Reversibly thermochromic bismuth-organic materials with tunable optical gaps

Article abstract of DOI:10.1039/c7tc03277c

Five reversibly thermochromic bismuth-based inorganic-organic hybrid materials were synthesized via facile one-step condensation reactions between triphenylbismuth and four arenedithiols and a diselenol. The materials were characterized by UV-vis and FT-IR spectroscopy, powder X-ray diffraction (PXRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), elemental analysis (EA), and thermogravimetric analysis (TGA). The bismuth-organic materials (Bi-OMs) are colored materials with optical HOMO-LUMO gaps that were readily tuned by systematic modification of the steric and electronic character of the π-conjugated organic linkers. All five Bi-OMs exhibit completely reversible thermochromism in the solid-state, and possess excellent thermal stability in air up to nearly 300 °C. This combination of physical properties, coupled with the ease of synthesis and the inexpensive, non-toxic nature of bismuth, make this class of inorganic-organic hybrid materials potentially promising for reversible smart window applications, camouflage coatings, temperature sensing, photocatalysis, color filters and displays.

Full text of DOI:10.1039/c7tc03277c

View Article Online
View Journal
Journal of
Materials Chemistry C
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: G. Zhang and J.
M. W. Chan, J. Mater. Chem. C, 2017, DOI: 10.1039/C7TC03277C.
This is an Accepted Manuscript, which has been through the
Volume
4 Number 1 7 January 2016 Pages 1–224
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry C
Accepted Manuscripts are published online shortly after
Materials for optical, magnetic and electronic devices
www.rsc.org/MaterialsC
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

Page 1 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
Reversibly Thermochromic Bismuth‒Organic Materials
with Tunable Optical Gaps
Guoxian Zhang† and Julian. M. W. Chan*†
†Department of Chemistry and Biomolecular Sciences,
University of Ottawa, 10 Marie Curie Pvt.
Ottawa, Ontario, K1N 6N5, Canada
*Eꢀmail: julian.chan@uottawa.ca
ABSTRACT: Five reversibly thermochromic bismuthꢀbased inorganicꢀorganic hybrid materials
were synthesized via facile oneꢀstep condensation reactions between triphenylbismuth and four
arenedithiols and a diselenol. The materials were characterized by UVꢀvis and FTꢀIR
spectroscopy, powder Xꢀray diffraction (PXRD), energy dispersive Xꢀray spectroscopy (EDS),
scanning electron microscopy (SEM), elemental analysis (EA), and thermogravimetric analysis
(TGA). The bismuthꢀorganic materials (BiꢀOMs) are colored materials with optical HOMOꢀ
LUMO gaps that were readily tuned by systematic modification of the steric and electronic
character of the πꢀconjugated organic linkers. All five BiꢀOMs exhibit completely reversible
thermochromism in the solidꢀstate, and possess excellent thermal stability in air up to nearly 300
°C. This combination of physical properties, coupled with the ease of synthesis and the
inexpensive, nonꢀtoxic nature of bismuth, make this class of inorganicꢀorganic hybrid materials
potentially promising for reversible smart window applications, camouflage coatings, temperature
sensing, photocatalysis, color filters and displays.
1

Journal of Materials Chemistry C
Page 2 of 26
View Article Online
DOI: 10.1039/C7TC03277C
INTRODUCTION
Purely inorganic materials that contain bismuth have been widely studied and used as
optoelectronic materials,1ꢀ9 thermoelectrics,10ꢀ13 photocatalysts,14ꢀ17 scintillators,18 lanthanide host
materials,19, 20 and biomedical Xꢀray contrast agents.21ꢀ24 This myriad of applications attests to the
versatility and technological importance of bismuth in advanced functional materials. In contrast,
hybrid organicꢀinorganic materials containing bismuth, such as Biꢀbased metalꢀorganic
frameworks (BiMOFs), are still relatively unexplored, with the vast majority of reported MOFs
being based on transition metals. Nevertheless, interest in bismuthꢀorganic materials (BiꢀOMs) has
been gaining momentum lately, with several systems having been published in recent years.25ꢀ33
Considering its high abundance, low cost, rich chemistry, nontoxic and nonꢀradioactive nature
(unusual for such a heavy element), bismuth makes for an attractive choice of metal for designing
novel hybrid materials with wideꢀranging technological applications. Incorporating bismuth into
inorganicꢀorganic systems would open the door to many new and interesting materials due to the
element’s flexible coordination geometry, stereoactive 6s lone pair, and relativistic spinꢀorbit
coupling effects. Of the BiꢀOMs reported so far, essentially all of them have been synthesized
through Bi‒O and Bi‒N bond formations between Bi(III) ions and coordinating carboxylate or
multidentate Nꢀheterocycleꢀbased πꢀlinker ligands. With the idea of promoting enhanced metalꢀ
ligand interaction, we envisaged that the use of larger coordinating atoms (e.g., S, Se) on the
ligands would mitigate the atomic size mismatch and foster better orbital overlap between Bi and
the πꢀlinker. To the best of our knowledge, BiꢀOMs made by iterative Bi‒S bond formations has
not yet been explored, in contrast to the recent surge in metalꢀdithiolene frameworks34ꢀ36
synthesized from transition metals and aromatic polythiols. Heightened interest in transitionꢀmetal
dithiolene networks has been primarily motivated by the unique optoelectronic properties of these
twoꢀdimensional πꢀd conjugated organometallic materials, in which πꢀconjugated organic linkers
2

