A new class of laser dyes, 2-oxa-bicyclo[3.3.0]octa-4,8-diene-3,6-diones, with unity fluorescence yield

Article abstract of DOI:10.1039/b604539a

A new class of highly fluorescent dyes, 4,8-diphenyl-2-oxa-bicyclo[3.3.0] octa-4,8-diene-3,6-diones (1a-c), have been synthesized, they all exhibit unity fluorescence quantum yield and short radiative lifetime (< 4 ns) in common organic solvents and have demonstrated remarkable amplified spontaneous emission with a gain efficiency of > 10. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b604539a

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A new class of laser dyes, 2-oxa-bicyclo[3.3.0]octa-4,8-diene-3,6-diones,
with unity fluorescence yield{
Chao-Yu Wang, Yu-Shan Yeh, Elise Y. Li, Yi-Hong Liu, Shiuh-Ming Peng, Shiuh-Tzung Liu* and
Pi-Tai Chou*
Received (in Cambridge, UK) 28th March 2006, Accepted 26th April 2006
First published as an Advance Article on the web 22nd May 2006
DOI: 10.1039/b604539a
polarity independent visible fluorescence in common organic
solvents. The emission peak wavelength can be tuned from green
(526 nm, 1a) to orange-red (y590 nm, 1c). Detailed synthetic
routes and unique photophysical properties such as the exploration
of amplified spontaneous emission are elaborated below.
A new class of highly fluorescent dyes, 4,8-diphenyl-2-oxa-
bicyclo[3.3.0]octa-4,8-diene-3,6-diones (1a–c), have been synthe-
sized, they all exhibit unity fluorescence quantum yield and
short radiative lifetime (, 4 ns) in common organic solvents
and have demonstrated remarkable amplified spontaneous
emission with a gain efficiency of . 10.
Compounds 1a–c were prepared according to a simple synthetic
route, depicted in Scheme 1.6 As for a prototypical example for
synthesizing 1a, an autoclave (100 mL) was loaded with phenyl-
acetylene and rhodium complex (or other metal complexes) in dry
toluene. The reactor was flushed with nitrogen several times and
then pressurized with carbon monoxide. The reaction mixture was
stirred at 110 uC. After that, the solvents were evaporated and the
residue was chromatographed on silica gel. The crude product was
thenrecrystallizedfromdiethylethertogive 1a asared-purplesolid.
To optimize the yield of the desired product 1a, screening the
reaction by using various rhodium complexes was performed,
and the results are summarized in Table 1. The best activity for
this transformation is the use of the rhodium carbonyl cluster
[Rh6(CO)16] (0.4 mol%), giving rise to compound 1a as a red solid
in 25% yield (entry 9). The products other than 1a were greasy
materials, which were believed to be certain polymeric species.
In addition to the 1H NMR and infrared spectroscopic data (see
ESI{), the 13C NMR and single crystallographic measurement
provide detailed structural information on 1a. 13C NMR shifts at d
191.3 (ketone) and 170.2 (ester) are characteristic of carbonyl
carbons, which are consistent with the infrared CLO stretching at
1752 and 1746 cm21. Single crystals of 1a were obtained by
recrystallization from chloroform.{ X-Ray analyses revealed that
there are two essentially identical molecules in the asymmetric unit
of compound 1a. Thus, this system is statistically in between
centrosymmetric and non-centrosymmetric space groups. The
structure was solved under a non-centrosymmetric space group.
