1960
K. A. Selanger, J. Falnes, and T. Sikkeland
of T2,we estimate EA = 22 kcal/mol for isotropic reorientation in the supercooled liquid. This value may be compared with isotropic reorientation3’ in benzene (16 kcal/mol) and hexamethylbenzene (28 kcal/mol), and with translational diffusion3’ in the rotator phase of cyclohexane (9.1 kcal/ mol). For 103/T> 8, it is possible to observe a rather weak “dipolar” signal in the supercooled liquid. TIDdecreases slightly with increasing temperature over the entire range of temperature in which a dipolar signal may be observed. It is not possible to attribute the TIDtemperature dependence to a specific motion. Inspection of Table I shows that the activation energy for methyl reorientation in the pyrrolidine is about 1 kcal/mol larger than has been found for methyl groups bonded to heterocyclic nitrogens in other systems.11,38This probably reflects the variation in hybridization at the nitrogen rather than packing forces in the solid. The low activation energy obtained for methyl reorientation in the pyrrole is consistent with a planar molecular structure. It is unfortunate that motion I1 in the pyrrolidine cannot be clearly assigned. The observed barrier is close enough to that for anisotropic molecular reorientation in the pyrrole that this possibility for motion I1 cannot be ruled out. If motion I1 is nitrogen inversion, it is of interest that the activation barrier is about 1 kcal/mol lower in the solid than in the liquid. Acknowledgment. B.A.S. acknowledges the support of NIH research Grant No. ROlNS10903 from National Institute of Neurological Diseases and Stroke. We thank McDonnell Douglas Astronautics Co. for the use of a storage oscilloscope. References a n d Notes (1) J. B. Lambert and W. L. Oliver, J. Am. Chem. SOC.,91, 7774 (1969). (2) W. J. Adams, H. J. Geise, and L. S. Barteli, J . Am. Chem. SOC., 92, 5013 (1970). (3) J. E. Kilpatrick, K. S. Pitzer, and R. Spitzer, J . Am. Chem. Soc., 69, 2483 (1947).
(4) K. S. Ptzer and W. E. Donath, J. Am. Chem. Soc., 81, 3213 (1959). (5) H. M. Seip, Acta Chem. Scand., 23, 2741 (1969). (6) Z. Nahlovska, B. Nahlovsky, and H. M. Seip, Acta Chem. Scand., 23, 3534 (1969). (7) Z. Nahlovska, B. Nahlovsky, and H. M. Seip, Acta Chem. Scand., 24, 1903 (1970). (8) J. P. McCullough, J . Chem. fhys., 29, 966 (1958). (9) N. D. Chizhikova, 0. S. Anisimova, Y. A. Pentin, and L. G. Yudin, Zh. Strukt. Khim., 10, 520 (1969). (10) J. B. Henderickson, J . Am. Chem. Soc., 83, 4537 (1961). (11) D. W. Larsen and J. Y. Corey, J. Am. Chem. Sac., 99, 1740 (1977). (12) A. J. Campbell, C. E. Cottrell, C. A. Fyfe, and K. R. Jeffrey, Inorg. Chem., 15, 1321, 1326 (1976). (13) T. C. Farrar and E. D. Becker, “Pulse and Fourier Transform NMR”, Academic Press, New York, N.Y., 1971. (14) J. Jeener and P. Broekaert, fhys. Rev., 157, 232 (1967). (15) A. Abragam, “The Principlesof Nuclear Magnetism”, Clarendon Press, Oxford, 1961, p 106. (16) J. G. Powles and J. H. Strange, Proc. fhys. SOC.,82, 6 (1963). (17) P. Mansfield, fhys. Rev., 137, A961 (1965). (18) G. P. Jones, Phys. Rev., 148, 332 (1966). (19) D. C. Ailion, Adv. Magn. Reson., 5, 177 (1971). (20) 0. Lauer, D. Stehlik, and K. H. Hausser, J . Magn. Reson., 6, 524 (1972). (21) N. Bloembergen, E. M. Purcell, and R. V. Pound, Phys. Rev., 73, 679 (1948). (22) H. S. Gutowsky and G. E. Pake, J. Chem. Phys., 18, 162 (1950). (23) J. H. Van Vleck, fhys. Rev., 74, 1168 (1948). (24) V. E. Lippert and H. Priggi, Ber. Bunsenges. fhys. Chem., 67, 415 ( 1963). (25) G. W. Smith, J . Chem. fhys., 42, 4229 (1965). (26) J. G. Powles and H. S.Gutowsky, J. Chem. Phys., 21, 1704 (1953). (27) J. M. Chezeau, J. Dufourcq, and J. H. Strange, Mol. Phys., 20, 305 (1971). (28) M. Goldman, “Spin Temperature and Nuclear Magnetic Resonance in Solids”, Oxford University Press, London, 1970, pp 57-60. (29) J. Angeru and E. Szczesniak, Institute of Nuclear Physics, Cracow, Report No. 819 PL (part I), 1973, p 58. (30) R. Van Steenwinkel, Z. Naturforsch. A , 24, 1526 (1969). (31) B. Bak, D. Christensen, L. Hansen, and J. Rastrup-Anderson, J. Chem. fhys., 24, 720 (1956). (32) P. S. Allen, J . Chem. fhys., 48, 3031 (1968). (33) P. S. Allen and S. Clough, fhys. Rev. Lett., 22, 1351 (1969). (34) S. Clough, J. fhys. C , 4, 1075, 2180 (1971). (35) J. Haupt and W. Muller-Warmuth, Z. Naturforsch. A, 23, 208 (1968); 24, 1066 (1969). (36) K. Luszczynski, J. A. E. Kail, and J. G. Powles, f r o c . fhys. Soc., 75, 243 (1960). (37) B. I. Hunt and J. G. Powles, R o c . fhys. Soc., 88, 513 (1966). (38) D. W. Larsen and T. A. Smentkowski, J . Magn. Reson., in press.
Fluorescence Lifetime Studies of Rhodamine 6G in Methanolt K. A. Selanger,+J. Falnes, and 1.Sikkeland” Instituff for eksperimentaifysikk, University of Trondheim-NTH-7034, Trondheim, Norway (Received April 29, 1977) Publication costs assisted by the Institutt for eksperimentalfysikk
Using the 1.06-km light from a passively mode-locked Nd:glass laser the two-photon induced fluorescence of rhodamine 6G in methanol at room temperature has been studied. When corrected for self-absorption the fluorescencelifetime decreases linearly from about 3.7 ns at 2 x M to about 2.5 ns at 9 X M indicating quenching by excimer formation. Above 9 X M there is a rapid decrease to about 0.9 ns at 2 X lo-’ M, which is attributed to quenching of excited monomers by aggregates. The molecular fluorescence lifetime of rhodamine 6G is found to be 3.7 f 0.4 ns.
I. Introduction The dye rhodamine 6G (R6G) is of interest to experiworking in the field of dye lasers. The emission ~ absorption spectraof R6G are well known, H few experiments have been reported in the literature on Taken in part from K. A. Selanger’s Lic. techn. thesis, University o f T r o n d h e i m - N T H , 1976. Present address: SINTEF, River and Harbour Lab., N-7034, Trondheim, Norway. The Journal of Physical Chemistry, Vol. 81, No. 20, 1977
its fluorescence lifetime in solution. Such measurements may give information on the kinetics for reactions involving excited molecules and other species in solution. In addition, the for the molecular ~ one obtains ~ ~ value ~ ~ ,fluorescence lifetime. Data for that lifetime show considerable ~pread.l-~ In the present work we measured the fluorescence lifetime of R6G in methanol. Since R6G is known to aggregate Strongly in water4 we chose methanol which is easy to obtain free from water.
