Photoinduced Intramolecular Electron Transfer Process Research Articles

Impact of Solvent-Solute Hydrogen Bonding on Ultrafast Electron Transfer in a Trimetallic Iron-Ruthenium Complex

Transition metal complexes (TMCs) are widely used as light absorbers and catalysts for photochemical energy conversion processes such as photocatalytic reactions in liquid environments. Of particular interest are TMCs which incorporate multiple bridged metal centres because they represent a step towards more complex supramolecular assemblies that can ultimately enable control of the charge flow in solution-based photo-redox processes. However, the development of design guidelines for such molecular assemblies is impeded by an incomplete understanding of the coupling between their electronic and nuclear dynamics, including the interplay of TMCs with their liquid environment upon photoexcitation. Probing the involved processes requires high spatial sensitivity towards electronic motion and the ability to capture atomic motion on ultrafast timescales – a combination which is not readily available for optical transient spectroscopic techniques.X-ray free electron lasers (XFELs) have revolutionized the field of ultrafast science in recent years. XFELs generate tuneable and extremely bright X-ray pulses with a typical pulse duration of <30 fs, which are well-suited as an element-specific probe of ultrafast electronic and atomic motion in molecules and materials.1,2 Applied to TMCs, XFEL probes are sensitive to the local electronic structure in such molecular systems as well as to intra- and inter-molecular structural reorganization at an atomic level.3 In this talk, I will present work on the interaction of a linear trimetallic cyanide-bridged iron-ruthenium complex (FeRuFe) with its solvent environment, carried out using the LCLS XFEL facility at SLAC National Accelerator Laboratory. The focus of this study lies on elucidating how photoinduced intramolecular electron transfer processes in FeRuFe are affected by hydrogen bonding interactions between its cyanide ligands and the surrounding solvent molecules. In water, such solvent-solute hydrogen bonding interactions are stronger for Fe(II) than Fe(III), leading to substantial changes in solute-solvent interactions upon photoinduced electron transfer between metal centres.The experiment is performed in a pump-probe geometry on a liquid jet, where a visible/near-infrared pump induces a metal-to-metal charge transfer (MMCT) between Ru and Fe which is followed by an ultrafast back-electron transfer (BET). The excited state dynamics are monitored using hard X-ray pulses as a function of time delay between the laser pump and the X-ray probe pulses. To resolve and correlate electronic and structural dynamics upon MMCT excitation, we simultaneously perform ultrafast Fe Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) measurements, where XES is sensitive to the oxidation and spin state of the Fe centres and XSS provides information on atomic motion in solute and solvent, as demonstrated in previous work on a similar system.4 To evaluate the effect of solute-solvent hydrogen bonding interactions on the electron transfer dynamics, we perform measurements in a series of solvents with different hydrogen bonding properties, including water, methanol, and acetonitrile. We find that the BET increases as a function of decreasing hydrogen bonding ability of the solvent, and that distinct signatures of solvation dynamics are present in the scattering data. The resulting molecular level understanding of solvent reorganization coupled to electron transfer demonstrates that the strength and type of solute-solvent interactions are a central factor in determining the outcome of photoinduced charge transfer processes in TMCs.References Bergmann, U. et al. Using X-ray free-electron lasers for spectroscopy of molecular catalysts and metalloenzymes. Nat. Rev. Phys. 3, 264–282 (2021).Schoenlein, R. et al. Recent advances in ultrafast X-ray sources. Philos. Trans. R. Soc. Math. Phys. Eng. Sci. 377, 20180384 (2019).Gaffney, K. J. Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering. Chem. Sci. 12, 8010–8025 (2021).Biasin, E. et al. Direct observation of coherent femtosecond solvent reorganization coupled to intramolecular electron transfer. Nat. Chem. 13, 343–349 (2021).

Synthesis and Physical Properties of Tetrathiafulvalene-8-Quinolinato Zinc(II) and Nickel(II) Complexes

To develop donor–acceptor–donor (D–A–D) type new photo-electric conversion materials, new tetrathiafulvalene (TTF)-Mq2-TTF complexes 1 and 2 were synthesized, where two bis(n-hexylthio)tetrathiafulvalene moieties were attached to the Mq2 part (1: M = Zn, 2: M = Ni, q = 8-quinolinato) through amide bonds. UV-Vis absorption spectra of these complexes showed strong and sharp absorption maxima at 268 nm and small absorption maxima around 410 nm, corresponding to those of Znq2 and Niq2 parts. Furthermore, complexes 1 and 2 exhibited absorption tails up to a much longer wavelength region of ca. 700 nm, suggesting the appearance of charge transfer absorption from TTF to the Mq2 parts. The photoelectrochemical measurements on the thin films of these complexes casted on ITO-coated glass substrates suggest that positive photocurrents can be generated by the photoinduced intramolecular electron transfer process between the TTF and Mq2 parts.

