J. Phys. Chem. 1994, 98, 7014-7021
7014
Thermochemical Profiles on Hydrogen Atom Transfer from Triplet Naphthol and Proton-Induced Electron Transfer from Triplet Methoxynaphthalene to Benzophenone via Triplet Exciplexes Studied by Laser Flash Photolysis Minoru Yamaji, Youichi Aihara, Toshiyuki Itoh, Seiji Tobita, and Haruo Shizuka' Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan Received: February 3, 1994; In Final Form: May 10, 1994" Temperature effects on hydrogen atom transfer (HT) and proton-induced electron transfer (p-ET) via triplet exciplexes in the 1-naphthol-benzophenone (ROH-BP (or >CO)) and methoxynaphthalene-BP ( R O M e B P ) systems in acetonitrile-HzO (ACN-H20; 4:l v/v) were studied by 355-nm laser flash photolysis. The H T rate in the ROH-BP system increased with an increase of temperature. The thermodynamic parameters for H T via the triplet exciplex 3(ROH-.>CO)* were obtained: the enthalpy change, AH1 = -2.0 kcabmol-l, and the entropy change, AS1 = -2.4 eu for formation of 3(ROH-.>CO)*; the frequency factor, A , = 7.7 X lo9 s-l, and activation energy, A,??,, = 3.5 kcal-mol-', for intraexciplex HT. The negatively small values of AH1 and AS1 suggest that 3(ROH.->CO)* has a loose sandwichlike structure. The corresponding parameters in the ROMe-BP system were obtained to be A H 1 = -2.2 kcal*mol-1and A S 1 = -2.8 eu for formation of the triplet exciplex 3(ROMe..->CO)*. In the presence of protons in the ROH-BP system, proton-enhanced H T (p-HT, electron transfer followed by proton transfer) occurs efficiently due to the protonated triplet exciplex,3(ROH*->C+OH)*. The p-HT rate increased with an increase of temperature. In the R O M e B P system with protons, formation of the protonated triplet exciplex 3(ROMe...>C+OH)* results in p-ET. The p-ET rate increased with an increase of temperature. The energy state diagrams for H T and p-ET are illustrated.
Introduction Since hydrogen atoml,Z*,b,3-1*and electron,13-l9 and protontransfer reactions2QC-28 in the excited state are fundamental processes in both chemistry and biochemistry, they have been studied theoretically and experimentally these past 4 decades. Especially, in order to elucidate the mechanism for hydrogen atom transfer (HT) in the excited state, much attention has been paid to photophysical and photochemical studies on benzophenone. By means of nano-, pico-, and femtosecond laser flash photolysis techniques, it has been revealed that triplet benzophenone undergoes HT or electron transfer (ET) followed by proton transfer from a variety of substrates, such as hydrocarbons, alcohols, and amines, to produce the benzophenone ketyl radical."i2 In the past 2 decades, we have had considerable interest in photochemical and photophysical properties of aromatic compounds in the presence of pr0tons:2~proton-transfer reactions in the excited state and proton-induced q ~ e n c h i n g , ~a ~one-way J~ proton-transfer reaction in the excited state of hydrogen-bonded complexes,31 and examples for the absence of excited-state prototropic equilibrium.32 These are photochemical and photophysical phenomena mainly in the excited singlet state upon direct excitation. On the other hand, recently, we have studied photochemical and photophysical processes in the triplet state of aromatic compounds produced by photosensitization in the presence of protons: proton-enhanced HT (p-HT) and protoninduced ET (p-ET).33-40 It has been found by nanosecond laser flash photolysis that HT occurs from triplet naphthylammonium ion (3RNH3+*) and triplet naphthol (3ROH*) to benzophenone (BP or >CO) to produce the naphthylamine cation radical (RNH2'+)36and naphthoxy radical. (RO*),34respectively, plus the benzophenone ketyl radical ( X O H ) . 3RNH3+*+ >CO 3ROH*
-
+ >CO
RNH," RO'
+ >COH
+ >COH
* Abstract published in Aduance ACS Abstracts, June 15,
(1) (2)
1994.
0022-3654 I 9 4 12098-7014%04.50/0
The mechanism for both HTs has been interpreted in terms of the triplet exciplexes, 3(RNH3+-->C0)*36 and 3(ROH->CO)*.34~37 In the presence of protons, the HT rate in the RNH3+-BPsystemis significantlyreduced, which is accounted for by considering the Coulombic repulsion in the protonated triplet exciplex, 3(RNH3+-.>C+OH)*.36 In contrast, the rate and efficiency for HT of 3ROH* are both enhanced due to formationof the protonated triplet exciplex, 3(ROH->C+OH)*, where the intraexciplexET (intra-ET) results in formation of the triplet radical pair, 3(ROH*+ + >COH) followed by rapid dissociation into ROO + H+ >COH without back-electron transfer.37
+
-
intra-ET
3(ROH-->C+OH)*
+
~(RoH*+ >COH)
-
+ >COH) ROO+ >COH + H+ 3(ROH'+
(3) (4)
+
Though 3(ROH*+ >COH) is not observed experimentally,its formation is supported by replacement of ROH into methoxynaphthalenes(ROMe),3*whose triplet state has an isoelectronic structure similar to that of 3ROH*. In the absence of protons, the triplet methoxynaphthalene (3ROMe*) produced by triplet energy transfer from triplet benzophenone (3BP*)forms the triplet exciplex 3(ROMe...>CO)*, which causes neither HT nor ET. However, in the presence of protons, the methoxynaphthalene cation radical (ROMe'+) and >COH were produced efficiently. This proton-induced ET (p-ET) was explained by considering intra-ET in the protonated triplet exciplex, 3(ROMe->C+OH)*.
