Borylated Arylamine-Benzothiadiazole Donor-Accep-tor Materials as Low LUMO, Low Band-Gap Chro-mophores Daniel L. Crossley, Rosanne Goh, Jessica Cid, Inigo Vitorica-Yrezabal, Michael L. Turner* and Michael J. Ingleson*School of Chemistry, University of Manchester, Manchester M13 9PL, United KingdomSupporting Information Placeholder

ABSTRACT: Fused and ladder type benzothiadiazole-arylamine donor-acceptor C,N-chelated boron complexes were synthesized through direct electrophilic C-H borylation. The frontier molecular orbital energy levels of the borylated materials then could be modulated through variation of the exocyclic boron substituents by trans-metallation with different diarylzinc reagents. The borylated materials possessed low band-gaps and low LUMO energy levels with a number of examples also showing significant absorbance > 700 nm; however, low photo-luminescence quantum yields were found for all these borylated compounds.

IntroductionDonor-acceptor (D-A) -conjugated systems have emerged as attractive materials for many applications, including or-ganic field effect transistors (OFETs), organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs).1,2 Whilst a plethora of D-A materials have been developed, there is still ongoing research into the modification of the donor and acceptor units to engineer new materials for specific applica-tions.3 One particularly topical area is the development of D-A materials that have small band-gaps and low LUMO en-ergy levels as these are desirable for use as ambipolar semi-conductors, electron acceptors in OPVs and as near-infrared (NIR) absorbing and emitting materials.4-6 Amongst D-A ma-terials that possess low band gaps, electron rich arylamines are prevalent as these donor units show low ionization poten-tials and good hole transport properties.7,8 A number of high performing far-red / near IR absorbing/emitting organic com-pounds combine arylamines with benzothiadiazole (BT) ac-ceptor units, or derivatives thereof (A and B, Figure 1).9,10

The further functionalisation of these materials to reduce the band-gap would be particularly desirable to red shift the ab-sorbance and shift the emission maxima to beyond 700 nm.

Figure 1: Two high performing D-A based far-red or-ganic emitters containing arylamine units and BT de-rivatives.

The incorporation of boron into -conjugated D-A mo-lecules by forming C,N-chelated boracycles was pioneered by Yamaguchi and Wakamiya.11 This approach has since be-come established as a useful means to modulate photophys-ical and electronic properties of conjugated materials.12-19

One effect of boracycle incorporation into D-A materials is a substantial decrease in the energy of the LUMO resulting in a red-shifted absorbance and emission relative to the un-borylated precursor.11-13 We have contributed to this area by using electrophilic C-H borylation to functionalise BT con-taining A-D-A and D-A-D materials to generate fused struc-tures with low LUMO energies.20-22 However, to date the electrophilic C-H borylation of BT containing D-A materials has been limited to the borylation of fluorene and thiophene substituents. Herein we report the extension of this methodo-logy to BT-arylamine containing D-A materials generating C,N-chelated fused and ladder type molecules (figure 2 bot-tom). These show smaller band-gaps than the previously re-ported borylated BT-fluorene analogues (Figure 2, top). Whilst the electrophilic borylation of 2-arylamine-substituted quinolines has been recently reported,19 the weaker acceptor

character of quinoline (relative to BT) results in materials with larger band gaps (absorption maxima < 502 nm) whereas materials reported herein have absorption maxima up to 680 nm and significant absorption beyond 700 nm.

Figure 2: Comparison of fluorene borylated D-A structures with the BT-arylamine borylated struc-tures reported herein.Results and DiscussionBorylated D-A-D materials: The addition of BCl3 to a DCM solution of compound 1 (Figure 3) resulted in the coordina-tion of boron to the basic nitrogen site of BT and the boryla-tion of a proximal C-H group of the triphenylamine unit in the position meta to the NPh2 moiety. Borylation was per-formed in the presence of 2,4,6-tri-tert-butylpyridine (TBP) to prevent any protonation of the amine units which would otherwise deactivate the system towards electrophilic sub-stitution. As previously observed with fluorene and thiophene analogues of 1 no C-H borylation of the second donor unit to form a doubly borylated D-A-D material occurs even in the presence of excess BCl3 and longer reaction times. This is in contrast to the report by Fang and co-workers where a thi-enyl-pyrazine-thienyl D-A-D materials underwent double electrophilic C-H borylation.13 This disparity is presumably due to the greater nucleophilicity of pyrazine relative to BT which enables coordination of a second boron moiety to the pyrazine unit post the initial borylation. Post borylation of 1, the initial product, 1-BCl2 which is sensitive to H2O, was transformed in-situ into the air and moisture stable materials, 1-BPh2 and 1-B(C6F5)2, by the addition of commercially available organozinc reagents, diphenyzinc or bis(penta-fluorophenyl)zinc, respectively. The products were isolated in >80 % yields by silica gel chromatography. The 11B NMR resonances for 1-BPh2 and 1-B(C6F5)2 were centred at 1 and -5 ppm, respectively, consistent with four coordinate boron centers and this with the 1H NMR spectrum showing only one species indicates that the four coordinate compound is the dominant species in solution suggesting minimal cleav-age of the dative bond occurs in solution, in contrast to BMes2 analogues.23

Figure 3: Top, the synthesis of 1-BPh2 and 1-B(C6F5)2. Bottom, the solid state structure of 1-BPh2, only one of two independent molecules in the

asymmetric unit is shown and disordered solvent molecules and hydrogen atoms are omitted for clar-ity, ellipsoids at 50% probability level. Blue = nitro-gen, grey = carbon, yellow = sulfur, pink = boron, red = centroid of C8, C7, C6, C1 and N1.

