LIGHT EMITTING ELECTROCHEMICAL CELLS AND COMPOUNDS

Abstract

Charged organic thermally activated delayed fluorescence (TADF) species are described. A light-emitting electrochemical cell (LEEC) includes the charged organic thermally activated delayed fluorescence (TADF) species and sufficient counter ions to balance the charge on the charged organic thermally activated delayed fluorescence species, as emitter material. Also disclosed are OLEDSs containing the TADF species.

Claims

1-35. (canceled)

36. An electroluminescent device comprising: a metal free charged organic thermally activated delayed fluorescence (TADF) species; and sufficient counter ions to balance the charge on the charged organic thermally activated delayed fluorescence species, as emitter material.

37. The electroluminescent device of claim 36, selected from the group consisting of: a light-emitting electrochemical cell (LEEC); and an organic light emitting diode (OLED).

38. The electroluminescent device of claim 36, wherein the emitter material comprises, consists of, or consists essentially of a compound according to formula I:
TADFY.sup.p).sub.nmA.sup.q  I wherein TADF is a metal free organic thermally activated delayed fluorescence moiety; Y is a metal free charged species bonded to the TADF moiety; n is at least 1; A is a counter ion; p and q are the charges on each Y and A respectively; and m is the number of counter ions A, wherein p multiplied by n=m multiplied by q.

39. The electroluminescent device of claim 38, wherein the compound of formula I takes the form of formula II:
TADFL-Z.sup.p).sub.nmA.sup.q  II wherein the metal free charged species Y is independently for each occurrence a non-metal charged group Z and optional linking group L; and wherein TADF, A, n, m, p and q have the same meaning as in formula I.

40. The electroluminescent device of claim 39, wherein the compound of formula II includes at least one linking group L and each linking group L comprises a hydrocarbylene chain, that may be substituted or unsubstituted, hydrocarbylene or unsaturated hydrocarbylene.

41. The electroluminescent device of claim 40, wherein the compound of formula II includes one linking group L, that comprises a hydrocarbylene chain, for each group Z.

42. The electroluminescent device of claim 40, wherein linking groups L are unsubstituted hydrocarbylene chains of the form: ##STR00053## wherein n is from 0 to 10 or even 0 to 5 and the hydrocarbylene chain optionally contains one or more unsaturations.

43. The electroluminescent device of claim 40, wherein linking groups L are selected from the group consisting of cyclopentane-1,3-diyl; cyclohexane-1,4-diyl; 1,4-phenylene; and 4,4′-biphenylene.

44. The electroluminescent device of claim 39, wherein the compounds of formula II take the form of formula III or formula IV: ##STR00054## wherein n is at least 1 and n=m.

45. The electroluminescent device of claim 44, wherein the compound of formula III is employed and the groups Z are independently for each occurrence selected from the group consisting of quaternary nitrogen cations and quaternary phosphorus cations.

46. The electroluminescent device of claim 44, wherein the compound of formula IV is employed and the groups Z are independently for each occurrence selected from the group consisting of carboxylate, sulfonate, sulfinate, phosphonate, cyanide and thiocyanate.

47. The electroluminescent device of claim 45, wherein the groups Z are selected from the group consisting of: ##STR00055## wherein -L indicates the position of bonding to a linking group L or directly to a TADF moiety; R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7, R.sup.8, R.sup.9 and R.sup.10 are, independently for each occurrence, selected from the group consisting of —H, substituted or unsubstituted primary, secondary or tertiary alkyl, that may be cyclic and may be unsaturated; substituted or unsubstituted aryl or heteroaryl, —CF.sub.3, —OMe, —SF.sub.5, —NO.sub.2, halo, aryl, aryl hydroxy, amino, alkoxy, alkylthio, carboxy, cyano, thio, formyl, ester, acyl, thioacyl, amido, sulfonamido, and carbamate.

