J. Phys. Chem. 1992,96,9353-9359
9353
Proton-Induced Electron-Transfer Reaction from Triplet Methoxynaphthalenes to Benzophenone via Triplet Exciplex'' MInoru Yamaji, Tetuhiro Sekiguchi, Mikio H o ~ h i n oand , ~ ~ Haruo Shizuka* Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan (Received: June 17, 1992; In Final Form: August 11, 1992)
A laser flash photolysis study at 355 nm has been carried out on acetonitrile-water (4:l v/v) mixtures of the methoxy-
naphthalene-benzophenone-H2S04 system. It is found that the proton-assisted photoionization reaction of triplet 1- and
2-methoxynaphthalenes (ROMe) produced by triplet sensitization of benzophenone (BP) effective1 occurs to produce the corresponding methoxynaphthalene cation radical (ROMe") and benzophenone ketyl radical (>$OH). The triplet energy-transfer reaction from triplet benzophenone ('BP*) to ROMe takes place as the primary event to produce triplet methoxynaphthalene ('ROMe*). The 1:l triplet exciplex '(ROMe->CO)*having a weak chargetransfer structure is readily formed between 'ROMe* and BP with the equilibrium constants Kl (10.1 M-' for 1ROMe; 4.0 M-l for 2ROMe at 290 K). In the presence of protons, the triplet protonated exciplex '(ROM+>C+OH)* is also produced by protonation to '(ROMa .>CO)*. Subsequently, the intraexciplex electron-transfer reaction in '(ROMa +C+OH)* takes place effectively, d t i n g in the formation of ROMe" and >COH. The presence of protons in the ROMe-BP system assists the photoionization of ROMe with the much lower energy (triplet energy of ROMe) than its ionization potential. The reaction mechanism is discussed in detail.
-
to yield the methoxynaphthalene cation radical (ROMe") and Introdllction the ketyl radical (>COH). It is noted that, in the presence of The acid-base reactions in the excited state of aromatic comprotons (H2S04),ROMe can be ionized at much lower energies pounds have been extensively studied since they are elementary (triplet energy of ROMe) than their ionization potentials. p"esin both chemistry and biochemistry."" In this decade, However, no mechanistic investigation on the proton-assisted there has been considerable interest in photochemical and phophotoionization of the ROMtBP-H2S04 system has been given tophysical properties of aromatic compounds in the presence of yet. protons:I2 proton-transfer reactions in the excited state and This paper reports the reaction mechanism for the proton-asproton-induced quenching,13-16a onaway proton transfer reaction sisted photoionization of methoxynaphthalenes produced by triplet in the excited state of hydrogen-bonded c o m p l e ~ e s , l ~ and - ~ ex~ sensitization of benzophenone in acetonitrile-water (4: 1 v/v) amples for the absence of excited-state prototropic equilibriumsm mixtures at 290 K by laser flash photolysis at 355 nm, considering On the other hand, hydrogen atom transfer reactions in the the proton effects on the decay features of 'ROMe* in detail. It triplet state of carbonyl compounds from a variety of hydrogen has been pointed out that the triplet exciplex with a weak atom donors such as alcohols, hydrocarbons, and amines are charge-transfer structure plays an important role in the protonwell-known. These reactions proceed by either hydrogen atom assisted photoionization of methoxynaphthalenes. transfer or electron transfer followed by proton transfer. A large number of studies on intermolecular and intramolecular hydrogen Experimental Section atom transfer reactions of carbonyl triplets have been r e p ~ r t e d . ~ l - ~ ~ 1-Methoxynaphthalene (G.R. grade, Wako) was purified by Thae pnxxsses are photochemical and photophysical phenomena vacuum distillation. 