8222
J. Phys. Chem. 1993,97, 8222-8228
Femtosecond-Picosecond Laser Photolysis Studies on the Mechanisms of Fluorescence Quenching Induced by Hydrogen-Bonding Interactions-1-Pyrenol-Pyridine Systems Hiroshi Miyasaka,'??Akihiro Tabata, Seishi Ojima, Noriaki Ikeda,# and Noboru Mataga.95 Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan Received: March 2, 1993; In Final Form: May 18, 1993
The mechanisms of strong fluorescence quenching of 1-pyreno1(PyOH) caused by hydrogen-bonding interaction with pyridine (P) and various methyl-substituted pyridines (MeP's) have been investigated by means of fluorescence and femtosecond-picosecond laser spectroscopic measurements and compared with those of 1-aminopyrene (AP)-P and -MeP systems studied p r e v i o ~ s l y . ~ J ~Femtosecond-picosecond J time-resolved transient absorption spectral measurements on PyOH-P and -MeP systems in hexane show rapid decay of locally excited (LE) state (D*-H-.A) without formation of detectable intermediate state, in contrast to the case of AP-P and -MeP systems where rapid establishment of equilibrium between LE and electron transfer (ET), (D+-H-A-), states occurs in the excited state. On the basis of the fact that the decay time of the LE state of PyOH-P and -MeP systems depends on the free energy gap for the LE ET reaction, it has been concluded that the decay of the LE state is due to the reaction LE ET, which is realized even in a nonpolar solvent assisted by hydrogenbonding interactions and by a slight shift of the D-H proton toward A, further facilitating ET. Moreover, immediately after ET a large scale and ultrafast proton shift takes place which induces a large destabilization of the ground state (GS)leading to an ultrafast nonradiative ET GS "switchover", contrary to the case of AP-P and -MeP systems where the proton shift in the excited state remains moderate.
-
Introduction It is well-known that the hydrogen-bonding interaction frequently leads to the fluorescence quenching of proton-donor or -acceptor in the bonded pair, especially when two conjugate a-electronic systems are directly combined by the hydrogenbonding interacti0n.I-3 We proposed the charge-transfer (CT) or electron-transfer (ET) interaction between the proton-donor and -acceptor *-electron systems via the hydrogen bond as a possible mechanism of the quenching.' That is, we suggested that a kind of nonfluorescent exciplex was formed in the course of this quenching process.' Hydrogen atom transfer from proton donor to acceptorin the hydrogen-bonding pair was also suggested as a possible mechanism of q~enching.~ In order to examine the mechanisms of the fluorescence quenching caused by the hydrogen-bonding interactions as described above, we have made picosecond laser photolysis studies on several systems and have obtained evidence which supports the ET m e c h a n i ~ m .Detailed ~~ quantitative studies have been madeon 132%and 7H-dibenzocarbazole(DBC)-pyridine (P)6,8,9 and 1-aminopyrene (AP)-P7J systems in nonpolar solvents. In the case of the AP-P and the 7-DBC-P systems in hexane solutions, it has been confirmed that immediately after pulsed excitation, the establishment of the equilibrium between the LE state, (D*-H.-A), and the ET state (D+-H-.A-) (where D-H is AP or DBC and A is P), takes place within the 10-ps time resolution of the a p p a r a t ~ s . 6 Because ~ the free energy gaps, -AG, for the ET reaction of these systems, (D*-H.-A)d (D+-H-A-), in nonpolar solvent estimated without taking into account the specific effect of the hydrogen bonding interaction is ca. -1 .O eV (AP-P) or more negative (7-DBC-P), the observed rapid formation of the (LE F? ET) equilibrium state indicates a remarkable effect of the hydrogen-bonding interaction in facilitating the ET. t Present address: Department of Polymer Engineering and Science, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606, Japan. 8 Present address: Department of Chemistry, Collegeof General Education, Osaka University, Toyonaka, Osaka 560 Japan. t Present address: Institute for Laser Technology,Utsubo-Hommachi 1-84, Nishiku, Osaka 550, Japan.
