the nucleophilic reagent. The existence of o-DCB makes the purification difficult. We tried to use several other solvents with lower boiling points at lower reaction temperatures, all of which generated the target compound NBNN-1 with similar yields around 70%. Usually, the presence of the additive i Pr 2 NEt was proven to be essential to the electrophilic borylation, but absence of i Pr 2 NEt did not show any influence on the borylation yield. To be our delight, the reaction temperature was finally lowered down to room temperature without i Pr 2 NEt in 78% yield (Scheme 2). In addition, the kind of nucleophiles was also probed, lithium reagent like phenyl lithium could produce compound NBNN-1 in slightly lower yield. While we are preparing the manuscript, the synthesis of NBNN-1 was reported under high temperature. With the optimal conditions in hands, ethynylmagnesium bromide and mesitylmagnesium bromide were used as nucleophiles to generate compounds NBNN-2 and NBNN-3 in 58% and 69% yields, respectively. Compound NBNN-4 was obtained when the reaction was quenched by water without adding any nucleophiles in one digit yield, however, the yield of NBNN-4 was significantly improved to 46% when i Pr 2 NEt was used to quench the reaction. We also tried other hydroxy group containing reagent like phenol, which did not give the corresponding product at all. Compound NBNN-5 was also synthesized under the optimal conditions. Other nucleophiles like isopropylmagenisium bromide, vinylmagenisium bromide, methyl lithium could also yield the corresponding tetracoordinate boron-doped PAHs, however, those compounds were slowly decomposing on silica gel or alumina. Bulky nucleophile like t butylmagenisium bromide did not react probably due to the steric hindrance. Compounds NBNN-1 to NBNN-5 were fully characterized by NMR and HRMS spectroscopies. The molecular structures of NBNN-1 and NBNN-3 were further determined by single-crystal X-ray diffraction analysis and shown in Figure . 3). Silica gel or alumina column chromatography was not compatible to NBNN-4f, which decomposed on silica gel or alumina and was confirmed by HRMS. NBNN-3 did not undergo oxidative coupling reaction probably due to the longer distance (3.356 Å, Figure ) between two carbon atoms. Unlike the tricoordinate boron-doped PAH analogue (E in Scheme 1), the crystal structure of NBNN-1f shows that the four coordinated boron atom adopts a tetrahedral geometry, which makes the molecular structure off-plane (Figure ). The B1-N1, B1-N2, and B1-N3 bond lengths are 1.640 Å, 1.523 Å, and 1.502 Å, respectively. Similar to tricoordinate boron-doped PAH E, the πstacking interactions of NBNN-1f are limited to two neighbouring molecules with longer separation distance than E. The absorption, emission and electrochemical data of these seven tetracoordinate boron-doped PAHs are shown in Figure and Table . The maximums of absorption of NBNN-1 to NBNN-5 are around 460 nm with the exceptions of NBNN-3 and NBNN-5, in which the absorption peaks are red-shit to 481 nm and blue-shift to 446 nm, respectively. The absorption peaks are in agreement with the colors of their THF solutions (Figure ). The compound NBNN-5 bearing indole units only has less than 10% quantum yield both in solution and in solid. Compared to NBNN-1 and NBNN-2, the emission peaks of the fused molecules NBNN-1f and NBNN-2f show remarkable red-shift up to 100 nm and appear indistinguishable fluorescent spectra (λ em = 660 nm, Φ < 1%), while their absorption spectra are also similar with different absorption extinction coefficients. The fluorescence peak of NBNN-1f in PMMA film was blue-shifted up to 39 nm compared to that in solution (see SI for details), while NBNN-2f in PMMA film exhibits two emission peaks at 624 nm and 659 nm. Both NBNN-1f and NBNN-2f show significant increase on quantum yields in solid up to 4%. The low quantum yields of NBNN-1f and NBNN-2f are likely due to the internal conversion between small S 0 -S 1 gaps. The fluorescence peaks in solution and solid are consistent with the emission colors of their THF solutions and PMMA films (Figure ). Molecules NBNN-1 to NBNN-5 display pseudo-reversible reduction peaks at ~-2.0 V (vs.Fc/Fc + ) and multiple reversible oxidation peaks with the first oxidation peaks at ~0.3 V (vs.Fc/Fc + ) (Table and To illustrate the aromaticity of these tetracoordinate boron-doped PAHs, the nucleus-independent chemical shift (NICS(1)) of two C3NBN rings and 7-memebered C4NBN ring (if exists) around boron atoms were calculated (Figure ). The anisotropy of the induced current density (ACID) calculations 24 that are based on the optimized structures further support the antiaromaticity of two C3NBN rings and C4NBN ring, showing an anti-clockwise diatropic ring current circuit. In addition, the 13-membered ring with a boron center of NBNN-1f is antiaromatic with an anticlockwise diatropic ring current circuit, as shown in Figure .