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Electropolymerizable Ir III Complexes with b-Ketoiminate Ancillary Ligands

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As eries of electropolymerizable cyclometallated Ir III complexes weres ynthesized and their electrochemical and photophysical properties studied. The triphenylamine electropolymerizable fragment was introduced by using
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  Electropolymerizable Ir III Complexes with  b -Ketoiminate AncillaryLigands Andreea Ionescu, [a, b] Rossella Caligiuri, [a] Nicolas Godbert,* [a, b] Angela Candreva, [a] Massimo La Deda, [a, b] Emilia Furia, [c] Mauro Ghedini, [a, b] and Iolinda Aiello* [a, b] Abstract:  A series of electropolymerizable cyclometallatedIr III complexes were synthesized and their electrochemicaland photophysical properties studied. The triphenylamineelectropolymerizable fragment was introduced by using tri-phenylamine-2-phenylpyridine and, respectively, triphenyla-mine-benzothiazole as cyclometalated ligands. The coordina-tion sphere was completed by two differently substituted  b -ketoiminate ligands deriving from the condensation of ace-tylacetone or hexafluoroacetylacetone with  para -bromoani-line. The influence of the -CH 3 /-CF 3  substitution to the elec-trochemical and photophysical properties was investigated.Both complexes with CH 3  substituted  b -ketoiminate wereemissive in solution and in solid state. Highly stable filmswere electrodeposited onto ITO coated glass substrates.Their emission was quenched by electron trapping withinthe polymeric network as proven by electrochemical studies.The -CF 3  substitution of the  b -ketoiminate leads instead tothe quenching of the emission and inhibits electropolymeri-zation. Introduction Due to their specific features induced by the presence of metal centers and, on the other hand, the processability of or-ganic polymers, metallopolymers based on transition metalcomplexes represent key functional materials in optoelectron-ics (electrochromics or energy converting devices). [1] The controlled deposition of homogeneous metallopolymer-ic thin films of transition metal complexes on different sub-strates represents a crucial issue in the device application. Inthis context, electropolymerization features some advantagesover other film deposition techniques (e.g.: drop casting, [2] spin-casting, [3] layer-by-layer assembly, [4] electrostatic bindingof polyelectrolyte films [5] ). Through electropolymerization pro-cess, the polymer formation and its deposition occur simulta-neously, avoiding limitations such as scarce solubility of poly-meric networks and leading to controllable film composition,thickness and surface coverages. [6] The electrodeposition of metallopolymeric thin films of or-ganometallic complexes (Ru, Os, Fe, Ir, Ni), [6,7] on conductingsubstrates has been achieved by using various electropolymer-izable groups such as thiophene, [8,9] pyrrole, [10] aromaticamines, [11–14] and triphenylamine. [15–18] In this context, a series of electropolymerizable cyclometal-lated square planar Pd II and Pt II complexes incorporating anelectropolymerizable triphenylamine-substituted Schiff base H(O^N)  as ancillary ligand (Figure 1), was recently report-ed. [19,20] Remarkably, the use of this Schiff base allowed theelectrodeposition of highly stable and homogeneous photo- Figure 1.  