Electronic Structure of Low-Dimensional Carbon π-Systems

J M García-Lastra,F J Himpsel,Idris Boukahil,Ruimin Qiao,Angel Rubio

doi:10.1021/acs.jpcc.6b02530

Abstract

X-ray absorption spectroscopy (XAS) is combined with density functional theory (DFT) to determine the orbitals of one- and two-dimensional carbon π-systems (lycopene, beta-carotene, retinal, retinol, retinoic acid, coronene, triphenylene). Considerable fine structure is observed for the transition from the C 1s level to the lowest unoccupied molecular orbital (LUMO) and explained by DFT. The wave functions of the one-dimensional chain molecules display the node structure of a vibrating string. The XAS transition energy is decomposed into contributions from the C 1s core level, the π* final state, and the electron–hole interaction. For the latter, we develop a simple model that accurately represents a full Δ-self-consistent field (ΔSCF) calculation. The distortion of the LUMO because of its interaction with the C 1s hole is investigated. These results illustrate the electronic states of prototypical π-bonded carbon structures with low-dimensional character, such as those used in molecular complexes for sol...

Highlights

Despite the complexity of these molecules, we find surprisingly sharp features in the X-ray absorption spectroscopy (XAS) spectra
The particular choice of the BHandHLYP functional among many other hybrid functionals for the time-dependent density functional theory (DFT) (TD-DFT) calculations is based on previous work, where the BHandHLYP functional required the smallest energy shift to match the threshold of the experimental XAS spectra
Detailed results about the nature of these orbitals are given in the Supporting Information

Summary

Introduction

Examples are transistors based on carbon nanotubes or graphene, organic light-emitting diodes (OLEDs), solar cells containing conducting polymers or organic dye molecules, single-molecule devices, and many other innovative structures. The electronic structure of such a complex has been investigated by X-ray absorption spectroscopy (XAS),[10] taking advantage of the element and orbital selectivity of this technique. The appeal of such structures is their atomic precision, which avoids having to deal with disordered, intermixed interfaces between different materials in conventional solar cells. As a step in this scheme, it has been proposed to connect two such absorber molecules by a molecular wire or diode into a tandem device in order to cover the solar spectrum more efficiently. Thousands of candidate molecules have been screened by density functional theory to see whether their energy levels are suitable for tandem arrangements.[11]

Methods

Results

Conclusion