Chain Length Dependent Excited-State Decay Processes of Diluted PF2/6 Solutions

The excited-state dynamics of a series of four poly­[2,7-(9,9-bis­(2-ethylhexyl)­fluorene] fractions, PF2/6, with different chain length (degrees of polymerization DP: 5, 10, 39, and 205) was investigated in dilute solutions by steady-state and time-resolved fluorescence techniques. Two decay components are extracted from time-resolved fluorescence experiments in the picosecond time domain: a chain length dependent, fast decay time (τ₂) for shorter emission wavelengths (ranging from 30 to 41 ps), which is associated with a rising component at longer wavelengths, and a longer decay time, τ₁ (ranging from 387 to 452 ps). The system was investigated with kinetic formalisms involving (i) a two-state system (A and B) involving conformational relaxation of the initially excited PF2/6 segment (A) under formation of a more planar (B) relaxed state and (ii) a time-dependent red shift of the emission spectrum using the Stokes shift correlation function (SSCF). In the case of (i), the kinetic scheme was solved considering the simultaneous excitation of A and B or only of A, and the rate constants for formation [<i>k</i>′_CR or <i>k</i>′_CR(α)], dissociation (<i>k</i>_–CR), and deactivation (<i>k</i>_B^*) were obtained together with the fraction of species A and B present in the ground state. The use of the SSCF in (ii) was found to be more adequate leading to a decay law with a 3.4 ps component (associated with the slow part of the solvation dynamics process) and a longer decay (43.3 ps) associated with the conformational/torsional relaxation process with a rate constant <i>k</i>_CR. This longer component of the SSCF was found to be identical to the short-living decay (τ₂) component of the biexponential decays, displaying an Arrhenius-type behavior with activation energy values in the range 5.8–8.9 kJ mol^–1 in toluene and 6.5–10.7 kJ mol^–1 in decalin. From the dependence of the fast decay component (<i>k</i>_CR ≡ 1/τ₂) on solvent viscosity and temperature, the activation energy for the conformational relaxation process was found to be distinctly dependent on the chain length, with the relaxation rate dependence with the solvent viscosity (<i>k</i>_CR ≈ η^– γ) displaying γ = 1 for the oligomer fraction with DP = 5 (i.e., <i>k</i>_CR is associated with a pure diffusion-controlled process) and γ < 1 for the higher molecular weight PF2/6 fractions (with DP = 10, 39, 205). This happens because of a decreased conformational barrier between nonrelaxed and relaxed states promoted by the polymer skeleton

Excited State Characterization and Energy Transfer in Hyperbranched Polytruxenes and Polytruxene-<i>block</i>-Polythiophene Multiblock Copolymers

A comprehensive investigation of the excited state characteristics of two hyperbranched truxene polymers [one end-terminated with poly­(3-hexylthiophene) blocks, P3HT] and a bistruxene model compound has been undertaken aiming to rationalize its inherent photophysical properties, including the energy transfer processes between the truxene (donor) and P3HT (acceptor) moieties. The study comprises qualitative absorption, emission, and triplet-singlet difference spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion and singlet oxygen formation) and fluorescence decay times. From the time-resolved data in solvents of different viscosity and as a function of temperature, it was established that with the P3HT-terminated hyperbranched polytruxene, the excited state deactivation mainly results from energy transfer and that conformational relaxation is absent in these systems, which gives further support for the rigidity of these polymers both in the ground and excited state. An energy transfer efficiency of 91% was obtained at room temperature. From a qualitative analysis of the data, it was also seen that radiationless processes (particularly the S₁∼∼→S₀ internal conversion channel) mainly contribute to the excited state deactivation of the hyperbranched polytruxenes, a behavior that is in contrast to what was observed for the bistruxene model compound. Spectral and fluorescence time-resolved data in thin films was also obtained and compared with the solution data

Photoinduced Energy and Electron-Transfer Reactions by Polypyridine Ruthenium(II) Complexes Containing a Derivatized Perylene Diimide