Alternatively, we made an attempt to solve the structure by using a
centrosymmetric space group but unfortunately failed. Fig. 1
shows an ORTEP plot of one of the two independent molecules of
1a. Selected bond distances, angles and dihedral angles are
summarized in Table S1 (see ESI{). All bond distances and bond
angles of 1a are in the normal range. All C–C single bonds except
Owing to their special photophysical properties such as high
luminescent yield, subjective to environmental perturbation, etc.,
aromatic compounds possessing a six membered-ring lactone
structure have been attracting much attention.1 Among these
derivatives, 2H-chromen-2-one and its analogues have been
ubiquitously applied as laser dyes, the categories of which have a
well-known common name, ‘‘coumarin dyes’’.2 Applications other
than laser dyes have also been explored in numerous territories
such as ion and molecular recognition,3 two-photon absorption,4
etc. In comparison to six-membered-ring lactones, lactones with
fused bicyclic five-membered rings are far less familiar.5
Particularly, to the best of our knowledge, there is no report on
the corresponding optical applications up to the present time. In
preparing polymers from the precursor 1-ethynylbenzene or its
derivatives catalyzed by various Rh (or Ir) catalysts,5 we
serendipitously explored a new series of 4,8-diphenyl-2-oxa-
bicyclo[3.3.0]octa-4,8-diene-3,6-diones denoted as compounds
1a–c (see Scheme 1). In contrast to the generally recognized weak
luminescent properties for five membered-ring lactones, 1a–c, with
a small, simple molecular structure, all exhibit near unity, solvent
˚
C(4)–C(5) and C(5)–C(6) are shorter than 1.54 A, presumably due
to the conjugation of p-bonds. A notable feature is the bond
lengths of C(6)–C(8) and C(2)–C(14) (bonding between the phenyl
ring and the bicyclic system), which might be due to the constraint
arising from the fused bicyclic [3.3.0] system. The fused ring
bicyclic system of 1a adopts a planar geometry, as evidenced by the
dihedral angles C(6)–C(7)–C(3)–C(2) [2179.5(6)u] and C(4)–C(3)–
C(7)–O(1) [179.7(5)u]. Similar planarity was also resolved for the
Scheme 1 The synthesis of compounds 1a–1c.
Department of Chemistry and Instrumentation Center, National Taiwan
University, Taipei 106, Taiwan. E-mail: chop@ntu.edu.tw;
stliu@ntu.edu.tw; Fax: +886 (2) 2369 5208; Tel: +886 (2) 3366 3894
{ Electronic supplementary information (ESI) available: Detailed synth-
eses, characterization and measurements. See DOI: 10.1039/b604539a
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Table 1 Results of metal catalyzed coupling of phenylacetylene and COa
Catalyst loading
Additive
C7H8/ml
Time/h
Yield (%)
1
2
3
4
5
6
7
8
9
[Rh(COD)Cl]2 1.2 mol%
RhCl(PPh3)3 10 mol%
RhCl(PPh3)3 10 mol%
[Rh(COD)Cl]2 1.2 mol%
Co(OAc)2 2.5 mol%
[Ir(COD)Cl]2 1.2 mol%
Pd(COD)Cl2 2.5 mol%
[RuCl2(C10H14)]2 2.5 mol%
Rh6(CO)16 0.4 mol%
Rh6(CO)16 1.6 mol%
Rh6(CO)16 0.4 mol%
—
—
5
5
5
50
50
50
5
16
16
16
24
48
16
24
24
24
24
24
3
Trace
0
10
4
0
0
AgBF4 10 mol%
dppe 1.2 mol%
—
—
—
—
—
—
5
0
50
50
50
25
11
18
10
11
a
2, 2.5 mol%
Phenylacetylene: 0.1 ml, CO: 500 psi, 2: pentacarbonyl 1,3-diethylimi-dazolidin-2-ylidene tungsten(0).
(3.2 ¡ 0.6) 6 104 and (2.8 ¡ 0.6) 6 104 M21 cm21 for 1a, 1b
and 1c, respectively, in THF suggests that the associated transition
is an allowed p A p* in character. The absorbance peak is
linearly proportional to the added 1a–1c concentrations from
1027–1024 M. This, in combination with the negligible change of
the absorption profile, eliminates the possibility of an aggregation
effect. 1a exhibits intensive emission with a peak wavelength at
526 nm, which is nearly unperturbed by the solvent polarity.
Rationally tuning of the transition gap, i.e. the peak wavelength, is
achieved by the substitution of electron donating –CH3 (1b) or
–OCH3 (1c) groups on the phenyl group, giving an emission
maximum at 544 and 586 nm, respectively. The quantum yield,
Wf, within experimental error, was measured to be ca. unity for
all 1a–c in both CH3CN and THF. The short radiative lifetime of
, 4 ns for 1a–c further supports the predominantly radiative
decay character.