-
intra-ET
3(ROMe...>C+OH)* 3(ROMe'+
+ >COH)
-
'(ROMe" ROMe"
+ >COH) + >COH
(5)
(6)
Through a comparison of eqs 3 and 4 with eqs 5 and 6, respectively, formation of the triplet radical pair is demonstrated, and it is obvious that the triplet exciplex and protonated triplet exciplex play important roles in promoting the p-HT and p-ET of triplet naphthalene derivatives. The electronic structures of triplet exciplexes are considered to have weak charge-transfer character 0 1994 American Chemical Societv
H Atom and Electron Transfers via Triplet Exciplexes because the triplet-triplet absorption spectra of the triplet exciplexes are similar to those of naphthalene derivatives.33-38.40 As to conformationsof triplet exciplexes, it was revealed with the use of methylene-chained chromophores, [(hydroxynaphthyl)propyl]benzophenone and [(methoxynaphthyl)propyl]benzophenone, that they have sandwichlikestructures upon HT and gET.39 By means of laser flash photolysis, new aspects of the reactions between triplet naphthalene derivatives and benzophenone have been revealed, as mentioned above. However, temperature effects on HT and p-ET have not yet been examined. It is of great interest to reveal the thermochemicalparameters to illustrate the energy diagramsfor the HT and p-ET mechanisms. In the present work, by means of laser photolysis, measurements of the temperature effects on HT and p-ET were carried out, and the energy diagrams for these reactions via triplet exciplexes were illustrated. Experimental Section 1-Naphthol (ROH) and benzophenone (BP or >CO) were sublimatedtwicein vacuo. 1-Methoxynaphthalene(ROMe) was purified by vacuumdistillation. Acetonitrile(G. R. grade; Wako) and deionized water were purified by distillation. An acetonitrile water mixture (4:l v/v) was used as the solvent. HzSO4 (97%; Wako) was used as supplied. H2SO4 was used as the proton source since it is known that the counterion (S042-) does not quench the triplet molecules.41 The concentrations of ROH and ROMe were usually 3.0 X M. BP was used as a triplet sensitizer in the range of concentrations of 6.7 X 10-3 to 0.15 M. All samples in quartz cells with a 1- or a 10-mm path length were degassed by freezepumpthaw cycles on a high-vacuum line. Spectraldata regarding transients were obtained by a fresh sample to avoid excessive exposure to the laser pulse. The kinetic measurements were carried out in the temperature range 275-320 K,which was restricted due to solubilities of the solutes at lower temperatures. The temperature of the samples in a quartz dewar was kept with hot water (1295 K)or a mixture of methanol and liquid nitrogen (COH) and naphthoxy radical (RO'), respectively." The decay rate, kowrfor 'ROH* at 430 nm was measured in the temperature range 275-320 K. Figure 1 shows the plots of kow vs [BPI obtained after laser photolysis in the ROH-BP system without H2SO4 in acetonitrilewater (4:l v/v) in the temperature range 275-320 K. Plots at the same temperature show a nonlinear quenchingby [BPI, which was evidence for exciplex formation.3' At the same [BPI, the koW value increased with an increase of temperature. In the absence of protons, the HT mechanism of the ROH-BP system
The Journal of Physical Chemistry, Vol. 98, No. 28, 1994 7015
,
I
I
0
0.1
0.2
[ B P I / M Figure 1. Plots of the decay rate (&a) at 430 nm as a function of [BPI obtained after laser pulsing in the ROH-BP system ([ROH] = 3.0 X l C 3M) with [HSO,] = 0 M in acetonitrilewater ( 4 1 v/v) in the range 275-320 K. The solid curyea were calculated by q 7. See text for
details.
SCHEME 1
. . ROH + >CO
kw = ka' + km K1 = ky
R O + >&OH
(3+d
I k-1
has been reported as shown in Scheme 1.3' Here, ko,kb,and km are respectively the decay rate constantsfor 3ROH*in the absence of BP, 3(ROH->CO)* to ROH plus BP in the ground state, and 3(ROH-*>CO)* to the production of >COH and RO'. The equilibrium constant, K1 consists of the rate constants kl and k-l for associationand dissociation of 3(ROH-+CO)*, respectively. A fast equilibriumbetween 3ROH* + >CO and 3(ROH.->CO)* is assumed.37 The decay rate, k0w for 'ROH* observed at 430 nm can be expressed as follows.
kow = (ko + K ~ ~ ~ X [ B P + I )K~CBPI)-' (~
(7)
where k,, = kb + kHT and KI = kt/k-l. On the assumption that ko is negligible compared to those of competitive processes, i.e. ko CO t H
RO
ROH" +>&OH)
+
+
+ > i O H t Hi
Figure 2. Therefore, k,, can be expressed by eq 10 with the use of the frequency factor, A,,, and the apparent activation energy, AE,,,for the decay of 3(ROH-.>CO)*.