0022-3654193f 2097-8222%04.00 f0
-
-
For the further elucidationof the photoinduced ET mechanism in the hydrogen-bonding systems, we have directly observed the formation process of the (LE a ET) equilibrium state from the LE state by means of the femtosecond laser photolysis method in the case of the AP-P as well as AP-methyl-substituted P(MeP) systems in nonpolar solvents.lOJ1 We have confirmed that the formation process of the equilibrium state in these systems can be expressed practically by a single exponential function with a time constant of ca. 5-10 ps. Thus, it has been proved that the hydrogen-bonding interaction in these systems must greatly stabilize the ET state in order to make possible such a rapid reaction. It has been suggested also that the ET in the excited hydrogen-bonded complexes is greatly facilitated by a little movement of the D-H proton toward A in the hydrogen bond,798J0J1although no radical formation by ultimate hydrogen atom transfer has been recognized.IOJ1 It has been demonstrated that the nonradiative deactivation process of the AP-P and AP-MeP systems from the equilibrium state to the ground state takes place in the time regions of several hundreds picoseconds and that the deactivation occurs predominantly through the ET state.lOJ1 It has been shown also that the deactivation rate constant decreases exponentially with increase of the energy gap between the ET (ion pair) state and the ground state of the hydrogen-bonded complex,ll which is somewhat similar to the energy gap dependence of the charge recombination deactivation of the strongly interacting ion pair (IP) (contact or compact IP) formed by excitation of typical CT complexes in solutions.12 Strong fluorescence quenching was observed also in the case of 2-naphthol-PIC and 1-pyreno1 (PyOH)-PSa systems, which suggested a similar quenching mechanism of ET from the excited proton donor to the acceptor in the hydrogen-bonded complex. However, previous picosecond laser photolysis studies on the PyOH-P system in hexane did not show the transient absorption band due to the PyOH cation but showed a weak absorption band, which might be ascribed to the 1-pyrenoxyradical PyO*.5a This result might indicate a hydrogen atom transfer mechanism for the fluorescence quenching: contrary to the ET mechanism in the case of the AP-P and DBC-P systems. Nevertheless, it seems to be possible that the successive multiphoton excitation 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8223
Mechanism of Fluorescence Quenching within the exciting picosecond laser pulse leads to the 0-H bond fission directly or to photoionization followed by proton detachment resulting in the pyrenoxy radical formation. Although the absorption band due to the PyOH cation was observed in the transient spectra of the PyOH-P system in acetonitrile solution,58 this result may be complicated also by the electron ejection from PyOH excited by multiphoton absorption in a polar solvent. In view of the above-described results and discussions on the PyOH-P system, we need more detailed studies on the excitedstate hydrogen-bonding interactions of this and related systems with measurements of higher time resolution by the femtosecond laser photolysis method. Results of such investigations will be given in the following and will be discussed in comparison with the results on AP-P and AP-MeP systems.
(4 1.5c
’
Experimental Section A picosecond laser photolysis system with a repetitive modelocked Nd3+:YAGlaser was used for transient absorption spectral measurements in the 10-ps to a few nanoseconds region.” The third harmonic pulse (355 nm) with 22 ps fwhm was used for exciting the sample. For the time-resolved transient absorption spectral measurements in the shorter time region, a femtosecond photolysis system was used.14-13 The second harmonic pulse (355 nm) of the pyridine 1 dye laser (710 nm) with 500 fs fwhm was used for the excitation. Fluorescence decay profiles were measured with a picosecond streak camera.” For the detection of relativelylong fluorescencedecay times, a MCP photomultiplier and a fast storage oscilloscopecombination were used. In order toobtain theabsorption spectrumof the 1-pyrenoxyradical, which is necessary for the analysis of the transient absorption spectra, conventional microsecond flash photolysis of a hexane solution of PyOH was performed. The absorption spectrum of PyOH cation radical, which was necessary also for the analysis of the transient absorption spectra, was obtained by producing the cation radical with W o y radiolysis ifi a sec-butyl chloride matrix at 77 K. PyOH was synthesized by the method given in the literature16 and purified by recrystallization from a methanol-water mixture and sublimation in a vacuum. Spectrograde pyridine (P) was used without further purification. GR grade reagent 2-methylpyridine (2MeP), 4-methylpyridine (4MeP), 2,6-dimethylpyridine (2,6DMeP), 3,5-dimethylpyridine (3,5DMeP), and 2,4,5trimethylpyridine (TMeP) (Tokyo Kasei) were carefully distilled before use. sec-Butylchloride (Tokyo Kasei) was passed through a column of alumina and distilled twice. Hexane and acetonitrile were spectrograde and were used without further purification. Sample solutions for measurements were deaerated by f r e e z e pumpthaw cycles.
