Phosphorescent Lifetimes and Spectra Brad Jackson and Henry Donato, Jr. College of Charleston, Charleston, SC 29424 Phosphorescent lifetimes and spectra have not often been studied in the nndereraduate laboratow. However, recently developed instrum&tation makes mkasnrement of these anantities oractical in a n undermaduate setting. A relatively simple computer-interfaced luminescent lifetime apparatus, described recently for use in physical chemistry laboratories and graduate-level computer interfacing courses, is capable of measuring the lifetimes of the relatively long-lived emissions of lanthanide complexes ( I ) . Also, the development of commercially available pulsed-source luminescence spectrometers allows measurement of lifetimes and phosphorescent spectra for a wide variety of experimental systems. This report describes the measurement of emission and excitation spectra and phosphorescent lifetimes using a commercially available pulsed-source luminescence specLS-50). Additionallv, a simolified trometer ~ - (Perkin-Elmer ~ ~ ~ ~ ~ ~ theoretical treatment suitable for se~ected-t&~eriAental svstems is uresented to aid in the interpretation of these experiments. The samole chosen to illustrate these techniques is an acidic aqueous solution of TbC18. This phosphor is stable does not photooxidize has well-defined emission and excitation soectra has rn,ission lifvtimei in thc range 0:lll to 1.30 ms, dcpcnding on the rumposition ofthe solut~ons2
These exercises should be suitable for introducing the concepts of emission decay kinetics and emission andkxcitation soectra in ohvsical chemistrr laboratow. Additionally, the operation i f a pulsed-source luminescence spectrometer and interpretation of the data could be the focus of investigation in the instrumental analysis laboratory. Measurement Using a Pulsed Spectrometer The theory of measurement of luminescent parameters using a pulsed spectrometer has been thoroughly investigated by Barnes and Winefordner (3).They consider an experimental situation in which the following parameters can be varied: the oulse width t. the delay timc ld:frorn the end of rhr cxcltlnx pulse until the rmiisum demy i~ mnnitnwd t h e gate iimr I,, the period uvrr which emlsslon decay is monitored the frequency of the pulses, f
The intensity of the emission signal is shown to be, in general. a function of t,,-. ta, . t.,-. and f. 1n the experiments discussed here, dependence of the emission intensitv on t, and f need not be considered. As an excitation source, the luminescence spectrometer used in this study had a pulsed xenon flash lamp with pulse width at half peak height of 10 ps. Because it is not possible to vary the to with this instrument, no dependence of emission intensity on pulse width can be observed. After the pulse, the emission intensity decays exponentially. The operator can set ta,t,, and f to maximize the emission intensity ofthe sample. However, because the minimum value of the period l l f (the time between pulses) that the instrument will accept 780
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is 16 ms, the emission intensity of the aqueous Tb(II1) sample will decay to zero in the interval between pulses. Hence for this sample, the emission intensity will not depend on f, the frequency of pulses. Measuring Luminescence Intensity With this particular choice of pulsed spectrometer and phosphorescent sample, emission intensity will depend on td and t p only. The intensity that one measures can be thought of as the area under the emission decay curve measured from the delav time t to the delav time nlus the gate time, td+ t,. Stated differentlv, the luminescence intensity P measured by the pulsedluminescence spectrometer i i given by %+Ip
P = Jl d t Ld
(1)
We use the substitution I = ~~e~~~
where I. is the intensity of the emission a t the end of the pulse; and z is the lifetime of the emitting excited state. Integrating, we obtain ( - t 1% -It + f )Il) P=Zore d +e d g
(2)
Choice of Gate Erne Experimentally, it is possible to hold td or t, constant while varying the other parameter to reveal the dependence of P on each parameter separately. We chose to investigate the case in which t, is held constant while td is varied. The opposite situation is left as an exercise for the student. If t. is chosen to be laree wmnared to z (for examnle. 10 ms 2 this case), the expinenti& term i&lving t, {n iq2 goes to zero. Then we may write Taking the natural logarithm of both sides we get
This shows that a plot of In P vs. Idyields a straight line with xlopc caual to -(I TI.The lifetime is calculated bv taking the negative reciprocal of the slope. Results The well-studied Tb(II1) ion emission consists of sharp peaks at 489 nm, 544 nm, and 585 nm. The major peak of excitation for aqueous Tb(II1)is in the near UV at 265 nm. From this data and the discussion above, one may make reasonable instrumental settings to obtain excitation and emission spectra of 0.075 M Tb(II1)in 0.1 M HCI. For measuring an emission spectrum in the region 45CL 650 nm, we may choose 2, = 265 nm
and set l / f= 16 ms
Figure 1. The emission spectrum of a 0.075 M solution of TbC13in 0.1 M HCI with the wavelength of excitation at 265 nm. The delay time is set at zero, and the gate time is set at 10 ms.