Compound 1-BPh2 crystallised with two molecules in the asymmetric unit with the bond distances and angles within the boracycles being closely comparable to previously repor-ted borylated BT-fluorene structures.21 The boracycle has only a minor deviation of the boron atom out of the plane generated by the other five atoms in the boracycle (as indic-ated by the out of plane para-C- (e.g. C6) centroid-B angles being 173o and 178°, Figure 3, and the distance between B and the plane of the other five atoms (e.g. C8-C7-C6-C1-N1) being 0.21 Å and 0.04 Å), this deviation combined with the compression of the endocyclic N-B-C (e.g., C8-B1-N1) angle (104.5(2)° and 105.1(2)°) indicates some degree of strain in the boracycle. This presumably arises from the longer bond distances to boron (C8-B1 = 1.629(4) and 1.627(4) and N1-B1 = 1.629(4) and 1.614(4) Å) relative to the C-C and C-N bonds in the boracycle (in the range 1.341(3) to 1.462(4) Å) combined with the annulated system containing one five membered ring orientating the nitrogen lone pair non-optim-ally for binding to boron as recently highlighted.23

Optoelectronic properties:As anticipated a large bathochromic shift in absorbance is observed upon borylation of 1 (Figure 4 and Table 1). Com-pound 1-BPh2 results in a bathochromic shift of 170 nm in the absorbance maximum and a reduction in the optical band-gap of 0.69 eV relative to unborylated 1. The absorbance maximum could be shifted further into the NIR region with a decrease in the absorbance maximum and the optical band-gap by 49 nm and 0.14 eV, respectively, upon installation of electron withdrawing C6F5 substituents on to boron in 1-B(C6F5)2. Based on previous work this decrease in the optical band-gap is attributed to the further reduction of the LUMO energy level of 1-B(C6F5)2 (relative to 1-BPh2). Comparison of 1-BPh2 with the related compound C (Figure 2 top), is notable as 1-BPh2 has an optical band gap 0.15 eV lower in energy than C confirming that replacement of fluorene for Ph-NPh2 has the desired effect of reducing the band gap. Whilst the borylated compounds all showed large Stokes shifts and NIR emission they were poorly emissive. Com-pound 1-BPh2 showed very weak emission with a maximum at 810 nm whereas 1-B(C6F5)2 was essentially non-emissive. Nevertheless, the significant absorption > 700nm, particu-larly for 1-B(C6F5)2, is notable.

Figure 4: UV-vis-NIR absorbance and emission spec-tra of 1, 1-BPh2 and 1-B(C6F5)2 (1 x 10-5 M toluene solutions). No emission is observed from 1-B(C6F5)2.Table 1: Comparison of the photophysical and redox properties of 1, 1-BPh2, 1-B(C6F5)2, and compound C.

Figure 5: Molecular orbital energy levels and molecular orbital contours (isovalue = 0.04) of the HOMO and LUMO of 1, 1-(BPh2)2 and 1-(B(C6F5)2)2 at the M06-2X/6-311G(d,p) level.

To gain further insight into the trends observed in the UV-vis absorbance spectra, DFT calculations (M06-2X/6-311G(d,p)) were performed (Figure 5). Calculations of 1

and 1-BPh2 show the expected significant lowering of the LUMO (by 0.70 eV) and the minor increase in the HOMO (by 0.07 eV) upon borylation and chelation of BPh2 which is consistent with observations of related borylated BT based D-A-D systems. There is an increased localization of the HOMO onto the borylated triphenylamine unit in 1-BPh2 rel-ative to 1 due to the positive inductive effect of four coordi-nate boron-C sigma bonds which raises in energy the orbitals associated with the borylated Ph-NPh2 unit relative to the non-borylated PhNPh2. Compound 1-B(C6F5)2 showed a fur-ther decrease in the LUMO (by 0.20 eV) with only a minor decrease in the HOMO level (by 0.07 eV) relative to 1-BPh2

which is consistent with the lower band-gap observed in the UV-vis absorbance spectra. Comparison of the calculated frontier orbitals of 1-BPh2 with compound C is of note and reveals minimal change in the LUMO (calculated LUMO en-ergy = -2.46 eV for C and -2.44 eV for 1-BPh2 with the LUMO localised on BT in both compounds) but a notable change in the HOMO which for 1-BPh2 is increased in en-ergy relative to C (-6.28 eV and -6.48 eV, respectively). The HOMO of C is highly localised on the non-borylated Ph-

Compoundmaxabs/

nm(M-1 cm-

1)a

maxem/nm (f

%b)a

Eoxonset

(V)dEred

onset

(V)dHOMO (eV)d

LUMO (eV)d

BandGap (eV)

1 461 (22200) 590 0.36 -1.87 -5.75 -3.52 2.35e, 2.23f

1-BPh2631

(12200)810

(<0.1) 0.29 -1.29 -5.68 -4.10 1.66e, 1.58f

1-B(C6F5)2680

(11000) ---c 0.36 -1.12 -5.75 -4.27 1.52e, 1.48f

Compound Cg

584 (13900) 753 (0.9) 0.45 -1.29 -5.84 -4.10 1.81e, 1.74

a1 x 10-5 M solution in toluene.. bRelative fluorescence quantum yield, estimated by using cresyl violet as standard (f = 54 % in methanol)24, estimated error ± 20 %. Fluorescence spectra were measured by exciting the solutions at their absorption maxima. cNon-emissive. dMeasured in DCM (1 mM) with [nBu4N][PF6] (0.1 M) as the supporting electrolyte at a scan rate of 50 mV s -1, potentials are given relative to the Fc/Fc+ redox couple which is taken to be 5.39 eV below vacuum.25 eOptical band-gap estimated from on-set of absorption. fElectrochemical Band-gap. g from reference 21

NPh2 unit as expected based on the greater “donor character” (higher energy of frontier orbitals) of triarylamines relative to fluorenes. However, in 1-BPh2 the HOMO is more local-ised on the borylated Ph-NPh2 again due to the positive in-ductive effect of four coordinate boron-C sigma bonds which leads to the higher HOMO energy of 1-BPh2 relative to C. An analogous effect was noted for borylated fluorene-BT-fluorene compound, with the borylated fluorene moiety con-tributing more character to the HOMO than the non-borylated fluorene unit.21

Cyclic voltammetry (CV) was employed in order to gain further insight into the effects of borylation and boron sub-stituents on the frontier orbital energy levels of the borylated arylamines (Figure 6 and Table 1). In all cases the reduction and oxidation waves are reversible and the trends observed by the DFT calculations and UV-vis absorption measure-ments correlated extremely well with those observed by CV. Significantly reduced reduction potentials of the borylated compounds were observed relative to 1. Furthermore, the ox-idation potential was less positive for 1-BPh2 relative to C consistent with the higher HOMO energy from a structure containing a borylated Ph-NPh2 unit.

