| Abstract [eng] |
Light harvesting molecular aggregates, such as pigment-protein complexes, are fundamental components of photosynthetic apparatus, utilized by plants, algae, and some bacteria to convert sunlight into chemical energy. Understanding the structure, function, and excitation quantum dynamics in these molecular complexes is crucial to unravel the fundamental principles of solar energy conversion in Nature and for developing efficient synthetic solar cells. During photosynthesis, these complexes collect sunlight energy in specialized light-harvesting antennas and supply energy to the “reaction centers”, where the excitation energy is used to separate the charges. Chlorophylls are the basic pigments that can be found in numerous structures. One of the most essential photosynthetic aggregates is the water-soluble chlorophyll-binding protein (WSCP), which is a ubiquitous molecular complex found in various photosynthetic organisms, including cyanobacteria, algae, and higher plants. Systems like WSCP, exhibit complex excitation quantum dynamics and are usually modeled using the Frenkel exciton model coupled to harmonic vibrational modes, representing the quantum system and its environment. A number of methods have been developed to describe quantum dynamics and one such method, which we will be applying here, is the Time-Dependent Dirac–Frenkel Variational Principle. This method is used in quantum mechanics to calculate the time-evolution of a quantum system based on the chosen trial wavefunction. Usually, trial wavefunction is one of the Davydov ansatze, which is a group of wavefunctions that utilize Gaussian wavepackets, also known as coherent states, to represent vibronic states of molecular aggregates. In reality, wavepackets are not as simple as the Gaussians, and the relationship between electrons and vibrations is often non-linear. Additionally, the accuracy of the method is also heavily dependent on the choice of the trial wavefunction. One approach to enhance the accuracy is to substitute the coherent state with squeezed coherent states, which have extra degrees of freedom (DOFs) that enable the wavepacket to contract and expand in its phase space along both coordinate and momentum axes. To comprehensively investigate the excitation dynamics in photosynthetic pigment-proteins aggregates, a diverse range of spectroscopic methods can be employed experimentally. One of the simplest and most widely used is linear absorption spectroscopy. The absorption spectra can provide valuable information about excitation energies and other photophysical properties. A more advanced technique called two-dimensional electronic spectroscopy (2DES) was developed relatively recently and has become an essential tool for studying various systems in physics, chemistry, and biology. 2DES provides several benefits over traditional one-dimensional spectroscopy, such as additional temporal resolution, enhanced signal-to-noise ratio, and the capability to distinguish overlapping transitions, among others. We have found that when considering quadratic electron-vibration coupling, it becomes necessary to use a more accurate representation of the wave function, such as the squeezed coherent state. The experimental WSCP absorption spectrum shows a blue shift when the temperature is lowered to 77 K compared to the absorption spectrum at room temperature. To replicate this effect, it is necessary to include squeezed coherent states in a trial wavefunction and quadratic coupling effect with 2% in frequency difference. 2DES spectrum allows to fine-tune static disorder parameter, which at 77 K was estimated to be 150 cm−1.Our calculations on WSCP 2DES spectrum reveal four distinct excitonic peaks only at 4.5 K temperature. The dynamics of our model accurately capture the experimental 2DES when we vary the waiting time. This indicates that the interaction with phonons is properly accounted for in our model. |
