Aromatic difluoroboron β-diketonate complexes: Effects of π-conjugation and media on optical properties

Article abstract of DOI:10.1021/ic300077g

Aromatic difluoroboron β-diketonate complexes (BF₂bdks) are classic fluorescent molecules that have been explored as photochemical reagents, two-photon dyes, and oxygen sensors. To gain a better understanding of their emissive properties in both solution and polymer matrices, BF ₂bdks with varying aromatic groups were synthesized and their photophysical properties were investigated in both methylene chloride and poly(lactic acid) (PLA). Absorption spectra showed systematic variations that are well correlated with structural features, including the size of the aryl substituent and the presence of a para electron-donating methoxy substituent. Computational modeling of the absorption spectra with the TD-B3LYP/6-311+G(d)// B3LYP/6-31G(d) formulation of density functional theory and a polarizable continuum model of dichloromethane solvent shows that all systems show intense π-π* one-electron excitations, usually from one of the highest occupied molecular orbitals (HOMO - k, k = 0, 1, 2) to the lowest unoccupied molecular orbital (LUMO). Emission properties are sensitive to the dye structure and medium. Based on spectroscopic and lifetime studies, BF₂bdks exhibit comparable fluorescence properties in both solutions and polymers when the diketonate group is functionalized with smaller aromatic ring systems such as benzene. For BF₂bdks with larger arene ring systems, such as anthracene, emission from a strong intramolecular charge-transfer (ICT) state was also noted in both solution and in PLA. There are differences in relative intensities of peaks arising from π-π* and ICT excitations depending upon dye loading in PLA. Substituent effects were also observed. Electron-donating methoxyl groups on the aromatic rings lead to enhanced fluorescence quantum yields. For certain dyes, phosphorescence is detected at low temperature or under a nitrogen atmosphere in PLA matrices.

Full text of DOI:10.1021/ic300077g

Article
pubs.acs.org/IC
Aromatic Difluoroboron β‑Diketonate Complexes: Effects of
π‑Conjugation and Media on Optical Properties
Songpan Xu, Ruffin E. Evans,‡ Tiandong Liu, Guoqing Zhang,† J. N. Demas, Carl O. Trindle,
and Cassandra L. Fraser*
Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, Virginia 22904, United States
S
* Supporting Information
ABSTRACT: Aromatic difluoroboron β-diketonate com-
plexes (BF2bdks) are classic fluorescent molecules that have
been explored as photochemical reagents, two-photon dyes,
and oxygen sensors. To gain a better understanding of their
emissive properties in both solution and polymer matrices,
BF2bdks with varying aromatic groups were synthesized and
their photophysical properties were investigated in both
methylene chloride and poly(lactic acid) (PLA). Absorption
spectra showed systematic variations that are well correlated
with structural features, including the size of the aryl
substituent and the presence of a para electron-donating
methoxy substituent. Computational modeling of the absorption spectra with the TD-B3LYP/6-311+G(d)//B3LYP/6-31G(d)
formulation of density functional theory and a polarizable continuum model of dichloromethane solvent shows that all systems
show intense π−π* one-electron excitations, usually from one of the highest occupied molecular orbitals (HOMO − k, k = 0, 1,
2) to the lowest unoccupied molecular orbital (LUMO). Emission properties are sensitive to the dye structure and medium.
Based on spectroscopic and lifetime studies, BF2bdks exhibit comparable fluorescence properties in both solutions and polymers
when the diketonate group is functionalized with smaller aromatic ring systems such as benzene. For BF2bdks with larger arene
ring systems, such as anthracene, emission from a strong intramolecular charge-transfer (ICT) state was also noted in both
solution and in PLA. There are differences in relative intensities of peaks arising from π−π* and ICT excitations depending upon
dye loading in PLA. Substituent effects were also observed. Electron-donating methoxyl groups on the aromatic rings lead to
enhanced fluorescence quantum yields. For certain dyes, phosphorescence is detected at low temperature or under a nitrogen
atmosphere in PLA matrices.
sensing and dynamic hypoxia imaging are possible in
tumors,25,26 the brain, and other contexts. BF2dbm analogues
have thus yielded promising preliminary results for cellular27,28
and hypoxia imaging with two-photon absorbing capability and
compatibility with multiphoton methods.28 Building upon these
early successes, dyes with emission profiles across the visible
region are important for multiplexing, and tissue penetration
depth can be increased with red-shifted dyes.29 Although the
emission wavelength of boron dye−polymer conjugate
BF2dbmPLA may be manipulated to a certain extent by
polymer molecular weight, the tuning range is limited.24 Also,
this method does not shift the BF2dbm absorption out of the
UV region, which can be damaging to biological systems. Thus,
the development of red-shifted BF2bdk derivatives can expand
their utility for in vitro cellular studies, ex vivo assays, and in
vivo imaging agents.
INTRODUCTION
■
Boron-containing materials possess impressive optical proper-
ties.1−4 Difluoroboron β-diketonate complexes (BF2bdks) are
classic fluorescent molecules with large molar absorptivities and
often high quantum yields5 that have been explored as
photochemical reagents,6−11 two-photon materials,12,13 con-
jugated polymers,14−16 semiconductors,17 photochromic mate-
rials,18 near-IR probes,19 and oxygen20 and mechanical
sensors.21 Extensively investigated targets among these
aromatic BF2bdks are difluoroboron dibenzoylmethane
(BF2dbm) and its derivatives, given that some of the parent
diketones are commercial products, such as dbm22 and
avobenzone.23 After boronation with BF3·OEt2 in noncarbonyl
solvents, most BF2dbm derivatives can be obtained as air-stable,
crystalline compounds. In solutions, they typically exhibit
intense blue fluorescence, and some give excimer or exciplex
emission9 or dye polarity24 effects at higher concentrations.
When BF2dbm fluorophores are dissolved in solid-state
media, such as poly(lactic acid) (PLA), also known as
polylactide, long-lived room-temperature phosphorescence
(RTP) is observed in the absence of oxygen.20 Through
further heavy atom perturbation, in vivo luminescence oxygen
Although BF2bdk luminescence has been investigated by
many groups,5,18,30−34 ours included, we observe interesting
emissive behaviors for the boron complexes in PLA compared
Received: January 12, 2012
© XXXX American Chemical Society
A
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Article
diethyl etherate (Aldrich, purified, redistilled) and all other reagents
and solvents were used as received without further purification.
Diketone ligands were prepared by Claisen condensation with NaH,
and boron complexes were prepared with BF3 etherate as previously
described. The data for complexes 1−4,39 5,31 6,13 9,13 and 1031 are in
accord with the literature. The synthesis of complexes 7 and 8 is
described below.
to solution, which have not been reported in the literature. For
example, for the naphthalene derivative, BF2nbmPLA,35 the
fluorescence emissions in both CH2Cl2 and in the solid state
(∼440 nm) were similar to those for the benzene derivative
BF2dbmPLA under the same conditions.20,36 However, the
phosphorescence of BF2nbmPLA showed a significant red shift
(544 nm) compared to that of BF2dbmPLA (509 nm), which
suggests that π-conjugation length affects singlet and triplet
states differently. Compared to the well-known difluoroboron
BODIPYs (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene),37,38
mechanistic studies of BF2bdk luminescence are more sparse.
Therefore to better understand the optical properties of BF2bdk
complexes, including emissive states, emission color range, and
media effects, it is important to conduct systematic structure−
property investigations.
