2940
J. Phys. Chem. 1980, 84, 2940-2946
Fluorescence Quenching of Carbazoles G . E. Johnson Webster Research Center, Xerox Corporatlon, Webster, New York 14580 (Recelved: February 19, 1980)
The fluorescence of carbazole and certain of its derivatives, observed in dilute fluid solution, is quenched with varying efficiency upon the addition of trichloroacetic acid and related compounds. Rate constants for this photophysical process are determined by measuring the change in fluorescence lifetime as a function of quencher concentration. The quenching mechanism is interpreted as involving a charge-transfer interaction between the electronically ekcited carbazole and the ground-state acceptor. Structure-reactivity patterns have been determined for a number of other carbonyl-containing compounds. Fluorescence quenching measurements have also been used to determine the excited state basicity of carbazole, N-isopropylcarbazole, and 3,6-dimethylcarbazole.
Introduction The interaction of an electronically excited molecule with a second species in its ground state often leads to new, interesting, photophysical or photochemicalprocesses.l In other cases, these intermolecular interactions lead to greatly altered rates for processes intrinsic to the isolated electronically excited molecule. Thus, in the case of fluorescent organic molecules in solution, for example, an increase in concentration or the introduction of a suitable impurity often leads to fluorescence quenching. The quenching of the fluorescence characteristic of the isolated species is, in many cases, accompanied by the appearance of a new, lower energy, structureless emission band.l This new emission has been assigned as that resulting from the formation of a transient excited-state complex, which is called an excimer or exciplex depending on whether the complex is formed from two of the same or two different species, respectively.2 Systems in which fluorescence quenching is accompanied by excimer or exciplex fluorescence lend themselves particularly well to detailed studies concerning the mechanistic aspects of the quenching process and to the properties of the excitedstate complex i t ~ e l f . ~ In many cases, however, one simply observes a decrease in the fluorescence intensity or lifetime of a dissolved solute upon addition of a quenching agent without the appearance of a new emission band. In these cases, the formation of a transient excited-state complex as an intermediate in the fluorescence quenching process is inferred on the basis of more indirect evidence. In any event, it seems intuitively clear that in those cases where the quencher possesses no low lying excited singlet state which can be populated by dipole-dipole resonance transfer of energy from the excited fluorophore, the two molecules must approach one another at least to distances where the mutual perturbation of the two species is effective. The initial step in the fluorescence quenching process thus involves the diffusion-limited approach of the excited fluorophore and quencher to form an encounter complex. This, in turn, is followed by a redistribution of energy and/or charge density to yield a species with some finite binding energy which is called the exciplex. The nature of the quenching mechanism is, in these cases, generally inferred from the structure-reactivity relationships which emerge from measurements of the quenching efficiency as the properties of the fluorescent solute and substrate are systematically varied. Perhaps no better example of this exists than the beautiful correlations between quenching rate constants and electrochemicalredox potentials of the fluorescent solute-quencher pair which lead unequivocally 0022-3654/80/2084-2940$01.0010
to an electron-transfer mechanism for the quenching pro~ess.~ It is the purpose of this paper t o report the results of measurements concerning the quenching of carbazole fluorescence by a variety of electron-deficient compounds. Quenchers related to trichloroacetic acid have been of particular concern since it was initially proposed that this and certain other organic acids quenched the fluorescence of N-isopropylcarbazole by a mechanism involving the protonation of the excited carbazole ring.5 On the basis of the results obtained by using compounds related to trichloroacetic acid, but containing no proton functionality, it appears that the quenching mechanism is one involving predominantly a charge-transfer interaction rather than protonation. To further substantiate this, the excited-state basicity of carbazole and two of its derivatives has been determined by using a solvent system of known Hammett acidity function. Experimental Section Materials. Solvents. The sources of the solvents were as follows: tetrahydrofuran (THF), Burdick and Jackson Laboratories, Inc., Muskegon, Mich.; benzene, Burdick and Jackson; acetonitrile, Burdick and Jackson; ethanol (absolute), US. Industrial Chemical Co., Division of National Distillers and Chemical Corp., New York, N.Y. The solvents were used as received. It was established that none of these solvents contained impurities which exhibited detectable fluorescence under the conditions of excitation and detection utilized here. These solvents were also free from any appreciable concentration of nonfluorescent impurities which could serve as potential quenching agents as evidenced by the values of the fluorescence lifetimes measured in the neat