1981
Studies of Rhodamine 6G in Methanol
TABLE I: Values for the Measured Fluorescence Lifetime r , the Quantity a q m , and the Fluorescence Lifetime T~ Corrected for Self-Absorption at Various Dye Concentrations m 2x 5 x 10-4 1.0x 10-3 2.0 x 10-3 3.0 x 10-3 5.0 x 10-3 7.0 x 10-3 8.0 x 10-3 9.0 x 10-3 1.0 x 1.2 x 1.5 X lo-' 2.0 x
8.2 * 9.5 c 10.1 i 10.3 i 10.0 i 8.6 i 6.9 -I 6.4 i 5.6 i 4.4 t 3.2 f 2.5 i 1.1i
0.2 0.2 0.3 0.3 0.3 0.2 0.3 0.2 0.1 0.1 0.1 0.1 0.1
0.55 0.64 0.70 0.75 0.78 0.81 0.83 0.84 0.84 0.85 0.86 0.88 0.89
3.7 i 0.4 3.6 f 0.4 3.5 i 0.4 3.3 i 0.4 3.2 i 0.4 3.0 * 0.4 2.7 f 0.3 2.6 ~t 0.3 2.5 f 0.3 2.2 * 0.3 1.8 i 0.2 1.6 i 0.2 0.9 It 0.2
To induce the fluorescence we used a passively modelocked Nd:glass laser. Such a laser produces a train of picosecond pulses of electromagnetic radiation at a wavelength 1.06 pm and an energy flux density of the order of GW/cm2. At this high flux density observable fluorescence can be induced in R6G by a two-photon absorption process5 since its So-S1 absorption spectrum has a maximum at 0.53 pm and the quantum yield is very high (nearly unity). The fluorescence lifetime of R6G is of the order of nanoseconds and is shorter than the time between the pulses in the laser pulse train. Furthermore, the widths of these pulses are in the picosecond region and hence the excitation pulses can be regarded as 6 pulses. Values for the lifetime may then be obtained directly from the fluorescence decay measured between the pulses. Another advantage in using the two-photon process is that the frequency of the primary beam is well separated from that of the fluorescence. 11. Experimental Section A. Materials. The dye R6G of spectroscopic purity was dissolved in methanol (p.a.) without further purification. The transmission spectrum of the solution shows that for a laser wavelength of 1.06 pm the dye is very nearly transparent, while methanol has an absorption coefficient with a value of 0.16 cm-l. Similarly, methanol is very nearly transparent for fluorescence emission. Preliminary experiments showed that the absorption of laser radiation by the R6G-methanol solution followed Beer's law with a value for the absorption coefficient equal to that of pure methanol. Hence, for the concentrations used, the twophoton absorption processes in the dye are negligible when compared to one-photon absorption processes in methanol. In column 1of Table I we have listed the various values of the molar concentration m used, as estimated on the basis that R6G is in the monomeric form. The uncertainty in m is about 5 % . B. The Mode-Locked Nd:Glass Laser. A detailed account of the laser has been given e1sewheree6 In the following we shall only give a brief description of its main features. The laser rod, a standard Brewster-Brewster ended ED-2 Nd:glass rod from Owens-Illinois has a diameter of 9.6 mm and an optical path length of 178 mm. The end parallelism is better than f6'. Optical pumping is achieved by two linear flash tubes and a cylindrical reflector of the double elliptical type. The rod and the flash tubes are water cooled. The resonator length can be varied between 0.6 and 2.1 m. This range in length corresponds to a pulse spacing (T) of between 4 and 14 ns. Pulse separation even longer
,c
rn
C
0 D2
u1 Figure 1. Experimental arrangement for the measurements of the fluorescence lifetime r . L is the laser, S a beam splitter, F, a red fiber, F2 a neutral filter, L a lens, D, and D2 are photodiodes, and C is the cell containing Rhodamine 6Gmethanol. D, and D2in conjunction with oscilloscopes register the laser emission and the fluorescence, respectively.
than 14 ns may be obtained but at that point alignment problems hamper the operation. As a combined Q-switch and mode locker we use an Eastman Kodak dye solution (No. 9860) placed in a 2 mm thick cell. This cell is in optical contact with the rear mirror. To prevent off-axial mode oscillations, a circular aperture with a diameter of 1.7 mm is placed in the resonator. A typical pulse train lasts for about 0.1 p s and consists of discrete pulses with a mean width measured to be about 5 ps.6 C . Photon Detection. The experimental arrangement for the measurements of the fluorescence lifetime (7) as shown in Figure 1 is somewhat different from that used by other experimentalist^.^,^ The laser light is split into two beams by a glass plate held 45O to the incident beam. The reflected light is monitored by an 1" F-4000 vacuum photodiode connected to a Tektronix 519 oscilloscope. The unreflected laser light passes through the center of a 5-cm long cylindrical cell with a radius r = 0.9 cm containing the R6Gmethanol. The fluorescence light is detected vertically to the laser beam by means of an ITT FW 114A photodiode (S20 cathode) in conjunction with a Tektronix 485 oscilloscope. This photodiode was chosen because of its short (0.5 ns) rise time, wide linear intensity range, and low noise. The 485 scope is externally triggered by a step signal produced by the 519 oscilloscope at the arrival of the laser pulse train. In addition, the fluorescence signal is synchronized to the sweep run of the 485 scope by a delay line from the photodiode. With this arrangement the oscilloscope recordings of the intensities of fluorescence emission and of the exciting laser light can be correlated in time. The results of the analysis of these recordings show that one can exclude excitation to higher states (e.g., S1-Sz) by subsequent pulses and that the observed fluorescence is due to a two-photon absorption process in R6G only. The fluorescence intensity increases sharply (risetime