48. The electroluminescent device of claim 45, wherein at least one group Z takes the form of structure 5: ##STR00056## wherein one of R.sup.8, R.sup.9, R.sup.10 and R.sup.11 bonds to a linking group L or directly to a TADF moiety and the others of R.sup.8, R.sup.9, R.sup.10 and R.sup.11, independently for each occurrence selected from the group consisting of —H, substituted or unsubstituted primary, secondary or tertiary alkyl, that may be cyclic and may be unsaturated; substituted or unsubstituted aryl or heteroaryl, —CF.sub.3, —OMe, —SF.sub.5, —NO.sub.2, halo, aryl, aryl hydroxy, amino, alkoxy, alkylthio, carboxy, cyano, thio, formyl, ester, acyl, thioacyl, amido, sulfonamido, and carbamate.

49. The electroluminescent device of claim 47, wherein the groups Z are selected from the group consisting of: ##STR00057## wherein -L indicates the position of bonding to a linking group L or directly to a TADF moiety.

50. The electroluminescent device of claim 45, wherein the groups Z are quaternary phosphorus groups Z of the form: ##STR00058## wherein R.sup.1, R.sup.2 and R.sup.3 are, independently for each occurrence, selected from the group consisting of —H, substituted or unsubstituted primary, secondary or tertiary alkyl, that may be cyclic and may be unsaturated; substituted or unsubstituted aryl or heteroaryl, —CF.sub.3, —OMe, —SF.sub.5, —NO.sub.2, halo, aryl, aryl hydroxy, amino, alkoxy, alkylthio, carboxy, cyano, thio, formyl, ester, acyl, thioacyl, amido, sulfonamido, and carbamate; and wherein -L indicates the position of bonding to a linking group L or directly to a TADF moiety.

51. The electroluminescent device of claim 50, wherein the groups Z are selected from the group consisting of: ##STR00059## wherein -L indicates the position of bonding to a linking group L or directly to a TADF moiety.

52. The electroluminescent device of claim 36, wherein the metal free charged organic thermally activated delayed fluorescence species is according to formula V, formula XIII or formula XVI: ##STR00060## wherein each D is a donor moiety independently selected from the group consisting of: ##STR00061## wherein each of A.sup.1, A.sup.2, and A are acceptor groups that may be same or different and are independently selected from the group consisting of —CN, —CO.sub.2.sup.−, —CO.sub.2R*, —SO.sub.3.sup.−, —PO.sub.4.sup.−, —NR.sub.3.sup.−, —PR.sub.3.sup.+, halogen, wherein R* and the substituents R on .sup.−, —NR.sub.3.sup.+ and —PR.sub.3.sup.+, are independently for each occurrence selected from H, alkyl, aryl or heteroaryl; and wherein when none of A.sup.1, A.sup.2, and A.sup.3 are charged, at least one of the occurrences of R.sup.11, R.sup.12, R.sup.13, R.sup.14, R.sup.15 and R.sup.16 represents the bonding position, either directly or via a linking group L, to a charged group Z; and wherein when none of A.sup.1, A.sup.2, and A.sup.3 are present, at least one of the occurrences of R.sup.13, R.sup.14, R.sup.15, R.sup.16, R.sup.17 and R.sup.18 represents the bonding position, either directly or via a linking group L, to a charged group Z; and wherein each group R.sup.11, R.sup.12, R.sup.13, R.sup.14, R.sup.15 R.sup.16, R.sup.17 and R.sup.18 not involved in bonding to an organic charged group Z is, independently for each occurrence, selected from the group consisting of —H, substituted or unsubstituted primary, secondary or tertiary alkyl, that may be cyclic and may be unsaturated; substituted or unsubstituted aryl or heteroaryl, —CF.sub.3, —OMe, —SF.sub.5, —NO.sub.2, halo, aryl, aryl hydroxy, amino, alkoxy, alkylthio, carboxy, cyano, thio, formyl, ester, acyl, thioacyl, amido, sulfonamido, carbamate, phosphine oxide and phosphine sulphide; and wherein groups Ar are independently for each occurrence substituted or unsubstituted aryl or heteroaryl and n ( ) indicates the optional presence of saturated —CH.sub.2— groups in the rings annelated to the benzene ring, wherein n is independently for each occurrence, 0, 1, or 2.

