Single molecule modulation spectroscopy of conjugated polymers

This paper describes a new single molecule spectroscopy approach for the investigation of triplet-triplet and singlet-triplet interactions in conjugated polymers. The technique involves the irradiation of isolated single, mulitchromophoric, conjugated polymer molecules by a repetitive sequence of variable-intensity microsecond time scale excitation pulses. The fluorescence intensity is synchronously time-averaged for thousands of cycles of the pulse sequence to yield a high signal-to-noise fluorescence transient on the microsecond time scale. The transient can be analyzed with kinetic models to obtain quantitative information about the kinetics of triplet-triplet exciton annihilation and the quenching of singlet excitons by triplet excitons in conjugated polymers.     

Infrared spectroscopy of electroluminescent conjugated polymers

The infrared spectra of the light-emitting diodes and the metal-insulator-semiconductor devices based on a poly(p-phenylenevinylene) derivative MEH-PPV have been measured in situ with a reflection configuration. The voltage-induced infrared spectra of these devices have been measured by the FT-IR difference-spectrum method. The observed bands have been attributed to the carriers injected into the polymer layers. The observation of positive carriers in the polymer light-emitting diode is probably related to the predominance of injected positive carriers, which is one of the factors in the low efficiency of the polymer light-emitting diode. In situ infrared reflective absorption measurements provide the information about injected carriers, which play a central role in the properties and the functions of polymer electronic devices.     

Molecular weight effects in the third-harmonic generation spectroscopy of thin films of the conjugated polymer MEH-PPV

Thin spin-cast films of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) were prepared from samples whose weight-average molecular weight (Mw) was varied in the range of 10-1600 kg/mol. We have characterized the films by means of transmission and reflection ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy to derive the linear optical constants, and third-harmonic generation spectroscopy with variable laser wavelengths to get the modulus and phase angles of the complex third-order nonlinear optical susceptibility chi(3). Increasing molecular weight yields films with significantly larger chi(3) values, absorption coefficients, and refractive indices. The chi(3) values of films from the largest and lowest Mw differ by a factor of 4, which is caused by chain orientation effects, local field effects, and changes of the effective conjugation length.     

An order-disorder transition in the conjugated polymer MEH-PPV

The poly(p-phenylene vinylene) derivative MEH-PPV is known to exist as two morphologically distinct species, referred to as red phase and blue phase. We show here that the transition from the blue phase to the red phase is a critical phenomenon that can be quantitatively described as a second order phase transition with a critical temperature T(c) of 204 K. The criticality is associated with the trade-off between the gain in the electronic stabilization energy when the π-system of a planarized chain can delocalize and the concomitant loss of entropy. We studied this transition by measuring the absorption and fluorescence in methyltetrahydrofuran (MeTHF) in two different concentrations as a function of temperature. The spectra were analyzed based upon the Kuhn exciton model to extract effective conjugation lengths. At room temperature, the chains have effective conjugation lengths of about five repeat units in the ground state (the blue phase), consistent with a disordered defect cylinder conformation. Upon cooling below the critical temperature T(c), the red phase with increased effective conjugation lengths of about 10 repeat units forms, implying a more extended and better ordered conformation. Whereas aggregation is required for the creation of the red phase, its electronic states have a predominant intrachain character.     

Single molecule nanoparticles of the conjugated polymer MEH-PPV, preparation and characterization by near-field scanning optical microscopy

We have developed a straightforward method for producing a stable, aqueous suspension of hydrophobic, fluorescent pi-conjugated polymer nanoparticles consisting primarily of individual conjugated polymer molecules. Features of the method are the facile preparation, purity, unique optical properties, and small size (approximately 5-10 nm) of the resulting nanoparticles. The results of TEM, scanning force microscopy, and near-field scanning optical microscopy of particles cast from the suspension indicate that the particles are single conjugated polymer molecules. The NSOM results yield estimates of the optical cross-sections of individual conjugated polymer molecules. The UV-vis absorption spectra of the nanoparticle suspensions indicate a reduction in conjugation length attributed to deformations of the polymer backbone. Fluorescence spectra of the aqueous nanoparticle suspensions indicate interactions between segments of the polymer chain and intramolecular energy transfer.     

Effect of sample preparation and excitation conditions on the single molecule spectroscopy of conjugated polymers

Extensive new single molecule spectroscopy (SMS) data on the conjugated polymer MEH-PPV at low temperature were obtained. In particular, the combined effects of sample preparation and excitation condition were explored in detail. The data confirm previous observations from this laboratory that (i) the distribution of emission maxima of single MEH-PPV molecules has a bimodal distribution and (ii) the single molecule emission spectrum of MEH-PPV exhibits few time-dependent fluctuations of the emission intensity, band shape, or spectral maxima. These data also help explain the discrepancy among the various published SMS data on this compound and suggest that environmental impurities, long irradiation times, nearby interfaces, and incomplete data sampling may account for some of the discrepancies among the published data.     

