Effect of Thermal Motions on the Structure and UV−Visible Electronic Spectra of Stilbene and Model Oligomers of Poly(p-Phenylene Vinylene)

Visualization of Polymer–Surfactant Interaction by Dual-Emissive Gold Nanocluster Labeling

Polymer-surfactant interaction decides the performance of corresponding complexes, making its rapid and intuitionistic visualization important for enhancing the performance of products and/or processing in related fields. In this study, the fluorescence visualization of the interaction between cationic hyperbranched polyethyleneimine and anionic sodium dodecyl sulfonate surfactant was realized by dual-emissive gold nanocluster labeling. The sensing mechanism was due to the interaction-induced polymer conformation change, which regulated the molecular structure and subsequent photoradiation process of the gold nanoclusters. All three inflection points of the interactions between the polymers and the surfactants were obtained by the change in fluorescence emission ratio of the designed dual-emissive gold nanoclusters. Moreover, these inflection points are verified by the hydrodynamic diameter and zeta potential measurements.

Enhanced E/Z-photoisomerization and luminescence of stilbene derivative co-coordinated in di-β-diketonate lanthanide complexes

A new tetradentate chelating ligand appending a stilbene derivative, E-N',N'-bis(pyridin-2-ylmethyl)-4-styrylbenzohydrazide (HL) was synthesized, together with two β-diketonates (4,4,4-trifluoro-1-phenylbutane-1,3-dionate, tfd), with or without the trifluoroacetate anion present as a ligand for coordination with lanthanide(iii) ions to form [Ln(tfd)2(HL)(CF3CO2)] (LnC49H36F9N4O7, Ln = La (1), Nd (2), Eu (3), Gd (4)) and [Yb(tfd)2(L)] (YbC47H35F6N4O5 (5), L = deprotonated HL). All five complexes were structurally characterized, and five crystals were obtained and analyzed by single-crystal X-ray diffraction. The quantum yield of trans-to-cis photoisomerization of the stilbene group in gadolinium complex 4 was enhanced about five-fold compared with that of HL itself. Other complexes showed slightly enhanced or depressed photoisomerization. The total luminescence quantum yield/sensitization efficiency of europium complex 3 in the solid state and acetonitrile solution were 22.1%/96.7% and 19.3%/97.9%, respectively. The transfer of ligand energy to the Eu3+ ion was highly efficient. This enhanced photoisomerization and luminescence of the stilbene group within complexes was found to be related to the energy level of lanthanide ions and whether a ligand-to-metal center or ligand-to-ligand charge transfer process was present. The interpretation of experimental results is rationally supported by time-dependent density-functional theory calculations. In complex 4, except for the intramolecular absorption transition of HL ligand itself (IL, πHL-π*HL), the presence of the ligand-to-ligand charge transfer transition from tfd to HL (LLCT, πtfd-π*HL) and the triplet state energy of HL being unable to transfer to the higher 6P7/2 excited energy level of the Gd3+ ion would facilitate HL photoisomerization. For complex 3, this was due to reversed ligand-to-ligand charge transfer transition from HL to tfd (LLCT, πHL-π*tfd) and its energy transfer to the metal center. Although the observed radiative lifetimes of NIR luminescent complexes 2 and 5 were around 10 μs, these systems contained only two diketone ligands, indicating that HL still had a certain promoting effect compared with tris(diketonate) lanthanide complexes. These results offer an important route for the design of new lanthanide-based molecular switching materials.

Self-Assembled Helical and Twisted Nanostructures of a Preferred Handedness from Achiral π-Conjugated Oligo( p-phenylenevinylene) Derivatives

The formation of chiral nanostructures from the self-assembly of achiral building blocks without external symmetry breaking inducing factors is believed to associate with the origin of chirality. Herein, we reported the synthesis and self-assembly of oligo( p-phenylenevinylene)- b-poly(ethylene glycol) (OPV₃- b-PEG₁₇, the subscripts represent the number of repeat unit of each block) in solution. We systematically examined the influence of solvent, heating temperature, and concentration of OPV₃- b-PEG₁₇ on the self-assembly of OPV₃- b-PEG₁₇ by UV/vis absorption and fluorescence spectrometry, circular dichroism technique, and transmission electron and atomic force microscopy. Interestingly, helical and twisted nanoribbons and nanotubes of a preferred handedness can be formed from achiral OPV₃- b-PEG₁₇ in the mixture of water/ethanol (v/v = 1/1) and the solution showed an obvious exciton-coupled bisignated signal, which indicated that symmetry breaking occurred during the formation of these nanostructures without external inducing factors. Our results showed that the occurrence of symmetry breaking is subtle to the experimental factors including solvent, heating temperature, and concentration of OPV₃- b-PEG₁₇. The directional π-π stacking along with steric repulsion between PEG domains should be the driving force for the formation of these chiral nanostructures. The occurrence of statistical fluctuations in the initial stage of self-assembly led to an accidental excess of helical or/and twisted structures, that is, symmetry breaking. Subsequently, the autocatalysis effect resulted in the formation of helical or/and twisted nanoribbons with a preferred handedness.

