Electron delocalization and electron density of small polycyclic aromatic hydrocarbons in singlet excited states

Abstract

The four lowest singlet electronic states of benzene, the acenes from naphthalene to pentacene, phenanthrene and pyrene were studied by means of theoretical methods. Their vertical excitation energies from the ground electronic states were computed at the CASPT2 approximation. As an attempt to explain the trends observed in the excitation energies, several descriptors based on the electron density were used and the similarity of these molecules with their ground state counterparts was analyzed. It was found that the changes of the topological properties at the C-C bond critical points do not explain the decreasing trends for the excitation energies with the increase of the number of rings, in part because the small changes that take place in the electron density occur above and below the molecular plane. A similarity index based on electron delocalization between quantum topological atoms was defined to compare a molecule in two different electronic states. It was found that, mainly for the acenes, this index goes in line with the excitation energies to the first excited state. Implications of the changes in electron delocalization on the aromatic character of these molecules are also discussed. In general, local aromaticity decreases upon excitation.

Energetic Analysis of Conjugated Hydrocarbons Using the Interacting Quantum Atoms Method

“…In the case of polycyclic hydrocarbons, the electron density gives a picture that in general is consistent with the classical description of the chemical bond with the advantage of adding quantitative microscopic information through the properties of the electron density at its critical points and the electron delocalization between topological atoms in the molecules . As a consequence, the QTAIM has been particularly relevant for the study of aromatic molecules in the ground and excited states …”

Why is Benzene Unique? Screening Magnetic Properties of C6H6Isomers

“…The name of benzene is almost an equivalent to the concept of aromaticity; aromaticity of benzene has been investigated extensively both in its ground and excited states. [1][2][3][4][5] Aromaticity is an elusive phenomenon that has been employed by chemists to explain exceptional stability [6][7][8][9][10][11][12][13][14][15][16][17][18][19] or justify particular reactivity of various molecules. [20][21][22][23][24][25] IUPAC recommends four criteria -structural, energetic, electronic, and magnetic (SEEM) -to characterize aromatic species.…”

Chemical bonding in excited states: Energy transfer and charge redistribution from a real space perspective

“…The H 2 molecule is used as a model for the making or breaking of single bonds in photophysical processes involving singlet or triplet electronic states. Although these tools have recently been applied in the study of molecules in excited states, [9][10][11][12][13][14][15][16][17] they are used here to describe basic reaction mechanisms of photophysical interest. LiF is taken as representative for charge transfer processes and because of its close relationship with NaI, a historical molecule in the development of femtochemistry.…”

“…[6,7] In addition, the energy partitioning provided by the interacting quantum atoms approach (IQA) [8] allows to carry out the energetic analysis beyond equilibrium geometries in terms of molecular fragment contributions. Although these tools have recently been applied in the study of molecules in excited states, [9][10][11][12][13][14][15][16][17] they are used here to describe basic reaction mechanisms of photophysical interest.…”