Highly Stable, Readily Reducible, Fluorescent, Trifluoromethylated 9‐Borafluorenes

Abstract Three different perfluoroalkylated borafluorenes (F Bf) were prepared and their electronic and photophysical properties were investigated. The systems have four trifluoromethyl moieties on the borafluorene moiety as well as two trifluoromethyl groups at the ortho positions of their exo‐aryl moieties. They differ with regard to the para substituents on their exo‐aryl moieties, being a proton (F XylFBf, FXyl: 2,6‐bis(trifluoromethyl)phenyl), a trifluoromethyl group (F MesFBf, FMes: 2,4,6‐tris(trifluoromethyl)phenyl) or a dimethylamino group (p ‐NMe2‐FXylFBf, p‐NMe2‐FXyl: 4‐(dimethylamino)‐2,6‐bis(trifluoromethyl)phenyl), respectively. All derivatives exhibit extraordinarily low reduction potentials, comparable to those of perylenediimides. The most electron‐deficient derivative F MesFBf was also chemically reduced and its radical anion isolated and characterized. Furthermore, all compounds exhibit very long fluorescent lifetimes of about 250 ns up to 1.6 μs; however, the underlying mechanisms responsible for this differ. The donor‐substituted derivative p ‐NMe2‐FXylFBf exhibits thermally activated delayed fluorescence (TADF) from a charge‐transfer (CT) state, whereas the F MesFBf and F XylFBf borafluorenes exhibit only weakly allowed locally excited (LE) transitions due to their symmetry and low transition‐dipole moments.

Yamaguchi and co-workers reporteda ir andm oisture-stable borafluorene derivatives employing either Tip( 2,4,6-tris(triisopropyl)phenyl) or the even bulkierM es* (2,4,6-tris(tert-butyl)phenyl) substituents. [86,87] It was foundt hat the Tipd erivatives could be used as turn-ont ype fluorides ensors, whereast he Mes* compounds showed no reactionw ith fluoride. Recently, Rupar and co-workersh ave studied these effects in detail. [88] They found that althoughT ip-substituted derivatives still decompose slowly (< 10 %d ecomp. over 24 h) in wet solvents, the corresponding F Mes derivativese xhibit highers tability (5 % decomp. over 24 h). Derivatives containing p-bonding moieties were found to be much more sensitive towards moisture (iPrN 2 :5 0% decomp. over 1h; tBuO:1 0% decomp. over 1h). ortho-Trifluoromethyl-substituted aryls exhibit as trong stabilizing effect on boranes. [33,79,[98][99][100][101][102] In additiont ot he steric effect, ad irecti nteractiono ft he lone pairs of the fluorine atomsw ith the empty po rbitalo ft he boron center is observed. This is supported by BÀFd istances which are much shorter than the sum of their van der Waals radii (3.39 ) [103] in crystal structures. The electronicp roperties of borafluorenes can be easily tuned to fit different applications by the introductiono fd ifferent substitution patternso nt he biphenyl backbone( Figure2, right). The introduction of methoxy groups at the 6a nd 12 positions leads to as mallh ypsochromic shift of both the absorp-tion and emission wavelength, [86] whereas elongation of the psystem with electron-rich conjugateds ystems attached at the 5a nd 13 positions leads to ab athochromic shift of both the absorption and emission wavelength. [87] The photophysical properties of borafluorenes can also be modifiedb yc oordination of Lewis bases. Both Yamaguchi and co-workers and Rivard and co-workerso bserved turn-on fluorescence upon adduct formation. [86,104] Wilson andG illard and co-workers observedt urn-off fluorescenceo faborafluorenium cation upon coordination of aL ewis base at low temperature, resulting in thermochromism. [105] Piers and co-workers investigated the properties of ah ighly electron-deficient perfluorinated borafluorene iii. [95,96] Although they only observed ar eduction corresponding to the perfluoroaryls in the cyclic voltammogram, ar eaction witht he relatively mild reducing agent CoCp 2 (CoCp 2 /CoCp 2 + + : À1.3 vs. Fc/Fc + + )w as observed, underlining the electron-deficient nature of the compound.I nc ompetition experiments with the strongL ewisa cid B(C 6 F 5 ) 3 ,p reference towards the borafluorene derivative waso bserved, especially   [87,88] It is important to note, that ortho-trifluoromethyl substituted aryls provide both steric as well as electronic stabilization.E xamples of the functionalization of the borafluorene backbonetot une the electronic properties (right). [86,[95][96][97] with sterically demanding Lewis bases. The (sp 2 -C)ÀFb onds in this compound, however,a re still reactive towards nucleophiles. In contrast, perfluoroalkyl groups are inert towards nucleophiles, provide as trong inductive electron-withdrawing effect and have been previouslye mployed in the synthesis of electron-deficient triarylboranes. [102] To the best of our knowledge, there have been no photophysical studies of borafluorenes with electron-deficient biphenyl backbones. We envisioned that judicious incorporation of trifluoromethyl groups both in the biphenyl core and at the exo-aryl moiety would provide as ignificant stability enhancement whiler etaining the low-lyingL UMO typical of non-annulated boroles.
