Enhancing Light‐Absorption and Luminescent Properties of Non‐Emissive 1,3,4,6,9b‐Pentaazaphenalene through Perturbation of Forbidden Electronic Transition by Boron Complexation

Abstract

We report an improvement of optical properties of the non‐emissive molecule through shuffling the molecular orbitals by boron complexation. The 1,3,4,6,9b‐pentaazaphenelene (5AP) derivative having a 2‐(dimesitylboryl)phenyl group was synthesized through a substitution reaction from a 2‐bromophenyl compound. The formation of the B−N dative bond was confirmed by 11B{1H} NMR spectroscopy and a single‐crystal X‐ray structure analysis. Most of the conventional 5AP derivatives hardly showed emission despite their planar and rigid molecular structures. The previous reports on 5AP derivatives ascribed the non‐luminescent behavior to the forbidden transition between frontier orbitals. On the other hand, the 5AP‐based boron complex in this work showed enhanced light absorption and luminescence from the HOMO−LUMO transition. Theoretical calculations suggested that the boron complexation should play a critical role in perturbing the forbidden character of the HOMO−LUMO transition. Because the nitrogen atom on the 5AP moiety formed the Lewis acid−base pair with the boron atom, the energy level of the HOMO of 5AP was downshifted. The character of the HOMO−LUMO transition of the boron complex was changed to afford the improved optical properties of the boron complex.

Organic molecules with inverted gaps between first excited singlet and triplet states and appreciable fluorescence rates

“…Furthermore, compound 38 and several other derivatives of 5 have been synthesized very recently and their optical properties characterized. [83][84][85][86] Based on our computational results, 38 has an inverted singlet-triplet gap and appreciable fluorescence rate. While the latter has been confirmed experimentally, 86 the inverted singlet-triplet gap has remained unrecognized, as the use of double-hybrid TD-DFAs capable to characterize their electronic properties is not widespread.…”

“…From these molecules, we selected four, marked in red in Figure 1 and depicted in Scheme 3, because of their favorable trade-off between the singlet-triplet gap and the oscillator strength, their distinct excitation energies and because synthetic procedures for compounds with these core structures have been reported. [72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88] State energy differences, oscillator strengths and estimated fluorescence rate constants of 1-6 are summarized in Table 2. Scheme 3.…”

“…130,143 It should be noted that a similar strategy to increase the fluorescence rate using boron complexation was demonstrated to be effective on substituted derivatives of 5 very recently. 85 However, the use of these strong intramolecular hydrogen bonds comes with the risk of potential ESIPT, which could introduce low-lying triplet states and break the INVEST property. [102][103][104][105][106][107][108][109][110] Nevertheless, in the set of validation compounds only 42 shows proton transfer to lead to a lower excited triplet state causing an increase of the singlet-triplet gap of 0.08 eV.…”

“…However, while systematic substitution of C-H by N is trivial in computational studies, it can strongly impact synthesizability. While substituted analogs of 5 have good literature precedence, 72,75,76,[81][82][83][84][85][86] derivatives of 3, 4 and 6 are less common. [72][73][74]77,78,80,165 Moreover, to the best of our knowledge, neither 21 itself nor any substituted analogs have been synthesized before, only derivatives of 18 have been reported.…”

Modulation of stimuli‐responsiveness toward acid vapor between real‐time and write‐erase responses based on conjugated polymers containing azobenzene and Schiff base moieties

“…25 By applying this azasubstitution protocol for lowering lowest unoccupied molecular orbitals (LUMOs) of conventional chromophores, light-absorbing materials in the longer wavelength region can be obtained. [26][27][28][29][30] The strategy for selective control of FMO was also suggested by Bunz et al by cross-conjugated cruciform fluorophores. 31 In general, extension and restriction of π-conjugation and electron-donating and withdrawing interaction are the conventional manners for modulating electronic properties of polymers, meanwhile significant changes are accomplished only by replacing carbon to heteroatom in our design scheme.…”