Here we synthesized a series of simple BF2bdk derivatives
(1−10) for a luminescence study in combination with
computational chemistry. The boron complexes all possess
aromatic hydrocarbons of different sizes. Compared to phenyl
molecules 1−4, the methoxyphenyl counterparts 5−8 have the
same chemical structures except that the latter series has an
electron-donating methoxyl group on the benzene ring to
explore substituent effects. The dimethoxyphenyl BF2bdk 9 and
the methyl-naphthyl complex 10 were also included for
comparison. Nomenclature for the complexes is indicated in
the structures shown below. We will also refer to complexes by
the hydrocarbon substituents on the difluoroboron diketonate
ring [i.e., Me-Ph = mbm, 1; Ph-Ph = dbm, 2; Ph-Np = nbm, 3;
Ph-An = abm, 4; Me-PhOMe = mbmOMe, 5; Ph-PhOMe =
dbmOMe, 6; Np-PhOMe = nbmOMe, 7; An-PhOMe =
abmOMe, 8; PhOMe−PhOMe = dbm(OMe)2, 9; Me-Np =
mnm, 10]. Fluorescence properties of these boron complexes
in CH2Cl2 were investigated via UV−vis and fluorescence
spectroscopies and quantum yield and fluorescence lifetime
measurements. Computational studies were also performed to
support and provide further insight into experimental findings.
Also, because many useful photophysical properties of BF2dbm
derivatives arise in a solid-state environment, both fluorescence
and phosphorescence were investigated for dye/PLA blends,
which can inform future work with dye−polymer conjugates for
imaging, sensing, and other uses.
Methods. 1H NMR (300 MHz) spectra were recorded on a Varian
UnityInova 300/51 instrument in CDCl3. 1H NMR spectra were
referenced to the signal for residual protiochloroform at 7.26 ppm.
UV−vis spectra were recorded on a Hewlett-Packard 8452A diode-
array spectrophotometer. Mass spectra were recorded on an Applied
Biosystems 4800 spectrometer with a matrix-assisted laser desorption
ioinization tandem time-of-flight (MALDI TOF/TOF) analyzer.
Elemental analysis was performed by Atlantic Microlab, Inc., Norcross,
GA. Steady-state fluorescence emission spectra were recorded on a
Horiba Fluorolog-3 model FL3-22 spectrofluorometer (double-grating
excitation and double-grating emission monochromators). Time-
correlated single-photon counting (TCSPC) fluorescence lifetime
measurements were performed with a NanoLED-370 (369 nm)
excitation source and DataStation Hub as the SPC controller. Lifetime
data were analyzed with DataStation v2.4 software from Horiba Jobin
Yvon. Fluorescence quantum yields, ϕF, for BF2bdks (1−10) in
CH2Cl2 were calculated versus anthracene in EtOH as a standard as
previously described40 using the following values: ϕF(anthracene)
=0.27,41 nD20(EtOH) = 1.360, and nD20(CH2Cl2) = 1.424. Optically
dilute CH2CH2 solutions of BF2bdks and EtOH solution of the
anthracene standard were prepared in 1 cm path length quartz cuvettes
with absorbances < 0.1. Quantum yield measurements were performed
with excitation at λex = 350 nm and emission integration range = 365−
695 nm. Phosphorescence spectra were recorded with the same
instrument except that a pulsed xenon lamp (λex = 369 nm; duration
<1 ms) was used for excitation and spectra were collected with a 2 ms
delay after excitation. Phosphorescence lifetimes were measured with a
500 ns multichannel scaler (MCS) excited with a pulsed xenon lamp
(λex = 369 nm; duration <1 ms).
Compounds 1−10 were modeled with the Gaussian 09 suite of
programs42 using density functional theory for the ground-state S0 and
time-dependent density functional theory for the electronic excited
states Sj, j = 1, ... . The S0 geometry optimization was conducted with
B3LYP/6-31+G(d) for the ground state in vacuo and with B3LYP/6-
31G(d) for the ground state in dichloromethane, modeled as a
polarizable continuum according to the default method in Gaussian
09.43,44 The optimized ground-state geometries are assured to be local
minima since all computed vibrational frequencies were real. Two
estimates of the triplet emission frequencies were possible; both used
the optimized geometry of the lowest-energy triplet state, called
T1(opt), obtained with B3LYP/6-31G(d) in vacuo and in dichloro-
methane. One estimate is provided by the energy difference between
T1(opt) and the S0 energy at the same geometry. Another estimate is
obtained by TD-B3LYP/6-311+G(d) calculations of vertical S0−T1
electronic transitions. Both included the solvent model.43,44 Graphic
representations for self-consistent field (SCF) orbitals for the ground
state were obtained with GaussView 5. Triplet structures were
obtained by geometry optimization B3LYP/6-31G(d).
Difluoroboron 2-Naphthoyl-4′-methoxybenzoylmethane
(BF2nbmOMe, 7). 4-Methoxyacetophenone (150 mg, 3.0 mmol),
methyl 2-naphthoate (230 mg, 1.2 mmol), and THF (20 mL) were
added sequentially to a 50 mL round-bottom flask. After the mixture
was stirred for 10 min, a suspension containing NaH (40 mg, 1.5
mmol) in THF (10 mL) was added dropwise at room temperature
under N2. The mixture was stirred for 20 h and then quenched with
saturated aqueous NaHCO3 (1 mL). THF was removed in vacuo, and
then 1 M HCl (20 mL) was added. The aqueous phase was extracted
with CH2Cl2 (3 × 20 mL), and the combined organic layers were
washed with distilled water (2 × 10 mL) and brine (10 mL) and dried
over Na2SO4 before filtration and concentration in vacuo. The residue
was purified by column chromatography on silica gel eluting with
hexanes/ethyl acetate (6:1) to give the diketone precursor, naphthoyl-
4′-methoxybenzoylmethane (nbmOMe) as a yellow solid (240 mg,
EXPERIMENTAL SECTION
Materials. Solvents CH2Cl2 and tetrahydrofuran (THF) were dried
and purified by passage through alumina columns. Boron trifluoride
■
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Table 1. Optical Properties of BF2bdks 1−10 in CH2Cl2
a
b
c
d
e
f
λmax
,
nm
330
365
ε, M−1·cm−1
λem, nm
τF, ns
ϕF
τrad
Stokes shift, cm−1
g
1
2
31 400
49 200
416
417
1.04
0.47
0.0031
0.26
1.81
9.38
1060
h
h
h
380
376
42 900
396
3
4
37 100
0.50
0.30
h,
i
h
(398)
(36300)
(482)
417
(4.69)
(4380)
1660
390
26 900
(8100)
39 900
57 000
66 300
29 700
(11 700)
73 000
34 300
(9900)
j
35
i
(467)
(596)
393
(10.5)
1.02
(4630)
2330
5
6
7
8
360
397
415
404
0.65
1.00
0.68
0.49
1.57
2.06
2.22
433
2.06
2090
449
1.51
1820
h
460
1.96
21.4
2330
i
(451)
(572)
437
(10.5)
1.79
(4590)
1450
9
411
344
0.78
0.63
2.29
15.2
10
h,i
(380)
(459)
(9.55)
(4530)
a
b
c
d
Absorption maxima. Extinction coefficients calculated at the absorption maxima. Fluorescence emission maxima. Fluorescence lifetime excited
with a 369 nm light-emitting diode (LED) monitored at the emission maximum. All fluorescence lifetimes are fitted with single-exponential decays
e
f
g
unless indicated. Relative quantum yield, with anthracene in EtOH as a standard. Radiative lifetime, where τrad = τF/ϕF. Possibly multiexponential
decay. Band is present as a shoulder or second maximum. Values in parentheses correspond to charge-transfer processes. Too weak.
h
i
j
1
79%). H NMR (300 MHz, CDCl3) δ 17.10 (s, 1H, COCHCOH),
RESULTS AND DISCUSSION
■
8.54 (s, 1H, 1′-ArH), 8.06−7.88 (m, 6H, 3′-, 4′-, 5′-, 8′-ArH, 3″-, 5″-
The boron complexes were synthesized by Claisen condensa-
tion with NaH to generate the β-diketones, followed by
ArH), 7.61−7.55 (m, 2H, 6′-, 7′-ArH), 7.03−6.99 (m, 2H, 2″-, 6″-
ArH), 6.95 (s, 1H, COCHCO), 3.90 (s, 3H, -ArOMe); MS (MALDI)
boronation with BF2·OEt2 in CH2Cl2. The rate of Claisen
condensation drops and the product solubility decreases with
increasing arene size. All of the complexes were obtained as
yellow crystals except for the anthryl compounds 4 and 8,
which are dark red powders. The synthesis and solid-state
fluorescence and mechanochromic properties for compounds
1−4 have been described elsewhere.39
m/z calcd for C20H16O3 [M + H]+ 305.11, found 305.07.
