Phosphorescence quenching by conjugated polymers

Energy transfer between phosphors and conjugated polymers was investigated using a fluorene trimer (F3) as a model conjugated material. The phosphors studied were bis-cyclometalated iridium complexes (FP, PPY, BT, PQ, and BTP), with triplet energies of 2.6, 2.4, 2.2, 2.1, and 2.0 eV, respectively (based on phosphorescence spectra). Stern-Volmer analysis of luminescent quenching shows that energy transfer from either FP or PPY to F3 is an exothermic process with Stern-Volmer quenching constants (kqSV) of near 109 M-1 s-1 while energy transfer from BT, PQ, and BTP is endothermic (kqSV = 107-106 M-1 s-1). On the the basis of above results, the triplet energy of F3 is estimated to be less than 2.3 eV (530 nm). This study suggests that conjugated polymers, which typically have lower T1 energies than F3, should also quench phosphorescent emission in thin films and organic light-emitting diodes (OLEDs) incorporating these and related phosphorescent dopants.     

Triplet-polaron quenching in conjugated polymers

We studied the triplet-polaron quenching in a platinum(II) porphyrin- (PtOEP-) doped polyspirobifluorene (PSF-TAD) copolymer. The copolymer contains a hole-transporting phenylenediamine unit (TAD) as a comonomer. Triplet-polaron quenching was probed by the change in PtOEP phosphorescence lifetime under an applied voltage in a unipolar device. The charge-induced reduction of the optically excited lifetime of PtOEP is one-third for the highest applied bias. The charge density can be obtained from current-voltage characteristics in the space-charge-limited (SCL) regime. The obtained hole mobility under SCL conditions is (7 +/- 2) x 10(-5) cm(2)/(V s). This result is in accord with recent mobility measurements of the time-of-flight mobility in our polymer. The triplet-polaron recombination constant was evaluated to be (4 +/- 1) x 10(-13) cm(3)/s, implying a triplet-polaron interaction radius of 2 x 10(-10) m. The results show that triplet-polaron annihilation cannot be neglected in device models for phosphorescent light-emitting diodes.     

Amplified quenching in metal-organic conjugated polymers

The luminescence from conjugated polyelectrolytes that contain pendant metal complex units is quenched very efficiently by oppositely charged electron acceptors.     

Photophysical studies of anion-induced colorimetric response and amplified fluorescence quenching in dipyrrolylquinoxaline-containing conjugated polymers

The dipyrrolylquinoxaline (DPQ)-containing monomer and polymers were synthesized and employed as chromogenic and fluorescent chemosensors for inorganic anions. We have found that in the presence of fluoride or pyrophosphate, the receptors do not form hydrogen bonds between the pyrrole protons and anions. The colorimetric responses and fluorescence quenching in these chemosensors are indeed the result of deprotonation of the N-H proton. The anion selectivity is primarily determined by the relative basicity of anions. The sensitivity of DPQ-based chemosensor was found to display a 34-fold enhancement by incorporation into the conjugated polymer. The anion-induced deprotonation generates low-energy, non-fluorescent trapping sites and is responsible for the signal amplification where the quenching of the excited state occurs from the deprotonated DPQ site in the network by rapid exciton migration along the polymeric backbone.     

Determination of proteins by fluorescence quenching of Magdala Red

Magdala Red (MR) binding to protein causes a decrease in the fluorescence intensity of MR at 556 nm. Based on this, a new quantitative determination method for proteins is developed. The linear range of this assay is 0.1-4.0 microg ml(-1) of Bovine Serum albumin (BSA). The measurements can be made easily on a common fluorimeter. The reaction between MR and proteins is completed in 1 min, and the fluorescence intensity is stable for at least 2 h. There is little or no interference from amino acids and most metal ions. The proposed method has been applied to the determination of protein in milk powder and soybean milk powder and the results are in agreement with the results by the other methods.     

