J. CHEM. SOC. FARADAY TRANS., 1994, 90(4), 533-539 Laser Flash Photolysis Studies on Hydrogen-atom Transfer from the Triplet Hydroxynaphthylammonium Ion to Benzophenone via a Triplet Exciplext Which Group is More Reactive for Hydrogen-atom Transfer, -OH or -NHS ? Minoru Yamaji, Ken-ichi Tamura and Haruo Shizuka" Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan Laser flash photolysis studies on hydrogen-atom transfer (HT) from the triplet hydroxynaphthylammonium ion (the protonated form of Saminonaphth-1-01, HORNH;) to benzophenone (BP or >CO)in methanol-water (9: 1 v/v) at 295 K have been carried out in order to elucidate which hydrogen atom of the substituent groups is more reactive for HT, and what is the mechanism. It was found that HT from triplet HORNH; (3HORNH,+*), produced by triplet energy transfer from 3BP*, occurs to BP to yield the hydroxynaphthylamine radical cation (HORNH?) and the benzophenone ketyl radical (>COH) with efficiencies of 0.92 and 0.48 in the presence of 0.015and 0.5 mol dm-3 H,SO, , respectively.The decay rate of 3HORNH,+* (kobs) decreases with increasing acid concentration, approaching a constant value at higher acid concentrations. This behaviour of kobs at higher acid concentrations cannot be explained by the HT mechanism alone for the naphthylammonium ion (RNH;)-BP system previously reported (S. Kohno, M. Hoshino and H. Shizuka, J. Phys. Chern., 1991,86,1297).With an increase of [BPI, kobs increases showing a levelling off at higher [BPI, which indicates the formation of a triplet exciplex between 3HORNH3+* and BP.The HT mechanism for the HORNHi-BP system was interpreted as a composite mechanism. The best-fit kinetic parameters were found to be k, = 5.0 x lo5 s-', k,, = k,, = lo7 s-', K, = 5 x lo2 dm3 mot-', K; = 3 dm3 mol-' and ki/kb = 6 x lo2 dm3 mol-'. It is concluded that (i) the hydrogen atom of the NH,' group (which is more protic than that of OH) is the more reactive for the HT reaction in the present system and (ii) the proposed composite mechanism including two different conformations of the protonated triplet exci- plexes, 3(HORNH;. -.>&OH)* and 3(H;NROH. . . $OH)*, is involved in the present HT. Acid-base reactions in the excited states of aromatic com- pounds have been extensively studied since they are elemen- tary processes in both chemistry and biochemistry.'-'' In this decade, we have paid much attention to photochemical and photophysical properties of aromatic compounds in the presence of protons in terms of proton-transfer reac-tions."-'8 In general, proton transfer occurs mainly in the excited singlet state of aromatic compounds, whereas in the triplet state, hydrogen-atom transfer is predominant.'' It is well known that the triplet carbonyl compounds produce their ketyl radicals by hydrogen-atom transfer from a variety of hydrogen-atom donors, such as alcohols, hydro- carbons and amines. A large number of studies on hydrogen- atom transfer reactions of triplet carbonyls have been rep~rted,'~-~'and it has been revealed by nano- and pico- second laser flash photolyses that these reactions proceed by either hydrogen-atom transfer or electron transfer followed by proton transfer.Until recently, little attention has been paid to the photo- chemical and photophysical processes of triplet aromatic compounds produced by triplet sensitization from carbonyl compounds. In particular, the hydrogen-atom transfer and electron-transfer reactions between triplet naphthalene deriv- atives and benzophenone and the effects of protons on these reactions have been of great interest to us.32-38 In the case of the naphthol (R0H)-benzophenone (BP or )CO) system, it has been revealed by nanosecond laser photolysis that the hydrogen-atom transfer (HT) reaction occurs from triplet naphthol ('ROH*) to BP to yield the naphthoxy radical (ROO) and benzophenone ketyl radical (>COH) via the triplet exciplex, 3(ROH.. )CO)*, which has a weak charge- transfer intera~tion.