Boron trifluoride diethyl etherate (81 μL, 0.64 mmol) was added to
a solution of nbmOMe (130 mg, 0.43 mmol) in CH2Cl2 (20 mL)
under N2. The mixture was refluxed for 12 h and then the solvent was
removed in vacuo. The residue was purified by column chromatog-
raphy on silica gel eluting with hexanes/ethyl acetate (6:1) to give 7 as
a yellow solid (123 mg, 81%). Anal. Calcd for C20H15BF2O3: C, 68.22;
1
Absorption Spectra in CH2Cl2. The optical properties of
compounds 1−10 (Table 1) were first studied in CH2Cl2
because these conditions do not result in boron−ligand
dissociation,45 exciplex formation,46,47 or photochemical
reactions7,8 seen for solvents containing Lewis bases, aromatic
rings, or double bonds. Absorption spectra show that all ten
complexes have high extinction coefficients (26 900−73 000
M−1·cm−1), which is typical of π−π* transitions (Figure 1).
Complex 9 with two methoxyl benzene rings has the strongest
absorption, whereas the weakest absorptions occur for
complexes containing the anthracene moiety, namely, 4 and
8. Compared with complexes 1−4, complexes 5−8 show
similar but stronger and red-shifted absorption spectra, since
the methoxyl substituent possesses electron-donating ability.
Another difference between the two sets of compounds is that
5−8 exhibit slightly sharper absorption spectra [i.e., smaller full
width at half-maximum (fwhm) for major absorption bands],
perhaps due to more rigid structures from the electron-
donating methoxyl groups (inferred from resonance struc-
tures). Figure 1C compares BF2dbm (2) with mono- (6) and
dimethoxy-substituted (9) derivatives to explore substituent
effects. As for 5−8, in this series too, increasing donor strength
correlates with stronger absorption and red-shifted emission.
The naphthylmethyl complex 10 is included because 2 and 10
have similar π conjugation lengths; however, the absorption of
10 is different from 2 in two aspects. First, 10 shows a bimodal
absorption spectrum (344 and ∼380 nm shoulder), as is also
noted for complexes 4 (390 and 467 nm) and 8 (404 and 451
nm); second, the absorption maximum of 10 is far more blue-
shifted than the rest. From the absorption spectra in CH2Cl2, it
H, 4.29; F, 10.79. Found: C, 68.31; H, 4.42; F, 10.52. H NMR (300
MHz, CDCl3) δ 8.74 (s, 1H, 1′-ArH), 8.19 (d, 2H, J = 9.0 Hz, 3″-, 5″-
ArH), 8.08−7.89 (m, 4H, 3′-, 4′-, 5′-, 8′-ArH), 7.66−7.60 (m, 2H, 6′-,
7′-ArH), 7.24 (s, 1H, COCHCO), 7.04 (d, 2H, J = 9.0 Hz, 2″-, 6″-
ArH), 3.94 (s, 3H, -ArOMe); MS (MALDI): m/z calcd for
C20H15BF2O3Na [M + Na]+ 375.10, found 375.03.
Difluoroboron 2-Anthracenoyl-4′-methoxybenzoylmethane
(BF2abmOMe, 8). This complex was prepared as described for 7.
Briefly, 2-acetylanthracene (101 mg, 0.45 mmol), methyl benzoate (91
mg, 0.55 mmol), and sodium hydride (46 mg, 1.82 mmol) in THF (20
mL) gave crude diketone anthroyl-4-methoxybenzoylmethane,
abmOMe, which was purified by column chromatography on silica
gel eluting with hexanes/ethyl acetate (6:1). The diketone was
1
obtained as a crude yellow solid (72 mg, 45%). H NMR (300 MHz,
CDCl3) δ 17.09 (s, 1H, ArCOH), 8.73 (s, 1H, 1′-ArH), 8.58 (s, 1H,
9′-ArH), 8.45 (s, 1H, 10′-ArH), 8.09−7.94 (m, 6H, 3′-, 4′-, 5′-, 8′-
ArH, 3″-, 5″-ArH), 7.54−7.51 (m, 2H, 6′-, 7′-ArH), 7.03−6.96 (m,
3H, 2″-, 6″-ArH, COCHCO), 3.88 (s, 3H, -ArOMe). (A second
species is suggested by minor peaks at 17.13 and 3.90 ppm.)
The boron complex was prepared via the reaction of crude
abmOMe (46 mg, 0.14 mmol) and BF3·OEt2 (36 μL, 0.28 mmol) in
CH2Cl2 (20 mL) and was purified via column chromatography on
silica gel eluting with hexanes/ethyl acetate (1:1) to give 8 (30 mg,
57%) as a red solid. Anal. Calcd for C24H17BF2O3: C, 71.67; H, 4.26;
F, 9.45. Found: C, 71.50; H, 4.20; F, 9.25. 1H NMR (300 MHz,
DMSO-d6) δ 9.30 (s, 1H, 1′-ArH), 8.91 (s, 1H, 9′-ArH), 8.70 (s, 1H,
10′-ArH), 8.44 (d, 2H, J = 8.7 Hz, 3″-, 5″-ArH), 8.27−8.14 (m, 4H,
3′-, 4′-, 5′-, 8′-ArH), 8.01 (s, 1H, COCHCO), 7.67−7.58 (m, 2H, 6′-,
7′-ArH), 7.23 (d, 2H, J = 8.7 Hz, 2″-, 6″-ArH), 3.94 (s, 3H, -ArOMe);
MS (MALDI) m/z calcd for C24H17BF2O3Na [M + Na]+ 425.11,
found 425.13.
C
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Figure 1. Absorption spectra of BF2bdk complexes 1−10 in CH2Cl2.
is reasonable to believe that complexes 4, 8, and 10 (which also
have the smallest extinction coefficient values) have significant
intramolecular charge-transfer (ICT) character compared to
the rest, as evidenced by a second structureless, red-shifted
absorption band in addition to the main absorption peak. One
common feature shared by the three molecules (4, 8, and 10) is
that they are the most unsymmetrically substituted in terms of
arene sizes on the diketonate. More definitive conclusions may
be reached after the emission spectral analysis and computa-
tional results are presented below.
