvement towards the most significant electronic transitions of the metal species and what strategies are used to exploit the potential of the molecules in medicinal applications. We begin by detailing the photophysics of the molecules; we then describe the three most common photoreactions of rhenium complexes as photosensitizers in H₂ production, photocatalysts in CO₂ reduction and photochemical ligand substitution. In the last part, we describe their applications as luminescent cellular probes and how photochemical ligand substitution is utilized in the development of photoactive carbon monoxide-releasing molecules as anticancer and antimicrobial agents.

Rylene- and diaza-rylene-derived cobalt clusters for solid-state pyrolysis towards undoped and N-doped carbon nanoparticles

2,7-Diazapyrene and 2,9-diazaperopyrene tetraalkynes (12 and 13) as well as related non-N-doped pyrene and peropyrene tetraalkynes (14 and 15) of the same shape were used as polyaromatic templates in their metalation by [Co₂(CO)₈]. Isolated cobalt-rich [(12, 13, 14, 15)Co₈(CO)₂₄] clusters were characterized by means of NMR, IR, UV-Vis spectroscopy and X-ray crystallography. Their thermogravimetric behaviour and products of solid-state pyrolysis (SSP) were investigated by TGA, DSC, and scanning electron microscopy (SEM). Despite the same precursor shape, different carbon nanoparticles and nanotubes were formed depending on the extension of the π-system and nitrogen content of the precursors. Diazapyrene and diazaperopyrene complexes formed cauliflower-shaped nanoparticles, and the pyrene complex formed spherical nanoparticles and the peropyrene complex led to multi-walled carbon nanotubes. These results elucidate that the carbon to cobalt ratio and the nitrogen dopant in the precursor have a significant impact on the products of the pyrolysis reaction.

Ligand-to-Metal Charge-Transfer Photophysics and Photochemistry of Emissive d0 Titanocenes: A Spectroscopic and Computational Investigation

Complexes with ligand-to-metal charge-transfer (LMCT) excited states involving d⁰ metals represent a new design for photocatalysts. Herein, the photochemistry and photophysics of d⁰ titanocenes of the type Cp₂Ti(C₂R)₂, where C₂R = ethynylphenyl (C₂Ph), 4-ethynyldimethylaniline (C₂DMA), or 4-ethynyltriphenylamine (C₂TPA), have been investigated. Cp₂Ti(C₂Ph)₂ and Cp₂Ti(C₂DMA)₂ have also been characterized by single-crystal X-ray diffraction. The two aryl rings in Cp₂Ti(C₂DMA)₂ are nearly face-to-face in the solid state, whereas they are mutually perpendicular for Cp₂Ti(C₂Ph)₂. All three complexes are brightly emissive at 77 K but photodecompose at room temperature when irradiated into their lowest-energy absorption band. The emission wavelengths and photodecomposition quantum yields are as follows: Cp₂Ti(C₂Ph)₂, 575 nm and 0.65; Cp₂Ti(C₂TPA)₂, 642 nm and 0.42; Cp₂Ti(C₂DMA)₂, 672 nm and 0.25. Extensive benchmarking of the density functional theory (DFT) model against the structural data and of the time-dependent DFT (TDDFT) model against the absorption and emission data was performed using combinations of 13 different functionals and 4 basis sets. The model that predicted the absorption and emission data with the greatest fidelity utilized MN15/LANL2DZ for both the DFT optimization and the TDDFT. Computational analysis shows that absorption involves a transition to a ¹LMCT state. Whereas the spectroscopic data for Cp₂Ti(C₂TPA)₂ and Cp₂Ti(C₂DMA)₂ are well modeled using the optimized structure of these complexes, Cp₂Ti(C₂Ph)₂ required averaging of the spectra from multiple rotamers involving rotation of the Ph rings. Consistent with this finding, an energy scan of all rotamers showed a very flat energetic surface, with less than 1.3 kcal/mol separating the minimum and maximum. The computational data suggest that emission occurs from a ³LMCT state. Optimization of the ³LMCT state demonstrates compression of the C-Ti-C bond angle, consistent with the known products of photodecomposition.