Efficient light harvesting in dye-endcapped conjugated polymers probed by single molecule spectroscopy

The development of sophisticated microscopic models of energy transfer in linear multichromophoric systems such as conjugated polymers is rarely matched by suitable experimental studies on the microscopic level. To assess the roles of structural, temporal, and energetic disorder in energy transfer, single molecule spectroscopic investigations of the elementary processes leading to energetic relaxation in conjugated polymers are desirable. We present a detailed study of energy transfer processes occurring in dye-endcapped conjugated polymer molecules on the single molecule level. These processes are mostly masked in ensemble investigations. Highly efficient intramolecular energy transfer along a single polyindenofluorene chain to a perylene endcap occurs in many instances and is resolved in real time. We further consider the spectral emission characteristics of the single molecule, the polarization anisotropy which reveals the chain conformation, the fluorescence intermittency, and the temperature dependence and conclude that the efficiency of energy transfer in the ensemble is controlled by the statistics of the individual molecules. The weak thermal activation of energy transfer indicates the involvement of vibrational modes in interchromophoric coupling. Whereas backbone-endcap coupling is strong, the rate-limiting step for intramolecular energy transfer is the migration along the backbone. The results are particularly relevant to understanding undesired exciton trapping on fluorenone defects in polyfluorenes.     

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.     

Single molecule spectroscopy of conjugated polymer chains in an electric field-aligned liquid crystal

Using single molecule polarization spectroscopy, we investigated the alignment of a polymer solute with respect to the liquid crystal (LC) director in an LC device while applying an external electric field. The polymer solute is poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (or MEH-PPV), and the LC solvent is 5CB. The electric field induces a change in the LC director orientation from a planar alignment (no electric field) to a perpendicular (homeotropic) alignment with an applied field of 5.5 x 103 V/cm. We find that the polymer chains align with the LC director in both planar and homeotropic alignment when measured in the bulk of the LC solution away from the device interface. Single molecule polarization distributions measured as a function of distance from the LC device interface reveal a continuous change of the MEH-PPV alignment from planar to homeotropic. The observed polarization distributions are modeled using a conventional elastic model that predicts the depth profile of the LC director orientation for the applied electric field. The excellent agreement between experiment and simulations shows that the alignment of MEH-PPV follows the LC director throughout the LC sample. Furthermore, our results suggest that conjugated polymers such as MEH-PPV can be used as sensitive local probes to explore complex (and unknown) structures in anisotropic media.     

Ultrafast vibrational spectroscopy of the flavin chromophore

Ultrafast time-resolved infrared (TRIR) spectra of flavin adenine dinucleotide (FAD) and the anion of lumiflavin (Lf-) are described. Ground-state recovery and excited-state decay of FAD reveal a common dominant ultrafast relaxation and a minor slower component. The Lf- transient lacks a fast component. No intermediate species are observed, suggesting that the quenching mechanism is internal conversion promoted by interaction of the adenine and isoalloxazine rings in FAD. Modes are assigned, and the potential for extension of the TRIR method to photoactive proteins is discussed.     

Effect of sub-Tg relaxations on chromophore reorientation in corona-poled polymers

Activation volumes for chromophore reorientation were measured for a series of guest–host polymeric materials, indicating a significant coupling between chromophore motion and the glassy α and β relaxation dynamics of the polymer host. The specific systems studied were formed by individually dissolving N,N-dimethyl-p-nitroaniline (DpNA), 4-(dimethylamino)-4′-nitrotolane (DMANT), 4-(diethylamino)-4′-nitrotolane (DEANT), and 1-((4-(dimethylamino)phenyl)ethynyl)-4-((4-nitrophenyl)ethynyl)benzene (DMAPEANT) in poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), and poly(isobutyl methacrylate) (PiBMA). In each of these systems, the isothermal, sub-T_g decay of the second-order optical susceptibility χ⁽²⁾ was monitored as a function of pressure using second harmonic generation. In each system, the observed decay of χ⁽²⁾ was represented by a stretched exponential equation from which the decay time τ₀ and decay distribution width β_KWW were determined. For each dopant molecule, the decrease in activation volume with the increasing size of the polymer host's alkyl side group and the pressure dependence of β_KWW were indicative of partial coupling between chromophore rotation and the glassy β relaxation dynamics of the polymer host. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1013–1024, 1998     

Chromophore localization in conjugated polymers: molecular dynamics simulation

Quantum chemical calculations of undistorted poly(phenylene vinylene) chains at zero temperature exhibit chromophores which are delocalized over the whole polymer. We demonstrate with molecular dynamics simulations that chromophore localization in agreement with experiment can be obtained if the system is simulated at finite temperature. The dependence of the chromophore localization on the temperature is investigated.     

