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Luminescent halogen-substituted 2-(N-arylimino)pyrrolyl boron complexes: the internal heavy-atom effect

Rodrigues, A. ; Krishnamoorthy, P. K. ; Gomes, C. S. B. ; Carmona, N. ; Paolo, R. ; Pander, P. ; Pina, J. ; Sérgio Seixas de Melo, J. ; Dias, F. ; Calhorda, M. ; Maçanita, A. ; Morgado, J. ; Gomes, P.

Dalton Transactions Vol. 49, Nº 29, pp. 10185 - 10202, May, 2020.

ISSN (print): 1477-9226
ISSN (online): 1477-9234

Journal Impact Factor: 3,580 (in 2008)

Digital Object Identifier: 10.1039/d0dt01845g

Abstract
A group of new boron complexes [BPh2{κ2N,N’-NC4H3-2-C(H)vN-C6H4X}] (X = 4-Cl 4c, 4-Br 4d, 4-I 4e, 3-Br 4f, 2-Br 4g, 2-I 4h) containing different halogens as substituents in the N-aryl ring have been syn- thesized and characterized in terms of their molecular properties. Their photophysical characteristics have been thoroughly studied in order to understand whether these complexes exhibit an internal heavy- atom effect. Phosphorescence emission was found for some of the synthesized halogen-substituted boron molecules, particularly for 4g and 4h. DFT and TDDFT calculations showed that the lower energy absorption band resulted from the HOMO to LUMO (π–π*) transition, except for 2-I 4h, where the HOMO−1 to LUMO transition was also involved. The strong participation of iodine orbitals in HOMO−1 is reflected in the calculated absorption spectra of the iodine derivatives, especially 2-I 4h, when spin–orbit coupling (SOC) was included. Organic light-emitting diodes (OLEDs) based on these complexes, in the neat form or dispersed in a matrix, were also fabricated and tested. The devices based on films prepared by thermal vacuum deposition showed the best performance. When neat complexes were used, a maximum luminance (Lmax) of 1812 cd m−2 was obtained, with a maximum external quantum efficiency (EQEmax) of 0.15%. An EQEmax of ca. 1% along with a maximum luminance of 494 cd m−2 were obtained for a device fabricated by co-deposition of the boron complex and a host compound (1,3-bis(N-carbazo- lyl)benzene, mCP).