Solid State Molecular Device Based on a Rhenium(I) Polypyridyl Complex Immobilized on TiO2 Films

Visible light-driven CO2 photoreduction by a Re(I) complex immobilized onto CuO/Nb2O5 heterojunctions

The immobilization of Re(I) complexes onto metal oxide surfaces presents an elegant strategy to enhance their stability and reusability toward photocatalytic CO2 reduction. In this study, the photocatalytic performance of fac-[ClRe(CO)3(dcbH2)], where dcbH2 = 4,4'-dicarboxylic acid-2,2'-bipyridine, anchored onto the surface of 1%m/m CuO/Nb2O5 was investigated. Following adsorption, the turnover number for CO production (TONCO) in DMF/TEOA increased significantly, from ten in solution to 370 under visible light irradiation, surpassing the TONCO observed for the complex onto pristine Nb2O5 or CuO surfaces. The CuO/Nb2O5 heterostructure allows for efficient electron injection by the Re(I) center, promoting efficient charge separation. At same time CuO clusters introduce a new absorption band above 550 nm that contributes for the photoreduction of the reaction intermediates, leading to a more efficient CO evolution and minimization of side reactions.

Dissecting conjugation and electronic effects on the linear and non-linear optical properties of rhenium(I) carbonyl complexes

Herein, we report a theoretical and experimental analysis of the conjugation and electronic effects on the one-photon (1PA) and two-photon absorption (2PA) properties of a series of Re(I) carbonyl complexes with terpyridine-based ligands. An excellent agreement was obtained between the calculated and experimental 2PA spectra of the κ2N-terpyridine tricarbonyl complexes (1a-b), with 2PA cross sections reaching up to ca. 40 GM in DMF. By stepwise lowering the conjugation length in the terpy ligand and changing the local symmetry around the metal centre, we show that conjugation and delocalisation play a major role in increasing 2PA cross sections, and that the character of the excited states does not directly enhance the non-linear properties of these complexes-contrary to the results observed in 1PA. Altogether, these results give valuable guidelines towards more efficient two-photon-absorbing coordination complexes of Re(I), with potential applications in photodynamic therapy and two-photon imaging.

Accessing the triplet manifold of naphthalene benzimidazole-phenanthroline in rhenium(I) bichromophores

The steady-state and ultrafast to supra-nanosecond excited state dynamics of fac-[Re(NBI-phen)(CO)₃(L)](PF₆) (NBI-phen = 16H-benzo[4',5']isoquinolino[2',1':1,2]imidazo[4,5-f][1,10]phenanthrolin-16-one) as well as their respective models of the general molecular formula [Re(phen)(CO)₃(L)](PF₆) (L = PPh₃ and CH₃CN) has been investigated using transient absorption and time-gated photoluminescence spectroscopy. The NBI-phen containing molecules exhibited enhanced visible light absorption with respect to their models and a rapid formation (<6 ns) of the triplet ligand-centred (LC) excited state of the organic ligand, NBI-phen. These triplet states exhibit an extended excited state lifetime that enable the energized molecules to readily engage in triplet-triplet annihilation photochemistry.

Coordination Environment Prevents Access to Intraligand Charge-Transfer States through Remote Substitution in Rhenium(I) Terpyridinedicarbonyl Complexes

Six rhenium(I) κ³N-dicarbonyl complexes with 4'-(4-substituted phenyl)terpyridine ligands were evaluated in their ground and excited states. These complexes, bearing substituents of different electron-donating strengths-from CN to NMe₂-were studied by a combination of transient IR (TRIR), electrochemistry, and IR spectroelectrochemistry, as well as time-dependent density functional theory (TD-DFT). They exhibit panchromatic absorption and can act as stronger photoreductants than their tricarbonyl counterparts. The ground- and excited-state potentials, absorption maxima, and lifetimes (250-750 ps) of these complexes correlate well with the Hammett σ_p substituent constants, showing the systematic effect of remote substitution in the ligand framework. TRIR spectroscopy allowed us to assign the lowest singlet and triplet excited states to a metal-to-ligand charge-transfer (MLCT) character. This result contrasts our previous report on analogous κ²N-tricarbonyl complexes, where remote substitution switched the character from MLCT to intraligand charge transfer. With the help of TD-DFT calculations, we dissect the geometric and electronic effects of coordination of the third pyridine, local symmetries, and increasing conjugation length. These results give valuable insights for the design of complexes with long-lived triplet excited states and enhanced absorption throughout the visible spectrum, while showcasing the boundaries of the excited-state switching strategy via remote substitution.

