1452
J. Phys. Chem. 1988,92, 1452-1458
Deacttvation Processes of Ultraviolet StaMlizers of the 2 4 Hydroxyphenyl)bentotrlazote Class with Intramolecular Hydrogen Bonds Gemot Goeller: Jochen Rieker: Andrew Maier,i John J. Stezowski,*Ewald Daltrozzo,s Manfred Neureiter,* Helmut Port,"Martin Wiechmann,ll and Horst E. A. Kramer*+ Institut fuer Physikalische Chemie and Institut fuer Organische Chemie, Biochemie und Isotopenforschung, Universitaet Stuttgart, Pfaffenwaldring 55, D - 7000 Stuttgart 80, Federal Republic of Germany, 3. Physikalisches Institut, Universitaet Stuttgart, Pfaflenwaldring 57, D- 7000 Stuttgart 80, Federal Republic of Germany, and Fakultaet Chemie, Universitaet Konstanz, D- 7750 Konstanz 1 , Federal Republic of Germany (Received: June 29, 1987)
The overall fluorescence quantum yield aFand the fluorescence decay time T' of crystalline TIN [2-(2'-hydroxy-5'methylphenyl)benzotriazole] (Ia, R = H) have been measured in the range from 350 to 12 K. The fluorescence originates from the SI' state of the proton-transferred species of TIN with an intact intramolecular hydrogen bond [TIN(intra)]. The isotope effect (Ia, R = D)of the fluorescence quantum yield [aF(D) > aF(H)] is nearly exclusively due to the isotope effect of the decay time #. The ratio @F/# increases rapidly with decreasing temperature; i.e., aFdepends more strongly on temperature than T'. The existence of additional quenching processes that influence the overall fluorescence quantum yield aFbut not 7'is inferred from this. In the boryl chelate (Ib) the proton Qf the intramolecular hydrogen bond is replaced by the B(C6U5), group. A crystal structure determination for the boryl chelate (Ib) is reported. The fluorescence quaptum yield of Ib is higher than that of the SI'fluorescence of TIN(intra). The fluorescence quantum yield and the fluorescence decay time of the boryl chelate are nearly independent of temperature. It is coocluded that internal vibrations in the TIN(intra) molecule (such as torsional vibration around the central N-C bond and vibrations where the H atom of the hydrogen bond is involved) are responsible for the rapid radiationless deactivation; these vibrations are severely hindered in the boryl chelate.
Introduction Since UV stabilizers such as 2-(2'-hydroxy-5'-methylphenyl)benzotriazole [Ia, R = H; trade name Tinuvin P (TIN)] are widely applied to diminish the photodegradation of polymers, considerable effort has been given to the elucidation of their photophysical deactivation mechanism. After the pioneering work of Heller' and Blattmann2*3and further studies were performed with picosecond spectroscopy (Gupta et al.,7-9 Flom and Barbara'O), phase fluorometry," and emission The excitation spectra of fluorescence and phosphorescence and the corresponding polarization spectra,I3-I6 the temperature dependence of the fluorescence, and the influence of deuteriawere measured. X-ray crystal structure determinations showed an almost perfect planar conformation for crystalline Tinuvin with an intramolecular hydrogen bond [TIN(intra)] " whereas for Methyltinuvin (MT; la, R = CH3)the dihedral angles between bonded benzotriazole and p-cresol rings are 54.9' and 56.3",respectively (two independent M T molecules).I6 M T can be considered as a model compound for those T I N molecules whose intramolecular hydrogen bond has been opened (see also refI8) and converted into an intermolecular hydrogen bond [TIN(inter)] to the solvent or polymer. TIN(inter) can initiate degradation processes of itself and/or of the p~lymer.'~''Further studies of the above-mentioned compounds and of other compounds with intramolecular hydrogen bonds have extended our knowledge about the intramolecular proton transfer in the excited state and of the mechanism of the UV stabilizer^;'^-^^ see also the papers cited by Shizuka et al." and by Woessner et al." The following mechanism is generally accepted: Upon photoexcitation, the acidityfbasicity of the intramolecular donor acceptor group increases in accordance with Forster's theory,)53Q thus promoting an intramolecular proton-transfer process which is the origin of the large Stokes shift in the fluorescence spectrum (A, = 638 nm).5,30311914315 The lifetime of the proton-transferred state SI' of crystalline TrN(intra) is very short (141 ps by phase fluorometry," 90 ps by time-resolved single-photon counting, 293 K). Similar results have been obtained by time-resolved The ' Author to whom correspondence should be addressed. Institut fuer Physikalische Chemie. Institut fuer Organische Chemie, Biochemie und Isotopenforschung. 8 Universitaet Konstanz. il 3. Physikalisches Institut.