~~,~~ In the presence of protons, the HT -f This work was presented at the 14th IUPAC Symposium on Photochemistry, Leuven, Belgium, July, 1992. rate of the ROH-BP system increases with an increase of proton c~ncentration.~~The mechanism of the proton-enhanced HT reaction of 3ROH* is interpreted as follows: protonation to 3(ROH- -)CO)* forms a protonated triplet exciplex, 3(ROH. * -)eOH)*, where intraexciplex electron transfer gives rise to a triplet radical pair, 3(ROH'+ + \eOH), which rapidly dissociates into RO', H+ and S(%H.'6 On the other hand, the proton effect on HT of the naphthylammonium ion (RNHl)-BP system differs from that of 3ROH*.It has been shown that HT from the triplet naphthylammonium ion (3RNH3f*) to BP proceeds via a triplet exciplex, 3(RNH,'. -. )CO)*, to produce the naphthyl amine radical cation (RNH;') and >cOH.35 The HT rate decreases markedly with an increase of proton concentration. This proton effect of suppressing the HT rate is interpreted by the mechanism in which protonation to 3(RNH,'... \FO)* forms a protonated triplet exciplex, 3(RNH,' -$OH)*, which rapidly decomposes into 3RNHl* + BP + H due to Coulombic repulsion.35 This demonstrates that + the effects of protons on the HT rate for the triplet naphtha- lene derivative-BP system depend on the substituent groups.On the basis of the above findings, the following questions arise. In the case of a triplet naphthalene derivative having both ammonium ion (-NH,') and hydroxy (-OH) groups, from which group does HT occur, and what is the reaction mechanism which includes proton effects on HT? In order to solve the above questions, the present study on HT from the triplet 5-hydroxy-1-naphthylammonium ion to benzophe-none was carried out by laser flash photolysis at 355 nm. Experimental 5-Aminonaphth- 1-01 (HORNH,) and benzophenone from Kanto Chemicals were purified twice by sublimation in uucuo. 534 Sulfuric acid (97%, Wako) was used without further purifi- cation. H2S04 was used as a proton source, since it is known that the counter-ion (SO:-) of H2S04 does not quench the triplet state of the molecules.39 Methanol (Spectrosol, Wako) was used as supplied.Deionized water was distilled. A meth-anol (Me0H)-water mixture (9: 1 v/v) was used as the solvent. The concentration of HORNH, was typically 8.0 x mol dm-3. Benzophenone was used as a triplet sensitizer in the concentration range 6.7 x 10-3-0.2 mol dm-3. The H2S04 concentrations used were in the range 0.015-1.0 mol dm-3. All samples were thoroughly degassed by means of freeze-pumpthaw cycles on a high-vacuum line in a quartz cell of 1 or 10 mm pathlength. The transient spectral data were obtained within a 10% error using fresh samples to avoid excessive exposure to laser pulses.Laser flash photoly- sis was carried out at 295 K. Absorption spectra were recorded on a Ubest-50 spectro- photometer from Jas. Co. A nanosecond Nd3+ : YAG laser system at 355 nm (JK Laser, HY-500;pulse width 8 ns, laser powder 70 mJ pulse-' at 355 nm) was used for sample excitation. The detection system has been reported else~here.