Page 3 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
were connected to the transition metals by strong metalꢀsulfur covalent bonds. In the case of
bismuth compounds, Bi‒S bonds are generally regarded as thermodynamically stable (favorable
softꢀsoft interactions) and less labile than Bi‒O bonds.37,38 Thus, synthesizing organobismuth
networks via iterative Bi‒S bond formations presented a viable strategy for the construction of
novel and robust BiꢀOM materials. Based on documented ligandꢀtoꢀmetal charge transfer (S→Bi)
effects observed in bismuthꢀthiolate complexes,39ꢀ43 it was anticipated that the use of electronꢀrich
aromatic dithiols44 as πꢀlinkers would give rise to interesting π→σ* interactions between the
organic components and the bismuth centers. Our interest in such systems was inspired in part by
the optoelectronic properties of bismuth chalcogenides45ꢀ48, thermochromic properties of select
organobismuth molecules,49 and recent advances in the tuning of bandgaps in metal‒organic
frameworks.50ꢀ54 By using an organicꢀinorganic hybrid system, we hoped to obtain materials with
interesting optical and chromic properties that can be fineꢀtuned by rational functionalization of
the organic linkers.55, 56 Herein we report the synthesis and characterization of five BiꢀOMs
(Figure 1). These materials were prepared from triphenylbismuth and various arenedithiols (and a
diselenol) through a polycondensation reaction that afforded volatile benzene as the only byꢀ
product. We demonstrate that the optical HOMOꢀLUMO gaps of the BiꢀOMs can be fineꢀtuned
through systematic functionalization of the πꢀconjugated organic linkers that connect the metal
atoms. In addition to color tunability, all five BiꢀOMs also exhibited intriguing reversible solidꢀ
state thermochromism and excellent thermal stability in air. Where suitably stable, such
thermochromic materials57ꢀ60 show great promise for utilization in technologies such as reversible
smart windows, camouflage coatings, photocatalysts, and nanothermometers.
3

Journal of Materials Chemistry C
Page 4 of 26
View Article Online
DOI: 10.1039/C7TC03277C
4

Page 5 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
Figure 1. Schematic representation of the synthesis of Bi-OM-1 through Bi-OM-5, and their
molecular structures.
EXPERIMENTAL SECTION
5

Journal of Materials Chemistry C
Page 6 of 26
View Article Online
DOI: 10.1039/C7TC03277C
Materials and methods. All synthetic procedures were performed in ovenꢀdried glassware under
anhydrous nitrogen atmosphere using standard Schlenk techniques. All reagents and chemicals
were purchased from SigmaꢀAldrich, Alfa Aesar, TCI America and Fisher Scientific, and used
without further purification. Reaction solvents were dried over activated 3Å molecular sieves and
rigorously degassed by sparging with nitrogen prior to use. Intermediates were purified by flash
column chromatography on silica gel (60–230 mesh). The NMR spectra of BiꢀOM precursors
were recorded on a Bruker AVANCE II 400 MHz NMR spectrometer. Solidꢀstate UVꢀvis diffuse
reflectance spectra (DRS) were obtained on a Varian Cary® 100 UVꢀvis spectrophotometer,
equipped with a diffuse reflectance accessory consisting of a 73ꢀmm diameter integrating sphere.
Infrared spectra were recorded on a Varian 640 FTIR spectrometer equipped with an ATR in the
500‒4000 cmꢀ1 range. Powder Xꢀray diffraction studies were carried out on a Rigaku Ultima IV
Diffractometer. Scanning electron microscopy and energy dispersive Xꢀray spectroscopy was done
on a JSMꢀ7500 FESEM (JEOL) instrument. Elemental analysis (EA) was performed at the
Analytical and Instrumentation Laboratory at the University of Alberta. Thermogravimetric
analysis (TGA) data were recorded on a Q5000 IR instrument (TA Instruments; New Castle, DE,
USA), and the samples were heated from 25 °C to 500 °C in air with a temperature rise of 10
°C/min. TGA data were analyzed using TA Instruments Universal Analysis 2000 Version 4.5 A.
Synthesis of triphenylbismuth (1). Triphenylbismuth was readily prepared using literature
protocols.61 To a cooled (ꢀ78 °C) solution of bromobenzene (3.3 mL, 31 mmol) in THF was added
1.6 M nꢀbutyllithium (19.4 mL, 31.0 mmol) and the mixture was stirred for 1 h before being
warmed up to 0 °C. Next, bismuth(III) bromide (4.49 g, 10.0 mmol) dissolved in THF was added
via syringe to the reꢀcooled (ꢀ78 °C) solution of phenyllithium. The stirred reaction mixture was
allowed to gradually warm up to room temperature overnight (16 h) before being quenched with
6

Page 7 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
methanol. The resulting mixture was partitioned between water and dichloromethane within a
separatory funnel, after which the triphenylbismuth was extracted (3 x 50 mL) into the organic
layer. After drying over anhydrous Na2SO4, the organic solution was rotary evaporated to dryness
and the crude product was recrystallized from ethanol to afford pure triphenylbismuth as white
crystals. Yield: 2.95 g, 67 %. (Note: Triphenylbismuth is not commercially available in certain
locations, and cannot be shipped internationally due to export regulations.)
Synthesis of Bi-OM-1. A solution of triphenylbismuth (0.409 g, 0.920 mmol) and benzeneꢀ1,4ꢀ
dithiol (0.192 g, 1.33 mmol) in anhydrous degassed toluene (70 mL) was refluxed with vigorous
stirring over 72 h. A reddishꢀorange precipitate developed over the course of three days. At the
end of the reaction, the mixture was cooled to room temperature, during which the reddish Bi-
OM-1 precipitate was observed to gradually turn yellowꢀorange in color. The product was isolated
by suction filtration, washed thoroughly with hot acetone, and then dried under high vacuum at
120 °C for 24 h to remove solvent traces. Yield: 0.293 g, 76.1 %. Anal. Calcd. for C12H9BiS2
(formula of one repeat unit): C, 33.81; H, 2.13; S, 15.04. Found: C, 33.96; H, 2.12; S, 15.18.
Synthesis of Bi-OM-2. This inorganicꢀorganic hybrid material was synthesized in the same way
as Bi-OM-1, but using benzeneꢀ1,3ꢀdithiol instead of the para isomer. The Bi-OM-2 product was
isolated as a yellow solid. Yield: 0.220 g, 56.1 %. Anal. Calcd. for C12H9BiS2 (formula of repeat
unit): C, 33.81; H, 2.13; S, 15.04. Found: C, 33.35; H, 2.18; S, 14.46.
Synthesis of Bi-OM-3. This material was synthesized in the same manner as Bi-OM-1, but using
biphenylꢀ4,4ꞌꢀdithiol in place of benzeneꢀ1,4ꢀdithiol. Bi-OM-3 was isolated as a yellow solid.
Yield: 0.271 g, 73.8 %. Anal. Calcd. for C18H13BiS2 (formula of repeat unit): C, 43.03; H, 2.61; S,
12.76. Found: C, 44.91; H, 2.65; S, 14.08.
7