Molecular structure of electropolymerized Pt II /Pd II complexes. [19,20] [a]  Dr. A. Ionescu, R. Caligiuri, Dr. N. Godbert, Dr. A. Candreva,Prof. M. La Deda, Prof. M. Ghedini, Prof. I. AielloMAT-INLAB (Laboratorio di Materiali Molecolari Inorganici) and LAS-CAMM—CR INSTMUnit    INSTM della CalabriaDipartimento di Chimica e Tecnologie ChimicheUniversit    della Calabria87036 Arcavacata di Rende (CS) (Italy)E-mail: nicolas.godbert@unical.it iolinda.aiello@unical.it  [b]  Dr. A. Ionescu, Dr. N. Godbert, Prof. M. La Deda, Prof. M. Ghedini,Prof. I. AielloCNR NANOTEC-Istituto di Nanotecnologia U.O.S.Cosenza, 87036 Arcavacata di Rende (CS) (Italy) [c]  Dr. E. FuriaDipartimento di Chimica e Tecnologie ChimicheUniversit    della CalabriaVia P. Bucci, Cubo 12/D, 87036 Arcavacata di Rende (CS) (Italy)Supporting information and the ORCID identification number(s) for the au-thor(s) of this article can be found under:https://doi.org/10.1002/asia.201900521. Chem. Asian J.  2019 ,  14 , 3025–3034    2019 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 3025 Full PaperDOI: 10.1002/asia.201900521  conductive and electrochromic Pd II and Pt II metallopolymers.For triphenylamine (TPA) containing compounds, the electro-polymerization process is promoted in the case of monosubsti-tution of the TPA fragment only when an electron withdrawinggroup is employed. Upon oxidation, the TPA fragment is oxi-dized and a single electron from the nitrogen atom is re-moved, affording a delocalized radical cation TPA + · . Afterwards,dimerization occurs to produce the corresponding tetraphenyl-benzidine (TPB) which further electropolymerizes. [21] TPB ismore readily oxidized than TPA, and upon cycling, chain exten-sion leads to a branched polymeric network. [22,23] In the field of transition metal-based optoelectronic devices,Ir III luminescent organometallic complexes represent a peculiaremerging class. [24–29] The high sensitivity of Ir III complexes toelectric stimuli (due to their charge-transfer excited states)makes them suitable for the application in optical data record-ing and security protection, which require time-resolved lumi-nescence technique. [30] Up to date, very few examples of Ir III or-ganometallic luminescent electrochromic complexes were syn-thesized and only one quasi-solid device was realized. [31] Forthis reason, in a previous recent work, we attempted to incor-porate our designed electropolymerizable triphenylaminesubstituted Schiff base  H(O^N) tpa ancillary ligand into Ir III complexes (Figure 2). [32] Unfortunately, the resulting (C^N) 2 Ir(O^N) tpa complexes did not display any luminescentproperties, but high quality thin films were obtained throughelectropolymerization process. [32] Following this itinerary, we decided to replace the  H(O^N) tpa Schiff base ancillary ligand with a ligand prompter to inducehigh luminescence in the resulting complexes. To this regard,differently substituted  b -ketoiminates have recently been suc-cessfully introduced as ancillary ligands, leading to highly lumi-nescent Ir III complexes. [33–35] Furthermore, up to 10 fold enhancement of photolumines-cence has been observed in a rigid PMMA matrix for com-plexes containing strongly  p -donor  b -ketoiminates. [36] We present herein a series of novel electropolymerizablecyclometallated Ir III complexes of general formula [(C^N) 2 Ir(O^N)] , wherein  H(C^N)  are triphenylamine-2-phenyl-pyridine ( H(ppy-TPA) ) or triphenylamine-benzothiazole ( H(bz-TPA) ) while  H(O^N)  are the substituted  b -ketoiminate shownin Scheme 1. Results and Discussion SynthesisH(O^N) ligands. H(O^N) 1 and  H(O^N) 2 were obtained by con-densation of acetylacetone (or its hexafluorinated analogous)with 4-bromoaniline through a microwave assisted proceduremodifying the classical synthetic pathway (Scheme 1). [37] Thechoice of these two differently substituted  b -ketoiminate li-gands was directed by the desire to evaluate the influence of the electronegative effect of the fluorine substitution onto theluminescent and\or electrochemical properties.The obtained  b -ketoiminate were characterized by IR, 1 H NMR spectroscopies and GC-MS. Moreover, potentiometricmeasurements of their acidic constants  K  a  were performedshowing their coordination potential as monoanionic chelatingligands and their relative stability towards hydrolysis in basicconditions. The protonation constants of   H(O^N) 1 and H(O^N) 2 were calculated from the data acquired by carryingout two titrations for each ligand. The experimental data re-ported in Table 1 were processed by numerical procedures (Ex-perimental). [38] H(C^N) ligands.  