14468
J. Phys. Chem. 1996, 100, 14468-14475
Investigations on Polymeric and Monomeric Intramolecularly Hydrogen-Bridged UV Absorbers of the Benzotriazole and Triazine Class Ju1 rgen Keck,† Horst E. A. Kramer,*,† Helmut Port,§ Thomas Hirsch,§ Peter Fischer,‡ and Gerhard Rytz⊥ Institute fu¨ r Physikalische Chemie und fu¨ r Organische Chemie und Isotopenforschung, UniVersita¨ t Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany; 3. Physikalisches Institut, UniVersita¨ t Stuttgart, Pfaffenwaldring 57, D-70569 Stuttgart, Germany; and Ciba-Geigy Limited, AdditiVes DiVision (AD 2.1), Research Center Fribourg, CH-1723 Marly 1, Switzerland ReceiVed: April 11, 1996; In Final Form: June 4, 1996X
Various copolymers of MA-TIN 1, 2-[2-hydroxy-3-tert-butyl-5-(O-[2-hydroxy-3-(2-methylpropenoyloxy)propyl]-2-carbonyloxyethyl)phenyl]benzotriazole, and MA-TZ 1, 2,4-bis(2,4-dimethylphenyl)-6-[2-hydroxy4-(2-hydroxy-3-[2-methylpropenoyloxy])propoxyphenyl]-1,3,5-triazine, with styrene, methyl methacrylate, and methacrylic acid have been synthesized by radical polymerization. Their absorption spectra in the longwavelength UV region appear unchanged compared to those of the monomeric UV absorbers, indicating the stabilizer chromophore to remain unimpaired in the course of the polymerization. Both the monomeric and the polymeric stabilizers exhibit a strongly Stokes-shifted, temperature-dependent, low-quantum-yield fluorescence which arises from an intermediate species formed by intramolecular proton transfer. The intramolecular hydrogen bond which is essential for the photostability of this type of UV absorbers thus is still intact in the copolymers. Activation energies for the radiationless deactivation process can be evaluated from the temperature dependence of the proton-transferred fluorescence. These energies lie between 4 and 5 kJ/mol for most of the benzotriazole and triazine stabilizers investigated and show hardly any matrix dependence. Fluorescence-decay measurements with crystalline MA-TIN 1 at different temperatures reveal a close correspondence of the temperature dependence between decay times and relative quantum yields. The radiationless process thence is concluded to originate from the proton-transferred level S1′. The decay time at room temperature is estimated at 70 ps, close to the value for crystalline TIN P. The proton-transferred fluorescence of MA-TIN 1, in contrast, exhibits a biexponential decay profile.
1. Introduction
CHART 1: Formulas and Designations
Compounds which incorporate an intramolecular hydrogen bond, such as methyl salicylates, o-hydroxybenzophenones, 2-(2hydroxyphenyl)benzotriazoles, and 2-(2-hydroxyphenyl)-1,3,5triazines, are widely used as UV absorbers for polymers.1-24 They absorb ultraviolet radiation which else would damage the polymer, transforming it into vibrational (i.e., thermal) energy through efficient radiationless deactivation pathways. The pivot for this mechanism is an excited-state intramolecular proton transfer (ESIPT) in the excited singlet state S1 which is followed by a fluorescence emission with a very large Stokes shift from the proton-transferred S1′ state.1-23 For 2-(2-hydroxy-5-methylphenyl)benzotriazole (Tinuvin P (TIN P), trade name of CibaGeigy Limited; see Chart 1), it has been shown by time-resolved emission and transient-absorption spectroscopy that the complete proton transfer cycle, including proton back-transfer, is a very rapid process in the picosecond time domain and sometimes even faster.15,18,25 The photophysical properties of several (hydroxyphenyl)-1,3,5-triazines were described by Bigger et al.26 and Stueber et al.27 who also discussed the influence of various functional groups on the quantum yield for the proton-transferred fluorescence26-29 (cf. section 3). Several detailed investigations on the radiationless deactivation mechanisms have been pub* To whom correspondence should be addressed. † Institut fu ¨ r Physikalische Chemie, Universita¨t Stuttgart. ‡ Institut fu ¨ r Organische Chemie und Isotopenforschung, Universita¨t Stuttgart. § 3. Physikalisches Institut, Universita ¨ t Stuttgart. ⊥ Ciba-Geigy Limited. X Abstract published in AdVance ACS Abstracts, August 1, 1996.
