Alkyl chain length effects on solid-state difluoroboron β-diketonate mechanochromic luminescence

Article abstract of DOI:10.1039/c1jm00067e

Solid-state difluoroboron β-diketonate dyes display reversible mechanochromic luminescence (ML). To test the effects of alkyl chain length on solid state photoluminescence and ML, a series of dyes, BF₂dbmOR, with dibenzoylmethane (dbm) ligands and alkoxyl substituents (-OR) were prepared, where R = C_nH_2n+1 and n = 1, 2, 3, 5, 6, 12, 14, 16, 18. Emission properties were investigated in solution and in the solid state. Fluorescence spectra and lifetimes were nearly identical for dyes in CH₂Cl₂ solution; whereas, in the solid state, as powders, thin films or spin cast films, emission maxima, and lifetimes were different among the samples. Solid-state ML emission spectra were monitored at room temperature as a function of time for smeared powders on quartz surfaces. The recovery time generally increased with alkyl chain length, ranging from minutes (n = 3) to days (n = 18). Longer chain analogues (n = 6, 12, 14, 16, 18) did not fully return to the original annealed emissive state even after months on quartz, though the dynamics are substrate dependent. Solid-state dyes were also investigated by XRD and DSC (powders), and by AFM (spin cast films).

Full text of DOI:10.1039/c1jm00067e

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PAPER
Alkyl chain length effects on solid-state difluoroboron b-diketonate
mechanochromic luminescence†
a
a
b
a
Nguyen D. Nguyen, Guoqing Zhang, Jiwei Lu, Arianna E. Sherman and Cassandra L. Fraser
a
*
Received 6th January 2011, Accepted 4th February 2011
DOI: 10.1039/c1jm00067e
Solid-state difluoroboron b-diketonate dyes display reversible mechanochromic luminescence (ML).
To test the effects of alkyl chain length on solid state photoluminescence and ML, a series of dyes,
BF2dbmOR, with dibenzoylmethane (dbm) ligands and alkoxyl substituents (–OR) were prepared,
where R ¼ CnH2n+1 and n ¼ 1, 2, 3, 5, 6, 12, 14, 16, 18. Emission properties were investigated in solution
and in the solid state. Fluorescence spectra and lifetimes were nearly identical for dyes in CH2Cl2
solution; whereas, in the solid state, as powders, thin films or spin cast films, emission maxima, and
lifetimes were different among the samples. Solid-state ML emission spectra were monitored at room
temperature as a function of time for smeared powders on quartz surfaces. The recovery time generally
increased with alkyl chain length, ranging from minutes (n ¼ 3) to days (n ¼ 18). Longer chain
analogues (n ¼ 6, 12, 14, 16, 18) did not fully return to the original annealed emissive state even after
months on quartz, though the dynamics are substrate dependent. Solid-state dyes were also investigated
by XRD and DSC (powders), and by AFM (spin cast films).
in-depth, predictive understanding of this phenomenon is still
limited. Further study of the relationship between molecular
structure and mechanochromic behavior is merited.1
Introduction
Mechanochromic luminescence (ML) refers to the phenomenon
whereby luminescent materials change their emission colors in
response to mechanical stimuli such as grinding, smearing, and
pressing.1 Such phenomena have been observed with liquid-crys-
talline2–4 and crystalline solids,5–12 as well as dye-doped poly-
mers.13–16 The causative mechanisms have been attributed to
mechanically induced structural changes in the solid-state molec-
ular assemblies. For example, Sagara et al. reported that a liquid-
crystal pyrene derivative has yellow emission in the cubic phase,
but switches to blue shifted emission upon mechanically induced
columnar phase formation, creating different hydrogen bonding
patterns and p–p stacking interactions.3 More commonly,
mechanical switching between ordered and amorphous states
results in changes in optical properties.5,9 In another study, alkyl
substituent effects on cyano-substituted oligo(p-phenylene vinyl-
ene) derivatives (cyano-OPVs) were noted. Cyano OPVs with C12
and C18 alkoxyl substituents possessed ML but a methoxy
analogue did not.6 Even though mechanisms for ML have been
proposed and compelling evidence has been shown in some cases,
Difluoroboron diketonates (BF2bdks) are classic dyes pos-
sessing impressive luminescent properties such as large extinction
coefficients,17–19 two-photon absorption cross-sections,20 high
fluorescence quantum yields,17–19 and unusual room-temperature
phosphorescence21 (RTP) in solid-state media (e.g. polymers).