Journal of Materials Chemistry C
Page 8 of 26
View Article Online
DOI: 10.1039/C7TC03277C
Synthesis of 2,3,5,6-tetrafluorobenzene-1,4-dithiol (2). The twoꢀstep synthesis of 2 was
performed according to a published procedure,62 starting from hexafluorobenzene (1.86 g, 10.0
mmol). Yield over two steps: 0.700 g, 32.7 %.
Synthesis of Bi-OM-4. This material was synthesized in the same manner as Bi-OM-1, but using
the perfluorinated dithiol 2 instead of benzeneꢀ1,4ꢀdithiol. Bi-OM-4 was isolated as a yellow solid
(which turns yellowꢀorange at elevated temperatures). Yield: 0.268 g, 84.8 %. Anal. Calcd. for
C12H5BiF4S2 (formula of repeat unit): C, 28.93; H, 1.01; S, 12.87. Found: C, 29.02; H, 1.07; S,
12.91.
Synthesis of benzene-1,4-diselenol (3). The synthesis of 3 starting from 1,4ꢀdibromobenzene was
adapted from a literature procedure,63 with several key modifications to work around the airꢀ
sensitive nature of the diselenol. First, 1,4ꢀdibromobenzene (1.65 g, 7.00 mmol) was first
dissolved in THF (60 mL) and cooled to ꢀ78 °C. A solution of 1.7 M tertꢀbutyllithium in pentane
(17.3 mL, 29.4 mmol) was then added via syringe to the cooled dibromide solution. The solution
was stirred for 2 h at ꢀ78 °C and then warmed to 0 °C for 30 min. Black selenium powder (1.10 g,
14.0 mmol) was then added over 30 min, before reꢀcooling the stirred reaction mixture down to ꢀ
78 °C. At this point, acetyl chloride (1.10 mL, 16.1 mmol) was introduced via syringe. The
resulting solution was allowed to warm to room temperature and stirred for 14 h. This was
partially purified by extraction (3 x 50 mL dichloromethane), evaporated to dryness to yield crude
product, which was purified by column chromatography on silica with hexane/dichloromethane
(2:1 v/v) as the eluent followed by recrystallization from hexane to give pure bisꢀselenoester as
pink crystals (yield: 1.21 g, 54.0 %). Next, a twoꢀneck roundꢀbottom flask was charged with the
bisꢀselenoester (0.096 g, 0.30 mmol) and pꢀtoluenesulfonic acid monohydrate (0.006 g, 0.03
mmol), thoroughly purged of air, and placed under nitrogen atmosphere. Rigorously degassed
toluene (25 mL) and absolute ethanol (0.040 mL, 0.72 mmol) were added, and the mixture was
8

Page 9 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
heated at 80 °C for 8 h. During this time, a yellow solution containing benzeneꢀ1,4ꢀdiselenol was
produced. The diselenol was not isolated, but used directly in the next step for the synthesis of Bi-
OM-5.
Synthesis of Bi-OM-5. Bi-OM-5 was synthesized from triphenylbismuth and benzeneꢀ1,4ꢀ
diselenol in a similar manner as Bi-OM-1, but using a lower reaction temperature (i.e. 25 °C). The
abovementioned yellow solution containing the in situ generated benzeneꢀ1,4ꢀdiselenol was first
cooled to room temperature, after which a solution of triphenylbismuth (0.088 g, 0.20 mmol) in
degassed toluene (10 mL) was introduced. The reaction mixture was then allowed to stir for 72 h
at room temperature. A red precipitate of Bi-OM-5 was formed over the three days of reaction.
The brickꢀred Bi-OM-5 was filtered off, washed thoroughly with hot acetone, and dried under
high vacuum at 120 °C for 24 h. Yield: 0.072 g, 64 %. Anal. Calcd. for C12H9BiSe2 (formula of
repeat unit): C, 27.71; H, 1.74. Found: C, 27.90; H, 1.78.
RESULTS AND DISCUSSION
The BiꢀOMs in the present study feature Bi(III) nodes connected by rigid aromatic
dithiolate πꢀlinkers to form a network structure. Aromatic dithiols were selected as the organic
linkers with the idea of creating a structure based on strong Bi‒S bonds and also in view of the
favorable πꢀconjugation and electronic delocalization between the sulfur lone pairs and benzene
rings.44 Owing to the affinity of sulfur for soft metals, such dithiols have been used as molecular
64
contacts in singleꢀmolecule conductance studies44,
and as πꢀconjugated linkers between
inorganic semiconductor nanocrystals.65 These dithiol building blocks are available commercially
or via straightforward syntheses. The bismuth source for the synthesis of our BiꢀOMs was
triphenylbismuth, which can be inexpensively purchased or conveniently synthesized in one step
from a bismuth(III) halide and phenyllithium. Employing triphenylbismuth as the metal source has
9