Biphenyl derivative 2 was synthesizedt hrough regioselective CÀHb orylation of 1 [106,107] ortho to the bromine and as ubsequent copper-catalyzed oxidative homocoupling. For the last step, 2 was dilithiated and subsequently reacted with the appropriate Ar-BF 3 Ks alt. Attempts to synthesize the haloborafluorene with different BX 3 (X = F, Cl, Br) sourcesf ailed. Attempts to use aryl boronates in place of the Ar-BF 3 Ks alt were also unsuccessful. The use of aryltrifluoroborate salts as boron source was previously reported by our group for the synthesis of boroles with enhanced stability, [79] and applied by others in the synthesis of boron polyaromatic hydrocarbons (PAHs) [108,109] and aryl borates. [110,111] Organic trifluoroborate salts are widely employed in cross-coupling reactions as they are readily accessible and very stable. [112,113] During the synthesis we observed that adding LiBr greatlyi mproves the reactivityo ft he Ar-BF 3 K salts. It is possible that ac ation-exchange reaction generates the more reactive Ar-BF 3 Li salt. The increased reactivity of the Ar-BF 3 Li salt is due to the thermodynamically favorable LiF elimination. It is also possible that LiBr stabilizes the aryllithium species towards decomposition in ethereal solvents. This decomposition also explains the low yields of isolated material. It is importantt on ote that the corresponding ortho-trifluoromethylarylboron halides( X = Cl, Br) are not stable due to halide exchange. [114] This mighta lso explain why the synthesis of the haloborafluorenes was not possible. The compounds F Mes F Bf, F Xyl F Bf,a nd p-NMe 2 -F Xyl F Bf were obtained after purification through sublimation and recrystallization. Both F Mes F Bf and F Xyl F Bf are bright-greens olids.I nc ontrast, p-NMe 2 -F Xyl F Bf is a red solid. All compounds exhibit 1 HNMR and 13 C{ 1 H} NMR signals consistentw ith their proposed structures. The 11 B{ 1 H} NMR shifts for all three borafluorene derivatives are around6 4ppm and differ only slightly.T he 19 F{ 1 H} NMR spectra display singlets and septets, the latter with a J FF coupling constant of 3-4 Hz. (Table 1) The singlets at about d = À63.5 ppm correspond to the two freely rotating para CF 3 groups on the borafluorene core. For F Mes F Bf,a nother singlet corresponding to the para CF 3 group on the exo-aryl is observed. The CF 3 groups ortho to the boron centerd isplay ac omplex coupling pattern of two septets with small coupling constants( J FF = 4Hz). This can be attributedt o through-space FÀFc oupling as previously observed at low temperature (243 K) for( F Mes) 2 BAr compounds. [100] The fact that the borafluorenes exhibit this phenomenona tr oom temperatureisa ni ndicatoro ft he high rigidity of the systems.
