Thienyl Difluoroboron β-Diketonates in Solution and Polylactide Media

Article abstract of DOI:10.1071/CH15750

Difluoroboron β-diketonates (BF₂bdks) have impressive optical properties in both solution and the solid state. In particular, both fluorescence and roomerature phosphorescence are present when the dyes are confined to a rigid matrix, such as po

Full text of DOI:10.1071/CH15750

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2016, 69, 537–545
Full Paper
http://dx.doi.org/10.1071/CH15750
Thienyl Difluoroboron b-Diketonates in Solution
and Polylactide Media*
A
,
B
,
A B
Milena Kolpaczynska,
Christopher A. DeRosa,
A
,
A C
William A. Morris, and Cassandra L. Fraser
A
Department of Chemistry, University of Virginia, McCormick Road,
Charlottesville, VA 22904, USA.
B
These authors contributed equally to this work.
C
Corresponding author. Email: fraser@virginia.edu
Difluoroboron b-diketonates (BF bdks) have impressive optical properties in both solution and the solid state.
2
In particular, both fluorescence and room-temperature phosphorescence are present when the dyes are confined to a
rigid matrix, such as poly(lactic acid) (PLA). To expand the current knowledge and colour range capabilities of this unique
type of multi-emitting chromophore, a series of thienyl-substituted BF bdk complexes have been synthesized. The
2
photophysical properties were investigated in dichloromethane solution and in the solid state as dye/PLA blends. By
varying donor ability, i.e. methyl, phenyl, methoxy, and thienyl substituents, and by changing the dye loading in the PLA
media (0.1–10 % dye loading) red-shifted emission was achieved, which is important for biological imaging applications.
In dilute CH Cl solution, complexes exhibited absorptions ranging from 350 to 420 nm, solid-state fluorescence in PLA
2
2
ranging from 390 to 500 nm, and oxygen sensitive phosphorescence ranging from 540 to 585 nm in PLA blends. Promising
candidates as dye/PLA blends serve as models for dye–polymer conjugates for application as biological oxygen
nanoprobes.
Manuscript received: 1 December 2015.
Manuscript accepted: 18 January 2016.
Published online: 7 March 2016.
Introduction
auto-fluorescence, and minimal photo-damage to biological
[
22,23]
The chemistry of boron-based materials has attracted significant
interest, as tetra-coordinated boron complexes are at the
forefront of advanced functional materials.
boron b-diketonates show strong emission in both solution and
materials.
to the deep red or to the near-IR region by modifying the
However, extending the emission properties
[
1–3]
For example,
p-conjugation has limitations, such as thermally activated
delayed fluorescence (TADF), which makes ratiometric oxygen
[
4,5]
[24]
the solid state.
(
two-photon cross-sections,
In solution, difluoroboron b-diketonates
BF bdks) display large molar extinction coefficients,
sensing more challenging.
Therefore, it is important to
[
6,7]
investigate new fluorophore scaffolds to gain meaningful
insights into the design of dual emissive materials, with the
hope of finding different ways to red-shift emission.
2
[
8,9]
and high quantum efficien-
Because of these desirable features, these materials
[
10]
cies.
have shown promise as molecular probes,
[
11,12]
organic light-
[15]
When BF bdks are confined to a rigid matrix, both fluores-
2
[
13,14]
emitting diodes (OLEDs),
stimuli-responsive materials.
memory devices,
and
cence, and oxygen sensitive, room-temperature phosphores-
[5,25]
[
16,17]
cence (RTP) can be present.
In particular, BF bdks
2
To tune the emissive properties of BF complexes, a broad
2
exhibit RTP in rigid poly(lactic acid) (PLA) media, a biode-
gradable polyester commonly used in biomedical applications.
Large Stokes shifts, controllable oxygen sensitivity, and high
colour tuneability make these single-component materials ideal
candidates for oxygen sensing, for example, for tumour hypoxia
variety of chemical modifications have been investigated. One
approach involves altering the ligand scaffold, such as extending
[
18]
the p-conjugation and/or the addition of heavy atoms.
Anciliary ligands coordinated to the boron centre have also
[
19]
[20]
[18,24,26,27]
been studied; phenyl,
[
pentfluorophenyl,
have been explored. Some of these modifications
or cyano
imaging.
21]
groups
Current BF bdk–PLA materials have colour limitations,
2
lead to a smaller HOMO–LUMO energy gap, providing
red-shifted absorption and emission. Such optical properties
can expand the utility of BF bdks in biological applications,
being that they emit mainly in the blue to yellow portion of
[
27]
the visible spectrum.
