Exploitation of Baird Aromaticity and Clar’s Rule for Tuning the Triplet Energies of Polycyclic Aromatic Hydrocarbons

Abstract

Polycyclic aromatic hydrocarbons (PAH) are a prominent substance class with a variety of applications in molecular materials science. Their electronic properties crucially depend on the bond topology in ways that are often highly non-intuitive. Here, we study, using density functional theory, the triplet states of four biphenylene-derived PAHs finding dramatically different triplet excitation energies for closely related isomeric structures. These differences are rationalised using a qualitative description of Clar sextets and Baird quartets, quantified in terms of nucleus independent chemical shifts, and represented graphically through a recently developed method for visualising chemical shielding tensors (VIST). The results are further interpreted in terms of a 2D rigid rotor model of aromaticity and through an analysis of the natural transition orbitals involved in the triplet excited states showing good consistency between the different viewpoints. We believe that this work constitutes an important step in consolidating these varying viewpoints of electronically excited states.

Functional group introduction and aromatic unit variation in a set of π-conjugated macrocycles: revealing the central role of local and global aromaticity

“…To interpret the data from Figure 6 in some more detail, it is important to note that the strength of the ring current is approximately proportional to the number of electrons contributing 36 and, accordingly, the diamagnetic shielding is proportional to the number of electrons divided by the radius of the ring. 37 In line with this, we find that due to the enhanced number of electrons the shielding for the anionic systems is slightly more negative than for the cationic systems. This is particularly true when comparing the dianions with the dications, a trend that is also seen for smaller annulenes.…”

“…At the UKS level, it is also straightforward to compute the shielding tensors relevant to the T 1 state. For the following analysis, it is worth noting that S 1 and T 1 are both dominated by the HOMO/LUMO transition and, thus, we expect that any statement made about the T 1 in terms of molecular geometry and electronic structure is also transferable to the S 1 (see Refs 37,[42][43][44] for more information on the similarities of excited-state aromaticity in singlet and triplet excited states).…”

Functional Group Introduction and Aromatic Unit Variation in a Set of π-Conjugated Macrocycles: Revealing the Central Role of Local and Global Aromaticity

“…To interpret the data from Figure 6 in some more detail, it is important to note that the diamagnetic shielding is approximately proportional to the number of electrons contributing to the ring current divided by the radius of the ring. 23 In line with this, we find that due to the enhanced number of electrons the shielding for the anionic systems is slightly more negative than for the cationic systems. This is particularly true when comparing the dianions with the dications, a trend that is also seen for smaller annulenes.…”

“…At the UKS level, it is also straightforward to compute the shielding tensors relevant to the T1 state. For the following analysis, it is worth noting that S1 and T1 are both dominated by the HOMO/LUMO transition and, thus, we expect that any statement made about the T1 in terms of molecular geometry and electronic structure is also transferable to the S1 (see Refs 23,[28][29][30] for more information on the similarities of excited-state aromaticity in singlet and triplet excited states).…”

Spin–Orbit Natural Transition Orbitals and Spin-Forbidden Transitions

“…Recent applications of the NTO method have been primarily in photochemistry and -physics and studies of excited states. A small selection of available studies is presented by refs − . Visualization of NTOs has been particularly useful for systems for which the descriptions of an electronic transition of interest involve a considerable number of pairs of occupied and unoccupied MOs with relatively low weights, which renders the assignment of the excitation difficult.…”