Tetramethoxybenzene is a Good Building Block for Molecular Wires: Insights from Photoinduced Electron Transfer

Excitation-Wavelength-Dependent Photoinduced Electron Transfer in a π-Conjugated Diblock Oligomer

Control of photoinduced electron transfer through selective excitation of a π-conjugated diblock oligomeric system featuring tetrathiophene (T₄) and tetra(phenylene ethynylene) (PE₄) donor blocks capped with a naphthalene diimide (NDI) acceptor (T₄PE₄NDI) is demonstrated. Each π-conjugated oligomeric segment has its own discrete ionization potential, electron affinity, and optical band gap which provides an absorption profile that has specific wavelengths that offer selective excitation of the PE₄ and T₄ blocks. Therefore, T₄PE₄NDI can be selectively excited to form a charge-separated state via ultrafast photoinduced electron transfer from the PE₄ segment to NDI when excited at 370 nm, but it does not produce a charge-separated state when excited at 420 nm (T₄). Picosecond transient absorption techniques were performed to probe the excited-state dynamics, revealing ultrafast charge separation (∼4 ps) occurring from the PE₄ segment to NDI when excited at 370 nm, followed by delocalization of the hole over the T₄ segment. On the contrary, electron transfer is suppressed with excitation at longer wavelengths (≥420 nm), where the spectrum is dominated by the T₄ unit. The rate of electron transfer and charge recombination was investigated versus the length of the PE bridge unit in oligomers featuring zero and two PE units (T₄NDI and T₄PE₂NDI). The rate of charge recombination decreases from 1.2 × 10¹¹ to 1.0 × 10⁹ s^-1 with increasing bridge length between the T₄ and NDI components (T₄NDI to T₄PE₄NDI). Furthermore, wavelength-dependent photoinduced electron transfer was not observed in either T₄NDI or T₄PE₂NDI due to an insufficient PE_n bridge length. This work demonstrates the ability to use optical wavelength to control photoinduced electron transfer in a fully π-conjugated oligomer.

Spin-crossover in iron(II)-phenylene ethynylene-2,6-di(pyrazol-1-yl) pyridine hybrids: toward switchable molecular wire-like architectures

Luminescent oligo(p-phenylene ethynylene) (OPE) and spin-crossover (SCO) active Fe(II)-2,6-di(pyrazol-1-yl) pyridine (BPP) systems are prominent examples proposed to develop functional materials such as molecular wires/memories. A marriage between OPE and Fe(II)-BPP systems is a strategy to obtain supramolecular luminescent ligands capable of metal coordination useful to produce novel spin-switchable hybrids with synergistic coupling between spin-state of Fe(II) and a physical property associated with the OPE skeleton, for example, electronic conductivity or luminescence. To begin in this direction, two novel ditopic ligands, namely L¹ and L², featuring OPE-type backbone end-capped with metal coordinating BPP were designed and synthetized. The ligand L² tailored with 2-ethylhexyloxy chains at the 2 and 5 positions of the OPE skeleton shows modulated optical properties and improved solubility in common organic solvents relative to the parent ligand L¹. Solution phase complexation of L¹ and L² with Fe(BF₄)₂·6H₂O resulted in the formation of insoluble materials of the composition [Fe(L¹)] _n (BF₄)_2n and [Fe(L²)] _n (BF₄)_2n as inferred from elemental analyses. Complex [Fe(L¹)] _n (BF₄)_2n underwent thermal SCO centred at T _1/2  =  275 K as well as photoinduced low-spin to high-spin transition with the existence of the metastable high-spin state up to 52 K. On the other hand, complex [Fe(L²)] _n (BF₄)_2n , tethered with 2-ethylhexyloxy groups, showed gradual and half-complete SCO with 50% of the Fe(II)-centres permanently blocked in the high-spin state due to intermolecular steric interactions. The small angle x-ray scattering (SAXS) pattern of the as-prepared solid complex [Fe(L¹)] _n (BF₄)_2n revealed the presence of nm-sized crystallites implying a possible methodology towards the template-free synthesis of functional-SCO nanostructures.

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.

Hole-transfer induced energy transfer in perylene diimide dyads with a donor–spacer–acceptor motif

Pump–probe spectroscopy, time resolved fluorescence, chemical variation and quantum chemical calculations reveal an efficient energy transfer mechanism enabled by a bright charge transfer state located on the spacer.