Considering the synthetic procedures of both  H(C^N)  ligands,  H(bz-TPA)  was prepared as previously re-ported [37] whereas the synthetic procedure of   H(ppy-TPA)  wasmodified with respect to the literature. [39] The structure of bothligands was confirmed by spectroscopic analyses (Experimen-tal). (C^N)Pt(O^N) complexes . The cyclometallated chloro-bridge intermediates,  [(C^N) 2 Ir(  m -Cl)] 2 , were obtained by a mi-crowave assisted reaction. [40] The synthesis of complexes  1 – 4 (Scheme 1) was achieved through a bridge splitting process,reacting the binuclear intermediate  [(ppy-TPA) 2 Ir(  m -Cl)] 2  and,respectively  [(bz-TPA) 2 Ir(  m -Cl)] 2  with the corresponding  b -ke-toiminate ligand  H(O^N) 1,2 in basic conditions.Complexes  1 – 4  were fully characterized by IR,  1 H NMR andMS spectroscopies. In particular, the structure of the expectedproducts was confirmed by their  1 H NMR spectra. Indeed, thecoordination of the  b -ketoiminate ligand occurred was con-firmed from the disappearance of the signal of the - NH   protonof the ancillary ligand and from the splitting of the aromaticsignals of the cyclometallated ligands owe to the loss in sym-metry induced by the  (O^N)  ligand coordination to the metalcentre. Electrochemical studies, Density Functional Theory calcula-tions and electropolymerization Complexes  1 – 4  were solubilized in ca. 3 mL of freshly distilledand degassed (Ar) dichloromethane solution, reaching the finalconcentration of ca. 1  10  6 M. To this solution, tetrabutylam-monium hexafluorophosphate (0.1 m ) was added as supporting Figure 2.  Molecular structure of electropolymerized Ir III complex [(bz-TPA)Ir(-O^N)]. [32] Chem. Asian J.  2019 ,  14 , 3025–3034  www.chemasianj.org    2019 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 3026 Full Paper  electrolyte. The typical voltammetry cell was composed as fol-lows: a Pt disk as working electrode, a Pt wire as counter-elec-trode and an Ag wire used as pseudo-reference electrode. Po-tentials applied varied from   1.3 V to 1.5 V, at a standard scanrate of 100 mVs  1 . Potential data and estimated energy levels(HOMO) were given with respect to the ferrocene/ferrocinium(Fc/Fc + ) redox couple used as internal reference (  4.8 eV forthe HOMO energy level [41] and   0.45 V vs. SCE [42] for the oxida-tion potential of the Fc/Fc + redox couple).Fc was indeed added in equimolar quantity with respect tothe analysed complex. The solution electrochemical data forcomplexes  1 – 4  obtained by cyclic voltammetry are collected inTable 2.As shown in Figure 3, complexes with the same ancillaryligand (Scheme 1) presented analogous cyclic voltammograms,which are irreversible for  1  and  3  and reversible in the case of  2  and  4 . The slight positive shift of the oxidation potential of complexes  2  and  4 , with respect to their corresponding analo-gous  1  and  3  respectively can be reasonably attributed to thefluorination of the  b -ketoiminate backbone. For all complexes 1 – 4  (Figure 4), the successive observed oxidation waves aretypical in features of the triphenylamine fragment embeddedonto the cyclometalled ligands. [32] While in the case of com-plexes  2  and  4,  two consecutive fully reversible