Pariser–Parr–Pople Model Based Configuration-Interaction Study of Linear Optical Absorption in Lower-Symmetry Polycyclic Aromatic Hydrocarbon Molecules

Abstract

The electronic and optical properties of various polycyclic aromatic hydrocarbons (PAHs) with lower symmetry, namely, benzo­[ghi]­perylene (C22H12), benzo­[a]­coronene (C28H14), naphtho­[2,3a]­coronene (C32H16), anthra­[2,3a]­coronene (C36H18), and naphtho­[8,1,2-abc]­coronene (C30H14), were investigated. We performed electron-correlated calculations using screened and standard parameters in the π-electron Pariser–Parr–Pople (PPP) Hamiltonian, and the correlation effects were included, both for ground and excited states, using multireference singles–doubles configuration-interaction (MRSDCI) methodology. The PPP model Hamiltonian includes long-range Coulomb interactions, which increase the accuracy of our calculations. The results of our calculations predict that, with the increasing sizes of the coronene derivatives, optical spectra are red-shifted, and the optical gaps decrease. In each spectrum, the first peak representing the optical gap is of moderate intensity, while the more intense peaks appear at higher energies. Our computed spectra are in good agreement with the available experimental data. For the purpose of comparison, we also performed first-principles time-dependent density-functional theory (TDDFT) calculations of the optical gaps of these molecules using Gaussian basis functions and found that they yielded values lower than our configuration-interaction (CI) results.

Why Does a B12H12 Icosahedron Need Two Electrons to be Stable: A First-Principles Electron-Correlated Investigation of B12Hn (n = 6, 12) Clusters

“…For this purpose, we employed a hybrid exchange–correlation functional, B3LYP, − and a localized Gaussian basis set, 6-31+G*, which is composed of a valance double-ζ set, augmented with s and p diffuse functions and d polarization functions for the non-hydrogen atoms. Although the B3LYP hybrid functional was mainly developed for ground-state calculations, it has also been found to yield accurate results on the excited states of both closed-shell and open-shell systems. , In some of the recent studies, it has been found that the above computational framework (B3LYP \ 6-31+G*) for both ground and excited states led to the high-accuracy results, in excellent agreement with the experiments for the π-conjugated aromatic hydrocarbon molecules. − We adopted the TDDFT approach to compute the optical properties of the stable structures because of the following two primary reasons: (i) earlier studies exhibit reliable performance and (ii) computing the optical absorption using wave function-based electron-correlated approaches is computationally quite prohibitive for the relatively larger clusters considered in this work. Therefore, we performed the ground-state calculations using the most accurate electron-correlated approach (coupled-cluster approach), while the excited-state calculations were carried out by employing the TDDFT methodology.…”

Computational Approaches for Organic Semiconductors: From Chemical and Physical Understanding to Predicting New Materials

“…In 1995, Hiruta et al introduced the concept of chemical softness for π-conjugated systems in computing the electron repulsion integral within the PPP method. With this approach, the calculated excitation energies of polycyclic aromatic hydrocarbons (PAHs) with up to seven acene rings showed improved agreement with experiments. , As the PPP method includes high-order CIs over a large active space, several works have used the PPP method to more recently investigate singlet fission in π-conjugated chromophores. , Bhattacharyya et al showed that the excitation energy of PAH using the PPP method with CI yields better results than the popular time-dependent density functional method . Because computational limitations in the early 1990s rendered a full CI approach with PPP infeasible for large polyacenes beyond anthracene, efforts were initiated to replace CI with the density matrix renormalization group (DMRG) method, which enables the accurate calculation of low-lying states for one-dimensional and quasi-one-dimensional systems with reduced computational cost compared to a full CI calculation.…”

Shining Light on Inverted Singlet–Triplet Emitters

“…In an effort to better understand the physics underlying the ST inversion, we undertook an extensive study of several molecular structures described by the celebrated Pariser–Parr–Pople (PPP) model Hamiltonian, the simplest model for correlated electrons in π-conjugated molecules. The model, proposed in the 1950s, − successfully described the behavior of several families of small organic molecules. ,− In the field of π-conjugated polymers, PPP was pivotal in understanding the anomalous behavior of polyacetylene, where correlated electrons are responsible for the appearance of a low-lying dark state, thus solving the puzzle of its nonfluorescent behavior. More recently, density matrix renormalization group (DMRG) approaches allowed to extend the PPP model to deal with graphene-based structures, to address singlet fission in polyenes, and to build models for dark states in polyene and carotenoid systems …”

“…The model, proposed in the 1950s, 36 − 38 successfully described the behavior of several families of small organic molecules. 37 , 39 − 47 In the field of π-conjugated polymers, PPP was pivotal in understanding the anomalous behavior of polyacetylene, 48 where correlated electrons are responsible for the appearance of a low-lying dark state, thus solving the puzzle of its nonfluorescent behavior. More recently, density matrix renormalization group (DMRG) approaches allowed to extend the PPP model to deal with graphene-based structures, 49 to address singlet fission in polyenes, 50 and to build models for dark states in polyene and carotenoid systems.…”