2770
The Journal of Physical Chemistry, Vol. 82, No. 26, 1978
M. M. Martin and W. R . Ware
Fluorescence Quenching of Carbazole by Pyridine and Substituted Pyridines. Radiationless Processes in the Carbazole-Amine Hydrogen Bonded Complext Monique M. Martini and William R. Ware* Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 567 (Received August 28, 1978) Publication costs assisted by the National Research Council of Canada
A study of the photokinetics of the quenching of carbazole by pyridine and 2,6-dimethylpyridine is reported. A set of rate constants has been obtained on the basis of the Mataga kinetic scheme for the reactions of hydrogen bond formation and decomposition in the excited state. Both steady-state and transient fluorescence experiments were carried out in order to determine rate constants. The formation of the hydrogen bonded complex is observed to be a very rapid, diffusion-controlled reaction. The rate constant for complex dissociation is found to be much smaller than that of the formation reaction, and the complex is shown to relax nonradiatively. Internal conversion rate constants have been determined for carbazole hydrogen bonded to pyridine, 2,6-dimethylpyridine (2,6-DMP),2,4-dimethylpyridine(2,4-DMP),3,4-dimethylpyridine(3,4-DMP),3,5-dimethylpyridine(3,5-DMP), and 2-benzylpyridine (2-BP). The hydrogen bond deuteration, pyridine substitution, and structural effects on the internal conversion are examined and the mechanism of quenching is discussed.
I. Introduction This paper describes a study of the kinetics of fluorescence quenching of carbazole by pyridine and substituted pyridines. From the work of Mataga et a1.1,2 it is well established that the quenching of carbazole occurs through the formation of a hydrogen bonded complex with pyridine. We have thus examined the steady-state and transient photokinetics of carbazole-pyridine systems, and checked the validity of the kinetic scheme for the reactions of hydrogen bond formation and decomposition postulated by Mataga et aL3as a general scheme for hydrogen bonded complexes. The deviation from the well-established kinetic scheme for exciplex formation and decaP5 comes from the fact that the carbazole-pyridine complex is formed in the ground state as well as in the excited state, and that a part of the excitation light is absorbed by the complex. Furthermore, the present paper deals with the nonradiative electronic relaxation of the carbazole-pyridine hydrogen bonded complex. Effects of hydrogen bonding on fluorescence yields have been widely studied for hydroxy and amino aromatic hydrocarbon derivatives, nitrogen heterocycles,6and dyesa7r8It is now well known that fluorescence is quenched by the formation of a hydrogen bond between two n-electronic systems3 but there are exceptions t o this observation, and the process of fluorescence quenching or fluorescence enhancement caused by hydrogen bonding cannot be described by a simple and unique mechanism. Delocalization of x electrons through the hydrogen bond due to a charge transfer interaction, proton or hydrogen atom transfer in the nonrelaxed hydrogen bonded complex, as well as inversion of n,x* and n , ~ states, * are the most often proposed mechanisms to explain the change of fluorescence properties of molecules due to hydrogen bond interactions. For the carbazole-pyridine system, the fluorescence quenching due to complex formation was a s ~ r i b e d *exclusively ,~,~ to the strong enhancement of the internal conversion rate constant. We have determined the rate constant for this +ContributionNo. 198 from the Photochemistry Unit, Department of Chemistry, University of Western Ontario, London, Ontario, Canada, N6A 5B7. Laboratoire de Photophysique Moleculaire du C.N.R.S., Batiment 213, Universite Paris-Sud, 91405, Orsay, France.
*
0022-3654/78/2082-2770$01 .OO/O
process and examined in further detail its mechanism by studying the effect of hydrogen bond deuteration and pyridine substitution on the nonradiative rate constant.
