Probing the Solvent Dependent Photophysics of fac-[Re(CO)3(dppz-X2)Cl] (dppz-X2 = 11,12-X2-dipyrido[3,2-a:2′,3′-c]phenazine); X = CH3, H, F, Cl, CF3)

Push-pull effect - how to effectively control photoinduced intramolecular charge transfer processes in rhenium(I) chromophores with ligands of D-A or D-π-A structure

Over the last five decades, diimine rhenium(I) tricarbonyl complexes have been extensively investigated due to their remarkable and widely tuned photophysical properties. These systems are regarded as attractive targets for design functional luminescent materials and performing fundamental studies of photoinduced processes in transition metal complexes. This review summarizes the latest developments concerning Re(I) tricarbonyl complexes bearing donor-acceptor (D-A) and donor-π-acceptor (D-π-A) ligands. Such compounds can be treated as bichromophoric systems with two close-lying excited states, metal-to-ligand charge transfer (MLCT) and intraligand-charge-transfer (ILCT). A role of ILCT transitions in controlling photobehaviour was discussed for Re(I) tricarbonyls with six different diimine cores decorated by various electron-rich amine, sulphur-based and π-conjugated aryl groups. It was evidenced that this approach is an effective tool for enhancement of the visible absorptivity, bathochromic emission shift and significant prolongation of the excited-state, opening up new possibilities in the development of more efficient materials and expand the range of their applications.

Photophysical and Chiroptical Properties of Pyrazino-Phenanthroline-Helicene Derivative and its Rhenium(I) Complex

A novel coordination motif comprising [4]helicene fused with pyrazino-phenanthroline (H4PP) has been synthesized and reacted with ReCl(CO)5 to yield its rhenium(I) complex (Re-H4PP). Absorption and emission spectroscopic analysis conducted in dichloromethane and 2-methyltetrahydrofuran reveals that combining pyrazino-phenanthroline with helicene visibly affects the photophysical attributes of both the resulting ligand and its Re(I) complex as compared to their non-helicene analogues, and even more importantly leads to relatively high photoluminescence quantum yield values, especially in the case of H4PP (29 %). Chiroptical studies through electronic circular dichroism and circularly polarized luminescence performed on enantiomerically enriched samples of Re-H4PP show the chiral nature of low-energy excited states affording notable glum values that amplify at cryogenic temperatures. Insights into experimental results are provided via first-principles quantum-chemical calculations showing important role of intra-ligand charge-transfer (ILCT) and metal-to-ligand CT (MLCT) states in determining photophysical features of these systems.

Further Insights into the Impact of Ligand-Localized Excited States on the Photophysics of Phenanthroline-Based Rhenium(I) Tricarbonyl Complexes

The present work shows the pivotal role of N-donor substituents attached to 1,10-phenanthroline at the 4,7-positions in perturbation of ground- and excited-state properties of fac-[ReCl(CO)3(R2phen)]. Excited-state processes occurring upon photoexcitation in the designed systems were thoroughly explored with a wide range of steady-state and time-resolved spectroscopic techniques, including transient absorption, as well as experimental results were complemented by theoretical studies based on the density functional theory (DFT). It was demonstrated that the attachment of six-membered heterocyclic amines (piperidine─ppr, morpholine─mor, and thiomorpholine─tmor) is a very effective tool for extending absorptivity and excited-state lifetimes of resulting fac-[ReCl(CO)3(R2phen)] due to the contribution of the excited state localized on the phenanthroline-based ligand. Both absorption and emission properties of these systems were attributed to configurationally mixed MLCT/IL excited states. Re(I) complexes with phenoxazine (pxz) and phenothiazine (ptz) substituents were shown to possess charge-separated excited states, clearly evidenced by the simultaneous presence of signals typical of phen-* and pxz+* or ptz+* in transient absorption spectra. Both complexes are rare examples of NIR light-emitting coordination compounds. The decoration of the phen framework with less polar 9,9-dimethyl-9,10-dihydroacridine (dmac) groups resulted in the formation of [ReCl(CO)3(R2phen)] with mixed 3MLCT/3ILCT triplet excited state.