*
0022-3654/88/2092-1452$01.50/0
molecule in its ground state Sd undergoes reverse proton transfer (now from N to 0 atom) to its original ground-state form So. The
Heller, H. J. Eur. Polym. J . Suppl. 1969, 105-132. Heller, H. J.; Blattmann, H. R. Pure Appl. Chem. 1972.30, 145-165. Heller, H. J.; Blattmann, H. R. Pure Appl. Chem. 1974,36,141-161. Williams, D. L.; Heller, A. J. Phys. Chem. 1970, 74, 4473-4480. Otterstedt, J.-E. A. J. Chem. Phys. 1973, 58, 5716-5725. Klbpffer, W. In Aduances in Photochemistry; Pitts, J. N., Jr., Hammond. G. s..Gollnick K.. Eds.: Wilev: New York. 1977: Vol. 10. DU 31 1-358. (7) Merrit, C.; Swtt, G. W.; Gupta, A,; Yavrouian, A. Chem:Phys. Lett. (1) (2) (3) (4) (5) (6)
1980,69, 169-173. (8) Huston, A. L.; Scott, G. 4978-4985.
W.;Gupta, A. J. Chem. Phys.
1982, 76,
(9) O'Connor, D. B.; Scott, G. W.; Coulter, D. R.; Gupta, A.; Webb, S. P.; Yeh, S. W.; Clark, J. H. Chem. Phys. Lett. 1985, 121, 417-422. (10) Flom, S. R.;Barbara, P. F. Chem. Phys. Lett. 1983, 94, 488-493. (1 1) Woessner, G.; Goeller, G.; Kollat, P.; Stezowski, J. J.; Hauser, M.; Klein, U. K. A.; Kramer, H. E. A. J . Phys. Chem. 1984, 88, 5544-5550. (12) Werner, T.; Kramer, H. E. A.; Kuester, B.; Herlinger, H. Angew. Makromol. Chem. 1976,54, 15-29. (13) Werner, T.; Kramer, H. E. A. Eur. Polym. J . 1977, 13, 501-503. (14) Werner, T. J . Phys. Chem. 1979,83, 320-325. (15) Werner, T.; Woessner, G.; Kramer, H. E. A. In Photodegradation and Photostabilization of Coatings; Pappas, S. P., Winslow, F. H., Eds.; ACS Symposium Series 151; American Chemical Society: Washington, DC, 1981; pp 1-18. (16) Woessner, G.; Goeller, G.; Rieker, J.; Hoier, H.; Stezowski, J. J.; Daltrozzo, E.; Neureiter, M.; Kramer, H. E. A. J . Phys. Chem. 1985, 89, 3629-3636. (17) Kramer, H. E. A. Farbe Lack 1986, 92,919-924. (18) Bocian, D. F.; Huston, A. L.; Scott, G. W. J . Chem. Phys. 1983, 79, 5802-5807. (19) Kuester, B.; Tschang, Ch.-J.; Herlinger, H. Angew. Makromol. Chem. 1976,54, 55-70. (20) Wwlfe, G. J.; Melzig, M.; Schneider, S.; Dorr, F. Chem. Phys. 1983, 77, 213-221. (21) Shizuka, H.; Matsui, K.; Okamura, R.;Tanaka, I. J . Phys. Chem. 1975, 79, 2731-2734. (22) Shizuka, H.; Matsui, K.; Hirata, Y.; Tanaka, I. J . Phys. Chem. 1976, 80, 2070-2072. (23) Shizuka, H.; Matsui, K.; Hirata, Y.; Tanaka, I. J . Phys. Chem. 1977, 81. 2243-2246. . ~- . (24) Shizuka, H.; Machii, M.; Higaki, Y.; Tanaka, M.; Tanaka, I. J. Phys. Chem. 1985.89, 320-326. (25) Barbara, P. F.: Rentzepis, P. M.; Brus, L. E. J . Am. Chem. SOC.1980, 102, 2786-2791. (26) Barbara, P. F.; Brus, L. E.; Rentzepis, P. M. J . Am. Chem. Soc. 1980, 102. 5631-563s.
