Controlling the efficiency of an artificial light-harvesting complex

Abstract

Adaptive femtosecond pulse shaping in an evolutionary learning loop is applied to a bioinspired dyad molecule that closely mimics the early-time photophysics of the light-harvesting complex 2 (LH2) photosynthetic antenna complex. Control over the branching ratio between the two competing pathways for energy flow, internal conversion (IC) and energy transfer (ET), is realized. We show that by pulse shaping it is possible to increase independently the relative yield of both channels, ET and IC. The optimization results are analyzed by using Fourier analysis, which gives direct insight to the mechanism featuring quantum interference of a low-frequency mode. The results from the closed-loop experiments are repeatable and robust and demonstrate the power of coherent control experiments as a spectroscopic tool (i.e., quantum-control spectroscopy) capable of revealing functionally relevant molecular properties that are hidden from conventional techniques. coherent control ͉ energy transfer ͉ quantum-control spectroscopy ͉ artificial photosynthesis A rtificial photosynthesis is an important challenge of science and technology today. Numerous applications include solar cells and other artificial power sources, light-emitting materials, sensor systems, and other electronic and photonic nanodevices that use the conversion of light energy into chemical potentials (1). Over the last decade, major technological advances have been made by using biomimicry, an approach that makes use of teachings from studies on nature's wide-ranging selection of highly efficient pigment-protein complexes (2). It has been shown that integrating light-harvesting antennae with electrontransfer relay systems is a potent way to emulate photosynthesis (3). Thus, biomimicry has inspired systems based on complicated natural light-harvesting complexes (LHCs) reduced to their basic elements, and efficient antenna systems based on polymer polyenes covalently attached to tetrapyrroles have been synthesized (4, 5).The antennae are responsible for the first step of photosynthesis, capturing energy of the sun and transferring it to subsequent photosynthetic structures where the energy is transformed in chemical potential. Within various natural and synthetic LHCs, blue-green photons are absorbed by carotenoid molecules, from which the energy is transferred to neighboring porphyrin molecules (6). This energy transfer (ET) step from the carotenoid donor to the accepting molecular species is the primary process in using energy in the 450-to 550-nm window and contributes significantly to the functioning of the complex. The efficiency of ET over competing loss processes, such as internal conversion (IC), is a crucial factor in the overall quantum yield of (artificial) photosynthesis. Hence, a high priority is given to understanding the mechanisms of energy flow and mediating processes to allow development of more efficient artificial systems.In this study, we use adaptive femtosecond pulse shaping in a learning loop (7, 8) to control the pathways of energy flow i...

Non-resonant dynamic stark control of vibrational motion with optimized laser pulses

“…In 1992 Judson and Rabitz 7 described a novel method of combining adjustable spectral pulse shapers 8,9 with evolutionary algorithms 10 in closed-loop schemes that could generate shaped laser pulses for driving molecular systems into desired states. While this technique has been demonstrated and employed successfully in a number of experimental applications, 5,[11][12][13][14][15][16] the complexity of the systems, and of the pulses that are generated via this kind of "black box" optimization approach, makes it difficult to ascertain the mechanisms behind the interaction dynamics.…”

Quantum Control of Population Transfer and Vibrational States via Chirped Pulses in Four Level Density Matrix Equations

“…A central aspect in both approaches, CC and QCS, is the control of population transfer between the ground and excited states, as well as the generation of vibrational coherence in both potential surfaces. By controlling the population transfer or by suppressing specific molecular vibrational coherences, a photochemical reaction channel can be selectively chosen [27,29,32,[37][38][39][40][41][42][43][44][45][46][47][48][49], or a certain mode in a multidimensional time-resolved signal can be suppressed [37,[50][51][52][53][54][55].…”

Searching for quantum optimal controls under severe constraints

“…Successful optimal control experiments (OCEs) have included selective control of molecular vibrational [10][11][12][13][14][15][16][17] and electronic states [18][19][20][21][22][23][24][25][26][27], preservation of quantum coherence [28,29], control of photoisomerization reactions [30][31][32][33][34][35], selective manipulation of chemical bonds [36][37][38][39][40][41][42][43][44], high-harmonic generation and coherent manipulation of the resulting soft X-rays [45][46][47][48][49][50][51], and control of energy flow in biomolecular complexes [52][53][54][55]. Optimal control theory (OCT) [7,9,…”