7 research outputs found
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Design Principles for Two-Dimensional Molecular Aggregates Using Kasha's Model: Tunable Photophysics in Near and Short-Wave Infrared
Technologies
which utilize near-infrared (700 – 1000 nm) and short-wave infrared (1000 –
2000 nm) electromagnetic radiation have applications in deep-tissue imaging,
telecommunications and satellite telemetry due to low scattering and decreased
background signal in this spectral region. It is therefore necessary to develop
materials that absorb light efficiently beyond 1000 nm. Transition dipole
moment coupling (e.g. J-aggregation) allows for redshifted excitonic states and
provides a pathway to highly absorptive electronic states in the infrared. We present aggregates of two cyanine dyes whose
absorption peaks redshift dramatically upon aggregation in water from ~800
nm to 1000 nm and 1050 nm respectively with sheet-like morphologies and high
molar absorptivities (e ~ 105 M-1cm-1). We use Frenkel exciton theory to extend
Kasha’s model for J and H aggregation and describe the excitonic states of
2-dimensional aggregates whose slip is controlled by steric hindrance in the
assembled structure. A consequence of the increased dimensionality is the
phenomenon of an intermediate “I-aggregate”, one which redshifts yet displays
spectral signatures of band-edge dark states akin to an H-aggregate. We
distinguish between H-, I- and J-aggregates by showing the relative position of
the bright (absorptive) state within the density of states using temperature
dependent spectroscopy. I-aggregates hold potential for applications as charge
injection moieties for semiconductors and donors for energy transfer in NIR and
SWIR. Our results can be used to better design chromophores with predictable
and tunable aggregation with new photophysical properties
Stochastically Realized Observables for Excitonic Molecular Aggregates
We show that a stochastic approach enables calculations of the optical
properties of large 2-dimensional and nanotubular excitonic molecular
aggregates. Previous studies of such systems relied on numerically
diagonalizing the dense and disordered Frenkel Hamiltonian, which scales
approximately as for dye molecules. Our approach scales
much more efficiently as , enabling quick study of
systems with a million of coupled molecules on the micron size scale. We
calculate several important experimental observable including the optical
absorption spectrum and density of states, and develop a stochastic formalism
for the participation ratio. Quantitative agreement with traditional matrix
diagonalization methods is demonstrated for both small- and intermediate-size
systems. The stochastic methodology enables the study of the effects of
spatial-correlation in site energies on the optical signatures of large 2D
aggregates. Our results demonstrate that stochastic methods present a path
forward for screening structural parameters and validating experiments and
theoretical predictions in large excitonic aggregates.Comment: 11 pages, 7 figures, as submitted to JP
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Stochastically Realized Observables for Excitonic Molecular Aggregates.
We show that a stochastic approach enables calculations of the optical properties of large 2-dimensional and nanotubular excitonic molecular aggregates. Previous studies of such systems relied on numerically diagonalizing the dense and disordered Frenkel Hamiltonian, which scales approximately as O(N3) for N dye molecules. Our approach scales much more efficiently as O(Nlog(N)), enabling quick study of systems with a million of coupled molecules on the micrometer size scale. We calculate several important experimental observables, including the optical absorption spectrum and density of states, and develop a stochastic formalism for the participation ratio. Quantitative agreement with traditional matrix diagonalization methods is demonstrated for both small- and intermediate-size systems. The stochastic methodology enables the study of the effects of spatial-correlation in site energies on the optical signatures of large 2D aggregates. Our results demonstrate that stochastic methods present a path forward for screening structural parameters and validating experiments and theoretical predictions in large excitonic aggregates
Thermodynamic Control over Molecular Aggregate Assembly Enables Tunable Excitonic Properties across the Visible and Near-Infrared
Bridging the gap between H- and J-aggregates: Classification and supramolecular tunability for excitonic band structures in two-dimensional molecular aggregates
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De novo design of proteins housing excitonically coupled chlorophyll special pairs
Natural photosystems couple light harvesting to charge separation using a 'special pair' of chlorophyll molecules that accepts excitation energy from the antenna and initiates an electron-transfer cascade. To investigate the photophysics of special pairs independently of the complexities of native photosynthetic proteins, and as a first step toward creating synthetic photosystems for new energy conversion technologies, we designed C2-symmetric proteins that hold two chlorophyll molecules in closely juxtaposed arrangements. X-ray crystallography confirmed that one designed protein binds two chlorophylls in the same orientation as native special pairs, whereas a second designed protein positions them in a previously unseen geometry. Spectroscopy revealed that the chlorophylls are excitonically coupled, and fluorescence lifetime imaging demonstrated energy transfer. The cryo-electron microscopy structure of a designed 24-chlorophyll octahedral nanocage with a special pair on each edge closely matched the design model. The results suggest that the de novo design of artificial photosynthetic systems is within reach of current computational methods