Abstract
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 domain-based 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 time-dependent 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.
Highlights
Chlorophylls are photosynthetic pigments that play crucial roles in the absorption of sunlight, excitation energy transfer, and primary charge separation in photosynthetic organisms.[1−4] Their low-energy electronic transitions comprise the Q band and the B band, known as the Soret band
The accurate calculation of the low-energy spectrum of the chlorophyll excited states has been a long-standing target of quantum chemistry.[20−22] A wide variety of computational investigations have been used for computing excitation energies and absorption profiles, ranging from time-dependent density functional theory (TD-DFT)[23−30] and DFT/multireference configuration interaction (MRCI)[31] to various wave function methods such as SAC-CI,[32] CC2,33,34 and ADC(2).[34]
Most theoretical studies of chlorophylls have dealt with solvated, or in general axially coordinated, systems.[16,99−108] to disentangle the two problems and be able to focus on the electronic structure itself, it is useful to have experimental gas-phase values that can serve as reference for quantum chemical calculations
Summary
Chlorophylls are photosynthetic pigments that play crucial roles in the absorption of sunlight, excitation energy transfer, and primary charge separation in photosynthetic organisms.[1−4] Their low-energy electronic transitions comprise the Q band (including the Qy and Qx transitions) and the B band, known as the Soret band. The Qy and Qx transitions (subscripts denote the idealized polarization direction within the macrocycle plane) are the most important for determining the biophysical properties of chlorophyll species in particular and chlorophyll-containing photosystems in general.[5−12] The transitions are conventionally described by the four-orbital Gouterman model,[13,14] which, has known limitations These arise from the fact that the electronic nature of actual excitations is more complex than that suggested by the Gouterman model, for the B band, and because of the possible mixing of transitions due to vibronic coupling.[9,11,12,15−19]. The inadequacy of TD-DFT to properly predict charge-transfer states, and be reliably applied in the context of photosynthetic charge separation, is well known.[25,27,38] Wave function-based approaches are able to deliver systematically better results than DFT, albeit at increased computational cost; it is important to seek new ways for their practical usage
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