A quantum molecular dynamics study of exciton self-trapping in conjugated polymers: Temperature dependence and spectroscopy (original) (raw)
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Computational and Theoretical Chemistry, 2013
In this work we adopt a model Hamiltonian approach to investigate exciton generation and its subsequent dynamics in conjugated polymers. By using a modified version of the Su-Schrieffer-Heeger model Hamiltonian to include temperature effects, interaction between polymer chains, and different photoexcitation processes, we study the dynamics of triplet excitons in the system. The temperature is incorporated in the method by means of a classical Langevin equation, and Ehrenfest molecular dynamics is used to describe the time evolution of the system. We were able to distinguish the response of the oscillating electric dipole for different excitations at nonzero temperatures based on the amplitude and frequency.
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Phys. Chem. Chem. Phys., 2016
A combination of classical molecular dynamics (MM/MD) and quantum chemical calculations based on the density functional theory (DFT) and many-body Green's functions theory (GW-BSE) was performed to describe the conformational and optical properties of diphenylethyne (DPE), methylated-DPE and poly para phenylene ethynylene (PPE).
Temperature Dependence of Exciton Diffusion in Conjugated Polymers
The Journal of Physical Chemistry B, 2008
The temperature dependence of the exciton dynamics in a conjugated polymer is studied using time-resolved spectroscopy. Photoluminescence decays were measured in heterostructured samples containing a sharp polymer-fullerene interface, which acts as an exciton quenching wall. Using a 1D diffusion model, the exciton diffusion length and diffusion coefficient were extracted in the temperature range of 4-293 K. The exciton dynamics reveal two temperature regimes: in the range of 4-150 K, the exciton diffusion length (coefficient) of ∼3 nm (∼1.5 × 10 -4 cm 2 /s) is nearly temperature independent. Increasing the temperature up to 293 K leads to a gradual growth up to 4.5 nm (∼3.2 × 10 -4 cm 2 /s). This demonstrates that exciton diffusion in conjugated polymers is governed by two processes: an initial downhill migration toward lower energy states in the inhomogenously broadened density of states, followed by temperature activated hopping. The latter process is switched off below 150 K.
Understanding excitons in optically active polymers
Polymer International, 2008
We review the solid-state physics approach to electronic and optical properties of conducting polymers, and bring together the languages of solid-state theory for polymers and the quantum chemistry of oligomers. We consider polymers as generic one-dimensional semiconductors with specific features of strongly correlated electronic systems. Our model combines the large distance electron-hole motion within an exciton, governed by long-range Coulomb attraction with strong intramonomer electronic correlations, which results in effective intramonomer electron-hole repulsion. We exploit the dielectric screening to go beyond the single chain picture and to compare excitons for polymers in solutions and in films. Our approach allows the connecting, explaining, exploiting and organizing of such different experimental and numerical findings as shallow singlet and deep triplet excitons in phenylenes, anomaly in singlet-triplet exciton formation ratio, A g-B u crossing in polyenes and common 1/N energy dependencies in oligomers.
Chemical Physics Letters, 2013
We investigate temperature effects on exciton dissociation dynamics in conjugated polymer systems. Using a modified version of the tight-binding Su-Schrieffer-Heeger model, the dissociation is studied under the influence of impurity effects with a nonadiabatic evolution method. Our results show that temperature effects reduce the critical electric field for the exciton dissociation. In the small temperature regime, the exciton is not trapped by the impurity and it is observed to perform a random walk, a fact not observed in the absence of temperature. This letter might enlighten the description of electroluminescence yields and charge transport efficiency in organic based electronic devices.
Excitation Trapping in Dynamically Disordered Polymers
Trapping of migrating incoherent electronic excitation by dynamically disordered substitutional traps in a 1-D polymer chain has been studied analyticaly and by means of Monte Carlo simulations. A closed-form analytical solution to the model is based on the assumption that the temporal changes in the spatial coordinates of the traps, due to conformational motion, can be mimicked by a global Poissonian renewal process of the polymer configuration as a whole. The excitation survival probability P(t) for this model of dynamic disorder hopping (DDH) obeys an Ornstein-Zernike-type integral equation, which can be solved analytically in the short- and long-time limits and numerically in the whole time domain. The DDH results are compared with Monte Carlo simulations using discrete and continuoustime random walks showing a good agreement. The relevance of our theoretical findings has been discussed and connections have been made to observations of migrative excitation trapping in aromatic vinyl polymers, where the trapssin the pair approximationsconsist of mobile excimer-forming sites (EFS) triggered by the local conformation of a chain.
