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

Article abstract of DOI:10.1002/chem.201905559

Three different perfluoroalkylated borafluorenes (^FBf) 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 (^FXyl^FBf, ^FXyl: 2,6-bis(trifluoromethyl)phenyl), a trifluoromethyl group (^FMes^FBf, ^FMes: 2,4,6-tris(trifluoromethyl)phenyl) or a dimethylamino group (p-NMe₂-^FXyl^FBf, 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 ^FMes^FBf 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-^FXyl^FBf exhibits thermally activated delayed fluorescence (TADF) from a charge-transfer (CT) state, whereas the ^FMes^FBf and ^FXyl^FBf borafluorenes exhibit only weakly allowed locally excited (LE) transitions due to their symmetry and low transition-dipole moments.

Full text of DOI:10.1002/chem.201905559

A Journal of
Accepted Article
Title: Highly stable, readily reducible, fluorescent, trifluoromethylated 9-
borafluorenes
Authors: Florian Rauch, Sonja Fuchs, Alexandra Friedrich, Daniel Sieh,
Ivo Krummenacher, Holger Braunschweig, Maik Finze, and
Todd B. Marder
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Highly stable, readily reducible, fluorescent, trifluoromethyl-
ated 9-borafluorenes
Florian Rauch1, Sonja Fuchs1, Alexandra Friedrich, Daniel Sieh, Ivo Krummenacher, Holger
Braunschweig, Maik Finze and Todd B. Marder*
Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron (ICB), Julius-
Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg (Germany)
*todd.marder@uni-wuerzburg.de
1Both authors have contributed equally to this work.
Abstract: Three different perfluoroalkylated borafluorenes 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 (FXylFBf), a trifluoromethyl group (FMesFBf) or a
dimethylamino group (p-NMe2-FXylFBf), respectively. All derivatives exhibit extraordinarily low
reduction potentials, comparable to those of perylenediimides. The most electron deficient
derivative FMesFBf was also chemically reduced and its radical anion isolated and
characterized. Furthermore, all compounds exhibit very long fluorescent lifetimes of ca. 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, while the FMesFBf and FXylFBf borafluorenes exhibit
only weakly allowed locally excited (LE) transitions due to their symmetry and low transition
dipole moments.
Introduction
Boron-containing organic π-systems, especially triarylboranes[1-13] and, more recently, boron-
containing polyaromatics are of much current interest.[3, 10, 14-20] Three coordinate boron is
isoelectronic with a carbonium ion, having an unoccupied p-orbital, making it inherently
electron deficient and Lewis acidic. Thus, three-coordinate boranes can be employed as π-
acceptors, single electron or electron pair acceptors. Such boranes have been used in linear[21-
40] and non-linear[41-53] optical materials, anion sensors,[8, 54-56] frustrated Lewis pairs (FLPs),[57-
63] as well as in organic light emitting diodes (OLEDs).[64-66] There are numerous examples, both
20]
aromatic and antiaromatic, of boron-containing conjugated cyclic π-systems.[16,
The
subclass of boroles is of special interest.[3, 18, 67-82] They are isoelectronic with the antiaromatic
cyclopentadiene cation, having 4π electrons; however, the introduction of the boron vertex
lowers the symmetry. This leads to a singlet ground state with a small HOMO-LUMO gap,
resulting in the intense color of boroles, even though the molar extinction coefficient (ε) is
quite small for their S1←S0 transitions.
1
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Figure 1: Frontier orbitals of the cyclopentadienyl cation (left) and borole (right).
Because of that, they are highly reactive towards nucleophiles and thereby unsuitable for
many applications. Steric shielding, which works well for triarylboranes,[33, 83, 84] only stabilizes
boroles to a certain degree.[79, 85] Via benzannulation, the stability of boroles can be greatly
increased,[86-89] but the anti-aromatic character is significantly decreased due to delocalization
of π electron density over the biphenylene backbone. This leads to a stabilization of the HOMO
as well as a destabilization of the LUMO resulting in a larger HOMO – LUMO gap and loss of
the characteristic strong color of boroles. This also results in a lower Lewis acidity and,
subsequently, significantly more stable systems.[16, 77] Detailed studies by Martin and co-
workers demonstrate, however, that sterically less hindered derivatives, in particular, retain
characteristic borole reactivity.[90-94] Compared with their triarylborane derivatives,
borafluorenes are usually more Lewis acidic and exhibit more positive reduction potentials.[95]
The stability of these systems can be further improved by sterically shielding[87, 88] or
electronically saturating the boron center,[86] through direct n to p conjugation or indirect F–
B interaction (Figure 2).
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Stability
Electronic properties
Figure 2: Selected examples of borafluorenes. A variety of exo-aryl substituents and the nature of their respective
influence on the stability of the borafluorene (left).[87, 88] It is important to note, that ortho-trifluoromethyl
substituted aryls provide both steric as well as electronic stabilization. Examples of the functionalization of the
borafluorene backbone to tune the electronic properties (right).[86, 95-97]
Yamaguchi and co-workers reported air and moisture stable borafluorene derivatives
employing either Tip or the even bulkier Mes* substituents.[86, 87] It was found that the Tip
derivatives could be used as turn on type fluoride sensors, whereas the Mes* compounds
showed no reaction with fluoride. Recently, Rupar and co-workers have studied these effects
in detail.[88] They found that while Tip-substituted derivatives still decompose slowly (<10%
decomp. over 24 h) in wet solvents, the corresponding FMes derivatives exhibit higher stability
(5% decomp. over 24 h). Derivatives containing π-bonding moieties were found to be much
more sensitive towards moisture (iPrN2: 50% decomp. over 1 h; tBuO: 10% decomp. over 1 h).
Ortho trifluoromethyl substituted aryls exhibit a strong stabilizing effect on boranes.[33, 79, 98-
102] In addition to the steric effect, a direct interaction of the lone pairs of the fluorine atoms
with the empty p orbital of the boron center is observed. This is supported by B–F distances
which are much shorter than the sum of their van der Waals radii (3.39 Å)[103] in crystal
structures. The electronic properties of borafluorenes can be easily tuned to fit different
applications by the introduction of different substitution patterns on the biphenyl backbone
(Figure 2, right). The introduction of methoxy groups at the 6 and 12 positions leads to a small
hypsochromic shift of both the absorption and emission wavelength,[86] whereas elongation
of the π-system with electron-rich conjugated systems attached at the 5 and 13 positions leads
to a bathochromic shift of both the absorption and emission.[87] The photophysical properties
of borafluorenes can also be modified by coordination of Lewis bases. Both Yamaguchi and
co-workers and Rivard and co-workers observed turn-on fluorescence upon adduct formation.
