J. Phys. Chem. B 2001, 105, 2757-2760
2757
Single-Pulse Measurements of Fluorescence Lifetimes: The Influence of Solvent on the Isomerization of trans-Stilbene Included in Zeolites E. H. Ellison and J. K. Thomas* Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: NoVember 14, 2000; In Final Form: February 26, 2001
The fluorescence lifetime (τf) of trans-stilbene has been evaluated in zeolites Y and ZSM-5 under various conditions, e.g., at low temperature and in the presence of coadsorbed solvents. To make these measurements, an indirect method was developed to measure short-lived (97% adsorbed from cyclohexane solutions, up to concentrations of 0.1 M in the zeolites. Loaded samples were removed from the cyclohexane solution, quickly transferred to 1 mm quartz cuvettes, and evacuated to millitorr pressures at 80 °C for 15 min to remove the cyclohexane and small amounts of H2O. All measurements in zeolites were carried out under vacuum, except when the zeolite was bathed in solvents. Measurements in solution were made under air-saturated conditions. Measurements of fluorescence in solution or solids employed 45° front-face excitation and collection. Time-resolved measurements of fluorescence employed 35 ps pulses from the third or fourth harmonic of a passively mode-locked Nd:YAG laser (Continuum PY-61). Decay transients were detected using a Hamamatsu R1328U-02 phototube. The rise time and fall time of the phototube are 60 and 55 ps, respectively. Either cutoff or interference (band-pass) filters were used to select the emission wavelength or to filter the excitation light. A Tektronix SCD-5000 transient digitizer (bandwidth ) 5 GHz) was used to capture signals from the phototube. A nanosecond photodiode was used to trigger the digitizer. Using this digitizer, a 1024point decay profile with a maximum sampling frequency of 200 GHz can be captured from a single laser pulse. From measurements of the instrument response function (IRF), the total rise n ti2)1/2 where ti is the rise time of the system, given by t ) (∑i)1 time or bandwidth of each component (e.g., the laser pulse, phototube, and transient digitizer) operating in the system, is 95 ps. In measuring time-resolved fluorescence, attempts to control trigger jitter and variation in the laser pulse intensity were unsuccessful. To overcome these problems, single-pulse measurements were made. To make comparisons of the initial intensity of sample decay profiles, a fraction of the excitation pulse was reflected using a quartz slide into a fiber optic cable such that the output of the cable was delayed a few nanoseconds downstream from the sample data. Using the same phototube as that used to measure the sample fluorescence, the delayed pulse profile was measured downstream from the sample data yet inside the measurement window of the scope. Thus, data from a single pulse contained both the sample fluorescence and delayed pulse data (see Figure 1). Adjustments of the initial intensity of various samples were made on the basis of the intensity of their delayed pulse relative to other samples. Through measurements of the intensities of the IRF pulse and reference peak, the two intensities were found to be highly correlated. Because the time separation between the sample and delayed pulse acquisition is constant, the delayed pulse could also be used to set the trigger position. This was achieved by precisely overlaying the delayed pulses of samples to be compared using a spreadsheet program so that the reference peaks overlapped (see the overlapping peaks in Figure 1). This lowered the trigger jitter to about 1 ps. The IRF was measured from scattered light and used to set the time zero position. Results and Discussion Calibration Methods. In Figure 1A, decay profiles of transstilbene in methanol, butanol, octanol, and carbon tetrachloride are illustrated. Each decay profile was collected from a single
Ellison and Thomas
Figure 1. Fluorescence decay profile of trans-stilbene in alcohols and carbon tetrachloride. Upper plot (graph A): solid lines at 250 ps from top to bottom are the decay profiles in octanol, butanol, methanol, and carbon tetrachloride; the dashed line is the IRF. The overlapped peaks at 4 ns are the normalized, reference pulse profiles collected downstream from each sample decay profile. Each profile is from a single laser pulse. The lower plot (graph B) is the variation in relative time shift at half-maximum of the decay profiles in alcohols versus the fluorescence lifetime of trans-stilbene as reported in ref 8. [c]TS ) 5.0 mM; λex ) 266 nm.
laser pulse. The IRF was measured from the scattered light of a vesicle solution. The position of the peak IRF intensity was set to t ) 0. Normalizing the data on the reference peak yields the relative intensities. Figure 1B illustrates how the measurement system can be calibrated to determine fluorescence lifetimes. The relative timeshift at half-maximum (τ1/2 - τ1/2,ref) is defined as the time at which the falling edge of the peak profile is at half-maximum minus the same value for the IRF. Linear correlation is observed from plots of these values in the alcohols versus the fluorescence lifetime of trans-stilbene (τf,TS) previously determined by Kim et al.8 The time shift of the CCl4 decay profile was essentially zero. Recent femtosecond approaches have estimated τf,TS in CCl4 to be 3 ps.10 Using the regression output from Figure 1B, estimates of τf,TS in other systems can be made. However, another more convenient approach to estimating the lifetime is to plot the full width at half-maximum (fwhm) versus τf,TS. This is shown in Figure 2 (see ref 11). Using this approach, a reference pulse is not collected. Thus, at the expense of convenience, information about relative peak intensities and time position is lost. The error in estimating the fwhm was determined to be 1.5% (SD/ mean × 100). The peak amplitudes of trans-stilbene in the alcohols were also highly correlated (R2 ) 0.999) with τf,TS (data not shown). The peak amplitude drops as τf,TS decreases because the fluorescence decay occurs at a rate faster than the measurement
Molecular Motion in Zeolites
Figure 2. Full width at half-maximum (fwhm) of the decay profile of trans-stilbene in the alcohols versus the fluorescence lifetime of transstilbene in the alcohols. The fwhm of IRF equals 138 ps.