2-Methoxynaphthalene (G.R. grade, Wako) of aromatic compounds upon direct excitation. was purified twice by recrystallization from ethanol. BenzoUntil recently, little attention has been paid to photochemical Sulfuric acid (9796, phenone was purified aa reported elsewhere!' and photophysical processes of triplet aromatic compounds proWako) was used without further purification. Since it is known duced by triplet sensitization of carbonyl compounds. Especially, that the counterion (SO4*) of H2S04does not quench the triplet photochemical reactions between triplet naphthalene derivatives molecules,44H2S04was used as the proton source. Acetonitrile As to and benzophenone are of our recent (G.R. grade, Wako) was purified by the usual method!s it is revealed by nanosecond laser photolysis that the hydrogen Deionized water was distilled. An acetonitrile-water mixture (4:l atom transfer (HT) reaction efficiently occurs from triplet nav/v) was used as a solvent. phthol ('ROH*) to benzophenone (BP or >CO) to produce a The concentrations of methoxynaphthalenes (ROMe) were naphthoxy radical (Rd) and a benzophenone ketyl radical usually 3.0 X M. Benzophenone was used as a triplet sen(>COH) via the triplet exciplex 3(ROH***>CO)*,which has a sitizer in the concentration range 0-0.2 M. The H2S04conweak charge-transfer structure, and in the presence of protons, centrations used were 0-1.0 M. All samples in quartz cells of the decay rate constant of 'ROH* (Le., HT rate constant) in1- or 1 0 " path length were thoroughly degassed by means of creases with an increase of proton concentration. Schematically, freezepumpthaw cycles on a high-vacuum line. Spectral data the proton-enhancedHT reaction of 3ROH*takes place as follows; regarding transients were obtained from a fresh sample to avoid the intraexciplex electron transfer in the protonated triplet exciplex excessive exposure to the laser pulse. gives rise to the triplet radical pair, 3(ROH'+ >COH), which A nanosecond Nd3+:YAGlaser system at 355 or 266 nm (JK readily dissociates into R6, H', and >COH. The proton effect Lasers HY-500; pulse width 8 ns, laser power 70 mJ/pulse for on the HT reaction from the triplet naphthylammonium ion 355 nm) was used for sample excitation. The detection system (3RNH3+*)to BP is quite different from that of 3ROH*?1 At was reported elsewhere!l In order to determine the triplet yields higher acid concentration, the triplet exciplex '(RNH3+.- ->CO)* of methoxynaphthalenes, the thermal lensing technique was emis decamped by protonation to give free 3(RNH3+)*and BP, ployed as reported by Kajii et a1.'6 resulting in suppression of the rate constant for the HT reaction. Therefore, the proton effects on the excited-state behaviors of Results d Discussion triplet naphthalene derivatives depend on their chemical properties. In a preliminary paper,@ it has been shown that the protonTriplet Eaergy Tramfer Reaction from Triplet assisted photoionization of methoxynaphthalenes (ROMe) sento M Upon laser excitation at 355 nm, triplet sitized by triplet benzophenone or acetophenone occurs effectively benzophenone ('BP*) having the T, Tlabsorption at 525 nm
+
-
0022-3654/92/2096-9353$03.00/0
Q 1992 American Chemical Society
9354
Yamaji et al.
The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
TABLE I: E x p " t d Jhta of the Jkcay of )BP* at [SP] = 6.7 X lW3M in Acctdtrik-Water ( 4 1 V/V) at 290 k w h b g Rate Constant (k,), Decay Rate c4ab.tof 3BP*in tk Ab#.ce of ROMe ( T : ~ ) - ~Efficiencies , of the Tripkt h e q y Trader (h) rad the BOMeInduced Quenchbg (#',), rad Rate Constants for the Tripkt T M w (km) rad the R O M C - M U CQpcnehg ~
4.03 0 -
(klb's
\
1 ROMe [HzSOII/M k,/M-I s-I (T;~)-'/S-'
-
0 0
2.0
1.0
#ETC
3.0
6'4
k E T / ~ -s-I l k\/M-I s-l
-
Figure 1. Stem-Volmer plot of the decay rate constant (rgp-') af )BP* as a function of [IROMe] containing [HgO,] 0 M (a) and 1.0 M (b) in acetonitrile-water (4:l v/v) at 290 K. I
I
r
llil
3.0
-
0 0
20
10
(2ROMe)
30
,/ lo3 M
Figure 2. Stem-Volmer plot of the decay rate constant ( ~ g p - ' ) of 'BP* as a function of [2ROMe] containing [HSO,] = 0 M (a) and 1.0 M (b) in acetonitrile-water (4:l v/v) at 290 K.