S0022-3654(96)01081-7 CCC: $12.00
lished: Barbara et al.,9,30,31 for example, suggested a slow largeamplitude torsional motion; Shizuka et al.22,23 were the first to describe time-resolved proton-transferred fluorescence in (hy© 1996 American Chemical Society
Polymeric and Monomeric UV Stabilizers droxyphenyl)triazines. The field of proton transfer in general has been covered in some comprehensive reviews.30,32-34 If the UV absorber is simply mixed with the polymer, problems may arise in long-term exposure due to migration and extraction processes.35-37 It may be advantageous, therefore, to employ polymerizable stabilizers that can be bound directly to the polymer backbone. The intramolecular hydrogen bond which is characteristic for this type of UV absorber must of course remain unaffected by the polymerization reaction. Numerous reports have been published on the synthesis and application of polymeric stabilizers, especially benzotriazole derivatives38-43 (for a particularly detailed discussion of polymeric stabilizers see refs 36 and 37). The present paper reports on the synthesis of polymeric benzotriazole and triazine stabilizer compounds; their absorption and emission spectra, both steady state and time-resolved, are discussed in terms of the individual stabilizing capacity. A number of polymeric and monomeric benzotriazole and triazine stabilizers are compared by analyzing the temperature dependence of their proton-transferred emission (for related prior work on benzotriazoles see refs 16 and 44-47). The fluorescence kinetics of a polymerizable benzotriazole are also investigated. 2. Experimental Section Chemicals. The following compounds were synthesized at Ciba-Geigy (Marly, Switzerland): MA-TIN 1, 2-[2-hydroxy3-tert-butyl-5-(O-[2-hydroxy-3-(2-methylpropenoyloxy)propyl]2-carbonyloxyethyl)phenyl]benzotriazole; MA-TZ 1, 2,4-bis(2,4dimethylphenyl)-6-[2-hydroxy-4-(2-hydroxy-3-[2-methylpropenoyloxy])propoxyphenyl]-1,3,5-triazine; TIN 2, 2-[2-hydroxy-3-methyl-5-(O-methyl-2-carbonyloxyethyl)phenyl]benzotriazole; TIN 3, 2-[2-hydroxy-3-tert-butyl-5-(O-methyl)2-carbonyloxyethyl)phenyl]benzotriazole; TTZ 3, 2,4-bis(2,4dimethoxyphenyl)-6-(2-hydroxy-4-methoxyphenyl)-1,3,5-triazine; TTZ 4, 2-(2,4-dimethoxyphenyl)-4,6-bis(2-hydroxy-4methoxyphenyl)-1,3,5-triazine; TTZ 5, 2-(2-hydroxy-4-methoxyphenyl)-4,6-diphenyl-1,3,5-triazine; TTZ 9, 2,4-bis[4-(3-butoxy2-hydroxypropoxy)-2-hydroxyphenyl]-6-phenyl-1,3,5-triazine. MA-TIN 1 and TIN 2 and 3 were recrystallized from hexane, MA-TZ 1 from methanol/water or cyclohexane, TTZ 3-5 from toluene or benzene, and TTZ 9 from ethanol. Styrene was distilled in vacuo over calcium hydride, methyl methacrylate at about 100 Torr over calcium hydride, and methacrylic acid in vacuo. The radical starter AIBN (azobisisobutyronitrile; Aldrich) was used without further purification. Synthesis of the Copolymers. 1. Styrene Copolymers. The monomeric stabilizer was dissolved in styrene (6.7-37 mmol/ L), AIBN was added (15 mmol/L), and the mixture was kept at 70 °C for approximately 90 min. The polymer was precipitated by pouring the viscous reaction mixture into methanol, purified by a repeated dissolving/precipitation process (toluene/methanol) and dried in vacuo at 50 °C for several hours. 2. Methyl Methacrylate Copolymers. The monomeric stabilizer was dissolved in methyl methacrylate (34.3-396.3 mmol/ L), AIBN was added (20.2 mmol/L), and the mixture was kept at 70 °C for 1 h. The highly viscous reaction mixture was diluted with toluene and poured into hexane. The precipitated polymer was purified by a repeated dissolving/precipitation process (toluene/hexane) and dried as described above. 3. Methacrylic Acid Copolymers. The monomeric stabilizer (0.05-0.4 mmol) and methacrylic acid (23-46 mmol) were dissolved in 50 mL of hexane. AIBN (0.12 mmol) was added and the mixture stirred at 50 °C. The reaction mixture soon turned turbid due to the insolubility of the polymer formed. After 90 min, the mixture was allowed to cool to room temperature,
