Exciton migration to chain aggregates in conjugated polymers: influence of side-chain substitution (original) (raw)
Related papers
Well-Packed Chains and Aggregates in the Emission Mechanism of Conjugated Polymers
The Journal of Physical Chemistry B, 2005
We synthesized dialkoxy-substituted poly[phenylene vinylene]s (dROPPV-1/1, 0.2/1, and 0/1) consisting of two repeating units with different side-chain lengths (methoxy and 3,7-dimethyloctyloxy). These polymers can serve as a model system to clarify roles of aggregates (the sites with ground-state interchain interactions) and the independent chain segments in the well-packed chains (the chain segments that are compactly packed without interaction) in the emission mechanism of conjugated polymers. Due to the packing of polymer chains, films of all of these polymers are accessible to interchain excitations, after which excitons can reform to result in delayed luminescence. Besides, some chains form aggregates so that the delayed luminescence is no more the ordinary single-chain emission but red-shifted and less structured. Not only the reformation of these indirect excitons but also the aggregation of chains are facilitated in the polymers with short methoxy side groups, revealing that both packing and aggregation of chain segments require a short spacing between polymer chains. However, the incorporation of other side chains such as the 3,7-dimethyloctyloxy group to dROPPVs is necessary for the formation of aggregates because these long branched side chains can reduce the intrachain order imposed by the short methoxy groups, which accounts for the absence of aggregate emission in the well-studied poly[2,5-dimethoxy-1,4-phenylene vinylene]. This study reveals that the wellpacked chains do not necessarily form aggregates. We also show that the photophysical properties and the film morphology of conjugated polymers can be deliberately controlled by fine-tuning of the copolymer compositions, without altering the optical properties of single polymer chains (e.g., as in dilute solutions).
Excitonic Energy Migration in Conjugated Polymers: The Critical Role of Interchain Morphology
Excitonic energy migration was studied using single molecule spectroscopy of individual conjugated polymer (CP) chains and aggregates. To probe the effect of interchain morphology on energy migration in CP, tailored interchain morphologies were achieved using solvent vapor annealing to construct polymer aggregates, which were then studied with single aggregate spectroscopy. We report that highly ordered interchain packing in regioregular poly(3-hexylthiophene) (rr-P3HT) enables long-range interchain energy migration, while disordered packing in regioran-dom poly(3-hexylthiophene) (rra-P3HT), even in aggregates of just a few chains, can dramatically impede the interchain mechanism. In contrast to rr-P3HT, interchain energy migration in poly(3-(2′-methoxy-5′-octylphenyl)thiophene) (POMeOPT), a polythiophene derivative with bulky side chains, can be completely inhibited. We use simulated structures to show that the reduction in interchain coupling is not due simply to increased packing distance between backbones of different chains, but reflects inhibition of stacking due to side-chain-induced twisting of the contours of individual chains. A competition from intrachain coupling has also been demonstrated by comparing POMeOPT aggregates with different polymer chain sizes.
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.
Chain Conformation and Exciton Delocalization in a Push-Pull Conjugated Polymer
arXiv (Cornell University), 2023
Linear and nonlinear optical lineshapes reveal details of excitonic structure in semiconductor polymers. We implement absorption, photoluminescence, and transient absorption spectroscopies in DPP-DTT, an electron push-pull copolymer, to explore the relationship between their spectral lineshapes and chain conformation, deduced from resonance Raman spectroscopy and from ab initio calculations. The viscosity of precursor polymer solutions before film casting displays a transition that suggests gel formation above a critical concentration. Upon crossing this viscosity deflection concentration, the lineshape analysis of the absorption spectra within a photophysical aggregate model reveals a gradual increase in interchain excitonic coupling. We also observe a red-shifted and line-narrowed steady-state photoluminescence spectrum, along with increasing resonance Raman intensity in the stretching and torsional modes of the dithienothiphene unit, which suggests a longer exciton coherence length along the polymer-chain backbone. Furthermore, we observe a change of lineshape in the photoinduced absorption component of the transient absorption spectrum. The derivative-like lineshape may originate from two possibilities: a new excited-state absorption, or from Stark effect, both of which are consistent with the emergence of high-energy shoulder as seen in both photoluminescence and absorption spectra. Therefore, we conclude that the exciton is more dispersed along the polymer chain backbone with increasing concentrations, leading to the hypothesis that the polymer chain order is enhanced when the push-pull polymers are processed at higher concentrations. Thus, tuning the microscopic chain conformation by concentration would be another factor of interest when considering the polymer assembly pathways for pursuing large-area and high-performance organic optoelectronic devices.
Measurement of interchain and intrachain exciton hopping barriers in luminescent polymer
Journal of Physics: Condensed Matter, 2012
The integrated photoluminescence intensity in thin films of 'Super Yellow' copolymer has been analyzed using a Mott-like temperature dependence. This has enabled us to observe contributions from two emission channels, indicative of exciton recombination proceeding from two distinct origins. At high temperature, interchain thermally activated exciton energy transfer and migration dominates, resulting in large scale quenching of the integrated emission intensity and hence the photoluminescence quantum yield. However, at relatively low temperature, an additional increase of the integrated emission intensity occurs. This new channel of emission has been attributed to recombination from excitons where intrachain exciton energy transfer between adjacent subunits of the copolymer backbone becomes hindered. The activation energy barriers that control both of these emission channels have been obtained and are correlated with chain backbone degrees of freedom.
