320
J. Phys. Chem. 1985,89, 320-326
Excited-State and Ground-State Intramolecular Proton-Transfer Reactions of 6-(2-Hydroxy-5-methylphenyl)-s-triazines in Poly(methy1 methacrylate)' Haruo Shizuka,* Masanori Machii, Yasunori Higaki, Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan
Masanao Tanaka, and Ikuzo Tanaka* Department of Chemistry, Tokyo Institute of Technology, Meguro- ku, Ohokayama, Tokyo 152, Japan (Received: August 29, 1984)
Excited-state and ground-state intramolecular proton-transfer reactions of the title compounds in poly(methy1methacrylate) (PMMA) have been studied by means of fluorometry and nanosecond and picosecond (streak camera) spectroscopy. The excited-state proton-transfer km occurs very rapidly (>2 X 10" S-I) at 77 and 300 K and the rate constant is estimated to be 2 X 10l2s-l from that of the nonradiative deactivation kd competitive with kpy. The km process has no potential barrier, while the kd process is temperature dependent. The fast kd process Seems to be the internal conversion in the slstate associated with the out-of-plane bending vibration of the hydroxy group, since the activation energies (several hundred cm-I) for kd are related to the strength of the intramolecular hydrogen bonding of the triazines. The absorption spectra due to the ground state of the tautomer So'[A, = 470 (00),490 (ON), -494 (NN) nm] are observed, which are assigned to the SI' So'electronic transitions. The ground-state proton-transfer kpTofrom So'to So (starting material) occurs relatively slowly (-lo3 s-l) at room temperature. The kmo process proceeds across a potential barrier [(00):1.8 kcal mol-'; (ON): 5.1 kcal mol-'; (NN): 10.5kcal mol-'] depending upon the strength of the hydrogen bonding. The nature of the proton-transferred species SI' is discussed with the aid of the usual MO calculations.
+
Since the original work of Weller2 (1956) on the excited state intramolecular proton-transfer reaction of methyl salicylate (MS), a large number of studies on the excited-state proton-transfer reactions of MS and related compound^,^-^^ salicylamides and 6-(2related compound^,^^-^^ o-hydroxybenz~phenones,~~-~~ ~
(1) This work was supported by a Scientific Research Grant-in-Aid (no. 58470001) from the Ministry of Education of Japan. (2) Weller, A. 2.Elektrochem. 1956,60, 1144. (3) Beens, H.; Grellmann, K. H.; Gurr, M.; Weller, A. Discuss. Faraday SOC.1965, 39, 183. (4) Hirota. K. 2.Phvs. Chem. I Wiesbaden) 1962. 35. 222. (Sj Klapffer, W.; Nhndorf, G. j . Lumin. 1974,8,457.' Klpffer, W. Adu. Photochem. 1977, 10, 31 1. (6) Catalan, J.; FernHndez-alonso, J. I. J. Mol. Struct. 1975, 27, 59. 17) Kosower. E. M.; Dodiuk, H. J . Lumin. 1975/1976 11. 249. (8) Sandros,'K. Acta Chem.-Scand., Ser. A 1976, 30, 761. (9) Smith, K. K.; Kaufmann, K. J. J . Phys. Chem. 1978.82, 2286. (10) Goodman, J.; Brus, L. E. J . Am. Chem. SOC.1978, 100, 7472. (11) Klwffer, W.; Kaufmann, G. J. Lumin. 1979, 20, 283. (12) Ford, D.; Thistlethwaite, P. J.; Woolfe, G. J. Chem. Phys. Lett. 1980, 69, 246. (13) Acuba, A. U.; Amat, F.; Catalan, J.; Gofizalez Tablas, F. J . Phys. Chem. 1980,84,629. Acuba, A. U.; Catalan, J.; Taribio, F. J . Phys. Chem. 1981, 85, 241. (14) Smith, K. K.; Kaufmann, K. J. J. Phys. Chem. 1981, 85, 2895. (15) Heimbrook, L. A.; Kenny, J. E.; Kohler, B. E.; Scott, G. W. J . Chem. Phys. 1981, 75, 5201. (16) Lopez-Delad, J.; Lazare, S . J . Phys. Chem., 1981, 85, 763. (17) Catalin, J.; Taribio, F.; Acuba, A. U. J. Phys. Chem. 1982,86, 303. (18) Toribio, F.; Catalan, J.; Amat, F.; Acuna, A. U. J. Phys. Chem. 1983, 87, 817. (19) Felker, P. M.; Lambert, Wm. R.; Zewail, A. H. J . Chem. Phys. 1982, 77, 1603. (20) Nagaoka, S.; Hirota, N.; Sumitani, M.; Yoshihara, K. J . Am. Chem. SOC.1983,105, 4220. (21) Schulman, S. G.; Underberg, W. J. M. Photochem. Photobiol. 