The role of topology in organic molecules: origin and comparison of the radical character in linear and cyclic oligoacenes and related oligomers

Abstract

We discuss the nature of electron-correlation effects in carbon nanorings and nanobelts using an analysis tool known as fractional occupation number weighted electron density (ρ) and the RAS-SF method, revealing for the first time significant differences in static correlation effects depending on how the rings (i.e. chemical units) are fused and/or connected until closing the loop. We choose to study in detail linear and cyclic oligoacene molecules of increasing size, and relate the emerging differences with the difficulties for the synthesis of the latter due to their radicaloid character. We finally explore how minor structural modifications of the cyclic forms can alter these results, showing the potential use of these systems as molecular templates for the growth of well-shaped carbon nanotubes as well as the usefulness of theoretical tools for molecular design.

From cyclic nanorings to single-walled carbon nanotubes: disclosing the evolution of their electronic structure with the help of theoretical methods

“…to yield the N FOD value estimating the number of strongly correlated electrons of the system. This cost-effective FT-DFT methodology has been applied before to a variety of chemical (bio)systems 41 and to small-size linear and cyclic oligoacenes, 26 with the latter results in good agreement with findings from the RAS-SF method, 36 showing thus its potential to deal with larger systems out of the scope of more costly methods.…”

“…These results from the tight-binding model qualitatively agree with the diradical and tetraradical character of even and odd ciclacenes, respectively, disclosed previously and confirmed quantitatively by previous multiconfigurational wavefunction calculations by some of the authors. 26…”

The Low Lying Double-Exciton State of Conjugated Diradicals: Assessment of TDUDFT and Spin-Flip TDDFT Predictions

“…The coefficient of determination R-squared(R 2 ) value is 0.93. In previous works, the N FOD values have been compared with the y 0 values for linear acenes [27] and have been used to demonstrate the poly-radical character of cyclacenes [27] and single-wall carbon nanotubes [28].…”

Emerging DFT Methods and Their Importance for Challenging Molecular Systems with Orbital Degeneracy

“…Finite-Temperature DFT, also called thermally assisted-occupation DFT [22], is a useful tool for systems with a complicated electronic structure [23,24]. Besides its utility to select active orbitals prone to partial occupation, as the first step for more sophisticated multiconfigurational or complete active space self-consistent field (MCSCF/CASSCF) treatments, it has been recently applied to a large set of radical and radicaloid systems [25][26][27][28][29]. It is also conceived as a low-cost tool to explore energy landscapes with varying biradical character, as it may happen in organic chemical reactions, and to discard pathological cases in datasets more effectively than using traditional descriptors [30].…”