Amplified fluorescence quenching of a conjugated polyelectrolyte mediated by Ca2+

The fluorescence of conjugated polyelectrolytes (CPEs) is quenched with very high efficiency by small molecule quenchers. This effect has been referred to as amplified quenching. In the present communication, we demonstrate that aggregation of a poly(phenylene ethynylene)-type CPE (PPE-CO2-) induced by Ca2+ has a pronounced effect on the amplified quenching of the polymer by the dication methyl viologen (MV2+). In particular, absorption and fluorescence spectroscopy of PPE-CO2- in methanol solution indicate that addition of a low concentration of Ca2+ induces aggregation of the polymer chains. The range of MV2+ concentrations within which linear Stern-Volmer quenching behavior is observed systematically decreases with increasing Ca2+ concentration to a point where superlinear quenching is observed immediately upon addition of MV2+. This finding is unequivocal evidence that the superlinear Stern-Volmer quenching behavior typically observed in CPE-quencher systems arises due to quencher-induced aggregation of the CPE chains.     

Piezochromic fluorescence in liquid crystalline conjugated polymers

Liquid crystalline diphenylacetylene polymer derivatives showed piezochromic fluorescence via order-to-disorder phase transition.     

Hole-induced quenching of triplet and singlet excitons in conjugated polymers

Quantitative information on the mechanisms and rates of hole (radical cation)-induced quenching of triplet and singlet excitons in the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] has been acquired by a new technique, fluorescence-voltage time-resolved single molecule spectroscopy (FV-TR-SMS). FV-TR-SMS measures the fluorescence intensity of a single conjugated polymer molecule that is embedded in a capacitor-like device while simultaneously modulating the bias on the device and the irradiation intensity. The results demonstrate that triplet excitons are efficiently quenched by holes in conjugated polymers for hole densities >10(16) charges/cm(3), while singlet excitons are quenched with a much lower efficiency. Detailed kinetic analysis shows that the greater efficiency for quenching of triplets by holes (compared to that for singlets) is due to a >10(6) times longer exciton lifetime for triplets. In fact, the results suggest that while singlet quenching is less efficient due to a much shorter singlet lifetime, the rate constant for the quenching of singlets by holes actually exceeds that for triplets by several orders of magnitude.     

Enhanced conjugated polymer fluorescence quenching by dipyridinium-based quenchers in the presence of surfactant

Poly[(2-methoxy-5-propyloxysulfonate)phenylene vinylene] (MPS-PPV) was synthesized directly from its bischloromethylated monomer, considerably reducing the total number of steps involved in the polymer preparation. For the first time, a simple technique of ultracentrifugation was employed for final purification of the polymer. The interactions among the polymer, surfactant, and quencher molecules, as well as amplified fluorescence quenching and fluorescence enhancement associated with the interactions, were investigated and discussed. When compared with methyl viologen [MV]2+, higher values of Stern-Volmer constant K(SV) values on the order of > or =10(7) M(-1) were observed for the newly synthesized N-(2-carboxyhexadecanoyl)-N'-methyl-4,4'-bipyridinium iodide bromide ([CHMB]2+) quencher in the presence of 1,2-dioleoyl-3- trimethylammonium propane (DOTAP) surfactant. Comparisons of surfactants demonstrated that the K(SV) of [CHMB]2+ was 10-fold higher in the presence of dodecyltrimethylammonium bromide (DTAB) surfactant than with DOTAP. Polymer fluorescence was totally recovered upon addition of DOTAP surfactant to a MV-quenched polymer system, whereas only 50% of fluorescence was recovered upon addition of DOTAP surfactant to the CHMB-quenched polymer solution. In contrast, no fluorescence was recovered when DTAB was added to either the MV- or CHMB-quenched polymer systems. Thus, fluorescence enhancement was observed for the polymer complex with DOTAP, whereas fluorescence quenching was predominant in the polymer complex with DTAB. Such studies will not only help to better understand the intrinsic properties of the ionic conjugated polymer and amplified fluorescence quenching and enhancement but also provide guidelines to develop the next generation of ionic conjugated-polymer-based biosensors.     