11. Experimental Section Carbazole (99%, Aldrich) was recrystallized several times from absolute alcohol, then purified twice by column chromatography on alumina and on a pyromellitic dianhydride-alumina mixture according to a method reported by Short and Young.'O The chromatographed carbazole was recrystallized from distilled absolute alcohol, then dried, and stored in a desiccator. Carbazole was deuterated (CI2H8ND) by the following procedure: Carbazole was ground with potassium hydroxide, the powder heated gently, then melted giving a soft yellow mixture. When cool, the mixture was hydrolyzed with D,O. A white precipitate of deuterated carbazole (insoluble in water) was obtained. The precipitate was filtered, washed with DzO, and then dried. The deuteration was confirmed by NMR. Pyridine, 2,6- and 2,4-dimethylpyridine (2,6- and 2,4DMP), purchased from Eastman, 3,4- and 3,5-dimethylpyridine (3,4- and 3,5-DMP),purchased from Aldrich, and 2-benzylpyridine (2-BP),purchased from Pfaltz and Bauer, were distilled from barium monoxide. 2,4-DMP was distilled twice and 2-BP three times under reduced pressure. Only the middle fraction of each distillation was collected. Solutions were prepared using cyclohexane as the solvent, which was passed through a column of basic alumina just prior each series of experiments. The carbazole concentration was 5.0-5.5 X M. The quencher concentration was varied in the 0-1.25 M range. The absorption and fluorescence spectra were recorded using respectively a Cary 118 spectrophotometer and an MPF-4 Hitachi Perkin-Elmer fluorimeter. For fluorescence measurements, the excitation monochromator was set at 3090 and 3185 p\ with a 15-p\ bandpass. The emission was recorded with a 30-p\ bandpass, filtering the light in the wavelength range below 3100 A. The spectra recorded at the highest sensitivity of the fluorimeter were corrected for distortions caused by Rayleigh and Raman scattering. No correction was applied for the photomultiplier response because the spectra to be compared 0 1978 American
Chemical Society
The Journal of Physical Chemistry, Vol. 82,
Fluorescence Quenching of Carbazole by Pyridine
No. 26, 1978 2771
2-
1
I
--A
3200
1
-
L
3LOO
Figure 1. Hydrogen bonding effect on the carbazole absorption spectrum in cyclohexane solutions: .) carbazole alone; (-) hydrogen bonded complex carbazole-pyridine, ([Q]= 1.125 M); (---) hydrogen = 1.125 M). The bonded bonded complex carbazole-2,6-DMP carbazole spectra were recorded using 1,125 M quencher in cyclohexane as the reference. (e.
([a]
were very similar and only slightly shifted. Fluorescence lifetimes were measured with the single photon counting technique described e1~ewhere.ll-l~ The flash lamp was operated at different voltages between 3.6 and 6.0 kV, with a repetition rate that we varied from 25 to 35 kHz. Deuterium at 0.5 atm pressure was used as the flash gas. The excitation wavelengths (3090, 3185, and 3360 A) were selected with a 1/4-m Jarrell-Ash monochromator. The entrance and exit slits of the monochromator were varied from 0.25 to 5 mm according to the fluorescence intensity of the sample. In the path of the emitted light, Corning 0-52 or Kodak 18 A filters were used. The samples were deaerated by freeze-pump-thaw cycles and thermostatted a t 24 “C. Lifetimes were obtained by iterative convolution.
111. Results and Discussion A. Kinetics of Fluorescence Quenching of Carbazole by Pyridine and 2,6-DMP. When pyridine or 2,6-DMP is added to a solution of carbazole in cyclohexane, a hydrogen bonded complex is formed between the carbazole and the amine. By varying the amine concentration from 0 to 1.25 M, one observes several isobestic points in the absorption spectra, which shows that both the “free” and hydrogen bonded carbazole are present in the the solution. From the observed spectral change when the amine concentration is increased, we determined, using eq 1,the equilibrium constant K g for the ground state equilibrium (eq A).
AH
+QS AH-Q K,
(A)
We obtained K , = 12 and K , = 13 when pyridine and 2,6-DMP, respectively, are used as the quencher. Thus, the absorption spectrum obtained for amine concentrations larger than 1 M is mainly due to hydrogen bonded carbazole. As shown in Figure 1,the absorption spectrum of carbazole is shifted toward the red and is slightly broadened, as a result of the hydrogen bonding interaction. The red shift of the absorption spectrum is caused by the fact that the hydrogen bonding ability of carbazole with pyridine and the substituted pyridine is stronger in the excited state than in the ground state. The equilibrium constant for reaction A in the excited state, K,, can be
Figure 2. Change in the carbazole fluorescence due to hydrogen bonding with pyridine. The excitation wavelength was 3185 A: (-) [Q]= 0 (X 1); (- X -) = 0.024 M (X3); (---) [Q] = 0.12 M (X30); (. * *) [a] = 1.22 M (X 100).