Scheme1.Synthesis of F Mes F Bf, F Xyl F Bf and p-NMe 2 -F Xyl F Bf. All three compounds are stable in the solid state andc an be stored under ambient conditions withoutd ecomposition. In wet CDCl 3 (1.5 equiv.H 2 Op er borafluorene) at room temperature, no decomposition of either F Mes F Bf or F Xyl F Bf was observed over 4days by NMR spectroscopy.T his is surprising given that for F MesBf and TipBf,b oth less electron-deficient compounds, decomposition rates of 5a nd 10 %r espectively in wet solvents over 24 hw ere reported. [88] This indicates that the CF 3 -groups ortho to the boron centero nt he borafluorenec ore have as tabilizing effect, likely due to steric shielding. However, p-NMe 2 -F Xyl F Bf shows very rapid decompositionw hen exposed to wet solvents. It is likely that the dimethylamine moiety is protonated first, thereby furtheri ncreasing the electrophilicity of the boron center and decreasing its stabilityt owards nucleophilic attack. The reactionw ith H 2 Ol eads to cleavage of one BÀCb ond of the borafluorene core, resulting in aB OH and CH moiety.T he same reactivity towards water and other EÀHb onds( E= N, O, S) was previously observed by Martin and co-workers. [94] Likely due to less steric hinderance in their system,asecond borafluorene reacts with the decompositionp roduct to form aB -O-B motif. The product of the hydrolysis of p-NMe 2 -F Xyl F Bf was isolated and studied by X-ray diffraction (compound D in Figure S41, Supporting Information). All three compounds are slightly soluble in non-polar solvents such as hexane or toluene and soluble in polar non-coordinating solvents such as CH 2 Cl 2 and THF.D issolving F Mes F Bf in acetonitrile gives ac olorless solution. Investigation of the solutionu sing 19 FNMR spectroscopy revealed the formation of an acetonitrile adduct, whichi sc onsistent with previous studies by Martin and co-workersf or less sterically hindered borafluorenes. [91] The para-CF 3 groups, both on the borafluorene backbone as well as the exo-aryl moiety,a re influenced only weakly by the coordination of acetonitrile, because both singlets in the 19 FNMR spectrum shift only slightly to lower field.
This suggests that only one exo-aryl trifluoromethyl moiety is coupling to the ortho trifluoromethyl groups on the borafluorene backbone. After evaporation of the acetonitrile and dissolution in C 6 D 6 ,o nly the borafluorene was observed through 19 FNMR spectroscopy.T hus, the formationo ft he adduct with acetonitrile is completely reversible.

Crystal and molecular structures
Single crystals of the three borafluorenes as well as the acetonitrile adduct of F Mes F Bf ( F Mes F Bf·MeCN)s uitable for X-ray studies were obtained ( Figure 4) and selected bond lengths, angles,t orsion angles and short BÀFc ontacts are listed in Ta ble 2. The single crystalso f F Mes F Bf and p-NMe 2 -F Xyl F Bf were obtained from as aturatedh exane solution at À30 8C, that of F Xyl F Bf was obtainedb ye vaporation of as aturated CH 2 Cl 2 solution and that of F Mes F Bf·MeCN was obtained from as aturated acetonitrile solution at À30 8C.