Figure 6: Cyclic voltammetry plots for 1, 1-BPh2 and 1-B(C6F5)2, measured in DCM (1 mM) with [nBu4N][PF6] (0.1 M) as the supporting electrolyte at a scan rate of 50 mV s-1 (potentials are relative to ferrocenium/ferrocene).Diborylated A-D-A Materials: With 1-BPh2 and 1-B(C6F5)2

being weakly emissive we targeted a more highly fused borylated-BT-arylamine structure in an attempt to prepare NIR emitters with an enhanced fluorescence quantum yield. Therefore the BT-carbazole-BT compound, 2, containing the “swallow tail” substituent (CH(C8H17)2) to provide solubility, was synthesized via standard methodologies. Carbazole was selected as it is a fused arylamine well documented to un-dergo high yielding electrophilic C-H borylation.26 Further-more, double borylation of 2 will generate analogues of pre-viously double borylated BT-fluorene-BT compounds (e.g., compound D, figure 2 top) allowing for structure-property re-lationships to be assessed.

The diborylation of compound 2 was achieved using the same methodology as described for compound D (Scheme 1) although the reaction time was significantly reduced (from 2 days to 4 hours) presumably due to the more nucleophilic

nature of carbazole at the 3 and 6 positions (relative to fluorene). Again four equivalents of AlCl3 are required due to the propensity to form the diborocation [2-(BCl)2][AlCl4]2

(Scheme 1) as previously observed for the fluorene ana-logue.20 The C-H borylation of compound 2 with BCl3 (with or without TBP) does not proceed, instead the addition of BCl3 to 2 led to a colour change to red and the formation of a sharp 11B resonance at 5.0 ppm, but no reduction in the num-ber of aromatic resonances (combined 12H integral relative to aliphatic resonances), this is consistent with coordination of BCl3 to the less hindered nitrogen positions as previously reported.20 Furthermore, attempts using Murakami’s condi-tions, BBr3 / Hünigs base,27 were also unsuccessful.

Scheme 1: The synthesis of 2-(BCl2)2

Whilst 2-(BCl2)2 could be produced in-situ in high con-version (it is the only carbazole containing species observed by in-situ NMR spectroscopy) it proved sensitive to protic species as expected therefore arylation at boron was attemp-ted. Unlike the fluorene analogue of 2-(BCl2)2 which under-goes transmetallation in DCM at 20oC within 3 hours with organozinc reagents (ZnPh2 or Zn(C6F5)2), 2-(BCl2)2 under-goes transmetallation much more slowly. The conditions that were successfully employed for formation of compound D in moderate isolated yield (53%) resulted in only trace amounts of 2-(BPh2)2. In our hands the best conditions for the trans-metallation of 2-(BCl2)2 with ZnPh2 proved to be heating the reaction mixture at 60oC in toluene for 2 hours, further heat-ing resulted in decomposition to unidentified species. Even under these optimized conditions 2-(BPh2)2 was only isolated in 17% yield (Scheme 2) and was characterised based on multinuclear NMR spectroscopy (including a broad 11B res-onance centred at 2 ppm) and mass spectroscopy. 2-(BPh2)2

is stable to silica and H2O, thus the low isolated yield is not from decomposition during isolation. The synthesis of 2-(BPh2)2 was also attempted via a borocation mediated boro-destannylation reaction,22 however, the addition of an excess (6 eq.) of PhnBu3Sn to [2-(BCl)2][AlCl4]2 only resulted in the formation of trace amounts of the desired product. Trans-metallation of 2-(BCl2)2 with Zn(C6F5)2 was also low yielding (24 % isolated yield). Attempts to improve the yield of 2-(B(C6F5)2)2 through extended heating times at 60oC or heat-ing at 100oC resulted in decomposition and only traces of the desired product.

Scheme 2: The synthesis of 2-B(Aryl2)2

To gain some insight into the transmetallation step the 19F{1H} NMR of the reaction mixture containing 2-(BCl2)2 in toluene was monitored (Figure 7). Upon addition of 4 equi-valents of Zn(C6F5)2 the 19F{1H} NMR spectrum showed an up-field shift and significant broadening of the resonances as-sociated with the para- (-153.4 ppm) and meta- (-161.1 ppm) substituted fluorines of Zn(C6F5)2 with no resonances associ-ated with the transmetallated product observed at short reac-tion times (longer reaction times at 20oC did not lead to a sig-nificant increase in transmetallation). Upon heating (for upto 16 hours) 19F{1H} resonances associated with the transmetall-ated product 2-(B(C6F5)2)2 are present but the majority of the Zn(C6F5)2 remains unreacted. The broad meta and para reson-ances suggest a dynamic interaction between Zn(C6F5)2 and a Lewis base that is retarding transmetallation. However, no in-teraction is observed when Zn(C6F5)2 is added to N-octylcar-bazole indicating that Lewis adduct formation between car-bazole and Zn(C6F5)2 is unlikely. There is also no significant production of HC6F5 (precluding reaction of Zn(C6F5)2 with TBP-H) and no evidence for deboronation (no BCl3 or B(C6F5)3 by 11B NMR spectroscopy). Thus, currently the reason for the low yields of 2-B(Aryl2)2 is unclear.

Figure 7: Partial in-situ 19F{1H} NMR spectra moni-toring the transmetallation of 2-(BCl2)2 with Zn(C6F5)2 (-153 – -164 ppm region in toluene with a d6-DMSO capillary insert at 298 K). Table 2: Comparison of the photophysical and redox properties of 2, 2-(BPh2)2, 2-(B(C6F5)2)2 and compound D.