Fluorescence in CH2Cl2. Emission spectra and fluores-
cence lifetimes of the complexes were also measured. The
results are given in Figure 2 and Table 1. As shown in the
photograph in Figure 2, under UV illumination, all the
complexes exhibit intense fluorescence ranging from violet to
orange-red except for 1, which only faintly fluoresces in the
violet-blue region. The Stokes shift, the difference in energy
between the emission-state energy and the energy of its
emitting-state absorption, is also included. In several cases the
lowest energy absorption or highest energy emission bands
were estimated from a shoulder or a weaker peak than the
absolute maximum (see Table 1, footnote h). For the dual-
emitting complexes 4 and 8, data are reported for both
emissions from each complex. The complexes are grouped into
two categories. One has large Stokes shifts (∼4500 cm−1) with
long radiative lifetimes (>9 ns); these include 3, 10, and the
low-energy emissions of 4 and 8. The other complexes and the
Figure 2. Photographs and steady-state emission spectra of BF2bdks
1−10 in CH2Cl2 (λex = 369 nm).
high-energy emissions of 4 and 8 have small Stokes shifts
(1200−2500 cm−1) and short radiative lifetimes (2 ns). As will
be shown, these emissions arise from different types of excited
states.
Theoretically, an increase in π conjugation length typically
brings about a red shift of emission and change in quantum
yield.48 The spectra roughly follow this rule but exceptions
were also observed. For example, the π-donating methoxyl
substituent extends the conjugation of 8 through π donation
compared to 4; however the emissions of 8 are blue-shifted
compared with 4. The same trend was observed for the
naphthalene-containing BF2bdks 3 (Ph-Np, 482 nm), 7 (Np-
PhOMe, 449 nm), and 10 (Me-Np, 459 nm), where the
greatest π conjugation (7) results in the most blue-shifted
emission in CH2Cl2. This observation indicates that con-
jugation length is not the only parameter that influences the
emissive state energy of the complexes. This signals that other
factors such as inductive effects or charge transfer are involved,
as we will discuss in greater detail below.
D
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The fluorescence lifetimes of the complexes were also
measured and the results are collected in Table 1. The radiative
lifetime for each molecule is given by
the π-donating methoxy group does not seem to make a
difference in terms of the fluorescence decay (τF = 10.5 ns for
both 4 and 8), the radiative lifetimes follow the expected trend,
given the differing quantum yields. For 4, τrad is nearly double
the value for 8, indicating a large long-lived contribution from
the ICT state. ICT from anthracene to the diketonate group is
significant regardless of whether benzene or methoxybenzene is
present as a competing donor group. In this process, the phenyl
ring may serve largely as a substituent on the acceptor. The
more electron-rich MeO-Ph ring (versus Ph) may slightly
weaken the acceptor ability of the boron diketonate ring, thus
increasing the energy of the transition. In comparison to
τrad = τF/ϕF
(1)
where τF and ϕF are the observed lifetime and luminescence
quantum yield, respectively. This value, τrad, which is also
known as the instrinsic lifetime, is the lifetime that would be
measured if the there were no radiationless processes
competing with the luminescence decay. The radiative lifetime
τrad is a direct measure of the allowedness of the emission.
Optically dilute complexes 2−10 exhibited single-exponential
decays in CH2Cl2. The decay of 1 was very weak (ϕF < 0.004)
with an estimated multiexponential lifetime of ∼1 ns. Given the
weak emission, the absence of a mirror-image relationship
between the absorption and emission, and the anomalously
long lifetime for the low quantum yield, we attribute the
emission to trace impurities, possibly a very low concentration
conformer, given that it persists from preparation to
preparation, and we consider 1 to be nonluminescent or
extremely weakly luminescent. The anthracene-containing
complexes 4 (Ph-An) and 8 (An-PhOMe) displayed especially
long fluorescence decay (10.5 ns), whereas BF2dbm exhibited
the shortest decay (0.47 ns) despite its reasonably large
fluorescence quantum yield (ϕF = 0.26). However, the addition
of one (6) or two (9) methoxy groups onto BF2dbm increases
the quantum yields of 6 and 9 considerably.
An interesting result was also observed for the fluorescence
lifetimes of the most unsymmetric molecules. Complexes 3, 4,
8, and 10, with different diketonate arenes (or Me), have the
longest radiative decay times (3, nbm, τrad = 9.38 ns; 4, Ph-An,
τrad = 35.0 ns; 8, An-PhOMe, τrad = 21.4 ns; and 10, Me-Np,
τrad = 15.2 ns). The remaining complexes, including symmetri-
cally substituted complexes such as 2 (Ph-Ph) and 9 (PhOMe−
PhOMe), all have much shorter radiative decays of about 2 ns.
The differences in lifetimes for 3, 4, 8, and 10 coupled with the
presence of weaker long-wavelength absorption bands for 4, 8,
and 10 indicate a fundamentally different type of excited-state
emission. Though not as distinct in the UV−vis spectrum for 3,
it could still be argued that the low-energy feature evident at
∼400 nm on the absorbance band also arises from (albeit
weaker) ICT. This is also supported by computational results
presented below.
anthracene dyes 4 and 8, the phenyl derivatives 2 (Ph-Ph, τrad
=
1.81 ns), 9 (PhOMe−PhOMe, τrad = 2.29 ns), and 6 (Ph-
PhOMe, τrad = 2.06 ns) are symmetrically substituted molecules
with respect to the arenes and they show relatively short
fluorescence decay. Within this series, where π−π* transitions
likely dominate the electronic transitions, the methoxy
substituents exert a subtle but measurable effect on fluorescence
decay.
Complexes 4 and 8 exhibit dual emissions (Table 1). There
is a weak high-energy emission and stronger low-energy
emission. The two emissions in spectra for species 4 and 8
clearly derive from distinct ground state species given different
excitation spectra, provided in the Supporting Information.
Both low- and high-energy emissions for 8 have a single-
exponential decay (Table 1). The same is true for 4, though the
high-energy emission is weak; a lifetime could not be obtained.
Furthermore, there is no negative preexponential factor for the
long-lived emission in either case, which establishes that the
low-energy emissions are not arising from the state responsible
for the high-energy emission. We suggest that there are two
ground-state conformers for these complexes. One has a
structure conducive to the π−π* state being lowest energy in
the excited state and gives rise to a short-lived (∼2 ns) π−π*
fluorescence. The dominant structure gives an ICT excited state
lowest in energy and is the source of the much longer-lived ICT
emission (10.5 ns).
In summary, absorption and fluorescence data indicate
moderate to strong ICT character for unsymmetrically
substituted difluoroboron diketonates 3, 4, 8, and 10, while
π−π* transitions dominate for symmetrically substituted
compounds (e.g., 2 and 9). This is consistent with known
solvatochromic properties of this family of dyes, as charge-
separated states are sensitive to the polarity of the surrounding
medium.13,49 The electronic transitions of the BF2bdks depend
on their molecular symmetries. A greater disparity in π-
electron-donating ability of the two arene rings seems to result
in a stronger ICT state dominated by the more electron-rich
aryl group. When the bdk arene ring sizes are comparable, in
more symmetrical structures, a delocalized π−π* model is
proposed. These experimental conclusions are supported by
computational studies discussed below.
As stated above, distinct low-intensity red features in the
absorption spectra for unsymmetrical complexes 4, 8, and 10
are ascribed to intramolecular charge transfer, ICT. Computa-
tional modeling discussed below supports this assertion; these
features are dominated by excitations in which charge is
transferred from the larger, more electron-rich arene toward the
boron diketonate center. The second and more intense feature
lying at higher energy is a π−π* transition where both π and π*
are delocalized throughout the molecule. Since both molecular
orbitals (MOs) occupy a common space, the transition has little
ICT character. In the other more symmetric structures such as
2 or 9, the π−π* transition is at lower energy than any ICT
transition and dominates the long-wavelength region of the
absorption spectrum. As a result, shorter radiative decay
lifetimes are expected.