solvents. Carbazoles. Carbazole, N-ethyl-2-ethylcarbazole, Nethyl.3-ethylcarbazole and 3,6-dimethylcarbazole were synthesized and purified by J. Yanus of these laboratories. The following compounds were obtained from commercial sources: N-isopropylcarbazole (NIPC), Eastman Kodak, white label, purified by multiple recrystallization from isopropyl alcohol; N-phenylcarbazole, Aldrich Chemical Co., Inc., purified by multiple recrystallization from isopropyl alcohol; N-(P-carboxyethyl)carbazole,Alfred Bader Chemicals, Division of Aldrich Chemical Co., Inc., purified by multiple recrystallization; N-(P-cyanoethyl)carbazole, Alfred Bader Chemicals, Division of Aldrich Chemical Co., Inc., used as received. Quenchers. A rather large number of quenchers were used and, rather than list them, their source, and method of purification, it is simply noted that all were obtained 0 1980 American Chemical Soclety
The Journal of Physical Chemistry, Vol. 84, No. 22, 1980 2041
Fluorescence Quenching of Carbazoles
through commercial supply houses with the exception of the methyl and ethyl esters of trichloroacetic acid. These two compounds were prepared via the acid-catalyzed reaction of trichloroacetic acid with methanol or ethanol. The esters were twice distilled with the middle fraction being retained each time. Boiling points agreed with the literature values. All other compounds were used either as received or subjected to purification by recrystallization or distillation. Experimental Methods The experimental apparatus used to measure fluorescence spectra and lifetimes has been described elsewhere.6 Fluorescence lifetirnes were determined by using the technique of time-correlated single-photon ~ounting.~ The quenching rate constants were all obtained from measurements of fluorescence lifetimes as a function of quencher concentration. All solutions were purged with high-purity nitrogen gas to remove dissolved oxygen immediately prior to measuring the fluorescence lifetime. Results and Discussion Prior to presenting the experimental results, it is useful to consider the following, frequently postulated kinetic scheme, which will Ibe used for subsequent discussion of the fluorescence qu~enching.~
The kinetic scheme (eq 1) can be simplified as follows: k
D*
t Q
-L 4
D
D
+
(5)
p k i E
D
+
hv,
D
D
+
P or products
where kf = Pfkd and k, = P,kcE. This simplified kinetic scheme can now be easily solved for the time dependence of the D* and (DQ)* fluorescence from the following pair of differential equations: d(D*)/dt
+ kr(DQ)*
- (1/70 + ~F[Q])(D)*
(6a)
d(DQ)*/dt = kf(D*)(Q)- (kr + CkiE)(DQ)* (6b) i
These solutions are of the form1
where
Y = k, D t hv,
(DO)*
+ ZkiE 1
Q
or products
Electronic excitation of the fluorescent species D to give D* is followed by the mutual diffusion of D* and the quencher Q to form the encounter complex (D*.-Q). The rate of this process is, of course, diffusion limited and occurs with the rate constant kd. The rate constant for formation of the exciplex (DQ)* from the encounter complex is designated kECwhile the rate constants for all other possible processes are designated similarly. Solution of eq 1 under steady-state conditions yields the following expression for the ratio of.,the fluorescence intensities of D* in the absence and presence of the quencher Q, T~ is the fluorescence lifetime of D* in the absence of Q.
Thus, it is seen that the fluorescence of D* should exhibit a two-component decay while that from (DQ)* should exhibit a growth and decay. Two points regarding the experimental results are of particular importance. First, the fluorescence decays of these carbazoles in the presence of the quenchers used here are first order and, second, the quenching rate constants obtained from steady-state experiments and fluorescence decay measurements are in good agreement. From this it is concluded that the rate constant kr is unimportant and the measured quenching rate constant has the form k, = kf = Pflzd
(8)
where
Pf = kEC/kEC + k-d and thus lZq
This equation can be written more simply by noting that kEC/(kEc + kd) is the probability, pf,that the encounter complex undergoes the energy and/or charge exchange interactions necessary to form the exciplex and k4/kEC + kd) is the probability, Pr, that D* and Q diffuse apart without reacting. Thus, eq 2 becomes
which has the form of the familiar Stern-Vdmer equation where the quenching constant K,, := kqro and the rate constant, k,, corresponds to PfkdCkiE
(4)
= kdkEC/kEC
+ k-d
Since kd and k-d (in the absence of hydrogen-bonding effects) should remain constant in any one solvent system, the measured variations in k, with different quenchers reflect changes in kEC, the rate constant for formation of the exciplex from the encounter complex, the process which results in fluorescence quenching. It should be noted that exciplex fluorescence was not observed for the majority of systems investigated. In those cases in which exciplex fluorescence was present, such as, for example, dimethyl terephthalate, this process was not studied. The work reported here is concerned solely with the quenching of the carbazole monomer fluorescence. The fluorescence quenching rate constants were obtained by measuring the fluorescence lifetime of the carbazoles first in the absence of and then in the presence of varying concentration of quencher. As noted above, all the carbazoles exhibited single exponential decay both in the absence and presence of all the quenchers utilized here.