Due to these many interesting properties, difluoroboron diket-
onate derivatives have been exploited as tumor hypoxia imaging
agents,22 anion receptors,23 and NIR probes for amyloid-
b deposits in Alzheimer’s disease.24 In addition, the solid-state
photophysical properties of BF2bdks have been studied by
Mirochnik et al.,25 who have shown that these dyes display
size-dependent luminescent and thermochromic properties.
Recently, we discovered that BF2AVB (AVB ¼ avobenzone)
possesses polymorphism and reversible mechanochromic lumi-
nescence. BF2AVB crystals possess different emission colors
depending on the crystal types and processing conditions: blue
(dendritic), cyan (needle) and green (prism). When crystals are
crushed and solid samples are smeared, the emission color shifts
significantly to yellow-orange. Surprisingly, smeared materials
can spontaneously revert back to the original green-blue emis-
sion at room temperature, or alternatively, the process may be
facilitated by thermally annealing (TA).26 Similar observations
have been noted for BF2dbmOC12H25 (C12) analogues.27,28
Smearing the C12 powder on weighing paper red-shifted the
emission color of the dyes, but upon thermally annealing, the
emission color significantly blue-shifted.27 Interestingly, a similar
dye, BF2dbmOCH3 (C1), does not possess mechanochromism;
aDepartment of Chemistry, University of Virginia, McCormick Road,
Charlottesville, VA, 22904, USA. E-mail: fraser@virginia.edu; Fax: +1
434-924-3710; Tel: +1 434-924-7998
bDepartment of Materials Science, University of Virginia, McCormick
Road, Charlottesville, VA, 22904, USA. E-mail: jl5tk@virginia.edu
† Electronic supplementary information (ESI) available: Synthesis, 1H
NMR data, detailed lifetime data, representative absorption and
emission spectra in CH2Cl2, additional solid-state emission spectra, and
DSC data. See DOI: 10.1039/c1jm00067e
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the emission spectra stayed the same upon smearing or
thermally annealing. Notably, the structures of BF2AVB,
BF2dbmOC12H25, and BF2dbmOCH3 only differ in the alkyl
substituent: tert-butyl, C12 alkyl chain and a methyl group.
These observations inspired us to conduct a systematic study of
the effects of the alkyl chain length on the solid state optical
properties and mechanochromic behavior of BF2dbm. In this
account, a series of BF2dbmOR derivatives (where R ¼ CnH2n+1
and n ¼ 1, 2, 3, 5, 6, 12, 14, 16, 18) were synthesized and the
luminescent properties in the solution and solid-state (powders,
thin films and spin cast films) were studied by fluorescence
spectroscopy and lifetime measurements. (Note: C1 and C12
were prepared and investigated again here for reference.) We also
investigate the solid-state luminescence and ML phenomenon
via XRD and AFM. For simplicity, we will refer to these dyes as
Cn, where n is the number of carbon units on the alkoxyl chain.
monochromator). Time-correlated single-photon counting
(TCSPC) fluorescence lifetime measurements were performed
with a NanoLED-370 (lex ¼ 369 nm) excitation source and
a DataStation Hub as the SPC controller. Lifetime data were
analyzed with DataStation v2.4 software from Horiba Jobin
Yvon. Fluorescence quantum yields, FF, in CH2Cl2 were calcu-
lated versus anthracene in EtOH as a standard as previously
described29 using the following values: FF anthracene ¼ 0.27,30
20
20
nD EtOH ¼ 1.360, nD CH2Cl2 ¼ 1.424. Optically dilute
CH2Cl2 solutions of Cn and the EtOH solution of the anthracene
standard were prepared in 1 cm path length quartz cuvettes with
absorbances <0.1 (a.u.). For ML recovery time measurements,
solid-state fluorescence spectra (lex ¼ 365 nm) were recorded on
a Horiba Fluorolog-3 Model FL3-22 spectrofluorometer equip-
ped with a Tiffen 58 mm neutral density 0.6 filter to maximize the
surface area while preventing photobleaching. The dyes (ꢂ3 mg)
were rubbed onto the outside surface of a quartz cuvette with
a cotton swab tip, and were thermally annealed in an oven at 110
ꢀC for 5 min. After cooling for ꢂ5 min, emission spectra were
recorded, the thin dye layers were smeared with a fresh cotton
swab tip, and emission spectra were recorded as a function of
time. X-Ray diffraction (XRD) (Smart-Lab, Rigaku Inc.)