Scientific Reports, 2019
The advent of multiple exciton harvesting schemes and prolonging exciton lifetimes to improve performance attributes of solar cells based on conjugated organic materials presents some interesting challenges that must be overcome in order to realize the full potential of these strategies. This is especially important for applications involving multi-chromophoric conjugated polymers where interactions between multiple spin-forbidden triplet excitons can be significant and are mediated by chain conformation. We use single molecule spectroscopic techniques to investigate interactions between multiple triplet excitons and emissive singlets by monitoring time-dependent fluorescence quenching on time scales commensurate with the triplet lifetime. Structurally related conjugated polymers differing by heteroatom substitution were targeted and we use a stochastic photodynamic model to numerically simulate the evolution of multi-exciton populations following photoexcitation. Single chains of poly(3-hexylthiophene) (P3HT) exhibit longer-lived triplet dynamics and larger steady-state triplet occupancies compared to those of poly(3-hexylselenophene) (P3HS), which has a larger reported triplet yield. Triplet populations evolve and relax much faster in P3HS which only becomes evident when considering all kinetic factors governing exciton population dynamics. Overall, we uncover new guidelines for effectively managing multi-exciton populations and interactions in conjugated polymers and improving their light harvesting efficiency. Conjugated organic polymers have demonstrated promise in solar cell applications but extreme heterogeneity and efficient intrinsic loss mechanisms 1,2 , such as rapid non-radiative excitation energy dissipation, are responsible for large disparities between measured and predicted efficiencies. There is now widespread interest for mitigating performance losses by generating multiple excitons per photon absorbed or extending exciton lifetimes 3-5. Singlet fission-the generation of two triplet excitons from one singlet exciton-and heavy atom substitution to increase triplet character, respectively, have attracted the most attention 4,6-11 , but applications involving conjugated polymers are limited 12,13. In fact, mechanistic studies of singlet fission have concentrated on crystalline solids and small molecule arrays with well-defined chromophore orientations. Furthermore, efforts to increase triplet exciton character and tune triplet interactions in polymers may be complicated by contributions from large vibrational displacements along high frequency modes that modulate spin-orbit coupling strength 14. Perhaps the most significant obstacle for effectively utilizing multi-exciton generation and harvesting strategies can be traced to the multi-chromophoric nature of polymers (i.e., many conjugated segments of varying length) and variable inter-chromophore coupling due to conformational heterogeneity 15. For example, the longer lifetimes of triplets creates complex photophysical scenarios due to the presence of multiple excitonic states of different spin on many chromophore segments that interact over a broad range of time scales (e.g., ~10 −12-10 −3 s). Interestingly, previous pulse radiolysis studies found that isolated conjugated polymer chains can support a large number of triplets (~30) simultaneously 16. While much of the current focus has emphasized elucidating triplet formation mechanisms on ultrafast time scales, relatively little is known about how populations of multiple triplet excitons evolve on longer time scales (i.e., comparable to triplet lifetimes). Unfortunately, resolving multi-excitonic interactions at the materials level in polymers is complicated from a myriad of competing decay channels arising from intermolecular interactions and aggregation 12,17. However, by
Influence of vibronic coupling on band structure and exciton self-trapping in α-perylene
The journal of physical chemistry. B, 2011
Exciton sizes influence transport processes and spectroscopic phenomena in molecular aggregates and crystals. Thermally driven nuclear motion generally localizes electronic states in equilibrium systems. Exciton sizes also undergo dynamic changes caused by nonequilibrium relaxation in the lattice structure local to the photoexcitations (i.e., self-trapping). The α-phase of crystalline perylene is particularly well-suited for fundamental studies of exciton self-trapping mechanisms. It is generally agreed that a subpicosecond self-trapping process in α-perylene localizes photoexcited excitons onto pairs of closely spaced molecules (i.e., dimers), which then relax through excimer emission. Here, electronic relaxation dynamics in α-perylene single crystals are investigated using a variety of nonlinear optical spectroscopies in conjunction with a Frenkel exciton model. Linear absorption and photon echo spectroscopies suggest that excitons are delocalized over less than four unit cells (1...
Theory of non-Condon emission from the interchain exciton in conjugated polymer aggregates
The Journal of chemical physics, 2007
The authors present here a simple analysis that explains the apparent strengthening of electron phonon interaction upon aggregation in conjugated polymer materials. The overall scheme is that of an intermolecular Herzberg-Teller effect whereby sidebands of a forbidden transition are activated by oppositely phased vibrations. The authors show that upon aggregation, the 0-0 emission becomes symmetry forbidden and the apparent redshift and remaining vibronic structure are due to sideband ͑0-1,0-2, etc.͒ emission. At higher temperatures, the 0-0 peak is due to thermal population in a higher lying even-parity vibronic state rather than direct emission from the odd-paritied lowest intermolecular vibronic state.