Charge-Transfer Dynamics in Nanorod Photocatalysts with Bimetallic Metal Tips

CdSe@CdS dot-in-rod nanostructures tipped with AuPt bimetallic nanoparticles as cocatalyst show increased photon-to-hydrogen conversion efficiency compared to their analogues with pure Au or Pt tips. The underlying charge-separation and recombination processes are investigated by time-resolved transient absorption spectroscopy, to unravel whether the observed enhancement of photocatalytic activity is due to charge-separation/recombination properties of the system or to higher reactivity for proton reduction at the surface of the metal nanoparticle. We find that in the catalytically active Pt- and AuPt-functionalized structures charge separation occurs with similar time constants (Pt 3.5, 35, and 49 ps; AuPt 2.6, 31, and 66 ps), and the charge-separated state shows a lifetime of ∼20 μs in both cases. Hence, these processes should not be regarded as a source of the increased catalytic efficiency in the AuPt-functionalized nanorods. The results indicate that the proton reduction at the metal nanoparticle surface itself determines the overall efficiency

Photophysical Dynamics of a Ruthenium Polypyridine Dye Controlled by Solvent pH

The photophysics of the novel ruthenium dye [Ru(tmBiBzIm)(dppz)(tbbpy)]²⁺ (tmBiBzIm = 5,5′,6,6′-tetramethyl-2,2′-bibenzimidazole, dppz = dipyrido[3,2-<i>a</i>:2′,3,3′-<i>c</i>]phenazine, tbbpy = 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine) is investigated, which might be suitable as a model compound for intracellular DNA and pH sensors. The combination of three different bidentate ligands allows for controlling the photophysics by two distinct mechanisms: (i) protonation and deprotonation of the tmBiBzIm and (ii) hydrogen bonding to the phenazine nitrogens of the dppz ligand. As will be reported, deprotonation of the tmBiBzIm ligand causes a bathochromic shift of the metal-to-ligand charge-transfer transition, although the tmBiBzIm ligand itself does not directly contribute to the light absorption. Furthermore, tmBiBzIm deprotonation shortens the overall excited-state lifetime of the complex significantly. Although the protonation stage of the tmBiBzIm directly impacts the excited-state properties of the dye, the overall photoinduced dynamics is dominated by the dppz ligand. Consequently, addition of water to the solvent affects the excited-state relaxation pathway as known from, for example, [Ru(phen)₂dppz]²⁺ (phen = 1,10-phenanthroline) complexes

Structural Control of Photoinduced Dynamics in 4<i>H</i>‑Imidazole-Ruthenium Dyes

The photoinduced dynamics of a series of terpyridine 4<i>H</i>-imidazole-ruthenium complexes, which constitute a new family of panchromatic dyes, is investigated. The dynamics involves two excited states localized within the 4<i>H</i>-imidazole sphere. Upon MLCT excitation, an excited state is populated, which is localized on the central part of the 4<i>H</i>-imidazole ligand caused by its nonplanar conformation. The population of the second excited state is connected to a planarization of the 4<i>H</i>-imidazole ligand as revealed by viscosity-dependent measurements, and the excess electronic charge on the ligand is shifted to the terminal rings. The impact on the photoinduced dynamics of the electronic character of the substituent at the terminal rings and the protonation state of the 4<i>H</i>-imidazole ligand is studied. Significant changes in the lifetime of the excitation and alterations of the decay mechanism are observed depending on the interplay of the electronic character of the substituent and ligand protonation. In a NMe₂ substituted complex, the character of the substituent is changed from electron donating to electron withdrawing upon stepwise protonation, resulting in pH switchable decay mechanism

Increased Charge Separation Rates with Increasing Donor–Acceptor Distance in Molecular Triads: The Effect of Solvent Polarity

Distance-dependent electron transfer in donor–spacer–acceptor systems is accepted to occur via two distinct mechanisms, that is, by coherent superexchange or incoherent hopping. In general, the rate of electron transfer (<i>k</i>_ET) decreases with increasing donor–acceptor distances, irrespective of the actual mechanism being responsible for the process. However, recently Wenger and his group showed that in the frame of the superexchange mechanism electron-transfer rates can pass a maximum when increasing the transfer distance. This manuscript presents an investigation of the forward electron transfer in a series of donor (<i>N</i>-methylphenothiazine)–photocenter (Ru­(II) bis­(terpyridine) complex)–acceptor (<i>N</i>-methylfulleropyrrolidine) triads that reveals the control of the electron-transfer rates by solvent variation to an extent that in acetonitrile an increasing electron-transfer rate is observed with increasing donor–acceptor distance, while in dichloromethane an increase in the separation causes the electron transfer rate to drop. This behavior is qualitatively rationalized based on a recently introduced model. Nonetheless, the quantitative mismatch between the results presented here and the theory indicates that nonexponential distance-dependent couplings will have to be considered in extending the theory