(Cu Ka source, 0.154 nm) was used to determine the crystallinity
of annealed and smeared samples and all the measurements were
performed at room temperature. The sample morphologies for
spin-cast films were characterized by Atomic Force Microscopy
(AFM) (Digital Image, DI 3000) in tapping mode. The scan area
was 20 mm ꢃ 20 mm with a scan rate of 0.8 Hz.
Experimental
Results and discussions
Materials
Synthesis and solution optical properties
Solvents CH2Cl2 and THF were dried and purified by passage
through alumina columns. 4-Ethoxyacetophenone, BF3$Et2O
(purified, redistilled), and all other chemicals were reagent grade
from Sigma-Aldrich and were used as received without further
purification. Synthesis and characterization data for C1–C18 are
provided in the ESI†.
BF2dbmOR were synthesized via standard Claisen condensation
of the appropriate alkoxyacetophenone with methyl benzoate,
followed by boronation in CH2Cl2, purification by column
chromatography and recrystallization with 1 : 1 CH2Cl2/
hexanes. All the boron dyes were obtained as yellow powders
with solid-state emission colors ranging from yellow to cyan
upon UV excitation (lex ¼ 365 nm) (Fig. 1a). In CH2Cl2 solution,
all the samples displayed absorption maxima at 399 ꢄ 1 nm and
exhibited high extinction coefficients, ranging from 50 100 to
66 200 (Mꢁ1 cmꢁ1), similar to related boron diketonate
dyes.17,18,21 They showed emission maxima at 435 ꢄ 1 nm (fwhm
z 52 nm), high quantum yields (FF > 0.82), and similar fluo-
rescence lifetimes (ꢂ2.0 ns) that were fitted to single exponential
decay (Fig. S1 and Table S1†). In summary, all these dyes
behaved similarly, thus indicating that increasing the alkyl chain
length on the alkoxyl group did not substantially affect the
photophysical properties in solution.
Methods
1H NMR (300 MHz) spectra were recorded on a UnityInova 300/
1
51 instrument in CDCl3 unless indicated otherwise. H NMR
spectra were referenced to the signal for chloroform residual
proton at 7.26 ppm and coupling constants are given in Hertz.
Melting and crystallization temperatures, Tm and Tc, were
recorded on a TA Instruments Modulated DSC 2920 (scanning
rate 5 ꢀC minꢁ1). A Laurel Technologies WS-650S spin-coater
was used to cast Cn (n ¼ 1, 2, 3, 5, 6, 12, 14, 16, 18) films at 3000
rpm for steady-state luminescence spectroscopy and AFM
studies. The films were fabricated by preparing 1 mg mLꢁ1
solutions of Cn in CH2Cl2, followed by adding 10–11 drops of the
solution onto a circular microscope cover glass (d ¼ 25 mm). The
spin cast films were then dried in vacuo for an additional ꢂ15 to
30 min before measurement. Photographs were taken with
a Canon PowerShot SD780 IS Digital ELPH. UV/vis spectra
were recorded on a Hewlett-Packard 8452A diode-array spec-
trophotometer. Steady-state fluorescence emission spectra were
recorded on a Horiba Fluorolog-3 Model FL3-22 spectrofluo-
rometer (double-grating excitation and double-grating emission
Solid-state optical properties
Powders. In contrast to the solution properties, the solid-state
photoluminescent properties varied significantly when altering
the alkoxyl chain length. First, emission properties of powders
obtained from synthesis after recrystallization from 1 : 1
CH2Cl2/hexanes were investigated. Fluorescence spectra and
lifetimes are shown in Fig. 1c and Table 1 respectively. The dye
series showed a trend from yellow to cyan with increasing alkyl
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However, for C2–C18, much shorter lifetimes were observed
32
(spwo ¼ 3–7 ns), suggesting different dye emissive species. Light
reabsorption or light scattering associated with the bulk
powder,33–35 and aggregation effects (e.g. ground state
structures)36 may be contributing to these differences. This is
supported by XRD studies for select powder samples that show
evidence of crystallinity (Fig. 4, green lines), with C6, C12, and
C18 displaying very similar features (i.e. strongest peaks at ꢂ25ꢀ,
ꢂ19ꢀ, and ꢂ6ꢀ 2q), and a different diffraction pattern for the
smaller C2, suggesting different crystal packing. The sample
thickness and processing can also play a role. This has been seen
previously for BF2AVB, where different fabrication methods and
associated solid-state morphologies give rise to different optical
properties.26 Thus, films fabricated by slow evaporation (SE) and
spin-casting (SC) methods were also investigated for C1–C18.