Ac omparison of the crystal structures of the three target compounds shows the following. Although all three BÀCb ond lengths are in as imilar range for F Mes F Bf (1.579(3)-1.591(3) ), the BÀC1 exo distances to the F Xyl groups of F Xyl F Bf and p- (3) )are slightly shorter than the re-spectiveB ÀC2 and BÀC3 bonds (1.591(3)-1.595(3) )within the borole moieties ( Table 2) (2) )a re typical for aromatic bonds, whereas the CÀCb ond (1.474(2)-1.482(2) )t hat is opposite to the boron atom hass ignificant single-bond character.The interplanar angle betweent he borafluorene (BC 12 )a nd the exo-aryl substituent is close to 908 in all three compounds (88.69(5)-89.54(6)8). This is due to the large steric demando ft he CF 3 groups in the ortho positions of both the exo-aryl moiety as well as the borafluorene core. In all three compounds, two BÀFd istances, each in the range of 2.366(2)-2.440(3) ,a re observed, which are significantly shorter than the sum of the van der Waals radii for boron and fluorine (3.39 ). [103] This was previously observed in boranes and boroles with ortho-CF 3 aryl moieties. [33,79,102,114,115] Given that the two respective fluorine atoms are directly above and below the boron center, it is most likely that the lone pair electrons of these fluorine atoms interactw ith the empty p-orbital of the boron center. The torsion angle C endo -C2 endo -B-C1 exo with the endo carbon atoms belonging to the borole moiety deviates slightly from 1808 (171.28(16)-178. 83 (17)8). This shows that the BÀC1 bond to the exocyclic moiety is tilteds lightly out of the borafluorene plane. The out-of-plane tilt increases from p- (16)8). The magnitude of the tilt is relatedt ot he molecular packing, which is similar in all three crystal structures because the borafluorene moieties are arranged in pairs, which are related by inversion symmetry and form weaki ntermolecular p···p interactions between the borafluorene backbones. The strongest p···p interaction is observed in F Mes F Bf, which shows the smallest centroid-distance, interplanar sepa-   (3) )t hat is elongated compared to F Mes F Bf,b ut still shorter than the sum of the van der Waals radii. The BÀN bond length is 1.591 (3) ,w hich is significantly shorter than that in the acetonitrile adduct of B(C 6 F 6 ) 3 (1.616 (3) ), [116] but similar to that in the previously reported MeCN adduct of PhBf (PhBf·MeCN)( 1.598 (4) ). [91] The NCb ond (1.129 (3) )i sa lso shorter than that in the acetonitrilea dduct of B(C 6 F 6 ) 3 (1.141 (2) )a nd very similar to that in PhBf·MeCN (1.128 (4) ).

Electrochemistry
Cyclic voltammogramso ft he three borafluorenesw ere recorded in dichloromethane with [nBu 4 N][PF 6 ]a st he electrolyte and as can rate of 250 mV s À1 ( Figure 5) in order to determine their reduction potentials. All measurements were referenced to the ferrocene/ferrocenium redoxc ouple (Fc/Fc + + ). The most electron-deficient borafluorene, F Mes F Bf,e xhibits ar eversible reductiona tÀ1.13 Va nd an irreversible reduction at À2.04 V.
For the slightly less electron-deficient F Xyl F Bf,areversible reductiona tÀ1.21 Vand an irreversible reduction at À2.12 Vare observed. Interestingly, p-NMe 2 -F Xyl F Bf shows ar eversibler eductiona tÀ1.28 V, an irreversible reduction at À2.15 Va nd a partially reversible oxidation at 0.95 V. The three borafluorenes exhibit much higher reduction potentials ( F Mes F Bf = À1.13 V; F Xyl F Bf = À1.21 Va nd p-NMe 2 -F Xyl F Bf = À1.28 V) than any of the previously reported borafluorenes and boroleso rt riarylboranes (Table3). The substitution patterns of compounds F MesBC 4 Ph 4 (E1 = 2 = À1.52 V) and F MesBf (E1 = 2 = À1.82 eV) allows ad irect comparison of the fluorinated borafluorene backbone( F Bf)w ith the unsubstituted borafluorene backbone (Bf)a nd the non-annulatedb orole. [88] The strong anodic shift of the F Mes F Bf as compared to  backbone. This is likely due to the planar geometry as well as the fact that the ortho CF 3 -groupso ft he borafluorene backbone do not display BÀFi nteractions, and thus do not increase electron density at the boron atom. The reduction potentials of the trifluoromethylatedb orafluorenes do not differ strongly from one another.T his indicates that the para substituent on the exo-cyclic aryl moiety does not have as ignificant influence on the electron accepting properties of these borafluorenes. This is best illustrated by the fact that the p-donating dimethylamino group only leads to ac athodic shift of 0.15 Vc ompared to at rifluoromethyl group. This is likelyd ue to the nearly perpendicular arrangement of the exo-aryl group with respect to the borafluorene backbone, that limits p-conjugation leaving only inductive effects of the exo-aryl moiety on the borafluorene core and the boron center.T oi nvestigate furthert he electronic properties of borafluorene F Mes F Bf,C oCp 2 was chosen as ar educing agent (E 0 '(CoCp 2 ) = À1.3 Vv s. Fc/Fc + + ). [122] Thus, after addition of CoCp 2 ,t he yellowish THF solution of F Mes F Bf turned dark purple and an ESR measurement confirmed the presence of the borafluorene radicala nion [ F Mes F Bf] ·À À (Figure 6).