~~ Results and Discussion Absorption Spectra in the Ground State The absorption spectra of HORNH, with [H2S04] = 0 and 1.5 x lo-, mol dm-3 in MeOH-water (9 :1 v/v) at 295 K are shown in Fig. l(a) and (b), respectively. Spectrum (b) is blue-shifted compared with spectrum (a),and shows no spec- tral change in the range 0.015 < [H,SO,J/mol dm-3 < 1.0. In the presence of protons, HORNH, is in the following equilibrium, HORNH, + H+eNORNHi (1) Therefore, spectrum (b) is assigned to be that of the 5-hydroxy-1 -naphthylammonium ion (HORNH;) in MeOH- water (9 :1 v/v) with [H,S04] 3 0.015 rnol dm-3.When benzophenone (BP) is added to a MeOH-water (9 :1 v/v) solution of HORNH;, the absorption spectrum of the HORNHi-BP system is identical to a superposition of those of HORNH; and BP at 295 K. Since the absorption spectrum of BP in MeOH-water (9: 1 v/v) exhibited no change in the range 0.015 < [H,SO,]/mol dm-3 < 1.0, it is concluded that no interaction between HORNH; and BP in the ground state occurs in MeOH-water (9 :1 v/v) under the concentrations of H,SO, employed at 295 K. 10 250 300 350 400wavelength/nm Fig.1 Absorption spectra of HORNH, (---) and HORNH; (-) in MeOH-water (9 : 1 v/v) at 295 K 1. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2.0 r I v) f-0 F2 1.0 I I 1A "0 1 2 3 [HORNH,+]/10-3rnol dm-3 Fig. 2 Plots of kobs us. [HORNH; J obtained by 355 nm laser pho- tolysis in the HORNHi-BP (6.7 x mol dm-3) systems with (a) 0.015 and (b) 0.5 mol dm-3 H2S0, in MeOH-water (9 :1 v/v) Since HORNH; has no absorbance at 355 nm, only BP is excited upon 355 nm laser excitation in the HORNHi-BP system. Triplet Energy Transfer from Triplet BP to HORNHf After the excited singlet state of BP ('BP*) is produced upon 355 nm laser excitation in the HORNHi-BP system, triplet benzophenone (3BP*) having triplet-triplet (T-T) absorption at 525 nm is formed via fast intersystem crossing (ca.10 ps)40.41 according to the El-Sayed rule.42 In the presence of HORNHi ,efficient triplet energy transfer (ET) occurs from 3BP* to HORNH; since the triplet energies of BP and HORNH; are known to be 69.243 and 58.3 kcal mol-',t respectively. In the present system, although hydrogen-atom abstraction (HA) of 3BP* from HORNH; is expected to occur in competition with ET, it is known that ET predomi-nates HA in polar media.44 The ET reaction from 3BP* to HORNH; was studied for the HORNH; (0-3.0 x mol dm-3)-BP (6.7 x mol dmh3) systems in MeOH-water (9 : 1 v/v) with 0.015 and 0.5 mol dm-3 H2S04. The first- order rates for the decay of 3BP* (k,Bbp,)observed at 525 nm are shown as a function of [HORNH;] for 0.015 and 0.5 mol dm-3 H2S04 in Fig.2(a) and (b), respectively. Since the plots ofk:!s us. [HORNH;] (f3.0 x mol dm-3) gave a straight line, quenching of 3BP* by HORNH; is demon- strated to follow a Stern-Volmer relationship. Therefore, kt:s is expressed as follows, kt:s= ktP+ kr[HORNHi] (1) where ktP and ky are the rate constants for the decay of 3BP* in the absence of HORNH; and the quenching of 3BP* by HORNH;, respectively. From the intercept and ~ t The triplet energy of HORNH; was determined from the phos- phorescence spectrum in a mixture of MeOH-water (9 :1 v/v) con-taining 0.015 mol dm-3 H,SO, at 77 K. