1956
D. W. Larsen and B. A. Soltz
Molecular Motion in Solid N-Methylpyrrolidine and in Solid and Supercooled Liquid N-Melhylpyrrole David W. Larsen" and Barbara A. Soitr Department of Chemistry, University of Missouri-St.
Louis, St. Louis, Missouri 63 121 (Received February 14, 1977)
NMR relaxation times for Zeeman relaxation, T I ,relaxation in the rotating frame, T1,, relaxation of dipolar order, TID,and second moments, M2, were measured as a function of temperature for solid N-methylpyrrolidine and for solid and supercooled liquid N-methylpyrrole. The spin-spin relaxation time T2was measured for the supercooled liquid N-methylpyrrole. The pyrrolidine exhibits two motions: methyl reorientation for which T , = 1.1 X lo-'* exp(3.4 X 103/RT)s and a second motion for which E A = 5-6 kcal/mol. The second motion may be inversion at nitrogen. Mz exhibits a discontinuity at 105 K due to a phase transition which may be associated with pseudorotation of the pyrrolidine ring. M 2 also exhibits a transition associated with methyl reorientation. The solid pyrrole exhibits two motions: methyl reorientation for which EA = 0.92 kcal/mol and a second motion which is probably anisotropic molecular reorientation for which EA = 7.5 kcal/mol. The supercooled liquid pyrrole exhibits isotropic molecular reorientation for which EA = 22 kcal/mol. Below about 125 K, a dipolar signal was observed in the supercooled liquid pyrrole.
Introduction We have currently been studying molecular motion in a variety of molecular solids by use of pulsed NMR. Recently, we chose to study solid N-methylpyrrolidine since it exhibits several interesting rate processes observable by NMR. First, for N-methylpyrrolidine, there is the possibility of inversion at nitrogen. Variable temperature NMR studies' in solution indicate that it may be possible to detect nitrogen inversion just below the melting point. N-Methylpyrrolidine is a small molecule and inversion at nitrogen will modulate a large portion of the total dipolar interaction, thus making the motion relatively easy to detect if present. Second, methyl group reorientation will be observable and the activation energy for this process will give an indication of the steric hindrance of the methyl group. Third, electron diffraction studies of cyclopentane suggest that the five-membered ring has a nonplanar skeletone2 The exact equilibrium geometry was found difficult to establish, but it appears that the conformation may lie somewhere between the half-chair (C,)and envelope (C,)models. From experimental data, it was concluded that cyclopentane in solution must exhibit free pseudorotation, which is an intramolecular motion, so that ring-puckering moves in a circle without the development of angular m ~ m e n t u m .The ~ stable conformation of 1methylcyclopentane has been established as an envelope form in which the methyl group occupies an equatorial position a t the tip of the e n ~ e l o p e . ~It has also been concluded that heterocyclic molecules, such as tetrahydrofuran,j C4H80, tetrahydrothiophene,6 C4H8S,tetrahydr~selenophene,~ C4H&e, and pyrrolidine,' C4H9N, exist in the half-chair conformation with the maximum puckering occurring a t the carbon atoms away from the sp3-hybridized heteroatom. An IR and Raman study of N-methylpyrrolidine has indicated that the molecule exists in a single stable conformation, both in the liquid and solid state.g Calculations'O for five-membered rings indicate that pseudorotation of the ring is a low energy process such that it may not be observable by high resolution NMR techniques. It is of interest to see if effects attributable to pseudorotation can be observed by pulsed NMR. It is the purpose of this study to characterize the motions exhibited by N-methylpyrrolidine as a solid. Pulsed NMR The Journal of Physical Chemistry, Vol. 81, No. 20, 1977
techniques are used because the method is sensitive to a wide range of motion frequencies, but especially to rapid motions with low activation energy barriers.11J2 As a matter of comparison, a study of solid N-methylpyrrole is also reported.
