J. Phys. Chem. 1995,99, 16925- 16931
16925
New Aspects on Fluorescence Quenching by Molecular Oxygen. 2. Inhibition of Long-Distance Electron Transfer in Acetonitrile Chika Sat0 and Koichi Kikuchi" Department of Physics, School of Science, Kitasato University, 1-15-1 Kitasato, Sagamihara 228, Japan
Kohji Okamura, Yasutake Takahashi, and Tsutomu Miyashi Department of Chemistry, Faculty of Science, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai 980, Japan Received: March 16, 1995; In Final Form: June 9, 1995@
Rate constants k, of fluorescence quenching by molecular oxygen 302(3Zg-)and the efficiencies of enhanced intersystem crossing @T and free-radical generation @R are measured for several aromatic hydrocarbons in acetonitrile. k, is the diffusion-controlled limit, kdiff (=(3.2-3.7) x 1O'O M-' SKI), when the free energy change AG of actual electron transfer (ET) from the first excited singlet fluorescer 'M* to 302 is more negative than -0.8 eV. In the region where AG > -0.8 eV, k, is slightly smaller than kdiff. @T decreases from 1.0 to 0.4 with decreasing AG, whereas @R is null except for 2,6-dimethoxynaphthalene(DMN). In the case of DMN, the actual ET is highly exothermic (AG = - 1.45 eV). Nevertheless, @R is as low as 0.003, while @T is as high as 0.43. As the lowest triplet energy of 2.70 eV for DMN is greater than the energy of 2.07 eV for the radical ion pair ('DM"' 20~'-), the triplet DMN cannot be produced by recombination of the radical ion pair. These facts indicate that fluorescence quenching by 302 is not induced by long-distance ET but rather by exciplex formation. Since the molecular size of 302 is considerably smaller than that of the aromatic molecule, the solvent reorganization energy of long-distance ET from an aromatic molecule to 3 0 2 is considered to be extremely high. For this reason, long-distance ET cannot compete with exciplex formation in fluorescence quenching by 302.
+
1. Introduction
SCHEME 1
Huorescence quenching by molecular oxygen 302(3Xg-) in nonpolar solvents such as cyclohexane and benzene has been considered to occur through exciplex formation in the encounter complex, 3(1M*...302).'-6 In the exciplex 3(M02)*, the charge transfer (CT) state, Le., the contact radical ion pair state 3(2M'+, 2 0 2 ' - ) , is mixed with the locally excited (LE) state such as 3(1M*, 3 0 2 ) , 3(3M*,302),3(3M,302), and 3(3M,'02) when the energy of the LE state is close to the energy of 3(2M'+,202'-). Here, 'M* and 3M are the first excited singlet and the lowest triplet state of the fluorescer, respectively. 3M* is the second excited triplet state or a higher excited triplet state close to 'M*. IO2 is the singlet oxygen, 'O2(lAg), with energy of 0.98 eV.2 It has been reported that the efficiency @T of enhanced intersystem crossing in fluorescence quenching by 302 is close to unity in general,6-'3 while the efficiency @A of ' 0 2 generation is less than ~ n i t y . ~ , ~ , The ' ~ - 'fluorescence ~ quenching by 3 0 2 in nonpolar solvent has been described by Scheme 1, processes 1-3. The exciplex 3(M02)* may deactivate to yield the following pairs: 3M 3 0 2 (process 3) and 3M ' 0 2 (process 1) when AEsl-~,2 0.98 eV, as well as 3M* 3 0 2 (process 2) when E('M*) 1. E(3M*). Here AEsl-~,is the energy gap between 'M* and 3M. E('M*) and E(3M*) are the energies of 'M* and 3M*, respectively. Recently Wilkinson et clearly demonstrated that process 2 takes precedence over process 3 when E(3M*) 5 E('M*). It is noted that the 3M* 3M internal conversion occurs too quickly for 3M* to be detected.I5 In a highly polar solvent such as acetonitrile, the exciplex may deactivate to yield the geminate (or solvent-separated) radical ion pair (RIP),which can dissociate into the free RIP or recombine to yield 'M 3 0 2 . Therefore, processes 4 and 5
+
+
+
-
+
@
Abstract published in Advance
ACS Abstracts,
October 15, 1995.
k
A 3 M + '02(lAg)
(I)
+ 302(32g-)(2) 3M + 3O2(3Zg-) (3) 3M'
2M'+
+ 20;-
AIM + 302
(4)
(5)
may compete with processes 1-3. In such cases, @T becomes smaller than unity, as confirmed by Potashnik et a1.6 According to Scheme 1, the overall quenching rate constant k, is given by eq I in the steady-state approximati~n,'~.''
