The radical character of the acenes: A density matrix renormalization group study

Abstract

We present a detailed investigation of the acene series using high-level wave function theory. Our ab initio density matrix renormalization group algorithm has enabled us to carry out complete active space calculations on the acenes from napthalene to dodecacene correlating the full -valence space. While we find that the ground state is a singlet for all chain lengths, examination of several measures of radical character, including the natural orbitals, effective number of unpaired electrons, and various correlation functions, suggests that the longer acene ground states are polyradical in nature.

Why 1,2-quinone derivatives are more stable than their 2,3-analogues?

“…The good performance of the UB3LYP method is confirmed by recent papers [25][26][27][28][29][30]. On the basis of previous results [31], spin contamination corrections were not included.…”

“…It should be noted that Clar structure shown in Scheme 5b is reinforced by all applied aromaticity indices (see Table S1 in Supporting Information). Summarizing, in 2,3-quinones for n > 2, the diradical singlet situation is favored, as in the case of the acenes [25,59,61,63] and in other polycyclic aromatic hydrocarbons [26][27][28][29][30][64][65][66][67][68][69] and graphene nanoflakes [70,71]. Therefore, for 2,3-isomers, only the results of the ground states (singlet closed shell for n ≤ 2 and singlet open shell for n ≥ 3) are presented below.…”

The ab-initio density matrix renormalization group in practice

“…Here, the DMRG has so far been applied in a complete active space setting, 8,38,[47][48][49][50][51] starting with the early work of Reiher and coworkers, 47 to the latest calculations on systems as large as the bioinorganic Mn 4 Ca core of photosystem II by Kurashige et al, 52 and the ubiquitous [4Fe-4S] biological iron-sulfur complexes by Sharma et al; 53 these have active spaces in excess of 50 orbitals. Finally, the internal structure of the MPS means that it is uniquely suited to pseudo-onedimensional correlation, and in the ab-initio chemical context, the DMRG has been used since its inception to study groundand excited-states of π-conjugated molecules, 27,54,55 such as the polyacenes 55 and graphene nanoribbons, 49 as well as other one-dimensional systems, such as atomic chains and rings. 54,56 The entanglement structure of the MPS has even generated a niche in interpretative quantum chemistry.…”

Effect of strong electron correlation on the efficiency of photosynthetic light harvesting

“…In reality, however, the chromophores are assembled from chlorophyll molecules that contain an extensive network of conjugated carbon-carbon bonds surrounding magnesium ions, from which significant strong electron correlation, including polyradical character, has been shown to emerge [17,20]. Recent experimental efforts, not yet published, provide further insights into the model developed here.…”

“…While the chromophores are chlorophyll molecules containing large networks of conjugated carbon bonds that surround a charged magnesium ion, they have largely been represented in theoretical studies [4][5][6][7][8][9][10][11][12][13] by one-electron models that neglect the effects of electron correlation and entanglement within chromophores. Two advanced methods in electronic structure, density-matrix renormalization group [17] and two-electron reduceddensity-matrix theory [18,19], have recently shown that networks of conjugated bonds as in acene chains [17,20], acene sheets [20], and chlorophyll are associated with polyradical character that cannot be adequately described without a strongly correlated many-electron quantum model. In this paper we examine the efficiency of light harvesting where we represent each chromophore by a correlated N-electron model to treat strong electron correlation.…”