Dibenzo[hi,st]ovalene as Highly Luminescent Nanographene: Efficient Synthesis via Photochemical Cyclodehydroiodination, Optoelectronic Properties, and Single-Molecule Spectroscopy

摘要

Dibenzo[hi,st]ovalene (DBOV), as a new nanographene, has demonstrated promising optical properties, such as red emission with a high fluorescence quantum yield of 79% and stimulated emission, as well as high thermal stability and photostability, which indicated its promise as a light-emitting and optical gain material. However, the previous synthetic routes required at least 12 steps. This obstructed access to different derivatives, e.g., to obtain crystals suitable for X-ray diffraction analysis and to tune the optoelectronic properties. Here, we report an efficient synthetic pathway to DBOV based on a sequential iodination-benzannulation of bi(naphthylphenyl)diyne, followed by photochemical cyclodehydroiodination (PCDHI). This protocol included a fused bischrysene as a key intermediate and furnished scalable amounts of meso-substituted DBOV derivatives with different substituents. DBOV with 2,6-dimethylphenyl groups could be used for single-crystal X-ray analysis, revealing the precise structure of the DBOV core. The optoelectronic properties of the DBOV derivatives were investigated by UV-vis absorption and fluorescence spectroscopy, cyclic voltammetry, and density functional theory calculations. Single-molecule spectroscopy at room and low temperatures provided novel insights into the photophysics of DBOV embedded in a polymer film. As a result of weak coupling of the optical transitions to the matrix, single-molecule emission spectra at 4.5 K showed narrow vibronic lines. The fluorescence autocorrelation function covering 9 orders of magnitude in time displayed high contrast photon antibunching and bunching, from which the fluorescence decay rate and the triplet population and depopulation rates could be retrieved. Remarkably, the intersystem crossing rate into the triplet state decreased by more than an order of magnitude at low temperature, demonstrating that temperature can be a crucial parameter to boost single photon emission of an aromatic hydrocarbon.