Thiophene-based tetra-coordinated
boron complexes have promising absorption and emission
properties, making them a logical extension of dual-emissive
2
given deeper tissue penetration, reduced interference from
*
Dedicated to the memory of Professor Brice Bosnich, Cassandra Fraser’s Ph.D. advisor at The University of Chicago, whose high standards, attention to detail,
forthrightness, and thoughtful approach continue to inspire.
Journal compilation Ó CSIRO 2016
www.publish.csiro.au/journals/ajc

5
38
M. Kolpaczynska et al.
[
28]
material design.
synthesis of a thienyl difluoroboron diketonate with near-IR
For example, Yam et al. have reported the
Experimental
Materials
[
29]
photochromic properties,
dual-boron core memory device.
and a solution processable,
[
˚
and THF were dried over 3 A molecular sieves
15]
Solvents CH Cl
2
2
Furthermore, Zhang et al.
[37]
that had been activated at 3008C. All other solvents were used
as received without further purification. Boron trifluoride diethyl
etherate (Aldrich, purified, redistilled) and other chemicals were
reagent grade from Sigma Aldrich (methyl benzoate, methyl
reported a synthetic strategy of forming b-diketone initiators via
a one-step Claisen condensation between 2-acetylthiophene and
e-caprolactone, followed by lactide polymerization from
the resulting hydroxy functionalized boron dye. The dye-
functionalized polymer thus obtained showed intense
[30]
4-hydroxybenzoate, methyl 4-methoxybenzoate, 1-bromodode-
cane) and Alfa Aesar (2-acetylthiophene) and were used without
further purification. Methyl 4-dodecyloxybenzoate was prepared
[7]
via a Williamson ether synthesis as previously described.
blue fluorescence (lmax ¼ 397 nm) and green phosphorescence
(
mainly oligo- and polythiophenes, are most versatile conjugated
lRTP ¼ 530 nm) under vacuum. Thiophene-based materials,
[
38]
[
31,32]
Ligands 1-(thiophen-2-yl)butane-1,3-dione (mtm),
[39]
-(thiophen-2-yl)propane-1,3-dione (tbm),
1-phenyl-
1-(4-methoxy-
materials.
Weak emission in these materials is typically
3
phenyl)-3-(thiophen-2-yl)propane-1,3-dione (tbmOMe),
attributed to the inclusion of the heavy sulfur atom, as well as the
efficient solid-state packing characteristic for thiophene-based
[40]
and
have been
previously reported and the H NMR data are in accordance with
[34]
[
33,34]
1,3-di(thiophen-2-yl)propane-1,3-dione (dtm)
materials, shown by Ono et al.
However, the presence of an
1
electron-rich thienyl ring can act as an electron-donating group,
and the heavy atom inclusion may assist favourable dual-
emissive properties when confined to a rigid matrix.
the literature. All BF bdks 1–5 were obtained via the reaction of
2
the appropriate ligand and boron trifluoride diethyl etherate in
anhydrous CH Cl solution, under nitrogen. Complex 5 was
2
2
Our investigation of boron b-diketonates and their structure–
property relationships continues with a set of thienyl-substituted
dyes presented herein. One aromatic unit is set as a thienyl
group, while the opposing side is varied (i.e. methyl, phenyl,
methoxyphenyl, and thienyl substituents), analogous to previously
[34]
prepared as previously reported.
1
The H NMR spectra of
previously reported ligands and dyes are in accordance with the
literature. PLA (,13 kDa; polydensity index (PDI) ¼ 1.02) was
[24]
synthesized from ethylene glycol as previously described.
[35]
designed dyes (Chart 1).
Photophysical properties of the dyes are studied in CH Cl
Methods
1
2
2
solution and supported by computational results utilising
density functional theory (DFT). To study the dual emissive
properties, thienyl dyes were blended with PLA. This models
how a dye–PLA conjugate would behave and serves as a
valuable test and precursor to dye–polymer conjugate materials.
Optical properties are tuned in two ways, by varying the dye
scaffold, and by varying dye loading in PLA matrices. The
purpose of attaching the C12 alkyl chain is to observe variations
H NMR spectra were recorded on a Unity Inova 300/51
instrument (300 MHz), or Varian VMRS/600 (600 MHz) in
CDCl . Resonances were referenced to the signal for the
3
1
residual protiochloroform peak at 7.26 ppm. H NMR peak
frequencies are reported in ppm (d) and coupling constants in
Hertz (Hz). Data are reported as follows: chemical shift,
multiplicity (s ¼ singlet, d ¼ doublet, t ¼ triplet, br ¼ broad,
m ¼ multiplet), coupling constant (Hz), and integration. Mass
spectrometry (MS) was performed on a Micromass Q-TOF
Ultima spectrometer using electrospray ionization (ESI) MS
techniques. UV-Vis spectra were recorded on a Hewlett-
Packard 8452A diode-array spectrophotometer in CH Cl .