Photophysical properties
To obtain furtheri nsighti nto the electronic structure of the borafluorenes, absorption and emission spectra as wella s quantum yields and excited-state lifetimesw ere measured in hexane (Figure 8a nd Ta ble 4). Furthermore, F Mes F Bf was also studied in CH 2 Cl 2 and in the solid state. Solvatochromic studies of p-NMe 2 -F Xyl F Bf could not be carriedo ut, because no emission was detected in more polar solvents. This is most likely due to af urtherr edshift of the emission, which in turn results in increased non-radiative decay processes and thus am uch lower quantum yield. Ap hotophysical investigation of F Mes F Bf·MeCN is included in the Supporting Information (see also Figure S50 and Ta ble S5).  All borafluorenes exhibit very small extinction coefficients for their lowest-energy absorption (e = 300-400 m À1 cm À1 ; log e = 2.48-2.60) whichc an be classified as weakly allowed transitions, [124] similar to those in previously reported boroles and borafluorenes. The lowest-energy absorption of F Xyl F Bf (l abs, max = 386 nm) appears to be slightly hypsochromically shifted compared to F Mes F Bf (l abs, max = 400 nm) and p-NMe 2 -F Xyl F Bf (l abs, max = 396 nm) but, due to the broad absorption bands, this is difficult to determine accurately.A ll three borafluorenes exhibit broad, structureless emission bands. The emission maximum of p-NMe 2 -F Xyl F Bf is strongly bathochromically shifted (l em, max = 627 nm) compared to the two nondonor-substituted borafluorene derivatives ( F Mes F Bf: l em, max = 521 nm; F Xyl F Bf: l em, max = 510 nm), which indicates that the emission arises from an intramolecular charge-transfer (ICT) transition. The quantumy ields of F Mes F Bf (F fl = 0.37;h exane) and F Xyl F Bf (F fl = 0.30;h exane) are higher than mosto ft he reported borafluorenes (ca. 0.1). In contrast, p-NMe 2 -F Xyl F Bf exhibits avery low quantumyield (F fl = 0.03;hexane).T oour surprise, F Mes F Bf and F Xyl F Bf exhibit very long fluorescent lifetimes in solution ( F Mes F Bf: t = 224 ns (hexane); t = 151 ns (CH 2 Cl 2 ); F Xyl F Bf: t = 249 ns (hexane)) as well as in the solid state ( F Mes F Bf: t = 173 ns). Similarf luorescence lifetimes (116-150 ns) of borafluorenesw ith bulky exo-aryl moieties were previously observed by Rupar and co-workers. [89] This results in exceptionally long natural lifetimes, t 0 ,u ncommon for organic molecules, for whichf luorescenceu sually takes place on a nanosecondt imescale. There are, however,s ome exceptions such as pyrene. [36,125,126] Thisi ndicates af orbidden process. It is very interesting that even thought he radiativer ate constants are small for organic chromophores, the non-radiativer ate constants are of the same order,r esulting in moderate quantum yields. This is likely ar esult of the high rigidity of the systems. In contrast, p-NMe 2 -F Xyl F Bf exhibits two different radiative decay processes, ap rompt (t = 9.2 ns) and ad elayed (t = 1.6 ms) one. This can be caused by different processes namely TTA( triplet-tripleta nnihilation) [127] or TADF (thermally activated delayedf luorescence). [66,[128][129][130][131][132][133][134][135][136][137][138][139][140][141][142] Due to the low concentrations ( 10 À5 m)ofthe compound employed, the lack of dependence on the concentration and the temperature dependence of the lifetime, we can attribute the observedb ehavior to TADF.T he mechanism forT ADF is based on areverseintersystem crossing process( rISC) between the lowest-energy triplet state (T 1 )a nd excited singlet state (S 1 )o fam olecule. In order for this to occur,t he energy differenceb etween S 1 andT 1 (DE S-T )h as to be sufficiently small. The mostc ommons tructures to exhibit this phenomenon are twisted dipolar systemsw ith spatially separated HOMO and LUMO such as D(donor)-p-A(acceptor) compounds. This structuralm otif is also found in p-NMe 2 -F Xyl F Bf.T he singlet-triplet gap (DE S-T )c an be easily determined experimentally if phosphorescence