Unravelling the effect of strand orientation on exciton migration in conjugated polymers
Computational Materials Science, 2013
The study of the average distance that singlet excitons travel during their lifetime in conjugated polymers has attracted considerable attention during the past decade, because of its importance in the functioning of many polymer-based optoelectronic devices, like solar cells and photodetectors. Intriguingly, different values of exciton diffusion length have been extracted from experiments on seemingly identical conjugated polymers. Here we use computer simulations to show that the observed discrepancies in the reported values of the exciton diffusion length may arise from differences in the orientation of conjugated polymer strands relative to the substrate surface, a factor which has been mostly overlooked. Our results show that, on pristine polymer nanodomains with conjugated strands perpendicular to the substrate surface, exciton migration length is approximately 30% and 40% lower than on those with parallel and random strand orientation relative to that surface, respectively, resulting from the different contents of physical traps present in nanodomains with different strand orientation. This work underlines the importance of molecular arrangement on exciton migration, and provides a 2 novel theoretical framework for estimating the dependence of the exciton diffusion length with the orientation of conjugated polymers strands within the nanodomains, as well as helping the design of more efficient polymer-based optical and optoelectronic devices, such as optical sensors, photodiodes, photovoltaic cells and white light-emitting diodes.
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
Interchain vs. intrachain energy transfer in acceptor-capped conjugated polymers
Proceedings of the National Academy of Sciences, 2002
The energy-transfer processes taking place in conjugated polymers are investigated by means of ultrafast spectroscopy and correlated quantum-chemical calculations applied to polyindenofluorenes end-capped with a perylene derivative. Comparison between the time-integrated luminescence and transient absorption spectra measured in solution and in films allows disentangling of the contributions arising from intrachain and from interchain energymigration phenomena. Intrachain processes dominate in solution where photoexcitation of the polyindenofluorene units induces a rather slow energy transfer to the perylene end moieties. In films, close contacts between chains favors interchain transport of the excited singlet species (from the conjugated bridge of one chain to the perylene unit of a neighboring one); this process is characterized by a 1-order-of-magnitude increase in transfer rate with respect to solution. This description is supported fully by the results of quantum-chemical calculations that go beyond the usual pointdipole model approximation and account for geometric relaxation phenomena in the excited state before energy migration. The calculations indicate a two-step mechanism for intrachain energy transfer with hopping along the conjugated chains as the ratelimiting step; the higher efficiency of the interchain transfer process is mainly due to larger electronic coupling matrix elements between closely lying chains. E nergy transfer is a key process in the working mechanism of a number of opto-electronic devices based on conjugated materials. This is the case for instance in electroluminescent displays where one can take advantage of energy transfer to tune the color of the emitted light when the active layer includes several materials with different optical gaps (1-5). In addition to providing an efficient technique for internal color conversion, polymer-polymer and polymer-dye blends have been shown also to lead to a significant improvement in photoluminescence (PL) and electroluminescence (EL) quantum efficiencies (6-8).
Control of luminescence in conjugated polymers through control of chain microstructure
Journal of Materials Chemistry, 2007
The development of semiconducting polymers with high solid-state luminescence efficiencies has enabled the fabrication of efficient polymer light-emitting diodes. Luminescence is often quenched in well-ordered molecular solids, as a result of inter-molecular dipolar coupling, and the general observation of efficient luminescence in semiconducting polymers is unexpected. We report here the synthesis and characterisation of a series of model 'glassy' poly(arylenevinylene)s where we control the cis to trans ratio about the vinylene linkage and also the phenylene linkage geometry. Photoluminescence efficiency is enhanced for more disordered materials, with highest values for 50 : 50 cis : trans ratios, when it exceeds 50%. We also find that the free volume associated with these glassy disordered polymers allows conformational relaxation of the excitonic state, via ring rotation at the vinylene linkage, causing a large Stokes' shift of the emission. We propose that high luminescence efficiency in these glassy polymers is due to emission from the more disordered regions, and that two effects due to disorder are simultaneously required: firstly that these regions are luminescent (prevention of aggregation by disorder), and, secondly, that the electronic excited state (exciton) can lower its energy below its value in more ordered regions by means of the ring-rotational coupling.
Effect of alkoxy side chains on intra and interchain exciton coupling in PPE-PPV copolymers solution
Synthetic Metals, 2017
The effects of alkoxy side chains on the optical and morphological properties of five poly (p-phenyleneethylene)-alt-poly (p-phenylene-vinylene)s (PPE-PPVs) solutions are investigated. By steady-state and time-resolved photoluminescence spectroscopy combined with Franck-Condon analyses, we show that the introduction of symmetrical side chains onto the PPE-PPV backbone increases the p-p stacking between the polymer chains leading to interchain interactions hence H-aggregate domains. Total dissymmetrical side chains lead to J-aggregate domains through intrachain interactions. On the other hand partial dissymmetrical side chain configurations lead to H and HJ-aggregate domains.