[
7]
in dye aggregation, since C12 is a good solubilizing group.
Dual emissive properties were analyzed for all dye/PLA blends,
in order to identify good candidates for dual emissive oxygen
[
36]
sensitive nanoprobes.
2
2
Steady-state fluorescence emission spectra were recorded on a
Horiba Fluorolog-3 model FL3-22 spectrofluorometer (double-
grating excitation and double-grating emission mono-
chromators). The data were analysed using FluorEssence v 2.1
software also from Horiba Jobin Yvon. Phosphorescence spec-
tra were recorded with the same instrument except that a pulsed
F
F
F
F
B
B
O
O
O
O
S
S
xenon lamp (l ¼ 369 nm; duration , 1 ms) was used for
ex
excitation, and spectra were collected with a 2 ms delay after
excitation. Fluorescence lifetime measurements (time corre-
lated single-photon counting, TCSPC) were performed with a
NanoLED-370 (369 nm) excitation source and DataStation Hub
as the SPC controller. Phosphorescence lifetimes were mea-
sured with a 500 ns multichannel scalar card (MCS) excited with
1
: BF mtm
2: BF tbm
2
2
F
F
F
F
B
B
O
O
O
O
S
S
a pulsed xenon lamp (l ¼ 369 nm; duration , 1 ms). Lifetime
ex
O
OC12H25
data were analysed with DataStation v 2.4 software from Horiba
Jobin Yvon. Thin films of dye/PLA blends were prepared on the
inner wall of vials by dissolving dyes and PLA in CH Cl to
3
: BF tbmOMe
4: BF tbmOC12
2
2
2
2
form a homogeneous solution and then evaporating the solvent
by slowly rotating the vial under a stream of nitrogen. The films
were dried under vacuum for 30 min before further measure-
ments. Lifetimes were fit to single exponential decays in solu-
tion (fluorescence) and to double or triple exponential decays in
solid-state films (fluorescence/total emission and phosphores-
F
F
B
O
O
S
S
5
: BF dtm
2
cence). Fluorescence quantum yields (F ) were calculated
F
Chart 1. Thienyl difluoroboron b-diketonates.
versus anthracene in EtOH as a standard using the following

Thienyl Difluoroboron b-Diketonates
539
[
41] 20
20
D
values: FF (anthracene) ¼ 0.27,
nD (EtOH) ¼ 1.361, n
BF tbm (2)
2
[
42]
(
CH Cl ) ¼ 1.424.
Optically dilute CH Cl solutions of BF
2 2 2
complexes 1–5 and EtOH solutions of the anthracene standard
were prepared in 1 cm path length quartz cuvettes, and absor-
2
2
Boron complex 2 was prepared as described for 1, using
-phenyl-3-(thiophen-2-yl)propane-1,3-dione (tbm) as the
1
ligand. The resulting complex was purified by recrystallization
bances (A , 0.1, a.u.) and emission spectra (l ¼ 350 nm;
ex
from hexanes to give a yellow powder: 61 mg (64 %). dH
(300 MHz, CDCl ) 8.14–8.10 (m, 3H, -ArH) 7.90 (d, J 4.8,
emission integration range: 365–650 nm) were recorded. BF
complexes 1–3 were modelled using the Gaussian 09 suite of
2
3
1
7
H, -ArH) 7.71–7.67 (m, 1H, -ArH) 7.58–7.53 (m, 2H, -ArH)
.30–7.27 (m, 1H, -ArH) 6.98 (s, 1H, -COCHCO-). HRMS (ESI,
[
43]
programs utilising DFT.
B3LYP/6–31þG(d) was employed
for ground state geometry optimization of all compounds with a
Tomasi polarized continuum for dichloromethane solvent.
Single point energy calculations were used to generate the
molecular orbital diagrams utilising 6–31G(d). Time-dependent
density functional theory (TD-B3LYP/6–311þG(d)) was used
for estimating the absorption spectra in dichloromethane at the
þ
TOF) m/z 259.0399; calcd for C H O SBF [M – F] 259.0400.
