Ultrafast Circular Dichroism Study of the Ring Opening of 7-Dehydrocholesterol

UV femtosecond time-resolved circular dichroism (TRCD) spectroscopy has been used to study the ultrafast changes of chirality in a small molecular biological paradigm sample, 7-dehydrocholesterol (7-DHC). Upon UV-photoexcitation, 7-DHC undergoes a ring opening to produce previtamin D₃, and two of the chiral centers of 7-DHC are removed, which impacts the overall chirality of the molecule. Here, measurements of this chirality change connected to the ring opening of 7-DHC with a time resolution of 280 fs in the UV are reported. With this method, a previously described discrepancy concerning the photophysics of 7-DHC was clarified. With our setup, the relaxation time of the chirality change was measured to be 1–2 ps, which corresponds to the shortest time constant in the transient absorption (TA) measurements, allowing us to assign that time constant to the ring opening

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

Fate of Photoexcited Molecular Antennae - Intermolecular Energy Transfer versus Photodegradation Assessed by Quantum Dynamics

The present computational study aims to unravel the competitive photoinduced intermolecular energy transfer and electron transfer phenomena in a light-harvesting antenna with potential applications in dye-sensitized solar cells and photocatalysis. A series of three thiazole dyes with hierarchically overlapping emission and absorption spectra, embedded in a methacrylate-based polymer backbone, is employed to absorb light over the entire visible region. Intermolecular energy transfer in such antenna proceeds via energy transfer from dye-to-dye and eventually to a photosensitizer. Initially, the ground and excited state properties of the three push–pull-chromophores (e.g., with respect to their absorption and emission spectra as well as their equilibrium structures) are thoroughly evaluated using state-of-the-art multiconfigurational methods and computationally less demanding DFT and TDDFT simulations. Subsequently, the potential energy landscape for the three dyads, formed by the π-stacked dyes as occurring in the polymer environment, is investigated along linear-interpolated internal coordinates to elucidate the photoinduced dynamics associated with intermolecular energy and electron transfer processes. While energy transfer among the dyes is highly desired in such antenna, electron transfer, or rather a light-induced redox chemistry, leading to the degradation of the chromophores, is disadvantageous. We performed quantum dynamical wavepacket calculations to investigate the excited state dynamics following initial light-excitation. Our calculations reveal for the two dyads with adjusted optical properties exclusively efficient intermolecular energy transfer within 200 fs, while in the case of the third dyad intermolecular electron transfer dynamics can be observed. Thus, this computational study reveals that statistical copolymerization of the individual dyes is disadvantageous with respect to the energy transfer efficiency as well as regarding the photostability of such antenna

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

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

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 Properties of Heteroleptic Cu(I) 4<i>H</i>‑Imidazolate Complexes

The excited state properties of three heteroleptic copper­(I) xantphos 4<i>H</i>-imidazolate complexes are investigated by means of femtosecond and nanosecond time-resolved transient absorption spectroscopy in dichloromethane solution. The subpicosecond spectral changes observed after excitation into the MLCT absorption band are interpreted as intersystem crossing from the singlet to the triplet manifold. This interpretation is corroborated by DFT and TD-DFT results, indicating a comparable molecular geometry in the ground state (and hence the nonrelaxed singlet state) and the excited triplet state. Population of the triplet state is followed by planarization of the <i>N</i>-aryl rings of the 4<i>H</i>-imidazolate ligand on a 10 ps time scale. The planarization strongly depends on the substitution pattern of the <i>N</i>-aryls and correlates with the reduced moment of inertia for the planarization motion. The triplet state subsequently decays to the ground state in about 100 ns. These results demonstrate that the excited state processes of copper­(I) complexes depend on the specific ligand(s) and their substitution pattern. Thus, the work presented points to a possibility to design copper­(I) complexes with specific photophysical properties

Self-healing Functional Polymers: Optical Property Recovery of Conjugated Polymer Films by Uncatalyzed Imine Metathesis

The implementation of a self-healing functionality into materials has become a prevalent approach for materials which require long-term reliability. As of today, the restoration of mechanical properties has dominated the research on self-healing materials, whereas research on healing of other functionalities (e.g., conductivity or optical properties) is still in its infancy. Here, the first conjugated polymer, which can restore its optical properties after photodamage is reported. The proposed self-healing mechanism relies on a thermally triggered imine metathesis between the conjugated polymer and additional macromolecular healing agents with no catalyst needed

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