Experimental Section Sample Preparation. Commercial samples of Nmethylpyrrolidine and N-methylpyrrole were dried over KOH and then distilled over CaH2. N-Methylpyrrolidine was collected at 83 "C and stored over molecular seives, while N-methylpyrrole was collected at 116 "C. The samples were then vacuum distilled into NMR tubes, which were sealed under vacuum. It was found that the pure N-methylpyrrole sample exists as a supercooled liquid down to 77 K. Polycrystalline samples were prepared by the addition of finely powdered Pyrex to the sample tube prior to degassing, followed by quenching of the sample at a temperature well below the melting point. NMethylpyrrole was studied both as a supercooled liquid and as a polycrystalline solid. N M R Measurements. Pulsed NMR measurements were made with a Polaron spectrometer (Watford, England) operating at 60 MHz. The radiofrequency field is continuously variable up to 60 G, corresponding to a 90" pulse width of 1ps. The spectrometer recovery time following a pulse is 4-5 ps. T1is the spin-lattice relaxation time, or the relaxation time for Zeeman order. T1 measurements were done utilizing a 90-7-90 pulse sequence.13 T1 values were obtained from a plot of log ( M , - M) vs. 7,where M is the intensity of the Bloch decay13immediately following the second pulse and M , is M when 7 = m. The values of M were obtained from the trace of Tektronix 549 oscilloscope which has a writing speed of 2 ps/cm. T,,is the relaxation time for "Zeeman" order in the rotating frame. TIPmeasurements were done with a pulse sequence of a 90" pulse immediately followed by a long "spin-locking" pulse of duration 7 that is phase shifted by 90" from the first pulse.13 T1,was obtained from a plot of log M vs. r , where M is the intensity of the Bloch decay immediately following the long pulse. For these studies, a radiofrequency field (H,) of 28.6 G was used. TIDis the relaxation time for dipolar order. T,D measurements were done using a three pulse sequence,14
NMR Study of Molecular Solids
1957
I
0 9
25 N v) v)
4 3
u c w z
6 : 20
\s
‘4\
z 0
”
IA w
a,
0.
15
I
I I
150
100 TEHPERATURE
Figure 2.
(OK)
Plot of second moment vs. temperature for solid N-
methylpyrrolidine.
10311
Figure 1. Plot of log relaxation times T I , TI,, and T I , vs. reciprocal temperature for solid N-methylpyrrolidine. The solid line for TI, is calculated as described in the text. The dashed lines are simply drawn through the data points.
90-r’-45-r-45 where r’ is less than T2,and the second pulse is phase shifted by 90” from the first pulse. The “read” pulse is the third pulse which is in phase with the first pulse. Values of T1D were obtained from a plot of log M vs. r , where M is the maximum intensity of the signal (dipolar signal) following the third pulse. T2is the spin-spin relaxation time. T2measurements were done using a Carr-Purcell-Meiboom-Gill sequence.13 Second momentI5 measurements were obtained from analysis of the Bloch decay following a single 90” pulse, as well as analysis of the “solid echo”16J7following a pulse sequence 90-7-90. Here r is slightly longer than the recovery time of the system (-5 ps) and the second pulse is phase shifted by 90” from the first pulse. Since the sample Bloch decay was found to be Gaussian within experimental error, then M exp(-M2t2/2) and M2 was obtained directly from a plot of log M vs. t2. Experimental Results a n d Discussion N-Methylpyrrolidine. Figure 1shows experimental TI, TIP,and T1D values, where the log of the relaxation times are plotted as a function of the reciprocal temperature. Figure 2 is a plot of experimental second moment vs. absolute temperature. The temperature interval for this study is below the freezing point of the pure N-methylpyrrolidine, in the range -103 to -177 “C. In Figure 2, there is a discontinuity in M2at about lo3/ T = 9.5 (105 K). There also appear to be slight discontinuities in T1, and TID a t the same temperature. These effects are characteristic of a phase transition. Over the entire range of temperature studied, Tl appears to be controlled by a single relaxation process. The linear behavior in Figure 1corresponds to the low temperature arm of the T1 minimum, which should occur between 103/T= 5.0 and 5.5. There is a well-defined T1, minimum a t 103/T = 7.70 (130 K) corresponding to the same relaxation process. The TIDvalues for 103/T> 7 also appear to be controlled by this relaxation process with a minimum
