František Hartl

The synthesis and photophysical properties of four [Re(R-NI-phen)(CO)₃Cl] complexes (Re-R), where NI = 1,8-naphthalimide and phen = 1,10-phenanthroline, are reported. The R-NI chromophores were substituted at their 4-positions with R = H (Re-H), ethynyl benzene (Re-EB), ethynyl thiophene (Re-ES), ethynyl trimethylsilane (Re-ET), yielding a series of metal-organic bichromophores. The R-NI-phen organic chromophores were investigated in parallel to better understand the photophysical properties exhibited in the Re-R complexes. The excited-state processes of these molecules were studied by transient absorption and time-resolved infrared spectroscopies, to unravel the kinetics of the energy transfer processes occurring in the Re-R series. Namely, the interplay between the singlet ligand-centered (¹LC), the triplet metal-to-ligand charge-transfer (³MLCT), and the triplet ligand-centered (³LC) excited states were studied using these time-resolved spectroscopic techniques, supported by quantum chemical calculations, to monitor the changing excited-state dynamics in the Re-R series. These studies reveal that the thermal equilibrium between the ³MLCT and ³LC excited states present in Re-H is modulated in favor of the ³LC excited state in the remaining Re-R series, giving rise to increasingly ligand-like photophysics as the electron-donating strength of the R-group increases.

Oxidation potentials of ferrocene–dichalcogenolate-bridged complexes, [Fe ₂ (CO) ₆ {μ-E(η ⁵ -C ₅ H ₄ )Fe(η ⁵ -C ₅ H ₄ )E}] (E = S, Se), in which the two redox-active centres lie in close proximity, have been tuned by carbonyl substitution.

While many [FeFe]-hydrogenase biomimetics are effective proton-reduction catalysts, few are active for H₂ oxidation, and examples containing both a pendant amine group, able to act as a proton relay, and a second redox center, both essential features of the enzymes, are rare. Here we report the preparation and oxidation chemistry of two ferrocene-functionalized amino-diphosphines (PCNCP), (CH₂PR₂)₂NCH₂Fc (R = Ph (1), Cy (2)), and their ethylenedithiolate (edt) diiron complexes, [Fe₂(CO)₄(μ-edt){κ²-(R₂PCH₂)₂NCH₂Fc}] (R = Ph (3), Cy (4)). Their crystallographic characterization shows that PCNCP occupies an apical–basal position. CV responses are slightly R-dependent, showing for 3 and 4 in three separate oxidative processes assigned to successive one-electron oxidation of the diiron core (quasireversible), appended Fc (reversible), and the amine–diiron moiety (irreversible), as confirmed by IR and UV–Vis spectroelectrochemical studies supported by Density Functional Theory (DFT) and Time-dependent Density Functional Theory (TDDFT) calculations. The first oxidation results in a structural rearrangement of the Fe(PNP)(CO) unit and the formation of a semi-bridging carbonyl. Slow protonation of 3 with HBF₄∙Et₂O affords the corresponding N-protonated cation in acetone, whilst μ-hydride products dominate for both 3 and 4 in CD₂Cl₂. A preliminary H₂ oxidation study was carried out with 3, and while there was some evidence of activity, it was much lower than reported for alkyl-functionalized PCNPC diiron derivatives.

Photosensitized damage by the mechanism of direct 1e^- transfer from a nucleobase to the metal complex is a complementary approach to type I and type II methods of photodynamic therapy. In this ultrafast spectroscopic study we report the ability of a nitrile infrared redox probe to report on the photo-oxidation of guanine-rich DNA, comprising persistent runs of guanine, by the dppz-10-CN containing complex [Ru(TAP)₂(dppz-10-CN)]²⁺ (1²⁺), dppz-10-CN = 10-cyano-dipyrido[3,2-a:2',3'-c]phenazine and TAP = 1,4,5,8-tetraazaphenanthrene. Our study reveals the ability of the enantiomers of 1²⁺ to photo-oxidize guanine in double-stranded and quadruplex DNA. Transient visible absorption reveals a high yield of the formation of the photoreduced metal complex due to photo-oxidation of guanine in the quadruplex-bound 1²⁺ systems, and that this is greater for the Λ enantiomer. Spectro-electrochemical and computational studies indicate the role of the dppz-10-CN as the preferred site of reduction, while time-resolved electronic absorption (TrA) spectroscopy highlights the impact of the enantiomers on the yield of photo-oxidation in the DNA systems. Notably, time-resolved infrared (TRIR) spectroscopy allows comprehensive tracking of the photo-oxidation dynamics by monitoring four key components, namely: (1) the transient band of the Ru/TAP-based lowest ³MLCT excited state, (2) bleach bands associated with DNA bases in close proximity to the excited state "site effect", (3) the guanine radical cation band at ca. 1700 cm^-1 and (4) the amplification of the red-shifted nitrile stretching vibration of the transient dppz-reduced complex. Together, these results allow detailed profiling of photoinduced electron transfer in DNA-bound ruthenium(II) polypyridyl complex systems and highlight the potential of such redox probes. Overall, this study presents an important insight regarding the nature of charge transfer in a Hoogsteen-bound guanine quadruplex compared to Watson-Crick GC base pairings.

