Exploitation of Luminescent Organometallic Rhenium(I) and Iridium(III) Complexes in Biological Studies

Rhenium(I) Complexes with Neutral Monodentate Coligands and Monoanionic 2-(1,2,4-Triazol-5-yl)pyridine-Based Chelators as Bidentate Luminophores with Tunable Color and Photosensitized Generation of 1O2: An Integrated Case Study Involving Photophysics and Theory

In this work, we describe the synthesis as well as structural, photophysical, and theoretical investigation of a new coordination chemical concept involving rhenium(I) complexes bearing monoanionic 1,2,4-triazolylpyridine-based bidentate chromophores. The X-ray diffractometric analysis of single crystals revealed particular packing features: the trifluoromethylated exemplar displayed two kinds of arrangements of the coordination centers, where the bidentate ligands at the edges of the unit cell are staggered parallel to each other, whereas those inside show antiparallel stacking with respect to the external ligands. On the other hand, the complexes bearing an adamantyl substituent yield a linear arrangement, where the bulky moiety of one luminophore points to the pyridine center of the adjacent ligand of the neighboring complex while including methanol molecules hydrogen-bonded to the triazolato unit. We observed that the photophysical properties of the complexes (photoexcited-state lifetimes, photoluminescence maxima and quantum yields) can be adjusted by tuning of the substitution pattern at the bidentate luminophore as well as by variation of the monodentate coligand. The photoluminescence spectra and photoexcited-state lifetimes of the crystalline phases were measured by phosphorescence lifetime micro(spectro)scopy. Interestingly, the vibrationally resolved emission spectra of the crystals closely resemble those of diluted frozen glassy matrixes at 77 K, in contrast with the broad bands observed in amorphous solids and in fluid solutions, where the charge-transfer character is enhanced. While the photoluminescence quantum yields (Φ_L) reach up to 15%, the complexes are able to attain up to 55% efficiency regarding the photosensitization of ¹O₂ (Φ_Δ), depending on the combination of luminophore and coligand. Theoretical calculations showed that the photoexcited triplet (T₁) state has a metal-ligand-to-ligand charge-transfer character, where promotion to the excited electronic configuration shortens the Re(I)-N bond involving the bidentate triazolylpyridine while stretching the three fac-CO-Re(I) bonds as well as the linkage to the axial monodentate coligand. The calculated vertical (E^vl) and 0-0 (E^(0-0)) radiative transition energies are in very good agreement with the experimental values (E_exp^lum).

Luminescent Metal Complexes as Emerging Tools for Lipid Imaging

Fluorescence microscopy is a key tool in the biological sciences, which finds use as a routine laboratory technique (e.g., epifluorescence microscope) or more advanced confocal, two-photon, and super-resolution applications. Through continued developments in microscopy, and other analytical methods, the importance of lipids as constituents of subcellular organelles, signalling or regulating molecules continues to emerge. The increasing recognition of the importance of lipids to fundamental cell biology (in health and disease) has prompted the development of protocols and techniques to image the distribution of lipids in cells and tissues. A diverse suite of spectroscopic and microscopy tools are continuously being developed and explored to add to the "toolbox" to study lipid biology. A relatively recent breakthrough in this field has been the development and subsequent application of metal-based luminescent complexes for imaging lipids in biological systems. These metal-based compounds appear to offer advantages with respect to their tunability of the photophysical properties, in addition to capabilities centred around selectively targeting specific lipid structures or classes of lipids. The presence of the metal centre also opens the path to alternative imaging modalities that might not be applicable to traditional organic fluorophores. This review examines the current progress and developments in metal-based luminescent complexes to study lipids, in addition to exploring potential new avenues and challenges for the field to take.

Electrochromic behavior of fac-tricarbonyl rhenium complexes

Tricarbonyl rhenium complex shows good electrochromic performance with a colored stage of green, rapid response and good switching stability.

