B(C6F5)3: A lewis acid that brings the light to the solid state

Article abstract of DOI:10.1002/anie.201508461

The straightforward coordination of the Lewis acid B(C₆F₅)₃ to classical, non-emitting aldehydes results in solid-state photoluminescence. Variation of the electronic properties of the carbonyl moieties lead to the modulation of the solid-state emission colors, covering the entire visible spectrum with quantum yields up to 0.64. Steady-state spectroscopy in combination with X-ray diffraction analysis and DFT calculations confirm that intermolecular interactions between the Lewis adducts are responsible for the observed luminescence. Alteration of the latter interactions induces, moreover, remarkable solid-state phenomena such as piezochromism. The versatility and simplicity of our approach facilitate the future development of solid-state emitting materials. Turn on the light: A straightforward methodology can be used to "switch on" the solid-state fluorescence of non-emitting carbonyl compounds. Subtle electronic changes in the aldehyde moieties lead to a variety of emission colors that cover the entire visible spectrum. The materials also display pressure-dependent luminescent properties (piezochromism).

Full text of DOI:10.1002/anie.201508461

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International Edition: DOI: 10.1002/anie.201508461
German Edition: DOI: 10.1002/ange.201508461
Solid-State Luminescence
Hot Paper
B(C6F5)3: A Lewis Acid that Brings the Light to the Solid State
Max M. Hansmann, Alicia López-Andarias, Eva Rettenmeier, Carolina Egler-Lucas,
Frank Rominger, A. Stephen K. Hashmi,* and Carlos Romero-Nieto*
Abstract: The straightforward coordination of the Lewis acid
B(C6F5)3 to classical, non-emitting aldehydes results in solid-
state photoluminescence. Variation of the electronic properties
of the carbonyl moieties lead to the modulation of the solid-
state emission colors, covering the entire visible spectrum with
quantum yields up to 0.64. Steady-state spectroscopy in
combination with X-ray diffraction analysis and DFT calcu-
lations confirm that intermolecular interactions between the
Lewis adducts are responsible for the observed luminescence.
Alteration of the latter interactions induces, moreover, remark-
able solid-state phenomena such as piezochromism. The
versatility and simplicity of our approach facilitate the future
development of solid-state emitting materials.
solid-state photophysical properties[8] (dual room-tempera-
ture fluorescence and phosphorescence). In that context,
examples of nonchelating boranes are, on the other hand,
rather scarce. Bazan et al.[9] reported the possibility to modify
the absorption properties of benzothiadiazole-containing
architectures by coordination of the B(C6F5)3 Lewis acid to
the nitrogen atoms of the benzothiadiazole units. The
formation of B–N adducts led to a reduction of the optical
bandgap and thus to absorption features in the near infrared.
Although the coordination properties of B(C6F5)3 were
reported by Piers et al. years ago,[10] the utilization of this
Lewis acid was, until now, restricted to frustrated Lewis pair
chemistry[11] and catalysis-related applications.[12] The latter
motivated us to investigate the impact of the B(C6F5)3
coordination on the photophysical properties of materials
capable of strongly interacting with boranes, that is, non-
chelating aldehyde-containing architectures.[10]
To conduct a systematic investigation, we designed
a series of aldehyde-containing materials meeting the follow-
ing requirements: a) straightforward preparation, b) gradual
variation of the optical bandgap, c) different electronic
communication between the p-system and the carbonyl
group, and d) the presence of double or triple bonds as
unsaturated carbon functional groups. Thus, we designed
a series of tolane and stilbene derivatives with electron-
donating or -accepting peripheral groups, and a carbonyl
moiety in ortho or para position (compounds 1b–k;
Scheme 1).