was produced via fast intersystem crossing ( b Hin Units of M-' cm-'a lROMeb 2ROMeb 435 nm 650nm 430 nm 600nm c('ROMe*)
10500 1990 620
c(ROMe'+) @OH)
14700
1850
3400
6500
610
"For details, see text. *The intersystem crossing yields for lROMe and 2ROMe are determined as 0.64 and 0.65, respectively. BP
>COH
M
3BP*
+ H+ e 'BP+H*
"-
(7)
-'E 8.0 U
p
6.0
r l
4.0
* 2.0 0
0
700
Wavelengt h/nm
(Cl
~OD520= @geszdab~A-l (8) Here, the Avogadro's number is expressed as N A . On the other hand, when an acetonitrilewater (4:l v/v) solution of ROMe has the same absorbance at 266 nm in a IO-" path length cell as that of an acetonitrile solution of BP and no change in laser intensity is used, the molar absorption coefficient of 3ROMe*, eAT,of the T-T absorption at the wavelength AT is expressed as
8
2
016-
(2)
-----z
012-
ai 5 4 5 nm
-01 - 0 01
- 0 001
(9)
Here, AOD, and @LOMe are the initial absorbance change at A, and the yield for the formation of 3ROMe*, respectively. The values of @LOMe were determined by the thermal lensing method as 0.64 for lROMe and 0.65 for 2ROMe.54 By the use of eqs 8 and 9 and the determined values of @E, eSZ0, and @ & O M ' , the = 10 500 M-' cm-' at 435 values of eATwere determined as nm for lROMe and e430 = 14700 M-' cm-I at 430 nm for 2ROMe. These values are listed in Table 11. Proton-Assisted Ionizntion of Triplet Metboxyrmphthalenes Produced by Triplet Sensitization of Benzophenone. Triplet methoxynaphthalenes, 'ROMe*, are produced by the triplet sensitization of BP within 120 ns after the start of 355-nm laser pulsing. No transient absorbance change at 435 nm, 31ROMe*, from 120 to 300 ns after a laser pulse was observed for the lROMe (3.0 X M)-BP (6.7 X M)-H2S04 (1.0 M) system. Figure 3a shows the transient absorption spectra on the microsecond time scale for the above system at 290 K. The T, T1 absorption of BP at 525 nm disappears within 100 ns after laser pulsing as stated above, resulting in the formation of a transient peak at 435 nm. The transient spectrum at 435 nm was assigned to that of the T, T1absorption of 1ROMe. since the spectrum was very similar to the T, T1absorption spectrum at 430 nm of 1-naphthol having isoelectronic structures.39 At 500 ns after laser pulsing, new transient peaks at 380,545, and 650 nm appear with a decrease of the 435-nm peak for 'lROMe* with an isos-
-
- -
:I
0.15/
In the presence of protons, the part of 'BP* in Scheme I should be replaced by eq 7. The prototropic equilibrium in eq 7 is known to be established within 20 ns.32 Therefore, regardless of the yresence of protons, the triplet state of methoxynaphthalene, ROMe*, is produced by triplet sensitization of BP under the experimental condition. The molar absorption coefficient of 'ROMe* in acetonitrilewater (4:l v/v) was determined by means of both the laser photolysis technique and the t h e d lensing method. The number of photons absorbed by ROMe in the absence of BP, lab [photons/cm2/pulse], at 266 nm (the excitation wavelength) was determined by comparing with the triplet-triplet absorption of and the molar abBP in acetonitrile whose triplet yield, @E, sorption coefficient, esm at 520 nm of the triplet-triplet absorption respectively. After laser are known as 1.0" and 6500 M-' pulsing to an acetonitrile solution of BP in a cell with 10-mm path length, the initial absorbance change, AODs20,observed for the formation of 3BP* at 520 nm is expressed as
AODAT = @LoPMC~AJabNA-'
1
435nm
rate constants determined above are also listed in Table I. The primary events of the laser flash photolysis of BP at 355 nm in the of ROMe can be depicted in Scheme I. When H2S04is added to the system, the prototropic reaction of 3BP* takes place as follows
I
( 3)
008 -
,
0
2
4
6
8
10
t/P.
