Luminescent Di‐ and Trinuclear Boron Complexes Based on Aromatic Iminopyrrolyl Spacer Ligands: Synthesis, Characterization, and Application in OLEDs

Abstract

New bis- and tris(iminopyrrole)-functionalized linear (1,2-(HNC4 H3 -C(H)N)2 -C6 H4 (2), 1,3-(HNC4 H3 -C(H)N)2 -C6 H4 (3), 1,4-(HNC4 H3 -C(H)N)2 -C6 H4 (4), 4,4'-(HNC4 H3 -C(H)N)2 -(C6 H4 -C6 H4 ) (5), 1,5-(HNC4 H3 C-(H)N)2 -C10 H6 (6), 2,6-(HNC4 H3 C-(H)N)2 -C10 H6 (7), 2,6-(HNC4 H3 C-(H)N)2 -C14 H8 (8)) and star-shaped (1,3,5-(HNC4 H3 -C(H)N-1,4-C6 H4 )3 -C6 H3 (9)) π-conjugated molecules were synthesized by the condensation reactions of 2-formylpyrrole (1) with several aromatic di- and triamines. The corresponding linear diboron chelate complexes (Ph2 B[1,3-bis(iminopyrrolyl)-phenyl]BPh2 (10), Ph2 B[1,4-bis(iminopyrrolyl)-phenyl]BPh2 (11), Ph2 B[4,4'-bis(iminopyrrolyl)-biphenyl]BPh2 (12), Ph2 B[1,5-bis(iminopyrrolyl)-naphthyl]BPh2 (13), Ph2 B[2,6-bis(iminopyrrolyl)-naphthyl]BPh2 (14), Ph2 B[2,6-bis(iminopyrrolyl)-anthracenyl]BPh2 (15)) and the star-shaped triboron complex ([4',4'',4'''-tris(iminopyrrolyl)-1,3,5-triphenylbenzene](BPh2 )3 (16)) were obtained in moderate to good yields, by the treatment of 3-9 with B(C6 H5 )3 . The ligand precursors are non-emissive, whereas most of their boron complexes are highly fluorescent; their emission color depends on the π-conjugation length. The photophysical properties of the luminescent polyboron compounds were measured, showing good solution fluorescence quantum yields ranging from 0.15 to 0.69. DFT and time-dependent DFT calculations confirmed that molecules 10 and 16 are blue emitters, because only one of the iminopyrrolyl groups becomes planar in the singlet excited state, whereas the second (and third) keeps the same geometry. Compound 13, in which planarity is not achieved in any of the groups, is poorly emissive. In the other examples (11, 12, 14, and 15), the LUMO is stabilized, narrowing the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO), and the two iminopyrrolyl groups become planar, extending the size of the π-system, to afford green to yellow emissions. Organic light-emitting diodes (OLEDs) were fabricated by using the new polyboron complexes and their luminance was found to be in the order of 2400 cd m(-2) , for single layer devices, increasing to 4400 cd m(-2) when a hole-transporting layer is used.

Synthetic Approaches to Star‐Shaped Molecules with 1,3,5‐Trisubstituted Aromatic Cores

“…Suresh et al. reported the synthesis of star‐shaped molecule 347 , which contained 2‐formiminopyrrolyldiphenylboron moieties . Their synthesis involved a two‐step sequence: condensation of 2‐formylpyrrole ( 344 ) with 1,3,5‐tris(4′‐aminophenyl)benzene (TAPB; 181 ) furnished imine derivative 345 , which, on subsequent treatment with BPh 3 ( 346 ) at reflux in toluene, gave triboron complex 347 in 84 % yield (Scheme ).…”

“…eties. [78] Their synthesis involved at wo-step sequence:c ondensation of 2-formylpyrrole (344)w ith1 ,3,5-tris(4'-aminophenyl)benzene (TAPB; 181)f urnished imine derivative 345,w hich, on subsequentt reatment with BPh 3 (346)a tr eflux in toluene, gave triboron complex 347 in 84 %y ield (Scheme 110). This triboron complex wasf ound to be ab lue-light emitter.M urugavel and co-workersa lso used buildingb lock 181 in the synthesis of severalt ris-benzaldimine and tris-salicylaldimine Schiffbase sensors.…”

Benzodipyrrole‐based Donor–Acceptor‐type Boron Complexes as Tunable Near‐infrared‐Absorbing Materials

“…11 B NMR spectra of L1B1 , L1B2 , and L1B2F showed signals at 5.55 ppm, 5.13 ppm, and 5.17 ppm, which appear at a much higher magnetic field than that of three‐coordinated triphenylborane (45.8 ppm). The higher magnetic field shifts indicate that the hybridization at the boron center changes from sp 2 to sp 3 , and the phenomena are observed in related N , N ‐chelating four‐coordinate boron species …”

Iodo‐Functionalized Salicylideneimine‐Boron Complexes: Synthesis and Photosensitized Degradation of Organic Water Pollutants

“…The overlap of the empty p‐orbital of boron with the organic moieties in such compounds leads to the delocalization of the electron cloud and planarization of the π‐systems, thereby resulting in interesting photophysical properties. Several tri‐ and tetra‐coordinated boron compounds have been synthesized by the complexation of boron with organic ligands containing hetero atoms . Four‐coordinate boron compounds based on an imine scaffold are reported to have higher stability compared with tricoordinate boron compounds .…”

“…Severalt ri-andt etra-coordinated boron compounds have been synthesized by the complexation of boron with organic ligands containing hetero atoms. [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] Fourcoordinate boronc ompounds based on an imine scaffold are reported to have higher stabilityc ompared with tricoordinate boron compounds. [18] Although the luminescentp roperties of boranils are well documented, [18] the photosensitizationp rop-ertieso fs uch molecules have rarely been explored.…”