Electronic energy self-exchange with macrocyclic chromium(III) complexes

Ferromagnetically Coupled Chromium(III) Dimer Shows Luminescence and Sensitizes Photon Upconversion

There has been much progress on mononuclear chromium(III) complexes featuring luminescence and photoredox activity, but dinuclear chromium(III) complexes have remained underexplored in these contexts until now. We identified a tridentate chelate ligand able to accommodate both meridional and facial coordination of chromium(III), to either access a mono- or a dinuclear chromium(III) complex depending on reaction conditions. This chelate ligand causes tetragonally distorted primary coordination spheres around chromium(III) in both complexes, entailing comparatively short excited-state lifetimes in the range of 400 to 800 ns in solution at room temperature and making photoluminescence essentially oxygen insensitive. The two chromium(III) ions in the dimer experience ferromagnetic exchange interactions that result in a high spin (S=3) ground state with a coupling constant of +9.3 cm-1. Photoinduced energy transfer from the luminescent ferromagnetically coupled dimer to an anthracene derivative results in sensitized triplet-triplet annihilation upconversion. Based on these proof-of-principle studies, dinuclear chromium(III) complexes seem attractive for the development of fundamentally new types of photophysics and photochemistry enabled by magnetic exchange interactions.

Complex-as-Ligand Strategy as a Tool for the Design of a Binuclear Nonsymmetrical Chromium(III) Assembly: Near-Infrared Double Emission and Intramolecular Energy Transfer

The chromium(III) polypyridyl complexes are appealing for their long-lived near-infrared (NIR) emission reaching the millisecond range and for the strong circularly polarized luminescence of their isolated enantiomers. However, harnessing those properties in functional polynuclear Cr^III devices remains mainly inaccessible because of the lack of synthetic methods for their design and functionalization. Even the preparation and investigation of most basic nonsymmetrical Cr^III dyads exhibiting directional intramolecular intermetallic energy transfer remain unexplored. Taking advantage of the inertness of heteroleptic chromium(III) polypyridyl building blocks, we herein adapt the "complex-as-ligand" strategy, largely used with precious 4d and 5d metals, for the preparation of a binuclear nonsymmetrical Cr^III complex (3d metal). The resulting [(phen)₂Cr(L)Cr(tpy)]⁶⁺ dyad shows dual long-lived NIR emission and a directional intermetallic energy transfer that is controlled by the specific arrangements of the different coordination spheres. This strategy opens a route for building predetermined polynuclear assemblies with this earth-abundant metal.

Efficient Triplet-Triplet Annihilation Upconversion Sensitized by a Chromium(III) Complex via an Underexplored Energy Transfer Mechanism

Sensitized triplet-triplet annihilation upconversion (sTTA-UC) mainly relies on precious metal complexes thanks to their high intersystem crossing (ISC) efficiencies, excited state energies, and lifetimes, while complexes of abundant first-row transition metals are only rarely utilized and with often moderate UC quantum yields. [Cr(bpmp)2 ]3+ (bpmp=2,6-bis(2-pyridylmethyl)pyridine) containing earth-abundant chromium possesses an absorption band suitable for green light excitation, a doublet excited state energy matching the triplet energy of 9,10-diphenyl anthracene (DPA), a close to millisecond excited state lifetime, and high photostability. Combined ISC and doublet-triplet energy transfer from excited [Cr(bpmp)2 ]3+ to DPA gives 3 DPA with close-to-unity quantum yield. TTA of 3 DPA furnishes green-to-blue UC with a quantum yield of 12.0 % (close to the theoretical maximum). Sterically less-hindered anthracenes undergo a [4+4] cycloaddition with [Cr(bpmp)2 ]3+ and green light.

Heteroleptic mer-[Cr(N∩N∩N)(CN)3] complexes: synthetic challenge, structural characterization and photophysical properties

The substitution of three water molecules around trivalent chromium in CrBr₃·6H₂O with the tridentate 2,2′:6′,2′′-terpyridine (tpy), N,N′-dimethyl-N,N′-di(pyridine-2-yl)pyridine-2,6-diamine (ddpd) or 2,6-di(quinolin-8-yl)pyridine (dqp) ligands gives the heteroleptic mer-[Cr(L)Br₃] complexes. Stepwise treatments with Ag(CF₃SO₃) and KCN under microwave irradiations provide mer-[Cr(L)(CN)₃] in moderate yields. According to their X-ray crystal structures, the associated six-coordinate meridional [CrN₃C₃] chromophores increasingly deviate from a pseudo-octahedral arrangement according to L = ddpd ≈ dpq ≪ tpy; a trend in line with the replacement of six-membered with five-membered chelate rings around Cr^III. Room-temperature ligand-centered UV-excitation at 18 170 cm⁻¹ (λ_exc = 350 nm), followed by energy transfer and intersystem crossing eventually yield microsecond metal-centered Cr(²E → ⁴A₂) phosphorescence in the red to near infrared domain 13 150–12 650 cm⁻¹ (760 ≤ λ_em ≤ 790 nm). Decreasing the temperature to liquid nitrogen (77 K) extends the emission lifetimes to reach the millisecond regime with a record of 4.02 ms for mer-[Cr(dqp)(CN)₃] in frozen acetonitrile.

