Positional Isomers of Isocyanoazulenes as Axial Ligands Coordinated to Ruthenium(II) Tetraphenylporphyrin : Fine-Tuning Redox and Optical Profiles

Two isomeric ruthenium(II)/5,10,15,20-tetraphe-nylporphyrin complexes featuring axially coordinated redox-active, low-optical gap 2- or 6-isocyanoazulene ligands have been isolated and characterized by NMR, UV-vis, and magnetic circular dichroism (MCD) spectroscopic methods, high-resolution mass spectrometry, and single-crystal X-ray crystallography. The UV-vis and MCD spectra support the presence of the low-energy, azulene-centered transitions in the Q band region of the porphyrin chromophore. The first coordination sphere in new L2RuTPP complexes reflects compressed tetragonal geometry. The redox properties of the new compounds were assessed by electrochemical and spectroelectrochemical means and correlated with the electronic structures predicted by density functional theory and CASSCF calculations. Both experimental and theoretical data are consistent with the first two reduction processes involving the axial azulenic ligands, whereas the oxidation profile (in the direction of increasing potential) is exerted by the ruthenium ion, the porphyrin core, and the axial azulenic moieties.Peer reviewe

Ferrocene-BODIPYmerocyanine dyads: new NIR absorbing platforms with optical properties susceptible to protonation

Ferrocene-BODIPYmerocyanine dyads 5 and 6 were prepared and characterized by a variety of spectroscopic, electrochemical, and theoretical methods. Experimental and theoretical data on these NIR absorbing compounds are suggestive of unusual susceptibility (for BODIPY chromophores) of the delocalized pi-system in 5 and 6 to protonation and low-potential oxidation of their pi-systems

Synthesis, Redox Properties, and Electronic Coupling in the Diferrocene Aza-dipyrromethene and azaBODIPY Donor–Acceptor Dyad with Direct Ferrocene−α-Pyrrole Bond

3,3′-Diferrocenylazadipyrromethene (<b>3</b>) and corresponding difluoroboryl (azaBODIPY) complex (<b>4</b>) were synthesized in several steps from ferrocenecarbaldehyde, following the well-explored chalcone-type synthetic approach. The novel diiron complexes, in which ferrocene groups are directly connected to the α-pyrrolic positions were characterized by a variety of spectroscopic techniques, electrochemistry, spectroelectrochemistry, and X-ray crystallography, while their electronic structure, redox properties, and UV–vis spectra were correlated with the density functional theory (DFT) and time-dependent DFT calculations

Tuning Electronic Structure, Redox, and Photophysical Properties in Asymmetric NIR-Absorbing Organometallic BODIPYs

Stepwise modification of the methyl groups at the α positions of BODIPY <b>1</b> was used for preparation of a series of mono- (<b>2</b>, <b>4</b>, and <b>6</b>) and diferrocene (<b>3</b>) substituted donor–acceptor dyads in which the organometallic substituents are fully conjugated with the BODIPY π system. All donor–acceptor complexes have strong absorption in the NIR region and quenched steady-state fluorescence, which can be partially restored upon oxidation of organometallic group(s). X-ray crystallography of complexes <b>2</b>–<b>4</b> and <b>6</b> confirms the nearly coplanar arrangement of the ferrocene groups and the BODIPY π system. Redox properties of the target systems were studied using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the first oxidation process in all dyads is ferrocene centered, while the separation between the first and the second ferrocene-centered oxidation potentials in diferrocenyl-containing dyad <b>3</b> is ∼150 mV. The density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to investigate the electronic structure as well as explain the UV–vis and redox properties of organometallic compounds <b>2</b>–<b>4</b> and <b>6</b>. TDDFT calculations allow for assignment of the charge-transfer and π → π* transitions in the target compounds. The excited state dynamics of the parent BODIPY <b>1</b> and dyads <b>2</b>–<b>4</b> and <b>6</b> were investigated using time-resolved transient spectroscopy. In all organometallic dyads <b>2</b>–<b>4</b> and <b>6</b> the initially excited state is rapidly quenched by electron transfer from the ferrocene ligand. The lifetime of the charge-separated state was found to be between 136 and 260 ps and demonstrates a systematic dependence on the electronic structure and geometry of BODIPYs <b>2</b>–<b>4</b> and <b>6</b>

Observation of the Strong Electronic Coupling in Near-Infrared-Absorbing Tetraferrocene aza-Dipyrromethene and aza-BODIPY with Direct Ferrocene−α- and Ferrocene−β-Pyrrole Bonds: Toward Molecular Machinery with Four-Bit Information Storage Capacity