Flgm 3. (a) Time-resolved transient absorption spectra in a time scale for the lROMe (3.0X lo-' M)-BP (6.7 X lO-' M)-H2S04 (1.0 M) system in acetonitrilewater (4:l v/v) observed after 355-nm laser pulsing at 290 K. (b) Reference absorption spectra for the transient species ('lROMe*, >eOH, and lROMe'+). (c) Time traces of the
absorbance changes for transient species monitored at 435 nm ('lROMe*) (l), at 545 nm (>COH) (2), and at 650 nm (lROMe'+) (3) after a 355-nm laser pulse to the lROMe (3.0X lr3 M)-BP (6.7 X lo-) M)-Hfi04 (1.0 M) system in acetonitrilewater ( 4 1 v/v) at 290
K. bestic point at 490 nm. The 545-nm transient is known as the ~ benzophenone ketyl radical, >COH (e = 3220 M-'~ m - ' ) ?Both transient peaks at 380 and 650 nm are ascribable to the 1methoxynaphthalene cation radical, 1ROMe'+.S6 The reference spectra of 31ROMe*, >COH, and 1ROMe" are depicted in Figure 3b. The molar absorption coefficient of 1ROMe" can
Yamaji et al.
9356 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
triplet energies (IROMe, 59.7 kcal mol-';5o 2ROMe. 61.9 kcal mol-' sl). The efficiency (Oh) for the ionization of ROMe in the present system can be obtained according to eq 11, where A[ROMe'+]
aoe-
ai,"= A[ROMe'+]/A[3ROMe*]
denotes the change in the ROMe" concentration between the delay times r2 and tl and A['ROMe*] that in the concentration of triplet methoxynaphthalene between tl and f2. The value of A[ROMe'+] can be obtained by eq 12. Here, AOD, represents
0.06-
L 3
a
0.04 -
A[ROMe'+] = AOD,/e,
A['ROMe*] = A O D , ( C ~ ~-~ '
I
600nm(E=6500)
1
Wavelength/nm
Hgm 4. (a) Timeresolved transient ahsorption spectra in a microsecond time scale for the 2ROMe (3.0 X lo-' M)-BP (6.7 X lo-' M)-H2S04 (1.0 M) system in acctonitrile-water (4:l v/v) observed after 355-nm lascr pulsingat 290 K. (b) Reference absorption spectra for the transient specica ('2ROMe*, >eOH and 2ROMe").
be readily determined from the spectral analysis, since the rate constant for the formation of 1ROMe" is the same as that of >COH. The rate constants for the rise of both transient peaks at 545 nm for >COH and 650 nm for 1ROMe" were the same as that for the decay, kObd= 8.9 X los s-I, observed at 435 nm for 'lROMe* within 5% experimental error as shown in Figure 3c. In the absence of protons, no photoionization upon 355-nm excitation took place in the l R O M t B P system. These results show that the ionization of 'lROMe* is assisted by both protons and BP, leading to the production of lROMe'+ and >COH, as discussed later. Similar transient spectra of '2ROMe* were obtained in the 2ROMe (3.0 X lo-' M)-BP (6.7 X lo-' M) system containing [H2S04]= 1.0 M by laser flash photolysis at 355 nm as shown in Figure 4a. The transient absorption at 430 nm for '2ROMe* (e = 14700 M-' cm-I) produced by triplet sensitization of BP decreases with an increase of the new absorption peaks 545 nm for >COH (e = 3220 M-' cm-')l" 380 (e = 8100 M-' cm-'), and 600 nm (e = 6500 M-' cm-') for 2ROMe". The decay rate constant, kObd= 1.0 X lo5 s-', at 290 K observed at 430 nm for '2ROMe' was the same as the rise rate at 545 nm for >COH or 600 nm for 2ROMe'+. The reference spectra of the transients are also shown in Figure 4b. These observations dunonstrate that, in the presence of protons, the ionization of '2ROMe* occurs to produce the 2-methoxynaphthalene cation radical, 2ROMe'+, and the benzophenone ketyl radical, >&H. No photoionization upon 355-nm excitation was observed for the 2ROMtBP system in the absence of protons. From these results, the ionization of 'ROMe* can be e x p r d as eq 10. The ionization potentials of lROMe and 2ROMe are
+ >CO + H+
-
ROMe"
+ >COH
(10)
known to be 179.4 and 182.6 kcal mol-', respecti~ely.'~ However,
the proton-assisted photoionization of ROMe occurs with lower
+
AT
I)-'
(13)