J. Phys. Chem., Vol. 100, No. 34, 1996 14469 and the polymer was isolated by filtration, washed repeatedly with hexane, extracted with hot cyclohexane for several hours to remove unreacted monomers, and dried as described above. The percentage of covalently bonded stabilizer in the respective polymers was evaluated by elemental analysis and UV spectroscopy, both methods giving almost identical results. Absorption spectra were recorded on a Perkin-Elmer Lambda 7 UV-vis absorption spectrometer. Fluorescence spectra, corrected for instrumental sensivity, were measured with an emission spectrometer described previously.13,15,18 The excitation wavelength was set at 313, 333, or 366 nm, corresponding to maxima of the high-pressure mercury lamp employed. Time-resolVed fluorescence spectroscopy in the picosecond time domain was performed using time-correlated single-photon counting after laser-pulse excitation (366 nm) by a synchronously pumped, cavity-dumped, and frequency-doubled dye laser. The time response of the detection system was approximately 30 ps. Corrections for light intensities and the spectral response of the apparatus were made (for details see refs 25 and 45). Fluorescence spectra were recorded using a front-face illumination geometry for crystalline powder or thin-film samples. These were produced by spreading a dilute solution of the polymer (in either toluene or ethanol) onto a quartz plate, allowing the solvent to evaporate, and drying the films in vacuo at 50 °C. 3. Results and Discussion 3.1. Absorption Spectra. The absorption spectra of MATIN 1 and MA-TZ 1 show the double-band structure in the long-wavelength UV region observed for many intramolecularly hydrogen-bridged UV absorbers.1-23 The longer wavelength band at about 350 nm can be attributed to a ππ* charge-transfer state; this is favored by the planar orientation enforced by the intramolecular hydrogen bond. The shorter wavelength band at about 300 nm arises from a local transition within the benzotriazole or the 1,3,5-triazine moieties, respectively (for details see refs 27 and 48). These spectral properties appear virtually unchanged in the spectra of the copolymers (Figure 1a,b). Benzotriazole and triazine stabilizers thus may be copolymerized via a radical process without detriment to the respective chromophores; especially, the intramolecular hydrogen bond remains unaffected by any radical attack. This behavior may be rationalized in terms of the diminished reactivity of a hydrogen-bonded compared to a free phenolic hydroxy group towards radicals; it has likewise been found for hydroxybenzophenones35 and other benzotriazoles.36-42 It is impossible, on the other hand, to polymerize benzotriazole stabilizers by an anionic process where deprotonation by the basic initiator would destroy the hydrogen bond. The same holds for group-transfer polymerization (GTP) employing basic agents.49 Figure 2 shows the absorption spectra of some methacrylic acid stabilizer copolymers in various solvents. Benzotriazole and triazine stabilizers differ, interestingly and distinctly, as to the influence of hydrogen-bond-breaking solvents, such as DMSO, on the relative intensity of the two long-wavelength UV absorption bands. The intensity of the long-wavelength band λ2 is reduced in the case of MA-TIN 1 and its copolymers upon addition of DMSO, due to formation of intermolecularly rather than intramolecularly hydrogen-bonded species.12-18,46,48 Opposite behavior is observed for MA-TZ 1 and its copolymers. Furthermore, the absorption spectrum of a methacrylic acid/
14470 J. Phys. Chem., Vol. 100, No. 34, 1996
Figure 1. Normalized absorption spectra (solid films; for extinction coefficients see ref 59) (a) of MA-TIN 1 (s) and MA-TZ 1 (- - -); for molecular formulas see Chart 1; (b) of styrene copolymers with MA-TIN 1 (4.3 wt %, - - -) and MA-TZ 1 (5.7 wt %, -‚-‚-), and of pure polystyrene (s) [RSTY ) OCH2CH(OH)CH2OC(O)C(CH3)(CH2∼∼)CH2CH(Ph)∼∼, Ph ) phenyl, ∼∼ ) polymer chain].
MA-TZ 1 copolymer appears less influenced in basic medium, e.g., methanolic KOH, than that of a corresponding MA-TIN 1 derivative (see Figures 2 and 3). From all this it may be concluded that the intramolecular hydrogen bonds of the triazine stabilizers are stronger than those of the benzotriazoles, in agreement with the interpretation of IR, NMR, and crystallographic data.27-29,48 3.2. Emission Spectra. The common feature of the fluorescence spectra of MA-TIN 1, MA-TZ 1, and all copolymers investigated here is a considerably Stokes-shifted, distinctly temperature-dependent red fluorescence with low quantum yield (approximately 10-3 at 77 K for crystallinic powders of MATIN 1 and MA-TZ 1) which can already be detected at room temperature when the samples are investigated in the solid state. The explanation for this large Stokes shift has already been given (cf. Section 1). Since MA-TZ 1 is capable of forming an intramolecular hydrogen bond too, it seems reasonable to interpret its emission likewise as a consequence of an ESIPT as established for 2-(2-hydroxyphenyl)triazines with either no or one additional aryl substituent in positions 4 or 6, respectively, such as compounds OO, ON, NN, and DTZ 2 (see Chart 1).22,23,26,50 3.3. Temperature Dependence of the Proton-Transferred Fluorescence. The four-level scheme depicted in Figure 5 (see
Keck et al.
