Session B25: Chemical Physics of Multichromophores I

Tensor network methods for the simulation of open quantum dynamics in multichromophore systems: Application to singlet fission in novel pentacene dimers

Singlet fission (SF) is an ultrafast process in which a singlet exciton spontaneously converts into a pair of entangled triplet excitons on neighbouring organic molecules. As a mechanism of multiple exciton generation, it has been suggested as a way to increase the efficiency of organic photovoltaic devices, and its underlying photophysics across a wide range of molecules and materials has attracted significant theoretical attention. Recently, a number of studies using ultrafast nonlinear optics have underscored the importance of intramolecular vibrational dynamics in efficient SF systems, prompting a need for methods capable of simulating open quantum dynamics in the presence of highly structured and strongly coupled environments. Here, a combination of ab initio electronic structure techniques and a new tensor-network methodology for simulating open vibronic dynamics is presented and applied to a recently synthesised dimer of pentacene (DP-Mes). We show that ultrafast (300 fs) SF in this system is driven entirely by symmetry breaking vibrations, and our many-body approach enables the real-time identification and tracking of the "functional' vibrational dynamics and the role of the "bath"-like parts of the environment. Deeper analysis of the emerging wave functions points to interesting links between the time at which parts of the environment become relevant to the SF process and the optimal topology of the tensor networks, highlighting the additional insight provided by moving the problem into the natural language of correlated quantum states and how this could lead to simulations of much larger multichromophore systems   

Ultrafast Nonlinear Frequency Generation in Excitonic Systems and the Dynamics of Novel Photosynthetic Pigment Analogs.

Efficient energy transfer in many light harvesting complexes is mediated by excitonic coupling and delocalization. By directly exploiting exciton-exciton interactions using frequency generation ultrafast electronic spectroscopy, we can connect the spatial, temporal and dynamic landscapes of these complex systems. These measurements reveal the relationship between delocalized excitations even in spectrally congested aggregates, providing a novel and generalizable means to understand relaxation in strongly coupled systems. In addition, we will discuss ultrafast nonlinear spectroscopy measurements of new synthetic analogs of highly conserved natural light harvesting pigments. These molecular systems are tunable and redox active, providing new pathways to controllable and efficient energy and charge transfer in artificial light harvesting systems.   

Coherent photoluminescence excitation spectroscopy of semicrystalline polymeric semiconductors

In polymeric semiconductors, the competition between through-bond (intrachain) and through-space (interchain) electronic coupling determines two-dimensional spatial coherence of excitons. The balance of intra- and interchain excitonic coupling depends very sensitively on solid-state microstructure of the polymer film (polycrystalline, semicrystalline with amorphous domains, etc.). Regioregular poly(3-hexylthiophene) has emerged as a model material because its photoluminescence (PL) spectral lineshape reveals intricate information on the magnitude of excitonic coupling, the extent of energetic disorder, and on the extent to which the disordered energy landscape is correlated. I discuss implementation of coherent two-dimensional electronic spectroscopy. We identify cross peaks between 0--0 and 0--1 excitation peaks, and we measure their time evolution, which we interpret within the context of a hybrid HJ aggregate model. By measurement of the homogeneous linewidth in diverse polymer microstructures, we address the nature of optical transitions within such hynbrid aggregate model. These depend strongly on sample processing, and I discuss the relationship between microstructure, steady-state absorption and PL spectral lineshape, and 2D coherent PL excitation spectral lineshapes.   

$\sigma$-SCF: A Direct Energy-targeting Method To Mean-field Excited States

The mean-field solutions of electronic excited states are much less accessible than ground state (e.g.\ Hartree-Fock) solutions. Energy-based optimization methods for excited states, like $\Delta$-SCF, tend to fall into the lowest solution consistent with a given symmetry -- a problem known as ``variational collapse". In this work, we combine the ideas of direct energy-targeting and variance-based optimization in order to describe excited states at the mean-field level. The resulting method, $\sigma$-SCF, has several advantages. First, it allows one to target any desired excited state by specifying a single parameter: a guess of the energy of that state. It can therefore, in principle, find \emph{all} excited states. Second, it avoids variational collapse by using a variance-based, unconstrained local minimization. As a consequence, all states -- ground or excited -- are treated on an equal footing. Third, it provides an alternate approach to locate $\Delta$-SCF solutions that are otherwise hardly accessible by the usual non-aufbau configuration initial guess. We present results for this new method for small atoms (He, Be) and molecules ($\textrm{H}_2$, HF).   

Complex basis functions for molecular resonances: Methodology and applications

The computation of positions and widths of metastable electronic states is a challenge for molecular electronic structure theory because, in addition to the difficulty of the many-body problem, such states obey scattering boundary conditions. These resonances cannot be addressed with naïve application of traditional bound state electronic structure theory. Non-Hermitian electronic structure methods employing complex basis functions is one way that we may rigorously treat resonances within the framework of traditional electronic structure theory. In this talk, I will discuss our recent work in this area including the methodological extension from single determinant SCF-based approaches to highly correlated levels of wavefunction-based theory such as equation of motion coupled cluster and many-body perturbation theory. These approaches provide a hierarchy of theoretical methods for the computation of positions and widths of molecular resonances. Within this framework, we may also examine properties of resonances including the dependence of these parameters on molecular geometry. Some applications of these methods to temporary anions and dianions will also be discussed.   