near 103/T = 10. The position is uncertain due to the phase transition. We believe that the relaxation process that gives rise to the above effects is methyl group reorientation. The second moment transition between 105 and 165 K in Figure 2 also results from this motion. . The relaxation data can be analyzed according to “weak-collision” theory.l* For solid samples:
rc/3 + 1+24w02rc2
1
where y is the gyromagnetic ratio, up = yHl, wg is the Larmor frequency, and r, is the correlation time for the motion, which is assumed to follow an Arrhenius expression:
r , = r o exp(E,/RT)
(2)
MZmdis that part of the second moment modulated by the motion and is given by Mzmod
= M ~ ( 1 o w) Mz(high)
where Mz(low) is the limiting value of M2 when the motion is too slow to affect the NMR line, and M2(high) is the limiting value of M2 when the motion is sufficiently rapid to cause the line to be motionally narrowed. Near the Tlp minimum, eq 1 reduces to
(3) and, at the T1, minimum, the correlation frequency is 2.45 X lo5 Hz. Equations 2 and 3 were used to fit the experimental TI, points in Figure 1. The calculated curve is shown as a solid line in Figure 1. The parameters used for the fit are shown in Table I. The activation energy is 3.4 kcal/mol for the motion. The same value, within experimental error, is obtained from the gradient of T1and from the gradient of the linear portion of T ~ between D 103/T = 7 and 9.5. We attribute this motion to methyl group reorientation, and we note that there are two unusual features of the relaxation data. First, between lo3/ T = 7 and 8, T ~ > D TIP,which is not accounted for by the The Journal of Physical Chemistiy, Vol. 81, No. 20, 1977
1958
D. W. Larsen and B. A. Soltz
TABLE I: Parameters for Molecular Motions Compound
Motion
N-Methylpyrrolidine
Methyl reorientation Motion I1 (nitrogen inversion) Methyl reorientation Anisotropic molecular reorientation Isotropic molecular reorientation
N-Methylpyrrole (solid) N-Methylpyrrole (supercooled liquid)
usual BPP type theories.lg Second, values of TI are appreciably shorter than expected to be consistent with T1, values. For solid samples, one expects
At the T1, minimum, r, = (2wJ1 and T1/T1, 3/32. (Ho/Hl)2= 2.2 X lo4 from eq 3 and 4. Experimentally, we observe Tl/Tl, 3.5 X lo3 at the Tlpminimum. Thus, T1values are substantially shorter than expected. Effects similar to these have been reported by Hausser et alS2Obut they are as yet unexplained, The fact that all gradients are identical within experimental error suggests that a single motion controls all three relaxation times but not in a manner consistent with the usual BPP expression^.^^^^^ Correlation frequencies can also be estimated from second moment transitionsz2such as observed in Figure 2. However, in this case, values of M2(lOw) and M2(high) cannot be clearly established and thus no estimate of EA was made from the second moment data. Below 103/T = 7, a second motion (motion 11)becomes the predominent relaxation mechanism. This gives rise to a negative slope in TIDbetween 103/T = 6 and 7. The activation energy for this motion is estimated to be 5-6 kcal/mol. I t is evident that TI and Tlpremain controlled by methyl reorientation in this temperature interval. Since there is no relaxation minimum or second moment transition associated with this motion it is not possible to assign the motion with certainity. However, a speculation about the motion may be made from activation energy considerations, and it is possible that this process is nitrogen inversion. Variable temperature solution NMR studied indicate that a t -100 “C, nitrogen inversion occurs slowly enough that separate signals for the a protons cis and trans to the N-methyl are observable. This indicates