Absorption spectrum of flexible conjugated polymers: the weak-disorder limit

Purely conformational contributions to dipole transition moments lead to a general new expression for the absorption spectrum of flexible conjugated polymers with persistence length ξ_c and alternation δ. Blue-shifts are found at rod-to-coil transitions without invoking rotational defects that break the conjugation. Conformational coupling in the weak-disorder limit increases the effective alternation.     

Selective adsorption of metronidazole on conjugated microporous polymers

Conjugated microporous polymers (CMPs) have recently received extensive attention in oil/organic solvent-water separation field as a kind of ideal porous absorbents with tunable porosity, large surface areas, and super-hydrophobicity. However, reports on the application of CMPs in adsorption of hydrophilic contaminants from water are very few. In this work, we studied the adsorption of metronidazole (MNZ), a polar antibiotic, by two kinds of CMPs. The adsorption characteristics of MNZ by the CMPs, including adsorption kinetics, mechanism, and isotherm parameters were calculated. The adsorption kinetics of MNZ was well expressed by the pseudo-second-order model, and the adsorption process was found to be mainly controlled by film diffusion. The adsorption isotherm data agreed well with the Langmuir isotherm model, and the values of free energy E indicated that the adsorption nature of MNZ on the CMPs was physisorption. Increasing dispersion degree of the CMPs in MNZ solution resulted in greater adsorption. This work may provide fundamental guidance for the removal of antibiotics by CMPs.     

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.     

On the spin gaps of conjugated hydrocarbon polymers

Many of the (ideal) infinite conjugated hydrocarbon polymers do not present a gap at the Fermi level in tight-binding calculations. However, due to the bielectronic interaction the excitation energy from the ground state to the lowest triplet state may be nonzero for some lattices (called spin gapped), while other lattices will keep a singlet-triplet degeneracy (spin-gapless lattices). This difference results in qualitative differences in their magnetic properties. Making use of the relevance of Heisenberg Hamiltonians for the study of the lowest states of conjugated hydrocarbons, this paper presents some qualitative arguments to predict the spin-gap character of various classes of such polymers. The arguments are based on real space renormalization group procedures, which considers fragments of the polymers as effective spins. Numerical evaluations, based on a renormalized excitonic method, confirm the qualitative predictions.     

Impedance spectroscopy of reactive polymers. 1

Dielectric measurements were utilized to follow the advancement of cure in an epoxy/amine formulation. In contrast to earlier studies, complex impedance was measured during cure and used to calculate ionic resistivity. By using complex impedance were able to separate according to their frequency dependence the contributions to overall polarization from electrode blocking layers, migrating charges, and dipole relaxations. At any stage of cure, there is a unique frequency at which ionic resistivity can be singularly measured. Our approach does not involve trial-and-error frequency search and is conducive to the development of phenomenological models based on equivalent circuits. Excellent agreement was reported between the calculated values of normalized degree of cure obtained by dielectric and calorimetric measurements. © 1994 John Wiley & Sons, Inc.     

Determination of the elementary rate constants of autoacceleration reactions on polymers

Equations describing the course of an autoacceleration reaction on a polymer in the low conversion region were derived and used to obtain relationships between the curvature of the conversion curve at the beginning and the elementary rate constants k₁ and k′₂. Calculations carried out by using our relationships are compared with the procedure suggested by Kawabe and Yanagita, and the applicability of the individual methods for the determination of the elementary rate constants is discussed.     

Interchain coupling effects on dynamics of photoexcitations in conjugated polymers

Within an extended Su-Schrieffer-Heeger model including interchain interactions and the extended Hubbard model, the dynamical relaxation of photoexcitations in two coupled conjugated polymer chains is investigated by using a nonadiabatic evolution method. Initially, one of the two chains is photoexcited and the other chain is in the dimerized ground state. Due to the interchain interactions, the electron and/or the hole can be transferred from one chain to the other chain. For weak interchain coupling, the dynamical evolution of the lattice on the photoexcited chain is similar to that found in an isolate single chain case. With interchain interactions increasing, the amplitude of the distortions on the photoexcited chain decreases, and simultaneously, that on the other chain gradually increases. Until stronger interchain coupling, the deformations of the two chains have almost the same amplitude. Besides intrachain polaron-excitons and intrachain oppositely charged polaron pairs as found in single chain case, interchain polaron-excitons and interchain separated charged polaron pairs are obtained. The results show that the yield of interchain products increases and that of intrachain products decreases with interchain coupling increasing. Totally, the yield of charged polarons (including intrachain oppositely charged polaron pairs and interchain oppositely charged polaron pairs) is about 25%, in good agreement with results from experiments.