Journal of Materials Chemistry C
Page 10 of 26
View Article Online
DOI: 10.1039/C7TC03277C
the advantage of generating a volatile byꢀproduct (i.e. benzene) that is easily removed from the Biꢀ
OMs. Our chosen protocol involving the reaction of aromatic dithiols with triphenylbismuth was
adapted from the preparation of bismuth(III) thiolates by Norman et al.66, 67 and Gilman.68, 69 As
pointed out, this method allowed for the construction of the target materials while producing
benzene as the sole byꢀproduct. This facilitates subsequent workup and purification as the
precipitated BiꢀOMs can be separated from the organicꢀsoluble precursors and byꢀproducts by
simple filtration. Remaining traces of benzene and toluene (reaction solvent) were completely
removed by first washing the BiꢀOMs in hot acetone, followed by drying at 120 °C under high
vacuum for at least 24 h. The elemental ratios of Bi-OMs 1‒5 were characterized by
microanalysis, and their compositions were further confirmed by energy dispersive Xꢀray
spectroscopy (EDS) (Figure S12‒S16).
Powder Xꢀray diffraction (PXRD) experiments revealed that all five BiꢀOMs possessed
bulk crystallinity (Figure 2). For example, Bi-OM-1 showed a wellꢀdefined diffraction pattern
with a large peak observed at 2θ (Cuκα radiation) of ~10.2°, mediumꢀintensity peaks at ~12.5°,
~15.0°, ~21.0°, and ~24.5°, along with several other lowerꢀintensity peaks (Figure 2a). The
experimental PXRD pattern was in good agreement with the simulated pattern (Figure S17).
Similarly, wellꢀdefined patterns were also observed with BiꢀOMs 3 and 4. In general, better
crystallinity and longꢀrange order was obtained when linear arenedithiol linkers were employed
(i.e. in BiꢀOMs 1, 3, and 4) instead of the bent metaꢀarenedithiols of Bi-OM-2 or the diselenolates
of Bi-OM-5. Scanning electron microscopy (SEM) studies were also carried out on Bi-OMs 1 ‒ 5.
The SEM images of Bi-OMs 1, 3, and 4 (Figures 2c, i, and l) revealed morphologies resembling
rigid, linear rodꢀlike crystallites. Among the more salient features of the highꢀmagnification SEM
image of Bi-OM-1 (Figure 2b) were the micrometerꢀscale rectangular rods with sharp faces,
edges, and corners. Bi-OM-2, which bears a structure isomeric to Bi-OM-1 but featuring bent
10

Page 11 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
metaꢀdisubstituted linkers rather than linear para connectors, displayed a morphology that was
instead more reminiscent of flexible bentꢀfibers. Finally, the SEM images of diselenolateꢀderived
Bi-OM-5 (Figure 2n, o) showed plateꢀlike morphologies with relatively smooth surfaces, as
opposed to the rod/beltꢀlike appearance of the sulfurꢀbased analogs.
11

Journal of Materials Chemistry C
Page 12 of 26
View Article Online
DOI: 10.1039/C7TC03277C
Figure 2. PXRD diffraction patterns and SEM images of BiꢀOMs 1 through 5.
The spectroscopic properties of the five BiꢀOMs were characterized by attenuated total
reflectance (ATR) FTIR spectroscopy and solidꢀstate UVꢀvis spectroscopy. The absence of signals
arising from SꢀH stretches (~2500ꢀ2600 cmꢀ1) in the FTIR spectra of Bi-OMs 1 through 4
indicated the absence of dithiol starting materials in the products (Figure 3). Absorption signals
around 3050 cmꢀ1 due to aromatic C‒H stretches were prominent in all the BiꢀOMs except Bi-
OM-4, where the peaks were less pronounced due to the high percentage of perfluorinated
aromatic rings. With Bi-OM-4, strong absorption peaks around ~1400 cmꢀ1 arising from the
aromatic C‒F bonds were observed instead. In all five samples, the absence of signals around
2900 cmꢀ1 due to aliphatic C‒H groups indicated the complete removal of residual toluene.
Figure 3. ATR FTIR spectra of Bi-OMs 1 through 5.
12

Page 13 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
The BiꢀOMs are colored materials ranging from yellowꢀorange to brickꢀred, with optical
HOMOꢀLUMO gaps measuring between 1.9 to 2.5 eV depending on the functionalization of their
organic πꢀlinkers. All five materials also exhibited reversible thermochromism in the solidꢀstate.
Bi-OM-1 is a yellowꢀorange solid at room temperature, and its UVꢀvis diffuse reflectance
spectrum revealed an absorption edge of about 530 nm, which corresponds to an estimated optical
gap of about 2.28 eV (Figure 4).
Figure 4. Solidꢀstate UVꢀvis (diffuse reflectance) spectra of Bi-OMs 1‒5.
The measured optical properties and observed colors of BiꢀOMs 1‒4 were consistent with
charge transfer (CT) behavior from electronꢀrich sulfur atoms to the bismuth centers (i.e.,
S→Bi).39ꢀ43 In particular, the absorptions between 350ꢀ450 nm are characteristic of S‒Bi ligandꢀ
toꢀmetal chargeꢀtransfer (LMCT) bands. When the benzeneꢀ1,4ꢀdithiolate linkers of Bi-OM-1
were replaced with the isomeric and crossꢀconjugated meta analogs, the resulting Bi-OM-2
showed a slight widening of its HOMOꢀLUMO gap to 2.4 eV due to the disruption in πꢀ
13