13 9 2
BF tbmOMe (3)
2
Boron complex 3 was prepared as described for 1 using 1-(4-
methoxyphenyl)-3-(thiophen-2-yl)propane-1,3-dione (tbmOMe)
as the ligand. The product was purified by column chromato-
[44,45]
optimized ground state geometries.
were depicted by GaussView 5 software.
Molecular orbitals
[
46]
graphy using CH
2
Cl as the eluent to give a yellow powder:
2
8
8
6 mg (80 %). d (300 MHz, CDCl ) 8.12 (d, J 9.3, 2H, -ArH),
H
.05 (d, J 3.9, 1H, -ArH), 7.83 (d, J 5.1, 1H, -ArH), 7.26–7.24
3
Synthesis
(m, 1H, -ArH), 7.03 (d, J 9.3, 2H, -ArH), 6.90 (s, 1H,
-COCHCO-), 3.94 (s, 3H, -ArOCH ). HRMS (ESI, TOF) m/z
1
1
-(4-(Dodecyloxy)phenyl)-3-(thiophen-2-yl)propane-
,3-dione (tbmOC12)
3
þ
289.0505; calcd for C H BO FS [M – F] 289.0506.
1
4
11
3
Acetylthiophene (1.0 g, 7.92 mmol), methyl 4-dodecyloxy-
benzoate (3.0 g, 9.51 mmol), and THF (25 mL) were added
sequentially to a 100 mL round-bottom flask. After the mixture
was stirred for 10 min, a suspension containing NaH (380 mg,
BF tbmOC12 (4)
2
The complex 4 was prepared as described for 1 using
1-(4-(dodecyloxy)phenyl)-3-(thiophen-2-yl)propane-1,3-dione
(tbmOC12) as the ligand. The resulting complex was purified by
recrystallization from hexanes to give a yellow powder: 178 mg
(80 %). d (600 MHz, CDCl ) 8.10 (d, J 9.0, 2H, -ArH), 8.04
1
temperature under N . The reaction was refluxed at 608C in a
5.8 mmol) in THF (20 mL) was added via cannula at room
2
nitrogen atmosphere, and monitored by TLC. Upon consumption
of the limiting reagent, 2-acetylthiophene (21 h), the reaction
mixture was removed from the heat and allowed to cool to room
temperature and quenched with 1 M HCl (1 mL, pH 6–7). The
THFwasremovedviarotaryevaporation. Organicswereextracted
with CH Cl (20 mL ꢀ 2), washed with H O (20 mL ꢀ 2) and
H
3
(d, J 3.9, 1H, -ArH), 7.81 (d, J 4.8, 1H, -ArH), 7.25–7.24 (m, 1H,
-ArH), 7.00 (d, J 9.0, 2H, -ArH), 6.87 (s, 1H, -COCHCO-), 4.06
(t, J 6.0, 2H, -ArOCH ), 1.84–1.79 (m, 2H, -OCH CH -), 1.26
2
2
2
(br m, 18H, -C H -), 0.88 (t, J 6.6, 3H, -CH CH ). HRMS
9
18
2
3
þ
2
2
2
(ESI, TOF) m/z 443.2231; calcd for C H O SBF [M ꢁ F]
25 33 3
brine (10 mL), and then dried over Na SO . After filtration
443.2228.
2
4
and concentration under vacuum, the residue was purified by
column chromatography on silica gel eluting with toluene/hexanes
Results and Discussion
(5 : 1) to give an ivory solid (1.35 g, 41 %). d (600 MHz, CDCl )
H 3
Synthesis
16.51 (s, 1H, –OH), 7.90 (d, J 9.0, 2H, -ArH), 7.77 (d, J 3.6, 1H,
-
(
-
ArH), 7.60 (d, J 5.1, 1H, -ArH), 7.16–7.15 (m, 1H, -ArH), 6.95
d, J 9.0, 2H, -ArH), 6.61 (s, 1H, -COCHCO-), 4.01 (t, J 6.6, 2H,
ArOCH -), 1.82–1.77 (m, 2H, -OCH CH -), 1.25 (br m, 18H,
The complexes 1–5 were obtained via a two-step synthesis.
Claisen condensation in the presence of NaH in THF generated
the b-diketone ligands followed by boronation with BF ꢂOEt in
2
2
2
2
2
-C H -), 0.87 (t, J 6.3, 3H, -CH CH ). HRMS (ESI, TOF) m/z
9
CH Cl . All of the ligands were obtained as tan solids and the
2 2
18
2
3
þ
1
4
15.2302; calcd for C H O S [M þ H] 415.2307.
complexes as yellow powders. H NMR and MS analysis
confirmed the structure and purity of the ligands and boron
complexes.
