Electron Transfer around a Molecular Corner

Intramolecular Electron Transfer in Multi-Redox Systems Based on Cyclic [3]Spirobifluorenylene Compound

Cyclic [3]spirobifluorenylene with bulky alkyl groups at the ends (1) was designed and synthesized to investigate the electron transfer phenomena in a π-conjugated system including orthogonal π-conjugated chains. The three bifluorenyl units in 1 are conjugated to each other via spiro-conjugation, resulting in the splitting of the HOMO levels to a small extent. Therefore, the SOMO-HOMO gap of the radical cation species is small, which is considered to allow the facile intramolecular electron transfer. The electronic properties of 1 and its partial structures were characterized by absorption and fluorescence measurements and electrochemical analysis. From the electrochemical oxidation, the interchain Coulombic repulsion was observed. In the TD-DFT calculations for the radical cation species of 1, the geometry-featured interchain electronic transitions were visualized by NTO calculations. The radical cation species of 1 generated by chemical oxidation with SbCl5 exhibited a broadened and lower-energy NIR absorption band exceeding 2000 nm. Considering the results of the TD-DFT calculations, the NIR band of the radical cation of 1 was attributed to the intramolecular electron transfer processes among the bifluorenyl units in the macrocycle. ESR experiments also indicated the delocalization of a spin of 1⋅+ in the whole molecule via hole hopping in the ESR time scale at room temperature. This work demonstrates the usefulness of spiro-conjugation as a bridging unit in molecular wires to facilitate smooth electron transfer.

Effect of positional isomerism on the excited state charge transfer dynamics of anthracene-based D-π-A systems

Understanding the dynamics of the back electron transfer (BET) rate of ion pairs from the electronically excited state of donor-acceptor systems is crucial for developing materials for organic electronics. The structure-property relationships in the organic molecular architectures play a key role in controlling the BET rate and have been utilized as a criterion to design systems with a reduced BET rate. Here, we examine the influence of isomerism on the BET rate in anthracene based systems, viz., (E)-2-(2-(anthracen-9-yl)vinyl)benzonitrile (ortho-CN) and (E)-3-(2-(anthracen-9-yl)vinyl)benzonitrile (meta-CN) with N,N-diethylaniline (DEA) in methylcyclohexane using time-resolved spectroscopy. The radical cation (DEA˙⁺) and the radical anion (ortho-CN˙^- or meta-CN˙^-) generated after photoexcitation show synchronous decay kinetics, and the rate constant of back electron transfer (k_BET) for the DEA/ortho-CN pair was 6.6 × 10⁴ s^-1, which is ca. 2 orders of magnitude lower compared with the DEA/meta-CN pair. The role of isomerism in providing resonance stabilization for the organic radicals is expected to have implications for strategies that retard charge recombination in photovoltaics. The role of the molecular structural features that dictate the kinetics for charge recombination has been further identified using quantum calculations.

Interplay between Förster and Dexter Energy Transfer Rates in Isomeric Donor-Bridge-Acceptor Systems

The ability to direct the flow of excitons enable molecular systems to perform highly advanced functions. Intramolecular energy transfer in donor-bridge-acceptor systems can occur by different mechanisms, and the ability to control the excited state energy pathways depends on the capacity to favor one process over another. Here, we show an anticorrelation between the rates of Förster and Dexter types of energy transfer in two isomeric donor-bridge-acceptor systems. Both dyads display intramolecular Förster triplet-to-singlet and Dexter triplet-to-triplet energy transfers. However, as the bridge-acceptor connection point changes, the rate of one energy transfer process increases at the same time as the other one decreases, allowing us to control the energy flow direction. This work shows how rational design can be used to tune excited state energy pathways in molecular dyads, which is of importance for advanced functions such as multiplicity conversion in future molecular materials.