Figure 2. Normalized absorption spectra of methacrylic acid copolymers with (a) MA-TIN 1 (9.7 wt %); (b) MA-TZ 1 (4.1 wt %) in ethanol (s), DMSO (-‚-‚-), water (- - -), and water/KOH (‚‚‚) [RMA ) OCH2CH(OH)CH2OC(O)C(CH3)(CH2∼∼)CH2C(CH3)(COOH)∼∼].
Figure 3. Changes in the absorption spectrum (c ) 0.5 g/L in methanol) of a methacrylic acid copolymer with MA-TIN 1 (1.8 wt %; for RMA see legend to Figure 2) upon addition of methanolic KOH (c ) 0.25 mol/L). For the respective copolymer of methacrylic acid with MA-TZ 1, the absorption spectrum does not change significantly upon analogous addition of methanolic KOH.
also Otterstedt, ref 4) is utilized to analyze the temperature dependence of the proton-transferred emission (PTE). With the assumption that (1) the proton-transfer quantum yield ΦPT is temperature-independent, at least in the region covered by our experiments (i.e., between 77 and 330 K), and (2) there is only
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J. Phys. Chem., Vol. 100, No. 34, 1996 14471
Figure 5. Four-level scheme for the deactivation characteristics of some ESIPT-type compounds (see e.g. refs 4, 8, 15, and 47).
knr′(T) ) A exp[-Ea/(RT)]
(4)
knr′(T0) ) A exp[-Ea/(RT0)]
(4a)
Inserting (4) and (4a) into (3) finally provides
A(exp[-Ea/(RT)] - exp[-Ea/(RT0)]) ) ΦPTkf′/Φ(T0)(Φ(T0)/Φ(T) - 1) (5) By rearranging eq 5, one obtains eq 6 where τ′(T0), the fluorescence lifetime at T0, is Φ(T0)/(kf′ΦPT) (see eq 2a).
Φ(T0)/Φ(T) - 1 ) Aτ′(T0)(exp[-Ea/(RT)] exp[-Ea/(RT0)]) (6) The last exponential expression in eq 6 may be neglected if T is sufficiently larger than T0. Thus, linearization of eq 6 provides a proper Arrhenius relationship.
ln(Φ(T0)/Φ(T) - 1) ) -Ea/(RT) + C
(7)
If the left-hand side of eq 7 is plotted vs 1/T, a straight line should result from the slope of which the activation energy Ea, and from the intercept of which the constant C may be derived, with
C ) ln(Aτ′(T0)) Figure 4. Temperature dependence of the emission spectra (λexc 333 nm) of solid films of (a) a copolymer of methyl methacrylate with MA-TIN 1 (7.0 wt %); (b) a copolymer of styrene with MA-TZ (5.7 wt %) [RMMA ) OCH2CH(OH)CH2OC(O)C(CH3)(CH2∼∼)CH2C(CH3)(C(O)OCH3)∼∼, for RSTY see legend to Figure 1].
one temperature-dependent radiationless deactivation process knr′(T), originating from the S1′ level, and obeying an Arrhenius equation (see below), the following relation may be derived.15,44,46,47
Φ(T) ) ΦPTkf′/(kf′ + knr′ + knr′(T))
(1)
Here, Φ(T) is the overall quantum yield of the PTE, kf′ the rate constant of the radiative deactivation, and knr′ the rate constant of the temperature-independent radiationless deactivation. Rearrangement of eq 1 gives eq 2, and for the lowest temperature T0 eq 2a.
kf′ + knr′ + knr′(T) ) ΦPTkf′/Φ(T)
(2)
kf′ + knr′ + knr′(T0) ) ΦPTkf′/Φ(T0)
(2a)
Subtracting (2a) from (2) results in eq 3. An Arrhenius-type
knr′(T) - knr′(T0) ) ΦPTkf′/Φ(T0)(Φ(T0)/Φ(T) - 1) (3) behavior may be assumed for knr′(T) and knr′(T0), where A is the Arrhenius constant and Ea the activation energy for the radiationless process.