The 1,3,7,9-tetraferrocenylazadipyrromethene (<b>3</b>) and the corresponding 1,3,5,7-tetraferrocene aza-BODIPY (<b>4</b>) were prepared via three and four synthetic steps, respectively, starting from ferrocenecarbaldehyde using the chalcone-type synthetic methodology. The novel tetra-iron compounds have ferrocene groups directly attached to both the α- and the β-pyrrolic positions, and the shortest Fe–Fe distance determined by X-ray crystallography for <b>3</b> was found to be ∼6.98 Å. These new compounds were characterized by UV–vis, nuclear magnetic resonance, and high-resolution electrospray ionization mass spectrometry methods, while metal–metal couplings in these systems were probed by electro- and spectroelectrochemistry, chemical oxidations, and Mössbauer spectroscopy. Electrochemical data are suggestive of the well-separated stepwise oxidations of all four ferrocene groups in <b>3</b> and <b>4</b>, while spectroelectrochemical and chemical oxidation experiments allowed for characterization of the mixed-valence forms in the target compounds. Intervalence charge-transfer band analyses indicate that the mixed-valence [<b>3</b>]⁺ and [<b>4</b>]⁺ complexes belong to the weakly coupled class II systems in the Robin–Day classification. This interpretation was further supported by Mössbauer spectroscopy in which two individual doublets for Fe­(II) and Fe­(III) centers were observed in room-temperature experiments for the mixed-valence [<b>3</b>]^<i>n</i>+ and [<b>4</b>]^<i>n</i>+ species (<i>n</i> = 1–3). The electronic structure, redox properties, and UV–vis spectra of new systems were correlated with Density Functional Theory (DFT) and time-dependent DFT calculations (TDDFT), which are suggestive of a ferrocene-centered highest occupied molecular orbital and chromophore-centered lowest unoccupied molecular orbital in <b>3</b> and <b>4</b> as well as predominant spin localization at the ferrocene fragment attached to the α-pyrrolic positions in [<b>3</b>]⁺ and [<b>4</b>]⁺

Tuning Electronic Structure, Redox, and Photophysical Properties in Asymmetric NIR-Absorbing Organometallic BODIPYs

Stepwise modification of the methyl groups at the α positions of BODIPY <b>1</b> was used for preparation of a series of mono- (<b>2</b>, <b>4</b>, and <b>6</b>) and diferrocene (<b>3</b>) substituted donor–acceptor dyads in which the organometallic substituents are fully conjugated with the BODIPY π system. All donor–acceptor complexes have strong absorption in the NIR region and quenched steady-state fluorescence, which can be partially restored upon oxidation of organometallic group(s). X-ray crystallography of complexes <b>2</b>–<b>4</b> and <b>6</b> confirms the nearly coplanar arrangement of the ferrocene groups and the BODIPY π system. Redox properties of the target systems were studied using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the first oxidation process in all dyads is ferrocene centered, while the separation between the first and the second ferrocene-centered oxidation potentials in diferrocenyl-containing dyad <b>3</b> is ∼150 mV. The density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to investigate the electronic structure as well as explain the UV–vis and redox properties of organometallic compounds <b>2</b>–<b>4</b> and <b>6</b>. TDDFT calculations allow for assignment of the charge-transfer and π → π* transitions in the target compounds. The excited state dynamics of the parent BODIPY <b>1</b> and dyads <b>2</b>–<b>4</b> and <b>6</b> were investigated using time-resolved transient spectroscopy. In all organometallic dyads <b>2</b>–<b>4</b> and <b>6</b> the initially excited state is rapidly quenched by electron transfer from the ferrocene ligand. The lifetime of the charge-separated state was found to be between 136 and 260 ps and demonstrates a systematic dependence on the electronic structure and geometry of BODIPYs <b>2</b>–<b>4</b> and <b>6</b>

Synthesis and Charge-Transfer Dynamics in a Ferrocene-Containing Organoboryl aza-BODIPY Donor–Acceptor Triad with Boron as the Hub

A <i>N</i>,<i>N</i>′-bis­(ferroceneacetylene)­boryl complex of 3,3′-diphenylazadiisoindolylmethene was synthesized by the reaction of an <i>N</i>,<i>N</i>′-difluoroboryl complex of 3,3′-diphenylazadiisoindolylmethene and ferroceneacetylene magnesium bromide. The novel diiron complex was characterized by a variety of spectroscopic techniques, electrochemistry, and ultrafast time-resolved methods. Spectroscopy and redox behavior was correlated with the density functional theory (DFT) and time-dependent DFT calculations. An unexpected degree of coupling between the two Fc ligands was observed. Despite a lack of conjugation between the donor and acceptor, the complex undergoes very rapid (τ = 1.7 ± 0.1 ps) photoinduced intramolecular charge separation followed by subpicosecond charge recombination to form a triplet state with a lifetime of 4.8 ± 0.1 μs

Redox and Photoinduced Electron-Transfer Properties in Short Distance Organoboryl Ferrocene-Subphthalocyanine Dyads