Phenyl-pyta-tricarbonylrhenium(I) complexes: combining a simplified structure and steric hindrance to modulate the photoluminescence properties

Strongly luminescent tricarbonylrhenium(I) complexes are promising candidates in the field of optical materials. In this study, three new complexes bearing a 3-(2-pyridyl)-1,2,4-triazole (pyta) bidentate ligand with an appended phenyl group were obtained in very good yields owing to an optimized synthetic procedure. The first member of this series, i.e. complex 1, was compared with the previously studied complex RePBO to understand the influence of the fluorescent benzoxazole unit grafted on the phenyl ring. Then, to gauge the effect of steric hindrance on the luminescence properties, the phenyl group of complex 1 was substituted in the para position by a bulky tert-butyl group or an adamantyl moiety, affording complexes 2 and 3, respectively. The results of theoretical calculations indicated that these complexes were quite similar from an electronic point of view, as evidenced by the electrochemical study. In dichloromethane solution, under excitation in the UV range, all the complexes emitted weak phosphorescence in the red region. In the solid state, they could be excited in the blue region of the visible spectrum and they emitted strong yellow light. The photoluminescence quantum yield was markedly increased with raising the size of the substituent, passing from 0.42 for 1 to 0.59 for 3. The latter complex also exhibited clear waveguiding properties, unprecedented for rhenium complexes. From this point of view, these easy-synthesized and spectroscopically attractive complexes constitute a new generation of emitters for use in imaging applications and functional materials. However, the comparison with RePBO showed that the presence of the benzoxazole group leads to unsurpassed mechanoresponsive luminescence (MRL) properties, due to the involvement of a unique photophysical mechanism that takes place only in this type of complex.

Understanding the photophysical properties of rhenium(I) compounds coordinated to 4,7-diamine-1,10-phenanthroline: synthetic, luminescence and biological studies

In the present study, the photophysical properties and preliminary time-dependent density functional theory (TD-DFT) data of new rhenium(i) polypyridyl compounds, fac-[Re(L)(Am2phen)(CO)3]0/+, where Am2phen = 4,7-diamine-1,10-phenanthroline and L = Cl and ethyl isonicotinate (et-isonic), provided new insights into excited-state deactivation through an unusual inversion between two metal-to-ligand charge-transfer excited states. In addition, their cellular uptake using breast cancer (MCF-7) and melanoma (SkMel-147 and SkMel-29) cell lines and bioactivity were investigated and their cell-killing mechanism and protein expression were also studied. Preliminary TD-DFT results showed that both compounds exhibited a strong and broad absorption band around 300-400 nm which corresponds to a combination of ILAm2phen and MLCTRe→Am2phen transitions, and a strong contribution of charge transfer transition MLCTRe→et-isonic for fac-[Re(et-isonic)(Am2phen)(CO)3]+ is also observed. In contrast to typical Re(i) polypyridyl complexes, the substitution of Cl with the et-isonic ligand showed a bathochromic shift of the emission maxima, relatively low emission quantum yield and fast lifetime. Photophysical investigation of the fac-[ReCl(et-isonic)2(CO)3] compound provided meaningful information on the excited state manifold of the fac-[Re(L)(Am2phen)(CO)3]0/+ complexes. As shown in the absorption profile, a remarkable inversion of the lowest-lying excited state takes place from the usually observed MLCTRe→Am2phen to the unusual MLCTRe→et-isonic. The lipophilicity of the positive-complex was higher than that of the non-charge compound and the same trend for the activity against cells was observed, in the absence of light. In addition, flow cytometry and Western Blot analyses showed an overexpression of pro-caspase-9, suggesting a caspase proteolytic cascade through an intrinsic-pathway apoptosis mechanism. The photophysical properties of these compounds reported herein provide new fundamental insights into the understanding of substituent groups on polypyridyl ligands which are relevant to practical development.