2942
The Journal of Physlcal Chemistty, Vol. 84, No. 22, 1980
Johnson
TABLE 11: Fluorescence Quenching Results for Trichloroacetic Acid-Carbazole Pairs in THF at 25 C fluorophore N-(P-cyanoethy1)carbazole N-phenylcarbazole N-(0-carboxyethy1)carbazole N-isopropylcarbazole carbazole N-ethyl-2-ethylcarbazole N-ethyl-3-ethylcarbazole
3,6-dimethylcarbazole
K ~ 10-9k ~ , M-' M - 1 s:' 45.5 3.2 44 3.8 64 4.2 79 5.2 73 6.5 92.5 6.7 82 5.7 97 7.0
7..
ns
14.3 f 11.3 f 15.1 f 15.2 f 13.7 f 16.2 f 14.51t 13.9 1-
0.5 0.5 0.6 0.5 0.5 0.5 0.5 0.5
TABLE I11 : Fluorescence Quenching Results for
Figure 1. Stern-Volmer plot for the quenchlng of NIPC fluorescence by trichloroacetic acid in THF at room temperature.
TABLE I : Fluorescence Quenching Results for
NIPC-Chloronated Methyl or Ethyl Acetate-Ethyl Acetate Pairs in THF at 25 " C quencher Ksv, M-' k,, M-'S-' methyl trichloroacetate 123 8 . o ~109 ethyl trichloroacetate 112 7.4 x 1 0 9 methyl dichloroacetate 17.3 1.1x 109 ethyl chloroacetate 0.21 1.4 x 107 ethyl acetate no quenching
MPC-Quencher Pairs in THF at 25 " C quencher chloroform 1,1,l-trichlorotrifluoroethane trichloroacetic acid ethyl trichloroacetate methyl trichloroacetate carbon tetrachloride trichloroacetaldehy de
trichloroacetonitrile
KSy,
M0.5 69 79 112 123 137 178 198
k,, M"
3.3 X 4.5 X 5.2 7.4 x 8.0 X 9.0 x 1.2X 1.3 X
s-l
lo7 lo9
~1 0 9 109
lo9 109
10" 10"
Plots of r0/7vs. the quencher concentration obeyed the Stern-Volmer expression (9) 7 0 / 7 = 1 + kq70[QI and the rate constant k, was obtained from the slope of these plots, since we knew the value of 70, the lifetime of the fluorophore in the absence of quencher. A typical plot is shown in Figure 1. The fluorophore in this case was NIPC and the quencher trichloroacetic acid. Trichloroacetic acid was one of the quenchers used by Pfister and Williams in their study of NIPC fluorescence q ~ e n c h i n g .The ~ value of the quenching constant obtained by these authors using steady-state fluorescence intensity measurements is in good agreement with that obtained here from lifetime measurements. A number of other NIPC-quencher combinationsinvestigated here have also been studied under steady-state conditions and indicate no discrepancy between the rate constants obtained under steady-state or transient conditions. Table I presents the results of similar measurements on a number of other quenchers, each containing the trichloromethyl group as a common structural element. The apparent trend in quencher effectiveness which emerges is an increase in rate constant with an increase in the electron-acceptor strength of the substituent attached to the trichloromethyl group. This trend suggests that the mechanism of fluorescence quenching by these compounds, including trichloroacetic acid, is one involving a chargetransfer interaction between the electronically excited NIPC functioning as an electron donor and ground-state quencher serving as the electron acceptor. This interpretation is placed on a semiquantitative basis by considering the trichloromethyl group as the reactive center and the attached group as a substituent which alters the electron density at this reactive center. Figure 2 is a plot of the log k,(X)/k,(H); where k,(X) is the value of
the rate constant for the quencher with substituent X divided by the value for chloroform vs. the value of the substituent constant F. The value of F reflects the field and inductive effects of the substituent as defined by Swain and Lupton.8 These authors have described the large variety of reaction-independent substituent constants, u, in terms of a linear combination of two basic sets, F and R, which are a measure of the field and resonance contributions of the substituent, respectively. The effect of the substituent on trichloromethyl group should be predominantly inductive and field, and thus F alone is used to check for a Hammett-type correlation between rate and substituent constants. Figure 2 indicates a correlation does exist although the number of substituents available and the variation in rate constants is quite limited. It is clear, however, that the rate constant does increase with increasing acceptor strength of the substituent and thus the is positive, reaction constant, p = A log kq(X)/k,(H)/AF indicating that the rate-determining step in the quenching reaction somehow involves the ability of the trichloromethyl group to accept negative charge. It is also clear from Figure 2 that the correlation is better if only the substituents CF3, C1, and CN are considered when chloroform serves as the standard. A reasonable interpretation of this effect would appear to be one based in the electron-accepting properties of the carbonyl group present in the other four compounds. The reaction center in these compounds is not the trichloromethyl group alone but rather the trichloroacetyl group as a whole with perhaps the carbonyl being dominant. Among these four compounds, trichloroacetaldehyde would serve as the standard. The lower curve in Figure 2 shows the variation in k , ( X ) / k (H) with the value of the substituent constant F with trichoroacetaldehyde as the standard. Here the quenching rate constant decreases as the electron-donating strength of the substituent group increases. This behavior is also consistent with an interpretation of the fluorescence quenching mechanism in terms of a charge-transfer interaction between the excited carbazole and quencher. The substituent effects in the carbonyl-containing quenchers suggests that the variation of the rate constants in these compounds reflects primarily the variation in the level of the antibonding ?r molecular orbital of the carbonyl. Fluorescence quenching investigations have also been carried out by using trichloroacetic acid as the quencher with a number of carbazole derivatives in which the sub-
The Journal of Physical Chemistry, Vol. 84, No. 22, 1980 2943
Fluorescence Quenching of Carbazoles
sidered on the basis of known electrochemical and spectral data, would be expected to be quenched with a diffusionlimited rate constant. The free-energy change for the reaction producing a solvent-shared radical ion pair from the encounter complex (eq 10) is given by eq 11, where E(D/D+) and E(A-/A) are (D,****A,)F? (2D,+-*2A[) (10)
AF
,/
L.