Metal-Catalyzed Hydride Transfer from Alcohols to Photoexcited Dipyridophenazine†

Dipyridophenazine (dppz) is known to react with alcohols upon photoexcitation into an n-π* transition at 378 nm to yield dihydrodipyridophenazine (dppzH₂ ). This process occurs via H-atom abstraction from alcohols and subsequent disproportionation of the dppzH^• radical species, to the final product. This reaction shows a primary kinetic isotope effect (KIE = 4.9) in methanol/perdeuteromethanol solvents, consistent with H-atom transfer processes. Addition of excess Zn(II) ions to the dppz solution not only accelerates the rate of photoreduction, but also lowers the KIE to 1.7, indicating a change in reaction mechanism. We postulate that the coordination of the alcoholic solvent to Zn(II) activates the alcohol α C-H bonds toward hydride transfer processes which would be consistent with the lowered KIE and faster overall reduction of the aromatic ligand. Interestingly, this appears to be an intramolecular process in which the Zn(II) is coordinated to both the dppz ligand and the reactive alcohol, as a sharp inflection in the overall rate increase is observed at a Zn:dppz ratio of 2:1. At this ratio, the dominant dppz species involves a Zn(II) bound to one dppz and several solvent molecules (methanol and water).

Carbazole effect on ground- and excited-state properties of rhenium(i) carbonyl complexes with extended terpy-like ligands

The ground- and excited-state properties of three novel complexes [ReCl(CO)₃(Lⁿ-κ²N)] bearing 2,2':6',2''-terpyridine, 2,6-di(thiazol-2-yl)pyridine and 2,6-di(pyrazin-2-yl)pyridine functionalized with 9-carbazole attached to the central pyridine ring of the triimine core via phenylene linkage were investigated by spectroscopic and electrochemical methods and were simulated using density functional theory (DFT) and time-dependent DFT. To get a deeper and broader understanding of structure-property relationships, the designed Re(i) carbonyl complexes were compared with previously reported analogous systems - without any groups attached to the phenyl ring and bearing pyrrolidine instead of 9-carbazole. The results indicated that attachment of the N-carbazolyl substituent to the triimine core has less influence on the nature of the triplet excited state of [ReCl(CO)₃(Lⁿ-κ²N)] than the pyrrolidine group. Additionally, the impact of the ligand structural modifications on the light emission of the Re(i) complexes under external voltage was preliminarily examined with electroluminescence spectra of diodes containing the synthesized new molecules in an active layer.

Optically Controlled Electron Transfer in a ReI Complex

Ultrafast optical control of intramolecular charge flow was demonstrated, which paves the way for photocurrent modulation and switching with a highly wavelength-selective ON/OFF ratio. The system that was explored is a fac-[Re(CO)₃ (TTF-DPPZ)Cl] complex, where TTF-DPPZ=4',5'-bis(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2',3'-c]phenazine. DFT calculations and AC-Stark spectroscopy confirmed the presence of two distinct optically active charge-transfer processes, namely a metal-to-ligand charge transfer (MLCT) and an intra-ligand charge transfer (ILCT). Ultrafast transient absorption measurements showed that the ILCT state decays in the ps regime. Upon excitation to the MLCT state, only a long-lived ³ MLCT state was observed after 80 ps. Remarkably, however, the bleaching of the ILCT absorption band remained as a result of the effective inhibition of the HOMO-LUMO transition.