53. The electroluminescent device of claim 52, wherein groups A.sup.1 and A.sup.2 are the same and A.sup.3 different.

54. The electroluminescent device of claim 52, wherein the donor D is: ##STR00062##

55. The electroluminescent device of claim 52, wherein the charged organic thermally activated delayed fluorescence species is according to formula Va or formula VI: ##STR00063## wherein at least one of the occurrences of R.sup.11, R.sup.12, R.sup.13, R.sup.14, R.sup.15 and R.sup.16 represents the bonding position, either directly or via a linking group L, to a charged group Z; and wherein each group R.sup.11, R.sup.12, R.sup.13, R.sup.14, R.sup.15 and R.sup.16 not involved in bonding to an organic charged group Z is, independently for each occurrence, selected from the group consisting of —H, substituted or unsubstituted primary, secondary or tertiary alkyl, that may be cyclic and may be unsaturated; substituted or unsubstituted aryl or heteroaryl, —CF.sub.3, —OMe, —SF.sub.5, —NO.sub.2, halo, aryl, aryl hydroxy, amino, alkoxy, alkylthio, carboxy, cyano, thio, formyl, ester, acyl, thioacyl, amido, sulfonamido, phosphine oxide, phosphine sulphide and carbamate.

56. The electroluminescent device of claim 55, wherein the metal free charged organic thermally activated delayed fluorescence species is according to one of formulas VII, VIII or Villa: ##STR00064## wherein the groups R.sup.11, R.sup.12, R.sup.13, R.sup.14, R.sup.15 and R.sup.16 take the same meaning as in claim 54, except R.sup.15 is not involved in bonding to an organic group Z; linking group L may be present or absent and Z is a monocationic metal free charged group.

57. The electroluminescent device of claim 56, wherein the metal free charged organic thermally activated delayed fluorescence species is according to one of formulas of formulas IX and X: ##STR00065## wherein for each occurrence L is absent or is independently selected from the group consisting of: ##STR00066## wherein n is from 0 to 10; ##STR00067## wherein for each occurrence Z is independently selected from the group consisting of: ##STR00068## ##STR00069## wherein -L indicates the position of bonding to linking group L or directly to the carbazole ring if L is absent.

58. The electroluminescent device of claim 57, wherein the counter ions employed with the metal free charged organic thermally activated delayed fluorescence species are selected from the group consisting of: PF.sub.6.sup.−, BF.sub.4.sup.− and F.sup.−.

59. The electroluminescent device of claim 52, wherein the metal free charged organic thermally activated delayed fluorescence species is according to formula XVIa or XVIb ##STR00070## wherein at least one of the occurrences of R.sup.13, R.sup.14, R.sup.15, R.sup.16, R.sup.17 and R.sup.18 represents the bonding position, either directly or via a linking group L, to a charged group Z, each L if present and each Z having, independently for each occurrence, the same meanings as discussed above for compounds of formula III or IV; and wherein each group of R.sup.13, R.sup.14, R.sup.15, R.sup.16 R.sup.17 and R.sup.18 not involved in bonding to an organic charged group Z is, independently for each occurrence, selected from the group consisting of —H, substituted or unsubstituted primary, secondary or tertiary alkyl, that may be cyclic and may be unsaturated; substituted or unsubstituted aryl or heteroaryl, —CF.sub.3, —OMe, —SF.sub.5, —NO.sub.2, halo, aryl, aryl hydroxy, amino, alkoxy, alkylthio, carboxy, cyano, thio, formyl, ester, acyl, thioacyl, amido, sulfonamido, phosphine oxide, phosphine sulphide and carbamate.

60. The electroluminescent device of claim 59, wherein the metal free charged organic thermally activated delayed fluorescence species is according to formula XVIc or formula XVId ##STR00071## wherein the groups R.sup.13, R.sup.14, R.sup.15, R.sup.1, R.sup.17 and R.sup.18 take the same meaning as in claim 58, except R.sup.15 is not involved in bonding to an organic group Z; linking group L may be present or absent and Z is a monocationic metal free charged group.