[a]
Figure 3. Change in the carbazole fluorescence due to hydrogen bonding with 2,6-DMP. The excitation wavelength was 3090 A: (-) [Q]= 0 (Xl); (---) [Q]= 0.10 M (X10); (- X -) [Q]= 0.20 M (X30); [Q]= 1.0 M (XIOO). ( a * . )
estimated from Kg and from the spectral shift due to the hydrogen bonding 8va, using the approximate equation log K , = log K , + (0.625/T)6va (2) The spectral shifts for pyridine and 2,6-DMP are 582 and 538 cm-l, and the corresponding K, values are 200 and 160, respectively. A better determination of K , is obtained by using the 0-0band shift for the So S1 transition in eq 2 and by assuming that the frequency of this band is equal to half the sum of the frequencies of the absorption and fluorescence peaks. This will be discussed below. The progressive change of the carbazole fluorescence spectrum, when the amine concentration is increased to 1.25 M, is illustrated in Figures 2 and 3. This change shows that the fluorescence spectrum can be assigned to the superposition of the fluorescence of both the free and the hydrogen bonded carbazole. The spectrum measured for amine concentrations larger than 1 M is attributed to the bonded carbazole alone. This spectrum displays a good image of the absorption spectrum obtained under the same conditions: it shows two peaks a t 3450 and 3580 A, and a shoulder around 3750 A. One observes, as in the absorption spectrum, that the fluorescence spectrum of carbazole is shifted toward the red as a result of hydrogen bonding. The change in shape of the fluorescence spectrum is accompanied by a strong fluorescence quenching. As observed earlier by Mataga et aL3 fluorescence quenching occurs for quencher concentrations as low as M whereas the absorption spectrum remains unchanged for
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2772
The Journal of Physical Chemistry, Vol. 82,
O,O
No. 26,
M.
1978
experimental data
0, f r e e solutions
absorbed by the solution. 6 = 1/[1 + AH- ( X e x c ) / t ~ ~ (h,,,))K,[Q]] is the fraction of light absorbedjy the free species, 1 - 6 is the fraction absorbed by the hydrogen bonded species. The total fluorescence emitted by AH* and (AH-Q)* may be written as F = kl[AH*] + h6[(AH-Q)+]. One can show3that for a system described by scheme I the change in the total fluorescence yield 4/&, due to increasing quencher concentration is given by eq k4r’(l - 6) kjr’(l - 6) 6+ 1 + k47‘ k170(1 + h4~’) 4_
+
-
$0
0
05
M. Martin and W. R. Ware
10
I
1
+
+ (-“-)[Q]
k5r’k3( 6
+
kl(l
1 + r’kq k4r’(l - 6) 1 k47’
+
+ k4r’)
)
+ k3k5~’(1 - 6) k l ( l + k47’)’
3 in which $ = F / I a b s and $o = F([Q] = o)/I,b:. l a b s and are the total light absorbed in presence and absence of quencher, respectively. T o check whether Mataga’s kinetic model can be applied to the carbazole-pyridine complex, we must determine the following parameters: Kg, 6, K,, kl, h5, ro, r’, k3, and k4. Once they are known, eq 3 can be computed for 0 < [Q] < 1.25 M and compared with the experimental ratio 4/qb0. K,, 6 , and K,. The equilibrium constant in the ground state K , has been reported at the beginning of this discussion. The fraction of light 6 absorbed by the free carbazole becomes 1.1 + K,[Q] under our experimental conditions since we irradiated the samples at 3090 and , 3185 A for which tA&Q/tAH is -0.95 and ~ 0 . 