+ + + +
where Cki = k1 k2 k3 k4 ks. According to eq I, the rate of fluorescence quenching by 3 0 2 becomes the diffusioncontrolled limit, kdiff, when k-diff -0.4 eV, and by long-distance ET when AG' < -0.4 eV. In the case of fluorescence quenching by 302, however, the switchover AG of the quenching mechanism has not been established yet. Potashnik et a1.6 evaluated the values of @T for several aromatic fluorescers in acetonitrile. All the @T values seem to be greater than 0.5, although the values of AG for actual ET from 'M* to 302 are as negative as -1.0 f 0.2 eV. In contrast, the efficiency (PR of free RIP (W+ 202*-) generation in fluorescence quenching by 3 0 2 is null or quite low, even when AG = - 1.0 f 0.2 eVS6T7 These facts suggest that when AG > -1.2 eV, fluorescence quenching by 3 0 2 is not induced by long-distance ET, but by exciplex formation. In this work, the AG dependence of @T and @R is studied in a wide range of AG, -0.25 to -1.45 eV, and the mechanism of fluorescence quenching by 3 0 2 is established.
*
+
2. Experimental Section 3,9-Dicyanophenanthrene (DCP) was synthesized according to the method described in the literature,26and it was purified by sublimation and recrystallized from chlorobenzene. 2,6Dimethoxynaphthalene(DMN; Aldrich) and 1,2-benzanthracene (BA; Nakarai) were used as received. Fluoranthene (Fl),pyrene (Py),and 1,12-benzoperylene (BPer) were the same as used in previous work."-24 9-Cyanophenanthrene (CP; Aldrich) was zone-refined and recrystallized from ethanol. 1,2;5,6-Dibenzanthracene (DBA; Aldrich) was treated by activated carbon in cyclohexane and was recrystallized twice from cyclohexane. Phenanthrene (Ph; Nakarai) was refluxed with maleic anhydride in xylene and pure crystals were deposited by the addition of sodium hydroxide solution. 9,lO-Dicyanoanthracene (DCA), 2,6,9,10-tetracyanoanthracene(TeCA), and N-methylacridinium tetrafluoroborate (AcMefBF4-) were the same as used in previous work.' Acetonitrile (SP grade, Nakarai) and methanol (GR grade, Wako) were used as received.
Sato et al. Absorption spectra were recorded on a Hitachi 330 spectrophotometer. Fluorescence and phosphorescence spectra were measured by a spectrophotometer built in the authors' laboratory. Fluorescence lifetimes were measured by a Horiba NAES-550 time-correlated single photon counting fluorometer. Oxidation potentials versus SCE were measured in acetonitrile with 0.1 M tetraethylammonium perchlorate as a supporting electrolyte. k, was determined from the Stem-Volmer plot for fluorescence lifetimes in deaerated, air-saturated, and oxygen-saturated solutions. The deaerated samples were degassed by repeated freeze-pump-thaw cycles. @T was determined by an emission-absorption (EA) method of nanosecond laser photolysis.",27 In this method, the transient absorption and the timeintegrated fluorescence intensity JZdA) dt during rise and decay of fluorescence are measured simultaneously. The former is used to determine the initial absorbance D(A) of transient species and the latter is used to evaluate the amount of light absorbed by a sample solution. The second harmonic of a Q-switched ruby laser (20 mJ,fwhm 30 ns) as the excitation light source was aligned normal to the round window of a cylindrical cell with an optical path length 10 cm and diameter 10 mm. Transient absorption was monitored using a xenon arc lamp, a Narumi R23 monochromator located behind the cell, and a Hamamatsu Photonics R955 photomultiplier. The time profile of fluorescence intensity was monitored from the side of the cell using a Jovin-Yvon H-20 monochromator and an RCA 1P28 photomultiplier modified for high-speed measurement. A Tektronix 2440 oscilloscope was used for storaging transient signals. @R was determined by an EA method of microsecond flash p h o t o l y ~ i s . ~To ~ .obtain ~ ~ the data shown in Table 4, the oxygen concentration was varied by bubbling with N2/02 mixtures. E('M*) was determined from the mirror-image relationship between the absorption and fluorescence spectra. E(3M) was determined from the 0-0 band of the phosphorescence spectrum at 77 K. I?(~M*) was evaluated from the fluorescence quenching rate constant kqtpby 1-iodopropane in benzene, using the relationship between kqtpand the energy gap AES~-T* = E('M*) It is noted that the value of E(3M*) for DMN has not been determined yet. The energy E(RIP) - , 5 1 1 2 ~ ~ ~of) free RIP was evaluated from the oxidation potential of the fluorescer and the reduction potential of 302 in acetonitrile, Le., = -0.94 V vs The oxygen concentration was reported to be 1.7 mM in aerated acetonitrile3* or 2.12 mM in aerated methanol.33 It is noted that the photophysical parameters and (PT for DCP were measured in methanol. In this work, the limit of experimental error is within 10%. All measurements were made at 298 K.
