Synthesis, light-harvesting and energy-transfer properties of regioregular silylene-spaced alternating [(donor-SiMe2-)(n=1-3)-(acceptor-SiMe2)] copolymers

A series of silylene-spaced alternating [(donor-SiMe2)(n=1-3)-(acceptor-SiMe2)] copolymers 4-6 was synthesized by rhodium-catalyzed hydrosilylation of bisalkynes with bissilyl hydrides. Monomeric reference compounds 7-10 with similar chromophore components were prepared for comparison. The ratio of donor to acceptor groups is well-controlled by the precise regiochemistry and nature of the repeat units. The silylene moieties serve as insulating spacers between chromophores. The polymers exhibit light-harvesting abilities, for which the intensity of the emission enhanced with larger donor-to-acceptor ratios. No emission originating from the donors was observed in fluorescence spectra, illustrating that intrachain energy transfer is highly efficient along the polymer chain.     

含荧光生色基团烯类单体及其聚合物的光化学行为

近年来关于电荷转移现象的研究因理论及应用方面的重要性而备受瞩目。缺电子性丙烯酰类单体可以和给电子性化合物形成激态电葆转移复合物进而引发光聚合。我们合成了一系列同一分子中既含有给电子性荧光生色团又含缺电子性双键的烯类单体,发现这类单体在相同生色团浓度下的荧光强度均无穷氏人相应的聚合物的荧光强度。我们将这咎现象称为荧光结构自猝灭效应（ＳＳＱＥ）,以区别于众所周知的浓度自猝灭现象。对于电子状态与之相反的     

Cysteine‐Containing Polyisocyanides as Versatile Nanoplatforms for Chromophoric and Bioscaffolding

The straightforward syntheses of polyisocyanides containing the alanine–cysteine motif in their side chains have been achieved. Detailed characterization of the polymers revealed a well‐defined and highly stable helical conformation of the polyimine backbone responsible for the formation of rodlike structures of over one hundred nanometers. The 4₁ helix is further stabilized by β‐sheet‐like interactions between the peptide arms. As a result, the cysteine sulfur atoms are regularly aligned along the polymer axis, which provides a unique platform for the scaffolding of various entities by using versatile click‐chemistry postmodification approaches. For instance, pyrene derivatives were introduced through thio‐specific reactions involving either maleimide, iodoacetamide, or thioester groups, leading to arrays of stacked chromophores with excimer‐like emission. A water‐soluble cysteine‐rich polyisocyanide was successfully biotinylated and coupled to streptavidin.     

Circularly Polarized Light Probes Excited-State Delocalization in Rectangular Ladder-type Pentaphenyl Helices

Ladder-type pentaphenyl chromophores have a rigid, planar π-system and show bright fluorescence featuring pronounced vibrational structure. Such moieties are ideal for studying interchromophoric interactions and delocalization of electronic excitations. We report the synthesis of helical polymers with a rigid square structure based on spiro-linked ladder-type pentaphenyl units. The variation of circular dichroism with increasing chain length provides direct evidence for delocalization of electronic excitations over at least 10 monomeric units. The change in the degree of circular polarization of the fluorescence across the vibronic side bands shows that vibrational motion can localize the excitation dynamically to almost one single unit through breakdown of the Born-Oppenheimer approximation. The dynamic conversion between delocalized and localized excited states provides a new paradigm for interpreting circular dichroism in helical polymers such as proteins and polynucleic acids.     

ALTERNATING COPOLYMERS CONTAINING AN AROMATIC CHROMOPHORE IN EVERY MONOMER UNIT. 2. EXCIMER FORMATION AND INTRAMOLECULAR ENERGY MIGRATION IN ALTERNATING COPOLYMERS OF NAPHTHYLALKYL METHACRYLATE AND VINYLNAPHTHALENE

A series of alternating and random copolymers of 2-naphthylal-kyl methacrylates (NpMMA and NpEMA) and 2-vinylnaphtha-lene (VNp) were synthesized, and their fluorescence properties in tetrahydrofuran (THF) were compared. All of the alternating copolymers containing NpMMA or NpEMA showed a weak ex-cimer emission, indicating that excimer formation between naph-thyl (Np) chromophores in the alternating methacrylate sequences cannot be perfectly inhibited. No distinct difference in the fluorescence from the NpMMA and VNp moieties in poly(NpMMA-alt-VNp) (a-NpVNp) suggests that practically the same chromophores are aligned along the whole polymer chain. This observation is sharp in contrast with the corresponding phenyl polymers exhibiting different spectral features. Fluorescence quenching studies, however, showed no enhancement of the quenching efficiencies for a-NpVNp among the pres-ent polymers. Therefore, energy migration between the nearest-neighboring Np chromophores would inefficiently occur, though this polymer has the highest density of chromophores.     