Phosphorescent multinuclear complexes for optoelectronics: tuning of the excited-state dynamics

Luminescent transition metal complexes have attracted a great deal of attention in the last two decades from both fundamental and application points of view. The majority of the investigated and most efficient systems consist of monometallic compounds with judiciously selected ligand sphere, providing excellent triplet emitters for both lab-scale and real-market light-emitting devices for display technologies. More recently, chemical architectures comprising multimetallic compounds have appeared as an emerging and valuable alternative. Herein, the most recent trends in the field are showcased in a systematic approach, where the different examples are classified by metal center and ligand(s) scaffold. Their optical and electroluminescence properties are presented and compared as well. Indeed, the multimetallic strategy has proven to be highly suitable for compounds emitting efficiently in the challenging red to near-infrared region, yielding metal-based emitters with improved optical properties in terms of enhanced emission efficiency, shortened excited-state lifetime, and faster radiative rate constant. Finally, the advantages and drawbacks of the multimetallic approach will be discussed.

2-Azabutadiene complexes of rhenium(I): S,N-chelated species with photophysical properties heavily governed by the ligand hidden traits

The reaction of [Re(CO)3(THF)(μ-Br)]2 or [Re(CO)5X] (X = Cl, Br, I) with the diaryl-2-azabutadienes [(RS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CAr2] containing two thioether arms at the 4,4-position forms the luminescent S,N-chelate complexes fac-[(OC)3ReX{(RS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CAr2}] (1a-h). The halide abstraction by silver triflate converts [(OC)3ReCl{(PhS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CPh2}] (1c) to [(OC)3Re(OS([double bond, length as m-dash]O)2CF3){(PhS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CPh2}] (1j) bearing a covalently bound triflate ligand. The cyclic voltammograms reveal reversible S^N ligand-centred reduction and irreversible oxidation waves for all complexes. The crystal structures of nine octahedral complexes have been determined along with that of (NaphtylS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CPh2 (L6). A rich system of weak non-covalent intermolecular secondary interactions through CHX(Cl, Br)Re, CHO, COπ(Ph), CHπCO, CHO and CHS contacts has been evidenced. The photophysical properties have been investigated by steady-state and time-resolved absorption (fs transient absorption, fs-TAS) and emission (ns-TCSPC and ps-Streak camera) spectroscopy in 2-MeTHF solution at 298 and 77 K. The emission bands are composed of either singlet (450 < λmax < 535 nm) and/or triplet emissions (at 77 K only, λmax < 640 nm, or appearing as a tail at λ > 600 nm), which decay in a multiexponential manner for the fluorescence (short ps (i.e. <IRF) < τF < 1.9 ns at 298 and 77 K) and monoexponentially for the phosphorescence (4.0 < τP < 7.0 ns at 77 K). The fs-TAS data reveal the presence of 2 to 4 transient species decaying in four narrow time windows (generally 125-165 fs, 370-685 fs, 3-6 ps, 30-45 ps). The complexity of these kinetics was explained by studying the photophysical behaviour of ligand L6. Its behaviour is the same as the complexes thus indicating that the ligand dictates the kinetic traits of the Re-species, except for the triplet emission as L6 is not phosphorescent. The triplet lifetime (4.0 < τP < 7.0 ns) is considered very short but not unprecedented. Furthermore, the nature of the lowest energy excited states of these chelate compounds and L6 has been addressed using DFT and TDDFT calculations and been assigned to metal-to-ligand (MLCT) and/or intraligand charge-transfer (ILCT).

Quenching of the phosphorescence of thermally reversible photochromic naphthopyran Re(i) complexes initiated by either visible or ultraviolet radiation

Re(i) complexes bearing thermally reversible photochromic naphthopyran axial ligands undergo highly efficient, reversible phosphorescence quenching actuated by either visible or UV irradiation. The photoinduced quenching of the triplet metal-to-ligand charge-transfer (³MLCT) emission is interpreted based on changes in the relative energies of the excited states.