61. The electroluminescent device of claim 60, wherein the metal free charged organic thermally activated delayed fluorescence species is according to one of formulas of formulas XVIe and XVIf: ##STR00072## wherein for each occurrence L is absent or is independently selected from the group consisting of: ##STR00073## wherein n is from 0 to 10; ##STR00074## wherein for each occurrence Z is independently selected from the group consisting of: ##STR00075## ##STR00076## wherein -L indicates the position of bonding to linking group L or directly to the carbazole ring if L is absent.

62. The electroluminescent device of claim 36, wherein the emitter material comprises a compound of any one of formulas XI XII, XVII, XVIII, XIX, XX, XXI, XXII, XXIII, and XXIV: ##STR00077## ##STR00078## ##STR00079##

63. The electroluminescent device of claim 36, wherein the metal free charged organic thermally activated delayed fluorescence species is charged by a substituent or substituents that is/are an integral part of the TADF species.

64. The electroluminescent device of claim 63, wherein the metal free charged organic thermally activated delayed fluorescence species is according to formula XIIIa: ##STR00080##

65. The electroluminescent device of claim 36, wherein the counter ion or counter ions are selected from the group consisting of: halide, PF.sub.6.sup.−, BF.sub.4.sup.−, BR.sub.4.sup.−; wherein R is an aryl group, selected from phenyl; OTf.sup.−, OTs.sup.−, SbX.sub.6.sup.− wherein X is halide, NTf.sub.2.sup.− NO.sub.3.sup.−, CO.sub.3.sup.2−; cations of first and second group elements in the periodic table and quaternary ammonium cations.

66. The electroluminescent device of claim 36, wherein the emitter material is provided in a luminescent layer that comprises an ionic liquid.

67. A metal free charged organic thermally activated delayed fluorescence (TADF) species and sufficient counter ions to balance the charge on the charged organic thermally activated delayed fluorescence (TADF) species, as defined in claim 38.

68. A method of producing light, the method comprising: providing a metal free charged organic thermally activated delayed fluorescence (TADF) species and sufficient counter ions to balance the charge on the charged organic thermally activated delayed fluorescence (TADF) species; or mixtures thereof; manufacturing an electroluminescent device including the charged organic TADF species and sufficient counter ions to balance the charge on the charged organic TADF species; or mixtures thereof as emitter material; and operating the electroluminescent device.

69. A method of producing light of claim 68, wherein the metal charged organic thermally activated delayed fluorescence species and counter ions are as defined in claim 38.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0131] FIG. 1 shows absorption and emission spectra of TADF compounds; and

[0132] FIGS. 2a and 2b show electroluminescent behaviour of light emitting electrochemical cells.

DETAILED DESCRIPTION OF SOME EMBODIMENTS AND EXPERIMENTAL RESULTS

Synthesis of Compounds

[0133] Synthesis of compounds of formulas XI and XII is illustrated in Scheme 1 below. A similar procedure may be employed when making TADF species incorporation donor moieties other than modified carbazole species.

##STR00045##

[0134] In the procedures shown in Scheme 1, the preparation of 3-bromocarbazole 1 using N-bromosuccinimide was contaminated by some starting material and 3,6-dibromocarbazole, both of which could be removed by fractional recrystallization from toluene. The preparation of 3 was accomplished in good yield by dropwise introduction of the lithiated TBDMS-protected 3-bromocarbazole 2 intermediate to excess 1,4-diiodobutane.

[0135] Compound 4 was obtained by S.sub.N2 reaction of sodium imidazolate with 3 followed by silyl deprotection using sodium hydride in a one-pot fashion.

[0136] Compounds 5 and 6 were synthesized by reacting 4 with sub-stoichiometric or stoichiometric 4,5-difluorophthalonitrile, respectively, under basic conditions via nucleophilic aromatic substitution reaction. The targeted charged TADF emitters XI and XII were obtained following methylation with iodomethane and anion metathesis with saturated NH.sub.4PF.sub.6 solution in 33% and 27% yield, respectively, over six steps. The solubilities of XI and XII in DCM were greatly improved after the anion metathesis, in comparison with that of the original iodo salts.