[86, 104] Wilson and Gillard and co-workers observed turn-off fluorescence of a borafluorenium
cation upon coordination of
a Lewis base at low temperature, resulting in
thermochromism.[105] Piers and co-workers investigated the properties of a highly electron
deficient perfluorinated borafluorene iii.[95,
While they only observed a reduction
96]
corresponding to the perfluoroaryls in the cyclic voltammogram, a reaction with the relatively
+
mild reducing agent CoCp2 (CoCp2/CoCp2 : –1.3 vs Fc/Fc+) was observed, underlining the
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electron deficient nature of the compound. In competition experiments with the strong Lewis
acid B(C6F5)3, preference towards the borafluorene derivative was observed, especially with
sterically-demanding Lewis bases. The (sp2-C)–F bonds in this compound, however, are still
reactive towards nucleophiles. In contrast, perfluoroalkyl groups are inert towards
nucleophiles, provide a strong inductive electron withdrawing effect and have been previously
employed 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 a significant stability
enhancement while retaining the low-lying LUMO typical of non-annulated boroles.
Results and discussion
Synthesis
In order to maximize the stability of the trifluoromethylated borafluorenes (FBf) we chose
three different meta-fluoroxylene (1,3 bis(trifluoromethyl)benzene) derivatives as the exo-
aryl moieties. To examine the influence of the exo-aryl and, specifically, substituents at the
para
position,
we
chose
2,6-bis(trifluoromethyl)phenyl
(FXyl),
2,4,6-
tris(trifluoromethyl)phenyl (FMes) and 4-(dimethylamino)-2,6-bis(trifluoromethyl)phenyl (p-
N(Me)2-FXyl) groups.
Scheme 1: Synthesis of FMesFBf, FXylFBf and p-NMe2-FXylFBf.
Biphenyl derivative 2 was synthesized via regioselective C–H borylation of 1[106, 107] ortho to
the bromine and a subsequent copper-catalyzed oxidative homocoupling. For the last step, 2
was dilithiated and subsequently reacted with the appropriate Ar-BF3K salt. Attempts to
synthesize the haloborafluorene with different BX3 (X = F, Cl, Br) sources failed. Attempts to
use aryl-boronates in place of the Ar-BF3K salt 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
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
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synthesis we observed that adding LiBr greatly improves the reactivity of the Ar-BF3K salts. It
is possible that a cation exchange reaction generates the more reactive Ar-BF3Li salt. The
increased reactivity of the Ar-BF3Li 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 isolated yields.
It is important to note that the corresponding ortho-trifluoromethylarylboron halides (X = Cl,
Br) are not stable due to halide exchange.[114] This might also explain why the synthesis of the
F
F
haloborafluorenes was not possible. The compounds MesFBf, XylFBf and p-NMe2-FXylFBf
were obtained after purification via sublimation and recrystallization. Both MesFBf and
F
FXylFBf are bright green solids. In contrast, p-NMe2-FXylFBf is a red solid. All compounds exhibit
1H NMR and 13C{1H} NMR signals consistent with their proposed structures. The 11B{1H} NMR
shifts for all three borafluorene derivatives are around 64 ppm and differ only slightly. The
19F{1H} NMR spectra display singlets and septets, the latter with a JFF coupling constant of 3 –
4 Hz. (Table 1)
Table 1: 11B{1H} and 19F{1H} NMR shifts of FMesFBf, FXylFBf, and p-NMe2-FXylFBf recorded in C6D6.
FXylFBf
63.2
FMesFBf
64.1
p-NMe2-FXylFBf
64.7
11B{1H} NMR (ppm)
19F{1H} NMR (ppm) singlet
‒63.4
‒62.0; ‒63.5
‒63.4
‒58.2 (JFF = 4 Hz)
‒59.6 (JFF = 4 Hz)
‒58.4 (JFF = 4 Hz)
‒59.6 (JFF = 4 Hz)
‒58.1 (JFF = 4 Hz)
‒59.5 (JFF = 4 Hz)
19F{1H} NMR (ppm) septets
The singlets at ca. δ = –63.5 ppm correspond to the two freely rotating para CF3-groups on the
F
borafluorene core. For MesFBf, another singlet corresponding to the para CF3-group on the
exo-aryl is observed. The CF3-groups ortho to the boron center display a complex coupling
pattern of two septets with small coupling constants (JFF = 4 Hz). This can be attributed to
through-space F–F coupling as previously observed at low temperature (243 K) for (FMes)2BAr
compounds.[100] The fact that the borafluorenes exhibit this phenomenon at room
temperature is an indicator of the high rigidity of the systems.
All three compounds are stable in the solid-state and can be stored under ambient conditions
without decomposition. In wet CDCl3 (1.5 eq. H2O/borafluorene) at room temperature, no
decomposition of either FMesFBf or FXylFBf was observed over 4 d by NMR spectroscopy. This
is surprising as for FMesBf and TipBf, both less electron deficient compounds, decomposition
rates of 5 and 10% respectively in wet solvents over 24 h were reported.[88] This indicates that
the CF3-groups ortho to the boron center on the borafluorene core have a stabilizing effect,
likely due to steric shielding. However, p-NMe2-FXylFBf shows very rapid decomposition when
exposed to wet solvents. It is likely that the dimethylamine moiety is protonated first, thereby
further increasing the electrophilicity of the boron center and decreasing its stability towards
nucleophilic attack. The reaction with H2O leads to cleavage of one B–C bond of the
borafluorene core, resulting in a BOH and CH moiety. The same reactivity towards water and
other E–H bonds (E = N, O, S) was previously observed by Martin and co-workers.[94] Likely due
to less steric hinderance in their system, a second borafluorene reacts with the decomposition
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product to form a B–O–B motif. The product of the hydrolysis of p-NMe2-FXylFBf was isolated
and studied by X-ray diffraction (compound D in Figure S41). All three compounds are slightly
soluble in non-polar solvents such as hexane or toluene and soluble in polar non-coordinating
solvents such as CH2Cl2 and THF. Dissolving FMesFBf in acetonitrile gives a colorless solution.
