11:10 - 11:35InvitedID: 154
COHERENT DYNAMICS AND ELECTRONIC STRUCTURE IN PORPHYRIN NANORINGS EXPLORED BY 2D ELECTRONIC SPECTROSCOPY
1Lund University, Lund, Sweden; 2Vilnius University, Vilnius, Lithuania; 3University of Oxford, Oxford, United Kingdom
Synthetized extended conjugation molecular structures provide excellent model systems for studying excitation delocalization, excitonic and vibronic coupling and internal exciton relaxation. All these phenomena play important roles in the energy-capture function of natural and artificial light-harvesting systems. Here we investigated a ring-shaped fully conjugated molecular system, where six porphyrin molecules are diacetylene-linked, forming a cyclic hexamer – a porphyrin nanoring. Porphyrin nanoring and a template, used in synthesis, form a very stable complex that shows a number of remarkable properties. Near infrared absorption, centered at 800 nm, features three strong well resolved peaks, while fluorescence is observed much further in the infrared region. There is no resemblance of a mirror image between the absorption and fluorescence spectra. Whereas fluorescence spectrum can be well explained in terms of Herzberg-Teller intensity borrowing, it was very little known about the electronic structure of the absorption bands.
Featuring richly structured absorption spectrum, porphyrin nanoring presents an excellent object for the two-dimensional electronic spectroscopy (2DES) studies. 2D spectra of porphyrin nanorings at 77 K temperature have numerous diagonal and off-diagonal peaks, providing direct indication of correlations between electronic/vibronic transitions. Besides population relaxation dynamics, multiple oscillating signals (coherences) were observed in the sequence of the 2D spectra. To explore the coherence signals we used Fourier analysis producing so-called oscillation maps, where each beating frequency is mapped on the excitation and detection frequency plots. By investigating these maps, we examined the nature of oscillating signals and found coherence signals of electronic, vibrational and mixed origin. Analysis of the beating signals with the aid of theoretical modeling allows as determining the origin of all transitions as seen in the linear absorption spectrum .
1. V. Butkus et al., J. Phys. Chem. Lett. 8, 2344 (2017).
11:35 - 12:00InvitedID: 278
QUANTUM THEORY OF TWO-FOLD CHARGE SEPARATION DYNAMICS IN ORGANIC SOLAR CELLS
Institute of Chemical Physics, Physics Faculty, Vilnius University, Lithuania
Photoinduced charge generation in blends of electron donating and accepting organic materials has recently gained special attention due to high energy efficiency. It has been observed that electrons and holes are very effectively generated on the inteface between two phases of the material even in very low externally applied electric fields [1,2]. While picosecond charge migration can be understood as an incoherent hopping process and can be modeled by Monte Carlo statistical process, early subpicosecond dynamics requires complete quantum description.
Stochastic Schroedinger Equation (SSE) approach has been applied to simulate charge coherent dynamics during primary charge separation events after photoexcitation process . The approach combines environment mediated statistical mixture of quantum propagation trajectories in the spirit of path-integral approach and allows to obtain distribution functions of physical variables. Weak system-environment coupling regime demonstrates the classical results of charge hopping. Experimental charge separation kinetics can be obtained in the intermediate coupling regime. We obtain two-fold charge separation kinetics: initial coherent charge separation and later transition into diffusive hopping motion.
The coherent process however finishes in 800 fs after photoexcitation. Efficient charge separation on a short distance at very low external optical fields can be then explained by internal electrostatic fields that may be present due to disordered nature of the blend at the interface. This is confirmed by Monte Carlo simulations.
1. Abramavicius V. et al. Carrier motion in as-spun and annealed P3HT:PCBM blends revealed by ultrafast optical electric field probing and Monte Carlo simulations. Phys. Chem. Chem. Phys. 16, 2686 (2014).
2. Amarasinghe Vithanage, et al. Visualizing charge separation in bulk heterojunction organic solar cells. Nat Commun 4, 2334 (2013).
3. Abramavicius V. et al. Role of coherence and delocalization in photo-induced electron transfer at organic interfaces Sci. Rep. 6, 32914 (2016).
12:00 - 12:25InvitedID: 119
HIGHER-ORDER AND FLUORESCENCE-DETECTED 2D SPECTROSCOPY AND MICROSCOPY OF OPTOELECTRONIC MATERIALS
Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Würzburg, Germany
Coherent two-dimensional (2D) electronic spectroscopy is now an established technique to investigate ultrafast quantum phenomena. It provides frequency resolution both for the excitation and the probe step and thus reveals, for example, state-to-state energy or electron transfer processes in multichromophore systems . Here, several new method variants will be presented that go beyond the conventional implementations and provide additional insight.