3. Results and Discussion Energy Levels. In order to establish the mechanism of fluorescence quenching by 3 0 2 in acetonitrile, it is essential to know the energies of IM*, 3M, 3M*, and the free RIP for every fluorescer. These are summarized in Table 1. Since the energy of ' 0 2 is 0.98 eV,2 the simultaneous generation of 3M and ' 0 2 * in fluorescence quenching by 3 0 2 is energetically prevented in the case of F1, DBA, Phen, and DMN. It should be noted that the energy of free RIP is lower than that of 3M only in the case of DMN. Rate Constants of Fluorescence Quenching by 3 0 2 . In Table 2 are shown the fluorescence lifetimes zfo and the fluorescence yields @fo in the deaerated solutions. The values for kq and AG are listed in Table 3. When AG < -0.8 eV, k, is at the diffusion-controlled limit, and kdiff = (3.5 f 0.3) x 1O'O M-' s-I. In the case of BA, BPer, and Py, three conditions (Le., (i) AG < 0 eV, (ii) AEs,-T, L 0.98 eV, and (iii) E('M*)
Fluorescence Quenching by Molecular Oxygen
J. Phys. Chem., Vol. 99, No. 46, 1995 16927
TABLE 1: Energies of First Excited Singlet (E('M*)), Lowest Triplet (E(3M)),Higher Excited Triplet close to IM* (WM*)), and Radical Ion Pair @(RIP)) in Acetonitrile, and Oxidation Potentials ElnoX fluorescer
E('M*) (eV)
E(3M) (eV)
E(3M*) (eV)
E(R1P) (eV)
(V vs SCE)
DCP F1 CP DBA BA Phen PY BPer DMN
3.38" 3.05 3.47 3.14 3.22 3.58 3.34 3.16 3.52
2.42" 2.29 2.52 2.27 2.05 2.69 2.10 2.01 2.70
3.21" 2.78 3.19 2.89 3.03 3.35 2.85 2.69
3.13" 2.58 2.86 2.33 2.25 2.56 2.18 1.97 2.07
2.17 1.64 1.92 1.39 1.31 1.62 1.24 1.03 1.13
a
In methanol; E(R1P) was evaluated by the Born eq~ation.~'
TABLE 2: Yields of Fluorescence (@#) and Intersystem Crossing (miseo),Lifetimes of Fluorescence (zfo)and Triplet (zTO) in Deaerated Acetonitrile, and Molar Extinction Coefficients for Triplets (cT(IZmm)) and Radical Cations (BR(3."))
fluorescer
@?
DCP" F1 CP DBA BA Phen PY BPer DMN
0.22 0.28 0.27 0.17 0.32 0.17 0.66 0.40 0.44
a
TO
t?
(ns) (ms) 0.48 19.3 0.25 46.0 0.68 21.6 0.84 30.6 0.48 42.1 0.72 54.8 0.46 338 0.29 127 0.51 10.7
0.46 1.33 0.25 0.74 0.69 0.1 1 1.35 0.36 0.50
(M-I cm-I)
(M-I cm-I)
6000 (480nm) 9300 (395 nm) 6800 (488 nm) 17100 (570 nm) 22400 (480 nm) 21 100 (480 nm) 31700 (412 nm) 35700 (460 nm) 18500 (460 nm)
4900 (780 nm) 4000 (980 nm) 6000 (515 nm) 11400 (860 nm) 5200 (870 nm) 47000 (450 nm) 39600 (515 nm) 21700 (630 nm)
In methanol.
