Metal complexes bearing photochromic ligands: photocontrol of functions and processes

Metal complexes associated with photochromic molecules are attractive platforms to achieve smart light-switching materials with innovative and exciting properties due to specific optical, electronic, magnetic or catalytic features of metal complexes and by perturbing the excited-state properties of both components to generate new reactivity and photochemical properties. In this overview, we focus on selected achievements in key domains dealing with optical, redox, magnetic properties, as well as application in catalysis or supramolecular chemistry. We also try to point out scientific challenges that are still faced for future developments and applications.

A Convenient DFT-Based Strategy for Predicting Transition Temperatures of Valence Tautomeric Molecular Switches

The ability to identify promising candidate switchable molecules computationally, prior to synthesis, represents a considerable advance in the development of switchable molecular materials. Even more useful would be the possibility of predicting the switching temperature. Cobalt-dioxolene complexes can exhibit thermally induced valence tautomeric switching between low-spin Co^III-catecholate and high-spin Co^II-semiquinonate forms, where the half-temperature (T_1/2) is the temperature at which there are equal amounts of the two tautomers. We report the first simple computational strategy for accurately predicting T_1/2 values for valence tautomeric complexes. Dispersion-corrected density functional theory (DFT) methods have been applied to the [Co(dbdiox)(dbsq)(N₂L)] (dbdiox/dbsq^•- = 3,5-di-tert-butyldioxolene/semiquinonate; N₂L = diimine) family of valence tautomeric complexes, including the newly reported [Co(dbdiox)(dbsq)(MeO-bpy)] (1) (MeO-bpy = 4,4'-dimethoxy-2,2'-bipyridine). The DFT strategy has been thoroughly benchmarked to experimental data, affording highly accurate spin-distributions and an excellent energy match between experimental and calculated spin-states. Detailed orbital analysis of the [Co(dbdiox)(dbsq)(N₂L)] complexes has revealed that the diimine ligand tunes the T_1/2 value primarily through π-acceptance. We have established an excellent correlation between experimental T_1/2(toluene) values for [Co(dbdiox)(dbsq)(N₂L)] complexes and the calculated lowest unoccupied molecular orbital energy of the corresponding diimine ligand. The model affords accurate T_1/2(toluene) values for [Co(dbdiox)(dbsq)(N₂L)] complexes, with an average error of only 3.7%. This quantitative and simple DFT strategy allows experimentalists to not only rapidly identify proposed VT complexes but also predict the transition temperature. This study lays the groundwork for future in silico screening of candidate switchable molecules prior to experimental investigation, with associated time, cost, and environmental benefits.

Structural flexibility versus rigidity of the aromatic unit of DNA ligands: binding of aza- and azoniastilbene derivatives to duplex and quadruplex DNA

The known azastilbene (E)-1,2-di(quinolin-3-yl)ethane (2a) and the novel azoniastilbene derivatives (E)-2-(2-(naphthalen-2-yl)vinyl)quinolizinium (2b) and (E)-3,3'-(ethane-1,2-diyl)bis(1-methylquinolinin-1-ium) (2c) were synthesized. Their interactions with duplex and quadruplex DNA (G4-DNA) were studied by photometric, fluorimetric, polarimetric and flow-LD analysis, and by thermal DNA denaturation studies, as well as by ¹H-NMR spectroscopy. The main goal of this study was a comparison of these conformationally flexible compounds with the known G4-DNA-binding diazoniadibenzo[b,k]chrysenes, that have a comparable π-system extent, but a rigid structure. We have observed that the aza- and azoniastilbene derivatives 2a-c, i.e. compounds with almost the same spatial dimensions and steric demand, bind to DNA with an affinity and selectivity that depends significantly on the number of positive charges. Whereas the charge neutral derivative 2a binds unspecifically to the DNA backbone of duplex DNA, the ionic compounds 2b and 2c are typical DNA intercalators. Notably, the bis-quinolinium derivative 2c binds to G4-DNA with moderate affinity (K_b = 4.8 × 10⁵ M^-1) and also stabilizes the G4-DNA towards thermal denaturation (ΔT_m = 11 °C at ligand-DNA ratio = 5.0). Strikingly, the corresponding rigid counterpart, 4a,12a-diazonia-8,16-dimethyldibenzo[b,k]chrysene, stabilizes the G4-DNA to an even greater extent under identical conditions (ΔT_m = 27 °C). These results indicate that the increased flexibility of a G4-DNA ligand does not necessarily lead to stronger interactions with the G4-DNA as compared with rigid ligands that have essentially the same size and π system extent.

