MLCT Excited State Research Articles

Photocatalytic H2 production from water based on platinum(II) Schiff base sensitizers and a molecular cobalt catalyst

Three platinum(II) Schiff base complexes were synthesized and were used as efficient photosensitizers (PSs) for the photocatalytic H2 production from mixed solvents under visible light irradiation with a molecular cobalt complex as catalyst. Their photophysical and electrochemical properties were investigated and catalytic activities were evaluated under different catalytic conditions. The turnover numbers (TONs) up to 601 for complex 1 was achieved under optimized conditions and the effects of different groups in the complexes on the photocatalytic activity were presented. The photocatalytic efficiencies of three PSs are in good agreement with obtained quenching rate constant of the MLCT excited state.

A Noble‐Metal‐Free System for Photocatalytic Hydrogen Production from Water

A series of heteroleptic copper(I) complexes with bidentate PP and NN chelate ligands was prepared and successfully applied as photosensitizers in the light-driven production of hydrogen, by using [Fe3(CO)12] as a water-reduction catalyst (WRC). These systems efficiently reduces protons from water/THF/triethylamine mixtures, in which the amine serves as a sacrificial electron donor (SR). Turnover numbers (for H) up to 1330 were obtained with these fully noble-metal-free systems. The new complexes were electrochemically and photophysically characterized. They exhibited a correlation between the lifetimes of the MLCT excited state and their efficiency as photosensitizers in proton-reduction systems. Within these experiments, considerably long excited-state lifetimes of up to 54 μs were observed. Quenching studies with the SR, in the presence and absence of the WRC, showed that intramolecular deactivation was more efficient in the former case, thus suggesting the predominance of an oxidative quenching pathway.

Stabilization of a Ruthenium(II) Polypyridyl Dye on Nanocrystalline TiO2 by an Electropolymerized Overlayer

The long-term performance of dye-sensitized solar and photoelectrochemical cells is strongly dependent on the stability of surface-bound chromophores and chromophore-catalyst assemblies at metal oxide interfaces. We report here electropolymerization as a strategy for increasing interfacial stability and as a simple synthetic route for preparing spatially controlled, multicomponent films at an interface. We demonstrate that [Fe(v-tpy)2](2+) (v-tpy = 4'-vinyl-2,2':6',2″-terpyridine) can be reductively electropolymerized on nanocrystalline TiO2 functionalized with a phosphonate-derivatized Ru(II) polypyridyl chromophore. The outer:inner Fe:Ru ratio can be controlled by the number of reductive electrochemical scan cycles as shown by UV-visible absorption and energy dispersive X-ray spectroscopy measurements. Overlayer electropolymerization results in up to 30-fold enhancements in photostability compared to the surface-bound dye alone. Transient absorbance measurements have been used to demonstrate that photoexcitation and electron injection by the MLCT excited state(s) of the surface-bound Ru(II) complex is followed by directional, outside-to-inside, Fe(II) → Ru(III) electron transfer. This strategy is appealing in opening a new approach for synthesizing surface-stabilized chromophore-catalyst assemblies on nanocrystalline metal oxide films.

Luminescent rhenium(I) complexes of substituted imidazole[4,5-f]-1,10-phenanthroline derivatives

Seven new chromophoric ligands (L1–L7), based upon the fused imidazo[4,5-f]-1,10-phenanthroline core, were dually functionalised with a variety of aryl substituents of varying character, and can be synthesised utilising a one-pot methodology. The resultant ligands are fluorescent (λem = 408–425 nm) and amenable to further reactivity with pentacarbonylbromorhenium to give coordination complexes of the form fac-[ReBr(CO)3(NˆN)] (where NˆN = L1–L7). The complexes have been fully characterised using a variety of spectroscopic and analytical techniques, and structurally confirmed for three examples using X-ray diffraction studies. These new rhenium complexes are all luminescent, revealing classical 3MLCT emission in aerated solution; the complexes show that the 3MLCT excited state is moderately sensitive to the nature of functionalisation of the chelated imidazo[4,5-f]-1,10-phenanthroline ligand.

Inverse Kinetic Isotope Effect in the Excited-State Relaxation of a Ru(II)–Aquo Complex: Revealing the Impact of Hydrogen-Bond Dynamics on Nonradiative Decay

Photophysics of the MLCT excited-state of [Ru(bpy)(tpy)(OH2)](2+) (1) and [Ru(bpy)(tpy)(OD2)](2+) (2) (bpy = 2,2'-bipyridine and tpy = 2,2':6',2″-terpyridine) have been investigated in room-temperature H2O and D2O using ultrafast transient pump-probe spectroscopy. An inverse isotope effect is observed in the ground-state recovery for the two complexes. These data indicate control of excited-state lifetime via a pre-equilibrium between the (3)MLCT state that initiates H-bond dynamics with the solvent and the (3)MC state that serves as the principal pathway for nonradiative decay.