[0137] Both XI and XII exhibit irreversible oxidation and reversible oxidation waves in MeCN solution by cyclic voltammetry.

[0138] The HOMO of XI (−5.93 eV) is slightly lower than that of XII (−5.87 eV) due to the presence of electron-withdrawing fluorine atom. The LUMO of XI (−2.92 eV) is slightly higher than that of XII (−2.99 eV), which is due to the increased conjugation imparted by the second carbazole moiety that lowers the LUMO in XII. These structure-property relationships are mirrored in the absorption spectra wherein the absorption profile of XII is slightly red-shifted compared with in XI (See FIG. 1).

[0139] In FIG. 1 the normalised absorption spectra in aerated acetonitrile for XI (line A) and XII (line B) are shown—at 298K. Also shown are the corresponding emission spectra for XI in deaerated acetonitrile (line A1) and thin film (line A2). Also shown are the corresponding emission spectra for XII in deaerated acetonitrile (line B1) and thin film (line B2).

[0140] The electrochemical and emission data for XI and XII are summarised in Table 1 below. λ.sub.em is the wavelength of maximum emission, φ is the photo luminescence quantum yield, T.sub.e is the fluorescence lifetime, E.sub.HOMO and E.sub.LUMO are the energy levels obtained from cyclic voltammetry with ΔE being the difference.

TABLE-US-00001 TABLE 1 Emission and electrochemical data of the TADF-LEEC dyes XI and XII. Emission Electrochemical λ.sub.em φ τ.sub.e E.sub.HOMO.sup.e E.sub.LUMO.sup.e ΔE Dye (nm).sup.a (%).sup.b (ns).sup.c (eV) (eV) (eV) XI Aerated 556 1.4 2.90 −5.93 −2.92 3.01 Degassed 558 2.9 3.26 Thin Film 526 9.1.sup.d 42.9 (83.4%), 4580 (16.6%) XII Aerated 574 2.1 4.44 −5.87 −2.99 2.88 Degassed 572 2.6 4.97 Thin Film 536 35.5.sup.d 35.2 (80.6%), 11700 (19.4%) .sup.aMeasured at 298 K; λ.sub.exc: 360 nm. .sup.bQuinine sulfate used as the reference (φ.sub.PL = 54.6% in in 1N H.sub.2SO.sub.4 at 298 K). λ.sub.exc: 378 nm. .sup.dMeasured using an integrating sphere at 298 K in ambient air. .sup.eThe HOMO and LUMO energies were calculated using the relation E.sub.HOMO/LUMO = −(E.sup.ox.sub.pa, 1/E.sup.red.sub.1/2 + 4.8) eV, where E.sup.ox.sub.pa, 1/E.sup.red.sub.1/2 are the oxidation and reduction peaks, respectively referenced vs Fc/Fc.sup.+.

[0141] The emission in MeCN solution and in a thin solid film for both in XI and XII is broad and unstructured, characteristic of CT (charge transfer) emission; the excitation and absorption spectra matched, pointing to a high level of purity. The emission spectra in MeCN are red-shifted by about 30-40 nm, respectively, compared to emission in the thin film. The thin film was deposited from a solution of acetonitrile in a film without any dopants or other additives present.

[0142] In solution, the emission for both XI and XII is weak and the observed lifetimes are in the nanosecond regime. There is little change in the photophysical properties upon degassing the sample, suggesting that in MeCN XI and XII act as fluorophores with no observed TADF. This is likely due to stronger stabilization of the triplet state by the solvent resulting in increased ΔE.sub.ST. By point of comparison, dicarbazolyldicyanobenzene was shown by Adachi to emit at 473 nm in toluene solution via TADF with a photoluminescence quantum yield, Φ.sub.PL, of 47% (reference 1a). In the neat film, XI and XII are much brighter with Φ.sub.PL values of 9.1 and 35.5%, respectively, under aerated conditions. Importantly, biexponential decay in the emission lifetimes is now observed, including both a short component and a long microsecond component, a hallmark of TADF emission.

