Synthesis and photophysical properties of imine borane adducts towards vapochromic materials

Article abstract of DOI:10.1039/c8dt03019g

A series of alkynyl aryl conjugated aldehydes and imines were prepared and their adducts with various Lewis acidic boranes have been studied via NMR, absorption, and luminescence spectroscopies in solution. The imine-B(C₆F₅)₃ adduct showed remarkable solution stability, and was then trialled in vapochromism experiments using simple impregnated paper strips to examine the fluorescence responses.

Full text of DOI:10.1039/c8dt03019g

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Synthesis and photophysical properties of imine
borane adducts towards vapochromic materials†
Cite this: DOI: 10.1039/c8dt03019g
Received 24th July 2018,
Accepted 8th August 2018
Yashar Soltani,
Paul D. Newman,
Samuel J. Adams, Jennifer Börger, Lewis C. Wilkins,
Simon J. A. Pope * and Rebecca L. Melen
‡
*
DOI: 10.1039/c8dt03019g
rsc.li/dalton
A series of alkynyl aryl conjugated aldehydes and imines were pre- borane catalysed transformations of alkynyl esters, we have fre-
pared and their adducts with various Lewis acidic boranes have quently encountered strong colouration of the solutions.7
been studied via NMR, absorption, and luminescence spectrosco- Often, reports of luminescence from such systems is confined
pies in solution. The imine-B(C6F5)3 adduct showed remarkable to the solid-state due to the weakness of the dative interaction
solution stability, and was then trialled in vapochromism experi- upon adduct formation. Previously, B(C6F5)3 has been shown
ments using simple impregnated paper strips to examine the fluo- to alter the optoelectronic properties of conjugated organic
rescence responses.
chromophores or polymers for potential applications in semi-
conducting and light harvesting materials.8,9 A recent report
has also suggested that the luminescence output from Lewis
acid–base adducts can be utilised to probe the molecular char-
acter of mesoporous organosilicates.10 Therefore we were inter-
ested to explore the scope of the photophysical properties of a
series of Lewis acid–base systems focusing on imine-borane
adducts. Secondly, we speculated that these adducts may have
applications as potential vapochromic materials with our
initial findings showing promise when these adducts are
impregnated into paper.
Firstly, a variety of aldehydes 1a–c were synthesised accord-
ing to literature methods as bases for reaction with boron
Lewis acids.9 Preliminary 11B-NMR studies in CDCl3 of 1a with
a range of boranes of differing Lewis acidity of the type BR3
(R = Ph, 2,4,6-F3C6H2, C6F5) showed that the tolane derivative
1a only forms adducts with the strongest Lewis acid tris(penta-
fluorophenyl)borane at room temperature (Fig. 1). The more
Typically, highly Lewis acidic boranes containing polyfluori-
nated aryl rings have been used in frustrated Lewis pair
(FLP)1,2 chemistry to activate substrates for catalysis.
Unfortunately, this can sometimes result in drawbacks such as
limited functional group tolerance as well as high moisture
sensitivity leading to higher catalyst loadings and longer reac-
tion times relative to transition metal-based systems. One
major advance in the last few years was the realisation that
weakly Lewis basic coordinating solvents could be used in
combination with a strong Lewis acid to affect the hydrogen-
ation of ketones.3 In addition, the design of moisture-tolerant
Lewis acids for FLP chemistry has been targeted4 with our-
selves in addition to others having previously showed that
B(C6F5)3 can be tolerant to water in metal-free catalytic reac-
tions.5 In relation to this, we were interested in gaining a
deeper understanding of adduct formation between various
Lewis bases in FLP substrates (ketones and imines) and sol-
vents with Lewis acidic boranes.6 Such experiments give an
insight into the use of Lewis acidic boranes in FLP chemistry
and may give an understanding into catalyst activity and
inhibition.
The fluorescence properties of Lewis pair adducts have
been rarely explored in detail and, in our previous studies into
School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT,
Cymru/Wales, UK. E-mail: PopeSJ@cardiff.ac.uk, MelenR@cardiff.ac.uk
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, NMR data, X-ray structures and UV-Vis measurements. CCDC 1836219.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8dt03019g
‡Current Address: Department of Chemistry, Texas A&M University,
College Station, Texas 77843, United States.