Sensitized Near-Infrared Luminescence From NdIII, YbIII and ErIII Complexes by Energy-Transfer From Ruthenium 1,3-Bis([1,10]Phenanthroline-[5,6-d]-Imidazol-2 -yl)Benzene

The luminescent ruthenium 1,3 -bis([1,10]phenanthroline-[5,6 -d]- imidazol-2 -yl)benzene (bpibH2) complex, a potentially useful bridging ligand with a vacant diimine site, has been used as 'metallo ligand' to make heterodinuclear d-f complexes by attachment of a {Ln(dik)3} fragment (dik = 1,3-diketonate) at the vacant site. When Ln = Nd, Yb, or Er the lanthanide centre has low-energy f-f excited states capable of accepting energy from the (3)MLCT excited state of the Ru(II) centre, there is quenching in the (3)MLCT luminescence of the Ru(II) centre, that affords sensitized lanthanide(III) based luminescence in the near-IR region. Nd(III) was found to be the most effective at quenching the (3)MLCT luminescence of the ruthenium component because of the high density of f-f excited states of the appropriate energy which make it as effective energy-acceptor compared to Er and Yb complexes.

Energy Dispersive XAFS: Characterization of Electronically Excited States of Copper(I) Complexes

Energy dispersive X-ray absorption spectroscopy (ED-XAS), in which the whole XAS spectrum is acquired simultaneously, has been applied to reduce the real-time for acquisition of spectra of photoinduced excited states by using a germanium microstrip detector gated around one X-ray bunch of the ESRF (100 ps). Cu K-edge XAS was used to investigate the MLCT states of [Cu(dmp)2]+ (dmp =2,9-dimethyl-1,10-phenanthroline) and [Cu(dbtmp)2]+ (dbtmp =2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) with the excited states created by excitation at 450 nm (10 Hz). The decay of the longer lived complex with bulky ligands, was monitored for up to 100 ns. DFT calculations of the longer lived MLCT excited state of [Cu(dbp)2]+ (dbp =2,9-di-n-butyl-1,10-phenanthroline) with the bulkier diimine ligands, indicated that the excited state behaves as a Jahn–Teller distorted Cu(II) site, with the interligand dihedral angle changing from 83 to 60° as the tetrahedral coordination geometry flattens and a reduction in the Cu–N distance of 0.03 Å.

Sensitized near-infrared luminescence of lanthanide complexes by energy transfer from a ruthenium antenna

The luminescent ruthenium tetrapyrido [3, 2-a:2′, 3″-c:3″, 2″-h:2‴, 3‴-j] phenazine(tpphz) complex acts as a potential bridging ligand with a vacant diimine site, and is used as ‘metallo ligand’ to make heterodinuclear d–f complexes by attachment of a {Ln(dik)3} fragment (dik=tta (trifluoro theonyl acetone)) at the vacant site. When Ln=Nd, Yb or Er, the lanthanide centre has low-energy f–f excited states capable of accepting energy from the 3MLCT excited state of the Ru(II) centre. Luminescence quenching of the 3MLCT Ru(II) center affords sensitized Ln(III) based luminescence in the near-IR region. Nd(III) is found to be the most effective quencher for the 3MLCT luminescence of the ruthenium component because of the high density of f–f excited states of the appropriate energy, which makes it a more efficient energy-acceptor compared to the Er and Yb complexes.

Sensitized near-infrared luminescence of NdIII, YbIII and ErIII complexes by energy transfer from a ruthenium antenna

A series of new ruthenium–lanthanide bimetallic complexes bridged by bibenzimidazole (BibzImH2) group were synthesized. The luminescent ruthenium bibenzimidazole complex, [(bpy)2Ru(BibzImH2)]2+, with a vacant diimine site, was used as a ‘complex ligand’ to prepare three new heterodinuclear d–f complexes by attaching a {Ln(dik)3} fragment (Ln = Nd, Yb, or Er); (dik = tta(trifluoro theonyl acetone)) at the vacant site. All of the prepared complexes were characterized by a variety of spectral techniques. Upon excitation at the MLCT absorption band of RuII, due to efficient energy transfer, sensitized emission occurs from lanthanide centre in the near-infrared region. The lanthanide centre has low-energy f–f excited states, which are capable of accepting energy from the 3MLCT excited state of the Ru(II) centre, leading to the quenching of 3MLCT luminescence of the Ru(II) centre and subsequent sensitized lanthanide(III)-based luminescence in the near-IR region. Because BibzImH2 is a short bridging ligand, it is able to assist in faster energy transfer from Ru(II) to Ln (III). The Nd(III) species was found to be the most effective for quenching the 3MLCT luminescence of the ruthenium component because of the high density of the f–f excited states of the appropriate energy, which make it as effective an energy-acceptor as Er(III) and Yb(III) complexes.