9 0respectively. Earlier in this paper, we gave values of the equilibrium constant in the excited state which we estimated from the absorption spectral shift due to hydrogen bond formation. Using both absorption and fluorescence shifts we estimated the GO band shift ( 6 ~ for ~ the ) So S1 transition. Then, we used 6 ~ instead ~ - of~ 6u, in eq 2. We obtained 724 and 702 cm-l for 6 ~ for~ carbazole- ~ pyridine and carbazole-2,6-DMP, respectively, and thus 395 and 356, respectively, for K, for these two systems. Although there is a noticeable discrepancy between the values of K , obtained using either 6v, or 6v0-,,, in all cases (160 < k , < 400) K , is found to be much larger than Kg. Furthermore, K, is likely to be almost the same for the two systems. kl and k5. We assumed that hydrogen bonding does not affect the carbazole radiative rate constant. This assumption is based on the fact that the slight red shift of the carbazole absorption spectrum due to hydrogen bonding is accompanied by a slight broadening and a slight decrease in intensity leading the whole spectral area and remaining nearly constant. ro and 7‘. ro was measured in the absence of quencher, as described in the Experimental Section, in aerated and 02-freesolutions; we obtained 8.0 and 14.8 ns, respectively. The fluorescence lifetime of the hydrogen bonded carbazole (7’) was determined by comparing the total fluorescence intensity of a solution of carbazole containing more than 1 mol of quencher with that of a solution of carbazole alone. Since kl and k j are assumed equal, r’ is given by T~ [$[&I > 1 M)/$[Q] = 011. We found r’ = 34 ps for the carbazole-pyridine complex and T’ = 59 ps for carbazole-2,B-DMP. Iabso
~ ~ ~ ~ ( 1 - 8 )
I0bS8
--
AH f hvf
ro = ( h ,
AH
+ k2)-’
(AH-Q)
t hvf’
(AH-(3)
r‘ = ( h , + k 6 ) - ’
quencher concentrations below M, in agreement with the fact that K , >> K,. (a) Steady-State Kinetics. The total fluorescence intensity emitted by the solution, in the presence ($) and absence (do)of quencher, was calculated from the area of the recorded fluorescence spectrum. For each chosen value of the concentration [Q] of pyridine and 2,6-DMP, we irradiated the samples a t 3090 and 3185 A for which cAH-$/~AH is e0.95 and -0.90, respectively. The measurements were repeated several times. The average values obtained a t both excitation wavelengths for the ratio $o/$ are given in Figures 1 and 5, for the carbazole-pyridine and the carbazole-2,ij-DMP systems, in oxygen free and air-saturated solutions, respectively. According to Mataga et ala3the rapid reactions of hydrogen bond formation and decomposition during the lifetime of the excited state may be described as shown in scheme I. In this scheme, Iabs is the total light quanta
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The Journal of Physical Chemistry, Vol. 82, No. 26, 1978
Fluorescence Quenching of Carbazole by Pyridine
of 1.3 upon pyridine substitution. From K, and k 3 , k4 is thus estimated to be 3.1 X lo7 < k4 < 7.9 X lo7 s-l'ind 2.4 X lo7 < k4 < 6.0 X lo7 when pyridine and 2,6-DMP, respectively, are used as the quencher. Equation 3 was used for computation, and the calculated ratio 40/+was compared with the distribution of the experimental values. Of all the parameters K , and T' are the most uncertain. We first checked that the calculated curve describes satisfactorily the experimental results. By varying K, and T', we found that the calculated curves are not changed at all when K, is varied between 160 and 400. This result is quite reasonable since the carbazole-pyridine hydrogen bonding in the excited state is a much more rapid reaction than complex dissociation (k3 >> k J . Optimum values were (Figures 4 and. 5) T' = 28 ps (instead of 34 ps) for the carbazole-pyridine complex and T' = 52 ps (instead of 59 ps) for the carbazole-2,B-DMP complex. Mataga's kinetic model is thus found to describe the quenching. The set of parameters obtained is given in Table 11. (b) Transient Kinetics. From the proposed kinetic scheme one expect^^,^ a simple two components fluorescence decay for [AH*], and a growth and a decay for [ (AH-Q)] *, i.e.
.,
[AH*] = cle-hlt + c&Zt
I 1 1 1 1 I 1 1 1 1 1 1 ' I 0 0.05 0.1 Flgure 6. Steady state plots (eq 4) for the quenching of the free carbazole by pyridine (black) and 2,6-DMP (white) in oxygen-free solution (circles) and in air-saturated solution (squares).
