Symmetry breaking and excitonic effects on optical properties of defective nanographenes

Abstract

We investigate optical properties of the nanographene family and predict a defect induced effect by utilizing the all-electron first-principles GW+Bethe-Salpeter equation (BSE) method based on the many-body perturbation theory. As an accuracy check of the GW+BSE, photoabsorption spectra are calculated for a grossly warped nanographene (C80H30), which was very recently synthesized [Kawasumi et al., Nat. Chem. 5, 739-744 (2013)]. The calculated spectra are found to faithfully reproduce the shape, height, and position of the measured peaks. Then the method is applied to the flat nanographene without defect (C24H12 and C38H16), the curved ones with single defect (C20H10, C28H14, and C32H16), and fragments of C80H30 with double defect (C36H16 and C42H20). The existence of the defects significantly changes the optical spectra. In particular, the interaction between the defects is found to break the symmetry of the atomic geometries and enhance the excitonic effect, thereby generating the extra peaks at the lower photon energy side of the main peak. The present results might help explain the origin of the first two peaks experimentally observed for C80H30.

Many‐body effects in optical response of graphene‐based structures

“…Based on the DFT‐RPA calculations this peak was attributed to the interband transitions determined by topology‐induced resonant states as well as *– * hybridization at the cone apex. A change in the optical absorption of a hydrogen‐terminated graphene fragment with the incorporation of various topological defects was studied using the GW‐BSE approach . One of the goals of that article was to interpret the spectrum measured for grossly warped nanographene C 80 H 30 .…”

“…A change in the optical absorption of a hydrogen-terminated graphene fragment with the incorporation of various topological defects was studied using the GW-BSE approach. [62] One of the goals of that article was to interpret the spectrum measured for grossly warped nanographene C 80 H 30 . [63] This structure was modeled by a fragment containing a pentagonal ring at the center and five heptagonal rings (inset in Fig.…”

Optical absorption of warped nanographenes tuned by five- and seven-membered carbon rings

“…It centers at a C5 ring that is surrounded by five C6 rings, and then by five C7 rings. The computed averaged r 56 , r 66 and r 67 are 1.405, 1.406, and 1.448 Å, respectively, which are quite different from the C-C distances in C 60 (1.45 AE 0.015 Å for r 56 It is apparent that the delocalization in the HOMO and LUMO orbitals of C 78 H 30 is more significant than that of C 80 H 30 . At the outermost of C 80 H 30 are ten C6 rings that are adjacent to the C7 rings.…”

“…The second (444 nm) and the strongest (435 nm) bands also match well with the measurement (452 nm and 418 nm), 33 respectively. The GW (Green's function G and screened Coulomb interaction W) + BSE (Bethe-Salpeter equation) approach was used by Noguchi and Sugino 56 to compute the absorption spectra of C 80 H 30 , giving the first three bands at 460, 422 and 393 nm, respectively. Therefore, B3LYP was selected to compute the optical absorption of the C 80 H 30 derivative in the subsequent TDDFT calculations.…”

High-Lying Triplet Excitons of Thermally Activated Delayed Fluorescence Molecules

“…We employ a standard one-shot GW+BSE method in which BSE is solved within GWA to simulate the optical properties of TADF molecules from first-principles. The GW+BSE method has been implemented into our original program code employing an all-electron mixed-basis approach ,− (the detailed calculation condition is given in Supporting Information S1). We follow the conventional scheme of adopting the Tamm–Dancoff approximation and convert BSE into an eigenvalue problem of the Hermitian BSE Hamiltonian ( H BSE ), The eigenvalue (Ω i ) corresponding to the i th excitation energy is denoted as S i for the singlet excitation energy without spin flip and T i for the triplet excitation energy with spin flip.…”