In k,, = In A,, - (hE,,/R)T'
+
>CO H+ with the rate constant k'b or undergoes intra-ET with a rate constant ET to produce the triplet radical pair 3(ROH'+ + >COH). The radical pair rapidly dissociates into RO' >COH + H+. We denote the decay rate constant of 3(ROH-.>C+OH)* as k, (=IC",, + ET). According to Scheme 2, kow is formulated as
(10)
From the slope and intercept of line b, the values of A&, and A,, were obtained to be 3.5 kcal-mol-I and 7.7 X lo9s-I, respectively. The obtained parameters of AH',MI,AE,,,and A , are listed in Table 1. In the presence of protons in the ROH-BP system, enhancement of the kow value for the decay of 3ROH* occurs proportionally to the acid con~entrations.3~ The HT mechanism of the ROHBP system in the presence of protons has been represented by Scheme 2.25 In addition to Scheme 1, 3(ROH-+CO)* forms the protonated triplet exciplex 3(ROH-.>C+OH)* with an equilibrium constant, Kz, in the presence of protons. Here, k2 and k-2 are the rate constants for protonation of 3(ROH->CO)* and deprotonation of 3(ROH..+C+OH)*, respectively. The protonated triplet exciplex 3(ROH...>C+OH)* decays to ROH
Since the activity of HzSO4 in acetonitrile-water (4:l v/v) is unknown, KZ and [H+] are replaced by K'z and [HzSO,], respectively. Thus, we obtain eq 11'.
Figure 3 shows typical plots of k o w obtained in the ROH-BP systems ([ROH] = 3.0 X lo-' M) with [H2S04] = 0,0.25, and 0.5 M as a function of [BPI at 295 K. Even in the presence of H2SO4, a leveling off at higher [BPI can be seen. With an increase of temperature, k o w values in the ROH-BP-HzSO, system ([ROH] = 3.0 X lk3;[H2S04] = 0.25 M), increase as shown in Figure 4. The solid curves in Figures 3 and 4 were calculated with the use of the obtained parameters as mentioned later. When it is assumed that 1 + KI[BP] >> K I K ~ [ B P ] [ H ~ S Oeq~ ]11' , is rewritten as
H Atom and Electron Transfers via Triplet Exciplexes
The Journal of Physical Chemistry, Vol. 98, No. 28, 1994 7017
I Equation 12 indicates that the kow value increases in proportion to [H2S04]. The typical plots of kow as a function of [HzSO~] are shown in Figure 5a. At all temperatures studied, the plots showed a straight line. Therefore, from the slope of the line, we determined the values of (L&K'2[BP])(1 + Kl[BP])-I in eq 12. With the use of the K I and [BPI values, we obtained the product value of k&'z at T. Figure 5b shows the logarithmic plots of the k&'z value vs T1. Plots showed a straight line. On the assumption that the rate constantk, for the decay of 3( ROH->C+OH)* can be expressed by the Arrhenius expression with the frequency factor AET,and apparent activation energy, MET, as follows. kp
=
exp(-m,/RT)
295K
3Y0
2A
'
0.2 '
'
0.4 '
'
0.6 '
'
'
0.8 '
(13)
It has been reported that k, N k@T(kET >> k'b).3' Therefore, k,, AET, and MET correspond to the rate constant, frequency factor, and apparent activation energy for intra-ET in 3(ROH.->C+OH)*, respectively. On the other hand, when the fast equilibriumfor protonation of 3(ROH.->CO)* is assumed3' to be expressed by the van't Hoff equation with the use of the enthalpy change, AH2, and entropy change, A S 2 , for formation of 3(ROH-+C+OH)*, the value of In k& should be expressed as follows. In k$r> = in A,
+ M,/R
- ( M E T + AH2)/RT
(14)
+
From the slope of the line in Figure 5b, the value of META H 2 is obtained to be 4.8 kcabmol-1. It was impossible to determine the net values of MET and A H 2 in the present system. A S 2 can be related to the change in configuration of 3(ROH-*>CO)* before and after protonation. On the assumption that the difference by protonation was little in the configuration of 3(ROH-->CO)*, we assumed A S 2 N 0. Therefore, the intercept of the line in Figure 5b gave AET = 8.6 X 1Olos-l. Temperature Effects on PET from jROMe* to BP. Upon 355-nm laser excitation in the ROMB-BP-H~SO~system, a transient absorption band at 435 nm due to the triplet-triplet (T-T) absorption spectrum of ROMe is produced by triplet sensitization from 3BP* in the nanosecond time scale.3s With an isosbesticpoint at 490 nm, the 435-nm band for 3ROMe* decreases in intensity, accompanying an increase in the intensities of the 545-nm band for the benzophenone ketyl radical (>eOH)43and the 380- and 630-nm ones for 1-methoxynaphthalene cation radical (ROMe*+),showing p-ET from 3ROMe* to BP.35J8 In the absence of protons, no ET takes place. In the present study, the decay rate, k , ~ for , 'ROMe* to 435 nm was measured in the temperature range 275-320 K. The mechanism for p-ET of the ROMe-BP system has been revealed as shown in Scheme 3,38 where 3ROMe*, 3(ROMe...>CO)*, and 3(ROMw>C+OH)* denote the triplet ROMe, triplet exciplex, and protonated triplet exciplex, respectively. ko,k,, and k'b are the decay rate constants of 3ROMe*, 3(ROMa+CO)*, and '(ROMe...>C+OH)*, respectively, to the ground-state; K1 ( = k l / k 1 ) , and K2 (=kz/k-z) are the corresponding equilibrium constants for complex formations; and k@T is the rate constant for intra-ET of 3(ROMe...>C+OH)* to produce the triplet radical pair, 3(ROMe*++ >COH). When fast equilibria amongSROMe* >CO H+, 3(ROMe...>CO)* + H+, and 3(ROMe...>C+OH)* are assumed, the decay rate, kow, for 3ROMe* observed at 435 nm can be expressed by eq 11'. In the absence of protons, k o w stands for the decay rate of the triplet intermediates, 3ROMe* and 3(ROMe...>CO)*, since no p-ET occurs. The kow value in the absence of protons can be expressed by eq 7.