Computational Studies. The Gaussian 09 software suite42
was used for all computational modeling. Optimized geometries
for species 1−10 were determined with a density functional
theory (DFT) method, B3LYP/6-31G(d). The absorption
spectra were characterized by time-dependent (TD) density
functional theory, specifically TD-B3LYP, in a more flexible
basis, 6-311+G(d). TD-DFT methods represent excited states
as combinations of the single excitations of an electron from an
orbital occupied in the SCF ground state to an orbital in the set
vacant in the ground state. The wavelength and intensity of
These observations about absorbance also inform the
interpretation of Ph-An (4) and An-PhOMe (8) complex
emissions, where 4 has a stronger ICT tendency than 8. From
the fluorescence spectra, the emission maximum of 4 is red-
shifted by 24 nm (704 cm−1) compared to that of 8. Though
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vertical absorption transitions correlate well with experimental
counterparts, with R2 = 0.91 between absorption wavelengths
computed with no consideration of solvent effects and the band
maxima as reported in Table 1. Incorporation of solvent effects
by the polarizable continuum model (Tomasi’s PCM) brings
predicted absorption wavelengths into close agreement with
observed values, increases R2 to 0.96, and also slightly improves
the estimates of absorption intensities. The calculations, in
vacuo or with solvent treatment, not only capture the general
form of the absorption curves but also track the intensity
enhancement and wavelength shifts attending structural
changes. The Supporting Information contains graphs of
computed spectra and pictures of relevant orbitals, which
may aid in comparison.
Figure 3. Orbitals involved in the absorption spectra for complexes 2
(Ph-Ph) and 4 (Ph-An) in methylene chloride medium.
Trends in the computed and observed absorption spectra can
be described most easily by considering subsets of related
molecules. The family 1−4 has a common Ph-R structure and
dye group 5−8 has a common structure of PhOMe-R, with R =
methyl, phenyl, naphthyl, and anthryl. The subset 1, 5, and 10
(Me-Ph, Me-PhOMe, and Me-Np respectively) allows
examination of the relative effects of methoxy substitution
and extending the aryl from phenyl to naphthyl. Finally the
subset 2, 6, and 9 (Ph-Ph, Ph-PhOMe, and PhOMe−PhOMe,
respectively) allows discussion of the cumulative effects of
methoxy substitution.
expressed in the discussion of the emission properties of these
systems.
When aryl substituents on the difluoroboron diketonate ring
are identical (2), spectra are dominated by HOMO to LUMO
excitations; both orbitals are delocalized throughout the
molecule. When the substituents are very different (4), the
spectrum has a broad weak red feature, derived from a HOMO
− LUMO ICT transition, and a higher-energy more intense
feature, in which the HOMO − 1 to LUMO and HOMO to
LUMO + 1 excitations are strongly mixed. Additional orbital
depictions and characterization of the transitions may be found
in the Supporting Information.
Modeling permits further remarks on the absorption spectra.
In most cases one can discern two strongly overlapping bands
in the leading red edge of the optical absorption spectra. The
two bands seem to grow more separate and distinct as the size
of the aromatic substituent grows, in either series 1−4 or 5−8,
until they become entirely separate in 4 and 8. This is evidently
another consequence of the systematic elevation of the highest
occupied MO of the aryl moiety, which eventually allows
distinct π−π* transitions: first the low-energy ICT from
HOMO to LUMO and then the fully delocalized HOMO − 1
to LUMO.
The spectra for subset 1, 5, and 10 (Me-Ph, Me-PhOMe, and
Me-Np, respectively) suggest that methoxy substitution on
phenyl elevates the highest occupied MO without blocking its
delocalization, while extending the aryl to naphthyl localizes the
highest occupied MO more thoroughly on the aryl fragment. In
the computed spectrum a weak red HOMO to LUMO
transition distinct from the more intense and higher energy
HOMO − 1 to LUMO feature is noted. This finds its
counterpart as a red shoulder in the first broad absorption band
of the experimental spectrum.
Finally the sequence of methoxy substitution in subset 2, 6,
and 9 (Ph-Ph, Ph-PhOMe, and PhOMe−PhOMe, respectively)
shows successive red shifts and intensity enhancements as first
one and then two p-methoxy substituents are introduced. Each
methoxy elevates its aryl highest occupied MO without going
so far as to impede delocalization. Again, computed results
correlate with experiment.
The families 1−4 and 5−8 show a red shift in the low-energy
edge of the optical spectrum as the aryl substituent increases in
size. Extending the size of the arene raises the energy of its
highest occupied molecular orbital (HOMO). If there is a
reasonable energy match between the HOMO for the aryl
fragment and MOs in the boron-containing ring, there will be
substantial mixing between them. This gives rise to
delocalization over the entire molecule, elevation of the highest
occupied MO, and a redder transition. If the energy match is
poor, the aryl MO remains localized, and the HOMO (aryl) to
lowest unoccupied molecular orbital (LUMO) (delocalized)
transition changes character to a charge transfer. The charge-
transfer band may have relatively weak intensity, owing to
reduction in Franck−Condon factors, and can be very red. It
has been noted that most DFT functionals, including B3LYP, is
generally inaccurate in predicting transition energies involving
long-range ICT states.50,51 Nonetheless, this feature of the
absorption spectra is captured by the modeling, and is a
significant part of what we see in the anthryl-substituted species
4 and 8, where contrast between aryl rings is greatest and the
effect on the spectrum is most pronounced. Similar but less
evident features are found for 3 and 10. For example, the
computed spectrum for complex 3 suggests the presence of a
weak absorption with some ICT character, to the red of a
stronger π−π* transition. The weaker red absorption computed
at 430 nm may not be clearly distinguishable from the stronger
blue transition computed at 372 nm owing to the breadth of
the absorption bands. As inferred from the experimental
observations, compound 4 appears to have the greatest ICT
character. See Figure 3 for a visualization of transitions for
molecules 2 and 4 determined with the solvent correction.
Table 2 contains a more detailed summary of these results.
It is interesting to note that species 4 and 8 display the
strongest Stokes shift in the emission. That is in accord with
with the substantial geometrical rearrangement to be expected
in the charge-transfer excited state. It is clear that the modeling
is entirely consistent with the interpretation of the experimental
absorption spectra and lends indirect support to the ideas
Even approximate treatment of solvent effects has had some
positive effects. For example, the excitation wavelength
calculated in vacuo is always blue-shifted compared to the
experimental value; the PCM version of the polar solvent
dichloromethane induces a red shift, toward the experimental
values. This behavior is consistent with the observed
solvatochromism of this class of compounds.13,49 The
calculation is still approximate however, and absorption
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Table 2. Comparison of Experimental and Calculated Optical Properties of BF2bdks 1−10 in CH2Cl2 and in Vacuo
a
b
c
d
e
f
λmax
,
nm
ε, M−1·cm−1
λcalc CH2Cl2, nm
f
λcalc in vacuo, nm
f
transition type
1
2
3
330
365
31 400
49 200
37 100
306
364
372
430
385
539
338
387
0.6111
0.9764
0.9648
0.2791
0.8325
0.2525
0.8090
1.0768
0.7417
0.6436
1.2386
0.3155
1.2893
0.7263
0.1541
306
342
353
0.4671
0.7761
0.8200
π−π*
π−π*
π−π*
ICT
376
g
g
398
36 300
4
390
467
360
397
415
26 900
8100
359
498
325
361
367
0.9262
0.1723
0.7202
0.8994
0.7811
π−π*
ICT
5
6
7
39 900
57 000
66 300
π−π*
π−π*
π−π*
hi
,
389
422
hi
,
8
404
451
411
29 700
11 700
73 000
34 300
9900
403
525
399
338
405
382
485
370
324
380
0.7828
0.2092
1.1035
0.5522
0.106
π−π*
ICT
9
π−π*
π−π*
ICT
10
344
g
380
a
b
c
Experimental absorption maxima in dichloromethane. Extinction coefficients calculated at the absorption maxima in dichloromethane. Computed
d
e
transition wavelength (in dichloromethane medium). Computed oscillator strength (in dichloromethane medium). Computed transition
wavelength (in vacuo). Computed oscillator strength (in vacuo). Low-energy ICT band is present as a shoulder. Two overlapping transitions: ε is
proportional to the sum of squares of f ’s. Complex 7 exhibits two transitions of roughly comparable energy. These each have some ICT character
f
g
h
i
but are predominantly π−π*.
intensities are poorly described. In particular, compounds 4 and
8 have much weaker absorption than their relatively high
oscillator strengths would predict. It may be that more
advanced methods52,53 can be employed to better estimate
the excitation energies of this family of complexes.