Direct Measurements of Intersystem Crossing Rates and Triplet Decays of Luminescent Conjugated Oligomers in Solutions

Photothermal calorimetry and fluorescence spectroscopy were used to determine the relaxations of the photoexcited singlet state of two PPV and polyfluorene oligomers, (E,E)-1,4-bis[(2-benzyloxy)styryl]benzene (PVDOP) and ter(9,9'-spirobifluorene) (TSBF). The decay rates of different S1 relaxation channels, which include intersystem crossing (ISC), radiative, and nonradiative decay can be determined by the combination of photoacoustic calorimetry (PAC) and the time-correlated single photon counting (TCSPC) technique. The triplet state energy level is determined by the phosphorescence (Ph) spectra recorded at 77 K. The ISC yields are approximately 3% and 6% for PVDOP and TSBF, respectively. The T1 to S0 transition decay rate is acquired by PAC and photothermal beam deflection (PBD) measurements. The triplet state decay rate is 17 and 21 ms(-1) at room temperature. The Ph intensity decay at 77 K shows that the triplet state lifetime increases by 4 orders of magnitude, as compared to room temperature.

Matrix-Induced Intensity Fluctuations in the Fluorescence from Single Oligo(phenylenevinylene) Molecules

Single molecule spectroscopy on oligo(phenylenevinylene) (OPV) chromophores shows that the fluorescence intermittency strongly correlates to the rigidity of the environment surrounding the molecules. For OPV single molecules, environmental rigidity inhibits twisting about the vinyl linkages, the molecular motion associated with the observed "off" (nonabsorbing) state. By increasing the rigidity of a single molecule's environment, we can tune its room temperature fluorescence from rapid, sub-millisecond "blinking" fluctuations (fluid polymer environment) to completely "on" with no blinking observed (molecules adsorbed to a rigid bare glass substrate). The difference in fluorescence intermittency from environment to environment is immediately apparent and explicit in single molecule intensity trajectories under cw (continuous wave) excitation, demonstrating the sensitivity of these chromophores to their surroundings and emphasizing the importance of morphological control in real-world applications involving phenylenevinylene-based materials.

A Theoretical, Spectroscopic, and Photophysical Study of 2,7-Carbazolenevinylene-Based Conjugated Derivatives

A combined theoretical and experimental study of the structure, optical, and photophysical properties of four 2,7-carbazolenevinylene-based derivatives in solution is presented. Geometry optimizations of the ground states of PCP, PCP-CN, TCT, and TCT-CN were carried out using the density functional theory (DFT/B3LYP/6-31G*). It is found that PCP and TCT are nearly planar in their ground electronic states (S0), whereas the cyano derivatives are more twisted. The nature and the energy of the first singlet-singlet electronic transitions have been obtained from time-dependent density functional theory (TDDFT) calculations performed on the optimized geometries. For all the compounds, excitation to the S1 state corresponds mainly to the promotion of one electron from the highest-occupied molecular orbital to the lowest-unoccupied molecular orbital, and the S1 <-- S0 electronic transition is strongly allowed and polarized along the long axis of the molecular frame. The optimization (relaxation) of the first singlet excited electronic state (S1) has been done using the restricted configuration interaction (singles) (RCIS/6-31G*) approach. It is observed that all four investigated compounds become more planar in their S1 relaxed excited state. Electronic transition energies from the relaxed excited states have been obtained from TDDFT calculations performed on the S1-optimized geometries. The absorption and fluorescence spectra of the carbazolenevinylenes have been recorded in chloroform. A good agreement is obtained between TDDFT vertical transitions energies and the (0,0) absorption and fluorescence bands. The change from phenylene to thiophene rings as well as the incorporation of cyano substituents induce bathochromic shifts in the absorption and fluorescence spectra. From the analysis of the energy of the frontier molecular orbitals, it is believed that thiophene rings and CN substituents induce some charge-transfer character to the first electronic transition, which is responsible for the red shifts observed. Finally, the fluorescence quantum yield and the lifetime of the compounds in chloroform have been obtained. In sharp contrast with many oligothiophenes, it is observed that TCT possesses a high fluorescence quantum yield. On the other hand, the CN-containing derivatives exhibit much lower fluorescence quantum yields, probably due to the combined influence of steric effects and charge-transfer interactions caused by the cyano groups.

Photodegradation of Substituted Stilbene Compounds: What Colors Aging Paper Yellow?

Photodegradation of lignin is one of the major postprocessing problems in paper production, as this renders yellowing of the paper and reduced paper quality. In this study, we have explored the photochemical properties of substituted stilbene derivatives believed to be key chromophores in the photodegradation of lignin derived from cinnamyl alcohol. In particular, the present work focuses on the computation of UV/vis electronic absorption spectra for different methoxylated stilbenes and their proposed photodegradation products. All calculations were performed using the time-dependent formalism of density functional theory (TD-DFT) and the B3LYP hybrid functional. It is concluded that the methodology employed is capable of reproducing not only the overall spectra, but also subtle features owing to the effects of different substitution patterns. For the strongly absorbing first excited singlet state (HOMO --> LUMO excitation) of the methoxylated stilbenes, the calculated transition energies are, albeit somewhat fortuitously, in excellent agreement with experimental data. The light-induced yellowing indirectly caused by the presence of stilbenes can be rationalized in terms of the absorption spectra of the resulting photodegraded o-quinones, for which distinct transitions in the 420-500 nm region of the visible spectrum lacking prior to degradation are observed.