1979, 29, 937. (22) Barbara, P. F.; Rentzepis, P. M.; Brus, L. E. J . Am. Chem. Soc. 1980, 102, 2786. (23) Woolfe, G. J.; Thistlethwaite, P. J. J . Am. Chem. SOC.1980, 102, 6917. (24) Beckett, A.; Porter, G. Trans. Faraday SOC.1963, 59, 2051. (25) Lamola, A. A.; Scharp, L. J. J . Phys. Chem. 1966, 70, 2634. (26) Otterstedt, J. E. A. J. Chem. Phys. 1973, 58, 5716. (27) KIBpffer, W. J. Polym. Sei, 1976, 57, 205. (28) Nurmukametov, R. N.; Betin, 0. I.; Shigorin, D. N. Dokl. Phys. Chem. (Engl. Trans.) 1977, 234. 590. (29) Hou, S.-Y.;Hetherington 111, W. M.; Korenowski, G.M.; Eisenthal, K. B. Chem. Phys. Lett. 1979, 68, 282.
0022-3654/85/2089-032OSOl SO10
hydroxy-5-methylphenyl)-~-triazines,~~-~~ o-hydroxyphenylb e n z o t r i a ~ o l e s , ~ ~o-hydroxyphenylbenzoxazoles,45~46 ~~~*~~-~~ ohydroxyphenylbenz~tiazole,~~~~~ and others26,4X,49 have been reported. Great attention to some compounds, such as ohydroxybenzophenones and o-hydroxyphenylbenzotriazoles,is given as polymer photostabilizers. Anomalous emissions with large Stokes shifts of these compounds are ascribed to those of the excited-state proton-transferred species. Recently, the kinetics and mechanism of the excited-state proton-transfer reactions have been extensively studies by means of picosecond spectr~scopy?.20,223,29,36,37,42,43,~,47 and the following facts have been revealed: (1) Excited-state intramolecular pro(30) Merritt, C.; Scott, G. w.; Yavrouian, A. Chem Phys. Lett. 1980,69, 169. (31) Gupta, A.; Yavrouian, A.; DiStefano, S.; Merritt, C. D.; Scott, G. W. Macromolecules 1980, 13, 821. (32) Huston, A. L.; Merritt, C. D.; Scott, G. W.; Gupta, A. In "Picosecond
Phenomena 11"; Hochstrasser, R. M., Kaiser, W., Shank, C. V., Eds.; Springer-Verlag: West Berlin, 1980; p 232. (33) Gupta, A,; Scott, G. W.; Kliger, D. ACS Symp. Ser. 1980, No. 151. (34) Scaiano, J. C. Chem. Phys. Lett. 1982, 92, 97. (35) Shizuka, H.; Matsui, K.; Okamura, T.; Tanaka, I. J . Phys. Chem. 1975, 79, 273 1. (36) Shizuka, H.; Matsui, K.; Hirata, Y.; Tanaka, I. J . Phys. Chem. 1976, 80, 2070. (37) Shizuka, H.; Matsui, K.; Hirata, Y.; Tanaka, I. J . Phys. Chem. 1977, 81, 2243. (38) Merrill, J. R.; Bennett, R. G. J . Chem. Phys. 1965, 43, 1410. (39) Werner, T. J. Phys. Chem. 1979, 83, 320. (40) Werner, T.; Whsner, G.; Kramer, H. E. A. ACS Symp. Ser. 1980, No. 151. (41) Werner, T.; Kramer, H. E. A. Eur. Polym. J . 1977, 13, 501. (42) Huston, A. L.; Scott, G. W.; Gupta, A. J . Chem. Phys. 1982, 76, 4978. (43) Flom, S. R.; Barbara, P. F. Chem. Phys. Lett. 1983, 94, 488. (44) Ding, K.; Courtney, S. J.; Strandjord, A. J.; Flom, S.; Friedrich, D.; Barbara, P. F. J . Phys. Chem. 1983, 87, 1184. (45) Mordzinski, A.; Grabowska, A. Chem. Phys. Lett. 1982, 90, 122. (46) Woolfe, G. J.; Melzig, M.; Schneider, S.; DBrr, F. Chem. Phys. 1983, 77, 213. (47) Barbara, P. F.; Brus, L. E.; Rentzepis, P. M. J. Am. Chem. Soc. 1980, 102, 5631. (48) Rossetti, R.; Raybord, R.; Haddon, R. C.; Brus, L. E. J . Am. Chem. SOC.1981,103,4303. Rossetti, R.; Haddon, R. C.; Brus, L. E. J . Am. Chem. SOC.1980, 102, 6914. (49) Bulska, H. Chem. Phys. Lett. 1983, 98, 328.