TABLE 3: Free Enery Changes of Actual Electron Transfer from IM* to 0 2 (AG) and from 3M to 3 0 2 (AG"), Energy Gap between lM* and 3M(AE~I-~I), Quenching Rate Constants for IM* (k,) and jM (k t), and Efficiencies of Enhanced Intersystem Crossing (&) and Free Radical Ion Pair Generation (@R) in Acetonitrile AG AES)-T~ kq fluorescer (eV) (eV) (M-ls-l) DCP" F1 CP DBA BA Phen PY BPer DMN a
-0.25 -0.47 -0.61 -0.81 -0.97 - 1.02 -1.16 -1.19 - 1.45
0.96 0.76 0.95 0.87 1.17 0.89 1.24 1.15 0.82
1.7 x 8.2 x 2.6 x 3.2 x 3.5 x 3.7 x 3.5 x 3.2 x 3.7 x
AG" @T
kq
(DR (eV) (M-Is-I)
1Olo 1.0 lo9 0.71 l o L o0.85 lolo 0.56 1OIo 0.79 l o i o 0.47 1Olo 0.45 lOlo 0.36 10" 0.43 0.003
+o.n +0.29 +0.34 +o.o6 +0.20 -0.13 +0.08 -0.04 -0.63
1.0 x 2.4 x 1.8 x 2.1 x 2.1 x 3.0 x 2.3 x 2.1 1.0 x
109 lo9 lo9 109 109 lo9 lo9 109 10"
In methanol; AG and AG" were evaluated by the Born equation3'
L E(3M*)), which are necessary for diffusion-controlled fluorescence quenching, are all satisfied. In the case of DBA and Phen, however, condition (ii) is not satisfied. In the case of the 9-aminoacridinium ion (AmAcH+) in acetonitrile (AG = -0.78 eV), condition (i) is satisfied, but not (ii) and (iii).I k, for AmAcH+ was determined to be k, = 2.2 x lo9 M-' s-',I which is 1 order of magnitude smaller than those for DBA and Phen. Therefore, it is can be assumed that condition (ii) is not necessarily required for diffusion-controlledfluorescence quenching when AG < -0.8 eV. Rate Constants of Triplet Quenching by 3 0 2 . Flashing of the deaerated solutions yields the transient absorption spectra shown in Figure 1. These spectra were assigned to the triplettriplet (T-T) absorption spectra by the triplet energy transfer, using anthracene (E(3M) = 1.85 eV) as the triplet energy donor or acceptor. The triplet lifetimes ZTO in the deaerated solutions
are given in Table 2. The rate constants k,, of triplet quenching by 3 0 2 were determined and are listed in Table 3. The values of free energy change AG't (=E1/zoX - E1lzRED - E(3M)) for actual ET in triplet (3M) quenching by 3 0 2 are shown in Table 3. Triplet quenching in nonpolar solvents is induced by an exchange energy transfer with the rate constant of (0.5-3.9) x lo9 M-' s - ' . ~ Therefore, k,, for the fluorescers except DMN can be regarded as the rate constant of the exchange energy transfer. In the case of DMN, k,, is greater than 3.9 x lo9 M-' s-I. Since the AGff for DMN is as negative as -0.63 eV, kqt for DMN would be increased by the CT interaction. Quantum Yields of 1M*-3M Intersystem Crossing. Molar extinction coefficients of T-T absorption at absorption maximum I,,, were determined by means of triplet energy transfer as listed in Table 2. ET(I,,~)for anthracene was used as the standard: 73 100 M-' cm-' at I,,, = 420 nm in acetonitrile. The quantum yields Qlsco for 1M*-3M intersystem crossing in the deaerated solution were determined by an EA method of microsecond flash photolysis28as listed in Table 2. A deaerated acetonitrile solution containing 0.5 mM anthracene was used as an actinometer solution. for anthracene was determined to be 0.68. Transient Absorption Spectra of Fluorescer Radical Cations. A nanosecond laser photolysis of either the aerated or the deaerated acetonitrile solution containing DBA, Py,BPer, or DMN yields an intense transient absorption in addition to the T-T absorption. In Figure 2 are shown the transient absorption spectra of these compounds. To assign the transient absorption, photoinduced ET reactions were used. In the case of DMN, for example, the transient absorption spectra were measured for (i) the aerated solution containing 1.5 mM DMN and 0.2 mM DCA and (ii) the aerated solution containing 1.5 mM DMN and 0.06 mM AcMe+BF4-, using a cutoff filter for selective excitation of DCA or AcMe+. The same transient spectrum, shown in Figure 2h, was observed for both solutions. E('M*) is 2.89 eV for DCA or 2.75 eV for AcMe+. is -0.95 V for DCA, -0.48 V for AcMe+, or -0.94 V for 3 0 2 . 3 1 E 1 1 2for ~ ~DMN is 1.13 V. Since AG is -0.82 eV for the pair of DCA-DMN or -1.14 eV for the pair of AcMe+-DMN, the following photoinduced ET reactions are expected to take place:
