Can Baird’s and Clar’s Rules Combined Explain Triplet State Energies of Polycyclic Conjugated Hydrocarbons with Fused 4nπ- and (4n+ 2)π-Rings?

Abstract

Compounds that can be labeled as "aromatic chameleons" are π-conjugated compounds that are able to adjust their π-electron distributions so as to comply with the different rules of aromaticity in different electronic states. We used quantum chemical calculations to explore how the fusion of benzene rings onto aromatic chameleonic units represented by biphenylene, dibenzocyclooctatetraene, and dibenzo[a,e]pentalene modifies the first triplet excited states (T) of the compounds. Decreases in T energies are observed when going from isomers with linear connectivity of the fused benzene rings to those with cis- or trans-bent connectivities. The T energies decreased down to those of the parent (isolated) 4nπ-electron units. Simultaneously, we observe an increased influence of triplet state aromaticity of the central 4n ring as given by Baird's rule and evidenced by geometric, magnetic, and electron density based aromaticity indices (HOMA, NICS-XY, ACID, and FLU). Because of an influence of triplet state aromaticity in the central 4nπ-electron units, the most stabilized compounds retain the triplet excitation in Baird π-quartets or octets, enabling the outer benzene rings to adapt closed-shell singlet Clar π-sextet character. Interestingly, the T energies go down as the total number of aromatic cycles within a molecule in the T state increases.

Is Excited‐State Aromaticity a Driving Force for Planarization of Dibenzannelated 8π‐Electron Heterocycles?

“…By also calculating the NICS‐ XY scan (Figure S18 in the Supporting Information) for the C 2v symmetric structure of 1 c (a structure with one imaginary frequency) we can attribute the slightly lowered excited state aromaticity of 1 c to the minute puckering around the S atom. The NICS plots also reveal a global aromaticity for these three molecules, similarly to dibenzocyclooctatetraene reported by Ayub et al ., although the central 8π‐electron ring makes a substantial contribution to the triplet state aromatic character.…”

A DFT Study of the Modulation of the Antiaromatic and Open‐Shell Character of Dibenzo[a,f]pentalene by Employing Three Strategies: Additional Benzoannulation, BN/CC Isosterism, and Substitution

“…It should be pointed out that the ring currents for the isomeric [ a , e ]DBP and its arene‐annulated derivatives, in the S 0 and T 1 states, as well as for its B 2 N 2 ‐substituted analogues, are discussed in Refs. and , respectively.…”

“…With this in mind, in the present work we had the aim to theoretically predict to what extent and in what direction the (anti)aromaticity and electronic state of [ a , f ]DBP can be affected by employing three strategies: 1) Fusion with one or two additional benzene rings, 2) the replacement of one CC unit by the isoelectronic, but polar BN unit (BN/CC isosterism), and 3) the introduction of substituents into the pentalene subunit. The first strategy has been recognized as useful in modulating the singlet–triplet energy gaps and paratropicity of 4 n π‐electron systems . BN/CC isosterism has also been widely exploited to tune various molecular properties and hybrid BN/CC compounds have been employed in materials science, medicinal chemistry, and synthetic chemistry .…”

Adaptive aromaticity in S0 and T1 states of pentalene incorporating 16 valence electron osmium

“…Recently, Kim and Osuka characterized the excited-state (anti)aromaticity in expanded porphyrins [17][18][19][20][21][22] . Ottosson and co-workers reported a series of "aromatic chameleons" that are prone to be aromatic in both the T 1 and S 1 states [23][24][25][26][27] . Note that Baird's rule can also be applied to the S 1 state [28][29][30] .…”