Phototriggered fluorescence color changes in azobenzene-functionalized conjugated polymers

We report fluorescence studies of phototriggered changes in spectral position and shape for two azobenzene-functionalized poly(p-phenylenevinylene) derivatives, poly(2-methoxy-5-(4-phenylazophenyl-4'-(1,10-dioxydecyl))-1,4-phenylenevinylene) (MPA-10-PPV) and poly(2-hexyloxy-5-(4-phenylazophenyl-4'-(1,10-dioxydecyl))-1,4-phenylenevinylene) (HPA-10-PPV). Upon trans --> cis azobenzene photoisomerization, small (ca. 1 nm) blue shifts in spectral position are observed for MPA-10-PPV in 100% toluene, a good solvent for this polymer. These shifts are reversed upon visible irradiation and can be cycled many times. To probe the dependence of these shifts on initial polymer conformation, a dichloromethane-methanol cosolvent study was performed in which the solvent quality was decreased incrementally to induce a reduction in polymer coil dimensions. Unirradiated dichloromethane solutions of both MPA-10-PPV and HPA-10-PPV showed a red shift and reduction in quantum yield with increasing methanol concentration as expected based on literature results for other poly(p-phenylenevinylene) derivatives. These changes have been attributed to a dramatic conformational collapse by others and occur for these azo polymers over the 30-60% (v/v) methanol range. While little or no light-induced spectral shifting was observed at low (or=70%) methanol concentrations, significant spectral shifts were observed for both polymers upon azobenzene photoisomerization in solutions with 30-60% methanol, the same range over which the polymer undergoes collapse to a highly coiled state. The largest shifts are visible to the eye, with a 65:35 (v/v) dichloromethane-methanol solution of HPA-10-PPV showing yellow-orange fluorescence when the azobenzenes are trans, green fluorescence when they are cis, and yellow-orange again after the azobenzenes are returned to the trans state. We attribute these color changes to a reversible UV-phototriggered expansion of polymer coil dimensions that occurs as a result of trans --> cis azobenzene side chain isomerization and provide temperature data to support this conclusion.     

Spectroscopy of conjugated polymers: phosphorescence and delayed fluorescence

We report on the observation of delayed fluorescence (DF) and phosphorescence (P) from films and dilute frozen solutions of various conjugated polymers of the PPP‐type. The materials differ with respect to the rotational freedom along the polymer backbone. Upon pulsed optical excitation into the S₁←S₀ transition of the materials, delayed emission occurs on a time scale of μs to ms in solid films at 80 K. The phosphorescence in dilute frozen solution decays monoexponentially with a radiative lifetime on the order of one second. The data analysis reveals that the DF is caused by recombination of geminate electron hole pairs rather than triplet‐triplet annihilation. This conclusion is supported by investigations of the response of the DF to an applied electric field.     

Enhanced fluorescence quenching in receptor-containing conjugated polymers: a calix[4]arene-containing poly(phenylene ethynylene)

A fluorescent poly(phenylene ethynylene) containing calix[4]arene-based receptor units has a sensitivity to quenching by the N-methylquinolinium ion that is over three times larger than that seen in a control polymer lacking calix[4]arenes.     

Regarding the origin of the delayed fluorescence of conjugated polymers

In the first part of this work we revisit and reevaluate the experimental data that lead to the assignment of the origin of the delayed fluorescence (DF) to triplet-triplet annihilation for polyfluorene and to geminate pair recombination in the case of the ladder-type polyparaphenylene (MeLPPP); the ambiguity of this classification is unveiled. Next, new data about the DF of MeLPPP under applied electric field are presented. Here, the DF intensity completely recovers once the field is turned off, which rules out geminate pairs as the origin of the DF and in turn provides clear evidence of the triplet-triplet annihilation picture. Finally, we show and discuss how recombination of space charge layers may also give rise to electric field induced delayed fluorescence, whereby the formation of these space charge layers strongly depends on device configuration and purity of the materials.     

A sensitive method for the detection of proteins by high-efficiency fluorescence quenching

A successful method for the detection of electron transfer proteins such as cytochrome c, hemoglobin and myoglobin has been developed based on the fluorescence quenching of semiconductor nanocrystals. High sensitivity and a good linear relationship are obtained.     