Shortcuts for Electron-Transfer through the Secondary Structure of Helical Oligo-1,2-Naphthylenes

Atropisomeric 1,2-naphthylene scaffolds provide access to donor-acceptor compounds with helical oligomer-based bridges, and transient absorption studies revealed a highly unusual dependence of the electron-transfer rate on oligomer length, which is due to their well-defined secondary structure. Close noncovalent intramolecular contacts enable shortcuts for electron transfer that would otherwise have to occur over longer distances along covalent pathways, reminiscent of the behavior seen for certain proteins. The simplistic picture of tube-like electron transfer can describe this superposition of different pathways including both the covalent helical backbone, as well as noncovalent contacts, contrasting the wire-like behavior reported many times before for more conventional molecular bridges. The exquisite control over the molecular architecture, achievable with the configurationally stable and topologically defined 1,2-naphthylene-based scaffolds, is of key importance for the tube-like electron transfer behavior. Our insights are relevant for the emerging field of multidimensional electron transfer and for possible future applications in molecular electronics.

Spirobifluorene Metallaaromatics

Spirobifluorene derivatives find use in many end-user applications. Therefore, further expansion of their scope is the focus of many research studies. However, although the optical properties of spirobifluorenes can be greatly tuned through incorporation of metal complexes, to date, spirobifluorene metallaaromatics remain unknown. Taking advantage of the versatility of our methodology for the synthesis of metallaaromatic systems, this work reports the first metallaaromatic spirobifluorene compound. The presence of an Ir atom was found to redshift the absorption maximum by ca. 1.1 eV compared to bare spirobifluorene. Additionally, X-ray analysis as well as anisotropy of the current-induced density calculations revealed this compound to be of aromatic nature. The high stability in solution, solid state, under air, and at high temperature, as well as distinct optical properties of this new class of compounds are expected to open new frontiers for chiroptical and optoelectronic applications.

Electron Transfer across o-Phenylene Wires

Photoinduced electron transfer across rigid rod-like oligo- p-phenylenes has been thoroughly investigated in the past, but their o-connected counterparts are yet entirely unexplored in this regard. We report on three molecular dyads comprised of a triarylamine donor and a Ru(bpy)₃²⁺ (bpy =2,2'-bipyridine) acceptor connected covalently by 2 to 6 o-phenylene units. Pulsed excitation of the Ru(II) sensitizer at 532 nm leads to the rapid formation of oxidized triarylamine and reduced ruthenium complex via intramolecular electron transfer. The subsequent thermal reverse charge-shift reaction to reinstate the electronic ground-state occurs on a time scale of 120-220 ns in deaerated CH₃CN at 25 °C. The conformational flexibility of the o-phenylene bridges causes multiexponential transient absorption kinetics for the photoinduced forward process, but the thermal reverse reaction produces single-exponential transient absorption decays. The key finding is that the flexible o-phenylene bridges permit rapid formation of photoproducts storing ca. 1.7 eV of energy with lifetimes on the order of hundreds of nanoseconds, similar to what is possible with rigid rod-like donor-acceptor compounds. Thus, the conformational flexibility of the o-phenylenes represents no disadvantage with regard to the photoproduct lifetimes, and this is relevant in the greater context of light-to-chemical energy conversion.

Exploiting Potential Inversion for Photoinduced Multielectron Transfer and Accumulation of Redox Equivalents in a Molecular Heptad

Photoinduced multielectron transfer and reversible accumulation of redox equivalents is accomplished in a fully integrated molecular heptad composed of four donors, two photosensitizers, and one acceptor. The second reduction of the dibenzo[1,2]dithiin acceptor occurs more easily than the first by 1.3 V, and this potential inversion facilitates the light-driven formation of a two-electron reduced state with a lifetime of 66 ns in deaerated CH₃CN. The quantum yield for formation of this doubly charge-separated photoproduct is 0.5%. In acidic oxygen-free solution, the reduction product is a stable dithiol. Under steady-state photoirradiation, our heptad catalyzes the two-electron reduction of an aliphatic disulfide via thiolate-disulfide interchange. Exploitation of potential inversion for the reversible light-driven accumulation of redox equivalents in artificial systems is unprecedented and the use of such a charge-accumulated state for multielectron photoredox catalysis represents an important proof-of-concept.