Photochemical conversion of CO2 to CO by a Re complex: theoretical insights into the formation of CO and HCO3− from an experimentally detected monoalkyl carbonate complex

Triethanolamine (TEOA) has been used for the photocatalytic reduction of CO₂, and the experimental studies have demonstrated that the TEOA increases the catalytic efficiency. In addition, the formation of a carbonate complex has been confirmed in the Re photocatalytic system where DMF and TEOA are used as solvents. In this study, we survey the reaction pathways of the photocatalytic conversions of CO₂ to CO + H₂O and CO₂ to CO + HCO₃⁻ by fac-Re(bpy)(CO)₃Br in the presence of TEOA using density functional theory (DFT) and domain-based local pair natural orbital coupled cluster approach, DLPNO-CCSD(T). Under light irradiation, the solvent-coordinated Re complex is first reduced to form a monoalkyl carbonate complex in the doublet pathway. This doublet pathway is kinetically advantageous over the singlet pathway. To reduce carbon dioxide, the Re complex needs to be reduced by two electrons. The second electron reduction occurs after the monoalkyl carbonate complex is protonated. The second reduction involves the dissociation of the monoalkyl carbonate ligand, and the dissociated ligand recombines the Re center via carbon to generate Re–COOH species, which further reacts with CO₂ to generate tetracarbonyl complex and HCO₃⁻. The two-electron reduced ligand-free Re complex converts CO₂ to CO and H₂O. The pathways leading to H₂O formation have lower barriers than the pathways leading to HCO₃⁻ formation, but their portion of formation must depend on proton concentration.

DFT and DLPNO-CCSD(T) calculations proposed a pathway for the conversion of the experimentally detected monoarkyl carbonate complex to tetracarbonyl complex.

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.

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.

Time-Resolved Femtosecond Stimulated Raman Spectra and DFT Anharmonic Vibrational Analysis of an Electronically Excited Rhenium Photosensitizer

Time-resolved femtosecond stimulated Raman spectra (FSRS) of a prototypical organometallic photosensitizer/photocatalyst ReCl(CO)₃(2,2'-bipyridine) were measured in a broad spectral range ∼40-2000 (4000) cm^-1 at time delays from 40 fs to 4 ns after 400 nm excitation of the lowest allowed electronic transition. Theoretical ground- and excited-state Raman spectra were obtained by anharmonic vibrational analysis using second-order vibrational perturbation theory on vibrations calculated by harmonic approximation at density functional theory-optimized structures. A good match with anharmonically calculated vibrational frequencies allowed for assigning experimental Raman features to particular vibrations. Observed frequency shifts upon excitation (ν(ReCl) and ν(CC inter-ring) vibrations upward; ν(CC, CN) and ν(Re-C) downward) are consistent with the bonding/antibonding characters of the highest occupied molecular orbital and the lowest unoccupied molecular orbital involved in excitation and support the delocalized formulation of the lowest triplet state as ReCl(CO)₃ → bpy charge transfer. FSRS spectra show a mode-specific temporal evolution, providing insights into the intersystem crossing (ISC) mechanism and subsequent relaxation. Most of the Raman features are present at ∼40 fs and exhibit small shifts and intensity changes with time. The 1450-1600 cm^-1 group of bands due to CC, CN, and CC(inter-ring) stretching vibrations undergoes extensive restructuring between 40 and ∼150 fs, followed by frequency upshifts and a biexponential (0.38, 21 ps) area growth, indicating progressing charge separation in the course of the formation and relaxation of the lowest triplet state. Early (40-150 fs) restructuring was also observed in the low-frequency range for ν(Re-Cl) and δ(Re-C-O) vibrations that are presumably activated by ISC. FSRS experimental innovations employed to measure low- and high-energy Raman features simultaneously are described and discussed in detail.

Optimization of aggregation-induced phosphorescence enhancement in mononuclear tricarbonyl rhenium(i) complexes: the influence of steric hindrance and isomerism

In order to improve the remarkable performance of a mononuclear tricarbonyl rhenium(i) complex (ReL1) that exhibits rare aggregation-induced phosphorescence enhancement (AIPE) behavior, two new complexes (ReL3 and ReL4) were prepared and investigated. They incorporate a 2-pyridyl-1,2,4-triazole (pyta) ligand connected to a 2-phenylbenzoxazole (PBO) moiety. Complex ReL3 differs from ReL1 by the presence of a bulky tert-butyl substituent, and ReL4 is an isomer where the PBO group is linked to the pyta ligand by its phenyl group. Theoretical calculations were in congruence with electrochemical and spectroscopic properties in solutions. Both new compounds exhibited strong AIPE and much better solid-state emission efficiency than ReL1, with photoluminescence quantum yields up to 55% for ReL4. Crystallographic data indicate that this increase in emission efficiency is due to optimum packing that prevents quenching. This work shows that minor structural changes may have major effects upon the solid-state spectroscopic properties and it provides a rational basis for accessing AIPE-active strongly emissive rhenium(i) complexes.

