Single-Pulse Measurements of Fluorescence Lifetimes: The Influence of Solvent on theIsomerization 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 variousconditions, 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 (<100 ps) fluorescence lifetimes with a single laserpulse. In dehydrated NaY,τf,TS was 52 ps compared to 66 ps in hexane and 32 ps in methanol. When theNaY sample was subsequently bathed in either water or cyclohexane,τf,TS increased by a factor of 4 and 5,respectively, reflecting decreased mobility oftrans-stilbene because of blocking effects of the solvent. However,with small amounts of coadsorbed water equivalent to 13 H2O/supercage,τf,TS decreased by a factor of 2,reflecting increased mobility oftrans-stilbene. Small amounts of coadsorbed cyclohexane only increasedτf,TS. Values ofτf,TS in the solvent-free zeolites were significantly smaller (higher degree of probe mobility)than those published a decade ago and contradicted the earlier conclusions regarding the mobility of stilbenein zeolites. The possible reasons for this are discussed.

Introduction

In zeolites, molecular motion is often limited by strongadsorption and confinement of molecules into narrow channelsor pores the diameter of which are sometimes only slightly largerthan that of the molecules themselves.1 Classically, theseconstraints on molecular motion and the restrictions imposedby the narrow pore openings on getting larger molecules intoand out of zeolites have been viewed as limitations to theapplication of zeolites in chemical synthesis. Nevertheless, formolecules which can be included in zeolites, the constraints onmolecular motion can sometimes be used strategically to gainan advantage. For example, increased selectivity of photoinducedradical reactions2 and the promotion of enantiomeric excesses3

have been demonstrated in zeolites.To gain a mechanistic understanding of chemistry in zeolites,

detailed information about the mobility and distribution ofadsorbates in zeolites is necessary. We have recently commentedon the motion of polycyclic aromatic hydrocarbons in zeoliteY from measurements of triplet quenching by energy transfer.This study focused on cage-to-cage migration of pyrene,phenanthrene, naphthalene, and ferrocene, showing large in-creases in mobility in the presence of small amounts ofcoadsorbed water.4 Similar results were demonstrated by Hash-imoto et al. for self-quenching of triplet anthracene in zeoliteY.5 We are presently engaged in fluorescence studies of motionand the rates of reactions in the supercage. Whereas the tripletquenching was evaluated on microsecond (or longer) time scales,these fluorescence studies rely on high-speed measurements onsubnanosecond time scales.

In the present study, the fluorescence lifetime oftrans-stilbene(τf,TS) has been evaluated in zeolites to gain information aboutmotion oftrans-stilbene in the adsorbed state. In fluid solution,

photoexcitation oftrans-stilbene yieldscis-stilbene at ratescorresponding to 1/τf,TS.6 In nonpolar solvents,τf,TS increaseswith solvent viscosity. The viscosity dependence originates fromfrictional forces which modulate the twisting motion necessaryto form a polar transition state,1p,7 where the phenyl rings areperpendicular. In polar solvents such as alcohols, the polartransition state is stabilized by rapid components of the solventdielectric response.8 Thus, in solvents of equal viscosity,τf,TS

is smaller in the more polar solvent.A previous study evaluated the motion oftrans-stilbene in

zeolites.9 Combined NMR and fluorescence lifetime studiesindicated more restricted mobility oftrans-stilbene in ZSM-5relative to NaY. This was explained by constraints on molecularmotion in the narrow channels of ZSM-5 relative to the largersupercages of zeolite Y. In the present study, the effectivenessof a measurement system developed for measuring picosecondfluorescence lifetimes is demonstrated. The system is used toevaluate the effects of temperature and coadsorbed solvents onthe mobility of trans-stilbene in zeolites.

Experimental Section

Chemicals.Anthracene andtrans-stilbene were purified byseveral recrystallizations from ethanol. Nitromethane wasobtained from Fluka Chemicals and used as received. Allsolvents were reagent grade or better. Cyclohexane was driedover activated molecular sieves (Linde 3A).

Zeolite NaY (Aldrich; Si/Al ) 2.5) was washed two timeswith 1M NaCl then rinsed thoroughly with water. KY wasprepared by exhaustive exchange of NaY with KNO3 at slightlyelevated temperatures. ZSM-5 (Si/Al) 60) was calcined in airat 500°C then rinsed with H2O.