Postfunctionalization of poly(propargyl methacrylate) using copper catalyzed 1,3‐dipolar Huisgen cycloaddition: An easy route to electro‐optic materials

A one‐pot synthetic route based on copper‐catalyzed Huisgen reaction has been developed to functionalize a methacrylate propargylic polymer with azido‐substituted moieties. This procedure was used for the preparation of electro‐optic materials containing well‐known Disperse Red One (DR1) chromophores along with bulky adamantyl moieties (Adam). The postfunctionalization of the propargylic polymer was successfully achieved using different molar ratios of DR1/Adam. These novel polymers exhibit high glass transition temperature owing to the rigid structure of adamantyl units. Moreover, the second harmonic generation measurements demonstrated the effectiveness of adamantyl groups to act as insulating shield limiting thus the electrostatic interactions between chromophores. Indeed, higher optimal chromophore concentration (50 mol %) than in conventional DR1‐containing polymers (30 mol %) allowed us to increase the d₃₃ coefficient up to 92 pm/V. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5652–5660, 2009     

Fluorescence Study of Energy Transfer in PMMA Polymers with Pendant Oligo‐Phenylene‐Ethynylenes

A spectroscopic characterization of polymers containing rigid π‐conjugated oligo(phenyleneethynylene) chromophores as well as oligo(phenyleneethynylene) and methyl methacrylate is presented. The polymers exhibit molar masses of up to 15 000 g mol^?1 and a degree of polymerization between 22 and 80. Emission measurements of the monomeric and polymeric species show that radiative as well as nonradiative rates are influenced by the degree of polymerization due to intramolecular interactions of chromophores pendant to the polymer backbone. Time‐resolved emission anisotropy measurements suggest that energy migrates within the polymers. Steady‐state emission anisotropy measurements also point to energy migration. Additionally, two oligo(phenyleneethynylene)s with different sizes of the conjugated system are copolymerized in order to enable energy trapping due to energy transfer. The shortened energy‐donor fluorescence lifetime within the donor–acceptor copolymers suggest energy transfer. Depending on the degree of polymerization, dispersion of the donor fluorescence lifetime is observed.     

Self-assembly of a hydrophobically modified naphthalene-labeled poly(acrylic acid) polyelectrolyte in water: organic solvent mixtures followed by steady-state and time-resolved fluorescence

The solution properties of two water-soluble polymers, poly(acrylic acid) (PAA), covalently labeled with the fluorescent hydrophobic dye naphthalene (Np), have been investigated in water:organic solvent mixtures. The naphthalene chromophores have been randomly attached, onto the polymer, with two different degrees of labeling. Fluorescence measurements (steady-state and time-resolved) have been used to follow the photophysical behavior of the polymers and consequently report on the self-association of the polymers in the mixed organic (methanol or dioxane):aqueous solutions. The emission spectra of the high-labeled Np PAA reveal the presence of monomer and excimer bands whereas with the low-labeled polymer only monomer emission is observed. The excitation spectra collected at the monomer and excimer emission bands show significant differences, depending on the water content of the mixture, which indicate the simultaneous presence of preformed and dynamic dimers as routes to excimer formation. The time-resolved data decay profiles of the high-labeled polymer in the mixtures were always triple exponential whereas in pure methanol and dioxane they follow biexponential laws. The data in the mixtures are consistent with two types of monomers and one excimer. Both monomers are able to give rise to excimer in the excited state, one type involving the movement of long distant Np chromophores and the other involving a local reorientation of adjacent Np chromophores. These correspond to different decay times: (1) a long which corresponds to the long distant approach of non-neighboring Np chromophores forming an excimer and (2) a short corresponding to the fast adjustment of two neighboring Np chromophores in order to have the adequate parallel geometry. An additional decay time corresponding to the excimer decay was found to be present at all wavelengths. All the decay times were dependent on the water content of the mixture. An estimation of the two excimer forming rate constants values is made for the mixed media considered in this work. On the whole, using both steady-state and time-resolved fluorescence parameters, and by comparing data for a polymer with a small number of hydrophobes with a more highly modified one, it is possible in great detail to demonstrate how association is controlled by solvent quality for the hydrophobes and by the distance between hydrophobes.     