Results and Discussion Hydrogen-Bonding Effects on the Ground-State Absorption Spectra and Fluorescence of PyOH. As an example, effects of
theadditionof 3,5DMePon the ground-state absorptionspectrum of PyOH in hexane solution are indicated in Figure lA, which shows typical characteristics of the spectral change due to the 1:1 complex formation. The absorption bands show a little red shift owing to the larger stabilization of the excited state due to the hydrogen-bonding interaction. This spectral change can be analyzed by eq 1 based on the 1:l hydrogen bonding and the
equilibrium constant can be obtained,’ where Kg = [D-H-A]/ [D-HI [A] for the equilibrium, D-H + A F? D-H-A, and 6,” are molar extinction coefficients of D-H and D-H-A at a wavelength A, respectively, and A i and AX are absorbances at A in the absence and presence of A, respectively. Plots according
ckH
’
I’
‘1
I
’
’
’
’
’
Wavelength / nm
,
-4°F
0
, , 0.2
4I
I
0.4
,
1 0.6
d
Figure 1. (A) Effects of the addition of 3,5DMeP on the ground state absorption spectra of PyOH in hexane solution. [3,5DMeP] = 0 and 1.44 X 10-3 to 1.01 X lo-* M. (B) Plots of the observed absorbance at several wavelengths according to eq 1.
TABLE I: Equilibrium Constants (
-
) of 1:l Hydrogen Bonding in the Ground State, Red Shi ts (h,) of the SI S, Absorption Band Caused by Hydrogen Bonding, and Fluorescence Quenching Rate Constants (4) of PyOH-P and -MeP Systems in Hexane Solutions’ acceptor Kg,M-1 bv,, cm-l &, 1010M-l s-l
9
P 2MeP 3MeP 4MeP 2,6DMeP 3,5DMeP 2,4,6TMeP
159 239 228 28 1 267 363 505
244 27 1 27 1 278 27 1 278 298
1.99 1.69 1.89 1.82 1.27 1.96 1.25
Experimental errors are less than i 5 % for K,,&4% for kq,and 1 1 0 cm-1 for 6va, respectively.
to eq 1 for the PyOHJ,5DMeP system in hexane solution are shown in Figure 1B. Very similar spectral changes have been observed also for the hydrogen-bonding interactions of PyOH with P and other MeP’s in hexane solutions. The obtained K, values at room temperature (27 f 2 “C)are indicated in Table I together with the amount of the red shift, b y a , of the longest wavelength absorption band. The PyOH fluorescence in hexane solution is strongly quenched by hydrogen bonding with P and MeP’s. As an example, the fluorescence spectra of the PyOH-3,5DMeP system in hexane solution are shown in Figure 2A. Since the hydrogen bonded systemsare practicallynonfluorescent, the fluorescencequenching can be expressed by the scheme of eq 2, where k’i >> k i , k4 and 6 is the fraction of exciting light absorbed by D-H and is given
8224 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 ,
'
,
'
,
'
I
7-
6-
-
5-
\
4t
X
=
:Y 1
I
0
I
I
I
I
0.004 0.008 [AI I M
I
I
0.012
Figure 2. (A) Effects of the addition of 3,5DMeP on the fluorescence spectra of PyOH in hexane solution. [3,5DMeP] = 0 and 1.44 X 10-3 to 1.01 X M. (B) Analysis of the quenching of PyOH fluorescence by added 3,5DMeP in hexane solution by means of q 3.
D*-H + A
D-H
+A
k,
7 (BH***A)* @-H***A)
by 6 = 1 / ( 1 + ( C L ~ . - ~ / & ~ ) K , [ Aleading ]), to the Stern-Volmer type of equation
+
uoa/o = 1 + kq70[Al
(3)
-
where 6 = (1 &[A])-' in the actual measurements because of the excitation at the isosbestic point. The (ZoS/T) [A] linear relation for the PyOH-3,5DMeP system is shown in Figure 2B. From the observed value of kqTo and 7 0 = 3 1.8 ns, kqwas obtained to be 1.96 X 1010 M-1 s-1, which is very near to the diffusioncontrolled bimolecular rate constant in hexane. Similar results have been obtained for other systems, and the kq values are collected in Table I. Roughly speaking, the K, and 6u, values in Table I increase a little with an increase in the number of the substituted methyl groups. Nevertheless, we can recognize also the effect of the position of the substitution on these quantities. That is, 2-, 6-, and 4-substitution are favorable for the stronger hydrogen-bonding interaction with PyOH because of the increaseof electron density