Here, AOD, represents the absorbance change between tl and the wavelength AT, where 'ROMe* has the peak of its abso tion band ('lROMe* 435 nm; '2ROMe*, 430 nm), and EAT AT RoMe'*, and &&OH' represent the molar absorption coefficients at XT of 'Rode* ('IROMe*, e4jS = 10500 M-' cm-'; '2ROMe*, 430 = 14700 M-' cm-'),ofROMe'+ (lROMe", 1990 M-' cm-'at 435 nm; 2ROMe'+, 1850 M-' cm-'at 430 nm), and of >COH (620 M-' cm-' at 435 nm; 610 M-' cm-'at 430 nm), rcapcctively. Bytheuseofeqsll, 12,and13withtheabovevalua of the molar absorption coefficients and the absorbance changes shown in Figures 3 and 4, the values of aimat 290 K were evaluated as 0.84 for lROMe (3.0 X 10'' M)-BP (6.7 X lo-' M)-HfiO, (1.0 M) system and 0.29 for 2ROMe (3.0 X lo-' M)-BP (6.7 X 10'' M)-H2S04 (1.0 M) system. The molar absorption coefficients for the transient species are also summarized in Table 11. Coaaatnth Effects Of rad H-4 011 Proton-~tedPhotdaaiptioaRtrctionofM~~; the Racoioll Md".The deactivation prooessesof )ROMe* produced by the triplet sensitization of BP have been studied in the ROMe-BP-H&O4 system under various concentrationsof BP and H2SO4 at 290 K. Upon laser excitation at 355 nm in the lROMe (3.0 X lo-' M)-BP (S0.2 M) systems with [H2S04]= 0,O.S. and 1.0 M at 290 K,the decay rate constants kobdof 'lROMe* at 435 nm were measured. The k, value corresponds to the rise rate constant of 1ROMe" or >FOH as described above. Figure 5 shows the plots of kobd vs [BPI obtained after laser pulsing to 1ROMeBP-H#04 systems. The value of kd increaseswith an increase of [BPI and [Hfi04] but not linearly. Especially in the eystem with [ H m 4 ] = 0, a leveling off is expected at higher [BPI (10.2 M). Similar results were obtained in the 2ROMe (3.0 X lo-' M)-BP (10.2 M) systems with [HzS04] = 0,O.l. and 1.0 M at 290 K as shown in Figure 6. Such a nonlinear quenching of 'Roble* shown in Figures 5 and 6 cannot be explaid by a simple quenching mechanism. Because the transient spectrum of 'ROMe. decays according to first-order kinetics, the probability is negligible that the nonlinear quenching is due to triplet-triplet annihilation of 'ROMe*. In order to account for these experimental results, we propose the proton-assisted photoionization mechanism as shown in Scheme 11, where 'ROMe*, '(ROMe+CO)*, and '(ROMa..>C+OH)* denote the triplet ROMe, the 1:l triplet exciplex, and the protonated triplet exciplex, respectively, k,, and k$ the decay rate constants of 'ROMe*, '(ROMe. .>CO)*, and '(ROMa .>C+OH)*, respectively, to the ground state, K l and K2 the corresponding equilibrium constants for complex formations, and kel the rate constant for the intraexciplex electron-transfer reaction of '(ROMo *>C+OH)*. The spectralchange in the T, TIabsorptions among 'ROMe., '(ROMa -.>CO)*, and '(ROMes ->C+OH)* was scarcely observed, suggesting that these triplet exciplexes have a weak t z at
1
'ROMe*
(12)
the absorbance change between t2 and tl at the wavelength &, where ROMe" has the peak of its absorption band (lROMe*+, X, = 650 nm; 2ROMe'+, X, = 600 nm), and e, the molar absorption coefficient of ROMe" at X, (1ROMe", = 3400 M-' cm-';2ROMe". = 6500 M-' cm-').On the other hand, the value of A['ROMe*] can be obtained by eq 13.
0.02-
I
(11)
w-
.
-
The Journal of Physical Chemistry, Vol. 96, No. 23. 1992 9351
Triplet Methoxynaphthalenes to Benzophenone
SCHEME 111 4
k2 3(ROll.
'
II*
,KO)'
3(ROll., .>tOll)*
3
r
lY)
CD
z
\
2 0
n Roll
Y
1 "ex
0
0
0.2
0.1 (6
P)
I
M
Figwe 5. Plots of the decay rate constant (kM) of 31ROMe* as a function of [BPI obsmed at 435 nm after laser pulsing at 355 nm in the 0.5 (A), and lROMe (3.0 X lW3 M)-BP systems with [Hfi04] = 0 (0), 1.0 M ( 0 )in acetonitrilewater (4:lv/v) at 290 K. The solid linea are calculated with eq 18. See text for details.
+
>CO
+
Rd
II*
kA '
kllT
K1
k1 1 k-1
=
(?