25 35 3
All BF bdks were obtained via the reaction of the appropriate
2
ligand 1–5 and boron trifluoride diethyl etherate in CH Cl
2
2
solution under nitrogen. A representative reaction is provided
for BF mtm (1).
Optical Properties in Solution
2
Optical properties of dyes 1–5 were first investigated in
ꢁ6
BF mtm (1)
2
dilute CH Cl solutions (10 M, A , 0.10) in air and at room
2
2
Boron complex 1 was prepared by dissolving 1-(thiophen-2-yl)
butane-1,3-dione (mtm) (80 mg, 0.48 mmol) in anhydrous CH Cl2
temperature. Highly dilute solutions prevent aggregation or
exciplex formation. Solution data are presented in Table 1 and
Fig. 1.
2
(25 mL) in an oven-dried 50 mL round bottom flask. Boron
trifluoride diethyl etherate (88mL, 0.71 mmol) was added via
syringe and the solution turned brown. The reaction mixture was
stirred at room temperature and monitored by TLC until the ligand
was consumed (20 h). The reaction mixture was filtered and the
solvent was removed by rotary evaporation. The product was
purified by column chromatography withhexanes/EtOAc (3 : 1) as
the eluent. A light yellow powder was obtained: 69 mg (67 %).
d (300 MHz, CDCl ) 8.01 (d, J 3.9, 1H, -ArH) 7.88 (d, J 5.1, 1H,
All boron complexes absorb light in the UV to violet range.
Stronger absorption and bathochromic emission were obtained
with increasing donor ability. The alkyl chain length in 3 versus
4 did not significantly affect the absorption bands (417 and
418 nm, respectively). The absorbance trend of dyes 1–4 was
similar to that observed for previously reported BF bdk
2
[
43]
complexes.
The exchange of a thienyl ring for a phenyl or
naphthyl aromatic group resulted in red-shifted absorption,
important for biological imaging. This may be assigned to the
sulfur atom acting as a donor, thus raising the energy of the
HOMO.
H
3
-ArH) 7.26–7.24 (br, 1H, -ArH) 6.36 (s, 1H, -COCHCO-) 2.38
(s, 3H, -COCH ). HRMS (ESI, TOF) m/z 197.0244; calcd for
C H BO FS [M – F] 197.0244.
3
þ
8
7
2

5
40
M. Kolpaczynska et al.
Table 1. Optical properties of boron complexes 1–5 in CH
2
Cl
2
The absorbances of dyes 3 and 4, containing terminal alkoxy
groups, were slightly blue-shifted compared with 5, as reported
A
l
B
e
ꢁ1
ꢁ1
C
D
F
E
F
[34]
Sample
abs [nm]
[M cm
]
l
em [nm]
t
[ns]
F
by Ono et al.
This may suggest that the alkoxy-substituted
phenyl rings and the thiophene moieties possessed similar
electrondonation and molecular geometry, for example, planarity
in their ground states. The thienyl complexes 1–5 have high
extinction coefficients (44000–61900) typical for p to p* transi-
tions, also characteristic for thiophene-based systems, such as
BF
BF
BF
BF
BF
2
2
2
2
2
mtm
1
2
3
4
5
354
401
417
418
420
44 000
46 400
56 000
61 900
57 200
392
416
441
445
438
0.18
1.83
2.06
2.08
1.02
0.04
0.31
0.38
0.71
0.39
tbm
tbmOMe
tbmOC12
dtm
[34]
oligothiophenes.
The fluorescence spectra for complexes 1–5 exhibited violet-
A
Absorption maxima.
B
C
Extinction coefficients calculated at the absorption maxima.
blue emission in CH Cl ranging from 392–445 nm, and sys-
2
2
Fluorescence emission maxima excited at 369 nm (except 1, excited at
tematically displayed red-shifts with an increase of conjugation
and donor ability (i.e. in CH Cl , l : 1 ¼ 354 nm, 2 ¼ 401 nm,
3
50 nm).
D
2
2
Abs
Fluorescence lifetime excited with a 369 nm light-emitting diode (LED)
3
alkoxy-phenyl (3 and 4) derivatives had similar optical proper-
¼ 417 nm, 4 ¼ 418 nm, and 5 ¼ 420 nm). Thienyl (5) and
monitored at the emission maximum. All fluorescence lifetimes are fitted
with single-exponential decay.
E
Relative quantum yield, with anthracene in EtOH as a standard.
ties in CH Cl , suggesting that the two rings have similar
2
2
electronic character.
