Selectively Fluorinated Furan‐Phenylene Co‐Oligomers Pave the Way to Bright Ambipolar Light‐Emitting Electronic Devices

Abstract

Linearly conjugated oligomers attract ever-growing attention as promising systems for organic optoelectronics because of their inherent lucky combination of high charge mobility and bright luminescence. Among them, furanphenylene co-oligomers (FPCOs) are distinguished by outstanding solubility, very bright luminescence, and good hole-transport properties; however, furan-containing organic semiconductors generally lack electron transport, which makes it impossible to utilize them in efficient light-emitting electronic devices, specifically, ambipolar light-emitting transistors. In this work, 1,4-bis( 5-phenylfuran-2-yl)benzene (FP5) derivatives are synthesized with the fully/partially fluorinated central and edge phenyl rings. It is shown that the selective fluorination of FPCOs lowers the energies of frontier molecular orbitals, maintaining the bandgap, solubility, and bright luminescence, dramatically improves the photostability, tunes the π-π stacked packing, and allows the first realization of electron transport in FPCOs. It is found that selectively fluorinated 2,2′-(2,3,5,6-tetrafluoro-1,4-phenylene)bis[5-(3,5difluorophenyl)furan] demonstrates well-balanced ambipolar charge transport and efficient electroluminescence in an organic light-emitting transistor (OLET) with external quantum and luminous efficiencies as high as 0.63% and 5 cdA −1 , respectively, which are among the best reported for OLETs. The findings show that "smart" fluorination is a powerful tool to fine-tune the stability and performance of linearly conjugated small molecules for organic optoelectronics.

Host–guest molecular doping guide for emissive organic semiconductor crystals

“…Anthracene, tetracene, 9,10-diphenylanthracene (DPA), pyrene, and perylene were purchased from commercial suppliers (Sigma Aldrich, Acros Organics, Merck and Alfa Aesar). 1,4-Bis(5phenylfuran-2-yl)benzene (FP5), 11 5,5 0 -bis(4-(5-phenylfuran-2yl)phenyl)-2,2 0 -bifuran (FP8), 10 2,5-bis(4-(5-phenylfuran-2-yl)phenyl)furan (FP7), 12 2,5-diphenylthiophene (PTP), 49 (5,5 0 -(perfluoro-1,4phenylene)bis(2-(3,5-difluorophenyl)furan) (F2-FPF5)), 28 1,4-bis(5-(4-(trifluoromethyl)phenyl)furan-2-yl)benzene (CF 3 -FP5), 50 1,4-bis(5-(ptolyl)furan-2-yl)benzene (Me-FP5) 29 and 1,4-bis(5-phenylthiophen-2yl)benzene (AC5) 51 were synthesized by literature procedures.…”

“…Powder X-ray diffraction patterns were recorded on a Stoe-MP diffractometer (Cu Kα1 radiation, curved Ge monochromator, and Mythen 1 K detector) at ambient temperature. The crystal data were taken from the Cambridge Structural Database, 52 the corresponding refcodes are as follows: anthracene (ANTCEN19 53 ), tetracene (TETCEN 54 ), 9,10-diphenylanthracene (α-form: DPANTR07, 55 β-form: DPANTR08 56 ), perylene (α-form: PERLEN04 57 ), rubrene (triclinic polymorph-I: QQQCIG14 58 ), 1,4-bis(5-phenylfuran-2-yl)benzene (AWOFUP 11 ), and (5,5′-(perfluoro-1,4-phenylene)bis(2-(3,5-difluorophenyl)furan) (WECKAU 28 )).…”

“…The variation in the number and type of conjugated units in TPCOs allowed the production of single crystals with high mobility 6 and luminescence efficiency. 7,18 The introduction of terminal substituents in TPCOs, 19,20 distyrylbenzenes, 5,[21][22][23][24][25][26][27] FPCOs 28,29 and 2,6-diphenylanthracene 30 was previously reported to be an efficient approach for solubility, charge carrier polarity and crystal morphology control. Crystallization control via solution processing has been proven to increase charge carrier mobility and aid the manipulation of the crystal structure and morphology without changing the chemical structure.…”

“…48 The generality of the phenomenon was also later highlighted for the furan-phenylene co-oligomer 10 and the series of selectively fluorinated FPCOs. 28 Therefore, the intentional and controllable molecular (both external and self-) doping of conjugated oligomer single crystals is an extremely useful approach in organic optoelectronics. Chart 1 summarizes previously reported host-dopant systems that are briefly discussed in the results and discussion section (from a host-dopant compatibility point of view, see below).…”

Synthetic approach for the control of self-doping in luminescent organic semiconductors

“…41 The generality of this phenomenon was shown in the pioneering work and was also highlighted later for the furan-phenylene co-oligomer 7 and for the series of selectively fluorinated furan-phenylenes. 42 So, one can generally expect the molecular self-doping to occur in materials synthesized by cross-coupling reactions allowing the formation of a luminescent by-product. The advancement of the molecular self-doping approach is also the improvement of the host luminescent properties due to host–dopant Förster resonant energy transfer (in case the dopant absorption spectrum overlaps with the host fluorescence spectrum) and balancing charge trap states by optimizing the dopant concentration which can be tuned by mixing of the samples with different dopant concentrations dependent on the purification degree.…”