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

Abstract

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.

Synthesis and helical supramolecular organization of discotic liquid crystalline dibenzo[hi,st]ovalene

“…Then Grignard reagent 4 was prepared by refluxing 3 and magnesium turning in tetrahydrofuran with I 2 as initiator. 5,14-Diformylbenzo 52,56 was subsequently treated with Grignard reagent 4 at room temperature followed by quenching with NH 4 Cl to give diol intermediate 6, which was subjected to BF 3 ÁOEt 2 promoted Friedel-Crafts reaction and oxidation by p-chloranil to afford DBOV-TDOP with a high yield of 85% in three steps. This compound showed high solubility in dichloromethane, tetrahydrofuran or toluene, which allowed unambiguous characterization of its structure in solution.…”

“…The synthesis of 5,14-diformylbenzo[a]dinaphtho[2,1,8-cde:1 0 ,2 0 ,3 0 ,4 0 -ghi]perylene (5) is described in our previously report, 52 and we also used an improved synthetic route, which we have more recently developed. 56 Synthesis of 5-bromo-1,2,3-tris(dodecyloxy)benzene 35-Bromo-1,2,3-trimethoxybenzene (1) (2.5 g, 10 mmol) was dissolved in dry dichloromethane (30 mL), and the solution was cooled down to À78 1C. BBr 3 (8.26 g, 33.0 mmol) was added dropwise to the solution then the reaction mixture was gradually warmed up to room temperature and stirred overnight.…”

Synthesis and Characterization of Dibenzo[<i>hi,st</i>]ovalene as a Highly Fluorescent Polycyclic Aromatic Hydrocarbon and Its π-Extension to Circumpyrene

“…A single crystal suitable for X ray diffraction analysis could thus be obtained for DBOV 15e with two 2,6 dimethylphenyl groups, revealing the precise structure of the DBOV core (Figure 8a b). 53 Interestingly, 15e exhibited a herringbone π stacking motif in the crystal, with a face to face distance between two DBOV cores of 3.13 Å (Figure 8c). The CH π interaction between the DBOV cores and the 4 position of the 2,6 dimethylphenyl groups was suggested by a distance of 3.25 Å, which might have facilitated the crystallization of DBOV 15e in comparison to DBOV 15b with mesityl groups.…”

“…Indeed, the photochemical cyclodehydroiodination of iodinated bichrysene 20 in acetone in the presence of triethylamine (TEA) gave fused bichrysene 14b in 86% yield ( Figure 7). 53 TEA was added as the base to trap the generated HI to prevent an intramolecular Friedel Crafts reaction of 14b to give an insoluble diketone byproduct. The yield of 14b from 10b was thus improved to 65% over two steps from 4.2% over six steps in the previous synthetic route via the Scholl reaction.…”

Spiers Memorial Lecture : Carbon nanostructures by macromolecular design – from branched polyphenylenes to nanographenes and graphene nanoribbons

“…While such open-shell NGs are typically unstable, dibenzo[hi,st]ovalene (DBOV) 35, also having zigzag edges, displays closed-shell properties with very high stability ( Figure 5A). [106][107][108] DBOV was revealed to have sharp absorption and emission peaks with a small Stokes shift of 12 nm ( Figure 5B). DBOV exhibited strong red emission with a high photoluminescence quantum yield of ~80%, which could later be improved up to 97% by substituting the bay regions with aryl groups.…”