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I I1 1 0.10 Q) c (I)e s9 (I) 0.05 0' ' L I I I 400 600 800 wavelengthfnm Fig.3 Time-resolved transient absorption spectra obtained by 355 nm laser photolysis at: 1, 80; 2, 200; 3, 350; and 4, 900 ns in the HORNH; (8.0 x mol dm-3)-BP (1.2 x mol dm-3)-H2S04 (0.015 mol dm-7 system in MeOH-water (9 :1 v/v) slope of the line, kEP and kr were found to be 3.9 x lo6 s-' and 5.8 x lo9 dm3 mol-' s-' for 0.015 mol dm-3 H2S04 and 7.4 x lo6 s-' and 3.0 x lo9 dm3 mol-' s-' for 0.5 mol dm-3 H2S04, respectively. Both values of k? for 0.015 and 0.5 mol dmP3 H2S04 are close to those of diffusion-controllcd processes, whereas ky at the former concentration is 1.9 times greater than k:' at the latter. As for ktP, the value for 0.5 mol dm-3 H2S04 is 1.9 times greater than that for 0.015 mol dm- H2S04. These results can be interpreted by considering the protonated 3BP* (3BPH+*), which has a small lifetime (ca.13 ns) due to the pK,* value of 3BP* (0.18).45-46 HT from Triplet HORNHZ to BP The ET reaction from 3BP* to HORNH: produces triplet HORNH; ,(3HORNH;*) in the nanosecond time region. In order to elucidate the deactivation processes of 3HORNH: * produced by ET, the transient absorption spectra in the microsecond time region were analysed. Fig. 3 and 4 show the transient absorption spectra observed after 355 nm laser photolyses in the HORNH; (8.0 x mol dm-3)-BP (1.2 x lop3mol dm-3) systems with 0.015 and 0.5 mol dme3 H2S04, respectively, in MeOH-water (9:l v/v). The transient absorption band at 430 nm observed at 70 ns in Fig.3 or at 150 ns in Fig. 4 after z laser pulse was ascribed to the T-T absorption spectrum of HORNH;, since it resembled the T-T absorption spectrum of naphth-1 -0134*36and the transient absorption spectrum which was quenched by introduction of air obtained by 266 nm laser photolysis of HORNH; in MeOH-water (9 :1 v/v) with 3 mol dm-3 H2S04. While the 430 nm band for 3HORNH3+* in both Fig. 3 and 4 decreases with isosbestic points at 490 and 610 nm, new transient absorption bands appear at 455, 545 and $00nm with time. The 545 nm band is known to be the benzophenone ketyl radical ( >eOH, with molar absorption coefficient, E = 3220 dm3 mol-' cm-' at 545 nm4'). The transient absorption bands at 455 and 800 nm are ascribable to the 5-hydroxy- 1-naphthylamine radical cation (HORNH;+) since they are similar to the absorption spectrum of HORNH;+ obtained by y-radiolysis of HORNH, in a PVC film.? Fig. 5 and 6 show the time traces of the transient absorb- ance changes at 430 nm for 3HORNHi*, 545 nm for )COH and 760 nm for HORNH;' after laser pulsing in the 7 Unpublished data.535 0.10 Q10 (I) 2% 0.05 0 400 600 800 wavelengt h/nm Fig. 4 Time-resolved transition absorption spectra obtained by 355 nm laser photolysis at: 1, 150 ns; 2, 700 ns; 3, 1.5 ps; and 4, 3.5 ps in the HORNH; (8.0 x mol dm-3)-BP (1.2 x mol dm-3)-H2S0, (0.5 mol dm-3) system in MeOH-water (9 : 1 v/v) HORNH: (8.0 x mol dm-3)-BP (1.2 x mol dm-3) system with 0.015 and 0.5 mol dm-3 H2S04, respec-tively, in MeOH-water (9 :1 v/v).The first-order rate con- stant (kobs) for the decay of 3HORNH3+* at 430 nm is almost identical with those for the increase of )tOH at 545 nm and of HORNH;+ at 760 nm within a 10% experimental error (kobs = 4.1 x lo6 s-' for 0.015 mol dm-3 H2S04 and 8.2 x lo5 s-' for 0.5 mol dm-3 H2S04). These results show that HT proceeds from 3HORNHl* to BP, resulting in the formation of HORNH;' and )eOH : bbr 3HORNH3+*+ >CO -HORNH;' + )tOH (2) 0.150 10.01 I I I I, (c)-0.1 -1--0.010.150 t 1O.OO' 0.075 1 ~I time/ps Fig. 5 Time traces of the absorbance changes for the transient species observed at 430 nm (3HORNH3+*) (a), 545 nm (>COH) (b) and 760 nm (HORNH;+) (c) obtained by 355 nm laser photolysis in the HORNH; (8.0 x mol dm-3)-BP (1.2 x mol dm-3)-H2S0, (0.015 mol dm-3) system in MeOH-water (9 :1 v/v).kobs= 4.2 x lo6 s-l (a),4.1 x lo6 sP1 (b),(c). 