Session L04: Electronic-Vibrational Coupling in Light Harvesting III. Singlet Fission, Upconversion, and Energy Transfer

Semiconductor Junctions for Photon Up and Downconversion: From Nanocrystals to Bulk Solids

Materials that repackage the energy of incoherent light, by either summing its photons together or dividing them into lower energy pairs, offer potential for new technologies for solar energy conversion, photon detection, catalysis, and quantum information science. Organic dimers, polymers, and extended solids that undergo singlet fission offer a potential means for achieving this goal as this process converts high-energy single spin-triplet excitons into pairs of low-energy spin-triplet exciton pairs. Likewise, singlet fission’s inverse, triplet fusion, can be used to combine low-energy exciton pairs into high-energy states. However, designing applications based on these materials necessitates design of both highly stable singlet fission/triplet fusion materials as well as hybrid organic:inorganic junctions that allow triplet excitons to interface with commercial semiconductor technologies. In this presentation, I will review our group’s efforts to produce covalently tethered structures for this purpose. The presentation’s first half will focus on the design of photostable single fission-capable solids based on perylenediimide dyes while its second half will focus on model semiconductor junctions that interface these and related materials with semiconductor quantum dots. Lessons learned from these studies will be used as a basis for designing silicon:organic structures that allow for spin-triplet exciton transfer across their junction.
  

Efficient Triplet Harvesting in an Air-stable Diketopyrrolopyrrole Singlet Fission Solar Cell

Diketopyrrolopyrrole (DPP) is an air-stable molecule which has recently been shown through spectroscopic investigation to host excitonic singlet fission (SF). Here we synthesize DPP-based solar cells using dicyano naphthalene diimide (dCN NDI) acceptor molecules and quantify SF generated carrier extraction efficiency. The characteristic dependence of the photocurrent as a function of magnetic field, which is the unambiguous signature of SF, is observed. Furthermore, we show that DPP-based photovoltaic devices are remarkably air-stable, unencapsulated devices do not exhibit a degradation of electrical or photovoltaic properties as a result of aging. Therefore, DPP is promising material for organic SF photovoltaic devices   

A Kinetic Description of Ultrafast Excitation, Relaxation, and Charge Transfer in Ru Dye-Semiconductor Systems

We describe a predictive kinetic framework for ultrafast photophysics of ruthenium polypyridyl dyes in solution and on solid surfaces to probe how kinetic processes such as absorption, relaxation, and intersystem crossing affect excited state lifetimes. Employing a form of kinetic Monte Carlo that produces an absolute time base, we compute transient absorption (TA) signals and find excellent agreement with experimental spectroscopic data. We compare dye photophysics in solution to that of sensitized metal oxide films, where charge injection from the excited states may occur. Dye molecules have similar excitation and decay kinetics in solution and on ZrO₂ films where there is no charge transfer. In contrast, charge transfer to the semiconductor competes with intramolecular transitions for dyes bound to TiO₂. Comparison of simulated TA spectra to experiment allow rate coefficients for charge injection to be estimated. The kinetic framework is readily integrated into multiscale models for dye-sensitized light harvesting systems.   

Fast algorithm for simulating nonlinear ultrafast spectroscopies

We present Ultrafast Ultrafast (UF²) spectroscopy, a fast algorithm for calculating perturbative n-wave mixing signals in nonlinear optical spectroscopies including arbitrary optical pulse shapes and overlaps. It was demonstrated for closed systems, and we present here the extension to open systems, including vibronic systems with Lindblad or Redfield evolution. UF² is more computationally efficient than any method of which we are aware when the dimension of the relevant system Hilbert space is sufficiently small, as in energy-transfer systems. The speed of UF² comes from working in the eigenbasis of the time evolution operator, which enables costless time evolution between pulses, and performing the time evolution due to the pulses using the FFT and convolution theorem. We characterize the computational cost for a range of system sizes, showing when UF² gives significant speed improvements over other methods.

UF² automatically generates all Feynman diagrams for a specified phase-matching condition. It is well-suited to calculating higher-order signals (6-wave mixing and above). UF² allows easy and quick prediction of nonlinear optical spectra for a wide range of system parameters and optical pulse shapes.
  