-~
Fioure 3. The excitation soectrum of a 0.075 M solution of TbCI, in 0.1 M HCI with theiavel>h of emission set at 544 nm. The diay time is 0.1 ms, and the gate time is 10.0 ms. ~
~
F ~ g ~2r eThe emss on spectrum of a 0 075 M sol~llonof TbCI, n 0 1 w ~ t hthe wavelength 01 exclatlon set a1 265 n m Tneoe ay I me 10 ms, and tne gate tlme is 10 0 ms hoo ng lne excltallon sl I w dth constant at 5 nrn we var eo the em ss on 5 I w olh from 2 5 to 5.0 to 7.5 nm. M &I s0
t , = 10 ma
Choice of Delay llme Remaining instrumental settings are slit width and ta. By arbitrarily setting the excitation and emission slit widths a t 5 nm, the student may investigate the effect of the choice of ta on the emission spectrum. If one chooses ta to be zero, that is, begins to monitor the emission decay signal immediately following t h e pulse, the spectrum shown i n Figure 1 is obtained. The peak a t 530 nm is the result of scattering of the 265nm excitation pulse. Peaks due to scattered exciting light can occur a t multiples of the original excitation wavelength. The distorting effects of scattering are removed by setting td = 0.1 ms, a s shown in Figure 2. The delay of 0.1 ms allows the excitation pulse sufficient time to decay before the monitoring of emission begins. This ensures minimal scattered exciting light. Choice of Slit Widths The choice of slit widths can now be examined. Both the excitation slit and the emission slit can be adjusted. Figure 2 shows the emission spectra of Tb(II1) taken with the excitation slit set a t 5.0 nm and the emission slit set a t several different values.
Figure 4. The emission intensity (in arbitrary units) of a 0.075 M solution of TbCI,in 0.1 M HCI monitored at 544 nm with excitation at 265 nm. The gate time is set at 10 ms, and the delay time is varied from 0.1-2.0 ms; (a) plot of intensity vs. delay time; (b)plot of In intensity vs. delay time. The emission slit width of 2.5 nm is the lowest possible instrumental setting, while 20.0 nm is the highest. As the slit widths increase, the emission intensity increases while the resolution, measured by a n increase in the peak width a t half height, decreases. Clearly there is a tradeoff between resolution of the spectrum and the intensity of the emission. Volume 70 Number 9 September 1993
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The Excitation Spectra a n d Emission Lifetime Excitation spectra of 0.075 M TbC13 i n 0.1 M HC1 can be recorded i n the wavelength range 2 4 0 4 4 0 nm, a s seen in Figure 3.
.
emission wavelength: 544 n m t~ " is 0.1 ms t, is 10.0 ms both slit widths set a t 5.0 nm
T h e lifetime of t h e Tb(II1) emission i s determined by measuring emission intensity (P) at 544 n m with t h e following instrumental settings.
.
, 1 = 265 nm tg = 10 ms llf = 16 ms various values of tdfrom 0.1-2.0 ms
Figure 4a is a plot of P vs. td.I n Figure 4b the ln (P) is plotted 7s. td, yielding a straight line. The lifetime calcu-
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lated according to eq 4 i s 0.42 ms, which is in excellent agreement with a literature value of 0.49.'
Suggested Exercises The following are suggested exercises for the student. (1.) Compare the excitation spectrum of 0.075 M TbC13 in 0.1 M HC1 to the absorption spectrum of 1.0 M TbCI3 in 0.1 M HC1. How do these spectra differ? How are they the same? (2.) Derive an expression for variation of emission intensity of Tb(II1) with t , while holding td constant. Verify that the equation predcts the measured dependence ofP on tp. Why must one know the lifetime of the phosphorescent sample to set tg to maximize emission intensity?
Literature Cited 1. Ballew, R. M.; Demas, J. N.;Ayda, N. P; Grubb. M.: Snyder. S. W. J Chem. E&c. 1991,68,222. 2. H o m k s , W Dew; Schmidt, G.F.; Sudnick, D. R.; KittrelL C.: Bemheim. R.A. J Amor Chem. Soe. 1977.99.2378. 3. Barnes, C. G.; Winefordner, J. D.Appl. Sppc 1481,38,214.
