Mechanochromic luminescence quenching: Force-enhanced singlet-to-triplet intersystem crossing for iodide-substituted difluoroboron-dibenzoylmethane- dodecane in the solid state

Article abstract of DOI:10.1021/ic902591s

A lipid derivative of difluoroboron-iododibenzoylmethane (BF ₂dbm(I)OC₁₂H₂₅) was synthesized via Claisen condensation and boronation. Green photoluminescence is observed for the complex in the solid state. Unlike the previ

Full text of DOI:10.1021/ic902591s

Inorg. Chem. 2010, 49, 10747–10749 10747
DOI: 10.1021/ic902591s
Mechanochromic Luminescence Quenching: Force-Enhanced Singlet-to-Triplet
Intersystem Crossing for Iodide-Substituted
Difluoroboron-Dibenzoylmethane-Dodecane in the Solid State
†
‡
,†
Guoqing Zhang, Jiwei Lu, and Cassandra L. Fraser*
†
‡
Department of Chemistry and Department of Materials Science and Engineering, University of Virginia,
Charlottesville, Virginia 22904
Received December 29, 2009
7
A lipid derivative of difluoroboron-iododibenzoylmethane (BF -
thermally reversible ML. A significantly red-shifted fluore-
2
dbm(I)OC H ) was synthesized via Claisen condensation and
boronation. Green photoluminescence is observed for the complex
scence spectrum was observed when solid samples were ground
or smeared. Crystalline or ordered BF AVB corresponds to
12 25
2
in the solid state. Unlike the previously reported difluoroboron-
higher energy emission, while the amorphous state is responsible
avobenzone (BF AVB) complex, which exhibited significantly red-
shifted fluorescence upon mechanical perturbation, the emission of
a BF dbm(I)OC H solid film is quenched when the sample is
for lower energy emission. Although the mechanism of BF AVB
2
2
ML is not entirely understood, these findings indicate that singlet
excited-state energy can be altered by mechanical processes.
2
12 25
smeared under air but becomes orange under nitrogen. Spectro-
scopic and lifetime studies suggest that smearing brings the singlet
excited state closer to the triplet state, thus increasing the coupling
between the two states. As a result, intersystem crossing from the
singlet to the triplet excited state is facilitated, and the total
luminescence intensity is quenched at room temperature.
BF dbm molecules display strong excited-state interactions
that can affect fluorescence. Previously, this phenomenon was
2
studied in a polymer matrix, where BF dbm was covalently
2
attached to a polylactide (PLA) chain. The singlet excited-state
energy dropped most dramatically for short BF dbmPLA
2
8
chains (i.e., stronger fluorophore-fluorophore interactions).
A material with tunable fluorescence-to-phosphorescence
(F/P) ratios was also achieved based on this model. When
the polymer chain is shorter, the excited-state interactions are
stronger, the singlet state energy decrease is more significant,
and the singlet-to-triplet intersystem crossing is enhanced9,
especially in the presence of an iodide internal heavy atom.
This material design concept was successfully applied to
ratiometric oxygen sensing for optical tumor hypoxia imaging
with the versatile boron dye-polymer system fabricated as
nanoparticles. If the singlet excited-state energy can also be
lowered after mechanical stimulation, enhanced intersystem
crossing is expected here too. Here we extend the F/P tuning
concept to nonpolymeric boron diketone complexes in the
solid state, where the triplet process is enhanced mechanically.
Piezochromism or mechanochromism refers to the color
change of materials in response to pressure or other mecha-
nical stimuli. These color changes are often highly dependent on
the solid-state morphology and can be induced by material
1
structural changes such as bond breaking or forming on the
2
molecular level or by larger-scale dye aggregation, or domain
3
,4
spacing effects. Specifically, if mechanical perturbation causes
a change in emission, it is called mechanochromic lumine-
5
,6
scence (ML). Recently, we discovered that a simple boron-
sunscreen complex, BF AVB, exhibits polymorphism and
2
*To whom correspondence should be addressed. E-mail: fraser@virginia.edu.