Compoundmaxabs/

nm(M-1 cm-

1)a

maxem/nm (f

%b)a

Eoxonset

(V)cEred

onset

(V)cHOMO (eV)c

LUMO (eV)c

BandGap (eV)

2 401 (24600) 491 0.75 -1.89 -6.14 -3.50 2.74d, 2.64e

2-(BPh2)2 540 727 (0.3) 0.50 -1.31 -5.89 -4.08 1.98d, 1.81e

(17000)2-

(B(C6F5)2)2

548 (15200) 719 (0.3) 0.67 -1.13 -6.06 -4.26 1.95d, 1.80e

Compound Df

538 (19500) 636 (18) 0.72 -1.28 -6.11 -4.11 2.02d, 2.00e

a1 x 10-5 M solution in toluene.. bRelative fluorescence quantum yield, estimated by using cresyl violet as standard (f = 54 % in methanol)24, estimated error ± 20 %. Fluorescence spectra were measured by exciting the solutions at their absorption maxima. cMeasured in DCM (1 mM) with [nBu4N][PF6] (0.1 M) as the supporting electrolyte at a scan rate of 50 mV s-1, potentials are given relative to the Fc/Fc+ redox couple which is taken to be 5.39 eV below vacuum.25 dOptical band-gap estimated from onset of absorp-tion. eElectrochemical Band-gap. ffrom reference 20

Optoelectronic Properties. The effect on the optical proper-ties of the incorporation of two B(Ar)2 units into the structure of compound 2 was investigated by UV-vis spectroscopy in toluene (table 2). It should be noted that due to the larger band gap of the parent compound 2 (relative to 1) the borylated congeners have blue shifted absorption and emis-sion relative to 1-B(Aryl)2. Again a large bathochromic shift in absorbance is observed upon borylation, for 2-(BPh2)2 and 2-(B(C6F5)2)2 the bathochromic shift in the absorbance max-ima are 139 nm and 147 nm, respectively (Figure 8). Further-more, there is a reduction in the optical band-gap of 0.76 and 0.79 eV, respectively, relative to unborylated 2. The similar optical band gaps for the two 2-(B(Aryl)2)2 compounds is at-tributed to the replacement of the Ph boron substituents with C6F5 having a similar effect on both the HOMO and LUMO energy levels, as observed in fluorene based A-D-A borylated ladder structures.12,20 In contrast, in the mono-borylated D-A-D systems the LUMO is reduced in energy by more than the HOMO on exchange of Ph for C6F5. 2-(BPh2)2

and 2-(B(C6F5)2)2 showed significantly red-shifted emission and larger Stokes shifts (max >700 nm) relative to their fluorene analogues, e.g. compound D, but significantly lower f values. Thus the increased degree of fusion in 2-(B(Ar)2)2

relative to 1-BAr2 has not significantly enhanced the f val-ues.

Figure 8: UV-vis absorbance spectra of 2, 2-(BPh2)2 and 2-(B(C6F5)2)2 (1 x 10-5 M toluene solu-tions).

Calculations on model compounds of 2, 2-(BPh2)2 and 2-(B(C6F5)2)2 (replacing N-CH(C8H17)2 with N-Me, referred to as 2’, 2’-(BPh2)2 and 2’-(B(C6F5)2)2, respectively) shows that upon diborylation there is a considerable reduction in the tor-sion angle from 46.3o to 3.0o between the carbazole and BT units. A significant lowering of the LUMO (by 0.70 eV) and an increase in the HOMO (by 0.37 eV) for 2’-(BPh2)2 relat-ive to the unborylated compound 2’ was found. However, this is not due to any increased orbital delocalisation on planarization with borylation actually decreasing the degree of delocalisation of the HOMO and LUMO (Figure 9). The energy changes observed on borylation are therefore attrib-uted to the electron withdrawing effect that boron dative bond formation to BT has on the LUMO energy and the pos-itive inductive effect of the four coordinate B-C bond raising the HOMO energy. The calculations also indicate that upon replacing the Ph boron substituents for C6F5 there is a de-crease in the energy of the HOMO and LUMO by approxim-ately the same degree (0.21 and 0.22 eV, respectively). This is consistent with the similar optical band-gaps observed in the UV-vis absorbance spectra. Additionally, the calculations indicate a complete localization of the LUMO onto the BT unit and a complete localization of the HOMO onto the car-bazole unit in 2’-(BPh2)2 and 2’-(B(C6F5)2)2. This is in con-trast to the calculated structure of a model of compound D where the HOMO is delocalised over both BT units and the fluorene unit.20 Again the change in the character of the HOMO is consistent with the stronger donor nature of the arylamine unit relative to fluorene. The greater spatial separ-ation of HOMO and LUMO in 2-(BPh2)2 relative to com-pound D is also consistent with the larger Stokes shift (187 nm) observed for 2-(BPh2)2 suggesting a greater degree of in-tramolecular charge transfer character. The absence of any spatial HOMO–LUMO overlap may also contribute to the observed low quantum yield values as this will result in a lower oscillator strength and thus a low radiative transition rate.10

Figure 9: Molecular orbital energy levels and molecular orbital contours (isovalue = 0.04) of the HOMO and LUMO of 2’, 2ʹ-(BPh2)2 and 2’-(B(C6F5)2)2 at the M06-2X/6-311G(d,p) level.

Again the trends in energies observed by the DFT calcu-lations and optical spectroscopy are confirmed by CV studies (Figure 10). Of note is that the comparable change in the cal-culated HOMO and LUMO energies on exchanging Ph for C6F5 is consistent with change in the observed oxidation and reduction onset potentials. Comparison of the onset potentials of 2-(BPh2)2 and compound D revealed that the replacement of fluorene with carbazole results in minimal change in the LUMO energy but a dramatic change in the HOMO (by 0.22 eV) as expected based on the nature of the respective frontier orbitals.