one-electronoxidation waves are observed that can be ascribed to the oxi-dation of TPA to TPA + ·  and then TPA 2 + , for the complexes  1 and  3  the situation is different. Indeed, the oxidation processof   1  and  3  becomes irreversible due to the formation of TPBthat occurs by dimerization of the radical cation TPA + · .For complexes  1  and  3 , upon repetitive oxidation scans, asignificant and constant increase in current can be observed.Such behavior is typical of the electropolymerization of elec-tron-withdrawing substituted triphenylamine fragment [32] and Scheme 1.  Synthesis pathway of   [(C^N) 2 Ir(O^N) n ]  complexes,  1 – 4  , reagents and conditions:  i)  H(O^N) 1 : acetylacetone, 4-bromoaniline, 300 W, 140 8 C, 90 min; H(O^N) 2 : Hexafluoroacetylacetone, 4-bromoaniline,  p -toluensulfonic acid (10%), 600 W, 140 8 C, 30 min;  ii)  H(ppy-TPA) : 2-EtOCH 2 CH 2 OH/H 2 O, 250 W, 110 8 C,1 h;  H(bz-TPA) : 2-EtOCH 2 CH 2 OH/H 2 O, 250 W, 200 8 C, 4 h;  iii  ) NaOH, EtOH, N 2 , 65 8 C, 2 h;  iv)  EtOH, N 2 , 65 8 C, 48 h. Table 1. H(ON) 1 , 2 log  K  a  values in 0.16 m  NaCl and at 60 8 C by numericalmethods. b -ketoiminanes pKa  3 s H(O^N) 1 8.98  0.03 H(O^N) 2 3.7  0.2 Table 2.  Electrochemical data. E  ox1 [V] [a] E  ox2 [V] [a] E  ox3 [V] [a] HOMO[eV] [b] HOMO[eV] [c] LUMO[eV] [c] EP [d] 1  + 0.21 (IR)  + 0.56 (QR)  + 0.67 (QR)   5.01   4.65   1.31 YES2  + 0.36 (R)  + 0.50 (R) –   5.16   4.84   1.73 NO3  + 0.26 (IR)  + 0.53 (QR) 0.64 (QR)   5.06   4.79   1.56 YES4  + 0.45 (R)  + 0.59 (R) –   5.25   4.96   1.76 NOE oxn ,  n = 1,2,3, is referred to the oxidation waves observed at growing po-tential. [a] Potentials were given versus ferrocene/ferrocinium (Fc/Fc + ).[b] estimated from the first oxidation potential (using   4.8 eV per Fc/Fc + ). [c] Calculated at the B3LYP/LANL2DZ(6-31G(d)) level of theory. (IR): irre-versible wave, (QR): quasi-reversible wave, (R): reversible wave. [d] EP = electropolymerization. Chem. Asian J.  2019 ,  14 , 3025–3034  www.chemasianj.org    2019 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 3027 Full Paper  the corresponding repetitive cyclic voltammograms for com-plex  1  and  3  are reported in Figure 5.The electropolymerization process was carried out bothonto the Pt working electrode as well as on ITO covered glassby application of 50 successive oxidation scans at a 100 mVs  1 .Both the washed modified Pt electrode and the covered ITOsubstrate were immersed in a freshly distilled electrolytic di-chloromethane solution and a cyclic voltammogram was re-corded in order to prove the film stability. The two scans werecarried out in a range of redox that varied to   1.5 V to 1.5 V.The presence of a complete reversible oxidation process, char-acterized by two consecutive oxidation waves, was indicativeof the effective electropolymerization deposition of thin filmsand their electrochemical stability. However, complex  3  exhib-its the phenomenon of electron trapping, as shown in Fig-ure 6a, as previously reported for  b -ketoiminate cyclometallat-ed Ir III complexes. [35] This process occurs when generated charges are trappedinto the polymeric network during reduction and afterwardsreleased upon oxidation leading to the appearance of a sharpintense peak. This characteristic phenomenon has also beenpreviously described for electropolymerized organometalliccomplexes. [43,44] Noteworthy, this charge trapping is rather per-sistent. Indeed, the film of   3  was reduced at   1.5 V and thepotential was subsequently returned to 0 V. The film was ex-posed to air for 30 minutes. The recorded cyclovoltammogramwas indeed still showing the sharp, although less intense, peak corresponding to this electron trapping.To provide a quantum chemical insight into