& 02
0.4
0.6
I
F
Figure 2. Correlation of the quenching rate constants with the field component of the substituent constant.
stituent groups or position of substitution is varied, as the fluorophore. These results are given in Table I1 and substantiate furtheir the above conclusions implicating the importance of charge-transfer interactions in the fluorescence quenching mechanism. Additional fluorescence quenching results have been obtained on a series of chlorinated methyl or ethyl acetates. These are given in Table 111and are of particular interest when compared to results obtained by Hammond et aL9 for the quenching OF indole fluorescence. The fluorescence lifetime of NIPC measured in nitrogen-saturated, neat ethyl acetate is within 0.1-0.2 ns of that measured in solvents such as saturated hydrocarbons (3-methylpentane, hexane, met hylcycllohexane, etc.) or alcohols (methanol, ethanol, etc.), anti thus this compound exhibits no quenching action on NIPC. Placement of a-chlorine substituents leads to increasing effectiveness as a quencher with nearly a 2 order of magnitude increase in k, in going from a single chlorine atom to two such substituents. The value of k, for the quenching of NIPC fluorescence by ethyl chloroacetate is only 1.4 X lo7 M-l s-l whereas Hammond et, ala9 report a value of 4.2 X lo9 M-l s-l for the quenching rate constant for indole by methyl chloroacetate in acetonitrile and 1.4 X 1O1OM-l s-l in hexane. Similarly, chloroform has been found to be a vrjry efficient quencher of indole fluorescence with k, ranging from -5 X lo9 to >lolo M-' s-' in numerous solvents compared to only 3.3 X lo7 M-' s-l for NIPC.lo It shoulld be noted that this profound difference between the reactivity of excited-state indole and NIPC is not the result of hydrogen bonding between the quencher and excited indole. Hydrogen bonding has a very dramatic effect on the ability of certain molecules to quench the fluorescence of carbazole as opposed to NIPC (see below), however, this is not the case with ethyl chloroacetate since the quenching rate constant is about the same for both carbazole and NIPC. The reason for this very large difference in reactivity is not understood, but perhaps could be due to the higher lying lowest excited singlet state of indole as compared to the carbazoles which more than compensatesfor the somewhat higher ionization potential of indole. However, more surprising than these reactivity differences in the low reactivity of the carbazoles themselves, which, when con-
= E(D/D+) - E(A-/A) - AEo,o- e 2 / v
(11)
the oxidation and reduction potentials of the donor and acceptor, respectively, AEo,ois the energy of the electronically excited donor and -e2/a corresponds to the Coulomb energy released upon transfer of an electron from D* to A at the encounter distance r in a solvent of dielectric constant e. The half-peak oxidation potential of NIPC in acetonitrile has been measured and found to be Epp(NIPC/NIPC+) = 1.14 V (vs. SCE).12 The reduction potentials of most of the compoynds utilized as quenchers do not appear to be available; however, those for chloroform13 and methyl chlor~acetate~ have been measured in 75 % dioxane, 25% water, and dimethylformamide, respectively, and are -1.67 and -1.84 V (vs. SCE). The differences between reduction potentials measured in acetonitrile and these two solvents are small. Using a value of AEo,o= 3.52 eV and e 2 / v = 0.1 eV (where r = 3.5 A and 6 = 37), we calculated AF = -19 kcal/mol for chloroform and AF = -14.3 kcal/mol for methyl chloroacetate. On the basis of the exothermicityof these reactions, quenching rate constants at or near the diffusion limit are expected, yet rates nearly three orders of magnitude less are observed.ll Presumably then, the small rate constants for the quenching of NIPC fluorescence by chloroform and ethyl chloroacetate must indicate a considerable activation energy for passage from the encounter complex to the exciplex in spite of the large exothermicity of the reaction or that the interaction between the fluorophore-quencher pair is of a highly specific nature. In this regard, it is im'portant to note that the preceeding thermodynamic calculations are based on passage from the encounter complex to a radical ion pair. These fluorescence quenching measurements alone do not, of course, reveal the electronic nature of the exciplex; that is, whether indeed it can be described as a radical ion pair and, thus, it is entirely possible that the exciplex binding energy arises from only partial charge transfer. In an attempt to gain understanding regarding this point, fluorescence quenching measurements were made in solvents