0 1985 American Chemical Society
Intramolecular Proton-Transfer Reactions
(00)
(ON) (NN) Figure 1. 6-(2-Hydroxy-5-methylphenyl)-s-triazines.
ton-transfer kPToccurs very rapidly ( > l o ” s-l) even at 4 K; (2) in most cases, the excited-state proton transfer has no potential barrier; (3) the isotope effect on kPTis scarcely observed, (4)there is a temperature-dependent nonradiative deactivation process kd competitive with the excited-state proton transfer. The excited-state proton transfer of 3-hydroxyflavone (3-HF) shows interesting features. Kasha’s g r o ~ p ~have ~ * shown ~ * that the excited-state proton transfer of 3-HF proceeds across a potential barrier in the double-minimum potential. In this excited-state proton transfer, the barrier height for the tautomerimtion was suggested to be dependent upon the viscosity of the solvent media, the origin of which was ascribed to the torsional motion of the phenyl group about its junction axis to the y-pyrone ring system. Therefore, the mechanism for the excited-state intramolecular proton-transfer reaction of 3-HF is different from that of the intramolecularly hydrogen-bonded compounds as described above. Further detailed kinetics in the excited state of 3-HF have been studied e x t e n s i ~ e l y . ~ ~Very - ~ ~ recently, Itoh’s reported that the ground-state proton transfer from the tautomer of 3-HF occurs with the lifetime 2.9 ps by measurements of a = 440 nm) of the tautomer. However, transient spectrum (A, little attention to the ground-state intramolecular proton transfer of the hydrogen-bonded compounds has yet been given. The excited-state and ground-state intramolecular proton-transfer reactions are very important processes not only in chemistry but also in industrial uses as photofunctional compounds. We report here the excited-state and ground-state intramolecular proton-transfer reactions of 6-(2-hydroxy-5-methylphenyl)-s-triazines in poly(methy1 methacrylate) (PMMA) by means of steady-state fluorometry and nanosecond and picosecond (streak camera) spectroscopy. The compounds are strong intramolecularly hydrogen-bonded molecules (that is, they have entirely “closed-ring” structures). The excited-state proton transfer of the triazine derivatives occurs effectively upon excitation to give green fluorescence with a large Stokes shift (- 10 X lo3 cm-I) and no normal emission of the compounds is observed, indicating - ~ ’PMMA sample that there is no “open-ring” c o n f ~ r m e r . ~ ~The made it possible to keep the volume change due to temperatures extremely small and to measure the precise fluorescence intensities at various temperatures. Furthermore, it was expected that molecular distortions in the excited states of the starting material (enol form) and the tautomer (keto form) might be avoided, revealing a typical mechanism for the excited-state and groundstate intramolecular proton-transfer reactions. We also discuss the nature of proton-transferred species whether it is a tautomer (keto form) or a zwitterion with the aid of the usual M O method. Experimental Section Sample Preparation. 6-(2-Hydroxy-5-methylphenyl)-s-triazines were prepared by the photochemical rearrangements of the corresponding aryloxy-s-triazines61 and were purified by recrystal-