that rc > s a t -100 “C. The activation enthalpy is estimated to be 7-9 kcal/mol. For motion I1 (in the solid phase), one estimates 7, 2 X s at -100 OC and EA 5-6 kcal/mol. These values correspond to a barrier to nitrogen inversion which is unconstrained by small valence angles or lone-pair bearing substituents. It is possible that the difference between solution and solid parameters may be attributable to the influence of packing forces on N bond angles. The above discussion assumes that motion I1 is inversion a t nitrogen. It is not possible to definitely make this assignment due to the lack of second moment data related to this motion. This motion could be an unidentified process peculiar to the solid state, e.g., a libration or anisotropic molecular reorientation. The principal motion controlling relaxation times in Figure 1 and that responsible for the second moment transition in Figure 2 is methyl reorientation. That this is the case can be demonstrated from the second moment data. The “rigid lattice” second moment can be calculated from the relationshipz3
-
-
-
The Journal of Physical Chemlstty, Vol. 81, No. 20, 1977
E A , kcal/mol 3.4 5 -6
70,
1.1 x
s
lo-’*
Mzrnod. G2
2.1
0.92 7.5 22
where n is the number of protons in the sample and rjk is the distance between protons j and k. MZcrl) can also be considered to be composed of the following SUM:
and M2(intra) is the contribution from the interaction of protons within a molecule and M2(inter) is that from the interaction of protons with neighboring molecules. Mz(htrt,) is evaluated utilizing electron diffraction dataz4for bond lengths in N-methylpyrrolidine and assuming tetrahedral angles. The calculated result is 20.0 G2. Since the crystal is a s s ~ m e d ~to~be v ~6 ~ structure is not available, M2(inter) f 2 Gz. Thus, the total rigid lattice second moment is calculated to be M2(rl)= 26 f 2 G2. The observed M2 a t the lowest temperature measured (96 K) is 27.9 G2. The two values agree within experimental error and the system could be considered a rigid lattice. However, within the uncertainty limits, the calculated M2 value could also be about 2 G2lower than the observed value, which could be due to our inadequate treatment of effects associated with pseudorotation. The discontinuity in Figure 2 a t 105 K lowers the observed second moment by 4-5 G2. We attribute this discontinuity to a phase transition (vide supra). It is difficult to rationalize this large a change in M2 solely upon the basis of a change in packing, since the total value of Mz(inteP) is expected to be no larger than 8 G2. Methyl reorientation and motion I1 (possibly nitrogen inversion) are both too slow at this temperature to contribute appreciably to this discontinuity. Thus, it is possible that a discontinuous change in the rate or nature of the pseudorotation of the ring occurs a t 105 K and this contributes to the observed M2 discontinuity. If this is the case, it is interesting to note that no other effects from pseudorotation were detected. The M2 transition between 105 and 165 K is indicative of considerable molecular motion since M2 is reduced to 165 K and is less than 18 G2. Thus, there is good agreement between observed and calculated M 2 values. This is somewhat fortuitous since effects due to pseudorotation were ignored. However, the total reduction in M 2 in Figure 2 between 105 and 165 K can be accounted for by methyl reorientation and a reduction in intermolecular contributions, and the observed reduction in M2 is much too large to be associated with pseudorotation or nitrogen inversion alone. N-Methylpyrrole (Solid). This compound was chosen for study to contrast its behavior with that of Nmethylpyrrolidine. The pyrrole is similar to the pyrrolidine, but pseudorotation and nitrogen inversion are not
NMR Study of Molecular Solids
1959
"\
I
I
I
I
I
10
9
8
7
6
-31 5
I I
10
I 10
9
7
8
6
5
1031~
Figure 3. Plot of log relaxation times T , and TII, vs. reciprocal temperature for solid N-methyipyrroie. The solid curve for TjI, Is calculated from the two linear portions indicated by dashed lines.