Figure 10: Cyclic voltammetry plots for 2, 2-(BPh2)2 and 2-(B(C6F5)2)2, measured in DCM (1 mM) with [nBu4N][PF6] (0.1 M) as the supporting elec-trolyte at a scan rate of 50 mV s-1 (potentials are rel-ative to ferrocenium/ferrocene). ConclusionsElectrophilic C-H borylation has been extended to BT-arylamine based D-A materials and represents a facile route to reduce the band gap of these popular low band gap materi-

als. The borylated arylamine compounds reported herein pos-sess lower band gaps than the fluorene analogues due to an increase in the HOMO energy achieved by incorporating the borylated arylamine donor units. The borylated D-A-D ma-terial 1-B(C6F5)2 shows a band gap of ca. 1.5 eV and has significant absorbance at greater than 700 nm. The low en-ergy LUMO and low band gap of these materials suggests that they (and derivatives thereof) have potential as ambi-polar semiconductors and this is currently being explored.

Experimental sectionMaterials and Instrumentation: Unless otherwise in-dicated all reagents were purchased from commer-cial sources and were used without further purifica-tion. 128 and 9-(9-Heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol) ester29 were prepared ac-cording to modified literature procedures. All appro-priate manipulations were performed using standard Schlenk techniques or in an argon-filled MBraun glovebox (O2 levels below 0.5 ppm). Solvents were distilled from NaK, CaH2, or K and degassed prior to use. Dichloromethane and THF were stored over ac-tivated 3 Å molecular sieves while toluene was stored over a potassium mirror. NMR spectra were recorded using a Bruker AV-400 spectrometer. Un-less otherwise stated all NMR spectra are recorded at 293 K. Carbon atoms directly bonded to boron are not always observed in the 13C{1H} NMR spectra due to quadrupolar relaxation leading to signal broaden-ing. Matrix assisted laser desorption/ionization time of flight (MALDI-TOF) measurements was performed by the Mass Spectrometry Service, School of Chem-istry, University of Manchester. MALDI-TOF analyses were performed using a Shimadzu Axima Confidence spectrometer using a 4k PPG as a calibration refer-ence. All UV-vis absorption spectra were recorded on a Varian Cary 5000 UV-vis-NIR spectrometer at room temperature in spectroscopic grade solvents. Emis-sion spectra were recorded on a Varian Cary Eclipse Fluorimeter at room temperature in spectroscopic grade solvents, the solutions were excited at their relative absorbance maxima. Cyclic voltammetry was performed using a CH-Instrument 1110C Elec-trochemical/Analyzer potentiostat under a nitrogen flow. The DFT Calculations were performed using the Gaussian09 suite of programmes30 Geometries were optimised with the DFT method using M06-2X functional31 and 6-311G(d,p) as a basis set with in-clusion of a PCM model for solvent correction (DCM).32 All stationary geometry optimizations were full, with no restrictions. Structures were confirmed as minima by frequency analysis and the absence of imaginary frequencies. Accurate combustion data was not obtainable, consistently low %C content were observed, and persisted even when V2O5 was used as an oxidation. Purity was indicated by multinuclear NMR spectroscopy in organic solvents (in which the sample fully dissolved) and is supported by MS analysis.

Compound 1-BPh2: BCl3 1M in DCM (0.35 mL, 0.35 mmol) was added to a solution of 1 (93 mg, 0.15 mmol) in DCM (~8 mL). The reaction mixture was stirred at ambient temperature for 1 hour under the

dynamic flow of nitrogen over which time period the reaction mixture had changed colour from orange to dark blue. 2,4,6-tritbutylpyridine (38 mg, 0.15 mmol) was then added to the reaction mixture. The solvent and other volatiles were removed under reduced pressure and the resulting blue residue was dis-solved in DCM (~10 mL). ZnPh2 (109 mg, 0.50 mmol) was added to the reaction mixture which was then stirred at ambient temperature for 3 hours. The reaction mixture was filtered through a plug of silica gel and the resulting solution was evaporated to dry-ness under reduced pressure. The resulting blue residue was purified via column chromatography [eluent: DCM:hexane (1:9 followed by 2:8)]. The de-sired product was obtained as a blue solid. (Yield 102 mg, 86 %)1H NMR (400 MHz, CDCl3) = 8.12 (d, 3JHH = 7.8 Hz, 1 H), 7.91 (d, 3JHH = 8.7 Hz, 1 H), 7.81-7.88 (m, 3 H), 7.28 - 7.39 (m, 5 H), 7.14 - 7.26 (m, 20 H), 7.04 - 7.14 (m, 6 H), 6.97 - 7.04 (m, 2 H), 6.93 ppm (dd, 3JHH = 8.6, 4JHH = 2.3 Hz, 1 H); 13C{1H} NMR (101 MHz, CDCl3) = 154.6 (br.), 154.0 (br.), 153.5, 148.4, 148.0, 147.8, 147.2, 147.1, 133.3, 130.4, 130.0, 129.5, 129.4, 129.1, 128.7, 128.0, 127.7, 127.4, 125.8, 125.0, 124.6, 124.4, 123.6, 123.2, 123.0, 122.9, 122.5, 120.8; 11B NMR (128 MHz, CDCl3) = ~1 (br.); MALDI-TOF: calc. for C54H39BN4S+ [M]+ = 786.3, found 786.3