Journal of Materials Chemistry C
Page 14 of 26
View Article Online
DOI: 10.1039/C7TC03277C
conjugation. Attenuation of its absorption coefficient relative to Bi-OM-1 was also consistent with
the diminished oscillator strengths of crossꢀconjugated πꢀsystems versus fully conjugated ones.70
The optical gap of the bismuthꢀorganic system could also be widened in alternative ways without
incurring the same large decrease in absorption coefficient. This was demonstrated in two ways,
the first employing a steric approach (i.e. with Bi-OM-3) and the second using an electronic
induction strategy (i.e. Bi-OM-4). In both cases, the linear, fully πꢀconjugated connectivity was
maintained. In the case of Bi-OM-3, employing the longer ditopic biphenylꢀ4,4ꞌꢀdithiolate πꢀlinker
widened the HOMOꢀLUMO gap to 2.5 eV. The larger energy gap of Bi-OM-3 compared to Bi-
OM-1 can be attributed to diminished πꢀconjugation resulting from stericallyꢀinduced interannular
twisting between the two rings of the biphenyl linker.44 The optical HOMOꢀLUMO gap could also
be widened using an inductive approach as in the case of Bi-OM-4. When electronꢀdeficient
perfluorinated benzeneꢀ1,4ꢀdithiolate linkers were used in place of the nonꢀfluorinated versions, a
lemon yellowꢀcolored Bi-OM-4 with a HOMOꢀLUMO gap of 2.5 eV was obtained. Here, the
strong inductive effect of the electronegative fluorines lowers the electron density on the
connector S atoms and reduces the degree of chargeꢀtransfer from sulfur to bismuth. Finally, we
investigated the possibility of narrowing the optical gap of the BiꢀOM by substituting the sulfur
atoms of Bi-OM-1 with selenium. An analogous trend is known from the bismuth(III)
chalcogenide series (i.e. Bi2X3 where X = S, Se, Te), where heavy atom substitution results in
narrowing of the HOMOꢀLUMO energy gap. Our asꢀsynthesized seleniumꢀbased Bi-OM-5
showed a pronounced bathochromic shift in its solidꢀstate UVꢀvis absorption compared with the
sulfur analogs. This was a red material with an optical gap measuring around 1.94 eV, the smallest
of the five BiꢀOMs. Comparing Bi-OM-5 and Bi-OM-1, one observes that replacing the sulfurs
with heavier chalcogens results in optical gap reduction. Modulating the optical and/or electronic
properties of functional materials by chalcogen replacement is a strategy that has also been
14

Page 15 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
successfully implemented with transition metal dichalcogenides, covalentꢀorganic frameworks,
and linear πꢀconjugated polymers.71, 72
Interestingly, all five BiꢀOMs displayed stable and reversible solidꢀstate thermochromism,
becoming more intensely colored at elevated temperatures (i.e. above 90 °C) and reverting to their
original appearance upon cooling to 25 °C (Figure 5). Starting off as a yellowꢀorange solid at
room temperature, Bi-OM-1 gradually turned red on heating to 100 °C, before reverting to its
original state when cooled. The other four BiꢀOMs also exhibited similar chromic phenomena
when heated: Bi-OM-2 (yellow to pale orange), Bi-OM-3 (yellow to orange), Bi-OM-4 (yellow to
orange), Bi-OM-5 (brickꢀred to dark/intense red). Reversible thermochromism was also observed
when the materials were cooled instead of heated. When the external temperature of the BiꢀOMs
was lowered to –78 °C in a dryꢀice/acetone bath, they gradually adopted lighter colorations. Upon
returning to 25 °C, their original colors were subsequently restored.
Figure 5. Reversible solidꢀstate thermochromism as exhibited by Bi-OMs 1‒5.
To rule out thermochromic behavior based on a dehydrationꢀrehydration mechanism,73, 74
we performed thermogravimetric analyses (TGA) on the five BiꢀOMs (Figures S7‒S11). When
heated in air, the TGA curves showed zero mass loss for any of the BiꢀOMs at temperatures below
260 °C. In particular, there was no weight decrease between 90ꢀ110 °C corresponding to water
loss. This was further supported by the complete absence of O‒H stretches in the IR spectra of the
15

Journal of Materials Chemistry C
Page 16 of 26
View Article Online
DOI: 10.1039/C7TC03277C
BiꢀOMs (Figure 3). Therefore, a thermochromic mechanism based on heatꢀinduced loss of bound
H2O and subsequent rehydration on cooling was not operative in these BiꢀOMs. The gradual
rather than abrupt heatꢀinduced color changes suggested that enhanced intramolecular CT at the
expense of intermolecular CT was taking place as a result of increasing separation between
molecular layers/chains. Such mechanisms involving the disruption of supramolecular
environments have been responsible for the gradual thermochromic transitions in some πꢀ
conjugated organic systems75 as well as metal complexes.76 Common intermolecular interactions
in organobismuth compounds include SꢁꢁꢁBi secondary bonding and/or areneꢀbismuth (π→σ*)
interactions.77 The former type of bonding has been frequently characterized in bismuth(III)
thiolates, where dimers are held together by weak intermolecular secondary bonding between S
lone pairs and Bi‒S σ*ꢀorbitals (Figure 6a).66 It is likely that the thiolateꢀbased Bi-OMs 1‒4 also
possess similar softꢀsoft interactions in the solidꢀstate (Figure 6b). The tendency of bismuth to
form secondary bonding interactions, coupled with its flexible coordination geometry and high
coordination number, often result in dense or layered nonporous BiꢀOMs.25 Through porosimetry
measurements, we ascertained that our system did indeed have low surface areas and small pore
sizes (Figure S23), which would be expected given the multitude of possible SꢁꢁꢁBi secondary
bonding modes. It is plausible that the thermallyꢀinduced disruption of weak interchain SꢁꢁꢁBi
interactions in these BiꢀOMs strengthens intrachain CT at the expense of interchain CT, leading to
the intensification of observed colors. Such competing CT interactions at the molecular level have
often been the basis of reversible thermochromic behavior in πꢀconjugated organic systems.73ꢀ75
Compared to chromic materials that are purely organic or inorganic, chromic organicꢀinorganic
hybrid materials with separate organic/inorganic components can generally be more readily fineꢀ
tuned to provide a set of tailorꢀmade properties.78
16