(7a)
Without this simplification, the relative quantum yield may be expressed as follows:
Φrel ) Φ(T)/Φ(T0) ) 1/[expC‚(exp[-Ea/(RT)] exp[-Ea/(RT0)]) + 1] (8) This equation can be evaluated by a least-squares fit procedure which was also done in addition to the straightforward linear regression according to eq 7 (see Figure 6). For all compounds investigated here, a good correspondence is obtained between the values for Ea and C determined both ways (see Table 1). The correlation coefficients for the linear regression are better than 0.99 in each case. From the data in Table 1 and from Figure 6, it becomes immediately apparent that all stabilizers investigated show an Arrhenius-type behavior for the PTE. The simple four-level scheme which our evaluation is based on thus may be used to satisfactorily describe the deactivation characteristics of these benzotriazole and triazine compounds.51 No significant differences are detected between benzotriazoles and triazines, whether in monomeric or in polymeric state. The values for Ea of 4-5 kJ/mol, and for C of 2-4, are in good agreement with those reported for TIN P and various benzotriazole copolymers (see refs 15, 46, 47, and 52). This supports the suggestion put forth in the literature that the temperature-dependent radiationless deactivation is an intramolecular process47 which, furthermore, seems to be alike for both classes of stabilizer. It was first proposed by Flom and Barbara9 for TIN P that it might be a low-frequency torsional vibration mode involving the phenol
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Figure 6. Temperature dependence of the relative PTE quantum yield for stabilizer copolymers MMA/MA-TIN 1 (2.3 wt %, +) and STY/ MA-TZ 1 (5.7 wt %, ]); experimental values and curves fitted to eq 7 (left) and to eq 8 (right; for parameter values see Table 1).
TABLE 1: Activation Parameters of Various UV Stabilizers Ea/kJ mol-1 substance
temp range/K
a
b
TABLE 2: PTEa Data for Some Mono- and Diaryltriazines at 77 K
C a
b
c
STY /MA-TIN 1 1.0%d 4.3% MMAe/MA-TIN 1 2.3% 7.0% MACf/MA-TIN 1 9.7% MA-TIN 1 in PS 3.5% MA-TIN 1 (powder) TIN 2 (crystals) TIN 3 (crystals) STY/MA-TZ 1 5.7% 42% MMA/MA-TZ 1 2% 54% MAC/MZ-TZ 1 1.7% MA-TZ 1 in PS 5.4% 38% MA-TZ 1 (powder) TTZ 3 (cryst in PMMA) TTZ 9 (cryst in PMMA) e
Figure 7. Emission spectra (77 K; λexc ) 333 nm) of TTZ 4 (s) [R1,3-5 ) OCH3, R2 ) OH], and TTZ 5 (- - -) [R1 ) OCH3, R2-5 ) H] crystals in PMMA and of a MA-TZ 1 powder (-‚-‚-) [R1 ) OCH2CH(OH)CH2OC(O)C(CH3) ) CH2) ) CH2, R2-5 ) CH3];
78-250 77-250
4.4 4.1
4.3 3.6
3.6 3.5
3.5 3.2
77-300 78-250
4.7 4.9
4.6 4.7
3.7 3.8
3.7 3.7
78-310
4.9
4.7
3.6
3.6
77-250 77-300 77-300 77-300
4.0 4.5 4.5 5.6
3.6 4.2 4.6 5.3
2.9 4.0 2.5 3.2
2.7 3.8 2.5 2.9
78-310 77-300
3.6 4.0
3.5 4.3
2.2 1.5
2.2 1.7
77-300 78-350
3.8 4.6
4.2 4.1
1.7 2.1
1.9 1.8
78-310
4.2
4.5
2.6
2.8
77-300 77-300 77-300 77-300 77-300
4.4 5.1 4.0 4.3 5.1
4.6 4.6 3.6 4.6 4.2
1.9 2.4 1.9 2.0 2.4
2.0 2.2 1.7 2.1 2.0
a According to eq 7. b According to eq 8. c Styrene. d Weight percent. Methyl methacrylate. f Methacrylic acid.
group and the heterocyclic moiety, and leading to a distortion of the intramolecular hydrogen bond (see also refs 16-20, 30, 31, 46, 47, and 52). On the basis of these considerations, certain correlations should hold between the activation energy, the quantum yield of the PTE, and the strength of the intramolecular hydrogen bond. The torsional vibration is expected to be the more hindered the stronger the hydrogen bond is in the excited state. This should lead to an increased activation barrier, and to an enhanced PTE quantum yield. An efficient UV absorber, operating by a proton-transfer mechanism, thence has to meet the following criteria: (1) it must incorporate an intramolecular hydrogen bond, strong enough in the ground state not to be broken by polar solvent molecules or polymer moieties, (2) in the excited state, the
substance
em λmax /nm
quantum yield
solvent
NN22 ON22 OO22 MTZ 150 DTZ 150 DTZ 250
476 493 520 510 550 515
0.64 0.54 0.21 0.24 0.009 0.002
3-MPb 3-MP 3-MP MCH/2-MBc MCH/2-MB MCH/2-MB
a Proton-transferred emission. b 3-Methylpentane. c Methylcyclohexane/2-methylbutane 1:1.