+
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0 1988 American Chemical Society
Deactivation Processes of Ultraviolet Stabilizers
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main result of this cycle (a sort of Forster cycle) is the conversion of the electronic excitation energy SI So into thermal vibrational energy by radiationless processes. According to Barbara et a1.,10,26 in the 2 4 2’-hydroxyphenyl)benzothiazole molecule radiationless processes originate from “large-amplitude internal motion” which depends on viscosity (e.g., rotation around a single bond). In the TIN(intra) molecule an additional process has to be considered “which is still rapid a t high viscosity and extremely low temperature”.I0 This can be attributed to electronic radiationless decay with hydrogen stretching vibrations as accepting modes for the electronic excitation energy.39 From investigations of the photostabilizer copolymerized with polystyrene, OConnor et al? concluded that “the nonradiative decay of 2-(2’-hydroxypheny1)benzotriazole chromophores is likely promoted by a molecular torsional mode”. Shizuka et alez4hold the out-of-plane bending vibration (of the intramolecular hydrogen bond) responsible for the rapid radiationless deactivation in the 6-(2-
The Journal of Physical Chemistry, Vol. 92, No. 6,1988 1453
9,2-
c2 05
hydroxy-5-methylphenyl)-s-triazines. From our previous measurements it follows that the ratio aF/# is not constant but depends on temperature where aFand f are
Figure 1. Conformation of crystalline TIN-BPh2 with the numbering related to Table I.
the overall quantum yield and the decay time, respectively, of the fluorescence of the proton-transferred species of TIN(intra). Since the ratio * F / f does not correspond to a temperature-independent emission rate, the simple reaction scheme of Otterstedts*” has to be expanded. On the other hand, the abovementioned or analogous vibrations (OH stretching and out-of-plane bending vibrations) are essentially nonexistent in the boryl chelate (Ib).I6 Furthermore, a torsional mode (vibration) of the p-cresol moiety around the central N-C bond is thought to be severely hindered. Measurements of the temperature dependence of the fluorescence quantum yield and the fluorescence decay time of the boryl chelate (Ib) were carried out to test these concepts. The corresponding experiments for TIN(intra) and its deuteriated compound (Ia, R = D) were extended to 12 K and together with the experiments for the boryl chelate should contribute to a better understanding of the deactivation mechanism. A crystal structure determination for the boryl chelate was carried out to elucidate its conformational properties and to facilitate interpretation of the spectral properties.
toluene solution. Fluorescence and phosphorescence corrected for instrumental sensitivity were measured with the spectrometer described previously.16 The phase fluorometer used to measure the fluorescence decay times is described in ref 11. A cryostat from Lake Shore Cryotronics, Inc., Westerville, OH (Mode DRC 80C), was used for the temperature range of 300-10 K (closed cycle of He). The temperature was measured with a calibrated Si diode fixed close by the probe. The probe chamber was evacuated to 3.15 ). 2. Absorption and Emission of the Boryl Chelate. The absorption spectrum of the boryl chelate TIN-BPh2 (Scheme Ib), the corrected fluorescence spectrum, and the corrected fluorescence excitation spectrum are presented in Figure 4 (see also ref 16). Originally, the boryl chelate was synthesized as a model compound of the planar TIN(intra). The conformation of the complex is similar to that of TIN(intra). The TIN-BPh2 molecule is not perfectly planar (dihedral angle = 1 1 .go) as revealed by crystal structure determination. Nevertheless, the ground2state absorption spectra of TIN(intra) (A, = 350 and 300 nm in methylcyclo= 395 hexane/isopentane a t 150 KI4) and of TIN-BPh2 (A, and 310 nm in cyclohexane a t 298 K; see Figure 4 ) are similar, or in other words, the whole molecule (benzotriazole ring and p-cresol ring) acts as absorbing system in the planar TIN(intra) and the slightly distorted TIN-BPh2, whereas the long-wavelength absorption of the distorted Methyltinuvin M T (Amx = 286 nm in Me2S0,16dihedral angles between bonded benzotriazole and p-cresol rings = 54.9O and 56.3O 16) and in the distorted TIN(inter)
w