Miyasaka et al. on N due to the hyperconjugationof the methyl group with the pyridine ring. However, there exists also a steric hindrance which cancels such an effect to some extent in the case of the 2- and 6-substitutions. Although the kq values in Table I are rather close to the diffusion-controlled bimolecular rate constant in hexane solution, those for 2,6DMeP and 2,4,6TMeP quenchers are appreciably smaller compared with the values for other quenchers, probably owing to the steric hindranceat theencounter between excited PyOH and those quenchers. We have examined also in acetonitrile solutions the effect of the hydrogen-bonding interaction upon the ground-state absorption spectrum of PyOH and its fluorescence quenching reaction by a hydrogen-bonding interaction at encounter for the same proton acceptors as used in hexane solutions. It has been confirmed that the ground state hydrogen bonding interaction becomes much weaker and the bimolecular rate constant of the fluorescence quenching reaction becomes much smaller (by an order of magnitude) in acetonitrilesolution,which may be ascribed to the hindering of the hydrogen-bonding interaction between PyOH and P as well as MeP's due to the solvation of both proton donor and acceptor molecules by stronglypolar acetonitrile. Quite similar results have been observed also in the case of the 2-naphthylamine (NA)-P systemsb and AP-P as well as APMeP ~ystems.~Jl As described already to some extent in the Introduction and will be discussed in detail later in this paper, ET from the *-electronic system of PyOH to P or MeP coupled with a little movement of the proton from donor to acceptor in the hydrogen bond seems to play an importantrole in the fluorescencequenching of this sort of hydrogen-bonding system where the hydrogen bond is directly connecting the conjugate a-electronic systems. In the usual outer-sphere ET process by weak interactions, such a large decrease of the reaction rate by increase of the solvent polarity cannot be expected. This shows the crucial importance of the specifichydrogen-bonding interaction in promoting the ET process and shows that the orientation polarization of the surrounding polar solvent cannot assist the ET process but rather disturbs such a specific effect. As reported in our previous paper? the fluorescence of PyOH was quenched also by 4-cyanopyridine (CP) in both hexane and acetonitrile solutions just as in the case of AP-CP' and NA-CP ~ystems.~ Contrary to thecase of the PyOH-Psystems, however, kq of the PyOH-CP system is closeto the diffusion-controlled values in both hexane and acetonitrile so1utions.S~ Since the hydrogen bonding interaction of PyOH with CP is much weaker than that with P (K,of the PyOH-CP system in hexane is about 1 order of magnitude smaller than that of the PyOH-P system, and hydrogen-bonded complex formation of PyOH-CP in acetonitrile can be neglected). Therefore, the PyOH-CP system in acetonitrile seems to undergo the direct electron transfer in the fluorescence quenching reaction without a hydrogen-bonding interaction. This conclusion seems to be supported by the estimation of the free energy gap for the charge separation (CS), -AGa. in the PyOH-P and PyOH-CP systems by means of
-AGa = AGp
+AEM
-AGp = E(D-H/D+-H) - E(A-/A)
- eZ/eR+ AG,
(4)
(5)
AGs = ( e 2 / 2 ) ( 1 / R + + l / K ) ( l / e - l/eJ (6) where AEw is the 0-0 transition energy between SIand SOstates of PyOH, E(D-HID+-H), and E(A-/A) are the oxidation potential of PyOH17 and reduction potentials of PI7 and CP18.19 measured in polar solvents with dielectricconstant cr, respectively, R is the center to center distance between cation and anion, assumed to be 7 A, and AGs is the sum of the correction terms of the solvation energies calculated with the Born formula for the cation and anion with radius R+ and R, respectively, in a solvent
Mechanism of Fluorescence Quenching F
.
l
I
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8225
-
I
I4
80.2
400
500
600 700 Wavelength / nm S1 absorption spectrum of PyOH in hexane solution
Figure 3. The S, observed at a 50-ps delay time by exciting with a 0.9-ps, 295-nm laser pulse. +
t. 400
:
. \ 500 600 700 Wavelength / nm
'
1
80
Figure 4. Absorption spectrum of PyOH cation radical produced by ~ C yOradiolysis in a s-BuC1matrix at 77 K. (The authors are grateful to Dr. M. Irie for his help in the measurements at the Institutefor Scientific and Industrial Research, Osaka University.)
n
t
1
II
a t
I
,
1
1
.