Opto-Spintronics: Photoisomerization-Induced Spin State Switching at 300 K in Photochrome Cobalt-Dioxolene Thin Films

Controllable quantum systems are under active investigation for quantum computing, secure information processing, and nonvolatile memory. The optical manipulation of spin quantum states provides an important strategy for quantum control with both temporal and spatial resolution. Challenges in increasing the lifetime of photoinduced magnetic states at T > 200 K have hindered progress toward utilizing photomagnetic materials in quantum device architectures. Here we demonstrate reversible light-induced magnetization switching in an organic thin film at device operating temperatures of 300-330 K. By utilizing photochromic ligands that undergo structural changes in the solid state, the changes in ligand field associated with photoisomerization modulate the ligand field and in turn the oxidation and spin state of a bound metal center. Green light irradiation (λ_exc = 550 nm) of a spirooxazine cobalt-dioxolene complex induces photoisomerization of the ligand that in turn triggers a reversible intramolecular charge-transfer coupled spin-transition process at the cobalt center. The generation of photomagnetic states through conversion between a low-spin Co(III)-semiquinone doublet and a high-spin Co(II)-bis-semiquinone sextet state has been demonstrated in both solution and the solid state and is described as a photoisomerization-induced spin-charge excited state (PISCES) process. The high transition temperature (325 K) and long-lived photoinduced state (τ = 10 s at 300 K) are dictated by the photochromic ligand. Theory provides effective modeling of the phenomenon and long-term strategies to further modulate the lifetimes of photomagnetic states for quantum information technologies at the single molecule level.

Modulation of the carboxamidine redox potential through photoinduced spiropyran or fulgimide isomerisation

Carboxamidines functionalized with either a spiropyran or fulgimide photoswitch were prepared on multigram scales. The thermal, electrochemical, and photochemical ring isomerizations of these compounds were studied and the results compared with related systems. The photochemical isomerisations were found to be reversible and could be followed by 1H NMR and UV-vis spectroscopy. The spiropyran/merocyanine couple was thermally active and an activation enthalpy of 116 kJ mol-1 was measured for ring-opening. These measurements yielded an enthalpy difference of 25 kJ mol-1 between the open and closed states which is consistent with DFT calculations. DFT calculations predicted a charge transfer to the carboxamidine group upon ring closure in the fulgimide and a charge transfer from the carboxamidine group upon switching the spiropyran to the merocyanine form. This was confirmed experimentally by monitoring the change in the oxidation potential assigned to the carboxamidine group. The potential of these molecules to therefore act as a new class of photoresponsive ligands that can modulate the ligand field of a complex is discussed.

Variable-temperature structural studies on valence tautomerism in cobalt bis(dioxolene) molecular complexes

A variable-temperature single-crystal structural study of five valence tautomeric cobalt molecular complexes, Co^II(3,5-DBSQ)₂(DBPy)₂ (1), Co^II(3,5-DBSQ)₂(DBPy)₂·1.33C₇H₈ (1S), Co^II(3,5-DBSQ)₂(DCPy)₂·C₇H₈ (2S), Co^II(3,5-DBSQ)₂(TBPy)₂ (3) and Co^II(3,5-DBSQ)₂(TCPy)₂ (4) (S = toluene, 3,5-DBSQ = 3,5-di-tert-butylsemiquinonate, DBPy = 3,5-dibromopyridine, DCPy = 3,5-dichloropyridine, TBPy = 3,4,5-tribromopyridine and TCPy = 3,4,5-trichloropyridine) is reported. The re-crystallization of (1S) in toluene at 277 K resulted in a concomitant formation of a solvent-free polymorph, Co^II(3,5-DBSQ)₂(DBPy)₂ (1). Thermally induced valence tautomerism (VT) is observed only in (1S), (1) and (2S) [hs-Co^II(3,5-DBSQ)₂L₂ ↔ ls-Co^III(3,5-DBSQ)(3,5-DBCat)L₂ (hs = high spin, ls = low spin, 3,5-DBCat = 3,5-di-tert-butylcatecholate)], whereas (3) and (4) remain locked in the hs-Co^II(3,5-DBSQ)₂ state during cooling of the sample. Multi-temperature single-crystal studies demonstrate the change in cobalt coordination environment during the VT conversion. The non-solvated compound (1) shows a sharp VT transition (T_1/2 ∼ 245 K with ΔT ∼ 10 K) from hs-Co^II(3,5-DBSQ)₂(DBPy)₂ to ls-Co^III(3,5-DBSQ)(3,5-DBCat)(DBPy)₂ oxidation state, whereas the other polymorph with lattice solvent (1S) results in a broad transition (T_1/2 ∼ 150 K with ΔT ∼ 100 K). This increase in the VT transition temperature for (1) relative to (1S) illustrates the effect of lattice solvent on the VT transition mechanism. Additionally, the influence of halogen substitutions on the pyridine ring is discussed with respect to observed VT behaviour in the studied compounds.