can be observed. However, for p-NMe 2 -F Xyl F Bf,e ven at 77 Ki nafrozen glass matrix of 2-MeTHF, no phosphorescence was observed. However, DE S-T can also be calculated once the rate constanto ft he rISC process (k rISC )i so btained,w hich is given by the Arrhenius Equation (1), or Equation (2) as derived by Dias et al. [143] with F DF , F PF and F rISC being the quantum yields of the delayed and prompt fluorescencea nd the reversei ntersystem crossing, respectively.G iven that no phosphorescence was observeda t7 7K,b ut delayedf luorescencew as, it can be assumedthat DE S-T is small and, as such, the rate of reverse intersystem crossing is very high, i.e., The ratio F DF /F PF can be ascertainedf rom time-resolved measurementsv ia Equation (4), The non-radiative rate constantsw erec alculated from k nr = (1ÀF fl ) = t.
[c]The radiative rate constants were calculated from k r = F fl = t.
with B DF and B PF being the pre-exponential fitting parameters of the time-resolved fluorescencel ifetimem easurements. As such, equation (1) can be written as Equation (5) where all parameters can be obtained from the time-resolved fluorescence decay.
Lifetimes were obtaineda tt emperatures between 300 Ka nd 230 Kinmethylcyclohexane. From the slope of aplot of ln(k rISC ) versus 1/T (Figure 9), we obtain DE S-T = 15 meV,w hich is comparable to values previously reportedf or TADF emitters. [143] DFT and time-dependent (TD)-DFT studies Using the crystal structures as the starting geometries, the ground-state (GS) structures were optimized using DFT calculations at the B3LYP/6-31G+ +(d) level of theory.F or F Mes F Bf and F Xyl F Bf,t he optimized ground-state structurese xhibit C s and C 2v symmetries, respectively.F or p-NMe 2 -F Xyl F Bf,o ptimization of the GS structure with C 2v symmetry did not lead to ag lobal minimum but rather to as addle point (1 imaginary frequency remained). However,g iven that the C 1 structure is very close to the C 2v symmetry one, in both geometry and energy,a nd exhibits almost the same transition dipole moments, the symmetry descriptors will be used as it simplifies the discussion. The optimized structures reproduce the geometries, bond lengths, angles, and shortest BÀFc ontacts of the crystal structures reasonably well. However,a sc ompared to the crystal structures, the optimized structures do not exhibit bending of the exoaryl out of the plane of the borafluorene backbone. Given that this torsion arises from solid-state interactions, this is to be expected. The highest occupied molecular orbital( HOMO)a nd lowest unoccupied molecular orbital( LUMO) energies increase from F Mes F Bf to F Xyl F Bf to p-NMe 2 -F Xyl F Bf (Figure 10 and Ta ble 5). The calculated LUMO energies fit well with the LUMO energiese stimated from the reduction potentials obtained throughc yclic voltammetry.D ue to the very broad nature of the lowest-energy absorption band, the HOMO energies of F Mes F Bf and F Xyl F Bf were not calculated from the experimental data.
The LUMOs are all localized on the borafluorene moieties with their largestc omponentso nb oron, and the energies differ by only 0.2-0.4 eV.F or F Mes F Bf and F Xyl F Bf the HOMOs are also localized on the borafluorene moieties with boron lying on an odal plane,a nd are energeticallys imilar (DE = 0.16 eV). The HOMO of p-NMe 2 -F Xyl F Bf is localized on the exoaryl moiety and lies about 1.4 eV higheri ne nergy than the HOMOso f F Mes F Bf and F Xyl F Bf.T hisi sd ue to the electron-donating effect of the para-dimethylamino-group that increases the energy of the exo-aryl fragment MO thereby raising it above the borafluorene-centered orbital which is now HOMOÀ1. For both F Mes F Bf and F Xyl F Bf,H OMOÀ1i sl ocalized on the exo-aryl moiety;h owever,d ue to the para-CF 3 group, the HOMOÀ1o f F Mes F Bf is about 0.5 eV lower in energy. Based on the optimized ground-statestructures the nucleus-independentc hemical shift (NICS) values of the borafluorenes were calculated (Table 6).
