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Rylenes are peri-fused naphthalene oligomers and possess a unique electronic structure, which makes them attractive materials as organic dyes and semiconductors. In particular, rylenes beyond terrylene are promising near-infrared dyes with high molar extinction coefficients. Recent studies have demonstrated an unusual open-shell diradical character for long rylene molecules and potential half-metallic properties of polyrylene ribbon. However, the synthesis of long rylenes and polyrylene is a very challenging task, with various strategies directed towards solving the critical molecular aggregation and stability problems having been developed since 1910. In addition, recent research has also shown that quinoidal rylenes have an irresistible tendency to become diradical, and so they constitute a new type of open-shell singlet diradicaloids with magnetic activity. This microreview mainly summarizes the synthesis of long rylene ribbons and quinoidal rylene diradicaloids, together with their unique electronic structures and physical properties. [ABSTRACT FROM AUTHOR]

Diindeno-fused bischrysene, a new diindeno-based polycyclic hydrocarbon (PH), was synthesized and characterized. It was elucidated in detailed experimental and theoretical studies that this cyclopenta-fused PH possesses an open-shell singlet biradical structure in the ground state and exhibits high stability under ambient conditions ( t1/2=39 days). The crystal structure unambiguously shows a novel saddle-shaped π-conjugated carbon skeleton due to the steric hindrance of the central cove-edged bischrysene unit. UV/Vis spectral measurements revealed that the title molecule has a very narrow optical energy gap of 0.92 eV, which is consistent with the electrochemical analysis and further supported by density functional theory (DFT) calculations. [ABSTRACT FROM AUTHOR]

A detailed electronic structure study of singlet and triplet states of a series of biradicaloid pleiadene-like molecules was performed using the DFT and complete active space methods. It was shown that the singlet-triplet gaps obtained with the B3LYP, B2PLYP, and CASSCF methods are not a monotone function of the number of annelated benzene rings. According to the CASSCF results, 9,10-naphthopleiadene is a triplet biradical, whereas all other members of the series are found to be singlet biradicaloids exhibiting very pronounced biradical character. On the other hand, the B2PLYP calculations revealed that 9,10-anthracenopleiadene is a triplet biradical molecule. A detailed analysis of the local aromaticity of singlet and triplet states of the examined molecules was done by means of the multi-centre delocalization indices, harmonic oscillator model of aromaticity index, and nucleus independent chemical shifts. It was shown that the triplet states are more aromatic than the corresponding singlet electronic states. The difference in aromaticity of the singlet and triplet electronic states becomes smaller along the series of benzannelated pleiadenes. The more pronounced aromatic character of the triplet states can compensate the instability coming from the presence of unpaired electrons, which can explain the fact that the mid-size members of the examined series are found to be triplet biradicals. Graphical abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]