Investigation of the solution via 19F NMR spectroscopy revealed the formation of an
acetonitrile adduct, which is consistent with previous studies by Martin and co-workers for
less sterically hindered borafluorenes.[91] The para CF3-groups, both on the borafluorene
backbone as well as the exo-aryl moiety, are influenced only weakly by the coordination of
acetonitrile, as both singlets in the 19F NMR spectrum shift only slightly to lower field. The
signals corresponding to the ortho CF3-groups, however, change dramatically. Instead of two
septets as observed for FMesFBf, one septet at –51.9 ppm (JFF = 10 Hz) one broad singlet at –
56.2 ppm and a quartet at –60.7 ppm (JFF = 10 Hz), are observed (Figure 3, middle).
Figure 3: 19F NMR spectra (188 MHz, 298 K) of FMesFBf in C6D6 (top), in CH3CN (middle) and in C6D6 after removal
of all volatiles (bottom).
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 C6D6, only the borafluorene was observed via 19F NMR spectroscopy. Thus,
the formation of the adduct with acetonitrile is completely reversible.
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Crystal and molecular structures
F
Single crystals of the three borafluorenes as well as the acetonitrile adduct of MesFBf
(FMesFBf•MeCN) suitable for X-ray studies were obtained (Figure 4) and selected bond
lengths, angles, torsion angles and short B–F contacts are listed in Table 2. The single crystals
F
of MesFBf and p-NMe2-FXylFBf were obtained from a saturated hexane solution at –30 °C,
F
that of XylFBf was obtained by evaporation of a saturated DCM solution and that of
FMesFBf•MeCN was obtained from a saturated acetonitrile solution at –30 °C.
F
F
Figure 4: Molecular structures of MesFBf (top left), XylFBf (top right), p-NMe2-FXylFBf (bottom left) and
FMesFBf•MeCN (bottom right) determined by single-crystal X-ray diffraction at 100 K. All ellipsoids are drawn at
the 50% probability level, and H atoms and solvent molecules are omitted for clarity.
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Table 2: Selected bond lengths [Å] and angles [°] in FMesFBf, FXylFBf, p-NMe2-FXylFBf and FMesFBf•MeCN. Atom
labels for the respective molecular structures are shown in Figure 4.
FMesFBf
FXylFBf
p-NMe2-FXylFBf
FMesFBf•MeCN
B−C1
B−C2
B−C3
B–N
1.579(3)
1.581(3)
1.591(3)
1.570(2)
1.591(3)
1.591(3)
1.570(3)
1.595(3)
1.591(3)
1.652(3)
1.645(3)
1.645(3)
1.591(3)
C3−C19/C18/C20
C2−C18/C17/C19
C18/C17/C19−C19/C18/C20
1.408(3)
1.410(2)
1.481(3)
1.410(2)
1.409(2)
1.474(2)
1.409(2)
1.408(2)
1.482(2)
1.401(3)
1.410(3)
1.475(3)
89.32(7)
89.43(6)
89.84(8)
89.54(6)
89.27(6)
88.69(5)
∠ BC3-Arylexo
82.12(6)
137.6(2)
∠ BC12- Arylexo
Torsion of Arexo out of
borafluorene plane
171.28(16)
176.45(16)
178.83(17)
∠ C1–B–C2–C18/C17/C19
5-ring:
103.95(15)
359.74(16)
103.07(13)
359.90(15)
102.94(14)
359.99(16)
99.74(18)
∠ C2BC3
338.43(18)
Sum ∠ CBC
Sum ∠ CNC
359.84(19)
2.392(3) 2.440(3)
2.379(2) 2.390(3)
2.366(2) 2.434(2)
2.853(3)
Shortest B−F contact(s)
A comparison of the crystal structures of the three target compounds shows the following.
F
While all three B–C bond distances are in a similar range for MesFBf (1.579(3)–1.591(3) Å),
the B–C1exo distances to the FXyl groups of FXylFBf and p-NMe2-FXylFBf (B–C = 1.570(3) Å) are
slightly shorter than the respective B–C2 and B–C3 bonds (1.591(3)–1.595(3) Å) within the
borole moieties (Table 2). The boron atoms have a nearly ideal trigonal planar configuration
with the sum of the C−B−C angles being 359.74(16) - 359.99(16)°. In all three compounds, the
C2‒B‒C3 angle (102.94(14)–103.95(15)°) within the borole moiety is much smaller than the
other two C–B–C angles. The borole moiety shows similar bond lengths and angles in all three
compounds. The angles increase from C–B–C to B–C–C (106.57(16)–107.52(15)°) and to C–C–
C (110.98(17)–111.41(15)°). The C2–C and C3–C bond lengths (1.408(3)–1.410(2) Å) are typical
for aromatic bonds, while the C–C bond (1.474(2)–1.482(2) Å) that is opposite to the boron
atom has significant single-bond character. The interplanar angle between the borafluorene
(BC12) and the exo-aryl substituent is close to 90° in all three compounds (88.69(5)–89.54(6)°).
This is due to the large steric demand of the CF3 groups in the ortho positions of both on the
exo-aryl moiety as well as the borafluorene core. In all three compounds, two B‒F distances,
each in the range of 2.366(2)–2.440(3) Å, are observed, which are significantly shorter than
the sum of the van der Waals radii for boron and fluorine (3.39 Å).[103] This was previously
observed in boranes and boroles with ortho-CF3 aryl moieties.[33, 79, 102, 114, 115] As 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 interact with the empty p-orbital of the boron
center. The torsion angle Cendo‒C2endo‒B‒C1exo with the endo carbon atoms belonging to the
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borole moiety deviates slightly from 180° (171.28(16)–178.83(17)°). This shows that the B–C1
bond to the exocyclic moiety is tilted slightly out of the borafluorene plane. The out-of-plane
F
F
tilt increases from p-NMe2-FXylFBf (1.17(17)°) to XylFBf (3.55(16)°) and MesFBf (8.72(16)°).
