E. Enciso a n d A. Cabello Departamento de ~u/mica ~/sica Universidad Complulense Madrid. Spain
Phosphorescence and Energy Transfer in Rigid Solutions
During the last decades, the importance of the role of the triplet state in the spectroscopy and photochemistry of polyatomic molecules has increased considerably. It is possible to find radiationless transfer of triplet energy in which a donor molecule called "sensitizer" in its lowest triplet state transfer energy to an acceptor or quencher molecule in its singlet ground state, raising it to its triplet level and quenching the donor to its ground singlet state. I t may he written.
A special requirement for such a proress is that the triplet state of theacceptor lies below or near the triplet stateof the donor. A highly influential work by Terenin and Ermolaev ( I ) elucidated the general aspects of intermolecular energy transfer between triplet states in rigid solutions of various orzanic comoounds solved in an ethanol-ether mixture a t "~ 7 7 O K The sensitized emission hy the acceptors was shown to he identical with that produced bv direct excitatim. A very good teaching paper h i Legg (2) showed several energy transfer processes in a qualitative way, demonstrating such radiationless transitions by the color of the emission. The aim of the present work is double. I I F n m nn experimentdl point oi wew, the scr u p is fundnmentsl. easy tc, use, and the uilfrrcnt pleccs arc rather cheap, and studrnta ran pcrform this iorr ot expermrnt withour mnny prddtmi.
+
Benzophmme states
+
2) To eet. in a semiuuantitative wav. ohosohareseence emission.
phenomena which are closely related. We chose henzophenone as the donor species which absorbs 3660 radiation and is raised to the first excited singlet state, SI,a '(n,II*)state. It undergoes SI TIwhose quantum yield is 0.99; the phosphorescence quantum yield being 0.74 (3).The triplet state 3(n,II*)has a relatively long radiative lifetime and during this period energy can he transferred to naphthalene molecules, raising them to their triplet states and quenching henzophenone:
-
Figure 1shows an energy level diagram for henzophenone and naphthalene. Observe that naphthalene does not absorb radiatibn a t 3660 & . and it cannotirap the phosphorescence emission from the donor, since the naphthalene singlet . lies above that for henzophenone. Experimental Note: all experiments must be performed in a dark room. A scheme of the experimental set up is shown in Figure 2. The reagents to be used (benzophenaneand naphthalene) must be analytical grade or
--
hdt-r .- -..
~lphtmM---c( sbtes
A low pressure mercury lamp model Pen Ray 11 SC-1, whose emission spectrum is shown in Figure 3, powered by a transformer model STC-1from Ultraviolet produds was used as the excitation source. As esn be seen in Firmre 3. the line at 254 nm shows the hiehest intensib. Fladiatkn at'this wavelengrh tau..- dmnnrr ro thr q r s . Thr lamp should hcusfd withanpropriatc pruleclion or fillrringofthnt wavelength. We eliminated danger by a filter model G-278 from Ultraviolet which transmits radiation of longer
.. .
with a 'icroammeter Hewsurements were lett-Packard model 425 A. Prepare solutions of benzophenone and benzophenone +naphthalene in a mixture of 2 "01. of ethanol with 1 vol. of diethyl ether. For the mixture, calculations of the true concentration of the solutions at 7 I 0 K were made by multiplying the concentration by the coefficient of thermal expansion ( 4 ) ,which at that tempersture is 1.3.
Figure 1. Energy level diagram showing abswption of radiation by benzophenone and triplet enwgybansferlonaphmalene. Radiative bansilions between states aregiven by solid ard boksn lines, radiationless process by wavy lines; ISGintersystem crossing. Vertical wavy lines are vibatiml relaxation process. V i b a t i i l and rotational levels are shown approximately squally spaced for convenience in presentation.
uimmtn Figure 2. Diagram of the experimental system. Volume 57, Number 4, April 1980 1 323
A (nm) Figure 3. Relative intensities ot the various spectral lines mined from an 17 SC-i Pen Ray mercury lamp. Photodiode
Quenchng of medona loemophanonelph06pmrescem aoa tmctson c~ncentratond the accepror (napnlhalene)at 77'< on an elnerelhanol moxture. mder rteadj lrrao at on at 3660 A IBenzophenona. 5 10 -'MI
Flgure 5 01
Gmnd
Figure 4. Scheme of the detector of light.