Living Long and Prosperous: Productive Intraligand Charge-Transfer States from a Rhenium(I) Terpyridine Photosensitizer with Enhanced Light Absorption

The ground- and excited-state properties of six rhenium(I) κ²N-tricarbonyl complexes with 4'-(4-substituted-phenyl)terpyridine ligands bearing substituents of different electron-donating abilities were evaluated. Significant modulation of the electrochemical potentials and a nearly 4-fold variation of the triplet metal-to-ligand charge-transfer (³MLCT) lifetimes were observed upon going from CN to OMe. With the more electron-donating NMe₂ group, we observed in the κ²N complex the appearance of a very strong absorption band, red-shifted by ca. 100 nm with respect to the other complexes. This was accompanied by a dramatic enhancement of the excited-state lifetime (380 vs 1.5 ns), and a character change from ³MLCT to intraligand charge transfer (³ILCT), despite the remote location of the substituent. The dynamics and character of the excited states of all complexes were assigned by combining transient IR spectroscopy, IR spectroelectrochemistry, and (time-dependent) density functional theory calculations. Selected complexes were evaluated as photosensitizers for hydrogen production, with the κ²N-NMe₂ complex resulting in a stable and efficient photocatalytic system reaching TON_Re values of over 2100, representing the first application of the ³ILCT state of a rhenium(I) carbonyl complex in a stable photocatalytic system.

Spectroscopic characterization of a new Re(i) tricarbonyl complex with a thiosemicarbazone derivative: towards sensing and electrocatalytic applications

A novel Re(i) complex with a thiosemicarbazone derivative is described and fully characterized. Its was further explored as CO₂ reduction electrocatalyst, being the first complex with a thiosemicarbazone derivative applied to this goal.

Excited-State Triplet Equilibria in a Series of Re(I)-Naphthalimide Bichromophores

We present the synthesis, structural characterization, electronic structure calculations, and the ultrafast and supra-nanosecond photophysical properties of a series of five bichromophores of the general structural formula [Re(5-R-phen)(CO)₃(dmap)](PF₆), where R is a naphthalimide (NI), phen = 1,10-phenanthroline, and dmap is 4-dimethylaminopyridine. The NI chromophores were systematically modified at their 4-positions with -H (NI), -Br (BrNI), phenoxy (PONI), thiobenzene (PSNI), and piperidine (PNI), rendering a series of metal-organic bichromophores (Re1-Re5, respectively) featuring variability in the singlet and triplet energies in the pendant NI subunit. Five closely related organic chromophores as well as [Re(phen)(CO)₃(dmap)](PF₆) (Re6) were investigated in parallel to appropriately model the photophysical properties exhibited in the bichromophores. The excited state processes of all molecules in this study were elucidated using a combination of transient absorption spectroscopy and time-resolved photoluminescence (PL) spectroscopy, revealing the kinetics of the energy transfer processes occurring between the appended chromophores. The spectroscopic analysis was further supported by electronic structure calculations which identified the origin of many of the experimentally observed electronic transitions.

Photophysical properties of rhenium(i) complexes and photosensitized generation of singlet oxygen

fac-[Re(ampy)(CO)₃(NN)]⁺ complexes (ampy = 2-aminomethylpyridine and NN = 1,10-phenanthroline (phen), and 2,2'-bipyridine (bpy)) were synthesized, purified and characterized by proton nuclear magnetic resonance (¹H NMR), UV-visible and Fourier-transformed infrared (FT-IR) spectroscopies, and their photophysical properties were investigated using steady state and time-resolved emission spectroscopies. The electronic absorption spectra exhibit two main absorption bands: the higher energy band, which was assigned to intraligand transition (IL), and the lower energy band assigned to metal-to-ligand charge transfer (MLCT). Both complexes showed emission at room temperature in a CH₃CN solution (λ_max = 560 nm, ϕ = 0.091, τ = 560 ns for fac-[Re(ampy)(CO)₃(phen)]⁺; λ_max = 568 nm, ϕ = 0.024, τ = 100 ns for fac-[Re(ampy)(CO)₃(bpy)]⁺) and rigid media (λ_max = 530 nm, τ = 3300 ns for fac-[Re(ampy)(CO)₃(phen)]⁺; λ_max = 530 nm, τ = 853 ns for fac-[Re(ampy)(CO)₃(bpy)]⁺) arising from the lowest lying ³MLCT_Re→NN excited state. Both complexes along with fac-[Re(L)(CO)₃(NN)]^+/0 complexes, L = Cl or pyridine, were capable of efficiently photosensitizing the generation of singlet oxygen with quantum yield in the range of 0.59-0.28. These results highlight the potential application of fac-[Re(L)(CO)₃(NN)]^+/0 complexes in the development of sensitizers for the generation of singlet oxygen.

Molecular engineered rhenium(i) carbonyl complexes to promote photoisomerization of coordinated stilbene-like ligands in the visible region

Novel fac-[Re(CO)₃(dmcb)(trans-stpyR)]⁺ complexes have been judiciously engineered to absorb at lower energies and sensitize trans-4-styrylpyridine (trans-stpy) or trans-4-(4-cyano)styrylpyridine (trans-stpyCN) photoisomerizable ligands up to 436 nm of irradiation.

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.

Mono- and di-nuclear Re(i) complexes and the role of protonable nitrogen atoms in quenching emission by hydroquinone

Using the simplest type of supramolecular architecture as an easy approach to understand the quenching mechanism of rhenium(i) compounds.