all the synthe-sized complexes, the Density Functional Theory (DFT) methodwas employed to investigate their ground-state electronicstructures and orbital configurations. DFT calculations havebeen performed following the standard B3LYP/LANL2DZ(6-31G(d)) level of theory used for Ir III complexes. [45–47] ComputedHOMO energy levels show a very good agreement with the ex-perimental data (Table 2 and Figure 7) confirming the oxidationof the triphenylamine fragment. Photochemical studies The photophysical properties of the complexes  1 – 4  were in-vestigated at room temperature in chloroform solution at theconcentrations of 1.15  10  5 m  for  1 , 0.84  10  5 m  for  2 , 1.45  10  5 m  for  3  and 0.95  10  5 m  for  4 , and the obtained data arereported in Table 3. Figure 3.  Cyclic voltammograms of complexes  1 – 4  in dichloromethane at a100 mVs  1 scan rate. Figure 4.  Comparison of the experimental and computed HOMO energylevels for complexes  1 – 4 . Figure 5.  Electropolymerization of complexes  1  (a) and  3  (b) on ITO coveredglass substrates in dichloromethane (50 scans at 100 mVs  1 ). Chem. Asian J.  2019 ,  14 , 3025–3034  www.chemasianj.org    2019 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 3028 Full Paper  In Figure 8 are shown the absorption spectra of all com-plexes. By comparing the spectral features of the complexesbearing the -CH 3  substituents (i.e.  1  and  3 ) with those of theirligands (reported in Figure S1 in ESI), it is possible to assignthe bands in the 250–330 nm range to LC( p – p *) transitions lo-calized on the cyclometallated ligand, the shoulder at 350 or380 nm (in  1  and  3 , respectively) to an excitation mostly local-ized on the  b -ketoiminate ligand, while bands in the 380–430nm range are attributed to MLCT-LC mixed transitions involv-ing the metal and the cyclometallated ligand, and, finally, the457 nm band of   1  and the 478 nm band of   3  are srcinatedfrom an MLCT excitation towards the cyclometallated ligand,red-shifted in  3  due to the presence of the extended aromaticstructure of the benzothiazole fragment. Absorption spectra of the complexes  2  and  4  show the same transitions observed in 1  and  3 , but the -CF 3  substituents prompt an electron attrac-tive effect on the metal and, consequently, onto the cyclome-tallated ligand. This effect induces an energy increase of thetransitions involving the metal and the cyclometallated ligand,and the corresponding bands are blue-shifted with respect tothe analogous transitions observed in  1  and  3 , resulting insome case outside of the examined spectral range and, inother case, merged with vicinal bands. In particular, the blue-shift of the low-energy band (corresponding to the HOMO/LUMO excitation) of   2  and  4  compared with  1  and  3 , is adirect consequence of lowering of the HOMO energy, as re-ported in Table 2. The differences that the -CF 3  substituentsexert on the examined complexes are more evident by com-paring the emission properties of   1  and  3  with those of   2  and 4 . While the first couple is luminescent, the second one is non-emissive (Table 3).Figure 9 shows that the emission profiles of   1  and  3  areidentical, but the spectrum of the benzothiazolate complex isbathochromically shifted. Both emissions derive from the 3 MLCT state decay, but in the case of   1  this state involves the Figure 6.  Two reduction scans after electropolymerization of complex  1  (a)and  3  (b) on ITO substrate. Figure 7.  Contour plots of the frontier orbitals of complexes  1 – 4  in theground state. Chem. Asian J.  2019 ,  14 , 3025–3034  www.chemasianj.org    2019 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 3029 Full Paper
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