+
~DCA* DMN
'AcMe+*
-
+ DMN
+
~DCA*- ~ D M R +'0, DCA -t202.--I- 2DMWS+(8) 2AcMe'
+ 2DMN'e
(9)
Both 20~'and 2AcMe' have no absorption in the region of 15 000-19 000 cm-I. Therefore, the spectrum shown in Figure 2h can be attributed to 2DMN'+. Transient absorption spectra of radical cations for DCP, Fl, CP, BA, and Phen were measured by photoinduced ET reactions from these compounds to DCA and/or TeCA and are shown in Figure 2. Molar extinction coefficients CR(I,,,,~) for zW+in acetonitrile were determined by photoinduced ET reactions from fluorescers to TeCA as listed in Table 2, using 6R(&,) = 6720 M-' cm-I at Amax = 720 nm for 2TeCA'- in acetonitrile. Since the apparent yield of the radical cation is linear with respect to the square of laser intensity in the case of DBA, Py, BPer, and DMN, the radical cation is thought to be generated by biphotonic ionization. In the case of Py, it was confirmed that the apparent yield does not depend on the 3 0 2 concentration, although fluoresence is efficiently quenched by 3 0 2 . This means that 2py'+is generated by the simultaneous absorption of two photons, as already pointed out by Richards et al. for the methanol ~ o l u t i o n .In ~
Sat0 et al.
16928 J. Phys. Chem., Vol. 99, No. 46, 1995
E l ! c l l-&lJ ':n I
50 i
5
10
0
-5 I
.
0
2
44
w'
4
I
,
I
-5
-5
6
0 40 4
I
C3il
I
h
m
z .
?.
d,
c
w'
20-1
L
Wavenumber I
lo3 cm.'
Wavenumber /
lo3 cm"
Wavenumber / l o 3 cm"
Figure 1. Triplet-triplet absorption spectra of fluorescers in acetonitrile: (a) DCP, (b) F1, (c) CP, (d) DBA, (e) BA, (0Phen. (g) Py, (h) BPer, and (i) DMN.
-6 a
a.
LL 20
wU
10
a
,
0
LJu "_ I
I
'1 m0
. wU
10
20
5
10
0 10
Wavenumber /
lo3 cm-'
15
20
25
30
0 10
15 Wavenumber 20 I
lo3 cm '25
30
Wavenumber 1 1o3 cm"
Figure 2. Transient absorption spectra of radical cations in acetonitrile: (a) F1, (b) CP, (c) DBA, (d) BA, (e) Phen, (0Py, (8) BPer, and (h) DMN. the case of DBA, BPer, and DMN, the apparent yield decreases with an increase in 3 0 2 concentration. Since the IM* population is reduced in the presence of 3 0 2 , this result indicates that a stepwise ('M IM* IM**) two-photon absorption participates in the generation of these radical cations. Efficiencies of Enhanced Intersystem Crossing in Fluorescence Quenching by 3 0 2 . Apparent deactivation processes of 'M* are summarized in Scheme 4,processes 10-16. Then, (PT is defined as follows:
- -
According to the EA method using nanosecond laser photolysis, @T can be determined on the basis of the following relation: 11.27
SCHEME 4 M' 'M' M'
'M'
+ 302 + SO2
1M'
+ 302
1M'
+ 302
'M'
: ' + h f I -
-
-
'M
kc
3M
kikisc
3M
+ '02
3M
+ 302
2M*+ + 202*-
'M
+ 302
(10) (11)
liq0
i
(12) (13)
kp[3o21 (14)
kqr[3021
(15)
(kq- kqisc - k4r)[3021
(16)
Here, &(A:) is the absorbance of T-T absorption immediately after laser excitation. The superscript "0" stands for the deaerated solution. Equation 111 is valid even when a biphotonic ionization arises, because both &(AI) and JZf(Ll) dt are linear with respect to the time-integrated concentration of 'M*: 4(A2)/CT(A&
+ @Tkq[3021)j['M*l
= (kjsc
lZ@,)
dt = akfl['M*] dt
dt
J. Phys. Chem., Vol. 99, No. 46, 1995 16929
Fluorescence Quenching by Molecular Oxygen I
1.20 i
-I
I
c?
