Quantum-coherence in energy transfer
Despite a century of study, researchers are still learning about basic physical chemistry by studying how the energy of absorbed light hops from one molecule to another. Our studies range from examining how proteins influence energy transfer at an atomic level of detail to the development of fundamental physical mode ls that help us address questions basic to quantum-mechanics, quantum information science, and organic photovoltaics.
Natural light-harvesting antenna systems are a prominent component in the photosynthetic machinery. Numerous highly absorptive molecules, bound onto a protein scaffold, capture sunlight and funnel that energy to power reaction centers—specialized biological solar cells. While reaction center architecture is basically conserved across a multiplicity of photosynthetic species, light-harvesting antennae exhibit remarkable diversity as well as the ability to adapt to local light conditions and to regulate the operation of reaction centers. Researchers are learning that arrangements of the light-absorbing molecules are not random, but are carefully positioned to optimize flow of energy from the point where sunlight is absorbed to a reaction center. Typically, that energy funnel comprises ~200 molecules and energy is transferred, via a sequence of quantum mechanical energy transfer processes, distances of ~20–100 nm with near unit quantum efficiency.
If researchers could learn how to move energy with such precision and efficiency over comparable distance, then enormous leaps in the development of cheap organic solar cell technology would ensue. This is a problem specific to organic photovoltaic research because excitation energy needs to be transferred to an interface before it can be dissociated into mobile carriers. Hence the active layer can only be as thick as the length scale over which excitation energy can be transferred during its nanosecond lifetime. That distance is known as the exciton diffusion length.
Recent discoveries in our group and at UC Berkeley suggest that light-harvesting in some photosynthetic proteins involves quantum-coherence. This has captured the attention of researches for several reasons. First, it means that quantum mechanical probability laws can prevail over the classical laws of kinetics, allowing the possibility that light-initiated processes can by controlled using the interference principles first described by Schrödinger, Dirac, Feynman, and others. Second, it raises the fascinating question: have these organisms developed quantum-mechanical strategies for light-harvesting to gain an evolutionary advantage?
(a) Gregory D. Scholes “Quantum-coherent electronic energy transfer: Did Nature think of it first?” J. Phys. Chem. Lett. 1, 2–8 (2010). Invited Perspective for the first issue.
(b)Elisabetta Collini, Cathy Y. Wong, Krystyna E. Wilk, Paul M. G. Curmi, Paul Brumer, and Gregory D. Scholes, “Coherently wired light-harvesting in photosynthetic marine algae at ambient temperature” Nature Vol 463, 4 February 2010: doi:10.1038/nature08811.
(c) Elisabetta Collini & Gregory D. Scholes, “Coherent dynamics in resonance energy transfer at room temperature: Quantum-mechanical energy migration along MEH-PPV chains” J. Phys. Chem. A, George Schatz Festschrift Special Issue 113, 4223–4241 (2009).
(d) Elisabetta Collini & Gregory D. Scholes, “Quantum coherent energy migration in a conjugated polymer at room temperature” Science 323, 369-373 (2009).
(e) Silvia E. Braslavsky, Eduard Fron, Hernán B. Rodríguez, Enrique San Román, Gregory D. Scholes, Gerd Schweitzer, Bernard Valeur, Jakob Wirz, “Pitfalls and limitations in the practical use of Förster’s theory of resonance energy transfer” Photochem. Photobiol. Sciences, Forum Article, Nicholas Tully Special Issue 7, 1444–1448 (2008).
Excitons in Nanoscale Systems
Nanoscale systems are forecast to be a means of integrating desirable attributes of both molecular and bulk building blocks into easily processed materials. Such materials would be used to make affordable technologies and even new kinds of devices to enrich our lives. Notable examples include plastic full-color displays, electronic clothing, lasers that can transmit information rapidly within a supercomputer, and organic solar cells to produce low-cost renewable energy. The development, operation, and improvement of these kinds of emerging technologies hinge on the control molecular-scale events that emit light or are initiated by light.
Excitons are of great interest in nanoscience because it has been discovered that their properties can be dictated by the size and shape of a material—as well as its constitution—when at least one dimension is smaller than the usual size of the exciton in an infinite-sized material. The new aspect of excitons that is prevalent—or even that defines—nanoscience is that the physical size and shape of the material is a parameter that significantly influences the properties of excitons. That interests us because (a) Excitons can be engineered in a material by the arrangement of building blocks. (b) The spatial compression of the exciton accentuates many of its interesting physical properties; exposing them for examination. (c) Our understanding is challenged: Perhaps we should not try to force-fit existing theories to model excitations in nanostructures as oftentimes they carry with them assumptions that we have forgotten, or that we erroneously ignore. Nano-scale materials thus provide both a test-bed and an inspiration for new quantum mechanical approaches to the calculation of electronic properties of large systems.