Films by slow evaporation (SE). Films of C1–C18 were formed
by slow evaporation of CH2Cl2 dye solutions (2 mg mLꢁ1) in
small glass vials. Visual inspection through a magnifying glass
revealed crystalline morphologies on the sides of the vials. The
C1 SE film exhibited yellow emission, C2 and C12–C18, blue,
and C3–C6, yellow-green upon UV excitation (lex ¼ 365 nm)
(Fig. 1b). Spectral data are provided in Fig. 1d and Table 1.
Previously Mirochnik showed that C1 crystals display excimer
fluorescence,31 which is similar to C1 data here. With the
exception of C2 which is considerably blue shifted relative to the
powder, all other Cn SE films had nearly identical emission
maxima to powder samples, with emission maxima blue shifting
with increasing alkoxyl chain length. Typical for heterogeneous
solid-state samples, lifetime measurements for C2–C18 show
multiexponential decays, with pre-exponential weighted lifetimes
(spw0)
32 ranging from 2–6.5 ns. Again, a much longer lifetime was
measured for C1 (spw0 ¼ ꢂ36 ns), ascribed to excimer forma-
tion31 (Tables 1 and S2†). In summary, luminescence properties
are similar for crystalline powders and SE films, with the
exception of C2 which displays sensitivity to these fabrication
conditions.
Fig. 1 Images of (a) powders obtained from recrystallization and (b)
thin films formed by slow evaporation (SE) of 2 mg mLꢁ1 solution of
Cn in CH2Cl2. Normalized emission spectra for powders are shown in (c)
and for thin films in (d) (lex ¼ 365 nm).
chain length; however, at C12, samples appeared to reach
a threshold, given that emission maxima of ꢂ473 nm were
observed for C12–C18, even as the alkoxyl chain length
increased. For C1, the long lifetime (30.94 ns) and broad,
structureless band have been attributed to excimer emission.31
Spin-cast (SC) films. To obtain even thinner, more amorphous
films, 1 mg mLꢁ1 CH2Cl2 dye solutions were spin-cast at 3000
rpm onto glass substrates. Images and steady-state fluorescence
spectra for representative as-spun (i.e. unannealed (UA))
samples are shown in Fig. 2 and a full set of data is included in
the ESI (Fig. S2†). All CnUA film spectra showed broad,
featureless bands with red shifted emission (lem z 510 nm,
except for C3, lem ¼ 523 nm) and long lifetimes (spw0 range ¼
15–32 ns) that could be fitted to multiexponential decays. Like
C1 powders and SE films and previously reported spin cast boron
bdk dyes,26,27 these observations point to excimeric species in the
amorphous UA films.37 Unannealed spin-cast film emission
maxima for C3–C18UA did not follow the chain length depen-
dent trend observed for crystalline powders and SE films. Here,
C5UA and C14UA were the most blue-shifted analogues.
Table 1 Fluorescence properties of solid-state Cn dyes
Spin-cast films
a
a
a
a
Powder lem
)
Thin films lem
)
UA lem
(spw0
TA lem
b
(spw0 )
b
b
b
(spw0
(spw0
)
C1
C2
C3
C5
550 (30.94)
507 (5.13)
500 (4.22)
496 (7.67)
489 (6.21)
473 (3.29)
476 (3.90)
471 (3.91)
473 (3.05)
544 (35.91)
473 (6.21)
501 (4.72)
491 (5.82)
483 (6.33)
474 (6.53)
480 (4.63)
475 (5.22)
479 (2.18)
544 (44.35)
499 (11.18)
523 (31.56)
511 (25.54)
506 (14.58)
504 (19.96)
510 (26.68)
507 (19.71)
518 (30.56)
540 (40.85)
497 (9.31)
482 (6.31)
453 (4.22)
478 (5.40)
466 (6.43)
457 (4.95)
470 (3.11)
472 (3.48)
C6
Because heating typically results in more ordered morphol-
ogies for this family of dyes,26–28,36 thermally annealed SC films
were also investigated. Heating was conducted at 110 ꢀC for
5 minutes (i.e. below the melting temperatures, Tm, for the dyes
which range from 126–188 ꢀC; Table S4†). A previous study
showed that C12 as-spun films (i.e. unannealed (UA)) displayed
C12
C14
C16
C18
a
b
Units are given in nm. Pre-exponential weighted lifetimes.30
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bandwidth emission corresponding to one of the BF2AVB crystal
forms (i.e. dendritic, lem ¼ 459 nm, fwhm ¼ 31 nm) reported
previously.26 Coincidentally, these two compounds possess the
same molecular weight.