(& = 290 nm16) has to be attributed to the benzotriazole moiety of the m o l e ~ u l e . ~ ~The J ~ torsional -~~ motion of the p-cresol ring therefore accelerates the radiationless deactivation in the excited TIN(intra) molecule but not in M T (see Table V of ref 16). According to the absorption spectra, TIN-BPh2 corresponds to TIN(intra) in the ground state. The Stokes shift of TIN-BPh2 amounts only to 7100 cm-I (Table 111) (MT Stokes shift = 9100 cm-I; see Figure 2 of ref 14), compared to 13 000 cm-I of TIN(intra); see also Figure 2 of ref 14. Thus the emitting state of TIN-BPh2 does not resemble the S1' state of TIN(intra) but rather the S,state. This seems reasonable if one considers the chemical structures. The BPh2 group is simultaneously bonded to both the N and 0 atoms and is in close contact with both. In contrast, the H atom is covalently bonded either to the 0 atom (ground state) or the N atom (excited state). Thus the lighter hydrogen atom can "hop" from one site to the other (S, SI')whereas the BPh2 group is confined in position (SI). Phosphorescence could be observed only for the distorted molecules TIN(inter) and M T but not for the planar TIN(intra) nor for TIN-BPh2 (dihedral angle = 11.8'). The absorption and emission spectra of TIN-BPh2 are not changed when going to 77 K. The fluorescence excitation spectrum reproduces well the form of the absorption spectrum (Figure 4) (the deviation of the intensity ratio of both bands is an artifact of the apparatus). The temperature dependence of the fluorescence decay time and of the fluorescence quantum yield of TIN-BPh2 is small in the temperature range from 10 to 300 K (Table 111), in contrast to TIN(intra),11s16vide infra. 3. Temperature Dependence of the Fluorescence Decay Time T' and of the Quantum Yield aF(rel)of the Fluorescence (Aem = 632 nm) of Crystalline TIN(intra). Figures 5 and 6 show the
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(45) Ghiggino, K.P.;Scully, A. D.;Leaver, I. 90, 5089-5093.
H.J . Phys. Chem. 1986,
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1456 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988
Goeller et al.
t
V/cm-'
A A A A A A A A
hlnm
10
50
100
150
200
250
300
-
A A
A A
0.5
TIK
Figure 6. Relative overall quantum yield @&el) of the SI'fluorescence = 366 of crystalline TIN(H) (A)and TIN(D) ( 0 )vs temperature: kXc nm; integration over the whole fluorescence band. Insert: Si' fluorescence spectrum of TIN(H) at 90 K.
I 10
50
100
150
200
250
300
T/K
Figure 7. Ratio @&el)/f relative overall fluorescence yield over decay time of the Si' fluorescence of crystalline TIN(H) (A)vs temperature; data from Figures 5 and 6 .
decay time 7' and the relative overall quantum yield cPF(rel) of the SI'fluorescence (excited state of the proton-transferred species) of crystalline TIN(H) and TIN(D) in the temperature range from 350 to 12 K (in previous experiments" only a minimum of 77 K could be reached). According to the simple reaction scheme, [seeref 11 and Figure 8, where So and S1represent the ground and the excited singlet state of the enol form of TIN(intra) and So' and SI'respectively, the corresponding states of the keto form [proton-transferred species of TIN(intra)]], an expression can be derived for the overall quantum yield of @F of the SI' fluorescence: @F
= @'rr'Pd= %Trk{T'
(1)
eTr being the proton-transfer yield from S1 to SI' and @{ the intrinsic fluorescence quantum yield of S1'and T' its decay time. There are no radiationless deactivation processes (kIc, kIsc) of the SI state of TIN(intra) to its ground state So which could compete effectively with the proton transfer" (ks,s,,> loll s-l). (In previous publications11T16 a different interpretation was given where the existence of such processes was assumed.) The overall fluorescence quantum yield aFmeans the number of quanta emitted by SI' related to the quanta absorbed by So, whereas the intrinsic fluorescence quantum yield @pd of the state SI' includes only deactivation processes of SI'. (i) The relative overall fluorescence quantum yield @,(rei) and the fluorescence decay time T' increase with decreasing temperature (Figures 5 and 6). aF(rel) and T', respectively, of the deuteriated compound TIN(D) (Ia, R = D) are higher by nearly the same factor when compared to cPF(rel) and T' of the nondeuteriated compound TIN(H) (Ia, R = H)," (2) (from Figure 5 and 6 ) . In other words
Although the relative overall quantum yield QF(rel) includes the H- or D-transfer quantum yield eTr from SI to SI' (see (l)), the main part of the isotope effect of the overall fluorescence quantum yield aF(rel) is located in the fluorescence decay time f of SI'. From this it follows that the transfer yield cPTr defined according to (1) has only a small isotope effect, if any, in agreement with previous measurements.' (ii) The ratio aF(rel)/f from Figures 5 and 6 for, e.g., TIN(H) is plotted versus temperature in Figure 7. (Note: The values for higher temperatures have been omitted. Very recent mea-
N . . , . H-0
")GO:;--tp 76 1
H
"TICT"
Figure 8. Extended reaction scheme; see text.
