An Element‐Substituted Cyclobutadiene Exhibiting High‐Energy Blue Phosphorescence

Abstract

1,3,2,4‐Diazadiboretidine, an isoelectronic heteroanalogue of cyclobutadiene, is an interesting chemical species in terms of comparison with the carbon system, whereas its properties have never been investigated experimentally. According to Baird's rule, Hückel antiaromatic cyclobutadiene acquires aromaticity in the lowest triplet state. Here we report experimental and theoretical studies on the ground‐ and excited‐state antiaromaticity/aromaticity as well as the photophysical properties of an isolable 1,3,2,4‐diazadiboretidine derivative. The crystal structure of the diazadiboretidine derivative revealed that the B2N2 ring adopts a planar rhombic geometry in the ground state. Yet, theoretical calculations showed that the B2N2 ring turns to a square geometry with a nonaromatic character in the lowest triplet state. Notably, the diazadiboretidine derivative has the lowest singlet and triplet states lying at close energy levels and displays blue phosphorescence.

An Air‐ and Water‐Stable B4N4‐Heteropentalene Serving as a Host Material for a Phosphorescent OLED

“…Compound 1 (Figure 1b), to which six sterically bulky mesityl groups are attached, could not be obtained with the procedure reported by Nçth and Nçlle. [11] Thus,wedeveloped ad ifferent synthetic route for 1 using known aminoborane (2) [3] (Figure 2a)asastarting material. [13] When 2 was reacted with anhydrous hydrazine (0.5 equiv) in the presence of triethylamine (1.0 equiv) in CH 2 Cl 2 at 25 8 8C, bis(aminoboryl)hydrazine 3 was obtained in 95 %y ield.…”

“…[11] To date,there have been no reports on the properties of B 4 N 4heteropentalene.Here we report the synthesis of anew B 4 N 4heteropentalene derivative (1)t hat is remarkably stable toward air and water (Figure 1b). X-ray crystallographic analysis together with theoretical calculations unveiled the detailed molecular and electronic structures of 1.S imilar to the case of diazadiboretidine II, [3] 1 features ahigh-energy T 1 state.B yv irtue of this excited-state property, 1 can serve as ag ood host material for ap hosphorescent organic lightemitting diode (OLED), where the external quantum efficiency (EQE) of atest device is as high as 15 %.…”

“…[17] Figure 3s hows the absorption and emission spectra of 1 (1.0 10 À5 M) in ad egassed ethanol solution at 298 K. The first absorption band was observed at l max = 271 nm with e = 1.4 10 4 M À1 cm À1 .U pon excitation at 271 nm at 298 K, 1 displayed fluorescence with l max = 344 nm (F FL = 16 %, t FL = 681 ps,F igure S10a). Unlike diazadiboretidine derivative II (Figure 1a), [3] 1 was not phosphorescent in solution at 298 K. However,a t7 7K in ag lassy matrix of ethanol, 1 displayed phosphorescence with l max = 372 (shoulder), 394, and 412 nm (t PHOS = 1.2 s, Figure S10b), which are longer than the fluorescence wavelength (l max = 344 nm). Notably,t he onset wavelength of the phosphorescence emission is as short as 350 nm (Figure 3), indicating that the T 1 state of 1 lies at ahigh energy.Meanwhile,there is apossibility that 1 consisting of polar BÀNbonds may exhibit solvatochromism, but the absorption and fluorescence wavelengths of 1 observed in ethanol (relative permittivity e r = 24.6) and methylcyclohexane (e r = 2.0) were virtually identical to one another (Figure S11).…”

“…[3] TheB 2 N 2 ring of II has as eparate HOMO/ LUMO feature,r esulting in as mall energy gap between the excited singlet (S 1 )a nd triplet (T 1 )s tates. [3] Based on this finding,B Nanalogues of antiaromatic compounds are expected to exhibit unique photochemical and photophysical properties.Here we focused our attention on pentalene (III) with ab icyclic8 p system (Figure 1b). Pentalene is ah ighly reactive species due to its antiaromatic character.W hile the parent molecule (C 8 H 6 )h as been observed only under lowtemperature argon-matrix conditions, [4] it is known that the incorporation of sterically bulky groups [5] and/or benzo-fused structures [6] can stabilize the antiaromatic compound enough for isolation.…”

“…Replacement of the carbon-carbon bonds of p-conjugated molecules with polar boron-nitrogen bonds is aviable way to develop functional molecules, [1] particularly for optoelectronic applications, [2] since the BN-substitution allows for the tuning of electronic structures and emission properties.V ery recently,w er eported that at etraarylated 1,3,2,, featuring ac yclic B 2 N 2 -4p system with an isoelectronic structure of cyclobutadiene (I), can emit blue phosphorescence in solution even at room temperature (Figure 1a). [3] TheB 2 N 2 ring of II has as eparate HOMO/ LUMO feature,r esulting in as mall energy gap between the excited singlet (S 1 )a nd triplet (T 1 )s tates. [3] Based on this finding,B Nanalogues of antiaromatic compounds are expected to exhibit unique photochemical and photophysical properties.Here we focused our attention on pentalene (III) with ab icyclic8 p system (Figure 1b).…”