Poly[(2,2′,5′,2″-tetramethoxy-p-terphenyl-5,5″-ylene)propylene]: Synthesis and physical properties of a novel amorphous regularly segmented conjugated polymer

A new regularly segmented conjugated polymer with methoxy-substituted p-terphenylene units tethered by their meta positions along the polymer main chain was synthesised using the Suzuki cross-coupling reaction. The small size of the connector and lack of long lateral chains lead to a high density of rigid electrooptically active moieties in the structure. However, this molecular architecture produces a very soluble and amorphous polymer with relatively high T_g. The optical properties of samples with different degrees of polymerisation were investigated using UV-visible absorption, steady state and time-resolved photoluminescence emission spectroscopies. Their absorption and emission properties are rather insensible to Mn after reaching a modest DP, to the aggregation state, either solution or neat film, and to annealing. The contorted polymer chain forms a very stable morphology and substantially hinders interactions between chromophores, thus diminishing the formation of aggregated species that are commonly observed in other electrooptically active polymers.     

Synthesis,characterization, and application of helical polymers

This review mainly describes the asymmetric synthesis of optically active polymers with helical conformation. Bulky methacrylates such as triphenylmethyl methacrylate and 1-phenyldibenzosuberyl methacrylate give one-handed helical and optically active polymers with almost perfectly isotactic main chain conformation by polymerization with chiral anionic initiators. The radical polymerization and copolymerization of these monomers under chiral conditions also afford optically active polymers with prevailing one-handed helicity. N, N-Disubstituted acrylamides also give optically active, helical polymers in the asymmetric anionic polymerization. Optically active polyisocyanates with a prevailing one-handed helical structure have been prepared in the copolymerization of an achiral isocyanate with a small amount of an optically active isocyanate and also in the polymerization of alkyl and aromatic isocyanates with optically active lithium alkoxide or amide compounds. The existence of a stable helical structure for polychloral has been successfully proved with the helical oligomers of chloral. One-handed helical polyisocyanides have been prepared by helix-sense-selective polymerization of bulky isocyanides and also by the cyclopolymerization of a 1, 2-diisocyanobenzene derivative with the Pd complex of a one-handed helical oligomer.     

Functionalizable polycyclic aromatics through oxidative cyclization of pendant thiophenes

We present a general strategy for obtaining large sulfur-containing polycyclic aromatics from thienyl precursors through iron(III) chloride mediated oxidative cyclizations. By placing thienyl moieties in close proximity to adjacent arenes, we have directed the oxidized intermediates into controlled cyclization pathways, effectively suppressing polymer formation. Utilizing these cyclized compounds and their thienyl precursors, we have studied cyclization/polymerization pathways of polymers such as poly(2). The unsubstituted positions alpha to the sulfur atoms within these aromatic cores allowed for efficient halogenation and further functionalization. As a demonstration, we prepared a series of arylene-ethynylene polymers with varying degrees of chromophore aromatization and used them to probe the effects of synthetically imposed rigidity on polymer photophysical behavior. The symmetries and effective conjugation pathways within the monomers play a key role in determining photophysical properties. We observed that rigid, aromatized chromophores generally led to increased excited-state lifetimes by decreasing radiative rates of fluorescence decay.     

Luminescent groups in very weakly degraded polystyrene

Unstabilized polystyrene samples from various producers, as well as carefully purified laboratory polymers, have been investigated by u.v.-absorption, fluorescence, phosphorescence and i.r. spectroscopy. The emission spectra of all samples indicate fluorescent and phosphorescent chromophores. In u.v.-absorption, the chromophores give rise to a long wavelength tail below the absorption edge of the phenyl groups. The phosphorescence spectrum is due to acetophenone-type end-groups of degraded polymer chains. Ultra-violet-fluorescence is due to monomeric styrene and trans-stilbene groups which probably can be formed at head-to-head sites of the polymer. Fluorescence in the blue spectral region may be due to chromophores formed by conjugated double bonds and phenyl groups. The chromophores are present in all commercial samples investigated and, at smaller concentrations, in the laboratory products also.     

How chromophore shape determines the spectroscopy of phenylene-vinylenes: origin of spectral broadening in the absence of aggregation

Single oligo(phenylene-vinylene) molecules constitute model systems of chromophores in disordered conjugated polymers and can elucidate how the actual conformation of an individual chromophore, rather than that of an overall polymer chain, controls its photophysics. Single oligomers and polymer chains display the same range of spectral properties. Even heptamers support pi-electron conjugation across approximately 80 degrees curvature, as revealed by the polarization anisotropy in excitation and supported by quantum chemical calculations. As the chain becomes more deformed, the spectral linewidth at low temperatures, often interpreted as a sign of aggregation, increases up to 30-fold due to a reduction in photophysical stability of the molecule and an increase in random spectral fluctuations. The conclusions aid the interpretation of results from single-chain Stark spectroscopy in which large static dipoles were only observed in the case of narrow transition lines. These narrow transitions originate from extended chromophores in which the dipoles induced by backbone substituents do not cancel out. Chromophores in conjugated polymers are often thought of as individual linear transition dipoles, the sum of which make up the polymer's optical properties. Our results demonstrate that, at least for phenylene-vinylenes, it is the actual shape of the individual chromophore rather than the overall chromophoric arrangement and form of the polymer chain that dominates the spectroscopic properties.     