Page 17 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
Figure 6. (a) Looselyꢀbound centrosymmetric bismuth(III) thiolate dimer featuring long SꢁꢁꢁBi
contacts.62 (b) Proposed structure of Bi-OM-1 featuring analogous secondary bonding
interactions.
In terms of stability, the BiꢀOMs appeared to have good aqueous stability at neutral pH.
Sonicating and suspending the materials in water at pH 7 (~2 mg/10 mL) over 24 hours did not
induce any decomposition. The materials were however, found to be unstable to mineral acids.
Exposing the BiꢀOMs to strong acids led to rapid decolorization of the materials due to hydrolysis
of their bismuth‒chalcogen bonds. Finally, in regard to the thermal stability of the five BiꢀOMs,
the TGA studies described previously showed them to be very thermally resilient. Heating the Biꢀ
OMs up to 280 °C in air resulted in a mere 5% weight loss (Figure S7‒S10), in contrast to the air
and moisture sensitivity of many simple bismuth(III) thiolate complexes. Overall, the optical
properties and ambient stability of these BiꢀOMs make them amenable to further studies that will
focus on the exploration of potential practical applications.
CONCLUDING REMARKS
In summary, we have developed a series of bismuthꢀcontaining inorganicꢀorganic hybrid
materials via facile oneꢀstep reactions between triphenylbismuth and various arenedithiols (and a
17

Journal of Materials Chemistry C
Page 18 of 26
View Article Online
DOI: 10.1039/C7TC03277C
diselenol). By varying the structural and electronic nature of the πꢀconjugated organic linkers, the
optical HOMOꢀLUMO gaps and chromic phenomena associated with the BiꢀOMs could be tuned
in a reliable manner. All five BiꢀOMs reported in this study exhibited completely reversible solidꢀ
state thermochromism, and were found to possess excellent thermal stability in air. Taken
together, the combination of physical properties makes these stimuliꢀresponsive materials
potentially useful for a diverse range of applications that include reversible smart windows,
camouflage coatings, photocatalysts, optoelectronics, and temperature sensors.
ACKNOWLEDGEMENTS
This work was financially supported by the Natural Sciences and Engineering Research Council
(NSERC Discovery Grant; RGPINꢀ2016ꢀ04614), the Canada Foundation for Innovation (CFI)
John R. Evans Leaders Fund (CFIꢀJELF project no. 34474), the Ministry of Research and
Innovation (MRI), and the University of Ottawa. We also thank Janire Matas for the videography
work on our thermochromic materials (Electronic Supporting Information).
NOTES AND REFERENCES
(1) A. Zeugner, M. Kaiser, P. Schmidt, T. V. Menshchikova, I. P. Rusinov, A. V. Markelov, W.
Van den Broek, E. V. Chulkov, T. Doert, M. Ruck, Chem. Mater. 2017, 29, 1321–1337.
(2)
J. Song, J. Yuan, F. Xia, J. Liu, Y. Zhang, Y. L. Zhong, J. Zheng, Y. Liu, S. Li, M. Zhao, Z.
Tian, R. A. Caruso, K. P. Loh, Q. Bao, Adv. Electronic Mater. 2016, 2, 1600077.
(3) N. H. Tu, Y. Tanabe, Y. Satake, K. K. Huynh, K. Tanigaki, Nat. Commun. 2016, 7, 13763.
18

Page 19 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
(4) S. K. Kushwaha, I. Pletikosić, T. Liang, A. Gyenis, S. H. Lapidus, Y. Tian, H. Zhao, K. S.
Burch, J. Lin, W. Wang, H. Ji, A. V. Fedorov, A. Yazdani, N. P. Ong, T. Valla, R. J. Cava,
Nat. Commun. 2016, 7, 11456.
(5) Z. Wang, A. A. Taskin, T. Frölich, M. Braden, Y. Ando, Chem. Mater. 2016, 28, 779–784.
(6)
(7)
B. Rasche, W. Van den Broek, M. Ruck, Chem. Mater. 2016, 28, 665–672.
S. Pecqueur, A. Maltenberger, M. A. Petrukhina, M. Halik, A. Jaeger, D. Pentlehner, G.
Schmid, Angew. Chem. Int. Ed. 2016, 55, 10493–10497.
(8)
(9)
A. J. Lehner, D. H. Fabini, H. A. Evans, C.ꢀA. Hébert, S. R. Smock, J. Hu, H. Wang, J. W.
Zwanziger, M. L. Chabinyc, R. Seshadri, Chem. Mater. 2015, 27, 7137–7148.
J. Song, F. Xia, M. Zhao, Y. L. Zhong, W. Li, K. P. Loh, R. A. Caruso, Q. Bao, Chem.
Mater. 2015, 27, 3471–3482.
(10) H. Goldsmid, H. Julian, Materials 2014, 7, 2577–2592.
(11) J. Kim, W. Shim, W. Lee, J. Mater. Chem. C 2015, 3, 11999–12013.
(12) W. Liu, K. C. Lukas, K. McEnaney, S. Lee, Q. Zhang, C. P. Opeil, G. Chen, Z. Ren, G.
Chen, Z. F. Ren, Energy Environ. Sci. 2013, 6, 552–560.
(13) Y. L. Pei, H. Wu, J. Sui, J. Li, D. Berardan, C. Barreteau, L. Pan, N. Dragoe, W.ꢀS. Liu, J.
He, L. D. Zhao, Energy Environ. Sci. 2013, 6, 1750ꢀ1755.
(14) Y. Liu, W.ꢀJ. Son, J. Lu, B. Huang, Y. Dai, M.ꢀH. Whangbo, Chem. Eur. J. 2011, 17,
9342–9349.
19