S
olid-state luminescent materials are of utmost importance
because of their application in light-emitting diodes
(LEDs),[1] lasers,[2] and luminescent sensors.[3] Thus, over the
last decades materials research has focused on defining the
key factors that lead to the best emission performances. As
a result, it is well established that: 1) Ideally in photolumi-
nescent materials, intermolecular interactions must restrict
nonradiative relaxation processes, namely, intramolecular
rotations.[4] The energy levels can then rearrange, generating
extremely efficient photoinduced emission. 2) The presence
of halogens, some transition metals, and carbonyl groups must
be avoided. Their n–p* transitions typically promote efficient
intersystem crossing and thus deactivate fluorescent decays.[5]
These findings have certainly limited the diversity of efficient
solid-state emitting materials. Most molecular dyes exhibit
reduced or quenched luminescence when the molecular
density is increased from diluted solutions to the solid state.
This is attributed to intermolecular interactions or intersys-
tem crossings that dissipate the absorbed energy.[6]
Recently, boranes demonstrated to successfully circum-
vent the aforementioned bottlenecks. Thus, a myriad of
chelating four-coordinate organoboron compounds have been
reported.[7] Fraser et al. described, for instance, the prepara-
tion of organoboranes from 1,3-diketones with extraordinary
[*] Dr. M. M. Hansmann, M. Sc. A. López-Andarias, Dr. E. Rettenmeier,
Dr. C. Egler-Lucas, Dr. F. Rominger, Prof. Dr. A. S. K. Hashmi,
Dr. C. Romero-Nieto
Organisch-Chemisches Institut
Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: hashmi@hashmi.de
carlos.romero.nieto@oci.uni-heidelberg.de
Prof. Dr. A. S. K. Hashmi
Chemistry Department, Faculty of Science, King Abdulaziz University
Jeddah 21589,(Saudi Arabia)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508461.
Scheme 1. Preparation of carbonyl–borane adducts 2a–k.
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DFT calculations (B3LYP/6-31G + (d) level of theory)
show a gradual variation of the HOMO and LUMO frontier
orbitals within the ortho and para series (Figure S1 and
Table S1 in the Supporting Information, SI). Moreover, we
included the benzaldehyde 1a as a model compound. The
preparation of the tolane and stilbene derivatives was carried
out by simple Sonogashira or Suzuki–Miyaura cross-coupling,
respectively, from commercially available reactants.
The synthesis of the R-CHO!B(C6F5)3 adducts turned
out to be highly selective and quantitative; a competing
reaction of the borane Lewis acid with the unsaturated double
or triple bonds was not observed. Thus, the addition of an
equimolar amount of B(C6F5)3 to 1a–k in dichloromethane
led instantly to the corresponding borane adducts 2a–k (see
SI). The products were fully characterized by NMR and FTIR
spectroscopy, mass spectrometry, elemental analysis, and X-
ray diffraction analysis.
First insights into the impact of the B(C6F5)3 coordination
came from DFT calculations (Figure S1 and Table S1). In all
cases, binding of the boron atom leads predominantly to
a lower LUMO level and, accordingly, to a reduction of the
optical band gap. In turn, the electronic densities rearrange so
that the LUMO contains the carbonyl moiety and the HOMO
is located on the borane group (Figure 1; and Figures S2–S5).
Figure 2. a) Normalized absorption spectra of 2a from concentrated
(solid line) and dilute (dashed line) solutions. Inset: 3D emission
spectrum of a concentrated solution of 2a in dichloromethane;
emission (gray) and excitation (red) spectra corresponding to the
maxima. b) Representative absorption spectra of 2k from concentrated
(solid line) and dilute (dashed line) solutions. Inset: Gradual fading of
the absorption band after the dropwise addition of methanol.
Figure 1. Representative LUMO (left) and HOMO (right) obtained by
DFT calculations on the adduct 2d.
Next, to evaluate the photophysical properties of the
borane adducts, we turned our focus to steady-state spectros-
copy. In general, carbonyl-containing compounds typically
exhibit rather low fluorescence quantum yields (QYs)(F <
0.05);[13,5] the n-p* singlet state funnels photoexcited electrons
into the p-p* triplet state, preventing efficient fluorescent
decay. Here, we expected the electronic rearrangement
caused by the O–B coordination to suppress the aforemen-
tioned photoinduced processes, promoting thus improved
luminescence. However, dichloromethane solutions of 2a–k
indicated that the O–B coordination has a negligible effect on
the spectroscopic features. Namely, the absorption spectra
from diluted solutions exhibit identical absorption maxima in
the UV range, between 260 and 341 nm (Figures S6 and S7).