It is apparent that the perfluoroalkylated borafluorenes exhibit lower NICS(1) zz values as compared to that of the borafluorene TipBf which does not contain CF 3 groups. This suggests ah igher degree of delocalization of the electron density over the borafluorene backbone in our compounds.T hus, the antiaromaticc haracter is less than in non-trifluoromethylated borafluorenes. The optimized structuresw ere then used for TD-DFT calculations to simulatet he absorption spectra.T ime-dependentD FT calculations on F Mes F Bf and F Xyl F Bf were carried out at the B3LYP/6-31+ +G(d) level of theory whereas for the donor substituted p-NMe 2 -F Xyl F Bf the Coulomb-attenuated functional CAM-B3LYP was employed using the same basis set (Table 7), because CAM-B3LYP is better suited to systems involvingc harget ransfer. [146,147] Furthermore, the optimized S 1state geometries of F Mes F Bf and F Xyl F Bf were obtained. In order to characterize the nature of the transition the overlap coefficients( L)w ere determined. [146] The calculated lowest-  [a] Determined from the half-wave potentials:H OMO = À(5.16+ +E1 =2 ,ox )eV, LUMO = À(5.16+ +E1 =2 ,red )eV. [122,144,145] Chem Mes F Bf and F Xyl F Bf exhibit weakly allowed lowest energy absorptions. However,t he calculated oscillator strengths of the S 1 ! S 0 transitions of F Mes F Bf and F Xyl F Bf are also very small. From the symmetries of the frontierm olecular orbitals it is possible to determine whether these transitions are forbidden by symmetry.T he symmetries of the HOMOs of F Mes F Bf (C s ) and F Xyl F Bf (C 2v )a re A'' and A 2 ,r espectively,a nd the LUMO symmetries are A'' and B 1 .T ransitionsa re allowed by symmetry if the initial and final states multipliedb yt he x-, y-, and z-characters of the electronicd ipole operator contain the totally symmetric irreducible representation (C s :A ' and C 2v :A 1 ). For F Mes F Bf (C s ), the lowest-energy transition is forbidden in the zdirection, whereas x and y are allowed, making the transition an allowed transition. For F Xyl F Bf (C 2v ), the x-a nd z-directions are forbidden,w hereas the y-direction is allowed, making the transition an allowed transition. However,f or both molecules, the dipole moment is oriented along the z-axis, resulting in a very small transition dipole moment in the x-a nd y-directions,   ! S 0 and S 6 ! S 0 transitions,r espectively.In both cases, these transitionsh ave predominantly HOMO to LUMO+ +1c haracter.B oth HOMO and LUMO+ +1a re delocalized over the borafluorene backbonew ithoutc ontributionsf rom the boron center. The optimized S 1 geometries of F Mes F Bf and F Xyl F Bf differ only slightly from their ground-state structures ( Figure 11).
In comparison, in the S 1 structure of F Mes F Bf,o nly the para-CF 3 groups on the borafluoreneb ackbone are rotated and in both F Mes F Bf and F Xyl F Bf the ortho-CF 3 groups on the exo-aryl are slightly benta way from the boron center.T he calculated emission maximao f F Mes F Bf and F Xyl F Bf,f it the experimental data in hexane and also exhibit very low oscillator strengths. Even though the optimizeds tructures do not exhibit ah igher symmetry it can be assumed that as imilar phenomenona sf or the absorption takesp lace and is the reason for the observed long lifetimes. Interestingly,a sp reviously discussed, the reasonably high quantum yields observed are due to extremely slow non-radiativedecay processes.