The Journal of Physical Chemistry, Vol. 89, No. 2, 1985 321 lizations from ligroin. The samples are shown in Figure 1. Methyl methacrylate (Tokyo Kasei) was washed for several hours with 2 M N a O H to remove the quinol stabilizer. The aqueous layer was removed, and the methyl methacrylate washed several times with distilled water and dried over anhydrous sodium sulfate. The sample triazine in methyl methacrylate (- lo4 M) was polymerized at 60-70 O C for 14 h after addition of 3 mg of azobis(isobutyronitrile) to the 25-mL solution, and then the solid sample was annealed at 110 OC for several hours before being cut and polished into appropriate shapes.62 Absorption and Emission Spectra. The absorption spectra were measured with a Hitachi 200 spectrophotometer. The fluorescence spectra were recorded with Hitachi M P F 2A and 850 spectrophotometers. The spectral corrections were made. Nanosecond Experiments. The fluorescence lifetimes were measured with a Hitachi nanosecond time-resolved spectrophotometer (pulse width 11 ns), and the convolution method was applied.63 The transient absorption measurements were taken with a nanosecond N 2 laser system (Japan Dynamic JS- 1OOOL; pulse width 5 ns; laser power 5 mJ) with a transient memory (Kawasaki Electronica, MR-50E).64 Picosecond Measurements. Picosecond measurements were carried out with a mode-locked Nd3+ glass laser (the third harmonic 351 nm, pulse width 5 ps). The third harmonic of the Nd3+ glass laser (351 nm) was used as the exciting light pulse, and the fluorescence buildup was time resolved by a transient analyzer (streak camera, HTV C1370), a SIT camera (HTV ClOOO-18), a transient analyzer (HTV C1098), and a personal computer (Sharp MZ-8OC). Details have been described e l s e ~ h e r e .The ~~ temperature of the sample was controlled by an Oxford DN-704 cryostat. Method of Calculations Semiempirical SCF MO CZ Method. The electronic structures of 6-(2-hydroxy-5-methylphenyl)-s-triazineswere studied by the variable & y procedure of the semiempirical S C F M O method combined with a singly excited CI calculation. The parameters were taken as proposed by Nishimoto and Forster.66 The onecenter electron-repulsion integrals y,, were estimated from the corresponding valence-state ionization potentials (I,) and electron Z$ and A,, affinities (A,) by the Pariser-Parr appr~ximation,~’ being determined from spectroscopic data by using the promotion energies of Hinze and JaffES6* The two-center electron-repulsion integrals yruwere estimated by the use of the NishimoteMataga a p p r o x i m a t ~ o n . ~The ~ two-center core resonance integrals prU were evaluated by the Nishimoto-Forster approximation.66 The C-C, C-0, C-N, and C-N(CH3)2 bond lengths were assumed to be 1.39, 1.36, 1.35, and 1.38 A, respectively. All bond angles were assumed to be 120’. The calculations were carried out with a HITAC 8800/8700 located at the Computer Center of the University of Tokyo. Results and Discussion Absorption and Emission Spectra of Triazine Derivatives. Absorption and fluorescence spectra of 6-(2-hydroxy-5-methylphenyl)-s-triazines [(00),(ON), and (NN)] in poly(methy1 methacrylate) (PMMA) at 293 K are shown in Figure 2. The normal fluorescence could not be observed, but only the green
(50) Sengupta, P. K.; Kasha, M. Chem. Phys. Lett. 1979,68, 382.