Reversible trans ⇌ cis photoisomerizations of [Re(CO)3(ph2phen)(stpyCN)]+ towards molecular machines

The fac-[Re(CO)₃(ph₂phen)(stpyCN)]⁺ complex is capable of switching through extended trans- and contracted cis-configurations upon irradiation under proper energies.

Improving the photocatalytic reduction of CO2 to CO through immobilisation of a molecular Re catalyst on TiO2

The photocatalytic activity of phosphonated Re complexes, [Re(2,2'-bipyridine-4,4'-bisphosphonic acid) (CO)3(L)] (ReP; L = 3-picoline or bromide) immobilised on TiO2 nanoparticles is reported. The heterogenised Re catalyst on the semiconductor, ReP-TiO2 hybrid, displays an improvement in CO2 reduction photocatalysis. A high turnover number (TON) of 48 molCO molRe(-1) is observed in DMF with the electron donor triethanolamine at λ>420 nm. ReP-TiO2 compares favourably to previously reported homogeneous systems and is the highest TON reported to date for a CO2-reducing Re photocatalyst under visible light irradiation. Photocatalytic CO2 reduction is even observed with ReP-TiO2 at wavelengths of λ>495 nm. Infrared and X-ray photoelectron spectroscopies confirm that an intact ReP catalyst is present on the TiO2 surface before and during catalysis. Transient absorption spectroscopy suggests that the high activity upon heterogenisation is due to an increase in the lifetime of the immobilised anionic Re intermediate (t50% >1 s for ReP-TiO2 compared with t50% = 60 ms for ReP in solution) and immobilisation might also reduce the formation of inactive Re dimers. This study demonstrates that the activity of a homogeneous photocatalyst can be improved through immobilisation on a metal oxide surface by favourably modifying its photochemical kinetics.

The triplet excited state of Bodipy: formation, modulation and application

The accessing of the triplet excited state of one of the most popular fluorophores, boron-dipyrromethene (Bodipy), was summarized.

Synthesis and characterization of monometallic rhenium(i) complexes and their application as selective sensors for copper(ii) ions

Monometallic Re(i) complexes, selective and sensitive sensor of Cu(ii) ion, show substantial enhancement in the emission intensity, quantum yield, and lifetime due to the restriction of CN isomerization.

Spectroscopic trends in a series of Re(I) α-diimine complexes as a function of the antenna/photoisomerizable ligands: a TD-DFT and MS-CASPT2 study

The absorption spectra of 11 rhenium(I) complexes with photoisomerizable stilbene-like ligands have been investigated by means of density functional theory (DFT). The electronic structures of the ground and excited states were determined for [Re(CO)3(N,N)(L)]+ (N,N = bpy (2,2′-bipyridine), phen (1,10-phenanthroline), Me4phen (3,4,7,8-tetramethyl-1,10-phenanthroline), ph2phen (4,7-diphenyl-1,10-phenanthroline), or Clphen (5-chloro-1,10-phenanthroline); L = bpe (1,2-bis(4-pyrydil)ethylene), stpy (4-styrylpyridine), or CNstpy (4-(4-cyano)styrylpyridine)) at the time–dependent (TD) DFT/CAM-B3LYP level of theory in vacuum and acetonitrile to highlight the effects of both antenna N,N and isomerizable L ligands. The TD-DFT spectra of two representative complexes, namely [Re(CO)3(bpy)(stpy)]+ and [Re(CO)3(phen)(bpe)]+, have been compared with MS-CASPT2 spectra. The TD-DFT spectra obtained in vacuum and acetonitrile agree rather well both with the ab initio and experimental spectra. The absorption spectroscopy of this series of molecules is characterized by the presence of three low-lying metal to ligand charge transfer (MLCT) states absorbing in the visible energy domain. The nature of the isomerizable ligands (bpe, stpy, or CNstpy) and the type of antenna ligands (bpy, phen, and substituted phen) control the degree of mixing between the MLCT and intraligand excited states, their relative energies, as well as their intensities.

fac-[1,2-Bis(pyridin-4-yl)ethane-κN]tricarbon-yl(1,10-phenanthroline-κ(2) N,N')rhenium(I) hexa-fluorido-phosphate aceto-nitrile monosolvate

The asymmetric unit of the title compound, [Re(C12H8N2)(C12H12N2)(CO)3]PF6.·CH3CN, contains one cation, one hexa-fluorido-phosphate anion and one aceto-nitrile solvent mol-ecule. The Re(I) ion is coordinated by two N atoms from the 1,10-phenanthroline ligand and one N atom from the 1,2-bis-(pyridin-4-yl)ethane ligand [mean Re-N = 2.191 (15) Å] and by three carbonyl ligands [mean Re-C = 1.926 (3) Å] in a distorted octa-hedral geometry. The electrostatic forces and weak C-H⋯F(O) hydrogen bonds pack cations and anions into the crystal with voids of 82 Å(3), which are filled by solvent mol-ecules. The crystal packing exhibits short inter-molecular O⋯O distance of 2.795 (5) Å between two cations related by inversion.

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.