Tricarbonyl rhenium(i) tetrazolato and N-heterocyclic carbene complexes: versatile visible-light-mediated photoredox catalysts

This study demonstrates that structurally-diverse, photoactive rhenium(i) tricarbonyl complexes can mediate representative atom-transfer radical addition, hydrodehalogenation, and α-amino C–H functionalisation reactions.

Functionalization and passivation of ultrathin alumina films of defined sub-nanometer thickness with self-assembled monolayers

Instability of ultrathin surface oxides on alloys under environmental conditions can limit the opportunities for applications of these systems when the thickness control of the insulating oxide film is crucial for device performance. A procedure is developed to directly deposit self-assembled monolayers (SAM) from solvent onto substrates prepared under ultra-high vacuum conditions without exposure to air. As an example, rhenium photosensitizers functionalized with carboxyl linker groups are attached to ultrathin alumina grown on NiAl(1 1 0). The thickness change of the oxide layer during the SAM deposition is quantified by x-ray photoelectron spectroscopy and can be drastically reduced to one atomic layer. The SAM acts as a capping layer, stabilizing the oxide thin film under environmental conditions. Ultraviolet photoelectron spectroscopy elucidates the band alignment in the resulting heterostructure. The method for molecule attachment presented in this manuscript can be extended to a broad class of molecules vulnerable to pyrolysis upon evaporation and presents an elegant method for attaching molecular layers on solid substrates that are sensitive to air.

Phosphine oxide-based tricarbonylrhenium(i) complexes from phosphine/phosphine oxide and dihydroxybenzoquinones

Neutral phosphine oxide (P[double bond, length as m-dash]O) donor-based organometallic complexes [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-DHBQ}{Re(CO)3O[double bond, length as m-dash]PCy3}] (1), [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-DHBQ}{Re(CO)3O[double bond, length as m-dash]PPh3}] (2), [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-THQ}{Re(CO)3O[double bond, length as m-dash]PCy3}] (3), [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-THQ}{Re(CO)3O[double bond, length as m-dash]PPh3}] (4), [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-CA}{Re(CO)3O[double bond, length as m-dash]PCy3}] (5), and [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-CA}{Re(CO)3O[double bond, length as m-dash]PPh3}] (6) were assembled from phosphine/phosphine oxide, a dihydroxybenzoquinone donor and Re2(CO)10via a one-pot solvothermal approach. The soft phosphine donor was transformed into a hard phosphine oxide donor during the formation of 1, 3, 4, 5, and 6. The complexes 1-6 were air and moisture stable and were soluble in polar organic solvents. The complexes were characterized by elemental analysis, FT-IR, and NMR spectroscopic methods. The molecular structures of 1, 2, 4, and 6 were analyzed by single-crystal X-ray diffraction analysis. The UV-Visible absorption studies indicated that 1-6 in THF display strong visible light absorption in the range of ∼350-700 nm.

Ligand substitution and conformational effects on the ultrafast luminescent decay of [Re(CO)3(phen)(L)]+ (L = imidazole, pyridine): non-adiabatic quantum dynamics

The C_s conformers of [Re(CO)₃(phen)(L)]⁺ (L = imidazole, pyridine) complexes are investigated by means of density functional theory (DFT/time-dependent DFT (TD-DFT)) electronic structure calculations and non-adiabatic quantum dynamics including spin–orbit coupling (SOC).

Photoisomerization of di-nuclear rhenium(i) bpe-based compounds

Photoisomerization processes of [{(NN)(CO)₃Re}₂(trans-bpe)]²⁺ complexes showed the contribution of the singlet pathway in addition to the usually observed triplet one. The luminescence observed for the cis-complexes in CH₃CN is able to efficiently photosensitize the generation of ¹O₂.