1
400 450 500 550 Wavelength / nm Figure 5. Absorption spectrum of 1-pyrenoxyradical in hexane observed at 65 ps by flash photolysis of an air-saturated hexane solution of PyOH. (The authors are grateful to Dr. A. Yoshimura and Dr. T. Ohno of the College of General Education, Osaka University for the use of flash photolysis apparatus.) with dielectric constant e. Results of the calculation using eq 4 for the PyOH-P system are as follows: - A G a = -1.46 eV in hexane and - A G a = 4 . 3 0 eV in acetonitrile. Those for the PyOH-CP system are as follows: - A G a = 4 - 1 3 eV in hexane and - A G a = 0.89 eV in acetonitrile. These results indicate that the sufficiently rapid outer-sphere ET leading to the diffusioncontrolled quenching reaction is possible in the case of the PyOHCP system in acetonitrile while such a mechanism is not possible in other cases. In the latter cases, the hydrogen bond formation and the ET assisted by the hydrogen-bonding interaction seems to cause the quenching of fluorescence. Femtosecond-Picosecond Time-ResolvedTransient Absorption Spectral Studies on the Fluorescence Quenching Mechanisms. Absorption spectra of the S1 state of PyOH in hexane, PyOH cation radical produced in low temperature sec-butyl chloride matrix at 77K by ~ C yOradiolysis, and 1-pyrenoxy radical in hexane are indicated in Figures 3-5, respectively. These spectra are necessary for the analysis of the time-resolved transient absorption spectra of PyOH-P and -MeP hydrogen-bonded systems with respect to the fluorescencequenching mechanisms. The S, S1 spectrum of PyOH in Figure 3 was observed by exciting PyOH in hexane with a 355-nm picosecond laser pulse.
-
In this measurement, in order to avoid the formation of 1-pyrenoxy radical by multiphoton excitation, we attenuated the exciting laser pulse intensity as far as possible. The absorption spectrum of the I-pyrenoxy radical in Figure 5 was obtained by microsecond flash photolysis of an air-saturated hexane solution of PyOH. This spectrum agreed with that reported previously.20 Although we have examined the transient absorption spectra of PyOH-P and -MeP systems in hexane by picosecond laser photolysis with attenuated exciting pulse intensity as in the case of the measurement of the S, SI spectrum of PyOH in Figure 3, only a very short-lived weak absorption with a peak around 500 nm which might be ascribed to S, S1 transition was observed. Accordingly, we need measurements with higher time resolution in order to observe clearly the spectra of the transient states. We show the time-resolved transient absorption spectra of PyOH-P and some PyOH-MeP hydrogen-bonded complexes in hexane solution obtained by exciting with the 500-fs pulse at 355 nm in parts A-C of Figure 6. In all of these figures, we can recognize only the absorption spectra that are very similar to the S, SIspectrum in Figure 3 and show simply decay within the 10-pstime region. We can observe neither the rise of the PyOH cation radical-like absorption band nor the rise of the pyrenoxy radical band in the course of the decay of the S, SI-likeband. The observed absorption spectra in Figure 6 may be assigned to thelocally excited (LE) S1 state of PyOH in the hydrogen-bonded complex (D*-H-A) which undergoes a rapid decay. The decay profiles of these spectra can be reproduced approximately by the single exponential function. As an example, we show the decay profile of PyOH-4MeP system in Figure 7,from which the decay time Td of the LE state has been evaluated. The obtained Td values are shown in Table I1 together with the - A G a values for some PyOH-acceptor pairs. The Td values are rather close to 10 ps for all of these systems. Nevertheless, close inspection of Table 11 indicates that Td increases slightly as the -AGcs becomes more negative. This suggests that the ET mechanism is responsible for the decay of the LE state. In the case of the AP-P and AP-MeP systems in hexane solution, we have confirmed directly the photoinduced ET from the LE state and the formation of the (LE F? ET) equilibrium state by observing clearly the absorption band due to the ET state (AP+ radical) with femtosecond transient absorption spectral measurements.lOJ In the present systems, however, we cannot detect the PyOH+ band in the time-resolved transient absorption spectra as shown in Figure 6. In spite of this, it might be possible that ET state, (D+-H--A-), produced from the LE state, (D*H.-A), undergoes much faster nonradiative degradation to the ground state than its formation process contrary to the case of the AP-P and -MeP systemswhere the ET state has a sufficiently long lifetime to establish the (LE F! ET) equilibrium state before its decay.lOJ1 If this is the case, we cannot detect the ET state in the transient absorption spectra even if the ET mechanism plays the most important role in the ultrafast nonradiative degradation. It should be noted here that this argument does not contradict with the fluorescence properties of AP-P and -MeP as well as PyOH-P and -MeP hydrogen-bondedsystemsin hexane solutions. In the case of the hydrogen-bonded AP, we can observe clearly the red-shifted broad fluorescence band of the hydrogen-bonded complex7.11 which is emitted by (LE s ET) equilibrium state with lifetimes of 200 ps to 2 ns," while practically no such fluorescence from the hydrogen-bondedcomplex can be detected in the case of the PyOH-P and -MeP systems in hexane. The reason for this difference between PyOH and AP with respect to the nonradiative degradation via the ET state will be discussed in the next subsection. Of course, such a large negative value of - A G a (--1.5 eV) as given in Table I1 is not compatible with the rapid ET reaction
-
-
-
-
Miyasaka et al.