The figure shows excitons and structural size variations on the nanometer length scale. (a) The photosynthetic antenna of purple bacteria, LH2, is an example of a molecular exciton. The absorption spectrum clearly shows the dramatic distinction between the B800 absorption band, arising from essentially “monomeric” bacteriochlorophyll-a (Bchl) molecules, and the red- shifted B850 band that is attributed the optically-bright lower exciton states of the eighteen electronically-coupled Bchl molecules [e.g. Scholes & Fleming, Adv. Chem. Phys. 2005. (b) The size-scaling of polyene properties, for example oligophenylenevinylene oligomers, derives from size-limited delocalization of the molecular orbitals. However, as the length of the chains increases, disorder in the chain conformation impacts the picture for exciton dynamics. Absorption and fluorescence spectra are shown as a function of the number of repeat units. [Reproduced courtesy of Dr. Johannes Gierschner.] (c) Single wall carbon nanotube (CNT) size and “wrapping” determine the exciton energies. Samples contain many different kinds of tubes, therefore optical spectra are markedly inhomogeneously broadened. By scanning excitation wavelengths and recording a map of fluorescence spectra, the emission bands from various different CNTs can be discerned, as shown. [The figure was kindly provided by Dr. Marcus Jones.]. (d) Rather than thinking in terms of delocalizing the wavefunction of a semiconductor through interactions between the unit cells, the small size of the nanocrystal confines the exciton relative to the bulk. Size-dependent absorption spectra of PbS quantum dots [Hines & Scholes, Adv. Mater. 2003] are shown.
(a) G. D. Scholes & G. Rumbles, Nature Materials (2006) 5, 683–696.
(b) Gregory D. Scholes, “Insights into Excitons Confined to Nanoscale Systems: Electron–hole Interaction, Binding Energy and Photodissociation” ACS Nano 2, 523–537 (2008).
Reaction mechanisms in organic chemistry and collective quantum-mechanical changes in electron motions
Mean-field models in chemistry, that is, theories based on orbital energies, symmetries, and shape, have proven to be enormously successful for predicting structure, explaining reactivity, and qualitatively describing spectroscopy. Important examples include the electronegativity principles proposed by Pauling and Mulliken and the rules for orbital symmetry devised by Woodward and Hoffmann. Nonetheless, the correlated motions of electrons in molecules are very significant and may one day be harnessed to expose new modes of chemical reactivity. The goal of our work is to begin to elucidate insights into chemistry beyond mean-field approximations. To this end, an experimental framework that will test and inspire theory needs to be established.
We have demonstrated the application and interpretation of a coherent two-dimensional electronic spectroscopy that measures changes in the electronic interactions among electrons and their many-body correlated response to optical excitation.
(a) Jeongho Kim, Vanessa M. Huxter, Carles Curutchet, Gregory D. Scholes, “Measurement of Electron-Electron Interactions and Correlations using Two-Dimensional Electronic Double-Quantum Coherence Spectroscopy” J. Phys. Chem. A 113, 12122–12133 (2009).
(b) Jeongho Kim, Shaul Mukamel and Gregory D. Scholes, “Two-dimensional Electronic Double-Quantum Coherence Spectroscopy” Acc. Chem. Res., Special Issue on Multidimensional Spectroscopy 42, 1375–1384 (2009).
Ultrafast exciton fine structure relaxation in quantum dots
Many spectroscopic investigations of quantum dots (QDs) are undertaken for colloidal samples, which are typically randomly oriented. In such samples the exciton fine structure is obscured by inhomogeneous line broadening, so individual states cannot be selected by tuning the frequency of the excitation light. Polarized excitation light may allow one to probe the fine structure exciton states with greater selectivity. In that case, resonant nonlinear optical spectroscopy has great potential; the higher the order of the spectroscopy, the more diverse the opportunities for using polarization to gather information from an isotropic ensemble.