[0143] Synthesis of compound of formula XVII is illustrated in Scheme 2 below and follows a similar route to that of Scheme 1.

[0144] Reaction of compound 4 from Scheme 1 above with bis(4-fluorophenyl) sulphone provides the intermediate 7 which is converted by methylation with iodomethane and anion metathesis with saturated NH.sub.4PF.sub.6 solution to the product XVII.

##STR00046##

[0145] Synthesis of compound of formula XVIII is illustrated in Scheme 3 below.

[0146] Reaction of 0.5 molar equivalents of carbazole with 4,5-difluorophthalonitrile affords compound 8 which is then reacted with 4 to provide intermediate 9. Hydrolysis of the nitrile functions provides dicarboxylic acid 10 on acidification. Methylation with iodomethane and anion metathesis with saturated NH.sub.4PF.sub.6 solution provides the product XVIII.

##STR00047##

[0147] Synthesis of compound of formula XIX is illustrated in Scheme 4 below.

[0148] Hydrolysis of compound 6 of Scheme 1 above provides dicarboxylic acid 11 on acidification. Methylation with iodomethane and anion metathesis with saturated NH.sub.4PF.sub.6 solution provides the product XIX.

##STR00048##

[0149] Synthesis of compound of formula XX is illustrated in Scheme 5 below.

[0150] Diphenylamine 12 is converted to the imidazole derivative 13 by a similar route to that shown for carbazole in Scheme 1 above. Reaction of 13 with bis(4-fluorophenyl) sulphone affords intermediate 14. Methylation with iodomethane and anion metathesis with saturated NH.sub.4PF.sub.6 solution provides the product XX.

##STR00049##

[0151] Syntheses of compounds of formulas XXI, XXII and XXIII are illustrated in Scheme 6 below.

[0152] Reaction of carbazole with 4,5-difluorophthalonitrile affords intermediate 15 which on hydrolysis provides potassium salt of the dicarboxylic acid 16. Reaction with n-Pr.sub.4NBr affords the product XXI. A similar approach using 3,6-di-tert-butyl-9H-carbazole 17 affords the intermediate 18 on reaction with 4,5-difluorophthalonitrile. Hydrolysis provides the potassium salt of the dicarboxylic acid XXII. Conversion to the product XXIII is made by reaction with n-Pr.sub.4NBr.

##STR00050##

[0153] Synthesis of compound of formulas XXV is illustrated in Scheme 7 below.

[0154] Reaction of imidazole with 1,4-dibromobutane affords 19 which on reaction with 11 (see Scheme 4) provides di-ester 20.

[0155] Methylation with iodomethane and anion metathesis with saturated NH.sub.4PF.sub.6 solution provides the product XXV.

##STR00051##

[0156] Emission and electrochemical data of the TADF-LEEC dyes XIII to XXIV are shown in Table 2 below.