'IIT)
+
Il*
>COll
kp K2
-
k6 =
+
kel ( e keel)
k2 / k-2
20 ns after laser pulsing.52 In Scheme 11, we neglected the proton-induced quenching of 3ROMe* for the following reason; as [BPI approaches 0 M, the values of kow at each [H2S04] in Figures 5 and 6 become close to the same. On the assumption that both equilibria for the formation of 3(ROMe..>CO)* and 3(ROMe- *>C+OH)*are attained within 200 ns, the following equation for the observed decay rate, kow,observed at 435 nm for 'lROMe* and at 430 nm for 32ROMe* can be derived kbsd
= (ko + Ki[BP]k,,
+ KiK2[BPl[H+Ikp)X (1 + Kl[BP] + KIK2[BP][H+])-' (14)
+
where k, = k,, k{. Since there is no information about the activity of H 3 0 4 in acetonitrilewater (4:l v/v), we use K; and [H2S04]instead of K2 and [H+], respectively. Therefore, eq 14 is reformed as
0.1 M
kobsd
0.1
0
At first, we consider the simplest case of the lROMe (3.0 X lo-' M)-BP (10.2M) and the 2ROMe (3.0 X lo-' M)-BP (10.2 M) systems in the absence of H2S04. For these cases, eq 14' can be simplified as
02
(BPI , M
Figure 6. Plots of the decay rate constant ( k a ) of '2ROMc' as a function of [BPI obsmed at 430 nm after laser pulsing at 355 nm in the 2ROMe (3.0X lW3 M)-BP system with [H+S04] = 0 (0),0.1 (A),and 1.0 M ( 0 )in acetonitrilewater (4:1 v/v) at 290 K. The solid linea are calculated with eq 18. See text for details. SCHEME 11
k
3ROMe*
+
>CO + H*
ROMe
K1
K2 kp
k1
>CO
-
-
'(ROMe...>CO)*
~
+
'H
H*
ROMe*'
2 \3(ROMe,..>tOH)'
+
>COH
+
H+
k1 1 k-1 k2 1 k-2
ki
+
= (ko + ~I[BPlk,, + K,K2'[BPI [H2S041kp) x (1 + Kl [BPI + KlK2'[BP] [H2SOJ)-' (14')
k,l
chargetransfer structure (Le., the locally excited state of ROMe), which will be discussed later. The acid-base reactions of 'BP* arc known to be very fast, whose equilibrium is established within
kow = (ko + Ki[BPIkex)(l + Ki[BPI)-' (15) On the assumption that the value of ko is small enough to be negligible, Le., ko &OH with a high efficiency. It should be noted that the pK, value in the ground state of BP is negatively large (-5.7),57 and no protonation to BP occurs in the ground state under the experimental condition. On the other hand, in the triplet state of ROMe, protonation to the triplet exciplex 3(ROMe...>CO)* between 3ROMe*and BP is formed as well as the case of the 3ROH* and BP system.42 In '(ROMe->CO)*, electron migration may slightly cccur from 3ROMe* to BP. That is, 3(ROMe*-->CO)*has a weak charge-transfer structure, which may have a sandwichlike structure. At higher acid concentrations, the protonated exciplex 3(ROMe. .>C+OH)* is, therefore, produced by protonation to 3(ROMe* .>CO)*. The electron affinity of the protonated BP (>C+OH) in the exciplex may become large compared to that of BP resulting in the intraexciplex electron transfer. When 1-naphthol (ROH) is used instead of 1ROMe. the proton-enhanced hydrogen atom transfer reaction occurs via the intracomplex electron-transfer reaction as follows:42 '(R0H.e .>C+OH)* 3(ROH'+ + >COH) (23) 3(ROH'+ + >6OH) ROH" + >6OH (24)
-
-
-- +
ROH'+ RO' H+ (25) where 3(ROH'+ + >COH) represents the triplet radical pair in a solvent cage. The 1-naphtholcation radical (kOH'+) codd not be detected experimentally. The result shows that ROH" dissociates very rapidly into the 1-naphthoxy radical, RO' and H+, since the pK, value of ROH" is considered to be very negative. Comparing eq 22 with eq 23, the present study strongly supports the mechanism on the proton-enhanced hydrogen atom transfer reaction. Conclusion It has been shown by means of laser flash photolysis at 355 nm that the proton-assisted photoionization reaction of ROMe sen-