The heteroleptic mer-[Cr(L)(CN)₃] (L = tpy, ddpd, dqp) complexes with their C_2v-symmetrical [CrC₃N₃] luminescent chromophores represent the missing links between pseudo-octahedral [CrN₆] and [CrC₆] units found in their well-known homoleptic parents.

Solvent Effects on the Spectroscopic and Photophysical Properties of the trans-(1,4,8,11-Tetraazacyclotetradecane)diisothiocyanatochromium(III) Ion, trans-[Cr(cyclam)(NCS)2]+

The spectroscopy and photophysics of trans-[Cr(cyclam)(NCS)2]+ (where cyclam is 1,4,8,11-tetraazacyclotetradecane) were studied in a range of solvents. The cyclam NH stretching vibration [nu(NH)] wavenumber correlates with the Gutmann donor number, whereas the thiocyanate CN stretching vibration [nu(CN)] wavenumber correlates with the Snyder solvent strength (P') scale. These results signify that there is a difference in the solvent interactions with the two types of ligands. The energy of the ligand-to-metal charge transfer absorption maximum between 310 and 320 nm and the energy of the spin-forbidden (doublet-quartet) absorption and emission bands above 700 nm correlate with the nu(CN) wavenumber. This establishes the dominant role of solvent effects at the NCS- ligand in "tuning" the energy of these spectroscopic features. Quantum yields phirx for photosubstitution are <0.02 at 54 degrees C and <0.002 at 22 degrees C, demonstrating that photochemical reaction is a very minor pathway. The effects of solvent and temperature on the nonradiative decay of the doublet excited-state were investigated by observing the time-resolved phosphorescence between 700 and 750 nm. Below 30 degrees C, the lifetimes are relatively temperature-independent, whereas at higher temperatures, a strong Arrhenius-type dependence is observed. Values for the preexponential factor (A) and the activation energy (Ea) are solvent-dependent and follow a Barclay-Butler-type correlation. These observations are consistent with a dominant back-intersystem crossing pathway for nonradiative decay in the higher-temperature region. From trends observed between ln(A) and the nu(CN) frequency, it appears that solvent effects at the thiocyanate ligand play a dominant role in influencing the rate of nonradiative decay in the high-temperature region.

Effects of Steric Constraint on Chromium(III) Complexes of Tetraazamacrocycles. 3. Insights into the Temperature-Dependent Radiationless Deactivation of the 2Eg (Oh) Excited State of trans-[Cr(N4)(CN)2]+ Complexes

Macrocyclic complexes of the type trans-[Cr(N4)(CN)2]+, where N4 = cyclam, 1,11-C3-cyclam, and 1,4-C2-cyclam demonstrate significant variation in their room-temperature excited-state behavior; namely, the lifetimes of the 2Eg (Oh) excited states are 335, 23, and 0.24 micros, respectively. The lifetimes of these complexes have been measured in acidified H2O/dimethyl sulfoxide over the temperature range between -30 and +95 degrees C. Arrhenius activation parameters were calculated from these data. There was very little variation in the values of the Arrhenius preexponential factor between these three complexes, whereas the value of Ea is 40.6 kJ/mol for the cyclam complex, 35.5 kJ/mol for the 1,11-C3-cyclam complex, and 22.3 kJ/mol for the 1,4-C2-cyclam complex. Thus, differences in the room-temperature excited-state lifetimes can be rationalized based on the competition between thermally independent nonradiative relaxation and a thermally activated channel. To test whether a photodissociation mechanism involving Cr-macrocyclic N bond cleavage is a plausible explanation for the thermally activated relaxation pathway, samples of the cyclam complex were photolyzed in acidified D(2)O. A marked increase in the lifetime after photolysis demonstrated the occurrence of photodeuteration and thus a likely photodissociation of a macrocyclic N.

Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles. Chemistry and excited-state behavior of 1,4-C2-cyclam complexes

The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,4-C(2)-cyclam = 1,4,8,11-tetraazabicyclo[10.2.2]hexadecane is reported. The ligand appears to form only trans complexes, and the structure of trans-[Cr(1,4-C(2)-cyclam)Cl(2)]PF(6) is presented. The constraint imposed by the additional C(2) linkage distorts the bond angles significantly away from the ideal values of 90 and 180 degrees. The effect of the distortion is to enhance the aquation rate of trans-[Cr(1,4-C(2)-cyclam)Cl(2)](+) (k(obs) for trans-[Cr(1,4-C(2)-cyclam)(H(2)O)(2)](3+) formation = 6.5 x 10(-)(2) s(-)(1), 0.01M HNO(3), 25 degrees C) by over 5 orders of magnitude relative to trans-[Cr(cyclam)Cl(2)](+). The complexes trans-[Cr(1,4-C(2)-cyclam)Cl(2)](+) and trans-[Cr(1,4-C(2)-cyclam)(CN)(2)](+) are found to have extinction coefficients four to five times higher than their cyclam analogues, owed to the lack of centrosymmetry caused by the steric constraint. The trans-[Cr(1,4-C(2)-cyclam)(CN)(2)](+) complex is a very weak emitter in aqueous solution with a broad room-temperature emission centered at 735 nm (tau = 0.24 micros). Extended photolysis (350 nm, 15 h) of trans-[Cr(1,4-C(2)-cyclam)(CN)(2)](+) in aqueous solution results in CN(-) ligand loss. This is in stark contrast to its unconstrained cyclam analogue, which is photoinert and has a room-temperature emission lifetime of 335 micros.