Photoluminescence enhancement of Re(i) carbonyl complexes bearing D-A and D-π-A ligands

Three Re(i) carbonyl complexes [ReCl(CO)3(Ln)] bearing 2,2'-bipyridine, 2,2':6',2''-terpyridine, and 1,10-phenanthroline functionalized with diphenylamine/or triphenylamine units (L1-L3) were synthesized to explore the impact of highly electron donating units appended to the imine ligand on the thermal and optoelectronic properties of Re(i) systems. Additionally, for comparison, the ligands L1-3 and parent complexes [ReCl(CO)3(bipy)], [ReCl(CO)3(phen)] and [ReCl(CO)3(terpy-κ2N)] were investigated. The thermal stability was evaluated by differential scanning calorimetry. The ground- and excited-state electronic properties of the Re(i) complexes were studied by cyclic voltammetry and differential pulse voltammetry, absorption and emission spectroscopy, as well as using density-functional theory (DFT). The majority of the compounds form amorphous molecular materials with high glass transition temperatures above 100 °C. Compared to the unsubstituted complexes [ReCl(CO)3(bipy)], [ReCl(CO)3(phen)] and [ReCl(CO)3(terpy-κ2N)], the HOMO-LUMO gap of the corresponding Re(i) systems bearing modified imine ligands is reduced, and the decrease in the value of the ΔEH-L is mainly caused by the increase in HOMO energy level. In relation to the parent complexes, all designed Re(i) carbonyls were found to show enhanced photoluminescence, both in solution and in solid state. The investigated ligands and complexes were also preliminarily tested as luminophores in light emitting diodes with the structures ITO/PEDOT:PSS/compound/Al and ITO/PEDOT:PSS/PVK:PBD:compound/Al. The pronounced effect of the ligand chemical structure on electroluminescence ability was clearly visible.

Dramatic Alteration of 3ILCT Lifetimes Using Ancillary Ligands in [Re(L)(CO)3(phen-TPA)] n+ Complexes: An Integrated Spectroscopic and Theoretical Study

The ground and excited state photophysical properties of a series of fac-[Re(L)(CO)₃(α-diimine)] ⁿ⁺ complexes, where L = Br^-, Cl^-, 4-dimethylaminopyridine (dmap) and pyridine (py) have been extensively studied utilizing numerous electronic and vibrational spectroscopic techniques in conjunction with a suite of quantum chemical methods. The α-diimine ligand consists of 1,10-phenanthroline with the highly electron donating triphenylamine (TPA) appended in the 5 position. This gives rise to intraligand charge transfer (ILCT) states lying lower in energy than the conventional metal-to-ligand charge transfer (MLCT) state, the energies of which are red and blue-shifted, respectively, as the ancillary ligand, L becomes more electron withdrawing. The emitting state is ³ILCT in nature for all complexes studied, characterized through transient absorption and emission, transient resonance Raman (TR²), time-resolved infrared (TRIR) spectroscopy and TDDFT calculations. Systematic modulation of the ancillary ligand causes unanticipated variation in the ³ILCT lifetime by 2 orders of magnitude, ranging from 6.0 μs for L = Br^- to 27 ns for L = py, without altering the nature of the excited state formed or the relative order of the other CT states present. Temperature dependent lifetime measurements and quantum chemical calculations provide no clear indication of close lying deactivating states, MO switching, contributions from a halide-to-ligand charge transfer (XLCT) state or dramatic changes in spin-orbit coupling. It appears that the influence of the ancillary ligand on the excited state lifetime could be explained in terms of energy gap law, in which there is a correlation between ln( k_nr) and E_em with a slope of -21.4 eV^-1 for the ³ILCT emission.

Photophysics and Photoredox Catalysis of a Homoleptic Rhenium(I) Tris(diisocyanide) Complex

Herein a homoleptic rhenium(I) complex bearing three chelating diisocyanide ligands and its photophysical properties are communicated. The complex emits weakly from a high-energy triplet metal-to-ligand charge-transfer excited state with an 8 ns lifetime in deaerated CH₃CN at 22 °C and is shown to act as an efficient photoredox catalyst comparable to [Ir(ppy)₃] (ppy = 2-phenylpyridine) in representative test reactions.