Directionality of Ultrafast Electron Transfer in a Hydrogen Evolving Ru–Pd-Based Photocatalyst

Directionality of electron transfer and long-lived charge separation are of key importance for efficient photocatalytic water splitting. Knowledge of the processes that follow photoexcitation is essential for the optimization of supramolecular assembly designs in order to improve the efficiency of photocatalytic hydrogen generation. Photoinduced intramolecular electron transfer processes within the hydrogen-evolving photocatalyst [Ru(bpy)2(tpy)Pd(CH3CN)Cl]2+ (RuPd; bpy = bipyridine, tpy = 2,2′:5′,2″-terpyridine) have been studied by resonance Raman, femtosecond transient absorption, and time-resolved photoluminescence spectroscopies. Comparison of the photophysical properties of RuPd with those of the mononuclear precursor [(bpy)2Ru(tpy)]2+ (Ru) enables establishment of a photophysical model ranging from the femtosecond to the submicrosecond domain. Optical excitation of Ru and RuPd populates both bpy- and tpy-based 1MLCT (metal-to-ligand charge transfer) singlet states, from where intersystem crossing (ISC) into corresponding 3MLCT triplet states occurs. Electron density localized on the peripheral bpy ligands can subsequently flow to the tpy bridging ligand by interligand electron transfer, which process occurs with a time constant of 32.5 (±1.5) ps for RuPd. Not all electron density undergoes this process, most likely due to a competing loss channel on the bpy ligand caused by vibrational relaxation occurring at a time scale of 9.1 (±0.4) ps. The relaxed 3MLCTbpy and 3MLCTtpy states have excited state lifetimes of 400 (±1) ns and 88 (±1) ns, respectively. Electron transfer from the tpy ligand to Pd may take place on a ∼100 ns time scale, but it is also possible that the final relaxed excited state is delocalized over the tpy ligand and the Pd center. The insight that optical excitation populates both the peripheral bpy ligands and the bridging tpy ligand, and that part of the electron density subsequently flows from the former to the latter, is important for the realization of efficient photocatalytic hydrogen generation. The next step is to make the interligand electron transfer process faster, by functionalizing the peripheral ligands with electron-donating moieties, and adapting the nature of the bridging ligand and the catalytic metal center.

A New Cyanofluorene–Triphenylamine Copolymer: Synthesis and Photoinduced Intramolecular Electron Transfer Processes

A new pi-conjugated copolymer, namely, poly{cyanofluore-alt-[5-(N,N'-diphenylamino)phenylenevinylene]} ((CNF-TPA)(n)), was synthesized by condensation polymerization of 2,2'-(9,9-dioctyl-9H-fluorene-2,7-diyl)diacetonitrile and 5-(N,N'-diphenylamino)benzene-1,3-dicarbaldehyde by using the Knoevenagel reaction. By design, diphenylamine, alkylfluorene and poly(p-phenylenevinylene) linkages were combined to form a (CNF-TPA)(n) copolymer which exhibits high thermal stability and glass-transition temperature. Photodynamic measurements in polar benzonitrile indicate fast and efficient photoinduced electron transfer ( approximately 10(11) s(-1)) from triphenylamine (TPA) to cyanofluorene (CNF) to produce the long-lived charge-separated state (90 mus). The finding that the charge-recombination process of (CNF(.-)-TPA(.+))(n) is much slower than the charge separation in polar benzonitrile suggests a potential application in molecular-level electronic and optoelectronic devices.

Photoinduced Electron Transfer in Dumbbell-type Fullerene Dyad

1,3-Dipolar cycloaddition of DTE-azomethine ylides (DTE: dithienyl-ethene) to C60 in refluxed toluene was used to synthesize novel dumbbell-type fullerene dimer 1. For the sake of comparison, the monoadduct 2 were also synthesized. The molecular geometries of these two compounds were determined by theoretical calculations with HF-3/21G method. UV-Vis and fluorescence experiments were carried out in solvents with different polarity at the room temperature. All the results indicated the existence of a photoinduced intramolecular electron transfer process between the donor and acceptor moieties.

Synthesis, charge-separated state characterization of N-methyl-2-(4′-N-ethylcarbozole)-3-fulleropyrrolidine and its derivatives

N-Methyl-2-(N-ethylcarbozole)-fulleropyrro lidine and N-methyl-2-(4′-N,N-diphenylaminophenyl)-fulleropyrrolidine were synthesized by 1,3-dipolar cycloaddition under microwave irradiation, which were characterized by MS, 1H NMR, IR and UV-Vis. Photoinduced intramolecular electron transfer process from C60 moiety to carbazole moiety has been studied by nanosecond laser flash photolysis. The charge-separated state C60•−-Cz•+ was observed in the near-IR region with a lifetime of 0.28 μs. The electronic spectrum of the C60-TPA was studied by using ZINDO method on the basis of the optimized geometrics with B3LYP/6-31G* program. The results show that the calculated absorption was beyond 440nm, essentially consistent with the experimental value 433 nm.