The film morphologies before and after heating were explored
by atomic force microscopy (AFM) for select samples with
notable emission behavior: C2UA/TA showed no difference,
C3UA displayed the most red-shifted maximum, C5TA was most
blue-shifted with the sharpest peak compared to other dyes, and
finally, C18 is the longest chain analogue, for comparison
(Fig. 3). For C2, there is little change upon thermal treatment;
the crystal size increased slightly and the shape remained similar
throughout, consistent with C2UA/TA emission data. On the
other hand, C3UA, C5UA and C18UA films all displayed largely
Fig. 2 Images and steady state fluorescence spectra of unannealed (UA)
and thermally annealed (TA) spin-cast films for (a) C2, (b) C3, (c) C5, (d)
C6, (e) C12, and (f) C18. Thermally annealing ¼ 110 ꢀC for 5 min.
red shifted emission maxima compared to thermally annealed
(TA) films.27 The difference in spectral data from UA to TA has
been ascribed to the transition from amorphous to ordered states
after heat treatment. Similar results were also observed for other
Cn dyes, with variations in spectral shift across the series.
The emission color and spectra of C1 and C2 did not change
when as-spun films were thermally annealed; however, the band
features were different among the two compounds. While C1
films displayed a broad, featureless band at ꢂ544 nm (spw0 ¼ ꢂ44
ns and close to single exponential), C2 films possessed narrower
band emission with blue-shifted maxima (ꢂ497 nm) and much
shorter lifetimes (spw0 ¼ ꢂ9 ns) (Table 1, Fig. 2 and S2†). In
contrast, for C3TA–C18TA, thermal annealing induced ꢂ30 to
60 nm hypsochromic shifts and TA samples possessed shorter
lifetimes (spw0 ¼ ꢂ3 to 6 ns) that could also be fitted to double or
triple exponentials. Although it is not clear why the C5 sample
should be unique, it exhibited distinctive luminescence proper-
ties, namely, a very sharp peak at 453 nm (fwhm ¼ 50 nm).
Interestingly, this is similar to the strongly blue shifted, narrow
Fig. 3 AFM images of unannealed (left-UA) and thermally annealed
(TA-right) spin-cast films of C2, C3, C5, and C18.
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amorphous morphologies that became much more crystalline
upon heating. C3TA crystals are equiaxed; whereas, C5TA and
C18TA are more needle-like in habit. The crystal size generally
increased with chain length for the samples explored. In
summary, AFM data support the notion that differences in
emission spectra between UA and TA spin-cast films are ascribed
to the transition from more amorphous, disordered states to
more ordered, crystalline states. This is similar to other difluor-
oboron diketonate dyes studied so far.26,27,36
surfaces with a cotton swab and then the thin transparent coat-
ings were thermally annealed. Starting spectra of the TA samples
were recorded, then the thin coatings were smeared and moni-
tored over time by luminescence spectroscopy. Compared to TA
samples, shearing red-shifted and broadened the emission and
much longer lifetimes with multiexponential decays were
observed for all Cn dyes but C1 (Table 2). As previously repor-
ted,27,36 luminescence properties of sheared samples were similar
to UA spin cast films (differing by only 1–12 nm), suggesting that
mechanical perturbation also produces amorphous states. Life-
time measurements also support this: comparable lifetimes are
obtained for CnUA spin-cast films and smeared powders (spw0
range ¼ 1–10 ns). Surprisingly, smeared C2 displayed significant
differences compared with its spin-cast film counterpart. For
example, even though there is little detectable difference between
C2UA and C2TA spin-cast films in emission and solid-state
morphology (Fig. 2a and 3), nonetheless when blue emitting
(ꢂ475 nm), thermally annealed C2 samples were smeared, they
displayed orange ML. Lifetimes also increased with smearing
(spw0 TA: 6 ns, smeared: 19 ns).