. 10 5
0,9 0.5
0.90
1 .o
1.5
2.0
0.0
( i / T f - infO) /
Figure 3. Plot of
YT
i o 7 s-'
vs l/tt-l/tp.
Figure 4. Plot of
TABLE 4: Data Necessary for Determining (PRof DMN in Acetonitrile: A1 = 440 nm,A2 = 460 nm, and A3 = 630 nm (mM)
[3021
SId1i)dr
DT0(12)
1.55 3.43 4.98 6.35 9.81 7.90 6.61 5.31 3.89 2.66 2.09
0.220 0.454
0
0 0 0 0.2
.o
1 1.7
2.4 4.2 6.6 8.5
D~(13)
0.714
0.990 0.005 1 0.0052 0.0052
0.0070 0.0076 0.0076 0.0085
Here, a is a constant that is determined by the experimental conditions. Figure 3 illustrates the plot of YT vs (l/zf - l/zfo) = 9.0 x M-' for DMN. Since the plot is linear, can be obtained from the slope. The value of kist for DMN is calculated to be 4.8 x lo7 S-' from ~ f and o @isco. Thus, @T = 0.43 is obtained. The @T values for the other fluorescers were similarly determined and they are listed in Table 3. In Table 3 , it should be noted that @T decreases up to about 0.4 with decreasing AG. When AG - 1.O eV, @T seems to be nearly constant regardless of AEs~-T~.It is surprising that @T is as high as 0.4, even in such a highly exothermic region of actual ET. If fluorescence quenching by 3 0 2 is induced by exciplex formation, 3M might be produced by Scheme 1, processes 1-3, and free RIP might be produced by Scheme 1, process 4. If fluorescence quenching by 3 0 2 is induced by long-distance ET, in contrast, the geminate RIP is produced according to Scheme 3 , process 7. Spin-allowed back ET within the geminate RIP may yield either 3M 3 0 2 or 'M 3 0 2 , when E(RIP) 1 E(3M). When E(IUP) < E(3M), however, 3M cannot be produced by the back ET. In the case of DMN, the value of 2.70 eV for E(3M) is considerably greater than the value of 2.07 for E(RIP), and hence, 3M cannot be produced by the back ET. If fluorescence quenching of DMN by 3 0 2 is induced by longdistance ET, then, @T for DMN should be null. Nevertheless, @T for DMN is as high as 0.43. Therefore, it can be concluded that fluorescence quenching by 3 0 2 is induced by exciplex formation, even in such a highly exothermic region of actual ET as AG = - 1.45 eV. Efficiencies of Free RIP Generation in Fluorescence Quenching by 3 0 2 . In Scheme 4, @R is defined as follows:
+
YR
vs
t302].
solution containing DCP, Fl, CP, BA, Phen, or AC, transient absorption due to 2M.+ was not observed. Apparent yield of 2 M + is linear with respect to the flash intensity in the case of DMN, but proportional to the square of flash intensity in the case of BPer. In the case of DBA and Py, the apparent yield was too low to study the flash intensity effect. Therefore, it can be concluded that @R % 0 except for DMN. In Table 4, the values of &O(A2) and JZfo(A1) dt were obtained by flashing the deaerated solution containing 1 mM DMN with various flash energies, and the values of DR(A,) and JZr(A1) dt were obtained by flashing solutions containing 1 mM DMN and various concentration of 302 with a constant flash energy. Here, &(A,) is the absorbance for 2DMN'+ just after flashing. Since the value of AG" for DMN is as negative as -0.63 eV, the ET reaction from 3DMN to 3 0 2 can be expected to take place. However, the decay of 3DMN was not accompanied by the production of 2DMN'+. Therefore, the ET reaction from 3DMN to 3 0 2 can be neglected. It was found that 2DMN'+ is generated even in the deaerated solution, and the yield of 2DMN'+ is linear with respect to the flash intensity. Thus the autoionization in IM* (process 17) should be included in Scheme 4:
'M*
-
+ esOlv-
2 ~ * +
kr
(17)
Then, @R can be determined on the basis of the following relation:
+
OR= k,'/k,
(IV)
In order to minimize the participation of biphotonic ionization in the generation of radical cations, an EA method of microsecond flash photolysis was used to determine @R. The transient absorption due to *M.+ was observed by microsecond flash photolysis of the aerated solution containing DBA, Py, BPer, or DMN. Apparent yield of 2M.+ increases as the 3 0 2 concentration increases in the case of DMN, whereas it decreases in the case of DBA, Py, and BPer. In the case of the aerated
Figure 4 shows the plot of YR vs [302]. Since the plot is linear, the following ratios are obtained from the intercept and slope:
~ ~ ( 6 nm) 3 0 kr --- 0.0028 ~ ~ ( 4 nm) 6 0 kisc ~ ~ ( 6 nm) 3 0 QRk -- - 2.75 M-' ~ ~ ( 4 nm) 6 0 kisc
'
( W
The values of ~ ~ ( 6 nm) 3 0 and ~ ~ ( 4 nm) 6 0 for DMN are given in Table 2. The values of kq, zfo, and @isco (&isczfo) for DMN are given in Tables 2 and 3. From eqs VI and VII, @R = 3.0 and k, = 1.1 x lo5 s-l are obtained. The quantum x yield of autoionization in the deaerated solution is calculated to be kzfo = 1.2 x which is consistent with the fact, @.fo @isco = 0.95. According to Scheme 1, @R is defined as follows:
+
OR= k 4 / C k i
(i = 1-5)
(VIII)
1
It is noteworthy that the efficiency (@R) of exciplex dissociation into free RIP is quite low, up to the highly exothermic region
Sat0 et al.