Triplet Methoxynaphthalenes to Benzophenone sitized by 'BP* occurs via the protonated triplet exciplex 3(ROMe-.>C+OH)* to produce effectively the methoxynaphthalene cation radical, ROMe", and the benzophenone ketyl radical, >COH. In the absence of protons for the R O M t B P system, the photoionization of ROMe does not occur upon 355-nm excitation. For the ROMe-BP-H2S04 system, the reaction mechanism for the proton-assisted photoionization of ROMe is interpreted by Scheme 11. It is also shown in the ROMe-BPHzSO4 system that the decay rate constant of 3ROMe* is proportional to the concentration of HzSO4. The kinetic parameters for the present ROMeBP-H2S04systems at 290 K are obtained as listed in Table 11. Since change in the T-T absorption spectrum among 'ROMe*, '(ROMe-.>CO)*, and 3(ROMe.-.>C+OH)* was scarcely observed, the triplet exciplex has a weak chargetransfer structure (that is, the locally excited state of ROMe). The protonation to the triplet exciplex, Le., the formation of '(ROMe. ->C+OH)*, induces the intraexciplexelectron-transfer reaction, resulting in the production of ROMe" and >COH with high efficiencies. Acknowledgment. We thank to Dr. Y.Kajii and Prof. K. Obi of Tokyo Institute of Technology for their experimental support by means of the thermal lensing method. This work was supported partially by a Scientiftc Research Grant-in-Aid from the Ministry of Education, Science and Culture of Japan. References and Notes (1) (a) The preliminary work was presented at the Symposium on Molecular structures, Yokohama, November, 1991. (b) The Institute of Physical and Chemical Research Wako, Saitama 351-01, Japan. (2) F6rster, Th. 2.Elektrochem. Angew. Phys. Chem. 1950,54,42,531. (3) Weller, A. Ber. Bunsenges.Phys. Chem. 1952, S6,662; 1956,66,1144. (4) Weller, A. Prog. Reuct. Kine?. 1961, I , 189. ( 5 ) Beens, H.; Grellmann, K. H.; Gurr, M.; Weller, A. Discuss. Furuduy Soc. 1%5,98, 183. (6) Van der Donckt, E. Prog. Reuct. Kine?. 1970, 5, 273. (7) Wehry, E. L.; Rogers, L. B. In Fluorescence und Phosphorescence Anulyses; Hercules, D. M.,Ed.; Wiley-Interscience: New York, 1966; p 125. (8) Schulman, S.G. In Modern Fluorescence Specrrmcopy;Wehry, E. L., Ed.; Plenum: New York, 1976; Vol. 2. (9) Schulman, S.G. In Fluorescence and Phosphorescence Spectroscopy; Pergamon: Oxford, U.K., 1977. (10) Ireland, J. F.; Wyatt, P. A. H. Adu. Phys. Org. Chem. 1976, 12, 131 and a number of references therein. (11) Klopffer, W. Adu. Photochem. 1977, 10, 311. (12) Shizuka, H. Acc. Chem. Res. 1985, 18, 141 and references cited therein. (13) Tsutpumi, K.; Shizuka, H. Chem. Phys. Lett. 1977,52,485; 2.Phys. Chem. (Wiesbuden) 1978, 1 1 1 , 129. (14) Shizuka, H.; Tobita, S. J. Am. Chem. Soc. 1982, 104, 6919 and references cited therein. (15) Shizuka, H.; Serizawa, M.; Kobayashi, H.; Kameta, K.; Sugiyama, H.; Saito, I. J . Am. Chem. SOC.1988, 110, 1726. (16) Shizuka, H.; Serizawa, M.; Shimo, T.; Saito, I. J . Am. Chem. SOC. 1988, 110, 1930. (17) Shizuka,H.; Kameta, K.; Shinozaki, T. J. Am. Chem.Soc. 1985,107, 3956. (18) Shizuka, H.; Serizawa, M. J. Phys. Chem. 1986, 90, 4573. (19) Shizuka, H.; Serizawa, M.; Okazaki, K.; Shioya, S.J . Phys. Chem. 1986, 90,4694. (20) Shizuka, H.; Ogiwara, T.; Kimura, E. J . Phys. Chem. 1985,89,4302. (21) Scaiano, J. C. J. Photochem. 1973/1974, 2, 81.
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