Spin-Forbidden Excitation: A New Approach for Triggering Photopharmacological Processes with Low-Intensity NIR Light

Exposure to low-intensity radiation in the near-infrared (NIR) spectral region matching the optically transparent "phototherapeutic window" of biological tissues can be applied to directly populate spin-restricted excited states of light-responsive compounds. This unconventional and unprecedented approach is introduced herein as a new strategy to overcome some of the major unresolved problems observed in the rapidly emerging fields of photopharmacology and molecular photomedicine, where practical applications in living cells and organisms are still limited by undesired side reactions and insufficient light penetration. Water-soluble and biocompatible metal complexes with a significant degree of spin-orbit coupling were identified as target candidates for testing our new hypothesis. As a first example, a dark-stable manganese carbonyl complex acting as a visible-light-triggered CO-releasing molecule (Photo-CORM) is shown to be photoactivated by NIR radiation, although apparently no spectroscopically evident absorption bands are detectable in this low-energy region. This quite remarkable effect is ascribed to a strongly restricted, but obviously not completely forbidden optical population of the lowest triplet excited state manifold of the diamagnetic complex from the singlet ground state.

Photofunctional multinuclear rhenium(i) diimine carbonyl complexes

In this review, we summarize the synthesis, photophysical properties and applications of a wide variety of multinuclear complexes consisting of Re(i)-diimine-carbonyl units.

Synthesis, spectroscopic, electrochemical and computational studies of rhenium(i) tricarbonyl complexes based on bidentate-coordinated 2,6-di(thiazol-2-yl)pyridine derivatives

The impact of structure modification of the 2,6-di(thiazol-2-yl)pyridine based ligand was investigated.

Theoretical study on mechanism of the photochemical ligand substitution of fac-[ReI(bpy)(CO)3(PR3)]+ complex

The mechanism of the CO ligand dissociation of fac-[Re^I(bpy)(CO)₃P(OMe)₃]⁺ has theoretically been investigated, as the dominant process of the photochemical ligand substitution (PLS) reactions of fac-[Re^I(bpy)(CO)₃PR₃]⁺, by using the (TD-)DFT method.

Synthesis, photophysical properties and application in organic light emitting devices of rhenium(i) carbonyls incorporating functionalized 2,2′:6′,2′′-terpyridines

Photophysics of [ReCl(CO)₃(4′-R-terpy-κ²N)].

Photophysical and photochemical studies of tricarbonyl rhenium(i) N-heterocyclic carbene complexes containing azide and triazolate ligands

Rhenium NHC complexes bound to azide anions readily react with alkynes to form the corresponding triazolate complexes, a new class of photochemically active species.

Luminescent rhenium(I) tricarbonyl complexes with pyrazolylamidino ligands: photophysical, electrochemical, and computational studies

New pyrazolylamidino complexes fac-[ReCl(CO)3(NH[double bond, length as m-dash]C(Me)pz*-κ(2)N,N)] (pz*H = pyrazole, pzH; 3,5-dimethylpyrazole, dmpzH; indazole, indzH) and fac-[ReBr(CO)3(NH[double bond, length as m-dash]C(Ph)pz*-κ(2)N,N)] are synthesized via base-catalyzed coupling of the appropriate nitrile with pyrazole, or via metathesis by halide abstraction with AgBF4 from a bromido pyrazolylamidino complex and the subsequent addition of LiCl. In order to study both the influence of the substituents present at the pyrazolylamidino ligand, and that of the "sixth" ligand in the complex, photophysical, electrochemical, and computational studies have been carried out on this series and other complexes previously described by us, of the general formula fac-[ReL(CO)3(NH[double bond, length as m-dash]C(R')pz*-κ(2)N,N)](n+) (L = Cl, Br; R' = Me, Ph, n = 0; or L = NCMe, dmpzH, indzH, R' = Me, n = 1). All complexes exhibit phosphorescent decays from a prevalently (3)MLCT excited state with quantum yields (Φ) in the range between 0.007 and 0.039, and long lifetimes (τ∼ 8-1900 ns). The electrochemical study reveals irreversible reduction for all complexes. The oxidation of the neutral complexes was found to be irreversible due to halido-dissociation, whereas the cationic species display a reversible process implying the ReI/ReII couple. Density functional and time-dependent density functional theory (TD-DFT) calculations provide a reasonable trend for the values of emission energies in line with the experimental photophysical data, supporting the (3)MLCT based character of the emissions.

