Cathy Y. Wong

Coherently wired light-harvesting in photosynthetic marine algae at ambient temperature

Photosynthesis makes use of sunlight to convert carbon dioxide into useful biomass and is vital for life on Earth. Crucial components for the photosynthetic process are antenna proteins, which absorb light and transmit the resultant excitation energy between molecules to a reaction centre. The efficiency of these electronic energy transfers has inspired much work on antenna proteins isolated from photosynthetic organisms to uncover the basic mechanisms at play. Intriguingly, recent work has documented that light-absorbing molecules in some photosynthetic proteins capture and transfer energy according to quantum-mechanical probability laws instead of classical laws at temperatures up to 180 K. This contrasts with the long-held view that long-range quantum coherence between molecules cannot be sustained in complex biological systems, even at low temperatures. Here we present two-dimensional photon echo spectroscopy measurements on two evolutionarily related light-harvesting proteins isolated from marine cryptophyte algae, which reveal exceptionally long-lasting excitation oscillations with distinct correlations and anti-correlations even at ambient temperature. These observations provide compelling evidence for quantum-coherent sharing of electronic excitation across the 5-nm-wide proteins under biologically relevant conditions, suggesting that distant molecules within the photosynthetic proteins are ‘wired’ together by quantum coherence for more efficient light-harvesting in cryptophyte marine algae.

Electronic coherence lineshapes reveal hidden excitonic correlations in photosynthetic light harvesting

The effective absorption cross-section of a molecule (acceptor) can be greatly increased by associating it with a cluster of molecules that absorb light and transfer the excitation energy to the acceptor molecule. The basic mechanism of such light harvesting by Förster resonance energy transfer (FRET) is well established, but recent experiments have revealed a new feature whereby excitation is coherently shared among donor and acceptor molecules during FRET. In the present study, two-dimensional electronic spectroscopy was used to examine energy transfer at ambient temperature in a naturally occurring light-harvesting protein (PE545 of the marine cryptophyte alga Rhodomonas sp. strain CS24). Quantum beating was observed across a range of excitation frequencies. The shapes of those features in the two-dimensional spectra were examined. Through simulations, we show that two-dimensional electronic spectroscopy provides a probe of the adiabaticity of the free energy landscape underlying light harvesting.

Ideal dipole approximation fails to predict electronic coupling and energy transfer between semiconducting single-wall carbon nanotubes

The electronic coupling values and approximate energy transfer rates between semiconductor single-wall carbon nanotubes are calculated using two different approximations, the point dipole approximation and the distributed transition monopole approximation, and the results are compared. It is shown that the point dipole approximation fails dramatically at tube separations typically found in nanotube bundles ( approximately 12-16 A) and that the disagreement persists at large tube separations (>100 A, over ten nanotube diameters). When used in Forster resonance energy transfer theory, the coupling between two point transition dipoles is found to overestimate energy transfer rates. It is concluded that the point dipole approximation is inappropriate for use with elongated systems such as carbon nanotubes and that methods which can account for the shape of the particle are more suitable.

Exciton Fine Structure and Spin Relaxation in Semiconductor Colloidal Quantum Dots

Quantum dots (QDs) have discrete quantum states isolated from the environment, making QDs well suited for quantum information processing. In semiconductor QDs, the electron spins can be coherently oriented by photoexcitation using circularly polarized light, creating optical orientation. The optically induced spin orientation could serve as a unit for data storage and processing. Carrier spin orientation is also envisioned to be a key component in a related, though parallel, field of semiconductor spintronics. However, the oriented spin population rapidly loses its coherence by interaction with the environment, thereby erasing the prepared information. Since long-lasting spin orientation is desirable in both areas of investigation, spin relaxation is the central focus of investigation for optimization of device performance. In this Account, we discuss a topic peripherally related to these emerging areas of investigation: exciton fine structure relaxation (EFSR). The radiationless transition occurring in the exciton fine structure not only highlights a novel aspect of QD exciton relaxation but also has implications for carrier spin relaxation in QDs. We focus on examining the EFSR in connection with optical spin orientation and subsequent ultrafast relaxation of electron and hole spin densities in the framework of the exciton fine structure basis. Despite its significance, the study of exciton fine structure in colloidal QDs has been hampered by the experimental challenge arising from inhomogeneous line broadening that obscures the details of closely spaced fine structure states in the frequency domain. In this Account, we show that spin relaxation occurring in the fine structure of CdSe QDs can be probed by a time-domain nonlinear polarization spectroscopy, circumventing the obstacles confronted in the frequency-domain spectroscopy. In particular, by combining polarization sequences of multiple optical pulses with the unique optical selection rules of semiconductors, fast energy relaxation among the QD exciton fine structure states is selectively measured. The measured exciton fine structure relaxation, which is a nanoscale analogue of molecular radiationless transitions, contains direct information on the relaxation of spin densities of electron and hole carriers, that is, spin relaxation in QDs. From the exciton fine structure relaxation rates measured for CdSe nanorods and complex-shaped nanocrystals using nonlinear polarization spectroscopy, we elucidated the implications of QD size and shape on the QD exciton properties as well, for example, size- and shape-scaling laws governing exciton spin flips and how an exciton is delocalized in a QD. We envision that the experimental development and the discoveries of QD exciton properties presented in this Account will inspire further studies toward revealing the characteristics of QD excitons and spin relaxation therein, for example, spin relaxation in QDs made of various materials with different electronic structures, spin relaxation under an external perturbation of QD electronic states using magnetic fields, and spin relaxation of separated electrons and holes in type-II QD heterostructures.