Improving the Photosensitizing Properties of Ruthenium Polypyridyl Complexes Using 4-Methyl-2,2′-bipyridine-4′-carbonitrile as an Auxiliary Ligand

We report in this work the synthesis and spectroscopic, electrochemical, spectroelectrochemical, and photophysical characterization of a novel series of ruthenium polypyridyl complexes with 4-methyl-2,2'-bipyridine-4'-carbonitrile (Mebpy-CN) as an auxiliary ligand of general formula [Ru(bpy)3-x(Mebpy-CN)x](PF6)2 (x = 1-3) (with bpy = 2,2'-bipyridine). A significant increase in the lifetime and quantum yield of emission of the lowest (3)MLCT excited state is disclosed when going from x = 1 to x = 3, evidencing an improvement of the photosensitizing properties with respect to [Ru(bpy)3](PF6)2. Furthermore, quenching by molecular oxygen of (3)MLCT excited states of the three complexes produced singlet molecular oxygen ((1)O2) with quantum yield values higher than that of [Ru(bpy)3](2+) in CH3CN. The structure of the complex with x = 1 has been determined by X-ray diffraction. The photoconductivity of ZnO nanowires covered with this same complex is increased by an order of magnitude, pointing to its feasibility as a component of a DSSC. A new dinuclear complex with Mebpy-CN as a bridging ligand has also been prepared and characterized by physicochemical techniques. The derived mixed-valent species of formula [(bpy)2Ru(II)(Mebpy-CN)Ru(III)(NH3)5](5+) displays a considerable metal-metal electronic coupling due to the delocalization effect of a nitrile group in the 4' position of the bpy ring.

Sensitized near-infrared luminescence of lanthanide complexes by energy transfer from rhenium(I) complexes bridged by bis(benzimidazole) and phenanthrolino-5,6:5′,6′-pyrazine ligands

A series of new rhenium-lanthanide heterobimetallic complexes bridged by bis(benzimidazole) (BiBzImH2) and 4,7-phenanthrolino-5,6:5′,6′-pyrazine (ppz) group were synthesized. The luminescent rhenium(I) complexes, [(CO)3Re(L)] having vacant diimine site, were used as ‘complex ligand’ to prepare six new heterodinuclear d–f complexes by attaching a {Ln(dik)3} fragment (Ln=Nd, Yb, or Er; dik=trifluoro theonyl acetone) at the vacant site. All prepared complexes were characterized by various spectral techniques. Upon excitation at the MLCT absorption band of Re, due to efficient energy transfer, sensitized emission occurs from lanthanide center in the near-infrared region. The lanthanide center has low-energy f–f excited states, which are capable of accepting energy from the 3MLCT excited state of the rhenium center, leading to the quenching of 3MLCT luminescence of the rhenium(I) and subsequent sensitized lanthanide(III)-based luminescence in the near-IR region. Because bis(benzimidazole) (BibzImH2) is a short bridging ligand compared to phenanthrolino-5,6:5′,6′-pyrazine (ppz), it is able to assist in faster energy transfer from rhenium(I) to Ln(III). The Nd(III) species is found to be the most effective for quenching the 3MLCT luminescence of the rhenium component because of the high density of the f–f excited states of the appropriate energy, which makes it as a more effective energy-acceptor compared to Er(III) and Yb(III) ions.

Photochemistry between a ruthenium(ii) pyridylimidazole complex and benzoquinone: simple electron transferversusproton-coupled electron transfer

A ruthenium(II) complex with two 4,4'-bis(trifluoromethyl)-2,2'-bipyridine chelates and a 2-(2'-pyridyl)imidazole ligand was synthesized and characterized by electrochemical and optical spectroscopic means. The respective complex has the potential to act as a combined electron-proton donor when promoted to its long-lived (3)MLCT excited state with visible light. The possibility of proton-coupled electron transfer (PCET) between the ruthenium(II) complex and 1,4-benzoquinone as an electron/proton acceptor was explored by steady-state and time-resolved luminescence spectroscopy, as well as by transient absorption spectroscopy in the nanosecond time regime. Excited-state deactivation is found to occur predominantly via simple oxidative quenching involving no proton motion, but a minor fraction of the photoexcited complex appears to react via PCET since there is spectral evidence for semiquinone as a photoproduct. Presumably, PCET is not kinetically competitive with simple electron transfer because the latter process is sufficiently exergonic and because there is little thermodynamic benefit from coupling proton transfer to the photoinduced electron transfer.