Procedures.The zeolite powders were pressed into thin (100µm thick) pellets using a Wilks Uni-die (13 mm barrel diameter)and an applied pressure (Carver laboratory press) of 4 tons. Thesample dry weight was approximately 20 mg. The pellets weredried in air at 500°C for at least 2 h. After cooling the pellets

* To whom correspondence should be addressed. Professor J. KerryThomas, Department of Chemistry & Biochemistry, 251 Nieuwland ScienceHall, University of Notre Dame, Notre Dame, IN 46556. Tel: (219) 631-7589. Fax: (219) 631-6652. E-mail: thomas.3@nd.edu.

2757J. Phys. Chem. B2001,105,2757-2760

10.1021/jp004210+ CCC: $20.00 © 2001 American Chemical SocietyPublished on Web 03/21/2001

under a flowing stream of dry N2, they were quickly transferredto 1 mL solutions oftrans-stilbene in cyclohexane. To achieveuniform loadings in the pellet, the samples were agitated at 150rpm for 12 h on an orbital shaker in 20 mL glass scintillationvials. From a quantitative analysis of the supernatant, thetrans-stilbene was>97% adsorbed from cyclohexane solutions, upto concentrations of 0.1 M in the zeolites. Loaded samples wereremoved from the cyclohexane solution, quickly transferred to1 mm quartz cuvettes, and evacuated to millitorr pressures at80 °C for 15 min to remove the cyclohexane and small amountsof H2O. All measurements in zeolites were carried out undervacuum, except when the zeolite was bathed in solvents.Measurements in solution were made under air-saturated condi-tions.

Measurements of fluorescence in solution or solids employed45° front-face excitation and collection. Time-resolved measure-ments of fluorescence employed 35 ps pulses from the third orfourth harmonic of a passively mode-locked Nd:YAG laser(Continuum PY-61). Decay transients were detected using aHamamatsu R1328U-02 phototube. The rise time and fall timeof the phototube are 60 and 55 ps, respectively. Either cutoffor interference (band-pass) filters were used to select theemission wavelength or to filter the excitation light. A TektronixSCD-5000 transient digitizer (bandwidth) 5 GHz) was usedto capture signals from the phototube. A nanosecond photodiodewas used to trigger the digitizer. Using this digitizer, a 1024-point decay profile with a maximum sampling frequency of 200GHz can be captured from a single laser pulse. From measure-ments of the instrument response function (IRF), the total risetime of the system, given byt ) (∑i)1

n ti2)1/2 whereti is the risetime or bandwidth of each component (e.g., the laser pulse,phototube, and transient digitizer) operating in the system, is95 ps.

In measuring time-resolved fluorescence, attempts to controltrigger jitter and variation in the laser pulse intensity wereunsuccessful. To overcome these problems, single-pulse mea-surements were made. To make comparisons of the initialintensity of sample decay profiles, a fraction of the excitationpulse was reflected using a quartz slide into a fiber optic cablesuch that the output of the cable was delayed a few nanosecondsdownstream from the sample data. Using the same phototubeas that used to measure the sample fluorescence, the delayedpulse profile was measured downstream from the sample datayet inside the measurement window of the scope. Thus, datafrom a single pulse contained both the sample fluorescence anddelayed pulse data (see Figure 1). Adjustments of the initialintensity of various samples were made on the basis of theintensity of their delayed pulse relative to other samples.Through measurements of the intensities of the IRF pulse andreference peak, the two intensities were found to be highlycorrelated.

Because the time separation between the sample and delayedpulse acquisition is constant, the delayed pulse could also beused to set the trigger position. This was achieved by preciselyoverlaying the delayed pulses of samples to be compared usinga spreadsheet program so that the reference peaks overlapped(see the overlapping peaks in Figure 1). This lowered the triggerjitter to about 1 ps. The IRF was measured from scattered lightand used to set the time zero position.

Results and Discussion

Calibration Methods. In Figure 1A, decay profiles oftrans-stilbene in methanol, butanol, octanol, and carbon tetrachlorideare illustrated. Each decay profile was collected from a single

laser pulse. The IRF was measured from the scattered light ofa vesicle solution. The position of the peak IRF intensity wasset tot ) 0. Normalizing the data on the reference peak yieldsthe relative intensities.

Figure 1B illustrates how the measurement system can becalibrated to determine fluorescence lifetimes. The relative time-shift at half-maximum (τ1/2 - τ1/2,ref) is defined as the time atwhich the falling edge of the peak profile is at half-maximumminus the same value for the IRF. Linear correlation is observedfrom plots of these values in the alcohols versus the fluorescencelifetime of trans-stilbene (τf,TS) previously determined by Kimet al.8 The time shift of the CCl4 decay profile was essentiallyzero. Recent femtosecond approaches have estimatedτf,TS inCCl4 to be 3 ps.10

Using the regression output from Figure 1B, estimates ofτf,TS

in other systems can be made. However, another more conve-nient approach to estimating the lifetime is to plot the full widthat half-maximum (fwhm) versusτf,TS. This is shown in Figure2 (see ref 11). Using this approach, a reference pulse is notcollected. Thus, at the expense of convenience, informationabout relative peak intensities and time position is lost. Theerror in estimating the fwhm was determined to be 1.5% (SD/mean× 100).

