J. Phys. Chem. 1982, 86, 4148-4156
4148
cording to reaction 2 the cation formed will deprotonate to molecule B by a proton jump along the hydrogen bond marked in the figure. A fraction of the electrons formed by reaction 1 become trapped in the lattice as we have observed. The main part, however, undergoes geminate recombination with the protonated cation according to reaction 4. The hydrogen atom in eq 4 is thus formed at the oxygen atom of molecule B and not at the oxygen atom of molecule A. It is reasonable to expect that this hydrogen atom would abstract a hydrogen atom in the a position on the nearest-neighbor carbon atom forming the first hydroxyalkyl radical of the pair. C(CH,OH), + H (CH,OH),CCHOH + H, (5) The second member of the pair is formed when the alkoxy radical (CH20H)3CCH20at molecule A converts to the hydroxyalkyl radical (CH20H),CCHOH according to reaction 3, restoring the hydrogen bond to molecule B. Such an isomerization of an alkoxy radical into a hydroxyalkyl radical has, for example, been shown by Toriyama and Iwasaki to take place in polycrystalline methanol below 77 K.6 For the abstraction reaction 5 there are two possibilities. The abstraction may occur in molecule B at carbon atom 7. This gives rise to a radical pair 1-7 connected by a hydrogen bond in the (001) plane. This pair has not been
-
observed, indicating that the hydrogen-bonded radicals recombine. The other possibility is that abstraction occurs at carbon atom 2 at the body-centered molecule, which gives rise to the observed radical pair 1-2. The reasoning implies that the sense of the orientation of the hydrogen bonds shown in Figure 1directs the formation of the radicals in the pair. If the orientation of the hydrogen bonds were in the opposite sense (i.e., the H atom is closer to the oxygen atom O2than to 01),the abstraction would take place at carbon atom 5 and radical pairs of the type 1-5 would be formed. In this pair the radical-radical distance is 4.05 A. The pairs present in low yield might be the 1-5 combinations as pointed out before. If our reasoning is correct, the majority of the hydrogen bonds would be oriented in the sense shown by Figure 1. This is also in accordance with what has been found by neutron diffraction studies of PET crystals.16 The conclusion is that, in a hydrogen-bonded system like PET, the hydrogen bonds are directional for the formation of the radicals in a radical pair, if an ionic mechanism for the radical-pair formation is accepted.
Acknowledgment. We thank Dr. Roland Tellgren and Mr. Sten &din for the neutron diffraction measurements and many fruitful discussions. Preliminary ESR data were obtained by Mr. Abdallah Sid Ahmed Magoub.
Picosecond Laser Photolysis Studies of Deactivation Processes of Excited Hydrogen-Bonding Complexes. 2. Dibenzocarbazoie-Pyridine Systemst Monlque M. Martin,*$ Norlakl Ikeda, Tadashl Okada, and Noboru Mataga' Department of Chemistry, Faculty of Engineering Science, Osaka Universlty, Toyonaka, Osaka 560, Japan (Received; May 3, 1982)
The mechanism of the fluorescence quenching observed when two conjugated r-electron systems are directly combined by hydrogen bonding has been studied in the case of 13H-dibenzocarbazole(13DBC)- and 7H-dibenzocarbazole (7DBC)-pyridine systems by means of picosecond laser photolysis. Time-resolved transient absorption spectra and fluorescence decay curves have been measured for both hydrogen-bonded systems in cyclohexane solutions at room temperature. Formation of a charge-transfer (CT) state D+-H. .A- from the excited-singlet hydrogen-bonded complex (D-H...A)* has been clearly observed. The formation-decomposition process (D-H. -A)*a D+-H.. .A- obeys the usual dynamics of exciplex formation over the 10 ps-1 ns region in the case of the 13H-dibenzocarbazole-pyridinesystem. However, rapid equilibrium between (D-He. -A)* and D+-H.. .A- is established within the time resolution of our picosecond apparatus in the case of the 7Hdibenzocarbazole-pyridine system. It has been established that the fluorescence quenching in these systems is due to the formation of a nonfluorescent charge-transfer state in the excited hydrogen-bondedpair and the mechanism of the deactivation of this charge-transfer state has been discussed on the basis of some theoretical considerations.