The magnitude of the tilt is related to the molecular packing, which is similar in all three crystal
structures as the borafluorene moieties are arranged in pairs, which are related by inversion
symmetry and form weak intermolecular π···π interactions between the borafluorene
F
backbones. The strongest π···π interaction is observed in MesFBf, which shows the smallest
centroid-centroid distance, interplanar separation, and offset shift (Table S3). Hence, these
F
F
pairs of molecules are closest in MesFBf, and the exocyclic Mes moiety is tilted out of the
plane the most, away from the center of the pair in order to avoid close F···F contacts between
F
the two molecules. In the crystal structure of the acetonitrile adduct of MesFBf, as the
hybridization at the boron is now sp3 rather than sp2, all of the B–C bonds are elongated. The
F
C2–B1–C3 angle of the borole ring (99.74(18)°) is decreased compared with MesFBf
(103.95(15)°). The torsion angle of the exo-aryl towards the borafluorene backbone is
decreased (82.12(6)°) and the bending of the exo-aryl out of the plane of the borafluorene
moiety (137.6(2)°) deviates significantly from 180°. In the adduct, there is only one short B–F
contact (F3‒B1 = 2.853(3) Å) that is elongated compared to FMesFBf, but still shorter than the
sum of the van der Waals radii. The B–N bond length is 1.591(3) Å, which is significantly shorter
than that in the acetonitrile adduct of B(C6F6)3 (1.616(3) Å),[116] but similar to that in the
previously reported MeCN adduct of PhBf (PhBf•MeCN) (1.598(4) Å).[91] The N≡C bond
(1.129(3) Å) is also shorter than that in the acetonitrile adduct of B(C6F6)3 (1.141(2) Å) and very
similar to that in PhBf•MeCN (1.128(4) Å).
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Electrochemistry
Cyclic voltammograms of the three borafluorenes were recorded in dichloromethane with
[nBu4N][PF6] as the electrolyte and a scan rate of 250 mVs-1 (Figure 5) in order to determine
their reduction potentials.
F
F
Figure 5: Cyclic voltammograms of the reversible redox events of MesFBf (top left), XylFBf (top right) and
p-NMe2-FXylFBf (bottom left). All samples are referenced against the Fc/Fc+ ion couple. For better comparison,
the reduction waves are plotted together (bottom right; FMesFBf (black), FXylFBf (blue), p-NMe2-FXylFBf (red)).
All measurements were referenced to the ferrocene/ferrocenium redox couple (Fc/Fc+). The
most electron deficient borafluorene, FMesFBf, exhibits a reversible reduction at ‒1.13 V and
an irreversible reduction at ‒2.04 V. For the slightly less electron deficient FXylFBf, a reversible
reduction at ‒1.21 V and an irreversible reduction at ‒2.12 V are observed. Interestingly,
p-NMe2-FXylFBf shows a reversible reduction at ‒1.28 V, an irreversible reduction at ‒2.15 V
and a partially reversible oxidation at 0.95 V.
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Table 3: Tabulated 1st reduction potential values of various three-coordinate boron species.
1st reduction potential
E1/2 in [V] vs. Fc/Fc+
1st reduction potential
E1/2 in [V] vs. Fc/Fc+
compound
compound
Borafluorenes
Free boroles
This work
a
FMesFBfa
FMesFBfa
p-NMe2-FXylFBfa
Mes*Bfd
TipBfe
FMesBfe
af
–1.13
–1.21
–1.28
–2.28[87]
–2.14[88]
–1.82[88]
–1.98[119]
PhBC4Ph4
–1.61[117]
–1.69[85, 117, 118]
–1.52[79]
a
MesBC4Ph4
FMesBC4Ph4
a
Diboraanthracene
Boron PAHs
bg
cg
–1.84[15]
–1.38[121]
da
ea
–1.15[120]
–1.07[120]
Triaryl boranes
b
c
c
c
B(Mes)3
–2.73[122]
B(3,5-(CF3)2C6H3)3
B(2,4-(CF3)2C6H3)3
B(2,5-(CF3)2C6H3)3
–1.61[102]
–1.79[102]
–1.85[102]
B(Mes)2(C6F5) b
–2.10[122]
–1.72[122]
–1.97[77]
b
B(Mes)(C6F5)2
a
B(C6F5)3
a Measured using a platinum electrode in CH2Cl2 using [nBu4N][PF6] as the electrolyte; b Measured using
a platinum electrode in THF with [nBu4N][B(C6F5)4] as the electrolyte; c Measured using a glassy carbon
electrode in CH2Cl2 with [nBu4N][B(C6F5)4] as the electrolyte; d Measured using a glassy carbon electrode
in THF with [nBu4N][ClO 4] as the electrolyte; e Measured using a glassy carbon electrode in CH2Cl2 with
[nBu4N][PF6] as the electrolyte; f Measured using a glassy carbon electrode in THF with [nBu4N][PF6] as
the electrolyte; g Measured using a platinum electrode in THF with [nBu4N][PF6] as the electrolyte.
The three borafluorenes exhibit much higher reduction potentials (FMesFBf = –1.13 V; FXylFBf
= –1.21 V and p-NMe2-FXylFBf = –1.28 V) than any of the previously reported borafluorenes
and boroles or triarylboranes (Table 3). The substitution patterns of compounds FMesBC4Ph4
F
(E1/2 = ‒1.52 V) and MesBf (E1/2 = –1.82 eV) allows a direct comparison of the fluorinated
borafluorene backbone (FBf) with the unsubstituted borafluorene backbone (Bf) and the non-
annulated borole.[88] The strong anodic shift of the FMesFBf as compared to FMesBC4Ph4 and
FMesBf demonstrates the strong electron withdrawing effect of the CF3-groups on the
borafluorene backbone. This is likely due to the planar geometry as well as the fact that the
ortho CF3-groups of the borafluorene backbone do not display B-F interactions, and thus do
not increase electron density at the boron atom. The reduction potentials of the
trifluoromethylated borafluorenes do not differ strongly from one another. This indicates that
the para substituent on the exocyclic aryl moiety does not have a significant influence on the
electron accepting properties of these borafluorenes. This is best illustrated by the fact that
11
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the π-donating dimethylamino group only leads to a cathodic shift of 0.15 V compared to a
trifluoromethyl group. This is likely due to the nearly perpendicular arrangement of the exo-
aryl group with respect to the borafluorene backbone, that limits π-conjugation leaving only
inductive effects of the exo-aryl moiety on the borafluorene core and the boron center. To
investigate further the electronic properties of borafluorene FMesFBf, CoCp2 was chosen as a
reducing agent (E0’(CoCp2) = –1.3 V vs. Fc/Fc+).[123] Thus, after addition of CoCp2, the yellowish
THF solution of FMesFBf turned dark purple and an ESR measurement confirmed the presence
of the borafluorene radical anion [FMesFBf]·− (Figure 6).
Figure 6: Experimental (black solid line) and simulated (red dashed line) continuous-wave X-band EPR spectra of
[FMesFBf]·− in a THF solution at room temperature.