16930 J. Phys. Chem., Vol. 99, No. 46, 1995
of actual ET as AG = - 1.45 eV. The sum of (PT and (PRtends to decrease with decreasing AG. In other words, Scheme 1, process 5 becomes more important as AG decreases.
4. Theoretical Consideration on Long-Distance ET between Aromatic Molecules and 3 0 2
"
.
Recently, Tachiya and M ~ r a t have a ~ ~ studied the first-order ET rate constant kel(R)in acetonitrile as a function of AG and of ET distance R between electron donor and acceptor according to the Marcus theory35
2n
ket = -
[
J2
h (4nkBTI,)
1/2
exp -
(AG
+ Q2
4kBT&
]
(Ix)
Here, is the solvent reorganization energy. n (=1.34 at 293 K) and E (=37.5 at 293 K) are the refractive index and the dielectric constant for acetonitrile, re~pectively.~~ rD and rA are the radii of the donor and acceptor, respectively. J is the transfer integral. In the case of the ET reactions between aromatic molecules, rD and rA may be assumed to be 3 8,. Tachiya and Murata have calculated the distance dependence of kel(R)for a variety of AG, assuming JO = 100 cm-' and /?= 1 A-l. The result is shown in Figure 5a.34 In the case of the ET reactions from aromatic molecules to 302, it can be assumed that r~ = 3 A and rA = 1 A, because the bond length of 3 0 2 is 1.21 8,. The result is shown in Figure 5b. Figure 5 shows that the distance dependence of kel(R) is drastically affected by changing rA from 3 to 1 8,. When rD = rA = 3 A, long-distance ET becomes more favorable with decreasing AG. When rD = 3 8, and rA = 1 A, in contrast, long-distance ET is not favored even at AG = -3.0 eV. Furthermore, kel(R)for rA = 1 8, is considerably smaller than that for rA = 3 8, at the corresponding values of R and AG in the region R 2 6 8, and AG 2 -2.0 eV. This is due to a large difference in 2s. In the case of fluorescence quenching by 3 0 2 , therefore, long-distance ET from 'M* to 3 0 2 in the encounter complex cannot compete with exciplex formation following contact collisions between the fluorescer molecule and 3 0 2 .