In turn, emission properties remain unaltered, that is, F <
0.02. A different scenario emerged when the concentration
was increased above 10À5 m.
exhibit absorption maxima at 248 and 288 nm (Figure 2a).
However, increasing the solutionꢀs concentration leads to
a new shoulder in the absorption spectrum at 316 nm
(Figure 2a). Surprisingly, photoexcitation of the newly
formed absorption band gives rise to emissive features that
maximize at 400 nm. Three-dimensional steady-state spec-
troscopy provides better insight into the origin of the new
emission band (Figure 2a, inset). Thus, a maximum at 265 nm
and a broad band centered at 310 nm appear responsible for
the photoluminescence of the benzaldehyde–B(C6F5)3 adduct
(see Figure 2a, excitation spectrum).
The observation of these new emissive properties from
a typically non-emitting carbonyl compound motivated us to
continue our studies systematically with substituted benzal-
dehydes. In all cases, increasing the concentration of 2b–k
induces the formation of a new absorption band of lower
energy (Figure 2b and Figures S8–S17). Importantly, this
band was found to be stable upon dilution on the exper-
Model compounds are fundamental to shed light on
spectroscopic behaviors. Thus, we first centered our inves-
tigations on compound 2a. Diluted solutions of the latter
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imental time scale (Figure S18). Again, these new emerging
absorption bands are responsible for a prominent luminescent
process; excitation spectra corroborate the latter (Figures S7–
S17). Concentrated solutions exhibit emission maxima that
range from 437 to 613 nm, as a function of the different
peripheral groups (Figure S19). One rationale infers the
formation of aggregates in the ground state, which lead to
the newly formed spectroscopic features.
To verify the role of the O–B coordination in the
formation of emissive aggregates, we progressively replaced
the aldehyde moieties of the adducts 2b–k with methanol.
Thus, when methanol is added dropwise to the concentrated
solutions, the new absorption bands gradually disappear
(Figure 2b, inset, and Figures S20–S24). The lack of shifts in
the absorption maxima is consistent with the formation of
discrete aggregates.[14] Massive molecular aggregation would
lead to a progressive displacement of the absorption bands as
the aldehydes are replaced.
In view of the photoluminescence of the concentrated
solutions, we investigated the solid-state spectroscopic prop-
erties of 2b–k. Surprisingly, we found the substituted benz-
aldehyde–borane adducts to exhibit a strong photolumines-
cence (Figures S25–S35). It is important to note that neither
the free Lewis acid B(C6F5)3 nor compounds 1b–k exhibit any
significant emission properties in the solid state. Remarkably,
the emission maxima of the borane adducts range through the
entire visible spectrum, that is, from 444 nm for 2b to 625 nm
for 2k (Figure 3a). Thus, the emission maxima are deter-
mined by the nature of the peripheral groups. Increasing the
electron-donating capacity leads to a red shift of the borane
adductsꢀ emission features. The color coordinates (Figure 3b
and Table S3) provide additional information on the observed
fluorescent colors (Figure 3c). Solid-state fluorescence quan-
tum yields of the borane derivatives differ from 0.64 for 2d to
0.15 for 2k (Table S4), presumably due to the stronger donor–
acceptor character of the latter adduct.
Figure 3. a) Normalized solid-state fluorescence of 2b–k. b) CIE color
coordinates from the solid-state emission of 2b–k. c) Solid-state
photoluminescence of compounds 2b–k.
Overall, emission maxima are bathochromically shifted in
the solid state with respect to those of concentrated solutions.