other than THF. The results of these measurements are presented in Table IV. Of principal interest is a comparison of the rate constants obtained in acetonitrile (viscosity = 0.342 cP; dielectric constant = 37.5) with those obtained in benzene (r = 0.603 cP, t = 2.27). Fluorescence quenching that occurs via an electron-transfer mechanism generally displays an increased efficiency in increasingly polar solvents. Here, however, there seems to be little if any effect of solvent polarity on the rate constants and the results are more in line with simply a viscosity effect. For trichloroacetic acid and trichloroacetonitrile this is not unexpected since the rate constants are approaching the diffusion limit and solvent viscosity effects should dominate. With chloroform the quenching rate constants are more than two orders of magnitude less than the diffusion-limited rate but even here the relative values of the quenching rate constants are more in keeping with the relative viscosity values than changes in solvent polarity. This result at first sight might appear to be evidence against quenching via an electron-transfer mechanism since this process is well-known to be solvent
2944
The Journal of Physical Chemistry, Vol. 84, No. 22, 1980
Johnson
TABLE IV : Fluorescence Quenching Results for NIPC-Quencher Pairs at 26 'C quencher
solvent
Ksv, M-I
k,, M-' s-'
:71
I
I
I
I
I
-0
I
2
3
cP
trichloroacetic acetonitrile 140 9.3 X l o 9 0.342 acid trichloroacetic benzene 117 7.7 X lo9 0.603 acid trichloroacetic THF 79 5.2 X l o 9 0.46 acid trichloroacetic ethanol 52.5 3.3 X l o 9 1.08 acid trichloroacetonitrile 234 1 . 5 X 10" acetonitrile trichloroTHF 198 1.3X 10"' acetonitrile chloroform acetonitrile 1.2 7.8 X l o 7 chloroform benzene 0.57 3.7 X l o 7 chloroform THF 0.5 3.3 x i o 7 a Viscosity values are from J. A. Ftiddick and W. B. Bunger, "Techniques of Chemistry", Vol. 11, 3rd ed, Wiley-Interscience, New York, 1970.
polarity dependent. When the quenching is the result of direct passage from the encounter complex in the radical ion pair state, this certainly is true; however, when the quenching results from formation of an excited state complex, only partially stabilized by charge transfer this solvent sensitivity is relaxed. Thus, as pointed out by Yip and co-workers,the absence of a dielectric constant effect on the quenching constant cannot be considered proof of the absence of electron transfer in the quenching process.14 On the basis of the preceding results, it is concluded that the fluorescence quenching mechanism is one involving a charge-transfer type of interaction between the electronically excited carbazole molecule serving as an electron donor and the various quenchers, including trichloroacetic acid, serving as an electron acceptor. This interaction, which does not occur between ground-state carbazole and quencher, as evidenced by the absence of static quenching, is a consequence of the lowering of the ionization potential of the donor molecule which invariably results upon electronic excitation. Next, the results of fluorescence quenching measurements designed to probe another property of electronically excited molecular states are presented. Basicity of the Lowest Excited Singlet State of Carbazole. In addition to the invariably lower ionization potential and increased electron affinity of excited electronic states, such states often exhibit profound differences in acidity or basicity from that of the ground state.15 Previous work regarding the basicity of carbazoles was primarily concerned with utilizing carbazole as a means of arriving at an estimate of the nitrogen basicity in the five-membered ring of pyrroles and indoles, compounds in which ground state protonation occurred at one of the carbons of the pyrrolic ring rather than the nitrogen. Carbazole, on the other hand, was found to protonate at the nitrogen in the ground state and the pK, for this process established an upper limit for the basic strength of nitrogen in pyrrole and indole.ls More recently, it has been established that in its lowest excited singlet-state carbazole is protonated at one of the ring carb0ns.l' Here the basicities of the lowest excited singlet states of carbazole, NIPC, and 3,6-dimethylcarbazole are determined in 20% ethanol-aqueous sulfuric acid, a solvent system for which the Hammett, Ho, acidity function has been determined.18 Figure 3 presents the results of fluorescence lifetime measurements of the three carbazoles in the 20% etha-
[H2so4]
I
1
M
FLgure 3. Change In the fluorescence lifetimes of carbazole (0),NIPC (A), and 3,8dlmethylcarbazole (0) in 20% ethanol-aqueous sulfuric acld at room temperature.