-
Introduction It is now well-known that hydrogen-bonding interactions can affect drastically the photophysical properties of a large number of organic molecules. Concerning the effect of hydrogen-bonding interactions upon the fluorescence yield, it has been frequently observed that, when two conjugate *-electronic systems are directly combined by hydrogen-bonding interaction, the fluorescence of the proton donor or the acceptor is strongly quenched.'-, Reference 6 will,hereafter, be referred to as part 1 of this series. !On leave of absence from the Laboratoire de Photophysique Mol6culaire du CNRS, Universit6 Paris-Sud, Orsay, France. 0022-3654/82/2086-4148$01.25/0
Possible mechanisms assumed for this quenching were charge-transfer (CT) i n t e r a c t i ~ n between ' ~ ~ ~ ~ the proton donor and acceptor r-electron systems via the hydrogen bond as well as hydrogen atom t r a n ~ f e r "from ~ the proton donor to the acceptor. (1)N. Mataga and S. Tsuno, Naturwissenschaften, 10,305 (1956); Bull. Chem. SOC.Jpn., 30,711 (1957);N.Mataga, ibid., 31,481 (1958). (2)(a) N. Mataga, Y. Torihashi, and Y. Kaifu, Z. Phys. Chem. (Frankfurt am Main), 34,379(1962);(b) N.Mataga, F. Tanaka, and M. Kato, Acta Phys. Pol., 34,733 (1968). (3)(a) K. Kikuchi, H. Watarai, and M. Koizumi, Bull. Chem. SOC. Jpn., 46,749 (1973); (b) S. Yamamoto, K. Kikuchi, and H. Kokubun, ibid., 49,2950 (1976). (4) M. M. Martin and W. R. Ware, J. Phys. Chem., 82, 2770 (1978). (5)D.Rehm and A. Weller, Isr. J. Chem., 8, 259 (1970).
0 1982 American Chemical Society
Excited Hydrogen-Bonding Complexes
The Journal of Physical Chemi$tty, Vol. 86, No. 21, 1982
In relation to this problem, we have detected for the first time the formation of a transient CT state from the fluorescent state of the 2-naphthylamine-pyridine hydrogen-bonded system, by means of the picosecond laser photolysis method.6 This result seems to give direct support to the CT mechanism of the fluorescence quenching due to hydrogen bonding. However, in order to establish a general mechanism of the quenching of fluorescence through hydrogen-bonding interaction, we need more detailed results of direct observations of the quenching processes. In view of the importance of this problem in elucidating the photochemical primary processes, especially in the case of biologically important molecules, we have undertaken a systematic picosecond laser photolysis study of a series of conjugate a-electronic hydrogen-bonding systems. Part of our studies is reported here for the particular case of some dibenzocarbazole-pyridine complexes, in view of the very well-known carbazole-pyridine s y ~ t e m . ~ ~ ~ Experimental Section 7H- and 13H-dibenzocarbazole (Aldrich, 98% ) were sublimated under vacuum. Pyridine (Kishida, spectrograde) was refluxed over calcium hydride and distilled. 4-Cyanopyridine (CP) (Nakarai, GR grade) was recrystallized twice from an ether-ligroin mixture and sublimated under vacuum. Triethylamine (TEA) (Nakarai, GR grade) was passed through a column of activated alumina and then distilled several times. The collected fraction was stored under nitrogen atmosphere. Solutions were prepared by using cyclohexane and acetonitrile as the solvents. Cyclohexane (Dotite, spectrograde) and acetonitrile as the solvents. Cyclohexane (Dotite, spectrograde) and acetonitrile (Merck, spectrograde) were used without further purification. The solutions were deaerated either by nitrogen bubbling or by freeze-pump-thaw cycles. The absorption and fluorescence spectra were recorded by using respectively a JASCO UVIDEC-1 type spectrophotometer and an Aminco-Bowman spectrophotofluorometer. (Part of the study was made in the Laboratoire de Photophysique MolBculaire, Orsay, France, by using a Cary 210 spectrophotometer and a MPF3 Hitachi-Perkin-Elmer fluorometer.) In the fluorescence intensity quenching experiments, no correction was applied for the distortions due to the photomultiplier response because the fluorescence spectra to compare were very similar and only slightly shifted. Picosecond transient absorption spectra and fluorescence decay curves were measured by using a microcomputer-controlled picosecond laser photolysis system with a repetitive mode-locked Nd3+:YAGlaser as the excitation source. A single pulse is selected from the pulse train by means of a Pockels cell, amplified and directed through KD*P crystals generating harmonics of the IR fundamental frequency. In these experiments, the third harmonic was generated, producing typically a 355-nm pulse of 25-ps duration (fwhm) and about 0.7-mJ output energy. This pulse is directed to the sample through a circular slit of 0.15-cm diameter bored through the cell holder. The transient transmission changes of the excited sample are monitored with a picosecond continuum produced by focusing the fundamental laser beam into a 10-cm quartz cell containing D20 (UVASOL, 99.5%). The generated picosecond continuum is passed through a 0-5-11s delay line and then divided into two beams through a dichroic beam splitter. One of the two beams is directed to the sample (6) N. Ikeda, T. Okada, and N. Mataga, Chem. Phys. Lett., 69,251 (1979);Bull. Chem. SOC.Jpn., 54, 1025 (1981).