Enhancing vibrationally assisted energy transfer via vibrational cooperativity and interference

Vibrationally assisted energy transfer has recently been engineered in a trapped-ion quantum simulator [Gorman el al., Phys. Rev. X 8, 011038 (2018)]. Motivated by this demonstration, we investigate how cooperativity and interference of multiple vibrations enhance energy transfer. This is a step in the quest to reveal the extent of vibrational or vibronic mechanisms behind the phenomenon of long-time coherences in photosynthetic light harvesting systems. By analyzing a donor-bridge-acceptor trimeric chromophore system coupled to two vibrations, we identify features of one-, two-, and even four-phonon transfer processes and signatures of cooperativity and interference that provide maximum enhancement of energy transfer. Our findings of vibrational cooperativity and interference in excited state energy transfer are verifiable in trapped-ion quantum simulators.   

Electrochemical tuning Localized Surface Plasmon Resonance of Au-Ag core-shell nanoparticle for Surface-Enhanced Raman Scattering

Active control of the localized surface plasmon in metallic nanoparticles is of fundamental importance for surface-enhanced Raman spectroscopy (SERS) application. By oxidation-reduction chemistry of Ag-AgCl shells or clusters on the surfaces of AuNP, we induced dielectric-conductive shell or clusters between AuNP and GME. Self-assembled monolayer modified on GME experienced different electromagnetic field produced by surface plasmon of Ag/AuNP or AgCl/AuNP, thus, induced extreme but reversible changes in SERS intensity. Meanwhile, the redox process of Ag-AgCl is confirmed by time-resolved electrochemical current (i-t). Simultaneously SERS and i-t measurement on Ag/AuNP-on-GME show our ability to manipulate the morphology of narrow inter AuNP-GME gaps. With the assistant of Ag shell or cluster on AuNP-on-GME, Raman enhancements can be tuned much more dramatically by electrochemical bias.   

Phonon-induced Spin State Relaxation in spin crossover molecule

Spin crossover (SCO) materials have drawn intense attention lately due to its unique property to switch from low-spin to high-spin state by external stimuli. The relaxation time of the spin state is a critical quantity in all application areas. Whether SCO molecules are in solution or in crystal form, it is crucial to understand the spin dynamics of a single molecule. This study describes a method development that goes beyond our previous work¹ on phonon-assisted electron relaxation processes where the total spin of the system remains unchanged. We discuss using spin-orbit coupling instead of kinetic energy of nuclei as a perturbation between states of different total spins. This method considers different nuclear positions in the configuration space generated by ab-initio molecular dynamics, which provides a suitable description for coupling between the vibrational modes of nuclei and the electron spins. We will demonstrate the dynamics of spin-dependent relaxation using our simulations

1 J. P. Trinastic et al. J. Phys. Chem. C, 119, 2015.   

Reveal the interplay between high frequency and low frequency intramolecular vibrational mode in ultrafast electron transfer reaction

We are presenting a theoretical study of an ultrafast electron transfer reaction by employing the Redfield theory of quantum dissipation dynamics and a model Hamiltonian involving three electronic states and two vibrational modes. Accompanying a pump-probe spectroscopy experiment of N, N'-Bis(2,6-methylphenyl)-3,4,9,10-perylenetetracarboxylic Diimide (PDI) emerging in an electron-donating solvent, we are able to identify the role of vibronic coherence and the timescale separation due to the interplay between the high-frequency mode and the low-frequency mode. We found that vibronic coherences provide a complete blueprint of a three-stage process: an ultrafast ET event, an impulsive response of nuclear coherences (truest manifestation of Born-Oppenheimer approximation), and the relaxation of coherently prepared hot vibrational states. The outcome of this work also provides a potential design principle for preventing detrimental charge recombination in organic photovoltaics.
  

Ultrafast Vibronic Dynamics of Singlet Fission: From Molecular Movies to Wavefunction Projection

The complex dynamics of ultrafast photoinduced reactions such as singlet fission are governed by their evolution along vibronically coupled potential energy surfaces. Here, I will decribe our recent work on both understading this coupling and manipulating it using ultrafast optical spectrscopy. Combining excited-state time-domain Raman spectroscopy and tree-tensor network state simulations, we construct the full 108-atom molecular movie of ultrafast singlet fission in a pentacene dimer, explicitly treating 252 vibrational modes on 5 electronic states. Our combined experimental and theoretical approach reveals the atomic- scale singlet fission mechanism and can be generalized to other ultrafast photoinduced reactions in complex systems. In other singlet fission systems, polydiacetylene and carotenoids, we experimentally demonstrate that S1 state (21Ag-) is a superposition state with strong contributions from spin-entangled pairs of triplet excitons (1(TT)). We further show that optical manipulation of the S1 (21Ag-) wavefunction using triplet absorption transitions allows selective projection of the 1(TT) component into a manifold of spatially separated triplet-pairs with lifetimes enhanced by up to one order of magnitude and whose yield is strongly dependent on the level of inter- chromophore coupling.