Past Work of PI

Two-dimensional electronic spectroscopy has been conventionally implemented in a non-collinear or partially collinear approach which typically spatially average over ~1E10 chromophores over a spatial area of ~1E4 micron square. This has limited the ability of multidimensional spectroscopies to characterize spatially heterogeneous samples or small energetic ensembles.

We built a variant of two-dimensional electronic spectroscopy which combines femtosecond time-resolution, sub-micron spatial resolution, and the sensitivity of fluorescence detection. The capabilities of the spectrometer were demonstrated on a spatially heterogeneous mixture of photosynthetic bacteria that are known to exhibit variations in electronic structure with growth conditions. The measured two-dimensional contour maps exhibited spatially varying peak intensities which mapped spatial variations in the constitution of mixed bacterial colonies, ultimately reflecting the structure of their excitonic manifold. The sensitivity of the technique was estimated to be orders of magnitude better than conventional spatially-averaged electronic spectroscopies.

Besides spatially-resolved measurements, strong time zero cross-peaks highlighted previous unobserved strong correlations between the protein excitonic manifolds, even before energy transfer between the manifolds occurs.

The spectrometer was also used to study a bacteriochlorin dyad, a toy model for photosynthetic energy and charge transfer. Room temperature quantum coherences were observed, which could be unequivocally assigned as purely vibrational coherences.

Other than photosynthetic cells, this approach can address long standing questions, pertaining to ultrafast exciton delocalization, in a wide variety of morphologically heterogeneous samples such as organic photovoltaic thin films and perovskite thin films.

During photosynthesis, antenna proteins position light absorbing pigments so that they couple and direct the transfer of electronic energy toward a reaction center that stores the energy chemically. This process of electronic energy transfer from an antenna towards a reaction center occurs within several tens of picoseconds with a remarkable quantum efficiency of near unity. Although significant advances in technology and quantum physics have helped researchers better understand this remarkably fast and efficient process, the underlying mechanisms and biological designs have proven elusive, with experiments even inspiring serious speculation that photosynthetic antennas function as quantum computers, exploiting quantum mechanical states in which the energy is simultaneously in more than one place (sort of like Schrodinger’s simultaneously dead and alive cat).

We examined the role of non-adiabatic dynamics in photosynthesis: energy transfer driven by “weak” vibrations present in photosynthetic pigments, such as the bacteriochlorophyll a pigments present in the FMO antenna complex of the green sulfur bacteria. Using a dimer model, which describes the physics of coupled nuclear and electronic motion, we showed that when the electronic energy gap matches a vibrational frequency, two coupled pigments can vibrate out of phase and give rise to a “mixing” of excited states. This resonant mixing gives rise to ground state vibrational wavepackets that have the characteristic signatures of quantum coherence and energy transfer as well as key features not reproduced by previous models. Furthermore, the wavepacket frequencies match the known electronic energy gaps in a number of photosynthetic antennas including the bacteriochlorophyll a pigment.

The findings argue strongly that resonant non-adiabatic coupling represents an important component of photosynthetic energy transfer, and are now widely accepted in the photosynthesis spectroscopy community. The above model of resonant vibronic mixing has also carried over to organic photovoltaic and singlet exciton fission communities.

Nature’s use of a vibration to drive energy flow between pigments on a femtosecond (1 fs = 1E-15 s = 1 millionth of a billionth of a second) timescale may be applicable to exciton delocalization and dissociation in artificial light harvesting systems such as organic solar cells. With a choice of chromophores with a large number of weakly coupled Franck-Condon active vibrations, an artificially designed resonance between the vibrations and electronic energy gaps can serve as a design parameter which exploits non-adiabatic dynamics to achieve ultrafast funneling of energy directed towards the lowest energy exciton.