surements with the picosecond-laser SPC system gave some indications that the straight line continues to room temperature.) On the other hand, Mordziiiski and GrabowskaZ9found that (in our notation) cPF/d= @TrkF'of the corresponding benzoxazoles decreases from 183 to 123 K a t least for the IH compound (no significant temperature dependence was observed for the deuteriated compoundz9). This result led us to extend our experiments to very low temperatures. Obviously, TIN(intra) shows a different behavior which we interpret in the following way: A detailed analysis of Figures 5 and 6 and especially of Figure 7 demonstrates that the overall fluorescence quantum yield aPF depends more strongly on temperature than T' (for both TIN(H) and TIN(D)). From this we have to conclude that additional quenching processes exist which influence the overall fluorescence quantum yield aFbut not the decay time f . As a hypothesis, an intermediate excited state C* between the SI and the S1' states is assumed; see Figure 8. The internal conversion k,(T) of C* should be very fast at high temperature
The Journal of Physical Chemistry, Vol. 92, No. 6, 1988 1457
Deactivation Processes of Ultraviolet Stabilizers
h d
al
lo5
L
v
h
.-c m
1 o4
C
aJ
c
.-C
lo3
al.52about indigo, where the N H stretching and the out-of-plane modes are considered a s decay channels for fast internal conversion, and thioindigo, where, in contrast, these modes do not exist and no such fast internal conversion is observed. The fluorescence yield and the decay time of the boryl chelate display very little temperature dependence. The above-mentioned vibrations may be considered so to speak as already hindered at room temperature for the boryl chelate, whereas they lose their effectiveness as deactivation channels for the TIN(intra) molecule only at lower temperature, which results in an increase of the fluorescence yield. The deactivation processes C* C are reflected by @F/T’ (transfer yield, aTr, (l), according to the simple reaction scheme; see ref 11 and Figure 4 there). Since @F/T’ displays only a very small isotope effect, torsional vibration is considered to contribute significantly to the deactivation process; due to the high masses involved no isotope effect is expected. On the other hand, the small isotope effect found experimentally might arise from hydrogen out-of-plane bending and/or hydrogen stretching vibrations of the intramolecular hydrogen bond. In this context the question should be discussed whether it is necessary to postulate an intermediate excited state C* instead of simply assuming a temperature-dependent rate constant kd(T ) for the radiationless deactivation of the SI state as in foregoing publications“>l6. As was already pointed out, the proton-transfer rate constant”,55 ks,slt is greater than 10” s-l. Therefore, the temperature-dependent rate constants of radiationless processes (kIc,kIsc) of the SI state of TIN(intra) should be much greater than 10” s-l to compete effectively with ksIsl.and to explain the temperature dependence of @&l)/~’. Such high values for kIc and kIsc seem to be improbable. The proton transfer does not exist as a deactivation channel for the distorted TIN(inter) and M T molecules, and therefore, fluorescence from the SI state arise^."^'^^^^ For TIN(inter) kIc = 2 X lo9 s-l and kIsc = 3.7 X lo8 s-l in Me2S0 at 296 K; for M T the rate constants are even sma1ler.I6 It is hard to understand why the deactivation rates (kIc, kIsc) in the SI (not proton-transferred) state should be much greater than 10” s-l if the molecule is planar [TIN(intra)] whereas k,c amounts only to 2 X lo9 s-l in the distorted TIN(inter) molecule. This contradiction can be explained by assuming the intermediate state C* where such rapid deactivation processes are feasible.