A THF solution of the radical anion [FMesFBf]·− shows a complex EPR signal centered at giso
=
2.004 consisting of hyperfine splittings to boron (a(11B) = 3.3 G; cf. non-annulated borole
derivatives (a(11B) = 3.4 − 3.7 G)[85, 124]), the CF3 fluorines (a(19F) = 11.3 G and 6.0 G) and the
hydrogen atoms (a(1H) = 6.1 G and 2.9 G) of the borafluorene core. The relatively small boron
hyperfine coupling together with the relatively large proton and fluorine hyperfine couplings
indicate significant spin delocalization onto the benzene rings, to a greater extent than in
other singly reduced borafluorenes.[87]
Crystals of the radical anion suitable for X-ray diffraction were obtained by slow diffusion of
pentane into a saturated THF solution (Figure 7).
12
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Figure 7: The solid-state molecular structure of [FMesFBf][CoCp2] determined by single-crystal X-ray diffraction
at 100 K. All ellipsoids are drawn at the 50% probability level. H atoms and THF solvent molecules are omitted
for clarity. Only half of the symmetrically non-equivalent molecules are shown. One of the CF3 groups on the
borafluorene core is rotationally disordered and only the part with the higher occupancy (64%) is shown here
(left). The relevant changes in bond lengths of [FMesFBf]•– compared to the neutral starting material are shown
at the right.
Upon reduction, a variety of changes in the structure are observed. As there are two
independent molecules in the unit cell for the radical anion, both molecules are taken into
account for comparison (Table S2). The negative charge is apparently localized on the
borafluorene core, as the exo-aryl moiety is only slightly influenced, with almost all changes
in bond length being within 3 esd’s. While the B–C1 bond length is slightly increased (Δ(B–C1)
= +0.008 and +0.015 Å), both borafluorene B–C bonds (B–C2 and B–C3) are shortened (Δ(B–
C2) = –0.034 and –0.025 Å; Δ(B–C3) = –0.043 and –0.040 Å). The neighboring borafluorene C–
C bonds are elongated (Δ(C3–C19) = +0.030 and 0.027 Å; Δ(C2–C18) = +0.019 and 0.022 Å) and
the C–C bond connecting the two borafluorene aryls is shortened (Δ(C18–C19) = –0.024 Å). In
summary, the bond lengths within the five-membered heterocycle equalize. This behavior was
previously observed in the structures of borole radical anions and dianions.[85] The one-
electron reduction also influences the shortest B–F distances which are elongated (mean Δ(B–
F) = –0.278 Å).
Photophysical properties
In order to obtain further insight into the electronic structure of the borafluorenes, absorption
and emission spectra as well as quantum yields and excited state lifetimes were measured in
F
hexane (Figure 8 and Table 4). Furthermore, MesFBf was also studied in CH2Cl2 and in the
solid state. Solvatochromic studies of p-NMe2-FXylFBf could not be carried out, as no emission
was detected in more polar solvents. This is most likely due to a further red shift of the
emission, which in turn results in increased non-radiative decay processes and thus a much
F
lower quantum yield. A photophysical investigation of MesFBf•MeCN is included in the
Supporting Information (see also Figure S50 and Table S5).
13
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F
F
Figure 8: Absorption (black) and emission spectra (red) in hexane of MesFBf (top left), XylFBf (top right) and
p-NMe2-FXylFBf (bottom left). For comparison, the emission spectra are plotted together (bottom right; FMesFBf
(black), FXylFBf (blue), p-NMe2-FXylFBf (red)).
Table 4: Photophysical data for borafluorenes FMesFBf, FXylFBf, and p-NMe2-FXylFBf.
apparent
λabs [nm]
λem
[nm][a]
Stokes
shift
(103 cm-1)
τ [ns]
(rel%)
τ0
[ns]
knr
kr
Compound
FMesFBf
FXylFBf
solvent (ε [103 M-1 cm-1];
Φfl
[107 s-1][b]
[107 s-1][c]
log ε)
290 (2.9; 3.46),
321 (0.8; 2.90),
400 (0.3; 2.48)
hexane
521
5.8
0.37
224
605
0.3
0.2
CH2Cl2
solid
395
405
540
527
6.8
6.0
0.18
0.06
151
173
835
0.1
0.5
0.005
0.04
2557
256 (54.2; 4.73),
290 (2.9; 3.46),
386 (0.4; 2.60)
hexane
510
627
6.3
9.3
0.30
0.03
249
820
0.3
0.1
9.2
268 (32.3; 4.51),
326 (2.7; 3.43),
396 (0.3; 2.48)
(64%),
1626
(36%)
p-NMe2-FXylFBf hexane
-
-
-
[a] Excited at the respective λabs,max of the S1←S0 transition. [b] The non-radiative rate constants were calculated from knr
(1-Φfl)/τ. [c] The radiative rate constants were calculated from kr = Φfl/τ.
=
14
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All borafluorenes exhibit very small extinction coefficients for their lowest energy absorption
(ε = 300 – 400 M-1 cm-1; log ε = 2.48 – 2.60) which can be classified as weakly allowed
transitions,[125] similar to those in previously reported boroles and borafluorenes. The lowest
energy absorption of FXylFBf (λabs, max = 386 nm) appears to be slightly hypsochromically shifted
F
compared to MesFBf (λabs, max = 400 nm) and p-NMe2-FXylFBf (λabs, max = 396 nm) but, due to
the broad absorption bands, this is difficult to determine accurately. All three borafluorenes
exhibit broad, structureless emission bands. The emission maximum of p-NMe2-FXylFBf is
strongly bathochromically shifted (λem, max = 627 nm) compared to the two non-donor-
F
substituted borafluorene derivatives (FMesFBf: λem, max = 521 nm; XylFBf: λem, max = 510 nm),
which indicates that the emission arises from an intramolecular charge transfer (ICT)
transition. The quantum yields of FMesFBf (Φfl = 0.37; hexane) and FXylFBf (Φfl = 0.30; hexane)
are higher than most of the reported borafluorenes (ca. 0.1). In contrast, p-NMe2-FXylFBf
F
F
exhibits a very low quantum yield (Φfl = 0.03; hexane). To our surprise, MesFBf and XylFBf
exhibit very long fluorescent lifetimes in solution (FMesFBf: τ = 224 ns (hexane); τ = 151 ns
(CH2Cl2); FXylFBf: τ = 249 ns (hexane)) as well as in the solid state (FMesFBf: τ = 173 ns). Similar
fluorescence lifetimes (116 – 150 ns) of borafluorenes with bulky exo-aryl moieties were
previously observed by Rupar and co-workers.[89] This results in exceptionally long natural
lifetimes, τ0, uncommon for organic molecules, for which fluorescence usually takes place on
a ns timescale. There are, however, some exceptions such as pyrene.[36, 126, 127] This indicates
a forbidden process. It is very interesting that even though the radiative rate constants are
small for organic chromophores, the non-radiative rate constants are of the same order,
resulting in moderate quantum yields. This is likely a result of the high rigidity of the systems.