Fig. 1 A view of one of the two independent molecules of 1a, with
ellipsoids drawn at the 30% probability level.
other independent molecule, as indicated by the dihedral angles
C(25)–C(26)–C(22)–C(21) [179.3(6)u] and C(23)–C(22)–C(26)–O(4)
[179.0(6)u] (see Table S1 and Fig. S7 in ESI{). The results clearly
demonstrate conjugation via p bonding. In addition, two phenyl
rings are also coplanar with the bicyclic system, which is revealed
by the dihedral angles of C(3)–C(2)–C(14)–C(15) [24(1)u] and
C(7)–C(6)–C(8)–C(9) [2(1)u]. These observations indicate that this
molecule is in a slightly distorted planar geometry.
Theoretical confirmation of the underlying basis for the
photophysical properties of 1a–1c was provided by the ab initio
MO and time-dependent density function theory (TDDFT, see
ESI{) calculations. As for 1a, Table 3 depicts the descriptions and
the S0–S1 energy gap as well as the features of the lowest
unoccupied (LUMO) and highest occupied (HOMO) frontier
orbitals, mainly involved in the S0 A S1 transition. Apparently,
the electron densities of the HOMO are located largely on the
delocalized p-orbital from the bicyclic core throughout the
diphenyl moieties, while that of the LUMO are distributed mainly
to the p-conjugated bicyclic core, indicating that the lowest
electronic transition is dominated by a pp* character. According to
the oscillator strength f y0.73, the S0 A S1 transition from the
HOMO is reasonably considered as an allowed process, for which
the energy gap of 472 nm correlates well with the strong y450 nm
absorption band with an extinction coefficient of 2.5 6
104 M21 cm21 in THF. The addition of an electron donating
–CH3 group on the phenyl ring should raise the HOMO energy,
reducing the HOMO–LUMO gap. Further red-shifted emission in
1c is expected due to the stronger –OCH3 donating property from
Fig. 2 shows the room-temperature absorption and emission
spectra of 1a–c in THF. Details of the photophysical properties of
1a–1c in THF are listed in Table 2. The observation of an S0 A S1
absorption band maximum at 446 (1a), 452 (1b) and 478 nm (1c)
with an extinction coefficient as high as e 5 (2.5 ¡ 0.5) 6 104,
Table 2 Photophysical properties of 1a–1c in THF (298 K)
Abs/nm
l
em/nm lASEa/nm Wf tf/ns Gain coefficientb
1a 446
526
544
586
526
543
563
1.0 3.9
1.0 3.7
1.0 3.5
10.2
13.2
10.8
1b 452
1c 478
a
b
The peak wavelength of amplified spontaneous emission. The
maximum gain coefficient of ASE gain spectrum.
Fig. 2 Steady state absorption and fluorescence spectra for 1a–c in THF
with an excitation wavelength of y400 nm.
2694 | Chem. Commun., 2006, 2693–2695
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The wavelength dependence of the ASE gain coefficient, i.e. the
gain spectrum, is obtained from the analysis of the spectroscopic
data recorded using the above equation. As shown in the insert of
Fig. 3, the experimental gain spectrum of 1a in THF at a pump
power of 10 mW covered a region from y500 to 560 nm with a
maximum at 526 nm and a gain efficiency of y10, which is
comparable with many other commercially available laser dyes.
The lack of a triplet state (T1) population, and hence no
interference from the T1–Tn (n . 1) absorption at the ASE region,
is believed to play a key factor for observing the stimulating
emission. Similar results were obtained for 1b and 1c, with ASE
peak wavelengths 543 and 563 nm, respectively (see Table 2). In
the case of 1b, a maximum gain efficiency of as high as 13.2 can be
obtained. As for a test of stability, for 1a–c, the ASE remained
unchanged with 10 mW (443 nm, 10 Hz) pumping within a period
of one hour. This, in combination with the tunability of ASE,
generates a new series of potential laser dyes in the green-orange
region.
