Does Cuta~ac)~ Quench Benzene Fluorescence? A Physical Chemistry Experiment Bronislaw Marciniak A. Mickiewicz University. 60-780 Poznah. Poland
The quenching of excited states plays an important role in many photochemical processes (1, 2). Some laboratory experiments have been recently described in THIS JOURNAL (3-9) that illustrate the quenching process and show students the special techniques used in the field of photochemistrv. " In this proposal, the auenchina of benzene fluorescence bv bis(acetyiac&nato) c o ~ p e r ( ~ ~ ) , ~ u ( ais c ainvestigated c)~, in a laboratory experiment. This experiment was chosen because, besides the "normal" quenihing process, corrections for the absorption of exciting light as well as the absorption of benzene emission by the quencher, sometimes called the inner filter effects (lo), are taken into account. Thus, students can see how many parameters should be taken into consideration in the solution of the quenching problem. In general, aromatic hydrocarbons show intense fluorescence in solution at room temperature. The kinetics of their fluorescence quenching can be illustrated by the following scheme: ~~~~
~
monomer and long-wavelength excimer fluorescence have been observed (12). Generally, both can he quenched by Cu(acac)~.However, the monomer-excimer equilibrium, D(SJ
+ D ( S J * Ex@,)
(8)
is rapidly established a t room temperature ("high temperature" region) and the quenching kinetics can be treated as if there was only one excited species present with the mean lifetime ro (130). Because of overlapping of benzene absorption and fluorescence with the Cu(acac)~absorption spectrum, additional corrections have to be included.
FLUORESCENCE
2-DETECTION
D(S,)
+ Q -Jquenching
k,[Q][S,]
(5)
where hr, h., and hrsc are the rate constants of fluorescence, internal conversion, and intersystem crossing, respectively, (s-'1, hq is the quenching rate constant (M-I s-'1, [Q] is the concentration of the quencher (M), and I, is the intensity of the absorbed light (Einstein dm-3 s-'1. Using the steadystate approximation and the definition of fluorescence quantum yield, the students can easily derive the SternVolmer relation and where Ksv is the Stern-Volmer quenching constant (M-I), and I r are the fluorescence intensities in the absence and presence of quencher, respectively, and, TO is the decay lifetime of the D(SJ species in the absence of quencher, ll(hr k . + htsc). The Stern-Volmer quenching constant Ksv can be obtained from the slope of (c/If) versus [Q]. If ro is known from literature or separate lifetime measurements, the quenching rate constant h, can be calculated from eq 7. In the experiment described here the quenching of henzene fluorescence by Cu(acac)2 is measured. The luminescence properties of benzene in the fluid media are well known (11). In neat benzene the fluorescence of benzene
+
998
Journal of Chemical Education
FLUORESCENCE
Figure 1. Experimental setups for measuring fluwescence spectra. The fluorescence (broken ray1is detected at me right angleto the excitation light (solid ray): a) in 0.2-cm X 1.0-cm cell for tluorescance, b) in 1.0-cm X 1.0-cm cell for fluorescence,c ) in 0.1-cm cell (''front-face" technique).
Experlrnent
Benzene (fluorescence grade, Merck) was used without further purification. Bis(acetylacetonato) copper(I1) was nrenared bv the standard method (14). . . The fluorescence spectra can be measured using a conventional spectrofluorimeter (right-angle viewing technique, Fig. la, l b ) or by using a spectrofluorimeter with a modified s a m ~ l eholder to accommodate a 0.1-cm cell ("front-face" view'ing technique, Fig. Ic). The effects of various geometric arrangements for observation of fluorescence have been presented and discussed by Y. R. Lakowicz (15). Two fluorescence quenching experiments are proposed. In the first one a 0.2-cm X LO-cm or a typical LO-cm X 1.0-cm fluorescence cell can he used in the geometry of excitation in Figure l a or lh, respectively. In order to minimize the absorption of the incident light by Cu(acac)n a wavelength of 265 nm is chosen for excitation. The emission spectra were recorded in the range of 270 nm to 400 nm. In the second quenching experiment the fluorescence spectra were recorded in the range of 260 to 400 nm in a 0.1cm cell using "front-face" viewing technique (Fig. Ic). Excitation of the sample occnred at a wavelength 250 nm. The excitation light was absorbed very strongly by the system-95% of exciting light was absorbed in 1X 10-"m. M in Quencher concentrations ranged from 0 to 1 X the first quenching experiment and from 0 to 3 X 10W M in the second one. Solutions were deaerated for 10 min with oxveen-free areon in the cell fitted with a stoocock. If the st;gent's t i m e k limited, the experiment can de done without deoxygenation of the system. However, in this case, the influence of oxygen on benzene fluorescence must he taken into account and the lifetime TO must he corrected. (The oxygen concentrntim in non-degassed benzene at 25 O C is ( O ) ]= 1.9 X 10.'' (161.)