(51) Taylor, C. A.; El-Bayoumi, M. A.; Kasha, M. Proc. Narl. Acad. Sci. U.S.A. 1969, 63, 253. (52) Woofe, G. J.; Thistlewaite, P. J. J . Am. Chem. SOC.1981, 103, 6916. (53) Itoh, M.; Tokumura, K.; Tanimoto, Y.; Okada,Y.; Takeuchi, H.; Obi, K.;’Tanaka, I. J . Am. Chem. SOC.1982, 104,4146. (54) Salman, 0. A.; Drickamer, H. G. J . Chem. Phys. 1981, 75, 572. (55) Itoh, M.; Kurosawa, H. Chem. Phys. Lett. 1982, 91, 487. (56) Itoh, M.; Tanimoto, Y.; Tokumura, K. J. Am. Chem. Soc. 1983, 105, 3339. (57) Strandjord, A. J. G.; Courtney, S.H.; Frierich, D. M.; Barbara, P. F. J . Phys. Chem. 1983,87, 1125. (58) Strandjord, A. J. G.; Barbara, P. F. Chem. Phys. Lett. 1983,98,21. (59) Wolfbeis, 0. S.;Knierzinger, A.; Schipfer, R. J . Photochem. 1983, 21, 67. (60) Tanimoto, Y.; Itoh, M. Chem. Phys. Lett. 1981, 83, 626.
(61) Shizuka, H.; Kanai, T.; Morita, T.; Ohoto, Y.; Matsui, K. Tetrahedron 1971, 27,4021. (62) El-Sayed, F. E.; MacCallum, J. R.; Pomery, P. J.; Shepherd, T. M. J . Chem. SOC.,Faraday Trans. 2 1979, 75, 79. (63) Shizuka, H.; Tobita, S . J . Am. Chem. SOC.1982, 104, 6919. (64) Shizuka, H.; Fukushima, K. Chem. Phys. Lett. 1983, 101, 598. (65) Shizuka, H.; Tsutsumi, K.; Takeuchi, H.; Tanaka, I. Chem. Phys. 1981, 59, 183. (66) Nishimoto, K.; Forster, L. S. Theor. Chim. Acta 1965, 3, 407. (67) Pariser, R.; Parr, R. G. J . Chem. Phys. 1953, 21, 446. Parr, R. G. “Ouantum Theory of Molecular Electronic Structure”; W. A. Benjamin: New Y&k, 1964. (68) Hinze, J.; Jaff6, H. H. J . Am. Chem. SOC.1962.84, 540. (69) Nishimoto, K.; Mataga, N. Z. Phys. Chem. (Wiesbaden) 1957, 12, 335; 1957, 13, 140. ~
Shizuka et al.
322 The Journal of Physical Chemistry, Vol. 89, No. 2, 1985
TABLE I: Absorption (As’* and AS’’*) and Fluorescence ( A L ) Maxima, Stokes Shifts (AD), Transition Energies (Us’-% and AES,,-), and of 6-(2-Hydroxy-5-methyipbenyl)-s -triazines in PMMA Oscillator Strengths ($$$%d AEs -so/ eV AEs,.-so,/eV
,
compd
AkTso/nm
obsd
calcdb
pAGs06
Afmax/nm
As/(lO’ cm-l)
(00)
343 336 328
3.61 3.69 3.78
4.169 4.188c 4.207
0.639 0.655 0.554
530 510 497
10.29
(ON) “)
10.l5 10.37
A~~~s;-SO”/nm obsd 470 490 494
2.64 2.43 2.49
-
calcd6
fdizsdb
2.914 2.919c 2.925
0.588 O.S9lc 0.593
“The peaks of the transient absorbances observed by nanosecond N2 layer photolysis, which correspond to the SI’ Sd transitions. bThe symbols obsd and calcd mean the observed and calculated data (by the usual SCF MO CI method), respectively. cThe values were obtained by averaging the calculated data of (ON) and (NO) as shown in the text. Wavelength
600
500
400
I nm
350
SO
Figure 3. Schematic energy-state diagram for the excited- and groundstate intramolecular proton-transfer reactions of the triazines. For details Figure 2. Absorption and fluorescence spectra of the triazine --rivatives
[(00), (ON), and (NN)] in PMMA. The transient absorption spectra at the delay time 50 ps are shown as dotted lines, which were observed by nanosecond N, laser photolysis of the triazines. For details see text.
emission with the maximum at 530 (OO), 510 (ON), or 497 (NN) nm. The green fluorescence with a large Stokes shift (- 10 X lo3 cm-I) is assigned to that of the corresponding intramolecular proton-transferred species (excited keto form, SI’)of the triazine derivative^.^^-^^ The excitation spectra were very similar to the corresponding absorption spectra of the starting materials. The lack of normal fluorescence suggests that (1) the lifetime of the fluorescent state SI (excited enol form) of the starting material ~ the negative free energy change is extremely short ( 77 K and T = 77 K, respectively, can be expressed as
1
6 U)
c
3 4 U
11 Q ‘
2
2 00
100
300
T/K F i 5. (a) Temperaturedependence of the green fluorescence intensity ratio I’/Id in PMMA (where I& is the value at 77 K). (b) Fluorescence lifetime T’ of the excited tautomer in PMMA as a function of temperature T.