The [Ru­(II) (phen)₂(pPDIp)]²⁺ complex, where pPDIp is the symmetric bridging ligand phenanthroline–perylene–phenanthroline, shows strong electronic absorption bands attributed to the pPDIp and {Ru­(phen)₂}²⁺ moieties in acetonitrile. The charge-separated intermediate {Ru­(III) (phen)₂(pPDIp^–•)} was detected by transient absorption spectroscopy upon electronic excitation in either the pPDIp or the complex moieties. The charge-separated intermediate species decays to generate the triplet state ³*pPDIp-Ru­(II) (τ_P = 1.8 μs) that sensitizes the formation of singlet molecular oxygen with quantum yield ϕ_Δ = 0.57. The dyad in deaerated acetonitrile solutions is reduced by triethylamine (NEt₃) to the [Ru­(II) (phen)₂(pPDIp^•–)] radical anion in the dark. The electron-transfer reaction is accelerated by light absorption. By photolysis of the radical anion, a second electron transfer reaction occurs to generate the [Ru­(II) (phen)₂(pPDIp^2–)] dianion. The changes of the color of solution indicate the redox states of complexes and offer a sensitive reporter of each stage of redox reaction from start to finish. The reduced complexes can be converted to the initial complex, using methyl viologen or molecular oxygen as an electron acceptor. The accumulation of electrons in two well-separated steps opens promising opportunities such as in catalysis

Triphenylamine–Benzimidazole Derivatives: Synthesis, Excited-State Characterization, and DFT Studies

The synthesis and comprehensive characterization of the excited states of four novel triphenylamine–benzimidazole derivatives has been undertaken in solution (ethanol and methylcyclohexane) at room temperature. This includes the determination of the absorption, fluorescence, and triplet–triplet absorption spectra, together with quantum yields of fluorescence, internal conversion, intersystem crossing, and singlet oxygen. From the overall data the radiative and radiationless rate constants could be obtained, and it is shown that the compounds are highly emissive with the radiative decay dominating, with more than 70% of the quanta loss through this deactivation channel. The basic structure of the triphenylamine–benzimidazole derivatives (<b>1a</b>) was modified at position 5 of the heterocyclic moiety with electron-donating (OH (<b>1b</b>), OCH₃ (<b>1c</b>)) or electron-withdrawing groups (CN, (<b>1d)</b>). It was found that the photophysical properties remain basically unchanged with the different substitutions, although a marked Stokes shift was observed with <b>1d</b>. The presence and nature of a charge-transfer transition is discussed with the help of theoretical (DFT and TDFT) data. All compounds displayed exceptionally high thermal stability (between 399 and 454 °C) as seen by thermogravimetric analysis

Multifaceted Regioregular Oligo(thieno[3,4‑<i>b</i>]thiophene)s Enabled by Tunable Quinoidization and Reduced Energy Band Gap

Thiophene-based materials have occupied a crucial position in the development of organic electronics. However, the energy band gaps of oligo- and polythiophenes are difficult to modulate without resorting to push–pull electronic effects. We describe herein a new series of monodisperse oligo­(thieno­[3,4-<i>b</i>]­thiophene) derivatives with well-defined regioregular structures synthesized efficiently by direct C–H arylation. These compounds show a unique palette of colors and amphoteric redox properties with widely tunable energy band gaps. The capacity to stabilize both cations and anions results in both anodic and cathodic electrochromism. Under excitation, these compounds can produce photoionized states able to interconvert into neutral triplet or form these through singlet exciton fission or intersystem crossing. These features arise from a progressive increase in quinoidization on a fully planar platform making the largest effective conjugation length among hetero-oligomers. Oligo­(thieno­[3,4-<i>b</i>]­thiophene)­s might represent the more distinctive family of oligothiophenes of this decade

Multifaceted Regioregular Oligo(thieno[3,4‑<i>b</i>]thiophene)s Enabled by Tunable Quinoidization and Reduced Energy Band Gap

Thiophene-based materials have occupied a crucial position in the development of organic electronics. However, the energy band gaps of oligo- and polythiophenes are difficult to modulate without resorting to push–pull electronic effects. We describe herein a new series of monodisperse oligo­(thieno­[3,4-<i>b</i>]­thiophene) derivatives with well-defined regioregular structures synthesized efficiently by direct C–H arylation. These compounds show a unique palette of colors and amphoteric redox properties with widely tunable energy band gaps. The capacity to stabilize both cations and anions results in both anodic and cathodic electrochromism. Under excitation, these compounds can produce photoionized states able to interconvert into neutral triplet or form these through singlet exciton fission or intersystem crossing. These features arise from a progressive increase in quinoidization on a fully planar platform making the largest effective conjugation length among hetero-oligomers. Oligo­(thieno­[3,4-<i>b</i>]­thiophene)­s might represent the more distinctive family of oligothiophenes of this decade