+
+
16'5'
312
'
'
'
T-1
'
I
3.4 '
'
'
'
3.6 '
10-3 K-1
Figure 5. (a, top) Plots of the decay rate ( k w )at 430 nm as a function of [H$304]obtained after laser pulsing in the ROH-BP-HzSOd ([ROH] = 3.0 X M; [BPI = 5.0 X M) in acetonitrilbwater ( 4 1 v/v) at 295 K. (b, bottom) Logarithmic plots of the k& value obtained for the ROH-BP-HzSO, system against T 1in the range 275-320 K.
SCHEME 3 ki
%OMe*t >CO
t
H'
e
k2 3(
ROMe,->CO)'t H ' S
3(
ROMe...>C+OH)'
3(
ROMe*'
k-2
ROMe t >CO
K1 =
t
H
ROMe''
t
t
>&OH)
>COH
k j / k-1
K2 = k2 / k-2
kp = b' t km ( Zkm)
Figure 6 shows the plots of k , , ~at 435 nm obtained after 355-nm laser photolysis in the ROMe-BP systems ([ROMe] = 3.0 X M) without HzSO4 in the temperature range 295-320 K as a function of [BPI. With an increase of temperature, the kMvalue increasesat the same [BPI. At all temperaturesstudied, a leveling off at higher [BPI is expected, which indicates formation of 3(ROMe.->CO)*. The obtained k w values were analyzed by the same method for the ROH-BP system, namely, by eq 8, and the values of k,, and K1 for the ROMe-BP system without protons were determinedSu The solid curves in Figure 6 were calculated with the use of eq 7 and the obtained values of k,, and K1 at T. Figure 7 shows the van't Hoff plots of the obtained Kl (In K1 vs PI)and the Arrhenius plots of the obtained k,, values. The
7018 The Journal of Physical Chemistry, Vol. 98, No. 28, 1994
Yamaji et al.
r
lY)
4-
z
(0
8
Y
0
0.1
I
0.2
[BP]/M Figure 6. Plots of the decay rate ( k o w )at 435 nm as a function of [BPI obtained after laser pulsing in the ROMe-BP system ([ROMe] = 3.0 x M) with [H2S04] = 0 M in acetonitrile-water (4:l v/v) in the range 275-320 K. The solid curves were calcualted by eq 7. See text for details.
.
.
,
.
0;l
0
0.2
[BP]/M A
320K
[ B P I = 0.1 M
I
I
0
I
I
I
I
0.4
0.2
I
I
0.6
I
0.8
HzS04 1/ M Figure 8. (a, top) Plots of the decay rate ( k o ~at) 435 nm as a function of [BPI obtained after laser pulsing in the ROMe-BP system ([ROMe] = 3.0 X M) with [HzS04] = 0 ( O ) , 0.25 (A),and 0.5 M (0)in acetonitrile-water (4:lv/v) at 295 K. The solid curves were calculated by eq 12. See text for details. (b, bottom) Plots of the decay rate ( k o ~ ) at 435 nm as a function of [HzSO,] obtained after laser pulsing in the ROMe-BP-HzSO4 system ([ROMe] = 3.0 X M; [BPI = 0.1 M) in acetonitrile-water (4:l v/v) in the range 275-320 K. T-1
10-3 K-1
Figure 7. van’t Hoff plots (0)of K1 and the Arrhenius plots (A)of k,, obtained for the ROMeBP system with [HzS04] = 0 M in acetonitrilewater (4:lv/v) in the range 275-320 K.
