Abstract
Regioselective peripheral hydrogenation of a nanographene molecule with 60 contiguous sp2 carbons provides unprecedented access to peralkylated circumbiphenyl (1). Conversion to the circumbiphenyl core structure was unambiguously validated by MALDI-TOF mass spectrometry, NMR, FT-IR, and Raman spectroscopy. UV–vis absorption spectra and DFT calculations demonstrated the significant change of the optoelectronic properties upon peripheral hydrogenation. Stimulated emission from 1, observed via ultrafast transient absorption measurements, indicates potential as an optical gain material.
Highlights
Large polycyclic aromatic hydrocarbons (PAHs),[1] i.e., with size >1 nm, can be regarded as nanographene molecules, and have attracted attention for their unique optical, electronic, and magnetic properties.[2]
Conventional synthetic strategies toward nanographene molecules are mostly based on πexpansion, mainly through planarization of tailor-made oligoarylene precursors, e.g., by oxidative cyclodehydrogenation,[4] photochemical cyclization,[5] and/or flash vacuum pyrolysis.[6] π-expansion of smaller PAH structures provides access to larger PAHs, for example through Diels−Alder cycloaddition[7] or a sequence of arylation and cyclization at the periphery.[8]
It is possible to modify the aromatic structures of PAHs by “shrinking” the π-conjugated cores, for example through peripheral hydrogenation
Summary
Communication reductive condensation of naphthanthrone (3).[12]. this first reported synthetic approach yielded a complex mixture and required multistep separation processes to isolate 2. Note that the other potential SE signals (550 and 600 nm) are overlapped with the strong and broad PA in the midvisible, which is typical for conjugated systems.[21] The reason why 1 displays SE likely lies in its improved solubility brought about by selective peripheral substitution, as we have recently reported that intermolecular interactions in nanographenes lead to the formation of aggregates with charge-transfer character, in which charge generation quenches SE and gain.[22] The TA dynamics (probe at 500 nm, Figure 5b) show a longer lifetime for 1 (τ1 = 0.4 ± 0.04 ps; τ2 = 420 ± 20 ps) than 4 (τ1 = 0.1 ± 0.01 ps ; τ2 = 120 ± 15 ps), a result that can correlate with the poorer solubility of 4 with respect to the hydrogenated molecule These findings highlight the potential of peralkylated circumbiphenyl 1 for applications in photonics and optoelectronics.
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