The crystallinity of TA and smeared samples was investigated
by XRD for C2, C6, C12 and C18 (Fig. 4, blue and red lines).
Spectra showed that crystallization occurs upon heat treatment
and mechanical perturbation leads to amorphous states. Crys-
tallinities were indicated by the small sharp peaks on the heat
treated sample spectra. Upon annealing, small peaks emerged at
ꢂ6ꢀ and 24ꢀ for C12, ꢂ8ꢀ and 24ꢀ for C6, and a very small peak
at 24ꢀ for C18 (Fig. 4), similar to XRD spectra for powder
samples. Even though ML is observed for C2, there are no
noticeable changes in the XRD spectra under these conditions.
Differential scanning calorimetry of powders revealed distinct
melting and crystallization events upon heating and cooling
(Fig. S4†). On the first heating cycle, the Tm endotherm is typi-
cally broader, and in many cases, a second, weaker, lower
temperature endothermic feature is also evident, suggesting
structural complexity in the ‘‘as isolated’’ powder. After thermal
conditioning, on the second and subsequent heating cycles, peaks
sharpen and the lower temperature feature typically disappears.
For 5 ꢀC minꢁ1 scan rates, only Tm and Tc are observed (Table
Because powders, slow evaporated and thermally annealed
spin cast films all possess crystalline morphologies, it is inter-
esting to compare their emission behavior, to gain insight into
processing effects (Table 1). All TA films show blue shifted
emission compared to bulk powders. The magnitude of the shift
varies across the series, but with no obvious trend. For example,
C5TA displayed the largest hypsochromic shift compared to the
powder (ꢂ43 nm), while C16 and C18 possessed the smallest
changes (i.e. 1 and 2 nm, respectively). One possible explanation
for these differences in bulk and thin samples is reabsorption and
emission due to differences in sample thickness. However, that
the differences are so small for some samples (e.g. C16, C18)
regardless of thickness, argues against this. Alternatively, longer
spacers may dilute the dyes and diminish dye–dye interactions,
even in the bulk, however, this is not a consistent trend across the
series (Table 1). What can be said with certainty is that some dyes
show dramatically different properties with certain fabrication
methods (e.g. C2 for SE films and C5 and C14 for thermally
annealed spin cast films), and the most dramatic differences are
between amorphous and ordered states.
Although the BF2dbmOR luminophores in this series are
identical throughout, with dipolar, H-bonding and arene stack-
ing interactions likely,26 even still, the alkyl chain substituents
influence solid-state emission in measurable ways. For the
shortest chain analogues, the addition of a single –CH2– can have
a dramatic effect on emission properties. As the chain length
increases, the hydrocarbon not only serves as a spacer,
decreasing dye–dye interactions, it can also induce self-assembly
and promote order, however, this effect tapers off at C12, above
which emission maxima show little change. Previous studies with
BF2dbmPLA polymer systems show blue shifted emission with
increasing dye dilution reaching monomer emission for high
molecular weight (20 kDa).38 This was attributed to decreasing
dye–dye interactions in high molecular weight materials. This is
similar to known solvatochromic effects for difluoroboron
b-diketonate dyes,17 but induced by dye concentration, not the
solid state polylactide solvent. A similar trend is observed for
Cn dyes as powders and C3–C18 films formed by slow evapo-
ration, where a blue shift is expected upon dye dilution and in
a non-polar hydrocarbon environment. However, the C18
substituent is considerably shorter than the shortest PLA chain
and interactions of polar dyes in the Cn series are considerably
stronger given closer dye-dye distances. Ordered assembly and
aggregate emission are dominant factors for the Cn dyes
compared to the amorphous polymers.
S4†). However, a slower scan rate (1 C minꢁ1) for C18 shows
ꢀ
two additional weak features at ꢂ86 ꢀC and ꢂ98 ꢀC, possibly
glass transitions (Fig. S5†).
Table 2 Mechanochromic luminescence properties of Cn powders
applied to quartz substrates
TAa lem (spw0
Smeareda lem (spw0
b
c
)
b
c
)
C1
C2
C3
C5
544 (30.61)
472 (6.04)
488 (9.12)
491 (10.14)
475 (5.24)
459 (7.10)
461 (5.80)
470 (4.13)
472 (3.12)
544 (31.53)
509 (18.70)
527 (12.46)
514 (21.62)
494 (13.17)
496 (23.94)
501 (19.95)
508 (19.94)
506 (23.04)
C6
C12
C14
C16
C18
Mechanochromic luminescence
a
b
c
TA ¼ thermally annealed. Units are given in nm. Pre-exponential
The mechanochromic luminescence behaviors of the dyes were
also investigated. Powders were applied to quartz cuvette
weighted lifetimes.30
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Fig. 4 XRD spectra of powder (green), thermally annealed (TA) (blue),
and smeared samples (red) for C2, C6, C12, and C18.