TABLE-US-00002 TABLE 2 Doped Solution.sup.a Film.sup.b Com- λ.sub.em.sup.c φ.sub.PL.sup.d τ.sub.e λ.sub.em φ.sub.PL Electro- pound (nm) (%) (ns) (nm) (%) chemistry.sup.e (eV) XVII 439 45.5 11.5, 440 44.1 HOMO: −5.65  (81) (30.2) 821  (78) (46.1) LUMO: −2.08 ΔE: 3.57 XX 447 46.7 6.7, 416 49.3 N/A  (81) (45.0) 684  (70) (48.3) XVIII 509 22.1 21.0, 462 50.6 HOMO: −5.88 (120) (18.2) 2490 (109) (37.4) LUMO: −2.66 ΔE: 3.22 XIX 515 19.5 17.7, 494 35.8 HOMO: −5.91 (126) (12.7) 1770 (122) (32.1) LUMO: −2.65 ΔE: 3.26 XXIV N/A XXI 441 51.4 16.6, 424 16.5 HOMO: −5.74  (86) (41.2) 600  (90) (19.5) LUMO: N/A ΔE: N/A XXII 455 33.6 17.0, 462 40.4 HOMO: −5.85 (125) (11.5) 4470 (111) (36.5) LUMO: N/A ΔE: N/A XXIII 465 32.8 19.0, 430 29.0 HOMO: −5.93  (96) (27.8) 1420  (96) (23.7) LUMO: N/A ΔE: N/A .sup.aIn DCM at 298 K. .sup.bDoped with PMMA (10 wt %) and spin-coated. .sup.cEmission maxima and full-width at half maximum (FWHM) are reported from degassed solutions. FWHM in parentheses. .sup.d0.5M quinine sulphate in H.sub.2SO.sub.4 (aq) was used as reference (PLQY: 54.6%). Values quoted are in degassed solutions, which were prepared by five freeze-pump-thaw cycles. Values in parentheses are for aerated solutions, which were prepared by bubbling with air for 5 minutes. .sup.eIn MeCN with 0.1M [nBu.sub.4N]PF.sub.6 as the supporting electrolyte and Fc/Fc.sup.+ as the internal reference. The HOMO and LUMO energies were calculated using the relation E.sub.HOMO/LUMO = −(E.sup.ox.sub.pa, 1/E.sup.red.sub.pc, 1 + 4.8)eV, where E.sup.ox.sub.pa and E.sup.red.sub.pc are anodic and cathodic peak potentials respectively. ΔE = −(E.sub.HOMO − E.sub.LUMO). N/A = not available.

Fabrication of a LEEC

[0157] The compounds of the invention can be utilised in the fabrication of a LEEC.

[0158] In general LEECs were prepared on top of a patterned indium tin oxide (ITO) coated glass substrate. Prior to the deposition of the emitting layer, a 80 nm of PEDOT:PSS was coated in order to increase the reproducibility of the cells.

[0159] For XII the emitting layer (100 nm) was prepared by spin-coating of an acetonitrile solution consisting of the emitting compound alone or with the addition of an ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate [Bmim][PF.sub.6] at a molar ratio of 4:1. After the deposition of the light-emitting layer the devices were transferred into an inert atmosphere glovebox. To complete the devices, a layer of 70 nm of aluminium that serves as the top electrode was thermally evaporated in a high vacuum chamber integrated in the inert atmosphere glovebox.

[0160] For XI a host guest matrix was employed as that salt did not produce light when in a pristine (additive free) solid film layer. Spin coating with the ionic host produced a functioning LEEC.

[0161] Compound XI was successfully fabricated into a LEEC with the compound prepared in a matrix of the known ionic host NS25, described in EP2733188 (which is incorporated by reference herein).

##STR00052##

[0162] In order to determine the performance of the LEECs, the devices were operated using a block-wave pulsed current driving method (1000 Hz and 50% of duty cycle) at different average current densities of 10, 25 and 50 Am.sup.−2. This operational mode was selected over constant voltage mode as it decreases the turn-on time and leads to a more sustained behaviour versus time.

[0163] The luminance and average voltage are depicted in FIG. 2 for LEECs using XII and XII: [Bmim][PF.sub.6] 4:1 mixture as the component(s) for the light-emitting layer.

[0164] FIG. 2a shows the luminance over time of the LEEC using XII: [Bmim][PF.sub.6] 4:1 mixture as the luminescent layer. The average voltage with time is shown on the insert graph. FIG. 2b shows similar graphs for the LEEC using XII only as the luminescent layer.

[0165] Different average current densities were employed as shown on FIGS. 2a and 2b:−50 Am.sup.−2 shown as lines A, 25 Am.sup.−2 shown as lines B and 10 Am.sup.−2 shown as lines C

[0166] The results for the different devices studied are summarized in Table 3 below. Initially, for both LEECs, the average voltage applied drops rapidly over the first seconds. Coinciding with the decrease in driving voltage, an increase in the luminance was observed. This reached a maximum value and then slowly decreased versus time.

[0167] This is typical for LEECs, as the injection barrier for electrons and holes reduces due to the migration of ions to the electrode interfaces and the subsequent formation of doped regions. The reduction of the injection barrier leads to the reduced driving voltage to sustain the set current density. With increasing operating time the doped regions expand leading to a slowly increasing quenching of the excitons, and as a result a (non-permanent) luminance reduction.