Folding of alternating dialkylsilylene-spaced donor-acceptor copolymers: the oligomer approach

A series of oligmers with donor-acceptor pairs separated by diisopropylsilylene (iPr(2)Si) spacers, composed of monomer 4b, dimer 5, trimer 6, and tetramer 7, were synthesized to scrutinize the folding behavior. Monomer 4a with a dimethylsilylene (Me(2)Si) spacer was also prepared for comparison. The 4-aminostyrene moiety was used as the donor and the stilbene moiety as the acceptor. Both steady-state and time-resolved fluorescence spectroscopic measurement were made. Regardless of the substituents on the silicon atom, the emission spectra of 4a and 4b exhibit both local excited (LE) emission of the acceptor chromophore and emission from the charge-separated state (CT emission), which are similar to that of the corresponding Me(2)Si-spaced copolymer 2a with the same donor and acceptor chromophores, but different from that of the copolymer with the iPr(2)Si spacer 2b. Dimer 5 behaves like 4 and 2a. As the chain length of the oligomers increases, the emission properties of the higher homologues become prone to that of 2b. Thus, tetramer 7 exhibits emission from the charge-transfer complex, which is essentially same as that of 2b. Moreover, charge-transfer absorptions emerge in 6 and 7. These results suggest that the folding nature of oligomers approaches that of the corresponding polymer, as the degree of oligomerization increases, and the electronic interactions between adjacent donor-acceptor pairs are controlled by the steric effect of the substituents on the silicon atoms and concomitant amplification of the stabilizing energy by extending the distance of the folding structure.     

Optically active helical polyisocyanides bearing chiral phosphine pendants: Facile synthesis and application in enantioselective Rauhut-Currier reaction

Three novel enantiopure phenyl isocyanide monomers with BH3-protected phosphine functional group were designed and synthesized.Polymerization of these monomers using a alkyne-Pd(Ⅱ) complex as a catalyst led to the formation of respective helical polyisocyanides in high yields with controlled molecular weights (Mns) and narrow molecular weight distributions (Mw/Mns).Removing the protecting BH3 groups afforded helical poly(phenyl isocyanide)s bearing phosphine pendants.Thanks to the chiral induction of monomer,the isolated helical polyisocyanides showed high optical activity,as revealed by circular dichroism (CD) and absorption spectroscopies and polarimetry.The helical structures of these polymers were quite stable in various organic solvents with different polarities and in a wide temperature range.Moreover,these helical polymers could be used as organocatalysts and showed good performance in enantioselective cross Rauhut-Currier reaction.The enantiomeric excess (ee) values of the isolated products of cross Rauhut-Currier reaction could be up to 90％.The polymer organocatalysts could be easily recovered from the reaction mixtures and reused at least five times in the reaction without significant loss of their enantioselectivities and catalytic activities.     

Synthesis and tunable emission of novel polyfluorene co-polymers with 1,8-naphthalimide pendant groups and application in a single layer-single component white emitting device

New luminescent polymers containing two individual emission species-poly(fluorene-alt-phenylene) as a blue host and variable amounts of 1,8-naphthalimide as red dopant have been designed and synthesized. Optical studies (optical absorption (OA) and steady-state photoluminescence emission (PL)) in diluted solutions and thin solid films reveal that the emission spectrum can be tuned by varying the content of 1,8-naphthalimide moieties. Although no significant interaction can be observed between both moieties in the ground state, after photoexcitation an efficient energy transfer takes place from the PFP backbone to the red chromophore, indeed, by adjusting the polymer/naphthalimide ratio it is possible to obtain single polymers which emit white light to the human eye in solid state. Energy transfer is more effective in the co-polymers than in physical mixtures of the two chromophores. We prepared single-layer electroluminescent simple devices with structure: ITO/poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT:PSS)/active layer/Ba/Al. With this single layer-single component device structure, white light with Commission Internationale de l’Eclairage (CIE) color coordinates (0.3, 0.42) is obtained for the electroluminescence (EL) emission with an efficiency of 22.62 Cd/A.     