Dynamics of rhenium photocatalysts revealed through ultrafast multidimensional spectroscopy

Rhenium catalysts have shown promise to promote carbon neutrality by reducing a prominent greenhouse gas, CO2, to CO and other starting materials. Much research has focused on identifying intermediates in the photocatalysis mechanism as well as time scales of relevant ultrafast processes. Recent studies have implemented multidimensional spectroscopies to characterize the catalyst's ultrafast dynamics as it undergoes the many steps of its photocycle. Two-dimensional infrared (2D-IR) spectroscopy is a powerful method to obtain molecular structure information while extracting time scales of dynamical processes with ultrafast resolution. Many observables result from 2D-IR experiments including vibrational lifetimes, intramolecular redistribution time scales, and, unique to 2D-IR, spectral diffusion, which is highly sensitive to solute-solvent interactions and motional dynamics. Spectral diffusion, a measure of how long a vibrational mode takes to sample its frequency space due to multiple solvent configurations, has various contributing factors. Properties of the solvent, the solute's structural flexibility, and electronic properties, as well as interactions between the solvent and solute, complicate identifying the origin of the spectral diffusion. With carefully chosen experiments, however, the source of the spectral diffusion can be unveiled. Within the context of a considerable body of previous work, here we discuss the spectral diffusion of several rhenium catalysts at multiple stages in the catalysis. These studies were performed in multiple polar liquids to aid in discovering the contributions of the solvent. We also performed electronic ground state 2D-IR and electronic excited state transient-2D-IR experiments to observe how spectral diffusion changes upon electronic excitation. Our results indicate that with the original Lehn catalyst in THF, relative to the ground state, the spectral diffusion slows by a factor of 3 in the equilibrated triplet metal-to-ligand charge transfer state. We attribute this slowdown to a decrease in dielectric friction as well as an increase in molecular flexibility. It is possible to partially simulate the charge transfer by altering the electron density moderately by adding electron donating or withdrawing substituents symmetrically to the bipyridine ligand. We find that unlike the significant electronic structure change induced by MLCT, such small substituent effects do not influence the spectral diffusion. A solvent study in THF, DMSO, and CH3CN found there to be an explicit solvent dependence that we can correlate to the solvent donicity, which is a measure of its nucleophilicity. Future studies focused on the solvent effects on spectral diffusion in the crucial photoinitiated state can illuminate the role the solvent plays in the catalysis.

Electron-transfer acceleration investigated by time resolved infrared spectroscopy

