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An example of the interest potential of such microemulsions is the following. Molecular liquids may usually not be obtained in the glassy state unless the melting point is low relative to the boiling point, Tb/T,,, > 2.738 0-Xylene, with Tb/T,,, = 1.7, therefore, usually does not vitrify but can be vitrified in ordinary emulsion form both because of the suppression of the heterogeneous nucleation phenomenonlggand of the reduction in homogeneous nucleation fluctuation probability associated with the smaller system size?+10 The microemulsion state takes this latter
factor close to the limit since critical nuclei, which must at least exceed the crystal unit cell in dimensions, are of the order 10-20 A in diameter. Thus, vitrification of a much wider range of liquids should be observable in microemulsified form. As a first example we have been able to vitrify p-xylene which has a very unfavorable Tb/Tmratio, viz. 1.43. Further work along these lines will be reported in a future articlee2
Acknowledgment. The support of this work by a National Science Foundation under Grant No. DMR 8007053 is gratefully acknowledged. (9) B. J. Mason, Ado. Phys., 7, 22 (1958); 17, 71 (1945). (10) D. Turnbull and J. C. Fisher, J. Chem. Phys., 17, 71 (1949).
The Proton-Transfer Laser. Galn Spectrum and Ampllficatlon of Spontaneous Emission of 3-Hydroxyflavone P.Chou, D. McMorrow, T.J. Aartsma, and M. Kasha* Institute of Molecular Biophysics and Department of Chemistry, Florida State University, Tallahassee, Florida 32306 (Received: July 27, 1984)
The efficient generation of amplified spontaneous emission (ASE) in 3-hydroxyflavonein methylcyclohexane and p-dioxane solutionsat 293 K is reported. This application of excited-state proton-transfer tautomerization approaches an ideal four-level laser system involving four different molecular electronic species in separate electronic states and constitutes a photoinduced chemical laser. The gain coefficient (a)for the ASE (530 nm) of 3-hydroxyflavone in methylcyclohexane (293 K) is calculated to be 10-15. Under similar conditions in our apparatus, a is observed to be in the range 7-9 for a proprietary coumarin laser dye (Molectron 70371-4 C485) and for rhodamine-6G. The tunable range for 3-hydroxyflavone is observed to be 518-545 nm. The peak laser power is comparable with that observed for the coumarin dye.
The Four-Level Proton-Transfer Laser The efficient generation of amplified spontaneous emission (ASE) in 3-hydroxyflavone (2-phenyl-3-hydroxychromone)via intramolecular proton transfer is reported here. This phenomenon was singled out in a recent discussion1 of four-level molecular electronic systems as an especially good candidate to act as an efficient laser “dye”. The rapidity of the excited-state protontransfer tautomerization (picoseconds),* the ease of achieving population inversion because the four levels each represent discrete molecular species with separate electronic states (Figure 1) (with the tautomer ground state at zero initial concentration), and the 10 000-cm-I band gap of the tautomer fluorescence band from the excitation band all serve to offer advantages over known dye lasers. The absence of primary interfering phenomena and the achievement of photostability in degassed systems in the 3hydroxyflavone case make it especially attractive as a molecular laser material. The first UV absorption band has maxima at 354 and 340 nm, suitable for excitation by a nitrogen laser, or by the third harmonic of the Nd:YAG laser, or by an excimer laser. The fluorescence of the tautomer at ca. 526 nm is the only emission observed at room temperature in liquid hydrocarbon solvent^.^.^ The intrinsic intramolecular excited-state proton-transfer rate SI (T, n*) SI’(n, T * ) (tautomer) has been measured to be less than 8 ps at room temperature in dry hydrocarbon solution.2 The tautomer excited state is populated with high quantum yield, and
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(1) A. U.Khan and M. Kasha, Proc. Nutl. Acud. Sci. U.S.A., 80, 1767 (1983). (2) D. McMorrow, T. P. Dzugan, and T.J. Aartsma, Chem. Phys. Lett., 103, 492 (1984). (3) P. K. Sengupta and M. Kasha, Chem. Phys. Lert., 68, 382 (1979). (4) D. McMorrow and M. Kasha, J . Am. Chem. SOC.,105, 5133 (1983); J . Phys. Chem., 88, 2235 (1984).