possible. In addition, methyl reorientation is expected to have a much lower activation barrier. The experimental relaxation times T1and T 1 are ~ plotted vs. reciprocal temperature in Figure 3. The temperature interval for this sample is below the freezing point of pure N-methylpyrrole in the range -176 to -89 "C. Between lo3/" = 7 and 10.5, T1exhibits linear behavior, and for 103/T < 7, T1 appears to be temperature independent, possibly due to paramagnetic impurities. The T1gradient above 103/T = 7 corresponds to an activation energy of 0.92 kcal/mol, and the motion is methyl reorientation. T ~ also D exhibits linear behavior for 103/T > 8, with a gradient identical within experimental error with that of T1.We attribute T1D relaxation in this region to methyl reorientation. The gradient indicates that the TI line corresponds to the high temperature arm of the T1 minimum, and in that case, T1/T1D = 2 is predicted.19p2* Experimentally, we observe T1/TlD 4. Below 103/T = 8, second relaxation mechanism contributes to TI=,resulting in a sharp decrease in T 1 with ~ increasing temperature. It is apparent that this new motion is too slow to affect T1in this region. The activation energy for this process is calculated to be 7.5 kcal/mol. The motion is probably anisotropic molecular reorientation. As a comparison, an activation energy of 3.2 kcal/mol has been reported29 for molecular reorientation in liquid pyrrole. Barriers for rotation about hexad axes in solids have been reported,3O 4.4 kcal/mol for benzene and 6.6 kcal/mol (low temperature phase), 4 kcal/mol (high temperature phase) for hexamethylbenzene. The large barrier observed for solid N-methylpyrrole probably reflects packing forces in the solid. The activation parameters for this sample are shown in Table I. We may confirm our assignment of methyl reorientation by the same procedure used above. The rigid lattice
-
Flgure 4. Plot of log relaxation times TI, TID, and T2 vs. reciprocal temperature for supercooled liquid N-methyipyrrole.
second moment is calculated utilizing electron diffraction data for bond lengths and angles in the planar molecule?l Assuming, as before, M2(inte1) = 6 f 2 G2,the total rigid = 16.7 lattice second moment is calculated to be 2 G2. Calculation of the second moment in the presence of rapid methyl reorientation, using the same reduction factors as above, yields a value M 2 ( c rot) ~ = 7.9 1 G2. Experimentally, we observe Mz = 7.0 G3 at -95 OC and 7.9 G2 a t -175 "C, which agrees well with calculated value. Comparison of the experimental results of N-methylpyrrolidine and N-methylpyrrole illustrates some differences in the motions of these molecules. For example, methyl reorientation in N-methylpyrrole has a lower activation barrier and occurs more rapidly a t a given temperature. At low temperatures, a contribution from tunneling in N-methylpyrrole is and the slope of the high temperature arm corresponds to the height of the barrier.35 The observed barrier may be compared with barriers reported,350.2 kcal/mol for toluene, 0.5 kcal/mol for 2-fluorotoluene, 0.3 kcal/mol for 3-fluorotoluene, 2.2 kcal/mol for o-xylene, and 1.1kcal/mol for p-xylene. N-Methylpyrrole (Supercooled Liquid). The relaxation times TI, T I D , and T2for supercooled liquid N-methylpyrrole are plotted vs. reciprocal temperature in Figure 4. The temperature range is from 22 to -177 "C, over which no discontinuities were observed. Between lo3/T = 8 and 10.5, TI slowly increases with increasing temperature. This low activation energy process is undoubtedly methyl group reorientation, as observed in the solid sample. Below 103T = 8, there is a contribution to 21 ' from a second motion, with a relaxation minimum a t 103/T = 5.85. This motion is responsible for motional narr0wing,3~*~~ as indicated by the large increase in T,; for 103/T < 6, T1 = Tz is observed. These effects are associated with isotropic reorientation with a possible contribution from translational diffusion. From the gradient
*
*
The Journal of Physical Chemlstty, Vol. 81, No. 20, 1977
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 fluorescence lifetime 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, ~ one ~obtains the ~ value~for the molecular ~ , 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.