Compound 1-B(C6F5)2: BCl3 1M in DCM (0.25 mL, 0.25 mmol) was added to a solution of 1 (69 mg, 0.11 mmol) in DCM (~8 mL). The reaction mixture was stirred at ambient temperature for 1 hour under the dynamic flow of nitrogen over which time period the reaction mixture had changed colour from or-ange to dark blue. 2,4,6-tritbutylpyridine (28 mg, 0.11 mmols) was then added to the reaction mixture. The solvent and other volatiles were re-moved under reduced pressure and the resulting blue residue was dissolved in DCM (~10 mL). Zn(C6F5)2 (73 mg, 0.33 mmol) was added to the re-action mixture which was then stirred at ambient temperature for 3 hours. The reaction mixture was filtered through a plug of silica gel and the resulting solution was evaporated to dryness under reduced pressure. The resulting blue residue was purified via column chromatography [eluent: DCM:petroleum ether (1:9 followed by 2:8)]. The desired product was obtained as a blue solid which was then washed with pentane. (Yield 87 mg, 82 %).1H NMR (400 MHz, CDCl3) = 8.22 (d, 3JHH = 7.8 Hz, 1 H), 7.90 (d, 3JHH = 7.8 Hz, 2 H), 7.79 - 7.87 (m, 2 H), 7.29 - 7.37 (m, 4 H), 7.16 - 7.26 (m, 10 H), 7.03 - 7.14 ppm (m, 10 H); 13C{1H} NMR (101 MHz, CDCl3) = 153.6, 149.0, 148.8, 147.6, 147.5 (br. d,1J (13C, 19F) = 240 Hz), 147.1, 146.9, 140.0 (br. d,1J (13C, 19F) = 250 Hz), 137.2 (br. d,1J (13C, 19F) = 250 Hz), 131.0, 130.2, 129.7, 129.5, 129.1, 128.1, 126.8, 126.3, 125.2, 124.9, 124.7, 123.8, 123.5, 123.2, 122.5, 122.3, 121.2; 19F{1H} NMR (376 MHz, CDCl3) = -131.91 (dd, 3JFF = 24.1, 4JFF = 8.6 Hz, 4 F), -156.90 (t, 3JFF = 21.1, 2 F), -162.66 (m, 4 F); 11B NMR (128 MHz, CDCl3) = ~-5 (br.); MALDI-TOF: calc. for C54H29BF10N4S+ [M]+ = 966.2, found 966.0

Compound 2: 9-(9-Heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol) ester (0.62 g, 0.94 mmol), 4-bromo-2,1,3-benzothiadiazole (0.46 g, 2.1 mmol), K3PO4.H2O (1.30 g, 5.62 mmol), degassed deionised water (1.35 mL), Pd2(dba)3 (42.9 mg, 0.05 mmol) and S-PHOS (38.5 mg, 0.01 mmol) were dissolved in THF (10 mL). The reaction mixture was stirred at 70°C for 24 hours. The cooled mixture was extracted with DCM (100 mL), washed with brine (1 x 100 mL), then water (1 x 100 mL) and dried with MgSO4. After evaporation of solvents, the resulting orange residue was purified on base treated silica gel (5 % NEt3 in hexane) column chromatography [eluent: chloro-form:hexane (0:1 followed by 2:8)]. The desired product was obtained as a yellow solid. (Yield 0.48 g, 77 %) 1H NMR (400 MHz, CDCl3) = 8.43 (br. s, 1 H), 8.31 (br. t, 3JHH = 8.4 Hz, 2 H), 8.20 (br. s, 1 H), 8.04 (dd, 3JHH = 8.8, 4JHH = 1.0 Hz, 2 H), 7.86 (dd, 3JHH = 6.8, 4JHH

= 1.0 Hz, 2 H), 7.83 (br. d 3JHH = 7.6 Hz, 2 H), 7.73 (m, 2 H), 4.82 (sept, 3JHH = 5.2 Hz, 1 H), 2.61 - 2.44 (m, 2 H), 2.14 - 1.98 (m, 2 H), 1.41 - 1.11 (m, 24 H), 0.87 - 0.76 (m, 6 H); 13C{1H} NMR (101 MHz, CDCl3) = 155.7, 153.7, 142.8, 139.3, 135.5, 135.4, 134.9, 134.3, 129.6, 127.7, 127.7, 123.6, 122.3, 120.5, 120.2, 120.1, 113.0, 110.2, 56.4, 33.8, 31.7, 29.4, 29.3, 29.1, 26.8, 22.5, 13.9; Hindered N-CH(C8H17)2 rotation leads to lower solution symmetry as previ-ously observed.33 MALDI-TOF: calc. for C41H47N5S2

+

[M]+ = 673.3, found 673.8

Compound 2-(BPh2)2: BCl3 1M in DCM (0.38 mL, 0.38 mmol) was added to a stirred solution of 2 (64 mg, 0.095 mmol) and 2,4,6-tritbutylpyridine (48 mg, 0.19 mmol) in DCM (3 mL). AlCl3 (26 mg, 0.19 mmol) was added to the reaction mixture which was then stirred for 2 hours at room temperature. An addi-tional quantity of AlCl3 (26 mg, 0.19 mmol) was ad-ded and the reaction mixture was stirred for 2 hours over which time period the reaction mixture had turned blue. The reaction mixture was evaporated to dryness and the residue was then dissolved in DCM (5 mL) and NMe4Cl (41 mg, 0.38 mmol) was added to the solution which instantly turned pink. After the removal of the solvent under reduced pressure, the reaction mixture was dissolved in toluene (20 mL) and ZnPh2 (105 mg, 0.49 mmol) was added to the reaction mixture. The reaction was stirred at 60°C for 2 hours which resulted in a purple solution. After the solvent was removed under reduced pressure, the desired product was purified by base treated (5 % NEt3 in hexane) preparative TLC [eluent: DCM:hexane (3:7)] to afford a purple solid. (Yield 16 mg, 17 %). 1H NMR (400 MHz, CDCl3) = 8.37 (d, 3JHH = 6.3 Hz, 1 H), 8.32 (d, 3JHH = 6.8 Hz, 1 H), 8.21 (s, 1 H), 8.12 (d, 3JHH = 6.8 Hz, 2 H), 8.03 (s, 1 H), 7.96 - 7.78 (m, 4 H), 7.13 - 7.31 (m, 20 H), 4.70 (sept, 3JHH = 4.8 Hz, 1 H), 2.44 (m, 2 H), 2.09 (m, 2 H), 1.43 - 1.08 (m, 24 H), 0.79 (t, 3JHH = 6.80, 6 H); 13C{1H} NMR (101 MHz, CDCl3) = 155.2, 155.1, 147.7, 142.8, 142.6, 139.4, 133.5, 133.0, 131.2, 128.6, 128.2, 127.5, 127.0, 126.7, 125.8, 124.6, 122.9, 118.7, 104.3, 101.6, 56.1, 33.9, 31.9, 31.7, 31.6, 29.7, 29.6, 29.3, 29.2, 27.1, 22.5, 14.0; Hindered N-CH(C8H17)2 rotation leads to lower solution symmetry as previously ob-

served.33 11B NMR (128 MHz, CDCl3) = ~2.0 (br.); MALDI-TOF: calc. for C59H60B2N5S2