We recently developed exciton–exciton-interaction 2D spectroscopy, a fifth-order technique that is sensitive to the interaction of excitons and reveals their spatial propagation . Now we have applied this to study squaraine polymers revealing differences in exciton delocalization and diffusion properties as a function of chain length.
In the interpretation of time-resolved spectroscopic data, one often has to take into account higher electronically excited states. We present multi-quantum fluorescence spectroscopy that explores energies and couplings of higher-lying states. This is achieved via phase cycling and shot-to-shot-based pulse shaping that allows rapid scanning. Apart from two-quantum 2D spectra , we acquire complete three-dimensional data sets within few minutes correlating various zero-, one-, two-, and three-quantum molecular coherences.
Combining fluorescence detection and optical microscopy in a high-NA objective, we realize 2D micro-spectroscopy with spatial resolution that is particularly attractive for heterogeneous materials . We analyzed 2D fluorescence spectra of a MoSe2 monolayer. The observed quantum beating of cross peaks reveals a theoretically predicted  but previously unobserved dark state near the electronically excited state and an additional, unpredicted one near the ground state, activated by exciton–phonon scattering.
 T. Brixner et al., Adv. Energy Mater. 2017, 1700236 (2017)
 J. Dostál et al., Nat. Comm. 9, 2466 (2018)
 S. Mueller et al., J. Phys. Chem. Lett. 9, 1965 (2018)
 S. Goetz et al., Opt. Express 26, 3915 (2018)
 D. Christiansen et al., Phys. Rev. Lett. 119, 187402 (2017)
12:25 - 12:40ID: 120
USING TWO-DIMENSIONAL SPECTROSCOPY TO PROBE RELAXATION, DECOHERENCE AND LOCALIZATION OF PHOTO-EXCITED STATES IN PI-CONJUGATED POLYMERS
University of Oxford, Oxford, United Kingdom
We use the coarse-grained Frenkel-Holstein model to simulate the relaxation, decoherence, and localization of photoexcited states in conformationally disordered pi-conjugated polymers. The dynamics are computed via wave-packet propagation using matrix product states and the time evolution block decimation method. The ultra-fast formation of an exciton-polaron causes exciton decoherence and relaxation, while external dissipation (modelled via quantum jumps) leads to exciton localization from higher-energy quasi-extended states onto local exciton ground states (or chromophores) [1, 2]. We determine the experimental realization of these phenomena by computing the two-dimensional electronic coherence spectroscopy .
 J. R. Mannouch, W. Barford and S. Al-Assam, J. Chem. Phys. 148, 034901 (2018)
 M. Marcus, O. R. Tozer, and W. Barford, J. Chem. Phys. 141, 164102 (2014)
 P. F. Tekavec, G. A. Lott, and A. H. Marcus, J. Chem. Phys. 127, 214307 (2007)
12:40 - 12:55ID: 182
FAST ALGORITHM FOR SIMULATING NONLINEAR ULTRAFAST SPECTROSCOPIES
1Department of Physics, University of Ottawa, Ottawa, Canada; 2School of Electrical Engineering and Computer Science, University of Ottawa, Ottawa, Canada
We present a fast algorithm for calculating the nth-order density matrices required for calculating n-wave mixing signals, which we call Ultrafast Ultrafast (UF2). The algorithm calculates spectra under the same assumptions as the well-known response function formalism, but we show that UF2 is more computationally efficient than any method of which we are aware for energy-transfer systems. The computational speed-ups of UF2 come from working in the eigenbasis of the time evolution operator, which enables costless non-perturbative time evolution at times when the optical pulses are negligible, and performing the perturbative time evolution due to the pulses using the fast Fourier transform and the convolution theorem. UF2 requires that the propagator be time-independent, as in closed systems or Markovian open systems. We also describe an efficient iterative sparse-matrix method for finding the eigenstates of the time evolution of coupled (an)harmonic vibronic systems, with or without a Markovian bath.
UF2 is built around the concept of Feynman diagrams, which provide useful visual representations of the signals that contribute to a given pulse configuration. The code we have released based upon UF2 is designed to allow users to translate Feynman diagrams directly into a few simple lines of code. It is therefore particularly well-suited to calculating higher-order perturbations than are typically considered (6-wave mixing and above). We therefore hope that UF2 will allow people to easily and quickly predict nonlinear optical spectra for a wide range of system parameters and optical pulse shapes.