Another interesting qualitative observation is that the force
required to elicit an optical response also varies across the series.
For example, longer alkyloxy chain analogues (e.g. C18)
required greater pressure for an observable color change,
whereas for shorter analogues (C3), even slight pressure suffices.
More systematic studies to quantify these force-induced effects
will serve as the subject of future reports.
Fig. 5 Emission spectra for C2, C3, C5, C6, C12, C18 sheared powder
films on quartz substrates monitored over time after smearing (lex ¼ 365
nm) (TA ¼ thermally annealed, S ¼ immediately after smearing,
m ¼ minutes, h ¼ hours, d ¼ days).
A unique feature of these boron dyes is their ability to spon-
taneously revert to the ordered emissive state after shearing at
room temperature. Recovery times of ꢂ3 hours for thin films of
C12 on glass27 and ꢂ1 day for BF2AVB26 on quartz were
observed by fluorescence spectroscopy. Note that visually, the
marks disappear even sooner than this on weighing paper
substrates and that effects are thickness dependent. Because
longer alkyl chains are capable of assembly, it is also interesting
to test for chain length effects on spontaneous ML recovery
processes (Fig. 5). Recovery time studies for Cn dyes were per-
formed by applying samples (ꢂ2 to 3 mg) to the surface of
a quartz cuvette. The resulting films were thermally annealed to
generate a starting ordered state and spectra were recorded. Then
the TA samples were smeared, and their recovery was monitored
over time at room temperature.
temperature recovery may not be possible. However, heating this
smeared sample did produce the original state TA, which is how
samples are fabricated originally, for testing. For C3–C18,
a consistent trend was observed; longer alkyl chain length
correlates with the increasing recovery time. On the other hand,
C2 recovers much more slowly than C3. As observed for C2
throughout this study, ML behaviors too, are very different than
the other dyes in the series.
Conclusions
In summary, the solid state and mechanochromic luminescence
properties of BF2dbmOR with different alkoxyl chain lengths
were investigated. Crystalline powders, slow evaporated films
and thermally annealed spin-cast film emissions displayed
narrow, hypsochromic-shifted peaks and shorter lifetimes
compared to the more amorphous unannealed films. Chain
length dependent emission was observed for C3–C18 in both
powders and slow evaporated films. Furthermore, solid-state
fluorescence spectroscopy studies suggested that shorter alkyl
chain lengths affect the optical properties more significantly than
longer alkyl chain length (n > 12). This is exemplified by spin-cast
C2, C3UA and C5TA films that differ by few carbons but all
exhibit unique behaviors. All Cn dyes but C1 are mechano-
chromic luminescent, showing bathochromic shifts, and broad,
structureless bands upon smearing. In addition, lifetime
measurements revealed longer decays for smeared powders,
suggesting that excimer emission is activated upon smearing.
Spontaneous recovery times on quartz surfaces generally
Blue emitting thermally annealed C2 films (475 nm) turned
orange (ꢂ510 nm) upon shearing, taking ꢂ18 h to spontaneously
recover to the stable TA state (ꢂ475 nm). (Upon visual inspec-
tion the dye appears to recover in <30 min.) The blue fluorescent
TA dyes C3 and C5 displayed quite similar behavior to C2,
becoming orange upon smearing, then spontaneously blue-
shifting back to the stable TA form, with C3 recovering faster
than C5 (25 min and ꢂ1 day, respectively). Smearing C6 and C12
also turned the emission orange, which was noted visually, but
these samples immediately recovered to green emission, and then
gradually further blue-shifted. But after 9 days, the C6 emission
stopped changing before full recovery to the original ordered TA
state, as monitored by fluorescence spectroscopy. In contrast,
C18 did not self-heal to green immediately and between 9 and 18
days, the emission had stabilized, suggesting that a full room
8414 | J. Mater. Chem., 2011, 21, 8409–8415
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reported experimental conditions and on quartz substrates.
Additional structure-property studies, further characterization
and quantification of force induced effects, and mechanosensor
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