-
the buildup curve of the S,,’ SI’a b ~ o r p t i o n .However, ~~ this was not so correct. The picosecond emission measurements by means of a streak camera method are much more accurate than those of the transient absorption. In addition, the transient S,,’ S l f absorption was masked by the intense green emission, making the rate slow. Similarly, fast intramolecular protontransfer reactions in the excited states of methyl salicylateg and are known (km > 1 X 10” SI). other compounds20~22J3~29~4~43~~~47 It can be said that in general the value of kpTin the excited state of the closed-ring conformer is greater than 1 X IO” s-l. Fluorescence Intensities I’ and Fluorescence Lifetimes 7; of the SIfState (Excited Tautomer) at Various Temperatures. The excited-state proton-transfer rate k , is very rapid (>2 X 10” s-l), which is beyond the limitation of our picosecond apparatus as described in the above section. In order to estimate the k , value, measurements of the fluorescence intensity I’ and the lifetime 7; of Sl’ at various temperatures were carried out. Figure 5 shows the plots of (a) Z’/Zd vs. temperature T and (b) 7; vs. T i n PMMA. The fluorescence intensity I‘of S1’ increased with
-
+
*,,’ = 4okfTm’
t 2)
where 4 [=km(km kd)-l] denotes the efficiency for the excited-state proton-transfer reaction and 4ois equal to unity at 77 K since k , > kd at 77 K. From eq 1 and 2, eq 3 is obtained. (3)
On the assumptions that the Arrhenius relations for kpTand kd may hold, eq 4 can be derived from eq 3 Ad
I‘
7”
Am
md-MF’T
2.303RT
(4)
where Am and Ad represent the frequency factors for k , and kd, respectively, AEm and hEd the activation energies for k , and kd, respectively, and AE, = 0 since the excited-state proton transfer proceeds across no potential barrier as stated above. The plot of the log [I,,’r{(Ifrmf)-I - 11 as a function of TI is shown in Figure 6 . The straight lines indicate that eq 4 fairly holds in can the present system. The kinetic parameters Ad/& and be obtained from Figure 6,whose data are listed in Table 11. The value of the activation energy for the nonradiative deactivation
The Journal of Physical Chemistry, Vol. 89, No. 2, 1985
324
Shizuka et al.
TABLE II: Frequency Factors ( A and A a) and Activation Energies (hE," and pEd)for the Excited-State Intramolecular Proton-Transfer Reaction of the Triazines in PMMA
'HNMR'
AEdb/(kCal
compd
(00) (OW
"(1
Ad/AF7 8.1 7.8 7.7
mol-' cm-I) 1.15 1.58 1.65
402 525 577
PK,f 11.71 11.94 12.23
I \ 951
*
\uv,
\
k
'OH
-2.31 -2.92 -3.13
-1.8
process kd competitive with kPT increases with increasing the strength of intramolecular hydrogen bonding of the starting materials. We have originally found that the fast kd process is competitive with the excited-state proton-transfer-reaction kpy.35 The AEd values of (NN), (ON) and (00)in PMMA were determined to be l .65, l&, and l . l 5 kcal mol-', respectively. The values agree with the order of the strength of the intramolecular hydrogen bond of the starting materials [(NN) > (ON) > (OO)]. As a result, the kd values are in the order (00) > (ON) > (NN). The intramolecular hydrogen bonding plays an important role for the excited-state intramolecular proton-transfer kpy. Rapid nonradiative deactivation kd in the SI state has been also noted in the cases of o-hydroxybenzaldehyde (OHBA),20 o-hydroxyand oacetophenone (OHAP),Zo o-hydroxyben~ophenone,~~ hydroxyphenylbenzotriazoles (AHBT).42 We have tentatively reported that the rapid internal conversion (IC) of the SI state is associated with the out-of-plane bending vibration of the hydroxy group of triazine^.