5. Conclusions In the region AG = -0.25 to -1.19 eV, it was confirmed that @T is greater than 0.36 while (PR is null and that (PT decreases with decreasing AG. Fluorescence quenching of electron donor and acceptor systems with AG < 0 eV in a highly polar solvent is induced by exciplex formation or long-distance ET. If the quenching is induced by long-distance ET, the primary quenching product is the geminate RIP. The geminate RIP can recombine to yield 3M 3 0 2 when E(R1P) > E(3M) and can dissociate to yield 2W+ 202'-. Then the decrease in (PT has to be accompanied by the increase in @R. However, @R is null regardless of @T. Therefore, it can be concluded that long-distance ET is not responsible for the quenching in this region of AG. In the case of DMN with AG = - 1.45 eV, one may suppose that the generation of free RIP with @R = 0.005 is an evidence for the quenching due to long-distance ET. In this case, however, E(3M) is greater than E(R1P) by 0.63 eV. If the quenching is due to long-distance ET, (PT can be expected to be null, because 3M cannot be generated by the geminate RIP recombination. Since @T = 0.45, the quenching
+ +
1
\ -3.0
lo4
n "
t'
bs \ 5
10
15
R I A Figure 5. Plots of k,,(R) vs R: (a) r D = rA = 3
A, (b) rD = 3 A and = 1 A. Numbers associated with the curves represent values of AG in units of eV. rA
is most likely induced by exciplex formation, even in this case. Consequently, fluorescence quenching by 3 0 2 is thought to be induced by exciplex formation in the region AG 2 - 1.45 eV. The switchover of the quenching mechanism from exciplex formation to long-distance ET occurs at AG < -1.45 eV. The Marcus theory predicts that the solvent reorganization energy increases as the radius of either the electron donor or acceptor decreases. In the normal region, therefore, the rate of long-distance ET is markedly reduced when the molecular size of the electron donor or acceptor is reduced. This is the reason why fluorescence quenching by 302is not induced by longdistance ET in the encounter complex, but by short-lived exciplex formation following contact collisions between the fluorescer molecule and 3 0 2 . Acknowledgment. We are greatly indebted to Professor H. Kokubun for his valuable comments and to Mr. N. Kawabe for determining the Q3M*) values. References and Notes (1) Kikuchi, K.; Sato, C.; Watabe, M.; Ikeda, H.; Takahashi, Y.; Miyashi, T. J . Am. Chem. SOC. 1993, 115, 5180-5184. (2) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London, 1970; pp 492-517. (3) Saltiel, J.; Atwater, A. In Advances in Photochemistry; Volman, D. H., Hammond. G.S., Gollnick, K., Eds.; Wiley: New York, 1988; Vol. 14 pp 1-38. (4) Kristiansen, M.; Scurlock, R. D.; Iu, K.-K.; Ogilby, P. R. J . Phys. Chem. 1991, 95, 5190-5197.
Fluorescence Quenching by Molecular Oxygen ( 5 ) Stevens, B.; Small, Jr., R. D. Chem. Phys. Lett. 1979, 61, 233238. (6) Potashnik, R.; Goldschmidt, C. R.; Ottlenghi, M. Chem. Phys. Lett. 1971, 9, 424-425. (7) Richards, J. T.; West, G.; Thomas, J. K. J . Phys. Chem. 1970, 74, 4137-4141. ( 8 ) Stevens, B.; Algar, B. E. J . Phys. Chem. 1969, 73, 1711-1715. (9) Tibilov, S. S.; Vember, T. M.; Ermolaev, V. L.; Cherkasov, A. S. Opt. Spectrosc. 1975, 39, 646-651. (10) Darmanyan, A. P. Chem. Phys. Lett. 1982, 86, 405-410. (11) Kikuchi, K. Chem. Phys. Lett. 1991, 183, 103-106. (12) Usui, Y.; Shimizu, N.; Mori, S. Bull. Chem. SOC.Jpn. 1992, 65, 897-902. (13) Wilkinson, F.; McGarvey, D. J.; Olea, A. F. J . Am. Chem. SOC. 1993, 115, 12144-12151. (14) Grewer, C.; Wirp, C.; Neumann, M.; Brauer, H.-D. Ber. Bunsenges. Phys. Chem. 1994, 98, 997-1003. (15) Fukumura, H.; Kikuchi, K.; Koike, K.; Kokubun, H. J . Photochem. Photobiol. A : Chem. 1988, 42, 283-291. (16) Rehm, D.; Weller, A. Ber. Bunsenges. Phys. Chem. 1969,73,834839. (17) Rehm, D.; Weller, A. lsr. J . Chem. 1970, 8, 259-271. (18) Kikuchi, K.; Niwa, T.; Takahashi, Y.; Ikeda, H.; Miyashi, T.; Hoshi, M. Chem. Phys. Lett. 1990, 173, 421-424. (19) Kikuchi, K.; Hoshi, M.; Niwa, T.; Takahashi, Y.; Miyashi, T. J . Phys. Chem. 1991, 95, 38-42. (20) Kikuchi, K.; Takahashi, Y.; Hoshi, M.; Niwa, T.; Katagiri, T.; Miyashi, T. J . Phys. Chem. 1991, 95, 2378-2381. (21) Kikuchi, K.; Takahashi, Y.; Katagin, T.; Niwa, T.; Hoshi, M.; Miyashi, T. Chem. Phys. Lett. 1991, 180, 403-408.
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