Switching of the π-electronic conjugations in the reduction of a dithienylethene-fused p-benzoquinone

Visible light unlocks the π-electronic conjugation of a dithienylethene-fused p-benzoquinone derivative to cause a light-driven oxidation reaction.

Fluorescence behaviour of an anthracene-BODIPY system affected by spin states of a dioxolene-cobalt centre

Dioxolene cobalt complexes, [Co(L)(TPA)]PF6 () and [Co(L)(Me3TPA)]PF6 (), were synthesized using a catechol with anthracene and boron-dipyrromethene (BODIPY), H2L, and tris(pyridin-2-ylmethyl)amine (TPA) or tris[(6-methylpyridin-2-yl)methyl]amine (Me3TPA) and characterised by UV-vis absorption, IR and NMR spectroscopy. Complexes and are a low-spin cobalt(iii) catecholate form and a high-spin cobalt(ii) semiquinonate form, respectively, in CH3CN at room temperature. The effect of the spin states of the dioxolene-cobalt centre on fluorescence was investigated in CH3CN at room temperature. The fluorescence intensity of was clearly lower than that of , although both complexes showed weaker fluorescence compared with that of H2L. The difference in the intensity between and is mainly due to the enhancement of nonradiative decay processes based on the stronger metal-ligand interactions for the cobalt(iii) catecholate form of compared with the cobalt(ii) semiquinonate form of .

Bidirectional photoswitching of magnetic properties at room temperature: ligand-driven light-induced valence tautomerism

Valence tautomeric (VT) metal complexes are highly promising bistable molecular compounds for applications as molecular switches in molecular electronics and spintronics. Although VT species can be switched with light, the photoswitching in all reported systems requires very low temperatures (usually below 20 K) because photoinduced states are highly unstable at room temperature. The thermal instability hinders any practical application of these complexes in genuine devices. In this report, for the first time we demonstrate photoswitching of VT species and associated magnetic properties at room temperature. The bidirectional photoswitching in solution is due to cis-trans photoisomerizable 4-styrylpyridine ligands deliberately integrated into cobalt dioxolene molecular complexes. The novel type of photoswitching has been coined Ligand-Driven Light-Induced Valence Tautomerism (LD-LIVT). The photoconversion of VT states of 28% has been achieved in solution at room temperature. The photoinduced states show extraordinary thermal stability for hours at room temperature, as compared to common nanoseconds reported previously. The switching proceeds at molecular level with the effective photoswitching rate of 3 × 1013 molecules per s under our conditions. Consequently, this work may open new horizons in applications of molecular switches based on VT metal complexes in molecular devices functioning at room temperature.

Preparation, electrochemical behavior, and variable-temperature magnetic properties of Co(3,5-DBSQ)2 complexes of imidazole- or pyrazole-substituted ligands

Herein we describe the preparation of four new Co(3,5-DBSQ)2 containing known 2-(2-pyridyl)benzimidazole 1, 2,2′-bisbenzimidazole 2, or 2,2-(1H-pyrazole-3,5-diyl)dipyridine 3. We investigate the electronic and variable-temperature magnetic susceptibility properties of these complexes. Complexes containing 2 and 3 are bimetallic and variable-temperature magnetic susceptibility data suggest observation of a stable intermediate (high-spin and low-spin) state for the bimetallic complex with 3, which does undergo valence tautomerism. Complexes 4–6 containing imidazole-based ligands 1 and 2 feature high-spin ground states and no valence tautomerism is observed in these materials. This experimental finding is corroborated with density functional theory calculations, which support the existence of a stable high-spin ground state in these complexes.

Enhanced bi-stability in a ruthenium alkynyl spiropyran complex

Attachment of a ruthenium alkynyl unit to a spiropyran ligand extends the lifetime of the merocyanine form of the complex more than twenty-fold.