4149
b 0
0
350
LOO
X Inm
Figwe 1. Hydrogen-bondingeffect on ttre absorption spectra of 13DBC M; pyridine: (0)0.0,(1) 0.01, (a) and 7DBC (b). 13DBC: 2.6 X (2) 0.04, (3) 0.1, (4) 0.3 M. 7DBC: 0.93 X lo-' M; pyridine: (0)0.0, (1) 0.05, (2) 0.1, (3) 0.3 M.
in such a way that only the very inner part of the 0.15-cm circular slit is analyzed. The light transmitted by the excited sample is detected by a multichannel photodiodes array (512 channel) (MCPD1) through a polychromator. The second continuum beam is directed to another polychromator-multichannel photodiodes arrangement (MCPDB) and is used as the reference for the spectral distribution of the analyzing light. Transient absorption spectra of 380-nm width can be obtained by two laser shots with and without excitation pulse. The absorbance A(X) of the transient species at the wavelength h is given by the following equation:
where I? and Ii are the intensities of the continuum beams detected by MCPDi with and without excitation, respectively. Both MCPDs are connected to a microcomputer system which calculates the transient absorbance at various delay times, averages the results obtained after a series of laser shots under the same conditions, stores the data on disk, and displays them on a plotter or a scope. The microcomputer is also used to control the picosecond laser. Fluorescence decay curves were measured with a streak camera (HTV C979). The streak image is detected by a SIT camera (HTV C1000-18X), the image of which is in turn analyzed by the microcomputer described above. Details of this microcomputer-controlled picosecond laser photolysis system are described el~ewhere.~
Results and Discussion A. Hydrogen Bonding in the Ground State. The change in the absorption spectra of 13H-dibenzocarbazole (7)H.Masuhara, N.Ikeda, H. Miyasaka, and N. Mataga, J. Spectrosc. SOC.Jpn., 31, 19 (1982).
4150
Martin et al.
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982
Scheme I 0-H
t
A
_______i a
= D-H.**A KG
IOb,a /Ilkf
+ h,)
IOb$it
14 + 4)
xq
D-H* t A
= (D-H..-A)* k- 4
K E = k,/k.,
(13DBC) and 7H-dibenzocarbazole (7DBC) caused by hydrogen bonding with pyridine in cyclohexane is shown in Figure 1, a and b, respectively. The observed red shift is about 174 cm-' for 13DBC and 300 cm-' for 7DBC. These spectra are also slightly shifted and broadened when triethylamine is added to the solution, whereas they remain unaffected by the addition of 4-cyanopyridine (up to 0.3 M) as previously observed for the 2-naphthylaminepyridine hydrogen-bonding system.6 The equilibrium constant KGfor hydrogen bonding with pyridine is found to be equal to 27 M-' for both 13DBC and 7DBC in cyclohexane at room temperature (22 f 2 OC). A spectral shift of 582 cm-' and a KGvalue of 12 M-' have been reported for c a r b a z ~ l e . ~Dibenzocarbazole ~!~ is almost totally hydrogen bonded to pyridine for a pyridine concentration of about 0.3 M, whereas a concentration as high as 1M must be used to isolate the hydrogen-bonded complex formed with ~arbazole.~ B. Hydrogen Bonding in the Excited State. Fluorescence Quenching. Fluorescence quenching is observed when pyridine is added to a solution of 13DBC or 7DBC in cyclohexane. It is indeed quite well-known that hydrogen-bonding photocomplexation can lead to fluorescence quenching. Therefore, steady-state kinetics of dibenzocarbazole fluorescence quenching has been studied on the basis of Scheme I,%in which D-H and A represent 13DBC or 7DBC and pyridine, respectively. This scheme is commonly used to describe fluorescence quenching through photocomplexation processes such as exciplex or charge-transfer complex formation and has been successfully applied to describe the strong fluorescence quenching of carbazole by pyridine through hydrogen bonding.