Here we investigate how structural changes stabilise (or destabilise) indolcarbazole-based diradical systems, demonstrating that they are very useful motifs for dynamic covalent chemistry.

Ruthenium(II) complexes [Ru(tap)₂(NN)]²⁺ (tap = 1,4,5,8-tetraazaphenanthrene, NN = 11-cyano-dipyrido[3,2-a:2',3'-c]phenazine (11-CN-dppz) and 11,12-dicyano-dipyrido[3,2-a:2',3'-c]phenazine (11,12-CN-dppz)) feature the C≡N groups as infrared (IR)-active redox markers. They were studied by cyclic voltammetry, UV-vis, and IR spectroelectrochemistry (SEC), and density functional theory calculations to assign the four 1e^- reduction waves R1-R4 observed in dichloromethane. Generally, the NN ligands are reduced first (R1). For [Ru(tap)₂(11,12-CN-dppz)]²⁺, R1 is sufficiently separated from R2 and delocalized over both tap ligands. Accordingly, IR SEC conducted at R1 shows a large red shift of the ν_s,as(C≡N) modes by -18/-28 cm^-1, accompanied by a 4-fold enhancement of the ν_s(C≡N) intensity, comparably with reference data for free 11,12-CN-dppz. The first tap-based reduction of spin-doublet [Ru(tap)₂(11,12-CN-dppz)]⁺ to spin-triplet [Ru(tap)₂(11,12-CN-dppz)] at R2 decreased ν(C≡N) by merely -2 cm^-1, while the intensity enhancement reached an overall factor of 8. Comparably, a red shift of ν(C≡N) by -27 cm^-1 resulted from the 1e^- reduction of [Ru(tap)₂(11-CN-dppz)]²⁺ at R1 (poorly resolved from R2), and the intensity enhancement was roughly 3-fold. Concomitant 1e^- reductions of the tap ligands (R2 and R3) caused only minor ν(C≡N) shifts of -3 cm^-1 and increased the absorbance by overall factors of 6.5 and 8, respectively.

In recent years, the field of organometallic chemistry has made a great progress and diverse types of metallaaromatics have successively been reported. In those studies, incorporation of ligated osmium centers into metallaaromatic systems played a prominent role. The reviewed literature documents that certain metallaaromatics with unconventional photophysical properties, redox and electronic transport properties and magnetism, have potential to be widely used in diverse practical applications, with selected examples of amino acid and fluoride anion identification, photothermal effects, functional materials, photodynamic therapy (PDT) in biomedicine, single‐molecule junction conductors, and electron‐transport layer materials (ETLs) in solar cells.

Three novel rhenium N−heterocyclic carbene complexes, [Re]−NHC−1−3 ([Re] = fac−Re(CO)₃Br), were synthesized and characterized using a range of spectroscopic techniques. Photophysical, electrochemical and spectroelectrochemical studies were carried out to probe the properties of these organometallic compounds. Re−NHC−1 and Re−NHC−2 bear a phenanthrene backbone on an imidazole (NHC) ring, coordinating to Re by both the carbene C and a pyridyl group attached to one of the imidazole nitrogen atoms. Re−NHC−2 differs from Re−NHC−1 by replacing N−H with an N−benzyl group as the second substituent on imidazole. The replacement of the phenanthrene backbone in Re−NHC−2 with the larger pyrene gives Re−NHC−3. The two−electron electrochemical reductions of Re−NHC−2 and Re−NHC−3 result in the formation of the five−coordinate anions that are capable of electrocatalytic CO₂ reduction. These catalysts are formed first at the initial cathodic wave R1, and then, ultimately, via the reduction of Re−Re bound dimer intermediates at the second cathodic wave R2. All three Re−NHC−1−3 complexes are active photocatalysts for the transformation of CO₂ to CO, with the most photostable complex, Re−NHC−3, being the most effective for this conversion. Re−NHC−1 and Re−NHC−2 afforded modest CO turnover numbers (TONs), following irradiation at 355 nm, but were inactive at the longer irradiation wavelength of 470 nm. In contrast, Re−NHC−3, when photoexcited at 470 nm, yielded the highest TON in this study, but remained inactive at 355 nm. The luminescence spectrum of Re−NHC−3 is red−shifted compared to those of Re−NHC−1 and Re−NHC−2, and previously reported similar [Re]−NHC complexes. This observation, together with TD−DFT calculations, suggests that the nature of the lowest−energy optical excitation for Re−NHC−3 has π→π*(NHC−pyrene) and d_π(Re)→π*(pyridine) (IL/MLCT) character. The stability and superior photocatalytic performance of Re−NHC−3 are attributed to the extended conjugation of the π−electron system, leading to the beneficial modulation of the strongly electron−donating tendency of the NHC group.

A conjugated diosma-octacyclic complex, with aromatic osmapentalene termini and the rigid naphthalene bridge linked by non-aromatic osmafuran rings, forms a singly reduced valence-trapped species characterised at low temperature.

Ferrocenyl diphosphine bridged diiron dithiolate complexes have been prepared and their redox properties investigated by CV and IR SEC supported by DFT calculations to give insight into their proton reduction and hydrogen oxidation activity.