J. Phys. Chem. B, Vol. 105, No. 14, 2001 2759
Figure 4. Variation of the fluorescence decay profile of trans-stilbene in dehydrated NaY. From top to bottom: at 77 K; at room temperature and bathed in cyclohexane; at room temperature. Dashed line is the IRF.
TABLE 1: Fluorescence Lifetime of trans-Stilbene in Hydrocarbon Solvents and in Zeolitesa
Figure 3. Variation of the fluorescence decay profile of anthracene in ethanol with added nitromethane. Dashed line is the ethanol solution not containing anthracene. λex ) 355 nm; [An] ) 3.2 mM.
system can respond to the rise in intensity. An extreme case is trans-stilbene in CCl4, where τf,TS is 3 ps and the relative amplitude is very small. The peak amplitude in CCl4 did not correlate well with those in alcohols probably because the response of the phototube is not linear at the low intensity or because measurements of amplitude at the low intensities are more prone to error. Further correlation of the amplitude method is shown in Figure 3, which illustrates the effect of nitromethane (NM) on quenching of anthracene fluorescence. In the absence of NM, the An fluorescence signal is adjusted for good signal-to-noise ratio on the oscilloscope screen. With a high concentration of NM in the solution, the signal drops to a level which depends on the rate of quenching and the propensity for complex formation. At 2.8 M nitromethane, the fluorescence lifetime of An is estimated from the fwhm to be 35 ps. This shows that even for long-lived components the speed of our measurement system is such that it is possible to observe amplitudes from dynamic quenching processes which lower τf to 35 ps. Thus, it can be conservatively estimated that if static quenching is observed by this method, then the static quenching corresponds to a dynamic process that is ultrafast with τ < 10 ps. This is important for later studies of fluorescence quenching by NM and other quenchers in zeolites.12 Motion of trans-Stilbene in Zeolites. Figure 4 illustrates some decay profiles of trans-stilbene in dehydrated NaY. A significant increase in τf,TS was observed when the sample temperature was lowered to 77 K or when the sample was bathed in cyclohexane. At 77 K, τf,TS is 1.4 ns, which is close to the natural decay rate of 1.7 ns reported in the literature.13
medium
fluorescence lifetime (ps)
hexane methylcyclohexane hexadecane glycerin
66 (70 ( 8)14 62 127 (157)15 580b
ZSM-5 ZSM-5 (77 K)
590b 1.5 nsb
NaY (5 mM trans-stilbene) solvent-free 52 4 H2O/sc 53 13 H2O/sc 22 H2O bath 210, 195b 1 cyclohexane/sc 71 2.5 cyclohexane/sc 105 cyclohexane bath 275, 258b solvent-free (77 K) 1.4 nsb NaY (100 mM trans-stilbene) solvent-free 28 2.5 cyclohexane/sc 54 cyclohexane bath 165 solvent-free 4 H2O/sc 13 H2O/sc
KY (5 mM trans-stilbene) 54 47 30
a Predetermined literature values are given in parenthesis. The transstilbene concentration in ZSM-5 was 10 mM. bValues determined by nonlinear least-squares fitting to a single-exponential minimizing χ2.
Some comments about the loading of trans-stilbene are necessary. In zeolite X and Y, there are 8 supercages per unit cell and the unit cell volume (from crystal cell constants) is about 15.20 nm3 (a ) 2.478 nm). Using these values, the supercage concentration is calculated to be 0.87 M. Thus, a loading of 1 molecule/sc corresponds to a concentration of 0.87 M. The molecular weight of trans-stilbene is 180.25 and that of the NaY unit cell is 12 740.7. Thus, a loading of 0.10 M (or 0.115/sc) trans-stilbene in NaY is calculated to be 1.3 wt % (g/g) trans-stilbene. By similar calculations, the loading in ZSM-5 at 0.10 M is determined to be 0.9%. From Figure 2 and values of fwhm, we have estimated τf,TS in various solvents and zeolites. These values are listed in Table 1. In hydrocarbon solvents, fair agreement is found between the values of τf,TS determined here and those previously reported. For cases out of the plot in Figure 2, extrapolation to longer lifetimes was used or the data was fit directly to a singleexponential function without deconvolution of the IRF. In most cases, the extrapolated and fitted data are close for lifetimes in
2760 J. Phys. Chem. B, Vol. 105, No. 14, 2001 the range 200-300 ps. In ZSM-5, trans-stilbene motion is more constrained than in NaY. This is in accord with previous studies,9,16 although the lifetimes reported here are significantly smaller than those reported in ref 9 (this earlier work gave 200 ps in NaY, 1.9 ns in ZSM-5, and 110 ps for methylcyclohexane). We believe the values of τf,TS reported here are more accurate, although good correlation between NMR and fluorescence lifetime measurements regarding the relative motion of transstilbene in ZSM-5 and NaY was reported in ref 9. The reason for these differences in τf,TS is not immediately clear but could be due to differences in loading or to the apparatus employed to measure τf,TS. In ref 9, single photon counting and deconvolution were used, but the excitation pulse width and IRF were not reported. Also, the trans-stilbene loading was 2.5% compared to