8226 The Journal of Physical Chemistry. Vol. 97, No. 31, 1993
400
600
500
Wavelength I nm Figure 7. Time profile of the transient absorbance of the PyOH4MeP system in hexane solution observed at 515 nm.
n
J
a -
g a --
8.0~s:
&-
12.0ps : e : A.
Wavelength / nm
(C)
1
TABLE Ik Decay Times ( 7 6 ) of the LE State of the Hydrogen-Bonded Complexes, (D*-H-.A), and the Free Energy Gap (-A&) for the (D*-H..*A)-(D+-H*..A-) Reaction of PyOH-P and -MeP Systems in Hexane Solutions acceptor 7d. -AGm eV P 8.8 1.0 -1.46 2MeP 9.6 i 1.1 -1.50 4MeP 14.2 i 0.8 -1.56 2,6DMeP 11.1 i 1.0 -1.55
"
"
1
'
'
'
'
1
'
*
'
0.5ps
n
.-> U
Systems in a Nonpolar Solvent. From the above results and discussions, it has beenconcluded that the LEstateof the PyOH-P and -MeP systems in hexane solution decays rapidly with time constant of ca. 10 ps without producing any detectable long-lived intermediate states, contrary to the case of AP-P and -MeP systems in hexane where the long-lived (several hundreds of picoseconds to nanoseconds) (LE e ET) equilibrium state formation takes place. Nevertheless, the dependence of the LE state decay time upon the - A G a for the (LE ET) reaction suggested an important role of the ET in the quenching of the LE state also in PyOH-P and -MeP systems in hexane solution. In addition to the important role of the specific hydrogen bonding interaction of such aromatic proton donor (D-H) molecules as DBC and AP with P and MeP, which lowers the ionization potential of the A-electron system of D-H by ca. 1 eV even in the ground state,2*there seems to arise a little shift of the D-H proton toward A in the excited state of the hydrogen-bonded system, which will further lower the ionization potential of the D-H r-electron system and enhance the electron affinity of the ?r-electron system of the acceptor, facilitating the ET process. Namely, the extent of the intramolecularCTfrom the substituentNH2 or >N-H group to the aromatic ring systems of those D-H molecules is much larger in the SIstate compared with the So state. The larger extent of the intramolecular CT will make the proton in the hydrogen bond shift a little toward the acceptor, which will further enhance the intramolecular CT of the D-H moleculeand lower the ionization potential of its r-electron system and, moreover, enhance the electron affinity of the acceptor ?r-electronicsystem due to the approach of the proton toward the ring nitrogen. These effects of proton movement induced by the photoinduced intramolecular CT in D-H will greatly facilitate the ET, (D*-H.-A) -.t (D+-H--A-).Although this coupled process of the proton movement and ET in the hydrogen-bonded system seems to stabilize enormously the (D+-H*-A-) state in a nonpolarsolvent, it willgreatlydestabilizetheground state, leading to the enhancement of the nonradiative transition to the ground state via the (D+-H-A-) state.- This enhancement of the nonradiativetransition may be the main cause for the fluorescence
-
\
e 9
0 v
8
es 8 2
400
600
500
Wavelength 1 nm Figure 6. Time-resolved transient absorption spectra of PyOH-P and -MeP systems in hexane solutions excited with a 500-fslaser pulse at 355 nm. Delay timea from the exciting laser pulse are indicated in the figures. Proton acceptors: (A) P; (B) 4MeP; (C)2,6DMeP. with the IO-ps time constant. However, as discussed already in our previous papers on AP-P7s8JoJ and DBC-P.*v9.21 systems, and also in the Introduction of this paper to some extent, the ET state (D+-H-.A-) seems to be greatly stabilized by the specific hydrogen-bonding interaction even in a nonpolar solvent (ca. 1 eV or more), facilitating the LE ET reaction. This specific and very important effect of the hydrogen-bonding interaction in stabilizing the ET state is not taken into account in the evaluation of - A G a values in Table 11. Although the - A G a values of PyOH-P systems are considerably more negative than those of AP-P systems, a similar mechanism for the specific hydrogen-bonding interaction promoting the ET process seems to be working as will be discussed in detail as well in the next subsection. Mechanismsof PhotoinducedET and Nmdiative Deactivation Process via the ET State of the PyOH-P Hydrogen-Bonding
-