TABLE V: pK,* Values for the 'L,,Excited State of Carbazoles
a
compd PKa * NIPC -1.06 carbazole -0.82 3,6-dimethylcarbazole - 0.22 Reported for N-ethylcarbazole.
PK, - 7.8a - 5.9
nol-aqueous sulfuric acid as a function of the acid concentration, Contrary to the results with the previously discussed quenchers, the Stern-Volmer plots are nonlinear in acid concentration. This, of course, is a consequence of the fact that, at these high concentrations, the proton activity is not given by the acid concentration. Instead, the fluorescence quenching results are described by a Stern-Volmer equation of the form TO/T = 1 kqTOhO (12)
+
where
since for the reaction HB+ P H+
+ B*
Since at these acid concentrations the decrease in fluorescence lifetime parallels the decrease in fluorescence intensity, the quenching is strictly dynamic in nature and, thus, the excited state pKa* is given by the value of Ho where log ( T ~ / T- 1) = 0. The data appropriately plotted as log ( T ~ / T- 1)vs. Ho is given in Figure 4. The excited state pK,* values, m determined from these plots, are given in Table V along with the published values for the ground state pKa.le It should be noted that the ground-state protonation occurs at the nitrogen, an atom which, in fact, becomes more acidic in the excited electronic state since it loses electronic charge density to the aromatic rings. Thus, the difference in pK, values between the ground and excited states is only a lower limit to the increased basicity of one of the aromatic ring carbons since, in the ground state, protonation at a ring carbon must occur at even lower values of Ho.A further cautionary note should also be stated; that is, these excited state pKa* values deter-
The Journal of Physlcal Chemlstty, Vol. 84, No. 22, 1980 2945
Fluorescence Quenching of Carbazoles
\ \
I A .
I
-1.0
TABLE VI : Fluorescence Quenching Results for NIPC in THF at 25 " C quencher K s v , M'' ha, M-'S-' no quenching acetone acetophenone effective benzaldehyde benzamide benzoic acid p-butoxy benzaldehyde p-butoxybenzoicacid butyl acrylate trans-cinnamaldehy de trans-cinnamic acid diacetyl
"\ '
c
-1.5
-1.0
-0.5
0
0.5
HO
Flgure 4. Change in the fluorescence Ilfetlms of carbazole (0),NIPC (A),and 3,6-dim1ethylcarbazole(0)in 20% ethanol-aqueous sulfuric acid vs. the acidity function.
mined here lack true thermodynamic significance since the slope of log ( T ~ / T- 1.) vs. Ho is not unity. The reasons for this can be many arid no detailed considerations for this are given here. In any event, the relative values of the pK,* for the three compounds are meaningful. Of particular interest is the value of the fluorescence quenching rate constants which are obtained from the values of ( T ~ / T1) at Ho = 0 (see eq 12). These are k = 5.8 X lo7,2.28 X lo7, and 1.44 X lo7 M-l s-l for 3,6-~krnethylcarbazole, carbazole, anrd NIPC, respectively, values which are 2 orders of magnitude less than the quenching rate constant for trichloroacetic acid. This would appear to form rather conclusive additional evidence that tlhe mechanism for the quenching of carbazole fluorescence by trichloroacetic acid is not excited state protonation but rather one involving a charge-transfer interaction. Miscellaneous Fluorescence Quenchers. The primary concern of this investigation has been to determine the mechanism with which compounds related to trichloroacetic acid quench the fluorescence of carbazole and certain of its derivatives. During the course of this study, a number of other materials were tested for their effectiveness as carbazole fluorescence quenchers. The materials tested are all, generally, recognized chemically as electron acceptors and, thus, the quenching mechanism appears to be one involving a charge-transfer interaction between the electronically excited carbazole and a ground state quencher mollecule. Table VI contains the quenching results obtainerd for these materials. The data, as before, were obtained from Stern-Volmer plots of 70/7 vs. quencher concentraltion. In all cases, these plots were linear and the fluorescence decay of the NIPC in the presence of quencher was first order. A number of materials are rated simply effective or no quenching based on qualitative observations of the concentration of quencher necessary to give a significant reduction in fluorescence lifetime. Each of these compounds has in common the presence of the carbonyl group. It is not the presence of this substituent group alone, however, but rather the nature of the