Substituent influence in phenanthroline-derived ancillary ligands on the excited state nature of novel cationic Ir(iii) complexes

Influence of ancillary ligands derived from phenanthroline on the nature of the deactivation pathways of novel cationic Ir(iii) cyclometalated complexes.

Contrasting photophysical properties of rhenium(i) tricarbonyl complexes having carbazole groups attached to the polypyridine ligand

The fac-[Re(CO)₃(cbz₂phen)(L)]^0/+1 complexes showed a remarkable presence of the ILCT_cbz2phen fluorescence in addition to the usually observed ³MLCT_{Re→cbz2phen}. In PMMA films the emission is completely turned into a triplet excited state manifold.

Rhenium(i) complexes with phenanthrolines bearing electron-withdrawing Cl and electron-donating CH3 substituents – synthesis, photophysical, thermal, and electrochemical properties with electroluminescence ability

Substituent effect was investigated.

Probing organometallic reactions by time-resolved infrared spectroscopy in solution and in the solid state using quantum cascade lasers

The photochemistry and photophysics of metal carbonyl compounds (W(CO)6, Cp*Rh(CO)2 (Cp* = η(5)-C5Me5), and fac-[Re(CO)3(4,4'-bpy)2Br] [bpy = bipyridine]) have been examined on the nanosecond timescale using a time-resolved infrared spectrometer with an external cavity quantum cascade laser (QCL) as the infrared source. We show the photochemistry of W(CO)6 in alkane solution is easily monitored, and very sensitive measurements are possible with this approach, meaning it can monitor small transients with absorbance changes less than 10(-6) ΔOD. The C-H activation of Cp*Rh(CO)(C6H12) to form Cp*Rh(CO)(C6H11)H occurs within the first few tens of nanoseconds following photolysis, and we demonstrate that kinetics obtained following deconvolution are in excellent agreement with those measured using an ultrafast laser-based spectrometer. We also show that the high flux and tunability of QCLs makes them suited for solid-state and time-resolved measurements.

Spin-vibronic quantum dynamics for ultrafast excited-state processes

Ultrafast intersystem crossing (ISC) processes coupled to nuclear relaxation and solvation dynamics play a central role in the photophysics and photochemistry of a wide range of transition metal complexes. These phenomena occurring within a few hundred femtoseconds are investigated experimentally by ultrafast picosecond and femtosecond transient absorption or luminescence spectroscopies, and optical laser pump-X-ray probe techniques using picosecond and femtosecond X-ray pulses. The interpretation of ultrafast structural changes, time-resolved spectra, quantum yields, and time scales of elementary processes or transient lifetimes needs robust theoretical tools combining state-of-the-art quantum chemistry and developments in quantum dynamics for solving the electronic and nuclear problems. Multimode molecular dynamics beyond the Born-Oppenheimer approximation has been successfully applied to many small polyatomic systems. Its application to large molecules containing a transition metal atom is still a challenge because of the nuclear dimensionality of the problem, the high density of electronic excited states, and the spin-orbit coupling effects. Rhenium(I) α-diimine carbonyl complexes, [Re(L)(CO)3(N,N)](n+) are thermally and photochemically robust and highly flexible synthetically. Structural variations of the N,N and L ligands affect the spectroscopy, the photophysics, and the photochemistry of these chromophores easily incorporated into a complex environment. Visible light absorption opens the route to a wide range of applications such as sensors, probes, or emissive labels for imaging biomolecules. Halide complexes [Re(X)(CO)3(bpy)] (X = Cl, Br, or I; bpy = 2,2'-bipyridine) exhibit complex electronic structure and large spin-orbit effects that do not correlate with the heavy atom effects. Indeed, the (1)MLCT → (3)MLCT intersystem crossing (ISC) kinetics is slower than in [Ru(bpy)3](2+) or [Fe(bpy)3](2+) despite the presence of a third-row transition metal. Counterintuitively, singlet excited-state lifetime increases on going from Cl (85 fs) to Br (128 fs) and to I (152 fs). Moreover, correlation between the Re-X stretching mode and the rate of ISC is observed. In this Account, we emphasize on the role of spin-vibronic coupling on the mechanism of ultrafast ISC put in evidence in [Re(Br)(CO)3(bpy)]. For this purpose, we have developed a model Hamiltonian for solving an 11 electronic excited states multimode problem including vibronic and SO coupling within the linear vibronic coupling (LVC) approximation and the assumption of harmonic potentials. The presence of a central metal atom coupled to rigid ligands, such as α-diimine, ensures nuclear motion of small amplitudes and a priori justifies the use of the LVC model. The simulation of the ultrafast dynamics by wavepacket propagations using the multiconfiguration time-dependent Hartree (MCTDH) method is based on density functional theory (DFT), and its time-dependent extension to excited states (TD-DFT) electronic structure data. We believe that the interplay between time-resolved experiments and these pioneering simulations covering the first picoseconds and including spin-vibronic coupling will promote a number of quantum dynamical studies that will contribute to a better understanding of ultrafast processes in a wide range of organic and inorganic chromophores easily incorporated in biosystems or supramolecular devices for specific functions.