Ultrafast electron transfer (ET) processes are important primary steps in natural and artificial photosynthesis, as well as in molecular electronic/photonic devices. In biological systems, ET often occurs surprisingly fast over long distances of several tens of angströms. Laser-pulse irradiation is conveniently used to generate strongly oxidizing (or reducing) excited states whose reactions are then studied by time-resolved spectroscopic techniques. While photoluminescence decay and UV-vis absorption supply precise kinetics data, time-resolved infrared absorption (TRIR) and Raman-based spectroscopies have the advantage of providing additional structural information and monitoring vibrational energy flows and dissipation, as well as medium relaxation, that accompany ultrafast ET. We will discuss three cases of photoinduced ET involving the Re(I)(CO)3(N,N) moiety (N,N = polypyridine) that occur much faster than would be expected from ET theories. [Re(4-N-methylpyridinium-pyridine)(CO)3(N,N)](2+) represents a case of excited-state picosecond ET between two different ligands that remains ultrafast even in slow-relaxing solvents, beating the adiabatic limit. This is caused by vibrational/solvational excitation of the precursor state and participation of high-frequency quantum modes in barrier crossing. The case of Re-tryptophan assemblies demonstrates that excited-state Trp → *Re(II) ET is accelerated from nanoseconds to picoseconds when the Re(I)(CO)3(N,N) chromophore is appended to a protein, close to a tryptophan residue. TRIR in combination with DFT calculations and structural studies reveals an interaction between the N,N ligand and the tryptophan indole. It results in partial electronic delocalization in the precursor excited state and likely contributes to the ultrafast ET rate. Long-lived vibrational/solvational excitation of the protein Re(I)(CO)3(N,N)···Trp moiety, documented by dynamic IR band shifts, could be another accelerating factor. The last discussed process, back-ET in a porphyrin-Re(I)(CO)3(N,N) dyad, demonstrates that formation of a hot product accelerates highly exergonic ET in the Marcus inverted region. Overall, it follows that ET can be accelerated by enhancing the electronic interaction and by vibrational excitation of the reacting system and its medium, stressing the importance of quantum nuclear dynamics in ET reactivity. These effects are experimentally accessible by time-resolved vibrational spectroscopies (IR, Raman) in combination with quantum chemical calculations. It is suggested that structural dynamics play different mechanistic roles in light-triggered ET involving electronically excited donors or acceptors than in ground-state processes. While TRIR spectroscopy is well suitable to elucidate ET processes on a molecular-level, transient 2D-IR techniques combining optical and two IR (or terahertz) laser pulses present future opportunities for investigating, driving, and controlling ET.

Methylated Re(i) tetrazolato complexes: photophysical properties and Light Emitting Devices

Methylated Re(i) tetrazolato complexes are suitable as phosphors for Light Emitting Devices.

Norharmane rhenium(i) polypyridyl complexes: synthesis, structural and spectroscopic characterization

Novel Re(i) polypyridyl complexes with norharmane as a ligand were obtained and characterized by different techniques. The nature of the electronic transitions was established by TD-DFT calculations.

Synthesis and Preliminary Characterization of a PPE-Type Polymer Containing Substituted Fullerenes and Transition Metal Ligation Sites

A substituted fullerene was incorporated into a PPE-conjugated polymer repeat unit. This subunit was then polymerized via Sonogashira coupling with other repeat units to create polymeric systems approaching 50 repeat units (based on GPC characterization). Bipyridine ligands were incorporated into some of these repeat units to provide sites for transition metal coordination. Photophysical characterization of the absorption and emission properties of these systems shows excited states located on both the fullerene and aromatic backbone of the polymers that exist in a thermally controlled equilibrium. Future work will explore other substituted polyaromatic systems using similar methodologies.

Photophysics of threaded sp-carbon chains: the polyyne is a sink for singlet and triplet excitation

We have used single-crystal X-ray diffraction and time-resolved UV-NIR-IR absorption spectroscopy to gain insights into the structures and excited-state dynamics of a rotaxane consisting of a hexayne chain threaded through a phenanthroline macrocycle and a family of related compounds, including the rhenium(I) chlorocarbonyl complex of this rotaxane. The hexayne unit in the rhenium-rotaxane is severely nonlinear; it is bent into an arc with an angle of 155.6(1)° between the terminal C1 and C12 atoms and the centroid of the central C-C bond, with the most acute distortion at the point where the polyyne chain pushes against the Re(CO)3Cl unit. There are strong through-space excited-state interactions between the components of the rotaxanes. In the metal-free rotaxane, there is rapid singlet excitation energy transfer (EET) from the macrocycle to the hexayne (τ = 3.0 ps), whereas in the rhenium-rotaxane there is triplet EET, from the macrocycle complex (3)MLCT state to the hexayne (τ = 1.5 ns). This study revealed detailed information on the short-lived higher excited state of the hexayne (lifetime ∼1 ps) and on structural reorganization and cooling of hot polyyne chains, following internal conversion (over ∼5 ps). Comparison of the observed IR bands of the excited states of the hexayne with results from time-dependent density functional calculations (TD DFT) shows that these excited states have high cumulenic character (low bond length alternation) around the central region of the chain. These findings shed light on the complex interactions between the components of this supramolecular rotaxane and are important for the development of materials for the emerging molecular and nanoscale electronics.