Conformation-Related Exciton Localization and Charge-Pair Formation in Polythiophenes: Ensemble and Single-Molecule Study

Single-molecule nano-optoelectronics: insights from physics

Single-molecule optoelectronic devices promise a potential solution for miniaturization and functionalization of silicon-based microelectronic circuits in the future. For decades of its fast development, this field has made significant progress in the synthesis of optoelectronic materials, the fabrication of single-molecule devices and the realization of optoelectronic functions. On the other hand, single-molecule optoelectronic devices offer a reliable platform to investigate the intrinsic physical phenomena and regulation rules of matters at the single-molecule level. To further realize and regulate the optoelectronic functions toward practical applications, it is necessary to clarify the intrinsic physical mechanisms of single-molecule optoelectronic nanodevices. Here, we provide a timely review to survey the physical phenomena and laws involved in single-molecule optoelectronic materials and devices, including charge effects, spin effects, exciton effects, vibronic effects, structural and orbital effects. In particular, we will systematically summarize the basics of molecular optoelectronic materials, and the physical effects and manipulations of single-molecule optoelectronic nanodevices. In addition, fundamentals of single-molecule electronics, which are basic of single-molecule optoelectronics, can also be found in this review. At last, we tend to focus the discussion on the opportunities and challenges arising in the field of single-molecule optoelectronics, and propose further potential breakthroughs.

Fluorescence enhancement induced by quadratic electric-field effects on singlet exciton dynamics in poly(3-hexylthiophene) dispersed in poly(methyl methacrylate)

The dynamics of the exciton generated by photoexcitation of a regioregular poly(3-hexylthiophene) (P3HT) polymer dispersed in a poly(methyl methacrylate) (PMMA) matrix was examined using electro-photoluminescence (E-PL) spectroscopy, where electric field effects on the photoluminescence (PL) spectra were measured. The quadratic electric-field effect was investigated using the modulation technique, with field-induced changes in the PL intensity monitored at the second harmonic of the modulation frequency of the applied electric field. Absorption and PL spectra indicated the formation of both ordered crystalline aggregates and amorphous regions of P3HT polymer chains. Although previous studies of electric field effects on π-conjugated polymers have generally shown that the PL intensity is decreased by electric fields, we report that the PL intensity of P3HT and PL lifetime increased with the quadratic electric-field effect. The magnitude of the change in PL intensity was quantitatively explained in terms of the field-induced decrease in the nonradiative decay rate constants of the exciton. We proposed that a delayed PL, originating from charge carrier recombination, was enhanced in the presence of electric fields. The rate constant of the downhill relaxation process of the exciton, which originated from the relaxation in distributed energy levels due to an inherent energetic disorder in P3HT aggregates, was implied to decrease in the presence of electric fields. The radiative decay rate constant and PL quantum yield of P3HT dissolved in solution, which were evaluated from the molar extinction coefficient and the PL lifetime, were compared with those of P3HT dispersed in a PMMA matrix.

Stark Effect and Environment-Induced Modulation of Emission in Single Halide Perovskite Nanocrystals

Organic-inorganic halide perovskites have emerged as promising materials for next-generation solar cells. In nanostructured form also, these materials are excellent candidates for optoelectronic applications such as lasers and light-emitting diodes for displays and lighting. While great progress has been achieved so far in optimizing the intrinsic photophysical properties of perovskite nanocrystals (NCs), in working optoelectronic devices, external factors, such as the effects of conducting environment and the applied electric field on exciton generation and photon emission, have been largely unexplored. Here, we use NCs of the all-inorganic perovskite CsPbBr₃ dispersed polyvinyl carbazole, a hole-conductor, and in poly(methyl methacrylate), an insulator, to examine the effects of applied electric field and conductivity of the matrix on the perovskite photophysics at the single-particle level. We found that the conducting environment causes a significant decrease of photoluminescence (PL) brightness of individual NCs due the appearance of intermediate-intensity emitting states with significantly shortened lifetime. Applied electric field has a similar effect and, in addition, causes a nonlinear spectral shift of the PL maxima, a combination of linear and quadratic Stark effects caused by environment-induced polarity and field-related polarizability. The environment and electric-field effects are explained by ionization of the NCs through hole transfer and emission of the resulting negatively charged excitons.

Effects of molecular architecture on morphology and photophysics in conjugated polymers: from single molecules to bulk

A definitive comprehension of morphology and photophysics in conjugated polymers at multiple length scales demands both single molecule spectroscopy and well-controlled molecular architectures.