+ [M - C6H5]+ = 924.4, found 924.6

Compound 2-(B(C6F5)2)2: BCl3 1M in DCM (0.6 mL, 0.6 mmol) was added to a stirred solution of 2 (100 mg, 0.15 mmol) and 2,4,6-tritbutylpyridine (73 mg, 0.30 mmol) in DCM (3 mL). AlCl3 (40 mg, 0.30 mmol) was then added to the reaction mixture which was then stirred for 2 hours at room temperature. An ad-ditional quantity of AlCl3 (40 mg, 0.30 mmol) was ad-ded and the reaction mixture was stirred for 2 hours whereupon the reaction mixture had turned blue. The reaction mixture was evaporated to dryness and the reaction mixture was then dissolved in DCM (5 mL) and NMe4Cl (32 mg, 0.30 mmol) was added to the solution which instantly turned pink. After the re-moval of the solvent under reduced pressure, the re-action mixture was dissolved in toluene (20 mL) and Zn(C6F5)2 (237 mg, 0.59 mmol) was added to the re-action mixture. The reaction was left to stir overnight at 60°C which resulted in a purple solu-tion. After the solvent was removed under reduced pressure, the desired product was purified by base treated (5 % NEt3 in hexane) preparative TLC [elu-ent: DCM:hexane (3:7)] to afford a purple solid. (Yield 48 mg, 24 %)1H NMR (400 MHz, CDCl3) = 8.50 (d, 3JHH = 6.7 Hz, 1 H), 8.54 (d, 3JHH = 6.7 Hz, 1H) 8.20 (s, 1 H), 8.09 - 7.87 (m, 7 H), 4.69 (sept, 3JHH = 5.0 Hz, 1 H), 2.46 - 2.35 (m, 2 H), 2.15 - 2.04 (m, 2 H), 1.40 - 1.07 (m, 24 H), 0.79 - 0.73 (t, 3JHH = 6.8 Hz, 6 H); 13C{1H} NMR (101 MHz, CDCl3) : 155.0, 147.6 (br. d,1J (13C, 19F) = 238 Hz), 147.1, 143.1, 140.0 (br. d,1J (13C, 19F) = 252 Hz), 139.8, 137.3 (br. d,1J (13C, 19F) = 250 Hz), 133.4, 130.2, 127.5, 127.1, 125.7, 125.4, 125.2, 124.9, 124.4, 121.6 (br. m), 119.4, 104.7, 102.0, 56.5, 33.9, 31.7, 29.5, 29.3, 29.2, 27.0, 22.5, 14.0; 19F{1H} NMR (376 MHz, CDCl3) = -131.89 (m, 8 F), -156.79 (m, 4 F), -162.80 (m, 8 F); 11B NMR (128 MHz, CDCl3) = ~-6.0 (br.); MALDI-TOF: calc. for C59H45B2N5F15S2

+ [M - C6F5]+ = 1194.3, found 1194.0

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Files avail-able include:Analytical data and crystallographic information (PDF)Crystallographic data (CIF)Calculated structures (MOL)

AUTHOR INFORMATIONCorresponding Author*Email for M. J. I. michael.ingleson@manch-ester.ac.uk*

Michael Ingleson ORCID 0000-0001-9975-8302

Author Contributions

The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTS The research leading to these results has received funding from Cambridge Display Technology Limited (Company Number 02672530, CDT/EPSRC Case Award to D.L.C.) the EPSRC (EP/K03099X/1) and the European Research Council (FP/2007–2013/ERC Grant Agreement 305868). M.J.I. acknowledges the Royal Society (for the award of a University Re-search Fellowship) and M.L.T. thanks InnovateUK for financial support of the Knowledge Centre for Mater-ial Chemistry. The authors acknowledge the use of the EPSRC UK National Service for Computational Chemistry Software (NSCCS) at Imperial College London in carrying out this work. Dr Martin J. Humphries at CDT is also thanked for useful discus-sions. Additional research data supporting this publication are available as supplementary information accompanying this publication.