Table 3 The calculated energy level of the lowest-lying transition and
the associated frontier orbitals for 1a
Assignments
l/nm
E/eV
F
S1 HOMO A LUMO(+72%)
HOMO
472
LUMO
2.62
0.73
In conclusion, we have synthesized a new class of highly
fluorescent dyes, 1a–c, that all exhibit unity fluorescence quantum
yield and short radiative lifetime (, 4 ns) in common organic
solvents. The pp* transition character has been fully justified
via theoretical approaches. 1a–c also demonstrate remarkable
amplified spontaneous emission with a gain efficiency of . 10.
This, in combination with the excellent photostability, makes
1a–c potentially novel laser dyes incorporating a bicyclic, fused
five membered-ring lactone structure. Future applications of
this series of dyes are wide ranging. For example, through a
strategic design of the substituents on the phenyl group, syntheses
of water-soluble fluorescent dyes can be achieved, such that
their applications toward fluorescent imaging and biomolecular
recognition become feasible. Work focusing on this issue is
currently in progress.
Fig. 3 Fluorescence and amplified spontaneous emission (ASE) of 1a
(2.1 6 1023 M), 1b (1.1 6 1023 M) and 1c (2.3 6 1023 M) in THF.
Insert: ASE gain spectrum of 1a in THF at a pump power of 10 mW. See
text for the definition and experimental set up.
the HOMO, the predictions of which are consistent with
experimental results.
Notes and references
{ Crystal data for 1a: C19H12O3 (M = 288.29), orthorhombic, space group
Pca21, a 5 11.7937(4), b 5 12.9224(5), c 5 18.4121(6) s, a 5 b 5 c 5 90u,
V 5 2806.1(2) s3, Z 5 8, m = 0.092 mm21, Dc 5 1.365 g cm23, T 5 295(2)
K, 2533 independent reflections (Rint 5 0.1298), final R indices
(398 parameters) [I . 2s(I)] are R1 5 0.0692, wR2 5 0.1411,
GOF 5 1.039. CCDC 602233. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b604539a
The high absorptivity, unity fluorescence yield of 1a–1c with a
fast radiative decay time of , 4 ns led us to examine the possibility
of lasing properties through the measurement of amplified
spontaneous emission (ASE) and its associated gain spectrum. In
this experiment, optical gain measurements were carried out using
the variable stripe length method.7 In this study, the second
harmonic (443 nm, pulse width 8 ns) of a Q-switched Nd:YAG
pumped Ti:Sapphire laser was focused into a stripe on the quartz
cell by a cylindrical lens. The stripe length was adjusted by a
barrier coated with opaque material. The stripe was aligned near
the edge of the square quartz cell, and the emission was collected
from the edge at a 90u angle with respect to the excitation beam.
The emission was analyzed using a polychromator coupled to an
intensified charge coupled detector.
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K. Yoshihara, J. Phys. Chem. A, 1998, 102, 3089.
Fig. 3 shows the ASE spectra of 1a–1c in THF. The exponential
dependence of the intensity of the ASE on the gain coefficient a (l)
leads to the observed band narrowing. By measuring the intensity
IL of ASE from the entire cell length L and the intensity IL/2
from the cell half-length, one can evaluate the ASE gain a(l),8
expressed as
4 W. G. Fisher, E. A. Wachter, F. E. Lytle, M. Armas and C. Seaton, Appl.
Spectrosc., 1998, 52, 536.
5 J. Kiji, S. Yoshikawa and J. Furukawa, Bull. Chem. Soc. Jpn., 1974, 47,
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8 (a) W. T. Silfvast and J. S. Deech, Appl. Phys. Lett., 1967, 17, 97; (b)
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J. Appl. Phys., 1971, 42, 1917; (c) C. V. Shank, Rev. Mod. Phys., 1975, 47,
649.
ꢀ
ꢁ
2
aðlÞ~ ln
L
IL
{1
IL
=2
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Chem. Commun., 2006, 2693–2695 | 2695