A Novel Ru(II) Polypyridine Black Dye Investigated by Resonance Raman Spectroscopy and TDDFT Calculations

The optical properties of a new (bipyridine)₂Ru­(4<i>H</i>-imidazole) complex presenting a remarkable broad absorption in the visible range are investigated. The strong overlap of the absorption with the solar radiation spectrum renders the studied complex promising as a black absorber and hence as a starting structure for applications in the field of dye-sensitized solar cells. The correlations between structural and electronic features for the unprotonated and protonated forms are studied by means of UV–vis absorption and resonance Raman (RR) spectroscopy modeled with the help of time-dependent density functional theory (TDDFT) calculations. The absorption spectra show two bands in the visible region, which TDDFT assigns to a metal-to-ligand charge-transfer (MLCT) state and to a superposition of three excited states with MLCT and intraligand charge-transfer character, respectively. Additionally, the analysis of the molecular orbitals and RR spectra in resonance with the first MLCT band shows that the effects of protonation favor a charge-transfer photoexcitation to the 4<i>H</i>-imidazole ligand. The RR spectra simulated for several excitation wavelengths covering the visible region are in excellent agreement with experimental data. In particular, it is noteworthy that the calculations are able to reproduce the wavelength dependence of the RR spectra provided that corrected excitation energies are employed. Interference effects between the electronic states contributing to the RR scattering are small for the investigated complex

Photoredox-active Dyads Based on a Ru(II) Photosensitizer Equipped with Electron Donor or Acceptor Polymer Chains: A Spectroscopic Study of Light-Induced Processes toward Efficient Charge Separation

A photosensitizer–multielectron-acceptor dyad (P–A_<i>n</i>) was synthesized via controlled nitroxide-mediated polymerization of styrenic naphthalene diimide (NDI) and subsequent functionalization with a [Ru­(dqp)₂]²⁺ photosensitizer (dqp is 2,6-di­(quinolin-8-yl)­pyridine) at the chain terminus. The optical and electrochemical analysis showed the preserved properties of the individual subunits, corroborated by the analysis of the related multielectron donor assembly (D_<i>n</i>–P) based on triarylamine (TARA). A detailed photophysical study of both dyads is presented to elucidate the primary light-induced energy- and electron-transfer events. While the D_<i>n</i>–P dyad displays the unchanged ³MLCT-based (MLCT is metal-to-ligand charge transfer) emission of the pristine photosensitizer, the P–A_<i>n</i> system revealed efficient emission quenching and the occurrence of the NDI radical anion signature. The time-resolved emission data revealed a nonmonoexponential decay attributed to the conformational freedom by the flexible linkage, while the transient absorption data confirmed the rapid formation of the reduced acceptor

Excited State Dynamics of a Photobiologically Active Ru(II) Dyad Are Altered in Biologically Relevant Environments

In this study femtosecond and nanosecond time-resolved transient absorption spectroscopy was used to investigate the influence of ionic strength and complexity on the excited state dynamics of a Ru­(II)-based metal–organic dyad. The bis-heteroleptic complex [Ru­(bpy)₂(ippy)]²⁺ (<b>1</b>), where bpy = 2,2′-bipyridine and ippy = 2-(1-pyrenyl-1<i>H-</i>imidazo­[4,5-<i>f</i>]­[1,10]­phenanthroline, is a potent photosensitizer for in vitro photodynamic therapy (PDT) and photodynamic inactivation (PDI) of microorganisms owing to a long-lived triplet excited state derived from a metal-to-ligand charge-transfer (³MLCT) state that is equilibrium with an intraligand (³IL) state. The prolonged lifetime provides ample opportunity for bimolecular quenching of this state by oxygen; thus singlet oxygen (¹O₂) sensitization is very efficient. In simple aqueous solution, fast cooling within the ³MLCT manifold is followed by energy transfer to an ³IL state, which is facilitated by rotation of a pyrenyl unit about the imidazo–pyrenyl (ip) coannular bond. For solutions of <b>1</b> in high ionic strength simulated biological fluid (SBF), a more physiologically relevant solvent that contains a complex mixture of ions at pH 7.4, femtosecond studies revealed an additional excited state, possibly based on an ion–ligand interaction. This new state appearing in high ionic strength SBF was not observable in water, simple buffers, or low ionic strength SBF. These photoinduced dynamics were also affected by the presence of biomolecules such as DNA in simple buffer, whereby relaxation on the picosecond time scale was accelerated from 39 to 18 ps with DNA intercalation by <b>1</b>. The increased rate of coplanarization of the pyrene and the imidazole units was attributed to DNA-induced conformational restriction of the pyrenyl unit relative to the ip bond. Quantitative changes to excited state decay rates of <b>1</b> in solutions of high ionic strength were also observed when probed on the microsecond time scale. Notably, the thermalized excited state decay pathways were altered substantially with DNA intercalation, with access to some states being completely blocked. Experimentally, this manifested in the absence of the slowest microsecond decay channel, which is normally observed for <b>1</b> in solution. The quantitative and qualitative observations from this study highlight the importance of employing biologically relevant solvents and potential biomolecule targets when the excited state dynamics and photophysical properties (under cell-free conditions) responsible for the potent photobiological effects are assessed in the context of photodynamic therapy and photodynamic inactivation

