The majority of today’s power is generated from fossil fuels such as coal, oil and natural gas. However, concerns over the environmental impact and finite availability of these resources have prompted the exploration of alternative energy sources. Solar-powered energy has emerged as a leading renewable energy source. As opposed to fossil fuels, solar-powered energy is more eco-friendly and has unlimited supply, because it draws from the planet’s most bountiful energy source — the sun.
Just as plants convert the sun’s energy to produce resources through photosynthesis, research is being conducted on “artificial photosynthesis” to replicate the natural conversion of solar energy. The goal is to use solar energy to facilitate chemical reactions for the production of solar fuels, such as H2 and CH4. One of the main drivers toward this technology is to look for ways to leverage solar energy that is collected during the day to generate fuels that can be used at night.
To find new approaches to artificial photosynthesis, researchers at the John M. Papanikolas Group in the Department of Chemistry at the University of North Carolina at Chapel Hill are studying the energy and electron transfer dynamics of molecular assemblies for improved solar energy conversion.
Exploring molecular assemblies and polymer structures
In one of the John M. Papanikolas Group’s chief research experiments, they explored different multichromophore light-harvesting assemblies and the complex photoinduced dynamics that occur when these assemblies are excited with light. They focused on intramolecular energy and electron transfer processes occurring within molecular assemblies composed of a polymer backbone, with attached isoindigo pendant chromophores. Isoindigo chromophores can absorb a significant amount of light, while the polymer backbones can fill in some of the gaps and enhance the light absorbing properties. The Group compared two different popular conjugated polymers attached with pendants — polyfluorene and isoindigo pendants (PF-iI), and polythiophene and isoindigo pendants (PT-iI).
The Group studied the energy and charge flow within these polymer assemblies by subjecting them to ultrafast laser pulses (less than 200 femtoseconds) directed at the polymer backbone. They then measured the rate of energy and electron transfer to the pendant chromophores to observe the speed of excitation and relaxation of the charge. They conducted the same process on unfunctionalized homopolymers, which have no pendants, to serve as comparison. The researchers averaged 2,000 pulses together with both the presence and absence of the laser pulse for over 100 different time-delays and then repeated this step about four times. This resulted in millions of laser pulses to the polymer backbones, which consequently produced millions of data points that were compiled by a third-party data collection software.
In order to fully understand these complicated dynamics and reveal the assemblies’ microscopic details, the researchers augmented the experiments with a theoretical model applied to polymer structures to simulate the photoinduced dynamics. Because these polymer structures consisted of multiple pendant chromophores, they wanted to examine how different designs, specifically variances in the donor-acceptor distance, orientation and accepter-acceptor distances (pendant density), influenced the rate at which the polymer excited state was quenched. The Group compiled millions of distance distributions and their probabilities of energy and electron transfer by applying a kinetic model to polymer structures obtained from molecular dynamics snapshots in simulation software.
Using ultrafast spectroscopic methods and computational tools, their studies in molecular assemblies resulted in large datasets containing millions of rows. Leaving the data in mountains of spreadsheets and their native platforms was not a viable way to interpret what happened within these molecular assemblies. To make the data more manageable, the Group’s researchers turned to advanced data analysis and graphing software.
Aggregating and visualizing the information
The data analysis and graphing software employed by the John M. Papanikolas Group offered connectivity with its data collection system and simulation software, allowing them to seamlessly drag-and-drop the data for streamlined post-acquisition presentation and analysis. The Group was able to aggregate the data into graphs in order to view the lifetimes of the excited states and extract physical information.
To visualize the transient absorption difference spectra following excitation, they utilized the graphing software’s nonlinear curve fitting functionality to fit the polymer excited state quenching data to mathematical models. The global fitting feature also allowed them to implement simultaneous curve fitting operations on multiple datasets. This can be seen in figure 1, which presents multiple wavelengths to show the change in excitation and relaxation occurring within PF-Hex homopolymers (in blue) and PF-iI polymer assemblies (in red) at various time points. They were also able to zoom in and isolate segments of graphs, as seen in graphs D and E which pull from A and B, respectively, to probe further and examine the shifts in the electronic bands across specific time points.
For comparison, figure 2 looks at the excitation lifetimes of the PT-Hex homopolymer (in blue) and the PT-il polymer assembly (in red).