Fig. 1 Aldehydes 1a–c and their adduct formation. Relative Lewis
acidity determined by the Gutmann–Beckett method.14
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electron rich methoxy-substituted aldehydes 1b and 1c form derivatives (2b and 2e). Indeed, adduct formation was observed
adducts with less Lewis acidic boranes, but not with BPh3 (see with the less Lewis acidic triphenylborane via 11B-NMR-spec-
ESI†).
troscopy (insert, Fig. 2). Slow evaporation of a saturated chloro-
In order to increase the donor ability of the Lewis base, 1b form/hexane solution of 2f and B(C6F5)3 at room temperature
and 1c were converted into a series of imines by condensation under inert atmosphere yielded crystals of the borane adduct
with PhNH2, iPrNH2 and nBuNH2 to yield 2a–f (Fig. 2). nButyl which was structurally characterised confirming the proposed
substituted imines 2c and 2f were anticipated to be the most formulation (Fig. 3). The solid-state structure of 2f·B(C6F5)3
promising Lewis bases due to their greater positive inductive was as anticipated with the exception of a slight shortening of
effect as well as being less sterically demanding than the iPr the N → B bond when compared with similar motifs (1.613(3)
Å vs. ca. 1.63 Å).11,12 This denotes a slightly stronger coordina-
tive interaction between the Lewis acid and base however, this
is still significantly longer than related formal N–B bonds
(ca. 1.39 Å).12,13 Additionally, despite there being the capacity
for extensive conjugation throughout the assembly, in the solid-
state this delocalisation is broken with the distal aryl ring lying
out of the plane of the proximal aryl ring of the imine residue
by 41.14(7)° presumably due to structure packing artefacts.
Based on the results above, Lewis acid adducts of imine 2f
were selected for further spectroscopic studies to elucidate
their solution-state electronic properties via UV-vis absorption,
and luminescence spectroscopies (Table 1).
For 2f, the UV-vis spectrum (Fig. 4) revealed three bands at
ca. 290, 310 and 390 nm, each of which is likely to have signifi-
cant π–π* contributions. This general pattern was replicated
Fig. 2 Imines 2a–f and their adduct formation. Relative Lewis acidity
determined by the Gutmann–Beckett method.14
Fig. 3 Solid-state crystal structure of 2f with B(C6F5)3. Thermal ellip-
soids set to 50% probability. H atoms omitted for clarity. C: Black, O:
red, N: blue, B: pink, F: green.
Fig. 4 UV-Visible absorption spectra of 2f and corresponding borane
adducts in chloroform solution.
Table 1 Photophysical properties of 2f and its adducts in chloroform solution
Compound
Absorption/nm (ε/dm3 mol−1 cm−1
)
Emissiona/nm
Lifetimeb/ns
2f
2f·BPh3
2f·B(2,4,6-F3C6H2)3
2f·B(C6F5)3
2f·B(3,4,5-F3C6H2)3
2f·BF3
313 (29 800), 329 sh (24 200), 395 sh (4000)
281 (46 300), 308 sh (22 300), 329 sh (17 000), 390 (14 000)
312 (28 600), 328 sh (23 000), 387 sh (10 000)
312 (24 500), 330 sh (13 200), 397 (7400)
294 (32 800), 317 sh (19 100), 332 sh (16 800), 381 (16 800)
317 (21 300), 332 sh (18 000), 401 (14 000)
289 (44 300), 399 (23 900)
490
491
493
501
490
496
493
495
1.5 (73%), 6.3 (27%)
2.0
1.2 (65%), 3.5 (35%)
1.1 (75%), 3.6 (25%)
1.7
1.3
1.6
1.5
2f·BCl3
2f·BBr3
289 (54 200), 402 (24 000)
a Using 400 nm excitation. b Using pulsed 295 nm excitation.
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for all the borane Lewis acid adducts of 2f, but with a notable
increase in intensity of the lowest energy band which was most
pronounced for 2f·BBr3 (ε ∼ 2.5 × 104 M−1 cm−1). This broad
band was also susceptible to subtle wavelength shifts depend-
ing upon the nature of the Lewis acid. This renders solutions
of these species as different colours to the naked eye, although
there was little correlation when comparing these adduct
colours to Gutmann–Beckett Lewis acidities.14 In comparison,
the more sterically hindered ortho-substituted imine 2c did
not display an absorption band at ca. 400 nm and its adducts
showed only very weak absorption in that region.
Luminescence studies in chloroform on imine 2f and its
borane adducts were then undertaken using excitation wave-
lengths of 330 nm and 400 nm (the latter data shown in
Table 1), thus correlating with the two absorption bands.
Excitation at 330 nm yielded two emission peaks at ca. 380 nm
and 500 nm (Fig. 5), while excitation at 400 nm selectively
reproduced the lower energy emission at ca. 500 nm (thus
Fig. 6 Solvatochromic fluorescence spectra of 2f·B(C6F5)3 (λex
400 nm).