536 0.1 0.01 0.001 0.1 0.0 0.1 02 0.01 al 0.001 C m e 0.12D I I I I 0.1 0.01 0.001 0.1o.21 0.01 I I I I 0 4 8 time/ps Fig. 6 Time traces of the absorbance changes for the transient species observed at 430 nm (3HORNHl*) (a), 545 nm (>COH) (b) and 760 nm (HORNH;') (c) obtained by 355 nm laser photolysis in the HORNH; (8.0 x lop3 mol dm-')-BP (1.2 x lo-' mol dm-3)-HZS04 (0.5 rnol dm-3) system in MeOH-water (9 :1 v/v). kobs = 8.0 x lo5s-' (a), 8.2 x 10' s-l (b),(c). Therefore, this indicates that HT of 3HORNHi* occurs not from the -OH group but from -NH,f .Proton Effects on the Efficiency and Rate of HT from 3HORNHi* to BP In order to estimate the efficiency for HT from 3HORNHl* to BP, it is necessary to determine the molar absorption coef- ficients of the transient species. Fig. 7 shows the reference spectra of 3HORNHi*, >COH and HORNH;+. The molar absorption coefficient of HORNH;' can be readily determined to be 2200 dm3 mol- ' cm-' at 455 nm by comparison with that of >COH (3220 dm3 mol-' cm-' at 545 nm47), since HORNH;+ and 6 430 nm mzz nL 0 400 600 800 wavel engt h/n m Fig. 7 Reference absorption spectra of 3HORNH:*, >eOH and HORNH;'. See text for details. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 >eOH are produced by HT from 3HORNHi* to BP in a 1 :1 ratio.The molar absorption coefficient of 3HORNH,f* was estimated to be 5600 dm3 mol-' cm-' at 430 nm in MeOH-water (9 :1 v/v) on the assumption that the oscillator strength of the T-T absorption of HORNH; is the same as that of naphth-1-01 (7300 dm3 mol-' cm-' at 430 nm in acetonitrile,,), since they have isoelectronic structures. With the use of the molar absorption coefficients of 3HORNH3+*, >eOH and HORNH;' shown in Fig. 7, the efficiency (4HT)for HT from 3HORNH3+* to BP was deter- mined as follows. &,* is defined by eqn. (11). 4HT = A[ >COH]/A[3HORNHi*] = A[HORNH;+]/A[3HORNH,f*] (11) where A[ )cOH], ACHORNH;'] and AC3HORNH,f*] are the concentration changes of )eOH, HORNH;' and 3HORNH:* induced by HT, respectively.On the other hand, the absorbance change, AA at the monitored wave-length, A (430, 545 or 760 nm) can be written as, AA = E( )eOH)A[ >eOH] + &(HORNH;+)A[HORNH;+] -E(~HORNH,~')A[~HORNH~*] (111) where E( >eOH), &(HORNH;+) and d3HORNH:*) are the molar absorption coefficients of >COH, HORNH;+ and 3HORNH,f* at the wavelength, A, respectively. From eqn. (11)and (111), we obtain AA = { ~HT[E( )COH) + &(HORNH;+)]-E(~HORNH:*)} x AC3H0RNH,f*] (IV) With the use of the observed AA in Fig. 3 and 4, the E values in Fig. 7 and eqn. (IV), we obtained $HT values of 0.92 and 0.48 for the HORNH; (8.0 x mol dmP3)-BP (1.2 x mol dm-3) systems with 0.015 and 0.5 mol dm-3 H2S04, respectively, in MeOH-water (9 : 1 v/v).These $HT values indicate that increased proton concentrations reduce the efficiency for HT in the HORNHl-BP system. As men-tioned above, the kobs value obtained for the system with 0.015 mol dm-3 H2S04 is five times greater than that obtained for the 0.5 mol dmP3 H2S0, system, which indi- cates that HT in the HORNHl-BP systems is obviously sup- pressed by protons. These proton effects of suppressing both the efficiency and rate for HT have been reported for the RNH,f-BP