REFERENCES(1) Shirota Y.; Kageyama, H. Chem. Rev. 2007, 107, 953–1010. (2) Guo, X.; Baumgarten M.; Müllen, K. Prog. Polym. Sci. 2013, 38, 1832–1908.(3) Sekine, C.; Tsubata, Y.; Yamada, T.; Kitano, M.; Doi, S. Sci. Technol. Adv. Mater. 2014, 15, 034203.(4) Dou, L.; Liu, Y.; Hong, Z.; Li, G.; Yang, Y. Chem. Rev. 2015, 115, 12633−12665.(5) Anthony, J. E.; Facchetti, A.; Heeney, M.; Marder, S. R.; Zhan, X. Adv. Mater. 2010, 22, 3876−3892.(6) Qian, G.; Wang, Z. Y. Chem. Asian J. 2010, 5, 1006–1029.(7) Wang, J.; Liu, K.; Ma, L.; Zhan, X. Chem. Rev. 2016, 116, 14675−14725.(8) Gudeika, D.; Bundulis, A.; Mihailovs, I.; Volyniuk, D.; Rutkis, M.; Grazulevicius J. V. Dyes Pigments 2017, 140, 431−440.(9) Liu, T.; Zhu, L.; Zhong, C.; Xie, G.; Gong, S.; Fang, J.; Ma D.; Yang, C. Adv. Funct. Mater. 2017, 1606384. DOI: 10.1002/adfm.201606384.(10) Yao, L.; Zhang, S.; Wang, R.; Li, W.; Shen, F.; Yang, B.; Ma, Y. Angew. Chem. Int. Ed. 2014, 53, 2119–2123.(11) (a) Wakamiya, A.; Taniguchi T.; Yamaguchi, S. Angew. Chem., Int. Ed., 2006, 45, 3170−3173. (b) Job, A.; Wakam-iya, A.; Kehr, G.; Erker, G.; Yamaguchi, S. Org. Lett. 2010, 12, 5470−5473. For a review see: (c) A. Wakamiya, S. Yamaguchi, Bull. Chem. Soc. Jpn. 2015, 88, 1357–1377(12) (a) Yusuf, M.; Liu, K.; F.; Guo, R.; Lalancette, A.; Jäkle, F. Dalton Trans. 2016, 45, 4580–4587. For a review see: (b) Ren. Y.; Jäkle. F.; Dalton Trans. 2016, 13996-14007.(13) Zhu, C.; Guo, Z.-H.; Mu, A. U.; Liu, Y.; Wheeler, S. E.; Fang, L. J. Org. Chem. 2016, 81, 4347–4352.(14) (a) Grandl, M.; Rudolf, B.; Sun, Y.; Bechtel, D. F.; Pierik, A. J.; Pammer, F. Organometallics 2017, DOI: 10.1021/ac-s.organomet.6b00916 (b) Grandl, M.; Kaese, T.; Kraut-sieder, A.; Sun, Y.; Pammer, F. Chem. - Eur. J. 2016, 22, 14373−14382. (c) Grandl, M.; Sun, Y.; Pammer, F. Chem. - Eur. J. 2016, 22, 3976−3980.(15) (a) Mellerup, S. K.; Yuan, K.; Nguyen, C.; Lu, Z.-H.; Wang, S. Chem. Eur. J. 2016, 22, 12464–12472. (b) Yang, D.-T.; Mellerup, S. K.; Peng, J.-B.; Wang, X.; Li, Q.-S.; Wang, S. J. Am. Chem. Soc. 2016, 138, 11513−11516. (c) Shi, Y.; Yang, D.; Mellerup, S. K.; Wang, N.; Peng, T.; Wang, S. Org.

Lett. 2016, 18, 1626−1626. (d) Yang, D.-T.; Mellerup, S. K.; Wang, X.; Lu, J.-S.; Wang, S.; Angew. Chem., Int. Ed. 2015, 54, 5498−5501. (e) Wang, S.; Yang, D.-T.; Lu, J.; Shi-mogawa, H.; Gong, S.; Wang, X.; Mellerup, S. K.; Wakamiya, A.; Chang, Y.-L.; Yang, C.; Lu, Z.-H. Angew. Chem., Int. Ed. 2015, 54, 15074−15078. (f) McDonald, S. M.; Mellerup, S. K.; Peng, J.; Yang, D.; Li, Q.-S.; Wang, S. Chem. - Eur. J. 2015, 21, 13961−13970. For a review of earlier work see: (g) Rao, Y.-L.; Wang, S.; Inorg. Chem. 2011, 50, 12263-12274.(16) Li, D.; Zhang, H.; Wang, Y. Chem. Soc. Rev., 2013, 42, 8416–8433.(17) Wang, J.-Y.; Pei, J. Chin. Chem. Lett. 2016, 27, 1139–1146.(18) Dou, C.; Ding, Z.; Zhang, Z.; Xie, Z.; Liu, J.; Wang, L. Angew. Chem. Int. Ed. 2015, 54, 3648–3652.(19) Shaikh, A. C.; Ranade, D. S.; Thorat, S.; Maity, A.; Kulkarni, P. P.; Gonnade, R. G.; Munshi, P.; Patil N. T. Chem. Commun. 2015, 51, 16115–16118.(20) Crossley, D. L.; Cade, I. A.; Clark, E. R.; Escande, A.; Humphries, M. J.; King, S. M.; Vitorica-Yrezabal, I.; Ingleson, M. J.; Turner, M. L. Chem. Sci. 2015, 6, 5144−5151. (21) Crossley, D. L.; Vitorica-Yrezabal, I.; Humphries, M. J.; Turner, M. L.; Ingleson, M. J. Chem. Eur. J. 2016, 22, 12439−12448.(22) Crossley, D. L.; Cid, J.; Curless, L. D.; Turner, M. L.; Ingleson, M. J. Organometallics 2015, 34, 5767−5774.(23) (a) Shimogawa, H.; Yoshikawa, O.; Aramaki, Y.; Murata, M.; Wakamiya, A.; Murata, Y.; Chem. Eur. J. 2017, 23, 3784-3791. (b) Nakamura, T.; Furukawa, S.; Nakamura, E.; Chem. Asian J. 2016, 11, 2016 –2020(24) Brouwer, A. M.; Pure Appl. Chem. 2011, 82, 2213−2228.(25) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater., 2011, 23, 2367−2371.(26) Bagutski, V.; Del Grosso, A.; Carrillo, J. A.; Cade, I. A.; Helm, M. D.; Lawson, J. R.; Singleton, P. J.; Solomon, S. A.; Marcelli, T.; Ingleson, M. J. J. Am. Chem. Soc. 2013, 135, 474−487.(27) Ishida, N.; Moriya, T.; Goya, T.; Murakami, M. J. Org. Chem. 2010, 75, 8709−8712.(28) Kato, S-I.; Matsumoto,T.; Shigeiwa, M.; Gorohmaru, H.; Maeda, S.; Ishi-I,T.; Mataka, S. Chem. Eur. J., 2006, 12, 2303-2317.(29) Zhang, Z.; Liu,Y.; Yang, Y.; Hou, K.; Peng, B.; Zhao, G.; Zhang, M.; Guo, X.; Kang E.; Li, Y. Macromolecules, 2010, 43, 9376-9383.(30) Gaussian 09, Revision C1, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian, Inc., Wallingford CT, 2009.(31) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241. (32) Mennucci, B.; Cancès, E.; Tomasi, J. J. Phys. Chem. B 1997, 101, 10506−10517.(33) Wang, K.; Firdaus, Y.; Babics, M.; Cruciani, F.; Saleem, Q.; Labban, A. E.; Alamoudi, M. A.; Marszalek, T.; Pisula, W.; Laquai, F.; Beaujuge, P. M. Chem. Mater. 2016, 28, 2200−2208.

11