effective no quenching no quenching effective no quenching no quenching
effective 125 effective g-diacetylbenzene 260 206 diethyl fumarate dfi_ethylacetylenedicarboxalate 177 dimethyl terephthalate 190 2,5-dimethylterephthalicacid effective diphenyl carbonate no quenching 9-fluorenone 309 1,3-indandione 21 6 mesityl oxide effective methyl benzoate no quenching N-methylphthalimide 208 phthalic anhydride 259 phthalimide 184 terephthalic acid effective
8.1 x 109 1.6 X 1.4 X 1.1x 1.2x
10" 10" 10'0 1O'O
1.9 x 10'0 1.4 X 10"
1.4 X 10" 1.7 X 10" 1.2 x 1 O ' O
groups bonded to the carbonyl or to the extent of conjugation between two carbonyl groups which determines the ability of the compound to quench NIPC fluorescence. As a starting point, consider the monosubstituted benzene derivatives. Benzaldehyde and acetophenone are both effective quenchers whereas benzoic acid, benzamide, and methyl benzoate do not quench. The later three compounds each contains a strong electron-donor group bonded to the carbonyl. The predominant effect of this is to raise the antibonding ?r molecular orbital localized on the carbonyl group with the result that the lowest-lying excited singlet state of these three compounds is (m*) as opposed to (n?r*) as in benzaldehyde and acetophenone. This suggests that it is the antibonding molecular orbital of the carbonyl group which is involved in the charge transfer interaction. It is interesting to note that benzoic acid which does not quench NIPC fluorescence is a rather effective quencher for carbazole fluorescence with k, = 9.3 X lo8 M-' s-l. A similar result was obtained by using benzamide as quencher. Both benzoic acid and benzamide were also effective in quenching the fluorescence of 3,6-dimethylcarbazole and, thus, it appears that the ability of the excited carbazole to hydrogen bond with benzoic acid or benzamide is the reason for this difference. These hydrogen-bonding effects have been observed before.19 While significant differences were noted in the ability of benzene derivatives containing a single carbonyl to quench NIPC fluorescence, depending on the substituent group adjacent to the carbonyl, no such difference was observed for compounds containing two conjugated carbonyl groups. For example, whereas benzoic acid or methyl benzoate did not quench NIPC fluorescence, both terephthalic acid and dimethyl terephthalate were effective, the latter, where quantitative data are available ( k , = 1.2 X 1O1O M-l d), must be at least 3 orders of magnitude more effective than its monosubstituted analogue. Similarly, the quenching rate constants for phthalimide, l-methylphthalimide, l,&indandione, and phthalic anhydride were all close to the rate expected for a diffusion-limited process. In this case, as with disubstituted benzene derivatives, it appears that, when there is conjugation between the two carbonyl groups, the nature of the group directly bonded
2946
J. Phys. Chem. 1980, 84, 2946-2952
to the carbonyl does not appreciably change the quenching efficiency. Additional information is obtained in the last few compounds. An essentially isolated carbonyl, as that in acetone, has no quenching ability. Two adjacent carbonyl groups (diacetyl) are effective quenchers. Two carbonyls which are conjugated through double or triple bonds are also effective quenchers as indicated by the rate constants obtained for diethyl fumarate and diethylacetylene dicarboxylate and, here again, as in the case of the benzene derivatives, the presence of the electron-donating ethoxy groups bonded to the carbonyls do not diminish the quenching efficiency. These results for the quenching of carbazole fluorescence by carbonyl-containing compounds are similar to the findings of Ricci and Nesta regarding the quenching of indole fluorescence. That paper and a number of references contained therein can be consulted for more detailed discussion.20 References and Notes (1) See, for example, J. B. Birks, "Photophysics of Aromatic Molecules", Wiley-Interscience, New York, 1970; N. Mataga and T. Kubuto, "Molecular Interactions and Electronlc Spectra", Marcel Dekker, New York, 1970; B. Stevens, Adv. Photochem., 8, 161 (1971). (2) Th. Forster, Angew. Chem., Int. Ed. Engl., 8, 333 (1969). (3) See, for example, W. R. Ware, D. Watt, and J. D. Holmes, J. Am. Chem. SOC.,96, 7853 (1974).