Influence of the Protonation State on the Excited-State Dynamics of Ruthenium(II) Complexes with Imidazole π-Extended Dipyridophenazine Ligands

Ruthenium(II) complexes, like [(tbbpy)₂Ru(dppz)]²⁺ (Ru-dppz; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, dppz = dipyrido-[3,2-a:2',3'-c]phenazine), have emerged as suitable photosensitizers in photoredox catalysis. Since then, there has been ongoing interest in the design of π-extended Ru-dppz systems with red-shifted visible absorption maxima and sufficiently long-lived excited states independent of the solvent or pH value. Herein, we explore the photophysical properties of protonation isomers of the linearly π-extended [(tbbpy)₂Ru(L)]²⁺-type complexes bearing a dppz ligand with directly fused imidazole (im) and methyl-imidazole units (mim) as L. Steady-state UV-vis absorption, resonance Raman, as well as time-resolved emission and transient absorption spectroscopy reveal that Ru-im and Ru-mim show desirable properties for the application in photocatalytic processes, i.e., strong visible absorbance and two long-lived excited states in the ³ILCT and ³MLCT manifold, at pH values between 3 and 12. However, protonation of the (methyl-)imidazole unit at pH ≤ 2 unit causes decreased excited-state lifetimes and an emission switch-off.

Cyanate Formation via Photolytic Splitting of Dinitrogen

Light-driven N2 cleavage into molecular nitrides is an attractive strategy for synthetic nitrogen fixation. However, suitable platforms are rare. Furthermore, the development of catalytic protocols via this elementary step suffers from poor understanding of N-N photosplitting within dinitrogen complexes, as well as of the thermochemical and kinetic framework for coupled follow-up chemistry. We here present a tungsten pincer platform, which undergoes fully reversible, thermal N2 splitting and reverse nitride coupling, allowing for experimental derivation of thermodynamic and kinetic parameters of the N-N cleavage step. Selective N-N splitting was also obtained photolytically. DFT computations allocate the productive excitations within the {WNNW} core. Transient absorption spectroscopy shows ultrafast repopulation of the electronic ground state. Comparison with ground-state kinetics and resonance Raman data support a pathway for N-N photosplitting via a nonstatistically vibrationally excited ground state that benefits from vibronically coupled structural distortion of the core. Nitride carbonylation and release are demonstrated within a full synthetic cycle for trimethylsilylcyanate formation directly from N2 and CO.

Irreversible Resistive State Switching in Devices with a Homoleptic Cobalt(II) Complex Active Layer

Molecules with bi-stable electronic transport behaviour have been in upfront research topics of the molecular semiconductor devices in the past few decades due to the use of such materials in resistive data storage devices. Transition metal complexes (TMC) are expected to be potential candidates in regard to the tunable and manifold redox behaviour expecting multiple bulk transport states. Finding alternate mechanisms in such devices with TMC as the active layer materials would revoke the multifaceted approach to the functional gain. We have succeeded in demonstrating write once-read many (WORM) type of resistive memory device using a homoleptic Cobalt(II) (Co(II)) complex with large on/off current ratio ensuring the easy readout process at lower voltage. The advantage of this device was the turn on voltage was found to be the low (<2.7 V) operational voltage and the success ratio of the devices were more than 83%. The durability of the stored data was found to be more than 35,000 seconds which ensures the stability of the bistable state in the fabricated devices. Such ambient stable, solution processable devices are important for the large-scale printable devices. The manuscript describes the preparation, optical and electrochemical characterisation of the metal complex used along with a detailed mechanistic investigations and electrical characterisation of memory device obtained from a stable cobalt complex.

Solvent Effects on the Phosphorescence of Gold(III) Complexes Chelated by β-Multisubstituted Corroles