Polymer Medium Effects: Fluorescence of BF2bdk/PLA
Blends. The optical properties of the BF2bdk series were also
measured in poly(lactic acid), a solid-state solvent, at both
room temperature and 77 K with two different dye loadings.
The PLA used in the experiment was synthesized via solvent-
free ring-opening polymerization.54 Boron complexes 1−10
and PLA were dissolved in CH2Cl2 to form a homogeneous
solution that was used to cast a thin film onto the inner wall of
a small vial. The film was then thoroughly dried in vacuo before
luminescence measurements. The measured optical properties
for these blends at room temperature are presented in Figure 4
and Table 3.
Compared to the emission data in CH2Cl2, for the more
dilute 0.05% (w/w) dye/polymer loading, the fluorescence
emission maxima at room temperature are typically blue-shifted
in the PLA substrate, possibly because PLA is a less polar and
more rigid medium than CH2Cl2. A combination of
solvatochromic and rigidochromic effects55 may explain the
blue shifts, which are greatest for emissions associated with ICT
transitions of complexes 4 (Ph-An) and 8 (An-PhOMe) (64
and 58 nm, i.e., 2018 and 1972 cm−1 respectively) with large
anthracene substituents. This too supports the hypothesis that
the strongest ICT states are present in these anthracene
complexes, because in the solid-state solvent PLA, the
presumed ICT “twisted configuration” may be hindered.56
The emission spectra from 4 and 8 in PLA are bimodal, with 4
possessing a more conspicuous high-energy shoulder at 419 nm
and 8 at 430 nm). In fact, even in CH2Cl2, both 4 and 8 have
additional high-energy bands around 417 and 460 nm,
respectively (Figure 2), which are weaker compared to those
recorded in PLA. It is not uncommon to observe dual emission
bands for donor−acceptor fluorescent molecules in polar
solvents. According to twisted intermolecular charge-transfer
(TICT) theory,29 the two distinct fluorescence emission bands
of complexes 4 and 8 might be ascribed to a TICT state (lower
energy) and a coplanar locally excited state (higher energy), as
discussed above for solution and computational studies.
Interestingly, the shoulder emission of complex 4 shares almost
the same emission maxima and vibronic features as those for
the dbm complex 2; similarly for complex 8, the higher energy
shoulder closely matches to the fluorescence of dbmOMe
complex 6.
The fluorescence lifetimes measured for the complexes in
PLA were all fitted to triple-exponential decays, except for the
Me-Ph derivative 1, which did not show strong enough
intensity for measurement with the current instrument setup.
As has been described previously, the solid-state polymeric
environments are heterogeneous, and therefore multiexponen-
tial decays are common for BF2bdks.20 Compared to
fluorescence lifetimes recorded in CH2Cl2, the values obtained
in PLA did not follow an obvious trend. However, we did
observe some reductions in τF in PLA versus CH2Cl2 for
molecules that exhibited strong ICT character (e.g., ICT band
in PLA vs CH2Cl2: 4, 10.1 vs 10.5 ns; 8, 9.12 vs 10.5 ns; and 10,
5.35 vs 9.55 ns), which again suggests that the ring twisting may
be partially inhibited or that there are reduced relaxation effects
in the rigid and less polar PLA matrix at room temperature.
From the steady-state fluorescence spectra, most of the
polymer films with higher dye loading exhibit slightly red-
shifted emission maxima (Table 3 and Figure 4), likely due to
the exciton coupling or “fluorophore−fluorophore interac-
tion”24 as previously reported. Methyl dyes 1 and 10 lacking a
methoxyl substituent and the dbm complex 2 serve as
exceptions. The fluorescence lifetimes, accordingly, are
consistently longer for molecules showing red-shifted emission
maxima at higher concentrations. The increase in lifetimes for
high dye loadings has also been discussed in terms of dye−dye
interactions, where excimeric species are likely to form. The
propensity of BF2bdk dyes to form H aggregates has also been
advanced as a possibility.57
Temperature effects were also studied by collecting steady-
state emission spectra at 77 K in liquid nitrogen for all BF2bdk/
PLA blends at two different dye loadings (Figure 5 and Table
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Figure 4. Photograph and steady-state emission spectra of BF2bdk/PLA blends at (A−C) 0.05% and (D−F) 0.5% dye loadings at room temperature.
(λex = 369 nm except for 1 and 5, where λex = 350 nm).
4). For dye blends at lower concentrations, most emission
maxima are slightly blue-shifted, probably due to the rigid-
ochromic effect at liquid nitrogen temperature. The blue shifts
are generally small; the low loading of compound 2 is even
slightly red-shifted, indicating that PLA at room temperature,
below the glass transition temperature of ∼60 °C, is already
rather rigid. At low temperature, the emission spectra also show
sharper vibrational structure for complexes 1−4. The spectra
are also narrowed for BF2bdks 5−8, which have methoxy
substitutions, and exhibit less prominent vibrational features.
Furthermore, distinct phosphorescence bands are observed for
certain complexes at low temperature (e.g., naphthyl complexes
3, 537 nm; 7, 538 nm; and 10, 523 nm). Further discussion of
the emissive triplet state for these materials is provided below.
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Table 3. Optical Properties of BF2bdk/PLA Blends at Room
Temperature
Table 4. Optical Properties of BF2bdk/PLA Blends at 0.05%
and 0.5% Dye Loadings at 77 K
0.05% (w/w) BF2bdk/PLA
0.5% (w/w) BF2bdk/PLA
0.05% (w/w) BF2bdk/PLA
0.5% (w/w) BF2bdk/PLA
a
b
a
b
a
b
a
b
λF, nm
τF, ns
λF, nm
τF, ns
λF, nm
τF, ns
λF, nm
τF, ns
c
c
1
2
3
4
4
5
6
7
8
8
9
415
415
c
413
415
c
1
2
3
4
4
5
6
7
8
8
9
412
d
391
d
c
1.82
2.31
d
1.67
3.66
1.81
(10.8)
0.89
3.79
1.83
1.98
(13.43)
3.71
6.58
421
441
2.11
2.40
e
413
2.19
3.33
1.54
(10.10)
0.84
2.12
1.39
2.03
(8.31)
1.75
7.81
448
453
449
418
419
428
418
e
f
(532)
389
(10.1)
0.61
2.36
1.44
1.22
(9.12)
2.44
5.35
(540)
392
(524)
385
(9.35)
0.64
2.06
1.42
1.85
(7.87)
1.77
(531)
387
427
435
422
435
437
443
434
425
430
430
427
431
e
f
(514)
429
(531)
435
(523)
425
(529)
432
c
10
429
413
10
426
6.59
409
a
b
a
b
λex = 369 nm. Pre-exponential weighted lifetimes. All fluorescence
Excited with 369 nm LED. Pre-exponential weighted lifetimes. All
c
lifetimes were fitted with triple-exponential decays. Signal too weak to
be detected with current LED excitation. Weak. Values in
parentheses correspond to charge-transfer processes.
fluorescence lifetimes were fitted with triple-exponential decays.
d
e
c d
Excited at 350 nm. Signal too weak to be detected with current
e f
LED excitation. Weak. Values in parentheses correspond to charge-
transfer processes.