The latter indicates the presence of additional intermolecular
interactions. Thus, fluorescence decays appear bi-exponential
in the solid state (see Table S4).[15]
To investigate the intermolecular interactions in the solid
state, we focused on single crystal X-ray analysis (Figure 4a
and Figures S36–S40). Analyses of the three-dimensional
arrangements reveal a strong preference of the B(C6F5)3
moieties to interact with one another. In all cases, the shortest
distances are found for F–F interactions between B(C6F5)3
and range from 2.83 to 2.94 Š. Thus, the boron center with an
increased electronic density, surrounded by highly polarized
C6F5 groups, drives the intermolecular interactions (Figure 4a
and Figures S36–S40). This leads, for example in 2d,[16] to
dimeric structures (Figure 4a). DFT calculations of the latter
assembly provide information about the resulting electronic
rearrangement (Figure S42). Thus, both the HOMO and
LUMO appear to be doubly degenerated; the HOMO is
centered on the diborane nucleus, whereas the LUMO is
located on both aldehyde moieties.
Figure 4. a) Packing of 2d deduced from X-ray crystallography. Short-
est F–F interactions are indicated in pink. b) Changes in the solid-state
photoluminescence of 2 f and color change from orange to red (inset)
upon increasing pressure.
These observations, together with our spectroscopic
investigations, indicate that: 1) The coordination of B(C6F5)3
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to carbonyl groups does not prevent radiationless processes in
individual aldehyde molecules: diluted solutions do not emit.
b) The intermolecular interactions lead to circumvent the
nonradiative processes and, ultimately, enable the photo-
luminescence. It is important to note that after aggregation
the energy distribution of the carbonyl–borane adducts is
altered (Figure S42) leading to the emissive state. A rationale
infers the latter redistribution to be responsible for preventing
the radiationless transitions.
Accordingly, altering the intermolecular interactions
would modify the photophysical properties of the solid
state. To verify our hypothesis, we reproduced our spectro-
scopic measurements while applying increasing pressure.
Compounds with narrower optical band gaps facilitate the
identification of batho- or hypsochromic changes. Thus,
increasing the pressure on solid 2 f induced piezochromism,
that is, a red shift of the emission maximum from 570 nm to
590 nm (Figure 4b), which correlates with a color change
from orange to red (Figure 4b, inset). Importantly, when the
sample was ground, the original spectroscopic features were
regenerated.
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In conclusion, we report a straightforward protocol to
efficiently turn on the solid-state photoluminescence of
typically non-emissive carbonyl materials by simple coordi-
nation of the Lewis acid B(C6F5)3. Our method appears
compatible with materials containing double or triple bonds.
Subtle modification of the carbonyl molecules leads to
emissive colors that cover the entire visible spectrum with
high quantum yields. Intermolecular interactions promoted
by B(C6F5)3 enable, moreover, intriguing phenomena such as
piezochromism. The commercial availability of B(C6F5)3 and
the simplicity of our method allow the facile preparation of
novel materials with enhanced solid-state fluorescence.
Investigations on superior carbonyl architectures are cur-
rently in progress.
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Acknowledgements
[14] B. Z. Tang, A. Qin, Aggregation-Induced Emission: Fundamen-
tals, Wiley-VCH, Weinheim, 2013.
[15] Fluorescence decays in the solid state are generally multi-
exponential due to the presence of multiple deactivation path-
ways.
[16] CCDC 1419191, 1419192, 1419193, 1419194, 1419195 and
1419196 contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
C.R.N. and A.L.A. thank the Fonds der chemischen Industrie
for a Liebig Grant and a fellowship, respectively. Moreover,
C.R.N., A.L.A., and C.E.L. thank Prof. Bunz and the
Organisch-Chemisches Institut of the University of Heidel-
berg for the outstanding support. M.M.H. is grateful to the
Fonds der chemischen Industrie for a Chemiefonds scholar-
ship and the Studienstiftung des deutschen Volkes.
Keywords: aldehydes · boranes · fluorescence · piezochromism ·
solid-state luminescence
Received: September 9, 2015
Published online: December 10, 2015
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