A set of gold corrole complexes containing four different β-substituent groups (Br/I/CF₃), namely, 4Br-Au, 4I-Au, and 4CF₃-Au, were investigated; all showed room temperature phosphorescence. The phosphorescence quantum yields of the corroles were determined using tetraphenylporphyrin as a reference: Φ_ph (4I-Au, 0.75%) > Φ_ph (4Br-Au, 0.64%) > Φ_ph (4CF₃-Au, 0.38%). 4CF₃-Au exhibited near-IR emission (858 nm, aerobic); absorbance intensity for the Q-band was higher than that for the Soret band. Complex 4I-Au showed a longer phosphorescence lifetime (82 μs) compared to those of 4Br-Au (53 μs) and 4CF₃-Au (28 μs; N₂, tol). Thermally activated delayed fluorescence (TADF) emission of 4I/Br-Au complexes was observed: stronger emission intensity correlated with increasing temperature. Good negative correlations for 4I/Br-Au were observed between the Soret band absorption energy and the solvent polarizability: excited states of 4I/Br-Au are more polar than their ground states. TD-DFT calculations revealed very fast intersystem crossing (ISC) rate constants, 2.20 × 10¹² s^-1 (4CF₃-Au) > 1.96 × 10¹¹ s^-1 (4Br-Au) > 1.15 × 10¹¹ s^-1 (4I-Au), and importantly, the reverse intersystem crossing (rISC) rate constants are determined as 1.68 × 10⁷ s^-1 (4I-Au) > 2.40 × 10³ s^-1 (4Br-Au) ≫ 8.09 × 10^-8 s^-1 (4CF₃-Au). The exceptionally low rISC rate constant of 4CF₃-Au is attributed to its more steric and deformed structure bearing a larger energy gap between the S₁ and T₁ states.

Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering

Light-driven chemical transformations provide a compelling approach to understanding chemical reactivity with the potential to use this understanding to advance solar energy and catalysis applications. Capturing the non-equilibrium trajectories of electronic excited states with precision, particularly for transition metal complexes, would provide a foundation for advancing both of these objectives. Of particular importance for 3d metal compounds is characterizing the population dynamics of charge-transfer (CT) and metal-centered (MC) electronic excited states and understanding how the inner coordination sphere structural dynamics mediate the interaction between these states. Recent advances in ultrafast X-ray laser science has enabled the electronic excited state dynamics in 3d metal complexes to be followed with unprecedented detail. This review will focus on simultaneous X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) studies of iron coordination and organometallic complexes. These simultaneous XES-XSS studies have provided detailed insight into the mechanism of light-induced spin crossover in iron coordination compounds, the interaction of CT and MC excited states in iron carbene photosensitizers, and the mechanism of Fe-S bond dissociation in cytochrome c.

Direct observation of coherent femtosecond solvent reorganization coupled to intramolecular electron transfer

It is well known that the solvent plays a critical role in ultrafast electron-transfer reactions. However, solvent reorganization occurs on multiple length scales, and selectively measuring short-range solute-solvent interactions at the atomic level with femtosecond time resolution remains a challenge. Here we report femtosecond X-ray scattering and emission measurements following photoinduced charge-transfer excitation in a mixed-valence bimetallic (FeⁱⁱRuⁱⁱⁱ) complex in water, and their interpretation using non-equilibrium molecular dynamics simulations. Combined experimental and computational analysis reveals that the charge-transfer excited state has a lifetime of 62 fs and that coherent translational motions of the first solvation shell are coupled to the back electron transfer. Our molecular dynamics simulations identify that the observed coherent translational motions arise from hydrogen bonding changes between the solute and nearby water molecules upon photoexcitation, and have an amplitude of tenths of ångströms, 120-200 cm^-1 frequency and ~100 fs relaxation time. This study provides an atomistic view of coherent solvent reorganization mediating ultrafast intramolecular electron transfer.

The impact of cyclometalated and phosphine ligands on the luminescence properties of cycloplatinated(ii) complexes: photophysical and theoretical investigations

The effects of various C^N cyclometalated and phosphine ligands on the photophysical properties of cycloplatinated(ii) complexes were investigated experimentally and theoretically.

Photophysics of Ruthenium(II) Complexes with Thiazole π-Extended Dipyridophenazine Ligands

Transition-metal-based donor-acceptor systems can produce long-lived excited charge-transfer states by visible-light irradiation. The novel ruthenium(II) polypyridyl type complexes Ru1 and Ru2 based on the dipyridophenazine ligand (L0) directly linked to 4-hydroxythiazoles of different donor strengths were synthesized and photophysically characterized. The excited-state dynamics were investigated by femtosecond-to-nanosecond transient absorption and nanosecond emission spectroscopy complemented by time-dependent density functional theory calculations. These results indicate that photoexcitation in the visible region leads to the population of both metal-to-ligand charge-transfer (¹MLCT) and thiazole (tz)-induced intraligand charge-transfer (¹ILCT) states. Thus, the excited-state dynamics is described by two excited-state branches, namely, the population of (i) a comparably short-lived phenazine-centered ³MLCT state (τ ≈ 150-400 ps) and (ii) a long-lived ³ILCT state (τ ≈ 40-300 ns) with excess charge density localized on the phenazine and tz moieties. Notably, the ruthenium(II) complexes feature long-lived dual emission with lifetimes in the ranges τ_Em,1 ≈ 40-300 ns and τ_Em,2 ≈ 100-200 ns, which are attributed to emission from the ³ILCT and ³MLCT manifolds, respectively.