(
1) Davis, D. A.; Hamilton, A.; Yang, J.; Cremar, L. D.; Van Gough, D.;
Potisek, S. L.; Ong, M. T.; Braun, P. V.; Martinez, T. J.; White, S. R.; Moore,
J. S.; Sottos, N. R. Nature 2009, 459, 68–72.
(
2) Crenshaw, B. R.; Burnworth, M.; Khariwala, D.; Hiltner, A.; Mather,
P. T.; Simha, R.; Weder, C. Macromolecules 2007, 40, 2400–2408.
3) Arsenault, A. C.; Clark, T. J.; von Freymann, G.; Cademartiri, L.;
(
Sapienza, R.; Bertolotti, J.; Vekris, E.; Wong, S.; Kitaev, V.; Manners, I.;
Wang, R. Z.; Sajeev, J.; Wiersma, D.; Ozin, G. A. Nat. Mater. 2006, 5, 179–184.
(
4) Caruso, M. M.; Davis, D. A.; Shen, Q.; Odom, S. A.; Sottos, N. R.;
White, S. R.; Moore, J. S. Chem. Rev. 2009, 109, 5755–5798.
The luminescent boron complex BF dbm(I)OC H
(1) was synthesized via Claisen condensation followed by
(
5) Recent review:Sagara, Y.; Kato, T. Nat. Chem. 2009, 1, 605–610.
6) Selected examples: (a) Lee, Y. A.; Eisenberg, R. J. Am. Chem. Soc.
2
12 25
(
2
003, 125, 7778–7779. (b) Ito, H.; Saito, T.; Oshima, N.; Kitamura, N.; Ishizaka,
S.; Hinatsu, Y.; Wakeshima, M.; Kato, M.; Tsuge, K.; Sawamura, M. J. Am.
Chem. Soc. 2008, 130, 10044–10045. (c) Sagara, Y.; Mutai, T.; Yoshikawa, I.;
Araki, K. J. Am. Chem. Soc. 2007, 129, 1520–1521. (d) Chung, J. W.; You, Y.;
Huh, H. S.; An, B.-K.; Yoon, S.-J.; Kim, S. H.; Lee, S. W.; Park, S. Y. J. Am.
Chem. Soc. 2009, 131, 8163–8172.
(
7) Zhang, G.; Lu, J.; Sabat, M.; Fraser, C. L. J. Am. Chem. Soc. 2010,
1
32, 2160-2162.
(8) Zhang, G.; Kooi, S. E.; Demas, J. N.; Fraser, C. L. Adv. Mater. 2008,
20, 2099–2104.
(9) Lower, S. K.; El-Sayed, M. A. Chem. Rev. 1966, 66, 199–241.
r 2010 American Chemical Society
Published on Web 03/10/2010
pubs.acs.org/IC

1
0748 Inorganic Chemistry, Vol. 49, No. 23, 2010
Zhang et al.
Figure 2. Normalized steady-state emission spectra of 1 solid film on a
piece of weighing paper at room temperature (RT) under air (A) and at
7
7 K (B) (λex = 369 nm). TA: thermally annealed. The image insets show
smearing-induced emission “turn off” (i.e., MLQ in dark region) at RT
under air (A) and red-shifted phosphorescence “turn on” (i.e., orange
region) at 77 K (λex = 365 nm).
Figure 3. AFM images of films of 1 spin-cast from dilute CH
2 2
Cl onto
glass before (A) and after heating (B) for 1 min in a 110 ꢀC oven.
