Multi-emissive difluoroboron dibenzoylmethane polylactide exhibiting intense fluorescence and oxygen-sensitive room-temperature phosphorescence

Article abstract of DOI:10.1021/ja0720255

Boron difluoride compounds are light emitting materials with impressive optical properties. Though their strong one- and two-photon absorption and intense fluorescence are well-known and exploited in molecular probes, lasers, and photosensitizers, phospho

Full text of DOI:10.1021/ja0720255

Published on Web 07/04/2007
Multi-Emissive Difluoroboron Dibenzoylmethane Polylactide Exhibiting
Intense Fluorescence and Oxygen-Sensitive Room-Temperature
Phosphorescence
Guoqing Zhang,† Jianbin Chen,† Sarah J. Payne,† Steven E. Kooi,‡ J. N. Demas,† and
Cassandra L. Fraser*,†
Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22904, and Institute for Soldier
Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received March 22, 2007; E-mail: fraser@virginia.edu
When boron difluoride dibenzoylmethane (BF2dbm) is coupled
with poly(lactic acid) (PLA), a biocompatible, biodegradable poly-
mer,1,2 the fluorescence quantum yield is enhanced and unexpected
optical properties emerge, namely, temperature-sensitive delayed
fluorescence and oxygen-sensitive room-temperature phosphores-
cence (RTP). Single-component, multi-emissive materials are rare,
and synthesis and fabrication of BF2dbmPLA are straightforward,
offering many advantages over existing materials for imaging and
ratiometric sensing.3
Fluorescent boron difluoride dyes such as “bodipy”4 and boron
diketonates5 exhibit large molar extinction coefficients and two-
photon absorption cross sections, high emission quantum yields,
Figure 1. Polymer synthesis and boron initiator and polymer luminescence
(λex ) 365 nm; room temperature in air unless indicated). BF2dbmOH:
(A) solid-state green-yellow fluorescence; (B) red phosphorescence (77 K);
and sensitivity to the surrounding medium.6 These features have
been exploited in molecular probes,4 photosensitizers,7 and lasers.8
Phosphorescence also offers advantages for optical sensing,3
(C) blue fluorescent CH2Cl2 solution. BF2dbmPLA: (D) short-lived blue
including larger Stokes shifts and minimal interference from short-
lived fluorescence and scattering. But for these boron systems,5,9
fluorescence and (E) long-lived green phosphorescence for a thin film (N2);
(F) blue fluorescent particle suspension in H2O after 45 days.
it is only observed at low temperatures in solid matrices, thus
limiting its utility. Though RTP is common for heavy-metal-based
luminophores with singlet-triplet state mixing and microsecond-
millisecond lifetimes, rigid or organized media and toxic heavy
atom assistance (e.g., Pb, Tl, halides) are often required for main
group triplet emission,3,9,10,11 which is longer lived (millisecond-
second) and more sensitive to oxygen quenching. Though a liability
for certain light emitting materials applications, this sensitivity can
be exploited for oxygen sensors3,12 and photodynamic therapy.7
Given the limitations of known sensor materials, systems with
strong singlet and triplet emission, easy synthesis and fabrication,
and good stability and biocompatibility are highly desired.
Hydroxyl-functionalized difluoroboron dibenzoylmethane, BF2-
dbmOH (1), was prepared for use as an initiator in the ring opening
polymerization of lactide to produce BF2dbm end-functionalized
polylactide, BF2dbmPLA (2). The boron complex, 1, was made
from dbmOH13 and BF3•Et2O in CH2Cl2 (60 °C, 1 h). After
recrystallization from acetone/hexanes, air-stable, bright yellow
needles were obtained in good yield (75%). The boron polymer, 2,
was generated from BF2dbmOH and DL-lactide using a tin octoate
catalyst (1/lactide/Sn(oct)2 ) 1:200:1/50; 130 °C). The solvent-
free reaction was stopped at ∼50% monomer conversion to avoid
higher polydispersity indices (PDIs) noted for longer reaction times,
due to transesterification and thermal depolymerization. After
precipitation, a pale greenish yellow polymer was obtained.
Molecular weight data determined by gel permeation chromatog-
raphy and 1H NMR spectroscopy are in good agreement: Mn(GPC)
) 8800, PDI ) 1.09; Mn(NMR) ) 8600. Furthermore, key proton
resonances associated with the boron dbm polymer end group are
shifted compared to the initiator (e.g., BF2dbmOCH2CH2OR: R
) H, 4.04 ppm; R ) PLA, 4.54 ppm).
The optical properties of the boron initiator and polymer were
investigated in CH2Cl2 solution and in the solid state (Figure 1).
UV/vis spectroscopic data are similar for BF2dbmOH (λmax ) 397
nm, ꢀ ) 53 000 M-1 cm-1) and BF2dbmPLA (λmax ) 396 nm, ꢀ )
50 100 M-1 cm-1), showing high molar absorptivities characteristic
of this family of compounds. Under black light illumination (λex
) 365 nm) in air, intense blue fluorescence is observed for BF2-
dbmOH (Figure 1C) and BF2dbmPLA (λem ∼ 436 nm), and in both
cases, fluorescence quantum yields, φF, are very high (1: 0.95; 2:
0.89; compare to BF2dbm, 0.20, and 1,3-di(4-methoxyphenyl)-
propane-1,3-dione, 0.85).6 Due to the near UV excited fluorescence,
solutions appear blue even in ambient light or upon flashlight