Redistribution of emitting state population in conjugated polymers probed by single-molecule fluorescence polarization spectroscopy

Fluctuations in the fluorescence polarization degree and direction are reported for the first time for single conjugated polymer molecules embedded in a polystyrene matrix at room temperature. The polymer molecule, a polythiophene derivative, clearly emits as a multi-chromophore ensemble showing that the energy does not funnel to any specific low-energy trap. The fluorescence instead originates from thermally populated exciton states with different relative orientations of the transition dipole moments. The fluctuations in the fluorescence polarization are explained in terms of changes in the relative contributions of the different exciton states to the signal due to conformational fluctuations of the molecule or selective exciton quenching by triplet states.     

Cationic conjugated polymers for homogeneous and sensitive fluorescence detection of hyaluronidase

The cationic charged water-soluble polyfluorenes containing 2,1,3-benzothiadiazole (BT) units (P1–3) have been synthesized and characterized. These polymers demonstrate intramolecular energy transfer from the fluorene units to the BT sites when oppositely charged hyaluronan is added due to the formation of electrostatic complexes, followed by a shift in emission color from blue to green or brown. Upon adding hyaluronidase, the hyaluronan is cleaved into fragments. The relatively weak electrostatic interactions of hyaluronan fragments with polyfluorenes keep their main chains separated and energy transfer from the fluorene units to the BT sites is inefficient, and the polyfluorenes recover their blue emissions. The complexes of conjugated polymers/hyaluronan can be utilized as probes for sensitive and facile fluorescence assays for hyaluronidase. The new assay method interfaces with the aggregation and light harvesting properties of conjugated polymers. Supported by the “100 Talents” Program of Chinese Academy of Sciences, the National Natural Science Foundation of China (Grant Nos. 20725308 & 20721061), and 973 Project (Grant Nos. 2006CB806200 & 2006CB932100)     

Effects of polymer aggregation and quencher size on amplified fluorescence quenching of conjugated polyelectrolytes

The fluorescence of conjugated polyelectrolytes (CPEs) is efficiently quenched by low concentrations of quenchers with opposite charges. We have reported the close correlation between this amplified quenching phenomenon and CPE chain aggregation. In this paper, we further demonstrate the profound correlation between the fluorescence quenching efficiency, CPE chain aggregation, and quencher molecular size. Aggregation of a poly(phenylene ethynylene)-type CPE (PPE-CO2-) is induced by the addition of either water or Ca2+ to methanol solution, as indicated by absorption, fluorescence, dynamic light scattering, and fluorescence microscope measurements. For quencher ions with a small molecular size, such as methyl viologen (MV2+), either the loose (induced by the addition of Ca2+) or the compact (induced by the addition of water) CPE chain aggregates are beneficial to the fluorescence quenching. For quencher ions with large molecular size, such as tris(4,7-diphenyl-1,10-phenanthroline)ruthenium (Ru(dpp)32+), however, the loose chain aggregates are found to be favorable for quenching, while the quenching efficiency is lower for the compact polymer aggregates present in aqueous solution.     

Pyrene fluorescence quenching by aromatic azides

Pyrene fluorescence quenching by phenylazide derivatives with donor and acceptor substituents has been studied by fluorescence spectroscopy and flash photolysis. The rate constants of quenching (k _q) in acetonitrile ((0.2–1.2) × 10¹⁰ l mol^?1 s^?1) are found to be close to a diffusion limit; the rate constants were somewhat higher for perfluoro-substituted arylazides. It is found that k _q does not depend on solvent polarity; the formation of the pyrene cation in the course of pyrene fluorescence quenching by tolylazide was not detected. Pyrene fluorescence quenching occurred by an energy-transfer mechanism; this is supported by the coincidence of the quantum yields of the direct and sensitized photodecomposition of tolylazide. As estimated, energy transfer in rigid media occurs at characteristic distances of about 10 Å.     

Advanced materials by functionalization of conjugated polymers

Applications of conjugated polymers to advanced materials are demonstrated with fabrications of functional molecular materials and devices by incorporation of functional molecules to conjugated and/or conducting polymer, ultrahigh anisotropic conducting polymer materials by topological polymerizations of LB multilayers of amphiphilic pyrrol, artificial conjugated polymer superlattice by the potential-programmed electro-polymerization and/or electrocopolymerization, and 1-dimensional and 2-dimensional porphyrin arrays connected with conjugated and/or insulating molecular wires with P(V) porphyrin and others, for molecular photonics and electronics.