^' Huston et aL4*suggested the importance of he rapid proton-transfer tautomerization between the vibrationally unrelaxed SI and SI'states with the internal conversion of the SI state of the triazoles. For triazines and triazoles, the SI state is the '(a,**)state, while the SIstate of OHBA or OHAP seems to be '(n,a*).20 Kasha7* suggested that the rapid internal conversion due to the distortions of OHBA or OHAP may be caused by the pseudo-Jahn-Teller effect, since the '(n,a*) and '(n,a*)states are close to each other. For the aromatic carbonyl compounds (OHBA and OHAP), Memtt et aL30considered that the radiationless deactivation other than kPTmight be due to the rapid intersystem crossing process (isc) from SI to the triplet state. This may be plausible since the isc processes in the carbonyl compounds are known to be very rapid.73-76 If the isc process takes place together with the kpy process, the phosphorescence from the triplet state may be expected in a rigid matrix at 77 K. No phosphorescence has been reported for such hydrogen-bonded ketones.20 In order to dissolve this contradiction, a possible explanation in which the triplet-state proton transfer may occur and no phosphorescence is given has been made by Nagaoka et aLzo It is noteworthy that the values of A.& are of the order of several hundred cm-l and the A& values are closely related to the strength of the intramolecular hydrogen bonding of the triazines. The strong hydrogen bonding in the triazine ( N N ) increases the h E d value, and a high efficiency for the SI' (tautomer, keto form) formation is yielded. This finding suggests that the out-of-plane bending vibration in the SI state is responsible for the rapid radiationless decay process kd. For the present system it is very likely that the rapid nonradiative decay kd competitive with kPT is not due to intersystem crossing, but internal conversion (IC) to the ground state So of the starting material, since no phosphorescence was observed in the present work. The SI state associated with the out-of-plane bending vibration may couple (71) Shizuka, H.; Matsui, K.; Tanaka, M.; Tanaka, I. Koen YoshishuBunshi Kozo Sogo Toronaki 1981, 540. (72) Kasha, M., quoted in ref 20. (73) Damschen, D. E.; Merritt, C. D.; Perry, D. L.; Scott, G . W.; Talky, L. D. J . Phys. Chem. 1978,82, 2268. (74) Anderson, Jr., R. W., Hoschstrasser, R. M.; Lutz, H.; Scott, G.W. J . Chem. Phys. 1974,61, 2500. (75) Anderson, Jr., R. W.; Hochstrasser, R. M.; Lutz, H.; Scott, G. W. Chem. Phys. Lett. 1978, 28, 153. (76) Turro, N. J. "Modern Molecular Photochemistry";W. A. Benjamin: Menlo Park, CA, 1978.
-2.0
Q:
-2.2
-0
cn
"The excited-state proton transfer proceeds with no potential barrier (AEm = 0). bErrors within f5%. 'Taken from the data of ref 70.
-1.8
I""
I ---/ 0
Y2 0.2 0.4 0.6 Time I m s
Figure 7. Time traces of the transmission (%) at 460 nm and 293 K in PMMA measured by nanosecond N2laser photolysis and the first-order kinetics of the plot of log A (A denotes the absorbance) vs. time.