Figure 1. Reversible MLQ (λex = 365 nm) of a thermally annealed solid
film of1onapieceofweighingpaper (A), “INORG”writtenwitha cotton
swab tip seen as shadow areas (B), background emission restored by
heating the film for 1 min in a 110 ꢀC oven (C), and rewritable MLQ
demonstrated by “CHEM” generated with a cotton swab tip (D). Steady-
state luminescence spectrum under air showing an intensity decrease after
smearing (E) and the fluorescence emission intensity monitored at 500 nm
under air versus smearing/thermal erasing of the cycle number (F).
boronation in CH Cl (Supporting Information). A bright-
2
2
yellow waxy solid (mp = 144-146 ꢀC) was obtained after
silica gel chromatography. The lipid derivative was chosen to
increase both the solubility of the iodide dye and the possible
organization of the boron complexes in the solid state, given
Figure 4. Normalized delayed emission spectra of a solid film of 1 on a
piece of weighing paper at room temperature (A) and 77 K (B) (λex =369nm,
delay time 1.0 ms, under nitrogen) TA: thermally annealed.
spontaneously at room temperature or much faster at ele-
vated temperature (∼3-5 s with a heat gun). The thermal
reversibility was also demonstrated by monitoring the lumin-
escence emission at 500 nm, where a cyclic drop and rise in
the intensity was recorded (Figure 1F). The recovery may be
related to the strong tendency of 1 to form ordered aggre-
gates, as evidenced by atomic force microscopy (AFM)
images (Figure 3).
5
that material ordering can play an important role in ML.
The optical properties of 1 were studied in a CH Cl solution.
2
2
The absorption (λmax = 409 nm) and emission (λ = 446 nm)
F
spectra (Figure S1 in the Supporting Information) are similar
1
0
to those for a reported BF dbm(I) derivative, but the
2
fluorescence lifetime (τ = 1.50 ns) and quantum (Φ =
F
F
0
.67) yield of 1 have increased values.
In the solid state, ML was observed for a film of 1 on a
If this MLQ behavior at room temperature is indeed due to
increased triplet quenching (e.g., oxygen or collisional), an
increase in the relative phosphorescence intensity would be
expected at low temperature, where these processes are
hindered. At room temperature, the normalized emission
spectra of 1 before and after smearing are similar. At 77 K,
however, the smeared sample shows a much stronger peak
2
piece of weighing paper (∼3 mg smeared on 2 ꢀ 2 in ). After
thermal annealing at 110 ꢀC for 1 min, the solid film showed
green luminescence under UV light. When the solid film was
rubbed with a cotton swab, the smeared regions turned
darker; with brief thermal treatment, the dark regions were
restored. This reversible ML quenching (MLQ) is illustrated
in Figure 1A-D. Despite the reduction in the emission inten-
sity from steady-state spectroscopy (Figure 1E), normalized
emission spectra of 1 before and after smearing are almost
superimposable. The smeared solid spectrum has a slightly
larger high-energy shoulder and a small shift in the maximum
intensity (Figure 2A; before, λ = 496 nm; after, λ = 500 nm).
11
around 570 nm (Figure 2). To confirm that the lower-
energy peak corresponds to phosphorescence, delayed emis-
sion spectra (Δt = 1 ms) were collected for the thermally
annealed and smeared films of 1 both at room temperature
under nitrogen and at 77 K (Figure 4). At room temperature,
F
F
Similar to BF AVB, the ML of 1 was found to recover
2
(11) MLQ caused by an increase in the surface area during smearing has
been ruled out. This was supported by the fact that the fluorescence loss at
room temperature is roughly 60% and the phosphorescence gain at 77 K is
also roughly 60%.
(
10) Zhang, G.; Palmer, G. M.; Dewhirst, M. W.; Fraser, C. L. Nat.
Mater. 2009, 8, 747–751.

Communication
Inorganic Chemistry, Vol. 49, No. 23, 2010 10749
the two delayed spectra are almost identical, with emission
maxima at 560 and 558 nm, respectively. At 77 K, however,
the maxima are red-shifted by ∼10 nm to 567 and 571 nm.