hydrogen bond has to be weak enough to permit effective radiationless deactivation but still sufficiently strong to promote a high-yield proton transfer, and (3) the molecule must of course have high extinction coefficients in the long-wavelength UV region to capture as much light as possible. On the other hand, it should be free of absorptions in the visible region which would thwart any application in fields where colourless polymers are required. 3.4. Substituent Effects on the Proton-Transferred Fluorescence of Triaryltriazines. Substituents on the aryl moieties show an interesting influence on the intensity of the PTE which up to now has been observed only with triaryltriazines bearing electron-donating groups such as methyl or methoxy in the nonphenolic aryl substituents. The low-temperature fluorescence spectra in Figure 7 illustrate these findings. TTZ 5, with two unsubstituted phenyl rings, shows no detectable PTE. For MATZ 1, on the other hand, with two xylene rings, a weak but distinct PTE is observed. TTZ 4, finally, with two dimethoxyphenyl groups, displays the most intense emission of all triaryltriazines investigated so far. This result may be rationalized as follows: electron-donating groups enhance the basicity of the heterocyclic nitrogen atoms and thus strengthen the hydrogen bond. So, proton transfer in the excited state is facilitated, and radiationless deactivation impeded.23,27,46,47,52 This influence of electron-donating substituents is also observed in mono-23 and diaryltriazines29,50 as is illustrated by the lowtemperature quantum yields compiled in Table 2 (for the respective structures, see Chart 1). 3.5. Kinetics of MA-TIN 1 Fluorescence. The kinetics of the PTE of solid MA-TIN 1 were measured in the 77-210 K range. A biexponential fit is required to properly describe the
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J. Phys. Chem., Vol. 100, No. 34, 1996 14473
TABLE 3: Lifetimes and Associated Preexponential Factors for MA-TIN 1 Fluorescence (λobs ) 666 nm) temp/K
τa′/ps
A(τa′)
τb′/ps
A(τb′)
77 120 140 170 210
110 75 70 52 ∼30
23 028 34 214 30 221 61 088 59 258
590 400 310 230 130
18 967 20 820 18 255 24 364 23 289
decay profiles. The respective lifetimes are listed in Table 3, with τb′ denoting the main component (with the longer lifetime). The time-resolved spectra show no difference in various time intervals (see Figure 8a-d). Extrapolation of the τb′ values to room temperature yields 70 ps, in good agreement with the 90 ps determined for neat crystalline TIN P.18 The temperature dependence of this lifetime gives the activation energy for the radiationless deactivation process. Only processes originating from the proton-transferred S1′ level are monitored here since they alone can influence the PTE lifetimes. Thus, activation parameters may be calculated in a manner analogous to that described above, by the well-known equation:
Φrel ) Φ(T)/Φ(T0) ) τ′(T)/τ′(T0) This equation applies if quantum yields and lifetimes have identical temperature dependence. Thus, eqs 9 and 10 are obtained from eqs 7 and 8, respectively.
ln(τ′(T0)/τ′(T) - 1) ) -Ea/(RT) + C
(9)
Φrel ) τ′(T)/τ′(T0) ) 1/(expC‚(exp[-Ea/(RT)] exp[-Ea/(RT0)]) + 1) (10) If the values for the main component τb′ are inserted into these equations and the results compared with the activation parameters evaluated from the steady-state emission spectra, a very close agreement is observed (see Tables 1 and 4). Thence, the radiationless deactivation for MA-TIN 1 may indeed be assumed to start from the proton-transferred level S1′. For crystalline TIN P, however, a different behavior is reported (see refs 18 and 45). With the activation parameters Ea and C, the Arrhenius constant A, the temperature-dependent rate constant knr′(T), and the sum of the temperature-independent radiative and radiationless rate constants (kf′ + knr′) may be calculated according to eqs 4 and 7a and eq 11.
kf′ + knr′ ) 1/τ′(T) - knr′(T) ≈ knr′
(ref 53)
(11)
In Table 4, the temperature-independent parameters for MATIN 1 are compiled, Table 5 gives the temperature-dependent rate constants. It becomes immediately apparent from these values that the temperature-dependent process dominates above 140 K. Analysis of the faster decay time τa′ in the same manner as for τb′ affords roughly the same values for the activation energy.52 Both components also appear to have the same fluorescence spectra: the spectra do not change for different time intervals as mentioned above. Thus, one may consider τa′ to characterize MA-TIN 1 molecules with a capacity for more rapid deactivation, perhaps because they are situated in amorphous regions of the solid matrix. These amorphous regions might be due to the long-chain substituent of this molecule hindering perfect crystallization. Solid TIN P, in contrast, displays only one single lifetime in the spectral region of the red fluorescence. This becomes understandable if one considers that TIN P bears no bulky groups and thus forms a purely
Figure 8. Fluorescence of MA-TIN 1 powder (77 K; λexc 366 nm) with different time intervals: (a) 0-0.122 ns; (b) 0.122-0.244 ns; (c) 0.854-1.831 ns; (d) -0.366-8.911 ns.