As shown in Table 1, the fluorescence quantum yields and
lifetimes increased with increasing electron donation for dyes 1–4.
The quantum yield of complex 5 (F ¼ 39.0 %) was similar to the
F
methoxy-substituted dye 3 (F ¼ 38.0 %). Overall, low quantum
F
yields were characteristic for thiophene-based materials, com-
pared with dbm-type boron dyes, which often have intense
emission and quantum yields near unity. This can be attributed
to fluorescence quenching due to the heavy atom effect of the
sulfur atom. Complexes 2–4 have fluorescence lifetimes between
(
a)
7
6
5
4
3
2
1
0000
0000
0000
0000
0000
0000
0000
0
1
2
3
4
5
[10]
1.82 and 2.08 ns, similar values to symmetrically substituted
phenyl BF bdks (,2.0 ns). The symmetric BF dtm dye 5 had a
2
2
much shorter lifetime, 1.02 ns. The lowest quantum yield and
lifetime was observed for 1, analogous to the phenyl-methyl
[35]
dye.
Computational Studies
300
350
400
450
500
For most compounds, the computed absorption maxima (labs) are
in good agreement with the experimental values (Tables S3–S4,
Supplementary Material). The one exception to this is complex 2,
for which the computed labs is blue-shifted ,20nm compared
with the experimental value. The absorption spectra are almost
entirely dominated by HOMO to LUMO transitions. Further-
more, the HOMO to LUMO transitions appeared to be p to p*
corresponding to a S - S transition, with no intramolecular
Wavelength [nm]
(
b)
0
1
charge transfer (ICT) bands evident from the experimental
absorption spectra. All of the dyes exhibit relatively small Stokes
ꢁ
1
shifts (i.e. 900–2739cm ). It is typical for fluorophores with
strong ICT character to exhibit large Stokes shifts corresponding
to a dramatic change in excited state geometry. The computa-
tionally generated molecular orbital diagrams also support this
qualitatively. Only a slight transfer of amplitude seems to occur
1
(
2
3
4
5
c)
1
1
0
0
0
0
.2
.0
.8
.6
.4
.2
0
1
from the aromatic rings to the BF bdk moiety when going from
2
2
3
4
5
the HOMO to the LUMO of these compounds (Fig. 2). This is an
interesting result. In the past, we have observed transitions pre-
dominantly p to p* in character when studying BF complexes
with symmetrical diarene ligands. However, when the compound
contains an unsymmetrical diarene ligand, the HOMO to LUMO
transition typically shows strong ICT. In this case, we observe
unsymmetrical compounds with very little ICT character evident.
Perhaps this is due to the thiophene ring being relatively similar in
size and electronic character to the phenyl ring, in contrast to a
naphthylene or anthracene moiety, for example in previous
2
3
50
450
550
650
Wavelength [nm]
[43]
studies. In the case of the thiophene-methyl compound 1, the
reason why no strong ICT is observed may be because the methyl
moiety is slightly electron donating and would, therefore, be poor
at accepting amplitude in any kind of charge transfer situation.
Fig. 1. Optical properties in CH
2
Cl
complexes 1–5 in CH
2
irradiation. (c) Steady-state emission spectra in CH Cl .
2
. (a) Absorption spectra in CH
2 2
Cl
ꢁ6
(
1 ꢀ 10 M). (b) Image of BF
2
2
Cl upon 365 nm light
2
2

Thienyl Difluoroboron b-Diketonates
541
F
F
F
F
F
F
B
B
B
O
O
O
O
O
O
S
S
S
O
1
: BF mtm
2: BF tbm
3: BF tbmOMe
2
2
2
LUMO
HOMO
Fig. 2. Calculated HOMO and LUMO molecular orbitals for 1–3 indicating p to p* character.
Table 2. Fluorescence properties of dye/PLA blends in air
Dye loading [%]
1/PLA
2/PLA
3/PLA
4/PLA
5/PLA
A
B
A
B
A
B
A
B
A
B
l
em [nm]
t
pw0 [ns]
l
em [nm]
t
pw0 [ns]
l
em [nm]
t
pw0 [ns]
l
em [nm]
t
pw0 [ns]
l
em [nm]
t
pw0 [ns]
C
C
0
.1
393
395
397
400
—
415
433
438
440
—
438
453
458
442
1.23
2.24
2.00
1.00
435
441
464
469
2.68
2.50
2.42
2.31
433
448
457
493
1.06
1.32
1.23
2.68
C
1
2
5
—
0.95
0.67
0.47
C
—
C
—
A
B
Fluorescence emission maxima excited at 369 nm (except for 1/PLA excited at 350 nm).
