On the Role of the Special Pair in Photosystems as a Charge Transfer Rectifier

Abstract

The special pair, a bacteriochlorophyll a (BChl) dimer found at the core of bacterial reaction centers, is known to play a key role in the functionality of photosystems as a precursor to the photosynthesis process. In this paper, we analyze the inherent affinity of the special pair to rectify the intrapair photo-induced charge transfer (CT). In particular, we show that the molecular environment affects the nuclear geometry, resulting in symmetry breaking between the two possible intrapair CT processes. To this end, we study the relationships of the intrapair CT and the molecular geometry with respect to the effective dielectric constant provided by the molecular environment. We identify the special pair structural feature that breaks the symmetry between the two molecules, leading to CT rectification. Excited state energies, oscillator strengths, and electronic coupling values are obtained via time-dependent density functional theory, employing a recently developed framework based on a screened range-separated hybrid functional within a polarizable continuum model (SRSH-PCM). We analyze the rectification capability of the special pair by calculating the CT rates using a first-principles-based Fermi's golden rule approach.

Vibronic structure of photosynthetic pigments probed by polarized two-dimensional electronic spectroscopy andab initiocalculations

“…The SRSH-PCM was recently shown to compare well with ionization energies measured in thin-film environments 62. Very recently we used the same protocol to explain the fine spectral splitting of the central pigments in bacterial reaction center64 and to calculate solvated charge-transfer state's energies 63. A related approach, where the α and β tuning was performed without the C-PCM environment, was recently successfully applied to calculate spectral properties 104…”

“…This SRSH-PCM approach has been shown to provide quantitatively more accurate orbital energies in the condensed phase than those by simpler RSH-PCM combinations 62. Recent benchmarking of using SRSH-PCM in time-dependent density functional theory (TDDFT) calculations of excited states also presented success in addressing condensed phase effects,63 in particular for the study of related pigments 64…”

“…62 Recent benchmarking of using SRSH-PCM in time-dependent density functional theory (TDDFT) calculations of excited states also presented success in addressing condensed phase effects, 63 in particular for the study of related pigments. 64 To elucidate the origin of spectral differences and to quantify the limitations of the Gouterman model, we calculate the excited states in both penta-and hexa-coordinated compounds. Through combined studies of P-2DES with TDDFT, a deeper understanding of the electronic structure and relaxation mechanisms underlying the Q band in both Bchl a and Chl a is obtained.…”

Intersystem Crossing in Tetrapyrrolic Macrocycles. A First-Principles Analysis

“…We employ the recently developed and well benchmarked polarization-consistent SRSH-PCM − scheme to calculate the potential energy surfaces used to obtain ISC rate constants at a fully quantum mechanical level of theory. The SRSH-PCM scheme is a relatively novel framework that was shown to achieve unique insight for several related pigments. − Importantly, we follow our well established protocol based on Fermi’s golden rule (FGR) rate theory that goes beyond the semiclassical limit of the widely employed Marcus theory. − In earlier studies, FGR ISC rates in porphyrinic rings have been found to be significantly larger than the ones obtained using approximated expressions …”

Electronic Spectra of C60 Films Using Screened Range Separated Hybrid Functionals

“…More recently, a range-separated hybrid (RSH) functional was used to study excited state properties of single C 60 and C 60 clusters . In this study we calculate the electronic spectra of C 60 thin films using a TDDFT approach in combination with a polarizable continuum model (PCM) that accounts properly for the effect of the electrostatic environment. , We use a dielectric environment screening protocol that combines a screened range separated hybrid (SRSH) functional with PCM . Below we show that single molecule based TDDFT calculations of the electronic spectra using SRSH-PCM reproduce the three intense spectral features that are associated with unit-localized excited states and that a model consisting of at least two units is needed to describe the red-shifted, low-energy, excitonic spectral feature.…”