Luminescent Rhenium(I)tricarbonyl Complexes Containing Different Pyrazoles and Their Successive Deprotonation Products: CO2 Reduction Electrocatalysts

Cationic fac-[Re(CO)₃(pz*H)(pypzH)]OTf (pz*H = pyrazole, pzH; 3,5-dimethylpyrazole, dmpzH; indazole, indzH; 3-(2-pyridyl)pyrazole, pypzH) were obtained from fac-[ReBr(CO)₃(pypzH)] by halide abstraction with AgOTf and subsequent addition of the corresponding pyrazole. Successive deprotonation with Na₂CO₃ and NaOH gave neutral fac-[Re(CO)₃(pz*H)(pypz)] and anionic Na{fac-[Re(CO)₃(pz*)(pypz)]} complexes, respectively. Cationic fac-[Re(CO)₃(pz*H)(pypzH)]OTf, neutral complexes fac-[Re(CO)₃(pz*H)(pypz)], and fac-[Re(CO)₃(pypz)₂Na] were subjected to photophysical and electrochemical studies. They exhibit phosphorescent decays from a prevalently ³MLCT excited state with quantum yields (Φ) in the range between 0.03 and 0.58 and long lifetimes (τ from 220 to 869 ns). The electrochemical behavior in Ar atmosphere of cationic and neutral complexes indicates that the oxidation processes assigned to Re^I → Re^II occurs at lower potentials for the neutral complex compared to cationic complex. The reduction processes occur at the ligands and do not depend on the charge of the complexes. The electrochemical behavior in CO₂ saturated media is consistent with CO₂ electrocatalyzed reduction, where the values of the catalytic activity [i_cat(CO₂)/i_cat(Ar)] ranged from 2.7 to 11.5 (compared to 8.1 for fac-[Re(CO)₃Cl(bipy)] studied as a reference). Controlled potential electrolysis for the pyrazole cationic (3a) and neutral (4a) complexes after 1 h affords CO in faraday yields of 61 and 89%, respectively. These values are higher for indazole complexes and may be related to the acidity of the coordinated pyrazole.

Iron's Wake: The Performance of Quantum Mechanical-Derived Versus General-Purpose Force Fields Tested on a Luminescent Iron Complex

Recently synthetized iron complexes have achieved long-lived excited states and stabilities which are comparable, or even superior, to their ruthenium analogues, thus representing an eco-friendly and cheaper alternative to those materials based on rare metals. Most of computational tools which could help unravel the origin of this large efficiency rely on ab-initio methods which are not able, however, to capture the nanosecond time scale underlying these photophysical processes and the influence of their realistic environment. Therefore, it exists an urgent need of developing new low-cost, but still accurate enough, computational methodologies capable to deal with the steady-state and transient spectroscopy of transition metal complexes in solution. Following this idea, here we focus on the comparison between general-purpose transferable force-fields (FFs), directly available from existing databases, and specific quantum mechanical derived FFs (QMD-FFs), obtained in this work through the Joyce procedure. We have chosen a recently reported Fe^III complex with nanosecond excited-state lifetime as a representative case. Our molecular dynamics (MD) simulations demonstrated that the QMD-FF nicely reproduces the structure and the dynamics of the complex and its chemical environment within the same precision as higher cost QM methods, whereas general-purpose FFs failed in this purpose. Although in this particular case the chemical environment plays a minor role on the photo physics of this system, these results highlight the potential of QMD-FFs to rationalize photophysical phenomena provided an accurate QM method to derive its parameters is chosen.