Tuning excited state isomerization dynamics through ground state structural changes in analogous ruthenium and osmium sulfoxide complexes

The complexes [Ru(bpy)2(pyESO)](PF6)2 and [Os(bpy)2(pyESO)](PF6)2, in which bpy is 2,2'-bipyridine and pyESO is 2-((isopropylsulfinyl)ethyl)pyridine, were prepared and studied by (1)H NMR, UV-visible and ultrafast transient absorption spectroscopy, as well as by electrochemical methods. Crystals suitable for X-ray structural analysis were grown for [Ru(bpy)2(pyESO)](PF6)2. Cyclic voltammograms of both complexes provide evidence for S→O and O→S isomerization as these voltammograms are described by an ECEC (electrochemical-chemical electrochemical-chemical) mechanism in which isomerization follows Ru(2+) oxidation and Ru(3+) reduction. The S- and O-bonded Ru(3+/2+) couples appear at 1.30 and 0.76 V versus Ag/AgCl in propylene carbonate. For [Os(bpy)2(pyESO)](PF6)2, these couples appear at 0.97 and 0.32 V versus Ag/AgCl in acetonitrile, respectively. Charge-transfer excitation of [Ru(bpy)2(pyESO)](PF6)2 results in a significant change in the absorption spectrum. The S-bonded isomer of [Ru(bpy)2(pyESO)](2+) features a lowest energy absorption maximum at 390 nm and the O-bonded isomer absorbs at 480 nm. The quantum yield of isomerization in [Ru(bpy)2(pyESO)](2+) was found to be 0.58 in propylene carbonate and 0.86 in dichloroethane solution. Femtosecond transient absorption spectroscopic measurements were collected for both complexes, revealing time constants of isomerizations of 81 ps (propylene carbonate) and 47 ps (dichloroethane) in [Ru(bpy)2(pyESO)](2+). These data and a model for the isomerizing complex are presented. A striking conclusion from this analysis is that expansion of the chelate ring by a single methylene leads to an increase in the isomerization time constant by nearly two orders of magnitude.

Photoswitchable DNA-binding properties of a photochromic spirooxazine derivative

An N-methylphenanthrolinium-annelated spirooxazine derivative 2(SO) whose DNA-intercalating properties are reversibly changed by a photochromic reaction was prepared. Upon irradiation at 350 nm the spirooxazine is transformed to the corresponding photomerocyanine 2(PM) that binds to DNA. After irradiation with visible light the spirooxazine 2(SO), which exhibits no significant DNA-binding properties, is regained. The association of 2(PM) with DNA was examined by CD and absorption spectroscopy, fluorescent intercalator displacement and viscometric titration.

Light-driven coordination-induced spin-state switching: rational design of photodissociable ligands

The bistability of spin states (e.g., spin crossover) in bulk materials is well investigated and understood. We recently extended spin-state switching to isolated molecules at room temperature (light-driven coordination-induced spin-state switching, or LD-CISSS). Whereas bistability and hysteresis in conventional spin-crossover materials are caused by cooperative effects in the crystal lattice, spin switching in LD-CISSS is achieved by reversibly changing the coordination number of a metal complex by means of a photochromic ligand that binds in one configuration but dissociates in the other form. We present mathematical proof that the maximum efficiency in property switching by such a photodissociable ligand (PDL) is only dependent on the ratio of the association constants of both configurations. Rational design by using DFT calculations was applied to develop a photoswitchable ligand with a high switching efficiency. The starting point was a nickel-porphyrin as the transition-metal complex and 3-phenylazopyridine as the photodissociable ligand. Calculations and experiments were performed in two iterative steps to find a substitution pattern at the phenylazopyridine ligand that provided optimum performance. Following this strategy, we synthesized an improved photodissociable ligand that binds to the Ni-porphyrin with an association constant that is 5.36 times higher in its trans form than in the cis form. The switching efficiency between the diamagnetic and paramagnetic state is efficient as well (72% paramagnetic Ni-porphyrin after irradiation at 365 nm, 32% paramagnetic species after irradiation at 440 nm). Potential applications arise from the fact that the LD-CISSS approach for the first time allows reversible switching of the magnetic susceptibility of a homogeneous solution. Photoswitchable contrast agents for magnetic resonance imaging and light-controlled magnetic levitation are conceivable applications.