In contrast, p-NMe2-FXylFBf exhibits two different radiative decay processes, a prompt (τ = 9.2
ns) and a delayed (τ = 1.6 μs) one. This can be caused by different processes namely TTA
(triplet-triplet annihilation)[128] or TADF (thermally activated delayed fluorescence).[66, 129-143]
Due to the low concentrations (≤ 10-5 M) of the compound employed, the lack of dependence
on the concentration and the temperature dependence of the lifetime, we can attribute the
observed behavior to TADF. The mechanism for TADF is based on a reverse intersystem
crossing process (rISC) between the lowest energy triplet state (T1) and excited singlet state
(S1) of a molecule. In order for this to occur, the energy difference between S1 and T1 (ΔES-T)
has to be sufficiently small. The most common structures to exhibit this phenomenon are
twisted dipolar systems with spatially separated HOMO and LUMO such as D(donor)-π-
A(acceptor) compounds. This structural motif is also found in p-NMe2-FXylFBf. The singlet-
triplet gap (ΔES-T) can be easily determined experimentally if phosphorescence can be
observed. However, for p-NMe2-FXylFBf, even at 77 K in a frozen glass matrix of 2-MeTHF, no
phosphorescence was observed. However, ΔES-T can also be calculated once the rate constant
of the rISC process (krISC) is obtained, which is given by the Arrhenius equation (1), or equation
(2) as derived by Dias et al.[144]
ꢀꢁꢂꢃꢄ = ꢅꢆꢇꢈ �− ∆ꢉꢊ−ꢋ�.
(1)
ꢌꢍ
ꢎ
ꢎ
�
+ꢎ
ꢕꢔ
ꢏꢐꢊꢑ
ꢀꢁꢂꢃꢄ
=
ꢓꢔ�.
(2)
15
ꢒ
ꢎ
ꢕꢔ
ꢓꢔ
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10.1002/chem.201905559
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with ΦDF, ΦPF and ΦrISC being the quantum yields of the delayed and prompt fluorescence and
the reverse intersystem crossing, respectively. As no phosphorescence was observed at 77 K,
but delayed fluorescence was, it can be assumed that ΔES-T is small and, as such, the rate of
ꢌ
ꢏꢐꢊꢑ
ꢋ
reverse intersystem crossing is very high, i.e., ꢖꢁꢂꢃꢄ
=
≈ 1 and, thus, equation
ꢕꢗ
ꢌ
+ꢌ +ꢌ
ꢐꢑ
ꢏꢐꢊꢑ
(2) can be modified to give equation (3).
1
ꢎ
�
+ꢎ
1
1
1
ꢎ
ꢓꢔ
�1 + ꢎꢎꢓꢔ� =
+
∗
.
(3)
ꢕꢔ
ꢀꢁꢂꢃꢄ
=
ꢓꢔ� =
ꢒ
ꢎ
ꢒ
ꢒ
ꢒ
ꢎ
ꢓꢔ
ꢕꢔ
ꢓꢔ
ꢕꢔ
ꢓꢔ
ꢓꢔ
ꢕꢔ
The ratio ΦDF/ΦPF can be ascertained from time-resolved measurements via equation (4),
ꢎ
ꢓꢔ
ꢎ
ꢕꢔ
ꢘ
ꢒ
ꢓꢔ ꢓꢔ
=
.
(4)
ꢘ
ꢒ
ꢕꢔ ꢓꢔ
with BDF and BPF being the pre-exponential fitting parameters of the time-resolved
fluorescence lifetime measurements. As such, equation (1) can be written as equation (5)
where all parameters can be obtained from the time-resolved fluorescence decay.
1
1
ꢙ
ꢒ
ꢀꢁꢂꢃꢄ = ꢅꢆꢇꢈ �− ∆ꢉꢊ−ꢋ� =
+
.
ꢙ ꢒ
ꢓꢔ ꢕꢔ ꢓꢔ
(5)
ꢓꢔ ꢓꢔ
ꢌꢍ
ꢒ
ꢒ
ꢓꢔ
Lifetimes were obtained at temperatures between 300 K and 230 K in methylcyclohexane.
From the slope of a plot of ln(krISC) vs. 1/T (Figure 9), we obtain ΔES-T = 15 meV, which is
comparable to values previously reported for TADF emitters.[144]
Figure 9: Arrhenius plot of ln(krISC) of p-NMe2-FXylFBf (determined from temperature-dependent lifetime
measurements) vs. 1/T.
DFT and TD-DFT studies
Using the crystal structures as the starting geometries, the ground state structures were
optimized via DFT calculations at the B3LYP/6-31G+(d) level of theory. For FMesFBf and FXylFBf,
the optimized ground state structures exhibit Cs and C2v symmetries, respectively. For
p-NMe2-FXylFBf, optimization of the GS structure with C2v symmetry did not lead to a global
minimum but rather to a saddle point (1 imaginary frequency remained). However, as the C1
structure is very close to the C2v symmetry one, in both geometry and energy, and exhibits
16
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10.1002/chem.201905559
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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 B–F contacts of the crystal structures reasonably well. However, as
compared to the crystal structures, the optimized structures do not exhibit bending of the
exo-aryl out of the plane of the borafluorene backbone. As this torsion arises from solid-state
interactions, this is to be expected. The highest occupied molecular orbital (HOMO) and
F
F
lowest unoccupied molecular orbital (LUMO) energies increase from MesFBf to XylFBf to
p-NMe2-FXylFBf (Figure 10 and Table 5). The calculated LUMO energies fit well with the LUMO
energies estimated from the reduction potentials obtained via cyclic voltammetry. Due to the
very broad nature of the lowest energy absorption band, the HOMO energies of FMesFBf and
FXylFBf were not calculated from the experimental data.
