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.
1 (a) R. Touzani and H. Alper, J. Mol. Catal. A: Chem., 2005, 227,
197; (b) C. Dong and H. Alper, J. Org. Chem., 2004, 69, 5011; (c)
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204–205, 97.
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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,
490.
6 C.-Y. Liu, D.-Y. Chen, G.-H. Lee, S.-M. Peng and S.-T. Liu,
Organometallics, 1996, 15, 1055.
7 K. L. Shaklee, R. E. Nahory and R. F. Leheny, J. Lumin., 1973, 7, 284.
8 (a) W. T. Silfvast and J. S. Deech, Appl. Phys. Lett., 1967, 17, 97; (b)
O. G. Peterson, J. P. Webb, W. C. McColgin and J. H. Eberly,
J. Appl. Phys., 1971, 42, 1917; (c) C. V. Shank, Rev. Mod. Phys., 1975, 47,
649.
ꢀ
ꢁ
2
aðlÞ~ ln
L
IL
{1
IL
=2
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 2693–2695 | 2695