5 , n w triplvr states are eff~rirntlyquenched in solutmn a t room 1emper:lturr 1.5, Ihr snmple has to be cooled to lrquid nilrugen temperature. Ailer cod~nx,thesample tuhr has t u he removed from the
quickly since thk phosphoresceice is quenched rapidly warms.)
the tube
Results and Discussion In spite of the simple set updescribed above, it is possible for students to carry out measurements of quenching and energy transfer processes. After excitation to S1, henzophenone undergoes a very efficient intersystem crossing t o T Iand phosphorescence appears as a bright blue emission which can be detected by the photadiode. The intensity of the current produced by the emitted light can bemeasured by the microammeter. Let us call lothe intensity of a 0.05 M solution of henzophenone. It can be observed that intensity diminished when naphthalene is added. This is shown in Figure 5 for several concentrations of naphthalene (0.05, 0.1, 0.2, and 0.4 M ) . The ratio I f l o , where I is the phosphorescence intensity of a solution of benzophenone naphthalene, becomes smaller as naphthalene concentration increases. Bars representestimated error ranges. I t can be seen that errors are bigger as intensity of emission is higher. The dominant mechanism in the quenching of henzophenone emission by naphthalene is a triplet-triplet energy transfer process. The sensitized quantum yields of naphthalene phosphorescence is 0.07 (3);therefore a very small fraction of naphthalene molecules radiates. Phosphorescence from naphthalene cannot be measured when we are observing emission from henzophenone. The lifetimes of phosphorescence for henzophenone and naphathalene are 4.7 msee and 2.3 see, respectively (3).If there is a time interval between excitation and observation of the emission, longer than 1msee and of the order of 1sec, it is possihle to measurephosphorescence intensity from naphthalene by taking advantage of their different time scales. This interval can he achieved blocking the lamp very quickly. Then the greenemission from the T I state of naphthalene can he seen. Using this procedure, sensitized emission from naphthalene is observable. Figure 6 shows the sensitized phospho-
+
324 / Journal of Chemical Education
Figure 6. Relative phosphorescence intensiw of naphthalene, sensitized by benzophenone against naphthalene concentration. (Experimentalconditions are the same as described in Figure 5.) rescence intensity referred to that of the solution of lowest caneentration of naphthalene as a function of acceptor concentration. Intensitv of sensitized ~hosohorescenceis hieher as concentration of naphthalene increares. Hraultj appear to t~ruonsisrcnt uifh those g,ht;rined by'l'erenin and K'.rmulae\ I , mdinly at louconcrnlrctionr. As shown above, errors are bigger at higher intensities which suggests that quenching of phosphorescence when the solution warms up is a more important effect at higher concentrations. A method toprevent this ~ r o b l e mis to use a transparent square Dewar ( 6 ) .The need to prevent this problem implies a more complex experiment. Consequently, it may be less appropriate far students.
.
.
Literature Clted (1) (a1 Trrenin, A , and Ermaleev, V., T ~ MFnrodoy . Soc. 52,1042 (1956).(h) Termin. A.,Pu~iko,E.sndAkimou,I.Discussin~FaradoySoe., 27.83 (1959).(c) Emolaev, V. L.. Uapekhi Fiz, Nauk 80.3 (19631.English transistion,Soviet Physier. Uspethi No".-Dec, p. 333,11363). (2) Legs, K. D.. J. CHEM. EDUC. J0.848 (19731. (3) Calven, Jack G., and Pittr,Jr., Jsma N.,"Phdoehhmiatry."Jobn Wilw Sam, hc.,New York, 1966, pp. %Ha. (0 Zanker, Z.,Physik. Chem., 2W.254 (1952). (5) Turro, N. J., "Molecular Photahemistry." W. A. Benjamin, Inc., New York 1965, p. 53.
(61 Cetorelli,J. J., McCsrthy. W. J., and Winefordner, J. D.. J. CHEM. EDUC. 45. 93
119681.