-
lo2
6
4
8
10
t I ns Figure 9. SI’fluorescence decay of crystalline TIN(H) (50 K) observed a t 600 nm after picosecond-pulse excitation with the single pulse apparatus described under Experimental Section; T’ = 1 ns.
and slow at low temperature. C* could be a configuration where the p-cresol ring is rotated around the central N-C bond ((90’) in the excited electronic state; C is the corresponding configuration in the electronic ground state. A configuration similar to TICT (Twisted Intramolecular Charge T r a n ~ f e r ~may ~ , ~be~ taken ) into consideration where, however, the angle of rotation should be smaller than 90°. The radiationless deactivation to the ground state C is well-known to be very fast in such twisted configurations. Furthermore, C* could be described by an hydrogen out-of-plane bending and/or an hydrogen stretching vibration of the intramolecular hydrogen bond;51ksls,,, and ksotsoare transfer rates of H (and D, respectively), and kd(T) and k,‘(T) are temperature-dependent radiationless deactivation processes (Figure 8). A similar model has been proposed by F O r ~ t e rand ~ ~ by Schulman and Liedke4’ (see also Hafner et al.48 and Tsutsumi et al.49) to explain the fluorescence quenching of naphthylammonium in strong acid solution. According to Weller” the electronic ground state of C should probably lie energetically high and thus the radiationless deactivation is favored by the small C. electronic energy difference C* The model readily explains that the overall fluorescence depends more strongly on temperature than quantum yield aPF T’, since the additional deactivation channel kslcr kd(T ) influences @F but not T’. The monoexponential decay of 7’ agrees with the proposed scheme. This monoexponential decay has been confirmed over three decades of intensity by using the SPC system (Figure 9; T’ = 1 ns, 50 K). If proton transfer (SI S I r ) were the first step followed by the population of state C* from SI’, a biexponential decay of T’ would be expected. In the boryl chelate (Ib), however, neither a large-amplitude torsional vibration around the central N - C bond nor an hydrogen out-of-plane bending and/or an hydrogen stretching vibration (of the intramolecular hydrogen bond) could act as accepting vibration modes for the electronic excitation energy (as in the TIN(intra) molecule) and could thus deactivate effectively the excited state of the boryl chelate (Ib). The moderately high fluorescence quantum yield (Table 111) of the boryl chelate [compared to the quantum yield of the SI’fluorescence (2 X 10-4)5of TIN(intra)] may be attributed to the absence of these efficient deactivation channels; see also the corresponding discussion by Elsaesser et
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(46) Forster, Th. Chem. Phys. Leu. 1972, 17, 309-311. (47) Schulman, S. G.; Liedke, P. 2. Phys. Chem. (Wiesbaden) 1973,84, 317-324. (48) Hafner, F.; Woerner, J.; Steiner, U.; Hauser, M. Chem. Phys. Lert. 1980, 73, 139-144. (49) Tsutsumi, K.; Sekiguchi, S.;Shizuka, H. J . Chem. SOC.,Faraday Trans. I 1982, 78, 1087-1101. (50) Weller, A.,private communication. (51) Ernsting, N. P. private communication, proposed a similar model where the proton transfer occurs when the N atom oscillates toward the H atom; this may be considered as a sort of pendulum oscillation of the benzotriazole moiety in the common plane.
Conclusion The effectiveness of the UV stabilizer 2-(2’-hydroxy-5’methylpheny1)benzotriazole (Ia, R = H) is attributed to, among other things, the fact that the intramolecular hydrogen bond remains intact [TIN(intra)] and is not converted into an intermolecular hydrogen bondl5-I7 (e.g., to the polymer). In the electronically excited TIN(intra) molecule there exist very fast and effective temperature-dependent deactivation processes where internal vibrations and librations of the molecule act as accepting modes for the electronic excitation energy. These internal motions cannot be effective in the boryl chelate TIN-BPh, (Ib), where the H atom of the intramolecular hydrogen bond is replaced by a B(C6H5)2group. This conclusion is drawn from the higher fluorescence quantum yield of the boryl chelate when compared to the S,’ fluorescence of the proton-transferred species of TIN(intra) and from the fact that both fluorescence quantum yield and fluorescence decay time of the boryl chelate show only a small temperature dependence. It is therefore proposed, in agreement with an earlier idea of Heller and Blattmannzs3 and further lite r a t ~ r e , ’ ~ . that * ~ *torsional * ~ ~ ~ ~l i b r a t i ~ n of ~~ the~ p-cresol J ~ ~ ~ ring ~ relative to the benzotriazole ring and hydrogen out-of-plane (52) Elsaesser, T.;Kaiser, W.; Liittke, W. J . Phys. Chem. 1986, 90, 290 1-2905. (53) Grabowski, 2. R.; Rotkiewicz, K.; Siemiarczuk, A,; Cowley, D. J.; Baumann, W. N o w . J . Chim. 1979, 3, 443-454. (54) Rettig, W. J . Phys. Chem. 1982,86, 1970-1976. (55) Shizuka et aLZ4found the proton-transfer rate constant to be k p =~ 2 X lo1*SK’ for their compounds.