Fluorescence lifetime excited with a 369 nm light-emitting diode (LED) monitored at the emission maximum. All fluorescence lifetimes are fitted with multi-
exponential decay.
C
Signal too weak to be detected with current LED excitation.
Table 3. Phosphorescence properties for dye/PLA blends under nitrogen
Dye loading [%]
1/PLA
2/PLA
3/PLA
4/PLA
5/PLA
A
B
A
B
A
B
A
B
A
B
l
RTP [nm]
t
pw0 [ms]
l
RTP [nm]
t
pw0 [ms]
l
RTP [nm]
t
pw0 [ms]
l
RTP [nm]
t
pw0 [ms]
l
RTP [nm]
t
pw0 [ms]
0
.1
543
543
546
550
92.6
82.8
58.2
44.7
557
555
569
571
109.7
105.5
77.6
558
568
563
570
59.6
72.4
62.9
76.4
567
560
567
562
73.9
38.3
26.8
3.45
572
576
572
584
137.0
123.2
110.9
38.8
1
2
5
60.1
A
B
2
Phosphorescence maxima under N excited at 369 nm.
Pre-exponential weighted lifetimes fitted to triple exponential decay.
As a result, we only observe a slight qualitative charge transfer in
the molecular orbital (MO) diagrams from the thiophene ring to
the BF moeity. Ono et al. have previously performed calculations
glass transition temperature) necessary to produce multi-
[
5,47]
emissive properties from boron dyes.
Solutions of dye plus
PLA mixtures were coated on the inner wall of vials, and dried
under vacuum overnight before the measurements were taken.
The solid-state photophysical properties, both fluorescence and
phosphorescence, were studied in air and nitrogen (i.e. deoxy-
genated) environments. The fluorescence properties are listed in
Table 2, and phosphorescence properties are presented in
Table 3.
2
on compound 5 using similar methods to our own. As would be
expected for the symmetrical BF bdk, the HOMO to LUMO
2
transition of this compound is also p to p* in character according
to their generated MO diagrams.
[34]
Optical Properties of Dye/PLA Blends
Optical properties of the dyes were also analysed in PLA
matrices. PLA is a biocompatible polymer commonly used in
biomedical research, and creates a more rigid environment (high
As shown previously in model studies of dye/PLA mixtures,
the fluorescence is highly dependent on dye loading. In dilute
dye/PLA blends (0.1 or 1.0 %), the fluorescence maxima are

5
42
M. Kolpaczynska et al.
[9]
often similar to the corresponding data in CH Cl (i.e. dye 3,
2
Supplementary Material). When analysing higher dye load-
ings (,10 %), a visible phase separation occurred and the
emission became unreproducible and random. Depending on
the analysed area on the vial, the fluorescence would change
colours (Fig. S1, Supplementary Material). For higher dye
loadings without phase separation, dye–polymer conjugates
are preferable to dye/polymer blends. As shown in Fig. 3, all
samples exhibited violet to blue emission, with a red shift in
fluorescence wavelength with increased conjugation and donor
2
lDCM ¼ 441 nm, l
¼ 438 nm). For dye blends at higher
PLA (0.1 %)
concentration, the emission maxima were significantly more
red-shifted when compared with CH Cl (Figs S1–S4,
2
2
(
a)
ability (i.e. at 2 % dye loading, l : 1 ¼ 397 nm, 2 ¼ 438 nm,
F
3
¼ 458 nm, 4 ¼ 464 nm, and 1 ¼ 457 nm).
Establishing structure–property relationships of BF bdks in
2
PLA is important for designing new dye materials. Previously, it
was observed that p-conjugation played a key role in molecular
weight (i.e. dye loading) colour tuneability. For example,
1
2
3
4
5
difluoroboron dibenzoylmethane (BF dbm) dyes reach a maxi-
2
[
9]
(
b)
mum blue-shift in ,20 kDa PLA polymers (,1.5 % dye),
while difluoroboron dinaphthoylmethane (BF dnm) dyes
1
.2
.0
.8
.6
.4
.2
0
1
2
3
4
5
2
[
24]
required ,30 kDa PLA polymers to maximally blue shift.