Evolution from the Plasmon to Exciton State in Atomically Precise Gold Nanoparticles

The evolution from the metallic (or plasmonic) to molecular state in metal nanoparticles constitutes a central question in nanoscience research because of its importance in revealing the origin of metallic bonding and offering fundamental insights into the birth of surface plasmon resonance. Here, we utilize the atomically precise gold nanoparticles protected by thiolate ranging in size from 1 nm to 3.5 nm (including Au$_{\mathrm{25}}$, Au$_{\mathrm{38}}$, Au$_{\mathrm{144}}$, Au$_{\mathrm{333}}$, Au$_{\mathrm{\sim 520}}$, Au$_{\mathrm{\sim 940}})$ and investigate the grand transition from metallic to molecular state by femtosecond transient absorption spectroscopy, as well as the impact of the transition on catalytic reactions. By directly probing the electron-phonon coupling of the gold nanoparticles, we have mapped out that the transition occurs between 2.3 nm (Au$_{\mathrm{333}})$ and 1.7 nm (Au$_{\mathrm{144}})$. This study paves the way for future exploitation of the grand transition and its impact on the physicochemical properties of metal nanoparticles, in particular the applications in energy transfer and utilization.   

A General Theory for Complete Destructive Interference in Molecular Electron Transport

Destructive interference effects in electric current through molecules provide unconventional routes to designing very good insulating materials. Theories developed for predicting these effects rely on knowing aspects of the molecular structure and/or only consider particular ways for the molecule to couple with the electrodes. Herein we present a general theory for describing the existence and robustness of destructive interference effects. Our results also relate interference effects to the isolated molecule’s electronic structure, helping us further develop physical intuition for these effects. Specific examples will be discussed.   

Dynamic and static disorder in supported Pt nanoparticles: when static is not static

Supported Pt nanoparticles (NPs) exhibit anomalous properties such as negative thermal expansion (NTE) and excessive disorder. Previous finite temperature DFT/MD simulations explain these properties,\footnote{F.D. Vila \textit{et al.}, Phys. Rev. B {\bf78}, 121404(R) (2008).} and show that they arise from bonding heterogeneity both near and far from the support. Pt NPs also exhibit large, so-called ``static'' or low T disorder, that decreases with increasing size. For small (0.9 nm) NPs, there is significant mean-square bond disorder $\sigma^2$, and a fit to an Einstein model results in an anomalously high Einstein temperature ($T_E=$298$\pm$25 K {\it vs} 179 K in bulk Pt), comparable to Pt-Pt bond strengths in the isolated Pt dimer, as well as an anomalous Gruneisen parameter. To resolve these puzzles, we decompose the $\sigma^2$ obtained from DFT/MD runs into ``static'', dynamic, and vibrational components. We find that the anomalous behavior stems from a decrease in the so-called ``static'' part with increasing temperature, while the vibrational $\sigma^2$ behaves normally with $T_E \approx $ 179 K. Finally, we discuss the origin of the pseudo-static $\sigma^2$ and Gruneisen parameter, and their temperature dependence, in terms of zero-frequency behavior.   

The Diffusion Process in Small Particles and Brownian Motion

Albert Einstein in 1926 published his book entitled "INVESTIGATIONS ON THE THEORY OF THE BROWNIAN MOVEMENT". He investigated the process of diffusion in an undissociated dilute solution. The diffusion process is subject to Brownian motion. Furthermore, he elucidated the fact that the heat content of a substance will change the position of the single molecules in an irregular fashion. In this paper, I have shown that in order for the displacement of the single molecules to be proportional to the square root of the time, and for $\frac{v2-v1}{\Delta }=\frac{dv}{dx}$, (where v1 and v2 are the concentrations in two cross sections that are separated by a very small distance ), $\int\limits_{-\infty }^\infty {\Phi (\Delta )d\Delta =I} $ and $\frac{I}{\tau }\int\limits_{-\infty }^\infty {\frac{\Delta {}^{2}}{2}} \Phi (\Delta )d\Delta =D$ conditions to hold, then equation (7a) $D=\sqrt {2D} \sqrt \tau $ must be changed to $\Delta =\sqrt {2D} \sqrt \tau $. I have concluded that $D=\sqrt {2D} \sqrt \tau $ is an unintended error, and it has not been amended for almost 90 years in INVESTIGATIONS ON THE THEORY OF THE BROWNIAN MOVEMENT, 1926 publication.   

Fully quantum analysis of photosynthetic coherent energy absorption and transfer

We simulate coherent energy transfer in photosynthetic light harvesting complexes with a diverse array of factors not previously considered together. We simulate both two level monomer and dimer systems being excited from the ground state through either thermal or coherent radiation. In doing so, we investigate the additional effects of a phonon bath on the energy transfer and rise time, simulated via the hierarchy equations of motion (HEOM). We incorporate an antiHermitian component to the Hamiltonian of varying magnitude, simulating excitation transfer to the unsimulated part of the extended system. We also investigate the population rise time and subsequent transfer. We find that the phonon bath can invert the order of excitation of dimeric eigenstates.