crystalline solid phase for which decay times are known to be slower than for amorphous states. If TIN P is dissolved in polystyrene, for example, deactivation was found to be almost 2 orders of magnitude faster than that of pure solid TIN P.25 This is possibly due to the stabilizer molecules being packed closer in the crystal than in the liquid state or a solid solution in a polymer matrix. The packing effect may decisively influence the torsional vibration mode and thus the rate of radiationless deactivation.54-56 Extrapolation of the decay time τa′ for MA-TIN 1 to room temperature gives about 20 ps, close to the values found for a number of (amorphous) benzotriazole copolymers with styrene and methyl methacrylate.46,47,52 4. Conclusions (Hydroxyphenyl)triazines such as MA-TZ 1 bearing appropriate substituents, e.g., methacrylic or vinyl groups, undergo radical copolymerization without detriment to the intramolecular
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Figure 9. Fluorescence decay profiles for MA-TIN 1 powder (λexc 366 nm), monitored at 666 nm: (a) T ) 210 K; (b) T ) 140 K; (c) T ) 77 K.
TABLE 4: Activation Parameters and TemperatureIndependent Rate Constants for MA-TIN 1 Ea/kJ mol-1
C
A/s-1
knr′ + kf′/s-1
4.4
3.7
6.7 × 1010
1.8 × 109
TABLE 5: Temperature-Dependent Radiationless Rate Constant for MA-TIN 1 temp/K
knr′(T)/109 s-1
77 120 140 170 210 300
0.07 0.8 1.5 2.8 5.9 12.0
hydrogen bond which is essential for the photostabilizing potential of these compounds. The same has been observed with polymerizable benzotriazoles like MA-TIN 1 (this paper) and other comparable structures.37-39 Both as monomers, and chemically bonded to a polymer backbone, these stabilizers exhibit a weak fluorescence from the proton-transferred state S1′ (φf ∼ 10-4 at room temperature). The quantum yield for the proton-transferred fluorescence of the (hydroxyphenyl)triazines appears strongly affected by the electron-donating capacity of further substituents on the triazine ring (electron density or energy aspect27), as well as by additional alkyl and particularly aryl moieties attached to the triazine ring. These are assumed to accelerate radiationless deactivation by offering additional vibration modes (concept of promoting and accepting modes,18-20,22,57,58 kinetic aspect27). Both factors may contribute
to the drastic decrease observed for the quantum yield of the proton-transferred emission (PTE) in the triaryl compared to the diaryl- and especially monoaryltriazines (cf. Introduction; see also Table 2). The differences observed for the PTE quantum yield within a series of triaryltriazines are held to be due mainly to the electron-density effect since these compounds all have the same number of aryl substituents, and the kinetic effect, i.e., the radiationless deactivation by vibrational modes, thus should be virtually the same. In general, a rather high PTE quantum yield, as e.g. in the mono and diaryltriazines, is contraproductive for good photostabilization since no rapid and thus efficient radiationless deactivation of the excited stabilizer molecule is possible in these cases. Only triaryltriazines with a limited number of electron-donating substituents in the aryl moieties display good stabilizing potential. Benzotriazoles and triaryltriazines show striking analogies in the deactivation mechanism and in spectral properties such as the values for the activation energy (Table 1) and the protontransferred emission. Differences are observed especially in the strength of the intramolecular hydrogen bridge in the ground state, as estimated from the solvent effect on the absorption spectra (cf. section 3.1), and the X-ray and the NMR data.27 The order of magnitude of the PTE lifetime is comparable for the polymerizable stabilizer MA-TIN 1 and for TIN P. It is interesting to note, though, that the PTE of solid MA-TIN 1 displays a nonmonoexponential decay characteristic (cf. the detailed discussion in section 3.5) which is in contrast to the findings for TIN P. The individual decay times of the proton-