van’t Hoff plots showed a straight line (a) in Figure 7. With the use of eq 9, we obtained the enthalpy change, AH], and the entropy change, AS],for formation of 3(ROMe-+CO)* to be AH1 = -2.2 kcalomol-l and PSI = -2.8 eu from the slope and intercept of line a, respectively. On the other hand, as the Arrhenius plots gave a straight line (b) in Figure 7, k , can be expressed by eq 7. From the slope and intercept of line b, the values of A&, of the apparent activation energy and A,, of the frequency factor for the decay of 3(ROMe...>CO)* were obtained to be 3.6 kcal-mol-I and 2.9 X lo9 s-I, respectively. The obtained parameters of M I AS,, , A&,, and A,, for the ROMe-BP system are also listed in Table 1. In the presence of protons, the kow value increases with an increase of [HzSO~] at the same [BPI. Figure 8a shows typical plots of k o u as a function of [BPI obtained in the ROMe-BP systems ([ROMe] = 3.0 X 10-3 M) with [H2S04] = 0,0.25, and 0.5 M a t 295 K. A leveling off is shown at higher [BPI even in the presence of protons. The solid curves in Figure 8a are the calculated values as mentioned later. Figure 8b shows the plots of k o u as a function of [HzSO~]obtained in the ROMe-BPH2SO4 system ([ROMe] = 3.0 X 10-3 M; [BPI = 5.0 X 10-2 M) at the temperature range studied. The kow value increases with an increase of temperature at the same [HzSO~].Since the plots showed a straight line at all temperatures, the k o w can be expressed by eq 12. Therefore, the product values of kpK4 at studied temperatures were determined by the same method as that for the ROH-BP-HzSO, system.
v
C -
-
2.5-
3.2
3.4 T-1
3.6
10-3 K-1
Figure 9. Logarithmicplots of the k&
value obtained for the ROMeBP-HzSO, system against T I in the range 275-320 K.
Figure 9 shows the logarithmic plots of the k& value vs TI. The plot shows a straight line. From the slope of the line, the value of A H 2 + MET. was determined to be 4.9 kcal-mol-1, where A H 2 and MET were, respectively, the enthalpy change for formation of 3(ROMe->C+OH)* and the apparent activation energy for thedecay of 3(ROMe->C+OH)*, It has been shown that k, kET (kET >> k’b).38 Therefore, k,, AET,and MET can be interpreted to be the rate constant, frequency factor, and apparent activation energy for intra-ET in 3(ROMe...>C+OH)*, respectively. It was impossible to determine the net values of AEETand A H 2 in the ROMeBP-H2S04 system. As well as for the case of the ROH-BP systems, when the difference in the configuration of 3(ROMe.+CO)* before and after protonation
The Journal of Physical Chemistry, Vol. 98, No. 28, 1994 7019
H Atom and Electron Transfers via Triplet Exciplexes
TABLE 2 Standard Heats of Formation, AHf, Used in the Present Study M AHAM)/ (kcalmol-l) BP 120 >C+OH 167.Ob >COH 36.lC ROH -7.1"
[H*]=OM 185.0 I
%OH'+
I
77-7
>co
RO'
25.96 -7.2' 52.1" 365.Jb
ROMe H'
ET
H+
AHR
ROH+>W
Reference 45. Reference 54. Evaluated by the Franklin additive rule. See. text for details. Evaluated by using substitution of the G H bond energy of phenol for that of ROH. See text for details.
(49)
[ H* ] = 0 M
AHR
= {AHARC) + AHf(>COH))(MAROH)
+ AHABP))
= 57.1 kcal-mol-'
I
It was assumed in the cases of neutral molecules and radicals that the difference in the total solvation energy between the initial and fiial states was small. On theother hand, the enthalpy change, AHHT,for formation of ROO + >COH from 3(ROH->CO)* was calculated as follows.
1 62.3 )
ROMet > C O
(4.8)
Figure 10. (a, top) Energy diagram for HT via 3(ROH-.>CO)* in the ROH-BP system without protons. See text for details. (b, bottom) Energy diagram for the decay of the ROMe-BP system without protons via 3(ROMc...>CO)*. See text for details.
was considered to be small, we assumed that the entropy change A S 2 = 0. From the intercept of the line in Figure 9, the frequency factor, ABT, for intraexciplex electron transfer was obtained to be 1.1 x 107 S-I.
Discussion Energy Profdes for HT and Relaxation Processesin the ROHBP and ROMe-BP Systems without Protons. In the absence of protons, HT occurs via 3(ROH..+CO)* in the ROH-BP system, whereas there was no reaction in the ROMe-BP system. In the present study, the heat of formation, AH1, the entropy change, AS1,for 3(ROH->CO)* and 3(ROM+>CO)*, and theapparent activation energy for the decay of 3(ROH-..>CO)* and 3(ROMe-->CO)* have been determined in both the ROH-BP and ROMe-BP system without protons, respectively. The energy diagram for HT in the ROH-BP system is illustrated in Figure loa. Here, ET is the triplet energy of ROH (58.6kcal-mol-1 37) in a polar solvent. The standard heat of formation, AHf (RO') for RO' was estimated with the use of those for ROH, A H T (ROH) = -7.1 kcal-mol-1,43 a hydrogen atom, AHdH') = 52.1 kcal.mol-1,45 and the O-H bond energy, D(O-H), of ROH. Since the D(O-H) value of ROH was not available, that of phenol was used for D(O-H) = 85.1 kcal-mol-l.46 Therefore, we obtained
AHf(R0') = AHAROH)
+ D(O-H)
- AHf(H*)
= 25.9 kcal-mol-'
The standard heat of formation, AHd>COH), for >COH was evaluated by means of the Franklin additive rule for the heat of formation.47 AH,-(>COH)
2AHAPh-)
+ AH&>C-)+ MA-OH)
= 2 x 22 + 34 - 41.9 = 36.1 kcalemol-' The enthalpy change, AHR,for HT from ROH + BP to RO* + >COH can be evaluated with the use of the AHdRO'), AH,(>COH), AHAROH), and AHI(BP) values listed in Table 2.