Enhanced Cytotoxicity through Conjugation of a "Clickable" Luminescent Re(I) Complex to a Cell-Penetrating Lipopeptide

Re(I) tricarbonyl polypyridine-based complexes are particularly attractive metal complexes in the field of inorganic chemical biology due to their luminescent properties, ease of conjugation to targeting biomolecules, and the possibility to prepare their "hot" (99m)Tc analogues for radioimaging. In this study, we prepared and characterized a novel, "clickable" complex, [Re(2,2'-bipyridine)(3-ethynylpyridine)(CO)3](BF4) ([Re(CO) 3 (bipy)(py-alkyne)](BF 4 )), exhibiting the characteristic luminescent properties and moderate cytotoxicity of this general class of compound. Using Cu(I)-catalyzed "click" chemistry, the complex was efficiently attached to a lipidated peptide known to increase cell permeability, namely, the myristoylated HIV-1 Tat peptide (myr-Tat), to give Re-myr-Tat. Fluorescence microscopy localization in human cervical cancer cells (HeLa) confirmed enhanced cellular uptake of Re-myr-Tat compared with [Re(CO) 3 (bipy)(py-alkyne)](BF 4 ), and cytotoxicity studies showed that this resulted in an increase in potency to a level comparable with cisplatin (13.0 ± 2.0 μM).

The photophysics of fac-[Re(CO)3(NN)(bpa)]+ complexes: a theoretical/experimental study

The influence of the polypyridyl ligand on the photophysics of Re(i) tricarbonyl complexes was investigated by steady-state and time resolved spectroscopy as well as by theoretical calculations.

Stretching the phenazine MO in dppz: the effect of phenyl and phenyl–ethynyl groups on the photophysics of Re(i) dppz complexes

Dppz complexes with extended conjugation show π,π* and MLCT excited states in solution. Raman spectral features for the short-lived π,π* state are observed in single-colour transient experiments.

Using metal complex reduced states to monitor the oxidation of DNA

Metallointercalating photooxidants interact intimately with the base stack of double-stranded DNA and exhibit rich photophysical and electrochemical properties, making them ideal probes for the study of DNA-mediated charge transport (CT). The complexes [Rh(phi)(2)(bpy')](3+) (phi = 9,10-phenanthrenequinone diimine; bpy' = 4-methyl-4'-(butyric acid)-2,2'-bipyridine), [Ir(ppy)(2)(dppz')](+) (ppy = 2-phenylpyridine; dppz' = 6-(dipyrido[3,2-a:2',3'-c]phenazin-11-yl)hex-5-ynoic acid), and [Re(CO)(3)(dppz)(py')](+) (dppz = dipyrido[2,3-a:2',3'-c]phenazine; py' = 3-(pyridin-4-yl)-propanoic acid) were each covalently tethered to DNA to compare their photooxidation efficiencies. Biochemical studies show that upon irradiation, the three complexes oxidize guanine by long-range DNA-mediated CT with the efficiency: Rh > Re > Ir. Comparison of spectra obtained by spectroelectrochemistry after bulk reduction of the free metal complexes with those obtained by transient absorption (TA) spectroscopy of the conjugates suggests that the reduced metal states form following excitation of the conjugates at 355 nm. Electrochemical experiments and kinetic analysis of the TA decays indicate that the thermodynamic driving force for CT, variations in the efficiency of back electron transfer, and coupling to DNA are the primary factors responsible for the trend observed in the guanine oxidation yields of the three complexes.