system in a previous paper.35 In order to elucidate the effect of protons on HT from 3HORNHl* to BP, kobs for the HORNHl-BP system was measured at various acid concentrations. Fig. 8 shows plots of kobs at 430 nm as a function of [H2S0,] obtained by 355 nm laser photolysis in the HORNHl (8.0 x mol dm-3)-BP (1.5 x mol dm-3) system in MeOH-water (9 : 1 v/v).Although kobs decreases markedly with increase of [H,SO,], it asymptotically approaches a constant value at higher [H,SO,] (>0.3 mol dm-3). This levelling off at higher [H2S0,] cannot be accounted for simply by the mechanism for HT in the RNHi-BP system.35 The solid curve in Fig. 8 was calculated according to a mechanism proposed for the HORNHl-BP system, as will be described in the following section. BP Concentration Effect on HT Kinetic studies on kobs for the HORNHl-BP system at various [BPI were performed. Fig. 9 shows the plots of kobs at 430 nm for the decay of 3HORNHl* as a function of [BPI (d0.2 mol dm-3) at 0.015, 0.1 and 0.5 mol dmP3 H,SO,, J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3-I si --% 82-& 1-nl I1 I L 1 I I I “0 0.3 0.6 0.9 [H 2S04]/mot dm-Fig. 8 Decay rate (kobJ of 3HORNH:* as a function of [H,SO,] observed at 430 nm, obtained by 355 nm laser photolysis in the HORNH: (8.0 x mol dm-’)-BP (1.5 x lo-’ mol dm-3) system in MeOH-water (9 :1 v/v). The solid curve was calculated by eqn. (V). See text for details. respectively, obtained by laser photolysis in the HORNH: (8.0 x mol dm-3)-BP system in MeOH-water (9: 1 v/v). kobs increases considerably with an increase of [BPI at all [H2S04] used, but not linearly. In particular, a levelling off at higher [BPI (>0.05 mol dm-3) is observed at 0.015 mol dmd3 H,S04.Such non-linear quenching behaviour is typical of reactions via triplet exciplexes formed between triplet naphthalene derivatives and BP.35-3 Therefore, HT in the HORNHi-BP system is expected to proceed via a triplet exciplex, 3(HORNH,’ -. . )CO)*, as reported for the RNHl-BP system.35 However, we were unsuccessful in fitting the experimental kobs values in Fig. 9 with the same mechanism as that for the RNHi-BP system.35 The solid curves in Fig. 9 were calculated according to the proposed mechanism, as will be described in the following section. ReactionMechanism for HT from 3HORNH3+* to BP In a previous paper on HT of the naphthylammonium ion (RNHl)-BP system, it has been shown that the HT rate decreases markedly with an increase of acid concentration owing to Coulombic repulsion in the protonated triplet exci- r I v) I z5 00 0.1 0.2 [BP]/mol dm-3 Fig.9 Decay rate (koh) of ’HORNH:* as a function of [BPI observed at 430 nm obtained by 355 nm laser photolysis in the HORNH: (8.0 x mol dm-3)-BP systems with 0.015 (O),0.1 (A) and 0.5 mol dm-3 HISO, (0)in MeOH-water (9 : 1 v/v). The solid curves were calculated by eqn. (V). See text for details. plex, 3(RNH,’. . * )60H)*.35 Similarly, in the present HORNHl-BP system, the HT rate decreases significantly with increase of acid concentration. We consider that the suppression of the HT rate is affected by the -NH; group in both cases. However, the behaviour of the HT rate at higher acid concentrations differs between the two naphthyl-ammonium ions.kobs in the HORNHl-BP system approaches a constant asymptotically, while that in the RNHl-BP system simply decrease^.