* For carbazole, it has been shown that the rate constants for the hydrogen-bonded complex formation (k,) and decomposition (k-) in the excited state are the same in aerated solution as well as in oxygen-free solution. That is, the ratio of the quenching rate constants (kQ = kq70), in aerated and deaerated solutions, is the same as that of the unbonded carbazole singlet-state lifetimes (70). Therefore, in this study, the steady-state fluorescence quenching experiments were carried out in nondegassed solutions. The excitation wavelength was set at the wavelength of an isosbestic point of the absorption spectra, and the change in total fluorescence intensity emitted by the sample with increasing pyridine concentration (I([A])) was calculated by comparing the fluorescence intensity of the maximum fluorescence peak. The results are given in Figure 2, a and b, for l3DBC and 7DBC, respectively, for increasing pyridine concentration up to 1 M. From the proposed reaction scheme, one expects a change in the total fluorescence intensity of the dibenzocarbazole given by eq 1,2aassuming that kf = kf in view 1 + 7 ' k q + 70kq[A] Io I([Al)- 7'/70 + 6(1 - 7'/70) + ~ ' ( k - +, k,[A]) (1) 70
=
(kf + ki)-'
7'
= (k;
+ k0-I
of the fact that the integrated absorbances of the bonded and unbonded dibenzocarbazoles are approximately the same. Io and 70 are respectively the fluorescence intensity
-. a
I
O --;,
/p-.
5v
0
\
-\I
1
--
b --o
0
_____- 4 1
02 5
c5
075
A M
1:
Figure 2. Experimental (-0-)and calculated (-) curves representing the change in fluorescence intenstly of 13DBC (a) and of 7DBC (b) with increasing pyridine concentration [A].
TABLE I: Equilibrium and Rate Constants Obtained from the Stationary-StateFluorescence Measurementsa
27 10 1-1.3 27 3.2 1.5-2 Solutions are not degassed. 13DBC 7DBC
a
250
>80
200 220
and the lifetime of dibenzocarbazole in the absence of pyridine, and 7' is the fluorescence lifetime of the hydrogen-bonded complex. 6 is the fraction of incident light absorbed by the unbonded dibenzocarbazole. The calculated ratio Io/I([A]) plotted against [A] is also shown in Figure 2, a and b, for l3DBC and 7DBC, respectively. It is seen that there is a good agreement between the experimental results and the calculated curve for pyridine concentrations below 0.3 M. For higher concentrations, eq 1 gives a constant value for the ratio Io/I([A]). This constant value represents the ratio between the fluorescence yield of the unbonded dibenzocarbazole and that of the hydrogen-bonded complex formed with pyridine. As a matter of fact, in this range of concentration the dibenzocarbazoles are almost totally hydrogen bonded in the ground state and only the fluorescence of the complex is observed. Moreover, the constant value of I,/I([A]) represents the ratio between the fluorescence lifetimes of the unbonded and bonded dibenzocarbazoles, since the radiative lifetimes of these two species are probably very close to each other in view of the small difference in their absorption spectra. On the other hand, experimentally for pyridine concentrations higher than 0.3 M the fluorescence intensity was found to increase slightly. This discrepancy between calculated and observed data may be due to some unknown specific solvent effect, which is not taken into consideration in eq 1, upon the relaxation properties ( k ; + ki') of the hydrogen-bonded complex. In the range of pyridine concentrations below 0.3 M, a good agreement between experimental and dculated data was found for the set of rate constants reported in Table I. These results indicate that the formation of the hydrogen-bonded complex is a diffusion-controlled process and that the equilibrium constant is larger in the excited state than in the ground state, as expected from the red shift of the absorption spectra with hydrogen bonding. The equilibrium constant in the excited state KEestimated from the Forster cycle is 63, 117, and 200 M-' for 13DBC, 7DBC, and carbazole,4respectively. It is interesting to note that eq 1 can fit experimental data for KE values of 250, 280, and 2200 M-' (ref 4) for these three molecules, re-
I
I
I
13
r
I
DBC IN
I
I
I
-
CYCLOHEXANE
-
.c1 0 c
: : : W
:L
3c