Sensing the glass transition in thin and ultrathin polymer films via fluorescence probes and labels

Fluorescence was used to characterize the glass transition in thin and ultrathin supported polymer films with common chromophores. The temperature dependence of the fluorescence intensity exhibits a transition or break upon cooling from the rubbery state to the glassy state, and this is identified as the glass transition. A variety of chromophores are investigated including pyrene, anthracene, and phenanthrene either as dopants, covalently attached to the polymer as a label, or both. The particular choice of the chromophore as well as the nature of the attachment, in the case of labels, have significant impact on the success of this method. Problematic cases include those in which the excited‐state chromophore undergoes significant photochemistry in addition to fluorescence or those in which the particular attachment of the chromophore as a label may allow for conformational interactions that affect the fluorescence quantum yield in a nontrivial way. Polymers that have an intrinsic fluorescence unit, for example, polystyrene, may allow for the fluorescence sensing of the glass transition without added dopants or labels. Finally, it is demonstrated that this technique holds promise for the study of the glass transition in polymer blends and within specific locations in multilayer films. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2745–2758, 2002     

Single chromophore spectroscopy of MEH-PPV: homing-in on the elementary emissive species in conjugated polymers.

Low-temperature, single-molecule spectroscopy can provide unparalleled access to the primary emissive species of conjugated polymers. We demonstrate this with the example of one of the most commonly studied polymers, poly(2-methoxy-5-(2'-ethylhexoxy)-1,4-phenylenevinylene), MEH-PPV, which is shown to exhibit sharp fluorescence signatures over one hundred times narrower than the ensemble. These unprecedented narrow emission features can be assigned to single chromophores on the polymer chain, which are selectively addressed by the narrow band excitation. As with organic dye systems, the emission from single chromophores is not static with time, but shows a substantial spectral fluctuation. We find that, for single chromophores, this spectral fluctuation always follows a universal Gaussian statistical distribution. High-resolution spectroscopy provides unique insight into low-energy vibrational modes in the polymer emission, which are generally inaccessible with conventional spectroscopic methods such as site-selective fluorescence or Raman spectroscopy. Interchromophoric coupling can also occur owing to the flexible nature of the polymer backbone. This leads to substantial spectral broadening and a loss of resolution in the vibronic progression. We observe reversible switching within one single molecule between narrow and broad emission, which directly correlates with a discrete switching in emission intensity. We conclude that one and the same single molecule can support aggregated and nonaggregated emission, that is, emission from isolated and aggregated chromophores in one single molecule, rather than the tendency for aggregate emission being intrinsic to the molecule.     

Synthesis, structure, and photophysical and electrochemical properties of a pi-stacked polymer

Dibenzofulvene (DBF) was polymerized using anionic initiators to afford a vinyl polymer. Oligo(DBF)s having from two to eight side-chain fluorene moieties bearing different chain-terminal groups were isolated by preparative size-exclusion chromatography. The structures of the isolated oligomers were revealed by single-crystal X-ray and (1)H NMR analyses. Both in solution and in crystal, the in-chain fluorene moieties stacked on top of each other, while the terminal conformation varied depending on the terminal group. These conformational characteristics were supported by molecular mechanics and dynamics calculations. The oligomers and polymers indicated hypochromism and red shift in UV absorption spectra and exclusive excimer emission in fluorescence spectra. In addition, reduced oxidation potentials were observed for the oligomers in electrochemical analyses, which suggests charge delocalization over the pi-stacked electron systems. The photophysical and electrochemical effects increased with the chain length of the oligomers and leveled off around the chain length of an oligomer consisting of five fluorene units.     

Helical self-assembled chromophore clusters based on DNA-like architecture

DNA duplexes were functionalized covalently by clusters of five adjacent chromophores consisting of 5-(pyren-1-yl)-2′-deoxyuridine (Py-U) and 5-(10-methyl-phenothiazin-3-yl)-2′-deoxyuridine (Pz-U). The chromophores form a regular helical π-array along the major groove of duplex DNA when the 5-fold chromophore-modified oligonucleotides are hybridized with an unmodified counter strand. As a result, these chromophores interact significantly and their fluorescence and absorption properties can be modulated by the sequence within the π-array. The 5-fold Py-U stack shows a strongly enhanced emission. The presence of intervening Pz-U groups quenches the fluorescence of the Py-U chromophores. Such modulation of the optical properties within a chromophore stack is potentially useful for optical nanodevices and as nucleic acid sensors for molecular diagnostics. The duplex architecture of DNA is suitable to provide the supramolecular structural scaffold for a directed arrangement of chromophores.