Moreover, the Group was able to take the millions of molecular dynamics simulation data points and make them comprehendible by presenting them in bar graphs, thereby facilitating their understanding of the relationship between the structure of the polymers and the rate at which the polymer excited state is quenched. This can be seen in figure 3. The donor-acceptor distance distributions (red bars) represent the distance between each monomer unit along the backbone and each pendant isoindigo. The acceptor-acceptor distribution (yellow bars) illustrates the overall density of the pendant isoindigo chromophores, and the lines at the base of the bars represent the “deconvolved” distance distribution (nearest acceptor, second nearest acceptor, third nearest acceptor, etc.), which was created using the deconvolution function in their graphing solution. Determining the nearest neighbor distribution was of particular interest to the researchers, because it describes the probability of finding a pendant at a specific distance from the polymer backbone.
Uncovering the quenching dynamics
The flexibility and prowess of the Group’s graphing software enabled them to expertly present their abundant, experimental data clearly and concisely in a variety of graphic displays. Being able to visualize the data, fostered easier analysis to make conclusive inferences as to how to develop adequate polymer molecular assemblies and structures for improved solar energy conversion.
The curve-fitting graphs produced by the Group allowed them to trace the decay of the rapid quenching of the polymer excited state over a transient period. It was evident that that electron transfer was occurring in both the PF-iI polymer and PT-il polymer assemblies, as both graphs C in figures 1 and 2 show the electron transfer products from photoinduced charge separation. The transient spectrum evolves into what indicates the charge separated states. In particular, for the PF-iI assembly, there is a new band that appears at 580 nm which is attributed to the oxidized PF polymer. Meanwhile, in PT-iI, the spectrum at 1500 picoseconds matches well with what the charge separated state should look like (red line).
The researchers then compared the transient absorption difference spectra graphs of the PF-iI polymer and the PF-Hex homopolymer. The absorption spectrum of the isoindigo overlaps significantly with the emission spectrum of the PF-Hex homopolymer (a condition for resonance energy transfer), which indicates a majority of the excited states decay by energy transfer to the pendants. In the case of the PT-iI polymer and PT-Hex homopolymer, the overlap is reduced but the electronic coupling (strength of interaction) for electron transfer is higher, as almost all the PT excited states decay by electron transfer.
By examining the kinetic simulation data in bar graphs and performing deconvolution, the Group was able to make important observations regarding the design of the polymers for both energy and electron transfer. The Group understood that both processes are fast when the pendant is close to a monomer when the polymer backbone is excited, and that energy transfer is long-range while electron transfer is short-range. As such, the Group could use the distances and orientations obtained from the molecular simulations to compute the energy and electron transfer rates and confirm the simulation data.
They found the density of the isoindigo pendants surrounding a monomer unit had a dramatic effect on the total electron and energy transfer rate. When there are multiple chromophores that are sufficiently close and capable of quenching the polymer excited state, the total energy and electron transfer rates are amplified and can be more than three times the rates found in a simple monomer and single chromophore system. The pendant density graph (yellow bars) illustrated to the Group that pendants tended to be closer in proximity in the PT-iI polymer than in the PF-iI polymer, which they were able to attribute to the shorter size of the thiophene monomer’s backbone units. The Group was able to determine that, for more compact structures with the pendants closer to the backbone, electron transfer dominates. In structures where the pendants are further from the backbone, energy transfer takes over. They determined that energy transfer dominates in PF-iI, and electron transfer is faster in PT-iI.
Continuing the quest for solar energy conversion
The molecular dynamics that the John M. Papanikolas Group was able to discover would not have been possible by solely looking at spreadsheets of data. Their spectroscopic studies into the molecular assemblies and structures for solar energy conversion applications were largely dependent on the graphs produced by their data analysis and graphing software. While the experiment described in this article is only the first iteration towards the development of molecular assemblies that exhibit broad solar absorption and conversion properties, the resulting publication-quality graphs will allow the Group to share their information with colleagues and others in the solar energy field to further additional work. Advanced data analysis and graphing will continue to be instrumental in the research and development of artificial photosynthesis technologies that will not only provide the world energy, but also minimize the environmental impact.
Easwar Iyer is vice president of technology at OriginLab Corporation.
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