=
resulting in a substantial Stokes’ shift). Time-resolved emis- lowest energy absorption band at ca. 400 nm is assigned to a
sion analysis of the 500 nm peak revealed lifetimes of <10 ns transition with a significant intramolecular charge transfer
in each case.
(ICT) contribution.
The emission spectra for the Lewis acid derivatives of 2f
The absence of similar observations for 2c may be due to
(Fig. 5) showed that adduct formation results in a dramatic the reduced conjugation throughout the chromophore caused
increase in the intensity of the 500 nm fluorescence band. The by the steric demands of this isomer; ortho substitution is
precise wavelength of the band was shown to be subtly depen- clearly unfavourable in this case. Interestingly, the observation
dent on the Lewis acid used for adduct formation (e.g. 2f·B of charge transfer bands in 2f at relatively low concentrations
(C6F5)3 gave the most bathochromically shifted emission peak) (10−5 M) is in contrast to the analogous aldehyde 1c (Fig. S83,
and was also found to be independent of the excitation wave- ESI†), which does not display any absorption or emission
length (330 nm or 400 nm). Solvatochromic luminescence charge transfer bands in chloroform solution, in agreement
studies on 2f·B(C6F5)3 (Fig. 6) showed that the band at ca. with previous studies.9
500 nm was clearly sensitive to solvent polarity, with hexane
To investigate the stability of the Lewis pair adducts in air,
giving the most hypsochromically shifted peak. The formation a 10−5 M chloroform solution of 2f·B(C6F5)3 was air-equili-
of Lewis acid adducts with 2f thus promotes charge transfer brated for 10 minutes. The fluorescence intensity of the charge
electronic states in these fluorophores, reflecting the donor– transfer emission band at 500 nm was then monitored over
acceptor electronic structure of the fluorophore that is induced 2 hours (Fig. S82, ESI†) and showed a surprisingly good reten-
by Lewis acid–base pair adduct formation. By inference, the tion of emission signal intensity over this period.
Further luminescence studies in the solid-state were under-
taken on selected species. The solid-state emission spectra for
2f and 2f·B(C6F5)3 revealed emission bands centred at 416 and
541 nm, respectively. Interestingly, these bands are signifi-
cantly bathochromically shifted from chloroform solution,
giving rise to a 40 nm shift in the case of 2f·B(C6F5)3 (Fig. 7). It
was noted that for 2f·B(C6F5)3 the higher energy emission
ascribed to the π–π* band for the adduct was absent with emis-
sion dominated by the proposed ICT band at 541 nm.
The above discussion revealed the apparent photo- and
solution-stability demonstrated by 2f·B(C6F5)3 as well as con-
trasting emission properties in the solid-state. This therefore
encouraged us to further explore the application of this
species as a potential vapochromic material. Firstly, small
strips of filter paper were impregnated with 2f·B(C6F5)3 using a
concentrated solution of the adduct. Fluorescence from these
strips was photographically recorded following excitation at
365 nm (Fig. 8).
For comparison, 2f·B(C6F5)3 was observed in a nitrogen
atmosphere, as well as in a vial open to air, and both filter
Fig. 5 Steady-state emission spectra of 2f and adducts in chloroform
solution (λex = 330 nm; C = 10−5 mol dm−3).
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from the other solvents. The blue emission is more consistent
with the solid-state fluorescence properties of 2f alone (Fig. 7).
Hexane showed the least effect, as might be expected for a
non-coordinating solvent.
In conclusion, we have synthesised a range of Lewis basic
imines and studied their adducts with Lewis acidic boranes by
various spectroscopic methods. In the case of the nButyl sub-
stituted imine, strong adduct formation was observed with
B(C6F5)3. The stability of this Lewis pair adduct allowed the
solution-state electronic properties to be determined showing
that ICT electronic transitions give rise to new visible region
absorption and fluorescence properties. For
a selected
example, sufficient air/moisture stability of the adduct facili-
tated an exploration of its vapochromic properties with a small
range of organic solvents. These findings suggest that with
further optimisation Lewis pair adducts may be applicable to
the design of new vapochromic materials via modulation and
discrimination of charge transfer fluorescence properties.
Fig. 7 Solid-state film fluorescence spectra of 2f and 2f·B(C6F5)3
(λex = 330 nm).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
S. J. A. P. thanks the Leverhulme Trust for financial support
(RPG-2015-359).
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