Exciton dynamics reveal aggregates with intermolecular order at hidden interfaces in solution-cast organic semiconducting films

Large-scale organic electronics manufacturing requires solution processing. For small-molecule organic semiconductors, solution processing results in crystalline domains with high charge mobility, but the interfaces between these domains impede charge transport, degrading device performance. Although understanding these interfaces is essential to improve device performance, their intermolecular and electronic structure is unknown: they are smaller than the diffraction limit, are hidden from surface probe techniques, and their nanoscale heterogeneity is not typically resolved using X-ray methods. Here we use transient absorption microscopy to isolate a unique signature of a hidden interface in a TIPS-pentacene thin film, exposing its exciton dynamics and intermolecular structure. Surprisingly, instead of finding an abrupt grain boundary, we reveal that the interface can be composed of nanoscale crystallites interleaved by a web of interfaces that compound decreases in charge mobility. Our novel approach provides critical missing information on interface morphology necessary to correlate solution-processing methods to optimal device performance.

Biexcitonic Fine Structure of CdSe Nanocrystals Probed by Polarization-Dependent Two-Dimensional Photon Echo Spectroscopy

The spectroscopy of colloidal CdSe nanocrystals is investigated using two-dimensional photon echo (2DPE) spectroscopy with copolarized and cross-polarized pulse sequences. Clearly resolved excited state absorption features are observed to beat at the frequency of the longitudinal-optical phonon, and the phase of this beating is found to be polarization-dependent. A simulation is performed using the excitonic and biexcitionic fine structure states predicted by theory, and the polarization-dependent beating allows each feature to be assigned to a particular excited state absorption pathway. Owing to their circularly polarized selection rules, the polarization-dependent 2DPE technique provides valuable insights into the spectroscopy of quantum dots. In particular, transient absorption features observed in pump-probe studies of CdSe quantum dots can now be assigned to specific fine structure transitions to the ground state biexciton.

The isobaric ions CH3OPO+ and CH3OPNH2+ and their neutral counterparts: a tandem mass spectrometry and CBS-QB3 computational study

Abstract In contrast to a previous report [J. Mass Spectrom. 171 (1997) 79], the dissociative electron ionization of acephate does not generate pure m / z 78 ions CH 3 OPO + . By combining the results of exact mass measurements, (multiple) high energy collision experiments and CBS-QB3 calculations, it is concluded that the m / z 78 ions consist of CH 3 OPO + and CH 2 OPOH + in admixture with isobaric CH 5 PON + ions. A computational analysis of the dissociation characteristics of the independently generated stable isomers CH 3 OPNH 2 + and CH 3 P(O)NH 2 + shows that the CH 5 PON + ions from acephate have the structure CH 3 OPNH 2 + . As a consequence, the recovery signal in the reported neutralization–reionization (NR) spectrum of acephate’s m / z 78 ions does not prove the stability of the CH 3 OPO neutral. Definitive evidence for its existence as a stable species in the dilute gas phase comes from a collision-induced dissociative ionization (CIDI) experiment on the reaction C 6 H 5 P(O)OCH 3 + →C 6 H 5 + +CH 3 OPO. The calculations indicate that CH 3 OPNH 2 is also a stable species, but attempts to confirm this by experiment have not yet been successful.

Three-Pulse Photon-Echo Peak Shift Spectroscopy and Its Application for the Study of Solvation and Nanoscale Excitons

An understanding of chemical reactivity begins with an understanding of the dynamics involved regarding system-bath interactions. Spectroscopic studies of these interactions in condensed-phase multichromophoric systems are intricate and contain much information. Photon-echo spectroscopy has proven to be a useful tool for probing these interactions. A description of three-pulse photon-echo peak shift spectroscopy (3PEPS)--theory, experiment, and application--for the study of solvation and dynamics of nanoscale excitons is presented. Also, we discuss how two-dimensional photon-echo spectroscopy (2DPE) relates to 3PEPS and show how 3PEPS data can be extracted from 2D photon-echo data.

Transmission of quantum dot exciton spin states via resonance energy transfer

The mechanism of resonance energy transfer between quantum dots is investigated theoretically. In order to incorporate explicit account of the selection rules for absorption of circularly polarized light, a quantum electrodynamical treatment of the electronic coupling is derived. The electronic coupling is mediated by the exchange of a virtual photon, which in the far zone limit acquires real character and is circularly polarized. The conditions by which quantum information, in terms of exciton spin orientation (total angular momentum quantum number), can be exchanged or switched through resonance energy transfer are discussed. Intrinsic exciton spin flip processes are shown experimentally to compete with typical energy transfer rates. Exciton spin flip times correspondingly range from <100 fs to 1.2 ps are reported.

Quantum-Coherent Energy Transfer in Marine Algae at Ambient Temperature via Ultrafast Photon Echo Studies

Experiments using two-dimensional photon echo spectroscopy reveal that electronic excitations are coherently coupled in a family of light-harvesting antenna proteins isolated from marine cryptophyte algae, thereby influencing energy transfer.