Figure 10: Frontier molecular orbitals of FMesFBf (left), FXylFBf (middle), and p-NMe2-FXylFBf (right) calculated at
the B3LYP/6-31+G(d) level.
F
F
Table 5: Frontier molecular p orbital energies [eV] and symmetries of MesFBf, XylFBf, and p-NMe2-FXylFBf
calculated at the B3LYP/6-31+G(d) level.
FMesFBf
FXylFBf
p-NMe2-FXylFBf
C2v
Symmetry
Cs
calc.
C2v
C1
exp.a
calc.
exp.a
calc.
calc.
exp.a
LUMO+1 [eV]
LUMO [eV]
HOMO [eV]
–2.66 (A‘‘)
–3.80 (A‘‘)
–7.65 (A‘‘)
–8.49 (A‘)
–2.50 (B1)
–3.60 (B1)
–7.49 (A2)
–8.03 (B2)
–2.34 (A1)
–3.41 (A1)
–6.17 (A1)
–7.32 (A1)
–2.32 (B1)
–3.42 (B1)
–6.16 (B2)
–7.41 (A2)
–4.02
–3.95
–3.88
–6.11
HOMO-1 [eV]
a
Determined from the half wave potentials: HOMO = –(5.16+E1/2,ox) eV, LUMO = –(5.16+E1/2, red) eV.[123, 145, 146]
17
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The LUMOs are all localized on the borafluorene moieties with their largest components on
boron, and the energies differ by only 0.2-0.4 eV. For FMesFBf and FXylFBf the HOMOs are also
localized on the borafluorene moieties with boron lying on a nodal plane, and are energetically
similar (ΔE = 0.16 eV). The HOMO of p-NMe2-FXylFBf is localized on the exo-aryl moiety and
lies about 1.4 eV higher in energy than the HOMOs of FMesFBf and FXylFBf. This is due 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
HOMO-1. For both FMesFBf and FXylFBf, HOMO-1 is localized on the exo-aryl moiety; however,
due to the para-CF3-group, the HOMO-1 of FMesFBf is about 0.5 eV lower in energy. Based on
the optimized ground state structures the nucleus-independent chemical shift (NICS) values
of the borafluorenes were calculated (Table 6).
Table 6: NICS(1)zz values of the borole moiety of FMesFBf, FXylFBf, p-NMe2-FXylFBf, and TipBf[125] calculated at the
B3LYP/6-311+G(d) level.
Compound
NICS(1)zz
Calc. Lit.
FMesFBf
FXylFBf
20.7
20.2
p-NMe2‒FXylFBf 20.0
TipBf
24.3 24.5[119]
It is apparent that the perfluoroalkylated borafluorenes exhibit lower NICS(1)zz values as
compared to that of the borafluorene TripBf which does not contain CF3-groups. This suggests
a higher degree of delocalization of the electron density over the borafluorene backbone in
our compounds. Thus, the antiaromatic character is less than in non-trifluoromethylated
borafluorenes. The optimized structures were then used for TD-DFT calculations to simulate
the absorption spectra. Time-dependent DFT calculations on FMesFBf and FXylFBf were carried
out at the B3LYP/6-31+G(d) level of theory whereas for the donor substituted p-NMe2-FXylFBf
the Coulomb-attenuated functional CAM-B3LYP was employed using the same basis set (Table
7), as CAM-B3LYP is better suited to systems involving charge transfer.[147, 148] Furthermore,
F
F
the optimized S1-state geometries of MesFBf and XylFBf were obtained. In order to
characterize the nature of the transition the overlap coefficients (Λ) were determined.[147] The
calculated lowest energy absorptions of the borafluorenes fit well with the experimental
values. For all compounds, the lowest energy transitions are predominantly of HOMO to
LUMO character.
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F
F
Table 7: Lowest energy and highest oscillator strength absorptions and emissions of MesFBf and XylFBf
calculated at the B3LYP/6-31+G(d) level of theory and lowest energy absorptions of p-NMe2-FXylFBf calculated at
the CAM-B3LYP/6-31+G(d) level of theory.
Compound
State E [eV]
λ
λexp
f
Symmetry Major contributions
Λ
[nm] [nm]
S1←S0 3.05
S2←S0 3.90
S7←S0 4.80
406
318
258
400 0.0005
0
257 0.8333
A'
A"
A'
HOMO→LUMO (99%)
HOMO-1→LUMO (99%)
HOMO-2->LUMO (24%),
HOMO->LUMO+1 (70%)
H-SOMO←L-SOMO (99%)
HOMO→LUMO (99%)
HOMO-1→LUMO (99%)
HOMO-3->LUMO (28%),
HOMO->LUMO+1 (68%)
H-SOMO←L-SOMO (99%)
HOMO->LUMO (92%)
HOMO-1->LUMO (97%)
HOMO->L+2 (70%),
0.65
0.25
0.73
FMesFBf
(Cs)
S1→S0 2.22
S1←S0 3.09
S2←S0 3.64
S6←S0 4.81
559
401
340
258
521 0.0038
385 0.0006
0
A
B2
A2
B2
0.66
0.23
0.74
FXylFBf
(C2v)
256 0.8413
S1→S0 2.34
S1←S0 3.25
S2←S0 3.46
S3←S0 4.25
531
382
358
292
510 0.0030
A
A
A
A
396
0
0.15
0.67
0.49
326 0.0006
270 0.0491
p-NMe2-FXylFBf
(C1)
HOMO->L+3 (24%)
S8←S0 5.11
243
256 0.6022
A
HOMO->L+4 (86%)
0.71
For both FMesFBf and FXylFBf these can be classified as locally excited (LE) transitions (Λ ≈ 0.65)
as both HOMO and LUMO are localized on the borafluorene backbone. This is surprising, as LE
transitions usually exhibit high extinction coefficients and FMesFBf and FXylFBf exhibit weakly
allowed lowest energy absorptions. However, the calculated oscillator strengths of the S1←S0
F
F
transitions of MesFBf and XylFBf are also very small. From the symmetries of the frontier
molecular orbitals it is possible to determine whether these transitions are forbidden by
F
F
symmetry. The symmetries of the HOMOs of MesFBf (Cs) and XylFBf (C2v) are A’’ and A2,
respectively, and the LUMO symmetries are A’’ and B1. Transitions are allowed by symmetry
if the initial and final states multiplied by the x-, y-, and z-characters of the electronic dipole
operator contain the totally symmetric irreducible representation (Cs: A’ and C2v: A1). For
FMesFBf (Cs), the lowest energy transition is forbidden in the z-direction, while x and y are
allowed, making the transition an allowed transition. For FXylFBf (C2v), the x- and z-directions
are forbidden, while the y-direction is allowed, making the transition an allowed transition.