Conjugated polymers (CPs) possess a wide range of desirable properties, including accessible energetic bandgaps, synthetic versatility, and mechanical flexibility, which make them attractive for flexible and wearable optoelectronic devices. An accurate and comprehensive understanding about the morphology–photophysics relations in CPs lays the groundwork for their development in these applications. However, due to the complex roles of chemical structure, side-chains, backbone, and intramolecular interactions, CPs can exhibit heterogeneity in both their morphology and optoelectronic properties even at the single chain level. This molecular level heterogeneity together with complicated intermolecular interactions found in bulk CP materials severely obscures the deterministic information about the morphology and photophysics at different hierarchy levels. To counter this complexity and offer a clearer picture for the properties of CP materials, we highlight the approach of probing material systems with specific structural features via single molecule/aggregate spectroscopy (SMS). This review article covers recent advances achieved through such an approach regarding the important morphological and photophysical properties of CPs. After a brief review of the typical characteristics of CPs, we present detailed discussions of structurally well-defined model systems of CPs, from manipulated backbones and side-chains, up to nano-aggregates, studied with SMS to offer deterministic relations between morphology and photophysics from single chains building up to bulk states.

The correspondence between the conformational and chromophoric properties of amorphous conjugated polymers in mesoscale condensed systems

For π-conjugated polymers, the notion of spectroscopic units or "chromophores" provides illuminating insights into the experimentally observed absorption/emission spectra and the mechanisms of energy/charge transfer. To date, however, no statistical analysis has revealed a direct correspondence between chromophoric and conformational properties-with the latter being fundamental to polymer semiconductors. Herein, we propose a "persistence length" calculation to re-evaluate chain conformation over a full conjugation length. The mesoscale condensed systems of MEH-PPV and MEH-PPV/C₆₀ hybrid (system size ∼10 × 10 × 10 nm³) are utilized as two prototypical model systems, along with a full range of segmental lengths (2-20-mer) and five lowest singlet excited states to hint at the generality of the features presented. We demonstrate, for the first time, that two properly re-defined conformational factors that characterize chain folding and planarity, respectively, capture excellently the population distribution of chromophores in both systems investigated. In contrast, the conventional strategy of utilizing two adjacent monomer units to characterize (local) chain conformation results in only an inconspicuous correlation between the two, as previously reported. It is further shown that chain folding-and not chain planarity-is more relevant in capturing the associated oscillator strength for the first excited state, where the transient dipole moments are known to align with the chain conformation, although the corresponding excitation energy and exciton size seem relatively unaffected. The observed effects of C₆₀ on the MEH-PPV adsorption spectra also agree with recent experimental trends. Overall, the present findings are expected to aid future multiscale computer simulations and spectroscopy-data interpretations for polymer semiconductors and their hybrid systems.

An electric field induced reversible single-molecule fluorescence switch

Based on the intramolecular electron transfer within a single molecule, we have achieved fluorescence switch induced by the electric field.

Energy transfer in PPV-based conjugated polymers: a defocused widefield fluorescence microscopy study

Both pendant and main chain conjugated MEH-PPV based polymers have been studied at the level of single chains using confocal and widefield fluorescence microscopy techniques. In particular, defocused widefield fluorescence is applied to reveal the extent of energy transfer in these polymers by identifying whether they act as single emitters. For main chain conjugated MEH-PPV, molecular weight and the surrounding matrix play a primary role in determining energy transport processes and whether single emitter behaviour is observed. Surprisingly in polymers with a saturated backbone but containing the same pendant MEH-PPV oligomer on each repeating unit, intra-chain energy transfer to a single emitter is also apparent. The results imply there is chromophore heterogeneity that can facilitate energy funneling to the emitting site. Both main chain conjugated and pendant MEH-PPV polymers exhibit changes in orientation of the emission dipole during a fluorescence trajectory of many seconds, whereas a model MEH-PPV oligomer does not. The results suggest that, in the polymers, the nature of the emitting chromophores can change during the time trajectory.

Effect of the side-chain-distribution density on the single-conjugated-polymer-chain conformation

The spatial arrangement of the side chains of conjugated polymer backbones has critical effects on the morphology and electronic and photophysical properties of the corresponding bulk films. The effect of the side-chain-distribution density on the conformation at the isolated single-polymer-chain level was investigated with regiorandom (rra-) poly(3-hexylthiophene) (P3HT) and poly(3-hexyl-2,5-thienylene vinylene) (P3HTV). Although pure P3HTV films are known to have low fluorescence quantum efficiencies, we observed a considerable increase in fluorescence intensity by dispersing P3HTV in poly(methyl methacrylate) (PMMA), which enabled a single-molecule spectroscopy investigation. With single-molecule fluorescence excitation polarization spectroscopy, we found that rra-P3HTV single molecules form highly ordered conformations. In contrast, rra-P3HT single molecules, display a wide variety of different conformations from isotropic to highly ordered, were observed. The experimental results are supported by extensive molecular dynamics simulations, which reveal that the reduced side-chain-distribution density, that is, the spaced-out side-chain substitution pattern, in rra-P3HTV favors more ordered conformations compared to rra-P3HT. Our results demonstrate that the distribution of side chains strongly affects the polymer-chain conformation, even at the single-molecule level, an aspect that has important implications when interpreting the macroscopic interchain packing structure exhibited by bulk polymer films.