Low-temperature fluorescence lifetimes were also examined for
comparison with room-temperature data. Most compounds
have moderately increased fluorescence lifetimes, as expected at
lower temperatures due to less collisional quenching. However,
more significant differences in both emission spectra and
lifetimes were observed for the 0.5% dye/PLA blends at 77 K.
Many of the differences are probably caused by inhibition of
excimer formation at liquid nitrogen temperature and a
significant reduction in fluorescence lifetimes (e.g., lifetime
Figure 5. Fluorescence emission of BF2bdk/PLA blends at (A−C) 0.05% and (D−F) 0.5% dye loading at 77 K (λex = 369 nm except for 1 and 2 at
0.5% loading and 5 for both loadings, where λex = 350 nm).
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Figure 6. Normalized emission spectra for BF2bdk complexes 2 (Ph-Ph), 6 (Ph-PhOMe), and 9 (MeOPh-PhOMe) under N2 (left, 0.05% dye
loading; right, 0.5% dye loading) at room temperature and 77 K. F, total emission spectrum under N2 (with dominant fluoresence); RTP, room-
temperature phosphorescence spectrum; LTP, low-temperature (77 K) phosphorescence spectrum. (λex = 369 nm.)
change for 9, PhOMe−PhOMe, from 3.71 ns at room
temperature to 1.75 ns at 77 K). In fact, fluorescence lifetimes
at 77 K for boron dyes at different loadings are quite close to
the lifetimes at room temperature, which confirms that the
lengthened lifetimes at the high loadings at room temperature
are not likely to be caused by trivial effects such as reabsorption
and reemission. The results suggest that even in a rigid polymer
matrix such as PLA, aggregate or excimer formation is possible
for smaller BF2bdks at room temperature.
Polymer Medium Effects: Phosphorescence of
BF2bdk/PLA Blends. As described above, in PLA matrices
under air, most of the complexes exhibit intense blue
fluorescence except complexes 4 and 8, which exhibit yellow
and greenish-yellow emission, as shown in Figure 4. When
steady-state emission spectra were acquired at 77 K for
BF2bdk/PLA blends, phosphorescence emission spectra were
observed for complexes 3 (Ph-Np), 7 (Np-PhOMe), and 10
(Me-Np) (e.g., Figure 5A−C). As previously reported,20 in the
solid state, amorphous PLA serves as a rigid medium that may
block thermal decay pathways and at the same time hinder
quencher molecules from interacting with the dyes in the
excited states.58 Thus, in PLA matrices even room-temperature
phosphorescence (RTP) is observed for BF2bdks. Because RTP
can be quenched by oxygen in ambient air, the BF2bdk/PLA
blends at two different dye loadings (0.05% and 0.5%) were
also studied under a N2 atmosphere, collecting both total
emission spectra (fluorescence plus phosphorescence), RTP
spectra (gated intensity), and low-temperature phosphores-
cence (LTP) spectra (77 K, in liquid nitrogen). Under a
nitrogen atomosphere, distinct green or yellow RTP was
observed for all BF2bdks/PLA blends except for complexes 1
(Me-Ph), 4 (Ph-An), and 8 (An-PhOMe). Phosphoresence for
5 was very weak; emission maxima could be determined but
lifetimes were not detectable (RT under N2 in PLA: for 0.05%
loading, 466 nm; for 0.5% loading, 487 nm). (Room- and low-
temperature emission spectra for 5 in PLA are provided in the
Supporting Information.)
The emission spectra of 2 (Ph-Ph), 6 (Ph-PhOMe), and 9
(PhOMe−PhOMe) (0.05% and 0.5% dye loading) are
provided in Figure 6. Compared to fluorescence, the relative
intensity of RTP is quite low; the total emission spectra (both
fluorescence and phosphorescence under nitrogen) are nearly
identical to the fluorescence spectra (under air) (Figure 4).
Phosphorescence spectra, both room temperature and 77 K,
were obtained by gated measurement with a delay time of 2 ms
after excitation. The shoulder band observed in certain
phosphorescence spectra (e.g., 2 at ∼420 nm, 6 at 427 nm, 9
at ∼430 nm), which matches the fluorescence emission, has
been ascribed to delayed fluorescence caused by the thermal
repopulation of the excited singlet state from the long-lived
excited triplet state.20,24 Complexes 2, 6, and 9 all have phenyl
moieties on both sides of the diketone but are non-, mono-, and
bis-methoxy-substituted, respectively. With the presence of
methoxyl substituents, phosphorescence emission maxima for 6
and 9 exhibit distinct red shifts compared to 2. Whether one or
two -OMe groups are present, however, seems to have little
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Table 5. Optical Properties of BF2bdk Complexes in PLA Matrices under Nitrogen
0.05% (w/w) BF2bdk/PLA
0.5% (w/w) BF2bdk/PLA
a
b
b
c
a
b
b
c
λt , nm
λp(RT), nm
λp(77 K), nm
τpw0, ms
λt , nm
λp(RT), nm
λp(77 K), nm
τpw0, ms
2
414
422
426
439
431
427
488
504
501
537
538
523
486
494
505
533
535
522
513
440
402
440
463
603
420
439
439
454
444
434
488
505
504
541
541
526
488
510
513
542
544
527
581
386
214
520
424
612
6
9
3
7
10
a
b
Total emission maxima (i.e., fluorescence peak) under N2; excited at 369 nm. Phosphorescence emission maxima under N2; excited at 369 nm.
c
Pre-exponential weighted room-temperature phosphorescence (RTP) lifetime fit to triple-exponential decay. Excitation source: xenon flash lamp.
Figure 7. Normalized emission spectra for BF2bdk complexes 3 (Ph-Np), 7 (Np-PhOMe), and 10 (Me-Np) under N2. (Left, 0.05% dye loading;
right, 0.5% dye loading) at room temperature and 77 K. F, total emission spectrum under N2 (with dominant fluoresence); RTP, room-temperature
phosphorescence spectrum under N2; LTP, low-temperature (77K) phosphorescence spectrum. (λex = 369 nm.)
effect on emission wavelength. Unlike the more significant red
shift in fluorescence emission in going from low to high dye
loading, the phosphorescence merely exhibits a slight red shift
in room temperature spectra [e.g., 6, Δλ = 8 nm (430 cm−1) for
fluorescence (Table 3) and Δλ = 1 nm (39 cm−1) for
phosphorescence (Table 5) from low to high dye loading]. The
delayed fluorescence band intensity increases and red-shifts for
the higher dye loading, consistent with previously reported dye
loading effects.24 At 77 K, where thermal back population of the
singlet state is inhibited, delayed fluoresence is not evident.20,35
In the absence of oxygen, the RTP decay of BF2bdk/PLA
blends is complicated and can be fitted with a triple-
exponential. Pre-exponential weighted lifetimes,59 τpw0, are
provided in Table 5. The phosphorescence lifetimes decrease
from 2 to 6 and 9 with increasing number of methoxy groups.
Figure 7 shows the total and phosphorescence spectra of 3
(Ph-Np complex), 7 (Np-PhOMe complex), and 10 (Me-Np
complex), with 0.05% and 0.5% dye loadings, respectively.