^' We consider that the difference in behaviour of the HT rates at higher acid concen- trations arises from the effect of the other substituent group (-OH) of HORNH;. It was previously reported that in the case of the naphthol (R0H)-BP system, the HT rate increases linearly with an increase of acid c~ncentration.~~ Therefore, we anticipate that the levelling off of kobs at higher acid concentration is due to cancellation of the opposing effects of both substituent groups (-NH; and -OH) on the HT rate. In order to explain the experimental results, we pro- posed a composite mechanism for the HORNHl-BP system, which contained elements of those for the RNHl-BP and ROH-BP systems, as shown in Scheme 1.Here, NH;3(R< OH . . >CO)* represents the triplet exciplex formed between 3HORNH,f* and BP with an equilibrium constant K,(=k,/k,), which undergoes an intraexciplex HT reaction to produce HORNH;’ and )COH with a rate constant (kHT). 3(HORNH,’. . -)tOH)* and ’(HlNROH. -->&OH)* are protonated triplet exciplexes with different configurations. The former is produced by protonation with a rate of k,[H+], and has a configuration favourable for decomposi- tion into 3HORNHi* + >CO + H+ due to Coulombic repulsion, with rate constant kdis. kdis was considered to be greater than k,[H+], since dissociation was rapid.This mechanism of decomposition of the protonated triplex exci- plex due to Coulombic repulsion is similar to that for the RNHi-BP The latter protonated triplex exciplex formed with an equilibrium constant K, (=k,/k-,) has a configuration that is less prone to decomposition. Here intraexciplex electron transfer occurs with a rate constant k,, to yield a triplet radical pair, 3[(H;NROH)” + >eOH], by the spin conservation rule. The hydroxynaphthylammonium ion radical cation (Hl NROH)” readily decomposes into HORNH;’ and H+, since the pK, of (H;NROH)*+ is con-sidered to be very negative. This mechanism involving intraexciplex electron transfer is similar to that for the ROH-BP system.36 The rate constants, k,, KO and Kh are for the non-radiative decay processes of 3(HORNH;)*, 3(R( 7;* >CO)* and 3(H,”ROH-.* )6OH)*.Here, we denote k,, = kb + k,, and k, = kg + k,,. According to the proposed mechanism, the decay rate of kobs can be formulated by eqn. (v). -1 k, + K,[BP] (1 + k&[HzS04]) kb kbx {1 + Kl[BP]( 1 + 5[H2S04]>’ Since the activity of H2S0, in MeOH-water (9 : 1 v/v) was unknown, we used k,[H+] = kd[H,S04] and K2[H+] = J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 HORNH3' + >CO k 3HORNH3*' + >CO 2'(Re::;------->COj kb 3[(HORNH3') .* + >;OH] \ / HORNHi* + Scheme 1 K;[H,SO,] in eqn. (V). When [H,SO,] + 0 and co,kobs in eqn. (V) approaches k,, and k,, respectively, as [BPI -,co.As shown in Fig. 9, both plots of kobs us.[BPI at 0.015 and 0.5 mol dmd3 H2S04 approach the same constant value (lo7 s-') as [BPI + co. Therefore, we assumed k,, = k, = lo7 s-'. k, was found to be 5.0 x lo5 sC1 by 266 nm laser pho- tolysis in an MeOH-water (9 : 1 v/v) solution of HORNH; with 3 mol dm-3 H,SO,. With the use of these values of k,, k,, and k,, we employed the best-fit method to our kobs results shown in Fig. 8 and 9, and obtained K, = 5 x lo2, K; = 3 and kdkb = 6 x lo2 dm3 mol-'. Using the deter- mined values of k, , k,, , k,, K,,K; and kd/kb,the calculated values of kobs are expressed as the solid curves in Fig. 8 and 9. Note that the calculated values of kobsfollow the experimen- tal ones very well (Fig. 8 and 9). Therefore, we conclude that the proposed mechanism shown in Scheme 1 can be applic- able to HT for the HORNHZ-BP system.The efliciencies (+HT) for HT from 