Unraveling the chromophoric disorder of poly(3-hexylthiophene)

The spectral breadth of conjugated polymers gives these materials a clear advantage over other molecular compounds for organic photovoltaic applications and is a key factor in recent efficiencies topping 10%. However, why do excitonic transitions, which are inherently narrow, lead to absorption over such a broad range of wavelengths in the first place? Using single-molecule spectroscopy, we address this fundamental question in a model material, poly(3-hexylthiophene). Narrow zero-phonon lines from single chromophores are found to scatter over 200 nm, an unprecedented inhomogeneous broadening that maps the ensemble. The giant red shift between solution and bulk films arises from energy transfer to the lowest-energy chromophores in collapsed polymer chains that adopt a highly ordered morphology. We propose that the extreme energetic disorder of chromophores is structural in origin. This structural disorder on the single-chromophore level may actually enable the high degree of polymer chain ordering found in bulk films: both structural order and disorder are crucial to materials physics in devices.

Massive enhancement of photoluminescence through nanofilm dewetting

Due to the rather low efficiencies of conjugated polymers in solid films, their successful applications are scarce. However, recently several experiments indicated that a proper control of molecular conformations and stresses acting on the polymers may provide constructive ways to boost efficiency. Here, we report an amazingly large enhancement of photoluminescence as a consequence of strong shear forces acting on the polymer chains during nanofilm dewetting. Such sheared chains exhibited an emission probability many times higher than the nonsheared chains within a nondewetted film. This increase in emission probability was accompanied by the emergence of an additional blue-shifted emission peak, suggesting reductions in conjugation length induced by the dewetting-driven mass redistribution. Intriguingly, exciton quenching on narrow-band-gap substrates was also reduced, indicating suppression of vibronic interactions of excitons. Dewetting and related shearing processes resulting in enhanced photoluminescence efficiency are compatible with existing fabrication methods of polymer-based diodes and solar cells.

Hydrodynamic and molecular characteristics of organosilane copolymers of low molecular weight

Molecular hydrodynamics and light scattering measurements have been carried out in dilute solutions of polysilanes copolymers containing segments with different degrees of rotational freedom. For this purpose, three low-molecular-weight linear polydiphenylsilane copolymeric structures containing various amounts of methylhydrosilylene or methylphenylsilylene flexible units have been synthesized and studied. The molecular and hydrodynamic properties of the diphenylsilane copolymers proved to be dependent on their composition and on the chemical structure of the second co-monomer.

Forbidden singlet exciton transitions induced by localization in polymer light-emitting diodes in a strong electric field

Through combining the electron transition process and dipole moment evolution as well as electron-phonon coupling, molecular dynamics calculations show that the radiative decay of singlet excitons in a conjugated polymer, such as a polymer light-emitting diode (PLED), is largely determined by the evolution of the dipole moment. Without an electric field, the decay life of a singlet exciton is about 1 ns. Once an electric field is applied and exceeds a critical value, with electron-phonon coupling, the original lattice structure evolves into two new localized lattice distortions, consistent with the experimental results. Owing to the new lattice structure and self-trapping, the dipole moment rapidly decreases to zero within 5 fs, eliminating the radiative decay of the singlet exciton.

Electrothermal manipulation of individual chromophores in single conjugated polymer chains: controlling intrachain FRET, blinking, and spectral diffusion

Single molecule spectroscopy of individual chains of a conjugated polymer opens up deep insight into electronic localization phenomena, which govern the underlying optical properties of these complex and disordered materials. We explore the nature of a single chromophore arising in a delocalized pi-electron system by applying periodic electrothermal perturbations at low temperatures. Brief heating of the chromophore leads to a dramatic increase in the transition line width and is generally accompanied by a random jump of the emission energy. This observation demonstrates that chromophores on a polymer chain are not only defined by structural disorder but also by the subtleties of the local dielectric environment. The effect of thermal perturbation becomes more complex when long polymer chains are considered, which can potentially support the formation of multiple chromophores. Here, a momentary increase in temperature can promote intrachain energy transfer to quenching sites, leading to a strong modulation of emission intensity with temperature. Unexpectedly, such energy transfer can serve to either raise or lower the transition line width and quantum yield of the ensemble with increasing temperature, depending on the specific energetics of the chromophores in the system, which in turn vary with time. The controlled perturbation of both the emission spectrum and the intensity by brief heating of the polymer chain offers insight into possible microscopic origins of fluorescence blinking and spectral diffusion, which ultimately impact on the efficiency and spectral purity of devices.

Quantitative mass determination of conjugated polymers for single molecule conformation analysis: enhancing rigidity with macrocycles

A combination of advanced synthetic and single molecule spectroscopic techniques allowed us to demonstrate that macrocycles, covalently bound to a conjugated polymer backbone, raise the effective chain persistence length by at least a factor of 5.