Biradicaloid with a Twist: Lowering the Singlet–Triplet Gap

Abstract

Fusing two phenalenyl units such that a Kekulé structure is obtained leads to 'biradicaloid' systems, which typically display low singlet-triplet (S-T) energy gaps. The value of the S-T gap in biradicaloids can be lowered by extending the conjugated system for an additional fused ring(s). Recently, we have described an alternative approach to decrease the S-T gap with the aid of through-space interactions that arise in helically twisted biradicaloids. Using [5]helicene-based biradicaloid 'cethrene' as an illustrative example, we have shown that the value of its S-T gap is substantially decreased when compared to that of its planar isomer heptazethrene, on account of the helical twist, inducing crucial intramolecular interactions.

Nature of the Ligand-Centered Triplet State in Gd3+β-Diketonate Complexes as Revealed by Time-Resolved EPR Spectroscopy and DFT Calculations

“…As far as the former point is concerned, a higher degree of delocalization in the wavefunction with respect to the other ligands (see Figure 6 ) results in a higher stabilization for the corresponding triplet state and therefore a larger energy gap between the excited S 1 and T 1 states, that is, a larger shift between the fluorescence and phosphorescence bands. 34 , 35 Indeed, while paired electrons mostly repel each other via Coulomb interaction, the exchange term, which characterizes electrons with the same spin, is less pronounced as the delocalization of the wavefunction increases, therefore stabilizing the corresponding triplet states for a relatively more delocalized triplet (pyrene) compared to a more localized one (diketone). Moving to the latter point, the low intensity of the phosphorescence band can be associated with relevant non-radiative triplet decay pathways.…”

Evaluation of the quasi correlated tight-binding (QCTB) model for describing polyradical character in polycyclic hydrocarbons

“…The zethrenes are of this type. 14,[50][51][52][53] We restrict ourselves to displaying densities of unpaired electrons for dibenzoheptazethrene (Fig. 6).…”

“…All zigzag-type borders, external and inner, accumulate most of the unpaired electrons. For comparison, the initial (53,16) small dynamical correlation effects, especially for very large networks. It would be interesting to find other examples of such a discrepancy between the linear, Eq.…”

Late-Stage Modification of Electronic Properties of Antiaromatic and Diradicaloid Indeno[1,2-b]fluorene Analogues via Sulfur Oxidation

“…The recent increase in reports of carbon-based diradicaloids has also been driven by their potential applications both in organic electronics 40,41 and as magnetic materials. 42−45 The desire to gain a better fundamental understanding of these compounds, which possess a unique set of properties (e.g., narrow HOMO− LUMO gaps, 40 low-lying doubly excited electronic energy absorptions, 46,47 and electronic spin resonance signals 48 ), has led to synthesis of various families of diradicaloids based on bisphenalenyls, 41,49−51 zethrenes, 52−55 indenofluorenes, 56,57 and closely related diindenoacenes, 58−65 structure in the ground state to relieve the antiaromaticity of the conjugated, closed shell core. 66 Utilizing the indeno [1,2-b]fluorene framework, our group in recent years has aspired to tune both antiaromaticity (in benzene core molecules) and diradical character (naphthalene and larger aromatic cores) through two different approaches, either by changing the fusion bond order on both sides of the s-indacene core 28,63 or by adding benzothiophene units in the same position (anti or syn to the apical carbon in the fivemembered ring, e.g., 1−4 in Figure 1) to modulate electronic properties.…”