Mechanism of Fluorescence Quenching quenching due to the hydrogen-bonding interaction directly coupled with the aromatic donor-acceptor *-electron systems.8 We proposed the above mechanisms for the ET process and nonradiative decay via the ET state in the excited hydrogenbonding systems many years ago mainly for the DBC-P and AP-P systems in nonpolar solutions.a Quite similar mechanisms may be applicable also for the present systems of PyOH-P and -MeP’s in hexane solutions. Moreover, the above concept may be related to such photoinduced reaction mechanisms of a wider range of systems than the typical hydrogen-bonding complexes as the hydrogen-transfer reaction due to the proton transfer from the secondary aromatic amine (D-H) to the aromatic hydrocarbon (A) in the exciplex (D+-H.-A-) formed in nonpolar solutions8qZz and also the hydrogen-transfer reaction due to the proton transfer within the aromatic amine cation-benzophenone anion ion pair in various nonpolar and polar soluti0ns.~3 It should be noted here that the above-described mechanism and concept for the photoinduced ET in hydrogen-bonding systems in nonpolar solutions are supported strongly by the ab initio MO CI studiesz4on model systems such as anilineP and phenol-P, results of which predict that one electron transfer from D*-H to the hydrogen-bonded A takes place when the D-H proton is moved to the vicinity of the midpoint between N-N or 0.-N. According to this calculation, the position of the shifted proton where the ET takes place corresponds to the barrier maximum of the potential energy curve plotted against the proton shift. Extensive CI calculations taking into account singly, doubly and triply excited configurations show a rather low barrier, indicating rapid ET. Moreover, the results of the calculation indicate that the amount of the proton shift necessary for the ET to occur is a little larger for the phenol-P system compared with the aniline-P system. This seems to be consistent with the fact that the -AGcs value for the ET (D*-H-A) (D+-H-A-) is more negative for the PyOH-P than the AP-P system, and a larger stabilization of the ET state by a more extensive proton shift is necessary for the photoinduced ET of the PyOH-P system to take place. Of course, the actual hydrogen-bonding systems we are studying by ultrafast laser spectroscopy are different from the model systems investigated by MO CI calculations. Nevertheless, their qualitative features of photoinduced ET and proton shift might be similar to each other. There is another important difference between PyOH-P and AP-P systems as we have pointed out in the beginning of this subsection. This is the rather long-lived (LE ~t ET) equilibriumstate formation and relatively slow decay from the ET state in the case of the AP-P system on the one hand, and, on the other hand, the ET reaction similar to the case of AP-P system but followed by ultrafast nonradiative transition to the ground state in the case of the PyOH-P system. Presumably, this difference may be related to the difference in the feasibility of proton movement or proton transfer in the excited state of the hydrogenbonded complexes. Aromatic hydroxy compounds such as 2-naphthoP and PyOHZ6 hydrogen bonded with relatively strong proton acceptors like aliphatic amine can undergo quite easily photoinduced proton transfer, (D*-H-A) (D-*-H-A-), even in nonpolar or only slightly polar solvents. For example, ultrafast laser photolysis studies on PyOH-triethylamine (TEA) hydrogen-bonded complexes in benzene solution show that the photoinduced proton transfer occurs with time constant shorter than 1 ps.Z6 This ultrafast excited-state proton transfer between the hydrogenbonded pair is induced by the intramolecular CT from the hydroxy group to the pyrene ring, which is enhanced considerably by photoexcitation. However, aromatic amines such as NA and AP hydrogen bonded with TEA in nonpolar or slightly polar solvents do not show such an excited-state proton transfer between the hydrogenbonded pair although the extent of the photoinduced intramo-
-
-
Th!e Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8227
II
1
(b)
1
Coordinate of the Proton Shift in the Hydrogen Bond Figure 8. Conceptual diagrams of photoinduced ET coupled with proton movement and nonradiative decay via the ET state in the PyOH-P as well as AP-P hydrogen-bonded complexes in nonpolar solvent: (a) APP;(b) F’yOH-P. The abscissa represents the proton shift in the hydrogen bond.