The peak amplitudes oftrans-stilbene in the alcohols werealso highly correlated (R2 ) 0.999) withτf,TS (data not shown).The peak amplitude drops asτf,TS decreases because thefluorescence decay occurs at a rate faster than the measurement

Figure 1. Fluorescence decay profile oftrans-stilbene in alcohols andcarbon tetrachloride. Upper plot (graph A): solid lines at 250 ps fromtop to bottom are the decay profiles in octanol, butanol, methanol, andcarbon tetrachloride; the dashed line is the IRF. The overlapped peaksat 4 ns are the normalized, reference pulse profiles collected downstreamfrom each sample decay profile. Each profile is from a single laserpulse. The lower plot (graph B) is the variation in relative time shift athalf-maximum of the decay profiles in alcohols versus the fluorescencelifetime of trans-stilbene as reported in ref 8. [c]TS ) 5.0 mM; λex )266 nm.

2758 J. Phys. Chem. B, Vol. 105, No. 14, 2001 Ellison and Thomas

system can respond to the rise in intensity. An extreme case istrans-stilbene in CCl4, where τf,TS is 3 ps and the relativeamplitude is very small. The peak amplitude in CCl4 did notcorrelate well with those in alcohols probably because theresponse of the phototube is not linear at the low intensity orbecause measurements of amplitude at the low intensities aremore prone to error.

Further correlation of the amplitude method is shown inFigure 3, which illustrates the effect of nitromethane (NM) onquenching of anthracene fluorescence. In the absence of NM,the An fluorescence signal is adjusted for good signal-to-noiseratio on the oscilloscope screen. With a high concentration ofNM in the solution, the signal drops to a level which dependson the rate of quenching and the propensity for complexformation. At 2.8 M nitromethane, the fluorescence lifetime ofAn is estimated from the fwhm to be 35 ps. This shows thateven for long-lived components the speed of our measurementsystem is such that it is possible to observe amplitudes fromdynamic quenching processes which lowerτf to 35 ps. Thus, itcan be conservatively estimated that if static quenching isobserved by this method, then the static quenching correspondsto a dynamic process that is ultrafast withτ < 10 ps. This isimportant for later studies of fluorescence quenching by NMand other quenchers in zeolites.12

Motion of trans-Stilbene in Zeolites.Figure 4 illustratessome decay profiles oftrans-stilbene in dehydrated NaY. Asignificant increase inτf,TS was observed when the sampletemperature was lowered to 77 K or when the sample was bathedin cyclohexane. At 77 K,τf,TS is 1.4 ns, which is close to thenatural decay rate of 1.7 ns reported in the literature.13

Some comments about the loading oftrans-stilbene arenecessary. In zeolite X and Y, there are 8 supercages per unitcell and the unit cell volume (from crystal cell constants) isabout 15.20 nm3 (a ) 2.478 nm). Using these values, thesupercage concentration is calculated to be 0.87 M. Thus, aloading of 1 molecule/sc corresponds to a concentration of 0.87M. The molecular weight oftrans-stilbene is 180.25 and thatof the NaY unit cell is 12 740.7. Thus, a loading of 0.10 M (or0.115/sc)trans-stilbene in NaY is calculated to be 1.3 wt %(g/g) trans-stilbene. By similar calculations, the loading inZSM-5 at 0.10 M is determined to be 0.9%.

From Figure 2 and values of fwhm, we have estimatedτf,TS

in various solvents and zeolites. These values are listed in Table1. In hydrocarbon solvents, fair agreement is found betweenthe values ofτf,TS determined here and those previously reported.For cases out of the plot in Figure 2, extrapolation to longerlifetimes was used or the data was fit directly to a single-exponential function without deconvolution of the IRF. In mostcases, the extrapolated and fitted data are close for lifetimes in

Figure 2. Full width at half-maximum (fwhm) of the decay profile oftrans-stilbene in the alcohols versus the fluorescence lifetime oftrans-stilbene in the alcohols. The fwhm of IRF equals 138 ps.

Figure 3. Variation of the fluorescence decay profile of anthracene inethanol with added nitromethane. Dashed line is the ethanol solutionnot containing anthracene.λex ) 355 nm; [An] ) 3.2 mM.

