Optical excitations of chlorophyll a and b monomers and dimers

Journal of Chemical Physics, 2019

A necessary first step in the development of technologies such as artificial photosynthesis is understanding the photoexcitation process within the basic building blocks of naturally-occurring light harvesting complexes (LHCs). The most important of these building blocks in biological LHCs such as LHC II from green plants are the chlorophyll a (Chl a) and chlorophyll b (Chl b) chromophores dispersed throughout the protein matrix. However, efforts to describe such systems are still hampered by the lack of computationally efficient and accurate methods that are able to describe optical absorption in large biomolecules. In this work we employ a highly efficient linear combination of atomic orbitals (LCAOs) to represent the Kohn-Sham (KS) wave functions at the density functional theory (DFT) level and perform time dependent density functional theory (TDDFT) in either the reciprocal space and frequency domain (LCAO-TDDFT-k-ω) or real space and time domain (LCAO-TDDFT-r-t) calculations of the optical absorption spectra of Chl a and b monomers and dimers. We find our LCAO-TDDFT-k-ω and LCAO-TDDFT-r-t calculations reproduce results obtained with a plane wave (PW) representation of the KS wave functions (PW-TDDFT-k-ω), but with a significant reduction in computational effort. Moreover, by applying the GLLB-SC derivative discontinuity correction ∆ x to the KS eigenenergies, with both LCAO-TDDFT-k-ω and LCAO-TDDFT-r-t methods we are able to semi-quantitatively reproduce the experimentally measured photoinduced dissociation (PID) results. This work opens the path to first principles calculations of optical excitations in macromolecular systems.

We present an approximate approach for the simulation of UV/vis spectra using conventional (non-TD) DFT computations. It uses Kohn-Sham orbitals and orbital energies to estimate both the excitation energies and the associated oscillator strengths. For a wide range of systems from small molecules to large molecular dyes used in electrochromic and solar-cell applications, reasonable UV/vis spectra are generated, each with just two conventional DFT computations. The accuracy is generally comparable to what one would expect from TD-DFT calculations. In comparison to TD-DFT, the protocol of the present study provides an intuitive and notably more rapid means for simulating electronic absorption properties. It enables efficient screening of materials for a wide range of relevant applications.

The ability to accurately compute low-energy excited states of chlorophylls is critically important for understanding the vital roles they play in light harvesting, energy transfer, and photosynthetic charge separation. The challenge for quantum chemical methods arises both from the intrinsic complexity of the electronic structure problem and, in the case of biological models, from the need to account for protein−pigment interactions. In this work, we report electronic structure calculations of unprecedented accuracy for the low-energy excited states in the Q and B bands of chlorophyll a. This is achieved by using the newly developed domainbased local pair natural orbital (DLPNO) implementation of the similarity transformed equation of motion coupled cluster theory with single and double excitations (STEOM-CCSD) in combination with sufficiently large and flexible basis sets. The results of our DLPNO− STEOM-CCSD calculations are compared with more approximate approaches. The results demonstrate that, in contrast to timedependent density functional theory, the DLPNO−STEOM-CCSD method provides a balanced performance for both absorption bands. In addition to vertical excitation energies, we have calculated the vibronic spectrum for the Q and B bands through a combination of DLPNO−STEOM-CCSD and ground-state density functional theory frequency calculations. These results serve as a basis for comparison with gas-phase experiments.

Physical Review Letters, 2000

The theory of dissipative exciton motion in chromophore complexes is applied to develop an approximate scheme for the simulation of frequency-domain linear absorption and circular dichroism. Besides lifetime broadening of the exciton lines and the inclusion of vibrational satellites in the spectra, the computations also account for static disorder. In applying the theory to a pigment protein complex of the photosynthetic light harvesting complex LHC-II of green plants the temperature dependence of linear absorption can be well reproduced. PACS numbers: 87.15.Aa In simulating the optical absorption of dye aggregates or chromophore complexes, one is confronted with the interplay of two distinct interactions: the coupling of electronic excitations to vibrational degrees of freedom and the interaction among the electronic excitations. Combining both couplings in a proper way should enable one to relate the absorption line shape to structural data.

Journal of Chemical Theory and Computation, 2015

We assess the performance of real-time timedependent density functional theory (RT-TDDFT) for the calculation of absorption spectra of 12 organic dye molecules relevant to photovoltaics and dye-sensitized solar cells with 8 exchange-correlation functionals (3 traditional, 3 global hybrids, and 2 range-separated hybrids). We compare the calculations with traditional linear-response (LR) TDDFT and experimental spectra. In addition, we demonstrate the efficacy of the RT-TDDFT approach to calculate wide absorption spectra of two large chromophores relevant to photovoltaics and molecular switches. RT-TDDFT generally requires longer simulation times, compared to LR-TDDFT, for absorption spectra of small systems. However, it becomes more effective for the calculation of wide absorption spectra of large molecular complexes and systems with very high densities of states.

ITB Journal of Sciences, 2012

Chlorophyll a is the most abundant pigment on Earth responsible for trapping light energy to perform photosynthesis in green plants. This molecule has been studied for many years from different points of interest with both experimental and theoretical methods. In this study, the Restricted Hartree-Fock /Configuration Interaction Single (RHF/CIS), Time-Dependent Density Functional Theory (TDDFT), and several semi-empirical methods (CNDO/S and ZINDO) calculations were carried out to reconstruct the UV-Vis absorption spectra of chlorophyll a. To some extent, the calculation results based on the single-molecule approach succeeded to reconstruct the absorption spectra, but they required to be rescaled to fit the experimental results. In general, the semiempirical methods provide a better energy scaling factor. However, they lack vertical transition fine features with respect to the spectrum obtained experimentally. In this case, the ab initio calculations provided more complete features, especially the TDDFT with high-level basis sets, which also has a good accuracy with regards to the transition energies. The contribution of the ground state and excited state orbitals in the main vertical transitions is discussed based on the delocalized nature of the wave functions and the presence of solvents using the polarizable continuum model (PCM).

Comptes Rendus Physique, 2009

Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics), 2009

Mechanism of photoswitching in diarylethenes involves the lightinitiated symmetry-allowed disrotatory electrocyclic reaction. Here we propose a computationally inexpensive Density Functional Theory (DFT) based method that is able to produce accurate potential surfaces for the excited states. The method includes constrained optimization of the geometry for the ground and two excited singlet states along the ring-closing reaction coordinate using the Slater Transition State method, followed by single-point energy evaluation. The ground state energy is calculated with the broken-symmetry unrestricted Kohn-Sham formalism (UDFT). The first excited state energy is obtained by adding the UDFT ground state energy to the excitation energy of the pure singlet obtained in the linear response Time-Dependent (TD) DFT restricted Kohn-Sham formalism. The excitation energy of the double excited state is calculated using a recently proposed (Mikhailov, I. A.; Tafur, S.; Masunov, A. E. Phys. Rev. A 77, 012510, 2008) a posteriori Tamm-Dancoff approximation to the second order response TD-DFT.

The Journal of Chemical Physics, 2004

Annual Reports in Computational Chemistry, 2019

Herein we highlight recent studies and active areas of interest in the ongoing challenge to model photochemical processes in a wide variety of molecules. We also discuss recent significant methodological improvements and developments that may aid future investigations. Studies using the wide range of techniques available in modern electronic structure software packages have been included, with their successes and shortcomings forming part of the discussion. This study should therefore aid in the design of future computational studies.