Synthesis, Characterization and Theoretical Calculation of the Fulleropyrrolidines Containing Triphenylamine

AbstractN‐Methyl‐2‐(4‐N,N‐diphenylaminophenyl)fulleropyrrolidine and N‐methyl‐2‐(4‐di‐p‐tolylaminophenyl)fulleropyrro‐ lidine were synthesized via the 1,3‐dipolar cycloaddition reactions under microwave irradiation. The molecular structures were identified and characterized by MS, UV‐Vis, FT‐IR, 1H NMR and fluorescence spectra. Photoinduced intramolecular electron transfer process from C60 moiety to triphenylamine moiety have been studied by nanosecond laser flash photolysis. The optimized structure and the distribution of the frontier molecular orbitals for C60‐TPA were obtained by using DFT method at B3LYP/6‐31G(d) level. The results indicated that the intramolecular photoinduced electron transfer could occur in these compounds, which were in excellent agreement with the nanosecond transient absorption spectra observed experimentally in polar solvent. The electronic spectrum of the compound C60‐TPA was studied by ZINDO method on the basis of the optimized geometrics, which was essentially consistent with experimental values.

60]Fullerene–perchlorotriphenylmethide anion triads. Synthesis and study of photoinduced intramolecular electron-transfer processes

For the first time an anionic donor, the perchlorotriphenylmethide anion (PTM−), has been covalently bonded to C60, generating the C60–(PTM−)2 triad that is reversibly oxidized to the corresponding C60–perchlorotriphenylmethyl radical triad C60–(PTM˙)2. For the triad C60–(PTM−)2, photoinduced charge-separation can be confirmed to occur via the excited singlet states of the C60 moiety and the PTM anion in polar and nonpolar solvents from quenching of their fluorescence intensities in the region of 700–750 nm and 560–630 nm, respectively. The charge-separation state was confirmed by the nanosecond transient absorption spectra in the visible and near-IR spectral regions. After charge-separation, back electron transfer takes place with a lifetime of about 80 ns. Steady-state concentration of the highly persistent PTM radical was observed after repeated laser light irradiation.

Perturbation of the PET Process in Fluorophore−Spacer−Receptor Systems through Structural Modification: Transition Metal Induced Fluorescence Enhancement and Selectivity

Several fluorescent signaling systems are built in the format fluorophore-spacer-receptor with ethylenediamine or N,N-dimethylethylenediamine as the receptor, anthracene as the fluorophore, and a methylene group as the spacer. The receptors are derivatized with different electron-withdrawing groups such as 4-nitrobenzene, 4-nitro-2-pyridine, and 2,4-dinitrobenzene, to perturb the photoinduced intramolecular electron transfer (PET) process from the nitrogen lone-pair to the fluorophore. The photophysical properties of these supramolecular systems and their fluorescence responses toward a number of quenching transition metal ions are reported. It is shown that the PET is highly efficient in the absence of a metal ion. With a metal ion input, the fluorescence can be recovered to a different extent depending on the nature of the metal and on the overall architecture of the system as well. Despite the possibility of strong interaction between the fluorophore and the metal ion, significant fluorescence enhancement is observed with quenching of paramagnetic transition metal ions. The complex stability data show that the stability constants for the metal ions showing fluorescence enhancement are of the order of 10(4) M(-1). This study shows that structurally simple fluorescent signaling systems for quenching transition metal ions can be built by maximizing the PET. It is also shown here that simple structural modification can make these systems highly specific for particular transition metal ions for potential applications in several contemporary areas of research.

The Fluorescence Properties of Hypocrellin B and its Amino‐substituted Derivative: Photinduced Intramolecular Proton Transfer and Photoinduced Intramolecular Electron Transfer¶

ABSTRACTWe report on the emission spectra and emission quantum yields of a newly synthesized hypocrellin dye, Z‐demethoxy‐ 2,3‐ethylenediamino hypocrellin B (EDAHB), and its parent HB in different solvents of varying polarity. Our results demonstrate that EDAHB is one of the few dyes that exhibit highly solvent polarity‐dependent fluorescence in the useful region (680–730 nm). Therefore, it offers some applications in the biomedical field as a fluorescent probe molecule. The solvatochromic effect of EDAHB is proposed to be due to a distinct change in the dipole moment of the dye on excitation. A photoinduced intramolecular proton transfer and a photoinduced intramolecular electron transfer process are considered relevant for the fluorescence properties of HB and EDAHB, respectively.

