Thio-Mayan-like Compounds: Excited State Characterization of Indigo Sulfur Derivatives in Solution and Incorporated in Palygorskite and Sepiolite Clays

Following what may mirror the recipe used by the ancient Maya civilization, consisting of a mixture of indigo and palygorskite, leading to the organic/inorganic pigment known as Maya Blue, we have switched indigo with thioindigo (<b>TI</b>) and a derivative (Ciba Brilliant Pink, <b>CBP</b>) and synthesized what was Christianized as “Maya Pink”. The spectral and photophysical behavior of <b>TI</b> and <b>CBP</b> was investigated in solution and solid state (in powder and incorporated in palygorskite clays). In solution, <b>TI</b> was investigated in different organic solvents (benzene, toluene, and dioxane) at room and low temperatures and further compared with indigo. <b>TI</b> displays a different spectral and photophysical behavior when compared to indigo: fluorescence dominating the deactivation of the first excited singlet state (50–70% depending on the solvent), with the S₁〰→S₀ internal conversion and the S₁〰→T₁ intersystem crossing deactivation channels representing the remaining <i>quanta</i> loss. Moreover, whereas in the case of indigo the proton transfer process in the excited state (between the N–H and CO groups), involving the formation of two excited species, leads to an efficient internal conversion radiationless process, with <b>TI</b> this process is precluded leading to a single exponential fluorescence decay. Incorporation of <b>TI</b> and <b>CBP</b> in palygorskite and sepiolite clays has showed different photophysical properties with decrease of the fluorescence quantum yields, a single exponential decay for <b>TI</b> and a multiexponential decay for <b>CBP</b> in palygorskite. Based on molecular fluorescence data, potential locations of <b>TI</b> and <b>CBP</b> in the clay are equated

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

Controlling the Fluorescence Behavior of 1‑Pyrenesulfonate by Cointercalation with a Surfactant in a Layered Double Hydroxide

Zn–Al layered double hydroxides (LDHs) containing solely 1-pyrenesulfonate (PS) or 1-heptanesulfonate (HS) anions, or a mixture of the two with HS/PS molar ratios ranging between ca. 7.5 and 82, were prepared by the direct synthesis method and characterized by powder X-ray diffraction, thermal and elemental analyses, scanning electron microscopy, and FT-IR, FT-Raman, and ¹³C­{¹H} CP MAS NMR spectroscopies. Well-ordered intercalates were obtained with basal spacings of 18.8 Å for the LDH intercalated by PS and 19.2–19.4 Å for the other materials containing HS. The photophysics of the solids, as well as the PS probe dissolved in water and common organic solvents (aiming to compare the behavior of the “isolated” molecule with that in the solid), were investigated by steady-state and time-resolved fluorescence techniques. The fluorescence spectra of the solid samples display two bands with maxima at 376 and 495 nm. Depending on the HS/PS ratios, the band intensity ratio (obtained at 375 and 520 nm) changes, reflecting different contributions from monomer and dimer species. The decays collected at 375 nm are biexponentials with a major component (∼97% of the total fluorescence) of 105 ns for the highest HS/PS ratio, which further loses importance with an increase in the PS content. When the decays are collected at 480 and 520 nm, the fits are triexponentials with a major component varying from 108 to 124 ns, attributed to an excimer. Steady-state and time-resolved measurements with PS in solution (ethanol, methanol, DMF, DMSO, and water) were also measured, and a comparison of the vibronic <i>I</i>₁/<i>I</i>₃ ratio and lifetimes in water (65 ns) with those in the LDHs indicates that the PS probe in the cointercalated LDHs is surrounded by the HS surfactant

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

Interactions and Supramolecular Organization of Sulfonated Indigo and Thioindigo Dyes in Layered Hydroxide Hosts

Supramolecularly organized host–guest systems have been synthesized by intercalating water-soluble forms of indigo (indigo carmine, IC) and thioindigo (thioindigo-5,5′-disulfonate, TIS) in zinc–aluminum-layered double hydroxides (LDHs) and zinc-layered hydroxide salts (LHSs) by coprecipitation routes. The colors of the isolated powders were dark blue for hybrids containing only IC, purplish blue or dark lilac for cointercalated samples containing both dyes, and ruby/wine for hybrids containing only TIS. The as-synthesized and thermally treated materials were characterized by Fourier transform infrared, Fourier transform Raman, and nuclear magnetic resonance spectroscopies, powder X-ray diffraction, scanning electron microscopy, and elemental and thermogravimetric analyses. The basal spacings found for IC-LDH, TIS-LDH, IC-LHS, and TIS-LHS materials were 21.9, 21.05, 18.95, and 21.00 Å, respectively, with intermediate spacings being observed for the cointercalated samples that either decreased (LDHs) or increased (LHSs) with increasing TIS content. UV–visible and fluorescence spectroscopies (steady-state and time-resolved) were used to probe the molecular distribution of the immobilized dyes. The presence of aggregates together with the monomer units is suggested for IC-LDH, whereas for TIS-LDH, IC-LHS, and TIS-LHS, the dyes are closer to the isolated situation. Accordingly, while emission from the powder H₂TIS is strongly quenched, an increment in the emission of about 1 order of magnitude was observed for the TIS-LDH/LHS hybrids. Double-exponential fluorescence decays were obtained and associated with two monomer species interacting differently with cointercalated water molecules. The incorporation of both TIS and IC in the LDH and LHS hosts leads to an almost complete quenching of the fluorescence, pointing to a very efficient energy transfer process from (fluorescent) TIS to (nonfluorescent) IC

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

Self-Assembly of Poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} with Oppositely Charged Phenylenevinylene Oligoelectrolytes

The interaction of the water-soluble conjugated polyelectrolyte (CPE) poly­{1,4-phenylene-[9,9-bis­(4-phenoxy-butylsulfonate)]­fluorene-2,7-diyl} (PBS-PFP) (degree of polymerization, DP, ∼3–6) with various concentrations of a homologous series of oppositely charged amphiphilic phenylenevinylene oligomers was investigated in water:dioxane mixtures and in aqueous micellar solutions of the non-ionic surfactant <i>n</i>-dodecylpentaoxyethylene glycol ether. The excellent spectral overlap between the CPE fluorescence and the conjugated oligoelectrolyte (COE) absorption indicates that energy transfer between these is a highly favored process, and can be tuned by changing the COE chain length. This is supported by time-resolved fluorescence data. The overall results provide support for different types of self-assembly, which are sensitive to the solvent environment and to the size of the phenylenevinylene oligoelectrolyte chain. It is suggested that large aggregates are formed in water:dioxane mixtures, while decorated core–shell structures are present in the surfactant solutions

Modulating the Self-Assembly of Calix[4]azacrowns to Design Materials with Improved Emission and Stimuli-Responsive Behavior

1,3-[Ethylene-bis­(aminocarbonylmethyl)]<i>-p-tert-</i>butylcalix­[4]­arene is analyzed and its optical properties are investigated, showing luminescence upon aggregation at low temperature or in the solid state described as aggregation-induced blue emission. The shift of the self-assembly from nanoporous to helical framework in the crystal is associated with a remarkable blue-shift and 33-fold increase in the fluorescence intensity. The purple-blue light emission is highly efficient (Φ_F ∼ 60%) and is switched to thermally activated yellow emission as a stimuli-responsive behavior of the helical assembly. The emitting material has been characterized by single crystal and powder X-ray diffraction, ATR-FTIR spectroscopy, and DSC and PLTM analysis, and its optical properties are investigated