Design, Synthesis, And Properties Of Λ-Shaped Organoboron Optoelectronic Functional Materials Based On Tr(o|¨)ger’s Base

Organic fluorophores with strong solid-state emission have aroused great interest for their important use in the fundamental research field of solid-state photochemistry and in the applied field of optoelectronic devices. However, most organic fluorophores exhibit "Aggregation-Caused Quenching (ACQ)" effect:showing high fluorescence in dilute solution but weak luminescence in the solid state due to the formation of close π-π stacking, excimer, and H-aggregation. The ACQ problem must be properly tackled because the luminescent materials are commonly used as solid films or crystalline states in their practical applications. Thus, design and searching of highly emissive solids, which can overcome this problem, would be very rewarding and significant for both theory and applications.Rencently, much effort has been devoted to developing effective solid-state emissive fluorophores, such as the introduction of bulky substituents to the original fluorophores, fluorescent J-Aggregates, aggreration-induced emission (AIE) and so on. Besides of these, another approach to alleviate the undesirable intermolecular close π-π stacking that occurs is to attach the conjugated backbones of the molecules to a severely twisted or noncoplanar scaffold. In this application, Troger’s base will serve as the scaffold. In theory, its special A-shaped steric configuration does not readily engage in close π-π stacking. Based on this assumption, our group has recently developed a class of TB analogues containing large π-conjugated aromatic groups or pyridinium salts. These compounds with strong fluorescence in aggregate states have been successfully applied in OLEDs and biosensor fields. However, new optoelectronic materials based on TB, the relationship between structures and properties, and the application in optoelectronic fields need to be explored deeply. Aiming at the functionality such as organic light-emitting, fluorescent sensor and ion recognition, we developed a series of new A-shaped organoboron compounds based on Troger’s Base which exhibit intense fluorescence in both dilute solution and solid state. We studied their photophysical properties, crystal structures and the relationship between structure and properties in detail. In addition, we also explore their applications in ionprobes filed. The main contents are as follows:1. Crystal structures and solid-state fluorescence of BODIPY dyes based on A-shaped Troger’s baseA strategy to prevent the close π-π stacking and thus enhance the solid-state fluorescence has been developed by introducing the twisted Λ-shaped TB skeleton into the BODIPY core. Based on this strategy, two A-shaped BODIPY dyes based on Troger’s base, namely2,8-Di(4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene)-(6H,12H-5,11-methanodibenzo[b,f][1,5]diazocineylene)(DFTMB) and2,8-Di(4,4-difluoro-2,6-diethyl-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene)-(6H,12H-5,11-methanodibenzo[b, f][1,5]diazocineylene)(DFDEB), were synthesized. Both compounds display intense fluorescence in dilute solutions and aggregation state. The analysis of the crystal structure indicates that molecules are stacked in an off-set fashion which is unfavourable for the formation of close π-π stacking. In addition, different substituted alkyl groups on the BODIPY core influence the molecular stacking modes such that the shortest distance between the closest overlapping nearparallel BODIPY wings in DFTMB (16.98A) is larger than that in DFDEB (13.26A). This result agrees well with the fluorescent behavior; that is, PL spectra of DFDEB powder exhibit a greater red shift compared with that of the solution. Moreover, we compared the powder PL spectra of the two TB-BODIPY dyes with another two solid-state emissive BODIPY dyes. It is noted that the emission of the two A-shaped BODIPY dyes, especially that of DFDEB, are much stronger than that of the reported BODIPY dyes. The results clearly indicate that our A-shaped BODIPY dyes can exhibit brighter emission compared with the bulky substituted BODIPY derivatives.2. Dual emission pathways in A-shaped triarylboranesEncouraged by the unusual dual emission pathways in the U-and V-shaped triarylboron molecules and their various potential applications, we developed a new class of A-shaped triarylboranes including two donor-acceptor triarylboron molecules (namely2-(4-(N, N-dimethylamino)-8-dimesitylboryl-6H,12H-5,11-methano-dibenzo [b, f][1,5]diazocine (TBBN) and2-(4-(N, N-diphenylamino)-8-dimesitylboryl-6H,12H-5,11-methanodibenzo[b, f][1,5]diazocine (TBBN2)) and two acceptor-only or donor-only molecules (namely2,8-Di(dimesitylboryl)-6H,12H-5,11-methanodibenzo [b,f][1,5]diazocine (TBB) and2,8-Di(4-N, N-dimethylamino)-6H,12H-5,11-methanodibenzo[b, f][1,5]diazocine (TBNN)), in which the twisted A-shaped TB scaffold is selected as the spacer. The first three compounds exhibit twisted structures and intense fluorescence in the aggregated states. For compounds TBBN and TBBN2, the A-shaped TB scaffold endows the amino substituents and the dimesitylboryl group with a nonplanar framework, by virtue of which their HOMO and LUMO are localized on the amine side and boryl side, respectively, thus charge transfer from the donor to acceptor most likely occurs through space rather than through the non-conjugated bridge. As a consequence, dual fluorescent pathways, namely through-space charge transfer and π*-π transitions, are observed to coexist in each molecule. However, the lowest electronic transitions (HOMO→LUMO transitions) possess quite small oscillator strength of0.0194and0.0055for TBBN and TBBN2, respectively, which are unfavorable for efficient fluorescence. While compound TBB exhibits intense donor-acceptor charge-transfer emission owing to the D-π-A geometry in both of two sides, thus the lowest electronic transition is consistent with the charge transfer from the N centers on TB scaffold to the two boron centers with the oscillator strength of0.2215.3. Through space charge-transfer emission in A-shaped triarylboranes and the use in Fluorescent Sensing for Fluoride and Cyanide IonsConsidering the characteristic through-space charge transfer emission of TBBN and TBBN2, we studied their abilities in fluorescence sensing of fluoride and cyanide. With absence of fluoride ion, the dual fluorescent pathways coexist in TBBN and TBBN2. While the binding of fluoride ions or cyanide ions to the boron center disrupts the through-space charge transfer, and the π*-π transitions become the lowest electronic transitions and gain intensity, leading to dramatic blue shifts (about80-140nm) and color changes in the fluorescence. Thus TBBN and TBBN2can be potentially applied as ratiometric and colorimetric "switch-on" fluorescent sensors for fluoride and cyanide. With both sides of boryl substituents, TBB is a "switch-off" fluorescence sensor. By intruding the twisted and nonplanar Λ-shaped TB scaffold to triarylboranes, we provide an efficient strategy to develop boryl compounds applied as visually colorimetric and ratiometric fluorescent sensors for fluoride and cyanide.