Journal of Materials Chemistry C
Page 20 of 26
View Article Online
DOI: 10.1039/C7TC03277C
(15) Y. Liu, B. Huang, Y. Dai, X. Zhang, X. Qin, M. Jiang, M.ꢀH. Whangbo, Catal. Commun.
2009, 11, 210ꢀ213.
(16) A. Walsh, Y. Yan, M. N. Huda, M. M. AlꢀJassim, S.ꢀH. Wei, Chem. Mater. 2009, 21, 547–
551.
(17) G. Li, D. Zhang, J. C. Yu, Chem. Mater. 2008, 20, 3983–3992.
(18) X. Jiayue, Y. Bobo, Z. Yan, Mater. Focus 2015, 4, 20–27.
(19) J. Heine, T. Wehner, R. Bertermann, A. Steffen, K. MüllerꢀBuschbaum, Inorg. Chem. 2014,
53, 7197–7203.
(20) S. Sarkar, A. Dash, V. Mahalingam, Chem. Asian J. 2014, 9, 447–451.
(21) A. L. Brown, A. M. Goforth, Chem. Mater. 2012, 24, 1599–1605.
(22) A. L. Brown, P. C. Naha, V. BenavidesꢀMontes, H. I. Litt, A. M. Goforth, D. P. Cormode,
Chem. Mater. 2014, 26, 2266–2274.
(23) P. C. Naha, A. Al Zaki, E. Hecht, M. Chorny, P. Chhour, E. Blankemeyer, D. M. Yates, W.
R. T. Witschey, H. I. Litt, A. Tsourkas, D. P. Cormode, J. Mater. Chem. B 2014, 2, 8239–
8248.
(24) O. Rabin, J. Manuel Perez, J. Grimm, G. Wojtkiewicz, R. Weissleder, Nat. Mater. 2006, 5,
118–122.
(25) A. K. Inge, M. Köppen, J. Su, M. Feyand, H. Xu, X. Zou, M. O’Keeffe, N. Stock, J. Am.
Chem. Soc. 2016, 138, 1970–1976.
20

Page 21 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
(26) M. Feyand, M. Köppen, G. Friedrichs, N. Stock, Chem. Eur. J. 2013, 19, 12537–12546.
(27) A. C. Wibowo, M. D. Smith, H.ꢀC. zur Loye, Y. Chen, Z. Wang, G. Ferey, M. Haouas, F.
Taulelle, N. Audebrand, M. Latroche, Chem. Commun. 2011, 47, 7371ꢀ7373.
(28) G. Wang, Y. Liu, B. Huang, X. Qin, X. Zhang, Y. Dai, C. H. Tan, M. Schröder, S.ꢀP. Yan,
H.ꢀC. Zhou, Dalt. Trans. 2015, 44, 16238–16241.
(29) B. J. Deibert, E. Velasco, W. Liu, S. J. Teat, W. P. Lustig, J. Li, Cryst. Growth Des. 2016,
16, 4178–4182.
(30) M. Feyand, E. Mugnaioli, F. Vermoortele, B. Bueken, J. M. Dieterich, T. Reimer, U. Kolb,
D. de Vos, N. Stock, Angew. Chem. Int. Ed. 2012, 51, 10373–10376.
(31) M. Savage, S. Yang, M. Suyetin, E. Bichoutskaia, W. Lewis, A. J. Blake, S. A. Barnett, M.
Schröder, Chem. Eur. J. 2014, 20, 8024–8029.
(32) G. Wang, Q. Sun, Y. Liu, B. Huang, Y. Dai, X. Zhang, X. Qin, Chem. Eur. J. 2015, 21,
2364–2367.
(33) A. Thirumurugan, A. K. Cheetham, Eur. J. Inorg. Chem. 2010, 2010, 3823–3828.
(34) T. Kambe, R. Sakamoto, K. Hoshiko, K. Takada, M. Miyachi, J.ꢀH. Ryu, S. Sasaki, J. Kim,
K. Nakazato, M. Takata, H. Nishihara, J. Am. Chem. Soc. 2013, 135, 2462–2465.
(35) X. Huang, P. Sheng, Z. Tu, F. Zhang, J. Wang, H. Geng, Y. Zou, C. Di, Y. Yi, Y. Sun, W.
Xu, D. Zhu, Nat. Commun. 2015, 6, 7408.
21

Journal of Materials Chemistry C
Page 22 of 26
View Article Online
DOI: 10.1039/C7TC03277C
(36) T. Kambe, R. Sakamoto, T. Kusamoto, T. Pal, N. Fukui, K. Hoshiko, T. Shimojima, Z.
Wang, T. Hirahara, K. Ishizaka, K. J. Am. Chem. Soc. 2014, 136, 14357–14360.
(37) H. A. Phillips, N. Burford, Inorg. Chem. 2008, 47, 2428–2441.
(38) G. G. Briand, N. Burford, Adv. Inorg. Chem. 2000, 50, 285–357.
(39) B. P. Kennedy, A. B. P. Lever, Can. J. Chem. 1972, 50, 3488ꢀ3507.
(40) A. I. Petrov, N. N. Golovnev, I. D. Dergachev, A. A. Leshok, Polyhedron 2013, 50, 59–65.
(41) A. I. Petrov, I. D. Dergachev, S. V. Trubina, S. B. Erenburg, M. A. Lutoshkin, N. N.
Golovnev, A. A. Kondrasenko, V. D. Dergachev, RSC Adv. 2014, 4, 52384–52392.
(42) A. I. Petrov, N. N. Golovnev, S. V. Trubina, S. B. Erenburg, I. D. Dergachev, J. Coord.
Chem. 2013, 66, 4188–4198.
(43) H. Sun, H. Li, I. Harvey, P. J. Sadler, J. Biol. Chem. 1999, 274, 29094ꢀ29101.
(44) G. C. Solomon, D. Q. Andrews, R. P. Van Duyne, M. A. Ratner, ChemPhysChem 2009, 10,
257–264.
(45) Y. L. Chen, J. G. Analytis, J.ꢀH. Chu, Z. K. Liu, S.ꢀK. Mo, X. L. Qi, H. J. Zhang, D. H. Lu,
X. Dai, Z. Fang, S. C. Zhang, I. R. Fisher, Z. Hussain, Z.ꢀX Shen, Science 2009, 325, 178ꢀ
181.
(46) D.ꢀX. Qu, Y. S. Hor, J. Xiong, R. J. Cava, N. P. Ong, Science 2010, 329, 821–824.
22