TABLE I: Quenching Rate Constants and Rate Constants for the Complex Formation Reaction Obtained by Steady-State (ss) and Transient (tr) Experiments quencher solution
pyridine
2,6-DMP
0 , free air satd 0, free
182 kQSS k3ssX10'0 M-'s-' 1.23 'Q,, M-' 158 k , , ~ 1 0 M-' ' ~s-' 1.07
137 0.93 122 0.83
103 1.29 84 1.05
air satd 79 0.99 64 0.80
k3BB (Steady-State)and k4. The quenching rate constant hass = k 3 , , ~ 0was determined from eq 4, in which 4 rep6 _4 --
40
(4)
(1 + h , , ~ o [ Q l )
2773
resents the fluorescence yield of the free carbazole. Equation 4 corresponds also to eq 3 in which the bonded carbazole is assumed to be nonfluorescent, Le., that T' N 0. We used eq 4 for quencher concentrations lower than 0.1 M. For this range of quencher concentration, the contribution of the bonded carbazole fluorescence to the total fluorescence is negligible. The (40/4)6vs. [Q] plots are shown in Figure 6. From the slopes of these plots we obtained k, then h3,,,which are given in Table I. The k388values obtained in oxygen free solution and in nondegassed solution are in good agreement for the two systems. These values show that hydrogen bond formation between carbazole and pyridine is a very rapid diffusion-controlled process. Yet, h389is decreased by a factor
(5)
[(AH-&)*] = ~ ~ ( e- -e-x2t) ~ l ~
(6)
where X1,2
=
l/,[k1 + kz + MQ1 + k4 + h5 + + + k , + k6 - hi - h2 - k3[Q1)2+ 4kMQl)1'21 ( 7 )
((k4
Experimentally, we observed a single exponential fluorescence decay irrespective of the emission and excitation wavelength (3100,3185, or 3360 A). Fluorescence lifetimes were determined for small quencher concentrations only. As matter of fact, the quenching is very efficient and the lifetimes become very difficult to measure with accuracy for a quencher concentration above 0.08 M. The change in the carbazole fluorescence lifetime due to increasing quencher concentration is shown by the plots T ~ / T vs. [Q] in Figure 7, in oxygen free solution as well as in nondegassed solution. The slopes of these plots give kQw = k3b~0since 7 0 / 7 = 1 h3tT~O[&]. The kQ, and k3,,values are reported in Table I. In this table, it can be seen that there is a small discrepancy between h3,, and h3,,, which will be discussed a t length later. We calculated X1 and X2 from eq 7 using k3t, and the whole set of data obtained in the steady-state experiments. X2 was found to correspond to a very short lifetime irrespective of [Q]. This very short lifetime represents 7' and is too short to be measured with our experimental setup. (7' = 28 ps for carbazole-pyridine and 52 ps for carbazole-2,6-DMP.) This result means that eq 5 and 6 can be reduced to eq 8 and 9, which explains why we find
+
[AH*] = cle-hlt
(8)
[(AH-Q)*] = c3e-xlt (10) a single exponential decay irrespective of the emission
TABLE 11: Set of Parameters Obtained for the Quenching of Carbazole by Pyridine and 2,6-DMP on the Basis of Mataga's Kinetic Scheme r0 = ( k ,
quencher pyridine 2,6-DMP
Kg
Ke
0 , free
12 13
160-400 16 0-400
14.8 14.8
+ k 2 ) - ' , ns air satd 8.0 8.0
= (ks+ k 6 ) - ' ps
k,= x 10"
28 52
1.26 0.96
T'
~
- s -1l
k , X I O 7 s-'
3.1-7.9 2.4-6.0
2774
The Journal of Physical Chemistry, Vol. 82, No. 26, 1978
M. M. Martin and W. R. Ware
TABLE 111: Hydrogen Bonding Effect o n the Branching Ratio for the Electronic Relaxation Processes of the First Singlet Excited State of Carbazole molecular system carbazole-pyridine carbazole- 2,6-DMP carbazole
@FI
@ISC
@IC
0.75 x 10-3 1.40 X 10-3 0.4
1.0 1.0 G0.15
kIc
(0.85-1.15) X (1.8-2.4) 0.45-0.60
k 1 s c s-'
S-'
(3-4) x l o 7 ( 3 - 4 ) x 10' ( 3 - 4 j x 10'
3.6 X 10" 1.9 x 1 O ' O
x