lecular CT from the substituent to the aromatic hydrocarbon ring seems to be larger in the case of the aromatic amines than aromatic hydroxy compounds. The intramolecular CT will certainly induce a little shift of the amino proton toward TEA but cannot form the ion pair state by complete proton transfer in such nonpolar or only slightly polar solvents. When the proton movement is coupled with ET in the case of the AP-P or NA-P systems, the extent of the proton shift will become larger. Nevertheless, the very large proton shift producing the neutral radical pair due to the ultimate hydrogen atom transfer does not occur, but only the ion pair formation due to ET with moderate amount of proton shift takes placeand the (LE i=ET) equilibrium state with fairly long life time of hundreds of picoseconds to nanoseconds is established. On the other hand, immediately after the ET coupled with the moderate amount of proton shift, an ultrafast large scale proton movement descending the potential curve toward the acceptor will take place in the case of the PyOH-P systems, which seems to induce a large destabilization of the ground state resulting in the ultrafast nonradiative crossing of the ET state to the ground state. We summarize the above-described mechanisms of photoinduced ET coupled with proton movement and nonradiative degradation via the ET state in the PyOH-P hydrogen-bonded system in conceptual potential energy diagrams against the coordinate of the proton movement in Figure 8 in comparison with those of the AP-P hydrogen bonded systems. In the PyOH-P hydrogen-bonded systems, the excited-state ET reaction coupled with a little proton shift seems to need a slight activation energy leading to a rate constant of ca. 10” s-l. Owing to the large scale proton shift in the ET state, the crossing of its potential curve with that of the ground state will take place near its bottom in the inverted region as indicated in Figure 8, leading to the ultrafast radiationless transition accompanied with back-ET charge recombination to the ground state (GS). The rate of this ET GS radiationless transition, an activationlessquantum mechanical tunneling, will be much larger in its order of magnitude than the excited-state ET reaction rate. Therefore, because the rate of the back-ET reaction is much greater than that of the photoinduced ET reaction, the detection of the intermediate ET state is practically impossible even with the femtosecond laser spectroscopy. As illustrated in Figure 8, a slight proton movement in the hydrogen bond can easily induce electron transfer from D*-H to the hydrogen-bonded A in nonpolar solvent without the assistance of the orientation of polar solvents. By selecting the proton donor (and probably also the proton acceptor), we can realize the case where the rapid forward and return electron transfer takes place by “proton switching”and also the case where the large scale ultrafast proton shift following electron transfer induces an ultrafast nonradiative “switchover” to the ground state.
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8228 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 Acknowledgment. The present work was partially supported by a Grant-in-Aid for Specially Promoted Research (No. 6265006) from the Ministry of Education Science and Culture of Japan to N.M. References and Notes (1) (a) Mataga, N.; Tsuno, S.Natunvissenschaften 1956,10,305. (b) Mataga, N.; Tsuno, S. Bull. Chem. Soc. Jpn. 1957,30,711. (c) Mataga, N. Ibid. 1958,31,481. (d) Mataga, N.; Torihashi, Y.; Kaifu, Y. Z. Phys. Chem. (FrankfurtlMain) 1962, 34, 379. (2) (a) Kikuchi, K.; Watarai, H.; Koizumi, M. Bull. Chem. SOC.Jpn. 1973,46,749. (b) Yamamoto, S.;Kikuchi, K.; Kokubun, H. Ibid. 1976,49, 2950. (3) Martin, M. M.; Ware, W. R. J. Phys. Chem. 1978, 82, 2770. (4) Rehm, D.; Weller, A. Isr. J . Chem. 1970, 8, 259. ( 5 ) (a) Ikeda, N.; Okada, T.; Mataga, N. Chem. Phys. Lett. 1980,69, 251. (b) Ikeda, N.; Okada, T.; Mataga, N. Bull. Chem. Soc. Jpn. 1981,54, 1025. (6) Martin, M. M.; Ikeda, N.; Okada, T.; Mataga, N. J . Phys. Chem. 1982,86, 4148. (7) Ikeda, N.; Miyasaka, H.; Okada, T.; Mataga, N. J. Am. Chem. SOC. 1983,105, 5206. (8) Mataga, N. Pure Appl. Chem. 1984, 56, 1255. (9) Martin, M. M.; Miyasaka, H.; Karen, A.; Mataga,N. J. Phys. Chem. 1985,89, 182. (10) Mataga, N.; Miyasaka, H.; Hirata, Y. In Ultrafast Processes in Spectroscopy 1991; Laubereau, A.; Seilmeier, A., Eds.;Institute of Physics: Bristol and Philadelphia, PA, 1992; p 561. (11) Miyasaka. H.; Tabata, A.; Kamada, K.;Mataga, N. J . Am. Chem. Soc., in press. (12) (a) Asahi, T.; Mataga, N. J . Phys. Chem. 1989.93.6575; 1991.95, 1956. (b) Segawa, H.;Takehara, Ch.; Honda, K.; Shimidzu, T.; Asahi, Y.; Mataga, N. Ibid. 1992, 96, 503.
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