The red shift in delayed emission spectra at lower tempera-
tures has been previous ascribed to a diminished contribution
from delayed fluorescence (blue-shifted relative to the phos-
phorescence peak), where thermal back-population from
8
triplet to singlet excited states is inhibited. Overlapped exci-
tation spectra at 77 K monitored at the two peak intensities
Figure 5. Fluorescence lifetime decays of a BF
solid film on a piece of weighing paper monitored at 466 nm (A) and
30 nm (B) (λex = 369 nm LED). TA: thermally annealed.
2
dbmOC12H25 (control)
(
λ = 500 and λ = 570 nm) also indicate that these features
F P
arise from the same species (Figure S2 in the Supporting
Information).
5
These results suggest that, for smeared solid films of 1, the
triplet excited-state population is indeed increased. As has
been discussed before, eq 1 explains how a decreased energy
gap in the presence of a heavy atom can enhance the singlet
wavelength. A consistent increase in the lifetime from 7.26 to
13.28 ns was also recorded when the emission was monitored
at 530 nm, but no initial rise in the decay profile characteristic
of exciton migration was observed (Figure 5). This does not
rule out exciton migration, however, given that our lifetime
instrument (>200 ps) may not reveal fast processes.
In summary, we have described a heavy-atom-substituted
difluoroboron-dibenzoymethane-lipid dye that exhibits
mechanosensitive intersystem crossing and reversible MLQ
in the solid state. Under UV excitation, the relative ratio of
singlet-to-triplet excited-state populations is decreased when
a solid film of the dye is smeared or scratched. One possible
explanation is that mechanical processes lower the singlet
excited-state energy level and thus increase the degree of
singlet and triplet state coupling according to perturbation
theory. Luminescence under nitrogen at room temperature
and 77 K shows that the relative intensity of phosphorescence
increases in the smeared solid film. This increase corresponds
nicely to the drop in the fluorescence intensity at room
temperature under air, suggesting near-complete singlet
quenching upon mechanical perturbation. This unique
force-induced method of altering the intersystem crossing
may be useful in the design of luminescent mechanical
sensors.
1
3
(
Ψ)-triplet ( Ψ) mixing and, thus, the intersystem cross-
9,10
ing.
For BF AVB, without a heavy-atom effect, only a
2
bathochromic shift in fluorescence was observed upon
mechanical perturbation. In the case of a solid film of 1,
mechanical force is likely to induce the same singlet energy
drop; however, with iodide substitution, this also corres-
ponds to an increased fraction of the singlet excited-state
species crossing over to the triplet excited state. At room
temperature, the triplet excited state for the dye-lipid deriva-
tive may be quenched by oxygen or may preferentially decay
by thermal pathways.
3
1
Æ ΨjHsoj Ψæ
δ ¼
ð1Þ
jE
1
-E
3
j
If this hypothesis is true, enhanced intersystem crossing
may also be reflected in a reduced fluorescence lifetime (τF)
for the smeared sample. Indeed, when monitored at the peak
intensity (500 nm), a reduction of τ from 1.63 to 1.31 ns was
F
recorded after smearing. However, the decrease in τ could
F
be due to enhanced intersystem crossing from increased
excited-state interactions or due to exciton migration. To
Acknowledgment. We thank the National Science
Foundation (Grant CHE 0718879) for support for this
work and Prof. J. N. Demas (Department of Chemistry,
University of Virginia) for helpful discussions.
better understand the mechanism, τ of BF dbmOC H , a
F
2
12 25
control compound of 1 without iodide substitution, was also
measured. Comparison of the τ values in CH Cl [1 (1.50 ns)
F
2
2
vs control (2.05 ns)] indicates a higher intersystem-crossing
efficiency for 1. When monitored at the emission maximum,
λ = 466 nm, for solid films, the control τ increases from
Supporting Information Available: Methods, experimental
details, and solution absorption and emission spectra (Figure
S1) and solid-state excitation spectra (Figure S2) of 1. This
material is available free of charge via the Internet at http://
pubs.acs.org.
F
F
6
.69 to 11.27 ns upon smearing, suggesting changes in the
excited-state energy. To explore whether exciton migration is
also involved, we measured τF of the control at a higher