Obvious phosphorescence contributions were observed in the
total emission spectra for these complexes because of the
relatively strong triplet emission. Similar to complexes 2, 6, and
9, the phosphorescence exhibits red-shifted emission as a result
of the increase of π conjugation length from complex 10 to
complexes 3 and 7 (e.g., 0.05% dye loading: 10, 523 nm; 3, 537
nm; and 7, 538 nm). At high dye loading, RTP lifetimes
decrease for this series from 10 to 3 to 7 (612, 520, and 424 ms,
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respectively; Table 5) with increased conjugation length.
Delayed fluorescence bands are also observed for complexes
3 and 7 at ∼450 nm and are slightly stronger at higher loading,
given a smaller singlet−triplet splitting, as previously
described.24 And again, delayed fluorescence is generally
inhibited at low temperature, given decreased thermal back
population.20,35
Computational studies were also performed for the triplet
states. We modeled the phosphorescence with the following
simplifying assumptions. Treating the PLA medium as a
polarizable continuous medium with the same dielectric
constant as dichloromethane, and optimizing the lowest triplet
with B3LYP/6-31G(d) to obtain E(T1), we can approximate
the energy of the phosphorescence as either (a) the energy
difference between that triplet energy E(T1) and the E(S0)
energy obtained with B3LYP/6-31G(d) at that same geometry
or (b) the transition energy for the singlet−triplet excitation as
computed by TD/6-311+G(d) at the B3LYP/6-31G(d)
optimized T1 geometry. Table 6 shows values of estimates of
population can occur by thermal excitation of the singlet state
from the triplet state if the thermal energy kT is comparable to
the S1−T1 energy gap.60 Therefore, the energy gap between S1
and T1 has an important influence on this reverse conversion.
Given the molecular structure of these complexes, we may draw
a preliminary conclusion. Complexes with greater π conjugation
length (e.g., 3 and 7 versus 10) and donor strength (e.g., 6 and
9 versus 2), are more inclined to undergo reverse intersystem
crossing, perhaps because a lower energy S1 level decreases the
S1−T1 energy gap. For higher dye loading, dye−dye distances
are shorter, aggregates may form, and the medium is more
polar. Under these circumstances, singlet-state energy also
decreases while phosphoresence energy is little affected,
facilitating intersystem crossing.
CONCLUSION
■
In summary, we have synthesized a class of aromatic
hydrocarbon-substituted difluoroboron diketonate complexes
(BF2bdks) ranging from methyl, phenyl, methoxyphenyl,
naphthyl, to anthryl groups on either side of the diketonate
ligands. The optical properties of these boron complexes were
studied in CH2Cl2 and in the solid-state solvent PLA. In
CH2Cl2, the absorption, fluorescence emission, and fluores-
cence lifetimes were investigated for all of the BF2bdks. It was
found that almost all complexes exhibit intense fluorescence
emission (ϕF 0.26−1) except for one molecule, which has
phenyl and methyl groups on the BF2bdk (1). While one might
think that simply extending the conjugation would lead to red-
shifted dyes, there appear to be two or more factors at play, and
properties in PLA environments are different than in solution.
The emission colors of the series of boron dyes are correlated
not only to the length of their π conjugation but also to the
tendency for intramolecular charge transfer (ICT) in solution;
larger π conjugation and stronger ICT result in more red-
shifted emission. In a lifetime study, we discovered that the
extent of the ICT processes for these molecules are determined
by the electron-donating ability of the arene groups on the two
sides of the diketonate complexes. A bigger difference in
electron-donating ability on the two sides correlates with
stronger ICT and longer fluorescence lifetime, which is
predicted by modeling and calculations. The optical properties
of these boron dyes were then investigated in PLA substrates at
two different dye loadings, 0.05% and 0.5%, at both room
temperature and 77 K. Most of the boron complexes exhibit
slightly blue-shifted emission and reduced fluorescence life-
times in PLA at lower dye loadings, suggesting the hindrance of
ICT processes by the polymer medium. At higher dye loadings,
however, both spectra and lifetimes representative of aggregate
or excimeric species were observed, which is consistent with the
previous report24 and has been ascribed to dye−dye
interactions in close proximity. These interactions were reduced
at 77 K, possibly due to the rigidity of the medium, which can
limit the configurations required for excimeric species. Delayed
emission, namely, phosphorescence and delayed fluorescence,
are also present for many dyes in PLA environments under
nitrogen. Studying the emission properties of these model
compounds may inform the rational design of BF2bdks for
practical applications in imaging and sensing.
Table 6. Phosphorescence Emission Maxima and
Computational Estimates of the Singlet−Triplet Gaps
emission maxima, nm
wavelengths, nm
0.05% in
PLA/N2
0.5% in
PLA/N2
from TD S−T
from B3LYP S−T
b
a
absorption
energy gap
1
2
3
4
5
6
7
8
9
10
540
564
701
1105
549
590
691
1095
590
697
501
554
621
899
510
547
611
886
548
616
486
533
488
542
466
494
535
478
505
544
505
522
504
527
a
Singlet−triplet excitation energy estimated by TD-B3LYP/6-
b
311+G(d), at the UB3LYP/6-31G(d) optimized T1 geometry. Direct
differences of B3LYP/6-31G(d) for singlet and UB3LYP/6-31G(d)
for triplet, all at the UB3LYP/6-31G(d) optimized T1 geometry.
both types. Linear regression shows a correlation statistic R2 =
0.89 between observed phosphorescence wavelengths and
values computed by direct difference of B3LYP/6-31G(d)
values, and R2 = 0.92 between observed values and estimates
from TD/6-311+G(d)//B3LYP/6-31G(d) calculations.
Though the linearity as measured by the R2 statistic is slightly
better for the TD calculations than for direct energy differences,
the TD estimates are more seriously shifted to the red. The
very red results for complexes 4 and 8 suggest that their
phosphorescence may lie outside the region monitored
experimentally with the present instrumentation and hence
would explain why phosphorescence was not observed for these
anthryl derivatives.
In summary, when the dye complexes were excited from the
ground state to a higher vibrational level of S1 (or S2), the
majority of the molecules readily relax (10−12 s) to the lowest
vibrational level, S1, through internal conversion. From the S1
state, excited molecules may decay radiatively in several
nanoseconds and exhibit fluorescence. They can also undergo
a spin conversion to the triplet state by intersystem crossing
and exhibit phosphorescence. Although this and the reverse
processes are spin-forbidden, under certain conditions, back
ASSOCIATED CONTENT
* Supporting Information
Seven tables listing B3LYP/6-31+G(d) and B3LYP/6-31G(d)/
■
S
PCM optimized ground-state structures and Gaussian log files
L
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Inorganic Chemistry
Article
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AUTHOR INFORMATION
Corresponding Author
*E-mail: fraser@virginia.edu.
■
Present Addresses
†G.Z.: CAS Key Laboratory of Soft Matter Chemistry,
Department of Polymer Science and Engineering, University
of Science and Technology of China, Hefei, Anhui, 230026,
China. E-mail: gzhang@ustc.edu.cn.
‡R.E.E.: Department of Physics, Harvard University, Cam-
bridge, Massachusetts 02138, United States. E-mail:
ruffinevans@fas.harvard.edu.
V. E. Russ. Chem. Bull. 2004, 53, 291.
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Notes
The authors declare no competing financial interest.
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Trans. 2010, 40, 377.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (CHE 0718879,
CHE 1213915) for support for this research, and the Goldwater
Scholarship Foundation for a scholarship to R.E.E. This work
was also supported in part by the National Cancer Institute of
the National Institutes of Health (R01 CA167250), and the
UVA Cancer Center through the Farrow Fellowship Fund and
the NIH NCI Cancer Center Support Grant (P30 CA44579).
We also thank Nguyen D. Nguyen and Alan D. Chien for their
assistance.
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