The fluorescence properties of hypocrellin B and its amino-substituted derivative: photoinduced intramolecular proton transfer and photoinduced intramolecular electron transfer.

We report on the emission spectra and emission quantum yields of a newly synthesized hypocrellin dye, 2-demethoxy-2,3-ethylenediamino hypocrellin B (EDAHB), and its parent HB in different solvents of varying polarity. Our results demonstrate that EDAHB is one of the few dyes that exhibit highly solvent polarity-dependent fluorescence in the useful region (680-730 nm). Therefore, it offers some applications in the biomedical field as a fluorescent probe molecule. The solvatochromic effect of EDAHB is proposed to be due to a distinct change in the dipole moment of the dye on excitation. A photoinduced intramolecular proton transfer and a photoinduced intramolecular electron transfer process are considered relevant for the fluorescence properties of HB and EDAHB, respectively.

Photoinduced Intramolecular Electron Transfer in a Bridged C60 (Acceptor)-Aniline (Donor) System; Photophysical Properties of the First "Active" Fullerene Diad

A covalently functionalized fullerene comprising an electron donating aniline group coupled to the fullerene unit by a saturated heterocyclic bridge is shown to undergo a photoinduced intramolecular electron transfer process that causes quenching of the fluorescence of the adduct and strong decrease triplet population in polar solvents. VIS-absorption, fluorescence and phosphorescence at 77 K, triplet-triplet absorption, time resolved fluorescence and redox potentials of the fullerene adduct are presented. Analysis of the solvent dependence of the energetics of the intramolecular electron transfer is given and is in good agreement with the experimental results. 17 refs., 6 figs., 3 tabs.

Photoinduced intramolecular electron transfer in bimetallic donor-acceptor assemblies

A series of bimetallic complexes containing an (α-diimine)(CO) 3Re 1 chromophore and a covalently attached ruthenium pentaammine moiety has been prepared and characterized. Ground state properties, including absorption spectra and redox potentials, are consistent with very weak metal-metal interaction in these systems. However, picosecond luminescence decay profiles indicate that emission from the Re → α-diimine 3MLCT excited state is strongly quenched by attachment of the Ru center for complexes in both Re I/Ru II and Re I/Ru III oxidation states. Such behaviour is consistent with extremely rapid photoinduced intramolecular electron transfer processes, and the data are interpreted in light of the ‘superexchange’ mechanism. No short-lived transients are observed for the Re I/Ru III bimetallic complexes, suggesting that electron transfer may occur from the 1MLCT state in these systems.

Effect of temperature, energy gap and distortion of potential surfaces on photoinduced intramolecular electron transfer

We report the quantitative examination of the effect of distortion of potential energy surfaces and the temperature effect on photoinduced intramolecular electron transfer (PIET) as a function of electronic energy gap. The results demonstrate the importance of distorted oscillators in determining the dependence of the rate of PIET on the energy gap. This phenomenon may in some cases lead to misinterpretations of experimental data, when undistorted oscillators are assumed to be involved in the PIET process. The condition for observing the linear (rather than parabolic) dependence of the ET rate will be determined using the multimode model. The inclusion of multivibrational modes in the nuclear factors results in energy gap behavior, which is markedly different from the single mode case. Finally, a comparison between PIET and internal conversion is discussed. 25 refs., 5 figs., 3 tabs.

Photoinduced intramolecular electron transfer: free energy and rigidity effects

The relative photoreactivities of several bridge-ring systems 2–4 have been studied. Compound 3 is photoreactive whereas 2 and 4 are photoinert when irradiated with 254 nm light. The systems contain aromatic electron donors and halogen electron acceptors which are held at well-defined geometries that vary in the degree of rigidity at the electron acceptor termini. The relative photoreactivities in the photoinduced intramolecular electron transfer processes cannot be explained by calculated relative free-energy or rate constant values for electron transfer. The discrepancies between the relative experimental photo-reactivities and those predicted based on calculated values of free energy (Weller equation) are rationalized by a rigidity argument.

Synthesis and electron-transfer efficiency of oligopeptide-bridged donor–acceptor molecules

The synthesis of anthracene–oligopeptide–N,N-dimethylaniline molecules is described. As the oligopeptide link, sequential oligomers, -(Gly-Aib)n-(n= 0–11) were used because they were easily prepared and highly soluble. A photoinduced intramolecular electron-transfer process in the oligopeptide bridged donor–acceptor molecules is described. The dependency of the estimated electron-transfer rate on the oligomerization number n supported the possible mediation of amido and/or hydrogen in the electron-transfer process.