TABLE-US-00003 TABLE 3 Performance of LEEC devices: ITO/PEDOT:PSS/XII/Al and ITO/PEDOT:PSS/XII:[Bmim][PF.sub.6] 4:1/Al biased with a block wave-pulsed current at a frecuency of 1000 Hz and a duty cycle 50% at different current densities. Electroluminescence Avg. Current Lum.sub.max PCE.sub.max EQE Photo- Density (cd .Math. (lm .Math. max luminescence LEEC (A .Math. m.sup.−2) m.sup.−2).sup.a W.sup.−1).sup.b (%).sup.c PLQY (%) XII only 10 13 0.7 0.39 16.2 25 24 0.4 0.29 50 26 0.2 0.16 XII:[Bmim][PF.sub.6] 25 10 0.2 0.12 20.7 50 17 0.1 0.1

[0168] The table shows luminance (Lum.sub.max), power conversion efficiency (PCE.sub.max), external quantum efficiency (EQE.sub.max) and photo luminescent quantum yield (PLQY) for the two LEECs tested.

[0169] In FIG. 2 it is clearly seen that the luminance decreases with decreasing current density, albeit this relationship is far from linear. A similar behaviour has been observed for ionic transition metal complex based LECs. This effect has been attributed to a reduced quenching of the excitions due to either a reduction of charge carriers, excited states or both. As the devices are operated at a fixed average current density, the efficiency of the devices is directly proportional to the luminance. With a fivefold decrease in current density, the luminance of the XII only (no ionic liquid) device drops only from approximately 26 to 13 cd m.sup.−2. Hence, the device power efficiency strongly increases to 0.7 lum W-1 with an external quantum efficiency (EQE) of 0.39% (assuming Lambertian emission.

[0170] The electroluminescent (EL) spectra for these LEECs are similar to those of the photo luminescent spectra of FIG. 1. All cells emitted homogenously from the active area. The spectra feature an unstructured green emission centred at 538 nm (CIE coordinates: 0.35, 0.57).

REFERENCES—THE FOLLOWING REFERENCES ARE FULLY INCORPORATED BY REFERENCE HEREIN

[0171] 1. (a) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Nature 2012, 492, 234; (b) Nakanotani, H.; Higuchi, T.; Furukawa, T.; Masui, K.; Morimoto, K.; Numata, M.; Tanaka, H.; Sagara, Y.; Yasuda, T.; Adachi, C. Nat Commun 2014, 5, 4016; (c) Zhang, Q.; Li, J.; Shizu, K.; Huang, S.; Hirata, S.; Miyazaki, H.; Adachi, C. J Am Chem Soc 2012, 134, 14706; (d) Zhang, Q.; Li, B.; Huang, S.; Nomura, H.; Tanaka, H.; Adachi, C. Nature Photonics 2014, 8, 326. [0172] 2. Reineke, S. Nature Photonics 2014, 8, 269. [0173] 3. a) Lee, S. Y.; Yasuda, T.; Yang, Y. S.; Zhang, Q.; Adachi, C. Angew Chem Int Ed Engl 2014, 53, 6402; (b) Mehes, G.; Nomura, H.; Zhang, Q.; Nakagawa, T.; Adachi, C. Angew Chem Int Ed Engl 2012, 51, 11311. [0174] 4. (a) Costa, R. D.; Orti, E.; Bolink, H. J.; Monti, F.; Accorsi, G.; Armaroli, N. Angew. Chem. Int. Ed. 2012, 51, 8178; (b) Hu, T.; He, L.; Duan, L.; Qiu, Y. J. Mater. Chem. 2012, 22, 4206. [0175] 5. Pertegás, A.; Tordera, D.; Serrano-Pérez, J. J.; Orti, E.; Bolink, H. J. J. Am. Chem. Soc. 2013, 135, 18008. [0176] 6. Tao, Ye; et al; Advanced Materials; 2014, Thermally Activated delayed fluorescence materials towards the Breakthrough of Organoelectronics, 17 Sep. 2014.