Probing the excited state dynamics of a new family of Cu(i)-complexes with an enhanced light absorption capacity: excitation-wavelength dependent population of states through branching

The ultrafast dynamics of six homoleptic Cu(I)-complexes and their respective ligands was examined through time-resolved electronic absorption spectroscopy in the subpicosecond time domain, in a variety of solvents, and at different excitation wavelengths. Results indicate that after excitation of the complexes in the blue part of the spectrum, the initially formed intraligand (IL) singlet excited state decays via two pathways yielding simultaneously both the lower-lying MLCT excited state and the ligand locally excited triplet state. The latter is also observed in the case of the free ligands and relaxes back to the ground state in a timescale of 40 ps. Excitation in the red part results in the formation of the MLCT excited state of the complexes which decays to the ground state through the same intraligand triplet excited state. The solvent viscosity does not affect the overall relaxation kinetics. The short time constant observed for the intersystem crossing of the MLCT singlet excited state is discussed in terms of the contribution of the d-orbitals of copper to the wavefunction of these states.

A single- and double-flash flash photolysis study of the sequential biphotonic photoprocesses of Cu(i) phenanthrolines. Comparison of the helicate complex, [Cu2(1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane)2]2+, and [Cu(2,9-dimethyl-1,10-phenanthroline)2]+ photoprocesses

Photochemical processes induced when two photons are sequentially absorbed by the helicate complex [Cu2(mphenpr)2](2+), where mphenpr = 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane, and [Cu(dmp)2](+) were investigated in CH2Cl2 containing solvents. A strong resemblance was observed between the fs-ns photophysics of [Cu2(mphenpr)2](2+) and Cu(I) phenanthroline complexes having a large steric hindrance. In the biphotonic regime, single-flash flash-photolyzed solutions were used for the determination of the product concentrations and quantum yields. The concentration of Cl(-) produced by the photoinduced decomposition of CH2Cl2 increases linearly with flash intensity as expected for a monophotonic process. In contrast, the concentration of a decomposed Cu(I) complex exhibits the quadratic dependence on flash intensity of a biphotonic process. Results of a sequential double-flash flash photolysis experiment are consistent with the decomposition of CH2Cl2 ahead of the flattened excited state formation and with the absorption of the second photon by the flattened MLCT excited state.

Syntheses, photophysical, electroluminescence and computational studies of rhenium(i) diimine triarylamine-containing alkynyl complexes

A series of triarylamine-containing rhenium(I) diimine alkynyl complexes has been synthesized and some of their X-ray crystal structures have been determined. Low-energy transition bands at 402–444 nm were observed in the electronic absorption spectra and were tentatively assigned as an admixture of [dπ(Re) → π*(diimine)] metal-to-ligand charge-transfer (MLCT) and [π(CC−C6H4–R) → π* (diimine)] ligand-to-ligand charge-transfer (LLCT) transitions. They were found to give emission upon photo-excitation in the solid state and in fluid solution at room temperature and their emissive origins were assigned as derived predominantly from the triplet 3MLCT excited state, mixed with 3LLCT character as well as the 3IL state. Electrochemical studies revealed that the first oxidation process was mainly alkynyl ligand-based with mixing of the metal-centred contribution, whereas the first reduction was attributed to the ligand-centred reduction of the diimine ligand. Computational studies have been performed to provide further insight into the electrochemical and photophysical properties. The photochemical properties with triphenylamine were also studied by quenching experiments and time-resolved transient absorption spectroscopy. Some of these complexes were able to act as emitters in OLED devices and their electroluminescence (EL) behaviour was investigated.