Page 23 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
(47) Y. Cao, M. Bernechea, A. Maclachlan, V. Zardetto, M. Creatore, S. A. Haque, G.
Konstantatos, Chem. Mater. 2015, 27, 3700–3706.
(48) K. Biswas, L.ꢀD. Zhao, M. G. Kanatzidis, Adv. Energy Mater. 2012, 2, 634–638.
(49) S. Patai, Ed. The Chemistry of Organic Arsenic, Antimony, and Bismuth Compounds; John
Wiley & Sons: West Sussex, UK, 1974; pp. 441–456.
(50) H. L. Nguyen, T. T. Vu, D. Le, T. L. H. Doan, V. Q. Nguyen, N. T. S. Phan, ACS Catal.
2017, 7, 338ꢀ342.
(51) P. Sippel, D. Denysenko, A. Loidl, P. Lunkenheimer, G. Sastre, D. Volkmer, Adv. Funct.
Mater. 2014, 24, 3885ꢀ3896.
(52) C. H. Hendon, D. Tiana, M. Fontecave, C. Sanchez, L. D’arras, C. Sassoye, L. Rozes, C.
MellotꢀDraznieks, A. Walsh, J. Am. Chem. Soc. 2013, 135, 10942ꢀ10945.
(53) C.ꢀK. Lin, D. Zhao, W.ꢀY. Gao, Z. Yang, J. Ye, T. Xu, Q. Ge, S. Ma, D.ꢀJ. Liu, Inorg.
Chem. 2012, 51, 9039ꢀ9044.
(54) T. Musho, J. Li, N. Wu, Phys. Chem. Chem. Phys. 2014, 16, 23646ꢀ23653.
(55) D. E. P. Vanpoucke, J. Phys. Chem. C 2017, 121, 8014–8022.
(56) H. Q. Pham, T. Mai, N.ꢀN. PhamꢀTran, Y. Kawazoe, H. Mizuseki, D. NguyenꢀManh, J.
Phys. Chem. C 2014, 118, 4567–4577.
(57) M. De Bastiani, M. I. Saidaminov, I. Dursun, L. Sinatra, W. Peng, U. Buttner, O. F.
Mohammed, O. M. Bakr, Chem. Mater. 2017, 29, 3367–3370.
23

Journal of Materials Chemistry C
Page 24 of 26
View Article Online
DOI: 10.1039/C7TC03277C
(58) S. Herrmann, J. T. Margraf, T. Clark, C. Streb, Chem. Commun. 2015, 51, 13702–13705.
(59) A. Seeboth, D. Lötzsch, R. Ruhmann, O. Muehling, Chem. Rev. 2014, 114, 3037–3068.
(60) P.ꢀC. Guo, T.ꢀY. Chen, X.ꢀM. Ren, Z. Chu, W. Jin, J. Mater. Chem. A 2014, 2, 13698ꢀ
13704.
(61) A. P. M. Robertson, N. Burford, R. McDonald, M. J. Ferguson, Angew. Chem. Int. Ed.
2014, 53, 3480–3483.
(62) T. Umemoto, L. M. Garrick, N. Saito, Beilstein J. Org. Chem. 2012, 8, 461–471.
(63) B. de Boer, H. Meng, D. F. Perepichka, J. Zheng, M. M. Frank, Y. J. Chabal, Z. Bao,
Langmuir 2003, 19, 4272ꢀ4284.
(64) X. Xiao, B. Xu, N. J. Tao, Nano Lett. 2004, 4, 267ꢀ271.
(65) M. A. Boles, D. Ling, T. Hyeon, D. V. Talapin, Nat. Mater. 2016, 15, 141–153.
(66) K. M. Anderson, C. J. Baylies, A. H. M. Monowar Jahan, N. C. Norman, A. G. Orpen, J.
Starbuck, S. J. Clark, T. B. Marder, Dalt. Trans. 2003, 26, 3270ꢀ3277.
(67) L. J. Farrugia, F. J. Lawlor, N. C. Norman, Polyhedron 1995, 14, 311–314.
(68) A. G. Davies, S. C. W. Hook, J. Chem. Soc. B 1970, 735ꢀ737.
(69) H. Gilman, H. L. Yale, J. Am. Chem. Soc. 1951, 73, 2880ꢀ2881.
(70) G. W. P. van Pruissen, J. Brebels, K. H. Hendriks, M. M. Wienk, R. A. J. Janssen,
Macromolecules 2015, 48, 2435–2443.
24

Page 25 of 26
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03277C
(71) S. Duhović, M. Dincă, Chem. Mater. 2015, 27, 5487–5490.
(72) S.ꢀH. Su, Y.ꢀT. Hsu, Y.ꢀH. Chang, M.ꢀH. Chiu, C.ꢀL. Hsu, W.ꢀT. Hsu, W.ꢀH. Chang, J.ꢀH.
He, L.ꢀJ. Li, Small 2014, 10, 2589–2594.
(73) T. Yuan, M. Vazquez, A. N. Goldner, Y. Xu, R. Contrucci, M. A. Firestone, M. A. Olson,
L. Fang, Adv. Funct. Mater. 2016, 26, 8604–8612.
(74) J. S. Moore, S. I. Stupp, Macromolecules 1986, 19, 1815–1824.
(75) J. H. Lee, P. Naumov, I. H. Chung, S. C. Lee, J. Phys. Chem. A 2011, 115, 10087–10096.
(76) G. Mehlana, C. Wilkinson, G. Ramon, S. A. Bourne, Polyhedron 2015, 98, 224ꢀ229.
(77) A. A. Auer, D. Mansfeld, C. Nolde, W. Schneider, M. Schürmann, M. Mehring,
Organometallics 2009, 28, 5405–5411.
(78) G. Mehlana, S. A. Bourne, CrystEngComm 2017, 19, 4238ꢀ4259.
25

Journal of Materials Chemistry C
Page 26 of 26
View Article Online
DOI: 10.1039/C7TC03277C
Table of Contents Entry:
Reversible thermochromism in color tunable bismuth‒organic hybrid materials