1458
J. Phys. Chem. 1988, 92, 1458-1464
bending and/or hydrogen stretching ~ i b r a t i o nof ) ~the intramolecular hydrogen bond in the excited TIN(intra) molecule are responsible for its rapid radiationless deactivation, since these vibrations are severely hindered in the boryl chelate. These rapid deactivation processes are the origin of the high efficiency of this UV stabilizer. Acknowledgment. We thank Dr. Helmut Mueller, Ciba-Geigy AG, Basel, Switzerland, for providing samples. We also thank Professor A. Weller, Goettingen, for a fruitful discussion concerning the rapid radiationless deactivation process in connection
with proton transfer. The financial support of the Deutsche Forschungsgemeinschaft and of the Fonds der Chemischen Industrie is gratefully acknowledged. Registry No. Ia (R = H), 2440-22-4; Ia (R = D), 85775-60-6; Ib, 97012-33-4.
Supplementary Material Available: Anisotropic temperature factors for B, C, N, and 0 atoms and complete tables of bond distances and bond angles (7 pages); calculated and observed structure factors (56 pages). Ordering information is given on any current masthead page.
Gas-Phase Organometallic Kinetics. 3. The Observation and CO Substitution Kinetics of cis-Cr(CO),(C,H,), by Time-Resolved Infrared Absorption Spectrometry Bruce H. Weilled and Edward R. Grant* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: July 6, 1987; In Final Form: August 25, 1987)
The new short-lived complex C ~ S - C ~ ( C O ) ~ ( Cis~ observed H , ) ~ by time-resolved IR absorption spectrometry after pulsed laser photolysis of gaseous mixtures of Cr(C0)5(C2H4)and C2H4. No evidence is found for associated production of transCr(CO),(C2H4),. C ~ S - C ~ ( C O ) ~ ( C ~reacts H , ) , with CO to re-form C T ( C O ) ~ ( C ~by H ~dissociative ) substitution. From the dependence of the observed exponential decay constants on CO and C2H4 pressures, the unimolecular decay constant of cis-Cr(C0),(C2H,), is found to be (6 & 2) X lo4 s-' and the ratio of the bimolecular rate constants for the reaction of Cr(CO),(C,H,) with CO vs C2H4is found to be 0.7 f 0.2 favoring reaction with C2H4.
Introduction The binding between olefins and transition metals occupies a place of central importance in the synthetic and catalytic chemistry of organometallic molecules. It has been the subject of numerous theoretical investigatiom2 An intriguing problem exists in the apparent trends in olefin coordination to the 14-electron carbonyl fragments of the iron and chromium groups. In group VIII, the metal tricarbonyls appear to prefer bonding with a conjugated diene in an 7, fashion over a nonconjugated diene (e.g., nonboradiene, 1,5-cyclooctadiene) or two individual olefins. For group VI the converse is suggested;2d metal tetracarbonyls prefer a nonconjugated diene or two olefins over a conjugated diene. Although many nonconjugated diene complexes of group VI tetracarbonyls are stable, the analogous bis(o1efin) complexes are rare. As noted above, such fundamental questions of electronic structure and bonding are important because olefin complexes are widely found in catalysis. Many important catalytic systems use olefins as substrates and must proceed by olefin c ~ o r d i n a t i o n . ~ Cr(C0)6 is a photoactive catalyst for the 1,4-hydrogenation of dienes4 and the isomerization of 01efins.