illumination.
In the solid state, in contrast, BF2dbmOH displays greenish
yellow emission that is sensitive to the solid form (λem, crystals:
540 nm, Figure 1A; powder: 512 nm).14 Association in the ground
state or excited state (e.g., as dimers or excimers) is likely involved.
Fluorescence lifetimes, τ, in air for the initiator 1 (in CH2Cl2: 2.0
ns; crystals: 8.9 ns) and polymer 2 (in CH2Cl2: 1.9 ns) fit to a
single-exponential decay, further verifying sample homogeneity.
In the solid state, emission spectra for BF2dbmPLA also exhibit
blue fluorescence (foam: 451 nm; film: 440 nm, Figure 1D; air),
but fluorescence lifetimes fit to double-exponential decay (foam:
90% 4.8 ns, 10% 20 ns; film: 90% 1.2 ns, 10% 4.4 ns) perhaps
due to heterogeneous polymer microenvironments or fluorophore
aggregation. Two-photon absorption with blue emission was also
confirmed for BF2dbmPLA using ∼800 nm excitation, demonstrat-
ing compatibility with bioimaging technologies that afford greater
tissue penetration, reduced cell damage, and minimal interference
from biological absorbers.15 Samples continue to emit strongly even
† University of Virginia.
‡ Massachusetts Institute of Technology.
9
8942
J. AM. CHEM. SOC. 2007, 129, 8942-8943
10.1021/ja0720255 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Normalized emission spectra for a BF2dbmPLA thin film. Room
temperature total emission under vacuum (442 nm), fluorescence under air
(442 nm), and phosphorescence (509 nm) with delayed fluorescence (∼450
nm) under vacuum after the excitation is turned off. The phosphorescence
spectrum at 77 K (517 nm) is also shown. (Note the absence of the ∼450
nm feature in the low-temperature spectrum.) Reported wavelengths
represent emission maxima and λex ) 365 nm.
Figure 3. Lifetime Stern-Volmer plot for a BF2dbmPLA film showing
oxygen quenching of room temperature phosphorescence.
tions from shorter lived (<10 ms) and very low amplitude long-
lived (>0.1 s) components with the present instrumentation. Further
investigation is merited.
By incorporating a classic boron dye into a common biopolymer,
a readily processable, single-component, multi-emissive material
exhibiting intense fluorescence, delayed fluorescence, and unusual
room-temperature phosphorescence is achieved. The optical proper-
ties of BF2dbmPLA are responsive to temperature, oxygen, and
the polarity and rigidity of the local medium. Further development
of these multifunctional biomaterials for imaging and sensing,
including emission color tuning, properties optimization, polymer
fabrication, and biological testing, serve as the subjects of future
reports.
after months in aqueous suspension (Figure 1F), conditions under
which PLA degradation is known to occur.
Upon deoxygenation, solid BF2dbmPLA also exhibits long-lived,
green room-temperature phosphorescence (RTP) (Figure 1E). (No
RTP is observed for solutions.) This is surprising because, normally,
phosphorescence is only seen at low temperatures for boron
compounds of this type.5,9 For example, BF2dbmOH crystals display
seconds long red phosphorescence at 77 K (Figure 1B). Triplet
thermal decay pathways may be restricted by the rigid poly-
mer medium as in the solid state and solvent glasses. Above the
glass transition temperature for BF2dbmPLA, Tg ) 52 °C, no phos-
phorescence is observed. Green RTP is seen for solid BF2dbmPLA
(Figure 1E), including in aqueous suspension, once the excitation
source is removed, and can last for as long as 5-10 s.
Room-temperature emission spectra for a BF2dbmPLA thin film
are provided in Figure 2, namely, total emission (fluorescence plus
phosphorescence) under vacuum, fluorescence in air where phos-
phorescence is quenched, and phosphorescence for an evacuated
sample following termination of excitation. The green phospho-
rescence spectrum at 77 K is also shown. Without gating, the
primary phosphorescence peak at 509 nm is buried under the much
more intense fluorescence peak. In addition to fluorescence and
phosphorescence, there is also a shoulder in the phosphorescence
spectrum around 450 nm, which closely matches the fluorescence.
Given that fluorescence lifetimes are very short and phosphores-
cence spectra are not collected until after fluorescence has ceased,
this feature may be ascribed to delayed fluorescence caused by
thermal repopulation from the excited triplet state to the excited
singlet state.16 The shoulder disappears at low temperature, where
repopulation may be blocked due to insufficient activation energy,
lending support to this proposal. Thus, BF2dbmPLA serves as a
temperature-sensitive material, too.
Acknowledgment. We thank R. C. Somers and Profs. D. G.
Nocera, and E. L. Thomas at MIT, and the NSF (CHE 0410061 to
J.N.D. and CHE 0350121 to C.L.F.) and Radcliffe Institute for
Advanced Study at Harvard University (C.L.F.) for support.
Supporting Information Available: Experimental details for the
synthesis and luminescence measurements for 1 and 2. This material
is available free of charge via the Internet at http://pubs.acs.org.
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