Reaction between ferrocene lithium or ethynylferrocene magnesium bromide and (chloro)­boronsubphthalocyanine leads to formation of ferrocene- (<b>2</b>) and ethynylferrocene- (<b>3</b>) containing subphthalocyanine dyads with a direct organometallic B–C bond. New donor–acceptor dyads were characterized using UV–vis and magnetic circular dichroism (MCD) spectroscopies, NMR method, and X-ray crystallography. Redox potentials of the rigid donor–acceptor dyads <b>2</b> and <b>3</b> were studied using the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) approaches and compared to the parent subphthalocyanine <b>1</b> and conformationally flexible subphthalocyanine ferrocenenylmethoxide (<b>4</b>) and ferrocenyl carboxylate (<b>5</b>) dyads reported earlier. It was found that the first oxidation process in dyads <b>2</b> and <b>3</b> is ferrocene-centered, while the first reduction as well as the second oxidation are centered at the subphthalocyanine ligand. Density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to probe the electronic structures and explain the UV–vis and MCD spectra of complexes <b>1</b>–<b>5</b>. DFT-PCM calculations suggest that the LUMO, LUMO+1, and HOMO-3 in new dyads <b>2</b> and <b>3</b> are centered at the subphthalocyanine ligand, while the HOMO to HOMO-2 in both dyads are predominantly ferrocene-centered. TDDFT-PCM calculations on compounds <b>1</b>–<b>5</b> are indicative of the π → π* transitions dominance in their UV–vis spectra, which is consistent with the experimental data. The excited state dynamics of the parent subphthalocyanine <b>1</b> and dyads <b>2</b>–<b>5</b> were investigated using time-resolved transient spectroscopy. In the dyads <b>2</b>–<b>5</b>, the initially excited state is rapidly (<2 ps) quenched by electron transfer from the ferrocene ligand. The lifetime of the charge transfer state demonstrates a systematic dependence on the structure of the bridge between the subphthalocyanine and ferrocene

Outsourcing Intersystem Crossing without Heavy Atoms: Energy Transfer Dynamics in PyridoneBODIPY–C₆₀ Complexes

The excited state dynamics in two fully characterized pyridoneBODIPY–fullerene complexes were investigated using time-resolved spectroscopy. Photoexcitation was initially localized on the pyridoneBODIPY chromophore. The energy was rapidly transferred to the fullerene, which subsequently underwent ISC to form a triplet state and returned the energy to the pyridoneBODIPY via triplet–triplet energy transfer. This ping-pong energy transfer mechanism resulted in efficient (>85%) overall conversion of the excited state pyridoneBODIPY constituent despite a complete lack of ISC in the pyridoneBODIPY in the absence of the fullerene partner. The small difference in attachment chemistry for the fullerene did not impact the initial singlet energy transfer. However, the N-methylpyrrolidine bridge did slow both the triplet–triplet energy transfer and the ultimate relaxation rate of the final triplet state when compared to an isoxazole-based bridge. The rates of each step were quantified, and computational predictions were used to complement the proposed mechanism and energetics. The result demonstrated efficient triplet sensitization of a strong chromophore that lacks significant spin–orbit coupling

Tuning Electron-Transfer Properties in 5,10,15,20-Tetra(1′-hexanoylferrocenyl)porphyrins as Prospective Systems for Quantum Cellular Automata and Platforms for Four-Bit Information Storage

Metal-free (<b>1</b>) and zinc (<b>2</b>) 5,10,15,20-tetra­(1′-hexanoylferrocenyl)­porphyrins were prepared using an acid-catalyzed tetramerization reaction between pyrrole and 1′-(1-hexanoyl)­ferrocencarboxaldehyde. New organometallic compounds were characterized by combination of ¹H, ¹³C, and variable-temperature NMR, UV–vis, magnetic circular dichroism, and high-resolution electrospray ionization mass spectrometry methods. The redox properties of <b>1</b> and <b>2</b> were probed by electrochemical (cyclic voltammetry and differential pulse voltammetry), spectroelectrochemical, and chemical oxidation approaches coupled with UV–vis–near-IR and Mössbauer spectroscopy. Electrochemical data recorded in the dichloromethane/TBA­[B­(C₆F₅)₄] system (TBA­[B­(C₆F₅)₄] is a weakly coordinating tetrabutylammonium tetrakis­(pentafluorophenyl)­borate electrolyte) are suggestive of “1e^– + 1e^– + 2e^–” oxidation sequence for four ferrocene groups in <b>1</b> and <b>2</b>, which followed by oxidation process centered at the porphyrin core. The separation between all ferrocene-centered oxidation electrochemical waves is very large (510–660 mV). The nature of mixed-valence [<b>1</b>]^<i>n</i>+ and [<b>2</b>]^<i>n</i>+ (<i>n</i> = 1 or 2) complexes was probed by the spectroelectrochemical and chemical oxidation methods. Analysis of the intervalence charge-transfer band in [<b>1</b>]⁺ and [<b>2</b>]⁺ is suggestive of the Class II (in Robin–Day classification) behavior of all mixed-valence species, which correlate well with Mössbauer data. Density functional theory–polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were applied to correlate redox and optical properties of organometallic complexes <b>1</b> and <b>2</b> with their electronic structures