0022-3654/84/2088-4596$01.50/0
this, combined with the zero population of the tautomer ground state Sd, leads to a relatively high gain factor of ASE. A schematic diagram for proton transfer in 3-hydroxyflavone in a four-level laser scheme is given in Figure 1, The lack of a temperature dependence for this proton transfer2v4in the temperature range 298-77 K indicates the absence of a significant energy barrier for excited-state proton transfer. The reverse proton transfer from the tautomer to the normal molecule ground state is found by our study also to be very rapid, with a barrier at 298 K of certainly less than kT. Experimental Section 3-Hydroxyflavone (Aldrich Chemicals) was recrystallized twice from methanol and then once from methylcyclohexane. Purity was confirmed by fluorescence excitation spectra. The spectral grade methylcyclohexane (J. T. Baker) and p-dioxane (Matheson Coleman and Bell) were used as received. A schematic diagram for the experimental arrangement is shown in Figure 2. The 3-hydroxyflavone was excited with the 337.1-nm output from a pulsed nitrogen laser (Molectron, UV-24) with up to 10 mJ/pulse and a pulse duration of 10 ns. The output of the nitrogen laser was focused to a narrow line in the dye cell in a transverse geometry. A laser dye cell of 0.6-cm optical length (Molectron DL 25 1) was used, which is designed to prevent optical feedback from the windows. The dye solution was stirred to prevent secondary processes (local heating, triplet population, etc.) from interfering with the experiments. The amplified spontaneous emission (ASE) was detected by an optical multichannel analyzer (OMA) system consisting of an intensified silicon photodiode array (EG&G/PAR, Model 1420), coupled to a 0.25-m polychromator, and analyzed by a system processor (EG&G/PAR, Model 1215). To reduce the noise, the detector was operated in a gated mode synchronously with the 0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4597
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fE WAVELENGTH, nm
Figure 3, Proton-transfer spontaneous emission (fluorescence) and amplified spontaneous emission (ASE) of 3-hydroxyflavone (1.64 X M) in liquid methylcyclohexane at 293 K. I
Normal
Tautomer
I
I
I
I
I
I
I
I
I
530 I
I
I
I
I
1
I
Figure 1. Schematic potential energy curves for the states involved in four-level laser action in proton-transfer spectroscopy of 3-hydroxy-
flavone. CYLINDRICAL LENS AXIS I
1-_____-_____-____ ________
!HALF-BEAM SHUTTER D t O L A S E R CELL MOLECTRON DL 251 , I
Ne LASER MOLECTRON UV-24
-1-
i TRIGGER
PIN HOLE DIAPHRAGM
?-l POLYCHROMATOR ARRAY
Figure 2. Block diagram for measurement of amplified spontaneous emission spectrum (1, vs. A) and gain spectrum (avs. A).
nitrogen laser. The gate width was typically set at 1 ~s to collect all the emission within the fluorescence lifetime. The ASE was detected through a pinhole (0.05-cm diameter) placed at a distance of 50 cm from the sample cell.
Photostability Tests A practical consideration for the usefulness of a laser dye is its photostability. The 3-hydroxyflavone undergoes efficient photooxygenation as studied by Matsuura et al.596 Recently, Itoh et ala7suggested that the labile species is the tautomer, which may react with oxygen under UV irradiation. The useful life of the dye solution in our experiments depends strongly on the method of preparing the solution. When the solvent was used as received, the ASE life of the dye solution in the cell was only a few minutes when exposed to the full power (10 mJ/pulse) of the nitrogen laser at a repetition rate of 20 Hz. After argon was slowly bubbled through the dye solution for 30 min, the ASE intensity dropped to half in about a half-hour. When the solution was degassed by several freeze-pump-thaw cycles, no significant decomposition ~
~
~
~~~
WAVELENGTH, nm
Figure 4. Gain spectrum for amplified spontaneous emission (ASE) for 3-hydroxyflavone (1.64 X M) in methylcyclohexane (293 K) at a
pump power of line.
500 kW:
experimental, solid; calculated (eq 3), dashed
was observed over a 6-h irradiation period in a stirring cell, indicating a good photostability in the absence of oxygen.