The thiophene complexes 1–5 follow this trend as well. For dye
(BF mtm), with a single thiophene ring, the shift in fluores-
1
0
0
0
0
1
2
cence maxima from 5 % dye loading to 0.1 % dye loading (Dl) is
only 7 nm (5 %: l ¼ 400 nm, 0.1 %: l ¼ 393 nm). In contrast,
dye 5 (BF dtm) has a Dl of 60 nm. For larger fluorophores with
2
a tendency to associate (e.g. p stack) the likelihood of forming
different aggregation states with different colours is a possible
explanation for the different colour ranges among the dyes
(
Figs S2–S4, Supplementary Material). The presence of the
alkyl chain in 4 had a surprising effect on the dye loading colour
tuneability. Initially, we theorized that with the increased
solubility of the fluorophore due to the alkyl chain, the propen-
sity for aggregation would decrease, resulting in a blue-shifted
emission. However, as seen in Fig. 4, the C12 chain in 4 resulted
in a visible red-shifted emission, whereas the C1 derivative (3)
only showed a weak red-shifted shoulder at ,600 nm. It is
3
60
460
560
660
Wavelength [nm]
Fig. 3. Dye dependent fluorescence of thienyl BF bdks in PLA. (a) Image
2
showing emission (excitation by UV lamp) of 2 % blends under ambient
conditions. (b) Total emission spectrum for 2 % blends (excitation ¼ 369 nm).
BF tbmOMe (3)
BF tbmOC12 (4)
2
2
5
%
2%
1%
0.1%
5%
2%
1%
0.1%
1.2
1.0
0.8
0.6
0.4
0.2
0
1.2
1.0
0.8
0.6
0.4
0.2
0
1
.1%
%
0.1%
1%
2%
2%
5
%
5%
0
400
450
500
550
600
650
700
400
450
500
550
600
650
700
Wavelength [nm]
Wavelength [nm]
Fig. 4. Total emission spectrum for PhOMe and PhOC12 dyes 3 and 4 respectively as PLA blends showing fluorescence changes
with dye loading (excitation ¼ 369 nm).

Thienyl Difluoroboron b-Diketonates
543
possible that the C12 chain acted as an organizing group within
the PLA, fostering assembly and microphase separation, as was
previously seen for Rhodamine B-C12 dye in PLA reported by
Zhang et al. Attaching organizing groups to the fluorophore may
be an important new strategy for achieving red-shifted emission
Data are collected in Table 3. Because the complexes do not
have a halide heavy atom substituent, such as bromide or iodide
typically used to enhance the phosphorescence properties of
[
24]
BF bdks,
2
there is little to no change in the total emission
spectra when going from an oxygenated to a deoxygenated
environment. In some thienyl dye/PLA blends, the total
emission spectra under nitrogen revealed red shoulders and
broadening peaks, attributed to phosphorescence. In dilute
blends (1 %) of compounds 1 and 5, phosphorescence shoulders
are evident. It is possible that sulfur is acting as the heavy atom
in this situation, as these two compounds have the highest
[
48]
within PLA matrices.
While the fluorescence is sensitive to dye structure and dye
loading, phosphorescence colour is only tuneable by dye struc-
ture. As previously reported, BF bdk dyes exhibit RTP in rigid
2
[25]
media such as PLA (T ,608C).
All complexes showed
phosphorescence in a nitrogen atmosphere (deoxygenated).
g
1.2
1.0
0.8
0.6
0.4
1
2
3
4
5
Air
1
N2
Phos
F
RTP
0.2
0
360 410 460 510 560 610 660
Wavelength [nm]
1.2
1.0
0.8
0.6
0.4
0.2
0
1.2
1.0
0.8
0.6
0.4
0.2
0
Air
Air
2
3
N2
N2
Phos
Phos
400 450 500 550 600 650 700
400 450 500 550 600 650 700
Wavelength [nm]
Wavelength [nm]
1.2
1.0
0.8
0.6
0.4
0.2
0
1.2
1.0
0.8
0.6
Air
Air
4
5
N2
N2
Phos
Phos
0.4
0.2
0
400 450 500 550 600 650 700
400 450 500 550 600 650 700
Wavelength [nm]
Wavelength [nm]
Fig. 5. Room-temperature phosphorescence properties of 1 % dye/PLA blends of thienyl BF
2
bdks. Image: (F) blends under N
(dashed lines)
2
(
l
ex ¼ 369 nm), (RTP) picture taken with light source turned off. Spectra: total emission in air (solid lines) and N
2
and delayed emission spectra (grey lines). Excitation of dyes ¼ xenon lamp set to 369 nm, except for 1 was excited at 350 nm.
Delayed emission spectra taken with a 2 ms delay.

5
44
M. Kolpaczynska et al.
percent sulfur content. However, the effect is much less dramatic
when compared with halide heavy atoms, such as bromide or
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