Outer-sphere effects on ligand-field excited-state dynamics: solvent dependence of high-spin to low-spin conversion in [Fe(bpy)3]2

In condensed phase chemistry, the solvent can have a significant impact on everything from yield to product distribution to mechanism. With regard to photo-induced processes, solvent effects have been well-documented for charge-transfer states wherein the redistribution of charge subsequent to light absorption couples intramolecular dynamics to the local environment of the chromophore. Ligand-field excited states are expected to be largely insensitive to such perturbations given that their electronic rearrangements are localized on the metal center and are therefore insulated from so-called outer-sphere effects by the ligands themselves. In contrast to this expectation, we document herein a nearly two-fold variation in the time constant associated with the 5T2 → 1A1 high-spin to low-spin relaxation process of tris(2,2'-bipyridine)iron(ii) ([Fe(bpy)3]2+) across a range of different solvents. Likely origins for this solvent dependence, including relevant solvent properties, ion pairing, and changes in solvation energy, were considered and assessed by studying [Fe(bpy)3]2+ and related derivatives via ultrafast time-resolved absorption spectroscopy and computational analyses. It was concluded that the effect is most likely associated with the volume change of the chromophore arising from the interconfigurational nature of the 5T2 → 1A1 relaxation process, resulting in changes to the solvent-solvent and/or solvent-solute interactions of the primary solvation shell sufficient to alter the overall reorganization energy of the system and influencing the kinetics of ground-state recovery.

Designing expanded bipyridinium as redox and optical probes for DNA

We report on the light-switch behaviour of two head-to-tail expanded bipyridinium species as a function of their interaction with calf thymus DNA and polynucleotides. In particular, both DNA and polynucleotides containing exclusively adenine or guanine moieties quench the luminescence of the fused expanded bipyridinium species. This behaviour has been rationalized demonstrating that a reductive photoinduced electron transfer process takes place involving both adenine or guanine moieties. The charge separated state so produced recombines in the tens of picoseconds. These results could help in designing new organic substrates for application in DNA probing technology and lab on chip-based sensing systems.

H/D solvent isotope effects on the photoracemization reaction of enantiomeric the tris(2,2′-bipyridine)ruthenium(ii) complex and its analogues

This work assessed solvent isotope effects on the photoracemization rate and emission lifetime for [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) in water.

Mediation of Electron Transfer by Quadrupolar Interactions: The Constitutional, Electronic, and Energetic Complementarities in Supramolecular Chemistry

Studies of intermolecular interactions enhance our knowledge of chemistry across molecular and supramolecular levels. Here, we show that host-guest quadrupolar interaction has a profound influence on the molecular system. With covalently bonded dimolybdenum complex units as the electron donor (D) and acceptor (A) and a thienylene group (C₄H₂S) as the bridge (B), the mixed-valence D-B-A complexes are shaped with clefts in the middle of the molecule. Interestingly, in aromatic solvents, the D-A electronic coupling constants (H_ab) and electron transfer rates (k_et) are dramatically reduced. Theoretical computations indicate that an aromatic molecule is encapsulated in the cleft of the D-B-A array; quadrupole-quadrupole interaction between the guest molecule and the C₄H₂S bridge evokes a charge redistribution, which increases the HOMO-LUMO energy gap, intervening in the through-bond electron transfer. These results demonstrate that a supramolecular system is unified underlying the characteristics of the assembled molecules through constitutional, electronic, and energetic complementarities.

Anion and solvent dependency of the electronic coupling strength in mixed valent class II systems

The influence of the coordination and ion pairing properties of electrolyte anions on electronic coupling in cationic class II mixed valent species was studied. In order to cover a range of electronic coupling strengths within the class II regime, weakly coupled 2,5-diferrocenyl-3,4-thiadiazol, moderately coupled 2,5-diferrocenyl thiophene and strongly coupled N-(4-dimethylaminophenyl)-2,5-diferrocenyl-1H-pyrrole were chosen as analytes. The electrochemical properties of these compounds were determined by cyclic and square wave voltammetry using electrolytes with varying ion pairing capabilities, such as [NBu₄][Cl], [NBu₄][PF₆] and [NBu₄][BArF] ([NBu₄][B(C₆F₅)₄]), as well as solvents with increasing dielectric constants (dichloromethane (ε_r = 8.93), acetone (ε_r = 20.56), acetonitrile (ε_r = 35.94) and propylene carbonate (ε_r = 64.92)). It is shown that the choice of electrolyte has a considerable impact on the electrostatic and the electron transfer features of the mixed valent compounds when solvents of low polarity and low relative permittivity such as dichloromethane are used. For the use of more polar solvents such as propylene carbonate the electrochemical and spectroscopic properties are almost electrolyte independent. The solvatochromic and ion-related changes in the spectroscopic properties are most pronounced for weakly coupled systems and decrease with an increase in the electron transfer coupling strength.