Amplified Spontaneous Emission Figure 3 shows the ASE proton-transfer fluorescence spectrum of 3-hydroxyflavone in methylcyclohexane, together with the spontaneous tautomer fluorescence ~ p e c t r u m . ~The , ~ exponential dependence of the intensity of the ASE on the gain coefficient .(A) leads to the band narrowing observed. The ASE maximum for the proton-transfer fluorescence of 3-hydroxyflavone is 530 nm in methylcyclohexane and 535 nm in p-dioxane. These maxima are red-shifted by ca. 4 nm from the maxima of spontaneous emission due to the h4 dependence of the ASE gain coefficient (vide infra). By measuring the intensity ZL of ASE from the entire cell length L and the intensity ZL from the cell half-length, one can evaluate the ASE gain a(h):clo
The wavelength dependence of the ASE gain coefficient, the gain spectrum, is obtained from an analysis of the OMA recorded spectra using this equation. The solid curve in Figure 4 shows the experimental gain spectrum of 3-hydroxyflavone in methylcyclohexane. Gain is observed over a region from 490 to 580 nm, with a maximum at 530 nm. The ASE gain coefficient for proton-transferfluorescence can be expressed as (normal molecule terms unprimed; tautomer terms primed)
~~~
(5) T. Matsuura, H. Matsushima, and R. Nakashima, Tetrahedron, 26, 435 (1970). (6) T. Matsuura, T. Takemoto, and R. Nakashima. Tetrahedron, 26. 3337 (1973). (7) M. Itoh and Y. Fujiwara, J . Phys. Chem., 87, 4558 (1983).
(8) W. T. Silfvast and J. S . Deech, Appl. Phys. Lett., 17, 97 (1967). (9) 0. G. Peterson, J. P. Webb, W. C. McColgin, and J. H. Eberly, J . Appl. Phys., 42, 1917 (1971). (10) C. V. Shank, Rev. Mod. Phys.f.47, 649 (1975).
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s’ Tk
7
--nI
this number is a lower limit. An upper limit of 23% can be estimated by dividing 7.7% by the fluorescence quantum yield ($4 = 0.36). The high gain in spite of the somewhat low quantum yield indicates the extreme efficiency of the proton-transfer lasing system. The calculated curve in Figure 4 was obtained by assuming that SI’ Sd and other reabsorptions can be ignored. This assumption seems reasonable on the basis of the shape of the gain spectrum. If SI’ SO’ reabsorption played a substantial role, one would expect an asymmetry of the gain spectrum, with a more or less sharp cutoff at the short-wavelength side. A clear example of such an asymmetric gain spectrum is that of rhodamine-6G (cf. ref 8-10), The symmetry of the gain spectrum of 3-ydroxyflavone suggests that the propulation of the tautomer ground state remains negligible during the entire excitation and stimulated emission process. In other words, the reverse proton-transfer rate (tautomer normal) in the ground state is faster than the excited-state decay rate. This statement is further corroborated by a novel observation of dual-frequency lasing” in which the one ASE peak appears a t the 3-hydroxyflavone fluorescence position and that of the second dye in the solution simultaneously appears at the position of the 3-hydroxyflavone tautomer absorption band.
-
s; T;
-
-
Tautomer
Normal
Figure 5. Schematicenergy level diagram for four-level proton-transfer lasing in 3-hydroxyflavone. Labels for radiative cross-section u and state population N are according to eq 2 terms.
where u,’Nsl’ represents the tautomer stimulated emission (measured by the cross section for stimulated emission times the number of molecules per unit volume in state SI’). A’ = usl‘Nso’ represents tautomer self-absorption (cross section times the number of molecules per unit volume in state Sd). [Bq = uT’NT1’ uSj’Nsl’represents losses due to triplet-triplet absorption and excited tautomer absorption. (C) = uslNso uTkNTl as,Nsl represents losses due to self-absorption by the normal ground state, normal molecule triplet-triplet absorption, and excited normal singletsinglet absorption. (The indices i j and k,l represent states which tentatively could give spectroscopic interference with the ASE and laser emission.) The respective terms are illustrated in Figure 5. After laser pulse excitation So SI, the
