Thiophene and its sulfur inhibit indenoindenodibenzothiophene diradicals from low-energy lying thermal triplets.

Many qualitative structure-property correlations between diradical character and emerging molecular properties are known. For example, the increase of diradical character further decreases the singlet-triplet energy gap. Here we show that inclusion of thiophenes within a quinoidal polycyclic hydrocarbon imparts appreciable diradical character yet retains the large singlet-triplet energy gap, a phenomenon that has no precedent in the literature. The low aromatic character of thiophene and its electron-rich nature are the key properties leading to these unique findings. A new indenoindenodibenzothiophene scaffold has been prepared and fully characterized by several spectroscopies, magnetic measurements, solid-state X-ray and state-of-the-art quantum chemical calculations, all corroborating this unique dichotomy between the diradical input and the emerging magnetic properties. New structure-property relationships such as these are not only extremely important in the field of diradical chemistry and organic electronics, but also provide new insights into the versatility of π-electron chemical bonding.

A Biradical Balancing Act: Redox Amphoterism in a Diindenoanthracene Derivative Results from Quinoidal Acceptor and Aromatic Donor Motifs.

The reduced and oxidized states of an open-shell diindeno[b,i]anthracene (DIAn) derivative have been investigated by experimental and theoretical techniques. As a result of moderate biradical character and the ability of cyclopenta-fused scaffolds to stabilize both positive and negative charges, DIAn exhibits rich redox chemistry with four observable and isolable charged states. Structural and electronic properties of the DIAn system are brought to light by UV-vis-NIR and Raman spectroelectrochemical measurements. Aromatization of the diindeno-fused anthracene core upon successive single-electron injections is revealed through single-crystal X-ray diffraction of radical anion and dianion salts. We present a rare case where the pseudoaromatic/quinoidal ground state of a neutral biradical polycyclic hydrocarbon leads to a stable cascade of five redox states. Our detailed investigation of the transformation of molecular structure along all four redox events provides a clearer understanding of the nature of charge carriers in ambipolar organic field-effect transistors.

Diindeno-fusion of an anthracene as a design strategy for stable organic biradicals.

The consequence of unpaired electrons in organic molecules has fascinated and confounded chemists for over a century. The study of open-shell molecules has been rekindled in recent years as new synthetic methods, improved spectroscopic techniques and powerful computational tools have been brought to bear on this field. Nonetheless, it is the intrinsic instability of the biradical species that limits the practicality of this research. Here we report the synthesis and characterization of a molecule based on the diindeno[b,i]anthracene framework that exhibits pronounced open-shell character yet possesses remarkable stability. The synthetic route is rapid, efficient and possible on the gram scale. The molecular structure was confirmed through single-crystal X-ray diffraction. From variable-temperature Raman spectroscopy and magnetic susceptibility measurements a thermally accessible triplet excited state was found. Organic field-effect transistor device data show an ambipolar performance with balanced electron and hole mobilities. Our results demonstrate the rational design and synthesis of an air- and temperature-stable biradical compound.

Multifaceted Regioregular Oligo(thieno[3,4-b]thiophene)s Enabled by Tunable Quinoidization and Reduced Energy Band Gap.

Thiophene-based materials have occupied a crucial position in the development of organic electronics. However, the energy band gaps of oligo- and polythiophenes are difficult to modulate without resorting to push-pull electronic effects. We describe herein a new series of monodisperse oligo(thieno[3,4-b]thiophene) derivatives with well-defined regioregular structures synthesized efficiently by direct C-H arylation. These compounds show a unique palette of colors and amphoteric redox properties with widely tunable energy band gaps. The capacity to stabilize both cations and anions results in both anodic and cathodic electrochromism. Under excitation, these compounds can produce photoionized states able to interconvert into neutral triplet or form these through singlet exciton fission or intersystem crossing. These features arise from a progressive increase in quinoidization on a fully planar platform making the largest effective conjugation length among hetero-oligomers. Oligo(thieno[3,4-b]thiophene)s might represent the more distinctive family of oligothiophenes of this decade.

An Unusually Small Singlet-Triplet Gap in a Quinoidal 1,6-Methano[10]annulene Resulting from Baird's 4n π-Electron Triplet Stabilization.

Within the continuum of π-extended quinoidal electronic structures exist molecules that by design can support open-shell diradical structures. The prevailing molecular design criteria for such structures involve proaromatic nature that evolves aromaticity in open-shell diradical resonance structures. A new diradical species built upon a quinoidal methano[10]annulene unit is synthesized and spectroscopically evaluated. The requisite intersystem crossing in the open-shell structure is accompanied by structural reorganization from a contorted Möbius aromatic-like shape in S0 to a more planar shape in the Hückel aromatic-like T1. This stability was attributed to Baird's Rule which dictates the aromaticity of 4n π-electron triplet excited states.