Polymeric and Monomeric UV Stabilizers transferred emission of various benzotriazoles in the solid state allow the following classification: pure stabilizers display the slowest rates (MA-TIN 1, 70 ps; TIN P, 90 ps18), followed by stabilizers incorporated into copolymers (30-40 ps47), with the fastest rates being found for physical mixtures of stabilizers with polymers (TIN P in polystyrene, 0.35 ps25,45). Acknowledgment. We thank Drs. J.-L. Birbaum and V. V. Toan, Ciba-Geigy Limited, Marly, Switzerland, for the synthesis and generous gift of substances, and Prof. Dr. W. Funke for helpful discussions. References and Notes (1) Heller, H. J. Eur. Polym. J. Suppl. 1969, 105-132. (2) Heller, H. J.; Blattmann, H. R. Pure Appl. Chem. 1972, 30, 145165; 1974, 36, 141-161. (3) Williams, D. L.; Heller, A. J. Phys. Chem. 1970, 74, 4473-4480. (4) Otterstedt, J.-E. A. J. Phys. Chem. 1973, 58, 5716-5725. (5) Klo¨pffer, W. AdV. Photochem. 1977, 10, 311-358. (6) Merrit, C.; Scott, G. W.; Gupta, A.; Yavrouian, A. Chem. Phys. Lett. 1980, 69, 169-173. (7) Huston, A. L.; Scott, G. W.; Gupta, A. J. Chem. Phys. 1982, 76, 4978-4985. (8) 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. (9) Flom, S. R.; Barbara, P. F. Chem. Phys. Lett. 1983, 94, 488-493. (10) Werner, T.; Kramer, H. E. A., Kuester, B.; Herlinger, H. Angew. Makromol. Chem. 1976, 54, 15-29. (11) Kuester, B.; Tschang, Ch.-J.; Herlinger, H. Angew. Makromol. Chem. 1976, 54, 55-70. (12) Werner, T.; Kramer, H. E. A. Eur. Polym. J. 1977, 13, 501-503. (13) Werner, T. J. Phys. Chem. 1979, 83, 320-325. (14) 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. (15) Woessner, G.; Goeller, G.; Kollat, P.; Stezowski, J. J.; Hauser, M.; Klein, U. K. A.; Kramer, H. E. A. J. Phys. Chem. 1984, 88, 5544-5500. (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) Goeller, G.; Rieker, J.; Maier, A.; Stezowski, J. J.; Daltrozzo, E.; Neureiter, M.; Port, H.; Wiechmann, M.; Kramer, H. E. A. J. Phys. Chem. 1988, 92, 1452-1458. (19) Kramer, H. E. A. In Photochromism-Molecules and Systems; Du¨rr, H., Bouas-Laurant, H., Eds.; Elsevier: Amsterdam, 1990; pp 654-684; and references cited there. (20) Kramer, H. E. A. In Book of Abstracts; 13th International Conference on Advances in the Stabilization and Degradation of Polymers, Lucerne, Switzerland, May 22-24; Patsis, A. V., Ed.; pp 59-78. (21) (a) Catala´n, J.; Fabero, F.; Guijarro, M. S.; Claramunt, R. M.; Santa Maria, M. D.; de la Concepcion Foces-Foces, M.; Cano, F. H.; Elguero, J.; Sastre, R. J. Am. Chem. Soc. 1990, 112, 747-759, and references cited there. (b) Catala´n, J.; Perez, P.; Fabero, F.; Wilshire, J. F. K.; Claramunt, R. M.; Elguero, J. J. Am. Chem. Soc. 1992, 114, 964-966. (c) Catala´n, J.; Del Valle, J. C.; Fabero, F.; Garcia, N. A. Photochem. Photobiol. 1995, 61, 118-123. (22) Shizuka, H.; Matsui, K.; Hirata, J.; Tanaka, I. J. Phys. Chem. 1977, 81, 2243-2246. (23) Shizuka, H.; Machii, M.; Higaki, Y.; Tanaka, M.; Tanaka, I. J. Phys. Chem. 1985, 89, 320-326. (24) Gormin, D.; Heldt, J.; Kasha, M. J. Phys. Chem. 1990, 94, 11851189. (25) (a) Wiechmann, M.; Port, H.; Laermer, F.; Frey, W.; Elsaesser, T. Chem. Phys. Lett. 1990, 165, 28-34. (b) Wiechmann, M.; Port, H.; Frey, W.; Laermer, F.; Elsaesser, T. J. Phys. Chem. 1991, 95, 1918-1923. (c) Wiechmann, M.; Port, H. J. Lumin. Chem. 1991, 48/49, 217-219. (26) Bigger, S. W.; Ghiggino, K. P.; Leaver, I. H.; Scully, A. D. J. Photochem. Photobiol. 1987, A 40, 391-399. (27) Stueber, G. J.; Kieninger, M.; Schettler, H.; Busch, W.; Goeller, B.; Franke, J.; Kramer, H. E. A.; Hoier, H.; Henkel, S.; Fischer, P.; Port, H.; Hirsch, T.; Rytz, G.; Birbaum, J.-L. J. Phys. Chem. 1995, 99, 1009710109. (28) Stu¨ber, G. J. Ph.D. Thesis, Universita¨t Stuttgart, 1994.
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