However, for both molecules, the dipole moment is oriented along the z-axis, resulting in a
very small transition dipole moment in the x- and y-directions, which results in very low
oscillator strengths. So, the lowest energy transitions are allowed, but exhibit only small
changes in dipole moment resulting in weak absorptions. This can be, in part, attributed to
the boron center, as its contribution to the LUMO gives the transition a π-n character. The
S2←S0 transitions, in both cases, are predominantly HOMO-1 to LUMO transitions. The
F
F
HOMO-1 of MesFBf and XylFBf are of A’ and B2 symmetry, respectively, and, thus, are
symmetry-forbidden. Both MesFBf and XylFBf, exhibit large oscillator strengths for their
S7←S0 and S6←S0 transitions, respectively. In both cases, these transitions have predominantly
HOMO to LUMO+1 character. Both HOMO and LUMO+1 are delocalized over the borafluorene
F
F
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backbone without contributions from the boron center. The optimized S1 geometries of
FMesFBf and FXylFBf differ only slightly from their ground state structures (Figure 11).
F
F
Figure 11: Overlap of the optimized ground state (blue) and S1 (red) geometries of MesFBf (left) and XylFBf
(right).
F
In comparison, in the S1 structure of MesFBf, only the para CF3-groups on the borafluorene
F
F
backbone are rotated and in both MesFBf and XylFBf the ortho CF3-groups on the exo-aryl
are slightly bent away from the boron center. The calculated emission maxima of FMesFBf and
FXylFBf, fit the experimental data in hexane and also exhibit very low oscillator strengths. Even
though the optimized structures do not exhibit a higher symmetry it can be assumed that a
similar phenomenon as for the absorption takes place and is the reason for the observed long
lifetimes. Interestingly, as previously discussed, the reasonably high quantum yields observed
are due to extremely slow non-radiative decay processes.
F
The photophysical properties of p-NMe2-FXylFBf differ strongly from those of MesFBf and
FXylFBf. This is, in part, due to the fact that the nature of the lowest energy absorption has CT
rather than LE character (Λ = 0.15). This is not surprising, as p-NMe2-FXylFBf is a donor-
acceptor system. The S1←S0 transition of p-NMe2-FXylFBf exhibits an oscillator strength of 0.
Using the optimized structure of C2v 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
respect to the borafluorene core. The HOMO is of B2 symmetry and the LUMO has B1
symmetry. This is the same as for the S2←S0 transitions of FMesFBf and FXylFBf, as the HOMO
and HOMO-1 are inverted compared to those of p-NMe2-FXylFBf. For p-NMe2-FXylFBf,
F
F
however, the S2←S0 transition is allowed, but analogously to the MesFBf and XylFBf cases,
exhibits a very low oscillator strength. This explains the low extinction coefficient observed for
the lowest energy absorption of p-NMe2-FXylFBf. Furthermore, we optimized the S1 structure
of p-NMe2-FXylFBf as well as its T1 structure in order to calcutate the S1-T1 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 S1 and T1. Comparing
the energies of both structures results in ΔES-T = 423 meV which is almost 30 times higher than
the experimentally determined gap. It is noteworthy that the experimental determination of
the gap is highly flawed due to approximations as well as unpredictable solvent effects at
20
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10.1002/chem.201905559
Chemistry - A European Journal
lower temperature. However, this should still give a good estimate, but the calculations fail to
match the experimental value at this level of theory, illustrating the difficulty of predicting
phenomena such as TADF accurately.
Conclusion
Herein, we report the synthesis and properties of three trifluoromethylated borafluorenes
FMesFBf, FXylFBf, and p-NMe2-FXylFBf. The copper-catalyzed homocoupling of boronate esters
provides a convenient route to 2,2’-dibromobiphenyl derivatives, which can be lithiated and
then reacted with stable and accessible aryl-BF3K salts for the synthesis of borafluorenes. All
of the borafluorenes exhibit a rigid geometry with the exo-aryl group lying perpendicular to
the borafluorene plane. All three borafluorenes exhibit exceptionally positively shifted
reduction potentials, emphasizing the electron withdrawing nature of the CF3-groups. This
allowed us to use a mild reducing agent (CoCp2 E0 = –1.3 eV vs. Fc/Fc+) to reduce the most
F
F
anodically shifted borafluorene MesFBf. The resulting radical anion of MesFBf exhibits a
strong 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 FMesFBf and FXylFBf, this is the result of the transitions being forbidden in the z-direction
which coincides with the dipole moment and the transition dipole moment being neglegible
in the x- and y-directions. The same is apparently true for their emissions, as both compounds
exhibit fluorescence lifetimes of τ > 200 ns in hexane. Even with small oscillator strengths, the
two compounds exhibit fluorescence quantum yields of 0.37 and 0.30, respectively, as their
rigidity results in exceptionally slow non-radiative decay. In contrast, the twisted donor-
acceptor system p-NMe2-FXylFBf has a symmetry-forbidden lowest energy transition and
exhibits TADF, with a singlet-triplet energy gap ΔES-T experimentally determined to be only 15
meV. FMesFBf and FXylFBf are highly stable towards hydrolysis, which makes them interesting
potential building blocks for organic materials.
Acknowledgements
We thank the Julius-Maximilians-Universität of Würzburg, the Deutsche
Forschungsgemeinschaft (MA4471/8-1) and the Bavarian State Ministry of Science, Research,
and the Arts for the Collaborative Research Network “Solar Technologies go Hybrid” for
financial support, and the Bavarian Academy of Science and Humanities for access to the
Linux-Cluster of the Leibniz Supercomputing Centre. We are grateful for a generous donation
of B2pin2 from AllyChem. Co. Ltd. The authors thank Christoph Mahler and Robert Ricker for
HRMS measurements, Dr. Rüdiger Bertermann for NMR measurements, and Dr. Jörn Nitsch
and Dr. Julia Merz for helpful discussions.
21
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Having your cake and eating it too: Functionalization of 9-borafluorenes with trifluoromethyl
groups makes them exceptionally easy to reduce while maintaining excellent stability towards
hydrolysis. The systems also exhibit long-lived fluorescence or thermally activated delayed
fluorescence (TADF) depending on the exo-aryl para-substituent.
28
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