with the vibrationally excited So state according to Hermi's golden r ~ l e , ' ~ - ~resulting O in rapid IC. If we assume that the value of Ad is the frequency of the out-of-plane bending motion, eq 5 may hold Ad = (I\Ed)(C)N 1.5 x i o i 3S-'
(5)
where c represents the velocity of light (3 X 10" cm s-I) and h E d = 500 cm-I. The excited-state proton-transfer rate constant kPT can be estimated approximately from eq 5 and the ratio &/APT (=8) in Table 11. k p ~ APT= Ad/8 = 2 x 10"
S-I
(6)
The reason that the lack of normal fluorescence of SI was observed is mainly due to the large value of kPT(-2 X 10l2s-'). The kpT value is constant regardless of temperature, whereas the value of k d is negligibly small at 77 K compared to that of kpy, and kd becomes appreciable at higher temperatures (> 100 K). Ground-State Proton- Transfer-Reaction k,, from S,' (Keto Form) to So (Enol Form). After the kPT process, vibrational relaxation in the S1' state having an excess vibrational energy occurs rapidly to yield the vibrationally relaxed fluorescent state SI' in PMMA. Subsequently, the S', state (ground keto form) is produced through the radiative k / or nonradiative kJ process as shown in Figure 3. The lifetime T / of the SI'state of (NN) is constant (5.7 ns). However, the values of T [ for (00)and (ON) decrease with increasing temperature, indicating the presence of activation energies for the nonradiative processes k i from SI' to So'. The activation energies for kd' are estimated to be 0 (NN), 1.2 (ON), and 2.6 (00)kcal mol-' from Figure 5b, on the assumption that the k / is constant at various temperatures. The ground-state proton-transfer kpTotakes place from the vibrationally relaxed S,' state (ground tautomer, keto form) to the ground state So of the starting material. The transient absorption spectra of So'were observed by means of nanosecond N, laser flash photolysis. The absorption spectra are shown in Figure 2, which is assigned to the S1' S,' transition since a mirror-image relationship between the transient absorption and green fluorescence spectra exists. The passibility that the transient spectra are due to the triplet-triplet transition is unlikely since no phos-
-
(77) Birks, J. B. 'Photophysics of Aromatic Molecules";Wiley: London, 1970. (78) Jortner, J.; Rice, S. A.; Hochstrasser, R. M. Ado. Photochem. 1969, 7, 149. (79) Henry, B. R.; Siebrand, W. In 'Organic Molecular Photophysics"; Birks, J. B., Ed.; Wiley: London, 1973; Vol. 1 , Chapter 4. (80) Rice, S. A. In 'Excited States"; Lim, E. C., Ed.; Academic Press: New York, 1975; Vol. 2, p 1 1 1 .
The Journal of Physical Chemistry, Vol. 89, No. 2, 1985 325
Intramolecular Proton-Transfer Reactions
TABLE III: Rate Constants @no), Frequency Factors ( A m0),and Activation Energies ( AEno) for the Ground-State Proton-Transfer Reactions of the Triazines in PMMA' kpT0(103s-1)
compd
273 K
283 K
289 K
293 K
298 K
(00)
1.85 0.87
2.0 1.2, 0.314
1.48
2.3 1.7 0.6
1.88
(ON) (") a
AEFTOl
300 K
ApTOls-'
(kcal mol-')
2.49
5.4 x 104 1.1 x 107 4.5 x 10'0
1.8 5.1 lo.,
1 .O'
Errors within f 5 % .
7r Formal Charges (Q,)" and Bond Lengths (IcdH and IC-) in the Ground and Excited States of 6-(2-Hydroxy-5-methylphenyl)-s-triazines Calculated by the Usual SCF MO CI Method
TABLE I V
compd
X' (00) OMe
Y"
QzO
QrN
IC-OH
Ql0
Q,"
IC4H
Qzo
Ic-0
QIN
Qro
Ic-0
QIN
OMe +0.1431 -0.3622 1.3407 +0.2768 -0.3922 1.3195 -0.5793 +0.4065 1.2874 -0.3913 +0.3703 1.2832 N(Me), +0.1432 -0.3704 1.3407 +0.2729 -0.3884 1.3209 -0.5837 +0.4024 1.2880 -0.3909 +0.3702 1.2834 (ON)c OMe +0.1432 -0.3724 1.3409 +0.2679 -0.3812 1.3218 -0.5846 +0.3967 1.2881 -0.3904 +0.3940 1.2837 (NO)c N(Me)2 OMe (NN) N(Me), N(Me)? +0.1423 -0.3800 1.3409 +0.2668 -0.4084 1.3221 -0.5888 +0.3929 1.2887 -0.3930 +0.3590 1.2838 "Ql0 and QINare the T formal charges of the proper oxygen and nitrogen atoms of the triazines concerned with the excited- and ground-state proton-transfer reactions. For details see text. b X and Y denote the substituents of the triazinyl groups: See Figure 3.
