Uncovering dark multichromophoric states in Peridinin–Chlorophyll–Protein

Abstract

It has long been recognized that visible light harvesting in Peridinin–Chlorophyll–Protein is driven by the interplay between the bright (S 2 ) and dark (S 1 ) states of peridinin (carotenoid), along with the lowest-lying bright (Q y ) and dark (Q x ) states of chlorophyll- a . Here, we analyse a chromophore cluster in the crystal structure of Peridinin–Chlorophyll–Protein, in particular, a peridinin–peridinin and a peridinin–chlorophyll- a dimer, and present quantum chemical evidence for excited states that exist beyond the confines of single peridinin and chlorophyll chromophores. These dark multichromophoric states, emanating from the intermolecular packing native to Peridinin–Chlorophyll–Protein, include a correlated triplet pair comprising neighbouring peridinin excitations and a charge-transfer interaction between peridinin and the adjacent chlorophyll- a . We surmise that such dark multichromophoric states may explain two spectral mysteries in light-harvesting pigments: the sub-200-fs singlet fission observed in carotenoid aggregates, and the sub-200-fs chlorophyll- a hole generation in Peridinin–Chlorophyll–Protein.

Optical Projection and Spatial Separation of Spin-Entangled Triplet Pairs from the S1 (21 Ag–) State of Pi-Conjugated Systems

“…Mixing CT character via donor-acceptor chemistry can lead to intramolecular-SF and one often needs to avoid the formation of ''dark'' 2 1 A g states to achieve good fission yields. 10,18,44,60,80,124,125 In contrast, our results show that by manipulating the 2 1 A g --state-efficient triplet production can in fact be achieved.…”

“…42,[56][57][58][59] Recent theoretical works have shown in these biosystems that in specific geometries the 2 1 A g wavefunction can have significant admixtures from other states and is not a pure 1 (TT) excitation. 60 Yet much about the electronic structure and ultrafast dynamics of S 1 (2 1 A g -) excited states remains unknown.…”

Overlap-Driven Splitting of Triplet Pairs in Singlet Fission

“…Initial-guess orbitals for the (4,4) active space were generated from a Hartree−Fock calculation, while for larger active spaces, they were obtained from smaller-active-space DMRG−SCF converged orbitals. 74,75 ■ RESULTS AND DISCUSSION Intermolecular Singlet Fission within the Weak-Coupling Regime. We focus on the crystalline unit cell of triisopropylsilyl (TIPS)−tetracene for which the triplet−triplet exchange interaction J underlying the pair has been experimentally resolved.…”

“…Total energies of all converged states were computed to within 10 –11 hartrees. Initial-guess orbitals for the (4,4) active space were generated from a Hartree–Fock calculation, while for larger active spaces, they were obtained from smaller-active-space DMRG–SCF converged orbitals. , …”

Twisted Carotenoids Do Not Support Efficient Intramolecular Singlet Fission in the Orange Carotenoid Protein

“…This is attributable in part to their complex manifold of low-lying triplet-pair states − and strong vibronic coupling , and also partly due to the sensitivity of the photophysics to conjugation length and molecular geometry. In polyenes, the lowest-lying 1 (TT) state that makes up the dominant contribution of S 1 ,,, contains tightly bound triplets that are unlikely to easily separate into free triplets without additional energy …”