Figure 4. Variation of the fluorescence decay profile oftrans-stilbenein dehydrated NaY. From top to bottom: at 77 K; at room temperatureand bathed in cyclohexane; at room temperature. Dashed line is theIRF.

TABLE 1: Fluorescence Lifetime of trans-Stilbene inHydrocarbon Solvents and in Zeolitesa

medium fluorescence lifetime (ps)

hexane 66 (70( 8)14

methylcyclohexane 62hexadecane 127 (157)15

glycerin 580b

ZSM-5 590b

ZSM-5 (77 K) 1.5 nsb

NaY (5 mM trans-stilbene)solvent-free 524 H2O/sc 5313 H2O/sc 22H2O bath 210, 195b

1 cyclohexane/sc 712.5 cyclohexane/sc 105cyclohexane bath 275, 258b

solvent-free (77 K) 1.4 nsb

NaY (100 mMtrans-stilbene)solvent-free 282.5 cyclohexane/sc 54cyclohexane bath 165

KY (5 mM trans-stilbene)solvent-free 544 H2O/sc 4713 H2O/sc 30

a Predetermined literature values are given in parenthesis. Thetrans-stilbene concentration in ZSM-5 was 10 mM.bValues determined bynonlinear least-squares fitting to a single-exponential minimizingø2.

Molecular Motion in Zeolites J. Phys. Chem. B, Vol. 105, No. 14, 20012759

the range 200-300 ps. In ZSM-5,trans-stilbene motion is moreconstrained than in NaY. This is in accord with previousstudies,9,16although the lifetimes reported here are significantlysmaller than those reported in ref 9 (this earlier work gave 200ps in NaY, 1.9 ns in ZSM-5, and 110 ps for methylcyclohexane).We believe the values ofτf,TS reported here are more accurate,although good correlation between NMR and fluorescencelifetime measurements regarding the relative motion oftrans-stilbene in ZSM-5 and NaY was reported in ref 9. The reasonfor these differences inτf,TS is not immediately clear but couldbe due to differences in loading or to the apparatus employedto measureτf,TS. In ref 9, single photon counting and decon-volution were used, but the excitation pulse width and IRF werenot reported. Also, thetrans-stilbene loading was 2.5% com-pared to<0.1% in the present study. As indicated from thevalues in Table 1, increased loadings lowerτf,TS, probably fromself-quenching due to energy transfer.

Adding cyclohexane to NaY increasesτf,TS. It appears thateven a small amount of cyclohexane (1/sc) restricts mobility.On the other hand, small amounts of H2O lower τf,TS to thelowest value observed. Recently, zeolites X and Y have beenclassified as “superpolar” mainly because of Coulombic forceswhich originate from interactions between aromatic moleculesand poorly shielded exchangeable cations.17 Other studies haveshown that small amounts of H2O added to these zeolitesincrease diffusion of aromatic species included in them, presum-ably by weakening theπ electron-cation interaction.4,5 Thus,the decrease inτf,TS could result from the combination of highpolarity of the zeolite medium favoring the polar transition stateand the effect that water has on mobility. At these low watercontents, it cannot be strictly stated that the small decreases inτf,TS are from an increase in mobility, because the effect thatH2O has on the polarity of thetrans-stilbene microenvironmentis unknown.

Bathing the sample in H2O appears to restrict motion. It hasbeen shown that certain nonpolar molecules such as anthracenemigrate to the exterior surface of the zeolite particles when H2Ois included in the zeolite.18 However, this was observed only atcertain loadings of H2O much higher than used here but lowerthan the maximum loading attainable by bathing the sample inH2O. It seems unlikely that molecules such astrans-stilbenewhich are essentially insoluble in H2O could migrate throughthe zeolite to the particle exterior when the zeolite voids arefilled with H2O. Thus, bathing the zeolite in H2O should traptrans-stilbene in the particle interior. Given that adsorption oftrans-stilbene should be considerably weakened by the presenceof H2O, then τf,TS in this sample may reflect bulk solventproperties in the zeolite.

Conclusion

A simple subnanosecond technique has been developed tofollow photoinduced events with one laser pulse. The techniqueis successfully used to monitor the picosecond motion oftrans-stilbene in the supercages of zeolite Y and in ZSM-5. Thenarrow channels of the latter severely restrict the twisting motionof stilbene. However, little inhibition to this motion is observedin solvent-free zeolite Y. The addition of small amounts of water

slightly enhances the stilbene motion, whereas saturation of thezeolite by either water or cyclohexane severely reduces thismotion. The data clearly show conditions that control limitedrapid motion within the channels and cages of zeolites. Thisinformation will be subsequently used to interpret the photo-induced reaction of two molecules in a supercage.