The photophysical properties of p-NMe 2 -F Xyl F Bf differ strongly from those of F Mes F Bf and F Xyl F Bf.T his is, in part, due to the fact that the nature of the lowest-energy absorption has CT rather than LE character( L = 0.15). This is not surprising, given that p-NMe 2 -F Xyl F Bf is ad onor-acceptor system.T he S 1 ! S 0 transition of p-NMe 2 -F Xyl F Bf exhibits an oscillator strength of 0. Using the optimized structureo fC 2v symmetry as an approximation, it becomes apparent that this transition is symmetry forbidden and, furthermore, the overlap between HOMO and LUMO is minuscule due to the nearly perpendicular arrangement of the exo-aryl group with respectt ot he borafluorene core. The HOMO is of B 2 symmetry and the LUMO has B 1 symmetry.T his is the same as for the S 2 ! S 0 transitions of F Mes F Bf and F Xyl F Bf,b ecause the HOMO and HOMOÀ1a re inverted compared to those of p-NMe 2 -F Xyl F Bf.F or p-NMe 2 -F Xyl F Bf,h owever,t he S 2 ! S 0 transition is allowed, but analogously to the F Mes F Bf and F Xyl F Bf cases, exhibits av ery low oscillator strength.T his explains the low extinction coefficient observed for the lowest-energy absorption of p-NMe 2 -F Xyl F Bf. Furthermore, we optimized the S 1 structureo fp-NMe 2 -F Xyl F Bf as well as its T 1 structure in order to calculate the S 1 -T 1 energy gap. Both optimizations were carried out using the PCM solvent-correction model due to the charge-transfer nature of the transitions and the high dipole moment of both S 1 and T 1 . Comparing the energies of both structuresr esults in DE S-T = 423 meV which is almost3 0t imes highert han the experimentally determined gap. It is noteworthy that the experimental determination of the gap is highly flawed due to approximations as wellasu npredictablesolvent effects at lower temperature. However, this should still give ag ood estimate, but the calculations fail to match the experimentalv alue at this level of theory,i llustrating the difficulty of predicting phenomena such as TADF accurately.

Conclusions
Herein,w er eported the synthesis and properties of three trifluoromethylatedb orafluorenes F Mes F Bf, F Xyl F Bf,a nd p-NMe 2 -F Xyl F Bf.T he copper-catalyzed homocoupling of boronate esters provides aconvenient route to 2,2'-dibromobiphenyl derivatives, which can be lithiated and then reacted with stable and accessible aryl-BF 3 Ks alts for the synthesis of borafluorenes. All of the borafluorenes exhibit ar igid geometry with the exo-aryl group lying perpendiculart ot he borafluorene plane. All three borafluorenes exhibit exceptionallyp ositively shifted reduction potentials, emphasizingt he electron-withdrawing nature of the CF 3 groups. This allowed us to use am ild reducing agent (CoCp 2 E 0 = À1.3 eV vs. Fc/Fc + + )t or educe the most anodically shifted borafluorene F Mes F Bf.T he resulting radical anion of F Mes F Bf exhibits as trong delocalization of the additional electron over the borafluorene backbone as evidenced by EPR spectroscopy and its solid-state structure. The trifluoromethylated borafluorenes exhibit unusually long excited-state lifetimes and weakly allowed lowest-energy transitions. For F Mes F Bf and F Xyl F Bf,t his is the result of the transitions being forbidden in the z-direction which coincides with the dipole momenta nd the transition dipole momentb eing negligible in the x-a nd y-directions. The same is apparently true for their emissions, as both compounds exhibit fluorescencel ifetimes of t > 200 ns in hexane. Even with smallo scillator strengths, the two compounds exhibit fluorescenceq uantum yields of 0.37 and 0.30, respectively,b ecause their rigidity results in exceptionally slow non-radiatived ecay.I nc ontrast, the twisted donor-acceptor system p-NMe 2 -F Xyl F Bf has as ymmetry forbidden lowest-energy transition and exhibits TADF,w ithasinglettriplete nergy gap DE S-T experimentally determined to be only 15 meV.T he compounds F Mes F Bf and F Xyl F Bf are highly stable towardsh ydrolysis, whichm akes them interesting potential buildingb locks for organic materials.