Synthesis, characterization and electrochemiluminescent properties of cyclometalated platinum(ii) complexes with substituted 2-phenylpyridine ligands

Two neutral cyclometalated platinum(II) complexes, Pt(DPP)(acac) and Pt(BPP)(acac) (DPP = 2,4-diphenylpyridine, BPP = 2-(4-tert-butylphenyl)-4-phenylpyridine, acac = acetylacetone), have been synthesized and characterized by (1)H NMR spectroscopy, mass spectrometry, elemental analyses and by X-ray crystallography for Pt(DPP)(acac). Electrogenerated chemiluminescence (ECL) of the two complexes in the absence or presence of coreactant tri-n-propylamine (TPrA) in different solvents (CH(3)CN, CH(2)Cl(2), DMF, CH(3)CN/H(2)O (V, 50 : 50)) has been studied. The ECL spectra are identical to their own PL spectra, indicating that ECL processes lead to the same metal-to-ligand charge-transfer ((3)MLCT) excited state that is generated by light excitation. The ECL potentials of Pt(DPP)(acac) and Pt(BPP)(acac)/TPrA in CH(3)CN and CH(3)CN/H(2)O solution were at ~0.75 V vs. SCE, and significantly negatively shifted by about 0.6 V compared to that of the Ru(bpy)(3)(2+)/TPrA system. The ECL quantum efficiencies of the complexes are comparable to that of the Ru(bpy)(3)(2+)/TPrA system. The significant increase of the ECL signal in the coreactant system is due to the formation of the strongly reducing intermediate TPrA˙. It is noteworthy that the ECL efficiencies of the synthesized compounds are much higher than that of the tridentate polypyridyl ligands.

Dynamics of the 3MLCT in Ru(II) Terpyridyl Complexes Probed by Ultrafast Spectroscopy: Evidence of Excited-State Equilibration and Interligand Electron Transfer

Ground- and excited-state properties of [Ru(tpy)(2)](2+), [Ru(tpy)(ttpy)](2+), and [Ru(ttpy)(2)](2+) (where tpy = 2,2':6',2″-terpyridine and ttpy = 4'-(4-methylphenyl)-2,2':6',2″-terpyridine) in room temperature acetonitrile have been investigated using linear absorption, electrochemical, and ultrafast transient pump-probe techniques. Spectroelectrochemistry was used to assign features observed in the transient spectra while single wavelength kinetics collected at a variety of probe wavelengths were used to monitor temporal evolution of the MLCT excited state. From these data, the excited-state lifetime of each complex was recovered and the rate limiting decay step was identified. In the bis-heteroleptic complex [Ru(tpy)(ttpy)](2+), photoexcitation to the (1)MLCT manifold generates both tpy-localized and ttpy-localized excited states. Accordingly, interligand electron transfer (ILET) from tpy-localized to the ttpy-localized (3)MLCT excited states is observable and the time scale has been measured to be 3 ps. For the homoleptic complex [Ru(tpy)(2)](2+), evidence for equilibration of the (3)MLCT excited-state population with the (3)MC has been observed and the time scale is reported at 2 ps.

Highly Efficient Ultrafast Electron Injection from the Singlet MLCT Excited State of Copper(I) Diimine Complexes to TiO2 Nanoparticles

Cu-Komplex als Photosensibilisator: Die Dynamik eines photoinduzierten Ladungstransfers von einem Kupfer(I)-Diimin-Komplex auf TiO2-Nanopartikel wurde mit einer Kombination aus mehreren zeitauflösenden Spektroskopiemethoden untersucht. Ein effizienter und sehr schneller Elektronentransfer aus dem MLCT-Singulettzustand resultiert aus einer Verzerrung der tetraedrischen Koordinationsumgebung und den sperrigen Gruppen an den Liganden.

Highly Efficient Ultrafast Electron Injection from the Singlet MLCT Excited State of Copper(I) Diimine Complexes to TiO2 Nanoparticles

Highly Efficient Ultrafast Electron Injection from the Singlet MLCT Excited State of Copper(I) Diimine Complexes to TiO₂ Nanoparticles

Coordination and Electronic Structure of Ruthenium(II)-tris-2,2′-bipyridine in the Triplet Metal-to-Ligand Charge-Transfer Excited State Observed by Picosecond Time-Resolved Ru K-Edge XAFS

Time-resolved X-ray absorption spectra of photoexcited ruthenium(II)-tris-2,2′-bipyridine ([RuII(bpy)3]2+) in the triplet metal-to-ligand charge transfer (3MLCT) state are measured and analyzed to investigate transient structural changes directly related to the photophysical properties of the complex. The results from visible (400 nm) and UV (267 nm) excitation indicate that electrostatic interaction between the oxidized Ru atom and the reduced bipyridine ligand is the dominant factor affecting the Ru–N bond contraction. This thus leads to two groups of Ru ligand distances, one exhibiting the ground-state Ru–N distance and another yielding a slightly decreased Ru–N distance due to the localized MLCT excited state. The EXAFS analysis of the photoexcited complex was analyzed toward one single Ru–N distance, yielding a contraction of 0.04 (0.01) A with an increased DW factor (corresponding to a 0.05 A mean increase).