Recently we have shown how cross linearly-polarized pulse sequences in three-pulse transient grating experiments form a polarization grating that monitors flipping among populations of quantum dot exciton spin states (those with total angular momentum F = +/-1 and +/-2). Spin flips among those states lead to a decay of the grating, and consequently the diffracted probe signal. The experiment was simulated by solving the equations of motion that dictate the temporal evolution of the third-order density matrix. In the microscopic picture elucidated from the simulations, destructive interference between the third-order polarizations radiated by populations of excitons with flipped and conserved spin states causes the signal decay. Hence this ultrafast laser experiment provides a means to detect the history of the quantum dot exciton states and to examine the relaxtion processes among the lowest exciton levels of quantum dots.
The results of new experiments based on heterodyne detection of the radiated polarization demonstrate the theory unambiguously. Three ultrafast laser pulses interact with the sample (a solution or film of QDs). Each pulse n the sequencehas a vertical (V) or horizontal (H) polarization, and the signal, which is radiated in a fourth direction, is detected through an analyzer set to either V or H. Data, reported for a CdSe quantum dot sample and shown in the figure, test the theory by confirming the sign-dependence of the VHHV signal compared to VHVH and VVVV. It was found that exciton relaxation occurs over at least two characteristic tiome scales, depending strongly on the quantum dot size.
(a) Gregory D. Scholes, “Selection rules for probing biexcitons and electron spin transitions in isotropic quantum dot ensembles,” J. Chem. Phys. 121, 10104-10110 (2004).
(b) Vanessa M. Huxter, Vitalij Kovalevskij, & Gregory D. Scholes, “Dynamics within the exciton fine structure of colloidal CdSe quantum dots,” J. Phys. Chem. B. Letters 109, 20060–20063 (2005).
(c) Gregory D. Scholes, Jeongho Kim, & Cathy Y. Wong, “Exciton spin relaxation in quantum dots measured using ultrafast transient polarization grating spectroscopy” Phys. Rev. B (2006) 73, 195325.
(d) Gregory D. Scholes, Jeongho Kim, Cathy Y. Wong, Vanessa M. Huxter, P. Sreekumari Nair, Karolina P. Fritz, & Sandeep Kumar, “Nanoscale shape and the mechanism of exciton spin relaxation ” Nanolett. (2006) 6, 1765–1771.
(e) Jeongho Kim, Cathy Y. Wong & Gregory D. Scholes, “Exciton Fine Structure and Spin Relaxation in Semiconductor Colloidal Quantum Dots” Acc. Chem. Res. 42, 1037–1046 (2009).
(f) Jun He, Shun S. Lo, Jeongho Kim, & Gregory D. Scholes, “Control of exciton spin relaxation by electron-hole decoupling in type-II nanocrystal heterostructures” Nano Letters 8, 4007–4013 (2008).
Semiconductor nanocrystal heterostructures and charge transfer
Inspiring work from a number of groups in recent years has paved the way to shape-control of colloidal semiconductor nanocrystals. The opportunity we see is that now theway that light-initiated dynamics can be controlled in heterostructures, and ths is a major thrust of our recent work.
Much is known about electron transfer reactions in supramolecular systems, but hardly anything is known about nanoscale analogs. We are designing, synthesizing, and studying light-initiated charge transfer reactions in sophisticated nanocrystal heterostructures (predominantly CdSe–CdTe systems) to study the mechanism and dynamics of charge transfer as well as applications in solar cells.
(a) Haizheng Zhong and Gregory D. Scholes, “Shape Tuning of Type II CdTe-CdSe Colloidal Nanocrystal Heterostructures through Seeded Growth” J. Am. Chem. Soc. (Commun.) 131, 9170–9191 (2009).
(b) Sandeep Kumar, Marcus Jones, Shun S. Lo, & Gregory D. Scholes, “Nanorod Heterostructures Showing Photo-induced Charge Separation” Small (2007) 3, 1633–1639.
(c) Shun S. Lo, Yaser Khan, Marcus Jones, Gregory D. Scholes, “Temperature and solvent dependence of CdTe/CdSe heterostructure nanorod spectra” J. Chem. Phys. 131, 084714 (2009).
(d) Gregory D. Scholes, Marcus Jones, and Sandeep Kumar, “Energetics of Photoinduced Electron-Transfer Reactions Decided by Quantum Confinement” J. Phys. Chem. C 111, 13777–13785 (2007).
(e) Marcus Jones, Shun S. Lo, & Gregory D. Scholes, “Quantitative modeling of the role of surface traps in CdSe/CdS/ZnS nanocrystal photoluminescence decay dynamics” Proc. Natl. Acad. Sci. USA 106, 3011–3016 (2009).