The AHHTvalue was expected to be negative, Le., exothermic, since intraexciplex HT occurs spontaneously. This discrepancy would be due to an overestimate of D(0-H). The bond energy of ROH is expected to be smaller than that of phenol, judging from the resonance stabilization of the ?r-electron system. Therefore, we illustrated AHHTC 0 in Figure IOa. The AHlvalue for formation of 3(ROH-->CO)* was negatively small (-2.0 kcabmol-I) as well as that of A S 1 (-2.4 eu). The former is considered to indicate that the binding energy between 3ROH* and BP for formation of 3(ROH*-.>CO)*is small, while the latter indicates that 3(ROH-->CO)* is structured not so tight. Therefore, the small values of AHl and AS1 are ascribable to a loose structure of triplet exciplexes, which is in agreement with the conclusion obtained previously." As for intraexciplex HT in the ROH-BPsystem, it has been revealed that the efficiency for production of RO' >COH via 3(ROH-*>CO)* is almost unity; Le., k,, H ~ H T . ~Therefore, ' the decay of 3(ROH.->CO)* with a rate constant, k,, leads only to yield RO' + >COH. In other words, AE, can be regarded as the apparent activation energy for intraexciplex HT in the ROH-BP system. The obtained value of AE, = 3.5 kcabmol-1 in the ROH-BP system is close to those obtained in the HT of 3BP* from neat alcohols (2.6-3.5 kcal-mol-l)a and hydrocarbons (2.5-3.9kcal.mol-1),48349 while the frequency factor, A, (7.7 X lo9 s-l), is greater than those from neat alcohols ((1 .&2.0) X lo9s - ~ ) . ~ *The HT of 3BP* from alcohols and hydrocarbons is considered to occur via collision ~omplexes.~ The apparent activation energies, Ma,obtained in the temperature range 32&275 K may correspond to the classical Arrhenius activation parameter. On the other hand, Formosinho has shown that HT from hydrogen-donor molecules to triplet ketones proceeds via the quantum mechanical tunneling in the triplet collision complex." One of criteria for tunneling is deviations from the simple Arrhenius plots of reaction rates at sufficiently low temperatures.% Unfortunately, the HT experiments at extremely lower temperatures were impossible due to thelow solubilitiesofthe solutes in the present system. Therefore, it was not straightforward to clarify the detailed HT mechanism for the present HT reaction. However, similarity in the apparent activation energiesbetween the usual HT of 3BP*and HT in the present system suggests that, presumably, the H T of
+
Yamaji et al.
7020 The Journal of Physical Chemistry, Vol. 98, No. 28, 1994
+
%OH'+
production of RO' >COH increases with an increase in the proton concentration. Figure 1l a indicates that the additional pathway for HT via 3(ROH-.>C+OH)* to the HT mechanism of the ROH-BP system without protons in Figure 10a increases the #RT value. On the other hand, in the ROMeBP system with protons, the enthalpy change, AH,,, for the p-ET of ROMe in the ground state expressed by eq 16 can be calculated.
>co + ti'
h
1 428.0 iAH 1
~ R O H>co).+H+
(427.01 ET
RO
+ >&OH + H+
.... ............. A b
ROH + >CO
1
iH*
( 370.4
ROMe
+ >CO + H+
-
ROMe"
+ >COH
The stabilizing solvation energies, Ps,for cations were estimated according to the Born equation.51
!
[Ht]>OM
P, = -e2/2r(1 - E-')
-
(16)
(17)
h
432
i AHz+AEET i 1 IE- I
............................................
ROMe"+ >COH
Figure 11. (a, top) Energy diagram for pHt via 3(ROH-.>CtOH)* in the ROH-BP system with protons. See text for details. (b, bottom) Energy diagram for pET via 3(ROMe-+C+OH)* in the ROMe-BP system with protons. See text for details.