New Ruthenium Bis(terpyridine) Methanofullerene and Pyrrolidinofullerene Complexes: Synthesis and Electrochemical and Photophysical Properties

A series of terpyridine (tpy) methanofullerene and pyrrolidinofullerene dyads linked via <i>p</i>-phenylene or <i>p</i>-phenyleneethynylenephenylene (PEP) units is presented. The coordination to ruthenium­(II) yields donor–bridge–acceptor assemblies with different lengths. Cyclic voltammetry and UV–vis and luminescence spectroscopy are applied to study the electronic interactions between the active moieties. It is shown that, upon light excitation of the ruthenium­(II)-based ¹MLCT transition, the formed ³MLCT state is readily quenched in the presence of C₆₀. The photoinduced dynamics have been studied by transient absorption spectroscopy, which reveals fast depopulation of the ³MLCT (73–406 ps). As a consequence, energy transfer occurs, populating a long-lived triplet state, which could be assigned to the ³C₆₀* state

Efficient Energy Transfer and Metal Coupling in Cyanide-Bridged Heterodinuclear Complexes Based on (Bipyridine)(terpyridine)ruthenium(II) and (Phenylpyridine)iridium(III) Complexes

We report a series of cyanide-bridged, heterodinuclear iridium­(III)–ruthenium­(II) complexes with the generalized formula [Ir­((R₂)₂-ppy)₂(CN)­(μ-CN)­Ru­(bpy)­(tpy-R₁)]­PF₆ (ppy = 2-phenylpyridine, bpy = 2,2′-bipyridine, and tpy = 2,2′:6′,2″-terpyridine). The structural, spectroscopic, and electrochemical properties were analyzed in the context of variation of the electron-withdrawing (e.g., −F, −Br, −CHO) and -donating (e.g., −Me) and extended π-conjugated groups at several positions. In total, ten dinuclear complexes and the appropriate model complexes have been prepared. The iridium­(III)-based emission is almost fully quenched in these complexes, and only the ruthenium­(II)-based emission is observed, which indicates an efficient energy transfer toward the Ru center. Upon oxidation of the Ru center, the fluorinated complexes <b>2</b> exhibit a broad intervalence charge-transfer transition in the near-infrared region. The complexes are assigned to a weakly coupled class II system according to the Robin–Day classification. The electronic structure was evaluated by density functional theory (DFT) and time-dependent DFT calculations to corroborate the experimental data

Photophysics of Ru(II) Dyads Derived from Pyrenyl-Substitued Imidazo[4,5‑<i>f</i>][1,10]phenanthroline Ligands

The photophysics of a series of Ru­(II) dyads based on the 2-(1-pyrenyl)-1<i>H</i>-imidazo­[4,5-<i>f</i>]­[1,10]-phenanthroline ligand was investigated. The ability of these metal complexes to intercalate DNA and induce cell death upon photoactivation makes them attractive photosensitizers for a range of photobiological applications, including photodynamic therapy. In the present study, time-resolved transient absorption and emission spectroscopy were used to interrogate the photoinduced processes that follow metal-to-ligand charge transfer excitation of the complexes in solution. It was found that energy transfer to pyrene-localized intraligand triplet states, facilitated by torsional motion of the pyrene moiety relative to the imidazo­[4,5-<i>f</i>]­[1,10]­phenanthroline ligand, was an important relaxation pathway governing the photophysical dynamics in this class of compounds. Biphasic decay kinetics were assigned to spontaneous (pre-equilibrium) and delayed emission, arising from an equilibrium established between ³MLCT and ³IL states. TDDFT calculations supported these interpretations