3HORNH;* to BP were experimentally determined as 0.92 and 0.48 for the HORNH; (8.0 x lob3 mol dm-3)-BP (1.12 x mol dm-3) systems in the presence of 0.015 and 0.5 mol dm-3 H2S04, respectively, as described above. On the other hand, $HT can be obtained according to Scheme 1 as follows: Using eqn. (VI) and the values of k,, k,, , k , K,,K; , kdkb and $HT, we obtained kH, w k,, and k,, x E,. These results indicate that once the triplet exciplex and the protonated triplex exciplex are established in equilibria, the intraexciplex HT and electron-transfer reactons occur to yield HORNH;' and )COH very efficiently. In Scheme 1, we proposed protonated triplex exciplexes with two configurations.The triplex exciplexes are considered to have a weak charge-transfer interaction since their absorp- tion spectra are similar to those of the normal triplet aro- andmatic c~mpounds,~~-~~ are revealed to have NH'sandwich-like str~ctures.~~ Since 3(R< od * )CO)* is also considered to be formed by a weak charge-transfer inter- action with a loose sandwich-like structure, it would be pos-sible in the present system for triplet exciplexes, such as 3(HORNH,'. -* )CO)* and 3(H,'NROH. -)CO)*, to alter their configurations in the loose sandwich-like structures. That is, in the former, the carbonyl group and -NH; may be situated at the same side, and in the latter, at the opposite side. Consequently, when protons attack \he configuration 3(HORNH,'.--)CO)*, 3(HORNH;. ->COH)* is pro-duced, resulting in rapid decomposition due to Coulombic repulsion. On the other hand, 3(H;NROH... )CO)* is attacked by protons to 5! roduce the protonated triplex exci- plex, 3(H;NROH... )COH)*, which gives rise to the intraexciplex electron-transfer reaction, as illustrated in Scheme 1. In the present study, it is revealed that the hydrogen atom of -NH; is more reactive than that of -OH for HT from 3HORNH,'* to BP. The difference in the HT reactivity between -NH; and -OH groups can be understood by considering the electronic structure of the triplex exciplex. The triplet exciplex has been reported to be formed by a weak charge-transfer intera~tion,~,-~~ which means that the BP site is comparatively electron-rich owing to a slight elec- tron migration from 3HORNH;* to BP.On the other hand, the hydrogen atom of -NH; is relatively more protic than that of -OH. Therefore, the protic hydrogen atom prefer- entially transfers to the electron-rich carbonyl group of BP due to Coulombic attraction. It can be said that the more protic hydrogen atom is the more reactive in HT via triplet exciplexes. Concluding Remarks It has been shown by 355 nm laser flash photolysis in the HORNHZ-BP system at 295 K that HT from 3HORNH3+* to BP occurs from the -NH; group to yield HORNH;' J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 539 and )COH. In other words, the more protic hydrogen atom is the more reactive in HT uia triplet exciplexes.A novel HT mechanism for the present HORNHi-BP system is proposed by combining those for the RNHi-BP and ROH-BP systems, as illustrated in Scheme 1, in order to account for the proton effect on the HT rate which signifi- 22 23 24 N. J. Turro, Modern Molecular Photochemistry, Benjamin/ Cummings, Menlo Park, 1978. S. G. Cohen, A. Parola and G. H. Parsons Jr., Chem. Rev., 1973, 73, 141. S. J. Formoshinho, J. Chem. SOC., Faraduy Trans. 2, 1976, 72, 1913; 1978,74, 1978; S. J. Formoshinho and L. G. Amout, Adu. 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