Prediction of Spin Density, Baird‐Antiaromaticity, and Singlet–Triplet Energy Gap in Triplet‐State Polybenzenoid Systems from Simple Structural Motifs

Abstract

Triplet‐state aromaticity has been recently proposed as a strategy for designing functional organic electronic compounds, many of which are polycyclic aromatic systems. However, in many cases, the aromatic nature of the triplet state cannot be easily predicted. Moreover, it is often unclear how specific structural manipulations affect the electronic properties of the excited‐state compounds. Herein, the relationship between the structure of polybenzenoid hydrocarbons (PBHs) and their spin‐density distribution and aromatic character in the first triplet excited state is studied. Although a direct link is not immediately visible, classifying the PBHs according to their annulation sequence reveals regularities. Based on these, a set of guidelines is defined to qualitatively predict the location of spin and paratropicity and the singlet–triplet energy gap in larger PBHs, using only their smaller tri‐ and tetracyclic components, and subsequently tested on larger systems.

Excited state character of Cibalackrot-type compounds interpreted in terms of Hückel-aromaticity: a rationale for singlet fission chromophore design

“…Our ambition is to develop rened design rules based on aromaticity as this can facilitate the identication of novel ways for the tailoring of molecules that function as chromophores for singlet ssion photovoltaics. Aromaticity comes in different forms, e.g., Hückel-, 18 Möbius-, 19,20 Baird-, [17][18][19][20][21][22][23][24][25][26][27][28][29] and spherical aromaticity. 30,31 Of relevance for the study presented herein are Hückel-aromaticity of closed-shell cycles with 4n + 2p-electrons and Baird-aromaticity of cycles with 4np-electrons in their triplet pp* states.…”

Revival of Hückel Aromatic (Poly)benzenoid Subunits in Triplet State Polycyclic Aromatic Hydrocarbons by Silicon Substitution

“…We pass to anthracene which mainly localizes triplet state antiaromaticity to the middle ring (NICS scans in Figures 2 and S1, the AICD plot in Figure S12, HOMA middle ring =0.494, EDDB F(middle ring) =2.493 e and EDDB F(α‐β) /EDDB F =0.160) having the highest spin density (Figure 1). We note that a detalied analysis of spin density distribution in polybenzenoid compounds and its relation to antiaromaticity was published recently [61] . The authors of this reference showed that spin localization on a specific subunit(s) allowed the remaining ones to recover their aromaticity.…”

“…This resembles the ground state properties of (poly)benzenoid compounds. • Substitution at α‐position of an outer ring creates more aromatic subunit than substitution at β‐position (substitution at the ring‐junction position only decreases antiaromaticity). There is no difference between the two different α‐positions, as, for example, in phenanthrene. • Two silicon atoms in 1,4‐position within one ring allow the formation of more aromatic carbocyclic subunits than one silicon atom, due to better unpaired electrons′ localization. • Though the extent of aromaticity of carbocyclic subunits of the same type appears as more pronounced in angular vs linear geometry and for synperiplanar (sp) vs antiperiplanar (ap) arrangement of the Si−H bonds in disilicon‐containing compounds, the difference is small. • Two isomeric carbocyclic subunits are equally efficiently created by putting the silicon atom(s) into various α‐positions of outer benzene rings within a polycycle. These results show that the spin localization in PAHs on particular subunit(s), enabling recovery of Hückel aromaticity of the others, [61] can be shifted to different rings by the silicon substitution allowing the fine tuning of S 0 −T 1 energy gaps, which nicely correlate with the extent of global aromaticity represented by ISE values (Figure S37f, R 2= 0.769). Formation of wings‐shaped sp structure upon excitation of the planar disila‐compound may be interesting for developing of photomechanical materials, featuring a kind of movement (change of shape) as a response to light. Having known that silicon substituted (poly)benzenoid compounds have been synthesized, we hope that the rules established in this work can be useful in future design of organic compounds‐based materials.…”

“…These data, along with the shape of NICS scans (Figure 2 and S1) and AICD plots shown in Figures S15 and S26, again predict angular geometry as slightly more favourable in formation of Hückel aromatic naphthalene subunit within the T 1 heterotricycle, aromaticity of which becomes close to that of S 0 naphthalene (94% and 98% according to HOMA and EDDB, respectively). Due to the small difference in the extent of naphthalene subunits′ aromaticity, the trend noticed for the parent hydrocarbons [61] remains: linear derivatives, generally, have smaller triplet state energies and smaller S 0 −T 1 energy gaps (Table 1).…”

“…in eV: benzene (3.95 [56] /3.8), naphthalene (2.63 [57] /2.7), anthracene (1.87 [57] /1.8), tetracene (1.2 [58] /1.2), phenanthrene (2.7 [59] /2.8) and benz[ a ]anthracene (2.0 [60] /2.1). Also, the B3LYP functional is frequently used to study poly(hetero)cyclic compounds in their excited triplet states and it shows good agreement with other functionals and levels of theory [24,26,37d,61] …”