. .
-
Figure 2. The absorption (broken lines)and fluorescence (solid lines) spectra of benzene and Culacad,. . .. For fluorescence roema the vertical axis reoresents relative omensty only. 1) aDsorpt on spectrum ol benzene In cyclohexane I 12). 2) fl~orescence specb~mof benzene n cyclohexane ( 12). 31IlLorescence specbum of neat benzene 112). 4) absorption spectrum of Cu(acac)~ in ~
~
~~~
cyclohexane
Results and Dlscusslon
First of all, the students must find proper experimental conditions for fluorescence measurements. Absorption and fluorescence spectra of benzene and UV-absorption spectrum of Cu(acac)z are shown in Figure 2. I t should be stressed that Cu(acac)? . .also shows a weak absorotion in the visihlr region (A,. 650 nm, c -z 40 h i ' cm-I). Emission of Cu(acach is not ohserved at all (17). Because of t h r a b s o r ~ tion of the excitation and theemission by the quench& corrections for hnth of these effects, expressed by eas 9 and 10, must be included.
-
Figure 3 The changes In uncorrected (onapparatus response) lluorescence spectra ol benzene with rariOuJ Cu(acac1, concentrations measured as in Figure la. Excitation wavelenglh = 265 nm.
where ly" and IY are fluoresrenre intensities observed and ~nrrectedfor the ahsorvtion of exciting lieht hv- the wench. er, respectively, r~ and rg are the molar absorption coefficients at the exciting wavelength for donor and quencher, respectively, c~ and cg are molar concentrations of donor and quencher, respectively, and 1 is the exciting pathlength. u
v
P"
p n =L lo-;QcQl'
(10)
ebs
where and ffW" are fluorescence intensities observed and corrected for the absorption of the emitted light by the quencher, respectively, tg is the molar absorption coefficient of the quencher a t the em~ssionwavelengthmonitored,and 1' is the effective uathlenpth for reahsor~tionof fluorescence.' Students can be asked to dertve eq 9 from expressions for the licht nhsorhed by a sinale component of n mulriabsorber er so1,ut~on(18)and e i l 0 using the ~ a m h e r t - ~ elaw.
I For the right-angle geometry estimated as 0.5 cm. For the frontviewing technique see discussion.
In the first experiment benzene fluorescence is wenched by various concentrations of Cu(acac)p (0-1 X 10-'1M) using the geometry of measurements as in Figures l a or lb. Typical results ohtained are presented in Figure 3. The changes in the observed benzene fluorescence in the short-wavelength rangeol'thespectrum (Figs. 2and 3) can hcrxplnined as a result of reabs(,rption of emitted radiation, somertmes alsu called inner filter effect (10). S i m p l ~rnlc~llntionsshow that, even fnr the highest Cu(acar12concentration used, the correction for thealwrvtion of inrident lieht - bv.theouencher can he neglected (eq 9). However, the correction'for the absor~tionof benzene emission bv auencher (ea . . 10) . must be taken into account. stern-volm& huenching constants as well as quenching rate constants obtained without this correction will only be "formal". The quenching plots are presented in Figure 4. The "formal" rate constant ohtained for the fluorescence monitored a t 287 nm gave an extremely high value of k , = 1 X 1012 M-I s-' (TO = 29.5 ns for neat benzene) (11). I t is important to note that a ground state complex between Volume 63 Number 11 November 1986
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Figure 4. Stern-Volmer plot for the quenching of benzene fluorescence by Cu(acach. Excitation wavelength = 265 nm, emission wavelengm = 287 nm; 0,with correction for the absorption of emitted light by the quencher, X. without correction (see text).
benzene and Cu(acac)~was not detected (19). The "formal" quenching constants, KSV, obtained for different monitoring emission wavelengths remain in the following relation:
which correlates with the changes of the absorption coefficients given for Cu(acac)z as follows:
Thus, the students can see that the observed quenching process is mainly due to the absorption of benzene fluorescence by Cu(acac)~and the nonradiative quenching cannot be observed under the conditions of the first experiment (Fie. 4). 'fhe correction for the absorption of henzene fluorescence by the quencher depends on the effective pathlength for reabsorption of the emitted radiation, 1' (eq 10). In the first experiment 1' was estimated as 0.5 cm (Figs. l a and lb). For the front-face technique the effective pathlength 1' can be explained as a weighted average over the degree of excitation penetration into the cell. In the second experiment, however, because of strong absorption of the exciting light, the effective pathlength 1' is very small (1'