3(ROH-.>CO)* mainly originates from the thermally activated HT process in addition to the H-tunneling. Analogous to the energy diagram for the ROH-BP system in Figure loa, Figure 10b shows the energy diagram for the deactivation processes of the ROMe-BP system via 3(ROMe.+CO)*. Here, ET is the triplet energy of ROMe (59.7 kcabmol-l 38)in a polar solvent. Comparing the AHl, ASI, and A&values in the ROMe-BP system with those in the ROH-BP system, respectively, shows that they are almost the same. It may be derived from the similarity in the electronic and molecular structures between 3(ROH-+CO) * and 3(ROM+>CO)* which have weak charge-transfer character with a loose sandwichlike structures.34-40 As for A,, in the ROMe-BP system, it can be interpreted as the frequency factor for the radiationless deactivation, Le., intersystem crossing from 3(ROMe->CO)* to ROMe BP. Energy Profdes For p-HT and p-ET. In the presence of protons in the ROH-BP and ROMe-BP systems, the HT rate in the former system as well as the ET rate in the latter increased proportionately to the proton concentration. In the present study, the sums (AH2 &ET) of the heat of formation, AH2, for protonated triplet exciplexes and the activation energy, &ET, for intra-ET were obtained as listed in Table 1. It was impossible to obtain the respective values of A H 2 and MET since only the product of k&'2 could be given. The A H 1 and A S 1 values in the presence of protons were applied with those in the absence of protons, assuming that little interaction of the protons was considered upon formation of triplet exciplexes. Parts a and b of Figures 11 show the energy diagrams for p-HT and p-ET, respectively, where the deactivation pathways with k,, of the triplet exciplexes are omitted. The energy states of 3(ROH-.>C+OH)* and 3(ROMe.+C+OH)* were indefinite since the A H 2 values were unknown, as mentioned above. In the ROH-BP system with protons, it is clear that the protons behave like a catalyst in HT.
+
where r and t are a radius of the cation and a dielectric constant of the solvent. In the present study, the value of t used was 42.9 for a mixture of acetonitrile-water (4:l V / V ) .The ~ ~ P8value for H+, P,(H+), was derived to be -258.0 kcal-mol-1 from the hydration enthalpy of H+, A H h y d = -260.8 kcal.mol-l,s3 by eq 17. For ROMe'+, the solvation energy, P,(ROMe*+), was obtained to be -54.1 kcal.mo1-l by eq 17, assuming r = 3 A. With the use of the standard heats of formation AHdROMe), AHdROMe*+), and AHdH+) (=365.5 kcal-mol-' S4) for ROMe, ROMe*+,and H+, respectively, the ionization potential, IP(R0Me) of ROMe (179.4 kcal./mol-l 29), the triplet energy of ROMe, ET (59.7 was obtained as follows. kcal-mol-l 3 9 , and the P,values, mion
+ P,(ROMe'+) + AH,(>COH)) {AH,(ROMe) + AHf(BP) + AHf(H+)+ P,(H+)) = {AHf(ROMe)+ IP(R0Me) + P,(ROMe'+) + AHf(>COH)) - (AH,(ROMe) + AH,(BP) +
AHion= (AH,(ROMe'+)
= IP(R0Me)
-
+ >COH (+H+)
(15)
In a previous paper,2swe showed that the efficiency, 4RT,for the
- AH,(BP) - AHf(H+) + P,(ROMe'+)-P,(H+)
= 41.9 kcal-mol-' Since Mion is endothermic, no electron transfer spontaneously occurs in the ground state. On the other hand, the total enthalpy change, ME, for p-ET of 3ROMe* can be evaluated as follows.
AHz = Mion - ET
= -17.8 kcal-mol-'
+
~ROH*+ BP (+H+) RO'
+ M,(>COH)
The enthalpy change, MET, for intra-ET leading to a yield of ROMe*+ >COH can be obtained from Figure 1lb.
+
AHET=ME-AH,-AH2
= - 20.9 kcabmol-' The obtained AHz and AHET values are highly exothermic. Therefore, once 3ROMe* is produced in the presence of BP and protons, ET proceeds according to the p-ET mechanism. The frequency factor, AET,for intra-ET in 3(ROH->C+OH)* is greater than that in 3(ROMe->C+OH)* by 3 orders of magnitude. The difference in AETmay be caused mainly by the following reasons: (1) formation of hydrogen bonding between water molecules and ROH in 3(ROH-+C+OH)* enhances the ET processss.56 in the former case and (2) the distance between ROH and >C+OH in 3(ROH-+C+OH)* with a sandwichlike structure is shorter than that between ROMe and >C+OH in