~ The stereospecificity of the 1,Chydrogenation reaction is likely the result of geometric constraints in an unstable g4-dieneintermediate. Cr(CO),(olefin), complexes are proposed as intermediates in the isomerization system but have not been observed.5c Complexes of the type M(CO),(olefin), (M = Fe, Cr) are interesting for their catalytic and theoretical importance but present experimental challenges due to their instability. For example, previous work in our laboratory found the gas-phase unimolecular decay constants for Fe(C0)3(C2H4)2and cis-Fe(CO),(C2H4), to be (2.9 & 0.6) X s-' and (3.6 f 0.9) X lo3 s-', respectively, where the decay channel in both cases is ethylene loss.'a,b Similar complexes for M = Mo and W are well-known,6 'Present address: Department of Chemistry, University of California, Berkeley, CA 94720
but for M = Cr they are quite rare. A few Cr(CO)5(olefin) complexes have been characterized, but only one Cr(C0)4(olefin)2has been found to be stable at room temperature.' In this case, the stability of the complex is greatly enhanced by relief of ring strain present in the free ligand, trans-cyclooctene. Only recently has C T ( C O ) , ( C ~ H ~been )~ studied in low-temperature solution.* The bis(o1efin) complexes of Cr(CO), are especially interesting because of their expected stability relative to 74-conjugated diene complexes as noted above. In this paper we present the gas-phase IR spectrum and decay kinetics of C~S-CT(CO),(C~H,)~, obtained by time-resolved IR (1) The first papers in this series are (a) Weiller, B. H.; Miller, M. E.; Grant, E. R. J . Am. Chem. SOC.1987, 109, 352. (b) Weiller, B. H.; Grant, E. R. J. Am. Chem. SOC.1987, 109, 1051. (c) Weiller, B. H.; Grant, E. R. J. Am. Chem. SOC.1987,109, 1252. (2) (a) Dewar, M. J. S. Bull. SOC.Chim. Fr. 1951, 18, C79. (b) Chatt, J.; Duncanson, L. A. J . Chem. SOC.1953, 2939. (c) Ittel, S. D.; Ibers, J. A. Adu. Organomet. Chem. 1976, 14, 33. (d) Elian, M.; Hoffmann, R. Inorg. Chem. 1975, 14, 1058. (e) Albright, T. A,; Hoffmann, R.; Thibeault, J. C.; Thorn, D. L. J. Am. Chem. SOC.1979,101, 3801. ( f ) Rosch, N.; Hoffmann, R. Inorg. Chem. 1974,13,2656. (g) Stockis, A,; Hoffmann, R. J . Am. Chem. SOC.1980, 102,2952 and references therein. (h) Bachman, C.; Demuynck, J.; Veillard, A. J . Am. Chem. SOC.1978, ZOO, 2366. (i) Basch, H.; Newton, M. D.; Moskowitz, J. W. J. Chem. Phys. 1978.69, 584. 0') Swope, W. L.; Schaefer, H. F., 111 Mol. Phys. 1977, 34, 1037. (k) Garcia-Prieto, J.; Novaro, 0. Mol. Phys. 1980, 41, 205. (3) Parshall, G. W. Homogeneous Catalysis; Wiley: New York, 1980. (4) (a) We have observed photocatalytic hydrogenation of dienes by gasphase Cr(C0)6 with quantum yields greater than one. Weiller, B. H. Ph.D. Thesis, Cornell University, 1986. (b) Wrighton, M.; Schroeder, M. A. J. Am. Chem. SOC.1973, 95, 5764. (c) Platbrocd, G.; Wilputte-Steinert, L. J. Mol. Catal. 1975176, 1, 265. ( 5 ) (a) Wrighton, M.; Hammond, G. S.; Gray, H. B. J . Orgunomet. Chem. 1974, 70, 283. (b) Wrighton, M.; Hammond, G. S.; Gray, H. B. J. Am. Chem. SOC.1970, 92, 6068. (c) Tumas, W.; Gitlin, B.; Rosan, A. M.; Yardley, J. T. J . Am. Chem. SOC.1982, 104, 5 5 . (6) Stolz, 1. W.; Robson, G. R.; Sheline, R. K. Inorg. Chem. 1963, 2, 1264. (7) Grevels, F. W.; Skibbe, V. Chem. SOC.,Chem. Commun. 1984, 681. ( 8 ) Gregory, M. F.; Jackson, S. A,; Poliakoff, M.; Turner, J. J. Chem. Commun. 1986. 1175.
0022-3654/88/2092-1458$01 .50/0 0 1988 American Chemical Society