(4) D. Rehm and A. Weller, Ber. Bunsenges. Phys. Chem., 73, 834 (1969), H. Knibbe, D. Rehm, and A. Weller, Ber. Bunsenges. Phys. Chem., 73, 839 (1969). (5) G. Pfister and D. J. Williams, J. Chem. Phys., 59, 2683 (1973); see, however, 0. PASter, D. J. Williams, and G. E. Johnson, J. phys. Chem., 78, 2009 (1974). (6) 0. E. Johnson, J . Chem. Phys., 61, 3002 (1974). (7) See, for example, W. R. Ware in "Creation and Detectlon of the Excited State", Vol. 1, A. A. Lamola, Ed., Marcel Dekker, New York, 1971, Part A, p 213. (8) C. G. Swain and E. C. Lupton, Jr., J . Am. Chem. Soc., 90, 4328 (19681. (9) A. T. McCall, G. S. Hammond, 0. Yonemitsu, and 8. Wltkop, J. Am. Chem. Soc., 92, 6992 (1970). (10) R. F. Steiner and E. P. Kirby, J . Phvs. Chem., 73, 4130 (1969). (11) D. Rehm and A. Weller, Sei. Bunsenges. Phys. Chem., 73, 834 (1969). (12) J. F. Ambroseand R. F. Nelson, J. Electrochem. SOC.,115, 1159 (1968); J. F. Ambrose, L. L. Carpenter, and R. F. Nelson, I M . , 122, 876 (1975). (13) C. K. Mann and K. K. Barnes, "Electrochemical Reactlons in NonAqueous Systems", Marcel Dekker, New York, 1970, p 231. (14) R. W. Yip, R. 0. Loutfy, Y. L. Chow, and L. K. Magdzinski, Can. J . Chem., 50, 3426 (1972). (15) J. F. Ireland and P. A. H. Wyatt, Adv. Phys. Org. Chem., 12, 131 (1976). (18) H. J. men, L. E. Hakka, R. L. Hinman, A. J. Kresge, and E. B. Whipple, J . Am. Chem. SOC.,93, 5102 (1971). (17) A. C. Capomaccha and S. 0. Schulman, Anal. Chim. Acta, 59,471 (1972). (18) D. Dolman and R. Stewart, Can. J . Chem., 45, 903 (1967). (19) M. M. Martin and W. R. Ware, J . Phys. Chem., 32, 2770 (1978), and references therein. (20) R. W. Ricci and J. M. Nesta, J . Phys. Chem., 80, 974 (1976).
Ionization and Dissociation Equilibria in Liquid SO2. 12. The Behavior of Tetrahedral Ions Norman N. Lichtin, * Bernard Wasserman, Edward Clougherty, June Wasserman, Department of Chemistry, Boston University, Boston, Massachusetts 022 15
and John F. Reardon Depaflment of Chemistry, Boston State College. Boston, Massachusetts 021 15 (Received:March 6, 1980)
Electrolytic conductance of their solutions in liquid sulfur dioxide over a wide range of concentrations was measured for the 21 ionophores, Me,NCl, Me4NC104,PhMe3NBr,PhMe3NC1,PhMe3NI,Et4NI, Pr4NC1,Pr4NBr, Pr,NI, Bu4NBr, Bu4NI,Bu4NPc, (i-Am)4NBr,(i-Am),NI, (i-Am)4NB(i-Am)4,(i-Am)3NHBr,Hex,NI, Ph4AsC1, Ph4AsI, Ph4AsPc,and Ph4PPc, at 273.15 K and other temperatures. Limiting equivalent conductances and dissociation constants were determined for these solutes by Shedlovsky's procedure. Utilizing the data of this and other investigations, we calculated thermodynamic quantities for the dissociation equilibria of many of the solutes. Values of Bjerrum's contact distance parameter, a, were calculated from the equilibrium data and compared to sums of estimated ionic radii. Limiting ionic conductances were evaluated by a Fuoss-Coplan division of the limiting equivalent conductance of (i-Am)4NB(i-Am)4.Stokes radii were calculated for the ions employed. The results of the measurements are interpreted in terms of ion-ion and ion-solvent interactions.
Introduction
Liquid sulfur dioxide dissolves a wide variety of organic and inorganic compounds and is one of the most thoroughly investigated aprotic nonaqueous solvents. Extensive reviews of its chemistry and solvent properties have been published by Lichtinl and Waddington.2 More recently, reviews have been contributed by Tokura3 on SO2 as an organic reaction medium, by Lichtin4 on carbonium ion stability in SOz, and by Burow5 and Zingaro6 on the solvent properties of SOz. The physical and thermodynamic properties of SOz have been summarized by Kuo7 and co-workers and a general review has been presented by Holliday and Nicholls.s 0022-3654/80/2084-2946$01 .OO/O
Recent contributions to the chemistry of liquid SO2 solutions have been by Lichtin and Wassermane on the reactivity of iodide and thiocyanate ions with p-nitrobenzyl bromide and TokuralO and associates on the apparent molar volume of a series of tetraalkylammonium halides in SOz at 298.15 K as well as on the electrical conductance of 34 electrolytes in SO2 a t 298.15 K.ll Very recently, interest has developed in electrochemical phenomena involving liquid SO2 as a s ~ l v e n t . ~ ~ . ' ~ In our earlier investigations in the series14J5concerning the kinetic and association behavior of ions in liquid SO2 we found that interpretation of the kinetic results must recognize ion-solvent interactions in this medium. The 0 1980 American Chemical Society