Charge photoinjection in intercalated and covalently bound [Re(CO)3(dppz)(py)]+-DNA constructs monitored by time-resolved visible and infrared spectroscopy

The complex [Re(CO)(3)(dppz)(py'-OR)](+) (dppz = dipyrido[3,2-a:2',3'-c]phenazine; py'-OR = 4-functionalized pyridine) offers IR sensitivity and can oxidize DNA directly from the excited state, making it a promising probe for the study of DNA-mediated charge transport (CT). The behavior of several covalent and noncovalent Re-DNA constructs was monitored by time-resolved IR (TRIR) and UV/visible spectroscopies, as well as biochemical methods, confirming the long-range oxidation of DNA by the excited complex. Optical excitation of the complex leads to population of MLCT and at least two distinct intraligand states. Experimental observations that are consistent with charge injection from these excited states include similarity between long-time TRIR spectra and the reduced state spectrum observed by spectroelectrochemistry, the appearance of a guanine radical signal in TRIR spectra, and the eventual formation of permanent guanine oxidation products. The majority of reactivity occurs on the ultrafast time scale, although processes dependent on slower conformational motions of DNA, such as the accumulation of oxidative damage at guanine, are also observed. The ability to measure events on such disparate time scales, its superior selectivity in comparison to other spectroscopic techniques, and the ability to simultaneously monitor carbonyl ligand and DNA IR absorption bands make TRIR a valuable tool for the study of CT in DNA.

Excited state dependent electron transfer of a rhenium-dipyridophenazine complex intercalated between the base pairs of DNA: a time-resolved UV-visible and IR absorption investigation into the photophysics of fac-[Re(CO)3(F2dppz)(py)]+ bound to either [poly(dA-dT)]2 or [poly(dG-dC)]2

The transient species formed following excitation of fac-[Re(CO)(3)(F(2)dppz)(py)](+) (F(2)dppz = 11,12-difluorodipyrido[3,2-a:2',3'-c]phenazine) bound to double-stranded polynucleotides [poly(dA-dT)](2) or [poly(dG-dC)](2) have been studied by transient visible and infra-red spectroscopy in both the picosecond and nanosecond time domains. The latter technique has been used to monitor both the metal complex and the DNA by monitoring the regions 1900-2100 and 1500-1750 cm(-1) respectively. These data provide direct evidence for electron transfer from guanine to the excited state of the metal complex, which proceeds both on a sub-picosecond time scale and with a lifetime of 35 ps, possibly due to the involvement of two excited states. No electron transfer is found for the [poly(dA-dT)](2) complex, although characteristic changes are seen in the DNA-region TRIR consistent with changes in the binding of the bases in the intercalation site upon excitation of the dppz-complex.

EPR parameters of radical ions and polarizability effect

The literature data on substituent influence on the g factors, hyperfine coupling (hfc) constants (a) of the EPR signals and the spin densities (ρ) have been analyzed for 25 series of the organic radical cations and radical anions as well as of the transition metal complexes. The g, a, and ρ values were first established to depend not only on the inductive and resonance effects but also on the polarizability of substituents which can be characterized by the σ(α) constants. The polarizability effect is caused by the partial charge on the radical center. This effect consists in an electrostatic attraction between the charge and the dipole moments induced by this charge in the substituents. The polarizability contribution ranges up to 92%.