H Atom and Electron Transfers via Triplet Exciplexes 3(ROMe-+C+OH)* since the methyl group is more bulky compared with the hydrogen atom. Conclusion By 355-nm laser flash photolysis in the ROH-BP and ROMeBP systems in acetonitrilewater (4:l v/v) with and without protons in the temperature range 275-320 K, thermodynamic parameters for HT and p E T via triplet exciplexes are obtained, as listed in Table 1. The negatively small values of M€,and AS1 are due to a loose structure of triplet exciplexes. On the basis of the obtained parameters, thermodynamic profiles on HT and p-ET are illustrated in Figures 10 and 11. Acknowledgment. This work was supported by a Grant-inAid on Priority-Area-Research: Photoreaction Dynamics (06239101)from theMinistry of Education, Scienceand Culture of Japan. References and Notes (1) Turro, N. J. Modern Molecular Photochemistry; Benjamin1 Cummings: Menlo Park, CA, 1978. Turro, N. J.; Dalton, J. C.; Dawes, K.; Farrington, G.; Hautala, R.; Morton, D.; Niemczyk, M.; Schore, N. Acc. Chem. Res. 1972,5,92.Dalton, J. C.; Turro, N. J. Annu. Reu. Phys. Chem. 1970,21,499. (2) (a) Formosinho, S.J. J. Chem. Soc., Faraday Trans. 2 1976,72, 1313; 1978, 74, 1978. Formosinho, S. J. Adu. Photochem. 1991, 16, 67. Arnaut, L. G.; Formosinho, S.J.; Da Silva, A. M. J. Photochem. 1984,27, 185. (b) Formosinho, S.J. J. Chem. Soc., Furuday Trans. 2 1976,72,1332. (c) Formosinho, S.J. J. Chem. Soc., Perkin Trans. 2 1987,61.Arnaut, L. G.; Formosinho, S. J. J. Photochem. Photobiol. A: Chem. 1993, 75, 1. Formosinho, S.J.; Arnaut, L. G. J . Photochem. Photobiol. A: Chem. 1993, 75,21. (3) Hoshino, M.; Shizuka, H. In New Aspects of Radiation Curing in Polymer Science and Technology; Fouassier, J. P., Rabek, J. F., Eds.; Elsevier: London, 1993;Vol. 2, p 637. (4) Wagner, P. J.; Hammond, G. S. Adu. Photochem. 1968, 5, 21. Wagner, P. J. Acc. Chem. Res. 1971.4, 168;Top. Curr. Chem. 1976,66,1. Wagner, P. J.; Truman, R. J.; Puchalski, A. E.; Wake, R. J. Am. Chem. Soc. 1986,108,7727. (5) Scaiano, J. C. J . Photochem. 197311974,2,81. (6) Cohen, S.G.;Parola, A.; Parsons, G. H., Jr. Chem. Reo. 1973,73, 141. Inber, S.;Linschitz, H.; Cohen, S . G. J. Am. Chem. Soc. 1980,102, 1419;1981,103,1048. Naguib, Y. M. A,; Steel, C.; Cohen, S.G.; Young, M. A. J . Phys. Chem. 1987,91,3033. (7) Okada,T.;Tashita, N.; Mataga,N. Chem. Phys. Lett. 1980,75,220. Okada, T.;Karaki, T.; Mataga, N. J . Am. Chem. Soc. 1982,104,7191. (8) Arimitsu, S.; Masuhara, H. Chem. Phys. Lett. 1973, 22, 543. Arimitsu, S.;Masuhara, H.; Tsubomura, H. J . Phys. Chem. 1975,79,1255. (9) Peters, K. S.;Freilich, S.C.; Shaefer, C. G. J. Am. Chem. Soc. 1980, 102,5701. Simon, J. D.; Peters, K. S.J. Am. Chem. Soc. 1981,103,6403; 1982,104,6542.Simon, J. D.; Peters, K. S.Acc. Chem. Res. 1984,17,277. Peten,K.S.;Pang,E.;Rudzki,J. J.Am. Chem.Soc.1982,104,5535.Manring, L. E.;Peters, K. S.J. Am. Chem. Soc. 1984,88, 3516; 1985, 107,6452. Goodman, J. L.; Peters, K. S.J. Am. Chem. Soc. 1986,108,1700. Shaefer, C. G.; Peters, K. S.J. Phys. Chem. 1987,91,714. Peters, K. S.;Lee, J. J . Phys. Chem. 1993,97,3761. (10) Hoshino, M.; Shizuka, H. J. Phys. Chem. 1987,91,714.Hoshino, M.;Seki,H.;Kaneko,M.;Kinoshita,K.;Shizuka,H.Chem. Phys.Lett. 1986, 132,209. Hoshino, M.; Kogure, M. J . Phys. Chem. 1989,93,728. (1 1) Devadoss, C.; Fessenden, R. W.J. Phys. Chem. 1990,94,4540,1991, 95,7253. (12) Miyasaka, H.; Mataga, N. Bull. Chem. Soc. Jpn. 1990,63,131. Miyasaka, H.; Morita, K.; Kamada, K.; Mataga, N. Bull. Chem. Soc. Jpn. 1990,63,3385. Miyasaka, H.;Morita, K.; Kamada, K.; Nagata, T.; Kiri, M.;Mataga,N. Bull. ChemSoc.Jpn. 1991,64,3229.Miyasaka,H.;Nagata, T.; Kiri, M.; Mataga, N. J . Phys. Chem. 1992,96,8060. (1 3) Marcus, R. A. J . Chem.Phys. 1956,24,966; Annu. Reu. Phys. Chem. 1964,15, 155; Angew. Chem., In?. Ed. Engl. 1993,32,1 1 11. (14) Rehm, D.; Weller, A. Isr. J . Chem. 1970,8,259, and references cited therein. (15) Mataga, N. In Molecular Interactions; Ratajczak, H., OrvilleThomas, W.J., Eds.; Wiley: Chichester, U. K., 1981;Vol.2.p 509. Mataga, N. Pure Appl. Chem. 1984,56,1255;1993,65,1605.Kakitani, N.; Mataga,
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