The numbers reported here indicate little or no restriction onthe motion leading to the isomerization oftrans-stilbene insolvent-free NaY. In this case, the mobility is equivalent to orgreater than that observed in butanol. On the other hand, onthe basis of actual fluorescence lifetimes oftrans-stilbene insolvents, the previous study9 indicated restricted motion in NaY,intermediate between that observed in hexadecane and glycerin.Thus, the present work, which directly measures fluorescencelifetimes, points out some of the difficulties of earlier measure-ments where extensive deconvolution of nanosecond excitationpulses was used to gain picosecond information.

Acknowledgment. This work was supported by the NationalScience Foundation. The authors thank Dr. Michael Wark forsupplying the ZSM-5.

References and Notes

(1) Reference textbooks on the structure and properties of zeolitesinclude the following: (a) Breck, D. W.Zeolite Molecular SieVes: Structure,Chemistry, and Use; John Wiley and Sons: New York, 1974. (b) Barrer,R. M. Zeolites and Clay Minerals as Sorbents and Molecular SieVes;Academic Press: New York, 1978. (c) Rabo, J. A.Zeolite Chemistry andCatalysis; ACS Monograph 171; American Chemical Society: Washington,DC, 1976.

(2) (a) Turro, N. J.Acc. Chem. Res.2000, 33, 637-646. (b) Turro, N.J. In Molecular Dynamics in Restricted Geometries; Klafter, J., Drake, J.M., Eds.; John Wiley and Sons: New York, 1989; Chapter 14. (c) Scaiano,J. C.; Garcia, H.Acc. Chem. Res.1999, 32, 783. (d) Blatter, F.; Sun, H.;Vasenkov, S.; Frei, H.Catal. Today1998, 41, 297.

(3) Joy, A.; Uppili, S.; Netherton, M. R.; Scheffer, J. R.; Ramamurthy,V. J. Am. Chem. Soc.2000, 122, 728.

(4) Ellison, E. H.J. Phys. Chem. B.1999, 103, 9314.(5) Hashimoto, S.; Miyashita, T.; Hagiri, M.J. Phys. Chem. B.1999,

103, 9149.(6) Sumitani, M.; Yoshihara, K.Bull. Chem. Soc. Jpn.1982, 55, 85.(7) (a) Saltiel, J.; Megarity, E. D.; Kneipp, K. G.J. Am. Chem. Soc.

1966, 88, 2336. (b) Saltiel, J.; Waller, A. S.; Sears, D. F.J. Am. Chem.Soc.1993, 115, 2453.

(8) Kim, S. K.; Courtney, S. H.; Fleming, G. R.Chem. Phys. Lett.1989, 159, 543.

(9) Ramamurthy, V.; Caspar, J. V.; Corbin, D. R.; Eaton, D. F.;Kauffman, J. S.; Dybowski, C.J. Photochem. Photobiol. A: Chem.1990,51, 259.

(10) (a) Iwata, K.; Hamaguchi, H.Bull. Chem. Soc. Jpn.1997, 70, 2677.(b) Iwata, K.; Hamaguchi, H. Bull.Chem. Phys. Lett.1996, 261, 208.

(11) The data in decanol was added to this graph in response to areviewer’s comment.

(12) Ellison, E. H.; Thomas, J. K.Langmuir2001, 17, in press.(13) Sumitani, M.; Nakashima, N.; Yoshihara, K.; Nagakura, S.Chem.

Phys. Lett.1977, 51, 183.(14) Fleming, G. R.Chemical Applications of Ultrafast Spectroscopy;

Oxford, New York: 1986; p 95.(15) Rothenberger, G.; Hegus, D. K.; Hochstrasser, R. M.J. Chem. Phys.

1983, 79, 5360.(16) (a) Gessner, F.; Olea, A.; Lobaugh, J. H.; Johnston, L. J.; Scaiano,

J. C.J. Org. Chem.1989, 54, 259. (b) Baldovi, M.; Corma, A.; Garcia, H.;Marti, V. Tetrahedron Lett.1994, 35, 9447.

(17) Uppili, S.; Thomas, K. J.; Crompton, E.; Ramamurthy, V.Langmuir2000, 16, 265.

(18) Hashimoto, S.; Ikuta, S.; Ashai, T.; Masuhara, H.Langmuir1998,14, 4284.

2760 J. Phys. Chem. B, Vol. 105, No. 14, 2001 Ellison and Thomas